United States
              Environmental Protection
             Office of Research
             and Development
             Washington, DC 20460
September 1996
Pollution Prevention in the
Paints and Coatings

                                                             September 1996

Pollution Prevention in the Paints and Coatings Industry
                   U.S. Environmental Protection Agency
                   Office of Research and Development
               National Risk Management Research Laboratory
                Center for Environmental Research Information
                            Cincinnati, Ohio
                                                           Printed on Recycled Paper

       The U.S. Environmental Protection Agency through its Office of Research and Development
funded and managed the research described here under Contract #68-3-0315 to Eastern Research Group,
Inc. It has been subjected to the Agency's peer and administrative review and has been approved for
publication as an  EPA document. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.

       The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency strives to
formulate and implement actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. To meet this mandate, EPA's research program is providing
data and technical support for solving environmental problems today and building a science knowledge
base necessary to manage our ecological resources wisely, understand how pollutants affect our health,
and prevent or reduce environmental risks in the future.

       The National Risk Management Research Laboratory is the Agency's center for investigation of
technological and management approaches for reducing risks from threats to human health and the
environment. The focus of the Laboratory's research program is on methods for the prevention and control
of pollution to air, land, water and subsurface resources; protection of water quality in public water systems;
remediation of contaminated sites and ground water; and prevention and control of indoor air pollution. The
goal of this research effort is to catalyze development and implementation of innovative, cost-effective
environmental technologies;  develop scientific and engineering information needed by EPA to support
regulatory and policy implementation of environmental regulations and strategies.

       This publication has  been produced as part of the Laboratory's strategic long-term research plan.
It is published and made available by EPA's Office of Research and Development to assist the user
community and to link researchers with their clients.

       This manual, Pollution Prevention in the Paints and Coatings Industry, funded through the Center
for Environmental Research  Information, is a pollution prevention guidance manual for processes and
waste reduction in paints and coatings industry.
                                    E. Timothy Oppelt, Director
                                    National Risk Management Research Laboratory

        The paints and coatings industry represents a significant source of multimedia pollution through the
wide use of solvent-based process materials and the extensive amounts of wastewater generated by the
operations. This manual presents recommended practices for minimizing the generation of pollution in this

        Regulations emphasizing source reduction of pollutants at the federal, state, and local level, are
driving facility operators to investigate the use of alternative cleaning formulations and paint systems.
Aqueous degreasers and powder coatings are two examples of efforts to reduce toxic air emissions and
control costs associated with the treatment of contaminated effluent.

        Many small and mid-sized facilities have few opportunities to take advantage of technology transfer
within the industry. The information in this manual can help operators assess operations and processes for
pollution prevention options in using "cleaner" technologies and more efficient management practices.
Suggestions contained within this manual can guide improvements in quality and efficiency, indirectly
impacting prevention in terms of reduced wastes.

        The manual has three general sections:
               •       An overview of the industry and an introduction to pollution prevention for paints
                      and coatings operations;

               •       Pollution prevention considerations;

               •       Case studies emphasizing approaches for reducing process waste.

Appendixes provide a list of suppliers of aqueous and semi-aqueous degreasers and equipment,
methodology for specified dilution ratio calculations, and a spreadsheet for factoring transfer efficiency
considerations into application processes.

        The audience for this document are facility operators and managers, manufacturing process
managers, painters, and environmental engineers.  Small and medium-size facilities that do not have
process engineers on staff have much to gain by implementing recommendations in this manual.

        This report was submitted in fulfillment of Contract #68-3-0315 by Eastern Research Group, Inc.
under the sponsorship of the U.S. Environmental Protection Agency. This report covers a period from
December, 1993, to September, 1996, and work was completed as of September 30,1996.

            Foreword	 iii
            Abstract	 iv
            Figures	 xiv
            Tables	 xvi
            Conversion Factors	 xviii
            Acknowledgments	 xix
SECTION 1  Overview	 1
Chapter 1   Introduction		2
            1.1   Pollution Prevention in the Paints and Coatings Industry	 2
            1.2   The Audience for This Document	 2
            1.3   The Organization of This Document	 2
Chapter 2   Overview of Paints and Coatings Operations	 4
            2.1   Introduction	 4
            2.2   Operations for Miscellaneous Metal Workpieces	 4
                 2.2.1   Priming Only	 4
                 2.2.2   Priming and Topcoating	 5
                 2.2.3   Surface Preparation	 7
                 2.2.4   Application of Paint Coating Systems	 9
                 2.2.5   Abatement Equipment	 9
            2.3   Operations for the Automotive Industry	 10
                 2.3.1   Process Overview	 10
                 2.3.2   Paint Coating Systems and Application Processes	 10
                 2.3.3   Abatement Equipment	 12
            2.4   Operations for Plastic Parts	 12
                 2.4.1   Surface Preparation	 12
                 2.4.2   Coatings Systems	 12
                 2.4.3   Application Equipment	 12
                 2.4.4   Abatement Equipment	 13
            2.5   Custom Coating Operations	 13

                                        Contents (continued)
             2.6  References	  13
             2.7  Additional Reading	  13
SECTION 2  Pretreatment Factors	  15
Chapter 3   Adhesion as a Critical Factor	  16
             3.1  Introduction	  16
                 3.1.1    Pollution Prevention Considerations	  16
             3.2  Corrosion of Metals and Alloys	  16
                 3.2.1    Basics of the Corrosion Process	  16
                 3.2.2    The Science Behind Corrosion	  17
                 3.2.3    Fundamentals of Corrosion Prevention	  18
             3.3  Preventing Corrosion by Ensuring Proper Adhesion	  18
                 3.3.1    Mechanisms of Adhesion	  18
                 3.3.2    The Importance of Proper Wetting	  18
                 3.3.3    The Role of Surface Contaminants	  19
             3.4  Adhesion Considerations Specific to Plastic Substrates	  21
             3.5  References	  21
Chapter 4   Considerations Regarding Vendor-Supplied Materials	  23
             4.1  Introduction	  23
                 4.1.1    Pollution Prevention Considerations	  23
                 4.1.2    Decision-Making Criteria	  23
             4.2  Raw Materials	  23
                 4.2.1    Protective Coatings and Treatments	  23
                 4.2.2    Storage	  24
             4.3  Components and Parts	  24
                 4.3.1    Protective Coatings and Primers	  24
                 4.3.2    Storage	  25
             4.4  Just-in-Time Delivery	  25
            4.5  References	  25
Chapter 5   Surface Degreasing: Alternatives  to Conventional Solvent-Based Methods	  26
            5.1  Introduction	  26
                 5.1.1    Pollution Prevention Considerations	  26
                 5.1.2    Decision-Making Criteria	  26
            5.2  Basic Practices and Regulatory Considerations	  26
                 5.2.1   Typical Oils and Grime on Substrates	  26

                                        Contents (continued)
                 5.2.2   Basic Cleaning Approaches	  28
                 5.2.3   Selecting a Cleaning Approach	  28
                 5.2.4   Regulatory Overview	  29
            5.3  Solvent-Based Methods	  29
                 5.3.1   Vapor-Solvent Degreasing	  29
                 5.3.2   Degreasing With Liquid Solvent (Cold Cleaning and Solvent Wiping)	  32
            5.4  Aqueous Methods	  34
                 5.4.1   Aqueous Degreasing	  34
                 5.4.2   Semi-aqueous Degreasing	  37
            5.5  Case Examples	  38
                 5.5.1   Frame Manufacturer	  38
                 5.5.2   Military Contractor	  39
                 5.5.3   Lift Truck Manufacturer	  39
            5.6  References	  40
Chapter 6   Phosphating Metal Surfaces:  Process Efficiency and Waste Minimization	  41
            6.1  Introduction	  41
                 6.1.1   Pollution Prevention Considerations	  41
                 6.1.2   Decision-Making Criteria	  41
            6.2  Process Basics and Best Management Practices	  41
                 6.2.1   Introduction	  41
                 6.2.2   Coating Quality and Basic Parameters	  44
                 6.2.3   Best Management Practices	  45
            6.3  Phosphating  Methods	  45
                 6.3.1   Iron Phosphating	  45
                 6.3.2   Zinc  Phosphating	  47
                 6.3.3   Wash Primers as an Alternative to Phosphating	  48
            6.4  Waste Minimization and Treatment	  48
                 6.4.1   Minimization	  48
                 6.4.2   Treatment	  49
            6.5  Additional Considerations Related to Phosphating	  49
                 6.5.1   Choosing a Phosphate Formulation and Qualifying the Phosphate Coating	  49
                 6.5.2   Degreasing Before Phosphating	  49
                 6.5.3   Design of an Immersion Tank System	  50
                 6.5.4   Design of a Spray Washer System	 50
                 6.5.5   Process and Quality Control Measures	  51

                                        Contents (continued)
             6.6   References	 51
             6.7   Additional Reading	 52
Chapter 7   Rinsing Process Efficiency and Alternatives to Chromate-Based Sealers	 53
             7.1   Introduction	 53
                  7.1.1    Pollution Prevention Considerations	 53
                  7.1.2   Decision-Making Criteria	 53
             7.2   Rinsing	 53
                  7.2.1    Rinsing Basics and Best Management Practices	 55
                  7.2.2   Counter-Flow Rinsing	 57
             7.3   Sealing	 59
                  7.3.1    Sealing Basics	 59
                  7.3.2   Chromate-Based Sealing Rinses Versus Nontoxic Alternatives	 60
             7.4   Case Example	 61
             7.5   References	 62
             7.6 'Additional Reading	 62
Chapter 8 {ซ Abrasive Blast Cleaning  of Metal Surfaces: Process Efficiency	63
             8.1   Introduction	 63
                  8.1.1    Pollution Prevention Considerations	 63
                  8.1.2    Decision-Making Criteria	 63
             8.2   Process Basics	 63
                  8.2.1    Introduction	 63
                  8.2.2    Abrasive Blasting Systems	 64
                  8.2.3    Media Recycling	 65
                  8.2.4    Blast Profile as a Critical Factor	 66
                  8.2.5    Types of Abrasive Media and Selection Criteria	 67
                  8.2.6    Blast Cleaning Standards	 68
             8.3   Best Management Practices	 69
             8.4   Process Variations  (With Case Examples)	 70
                 8.4.1    Abrasive Blasting Preceded by Degreasing	 70
                 8.4.2    Abrasive Blasting Followed  by Phosphating	 71
             8.5   References	 71
SECTION 3  Application Process Factors	 73
Chapter 9   Transfer Efficiency as It Affects Air, Water, and Hazardous Waste Pollution	 74
             9.1  Introduction	 74

                                         Contents (continued)
                  9.1.1   Pollution Prevention Considerations	  74
                  9.1.2   Decision-Making Criteria	  74
             9.2   Benefits of Improved Transfer Efficiency	  74
                  9.2.1   Reductions in Pollution and Related Factors	  75
                  9.2.2   Reduction  in Costs	  76
             9.3   Methods for Measuring Transfer Efficiency	  77
                  9.3.1   Defining Parameters Before Commencing the Transfer Efficiency Test	  77
                  9.3.2   Using the Weight (Mass) Method	  78
                  9.3.3   Using the Volume Method	  79
             9.4   The Effects of Common Spray Guns on Transfer Efficiency	  79
                  9.4.1   Conventional Air Atomizing Spray Guns	  79
                  9.4.2   High  Volume, Low Pressure Air Atomizing Spray Guns	  79
                  9.4.3   Airless Spray Systems	  80
                  9.4.4   Air-Assisted Airless Spray Guns	  80
                  9.4.5   Electrostatic Spray Guns	  81
             9.5   Pollution Prevention Strategies To Improve Transfer Efficiency	  81
                  9.5.1   Strategies That Require No Capital Expenditure	  81
                  9.5.2   Strategies That Require Nominal Capital Expenditure	  83
                  9.5.3   Strategies That Require Moderate or Significant Expenditure	  85
             9.6   References	  85
             9.7  Additional Reading	  85
Chapter 10  Liquid Compliant Coating Technologies	  86
             10.1  Introduction	;	  86
                  10.1.1   Pollution Prevention Considerations	  86
                  10.1.2   Decision-Making Criteria	86
             10.2 Guidelines for Choosing Best Management Practices	  86
                 10.2.1   Liquid Versus Powder Coatings	  86
                 10.2.2   Water-Borne Versus Solvent-Borne Coatings	  90
                 10.2.3   Air/Force Dry Versus Bake	—	  90
                 10.2.4   Single-Component Versus Plural-Component	  90
             10.3 Water-Borne Coatings	  94
                 10.3.1   Overview	  94
                 10.3.2   Water-Borne Air/Force Dry Alkyds, Acrylics, Acrylic-Epoxy Hybrids	  95
                 10.3.3   Water-Borne Epoxy Water-Reducible Air/Force Dried Coatings	  97
                 10.3.4   Polyurethane Dispersions	  98

                                        Contents (continued)
                 10.3.5 Water-Borne Baking Finishes—Alkyd, Alkyd-Modified, Acrylic, Polyester	  99
             10.4 Solvent-Borne Coatings	  100
                 10.4.1 Overview	  100
                 10.4.2 Solvent-Borne Alkyds and Modified Alkyds That Air or Force Dry	  100
                 10.4.3 Alkyd Derivative Combinations That Cure by Baking	  101
                 10.4.4 Catalyzed Epoxy Coatings	  102
                 10.4.5 Catalyzed Two-Component  Polyurethanes	  104
                 10.4.6 Moisture Curing  Polyurethanes	  105
             10.5 Specialized Coatings	  105
                 10.5.1 Overview	  105
                 10.5.2 Autodeposition	  105
                 10.5.3 Electrodeposition	  107
                 10.5.4 Radiation Cured Coatings	  108
                 10.5.5 Vapor Injection Cure	  110
                 10.5.6 Supercritical CO2 for Paints and Coatings	  110
             10.6 Emerging Technologies	  111
             10.7 Selecting the Best Technology for Specific Applications	  112
             10.8 References	  112
             10.9 Additional Reading	  112
Chapter 11   Powder Coatings	  114
             11.1  Introduction	  114
                 11.1.1  Pollution Prevention Considerations	  114
                 11.1.2 Decision-Making Criteria	  114
             11.2  Suitability for Specific Applications	  114
                 11.2.1  Suitable Applications	  114
                 11.2.2 Unsuitable Applications	  114
             11.3  The Powder Coating Process	  115
                 11.3.1  Applying the Coating	  116
                 11.3.2  Curing the Coated Part	  116
             11.4  Costs Associated With Powder Coating	  117
                 11.4.1  Profiles of Economic Impact of Switching to  Powders	  118
            11.5  Advantages and  Limitations of Powder Coatings	  118
                 11.5.1  Advantages	  118
                 11.5.2  Limitations	  119
            11.6  References	  119

                                        Contents (continued)
Chapter 12  Viscosity Management for Pollution Prevention	 121
             12.1  Introduction	 121
                  12.1.1  Pollution Prevention Considerations	 121
             12.2  Description of Viscosity.	 121
             12.3  Measuring Viscosity	 122
                  12.3.1  Zahn Cup	 122
                  12.3.2  Ford Cup	 123
                  12.3.3  Brookfield Viscometer	 124
             12.4  Guidelines for Best Management Practices (BMPs)	 125
                  12.4.1  Measuring Viscosity and Temperature	 125
                  12.4.2  Specifying a Viscosity Range	 125
                  12.4.3  Developing Acceptable Alternatives	 126
                  12.4.4  Using Heat To Reduce Viscosity	 126
                  12.4.5  Minimizing Waste Disposal	 126
                  12.4.6  Recognizing Thixotropic Properties	 126
             12.5  Managing Viscosity Differences for Different Coatings	 127
             12.6  Problems Associated With Viscosity Mismanagement	 128
                  12.6.1  Effect of Film Thickness Variations on Color, Gloss, and Drying Time	 128
                  12.6.2  Effect of Viscosity Differences on Metallic Colors	 128
                  12.6.3  Effects of Too Low a Viscosity	 128
             12.7  Strategies That Optimize Factors Affecting Viscosity	 129
                  12.7.1  Effect of Plural-Component, In-Line Mixing	 129
                  12.7.2  Effect of Dilutant (Reducer or Thinner) on Viscosity	 129
                  12.7.3  Effect of Temperature on Viscosity	 129
                  12.7.4  Effect of Batch Mixing of Plural-Component Coatings	 131
                  12.7.5  Methods for Increasing the Pot-Life of Batch-Mixed Plural-Component
                         Coatings	 132
             12.8  References	 133
Chapter 13  Minimizing Solvent Usage for Equipment Clean-Up	 134
             13.1  Introduction	 134
                 13.1.1   Pollution Prevention Considerations	 134
                 13.1.2  Decision-Making Criteria	 134
             13.2 Solvent  Recycling	 134
             13.3 Minimizing Emissions of Hazardous Air Pollutants	 135
                 13.3.1   Strategies To Minimize HAP Emissions	 136

                                        Contents (continued)
            13.4 Regulatory Provisions	 136
                 13.4.1  South Coast Rule 1107, (b)(3-7)	 136
            13.5 Process for Cleaning Spray Guns and Fluid Hoses	 137
            13.6 References	 138
Chapter 14  Paint Stripping: Alternatives to Solvent-Based Methods	 139
            14.1 Introduction	 139
                 14.1.1  Pollution Prevention Considerations	 139
                 14.1.2  Decision-Making Criteria	 139
            14.2 Process Basics	 139
            14.3 Solvent-Based, Aqueous, and Semi-aqueous Methods	 141
                 14.3.1  Solvent-Based Methods	 141
                 14.3.2  Aqueous Methods	 141
                 14.3.3  Semi-aqueous Methods	 142
            14.4 "Cleaner" Technologies: Alternatives to Conventional Methods	 142
                , 14.4.1  Impaction Methods	 142
                 14.4.2  Abrasion Method	 144
                                       Contents (continued)
                 15.5.3  Selecting the Appropriate Chemicals	 157
                 15.5.4  Methods for Treating Water From Water-Wash Booths	 157
            15.6 Baffle Booths	 158
            15.7 Best Management Practices To Minimize Coating Defects in the Spray Booth	 158
                 15.7.1  Poor Wrap	 158
                 15.7.2  Dust and Dirt in the Finish	 158
                 15.7.3  Water Spots in the Finish	 158
                 15.7.4  Haziness That Detracts From the Gloss	 159
                 15.7.5  Dry Overspray on the Finish	 159
                 15.7.6  Non-uniform Coating Finish With Gloss Patches, Orange Peel, Voids, etc	 159
            15.8 References	 159
            15.9 Additional Reading	 159
SECTION 4 Problem Solving	 161
Chapter 16 Problem Solving: Case Studies of Some Typical Paint Facilities	 162
            18.1 Introduction	 162
            16.2 Case Study #1: Flaking Paint on Tool Boxes	 162
        f'        16.2.1  Background of Problems	 162
                 16.2.2  Possible Solutions	 162
                 16.2.3  Pollution Prevention Opportunities	 163
            16.3 Case Study #2: High Reject Rate and VOC Emissions From Aluminum Lamp Housings..... 164
                 16.3.1  Background of Problems	 164
                 16.3.2  Possible Solutions	 164
                 16.3.3  Pollution Prevention Opportunities	 166
            16.4 Case Study #3: Start-Up Problems for Automotive Component Manufacturer	 166
                 16.4.1  Background of Problems	 166
                 16.4.2  Possible Solutions		 167
                 16.4.3  Pollution Prevention Opportunities	 169
            16.5 Conclusion	 169
Appendix A Selected List of  Suppliers of Aqueous and Semi-aqueous Degreaser
            Formulations and Equipment	 172
Appendix B How To Calculate the Flow Rate of Rinse Water Required To Achieve a
            Specified Dilution Ratio	 178
Appendix C Spreadsheet Model To Estimate Transfer Efficiency		 180
Index	 183

Figure                                                                                            Page

2-1    Schematic of a typical process for priming metal parts	5
2-2    Schematic of a process for two-stage application of a primer-topcoat system	7
2-3    Schematic of a process for single-stage application of a primer-topcoat system	8
2-4    Schematic of a three-stage iron phosphating process	8
2-5    Schematic of a five-stage iron or zinc phosphating  process	9
2-6    Schematic of a typical conversion coating process  for aluminum workpieces	9
2-7    Schematic of a typical process for applying a primer-topcoat system in the automotive industry.	11
2-8    Schematic of a typical process for applying a zinc phosphate coating in the automotive industry.	11
3-1    Movement of electrons and ions in corrosion process involving a galvanic couple	17
3-2    Mechanism of corrosion on a steel substrate	17
3-3    Coating contact angle relative to wetting of surface	19
3-4    Cross-sectional view of surface wetting	20
3-5    Cross-sectitfnal view of surface spading caused by scale	20
3-6    Cross-sectional view of compromising effect of weld slag and spatters on a coating	21
5-1    Schematic of a typical solvent vapor degreasing process	31
5-2    Schematic of a typical cold cleaning degreasing process	33
6-1    Cross-sectional view of conversion coating  process using iron or zinc phosphate	43
6-2    Immersion rinse system schematic	50
6-3    Schematic of a conveyorized paints and coatings operation	51
7-1    Schematic of three-step post-degreasing rinse stage	56
7-2    Schematic of counter-flow rinsing	58
7-3    Dilution ratio as a function of time for different tank sizes	58
7-4    Graph of rinse water flow rate required to dilute drag-in  stream at 1 gal/min for first rinse bath only..... 60
7-5    Graph of counter-flow rinse water flow rate  required to dilute drag-in stream at 1 gal/min for
       subsequent rinse baths	60
7-6    Schematic of post-phosphating rinsing process with sealing rinse bath	60
8-1    Schematic of an abrasive blasting operation with a  media recovery system.	65
9-1    Effect of transfer efficiency on VOC emissions	.77
9-2    Effect of fluid flow rate on residence time in gun	82
9-3    Effect of fan width	82
9-4    Effect of leading and trailing edges on transfer efficiency	83
9-5    Deliberate overspray at top of first stroke and bottom of last stroke	84
10-1   VOCs in water-borne coatings	95
10-2   Hardness scale for solvent-borne coatings	102
12-1   The concept of viscosity (2)	121
12-2   Thixotropy.	122

                                         Figures (continued)

Figure                                                                                            Page

12-3   Zahn cups	123
12-4   Ford viscosity cups	124
12-5   Brookfield viscometer	124
12-6   Effect of solvent reduction on viscosity.	127
12-7   Effect of reduction on viscosity for water-borne coatings	127
12-8   Plural-component proportioning system	130
12-9   Effect of solvents and diluents on viscosity.	130
12-10   Effect of temperature on viscosity.	131
12-11  Effect of viscosity on single- and plural-component coatings	132
12-12  Effect of temperature on pot-life of plural-component coatings	132
13-1   Typical solvent distillation unit	135
13-2   Typical spray gun cleaner	137
15-1   Spray booth design concepts	149
15-2   Cross-draft  spray booth	150
15-3   Side-by-side cross-draft booths	150
15-4   Down  draft  spray booth	151
15-5   Semi-down  draft spray booths	152
15-6   Cost of filter disposal based on holding capacity.	156
16-1   Example of  power-and-free conveyor.	164
B-1    Schematic of counter-flow rinsing	179

Table                                                                                           Page

2-1    Major Group 33: Primary Metal Industries	4
2-2    Major Group 34: Fabricated Metal Products, Except Machinery and Transportation Equipment	4
2-3    Major Group 35: Industrial and Commercial Machinery and Computer Equipment	6
2-4    Major Group 36: Electronics and Other Electrical Equipment and Components, Except
       Computer Equipment	 6
2-5    Major Group 37: Transportation Equipment	6
2-6    Major Group 38: Measuring, Analyzing, and Controlling Instruments; Photographic, Medical,
       and Optical Goods; Watches and Clocks	6
2-7    Major Group 39: Miscellaneous Manufacturing Industries	7
2-8    Typical Coating Technologies for Miscellaneous Metals Parts	9
2-9    Most Common Manual Spray Guns	9
2-10   Most Common Automated Coating Processes	9
2-11   Common Spray Booth Designs	10
2-12   Typical Abatement Control Devices for Painting Facilities	10
3-1    Electromotive Force Series	17
3-2    Approximate Surface Tension of Substances in Contact With Their Vapor	19
3-3    Approximate Surface Tension of Metallic Elements in Inert Gas	19
3-4    Surface Tensions of Coating Ingredients Versus Plastic Substrates	22
4-1    Decision-Making Criteria Regarding Vendor-Supplied Materials	23
5-1    Decision-Making Criteria Regarding Surface Degreasing Process Efficiency and Alternatives to
       Conventional Solvent-Based Methods	27
5-2    Relative Boiling Points of Principal Degreasing Solvents	31
5-3    Typical Organic Solvents Used in Degreasing Operations	33
5-4    Considerations for Aqueous Degreasing	35
5-5    Selected Aqueous Degreasers	35
5-6    Typical Organic Constituents in Semi-aqueous Degreasers	38
6-1    Decision-Making Criteria Regarding Phosphating of Metal Surfaces	42
6-2    Typical Spray Phosphating Production Rates in the Appliance Industry	44
6-3    Process Line for Pretreatment of Complex Workpieces in Electrocoating Operation	47
6-4    Process Line for Pretreatment of Simple Workpieces in Electrocoating Operation	47
6-5    Corrosion Resistance of Zinc Phosphate Coatings on Steel and Electrogalvanized Steel	48
6-6    Pretreatment Standards for Existing Sources That Electroplate Common Metals and Discharge
       38,000 Liters or More of Wastewater per Day	49
6-7    Pretreatment Standards for Existing Sources Involved in Metal Finishing Operations.	49
7-1    Decision-Making Criteria Regarding Rinsing Processes	54
7-2    Counter-Flow Rates for Workpieces With a 1 gal/min Drag-In	59

                                         Tables (continued)


7-3    Total Percentage Reduction in Flow Rate From One Rinse Tank to the Next for Workpieces With
       a 1 gal/min Drag-In	59
7-4    Counter-Flow Rates for Workpieces With a 2 gal/min Drag-In	59
7-5    Counter-Flow Rates for Workpieces With a 0.5 gal/min Drag-In	59
8-1    Decision-Making Criteria Regarding Abrasive Blasting Processes	64
8-2    Recycle Frequency of Abrasives  	65
8-3    Selected Screen Sizes	66
8-4    Guide for Selected Abrasive Media	68
8-5    Sample Specification Sheet for Steel Shot)	68
8-6    Sample Specification Sheet for Steel Grit  	69
8-7    Comparison of Designations for Blast Cleaning Finishes	69
9-1    Decision-Making Criteria Regarding Transfer Efficiency	 75
9-2    Effect of Transfer Efficiency on VOC Emissions	76
9-3    Annual Cost Savings Due to Transfer Efficiency (TE) Improvement From 30% to 45%	77
10-1   Decision-Making Criteria Regarding Liquid Compliant Coatings	87
10-2   Advantages of Liquid Over Powder Coatings	87
10-3   Advantages of Powder Over Liquid Coatings	88
10-4   Advantages of Water-Borne Over High Solids Solvent-Borne Coatings	91
10-5   Advantages of High Solids Solvent-Borne Coatings Over Water-Borne Coatings	92
10-6   Air/Force Dry Versus Bake	93
10-7   Typical RACT Limits for Miscellaneous Metal Parts Coatings	93
10-8   Single-Component Versus Plural-Component Coatings	94
11-1   Decision-Making Criteria Regarding Powder Coating	..	115
12-1   Zahn Cup Orifice Sizes	123
13-1   Decision-Making Criteria Regarding Minimizing Solvent Usage for Equipment Clean-Up	135
13-2   High-Boiling Solvents	136
14-1   Decision-Making Criteria Regarding Paint Stripping  Operations	140
15-1   Decision-Making Criteria Regarding Minimizing Pollution in Spray Booths		148
15-2   Efficiency and Holding Capacity of Dry Filters	154
15-3   Cost of Waste With 65 Percent Transfer Efficiency		154
15-4   Cost of Waste With 30 Percent Transfer Efficiency	155
C-1    Table of Assumptions	180
C-2    Calculation of Costs (TE = 30%)	180
C-3    Calculation of Costs (TE = 45%)	181
C-4    Formulas Used To Perform Calculations	181

                                  Conversion Factors
Units of measurement used throughout this document can be converted to SI units using the
following conversion factors:
To convert...
multiply by...
cubic feet
degrees Fahrenheit
quarts, liquid
pounds per,cubic foot
pounds per cubic foot
square inches
U.S. gallons
cubic meters
degrees Celsius
to liters
kilograms per cubic meter
to liters
square inches
metric tons
2.831685 x10'2
t,c=(t,F- 32)71.8

Doug Williams of EPA's Office of Research and Development, Center for Environmental Research
Information, was responsible for the development and review of this document.  Ron Joseph, of
Ron Joseph and Associates, Inc., Saratoga, California, served as the document's author under a
consulting agreement with Eastern Research Group, Inc. (ERG), of Lexington, Massachusetts. Jeff
Cantin was ERG's project manager for the task. ERG also edited the document and prepared it for

Section 1

                                              Chapter 1
 1.1    Pollution Prevention in the Paints
       and Coatings Industry

 Given the wide use of solvent-based process materials
 and the extensive amounts of wastewater generated by
 paints and coatings operations, this industry represents
 a significant source of multimedia pollution. This manual
 presents  recommended  practices  for minimizing the
 generation of pollution in paints and coatings operations.

 Many facility operators are actively investigating the use
 of alternative cleaning formulations and paint systems,
 such as aqueous degreasers and powder  coatings, in
 an effort to reduce toxic air emissions and control costs
 associated with the treatment of contaminated effluent.
 These efforts are being driven in part by regulations at
 the federal, state,  and  local level  aimed at preventing
 pollution at its source. In particular, the paints and coat-
 ings and other industries must achieve compliance with
 the Clean Air Act and amendments. Along with prevent-
 ing pollution at its source, companies are increasingly
 encouraged to limit the  generation of waste through
 recycling and enhanced management practices.

 Because of the diversity in the types of  paints and
 coatings operations, many operators of small and mid-
 sized facilities have few opportunities to take advantage
 of technology transfer within the industry. The  informa-
tion in this manual should help operators to perform a
complete investigation  of pollution prevention  (i.e., P2
 as referred to by government and  industry) factors in
their processes and to consider using "cleaner" tech-
 nologies and more efficient management practices.

Additionally, this manual  presents  numerous  sugges-
tions concerning management practices that may ap-
pear  to  have  no direct  connection  with  pollution
prevention. Nonetheless,  many operators in this indus-
try have found that by making improvements in the
name of quality and efficiency, additional benefits can
be realized in terms of reduced waste.

The manual covers all basic aspects of a paints and
coatings operation. Pollution prevention strategies dis-
cussed lead both directly and indirectly to waste minimi-
zation.  The  majority  of these  strategies   can  be
implemented without the need for major capital expen-
ditures. Often by modifying the approach to a conven-
tional practice, considerable waste and cost reduction
benefits can be realized.

1.2  The Audience for This Document

As presented, the suggestions in this document  are
directed primarily to  facility operators and managers,
regardless of whether their paints and coatings proc-
esses are conducted on an intermittent or continuous
basis.  Nonetheless, the material  also is intended for
manufacturing process managers, environmental engi-
neers, and painters themselves. Operators of small and
medium-sized facilities likely will have the most to gain
by implementing  recommendations presented  in this
document, particularly facilities that do not have a full-
time paints and coatings process engineer on staff. Most
large operations, such as original equipment manufac-
turers with in-house expertise, already will have systems
in place that  incorporate most of these strategies. Al-
though many aspects of paints and coatings processes
are chemical specific, the  vast majority of information
presented in  this document can  be understood and
acted upon  regardless of whether the reader has a
science background.

1.3  The Organization of This Document

This manual is divided into four sections. This first sec-
tion provides a general introduction to pollution preven-
tion in relation to paints and coatings operations along
with an overview of the industry (Chapter 2). The sec-
tions that follow address pollution prevention considera-
tions in the context of the basic process flow for paints
and coatings operations. Thus, the discussion proceeds
from pretreatment stages, such  as  degreasing  and
phosphating,  to the various methods of paint applica-
tion. The final section  presents a selection of case stud-
ies that emphasize approaches for reducing process

Section Two on pretreatment factors begins with a gen-
eral discussion about the importance of proper adhesion
of the coating to the substrate for minimizing pollution in
paints and coatings operations (Chapter 3). The chapter
introduces the concept of "right-first-time" processing as

a  management practice that focuses on  avoiding re-
works of coatings that fail because the workpiece was
inadequately prepared to receive a  paint system. As
described in Chapter 4, a comprehensive approach to
ensuring proper adhesion  of applied coatings begins
with the appropriate handling and storage of raw mate-
rials and vendor-supplied component parts.

Chapters 5, 6, and 7 address pollution prevention in
regard to the  fundamental pretreatment  processes of
degreasing, phosphating, and rinsing, respectively. For
many operations, conventional approaches to cleaning
and otherwise preparing workpiece surfaces for coating
application  generate large amounts  of  wastewater,
much of which must be handled expensively as hazard-
ous waste. These chapters suggest alternative  ap-
proaches to performing these  pretreatment  steps that
can, for instance, minimize water usage (i.e., by using
counter-flow rinsing) and reduce the use of toxic, sol-
vent-based  materials  (e.g.,  by  using  aqueous  de-
greasers). Although degreasing, phosphating, and rinsing
often are conducted in an integrated process line,  they
are addressed separately in this document as a means
of highlighting  specific best management practices.

The final pretreatment chapter (Chapter 8) addresses
pollution prevention in regard to abrasive blast cleaning.
A primary consideration is the recyclability of the abra-
sive media; however, water-use reduction as an inciden-
tal benefit of blasting also is addressed.

Section Three  on application  process factors begins
with a  discussion of transfer efficiency—of the coating
to the workpiece substrate—as a fundamental consid-
eration for pollution prevention (Chapter 9). Of the many
strategies recommended in this manual, transfer effi-
ciency improvement is likely to yield the greatest pollu-
tion  and  process cost  reductions.  Several of the
practices discussed can be implemented immediately,
without the need for either capital expenditure or proc-
ess-line reconfiguration.
 Chapters 10 and 11 focus on the two types of coating
 systems, liquid compliant and powder coatings, respec-
 tively, in terms of selection criteria related to pollution
 prevention. The discussion on liquid coatings, for exam-
 ple, presents a basis for considering the use of coatings
 that are low in volatile organic compounds (VOCs),
 while the powder coatings discussion considers appro-
 priate situations for the  use of these  low-pollutant-gen-
 erating systems.

 Although the pollution prevention benefits of controlling
 the viscosity of an applied coating are somewhat indi-
 rect, the management practices suggested in Chapter
 12 can be essential for ensuring right-first-time process-
 ing. As this chapter explains, by altering the viscosity of
 a coating to achieve better substrate coverage for par-
 ticular workpieces,  superior finishes  can be achieved,
 thus minimizing the need for reworks.  Several strategies
 are  suggested  for  maintaining a constant viscosity
 throughout the  application process to improve the con-
 sistency of color, gloss, and texture in a coating system.

 Chapters 13,14, and 15 speak to practices that can have
 a more direct effect on pollution prevention. For exam-
 ple, recommended practices include minimizing solvent
 usage when cleaning equipment (e.g., through recycling
 cleaning formulations) and minimizing pollution in spray
 booths (e.g., by controlling particulate emissions).

 Section Four provides case studies  that illustrate ap-
 proaches to addressing typical paints  and coating prob-
 lems (Chapter 16).

 Appendixes to the document provide  a list of suppliers
 of aqueous and semi-aqueous degreasers and equip-
 ment (Appendix A), a methodology for calculating the
 rinsing flow rate required to achieve a specified dilution
 ratio (Appendix B),  and  a spreadsheet for factoring
transfer efficiency considerations into  a coating applica-
tion process (Appendix C).

                                                Chapter 2
                         Overview of Paints and Coatings Operations
 2.1   Introduction

 The paints and coatings industry is made up of many
 different types of operations, ranging from large-volume
 original equipment  manufacturers  (OEMs)  that  run
 highly automated, closely monitored systems to custom
 shops performing a range of contract work with manually
 operated equipment. Nonetheless, because certain ba-
 sic practices are common to the industry, pollution pre-
 vention measures discussed in this document will have
 relevance for many facility operators.

 Throughout this document, pollution prevention consid-
 erations are  raised in the context of best management
 practices  recommended for individual  stages  in  the
 paints and coatings process. This chapter introduces
 those  that follow by providing brief descriptions of the
 general types of operations that constitute this industry.
 Process-specific  terms used  in  this chapter are  ex-
 plained in subsequent discussions on pretreatment and
 application processes.

 2.2   Operations for Miscellaneous Metal

 2.2.1   Priming Only

 Most manufactured products, or parts included in those
 products, are not required to receive a coating beyond
 the primer coating.  For instance, a topcoat may be
 unnecessary if such products or parts in their intended
 use will never be exposed to corrosive environments. In
 other cases, the useful life of the product or part may be
 sufficiently short that applying a finish coat adds little or
 no value. Additionally, some parts may receive a  primer
 coating in conjunction with the original fabrication, and
 then they may or may not receive a finish coating when
 the end-product is assembled. Examples of products
 and parts manufactured in  the  metals industries that
 might receive only a primer coating are listed according
to Standard Industrial Classification groups  and  codes
 in Tables 2-1 and 2-2.

 Figure 2-1 presents a schematic of a typical process line
 in which fabricated metal parts receive only a primed
coating before being shipped.  This type of operation
 might involve removing surface contaminants such as
Table 2-1.  Major Group 33: Primary Metal Industries

SIC Code      Example of Industry

3122-3399      Axles, rolled or forged
              Car wheels
              Railroad crossings
              Sheet steel
              Steel baskets, made in wire drawing plants
              Chain link fencing, made in wire drawing plants
              Steel wire cages
              Wire carts, household, grocery, made in wire
              drawing plants
              Wrought pipe and tubes
              Cast iron cooking utensils
Table 2-2. Major Group 34: Fabricated Metal Products,
         Except Machinery and Transportation Equipment
SIC Code
Example of Industry
3411-3499     Shipping containers
             Drums and pails
             Hedge shears and trimmers
             Hand and edge tools
             Saw blades and handsaws
             Fabricated iron and steel brackets
             Fireplace equipment
             Ice chests or coolers
             Ladder jacks
             Trunk hardware
             Bathroom fixtures
             Lawn sprinklers
             Room gas heaters
             Swimming pools heaters
             Wood and coal burning stoves
             Door and jamb assemblies
             Liquid oxygen tanks
             Sheet metal hoods
             Bombs and parts
             Mortar fin assemblies
             Industrial gate valves
             Torsion bars

                                Solvent Washing,
                                Solvent Wiping, or
                              Aqueous Degreasing
                                                       Welding and Fabrication
                     Raw Material
                        Spray, Flow, or Dip

 Figure 2-1.  Schematic of a typical process for priming metal parts.

 oil and grease by washing or wiping the workpieces with
 a solvent or applying an aqueous degreaser with  high-
 pressure  hot water. Because the quality of the surface
 finish is not critical for such parts and  products, the
 primer coating can be applied either in a dip tank or with
 a flow coater.
The use of dip tanks involves immersing the workpiece
into a vat of paint, after which the piece  is suspended
over the vat so that excess primer can run off. The flow
coating process is similar to dip coating, although the
paint is  poured  onto the workpiece; the  piece  is then
suspended over a collection area so that excess paint
can drain and be recycled into the process. An alterna-
tive to dipping -and flow coating  is spray  application.
Primer-only operations in which spray guns are used
tend to be, fairly basic, without sophisticated equipment
or procesfs-line automation.

Historically,  paints and coatings facility operators have
used these approaches to apply solvent-borne primers
that are high in  volatile  organic  compounds (VOCs).
Although such coatings were favored for their extremely
fast drying properties, they are known to emit significant
quantities of VOCs, of which some also may be hazard-
ous air pollutants and/or ozone depleting compounds.

In recent years,  water-borne primer coatings have be-
come  available that offer dramatic reductions  in VOC
content.  These can be used for  dip, flow,  and spray
applications. For some operations, however, switching
to these alternative formulations  may  be problematic
because they require longer drying times. Moreover,
some formulations are highly sensitive to the degree of
surface cleanliness. For instance, whereas the cohesion
of some solvent-borne coatings might be unaffected by
traces of oil  and grease  on a metal substrate, water-
borne coatings might pull away and form  craters. Nev-
ertheless, many  paints and coatings  operations  are
moving toward water-borne primers because they  are
less harmful to the environment.

Typically for such operations, process-line operators
could benefit from additional training, and abatement
equipment for reducing hazardous emissions is some-
what inadequate. Thus, pollution prevention programs
can be beneficial.

2.2.2  Priming and Topcoating
 Many  manufactured  products must receive both a
 primer and a finish coating. Such products  might be
 used in applications in which corrosion resistance is an
 important, if not critical, property. Also, the value of the
 product might be significantly enhanced  if its useful life
 can be extended by  its ability to resist  the assault of
 corrosive elements. Additionally, the value of countless
 products can be enhanced by a primer-topcoat system
 that provides general visual appeal while adding to over-
 all quality and durability. Examples of products  in  the
 metals and metals-related industries that might receive
 a primer-topcoat system are listed according to Stand-
 ard Industrial Classification groups and codes  in Tables
 2-3 to 2-7.

 Primer and finish coatings are applied either separately
 or in a single process line, as described below.   Priming and Topcoating as a Two-Stage

 Typically, heavy equipment and machinery (e.g., exca-
 vators, army tanks) receive a primer-topcoat system in
 two -stages. In the first stage, the various parts and
 components of the products are primed.  In the second
 stage, following assembly, the topcoat is  applied.

 Figure  2-2  shows a schematic of a  process  in which
 workpieces receive a primer coating in a first stage, then
 a finish coating following product assembly. In such a
 process, incoming raw material often is  cleaned (e.g.,
 degreased or steam-cleaned) before being moved along
for welding  and fabrication operations. This initial clean-
 ing removes surface contaminants that could undermine
the integrity of welding bonds on sub-assemblies. After
fabrication,  sub-assemblies and  component parts un-
dergo pretreatment (e.g., additional  cleaning)  before
priming. Once applied, typically the primer is allowed to
dry and cure at ambient temperature, although  some
operations  use dry-off ovens. The primed  piece then

 Table 2-3.  Major Group 35: Industrial and Commercial
           Machinery and Computer Equipment
 SIC Code
Example of Industry
 3511-3599       Windmills for generating power
                Steam engines, except locomotives
                Engine and engine parts
                Marine engines
                Agricultural implements and machinery
                Blowers and cutters
                Farm elevators
                Greens mowing equipment
                Combines (harvesters and threshers)
                Spraying machines
                Construction cranes
                Road graders
                Logging equipment
                Vibrators for concrete construction
                Mining machinery and equipment
                Elevators and moving stairways
                Conveyors and conveying equipment
                Machine tools
                Power-driven hand tools
                Textile machinery
                Woodworking machinery
                Printing trade machinery
Table 2-4.  Major Group 36: Electronics and Other Electrical
           .Equipment and Components, Except Computer
 Table 2-5.  Major Group 37: Transportation Equipment
 SIC Code       Example of Industry
 3712-3799      Ambulances
                Car bodies
                Fire department vehicles
                Motor homes
                Personnel carriers
                Motor vehicle parts and accessories
                Oil, air, and fuel filters
                Motor vehicle horns
                Exhaust mufflers
                Motor vehicle radiators
                Patrol boats
                Floating radar towers
                Steam engines (locomotives)
                Trolley buses
                Bicycles and parts
                Motor scooters  and parts
                Campers for mounting on trucks
                Military tanks
                Trailer hitches
                Wheel barrows
                                                              Table 2-6.   Major Group 38: Measuring, Analyzing, and
                                                                         Controlling Instruments; Photographic, Medical,
                                                                         and Optical  Goods; Watches and Clocks
                                               SIC Code
               Example of Industry
SIC Code
Example of Industry
3612-3699      Power distribution and specialty transformers
               Switchgear and switchboard apparatus
               Motors and generators
               Relays and industrial controls
               Battery chargers
               Barbecues, grills, and braziers
               Electric dehumidifiers
               Household fans
               Electric wall heaters
               Vacuum cleaners
               Floor waxers and polishers
               Electric wiring boxes
               Electric conduits and  fittings
               Residential electric lighting fixtures
               Commercial, industrial, and
               institutional lighting fixtures
               Household audio and video equipment

may be stored for a time as inventory before it is used
in end-product assembly.

The component parts of an end-product can become
scuffed and soiled during assembly and product testing
operations.  In many cases, the  product becomes suffi-
ciently marred and soiled that  it must undergo some
3812-3873      Air traffic control radar systems
               Distance measuring equipment
               Nautical instruments
               Laboratory balances
               Laboratory hot plates
               Laboratory furniture
               Clothes dryer controls
               Computer interface equipment
               Differential pressure instruments
               Magnetic flow meters
               Sparkplug testing equipment
               X-ray equipment
               Photographic developing machines
               Photographic enlargers
               Appliance timers

surface preparation (e.g.,  selective  scuff sanding, sol-
vent wiping, hot-water spray) before  the finish coating is
applied. On occasion, surfaces may have become suf-
ficiently damaged  overall  that the  assembled product
must be prepped and reprimed either extensively or in
selected areas.

 Factors that can contribute to  the need  for remedial
 preparation  before applying a  topcoat, unnecessarily
 generating pollution and adding to costs, include:

 • Inadequate initial surface preparation.

 • Use of a low-quality primer.

 Table 2-7.  Major Group 39: Miscellaneous Manufacturing

 SIC Code

 3911-3999      Electronic musical instruments
              Music stands
              Games, toys
              Fish and bait buckets
              Exercising machines
              Rowing machines
              Pen holders and parts
              Artist frames
              Stamp pads
              Hand stamps (e.g., time, date)
              Costume jewelry
              Costume ornaments
              Paint rollers
              Street sweeping brooms
              Advertising displays
              Name plates
              Neon signs
                •  Inadequate storage procedures (e.g., outdoors and

                •  Improper material handling procedures.

                As with the primer coating, following topcoat application
                the finished product is dried either at ambient tempera-
                ture or in a dry-off oven.

         Priming and Topcoating as a Single-Stage

                For smaller products that require little or no assembly
                before shipping (e.g., wheel barrows, music stands) and
                for some component parts, a primer-topcoat system is
                applied  in  a single process line. Such a  process is
                similar to the two-stage process, except that the painting
                operation is not interrupted for assembly.

                Figure 2-3 shows a schematic of a process in which
                workpieces receive a primer coating and a finish coating
                in a single-stage operation. Typically, for such a process
                the entire operation is conveyorized, from cleaning the
                incoming raw materials  to  applying the topcoat. After
                drying and curing, the workpiece is removed from the
                conveyor and prepared for shipping or stored for assem-
                bly operations.

                2.2.3   Surface Preparation

                The  amount of surface  preparation included in paints
                and coatings operations  for miscellaneous metal work-
                               Welding and Fabrication
                                                                Vapor Degreasing
                               Metal Pretreatment
                                                    Steam Cleaning
            Dry-Off Oven (400ฐF)
                                     Raw Material
                                          Assembly Area
                                                                                  Priming Spray
                                                                                   Curing Oven
                      Propping Area
 Top Coat
Spray Booth
Figure 2-2.  Schematic of a process for two-stage application of a primer-topcoat system.

                                   Welding and Fabrication
                                                                 Vapor Degreasing
                                                                             Raw Material
                                    Metal Pretreatment       Dry-Off Oven (400ฐF)
                                                                                 Priming Spray
                                                                                 Curing Oven
                                                      Finished Product Is Shipped

Figure 2-3.  Schematic of a process for single-stage application of a primer-topcoat system.
pieces spans a broad  range.  For example, low-value
products for price-sensitive markets may undergo little
or no preparation before a paint coating is applied, while
metal components for industrial machinery that will be
operated in a corrosive environment may receive exten-
sive pretreatment. Typical pretreatments for metal work-
pieces include phosphating and abrasive blasting, both
of which are discussed  briefly below.   Phosphating

Phosphating (i.e., iron and zinc phosphating) is a proc-
ess  of depositing a conversion coating onto steel or
galvanized steel  to enhance the paint coating's adhe-
sion to the metal surface. This strengthened bond en-
hances the coatings' ability to resist corrosion. Typically,
iron  phosphating  is conducted  using a three-step proc-
ess, as shown in Figure 2-4,  that includes two rinse
steps. To achieve a primer-topcoat  system with en-
hanced corrosion resistance, facility operators often rely
on a five-step process, as shown in  Figure 2-5, that
comprises three rinse steps. Although either iron or zinc
phosphate can be used in such  a process, usually zinc is
specified when superior corrosion resistance is required.
                                      Seal Rinse
Degrease/ Water Rinse
on Phosphate (Ambient)

Figure 2-4.  Schematic of a three-stage Iron phosphating
 Depending  on their size and the volume throughput
 requirements, workpieces undergo phosphating either
 in batches by immersion or as individual pieces that are
 sprayed as they are moved through the process by
 conveyor. In spray processing, workpieces are trans-
 ported through the various spray zones. To the extent
 possible, solutions are captured and recycled.

 Chromate oxide formulations should be used to apply a
 conversion coating to aluminum workpieces.  For low-
 value end-products,  aluminum workpieces often  are
 pretreated using an aqueous (i.e., nonchromate) formu-
 lation. A typical process for applying a conversion coat-
 ing to aluminum workpieces with either a chromate or
 nonchromate formulation is shown in Figure 2-6.

 For most pretreatment processes, the phosphating stage
 is followed immediately by a dry-off oven at a temperature
 that will evaporate water as quickly as possible, to prevent
 flash  rusting. For ovens used  to dry particularly bulky
 pieces, the temperature may be as high as 400ฐF.   Abrasive Blasting

 Abrasive blasting is a method of both cleaning corrosion
 and other surface contaminants from metal workpieces
 and giving the substrate a textured profile. The combi-
 nation of a clean surface and a textured profile  enhance
 coating  adhesion, providing corrosion-resistance prop-
 erties. Facility operators generally opt for this approach
 when workpieces are too bulky and heavy (e.g., metal
frames) to be effectively cleaned and phosphated in
 spray or immersion processes.

 If oil or grease is on the surfaces of the workpieces, the
facility operator typically  will degrease them prior to
abrasive blasting by spraying them to the extent possi-

             Water Rinse
                                         Iron or Zinc
                Water Rinse
Seal Rinse
 Figure 2-5.  Schematic of a five-stage Iron or zinc phosphatlng process.
  Water Rinse
              Chromate or
Water Rinse    Conversion    Water Rinse    Seal Rinse
 (Ambient)      Coating      (Ambient)       (Ambient)
 Figure 2-6.  Schematic of a typical conversion coating process for aluminum workpleces.
ble with super-heated steam or high-pressure hot water.
This minimizes the likelihood that the blasting media will
transfer contaminants between workpieces.
                                    spray  booth designs are listed in Table  2-11.  Spray
                                    booths in these designs are available off-the-shelf or as
                                    custom equipment.
2.2.4  Application of Paint Coating Systems       225  Abatement Equipment
The types of paint coatings and application systems
used in paints and coatings operations for miscellane-
ous metal workpieces also span a broad  range. A se-
lected list qf paint coatings that includes both water- and
solvent-borne systems is presented in Table 2-8. Typical
spray and automated applications equipment is listed in
Tables 2-9 and 2-10, respectively. Because many paints
and coatings operations use spray application, common
Table 2-8. Typical Coating Technologies for Miscellaneous
          Metals Parts
Resin Technology
Water borne (air or
force dry)
Water borne (bake)
Solvent borne (air or
force dry)
Solvent borne (bake)
Specialized coatings
Alkyd and modified alkyd (water
Acrylic latex
Epoxy (water based)
Alkyd and modified alkyd (water
Alkyd and modified alkyd
Epoxy catalyzed (two component)
Poiyurethane (single or two
Alkyds and modified alkyds
Polyester (oil free)
Ultraviolet curable
                                               Although  emission abatement devices are not widely
                                               used in operations applying coatings to miscellaneous
                                               metal products, several types of equipment are avail-
                                               able. Typical devices are listed in  Table  2-12. Indeed,
                                               Table 2-9.  Most Common Manual Spray Guns
                                               Conventional air atomizing
                                               Air-assisted airless
                                               High volume, low pressure (HVLP)
                                               Electrostatic (low voltage)
                                                Conventional air atomizing
                                                Air-assisted airless
                                               Electrostatic powder application

                                               Table 2-10.   Most Common Automated Coating Processes
                                               Dip coating
                                               Row coating
                                               Autodeposition (primarily for priming steel)
                                               Electrostatic turbo bells and discs
                                               Automatic spray guns8
                                               a Using any of the  delivery and atomization mechanisms listed in
                                                Table 2-9, except that electrostatic guns will usually be of the high-
                                                voltage type.

Table 2-11.  Common Spray Booth Designs

Direction of air flow

  Cross draft
  Down draft
  Semi-down draft

Filtering mechanism
  Dry filter
  Water wash
 Table 2-12.  Typical Abatement Control Devices for
           Painting Facilities

 Thermal oxidation (regenerative)

 Thermal oxidation (recuperative)

 Catalytic incineration (regenerative)

 Carbon adsorption (alone or in combination with thermal oxidation)

 Zeolyte adsorption (alone or in combination with thermal oxidation)

 Ultraviolet oxidation



indications are that less than 20 percent of paints and
coatings facilities operate with abatement equipment for
capturing  VOC emissions. The use of such devices is
low in the  industry because most facilities operate below
threshold  limits established by regulation. These regu-
latory limits can vary from state to state, or even from
one community to another. Relatively few facilities,  par-
ticularly those with  VOC emissions  exceeding  100
tons/year, are required by federal, state, or local regula-
tions to abate emissions.

2.3   Operations for the Automotive

2.3.1   Process Overview
Paints and coatings operations for the automotive indus-
try generally differ from those for miscellaneous metals
because the finish coating on products must be of su-
perior quality and appearance. Typically, the process for
applying a primer-topcoat system in the automotive in-
dustry includes multiple stages, as shown in Figure  2-7.
Moreover, individual stages in the process can include
multiple steps.
For instance, industry standards call for the use of  zinc
phosphating, which typically is conducted in a multistep
process that is closely monitored. Figure 2-8 shows a
10-step phosphating process, typically used in the auto-
motive industry, that includes six rinse steps, half of
which use deionized water.
After a car body, for instance, has passed through the
phosphating stage, it is immersed in a large electrode-
position tank, in which a cathodic or anodic primer is
applied. This electrodeposited primer is then cured in an
oven at temperatures ranging from 300ฐ to 400ฐF. The
underside of the body then receives a polyvinyl chloride
(PVC) coating that provides sound-proofing attributes.
Also, all seams and mating surfaces receive a sealer to
prevent moisture penetration.

Next, the car body may undergo light sanding before a
primer  is applied.  In some facilities,  a wet-on-wet top-
coat also is applied at this point to the underside of the
hood and the inside of the trunk. The primer and interior
topcoats then are dried and cured in a baking oven, after
which the body enters the topcoating spray booth. De-
pending on the color to be achieved, a solid color top-
coat  may be  applied  or  a basecoat may be applied
followed by a wet-on-wet clearcoat. After topcoating, the
car body enters the final baking oven in which the top-
coat is cured.

At various locations along the process line, the car body
may be moved aside so that line operators can inspect
for defects in either the primer or the topcoat. When a
defect  is  discovered,  the area  is  scuff  sanded and
touched up.

2.3.2  Paint Coating Systems and Application
        Processes   Types of Coatings

During the 1970s, the automotive industry  made a con-
certed effort to use water-borne primers and topcoats.
These  included  acrylics, epoxies,  polyesters,  mela-
mines,  and  oil-modified alkyds. Most of the pigments
were  compatible with water-soluble resin  systems.  In-
itially, however, problems arose because adding alumi-
num  pigments to these  high-pH  range  (8.0 to 9.0)
formulations generated hydrogen  gas (1). As a result,
specially treated  aluminum  pigments  were  manufac-
tured  to solve this problem.

Other problems  included  the requirement that water-
borne coatings be applied in highly controlled environ-
ments (e.g.,  temperature ranging from 70ฐ  to 80ฐF,
relative humidity ranging from 40 to 60 percent). Also, to
prevent rupturing or blistering of the coating, finished
parts  had to be dried initially  in a low-temperature zone
(i.e., 150ฐF) of the oven. Only after all water had been
evaporated, could the part safely enter a high-tempera-
ture zone (i.e., greater than 250ฐF).

Later, when basecoat/clearcoat systems providing an
enhanced finish  and  greater durability became  avail-
able, the industry embraced  these solvent-borne coat-
ings in favor of water-borne alternatives. Additionally, the
industry found it easier to formulate and apply high-pig-

                 Metal Pretreatment
                    Bake Oven
 Dry-Off Oven    Electrodeposltlon
  (Optional)          Tank
              Bake Oven
Topcoat and/or
Bake Oven
                                                                                    Sealers and
                                                                                  Sound Deadening
Figure 2-7.  Schematic of a typical process for applying a primer-topcoat system in the automotive industry.

' < r - "- *
Dry-Olf Oven


01 Mist Spray

Dl Rinse

Dl Rinse

Zinc Phosphate

' -"- '

Chromate or Non
chromate Sealer



                                                                                    Dl Rinse
                    Dl = delonized water

Figure 2-8.  Schematic of a typical process for applying a zinc phosphate coating In the automotive industry.
ment-loaded basecoats, particularly those containing
metallic pigments, in solvent-borne systems.

The industry returned to water-borne basecoats in the
1980s when improved formulations  became available.
Water-borne basecoats also are  used extensively on
automotive plastics.  Coating Systems

Primer-topcoat systems for the automotive industry can
include any of the following components:

• Primers: Most primers are applied by electrodeposi-
  tion  and many are based on anodic or cathodic for-
  mulations, although cathodic epoxy is the most popular.
                       New electrodeposited primers tend to be low in VOCs
                       and heavy metals and they yield good coverage and
                       corrosion resistance (2). Water-borne  primer surfac-
                       ers also are being tested by the industry.

                     • Basecoats: Both  conventional  solvent-borne base-
                       coats and the newer water-borne systems are acrylic
                       melamine formulations.

                     • Clearcoats: These finish coatings are available in
                       many forms:
                       - Conventional solvent-borne acrylic melamine.
                       - New water-borne acrylic melamine.
                       - Two-component polyurethane.

   - One-component polyurethane.
   - Scratch-resistant  clearcoats  based  on  silane
   - Scratch-resistant clearcoats based on acid-epoxy
   - Powder coatings.

 • Monocoats: These coatings combine a basecoat and
   a clearcoat. Although most of them are solvent-borne,
   the industry is moving toward the water-borne base-
   coat/clearcoat systems.

 These coatings are likely to gain greater popularity when
 baking  temperatures can be reduced to  the 250ฐ to
 285ฐF range.  In  particular,  the  industry  is becoming
 increasingly interested in powder coatings (3).   Application Equipment

 The automotive industry  relies on sophisticated spray
 application systems to achieve superior coatings. Facili-
 ties (e.g., OEMs) typically operate down-draft,  wash-
 water systems that are totally enclosed to minimize dust
 generation and overspray. Most operations apply auto-
 motive coatings using  both high-voltage, electrostatic
 turbo-bell systems and manual electrostatic or high vol-
 ume, low pressure  (HVLP) spray guns. Generally, air-
 less or air-assisted  airless spray guns are used on the
 finishing line exclusively for the application of sealers
 and sound-proofing  coatings.

 2.3.3   Abatement Equipment

 Given the importance of paints and coatings application
 in the automotive industry, pollution control equipment
 is an important aspect of operations generally. Facilities
 typically use thermal  oxidizers,  catalytic  incinerators,
 and carbon adsorbers, or a combination of these tech-
 nologies, to control hazardous emissions. Moreover, the
 industry has pioneered the development of many low-
 emission coating systems. As a result of its prominence
 in the paints and coatings area, the automotive industry
 plays a leadership role in pollution prevention technolo-
 gies, and many of the approaches detailed in this docu-
 ment are based on these  innovations.

 2.4  Operations for Plastic Parts

 2.4.1   Surface Preparation

 Paints and coatings are  applied to plastic parts and
 components primarily for the automotive and elec-
tronics industries (e.g., business machines). The most
 notable  difference between plastic  and  metal  work-
 pieces regarding paints and coatings operations is that
surface  preparation processes  primarily  rely on  de-
greasing. Plastic workpieces are not subjected  to
phosphating, although in  some operations pieces  are
 scuff sanded to  achieve a surface that will enhance
 coating adhesion.

 Most plastic workpieces must be subjected to degreas-
 ing operations to remove contaminants, such as mold
 release agents. Because the characteristics of  plastics
 can be quite varied, surface cleaning formulations must
 be carefully selected. For example, whereas some plas-
 tics are solvent sensitive, others are inert. Thus, when
 selecting a degreaser the facility operator must consider
 both the basic nature of the  particular plastic material as
 well as the method by which it was manufactured.

 Typically,  high-volume production operations degrease
 plastic workpieces using a  conveyorized spray washer
 process that includes rinsing with deionized water. Few
 operations clean plastic pieces with the vapor degreas-
 ing method. Regardless of the particular approach, the
 operator must guard against the tendency of some plas-
 tics to take on an electrostatic charge that can attract
 dust and undermine coatings.

 Plastics hold some advantage over metal workpieces in
 terms of pollution prevention because phosphating is
 never part of the paints and  coatings process. The more
 distinct advantage in this regard, however, is  that be-
 cause plastics do not corrode as metal does, less paint
 needs to be applied to the surface. Thus, the generation
 of pollutants is reduced.

 2.4.2  Coatings Systems

 The most widely used coating system for plastics is
 two-component polyurethane, which provides superior
 adhesion and exhibits outstanding durability. Moreover,
 this type of system can be formulated for application on
 both rigid  and flexible plastics. In situations where the
 plastic in a workpiece is not compatible with polyure-
 thane, epoxy formulations  present an alternative that
 provides good adhesion and excellent  performance

 Because most plastics are heat sensitive, coatings
 must be air- or force-dried at relatively low temperatures
 (i.e., below 180ฐF). Thus, coating systems that must
 be baked on  at temperatures above 250ฐF, such as
 acrylics, melamines, and polyesters, generally  cannot
 be used on plastic workpieces.

 2.4.3  Application Equipment

 Coating systems are applied to plastic workpieces using
 both manual and automated spray gun systems. Facili-
ties typically use conventional air-spray, air-assisted air-
 less,  and  HVLP  spray  guns.  Electrostatic guns  are
 preferred when the plastic has been formulated to be
 moderately conductive or if a conductive primer has
been applied.

Generally, requirements concerning the appearance of
finished pieces cannot be met using airless spray guns.
For similar reasons, dip or flow coatings are rarely used
on plastic surfaces.

The type of spray booth used in plastic coating opera-
tions  depends on the specifications for the  finished

2.4.4  Abatement Equipment

The use of emission abatement equipment for facilities
applying paints and coatings to plastic workpieces var-
ies widely. In general, large operations with high VOC-
emission  rates often are  required  to  add  control
equipment, whereas smaller facilities with lower emis-
sion rates may be allowed to exhaust VOCs into the air
without abatement.

2.5    Custom  Coating Operations

Because custom coating operations work on a contract
basis, the types of workpieces a particular facility proc-
esses  can vary widely.  For instance,  a custom shop
might shift coating operations from metal to plastic work-
pieces within a short period. In general, such operations
are less sophisticated than the paints and coatings op-
erations of OEMs and are capable of applying  either
liquid  or powder coatings but not both. A very few cus-
tom houses  (primarily  in the Midwest) have the facilities
for applying  liquid, powder, and electrocoatings.

Typically, custom shops are required to use the coatings
specified by the customer. In some locations, however,
facility operators are encouraged by the stringency of
environmental regulations (e.g.,  in California) to use
water-borne materials when feasible.
Most custom shops apply paint exclusively with manu-
ally operated spray guns. If an operation handles large
quantities of throughput for individual contract jobs,
however, it is likely to have an automated process.

The general trend  among  custom  shops  is away
from water-wash  spray  booths and  toward dry-filter
units, which are less expensive and easier to maintain.
Additionally, with dry-filter spray booths,  the operator
does not need to use chemicals to detackify the coating
overspray; thus, disposal of the paint waste sludge
and  contaminated water in the spray  booth water
trough are eliminated. A few of the larger shops  are
equipped with drive-in spray booths, with either cross-
or down-draft capabilities. Most, however, operate with
the cross-draft, walk-in type of booth, which can be three
sided or totally enclosed.

In general, the volume of throughput at individual cus-
tom coating shops is sufficiently low that facility opera-
tors  are not required to  install emission  abatement
equipment. Exceptions are the few larger  operations in
this industry sector.

2.6   References
1. Jamrog, R. 1993. Automotive water-borne coatings. Products Fin-
  ishing  93:56-62.
2. Bailey, J.M. 1992. Automotive coating trends. Industrial Finishing
3. Schrantz, J. 1993. Polyurethane automotive coatings. Industrial
  Finishing 69:34-35.

2.7   Additional Reading
U.S. EPA. 1991. Report on compliance coatings for the miscellaneous
  metal  parts industry.  Stationary Source Compliance Division.

     Section 2
Pretreatment Factors

                                               Chapter 3
                                  Adhesion as a Critical Factor
3.1    Introduction

3.1.1   Pollution Prevention Considerations

Adhesion is a critical factor for ensuring the integrity of
a coating. Only if a  firm bond is established  with a
substrate can a coating provide the surface protection
required by its product application. Many factors, how-
ever, can undermine  the ability to thoroughly cover a
surface. For metals and alloys, the principal threat to
good adhesion is corrosion, which can result in  degra-
dation products, such as rust, that eventually push the
coating away from the substrate. For plastics, the pri-
mary threat  is from release agents,  such as  wax or
silicone oil, that can remain on the surface after molding
of the workpiece.

By anticipating these factors and then implementing
process steps that guard against such threats to good
adhesion, a facility operator can significantly extend the
useful life of applied coatings. Typically, such measures
result in process efficiency enhancements that save on
operational inputs  such as materials and labor.  At the
same time,  because  an efficient process is one that
minimizes wastes, process enhancements usually will
yield significant contributions to  pollution prevention.

Right-first-time processing, a concept directly related to
good adhesion, should be the overriding objective of the
coatings operator seeking to reduce costs and minimize
waste generation.  Ensuring that all process steps in a
paints and coatings operation are carried out thoroughly
and consistently can yield considerable benefits in terms
of avoided costs. As well as being labor intensive, cor-
rective measures such as coating adjustments and re-
works  tend  to require extensive use of  solvents. A
longer-term pollution prevention  consideration concern-
ing right-first-time  processing is that when operations
achieve good  initial coating adhesion, a workpiece
can be in service  for a longer  time before it requires
refurbishing. The recoating of workpieces returned from
service, such as truck bodies, requires extensive proc-
essing to achieve proper adhesion. Thus,  by reducing
the volume of recoating work, the paints and coatings
industry can make considerable strides  in terms of
pollution prevention.
 Good adhesion is presented in this chapter as a funda-
 mental  concept for all pretreatment and  application
 steps in the paints and coatings process.  Evaluating
 each process step in terms of how it promotes adhesion
 increases the likelihood that opportunities  for opera-
 tional efficiency and waste reduction can be identified.

 Decision-making criteria relevant to adhesion are high-
 lighted in subsequent chapters.

 3.2  Corrosion of Metals and Alloys

 Because even superior coatings are microscopically po-
 rous, metals and alloys are vulnerable to the ravages of
 corrosion despite good adhesion. Over time, atmos-
 pheric moisture and oxygen, which are extremely low in
 density  in relation to paint molecules,  can penetrate a
 coating.  How  quickly  this  migration occurs, however,
 depends on many factors, including the coating's thick-
 ness and its porosity, which varies with resin type. Once
 water and oxygen reach vulnerable sites on the sub-
 strate, the corrosion process can begin.

 Nonetheless, corrosion, which is  the principal cause of
 coating  failures on metal substrates, can be controlled
 to a significant degree with conscientious surface prepa-
 ration and coating  application (1). These  processes
 should be based on an understanding of the mecha-
 nisms of corrosion and how to prevent it.

 3.2.1   Basics of the Corrosion Process

 Corrosion is the electrochemical  process by which the
 material integrity of a metal or alloy  is gradually de-
 graded.  The process involves two physical  mecha-
 nisms:  a chemical  reaction and the  flow  of electric
 current.  Thus,  when subjected to  humidity and oxygen,
steel will corrode as microscopic condensation forms
and conducts electricity between  reactive areas on the

More specifically, condensation acts as an electrolytic
solution  in  which soluble compounds  such as salts,
acids, or alkalis conduct electricity via the movement of
ions. Rain, sea mist, and tap water all contain these
soluble compounds. When subjected to an electrolyte,
the more reactive areas of the steel's surface  (the an-

odes)  dissolve into the  solution, generating electrons
that flow through the steel to less-reactive areas (the
cathodes). At these sites, oxygen and hydroxyl ions
combine to form rust.

3.2.2   The Science Behind Corrosion

The process known as galvanic corrosion occurs when
two metals that have different oxidation potentials are
connected electrically and immersed in an electrolyte.
Table 3-1 lists the  most common metals and their re-
spective oxidation potentials. Those higher up in the list
are generally more  reactive; elements with the lowest
oxidation potential appear at the bottom of the list (i.e.,
platinum and gold, the "noble metals").

If two dissimilar metals, such as copper and iron, were
connected with  a piece of wire and immersed in an
aqueous electrolyte, the more reactive of the two metals
would dissolve, in this case the iron (Figure 3-1). In such
a galvanic couple, the metal that dissolves is called the
anode.  As this dissolves, it discharges  an excess of
electrons to the remaining solid metal, giving it a nega-
tive charge. The wired connection between the two elec-
trodes   allows  oxygen  and hydroxyl  ions  from the
electrolyte  to be drawn  to the less reactive of the two

Table 3-1. Electromotive Force Series (2)
                                                                             Electrons e'

Li = Li+ + e
Mg = Mg+2 + 2e
Al = Al+3 + 3a
Ti + Ti+* + 2e
Mn = Mn*2 + 2e
Zn = Zn+2 + 2e
Cr = Cr"3 + 3e
Fe = Fe+2 + 2e"
Cd = Cd+2 -i- 2s
Co = Co+2 + 2e
Ni = Ni*2 + 2e
Mo = Mo*3 + 3e"
Sn = Sn*2 + 2e
Pb = Pb+2 + 2e
H2 = 2H* + 2e
Cu = Cu+2 + 2e
Ag = Ag+ + e
Hg = Hg+2 + 2e
Pt = Pf 2 + 2e
Au = Au+3 + 3e
E8 (volts), 25ฐC
              Iron (Fe)
Copper (Cu)
               > Iron Ions
                              Gas \.
                       Oxygen + Water
                       = Hydroxyl Ions"
                   Aqueous Electrolyte
Figure 3-1.  Movement of electrons and ions In corrosion proc-
           ess Involving a galvanic couple.

metals, known as the cathode. Here they take on excess
electrons and form new hydroxyl ions. Ions are atoms
carrying either a positive or negative charge (e.g., when
an atom of iron loses two electrons, the iron becomes a
positively charged iron ion).

The newly formed hydroxyl ions then move through the
electrolyte toward the iron surface where the iron ions
(Fe2+)  react with the hydroxyl ions (OH") to form iron
oxide,  or rust.  This  process is considered an electro-
chemical  reaction because it cannot occur unless a
chemical  reaction takes place along with the flow  of
electric current.

To illustrate the science of corrosion, Figure 3-1 portrays
an electrical connection between anodes and cathodes
on separate pieces  of metal  connected by a wire.  In
contrast, Figure 3-2 illustrates how corrosion occurs on
a  single piece of steel. Although steel is  composed
primarily of iron, depending on the type of alloy, steel
also comprises small amounts of carbon, magnesium,
copper, silicon, and other elements. On a single piece
of steel, the base metal of the alloy conducts the electric
current between the anodes and cathodes on the surface.

Once atmospheric moisture and oxygen come into con-
tact with the steel surface, iron will dissolve at the an-
                                                                                  Oxygen + Water + Electrons
                                                                                       = Hydroxyl Ions
                   Iron Oxide
                                                              Anodic Area
                                 Cathodic Area
                                                                        Aqueous Electrolyte
                                                       Figure 3-2.  Mechanism of corrosion on a steel substrate (3).

 odes to form iron  ions. The electrons given up by the
 iron ions then will flow through the metal to the cathodes,
 where they are taken up by water and oxygen to form
 hydroxyl ions. Finally, a reaction between the positively
 charged iron ions  and  the negatively charged hydroxyl
 ions occurs, forming rust.

 3.2.3  Fundamentals of Corrosion Prevention

 It is known that when two metals with different oxidation
 potentials are connected and subjected to an electrolytic
 solution, corrosion of the more reactive metal is accel-
 erated. For instance, if a piece of magnesium, which is
 relatively  high in the electromotive force series (Table
 3-1), and a piece  of iron, which is lower in the series,
 are connected and immersed in a mild acid bath, the
 magnesium will  corrode more rapidly  than if it were
 immersed alone. The piece of magnesium would cor-
 rode at an even faster rate, however, if it were connected
 to a piece of copper, which has a lower reaction rate than
 iron. Thus, the greater the difference in oxidation poten-
 tial between two  pieces of metal, the faster the corrosion

 The relative rate of degradation for various metals is
 fundamental to the concept of sacrificial,  or cathodic,
 protection against corrosion. This concept relates to the
 converse of accelerated corrosion, which is that the less
 reactive of two metals will degrade at a slower rate than
 if the two metals  were not  in contact. Based on this
 principle, iron will corrode more slowly when it is  con-
 nected with lithium, which has the highest oxidation
 potential, than when coupled with magnesium.

 Sacrificial protection is  used  extensively throughout the
 world to control the corrosion of metals  and alloys. For
 instance, the steel beams in San  Francisco's Golden
 Gate Bridge are regularly painted with a zinc-rich primer
 to protect the structure against the continual assaults of
 fog and salt air. This galvanic coupling prevents corro-
 sion of the iron while sacrificing the zinc, which has  a far
 higher reaction rate.

 Of the naturally occurring elements listed in Table 3-1,
 lithium is the  most reactive,  while gold  has the lowest
 oxidation potential. Hydrogen, which is  the only non-
 metal in this selected list, has a reaction rate of zero and
thus functions as a  point of reference between elements
with a positive or negative oxidation potential.

 More generally, corrosion can be prevented by control-
 ling any one of the  following factors:

• Dissolution of the metal at the anode.

 • Conduction of  charged ions via the aqueous electro-

• Conduction of  electrons via the metal surfaces.
 • Conjoining of chemical species formed at the anode
   and cathode.

 3.3   Preventing Corrosion by Ensuring
       Proper Adhesion

 The ultimate objective of a paints and coatings operation
 is for the finish on a workpiece to adhere so thoroughly
 that moisture  and oxygen will be prevented from con-
 tacting the metal substrate and initiating  the chemical
 reactions that lead to corrosion. Adhesion is critical be-
 cause, even when a  superior bond between  the sub-
 strate and the finish is achieved, over time electrolytes
 will diffuse to  the metal surface through micropores in
 the coating. Thus, the primary role of coatings for pre-
 venting the corrosion of metal is in restricting the move-
 ment of ions in the electrolyte from cathode to anode.
 Only through proper adhesion to the substrate can coat-
 ings  present an effective impediment  to this flow of

 3.3.1 Mechanisms of Adhesion

 The four mechanisms by which a primer coating can
 successfully adhere to a substrate are as  follows (4):

 •  Primary bonding involving covalent or  ionic  interac-
   tion (e.g., chemical  reactions). (Since most  primers
   are  formulated to have an excess of hydroxyl ions,
   adhesion is improved when the substrate  has an ex-
   cess of hydrogen ions. Thus, metal surfaces should
   be slightly acidic [i.e., a pH of 5 to 6].)

 •  Secondary bonding  involving dipole-dipole  interac-
   tions, induced dipole  interactions, and  dispersion
   forces (e.g., Van der Waal's forces).

 •  Chemisorption involving the formation by  adsorption
   of chemical bonds between liquid  molecules and a
   solid surface.

 •  Mechanical  adhesion involving  roughening of  the
   substrate (e.g., abrasive blasting).

 Although all four mechanisms can occur at the same
time, each exhibits a different degree of effectiveness.
 In most cases,  primary bonding, which relies on  the
composition of the primer to provide covalent or ionic
 interaction, is the most important of these mechanisms.
When the substrate  is especially smooth, such as a
 polished surface, mechanical adhesion is usually a criti-
cal mechanism.

 3.3.2  The Importance of Proper Wetting

Superior wetting of the primer to the substrate is essen-
tial if good adhesion is to occur. For a liquid coating to
spread over a solid surface, the critical surface tension
of the solid must be greater than the surface tension of
the liquid. Thus, as illustrated in Figure 3-3, a drop of

                      High Contact Angle
                        Poor Wetting
                      Low Contact Angle
                        Good Wetting
                    Very Low Contact Angle
                      Excellent Wetting
 Figure 3-3.  Coating contact angle relative to wetting of surface.

 liquid with a high contact angle relative to a substrate
 with a low surface tension will wet a smaller area than
 a drop with a low contact angle. While  a drop of liquid
 with a contact angle even slightly below 90 degrees will
 provide relatively good wetting, a contact angle close to
 45 degrees can make a considerable difference in sur-
 face coverage.

 Depending on whether molecules are similar in charac-
 ter, the tension forces that hold them together are either
 cohesive or adhesive.  Molecules of  similar character
 (e.g., water molecules) are held together by cohesive
 forces, whereas unlike molecules (e.g., water and glass)
 are held together by adhesive forces. The relevance of
 this distinction in regard to surface tension can be illus-
 trated using droplets of different  liquids placed on a
 piece of glass. A drop of mercury will bead up rather than
 wet the glass because the  cohesive  forces within the
 mercury are stronger than the adhesive forces between
 the mercury and the glass surface. In contrast, a drop
 of water will spread out on  the glass surface because
 the adhesive forces between the water and the glass are
 slightly  stronger than the cohesive forces within  the
water droplet. Thus, between the two liquids, water dem-
onstrates the better wetting properties on  glass. If a
surfactant such as soap were applied to the glass, the
water would wet the surface even  more thoroughly be-
cause the  droplet's adhesive  properties  would  be
strengthened over its cohesive properties.
 Surface tensions for water and mercury are 73 dyne/cm
 and 465 dyne/cm, respectively. In contrast, most of the
 common solvents, such as acetone,  n-butyl  alcohol,
 toluene, and xylene, have surface tensions in the range
 of 20 to 30 dyne/cm (Table 3-2). Steel has a surface
 tension  in the range of 1,700 to 1,800 dyne/cm (Table
 3-3). Solvents with surface tensions of 20 to 30 dyne/cm
 will wet  a clean piece of steel more easily than water.

 For powder coatings, surface tension  becomes a critical
 factor when the applied powder melts and  liquifies as it
 is heated  in a  high temperature oven (>250ฐF).  If  its
 wetting properties are good, the powder will easily flow
 over the substrate.

 As indicated by Figure 3-4, at the microscopic level a
 typical substrate has considerable variation. Poor wet-
 ting (as  shown  in Figure 3-4a) leaves a gap, making it
 easier for corrosion to push the coating away from the
 substrate.  When proper wetting is achieved (as shown
 in Figure 3-4b),  the corrosion process is impeded.

 3.3.3   The Role of Surface Contaminants

 The inability to sufficiently wet a surface can be due to
 the presence of  contaminants such as oil and grease on

 Table 3-2.  Approximate Surface Tension of Substances in
          Contact With Their Vapor (5)
                                  Surface Tension

n-Butyl alcohol
Ethyl acetate

Methylene chloride










Table 3-3. Approximate Surface Tension of Metallic Elements
         in Inert Gas (5)
Surface Tension
1 ,200-1 ,300

                   (a) Poor Wetting
                   (b) Excellent Wetting

Figure 3-4.  Cross-sectional view of surface wetting.

the substrate. Other contaminants such as scale and
weld slag may initially accept a coating but cause it to
fail prematurely. The thorough cleaning of workplaces
before applying coatings  can remove  such contami-
nants and ensure long-term durability.   Qil and Grease

The presence of oil or grease on a substrate can prevent
a coating from thoroughly wetting the surface, especially
if the surface tension of the coating is slightly higher than
that of the surface contaminants. For example, consider
how water beads up on the surface of  a greasy plate
when held under a faucet. This occurs because water
droplets  have a surface tension of  approximately 73
dyne/cm, while grease can have a tension in the 20 to
50 dyne/cm range.  Washing the grease from the plate
would raise the surface tension above that of the drop-
lets, facilitating thorough wetting. Water flowing across
the clean plate in sheets would indicate  that the contact
angle is extremely low (i.e., well below 90 degrees).

This example  illustrates  that the  coating  (e.g., the
primer) will not adhere if it cannot make direct contact
with the substrate. For instance,  hydroxyl  ions in a
primer may not have an  opportunity to react  with a
slightly acidic metal surface. Although some degree of
mechanical adhesion may occur if the surface has been
roughened, overall adhesion is likely to  be poor.

Another important reason to remove oil and grease from
a substrate concerns the integrity of the coating. Con-
sider that primer coatings, for instance, are precisely
formulated to provide specified performance properties.
When a  primer is applied  over a film of oil or grease,
 solvents in the primer can dissolve the contaminants,
 incorporating them into the coating. The dissolved con-
 taminant can  in effect change the coating formulation
 and undermine its performance properties.  Scale (Oxides)

 Scale is a  flaky oxide film that forms on metal that has
 been heated to high temperatures. For instance, a type
 of scale known as iron oxide forms on steel when it is
 heated in the rolling process. Although iron oxide is inert
 to corrosion,  its  brittleness and  tendency to form in
 multiple layers of varying physical characteristics  can
 seriously   compromise coating adhesion.  Moreover,
 scale can act as a cathode to the adjacent metal anode;
 thus, as moisture penetrates the  pores of the coating,
 corrosion occurs at the  edge of the scale formation,
 where the  galvanic couple is established. Moisture also
 can activate corrosive salts (e.g., ammonium salts, chlo-
 rides,  and  sulfates)  that  can be bound  up in scale or
 generally in the atmosphere in industrial process  set-
 tings. Eventually,  the corrosion spreads under the scale
 and  lifts it from the substrate (Figure 3-5).
            Moisture and Oxygen
                Rust Lifts Scale
Figure 3-5. Cross-sectional view of surface spalling caused by
Depending on the end-use of the workpiece, many com-
panies apply finishes directly over scale. When such
coatings are exposed  to the elements, particularly in
humid or marine environments, they tend  to degrade
rapidly. The result of such adhesion failures is that the
coating flakes,  or spalls.  For example, consider  how
rapidly paint applied to steel  handrails  and stanchions
tends to fail when constantly exposed to ocean winds.
When  applied directly over corrosion, the coating is
likely to fail within a few months and require repainting.
Proper surface preparation could extend the life of such
coatings considerably.
In contrast to iron oxide, oxide on aluminum forms a thin,
transparent film on the substrate when  it is  exposed to
oxygen at ambient temperatures. As with other surface
contaminants, this  film should be removed from  the
substrate before a coating system is applied.   Welding By-Products

Adhesion also can  be undermined by weld slag and
spatters  in the  area of a  welded seam. Because the

seam itself is highly  prone to corrosion  and is often
where coating failure begins, thorough preparation of
such areas is particularly important. Like scale, weld
slag can  include corrosive substances that undermine
surface adhesion when activated by moisture. In con-
trast, spatters encourage premature corrosion by their
irregular profiles,  often with  sharp peaks, which make
them difficult  to cover and likely to protrude from  the
coating (Figure 3-6). Another concern is that the area
around a weld seam can be slightly alkaline. This can
cause a compatibility problem with the primer, which
should be applied to metal substrates that are slightly
acidic (i.e., a pH of 5 to 6).

The most effective  approach for preparing a  welded
surface before painting calls for removing all spatters
and slag  material, either through grinding or abrasive
blasting. The weld seams should then be thoroughly
wiped down using a cloth moistened with a low concen-
tration phosphoric acid to adjust the pH.

Additional preventive measures include brushing  the
weld seam with  a  corrosion-resistant primer  before
spraying  the  entire  piece with the  primer coat. This
additional step ensures that the primer covers most if
not all surface irregularities.  One company using this
labor-intensive approach reports that an earlier problem
with paint failures  around weld seams has been essen-
tially eliminated.

3.4   Adhesion Considerations Specific to
       Plastic Substrates

Plastics are complex organic composites that present a
particular challenge to paints and coatings operations.
For example, most plastics have a surface tension in the
same range as organic coatings, making adhesion gen-
erally  problematic (Table  3-4). To  some  degree,  this
challenge can be addressed with the use of coatings
specifically formulated for a lower surface tension.
Because the range for adjustment is quite narrow, how-
ever, ensuring that the substrate is free of contaminants
is even  more important for coating  plastics  than for
metals. Of particular concern are release agents (e.g.,
wax or silicone  oil), which are used during  molding
operations to keep the surface of the workpiece from
adhering to the form. Additionally, plasticizers, which are
added to the plastics  blend to enhance  flexibility,  can
contaminate the  substrate.  In some cases, plasticizers
migrate to the surface over time to undermine a work-
piece's long-term durability.

For most plastic  workpieces, thorough cleaning of the
surface ensures  that coating adhesion meets  end-use
specifications. Certain  plastics,   however, such  as
polypropylene, are so inert  that additional pretreatment
may be  required. Recommended  approaches include
light abrasion of the surface or heating the workpiece to
alter the chemical characteristics of the substrate (e.g.,
using hot flame or gas plasma technology).

3.5    References
1. Joseph, R. 1993.  Pollution prevention for paints and coatings fa-
  cilities: Why the need for surface preparation? Presented at  train-
  ing seminar on Liquid and Powder Coating Operations,  November
  16-18, New Orleans, LA. Sponsored by University of  California-
2. Uhlig, H.H. 1965.  Corrosion and corrosion control. New York, NY:
  John Wiley and Sons.
3. Joseph, R. 1988.  How paints  and coatings reduce corrosion: A
  short introduction. Saratoga, CA: Ron Joseph & Associates.
4. Wegman, R. 1989. Surface preparation techniques for adhesive
  bonding. Park Ridge, NJ: Noyes Publications.
5. Lide, D.R. 1992. Handbook of chemistry and physics, 72nd ed.
  Boca Raton, FL: CRC Press.
6. Hajas, J., K. Haubennestel, and A.  Bubat. 1994.  Improvements of
  substrate wetting  of waterborne coatings on plastics and related
  surfaces.  In: Proceedings of the 24th Waterborne, Higher Solids,
  and Powder Coating Symposium,  February 9-11,  New Orleans,
  LA. Sponsored by the University of Southern Mississippi, Depart-
  ment of Polymer Science, and  the  Southern Society for Coatings
                                     Weld Spatter
Figure 3-6. Cross-sectional view of compromising effect of weld slag and spatters on a coating.

Table 3-4.  Surface Tensions of Coating Ingredients Versus Plastic Substrates (6)

Solvents                  dyne/cm      Resins                       dyne/cm
                                        Polyester resin solution

Butyl cellosolve
Isopropyl alcohol
Propylene glycol
methyl ether (PM)
Dipropylene glycol
methyl ether (DPM)


Acrylic latec
Acrylic resin solution
Polyurethane emulsion
Polyurethane resin solution
PV Ac latec
Melanine resin

PVC (nonplasticized)
PVC (plasticized)
Polyester SMC, BMC


                                                                                     Waterbome primers           29-40

                                                                                     Waterborne topcoats          27-38
PVC = polyvinyl chloride
PP = polypropylene
SMC = sheet molding compound
BMC = blow molding compound
PV Ac = polyvinyl acetate
PTFE = polytetrafluoroethylene
ABS =  acrylonitril-butadiene-styrene

                                                 Chapter 4
                    Considerations Regarding Vendor-Supplied Materials
4.1    Introduction
4.1.1   Pollution Prevention Considerations

Often, the  earliest  opportunity for the  manager of a
paints and  coatings operation to avoid  extensive pre-
treatment of workpieces—and thus prevent the genera-
tion of excess wastewater,  residual  pollutants,  toxic
emissions, or other wastes—is when taking delivery of
vendor-supplied materials. To the degree possible, op-
erators should stipulate to vendors that delivered mate-
rials must be free of corrosion and contaminants.  It is
then contingent upon the operator to maintain the sup-
plied materials in the same "coating-ready" condition in
which they arrived.

Delivered materials  should be stored indoors whenever
possible to protect them from the elements. When floor
space is not available for holding inventory,  materials
should be thoroughly covered for  outside storage and
kept above  ground level. More streamlined operations,
however, minimize the likelihood that materials will cor-
rode during storage by relying on a just-in-time delivery
system. Such systems have been used in most industry
sectors to control  inventory costs.  In the paints and
coatings sector, they can afford additional benefits as-
sociated with pollution  prevention.

The potential for  vendor-supplied materials to under-
mine the long-term durability of a finished workpiece is
easily overlooked. Corrosion on raw materials or on a
component  or part,  however, can  significantly shorten
the service life of an otherwise high-quality product. This
chapter considers various options for working with sup-
pliers to reduce this likelihood.
Table 4-1.  Decision-Making Criteria Regarding
          Vendor-Supplied Materials
Are raw materials
and components
supplied by the
vendor with an
application of rolling
oils and/or corrosion
preventive coatings?
Can pretreated
materials be
substituted for
standard materials?
Are some raw
materials and
components stored
Such coatings can be effective in
preventing corrosion; however, they can
be difficult to remove prior to fabrication or

Consideration should be given to requiring
the vendor to use oils and preventative
coatings that can be easily removed using
an aqueous degreaser or detergent

Consideration should be given to
purchasing raw materials and components
without a coating of oil or a corrosion
preventative, thus minimizing the
generation of wastewater and emissions
associated with cleaning operations.

If so, a cost-benefit analysis of this
approach should be conducted.

This approach can minimize the
generation of wastewater and emissions
associated with cleaning operations.

If so, materials should be stored under
cover, even if this means covering them
with a tarpaulin.

Additionally, consideration should be given
to treating materials with a rust converter
before application of a primer-topcoat

Consideration should be given to
implementing a program for just-in-time
(JIT) delivery of materials to minimize
corrosion of materials on site.
4.2    Raw Materials
4.1.2   Decision-Making Criteria

Decision-making criteria relevant  to  vendor-supplied
materials, as addressed in this chapter, are highlighted
in Table 4-1.
4.2.1   Protective Coatings and Treatments

For most operations that both fabricate products and
apply paints and coatings, steel represents the largest
portion  of vendor-supplied raw materials. Aluminum is

 also widely used in fabrication because it is lightweight
 and less susceptible to corrosion. Depending on how it
 will be used in the manufacture of a workpiece, the raw
 material may be delivered in the form of plates, sheets,
 or extrusions. Milling operations typically involve appli-
 cation of one of the following types of coatings:

 • Rolling  oils, which  are lubricants used to  minimize
   friction between the metal and the pressing machin-
   ery; also, these oils provide some corrosion protec-
   tion,  primarily  during transportation  and short-term

 • Corrosion preventatives, which are organic formula-
  tions used specifically to protect the  substrate in the
   longer term (e.g., by displacing condensed moisture).

 Although important for minimizing the corrosion of metal
 between milling and the application of a finish coating,
 protective coatings can be difficult to remove, especially
 if they  have been on the substrate for  an extended
 period.  Whereas  some of the coating may be removed
 incidentally during fabrication,  manufactured   work-
 pieces are likely to require extensive washing or abra-
 sive blasting before surfaces are sufficiently clean to
 receive paint. Thorough washing can  consume large
 volumes of water, and many of the degreasers in use
 are solvent based, raising process management issues
 involving toxic emissions and contaminated wastewater
 (see Chapter 5).  Some situations may  require the use
 of several  solvents to remove protective coatings, fur-
 ther complicating  the overall process. Abrasive blasting
 can  raise  other  pollution  prevention  considerations,
 such as dust generation (see Chapter 8).

 To minimize process  demands  and wastewater out-
 flows, the facility  operator should specify that vendors
 only use protective coatings that can be readily removed
 by washing with one of the following:

 • Ambient  water  and an aqueous degreaser

 • Hot water and a detergent solution

 • Steam or high-pressure water

 Alternatively,  the facility  operator could  purchase
 specially treated  raw materials that would not require
 application of a corrosion preventative before delivery.
 Galvanized steel, for  instance, receives  a deposition
 coating of  zinc during the milling process to provide
 corrosion   resistance.  Similarly, stainless  steel  in-
 cludes other elements (e.g., chromium, nickel, molyb-
 denum) that make the alloy nearly immune to ordinary
 rusting. While these alternatives can be more expen-
 sive, the cost should be weighed against savings in
terms of avoided process steps and reduced  waste

Another alternative is for the fabricator to  use raw ma-
terials that have  been precoated by the vendor. Coil
 coating, powder coating, and electrodeposition opera-
 tions all generally yield a vendor-applied finish that is
 sufficiently resilient for the fabricator to post-form work-
 pieces from the stock material. For instance, often sheet
 steel or aluminum undergoes coil coating operations in
 which the surface is thoroughly cleaned  before a white
 or neutral-tone finish is applied. This material can be cut
 and punched in forming operations with little or no dam-
 age to  the  surface.  Usually there is  no need for the
 fabricator to apply a topcoat to the workpiece after form-
 ing operations.

 4.2.2  Storage

 Vendor-supplied  raw  materials should  be carefully
 stored so that they will not be subjected to moisture and
 contamination. This is especially important for metal that
 has received neither a protective coating or undergone
 some type of pretreatment. Because steel is particularly
 vulnerable to corrosion, it should be stored indoors when
 possible. When outside storage is the only option, ma-
 terials should be well  covered  and raised above the
 ground. Protection from  the elements is of particular
 concern in humid or marine environments.
 If steel begins to corrode while in storage, the operator
 may be able to arrest the process with a rust converter,
 a chemical formulation that converts iron oxide to inert
 matter. Depending on the durability requirements of the
 workpiece,  a primer can be applied  directly over the
 treated  substrate, which with most converter formula-
 tions turns black within minutes. For long-term durability,
 the chemicals and oxides should be cleaned from the
 steel before a coating system is applied, either through
 surface degreasing or abrasive blasting.

 4.3  Components and Parts

 4.3.1   Protective Coatings and Primers

 Operations  that apply paints and coatings to work-
 pieces assembled on site using vendor-supplied com-
 ponents and parts should be attentive to the condition
 of delivered materials. Often, an establishment will go to
 great lengths to ensure that the surfaces of its fabricated
 pieces are thoroughly prepared for  finish coating while
 overlooking  the substrate quality of supplied compo-
 nents. A component or part that corrodes prematurely,
 however, can undermine the overall quality of an assem-
 bled product.
To avoid problems with supplied components and parts,
the  operator should require that the vendor supply ma-
terials with  a protective coating  that is consistent with
the  primer-finish coating system that will be applied. The
operator may want to specify the use  of compositions
that can be  removed using  a nonsolvent degreaser or

 For some materials, a primer coat will need to be applied
 by the vendor. When the vendor is supplying assembled
 components  that include sophisticated electronics  or
 computer circuits, for instance,  the operator should be
 directly involved in the selection  of the undercoating.
 Such sensitive and expensive components cannot be
 readily cleaned and reprimed after delivery if the under-
 coating is found to be incompatible with the finish coat-
 ing. Moreover, whenever components and parts must be
 cleaned and reprimed,  the operator incurs added costs
 and  generates unnecessary wastes. Operators  should
 always  specify the use of corrosion-resistant primers
 that are in keeping with  workpiece quality specifications.
 Additionally, operators  should  require the use of pre-
 treated  (e.g.,  galvanized steel) or finish coated (e.g.,
 electrocoated) materials when appropriate.

 4.3.2   Storage

 Storing  components and parts to protect  them from
 moisture and contaminants often is even more important
 than it is for raw materials. The  substrate of a sophisti-
 cated assembly that begins to corrode while in storage
 may be impossible to thoroughly clean. Similarly, parts
 may have intricate geometries that hide contaminants or
 the beginnings of corrosion from  view. For these rea-
 sons, along with their generally high value, components
 and parts should be stored indoors whenever possible.
 When  stored  outdoors,  they  should  be  completely
 wrapped for protection and inspected routinely.

 4.4   Just-in-Time  Delivery

Just-in-time delivery of supplies is practiced by many
 companies to control costs through the careful manage-
 ment of inventory. For paints and coatings operations,
 however, this technique can also present opportunities
for avoiding the cost of additional pretreatment for ma-
terials that have begun  to corrode while stored on site.
The degradation of vendor-supplied materials  is a par-
ticular concern for establishments that increasingly dedi-
cate available floor space to operations in an effort to
 remain competitive, while resorting to yard storage of

 At the least, implementing such a program will free up
 floor space and minimize the contamination of raw ma-
 terials and components. In the best case, close control
 of inventory might eliminate the need to receive vendor-
 supplied materials with a corrosion-prevention coating
 that ultimately must be cleaned from the  substrate.

 To  implement a successful  just-in-time  inventory pro-
 gram, the operator must work in close coordination with
 suppliers. Generally, this requires establishing computer
 links that enable the operator and principal suppliers to
 share inventory data so that they can work together in
 the tracking and in-time delivery of materials. This link
 is often established using a computer networking sys-
 tem called electronic data interchange, or EDI. In a
 highly sophisticated undertaking, computers also can be
 used to model material consumption patterns, providing
 additional data for refining inventory needs.  In some
 industry sectors, cooperative efforts between producers
 and suppliers have evolved  into strategic "partnerships"
 in the management of inventory, significantly  reducing
 the amount of time materials remain on site before they
 are needed (1).

 Even  without computer links, operators should be in
 regular contact with their principal suppliers in an effort
 to minimize the need to manage excess  inventory that
 is prone to corrosion. Frequent communication with sup-
 pliers will reduce the potential for misunderstandings
 about the need for coating-ready materials. Moreover,
 regular contact will afford an opportunity for the operator
to implement and oversee a policy according to which
 materials  would  only be accepted  if delivered in their
 agreed-upon condition.

4.5  References

 1. The Economist. 1995. Survey on retailing: Stores  of value. March 4.
  pp. 5-6.

                                             Chapter 5
      Surface Degreasing: Alternatives to Conventional Solvent-Based Methods
5.1    Introduction

5.1.1   Pollution Prevention Considerations

Thorough degreasing of  a workpiece is an essential
pretreatment step in the paints and coatings process for
ensuring proper adhesion. Even exemplary application
processes and superior coatings cannot provide a rea-
sonable  measure of durability if the various oils and
greases, corrosion products, waxy films, and tars that can
become attached to a substrate are not first cleared away.

Unfortunately, many of the chemical agents that are the
most effective for removing such contaminants from a
workpiece raise significant concerns about pollution. In-
deed, solvents in  several  conventional degreasers are
strictly regulated at the federal level and are scheduled
to be phased out of use early in the next decade under
an  international  agreement.  Facility operators that
choose to use  degreasers based on these  particular
solvents in the interim will incur the additional costs
associated with controlling hazardous air emissions. In
some cases, the  cost of the solvents themselves is
being driven up by taxes that create an incentive for
facility operators to seek out less-toxic alternatives. Be-
cause solvent-based degreasers are generally easy to
recycle, however,  operators will have opportunities to
maximize the use  of currently available stocks.

At present, aqueous degreasers represent the best al-
ternative to solvent-based  formulations in regard to pol-
lution  preventions considerations.  These water-based
solutions are already widely  used in  the industry to
remove an array of surface contaminants—from corro-
sion to waxy films. Because they are less  volatile and
do not pollute the atmosphere, aqueous degreasers are
generally less expensive to use. Nonetheless, certain
aqueous cleaning  approaches can generate  consider-
able volumes of wastewater that must be treated before
being  released  to a publicly owned treatment works.
Moreover, the use  of aqueous formulations necessitates
the addition of a rinse step to the degreasing stage.

For surface contaminants that are particularly difficult to
remove, such as heavier grease and tar, semi-aqueous
degreasers present an alternative that lies between sol-
vents  and aqueous formulations. Whereas the organic
compounds in semi-aqueous degreasers are effective
cleaning agents, they are also considered hazardous air
pollutants (HAPs).  Because semi-aqueous degreasers
are less toxic than solvents, however, they are easier
and less expensive to use.
A potential third alternative is still in development. Re-
searchers are working  on hydrofluorocarbons (MFCs)
that promise effectiveness in removing stubborn surface
contaminants and pose little or no threat to air quality.
Current indications are that the first of these may be-
come available by the end of the decade.
These  pollution prevention  considerations  are pre-
sented  in this chapter in the  context of  the various
approaches currently used to degrease workpieces.
Conventional solvent methods are discussed first, fol-
lowed by aqueous alternatives.

5.7.2   Decision-Making Criteria
Decision-making criteria relevant to surface degreasing
process efficiency and alternatives to conventional sol-
vent-based methods, as addressed in this chapter, are
highlighted in Table 5-1.

5.2   Basic Practices and Regulatory

5.2.1   Typical Oils and Grime on Substrates
The operator of a  paints and coatings  facility should
determine the best approach for cleaning workpieces
based on an assessment of the particular types of con-
taminants  on the substrate. Typically, contaminants fall
into one or more of the following categories:
• Oil and grime with a relatively low viscosity such that
  it easily flows at ambient temperatures.  These con-
  taminants may contain chlorinated  paraffins or sul-
  phurized  oils. Generally,  such  material  can be
  removed with either a solvent-based or an aqueous
• Grime with a relatively high viscosity such that it does
  not flow. These  contaminants  may include waxes,
  oxidized  resins, and  pastes or other  soft and filmy
  matter. Generally, such material can only be removed

 Table 5-1.  Decision-Making Criteria Regarding Surface Degreasing Process Efficiency and Alternatives to Conventional
            Solvent-Based Methods
 Are the workplaces that need to be
 cleaned large (e.g., assembled

 Have the workpieces already received
 a primer coating and will the cleaning
 be conducted to prepare surfaces for
 topcoat application?
 Are workpieces uncoated and will the
 cleaning be conducted to prepare
 surfaces for a primer-topcoat system?
 Are the workpieces that need to be
 cleaned small enough for vapor
 degreasing, cold cleaning, or
 conveyorized spray washing or for
 immersion in a tank?

 Are workpieces already being cleaned
 in a vapor degreaser using 1,1,1
 trichloroethane or CFC-113?
Is the use of a solvent-based
degreaser necessary, although some
degree of residue can be tolerated?
Can pretreatment specifications be met
with the use of an aqueous degreaser?
Are the workpieces that need to be
cleaned too heavy to be cleaned in a
conveyorized spray process?
Do the workpieces that need to be
cleaned have complex geometries
(e.g., channels, box sections,
crevices), making spray washing an
ineffective approach?

For the workpieces that need to be
cleaned, is the production rate
sufficiently low that continuous
degreasing operations would not be
cost effective?
 • If yes, then the most effective method would be to use high-pressure, super heated steam or
   high-pressure hot water.

 • If yes, then cleaning with high-pressure hot water is recommended.

 • Only a low concentration of detergent may be necessary (consult degreasing formulation

 • A final rinse with  hot tap water should follow the cleaning.

 • If yes, then the most effective method would be to use high-pressure, super heated steam or
   high-pressure hot water.

 • Only a low concentration of detergent may be necessary.

 • A hot tap-water rinse with a small concentration of phosphoric acid should follow the
   cleaning; this will give the substrate a slight etch and lower its pH (making it more acidic),
   resulting in enhanced coating adhesion.

 • If yes, then the use of high-pressure steam or high-pressure hot water might not be the most
   effective cleaning method.
 •  If yes, consider substituting such solvents with an aqueous degreasing system.

 •  Otherwise, consider near-term strategies such as substituting with methylene chloride,
   perchloroethylene, or trichloroethylene.

 •  Other possible temporary substitutes to consider would include alternative HCFCs.

 •  Factors to consider when selecting an alternative degreaser Include: the nature of the grime
   on workpieces, the thoroughness of cleaning required for the particular end-producfs
   application, and workpiece drying considerations.

 •  If yes, consider using a solvent that has a high boiling point and low vapor pressure to
   prevent unnecessary toxic air emissions.

 •  Regardless, avoid the use of listed hazardous air pollutants and ozone-depleting compounds.

 •  Experiment with alternative solvents to achieve  the required substrate cleanliness (e.g., some
   cold cleaning approaches, in which the  workpiece is immersed in a bath, can leave a

 •  Give strong consideration to the use of  a semi-aqueous formulation (i.e., an emulsion
   comprising solvents and water).

 •  Use of a semi-aqueous formulation in a degreasing process should include multiple rinses,
   using deionized water for the final rinse; additionally, workpieces should be dried with forced

 •  If yes, consider using these less-toxic formulations, many of which have been proven
   effective through widespread use by the industry.

 •  Aqueous degreasing processes should be given particularly close consideration for new

 •  If yes, consider a system of one or more immersion tanks.

 •  For enhanced, cost-effective cleaning, consider a system  in which the workpiece is immersed
   first in a bath of aqueous degreaser (i.e., water, detergent, surfactants, and other chemicals)
   followed by at least a tap-water rinse.

•  If yes, same as above.
• If yes, same as above.

• As a rule of thumb, a degreasing operation that cleans less than 2 feet of production per
  minute is considered too slow to be cost-effective as a continuous operation.

Table 5-1.  Decision-Making Criteria Regarding Surface Degreaslng Process Efficiency and Alternatives to Conventional
          Solvent-Based Methods (continued)
For the workplaces that need to be
cleaned, is the production rate high
enough to justify using a continuous

Regardless of the degreasing
approach used, must the cleaned
workpieces be dried rapidly to avoid
the onset of flash rusting?
• If yes, consider a conveyorized spray process.
• For enhanced, cost-effective cleaning, consider a system in which the workpiece is sprayed
  with an aqueous degreaser (i.e., water, detergent, surfactants, and other chemicals) followed
  by at least a tap-water rinse.

• If yes, it is likely that a high-temperature oven (at 230ฐ to 400ฐF) will need to be included in
  the process line.
  at higher temperatures or by using stronger solvents
  or higher-concentration aqueous degreasers.

• Grime that may contain abrasives, carbonized films,
  buffing compounds, welding smut, metal or plastic chips
  and fines,  dust, and even rust and scale (i.e., oxides
  formed during  hot  working of the metal). Generally,
  such material can only be removed using particularly
  strong inorganic acids or specialty chemicals.

5.2.2   Basic Cleaning Approaches

Workpieces can be cleaned  using any combination of
the following basic approaches (1):

• Cleaning by  mechanical or physical means, such as
  machining, abrading, pressure spraying, brushing, or
• Dissolvirig/washing  by application  of  a  chemical
• Washing/dissolving by application of an aqueous so-
• Displacing/washing by application of a detergent (i.e.,
  applying surface-active materials that displace  the
For general purpose workpieces, most cleaning opera-
tions involve either immersion of the piece in a tank of
degreasing solution (batch operations) or spraying the
piece with solution at low pressure (continuous, or con-
veyorized,  operations). Immersion is generally recom-
mended for smaller workpieces (i.e., component parts
without electrical wiring), especially those with complex
geometries (1). Whether to agitate the immersion solu-
tion can depend partially on the type of degreaser used
(see Section 5.3.2 on Degreasing with Liquid Solvent).
Spraying may be required for large workpieces, such as
truck bodies, or when the additional contaminant re-
moval afforded by impingement is an advantage.

5.2.3  Selecting a Cleaning Approach

A facility operator should follow the recommended steps
outlined below when selecting an approach for cleaning
particular types of workpieces (2):
                      1.  Determine the level of part cleanliness required.
                         Such an assessment must be based on the process-
                         flow design of the particular operation  (e.g., will a
                         high level  of  cleanliness  extend the useful life of
                         subsequent baths?)  as well as the quality require-
                         ments of the finished workpiece (e.g., do specifica-
                         tions call  for a coating with long-term durability in
                         extreme use conditions?).

                      2.  Research and make preliminary selections of the
                         most appropriate degreasers and associated equip-
                         ment for achieving  the required level of cleaning.
                         This involves reviewing vendor literature and consid-
                         ering the cost and waste-generation implications of
                         various  options.

                      3.  Test run selected degreasers and associated equip-
                         ment to confirm satisfactory performance under all
                         anticipated operating conditions.  Operators should
                         test similar degreasers from more than  one vendor
                         because a slight variation in formulation can result
                         in a higher level of effectiveness. Even  generic de-
                         greasers can vary in their formulations.

                      4.  Negotiate price with vendors of degreaser and asso-
                         ciated equipment. It pays to shop around, particularly
                         when the  operator has tested similar products that
                         yield nearly the same  results. In negotiating, the
                         operator may  want to establish that the vendor will
                         provide  training  and support  in use  of the product
                         and any associated equipment.

                      5.  Make final selections and apply for any operational
                         and waste-related permits required by federal, state,
                         or local  authorities. The operator may need to estab-
                         lish or modify recordkeeping procedures  based on
                         permit  requirements  (e.g., for reporting  on emis-
                         sions, water discharges, and waste disposal).

                      6.  Implement the  cleaning  approach.  The operator
                         should allow sufficient startup time for training em-
                         ployees and to  refine the process. Quality control
                         procedures should be developed and distributed.

5.2.4   Regulatory Overview

When  assessing the appropriateness of various de-
greasers for a particular process, the facility operator
should investigate the applicability of federal, state, or
local regulations concerning the use of specific cleaning
agents. The costs associated with some requirements
can make the use of some solvent-based degreasers
prohibitive, particularly for smaller operations. Most con-
ventional solvent-based degreasers used in paints and
coatings operations  come  under the following regula-

•  Title III of the Clean  Air Act Amendments of 1990:
  This federal regulation establishes limits on the emis-
  sion of HAPs, including those from certain degreas-
  ing solvents. Operations whose solvent emissions
  exceed these limits may be required to perform risk
  analyses and to install Maximum Achievable Control
  Technology (MACT).

• Occupational  Safety  and  Health Administration
  (OSHA) regulations: These federal  regulations estab-
  lish limits on emissions of HAPs from materials con-
  sidered particularly hazardous, including those from
  certain degreasing solvents.

• The  Montreal Protocol: This international agreement
  and  subsequent related  federal regulations require
  that certain ozone-depleting compounds (ODCs) be
  phased out within the next  several  years. Under this
  agreement, the use of chlorofluorocarbon 113 (CFC-
  113) and  1,1,1 trichloroethane (methyl chloroform),
  the two most commonly used compounds  in vapor
  degreasing operations, will be banned  by 2000 and
  2002, respectively. In the United States, the manu-
  facture of both compounds will cease after 1995,
  leaving several years for inventories to be exhausted.
  Also under this agreement, fluorinated hydrocarbons
  (HCFCs), some of which also are included in solvent-
  based formulations, are expected to be phased out
  between 2020 and 2040.

• State permit rules: Under the Clean Air Act Amend-
  ments (i.e., the Title V Permit Rule), states are re-
  quired to monitor "major" source categories of various
  pollutants, including compounds found in many de-
  greasing solvents. Thus, facility operators must apply
  for a state permit  before using solvent degreasers
  that include regulated compounds. Applications for
  Title V permits are  required as of 1995;  state compli-
  ance officials notify facility operators directly about
  the deadline for submitting  an application. State offi-
  cials may determine that an operation comes within
  the "minor" source category based on an assessment
  of the concentrations of listed compounds the appli-
  cant expects to use and the effectiveness of emission
  control equipment. The advantage of  being desig-
   nated a minor source is that applicable requirements
   are less stringent.

 • State Implementation Plans (SIPs): These programs
   monitor emissions of volatile organic compounds
   (VOCs), including those from solvent degreasers and
   solvent cleaning operations. Facility operators are ad-
   vised to become familiar with VOC regulations in the
   state in which their facility is located.

 More detailed information on  regulatory considerations
 specific to the paints and coatings industry is available
 in the literature (3-5).

 5.3  Solvent-Based Methods

 Solvent-based  methods for degreasing and cleaning1
 workpieces  have been widely used throughout the in-
 dustry for many  years  because they  are particularly
 effective for removing surface contaminants from metals
 and high-performance plastics. Moreover, because they
 clean thoroughly and then evaporate in the ambient air
 without  leaving surfactant residues on the substrate, no
 rinsing steps or oven drying is required. Another advan-
 tage is that, given the effectiveness of solvents in both
 their vapor and liquid forms, facility operators can use
 this method for many different types of cleaning applica-
 tions. For example, vapor degreasing is widely used in
 the aerospace  and electronics industries  for cleaning
 entire pieces with complex geometries. In contrast, wipe
 cleaning with  liquid solvents at ambient  temperature
 (cold cleaning) enables line operators to degrease spe-
 cific sections of workpieces that integrate sophisticated

 Solvent use, however, generates emissions that are
 considered hazardous to the  atmosphere and  pose a
 threat to  human  health. As a  result,  the paints and
 coatings industry is investigating alternative degreasing
 and cleaning methods as well as ways  to use solvents
 more efficiently while controlling emissions. This section
 discusses the advantages and disadvantages of sol-
vent-based  degreasing  and cleaning  methods along
with recommended practices.

 5.3.1   Vapor-Solvent Degreasing   Introduction

Over the years, vapor degreasing has been widely used
in paints and coatings operations to clean the surface of
various  metals, ceramics, high-performance plastics,
and electric and electronic components (e.g.,  printed
1 In this document generally, "degreasing" refers to the various liq-
 uid/vapor methods used in paints and coatings operations to dean
 substrates. The author recognizes, however, that some facility op-
 erators use the term degreasing to refer specifically to vapor de-
 greasing. Thus, this particular chapter discusses vapor-solvent degreasing
 and cold-solvent cleaning as distinct pretreatment methods.

 circuit boards). The process involves subjecting work-
 pieces to the vapor form of a chlorinated solvent, typi-
 cally 1,1,1 trichloroethane, CFC-113, trichloroethylene, or
 perchloroethylene (perc).

 Vapor degreasers are highly effective in removing sub-
 strate contaminants without leaving  a film of residue,
 making them particularly well suited  to the demanding
 requirements of the electronics and  aerospace indus-
 tries. Additionally, capital and operating costs are low
 because this fairly simple, one-step cleaning approach
 requires only minimal floor space and  limited line opera-
 tor training; moreover, the process can be readily auto-
 mated.  Another advantage is that, given the vapor's
 rapid evaporation rate, workpieces  can be air dried,
 thereby avoiding the cost of a drying  oven.

 The principal limitation of this approach is that emissions
 from solvents in conventional vapor degreasers can be
 damaging to the environment. Moreover, some evi-
 dence indicates that long-term exposure to certain con-
 centrations of these compounds can pose human health
 risks, a particular concern for line operators. Because
 CFC-113  and  1,1,1  trichloroethane are  considered
 ODCs, the United States and numerous other countries
 have agreed to phase out their use within the next 10
 years under the Montreal Protocol and the Clean Air Act.

 Other solvents used in vapor degreasing are being con-
 sidered for further regulation at various levels of govern-
 ment. Perc,  for example, is a listed  toxic  air pollutant
 under the Clean Air Act and is covered in a proposed
 National Emissions Standard for Hazardous Air  Pollut-
 ants (NESHAP) (also known as the MACT Standard for
 Halogenated Solvent Cleaning Operations, 40 Code of
 Federal Regulations Part 63, Subpart T). Thus, despite
 the advantages associated with conventional vapor de-
 greasers, alternative formulations  are likely to be more
 cost effective over the long term for most operations.   Process Basics and Best  Management

 In vapor degreasing, workpieces are suspended at am-
 bient temperature in the headspace of a tank of heated
 degreaser solution, where they are subjected to chlorin-
 ated solvent in a vapor form. As the solvent vapor comes
 in contact with  the cool surface of the  workpieces,  it
condenses into a liquid, dissolving contaminants  and
carrying them off  into the degreaser tank as drainage.
There the heavier contaminants gradually sink  to the
bottom. Because vapor degreasing works on the basis
of condensation, the cleaning action slows as the tem-
perature of the substrate rises. Typically, workpieces are
suspended in the degreaser tank headspace until the
substrate temperature rises to that  of the vapor, at which
point condensation stops.
 For the most part, the vapor degreasing tank is a closed-
 loop system in which vapor that does not condense on
 the workpiece collects on chiller  coils that run  up the
 walls of the tank. Figure 5-1  illustrates a typical vapor
 degreasing process. In such a system, condensate that
 forms on the chiller coils runs off into a separator, which
 removes water and allows solvent to drain back into the
 tank. Contaminants captured by filters during recycling
 are disposed of as sludge. The small amounts of vapor
 that do escape from the degreasing operation are either
 recycled or, if permitted, exhausted to the atmosphere.
 Inadequately recycled or exhausted vapors can pose a
 hazard to line operators.

 Best management practices for enhancing  process effi-
 ciency in the degreasing operation include the following
 (2, 7):

 •  For thorough cleaning, workpieces should be kept in
   the vapor zone until condensation  has ceased.

 •  To control drag-out, workpieces should  be removed
   slowly, allowing vapors to be drawn off  into the ex-
   haust system (i.e., a minimum of about  15 seconds
   or until parts are visibly dry). Workpieces  that have
   porous  substrates, which tend to  entrap solvents,
   should be degreased by an aqueous or semi-aque-
   ous method.

 •  To minimize emissions and ensure efficient solvent
   use, degreasing operations should  be conducted in
   an enclosed area and the temperature of the de-
   greaser solution should be monitored to control the
   rate at which vapors rise to the workpiece. Also, to
   minimize  turbulence  in vapor zone,  workpieces
   should be moved in and out slowly.

 • To control fugitive emissions and enhance  recycling,
  vapor tanks should have a minimum freeboard ratio
   (i.e., depth  to vapor zone relative to width  of the
  tank's opening) of 0.75,  although  a ratio  of  1.0 or
  greater is preferable. This step can be enhanced fur-
  ther with the addition of refrigeration. With a  higher
  freeboard, vapors can  be more effectively captured
   by chiller coils for recycling.

Other suggested practices include:

• Turning off the  unit's exhaust system when the de-
  greaser is covered so that vapors are not  unneces-
  sarily drawn from the tank.

• Ensuring that when adding solvent the flow is slow
  enough that splashing is prevented.

• Being careful to avoid overloading the  degreasing

• Racking parts for thorough drainage.

• Storing  both fresh  and used solvent in closed con-

                                                  Diffusion and
Exhaust -, Wq|
- 7- ^T \ i
/ , Air
Of 0
o ป o
o 	 t 	 | 	 	 _o
"""" "" " i "" ^"™" . ." ^ ^~
• ( ' . \ ' '• 1
' i • i 1 . . I • .
'.',(••"' •!•/•/
. / (J; vapor ;!.-'•


Figure 5-1.  Schematic of a typical solvent vapor degreasing process (6).
• Adding a refrigerated freeboard chiller, either above
  or below freezing, which in some situations can yield
  control efficiencies of 40 percent.

• Ensuring that the degreasing tank is not undersized.

• Minimizing agitation of the liquid solvent.

• Designing the degreaser cover to be below the cross-
  ventilation ports at the top of the tank.

• Covering the degreaser tank whenever possible, par-
  ticularly when not in use; in some cases, keeping the
  tank covered while the parts are suspended in the
  vapors by be feasible.   Operational Strategies Involving the  Use
         of Conventional Vapor-Solvent

Depending on the particular coatings  operation,  some
facilities may be able to comply with near-term air quality
regulations by using perc, methylene chloride, ortrichlo-
roethylene as a vapor degreasing solvent (8). All three
are cost-effective  alternatives to. CFC-113 and  1,1,1
trichloroethane, which are ODCs, and none of them is
currently being considered for phasing out. Moreover,
they can  be used in conventional degreasing equipment
with little or no retrofitting (2). Table 5-2 presents chemi-
cal  formulas of vapor degreasing  solvents along with
their respective boiling points. Solvents with a higher
Table 5-2. Relative Boiling Points of Principal Degreasing
         Solvents (9)
Methylene chloride
1,1,1 Trichloroethane
Point (ฐF)
boiling point condense faster when they enter the lower
temperature of the degreasing tank headspace.

Although emissions from these alternative solvents are
generally considered less damaging to the atmosphere
than conventional formulations, their use is controlled
under various regulatory standards:

• Perc is considered a VOC as  well as a HAP, and
  restrictions on its  use have been proposed by the
  EPA under the Clean Air Act. Perc is recommended
  as a degreasing solvent over both methylene chloride
  and trichloroethylene because it has a higher boiling
  point, making vapor emissions easier to control. Gen-
  erally, facility operators that use perc can keep emis-
  sions below 50 ppm, the threshold limit value  (TLV)
  established by the American Council of Governmen-
  tal Industrial Hygienists (ACGIH).

 • Methylene chloride, a suspected carcinogen, is regu-
   lated as a HAP under the Clean Air Act. Although not
   considered a VOC due to its negligible photochemical
   reactivities, OSHA is seeking to lower its permissible
   exposure level (PEL) from 500 ppm to 25 ppm. More-
   over, methylene chloride is covered  along with perc
   by EPA's proposed NESHAP for solvent degreasing.

 • Trichloroethylene is regulated as a VOC and a  HAP
   under the Clean Air Act.

 More generally,  if the state permitting  authority deter-
 mines that an operation submitting  an application for
 one of these solvents is likely to exceed federal or state
 TLVs, the facility may be considered to  come within the
 "major source" category under Titles III and  V of the
 Clean Air Act.2 Air quality control requirements for facili-
 ties in this category can increase the cost of operation.
 For instance, a major source facility  might be required
 to install emission abatement devices such as thermal
 or  catalytic oxidizers, zeolite adsorbers, or biofilters.
 Thus, facility operators should perform  a thorough
 analysis of the "potential to emit," as defined in Titles III
 and V, before switching to one of these alternatives.

 Some of the available alternative degreasers include:

 • HCFC-141b: Although this solvent, manufactured by
  Allied Signal, is a VOC, it has a low ozone-depleting
  potential. Nonetheless, it can only  be used in clean-
  ing operations through 1996 and only at facilities
  where it (pas been in use since late in 1994. Complete
  phaseout of the solvent is scheduled for 2002.

 • HCFC-225: This  solvent,  manufactured  by AGA
  Chemicals, has an even lower ozone-depleting po-
  tential than HCFC-141b and can be used until 2020,
  at which time  it will be banned from use.

 • HCFC-123: This solvent, manufactured by DuPont,
  appears to offer low toxicity; however, it is not in wide

Additional possible interim strategies include:

• Use HFCs for vapor degreasing and drying until De-
  cember 31, 1999, after which the HFCs must be

• Use a relatively nonvolatile solvent for cleaning and
  an HFC solvent for drying until December 31, 1999,
  after which the HFCs  must be replaced.

The best long-term strategy may be to  switch to a de-
greaser that does not emit HAPs. Numerous aqueous
 Under Title III, a major source is one that has the potential to emit
 greater than 10 tons per year (tpy) of a single HAP or greater than
 25 tpy of more than one HAP. Under Tilte V, a major source is one
 that has the potential to emit greater than 100 tpy of VOCs, greater
 than 10 tpy of a single HAP, or greater than 25 tpy of more than one
 HAP. Under both Title III and V, other conditions also can apply to
 qualify a source as "major."
 and semi-aqueous degreasers are currently available,
 and others are in development. Although for certain
 high-value processes the effectiveness of present for-
 mulations as replacements for solvent degreasers has
 yet to be demonstrated, many facility operators are likely
 to find them well suited to their needs. One limitation is
 that aqueous  degreasers generally require a multiple-
 step process  (i.e., cleaning then  rinsing) followed by
 drying in a high-temperature oven. As a  result, capital
 costs can be higher. Aqueous and semi-aqueous formu-
 lations are discussed in detail in Section 5.4.

 Also in development are HFCs that neither deplete the
 ozone nor are  considered  to be  VOCs due to their
 negligible photochemical reactivity to the atmosphere.
 The challenge for researchers will be to formulate a
 degreaser that has both good substrate  cleaning and
 thorough drying characteristics. Some of these alterna-
 tive solvents are expected to be available before the end
 of the decade.

 One encouraging development concerns perfluorinated
 carbon compounds (PFCs) that contain only carbon and
 fluorine and are considered to be neither  VOCs (smog
 formers) nor ODCs. These compounds may be devel-
 oped for use  as alternative drying agents. PFCs  are
 more volatile than 1,1,1  trichloroethane and CFC-113
 and thus would serve as an ideal  replacement for op-
 erations  in which fast drying  is mandatory (e.g., for
 workpieces with complex geometries). Although there
 are concerns that these compounds contribute to global
 warming,  EPA  has approved them for the Significant
 New Alternatives Program (SNAP)  1.

 More generally, if facility operators  follow the proposed
 NESHAP for halogenated solvents,  they should be able
 to run their processes  well within OSHA requirements
 and easily meet permit limits.

 5.3.2  Degreasing With Liquid Solvent (Cold
       Cleaning and Solvent Wiping)   Introduction

 Solvents in liquid form  are widely used for degreasing
 workpieces before applying  a primer-topcoat system.
 This method—often called cold cleaning  because the
 solvent is  unheated, in contrast to vapor degreasing—
 involves bringing workpieces into direct contact with a
 solvent, such as methyl isobutyl ketone (MIBK), methyl
 ethyl ketone (MEK), or 1,1,1 trichloroethane (Table 5-3).

The great advantage of degreasers  in  liquid form is their
 versatility. They can be used to clean entire workpieces
 by immersion or spray washing (i.e., cold cleaning), for
 instance, or to  clean selected areas of a component
 using rags, brushes, or cotton swabs  (i.e., solvent wip-
ing). Figure 5-2 illustrates a typical cold-solvent cleaning

Table 5-3.  Typical Organic Solvents Used in Degreasing
 Solvent Group
 tions off site to commercial operations. These vendors
 typically recycle the spent solution and sell the recycled
 solvent at low cost.


Ester solvents

Aliphatic solvents
Aromatic solvents

Chlorinated solvents

Fluorinated solvents
Isopropanol, methanol, ethanol,

Acetone, methyl isobuty! ketone (MIBK),
methyl ethyl ketone (MEK)

Ethyl acetate, isobutyl isobutyrate,
isopropyl acetate, glycol ether acetate

Hexanes, mineral spirits (made up of
many different aliphatic petroleum
fractions), heptane and higher
molecular-weight fractions

Toluene, xylene

Methylene chloride, trichloroethylene,
1,1,1 trichloroethane, perchloroethylene

Freons (chlorofluorocarbons) (a wide
range is available; CFC-113 is the most
widely used for degreasing)
Figure 5-2.  Schematic of a  typical cold cleaning degreasing
           process (6).

In general, these approaches are effective for dissolving
a wide range of oils, greases, and waxes, particularly on
metal substrates but also on certain high-performance
plastic workpieces with solvent-insensitive components.
Like vapor degreasing, capital costs for cold-solvent
cleaning operations are generally low,  given  minimal
requirements for equipment, floor space, and  training.
Additionally, spent solvent can be easily  distilled  and
recycled on site. In states where typical cleaning  sol-
vents are regulated as a hazardous material, however,
most facility operators send exhausted  cleaning solu-
 As with vapor degreasing, the principal limitation of cold
 cleaning is that emissions  from conventional solvents
 can be damaging to the environment and may pose a
 threat to human  health.  Other  limitations of this ap-
 proach include:

 •  If the solvent evaporates  from a metal workpiece too
   quickly, atmospheric moisture can condense on the
   substrate and promote corrosion.

 •  Some solvents, especially after they have been re-
   cycled, leave a residue  on the substrate that can
   undermine coating adhesion.

 •  Solvents with low flashpoints can cause fires or ex-

 Given that vapor degreasing is generally more thorough,
 facility operators typically opt for the cold-solvent clean-
 ing approach when residues on  the workpiece can be
 tolerated and costs are a critical factor.  Process Basics and Best Management
 Typically, cleaning workpieces with a  liquid solvent in-
 volves one of the following approaches:
 •  Immersing the workpiece into a solvent bath.
 •  Spraying the workpiece with solvent at low pressure.
 •  Wiping/scrubbing the workpiece with a brush/brush
   dipped in solvent.
 Facility operators also use liquid solvent to clean  coat-
 ings application equipment, such as spray guns. The
 cold cleaning method is used predominantly, however,
 to clean small workpieces,  such  as parts,  rather than
 workpieces with expansive and complex geometries.

 Cold-solvent cleaning systems should be configured to
 catch as much solvent as possible as it drains from the
 workpiece. Thus,  when the operation involves immer-
 sion or spraying, the  workpiece  should be allowed to
 drain over the solvent tank for a minimum of 15 seconds
 or until it is visibly dry. Wiping or brushing operations
 should be carried  out such  that solvent drains back to
the tank for reuse.
As in vapor degreasing,  solvent  emissions should be
 kept to a minimum in cold cleaning operations so that
the cleaning formulation is not exhausted unnecessarily.
 For this reason, solvents with low vapor pressures and
 high boiling points are preferred. Also, the solvent tank
 should be covered when not in use and the tank should
 be regularly checked for leaks using a halon detector.
 Facility operators must weigh the cleaning effectiveness
 afforded by either adding agitation to the immersion step

 or increasing the spray impingement against the result-
 ing loss of solvent to evaporation.

 Solvent vapors that are emitted during cleaning opera-
 tions can be captured using an exhaust system with low
 vacuum pressure, to avoid drawing vapors off the sur-
 face of the tank. When substrate cleaning specifications
 necessitate the use of a relatively volatile solvent (e.g.,
 toluene and xylene in wipe cleaning operations),  the
 tank  should be equipped with  chiller coils that capture
 vapor and drain the condensed solvent back to the tank.

 Best  management practices for enhancing process effi-
 ciency in the cold-solvent cleaning operations include
 the following:

 •  For thorough  immersion cleaning, a facility operator
   should provide additional solvent tanks rather than
   overload a single tank.

 •  To  minimize emissions and  ensure efficient solvent
   use, cleaning  operations should be conducted in an
   enclosed area; if solvent is  heavier than water and
   not miscible, a water cover  (i.e., a shallow layer of
   water on top  of the solvent) should be used as a
   vapor barrier; tank solvent should be replenished us-
   ing an enclosed pumping system.

 •  To  manage contaminated cleaning materials effec-
   tively, any  solvent-laden rags should be stored in
   closed containers and specially permitted laundries
   should be fhired to recycle solvent from rags; when
   disposing of rags as hazardous waste,  they should
   be  kept separate from other wastes for cost advan-

 •  To  control  drag-out, workpieces that have porous
   substrates, which tend to entrap solvent, should be
   degreased by aqueous or semi-aqueous methods.  Operational Strategies Involving the Use
        of Conventional Liquid Solvents

 Facility  operators are strongly  advised  to consider
 switching  to  aqueous  degreasers when workpiece
 specifications make such alternatives feasible. For situ-
 ations where the use of aqueous formulations would not
 be appropriate, operators should investigate the effec-
tiveness of solvents that have a high boiling point (i.e.,
 low vapor emissions) and that are not VOCs, HAPs, or

5.4   Aqueous Methods

 Degreasing with aqueous-based solutions represents
an attractive alternative to solvent-based methods. Both
aqueous and semi-aqueous formulations are less toxic
than conventional solvents and their ability to remove
stubborn surface contaminants has been well estab-
lished throughout the industry.  (Appendix A presents a
 selected list of aqueous and semi-aqueous products on
 the  market,  along with information on their recom-
 mended use.) Despite the need for facility operators to
 include rinsing and drying steps for aqueous cleaning,
 many have found these formulations to be cost-effective
 alternatives because capital outlays associated with pol-
 lution prevention can  be minimized. Moreover, like sol-
 vents, the versatility of  aqueous solutions make them
 adaptable to a variety of degreasing approaches (e.g.,
 in an immersion tank;  in  a heated, high-pressure spray).

 To achieve maximum effectiveness when using aqueous-
 based formulations, it  is  particularly important for facility
 operators to fully understand process basics and recom-
 mended practices. For instance, even when using these
 less-toxic degreasers, facility operators will need to ad-
 dress some waste management and pollution preven-
 tion  issues. Thus, this section discusses aqueous and
 semi-aqueous methods  in thes context of process effi-
 ciency, while touching  on potential limitations associated
 with these alternative  formulations.

 5.4.1  Aqueous Degreasing   Introduction

 Aqueous degreasing is by far the most common method
 for cleaning small parts and large workpieces before
 they are painted. Numerous facilities that for many years
 have relied on vapor degreasing and cold-liquid  clean-
 ing methods have  converted  to aqueous and  semi-
 aqueous  methods,  primarily because  they  minimize
 concerns about pollution.

 Aqueous degreasers include a base (e.g., sodium hy-
 droxide), water, and one or more other ingredients (i.e.,
 saponifiers, surfactants,  chelating agents, corrosion in-
 hibitors, or acidic or alkaline agents). By enhancing the
 properties of  water that make it a  universal inorganic
 solvent, these formulations  are  able to remove oils,
 greases,  waxes,  and  similar  organic compounds
 through solvation, detergency, and/or chemical reaction.

 Because chemical compounds used in aqueous de-
 greasers are less volatile and for the most part are not
 considered VOCs or HAPs, these cleaning formulations
 are  subject to less-stringent  regulatory constraints.
 Given that less, if any,  air pollution is  generated  by
 aqueous degreasing operations, this cleaning approach
 is regarded as a cost-effective alternative for the longer
 term. A list of general advantages and limitations asso-
ciated with aqueous degreasing is presented in Table 5-4.

The  primary distinction  between various aqueous de-
 greasing formulations  is  whether they are acid or alka-
line based. A selected list of both types  of cleaners is
presented in  Table  5-5.  Generally, acid-based  de-
 greasers are more active formulations  and thus are
preferred for removing corrosion and scale from metal

 Table 5-4.  Considerations for Aqueous Degreasing

 Advantages              Disadvantages
 Does not emit solvent
 vapors (VOCs, HAPs, or
 ODCs) into the air

 Removes most contaminants
 (e.g., oils, greases, hydraulic
 fluids) and more stubborn
 contaminants (e.g., smut,
 metal fines) if agitation is

 Can be used in batch or
 continuous operations

 Well suited to cleaning
 processes that will be
 followed by a phosphate

 A dry-off oven may not be
 necessary if the parts will be
 coated with a water-borne
 coating (e.g., electrocoating)

 Monitoring of chemicals is
 not complicated; process
 includes a pH check and
 control of temperature,
 processing time, agitation in
 the form of air sparging (for
 immersion tanks); good
 Impingement (for tunnel

 Can be used for all types of
 parts, regardless of whether
 they are solvent sensitive
 Requires more floor space than
 vapor degreasing or cold-solvent
 cleaning operations

 Cannot be used to clean parts that
 are moisture sensitive (e.g.,
 assembled electronic components)

 Usually requires a dry-off oven,
 which consumes energy; inadequate
 drying can promote flash rusting

 Large parts may be more difficult to

 Operator may need to experiment
 with various degreasing chemicals if
 stubborn deposits are not easily

 Poor rinsing can contribute to paint

 Additional  quality control is required
 (in contrast to vapor degreasing) if
 surfaces must be especially clean

 Water may collect in channels and
 pockets, from where it may not
 thoroughly evaporate

 Water and degreaser may get
 between overlapping joints on certain
 workpieces and later seep out and
 mar the coating if inadequate oven
 drying is carried out

 Metal surfaces, which are slightly
alkaline after degreasing, must be
 neutralized with an acidic solution
 (e.g., a phosphate coating) before
 paint can be applied

Wastewater must be treated before it
can be disposed
Table 5-5.  Selected Aqueous Oegreasers (1)

• Ammonium hydroxide, potassium hydroxide, sodium hydroxide

• Diethylene glycol monobutyl ether

• Dodecanedionic acid

• Ethylenediaminetetra-acetic acid (EDTA) and its tetrasodium salt

• Monoethanolamine,  diethanolamine, triethanolamine

• Borax

• Sodium carbonate

• Sodium gluconate

• Sodium silicate, sodium metasilicate

• Sodium tripolyphosphate, trisodium phosphate, tetrasodium
  phosphate, tetrapotassium pyrophosphate

• Sodium xylene sulfonate

• Water (tap, deionized, steam)
 workpieces.  In contrast, because they are somewhat
 milder,  alkaline  formulations  are  recommended for
 cleaning plastics as well  as certain metal substrates,
 such as aluminum, particularly when the corrosivity of
 acid degreasers is a concern. Alkaline solutions can
 effectively  remove such contaminants as  oil, grease,
 and waxy films. Because both types of formulations are
 corrosive, cleaning system operators must take precau-
 tions, such as wearing  protective equipment, to avoid
 sustaining  chemical burns.

 Although acid degreasing is more effective for certain
 substrates, the corrosivity of acid compounds necessi-
 tates the use of more expensive containment equipment
 and additional maintenance. For  instance,  to prevent
 corrosion of immersion  tanks, they must be lined  with
 rubber or plastic or made of stainless  steel. Indeed,
 inhibitors are often added  to the degreasing solution to
 prevent the corrosion of tanks. These formulations  also
 solubilize heavy metals from substrates and etch steel,
 thus generating more sludge that must be disposed  of
 as a hazardous waste. Moreover, because acid cleaners
 can cause hydrogen embrittlement of the substrate, this
 approach should not be used for workpieces  made  of
 high-tensile steel. Finally, without thorough rinsing or the
 incorporation of inhibitors,  acids in the cleaning solution
 can promote  corrosion of the finished workpiece.

 Alkaline formulations are  not without their limitations,
 however. For example, trace alkalinity may be difficult to
 rinse from the workpiece. Also, certain substrates,  par-
 ticularly on  some electrical components, may be subject
 to corrosion under alkaline,  rather than acidic, conditions.

 Aqueous degreasing generally allows facility operators
 to avoid costs associated with pollution prevention,  par-
 ticularly air emissions control devices. Capital equip-
 ment and process  requirements, however,  ca'n add  to
 operation costs.  In contrast to one-step solvent  ap-
 proaches, aqueous degreasing  involves at least a two-
 step process in which  acidic or alkaline residues are
 rinsed from the workpiece following degreasing. More
 often, however, operators use a three-step system  that
 includes drying the rinsed  workpiece in an oven before
applying paint or a pretreatment coating. (For detailed
discussions about phosphating  and rinsing, see Chap-
ters 6 and 7,  respectively.)   Process Basics and  Best Management

Typically, aqueous  degreasing operations involve sub-
jecting  workpieces to   the cleaning solution  either
through immersion or pressure spraying. The most basic
process includes a cleaning step followed by rinsing  that
adjusts the  pH level of the  substrate by removing acidic
or alkaline  residues. The system should be configured
to allow the degreasing solution to thoroughly drain from
the workpiece, thus minimizing drag-out into the rinsing

 bath. Proper draining of workpieces also facilitates effi-
 cient use of the cleaning solution. Following immersion,
 a workpiece should be allowed to drain while suspended
 over the tank; in spray operations, a containment system
 should  be  used to channel drainage back to the feed
 source. Both immersion and spraying lend themselves
 to automation.

 An advantage of aqueous degreasing  over the liquid
 solvent method is that agitation can be readily added to
 the immersion process, given that the cleaning solution
 is less  volatile and therefore less  likely to evaporate.
 Agitation is particularly recommended for cleaning either
 workpieces with  complex  geometries  (e.g., with  re-
 cesses or threaded sections) or numerous small parts
 at one time. The immersion approach in general is ad-
 vantageous when floor space is limited.
 Some facilities enhance the effectiveness of conventional
 spray cleaning operations by using  either super-heated
 steam or high-pressure hot water. Both approaches,
 loosely referred to as steam cleaning, involve a pumping
 system that mixes heated water with the cleaning solu-
 tion  and delivers it via  a hose to  the spray wand. In
 general, steam cleaning  is used on  workpieces that are
 too large to fit in an immersion tank or to pass through
 a conveyorized spray system. The  major disadvantage
 of such cleaning  methods is  that they consume large
 amounts of water, which must be treated before being
 discharged to a publicly  owned treatment works.
 For  true steam cleaning, water is typically  heated to
 approximately 230ฐF (i.e., well above the boiling point of
 water) and  the super heated steam is sprayed at a
 pressure of 50 to  150 psi. Steam can be effective  for
 removing particularly stubborn contaminants. This ap-
 proach  also is recommended for minimizing  water us-
 age  and promoting rapid drying of the  substrate. The
 principal disadvantage of this approach is that line op-
 erators can be scalded easily by super heated steam, in
 part  because it is nearly invisible as it comes off the
 spray wand.
 High-pressure hot water spraying reduces the likelihood
 of worker injury because the water is heated to a tem-
 perature below the boiling point and sprayed at pres-
 sures ranging from 50 to 100 psi. Despite the lower
 temperature of the water, this approach, which includes
 use of a soap detergent typically drawn from a 55-gallon
 drum, can be highly effective for removing many of the
 same deep-seated contaminants from a substrate. (The
 appropriate  concentration  of the detergent should  be
 based on the manufacturer's recommendation.) Addi-
tionally, a system can be set up in which  a single spray
 wand is used to clean and rinse a workpiece  and then,
for a metal substrate, apply a mild  phosphate coating.
 In such a system, the process operator can control a
valve that shuts off the degreaser formulation feed and
turns on the phosphate feed. As with the degreaser, the
 phosphate typically is siphoned from a 55-gallon drum.
 After applying a low-concentration phosphoric acid (e.g.,
 2 oz/gal) to the workpiece and allowing for a 45  to 60
 second contact time, the operator can give the piece a
 final rinse with clean, hot water.

 The following factors apply with this approach to phos-
 phating (see also Chapter 6 for an extensive discussion
 of phosphate deposition considerations):

 • Phosphoric acid should be syphoned directly to the
  wand rather than to the hot water  heater, where it
  might encourage corrosion of the heating coils.

 • The light  phosphate coating  deposited  with this
  method can provide only short-term protection  (sev-
  eral hours) against flash  rusting; it should  not  be
  compared with conventional iron or zinc phosphates,
  which provide conversion  coatings with significantly
  greater corrosion resistance.

 • Despite  deposition of the phosphate  coating, the
  workpiece should be dried quickly to avoid potential
  flash rusting,  especially on workpieces with complex

 • Whereas blow drying  is recommended, the process
  operator should ensure that moisture or oil is not
  conveyed to the workpiece with the ambient air  com-
  ing from the compressor. The blower system's oil and
  moisture traps should be checked frequently.

 • Because phosphate cannot form over scale or rust,
  in some cases the deposited coating will provide little
  corrosion protection for hot rolled steel with such con-
  taminants on  the substrate. The acid will  neutralize,
  however, any alkalinity that may remain on the sub-
  strate after alkaline degreasing—a critical parameter
  for adhesion of  the  primer coat.

 Regardless  of the aqueous  cleaning  approach  used,
such operations generate wastewater that must  be
treated before being  exhausted to a  publicly  owned
treatment works. Generally, spent washwater is dumped
or drained into a settling tank. Oil and  grease that rise
to the top are skimmed off and usually either 1) sent off
site to be blended into  a fuels that can be thermally
oxidized or 2) disposed of as a liquid hazardous waste.
Contaminants pumped out from the bottom often are
passed through a filter press, dried into a cake, and then
disposed of as a solid hazardous or nonhazardous
waste, depending on the characteristics. In some cases,
the dried sludge is used as an inert filler in other opera-
tions. The remaining water is treated for pH adjustment
and  then either discharged to the treatment works or
dumped into a shallow holding pond, where it is allowed
to evaporate. Many large facilities have begun recycling
all of their process water to the cleaning operation fol-
lowing onsite treatment. In this way, many such facilities
are seeking to achieve closed-loop operations.

 Best management practices for enhancing process effi-
 ciency in the aqueous degreasing operation include the

 • For thorough cleaning:
   - Experiment with different aqueous degreaser prod-
     ucts  and contact  several vendors to identify the
     formulation best suited to particular workpieces.
   - Test the entire range of degreasers recommended
     by a vendor because solution formulation and tem-
     perature can alter cleaning efficiency.
   — Ensure that batches of parts immersed in the de-
     greaser are properly positioned to  avoid  overlap
     and to minimize drag-out.

 • For cost effectiveness:
   - Raise the temperature of the degreasing solution
     and the rinse water to avoid the expense associ-
     ated  with removing flash rusting, especially in op-
     erations that do not include a  drying oven. (The
     facility operator should keep in mind, however, that
     heating  the  degreaser will add to energy  costs
   - Compare costs of  powdered and liquid degreasing

 • For process efficiency:
   - Purchase degreaser and phosphate formulations
     from   the same  vendor to avoid compatibility
     problems.   Process Variations

 Two-Step Process

 A two-step aqueous degreasing process involves clean-
 ing and then  rinsing the workpiece, usually with tap
 water.  In  a paints and coatings operation, this  basic
 system for degreasing might be used, for example, be-
 tween application of the primer and the finish coat.  Such
 a process might be used when primed workpieces will
 be stored outdoors for weeks or months before being used
 in the assembly of a finished product. Degreasing would
 be performed immediately before application of the top-
 coat to remove any fingerprints and  general grime, in-
 dustrial oils, or hydraulic fluids deposited on workpiece
 surfaces. After degreasing, the workpieces then could
 be left to  dry in  the ambient environment, particularly
 pieces that incorporate electronic components or  heat-
 sensitive materials. Flash rusting is not a concern at this
 stage because the piece has already  received its primer
 coat. Alternatively, the workpieces may be dried using
 air knives (i.e., targeted  jets of warm air) or by subjecting
the pieces to blasts  of clean, dry compressed air.

 Contaminants in tap water, however, can undermine the
 long-term corrosion-resistance of a finished piece. Thus,
 a single tap-water rinse is recommended only for low-
 value products in price-sensitive markets or products
 that will not be used in humid or corrosive environments.

 Three-Step Process

 In a three-step process, the degreaser is followed by at
 least one tap-water rinse and then rinsing with deionized
 water. The use of deionized  water is  recommended
 when the workpiece will undergo phosphating after de-
 greasing and a high-quality phosphate coating must be
 achieved to ensure a high-value finished piece.

 If a  high-value workpiece will  not receive phosphating
 pretreatment, the piece might need to be dried, following
 rinsing, in an oven at a temperature ranging from 260ฐ
 to 400ฐF. The higher end of the temperature range for
 dry-off is not  recommended, however, for alloys  that
 might undergo a phase transformation, for machined
 parts that must meet especially high tolerances, or for
 components that include heat-sensitive materials. Other
 considerations include the cost of firing the drying oven
 at sustained high temperatures and the time required for
 workpieces to cool, especially heavy castings, before
 being moved along in the process.

 Four- and Five-Step Processes

 Operations applying a primer-topcoat system to high-
 value workpieces that must be thoroughly cleaned be-
 fore   a  phosphate  coating is applied  often provide
 additional rinse steps at the degreasing stage. For ex-
 ample, operations  in the automotive and appliance in-
 dustries typically rinse workpieces in one or two baths
 of deionized water after the tap-water rinse step. Along
 with  ensuring proper adhesion of coatings by minimizing
 surface contaminants, these additional rinse steps also
 extend the useful  life of conversion  coating  baths by
 minimizing  degreaser drag-out. For superior corrosion
 resistance, the conversion coating must be deposited on
 a slightly acidic surface (i.e., in the range of 5 to 6 pH).
 (For a detailed discussion  of  rinsing operations,  see
 Chapter 7.)

 5.4.2 Semi-aqueous Degreasing   Introduction

 Semi-aqueous degreasers represent a  middle ground
 between  the  use of solvent-based  and aqueous  ap-
 proaches. They are more effective than strictly aqueous
formulations for  removing heavier grease,  wax, and
even tar from a  variety of substrates (i.e., metal,  ce-
 ramic, plastic, and elastomer); however,  because these
formulations include volatile ingredients—albeit with low
vapor pressures and high boiling points—they are regu-
 lated as  VOCs, HAPs, or ODCs. Semi-aqueous  mix-
tures are  based on  organic  compounds,  such  as
terpenes and alcohols,  and thus are somewhat less
threatening to the environment and human health than

most conventional solvent degreasers. The cleaning
mechanism for semi-aqueous degreasers is essentially
the same as for aqueous degreasers. Table 5-6 lists
typical organic constituents used in semi-aqueous de-

An additional advantage of semi-aqueous over solvent
degreasers is that they generally have a higher flash-
point and lower volatility than organic solvents; thus,
they are less prone to  combustion and evaporation,
making them applicable in both spray and immersion
processes. Moreover, because such formulations  tend
to be characterized by low surface  tension, they are
particularly effective on  workpieces  with surface fea-
tures that are difficult to clean, such as small holes and

A principal limitation of semi-aqueous degreasers is that
they are highly flammable when in a concentrated gase-
ous form, especially formulations based on terpenes. For
this reason, they should never be heated above 90ฐF.
Flammability can be minimized, however, by formulating
or using them in their emulsion form. Additionally,  certain
formulations can cause plastics and elastomers to swell.   Process Basics and Best Management

Degreasing operations using semi-aqueous formula-
tions are conducted in the same way as aqueous clean-
ing. The basic process involves two steps—degreasing,

Table 5-6. Typical Organic Constituents in Semi-aqueous
         Degreasers (3)
Terpenes          Derived from citrus and pine oils; can be
                 formulated into emulsions; new formulations
                 raise flashpoint to >144ฐF providing cleaning
                 effectiveness and reducing danger of fire or
                 explosion; effective at low temperatures;
                 often can be recycled

Esters  -          Most common are aliphatic mono esters
                 (primarily alkyl acetates) and di-basic esters
                 (DBEs); can be used cold or heated;
                 favorable solvent properties but poor
                 solubility in water; flashpoint usually >200ฐF;
                 can be slow drying

Glycol ethers       Generally divided between e- and p-series,
                 with neither considered a HAP; favorable
                 solvent properties and  effective as emulsion
                 in water; can remove polar and nonpolar
                 contaminants; easy to  recycle; flashpoint
                 usually to >200ฐF

N-methyl-2-pyrro-    High solvency and effective on many
lidone (CsHgNO)     contaminants; completely soluble in water
                 and other liquids; can be used cold or
                 heated; flashpoint is approximately to 199ฐF

Ethyl lactate        Can be used as for cold-liquid degreasing; a
                 VOC, but not considered a HAP or an ODC;
                 has a favorable evaporation rate
either by immersion or spraying, followed by a tap-water
rinse to remove residues.

For a more extensive discussion of semi-aqueous de-
greasers, see  EPA's Guide to Cleaner Technologies:
Alternatives to Chlorinated Solvents for Cleaning and
Degreasing (3) (see also Reference 5).

5.5   Case Examples

5.5.1  Frame Manufacturer

A large manufacturer purchased oil-free  and pickled
steel  for fabricating frames to be used in heavy machin-
ery. Despite the higher cost,  managers believed they
would be able to  produce a  better and longer-lasting
product. A few  months after switching to the treated
steel, however, they experienced a spate of catastrophic
paint  failures. It appeared that the frames would have to
be recalled and then stripped, cleaned, and repainted.

In the original process, after fabrication the frames were
moved into a  washing room where all workpiece sur-
faces were  thoroughly  cleaned  with  a  high-pressure
hot-water degreaser using a wand. Because the frames
were  long and wide, the left side was cleaned before the
right.  In the first pass along the left side, the hot water
incorporated a soap solution. In the second  pass,  the
frame was rinsed  with municipal  tap water at ambient
temperature. The line operator then  repeated the proc-
ess on the right side of the frame.  After the entire clean-
ing operation  was completed, the  frame  was stored
outside where the surfaces were left to dry at ambient

Due to the size of the frames, the entire degreasing
process took 1 to 1.5 hours. By this time, the entire
workpiece was covered  in flash  rust. The production
manager,  unaware of the situation, allowed the primer
and topcoat to be applied over the rusted surfaces. Only
after  several frames were rejected because of  cata-
strophic failures in the field did managers call in a con-
sultant to investigate the  cause of the problem.  They
found that several poor practices contributed to the paint

• During the first pass along the right side of the frame,
  the fine overspray of detergent  solution  from the
  spray wand was contaminating the  already-cleaned
  surfaces on the  left side.

• Because  the first stage  was hot  (approximately
  180ฐF), the detergent solution from the first stage
  evaporated from the frames, leaving a residue of al-
  kaline soap  on the surface.

• The frames were rinsed with municipal tap water that
  had a high concentration of minerals (i.e., dissolved
  salts).  When  the  water  evaporated,  the  minerals

   remained on the surface and thus were available to
   promote corrosion under the topcoat of paint.

 • Because the original milling oils had been removed
   from the steel surfaces during the degreasing opera-
   tion, the metal was more sensitive to flash rusting.
   This was aggravated by the slow evaporation rate of
   water at ambient temperatures.

 The managers were advised to abrasive-blast clean the
 metal surfaces to a near-white finish, (see Chapter  8)
 and then apply a corrosion-resistant primer within 4
 hours. The company, however, could not justify the ex-
 pense of installing a blast cleaning room. Instead, the
 managers made the following changes to their process:

 • Two workers were assigned to perform the degreas-
   ing operation so that the metal surfaces at the front
   end of the frame would not dry while the  back end
   was still being degreased.

 • Surfaces were kept wet until the final rinse had been

 • Deionized rinse water was used to avoid contamina-
   tion by dissolved salts in the municipal tap water.

 • The deionized  water was  heated  to approximately
   180ฐF  to  accelerate  the drying process  and thus
   avoid the need for a drying oven.

 • To minimize water usage and the disposal  of excess
   contaminated water,  the rinse  stage was  recycled
   through an ion exchange resin in the deionized water

 • The company's production office  rescheduled  work
   so that the frames could be moved directly from the
   washing room into the primer spray booth. (Cleaned
   frames were not stored outside unless they had been

The frames that were rejected due to flash rusting un-
derwent paint stripping  operations and then were re-
painted. The cost  of this  approach was quite  high
because contractors were required to disassemble the
end-products to process the failed frames.

5.5.2  Military Contractor

A large military contractor in the Midwest was using
approximately  250,000 pounds  per year  of 1,1,1
trichloroethane to degrease primarily aluminum parts
prior to welding. The company was participating in EPA's
33/50 program, which encourages a use reduction for
selected hazardous chemicals by 33 percent by the end
of 1992 and a further 50 percent  by the end of 1995.
Thus, managers decided to totally eliminate their use of
1,1,1 trichloroethane by installing an aqueous degreas-
ing washing cabinet.
 Given the vast number of part configurations needing to
 be cleaned,  a highly  sophisticated system was  pur-
 chased. The cabinet included a rotating table, high-pres-
 sure spray nozzles, the ability to add an inhibitor to the
 rinse water, and the option to add more than one rinse,
 depending on the complexity and configuration of the
 parts. The system  is a closed loop, ensuring that the
 large amount of water used is treated in an ultrafiltration
 unit and then recycled.

 Military specifications  needed to be  followed in the
 manufacture of the end-product, requiring that the con-
 tractor obtain approval  before changing the degreasing
 process. The military client, however, also was inter-
 ested in eliminating the use of 1,1,1 trichloroethane and
 readily approved the change.

 5.5.3  Lift Truck Manufacturer

 A lift truck manufacturer with a solvent-based degreas-
 ing operation for cleaning cutting oils and metal fines,
 primarily from aluminum parts, decided to switch to an
 aqueous degreaser. The incentive, in part, was concern
 about exposure of line operators to harmful emissions.

 The principal solvent being used by the company for
 vapor and cold cleaning was 1,1,1 trichloroethane. Be-
 fore switching degreasing formulations,  the company
 tested approximately 30 different aqueous degreasers,
 comparing their effectiveness to the 1,1,1 trichloroethane.
 Eventually, the company identified an aqueous degreaser
 that was more effective for removing stubborn surface
 contaminants than the 1,1,1 trichloroethane.

 When the company evaluated  their cleaning operations
 for both small parts (primarily screw machine parts)  and
 larger workpieces with complex geometries, managers
 found that both types of workpieces could be effectively
 cleaned by immersion  in a tank of agitated aqueous
 degreaser. For the smaller parts, an additional advan-
tage of the process change  was that it allowed  the
company to combine degreasing with  the  removal of
 burrs as a result of bath agitation. For the larger parts,
 managers were able to identify a degreaser that would
 be effective on the aluminum workpieces as well as the
occasional copper and  cast iron pieces. Following de-
greasing, the workpieces were rinsed in a solution con-
taining a corrosion inhibitor and then were blown dry.

Conversion to aqueous degreasing reportedly saved the
company about $102,000 per year, at the same time that
toxic emissions were essentially eliminated. The major-
ity of  savings resulted  from more efficient use of the
cleaning formulation, given that a batch of aqueous
degreaser includes only 5 to 10 percent cleaning solu-
tion, with the balance being water.

Over the past decade, EPA and state officials have
been  encouraging companies to evaluate  their proc-
esses and consider switching to degreasing approaches

that generate less pollution. As a result,  many compa-
nies have discovered that they have been clinging to old
and inefficient practices. By updating their operations,
many realized significant process efficiencies and even
enhanced the quality of their finished products.

This case example emphasizes the importance of test-
ing numerous degreasers  from more than one vendor
before making  a change.  Because  no  universal de-
greaser exists—solvent or aqueous—that will remove all
surface contaminants, often  a  degreaser  must  be
matched to the specific operation. Thus, when switching
degreaser  formulations,  a  facility operator should  al-
ways allow sufficient time to test available products.
5.6   References
1.  Foecke, T. 1993. Principles of cleaning. Teleconference on water-
   based alternatives to solvent cleaning. Presented by the Cleveland
   Advanced Manufacturing Program, February 11. Sponsored by the
   Great Lakes Protection Fund and the Joyce Fund, Cleveland, OH.
2. Burch, DJ. 1993. Chlorinated solvent vapor degreasing: The clock
   is ticking. Teleconference on water-based alternatives to solvent
   cleaning. Presented by the Cleveland Advanced Manufacturing
   Program, February 11. Sponsored by the Great Lakes Protection
   Fund and the Joyce Fund,  Cleveland, OH.

3. U.S.  EPA. 1994. Guide to cleaner technologies: Alternatives to
   chlorinated solvents for cleaning and degreasing. Office or  Re-
   search and Development, Washington, DC. EPA/625/R-93/016.

4. U.S.  EPA. 1994. Guide  to cleaner technologies: Cleaning and
   degreasing process changes. Office or Research and Develop-
   ment, Washington,  DC. EPA/625/R-93/017.

5. Cleveland  Advanced  Manufacturing  Program (CAMP).  1993.
   Water-based alternatives to solvent cleaning. Teleconference on
   water-based  alternatives to solvent cleaning.  Presented by  the
   CAMP, February 11. Sponsored  by the Great Lakes Protection
   Fund and the Joyce Fund, Cleveland, OH.

6. U.S. EPA.  1985. Stationary point and area sources. In: Compila-
   tion of Emission Factors, 4th ed., vol.1. EPA/AP-42 (September).

7. Joseph, R. 1992. Pollution  prevention for paints and coatings fa-
   cilities: Training  Program. Saratoga, CA: Ron Joseph & Associates.

8. Durkee, II,  J.B.  1994. The parts cleaning handbook without CFCs:
   How to manage the change. Cincinnati, OH: Gardner Company.

9. Merck. 1989. The Merck Index, 11th ed. Pathway, NJ: Merck Re-
   search Laboratories.

                                              Chapter 6
       Phosphating Metal Surfaces: Process Efficiency and Waste Minimization
6.1    Introduction
 6.1.1   Pollution Prevention Considerations

 For many paints and coatings operations, workpiece spe-
 cifications do not require the superior adhesion and corro-
 sion-resistance characteristics that can be achieved with
 a phosphate pretreatment step (i.e., depositing a conver-
 sion coating on a  metal  substrate in preparation  for a
 primer-topcoat system). In such cases, phosphating may
 add costs that make the finished product less attractive to
 consumers in price-sensitive markets. Operations  proc-
 essing higher-value metal workpieces, however—for such
 products as automobiles, appliances, office furniture, and
 outdoor equipment—must include this step so that coat-
 ings meet requirements for long-term corrosion resistance.

 Phosphating can add unavoidable costs associated with
 the operation and maintenance of an extended process
 line. The facility operator can minimize the generation of
 pollutants, however,  and thus the cost of managing
 wastewater  and sludge, if the  phosphating process is
 conducted efficiently. Indeed, efficient phosphating not
 only minimizes waste generation and maximizes chemi-
 cal use, it also ensures optimum deposition weight. This
 ultimately lengthens the life of  the product. A principal
 consideration in phosphating is that formulations be ap-
 propriately matched to the  particular metal substrate.
 Otherwise, the process will result in less-desirable pre-
 treatment coatings and will generate an excess of heavy
 metal sludge. The expense  of collecting and disposing
 of  these hazardous materials can add  significantly to
overall processing costs.

 Iron and zinc phosphating  are the most  widely  used
conversion  coating approaches for steel  substrates.
Wash primers represent an  alternative approach  when
conventional phosphating is not possible.  Whereas
these pretreatment-primer coatings can be used with
 minimal process costs, conventional high-VOC  wash
 primer formulations raise significant concerns about air
emissions. The less-volatile water-borne wash primers
that have become  available in recent years,  however,
represent a cost-effective alternative for certain types of
 Various approaches for phosphating are discussed in
 this chapter  in the context of  the  process  efficiency
 considerations that are critical  to waste minimization.
 Although the emphasis in this discussion is on phos-
 phate coatings for steel substrates, many of the recom-
 mended practices also apply to other metals.

 6.1.2   Decision-Making Criteria

 Decision-making criteria relevant to phosphating process
 efficiency and waste minimization, many of which are
 addressed in this  chapter, are highlighted in Table 6-1.

 6.2  Process  Basics and Best
      Management Practices

 6.2.1   Introduction

 Phosphating (i.e.,  iron and zinc phosphating)  is a proc-
 ess of depositing  a conversion coating  onto  steel and
 galvanized  steel to prepare the surface to receive a
 liquid, powder, or electrodeposited coating. The phos-
 phate deposit is  referred to as a conversion coating
 because it converts the surface of the virgin steel (no
 oxide present) to a roughened amorphous or crystalline
 phosphate composite (Figure 6-1). A  phosphate deposit
 can enhance a paints and coatings application in essen-
 tially three ways:

 • Serving as a barrier to keep atmospheric oxygen and
  moisture from attacking the base metal.

 • Acting as a dielectric film that electrically  insulates
  the substrate from the paint or other coating, slowing
  the process of galvanic corrosion.

 • Providing a rough surface for mechanical gripping of
  the paint or other coating for an improved bond.

 Establishing a strong bond between the  primer-topcoat
 system and the substrate enhances the corrosion-resis-
 tance of the workpiece as well as the general resilience
 of the surface (1). Along with providing  the foundation
 for this bond, however, this pretreatment  step also plays
 another important  role in promoting the durability of the
finished piece. The phosphate coating acts as a secon-
 dary barrier against moisture and oxygen, inhibiting the

 Table 6-1.  Decision-Making Criteria Regarding Phosphating of Metal Surfaces

 Issue                               Considerations
 Is the workplace too large or is
 its geometry too complex for
 pretreatment in  an immersion
 tank or a spray  system?
 Is the workpiece small enough
 to go through either an
 immersion tank system or a
 spray washer system?
 Will the topcoated workpiece be
 exposed primarily to
 noncorrosive environments?


 Is the workpiece a low-cost
 product for a price-sensitive
 market such that adding the
 cost of pretreatment would
 undercut sales?  '
Will the topcoated workpiece be
exposed to aggressive corrosive


Is the workpiece a high-cost
product (e.g., for the computer,
automobile, or large appliance
industry) sold in a market that is
not especially price-sensitive?

Do workpiece specifications
emphasize corrosion resistance
and long-term appearance?
Will phosphating be conducted
with a zinc phosphate
formulation, and will the
workpiece require a super
high-gloss finish (i.e., registering
>95 percent on a 60-degree
gloss meter)?

Do workplaces currently
undergo phosphating and then
receive a wash primer?

Does a line operator manually
move workpieces from one tank
to the next?
 • If yes, consideration should be given to the following approaches:

    #1 Use wand-operated steam cleaning with a detergent, followed by wand application of a mild
       phosphoric acid solution; a final rinse with clean municipal tap water may be necessary.

    #2 If the above approach  is  not feasible, consider wipe cleaning with an aqueous degreaser,
       followed by a second wipe cleaning with clean tap water; use of solvents should be avoided
       because they can cause  unnecessary air and water pollution.

    #3 As a last resort: After degreasing metal surfaces, apply a thin coat of wash (acid etch)
       primer; the coating film thickness is usually controlled at 0.3 to 0.5 mil. This approach should
       be avoided because most wash primers contain high concentrations of solvents (typically
       with a VOCs content of 6.5 Ib/gal, or 780 g/L) and thus raise air pollution concerns.

 • If yes and  the operation's production rate is relatively low (i.e., workpieces would proceed at
   about 2 ft/min), an immersion system should be considered.

 • If yes and  the operation's production rate is higher (i.e., workpieces would proceed at a speed
   greater than 2 ft/min), a spray washer system should be considered.

 (Note: A rate of 2 ft/min is a guideline only. When designing a system, a facility operator should
 consult with an equipment vendor and conduct a process cost analysis.)

 • If yes, then consideration should be given to the following approaches:

    #1 Use a three-step process in which the first step combines cleaning and phosphating, the
       second step is a tap-water rinse, and the third step is a rinse that includes a nonchromate
       rinse sealer. (A two-step process with a combined cleaning and phosphating step followed
       by only one rinse is ill advised.)

    #2 Use wand-operated steam cleaning with a detergent, followed by wand application of a mild
       phosphoric acid  solution;  a final rinse with clean municipal tap water may be necessary.

    #3 If the above approach is  not feasible, consider wipe cleaning with an aqueous degreaser,
       followed by a second wipe cleaning with clean tap water; use of solvents should be avoided
       because they can cause  unnecessary air and water pollution.

 (Note: None of these approaches yields a high-quality surface on which to apply paint.)

 • If yes, then at the least consideration should be given to a three-step pretreatment process;
   however, a process with five or more steps would be preferable. In these multistep processes,
   degreasing and phosphating are separate steps and each is followed by rinsing.

 (Note: In general, the quality  and corrosion-resistance characteristics of a primer-topcoat system
 will improve as rinse steps are added.)
•  If yes, then considerations should be given to the following approaches:

   #1 Apply a phosphate coating using zinc phosphate rather than iron phosphate for greater
      corrosion-resistance and appearance characteristics. This pretreatment process will require
      at least five steps. (A decision to use zinc phosphate should be well researched because
      this approach is more expensive and complex than alternatives.)

   #2 Apply a phosphate coating using iron phosphate; to achieve the specified quality, additional
      rinse steps may be required, with at least one deionized water rinse at the end.

•  If yes, then consideration should  be given to using a microcrystalline zinc phosphate because
   small crystals will not detract from the gloss.
• If yes, then the wash primer can be eliminated since it is both unnecessary and may be harming
  the topcoat (e.g., causing blistering or corrosion under the paint film). Elimination of the wash
  primer step will dramatically reduce VOC emissions.

• If yes, the worker should be instructed to allow each workpiece to drain over the process tank
  before moving it to the next tank; training should also cover the  importance of keeping draining
  time to a minimum to avoid the onset of flash rust'ng.

 Table 6-1.  Decision-Making Criteria Regarding Phosphating of Metal Surfaces (continued)

 Issue                              Considerations
 Are workpieces automatically
 moved from one tank to the
 next via a computerized hoist

 Do workpieces pass through a
 conveyorized spray washer?
 Does the spray booth operator
 have access to the conveyor
 system on/off switch? And does
 the operator on occasion stop
 the system while workpieces
 are still wet?
What approach is
recommended for selecting a
phosphate formulation from the
many that are available?
Is it better to use a low- or a
high-temperature phosphate

What approach is
recommended for selecting
between powder and liquid
phosphate formulations?
•  If yes, the system should be programmed to allow workpieces to drain over immersion tanks,
   while avoiding the onset of flash rusting.

•  If workpieces span a wide range of geometries, consideration should be given to programming
   the system for various groupings of workpieces.

•  If workpieces span a wide range of geometries, consideration should be given to programming
   the system for various groupings of workpieces.

•  The facility operator should conduct tests to determine the optimum conveyor system speed for
   allowing adequate workpiece draining {as opposed to changing the speed for different workpiece

•  If yes, then consideration should be given to the following approaches:

   #1 The operator should be instructed not to stop the conveyor system until all workpieces have
      passed through the spray washer and the dry-off oven; it is likely that allowing a workpiece
      to remain above a tank  or between stages will ultimately cause a paint coating failure.

   #2 Establish two separate conveyor systems: one that makes a loop around the spray washer
      and another that passes through the spray booths and the dry-off oven. The disadvantage of
      this approach is that the line operator must offload workpieces from the first conveyor and
      then load them onto the second system.

   #3 Install a power-and-free conveyor so that the speed of the conveyor as  it passes through the
      spray washer can be faster than the speed of the second conveyor that passes through the
      paint booths and dry-off oven. This approach is more expensive  than the others, but it allows
      workpieces to accumulate after leaving the spray washer and avoids the need for a line
      worker to offload and load workpieces,  as required in approach #2.

• The best approach is for the operator to test different formulations in the existing process line.
  Since this is usually not feasible, an alternative is to have several vendors phosphate test
  pieces, immediately after which a primer should be applied. Once the primer has cured, the
  coating should be tested for adhesion and then for corrosion-resistance characteristics in a salt
  spray (i.e., fog) chamber. These tests will identify the best formulation.

• Only by testing a formulation in the actual process line can the operator determine the typical
  useful life of a phosphating immersion bath.

(Note: Generally it is not possible to make an  assessment regarding the most appropriate
phosphate formulation by reviewing vendor data sheets.)

• In general, low-temperature formulations do not provide the same quality phosphate coating as
  high-temperature formulations. Thus, the tradeoff is between quality and energy costs.

• A decision usually can be made on the basis of cost. Although powder formulations are generally
  less expensive, the operator must mix the phosphating solution according to vendor literature. In
  contrast, liquid formulations come ready for use, although some dilution with water may be
      Iron Phosphate
       25-80 mg/ft2
      Zinc Phosphate
      100-1,000 mg/ft2
Figure 6-1.  Cross-sectional view of conversion coating proc-
            ess using Iron or zinc phosphate.
electrochemical process that leads to galvanic corrosion
of the metal substrate.

This  pretreatment step is specific to metal substrates.
The phosphate coating process  is not used on plastics
or ceramics because neither can participate in an elec-
trochemical reaction  as can metals. The  deposition of
phosphates only  takes place if an electric current can
flow through the substrate/liquid  system (see Chapter 3
for a discussion  of the  electrochemical  reaction that
takes place in the corrosion process).

The  discussion  in  this chapter primarily focuses on
methods for  applying a  phosphate coating to steel,
which typically  is accomplished by bath immersion or
spraying of the workpiece with an iron or zinc phosphate
solution. These same phosphating methods also can be
used  on several  other metals.  For some  substrates,

 however, processes specific to the particular metal must
 be used. Indeed, studies have shown that steel sub-
 strates are the most conducive to phosphating (2). Using
 a phosphating process that is inappropriate fora particu-
 lar substrate can result in both a poor conversion coating
 and an excess of heavy metal sludge (see discussion
 on waste minimization in Section 6.4).

 In the case of aluminum, phosphating chemicals occasion-
 ally are used to clean the substrate rather than to establish
 a conversion coating. The most popular conversion coat-
 ings for aluminum are based on chrome oxides. Histori-
 cally, such coatings have provided corrosion resistance
 that is superior to that achieved with other aluminum pre-
 treatments. Unfortunately, however, they contain hexava-
 lent chromium (Cr6+), which is a hazardous heavy metal.
 For many years, the industry has sought to replace chrome
 oxides  with  less-hazardous  pretreatments, and  non-
 chrome alternatives are available for applications in which
 corrosion resistance is  not critical for the finished piece.
 These alternative formulations have been  slow to gain
 acceptance, however,  from  some  operations. For  in-
 stance, the U.S.  Department of Defense (DOD) has only
 recently tested and found some of  these nonchromate
 alternatives environmentally acceptable (3). Thus, before
 a particular nonchromate alternative is used  on work-
 pieces being finished under a DOD contract, the process
 operator should check to confirm that the formulation has
 been specifically approved (especially when the pieces are
 for the Air Force).
 Despite the enhanced durability afforded by application
 of a phosphate coating,  for many paints and coatings
 operations  the addition of this pretreatment step is not
 cost effective.  Many steel products for the  building and
 construction industry (e.g., metal  ties, brackets), for  in-
 stance, are not required to have a high-quality organic
 finish.  Indeed, the higher price that manufacturers of
 such products would need to charge to recoup the cost
 of additional pretreatment might undermine sales in this
 price-sensitive market. Similarly, the cost of phosphating
 particularly large workpieces can be  preclusive. The
 alternative  pretreatment  approach for large structural
 members such as I-beams is  abrasive blasting  (see
 Chapter 8).

 Phosphate coatings are applied primarily to higher-
 value goods or to products  designed to provide long-
term  performance.  In  the  appliance  manufacturing
 industry,  for example, both iron and zinc phosphating
 are used extensively to achieve  high-quality primer-top-
coat systems.  As shown in Table 6-2,  large and small
parts alike  receive this pretreatment at relatively high-
production  rates.

 6.2.2  Coating Quality and Basic Parameters

The quality of a phosphate coating is determined primar-
 ily by its weight (in milligrams per square foot) rather
 Table 6-2.  Typical Spray Phosphating Production Rates In
          the Appliance Industry (1)
Area (ft2)      Pieces per Hour
 Zinc Phosphate (150-200 mg/ft2)
Dryer shell
Cabinet backpanel
Base pan assembly
Timer mounting bracket
Iron Phosphate (40-80 mg/ft2)
Washing machine shell
Dryer top
Motor access panel
Conduit cover plate


than  its thickness.  For optimum process efficiency,
phosphate coatings  should be weighed regularly and
the results  tracked  over  time. Allowing too heavy  a
phosphate coating to form on a substrate can ultimately
lead to failure of the  primer-topcoat system. For exam-
ple, an excessive coating can eventually split and cause
delamination of the topcoat.

Coating  weight  can be determined  by immersing  a
preweighed, coated panel in a beaker containing heated
chrome oxide. Results can usually be obtained within a
few hours. Such a test, however, should only be con-
ducted by a trained  technician in a  laboratory that is
properly equipped  with a fume hood.

For high-quality workpieces,  some operators also test
the quality of phosphate coatings for corrosion  resis-
tance. Atypical test involves subjecting a panel that has
received a phosphate coating and then a primer to salt
spray  in a  laboratory chamber. The  results are then
compared with the corrosion resistance demonstrated in
the same test using  a panel  of known quality (several
such  test panels  are commercially  available). Some
operations also test  phosphate coatings for electrical
resistance (i.e., the ability to  resist galvanic corrosion)
with an impedance test.

The key  parameters that must be controlled to achieve
a quality phosphate coating are concentration, tempera-
ture, pH, and dwell time.

Concentration: Within a narrow range, deposition of iron
phosphate tends to increase as the concentration of the
purchased material in the  phosphating solution  is in-
creased. That range tops out at 5 percent, beyond which
the degree of deposition achieved on  the substrate re-
mains  essentially unchanged. At concentrations above
5 percent, the process operator is likely to be wasting
the phosphating chemicals. A concentration below 3
percent usually will deposit a coating that is too thin to
achieve  desirable adhesion  or  corrosion-resistance

 characteristics. Thus, process operators generally mix
 phosphating solutions with a 3 to 5 percent concentration
 of the purchased material. Similarly, specific parameters
 apply for zinc phosphate. Thus, process operators are
 strongly advised to follow vendors' recommendations.

 Temperature: As  with  concentration;  within a  given
 range deposition increases as the temperature of the
 phosphating  solution is  raised. Process operators typi-
 cally maintain iron phosphate solutions at 120ฐ to 140ฐF,
 in keeping with vendor recommendations. An exception
 applies when using low-temperature phosphate materi-
 als, for which the phosphating solution is maintained at
 90ฐF. At temperatures above or below the vendor's rec-
 ommendation, the phosphating reaction might be too
 fast or too slow,  respectively. For  instance, if phos-
 phating occurs too quickly, the deposition may be ex-
 tremely  porous.  Moreover,  phosphating at a high
 temperature  raises energy  costs and increases the
 evaporation  of water from the phosphating solution.
 Similarly, specific parameters apply for zinc phosphate.

 Phosphate baths and spray feeds are generally heated
 by  either a burner-and-tube system or by a heat ex-
 changer that  incorporates steam.  Although the burner-
 and-tube method has been in  use for many years, the
 more recently installed phosphating equipment tends  to
 incorporate heat exchanger technology, which is more
 energy efficient and does not generate fumes.
 pH: The pH of an iron phosphate  bath gives an indica-
 tion of the acidity. Typical iron phosphating solutions are
 slightly acidic, in a pH range of 3 to 6 for both immersion
 and spray applications; zinc phosphating solutions gen-
 erally are more acidic, in a pH range of 1.8 to 2.4 for
 immersion and up to  3.0 for spray application (4). Con-
 trol of pH is critical because the phosphate precipitates
 out from the solution only when the pH  at the sub-
 strate/liquid interface is  in the correct range. Because
 the pH range is  specific to the  particular phosphate
 formulation, vendor recommendations must be followed

 Dwell Time: For both iron and zinc phosphating, the dwell
 time required to achieve an adequate conversion coat-
 ing differs significantly between immersion and spray
 application. Depending on the concentration of the pur-
 chased material, a workpiece immersed in an agitated
 bath of phosphating solution generally requires a dwell
time of 3 to 5  minutes. Agitation brings the fresh chemi-
cal in the bath to the substrate/liquid interface, where the
electrochemical reactions occur. Thus, agitation is rec-
ommended to achieve uniform deposition  and to maxi-
 mize  chemical use.  In sharp contrast,  with  spray
application the phosphating solution only has to make
contact with the workpiece for 60 to 90 seconds. The
conversion  reaction occurs faster because the  spray
solution continuously supplies fresh  chemicals to the
surface of the workpiece.
 6.2.3  Best Management Practices

 Recommended  practices that  enhance  process effi-
 ciency as well as the quality of the phosphate coating
 include the following:

 • To promote  proper  adhesion  for  high-durability
   primer-topcoat systems and to extend the life of im-
   mersion baths, process operators should ensure that
   workpieces are  thoroughly rinsed before and after

 • To maximize the effectiveness of phosphating formu-
   lations, operators should  confer regularly with ven-
   dors  and thoroughly test various combinations of
   acids, accelerators, and surfactants. For many opera-
   tions, it may be necessary to customize the phos-
   phating  formulation  to  the  specifications  of the
   particular coating system.

 • To ensure the cost-effective use of chemicals, facility
   operators should automate the addition of the phos-
   phating formulation  to  processing tanks. Although
   capital cost outlays for the installation of flow control-
   lers can be somewhat high, this measure can yield
   process input  savings in the near term.

 • To avoid contaminating the phosphated surface with
   perspiration, skin oils,  or  general grime,  facility op-
   erators should require  process line workers to wear
   clean gloves when handling freshly phosphated work-
   pieces. Such contaminants can  undermine adhesion
   of the primer-topcoat system and mar the finish by
   photographing through.

6.3  Phosphating Methods

6.3.1   Iron Phosphating

For most operations that apply a conversion coating to
steel workpieces, iron phosphating is the preferred ap-
proach because it is easier to control, less expensive,
and  generates  less sludge  than  the zinc  phosphate
method. Iron phosphate yields a conversion coating that
generally has less weight than that achieved with zinc
phosphating,  however,  and thus the coating provides
less corrosion resistance. Nonetheless, the quality of
the deposition is sufficient to meet specifications for the
majority of finished workpieces. Although iron  phos-
phate can be used on most steel substrates, it is incom-
patible with galvanized  steel, for which zinc  phosphate
is recommended.

The iron phosphate process is essentially the pickling of
steel in phosphoric acid. The surface of the steel is
made up of numerous  anode and cathode  sites. The
acid attacks the steel at the anodes, liberating iron ions
into the bath and generating hydrogen gas. An accelera-
tor (i.e., oxidizing agent) is required to oxidize the iron
ions and use up the hydrogen at the metal surface. This

 lowers the acid content, or pH, at the cathodic sites to
 the point at which iron phosphate naturally precipitates
 onto the steel surface. This process continues until all
 cathodic sites (i.e., all surfaces) are coated.

 Iron phosphate formulations generally contain a combi-
 nation of ferrous phosphate, ferric phosphate, and ferric
 oxide. Process operators  typically use  solutions that
 include phosphoric acid, an  accelerator, and  one or
 more surfactants (ironically, iron phosphate solutions do
 not actually contain iron). The surfactants  help to wet the
 substrate, enhancing adhesion of the phosphate coating.

 The four types of available  iron phosphates are  catego-
 rized by the accelerator added:

 •  Chlorate (yielding a gold-blue-gray deposition).

 •  Molybdate (yielding a  blue deposition).

 •  Sodium metanitrobenzene sulfonate (SNIBS) (yield-
   ing a grayish  blue deposition).

 •  Bromate (yielding a blue to bluish gray deposition).

 The color of the phosphate coating should be consistent
 from  workpiece to workpiece. A change in color can
 indicate a problem with the deposition (e.g., the immer-
 sion bath is exhausted).

 The four types of iron phosphates are sold in both liquid
 and powder form. The liquid form is generally preferred
 because it is easier to mix into  an immersion  bath or
 spray tank.  Powders can be difficult to mix thoroughly
 into an aqueous form and can generate  housekeeping
 problems. Also, the mix-and-feed  of powdered formula-
 tions cannot be automated  as easily as liquid forms.

 Typical deposition  weights achieved with iron phos-
 phating range from 25 to 80 mg/ft.1 Deposition weight
 depends not only on the control of phosphating process
 parameters, but also on the type of steel  or alloy being
 treated. Some steels are  particularly difficult to treat. On
 such substrates, deposition weights may be low  regard-
 less of how well the phosphating system  is controlled.   Process Variations

 Wand Application

 One method for applying an iron  phosphate coating is
 to subject the workpiece to the phosphating solution with
a spray wand. Often the workpiece can be degreased
 before phosphating and  then rinsed afterward using a
 single wand equipped with an operator feed-source con-
trol (see  discussion in Chapter 5 on aqueous degreas-
 ing). Wand application is primarily used on particularly
1 As noted in Chapter 5, "degreasing" is used generally in this docu-
 ment to refer to the various liquid/vapor methods used in paints and
 coatings operations to clean substrates. The author recognizes that
 some operators use the term degreasing to  refer specifically to
 vapor degreasing.
 large workpieces  being  processed at low volume. In
 general, this  approach does not yield a  high-quality
 surface for application of a primer-topcoat system.

 Two-Step Process With Immersion

 Another approach  involves immersing workpieces into a
 bath that contains  a formulation that combines degreas-
 ing and phosphating. The workpiece is then rinsed in the
 second step in this process. Although economical, this
 approach tends to leave many contaminants  on the
 substrate, and thus the resulting phosphate coating pro-
 vides only minimal corrosion resistance. Generally, this
 approach is used to phosphate workpieces that will not
 be exposed to corrosive conditions during most of their
 useful life.

 Three-Step Process With Immersion or Spray Washing

 The most widely used iron phosphating approach in the
 general metals industry involves an immersion  bath or
 spray washing step that combines degreasing and iron
 phosphating followed by two rinse steps. Rinsing can be
 carried out with municipal tap water, although deionized
 water is recommended for the second rinse as a way of
 controlling for residual contaminants. Some operations
 also add a sealer to the second rise that fills pores in the
 phosphate coating (see discussion on sealers in Chap-
 ter 7). Whereas the three-step process minimizes phos-
 phating costs, the corrosion resistance yielded is not of
 sufficient quality to meet specifications for higher-value
 workpieces  (e.g., appliances and many other durable
 goods). (Operations generally do not use a four-step

 Processes With Five or More Steps

 Operations applying paints and coatings that  require
 high-grade corrosion  resistance thoroughly clean and
 rinse workpieces before and after phosphating. In these
 systems, phosphating as well as degreasing and rinsing
 are carried out in  dedicated  immersion baths or with
 spray washers.

The five-step approach (i.e., degreaser, tap-water rinse,
 phosphating, tap-water  rinse,  and  deionized water
sealer rinse) often  is used for phosphating workpieces
that will be  put into service  outdoors  or in generally
corrosive environments. Operations coating workpieces
with specifications  for superior durability (e.g., for large
appliances) often use seven or more process steps that
 include additional rinsing, either by impingement or im-
mersion. Tables 6-3 and 6-4 present examples of two
 multiple-step process  lines for high-quality workpieces.
 Iron phosphating using such extensive processes yields
conversion coatings of quality similar to that achieved
with zinc  phosphate.  Limitations of such approaches
concern process costs related to worker training, opera-
tion of the system,  and floor-space needs.

Table 6-3. Process Line for Pretreatment of Complex
Workpleces In Electrocoatlng Operation (5)
Stage Description Process Time
1 Alkaline cleaner Spray 60 sec
2 Alkaline cleaner Immersion 30 sec
3 Water rinse Spray 30 sec
4 Water rinse Immersion 30 sec
5 Phosphate Immersion 60 sec
6 Water rinse Immersion 30 sec
7 Sealer Immersion 30 sec
8 Deionized rinse Immersion 30 sec
8a Deionized make-up Spray Variable
Drain and flash 5 min
Table 6-4. Process Line for Pretreatment of Simple
Workpieces In Electrocoatlng Operation (5)
Stage Description Process Time
1 Alkaline cleaner Spray 60 sec
2 Alkaline cleaner Spray 30 sec
3 Water rinse Spray 30 sec
4 Water rinse Spray 30 sec
5 Phosphate Spray 60 sec
6 Water rinse Spray 30 sec
7 trSealer Spray 30 sec
8 Deionized rinse Spray 30 sec
8a Deionized make-up Spray Variable
Drain and flash 5 min
6.3.2 Zinc Phosphating
In most operations where the corrosion resistance of
finished workpieces must be especially high, conversion
coatings are applied using zinc phosphate. This ap-
proach is widely used in the automotive industry and in
certain sectors of the appliance and electronics indus-
tries. Similarly, zinc phosphating is often specified by the
armed services, especially for equipment that may be
exposed to severe environments. Moreover, many op-
erations using electrocoating or powder coatings, par-
ticularly when a one-coat finish will be exposed to the
weather, pretreat workpieces with zinc phosphate.
The electrochemical process whereby zinc phosphate
deposits on a substrate is similar to the iron phosphating
process. As soon as the workpiece is subjected to the
acidic solution, metal dissolves at anodic sites.
As in iron phosphating, accelerators (i.e., oxidizers) are
the acid content, or pH, at the cathodic sites to the point
at which zinc phosphate naturally precipitates onto the
steel surface. This process continues until all cathodic
sites are coated.
The accelerator performs two basic functions:
• The excess ferrous ions in the solution tend to slow
down the phosphating process. The accelerator
speeds up the process by oxidizing the excess iron
ions, causing them to precipitate out as a ferric phos-
phate sludge, which extends the life of the bath. (The
sludge must later be filtered out of the solution and
disposed of as a hazardous waste.)
• By reacting with hydrogen as it is formed at the an-
odic sites, the accelerator prevents hydrogen gas for-
mation. If an oxidizer were not used, the formation of
gas would interfere with the deposition of the phos-
phate. Thus, addition of an oxidizer (also known as
a depolarizer) frequently prevents hydrogen embrit-
tlement of high-strength steel.
Accelerators specifically used with zinc phosphate
range in reactivity from mild nitrates to the fairly aggres-
sive chlorates and peroxides. Calcium compounds are
particularly favored as accelerators for the low coating
weights and compact grain sizes they yield. These typi-
cally are used when higher temperature phosphating
solutions (i.e., 112ฐ to 130ฐF) would otherwise slow
conversion kinetics (6). They can also be used in lower
temperature baths when accelerators or initiators such
as nickel, iron, manganese, and borium are also used.
In general, the crystals that result from zinc phosphating
have low porosity and provide a strong base for adhe-
sion of the primer-topcoat system and superior long-
term corrosion resistance. Table 6-5 presents the
corrosion resistance of various zinc phosphate coatings
when subjected to salt spray.
The performance of zinc phosphate formulations in-
creases in the following order:
• Zinc phosphate
• Zinc-calcium phosphate
• Zinc-nickel-fluoride phosphate
• Zinc-nickel-magnesium-fluoride phosphate
Typical conversion coatings deposited on the substrate
in zinc phosphating include:
Phosphopyllite Zn2Fe(PO4)2.4H2O
Hopeite Zn3(PO4)2.4H2O
Scholzite Ca2Zn(PO4)2.2H2O
Brushite CaHPO4.2H2O
Monetite CaHPO4
an important addition to zinc phosphating solutions. In
zinc formulations, accelerators oxidize the iron ions and
use up the hydrogen at the metal surface. This lowers
Crystal  size also is  affected by the method used for
subjecting the workpiece  to the phosphating  solution

Table 6-5.  Corrosion Resistance of Zinc Phosphate Coatings
          on Steel and Electrogalvanized Steel (1)

                                   Temperature Water
                      Salt Spray        Condensate
Zinc phosphate
(DIN 50021 SS)

(DIN 50017)

Zinc phosphate              3
(nitrate accelerated)8

Zinc phosphate             250
(nitrate accelerated with
nickel and polycarbonic
acid additions)3

Zinc phosphate             250
(nitrate accelerated +
corrosion protection oil)8
Electrogalvanized Steel
Untreated                   1

Zinc phosphate             50
(nitrate + nitrite



8 Approximate coating weight was 25 g/m2 (2,322 mg/ft2).
b Approximate coating weight was 2 g/m2 (186 mg/fr).
Note: These coatings were intended to provide corrosion projection
without the benefit of an organic paint or coating.
and by the thoroughness of the rinsing steps. The par-
ticular phosphating method used, however, often de-
pends on the type of workpiece. Typical zinc phosphate
coating  weights are 100  to 1,000  mg/ft2 using spray
application, whereas coatings can  range from 150 to
4,000 mg/ft2 using immersion tanks  (4).

Fewer process variations  are applicable to zinc phos-
phating, given that process operators must thoroughly
rinse  drag-out and contaminants from workpieces be-
fore and after phosphating. Also, each spray or immer-
sion step must be specific to the  particular  process
stage. Thus, for instance, the degreasing and phos-
phating  steps cannot be combined,  as they are some-
times in the iron phosphating approach. For operations
using zinc phosphating, the process  line includes five or
more  steps in which workpieces are degreased, tap-
water rinsed, phosphated, tap-water  rinsed, and then
rinsed with deionized water.

6.3.3   Wash Primers as an Alternative to

Wash primers represent an alternative means of etching
a substrate in preparation for receiving a topcoat. These
coatings are used primarily on particularly large work-
pieces that cannot be treated in tanks. The conventional
approach for this low-cost pretreatment step, which
dates back to the 1940s, involves priming the workpiece
with a high-VOC coating formulation that slightly etches
the substrate; this approach is also known as acid etch.
A typical wash primer is a vinyl butyryl organic coating
formulated with solvents (e.g., ethyl alcohol and/or iso-
propyl alcohol), vinyl butyryl resin, phosphoric acid, zinc
chromate, water, and an extender pigment.

The high VOC content of conventional wash primers in
contrast to other primer coatings represents a significant
disadvantage of  this approach. The VOC  content  in
typical formulations is about 6.5 Ib/gal (780 g/L). Thus,
the use of wash primers is an inexpensive but low-qual-
ity alternative to phosphating. Typically, facility operators
resort to this approach only when a phosphating process
line is not an option.

Because most states now require that the VOC content
of wash primers not exceed 3.5 Ib/gal, facility operators
that favor this approach have been experimenting with
the less-volatile water-borne wash primers that  have
become available in recent years. Some of these  alter-
native formulations  may meet the  military's stringent
specifications for quality and pollution prevention (7).

6.4    Waste Minimization and Treatment

6.4.1   Minimization

The key to waste minimization in the phosphating stage
of a paints and coatings operation is process efficiency.
Applying conversion coatings to workpieces with phos-
phating chemicals that are appropriate for the particular
metal substrate can minimize the generation of heavy
metal sludge in immersion baths or  from phosphating
spray  operations. If the color of a deposited coating
varies from the coloration  associated with particular
phosphating formulations, the process operator should
check for problems  such as exhaustion of the  phos-
phating solution.  Both the monitoring  of phosphating
operations and the  replenishing of chemicals can  be
automated to ensure maximum process efficiency.

In general, some amount of heavy metal sludge is gen-
erated in all phosphating, with greater amounts associ-
ated with zinc phosphating. In the worst case, the use
of phosphating chemicals that are not well suited to a
workpiece's metal substrate will fail to deposit a coating
and will generate an excess of heavy metal sludge. For
example,  iron phosphate  cannot be used to  apply a
conversion coating to galvanized steel because the acid
will react with the zinc in the substrate but not the iron,
resulting in an excess of  zinc sludge.  Instead, a zinc
phosphate formulation should be used to apply a con-
version coating on galvanized steel. Similarly, an alumi-
num substrate will not receive a conversion coating from
iron phosphate and will generate an excess of aluminum
sludge. Aluminum phosphate should be used to apply
conversion coatings to aluminum workpieces. Some

 nonchromate formulations are used on aluminum work-
 pieces that have low corrosion-resistance requirements
 (see Section 6.2.1).

 Discharges for such operations are regulated under the
 Clean Water Act at both the federal and state level, and
 local requirements may apply; also, industry-specific
 effluent  guidelines have been established. Relevant
 effluent  standards established by EPA are  specific to
 metal finishing and electroplating operations (40  CFR
 Part 413 and Part 433, respectively). These standards
 stipulate general limitations on heavy metals as shown
 in Tables 6-6 and 6-7.

 6.4.2   Treatment

 Contaminated phosphate baths or rinses can be treated
 in various ways. Raising the pH of an exhausted phos-

 Table 6-6.  Pretreatment Standards for Existing Sources That
          Electroplate Common Metals and Discharge 38,000
          Liters or More of Wastewater per Day
                                   Average of Dally
                                    Values for 4
 Pollutant or        Maximum for   Monitoring Days Shall
 Pollutant Property Any 1 Day  (mg/U)    Not Exceed (mg/L)
Cyanide, total
Copper '
Total metals
Source: Electroplating of Common Metals, 40 CFR Section 413.14.
Table 6-7. Pretreatment Standards for Existing Sources
         Involved in Metal Finishing Operations (for All
         Facilities Except Circuit Board Manufacturers)
Pollutant or
Pollutant Property
Cadmium, total
Chromium, total
Copper, total
Lead, total
Nickel, total
Silver, total
Zinc, total
Cyanide, total
Total toxic organics
Maximum for
Any 1 Day (mg/L)
Monthly Average
Shall Not Exceed

Source:, Metal Finishing Point Source Category, 40 CFR Section
 phate bath or of collected spray drainage will precipitate
 out any heavy metal sludge. The wastewater can then
 be run  through a centrifuge to collect the sludge into a
 cake, which must be disposed of as a hazardous waste.

 A growing trend in phosphate waste treatment is to use
 ultrafiltration to maintain  clean  rinses.  Ultrafiltration
 pumps  the rinse water through  membranes and allows
 the return of concentrates to the phosphate bath and
 purified water to the rinse tank. This additional step
 maximizes water use and reduces the amount of waste-
 water discharged to local treatment works.

 6.5  Additional Considerations Related to

 6.5.1   Choosing a Phosphate Formulation
        and Qualifying the Phosphate Coating

 Paints and  coatings facility operators typically confer
 with chemical vendors in the selection of a phosphate
 formulation. Indeed, one vendor may be able to offer a
 better formulation than another vendor, especially if the
 performance requirements are unique.

 The choice of formulation can be significant in terms of
 achieving optimum coating properties. It is especially
 prudent for the operator to discuss special requirements
 with a chemical vendor, particularly if the finished  work-
 piece will be subjected to aggressive environments.  In
 some situations, laboratory tests may need to be con-
 ducted to verify that the selected coating will be able to
 provide  the  required finish. In general, choosing  a for-
 mulation on the basis of price is inadvisable.

 5.5.2   Degreasing Before Phosphating

 Degreasing  formulations are varied and  must be se-
 lected according to the types of contaminants that need
 to be removed from workpieces (8). The most common
 types are alkaline and acid cleaners. (For a more exten-
 sive discussion of degreasing, see Chapter 5.)

 Degreasers should have the following characteristics (9):

 • Sufficient  detergency to  remove a wide variety  of

 • Capability to be easily rinsed,  so that residues do not
  contaminate the phosphating  stage.

 • Sufficiently mild that components of the formulation
  do not attack zinc and aluminum, which may be proc-
  essed together with steel in the workpieces.

Also, degreasers used in spray cleaning systems must
have controlled foaming characteristics.

A rinsing step after degreasing can be used to accom-
plish the following:

• Remove trace contaminants from the workpiece.

 • Minimize the likelihood  of alkaline salts and grime
   contaminating the phosphate bath.

 • Prevent the alkaline salts from raising the pH of the
   phosphate bath.

 The cleanliness of the substrate as the workpiece enters
 the phosphating step or as it leaves the final rinse tank
 should pass the water break-free or the towel-wipe test.
 In the water break-free test, a squirt bottle is used to
 pour deionized  water over a cleaned  substrate. The
 water should  run  off in a  sheet rather than bead up.
 While the test may demonstrate that oils and  greases
 have been removed from the workpiece, it will  not con-
 firm that the surfactants from the degreaser have also
 been removed.  To do this,  one needs to rinse the part
 with a small quantity of deionized water and then deter-
 mine the pH of the water. This can easily be done using
 pH papers.

 To determine that metal fines, smut, and other contami-
 nants have  been removed, a clean paper towel should
 be  wiped across  the wet surface of the  workpiece.
 Whereas the test may not always result in a perfectly
 clean towel, relative changes in cleanliness can be as-
 sessed (8).

 If the degreasing formulation is properly selected for an
 immersion process, contaminants from workpieces will
 either sink to the bottom of the tank  or float to the top
 (i.e., the oils will float rather than emulsify).  The line
 operator can then easily filter out the insoluble  sludges
 or separate off the oils. Sludge material can be dried and
 then disposed of as hazardous waste, whereas the oils
 can be sent off site for fuels blending.

 6.5.3 Design of an Immersion Tank System

 Rinsing by immersion is ideal for situations in which:

 • The production flow through the process is relatively
  slow (i.e., less than 2 ft/min on  a continuous basis).

 • Production is  intermittent.

 • The configuration  of the workpieces  is such that a
  spray washer  could not thoroughly wet all parts (e.g.,
  boxed and channel sections).

 • Available  floor space  would not  accommodate a
  spray washer system.

 • Parts to be processed can be placed in baskets more
 . easily or cost effectively than if hung on a conveyor line.

 • Workpieces are so large that a spray washing system
  would be  prohibitively expensive.

 A facility operator considering the  installation of an im-
 mersion system  should consult with a specialized con-
tractor about design and layout.
 Figure 6-2 illustrates two typical immersion system lay-
 outs: Figure 6-2(a) shows the more common layout for
 a typical batch operation; Figure 6-2(b) shows a less-
 common layout that would rely on  a conveyor to carry
 workpieces in and out of the tanks  in a continuous
 process. (For a detailed discussion  of rinsing opera-
 tions, see Chapter 7.)
      (a) Immersion tanks laid out for batch operation.

      I Tank #1 fl[	B Tank #2H—|lTank#3B|—Blank #4 j

  (b) Immersion tanks laid out for continuous conveyorized operation.

Figure 6-2.  Immersion rinse system schematic.

6.5.4   Design of a Spray Washer System

A spray rinse system is often recommended for a paints
and coatings operation that has a conveyor line with a
speed greater than 2 ft/min.

Advantages of the spray washer approach include:

• Increased impingement afforded by  high-pressure
  nozzles, providing more  efficient cleaning and  uni-
  form phosphate coating deposition.

• Increased production, given the ability to effectively
  pretreat thousands of tons per year of metal work-
  pieces on a continuous basis.

Limitations of the spray washer approach include:

• Inability to apply the phosphate coating uniformly on
  workpieces with complex geometries  (e.g.,  with  re-
  cesses  and crevices), particularly on  spray washer
  "shadow" areas.

• Inability to provide the same  wash effectiveness to
  parts, particularly if some are very large and others
  very small (i.e., smaller parts will be farther from the
  spray nozzle  and thus subjected to lower impinge-
  ment pressure).

• Greater floor-space  requirements,  particularly  for
  fast-moving conveyors.

• Greater energy losses due to the high evaporation
  rate of hot water.

• Higher equipment costs (e.g., for pumps, motors).

•  Higher maintenance costs due  to the need for fre-
   quent cleaning and replacement of nozzles, as well
   as the need to lubricate the conveyor system, which
   is continuously subjected to a moist environment.

A facility operator considering the installation of a spray
washer  line would be well advised to consult with a
specialized contracting company. In general, when plan-
ning for a spray washer, the  facility .operator needs to
consider how the layout will affect process flow. The
spray washer system must be designed  so that work-
pieces can easily pass through the pretreatment  proc-
ess, allowing adequate time  for the solutions to  drain
between each tank.

A spray washer system cannot be properly designed
unless the conveyor  line speed and the part sizes are
known. The dimensions of the spray  tunnel  must be
based on the silhouette of the maximum part size. The
spray  nozzles inside the  tunnel  must be located on
risers so that they are only a few inches away from the
largest part.

When possible, a system should be designed so that spray
rinses precede every process tank. Although the rinses are
at low pressures, they enhance pretreatment by prevent-
ing the contamination of tanks with chemicals from a
preceding tank. Operation of such spray washers is rela-
tively inexpensive because low volumes of water are used.

Given the vast number of workpieces and parts of vary-
ing size that can pass through a spray system each day
for certain operations, nozzles can often be misdirected.
Thus, a maintenance  engineer should routinely check to
see that spray nozzles are pointing in the correct direction.

A  design feature  often overlooked regards conveyors
that pass workpieces through the tunnel, dry-off oven,
and spray booths, as shown in Figure 6-3. The advan-
                 tage of such designs is that line workers are only needed
                 for hanging and offloading workpieces.

                 6.5.5  Process and Quality Control Measures

                 Specification TT-490-D  (7) is the  military specification
                 that covers cleaning and pretreating ferrous surfaces for
                 organic coatings.  This document is useful even for op-
                 erations not performing work for the military because it
                 provides  excellent guidelines for  process and quality
                 control (see also Reference 10).

                 Beyond following general guidelines, it is imperative that
                 facility operators conduct process  control tests recom-
                 mended by the vendor on a regular basis. These include
                 tests relating to pH, concentration, total acid, tempera-
                 ture, and dwell  time. Also, operators should be careful
                 that processing tanks do not become over contaminated
                 because  the effectiveness  of  pretreatment can be

                 6.6   References

                  1.  Rausch, W. 1990. The  phosphating of metals. ASM International
                    and Finishing Publications. American Society of Metals Interna-
                    tional, Metals Park, OH. p. 140.
                  2.  Yaniv, A.E. 1979. The influence of surface preparation on the
                    behavior of organic coatings. Metal Finishing (Nov.), pp. 55-61.
                  3.  Hurd, J. et al. Chromate  conversion  coating elimination from
                    5,000-series armor-grade aluminum. Metal Finishing (in press).
                  4.  American Society of Metals (ASM). 1982. Nonmetallic coating
                    processes. In: Metals Handbook (9th ed.), vol. 5,  p. 434. ASM
                    International, Metals Park, OH.
                  5.  Kraft, K. 1994. Electrocoat system design. Symposium Proceed-
                    ings for Electrocoat '94, Orlando, FL, March 23-25. Sponsored
                    by Products Finishing, Gardner Publications, Clough Pike, OH.
                  6.  Nair, U.B. 1995. Calcium as a phosphating additive: An overview.
                    Metal Finishing 93(3):40 (March).
             Load Station
                           Metal Pretreatment
             Dry-Off Oven (400'F)
               Unload Station
                                                                                       Priming Spray
                   Curing or
                  Baking Oven
 Top Coat
Spray Booth
Prepping Area
Figure 6-3. Schematic of a conveyorized paints and coatings operation.

 7. General Services Administration. No date. Federal Specification
    TT-490-D. Cleaning methods and pretreatment of ferrous sur-
    faces for organic coatings.

 8. Gotoff,  D.M. 1995.  Troubleshooting pretreatment systems.  In:
    The Finishing Line, vol. 10(1). Society of Manufacturing Engineers.

 9. Witke, W.J.  1994. Product Finishing Directory. Clough Pike, OH:
    Gardner Publications, p. 74.

10. Menke, J.T.  1994. Process control for phosphate coating. Prod-
    ucts Finishing 58:57-61 (January).
6.7   Additional Reading
Errikson, M. 1995.  Zinc phosphating. In:  Metal Finishing Organic
   Guide Book and Directory, vol. 93 (No. 4A). New York,  NY: El-
   sevier Science Publishers.

Gruss, B. 1995. Cleaning and surface preparation. In: Metal Finishing
   Organic Guide Book and Directory, vol. 93 (No. 4A). New York,
   NY: Elsevier Science Publishers.
Gruss, B. 1995. Iron phosphating. In: Metal Finishing Organic Guide
   Book and Directory, vol. 93 (No.  4A). New York, NY: Elsevier
   Science Publishers.

Rausch, W. 1990. The Phosphating of Metals. Teddington, England:
   Finishing Publications.

Woods, K. and  S. Spring. 1979. Zinc phosphating. Metal Finishing
   77:24-60 (March).

Wood, W.G., ed. 1982. Surface cleaning, finishing and coating. In:
   Metals Handbook (9th ed.), vol.  5. American Society for Metals.
   Metals Park, OH.

The author wishes to acknowledge Joe Schrantz, former
editor of Industrial Finishing Journal, for his contribution
to the information in this chapter.

                                              Chapter 7
       Rinsing Process Efficiency and Alternatives to Chromate-Based Sealers
 7.1    Introduction
 7.1.1   Pollution Prevention Considerations

 Thorough rinsing is the  most important  factor in the
 paints and coatings process for ensuring  that work-
 pieces  receive a  high-quality finish. Without rinsing
 away residual surfactants, excess alkalinity,  or unre-
 acted acids, for example, a finish can degrade prema-
 turely, if not fail catastrophically. Sealing the phosphate
 coating can be equally important for ensuring the quality
 of the finished piece.

 Pollution prevention is a critical consideration for these
 process steps because many operations generate high
 volumes of wastewater when rinsing and sealing work-
 pieces.  Additionally,  the chromate-based  formulations
 traditionally used in sealing rinse  baths generate toxic
 residues, some of which must be handled as hazardous

 Often, however, these wastes can be minimized through
 process modifications that can yield overall efficiencies
 and cost  savings.  The volumes  of rinse wastewater
 generated, for instance, can be dramatically reduced at
 the same time that rinsing efficiency  is enhanced by
 using a multiple-bath method called counter-flow rins-
 ing. Similarly,  the generation of hazardous chromate
 residuals can be controlled,  and in some cases elimi-
 nated, by switching to nonchromate formulations. While
 nontoxic sealers are not considered as effective as chro-
 mate-based formulations in all operations, many proc-
 esses may realize cost and process benefits from using
these alternatives.

These pollution prevention approaches are discussed in
this chapter in the context of best management practices
associated with the  rinsing and sealing process steps.
 In a general  sense, any process operated efficiently
controls the unnecessary generation of pollution to the
degree that the operation minimizes overall waste and
the number of workpieces that  must be  rejected and
disposed of or reprocessed. Indeed, the  pretreatment
process stages of rinsing  and sealing are particularly
important in a right-first-time approach to applying paints
and coatings. By ensuring thorough rinsing and sealing,
 an operator can avoid corrective measures, which tend
 to be both chemically intensive and expensive.

 7.7.2  Decision-Making Criteria

 Decision-making criteria relevant to rinsing process effi-
 ciency and alternatives to chromate-based sealers, as
 addressed in this chapter, are highlighted in Table 7-1.

 7.2    Rinsing

 The primary purpose of the rinsing step in the paints and
 coatings process is to  clean  contaminants from the
 workpiece before  it moves on to the next stage in the
 sequence. Depending on where rinsing takes place in
 the overall process, contaminants can include dirt, sand-
 ing dust, metal fines, or any other particulates as well as
 chemicals, solvents, or residues that may adhere to the
 workpiece. Thorough rinsing can both enhance the ulti-
 mate  quality  and  durability of the  finished piece  and
 minimize contamination of downstream steps  in the
 process flow.

 The number of rinsing steps in a process, as well as the
 number of baths in a given step, primarily depends on
 the quality requirements for the finished workpiece. In-
 deed, rinsing might be left out entirely from  the paints
 and coatings process for a particularly low-value piece;
 however, best management practice would  argue in
 favor  of a minimum  of one rinsing stage to maximize
 process efficiency by controlling drag-out from one bath
 to another. The most effective method of rinsing is the
 counter-flow approach, which relies on multiple baths to
 provide thorough rinsing of the workpiece while minimiz-
 ing the volume of rinse water used (see  Section 7.2.2).

The typical process  flow for a high-value paints  and
 coatings operation includes a step for rinsing the clean-
 ing-formulation residues from the workpiece after de-
 greasing and then rinsing  the  piece  again  after
 phosphating to remove unreacted acids. These two rins-
 ing steps are described below following a brief discus-
sion of the basics of the  rinsing process. This section
also includes a discussion of wastewater minimization
 using  the counter-flow rinsing approach.

 Table 7-1.  Decision-Making Criteria Regarding Rinsing Processes

 Issue                                Considerations
 Does the pretreatment system
 include degreasing and
 phosphating as a single stage?
 Does the pretreatment system
 include degreasing and
 phosphating as separate stages?
 Will the paint finish on workpieces
 be solely for appearance (i.e.,
 corrosion and other physical
 properties have little significance)?
 Will the finished workpieces be
 required to have only low
 corrosion resistance (i.e., be able
 to withstand between 96 and 168
 hours of salt fog exposure per
 ASTM B-117 [see Reference 1])?
Will the primed workpieces be
required to have moderate
corrosion resistance (i.e., be able
to withstand between  168 and 500
hours of salt fog exposure per
ASTM B-117 [see Reference  1])?
Will the finished workpieces be
required to have superior
corrosion resistance (i.e., be able
to withstand at least 500 hours of
salt fog exposure)?
Is the useful life of the phosphate
bath shorter than what is projected
in vendor literature?
  If yes, then rinsing before phosphating is not a consideration.
  Regardless of whether these stages are separate, at least one rinse with municipal tap water
  should follow phosphating.
  Addition of a sealing rinse in a static tank should also be considered.
  If yes, then rinsing with tap water after degreasing should be included.
•  Many low-value workpieces (i.e., household products) for price-sensitive markets are in this
  category. Manufacturers often cannot justify improvements in the coatings process on a
  value-added basis.
•  When corrosion-resistance requirements are low, it may be cost effective to conduct degreasing
  and phosphating in one step, followed by at least one municipal tap water rinse.
>  Addition of a sealing rinse in a static tank should also be considered.
  Most finished metal products are in this category because they might be subjected to a
  moderate degree of outdoor exposure (i.e., not particularly corrosive elements).
  It may be cost effective to conduct degreasing and phosphating in one step.
  Consideration should be given to separate stages, with at least one tap water rinse between the
  two stages.
  If separate stages are used, a second post-degreasing rinse with deionized water should be
  considered for extending the useful life of the phosphating bath. The second bath is particularly
  important if the process line does not allow time for sufficient draining  before phosphating.
  A sealing rinse should be considered mandatory.
  If it can be shown that the primer-topcoat system will provide the required corrosion  resistance,
  consideration should be given to using a nonchromate sealer.
  Rinse drainage should be collected and recycled.
  If two or more rinse tanks follow degreasing or phosphating, consideration  should be given to
  using a counter-flow system.
  If one or more of these rinses use deionized water, consideration should be given to installing
  automatic flow controllers, which monitor the concentration of chemicals in  the rinse tank.
  If degreased and phosphated workpieces will be stored outdoors for several days prior to
  application of a primer-topcoat system, consideration should be given to  using a chromate
  sealer for enhanced corrosion resistance. Expectations are that nonchromate sealers eventually
  will be proven fully equivalent to conventional sealers in corrosive environments.
  This might apply to products that are subjected to outdoor exposure in all types of weather,  to
  marine environments, or to chemical vapors.
  Same considerations as for low-corrosion resistance  requirements above, although additional
  emphasis should be placed on multiple rinse steps.

  This usually applies to workpieces in automotive manufacturing. Electrocoating is used on most
  automotive parts, a process that cannot tolerate any  drag-in  from pretreatment steps.
  Following degreasing, the workpiece must undergo at least one tap water rinse followed by a
  deionized water rinse.
  At least two tap  water post-phosphating rinses must be included, followed by rinsing with
  deionized water.
  A sealing rinse should be considered mandatory.
  If it can be shown  that the primer-topcoat system will provide the required corrosion resistance,
  consideration should be given to using a nonchromate sealer.

  Rinse drainage  should be collected and recycled.
  If two or more rinse tanks follow degreasing or phosphating, consideration should be given to
  using a  counter-flow system.
  If one or more of these rinses use deionized water, consideration should be given to installing
  automatic flow controllers, which monitor the concentration of chemicals in the rinse tank.
  If degreased and phosphated workpieces will be stored outdoors for several days prior to
 application of a primer-topcoat system, consideration  should  be given to using a chromate
  sealer for enhanced corrosion resistance.
  Additional emphasis should be placed on rinsing after degreasing.
 Also, rinse drainage should be  collected and recycled.

 7.2.1  Rinsing Basics and Best Management

 By monitoring and controlling basic aspects of the rinse
 stages of the paints and coatings process, an operator
 can enhance overall efficiency, while minimizing  the
 amount of wastewater discharged by extending the use-
 ful life of rinse baths. The most important of these con-
 siderations  are water quality,  immersion  time, rinse
 temperature, agitation  or  impingement, workpiece  ge-
 ometry, system loading, and rinse water dumping.

 Water quality. Most operations that include a rinse stage
 use municipal tap water, which typically is slightly acidic
 (i.e., pH of 5.0 to 5.3) and with low resistivity (i.e., about
 5 megohm/cm). Although the quality of tap water can
 vary depending on its source, it generally includes any
 number of impurities, such as ions of sodium, magne-
 sium, iron, calcium, potassium, chlorine, sulfates, car-
 bonates, and nitrates. As tap water evaporates  from a
 rinsed workpiece, ions are left on the surface (i.e., only
 the volatile molecules will evaporate). Because the ions
 can conduct an electrolytic current (see Chapter 3), they
 can cause corrosion to occur,  even after a primer or
 topcoat has  been applied to the workpiece.

 For many workpieces, this corrosion potential is not a
 paramount issue. For high-value pieces, however, most
 operators enhance long-term durability  by using deion-
 ized water for final rinsing (e.g., the second and third
 step in multiple rinse stages), which removes corrosive
 residues from workpiece surfaces.  Tap water can be
 deionized using a sophisticated ion-stripping technology
 (e.g., ion  exchange resins). Deionized water typically
 has a relatively high  resistivity (i.e., about 18.3 me-
 gohm/cm) (2) and a neutral pH (i.e., 7). The purer the
 rinse water, the longer its useful life. Chemical vendors
 usually are willing to provide log sheets to assist the
 operator in determining the degree of contamination that
 rinse water can withstand. Some operators also rely on
 instrumentation for monitoring the pH and conductivity
 of rinse water as a way of gauging its useful life.

 Immersion time. To ensure removal of as much contami-
 nant as possible, the workpiece must remain in the rinse
 bath long enough for all residues to be removed.  Allow-
 ing a steel piece to remain in the bath for an excessive
 period of time, however, can encourage flash rusting
 (i.e., the formation of ferrous hydroxide  [rust] on the
 surface of the steel). Steel is particularly prone to flash
 rusting after  it has  undergone degreasing and before it
 has received a phosphate coating. Because at this point
the surface has been cleared of protective oils, flash
 rusting can easily occur if the steel remains wet for more
than a few seconds.

 Rinse temperature. Rinsing is typically carried out using
water at ambient temperature. Heated rinse water, how-
ever, can  enhance the capacity of the rinse stage to
 remove  certain  types of contaminants from the work-
 piece. More specifically, for rinsing after phosphating,
 the  use of heated water can  expedite drying of the
 phosphated piece.

 Agitation or impingement (spray washing). Efficient re-
 moval of  contaminants from  a  workpiece  can be
 achieved by subjecting the piece to agitation or impinge-
 ment during the rinsing  step. For systems  in which the
 workpiece  is immersed in  the  rinse bath, agitation is
 typically provided by air  sparging, using compressed air
 at low pressure (i.e., 10 to 20 psi). In contrast,  the
 impingement  approach  involves  spray washing  the
 workpiece  with  100 to  150 psi of pressure while the
 piece is  suspended above  the  rinse  bath.  For a spray
 washing system to be  effective, the nozzles must be
 correctly configured and directed to wet all surfaces of
 the workpiece. Nozzles should  be checked and  main-
 tained regularly. Spray washing often is used either in
 addition to  immersion  rinsing for high-value workpieces
 or in place of immersion rinsing when floor space is

 Workpiece geometry. Large workpieces and pieces with
 complex geometries (e.g., with  channels and box sec-
 tions that are difficult to reach with  rinse water) can
 make efficient rinsing difficult. For such pieces, racking
 or suspension from conveyors may be necessary to
 allow for thorough drainage before and after the rinse
 step. For pieces with  particularly complex  geometries,
 drilling small drainage holes in workpiece sections might
 be necessary. In  immersion operations, most rinse water
 drainage can be captured by allowing the workpiece to
 remain suspended over the rinse tank for a few minutes.
 Also, many conveyor  systems include a sloped metal
 tray that  collects drainage and channels it back to the
 rinse tank.

 System loading. An operator can boost production by
 tightly loading a conveyor or rack  system  that moves
 workpieces through the  rinse stage. Excessive  loading
 of rinse baths relative to  the dilution ratio, however, can
 undermine  the efficiency of this stage of the process,
 and ultimately the quality of the finished piece. Thus, the
 system loading rate needs to be balanced against the
 performance requirements of the workpiece.

 Rinse water dumping.  Generally a rinse bath is kept at
 equilibrium  by discharging effluent as the tank is infused
 with  fresh makeup water. The rinsing process  can be
 optimized, however, by periodically dumping  the entire
 rinse bath into the wastewater treatment system. The
frequency of dumping  should be determined  based on
such factors as rinse tank volume and workpiece size.
Titrations (i.e., tests for determining  the concentration of
contaminants in the rinse water) performed on site and
 laboratory  analysis  can provide  qualitative data for
scheduling the routine dumping of a system's rinse water.
 Pretreatment chemical  vendors can supply titration

 equipment and training as well as advice about testing
 frequency.   Rinsing Following Degreasing

 Before receiving a phosphate coating, a metal work-
 piece should be thoroughly rinsed to remove any surfac-
 tant residues from the  degreasing  step.  While the
 surfactants in degreasing formulations are essential for
 removing contaminants from a workpiece, their typically
 low surface tensions make them extremely difficult to
 remove without thorough rinsing. Surfactants and other
 contaminants that remain on the surface of the work-
 piece following degreasing can undermine the integrity
 of the phosphate deposition and ultimately the quality of
 the finished piece;

 An additional reason for including a rinsing step at this
 stage of the process is to minimize the amount of drag-in
 from the  high-alkaline  degreasing bath  (i.e.,  typically
 with a pH greater than 10) to the near-neutral  phos-
 phating bath (i.e., a pH of 5 to 6, depending on the
 composition of the bath). Drag-in from a degreasing bath
 or from an exhausted post-degreasing rinse will gradu-
 ally  neutralize the  phosphating bath until little  or no
 phosphate deposits on the workpiece. Even before a
 phosphating  bath  reaches this point,  it  should  be
 dumped. Thus, eliminating this rinsing step can dramati-
 cally shorten the useful life of the phosphating bath.

 Although many operations rinse their degreased work-
 pieces in a  single  bath before the  pieces  receive  a
 phosphate coating,  companies that produce high-value
 finished pieces typically include a multiple-bath rinse
 step following degreasing. For instance, companies that
 apply paints and coatings to automotive parts, large
 appliances,  exterior-use coils, and office furniture, as
 well as in many electrocoating  operations, rinse  work-
 pieces after degreasing especially thoroughly to meet
 demanding durability and performance specifications.

 Figure 7-1 presents a  schematic of a post-degreasing
 rinse stage that includes two baths—the first using mu-
 nicipal tap  water and the second  using  deionized
water—and an optional spray rinse. This type of rinsing
system would be used in an operation finishing  work-
pieces with particularly  high-performance requirements.

 In contrast, many operations can meet less-demanding
requirements for coatings without including a phos-
phating step following  degreasing or alternatively by
using a single rinse bath. The tradeoff in terms of the
finished piece yielded by an abbreviated process such
as this is that the coating can fail catastrophically.  Be-
cause many coatings are sensitive to alkalinity, they can
break down to form soaps by means of a saponification
reaction. When this occurs, large areas of the coating
may flake, or spall, from the surface.
                                      Deionized Waier
                                         Spray Rinse
 Figure 7-1.  Schematic of three-step  post-degreasing rinse
 Operations that include multiple-bath rinse stages often
 use municipal tap water in the first bath for removing the
 highest  concentrations of contaminants,  ending  the
 rinse stage with a deionized water bath that removes tap
 water impurities left on the workpiece surface. Because
 tap water is generally inexpensive and readily available
 at high flow rates, operators try to use it for rinsing where
 appropriate. Although deionized water is not particularly
 expensive, it must be generated on site and flow rates
 tend to be limited. Also, the ion exchange resins typically
 used to  deionize water eventually become exhausted
 and must be regenerated or replaced at additional cost.
 By using the counter-flow rinsing approach, operators
 can minimize the volume of deionized water required to
 perform superior rinsing (see Section 7.2.2).

 Operators who apply a deposition  coating using zinc
 phosphating can enhance process efficiency by adding
 a low concentration of a titanium salt to the rinse stage
 immediately  preceding the phosphating  tank. Titanium
 salt acts as an activator in initiating nucleation of the zinc
 phosphate crystals. For this rinsing step, chemical ven-
 dors strongly recommend the  use of  deionized rather
 than municipal tap water.   Rinsing Following Phosphating

 For certain types of operations, a second rinse stage is
 included  to remove drag-out of unreacted acids, sludge
 deposits, corrosive  salts,  and  other contaminants that
 remain on the workpiece following phosphating. To
 achieve a quality finish, the primer and topcoat must be
 applied to a workpiece that is as free as  possible  of
 contaminants. Without thorough rinsing at the end of the
 pretreatment process, the ability of the organic coating
 system to provide the designed-in corrosion resistance
 and other physical properties can be undermined. More-
 over, contaminants that remain on the workpiece after
 phosphating can "photograph" through or stain the top-
coat, marring the finished piece.

Alternatively, some operators rinse workpieces after the
phosphating  step primarily to arrest or slow the phos-
phating process at a certain point. In processes in which
the thickness of the conversion coating is a critical pa-
rameter, operators typically include a stage for rapid and
thorough rinsing of the workpiece.

 In contrast to the detrimental effect that surface alkalinity
 can have on a primer-topcoat system, a slightly acidic
 surface enhances initial adhesion of the primer as well
 as long-term corrosion resistance. Thus, for many op-
 erations, municipal tap water can be used for rinsing at
 this stage. Although tap water has a slightly higher pH
 than the phosphating chemicals, the rinse does not need
 to raise the pH of the workpiece surface to neutral  (i.e.,
 pH 7.0).  Most municipal water is  unsuitable for use
 directly from the tap for operations  coating workpieces
 of especially high value. This is due to the presence of
 impurities  (e.g.,  soluble  and  insoluble metal  salts).
 Often, however, tap water is used for the initial step in a
 multiple-bath  rinse stage. For high-value operations,
 deionized water is preferred for all subsequent rinse

 Whereas  a single  rinse bath following degreasing  may
 be sufficient for some operations, the use  of multiple
 baths following phosphating is recommended for most
 workpieces. In general, a single post-phosphating rinse
 would leave considerable residue on the workpiece as
 it passes through the dry-off oven and enters the primer-
 topcoat application stage. Thus, at a minimum, a second
 rinse stage, preferably one that also functions as the
 sealing rinse (see Section 7.3), should be included for
 most paints and coatings  processes. As  with the de-
 greasing rinse, the counter-flow rinsing approach is an
 effective method at this stage for maximizing process
 efficiency (see Section 7.2.2). Operations finishing high-
 value workpieces typically  include a system of at least
 three post-phosphating rinses, two  of which bathe the
 piece  in deionized water, followed  by spray or mist
 rinsing with deionized water.

 7.2.2   Counter-Flow Rinsing

 As well as  being  an effective  method for thoroughly
 washing contaminants  from workpieces after d^greas-
 ing or phosphating, counter-flow rinsing is a particularly
 effective method for minimizing water usage. Nonethe-
 less, few  managers  of paints and coatings operations
 have a sufficient understanding of this rinsing method
 as a process control  strategy.

 Fundamentally, a counter-flow rinsing system is a se-
 quence of baths (i.e., two or more) in which replenished
 rinse water moves in the opposite direction of the proc-
 ess flow. Thus, the workpiece progresses from dirtier to
cleaner rinse water (Figure 7-2). The system maximizes
water use by replenishing the rinse at the final bath  in
the sequence; overflow from each bath in the sequence
 in  turn replenishes rinse water in the preceding bath.
 Rinse water effluent is ultimately released to the waste-
water treatment system as overflow from the first (dirti-
 est)  bath  in the sequence. The basic concept behind
counter-flow rinsing is that the makeup water in the first
 bath in a rinsing sequence does not need to be as clean
 as that in the last.

 The key to an effective rinse  system based on this
 approach is maintaining the dilution ratio from the first
 to the last bath in the counter-flow sequence. The dilu-
 tion ratio is primarily a factor of the system's rinse water
 flow rate versus  the workpiece drag-in  rate. For in-
 stance,  if the degreasing tank has a chemical concen-
 tration of 1  Ib/gal, then  the workpiece will drag 1  Ib/gal
 of chemical into the first rinse  tank.  If that rinse tank
 holds 99 gallons of uncontaminated water, the chemical
 concentration of the tank with the 1 gallon of drag-in will
 be 1 pound of chemical per 100 gallons of water; thus,
 the chemical concentration will  be 0.01 Ib/gal and the
 dilution ratio will be 100:1.

 Related considerations, however, include the concentra-
 tion of contaminant in the makeup water replenishing a
 bath and the contaminant concentration in the bath itself.
 Equations for calculating the rinse water flow rate and
 number of  rinse baths  required to achieve a specified
 dilution ratio are provided and explained in  Appendix B.

 Controlling  a system's dilution ratio allows the Operator
 to take  advantage of one of the principal benefits of
 counter-flow rinsing: reducing  the overall volume of
 water required for cleaning a workpiece by adding baths.
 A single rinse bath quickly loses  its effectiveness unless
 relatively large volumes of water are added to maintain
 the  dilution ratio.  Figures  7-2 and 7-3  illustrate water
 usage needs relative to time for maintaining  a dilution
 ratio in a one-step rinse system of 1,000:1 gallons of
 rinse water to contaminant. Based on this illustration, a
 one-step rinse for a workpiece with a drag-in rate of 1
 gal/min would consume large amounts  of water. To be
 effective, the process would require either a  large-vol-
 ume tank or a small tank with rinse water changed (i.e.,
 dumped) frequently.
 By comparing  gallon-per-minute flow ra*tes required to
 clean a workpiece with a 1 gal/min contaminant drag-in
 rate, Table 7-2 indicates the reduction in water use that
 can be realized by increasing the number of baths in a
 counter-flow rinsing system. Thus, Table 7-2 shows that
 when a process's dilution ratio is  100:1, adding a second
 bath reduces the flow rate requirement from  99 to 9.5
 gal/min. The operator of this system would reduce the
 water requirement to 2.3 gal/min by adding a fifth rinse
 bath. Note that for a dilution ratio of 20,000:1  (required
 for some particularly high-value workpieces), the flow
 rate specified for a five-bath system is 7.0 gal/min, which
 is only about three times the rate for a 100:1 dilution rate.
 Table 7-3 provides another perspective  on  the process
"efficiency advantages of  counter-flow  rinsing by pre-
 senting water flow  rates in terms of percentage reduc-
 tions between additional baths. For example (based on
 the data in Table 7-2), with a dilution ratio of 100:1 for a
 workpiece with a 1  gal/min drag-in, the flow rate reduc-

         y gpm, d Ib/ga!
                  y gpm, c Ib/gal
y gpm, b Ib/gal
y gpm, a Ib/gal
     y gpm, c Ib/gal

      To WWT
                                 y gpm.b Ib/gal
                      Rinse Stage #1
               c Ib/gal
                                                       y gpm, a Ib/gal
                                                 Process Flow
                                        Counter Current Rinse Flow
   a = concentration of chemical in Bath 13 (Ib/gal)
   b = concentration of chemical in Bath ttl (Ib/gal)
   c = concentration of chemical in Bath #1 (Ib/gal)
   d = concentration of chemical in process bath (Ib/gal)
   x = flow rate of counter-flow rinse (gal/rain)
   y = flow rate of drag-in (gal/min)

Figure 7-2.   Schematic of counter-flow rinsing.
                                                                                            200 gals

                                                                                            500 gals

                                                                                            1,000 gals

                                                                                            2,000 gals
                                                         Time (mnutes)
Figure 7-3.
Dilution ratio as a function of time for different tank sizes (based on a process as Illustrated in Figure 7-2 and assuming
a 1,000 gallon process tank and a 1 gal/min drag-out to the first rinse tank).
tion that can be achieved by adding a second rinse bath
is 90.4 percent (as shown in Equation 7-1); addition of
a third bath would reduce the flow rate to 95.7 (as shown
in Equation 7-2).
                                                (Eq. 7-1)
                                                           % Reduction =    „.  ; * 100 = 95.7%
                                                                                                (Eq. 7-2)
                                                 Comparison of Figures 7-4 and 7-5 illustrates the point
                                                 that significant reduction in water usage can be realized
                                                 with the counter-flow, multiple-bath rinsing method.

Table 7-2.  Counter-Flow Rates for Workpleces With ;
          1 gal/min Drag-In
Table 7-4.  Counter-Flow Rates for Workpieces With a
          2 gal/min Drag-In
Flow Rates
Stage 1
Stage 2
Stage 4
Stage 5
Stage 1
Stage 2
Flow Rates
Stage 3
24.5 .
Stage 4
Stage 5
Table 7-3.  Total Percentage Reduction In Flow Rate From
          One Rinse Tank to the Next for Workpleces With
          a 1 gal/min Drag-In
Table 7-5.  Counter-Flow Rates for Workpieces With ,
          0.5 gal/min Drag-In


Stage 1 > 2
99 ,
Stage 2 > 3
Stage 3 > 4
Flow Rates

Stage 4 > 5

Stage 1

Stage 2
Stage 3

Stage 4

Flow rate requirements in a  counter-flow system are
influenced, however, by the rate of drag-in for the work-
piece. As indicated by comparing Table 7-2 with Table
7-4, if the drag-in rate for a workpiece increases from 1
to 2 gal/min, the counter-flow rate requirement will in-
crease by a factor of 2. Conversely,  as indicated by
comparing Table 7-2 with Table 7-5, if the drag-in drops
to 0.05 gal/min,  the flow rate needs will be cut in half.

The flow rate between tanks in a counter-flow system
should be set and monitored using automatic flow con-
trollers. This ensures that the rinsing system runs at
optimal efficiency and avoids the possibility that the rate
will be altered with each work  shift.

7.3   Sealing

Some operations subject workpieces to a final rinse bath
after  phosphating to harden the deposited  phosphate
coating, providing enhanced long-term corrosion resis-
tance. This process step is included in operations for a
wide  range of industries, most of which apply coatings
to  high-value workpieces.  Typically,  workpieces  are
sealed using a  rinse of  deionized water mixed with a
small concentration of chromate or nonchromate addi-
tive (Figure 7-6). Information on specific formulations is
generally available from pretreatment chemical suppli-
ers. Pollution prevention considerations regarding the
use of chromate rinses are addressed below following a
discussion of the basics of the sealing process.

7.3.1   Sealing Basics

With chromate-based sealing rinses, chemicals in the ad-
ditive seek out areas of the coating (i.e., porosities and
voids) where the phosphate failed to convert the base
metal. The chemicals then react with the exposed sub-
strate, in much the same way as the phosphating process
itself, to form a corrosion-resistant film. Nonchromate seal-
ers (e.g., polymer sealers) also form a protective film over
exposed areas of the substrate, although not through a
chemical reaction with the base metal.

The protective film yielded by a sealing rinse provides a
barrier between  the exposed base metal and the envi-
ronment.  Shielding the substrate from atmospheric
moisture  and oxygen  prevents  electrolytic corrosion
from occurring.  The superior corrosion  resistance af-
forded by chromate sealers is particularly important for
operations that  store unprimed  steel workpieces out-
doors for several days or more before applying a coating

    20,000 ,-
 -=. 15,000 -
    10,000 -
     5,000 -
            100   1,000   2,000  5,000   10,000 20,000
                        Dilution Ratio Required

Figure 7-4.  Graph  of rinse water flow rate required to dilute
           drag-in stream at 1 gal/mln for first rinse bath only.
 S 100

 ง  50
H 2nd Rinse Stage
Q 3rd Rinse Stage
• 4th Rinse Stage
S 5th Rinse Stage
        100    1,000   2,000   5,000   10,000  20,000
                   Dilution Ratio Required

Figure 7-5.  Graph of counter-flow rinse water flow rate required
          to dilute drag-in stream at 1 gal/min for subsequent
          rinse baths.
                                                                                     Sptay Rinse'
 Figure 7-6.  Schematic  of post-phosphating  rinsing process
           with sealing rinse bath.

 Many companies omit the sealing stage, lowering the
 corrosion resistance provided by the phosphate coating.
 Typically, however, sealing rinses  are a cost-effective
 addition to a pretreatment process line, given that rinse
 additives are inexpensive to use in low concentrations
 (i.e.,  a  few ounces per gallon of rinse water) and the
 rinse stages are static (i.e., no overflow from the  bath).
 Depending on the volume throughput of workpieces and
 the condition of drag-in from the previous stage, a seal-
 ing tank can have a useful life of several weeks before
 it must  be replaced.

 7.3.2  Chromate-Based Sealing Rinses
        Versus Nontoxic Alternatives

 The sealing rinse stage in the paints and  coatings proc-
 ess raises important considerations in terms of pollution
 prevention.  The  operator must balance tradeoffs  be-
 tween the use of chromate additives (i.e., hexavalent
 and trivalent chromium), which can be highly toxic, and
 nonchromate alternatives, which at present are gener-
 ally less effective.   Chromate-Based Sealing Rinses

 Operators have used chromate-based rinses for  many
 years as an effective means of sealing the phosphate
 coating  on the workpiece. Chromate rinse additives are
 based on either a hexavalent or trivalent chromium (i.e.,
 Cr6* or Cr3*). While both forms are pollutants of concern,
 hexavalent chromium is particularly toxic and is a sus-
 pected carcinogen; thus,  residuals must be disposed of
 as hazardous waste, which can add significant costs to
 the paints and coatings process.

 Consider, for example, a situation in which  all of the
 exhausted chromate-containing rinse water held by a
2,000-gallon immersion tank must be disposed of as a
 hazardous waste  in 55-gallon drums, unless the waste-
water is first treated. In 1995, the cost of disposing of a
55-gallon drum of liquid hazardous waste approached
$600. Thus, the total  cost to dispose of the entire tank
of rinse  water could exceed $21,000. If the operation's
 rinse  water is replaced frequently,  the annual cost of
disposal could be significant. Moreover, the operator is
responsible for tracking the hazardous waste from the
"cradle to the grave."

 Alternatively, the wastewater could be discharged to an
 onsite treatment plant for removal of the chromates and
 other contaminants by precipitation and filtration.  The
 resulting sludge material would then need to be properly
 disposed of. The treated water could be recycled to the
 rinsing operation. While this approach  is usually cost
 effective for large operations, most medium- and small-
 sized  operations cannot afford  the cost of an  onsite
 treatment plant.

 Another limitation of chromate use is that some formu-
 lations require that the workpiece be rinsed with clean
 water after the sealing rinse to remove unreacted chro-
 mate salts. Along with the cost of any equipment asso-
 ciated  with adding  this process step, costs associated
 with the generation of additional wastewater must be
 considered. One approach to minimizing the cost of this
 rinse step is to spray wash the workpiece while  it is
 suspended over the sealing rinse bath. The tradeoff with
 this approach is that the spray rinse water is likely to
 gradually alter the  chromate dilution ratio,  limiting the
 useful  life of the bath.

 Given  the limitations associated with  the use of chro-
 mate-based rinse formulations, operators need to care-
 fully weigh  tradeoffs  in  terms of  costs,  pollution
 prevention,  and the durability  requirements of the  fin-
 ished workpiece. The determination of which sealing
 formulation to use'must be made on a process-specific
 basis after thorough testing of various options.
           t?  Nonchromate  Sealing Rinses

 Although  several nonchromate sealing formulations
 have been developed, their effectiveness for enhancing
 the durability of a finished workpiece as compared with
 chromate-based sealers  has yet to be fully established
 (3). Nonetheless, when the finished workpiece will be used
 in applications requiring  less-demanding corrosion re-
 sistance, nonchromate sealers can present an attractive
 alternative. Also, available high-performance  coatings
 (e.g., epoxies and polyurethanes) have corrosion-resis-
 tance properties that allow operators to offset  potential
 deficiencies associated with nonchomate sealers.

 The great advantage that  nonchromate sealers hold
 over chromate-based formulations is that they are non-
 toxic. Thus,  an operator can realize significant benefits
 by reducing or eliminating the  need to dispose of haz-
 ardous residuals.

A related advantage is that often no clean-water rinsing
of the workpiece needs to be performed after  use of a
 nonchromate sealer. Indeed, post-sealing rinsing may
 harm the  workpiece because  it can wash away  the
protective film on the piece's surface. Thus, an  operator
can  realize  process savings in  terms of wastewater
 When  determining  whether to  use a  nonchromate
 sealer, the operator needs to weigh these potential ad-
 vantages against the  quality requirements of the fin-
 ished workpiece. Before incorporating a nontoxic sealer
 into a paints and coatings process, an operator should
 thoroughly test  the  formulation  in terms of the work-
 piece's specifications.

 7.4   Case Example

 Navistar International Transportation Corp., a manufac-
 turer of truck cabs, has  reported  on  its program to
 minimize pollution of all media (4). The truck cabs enter
 the pretreatment process via a two-stage alkaline de-
 greaser. Stage 1 operates optimally at a pH of 10.5 and
 with  an alkalinity range  of  6 to 10. When analytical
 testing  finds  the total alkalinity to fall below 6, the de-
 greaser is no longer considered effective. Formerly at
 this  point, a  portion of the tank  would be dumped. To
 optimize the  performance of Stage 1, the manufacturer
 would allow 2.5 gal/min of contaminated tap water from
 Stage 2 to overflow into Stage 1 while allowing an equal
 amount of water displaced from  Stage 1 to overflow to
 the wastewater treatment system. In addition, the manu-
 facturer would flush approximately 2,000 gal of water
 from the tank every 7 days, discharging it to the waste-
 water treatment system. Finally, every 45 days, Navistar
 dumped the entire contents of Stage 1, rinsed the tank
 with  up to 17,000 gal of water, and then filled it with half
 the contents  in Stage 2.

 After examining the system further, Navistar discovered
 that it was not cost effective to cross-contaminate Stage
 1 with water in Stage 2. Rather, after dumping, Navistar
 used fresh chemicals and water, extending the life of
 both stages from 45 to 90 days.  Total cost savings for
 these modifications amounted to  $9,384 per month.

 Stage 3 of its pretreatment process comprises a munici-
 pal tap  water rinse, which  is  contaminated with drag-in
 from the alkaline  degreasing Stages 1 and 2. Navistar
 discovered that by allowing the cabs to drip drain over
 Stage 2 for  an additional 1/2 minute, it could  realize
 significant savings in the tap water rinse of Stage 3.
 Previously, Navistar had dumped this stage arbitrarily on
 a  14-day schedule.  After performing process control
 laboratory tests on the alkalinity of the bath, however, it
was able to decrease the dumping schedule to every 30
days. Apparently, this resulted in a 50-percent reduction
of contaminated water sent to the wastewater treatment

Navistar performed a similar  modification regarding the
post-phosphating  rinse stage. Process line operators
had  been arbitrarily dumping Stage 6  on a  14-day
schedule. They found, however, that by monitoring the
level of contaminants in this stage, they could decrease
the frequency of dumping to between 90 and 120 days.

In the past, the sealing rinse was conducted with mu-
nicipal tap water,  and Navistar dumped this tank every
30 days because of unacceptably high levels of water
contamination. The company found that by making up this
bath with a 50:50 mix of  municipal  water to deionized
water, it could increase the bath life by 50 percent, resulting
in a cost savings of approximately  $8,000 per month.
This process modification was made based on informa-
tion on the minimal cost of generating deionized water.

Navistar uses an electrocoating tank to apply primer. A
considerably  more  expensive  batch process treats
some of the wastewaters from the primer process. Pre-
viously, the deionized water from the  two rinse stages
preceding the electrocoating line also overflowed to the
same  waste treatment process as that used for the
electrocoat  wastewater (analyte). Navistar discovered
that it was not necessary to route the waste deionized
water  to  this  more expensive  treatment process. In-
stead, it  dumps water to its more general industrial
wastewater treatment system. This  minor modification
further reduced treatment costs  by $306 per month.
7.5   References

1. American Society for Testing and Materials. 1984. ASTM B-117,
   Salt fog testing. In: Annual Book of ASTM Standards: Paint Related
   Coatings and Aromatics, vol. 6.01. ASTM, Philidelphia, PA.

2. Angell, K. 1993.  High purity water pH. Industrial Finishing 69:42
3. G6recki, G.  1995. Importance of rinsing during pretreatment. In:
   Metal Finishing Organic Guide Book and Directory, vol. 93(4A).
   New York, NY: Elsevier Science Publishers.

4. C6te, K.H. and P.L Bradley. 1994. Physical and methodological
   modifications benefiting Navistar International's finishing plant in
   Springfield, Ohio. Presented at the 87th Annual Meeting & Exhibi-
   tion, Air & Waste Management  Association, Cincinnati, OH (June

7.6   Additional Reading

Foley, D. 1993. Automated rinsing/drying of truck bodies to improve
   final paint finish. Metal Finishing 91:51 (October).
Wittke, W.J. 1993. Finish world class! Liquid coatings. Presented at
   PaintCon '93.
Yates, B. 1991. Ten  minutes to better rinsing. Products Finishing 55

                                               Chapter 8
             Abrasive Blast Cleaning of Metal Surfaces: Process Efficiency
8.1    Introduction

8.1.1   Pollution Prevention Considerations

Abrasive blasting is widely used in the paints and coat-
ings industry as a means of cleaning metal workpieces
and  preformed materials. If blast cleaning operations
are not carefully implemented and monitored, however,
quality control problems can result that undermine proc-
ess efficiency and lead to excess waste. A critical factor
in blast cleaning is selection of an abrasive media that
will yield a blast profile appropriate to the thickness of
the primer coating. When an abrasive raises peaks on
the substrate that protrude  through the coating, flash
rusting can result, especially if primed workpieces are
temporarily stored outdoors. Rusting generally necessi-
tates  the  reworking  of pieces,  adding  process costs
associated with material, labor, and waste management.

An efficiently run blasting operation also can yield proc-
ess savings  related  to the cleaning media. Similar to
aqueous degreasing operations, most of the dry media
used in abrasive blasting can be recycled. Indeed, many
operations include a degreasing step in the process line
to maximize the blast media's reuse potential. By reus-
ing abrasives, an operator can minimize the generation
of the significant amounts of waste represented by spent
media. Other variations include adding a  phosphating
step for further enhancing mechanical adhesion of the
coating system.
Compared with degreasing, the abrasive blasting proc-
ess can be time consuming  and labor intensive; more-
over,  blasting can  involve the  risk of warping the
workpiece. Thus, facility operators generally opt for this
cleaning approach only when workpieces are too large
to be immersed or effectively sprayed with a degreasing
formulation.  An incidental benefit of abrasive blasting,
however, is that the considerable volume of wastewater
generated with other cleaning methods is avoided.

These pollution  prevention considerations are  pre-
sented in  this chapter in  the  context of process effi-
ciency. An important overriding consideration  in this
discussion is right-first-time processing, which calls for
designing  and monitoring operations to ensure that re-
works, and associated costs and pollutants, are mini-

8.1.2  Decision-Making Criteria

Decision-making criteria  relevant to process efficiency
in the abrasive blast cleaning of metal surfaces, as
addressed in this chapter, are highlighted in Table 8-1.

8.2   Process Basics

8.2.1   Introduction

Abrasive blasting is a method of cleaning corrosion and
other contaminants from previously uncoated metal sub-
strates before applying a primer-topcoat system. Blast
cleaning also is used to remove failed or aged coatings
from substrates  before repainting (i.e., paint stripping),
as discussed in Chapter 14.

In abrasive blasting, mineral and metallic abrasives,
such as steel  shot or mineral grit, are directed or pro-
pelled from a hose at a substrate using a high-pressure
pneumatic system (Figure 8-1). The line  operator holds
the blasting nozzle a few inches from the substrate while
directing the blast to all areas of the workpiece.

As  a cleaning approach  for substrates  that have  not
been previously painted,  abrasive blasting is used  pri-
marily for workpieces that are too large and heavy to be
pretreated using immersion x>r spray degreasing proc-
esses.1 Nonetheless, many operations degrease work-
pieces to the extent possible before subjecting them to
blasting in order to minimize contamination of an abra-
sive media that  will be recycled. Even when  abrasive
blasting is  used in conjunction with a degreasing or a
phosphating stage  (see  Section 8.4 on process vari-
ations), the operator is likely to realize some incidental
benefits in terms of lower water-use requirements and
thus lower wastewater generation.
1 As noted in Chapter 5, "degreasing" is used generally in this docu-
ment to refer to the various liquid/vapor methods used in paints and
coatings operations to clean substrates. The author recognizes that
some operators use the term degreasing to refer specifically to vapor

 Table 8-1.  Decision-Making Criteria Regarding Abrasive Blasting Processes

 Issue                         Considerations
 Do workplaces have a steel
 substrate of sufficient thickness
 (>14 gage) to allow for abrasive
 blasting without warping surface?

 Do workpieces need to be blast
 cleaned even though they have
 sections of thin steel substrate
 (<14 gage)?

 Will a liquid primer be applied?
 Will a zinc-rich primer be applied
 to workpieces?
 Can the operator select from a
 range of abrasives?

 Are workpieces currently
 degreased prior to abrasive
If so, consideration should be given to this approach as an alternative to pretreating workpieces with
a chemical process.
If so, it might be necessary to use a fine-mesh abrasive and to experiment with different blasting
pressures to avoid warping the workpiece.
If so, select an abrasive (or blend of abrasives) that will yield a blast profile that can be completely
covered by the film thickness of the coating. An angular profile, for instance, can be particularly
difficult to cover and may require a second coat of primer.

Workpieces should be primed within 4 hours (but not longer than 8 hours) after abrasive blasting,
depending on the ambient environment.  For example, if blasted workpieces will  be exposed to a
marine or chemical environment, the interval should be shortened to avoid the onset of corrosion.

Consideration should be given to degreasing workpieces before blasting so that the abrasive media
can be kept clean for recycling in the blasting process.

The use of wash primers, which tend to have a high VOCs content, should be avoided as a pollution
prevention measure.

If so, workpieces should not undergo phosphating or wash priming; to be effective the zinc-rich
primer must be in direct contact with the metal surface.

Given the importance of establishing direct contact between  the primer and the substrate,
workpieces should be degreased prior to blasting.

The blast  profile should be sufficient (1.5 to 2.5 mils) to facilitate good mechanical adhesion between
the primer and the substrate.  (It is strongly recommended that the operator consult with a vendor
when establishing the profile specification.)

Because workpieces receiving a zinc-rich primer are likely to provide corrosion resistance in
aggressive environments, they should be cleaned to a near-white or white metal  finish.

If so, an abrasive with the lowest dusting characteristics and the highest recycle rate should be
selected.  (It may be necessary to consult with a vendor when choosing an abrasive.)

Consideration should be given to degreasing workpieces before blasting so that the abrasive media
can be  kept clean for recycling in the blasting process.

Without including a degreasing stage, abrasives can transfer contaminants from one workpiece to
another and even imbed them in the substrate.
 Abrasive blasting is used primarily to remove such sur-
 face contaminants  as carbon deposits, scale, chemical
 impurities, and rust as well as oil and grease. It also can
 be used, however, to physically alter the  surface of a
 workpiece to  encourage good  adhesion between the
v coating system and the substrate.  For example, an op-
 erator might blast a metal  surface with an abrasive to
 accomplish the following:

 • Create a surface  profile for optimum coating adhesion.

 • Reduce design weights,  porosity, friction, or suscep-
   tibility to corrosion.

 • Strengthen the surface by peening.

 • Add fatigue resistance.

 • Remove surface irregularities.

 • Correct distortions.

 The  discussion in  this chapter, however,  primarily fo-
 cuses on blast cleaning. For more  detailed discussions
                             about other uses of abrasive blasting, see References
                             1,2, and 3.

                             8.2.2   Abrasive Blasting Systems   '

                             Blast cleaning is conducted using a pneumatic system
                             that  mixes the  abrasive  media  and pressurized air
                             through a  valve at the  base of the unit. Typically,  sys-
                             tems force the media out  the  blasting nozzle with 100
                             psi of pressure (4); the  speed at which the media travel
                             is directly  related to  particle mass. Thus, the blasting
                             efficiency of a particular media can be  determined using
                             the  following  equation, which  relates mass to kinetic
                             energy and velocity (5):

                                      Impact energy = 1/2 mass x velocity2

                             Based on  this  equation,  if mass is  doubled, impact
                             energy  is also doubled. Similarly, if air pressure is in-
                             creased, velocity also  increases. Thus, if the media's
                             velocity is  doubled,  the impact energy is quadrupled.
                             Given that the  production rate is proportional to the
                             impact energy, if the impact energy is  quadrupled, then

                                                                                        Clean Air
                                                              Air + Particulates
                        Ambient Air
                     Enters Through Doors
                    Compressed Air
                    for Ventilation
                                                                (for Fine Mesh Sizes)
            Air to Blast Hose
           Nozzle (90-100 psi)
                                           Abrasive Blast Room
                                                                                         Dust Collector
                                                                                    Waste Drum
                     Sieves Eliminating
                     the Fines (Optional)
                                                       Recycled Usable Abrasives
                                                  Blast Machine
                                               Metering Valve
               Compressed Air Inlet
              	 (90-100 psi)
Figure 8-1.  Schematic of an abrasive blasting operation with a media recovery system.
production rate (i.e., the blasting speed) increases by
the same amount.

Depending  on the  size  of the  piece to  be cleaned,
abrasive blasting operations can be conducted within a
cabinet or in a blast room. Cabinets, which are used for
cleaning small parts, allow the line operator to manipu-
late the blasting nozzle from  outside the enclosure by
inserting his hands into protective gloves attached to the
inside of the unit. Blast rooms are large enclosures that
can accommodate both a full-size workpiece (e.g., weld-
ments,  subassemblies) and the line operator wearing
protective gear. Both cabinets and blast rooms can be
relatively  simple enclosures or they can be  equipped
with powered turntables, media recovery systems, and
dust filtering mechanisms (i.e., cyclone separators).

Conventional pneumatic blasting systems facilitate de-
livery of the media with a configuration that allows grav-
ity feed from the hopper. Because such systems allow
the line operator to precisely control air pressure, clean-
ing of the substrate tends to be more uniform and higher
production rates can be achieved,  particularly when
heavier abrasives are used. Nonetheless, favorable re-
sults can be achieved with a lightweight abrasive, given
that conventional systems can deliver most media to the
substrate with high-impact energy.

In contrast,  induction feed systems include a venturi at
the nozzle to create a suction that draws the abrasive
media from the feedstock without the benefit of gravity.
Such  systems generate less-constant blasting pressure
and thus generally yield lower production rates. None-
theless, they have certain advantages over conventional
systems.  For example,  they are less expensive  and
require minimal maintenance; they are recommended
for operations with space limitations because the units
are smaller;  and they can be readily modified for con-
tinuous operation, eliminating the need to occasionally
stop operations to refill the media hopper.

8.2.3   Media Recycling

Wastes generated by the abrasive blasting process can
be significantly controlled if a recyclable media is used.
Typically, the spent media itself represents the greatest
volume of waste from blasting operations. Of the various
angular grit media, steel grit has the'highest recycle  rate
and is  less expensive than, for instance, sand and  alu-
minum oxide (Table  8-2).  The media with  the lowest
recycle rate is sand, which is generally discarded after
one use. In cases where an abrasive is used in conjunc-

Table 8-2. Recycle Frequency of Abrasives (6)
Type of Abrasive                        Recycle Times

Aluminum oxide

Steel grit

Chilled cast iron





tion with a toxic pretreatment chemical, the media can
become contaminated and require special handling.

Grit abrasives  are  recycled into the  blasting  process
after dust and fines have been removed via airwashing
in an abrasive recovery system. In a typical blast clean-
ing operation with a media recovery system (i.e., based
on screw conveyor, elevator, scraper floor, or pneumatic
technology), waste  can be reduced  by more  than 80
percent when steel grit is used (7).

Pneumatic abrasive recovery systems are one of the
most effective approaches for removing dust and other
contaminants from  blasting media (6). Typical pneu-
matic systems draw up the media from the blasting room
or cabinet floor with vacuum hoses by means of electri-
cally powered impellers. The media is fed  into a cham-
ber where particles  are separated out via centrifugal
force. Heavier particles and debris that are thrown to the
outer perimeter of the chamber  swirl  downward  to a
mesh screen, through which the abrasive passes to a
hopper (Table 8-3). Dust and lightweight particles circle
around the center of the chamber where they are  cap-
tured by a suction tube.

8.2.4   Blast Profile as a Critical Factor

Many abrasive blasting media cut into the substrate
somewhat as they clear away hard mill scale and corro-
sion products (i.e., rust). This gives the workpiece sur-
face a  rounded or angular profile (i.e., the blast profile).
As a result, the applied primer coating usually can es-
tablish a firm mechanical bond with the substrate  (see
Section 8.4 on process variations). This profile must be
appropriate to the dry-film thickness of the primer coat-
ing. If the profile is too coarse, flash rusting can occur
where  the  peaks of the profile  protrude  through the
primer.  For example, the blast  profile of a substrate
should be well  below 2 mil (i.e., 2 thousandths of an
inch) if the primer that will be applied has a dry-film
thickness in range of 1.0 to 2.0 mil. The ability of a primer
coating to thoroughly cover a blast profile also relates to
the primer's drying time. A fast-drying formulation  may
set up before the coating can flow off the peaks and into
the valleys of  the profile. Conversely, a slow-drying
primer can run  off the peaks entirely and well up in the

If a primer coating does not thoroughly cover the peaks
of a substrate's blast profile, flash rusting can occur,
especially on large weldments and workpieces that must
be  stored  outdoors  where they  may  be exposed to
excess moisture. Often, the line operator will return such
rusted  pieces to the blast  room for  either partial or
complete reworking, resulting in excess costs and waste
generation. If the flash rusting is not removed before the
topcoat is applied, the coating system is likely to fail
prematurely, possibly by delaminating from the surface.
Table 8-3.  Selected Screen Sizes (2)

NBS Screen No.       Screen Size (mm)
Screen Size (In.)
7.0 X10"3
4.9 x10'3
2.9 x 10'3
NBS = National Bureau of Standards

The blast profile is controlled by the size and shape of
the abrasive, the size  of the blasting nozzle, and the
blasting velocity and air pressure. During abrasive blast-
ing, the line operator should occasionally check that the
appropriate blast profile is being achieved. This can be
done  either visually using a surface profile comparator
(i.e., a profilometer) or by measuring the profile with a
roughness gauge', both of which are available from in-
dustry sources (8, 9).   Case  Example: Coating Failure Due to
         an Extreme Blast Profile

A company in Florida installed a new coating facility and
purchased equipment for abrasive blast cleaning all of
its large steel weldments. Because of the high humidity
in  Florida, the company applied a relatively expensive,
high-quality epoxy primer directly over the  abrasive-
blasted steel. After the primer had cured, the steel weld-
ments were taken to outside  storage,  where  they
awaited final assembly and testing.

Within 24 to 48 hours after the weldments were exposed
to  the outside environment, entire surfaces  began to
show signs of flash rusting. The rusted weldments were
reworked in the blasting room and a fresh coat of primer
was applied.

 An inspection of the substrate using a low-power mag-
 nifying glass showed that the peaks of the blast profile
 were  protruding through the primer.  The profile was
 measured at 2.5 to 3.5 mil and the primer dry-film thick-
 ness at only 0.8 to 1.0 mil.

 The problem was then solved by changing the applica-
 tion parameters, which favored  a shallower profile. Al-
 though this process change made the blasting operation
 more time consuming and labor intensive, costs were
 more than offset  by avoiding the reworking of rusted
 weldments. Changing the application parameters re-
 quired no major management decision.

 Although the company had carefully planned operations
 at the new facility and specifications  had  called for a
 blast profile less than 1.5 mil, no quality control checks
 of the blasting process were conducted. Thus, no one
 noticed that the profile was out of specification. The
 problem could have been avoided had better manage-
 ment practices been enforced.

 The corrective  action  was taken to resolve a quality
 control problem, but  the operation  also benefited by
 minimizing pollution generation associated with rework-
 ing the weldments. This was an important unanticipated
 benefit, especially because the company operated in an
 area with strict Ibcal environmental requirements.

 8.2.5  Types of Abrasive Media and Selection

 The six most commonly used abrasive  media are (6):

 • Steel shot: Small, spherical particles of hypereutec-
  toid steel (i.e., containing more than 0.8 percent carb-
  on) in its fully heat-treated condition. Steel shot has
  a uniform structure of finely tempered martensite (i.e.,
  the hard constituent of quenched steel), which pro-
  vides optimum resilience  and  resistance to fatigue.
  Thus,  it  is particularly suited to shot peening and
  considered an optimum abrasive for wheel blast ap-

 • Cast steel grit: A high-carbon content, angular pellet.
  Depending on the hardness selection, this abrasive
  is effective, for instance, in removing  scale or etching
  the substrate to enhance its profile. Steel grit is one
  of the  most commonly used abrasives for preparing
  steel substrates to receive a coating (6).

• Aluminum oxide: Fused alumina grains that are an-
  gular and characteristically hard and resilient, provid-
  ing  particularly fast cutting action.

• Garnet grit: A mineral abrasive with sharp angular
  characteristics that provides fast cutting  action and
  has a long service life.
 • Mineral slag: A diamond-like, angular abrasive that is
   without free silica and does not attract moisture, pro-
   viding fast cutting action.

 • Chilled iron grit: The lowest cost mineral abrasive. It
   is particularly recommended for difficult cleaning jobs.

 • Glass beads:  Small, lightweight,  spherical  media
   used primarily on nonferrous metals for shot peening
   and surface finishing. Predominantly used in the air-
   craft and automotive industries.

 Table 8-4 lists selection criteria specific to various abra-
 sive blasting media. For some blasting  operations, as-
 sorted abrasives are mixed so that the media include
 different grit sizes. More general factors that the opera-
 tor should consider when choosing  an abrasive include
 the following:

 • Compatibility: The mineral or metallic abrasive should
   have characteristics similar to the metal substrate to
   avoid the likelihood that galvanic  corrosion will result
   if some of the  blasting material becomes imbedded
   in the surface of the workpiece. For example, a steel
   abrasive should not be used on an aluminum sub-
   strate. Moreover, too hard an abrasive can  result in
   distortion of the workpiece surface.

 • Shape: The shape of the abrasive relates to its cutting
   ability and therefore the blast profile. Thus,  because
   steel shot is  round in shape, it will  produce  a profile
   characterized by rounded valleys. In contrast,  an abra-
   sive with an angular shape will yield a sharper profile.
   For example, an operator might use cast steel grit when
   a zinc-rich primer will be applied, because such primers
   rely on a mechanical bond with the substrate.

 •  Size: The grain size of the abrasive  media used must
   be consistent with the specified blast profile so that
   the primer coating will thoroughly cover the substrate.
   Smaller grain sizes are used to avoid either cutting
   too course a profile in the substrate  or warping work-
   piece areas made with a thin  metal. Tables  8-5 and
   8-6 are examples  of size  specification sheets avail-
   able from media vendors.

 •  Low dust generation: The  amount of dust caused by
   fragmentation of the abrasive should be minimal to
   reduce pollution of the ambient air with particulate
   matter. When a metallic abrasive is used, dust should
  also be minimized to avoid encouraging galvanic cor-
   rosion from particulate left on the substrate. Addition-
  ally, an excess  of  metal fines  mixed into a  recycled
   medium can undermine blasting efficiency.

•  Recyclability: Preferably, an abrasive will have a high
   reuse rate, minimizing process costs and waste gen-

•  Cost: Cost comparisons should include consideration
  of all process factors, including the  cost of rejected/

Table 8-4. Guide for Selected Abrasive Media

Surface profiling (Etch)
Working speed
Probability of metal removal
Hardness, MOH scale
(Rockwell Re)
Bulk density (Ib/cu ft)
Mesh sizes
Typical blast pressures (psi)
Source: Industry literature.
Cut Wire

Table 8-5. Sample Specification Sheet for Steel
Product 7 8 10
S780 0.85
S660 •
Screen number 7 8 10
Screen size mm 2-8 2-4 2
Screen size Inches 0-1 0-1 0-1
12 14
85% 97%
min min
5% max
Very Lo

Shot (2)
16 18

85% 96%
min min
5% max 85%

12 14
1.7 1.4
0.1 0.1
5% max
5% max

16 18
1.2 1
0 0
Steel Grit

20 25 30

85% 96%
min min
85% 96%
min min
10% 85%
max min

20 25 30
0.9 0.7 0.6

35 40


35 40
0.5 0.4
0 0

45 50 80

80.5 90%
min min
10% 80.5
max min
45 50 80
0.4 0.3 0.2
0 0 7.0e-03

120 200

120 200
0.125 0.075
4.9e-03 0.0029
reworked pieces and recyclability of the media. Re-
search has shown, for instance, that overall opera-
tional costs when using a non-recyclable abrasive
such as slag can be seven times higher than when
using a recyclable media such as steel  grit, even
though the per pound cost of the grit may be nine
time higher than the slag (10).
8.2.6  Blast Cleaning Standards

Industry standards have been established regarding the
cleanliness of a substrate following blast cleaning opera-
tions. The cleanliness coding of various rating systems are
presented in Table 8-7. The ratings are portrayed pictorially
in  standards compilations  and based on the following
paraphrased industry-wide definitions:

Table 8-6.  Sample Specification Sheet for Steel Grit (2)

Product         7    8    10   12   14    16    18
20   25   30   35   40   45   50   80   120  200  325
G.12 0-8 0.9
G.14 80%
Screen number 7 8 10 12 14
Screen size mm 2.80 2.4 2 1 .7 1 .4
Screen size inches 0.111 0.09 0.08 0.07 0.06
Table 8-7. Comparison of Designations for Blast

Steel Structures Painting Council (USA)
National Association of Corrosion Engineers (USA)
Swedish Standards Organization
United Kingdom Standards (BS 4232)
0.75 0.85

16 18 20
1.2 1 0.9
0.05 0.04 0.03
Cleaning Finishes
NACE No. 4


25 30 35
0.7 0.6 0.5
0.03 0.02 0.02

NACE No. 3
3rd Quality
0.7 0.8
0.65 0.75

40 45 50 80
0.4 0.4 0.3 0.2
0.02 0.01 0.01 0.01

Near-White Metal
NACE No. 2
v 2nd Quality
0.6 70%
120 200 325
0.1 0.075 0.045
0.0049 0.0029

White Metal
NACE No. 1
1st Quality
• Brush-off: The cleaned surface, when viewed without
  magnification, must be free of all visible oil, grease,
  and  dirt as  well as loose mill scale,  rust,  and pre-
  viously applied coatings. Adherent mill scale,  rust,
  and  old coatings may remain on the  surface. Such
  contaminants are considered  adherent if they cannot
  be lifted with a dull putty knife.

• Commercial: The cleaned surface must be free of all
  visible oil, grease, dirt, and dust as well as mill scale,
  rust,  and previously applied  coatings. Generally,
  evenly dispersed, very light shadows, streaks, and
  discolorations caused by stains of mill scale, rust, and
  old coatings may remain on no more than 33 percent
  of the surface. Also, slight residues of rust and old
  coatings may be left in the craters of pits if the original
  surface is pitted.

• Near-white metal: The cleaned surface must be free
  of all visible oil, grease, dirt, and dust  as well as mill
  scale, rust, and previously applied coatings. Gener-
  ally,  evenly  dispersed, very light shadows,  streaks,
  and discolorations caused by stains of mill scale, rust,
  and  old coatings may remain on no more than 5
  percent of the surface.

• White metal: The cleaned surface must be free of all
  visible oil, grease, dirt, and dust as well as mill scale,
  rust, and previously applied  coatings. No traces of
  contaminants may remain on the surface.
    Pictorial portrayals of abrasive cleaning standards are
    compiled in the following trade association documents:

    •  Steel  Structures Painting  Council  Visual  Standard
      (SSPC-VIS-1-89), Steel Structures Painting Council,
      Pittsburgh, PA.

    •  NACE  Visual Standard for Steel Surfaces Airblast
      Cleaned With Sand Abrasive; NACE Standard TM-
      01-07,  National Association of Corrosion Engineers,
      Houston, TX.

    •  Swedish Standard (05/5900/67), Swedish Standards
      Organization (available from American  Society for
      Testing Materials, Philadelphia, PA).

    •  United  Kingdom Standards (BS 4232).

    The total cost of a standards compilation and  a blast
    profile comparator (see Section 8.2.3) is approximately

    8.3   Best Management Practices

    The following management practices are recommended
    for enhancing abrasive blasting process efficiency:

    •  To maintain quality control, facility operators should
      periodically inspect surfaces to ensure that industry
      blast cleaning  standards are being  met; also, they
      should  occasionally  measure  the  blast profile to
      guard against the potential for flash  rusting.

 • To avoid flash rusting generally, line operators should
   apply a primer coating to  clean  surfaces within 8
   hours of abrasive blasting; in high-humidity environ-
   ments, a primer should be applied within 4 hours. If
   near-term priming is  not  feasible,  line operators
   should wrap and/or store the cleaned workpiece un-
   der cover or apply a temporary corrosion preventive,
   even though this coating will need to  be  removed
   through degreasing  before  application of a topcoat
   (see  Chapter 5).

 • To avoid the deposition of fingerprints and other inci-
   dental  contaminants when processing white-metal
   workpieces, operators should require workers to wear
   latex gloves when handling the pieces after blast

 • To ensure consistent control of the blast cleaning
   process, operators should thoroughly train relevant
   workers,"even though turnover tends to be high  for
   such operations. Training materials (e.g., videos) are
   available from both  the  Steel  Structures Painting
   Council and the  National Association of Corrosion

 • To avoid contamination of the media feedstock, fac-
   cility  operators should  ensure that  moisture is  not
   condensing on the hopper surfaces.  Also, air supply
   lines  should  be equipped with oil and water traps.

 • To ensifre optimum mixing of pressurized air and the
   abrasive media, operators should equip the blasting
   system with  a hopper that has a concave head and
   a cone-shaped bottom to facilitate feed flow.  Similarly,
   hose  couplings should be flush with the inside of the
   hose  and sized to minimize obstruction and leakage
   of pressure. Additionally, the mixing valve should  be
   periodically cleaned.

 8.4   Process Variations (With Case

 8.4.1   Abrasive Blasting Preceded by

 Many paints and  coatings operations subject work-
 pieces to abrasive blasting as well as degreasing in one
 process  line. In most cases,  these  two pretreatment
 stages are used in conjunction to minimize contamina-
tion of an abrasive that  will be recycled in the blasting
operation. If  a  relatively expensive abrasive  is being
 used, such as steel shot, the facility operator will have
a strong  incentive to optimize its useful  life. With a
less-expensive  media, such as sand, the operator will
 need to weigh the tradeoff between the cost of replacing
the media more frequently and the water-use and waste-
water-handling  costs associated with degreasing (see
Chapter 5).
 If the facility operator chooses to recycle the blast media
 without degreasing workpieces, the recycled abrasive is
 likely to entrain grease, metal fines, and other contami-
 nants and then deposit them on the surface of the next
 uncoated piece.  Coatings that are applied over such
 contaminants will have a high  potential for premature
 failure,  either gradually (by spalling) or catastrophically
 (by delaminating). Whereas  the cost of taking steps to
 prevent such failures  may be preclusive for some low-
 value end products  sold in price-sensitive  markets,
 achieving a reasonably durable coating is likely to be a
 requirement for many operations.

 In most situations, the roughening of the metal substrate
 that can be achieved  in'abrasive blasting is particularly
 important for enhancing adhesion. With marine coating
 systems, for example, the zinc-rich primers specified by
 industry standards provide superior corrosion resistance
 but have poor adhesion properties. Thus, the substrate
 profile resulting  from abrasive  blasting enhances  the
 ability of the epoxy and polyurethane enamel coatings
 applied  over the primer for marine workpieces to estab-
 lish a strong mechanical bond. The danger is that  too
 high a blast profile would lead to premature corrosion of
 the substrate when subjected to marine environments.

 Factors a facility operator should consider when decid-
 ing on whether to add a degreasing stage include:

 • Regulations concerning VOC emissions and waste-
  water treatment.
 • Equipment and floor-space requirements.

 • Costs versus benefits in terms of the overall operation.   Case Example: Coating Failure  Due to
         Contamination of  Recycled Media

 A major fabricator of railcars installed an automatic sys-
 tem for  blast cleaning steel plates upon delivery to  the
 facility. Immediately after blasting, a corrosion-resistant
 primer was applied using an  airless spray gun. The
 primed  plates then were stored until required for fabri-
 cation. After  assembly, a second coat of  primer was
 applied, followed by a colored, decorative topcoat.

 Occasionally, the  operator discovered craters  in  the
 paint film,  requiring that certain areas of painted pieces
 be reworked (i.e., scuff sanded followed by repainting).
One day, however, the paint  operator found that entire
sides  of several finished railcars had thousands of cra-
ters on the surface. The coatings on these cars had to be
stripped and then reblasted, reprimed, and refinished.

On close analysis of the process, it was found that  the
abrasive media was picking up so much oil and grease
over several  months  of  recycling that the  substrates
were  being recontaminated.  The  problem  was easily
solved by  replacing the abrasive with new material. To
prevent  such coating failures  in the future, the company

 added a pretreatment step for subjecting all steel plates
 to high-pressure, hot-water degreasing. In addition, kao-
 lin  powder was added to the abrasive to absorb  any
 traces of oil or grease that might become entrained in
 the media.

 The most important change to the process was  the
 addition of aqueous degreasing. While this added to
 process costs, it prevented  further failures and  thus
 reduced the cost  of  labor and materials required to
 rework rejected paint finishes. With the degreasing step,
 the company needed to handle the large quantities of
 wastewater. This was accomplished, however, by direct-
 ing the spent water to a settling tank, then skimming off
 oil and grease and adjusting the pH before discharge.

 The problem could have been avoided had the company
 initially used better management practices. Until  the
 catastrophic failures brought production to a halt, no one
 at the company had  fully assessed the unnecessary
 costs and additional pollution generation incurred during
 the earlier months when sporadic failures had occurred.

 8.4.2  Abrasive Blasting Followed by

 For some  situations, subjecting workpieces to a phos-
 phating stage after abrasive blasting is  recommended.
 Although few operations use both of these stages in
 conjunction, this approach  can yield a superior mechani-
 cal bond between the substrate and the  coating system
 and thus improved corrosion resistance.
 When workpieces are subjected to both abrasive blast-
 ing and  phosphating,  the  operator should monitor the
 blast profile closely. Phosphate deposition can vary sig-
 nificantly  depending on  the profile of  the  substrate
 yielded by the blasting abrasive. A more  pronounced
 blast  profile will result in a heavier phosphate coating.
 For example, deposition of an iron phosphate can vary
 from 30 to 220 mg/ft2 depending on the type of media
 used  in blast cleaning.   Case Example:  Coating  Failure Due to
         Peening  of the Substrate

 A fabricator of  steel cabinets intended to be used in all
types of outdoor environments selected a powder coat-
 ing process for finishing  the workpieces.  Because of
 major contamination on the substrate, the facility opera-
tor  abrasive blasted the workpiece surfaces before  ap-
 plying an  iron phosphate. The  operator felt that  the
combination of the blast  profile plus the  phosphate
deposition would benefit adhesion of the powder coating
and provide enhanced corrosion resistance.

During accelerated environmental tests of the cabinets,
however, the powder coating failed catastrophically due
to poor  adhesion. Extensive experimentation showed
that a heavy  phosphate coating was  required for the
cabinets to pass the tests. Further experimentation indi-
cated  that  the  shape and hardness  of the abrasive
selected were critical for accomplishing the appropriate
phosphate deposition on the substrate; the steel sur-
faces were being peened, and this hindered the phos-
phate from adequately depositing on the surface. After
a change to angular grit, which  yielded a more active
surface,  the phosphate  coating  weight increased ap-
proximately threefold. With this pretreatment modifica-
tion, the powder coating passed the accelerated testing.

The change of abrasive was accomplished within a few
days and at little expense, and the problem of workpiece
rejects was essentially eliminated.

8.5   References

 1.  Bennett,  P.J. 1994. Abrasive air blast cleaning. In: Keane, J.D.,
    et al. (eds.), Steel Structures Painting Council, vol. 1, pp. 52-67.
   Third edition. Pittsburgh, PA.
 2.  Borch, E.A. 1994. Metallic abrasives. In: Keane, J.D., etal. (eds.),
    Steel Structures Painting Council, vol. 1, pp.  32-51. Third edition.
    Pittsburgh, PA.
 3.  Mallory, A.W. 1994. Mechanical surface preparation; Centrifugal
    Blast Cleaning. In: Keane, J.D., et  al. (eds.), Steel Structures
   Painting Council, vol. 1, pp.22-31. Third edition. Pittsburgh, PA.
 4.  Kerr, R. 1995.  Personal communication between R. Joseph, of
    Ron Joseph  &  Associates, Saratoga, CA, and Robert Kerr, of
   Clemco Industries.
 5.  Hanna, M.R. 1993. Versatile technique for surface preparation.
    Industrial Finishing 69(5):22 (May).
 6.  Hansel, D. In press. Industrial abrasive blasting systems. Metal
   Finishing Journal. (Scheduled for publication in 1996.)
 7.  Lampkin, W. 1995. Personal communication  between R. Joseph,
   of Ron Joseph & Associates, Saratoga, CA, and Walter Lampkin,
   of Baghouse Services, Los Alamos, CA.
 8.  Murphy, M., and R. Joseph, eds. 1995. Metal Finishing Organic
   Guide  Book and Directory, vol. 93. No. 5A (May). New York, NY:
   Elsevier Science Publishers.
 9. Robison,  T.G., ed. 1994. Products Finishing  1994 Directory and
   Technical Guide, vol. 58 (2A). Cincinnati, OH: Gardner Publica-
10. Griese, II, J.  1988. Using  recyclable steel grit for portable field
   applications. J.  Coatings and Linings (October), pp. 35-37.

         Section 3
Application Process Factors

                                              Chapter 9
      Transfer Efficiency as It Affects Air, Water, and Hazardous Waste Pollution
9.1    Introduction

9.1.1   Pollution Prevention Considerations

Of all the strategies available to minimize pollution in a
paints and coatings facility, improving transfer efficiency
is perhaps one of the most effective. Slight increases in
transfer efficiency can result in significant pollution re-
ductions as well as guaranteed cost reductions.

The concept of transfer efficiency is extremely simple: it
is the ratio of the mass of solid coating deposited on a
substrate to the mass of solid coating used during the
application. It can also be defined in terms of volume.
The following equations express these definitions:
 Transfer Efficiency =
Mass solid coating deposited
  Mass solid coating used
T    ,   ._„.  .       Volume solid coating deposited
Transfer Efficiency = —rr-.	rrj	-r—c—-;—
                      Volume solid coating used

To illustrate the importance of this concept, suppose that
a spray painter applies a coating to a metal filing cabinet
using a conventional air atomizing spray gun. The spray
gun deposits  much of the coating on the metal cabinet,
but a significant amount  of overspray is directed toward
the spray booth filter or  drops to the spray booth floor.
Clearly, the overspray is  wasted and  represents the
inefficiency of the spray  application. Wasted overspray
contributes to air, water, and hazardous waste pollution.
It is evident, then, that making the process more efficient
can directly benefit pollution prevention.

The definition  of transfer efficiency does omit a couple
of important related factors. First, transfer efficiency ac-
counts for only the amount of solid coating (i.e., resins,
pigments, extenders, and additives) that remains on the
steel  cabinet  after the solvents have evaporated, and
relates this to the total amount of  solids that the spray
gun applied. In both the  numerator and  denominator of
the equation,  therefore,  the  amount  of  solvent  in the
coating is not relevant.

Secondly, the  definition  of transfer efficiency does not
account for the dry film thickness of the substrate coat-
ing. The following, which builds on the previous exam-
ple, illuminates this distinct weakness in the definition:

    Suppose the spray painter who applied the previous
    coating to the metal filing cabinet applies a coating
    of 1 mil (1 mil = 0.001 inches) dry film thickness to
    the  substrate. If the spray painter deposits  80 per-
    cent of the solid  content of the coating onto the
    metal surfaces and wastes 20 percent in  the spray
    booth, then transfer efficiency is 80 percent.  Now,
    suppose that a second  spray painter who is less
    experienced than the first applies the same coating
    to an identical filing cabinet. If he deposits twice as
    much coating (i.e., 2 mil  dry film thickness), but he
    too deposits 80 percent of the solids to the surfaces,
    transfer efficiency  would  also be 80 percent.

    Thus, despite the fact that the second spray painter
    uses twice as  much coating as did the first spray
    painter, the transfer efficiency for both spray paint-
    ers  is the same.

It is unfortunate that the definition does not encompass
dry film  thickness or the amount of solvent used. This
chapter, however, explores many strategies for improv-
ing not only transfer efficiency, but overall efficiency of
the coating application.

9.1.2   Decision-Making Criteria

Decision-making criteria relevant to transfer efficiency,
as addressed in this chapter, are highlighted in Table 9-1.

9.2   Benefits of Improved Transfer

Benefits associated with improving  transfer  efficiency

• Reduced air pollution (volatile organic compounds—VOCs).

• Reduced hazardous waste.

• Less frequent cleaning of guns, spray booths, and filters.

• Reduced use of chemicals in water-wash spray booths.

• Reduced discharge/treatment of water.

• Reduced costs.

 Table 9-1.  Decision-Making Criteria Regarding Transfer Efficiency
 Issue                             Considerations
 Which spray guns are most
 efficient for specific workpieces?
What easy-to-implement
strategies can improve transfer
How should the transfer
efficiency of the process be
•  If workpieces are small (can fit into 12-inch cube) and require high or medium quality finishes,
   choose electrostatic, HVLP, conventional air atomizing, or air-assisted airless guns.
•  If workpieces are medium-sized (can fit into 24-inch cube) and require high quality finishes, choose
   electrostatic, HVLP, or conventional air atomizing guns.
•  If workpieces are medium-sized and require medium quality finishes, choose electrostatic, HVLP,
   conventional air atomizing, air-assisted airless, or, in some cases, airless guns.
•  If workpieces are large (cannot fit into 24-inch cube) and require high quality finishes, choose
   electrostatic, HVLP, or conventional air atomizing guns.
•  If workpieces are large and require medium quality finishes, choose electrostatic, HVLP,
   conventional air atomizing, air-assisted airless, or airless guns.
•  Stand closer to the workpiece.
•  Select the most efficient spray gun for the intended  application.
•  Reduce fan width, as well as the extent of overspray due to fan width during first and last stroke.
•  Reduce atomizing air pressure (where applicable) and fluid pressure.
•  Space workpieces closer together.
•  Reduce air velocity in spray booth but not below OSHA recommended limits.
•  Avoid air turbulence in spray booth.
•  Reduce leading and trailing edges.
•  Optimize parameters when using electrostatic guns.
•  Do not apply thicker coating than is specified.
•  If workpieces are small and lightweight (less than 70 pounds each), use the weight (mass) method.
•  If workpieces are small and heavy (greater than  70  pounds each) with simple geometry, use weight
   method by "wallpapering" with aluminum foil.
•  If workpieces are small with complex geometry but surface area can still be calculated, use volume
•  If workpieces are small with complex geometry but one cannot calculate surface area, a special
   protocol may need to be designed.
•  If workpieces are too large to fit onto balance and have simple geometry, use weight method by
  "wallpapering" with aluminum foil.
•  If workpieces are too large to fit onto balance and have complex geometry but surface area can
  still be calculated, use volume method.
• If workpieces are too large to fit onto balance and have complex geometry but surface area cannot
  be calculated, a special protocol may need to be designed.
9.2.1   Reductions in Pollution and Related
Small increases in transfer efficiency can result in great
reductions in pollution. Table 9-2 presents the emissions
of VOCs from a painting operation that uses a coating with
a VOC  of 3.5 Ib/gal. The painters apply this coating to
achieve a dry film thickness of 1.0 mil on the substrate.
To understand the significance of the calculations, con-
sider only the first column, namely 'Transfer Efficiency,"
and the last column, "Emissions of VOC/1,000 ft2 of
Coated  Surface." The table  includes the middle column
because several VOC regulations are written in terms of
Ib VOC/gal Solids Applied.
Figure 9-1  is a graph based on the calculations of
Table 9-2.
                           Table 9-2  and  its corresponding graph can apply to a
                           factory that  must coat 1,000 ft2 of metal surface each
                           day. A novice painter who poorly handles the spray gun
                           achieves a transfer efficiency of only 5  percent. He is
                           able to deposit a coating  film of 1 mil dry film thickness.
                           In order to coat 1,000 ft2  of surface, he emits 83.2 Ib of
                           VOC into  the air.  Suppose another novice painter can
                           achieve a transferefficiency of 10 percent. Although this
                           is hardly better than  the first painter's  5  percent, the
                           second painter's emissions for the 1,000 ft2 of coated
                           surface is  only 41.6 Ib. Even though transfer efficiency
                           increases  by a very small amount, emissions are cut in
                           half. As Figure 9-1 indicates,  an  increase  in transfer
                           efficiency  from  5  percent to 10  percent is  really very
                           small and  not difficult to achieve. As transfer efficiency
                           continues  to improve, probably with the use of more
                           experienced painters  or  better equipment, VOC emis-

Table 9-2.  Effect of Transfer Efficiency on VOC Emissions
Efficiency (%)
90 '
100 V
VOC of Coating
Dry Film Thickness
Emissions of VOC
In Ib VOC/gal
Solids Applied
1.0 mil
Emissions of
VOC/1,000 ft2 of
Coated Surface

sions quickly drop to very small values (see Table 9-2
and Figure 9-1).

Moreover, if one of the painters uses a paint brush to
apply the coating and achieves a transfer efficiency of
100 percent, yet can also apply the coating at a uniform
film thickness of 1 mil, his total VOC emissions for coat-
ing the same surface area is only 4.2 Ib. While the first
spray painter, whose transfer efficiency was 5 percent,
emitted 83.2 Ib of VOC into the air, the last painter who
used a paint brush, emitted only 4.2 Ib to do exactly the
same job.

Of course, it is  impractical to use a paint brush to apply
all coatings. It is clear, however, that if using an efficient
spray gun or other method  of coating application can
maximize transfer efficiency, an enormous reduction in
pollution will result.

While it may not be practical or cost-effective to achieve
transfer efficiencies of 80 percent or more under most
circumstances, spray painters can often achieve trans-
fer  efficiencies  in excess of 50 percent. As Table 9-2
notes, even a transfer efficiency of 50 percent causes
 emissions to drop to  only 8.3 Ib of VOC/1,000 ft2 of
 coated surface.

 While Table 9-2 and Figure 9-1 demonstrate only reduc-
 tions of  emissions into the  air,  obviously as transfer
 efficiency improves, the amount of overspray  in  the
 spray booth drops significantly. This translates into less
 frequent cleaning of the spray booth, as well as a reduc-
 tion in the disposal of used dry filters (in dry filter spray
 booths) or of paint sludge (in water-wash spray booths).
 For water wash spray booths, improved transfer effi-
 ciency also reduces the use of  chemicals needed to
 detoxify  the paint sludge, and the discharge and treat-
 ment of water from the water trough.

 9.2.2  Reduction In Costs

 While increased transfer efficiency and reduced waste
 contribute to  preventing  pollution, they also  result in
 reduced  costs. In order to fully appreciate the impact
 transfer efficiency has on air, water, and waste pollution,
 as  well as on costs,  consider a spreadsheet that  ac-
 counts for all factors. The tables in  Appendix C serve
 this purpose.

 The tables of this appendix present assumptions and
 calculations based on a relatively small operation which
 coats 100 widgets per day. Table C-1 of the appendix
 provides a list of assumptions that are required to cal-
 culate cost savings due to improved transfer efficiency.
 Table C-2 provides the results of calculations that reflect
 the total  cost for waste, filters, labor,  and wasted paint
 when the transfer efficiency is 30 percent. Table C-3 is
 identical  to Table C-2, except it lists the results when the
 transfer efficiency is 45 percent. Table C-4 provides  the
 formulas that are used to perform the calculations.

 Using the spreadsheet structure and calculations model
 presented in this  Appendix,  the reader can estimate
 transfer efficiency at his or her own facility. The reader
 can change any of the assumptions to see how effective,
 minor changes in  the  coating application which affect
 transfer efficiency can provide dramatic benefits.

 Table 9-3 of this chapter presents the total waste costs
 of this same relatively small operation which coats 100
 widgets  per day. When  the transfer efficiency of this
 operation is  30 percent, its  annual  waste costs are
 $102,750.62. If all of the assumptions remain the same
 but transfer efficiency increases  reasonably from 30 to
45 percent, the operation can realize great savings (see
Table 9-3) namely $48,928.87.

 It is entirely possible to realize this cost savings without
spending a single dime on spray or  other equipment.
With training, painters can probably achieve this conser-
vative increase in transfer efficiency. In addition, along
with the cost savings, a 15 percent increase in transfer
efficiency contributes considerably to pollution prevention.

                                          Assume coating of 3.5 Ib VOC/gal, and dry film
                                                    thickness 1.0 mil
                                                 Transfer Efficiency
Figure 9-1.  Effect of transfer efficiency on VOC emissions.

Table 9-3.  Annual Cost Savings Due to Transfer Efficiency
          (TE) Improvement From 30% to 45%

Cost of waste +
filters + labor
Costs With
TE = 30%
Costs With
TE = 45%
Savings Due
Cost of wasted paint   $73,101.44

Total cost of waste    $102,750.62
$38,291.23  $34,810.21

$53,821.75  $48,928.87
9.3    Methods for Measuring Transfer

9.3.1   Defining Parameters Before
        Commencing the Transfer Efficiency

Before deciding on whether an operation needs to im-
prove its transfer efficiency, it is helpful to determine its
current transfer efficiency. This section describes vari-
ous testing methods available. Before conducting any
transfer efficiency test, several parameters need to be

• Upon which parts will the test focus.

• Which coatings and spray guns will the test employ.

• Who will apply the coatings.

• How will the test simulate day-to-day production con-

After identifying the basic parameters, the paint operator
must establish a fluid flow rate that is representative of
day-to-day production. The operator needs to set the
optimum air  pressure for correct coating atomization
and to adjust the coating viscosity and temperature to
be representative of typical application conditions.

If using electrostatic equipment, the operator must con-
firm that the parts to be coated are properly grounded,
that the coating has been adjusted so that its resistivity
meets the manufacturer'arecommendation, and that the
air velocity through the spray booth is neither too high
nor too turbulent.

 Another decision to make before starting a transfer effi-
 ciency test is whether to use the weight (mass) method
 (most common) or the volume method.

 9.3.2  Using the Weight (Mass) Method

 Determining  transfer efficiency on  a weight or mass
 basis, as is  usually the  case, requires purchasing  or
 renting an electronic balance capable of measuring to
 within 0.5 g. (In this document, the words weight and
 mass are synonymous. While scientists use mass, most
 others prefer to use weight.)  Available balances can
 weigh as much as  154 kg  (70 Ibs)  with this accuracy.
 The balance must sit on a hard surface such as a metal
 table, concrete  floor, or  cement slab. Never place a
 piece of cardboard  under the balance as it will lead to
 erroneous results.

 In addition, the operator  must shield the  balance from
 all drafts that may occur  on a factory floor, perhaps by
 surrounding the scale with large  pieces of cardboard.
 The operator must also ensure that the pressure pot or
 coating reservoir is not too heavy for the balance and
 that the individual parts to be coated also fall within the
 maximum limit of the balance.

 The balance should be set so that the air bubble in the
 bubble  glass falls within the  center  of the glass.  In
 addition, all four feet below  the balance must be in firm
 contact with the ground or surface. Finally, the operator
 must calibrate  the  balance  using  standard weights
 which are often supplied  by the balance  manufacturer
 or rental company.

 The cost  to conduct a transfer efficiency test can be
 minimal. Companies  can usually rent electronic  bal-
 ances for less  than $300/week.  A  laboratory charge
 might run approximately  $150/sample. The only other
 real expense involves in-house labor. Of course, if a
 company retains a consultant to conduct the test, costs
 might range from $3,000 to $5,000, depending on the
 complexity of the operation.   Measuring the  Weight of Coating During

The paint operator  should  follow the steps below to
 determine the weight (mass) of coating used during the
 application. This process begins by measuring the liquid
coating, then uses  the  information to  calculate the
weight (mass) of the solid coating.

 1.  Prior to commencing the transfer  efficiency test, ap-
   propriately label  each part  to  be coated  and then
   accurately weigh each part on the electronic  bal-
   ance. Record all  of the weights.

2.  Place the  pressure pot or coating reservoir on the
   balance and slowly fill with  coating, ensuring not to
    exceed the limit of the balance even after tightening
    the pressure pot cover.

 3.  Before commencing the actual test, apply the coating
    to several  dummy parts to ensure that the coating
    application is representative of day-to-day production

 4.  To commence the test, disconnect the fluid and air
    hoses from the pressure pot.  Do not allow any paint
    to drip to the floor as it is imperative that the coating
    fills the  line all the way up to the spray gun. Record
    the coating weight and then replace the air and fluid
    hoses and commence the spraying operation.

 5.  For accurate results, continue spraying until at least
  ' 1 qt of the paint has been used (equivalent to ap-
    proximately 2.2 Ib or 1 kg). After applying the coating
    to the selected parts, immediately  disconnect the
    fluid and air hoses from the pressure pot and record
    the second reading. Repeating this entire procedure
    at least three times can help in determining an aver-
    age transfer efficiency at the end of the trials.

 At any time during the test, take a small grab sample,
 approximately 1 pt of the coating, directly out of the
 pressure pot.  Be sure to close the container to prevent
 solvent evaporation. Then send the sample to an ana-
 lytical laboratory which  will conduct  a  percent weight
 solids test in accordance with ASTM D2369. The ASTM
 D2369 is a standard test method for volatile coatings (1).

 Do not bypass the sampling procedure by simply calling
 the coating manufacturer to request information on the
 percent weight solids or referring to the Material Safety
 Data Sheet (MSDS). Even a small discrepancy between
 the manufacturer's value and the actual value obtained
 from the pressure  pot sample will make a large differ-
 ence to the transfer efficiency calculations.

 The weight (mass) of solids used is calculated by the
 following equation:

              Wt.  (mass) SolidsUsed =

    Wt. (mass) LiquidCoating* PercentWt. Solids
                        Too   Determining the Weight or Mass of Solid
         Coating Deposited

As  noted earlier, before starting the transfer efficiency
test, each part was labeled and weighed. After applying
the coating, it must thoroughly cure before  weighing the
 part again.  If the coating is normally air or force-dried,
 allow  extra time for all of the solvent to evaporate.
 Curing the parts in an oven set at 230ฐF will result in a
 more accurate transfer efficiency  reading, even if this is
 not the  normal  method for curing. This  oven curing
schedule is identical to what the laboratory will use to

 determine the percent weight solids of the one pint
 sample taken earlier during the test.

 After the coating has thoroughly cured, weigh the parts.
 The difference between the weights of coated and un-
 coated parts represents the weight of solid coating de-
 posited.  Knowing  the weight  (mass) .of solid coating
 used, and the weight or mass of solid coating deposited,
 calculate  the transfer efficiency as follows:

   T    ,   ._„. .       Mass solid coating deposited
   Transfer Efficiency = —7-.	r-:	f—c—-—
                        Mass solid coating used  Increasing Test Credibility

 The credibility  of the results depends entirely on the
 accuracy of all the weighings. If the factory has drafts or
 if vibrations from the floor affect the balance, the opera-
 tor may wish to take two or three readings before record-
 ing  any  one  weight.  In  addition,  the  laboratory
 determination of percent weight  solids must be accu-
 rate. Finally, the accuracy of the  results will increase if
 coating many parts during any one test, due to the larger
 sample size.

 9.3.3   Using the Volume Method
 The volume  method is not as accurate as the weight
 method. Facilities reserve this method for parts that are
 too large or heavy to accurately weigh. For example, a
 facility operator might use the volume method to meas-
 ure the transfer efficiency of a large transformer, street
 sweeper, forklift, engine block (which is too heavy but
 not too large  for the balance).

 When  the  object is large but has a relatively simple
 shape, a facility operator can often still use the weight
 method by "wallpapering" the surface with preweighed
 aluminum foil. At the conclusion of the test, weighing the
 dried coating  on the foil completes the calculations.

 To measure transfer efficiency using the volume method,
 a laboratory must determine the percent volume solids
 of the coating, as applied. To determine the volume of
 solid coating deposited, a lab measures the average film
 thickness of the deposited coating, as well as the total
 surface area of the coated parts.
9.4   The Effects of Common Spray Guns
      on Transfer Efficiency

The  most important equipment to affect transfer effi-
ciency, and thus  pollution prevention, in a paint and
coating facility is the spray gun. This section, therefore,
describes available  types of spray guns and discusses
their effects on transfer efficiency.
 9.4.1  Conventional Air A tomizing Spray Guns

 These guns are still the most popular for providing high
 quality finishes on a wide variety of substrates. The
 spray guns work on the following principles.

 The operator pumps fluid from a pressure pot to the
 spray gun under  relatively low pressure, usually  10 to
 20 psi. Sometimes, a cup contains the coating which is
 then siphoned directly to the gun.

 The operator then feeds compressed air into the gun
 which mixes with the coating, finely atomizing it into very
 small particles. For most applications, the atomizing air
 pressure is 40 to 80 psi. One of the primary reasons for
 the gun's  popularity is that the operator can adjust both
 the atomizing air pressure  and the fluid delivery rate
 because both controls are on the gun body itself.

 Unfortunately, many  operators set  the  atomizing air
 pressure considerably higher than what is necessary to
 produce an acceptable finish. For instance, while an air
 pressure  of 40 psi may  be adequate to produce the
 desired finish, the operator may choose to apply the
 coating at the maximum shop or line pressure of 80 psi
 or more. This, of course, can increase VOC emissions,
 waste, and clean-up efforts. Because of the high atomizing
 pressure,  the finely divided spray particles form a fog in
 the spray  booth. Moreover,  as the particles travel at a
 relatively  high speed from  the gun  to the target, the
 opportunity for the particles to bounce off  the surface
 and  rebound into  the spray booth increases. Conse-
 quently, the transfer efficiency for this type of spray gun
 is usually  fairly low relative to the other types. For this
 reason, the South Coast Air Quality Management Dis-
 trict (SCAQMD), among other jurisdictions,  have highly
 limited the conventional air atomizing gun. The actions
 of SCAQMD  are important because  the industry looks
 to SCAQMD to assess future regulatory trends regard-
 ing transfer efficiency and spray guns.

 A general  perception exists that the  transfer efficiency
 for this gun is always low, perhaps around 25  percent.
This is absolutely not so. When operators use the con-
 ventional air atomizing spray gun at low air pressures
 (less than 40 psi), transfer efficiency can be consider-
 ably higher than 25  percent,  and, depending on part
 size, can even exceed 65 percent.

 9.4.2  High Volume, Low Pressure Air
       Atomizing Spray Guns

The high volume,  low pressure (HVLP) spray gun was
 introduced to the United States market in the mid-1980s.
 It is very similar to the conventional air atomizing  gun.
While the  conventional gun atomizes the coating  at
pressures  of  40 to 80 psi, HVLP guns use higher vol-
 umes of air at pressures less than 10 psi to perform the
same function. Many regulations, such as those written
by the South Coast  Air Quality Management District,

 limit the air atomizing pressure to 10 psi to ensure the
 realization of transfer efficiency benefits from  low  air

 Several methods are available for generating the high
 volume, low pressure air. During the mid-1980s the most
 common method was  using a high speed turbine that
 draws large volumes of air directly from the surrounding
 space. The  turbine pushes this high volume  of  air
 through a large diameter hose to the spray gun,  but the
 air pressure can range from as low as 0.5 to 10 psi. The
 key to atomizing the coating with this method is the high
 volume of air that mixes with the coating inside the gun.
 In addition, the turbine tends to heat the air to  a tem-
 perature of  approximately  110ฐF,  which appears  to
 benefit the application of the coating.

 Historically, the turbine HVLP guns have been relatively
 expensive, with costs in the $2,000  to $15,000 range.
 More recently, spray gun vendors have introduced ver-
 sions that do not require a turbine to generate the high
 volume air. Instead, they directly convert low volume,
 high pressure shop air, to high volume,  low pressure by
 means of venturies or regulators. Typically, the incoming
 shop air is at 80 to 100 psi, while the air emerging from
 the cap of the spray gun is less than 10 psi. The volume
 of air for this gun is considerably less than that emerging
 from the turbine gun.

 The major advantage of these newer pressure conver-
 sion guns is 1Jiat they can immediately replace conven-
 tional  air at6mizing spray guns without requiring any
 other major capital purchases. The conversion units do
 not automatically heat the air as do the turbines, but
 several vendors provide in-line heaters with the option
 of heating the air if desired. Currently, all types of HVLP
 guns are popular, even if regulations do  not require their
 use, because they have been  marketed as high effi-
 ciency guns. Operators can use these guns to apply
 coatings to small, medium,  and large targets.  Some
 reports claim that the guns cannot  keep up with high
 production-line speeds, but facilities must determine this
 on a case-by-case basis.

 Generally, HVLP guns have been successful in atomiz-
 ing a wide range of coatings, although some Theologies
 do not atomize well.  Although the turbine-operated
 HVLP guns are more expensive than the pressure-con-
 version HVLP guns, the turbine types are generally more
 efficient at  atomizing a wider range  of coatings; there-
 fore, in some cases, they are the most cost-effective

Transfer efficiency trials, which numerous companies
and agencies have conducted, have demonstrated that
the HVLP guns are generally more efficient than other
gun types,  and in some instances even more efficient
than electrostatic spray guns. Each operating scenario
determines how efficient one gun type will be relative to
 the other types. One should not be misled by advertise-
 ments which claim that  HVLP guns are always more
 efficient than other gun types. Only on-line testing can
 provide the answer.

 9.4.3  Airless Spray Systems

 The airless spray system works much like a home water
 system. When  turning on the faucet at home to take a
 shower, high pressure from the city's pumping  station
 forces water through small  orifices in the shower head.
 Depending on the size of the orifices, the spray is either
 fine or coarse.

 With an airless  spray system, a hydraulic pump siphons
 the coating out  of a reservoir such as a 55-gallon drum,
 and then pumps the coating, usually under pressures of
 1,000 to 3,000  psi, to the spray gun. The coating atom-
 izes as it passes through the small orifice (0.011 to 0.074
 inches) in the cap of the gun. The size and shape of the
 orifice determine the  degree  of atomization and the
 shape and width of the fan pattern. Moreover, a large
 orifice permits a higher fluid  flow rate than a small orifice.

 Unlike the conventional  air atomizing  spray gun, the
 airless spray gun does not permit the operator the same
 flexibility in setting spraying parameters. Further, because
 of the high fluid pressure,  operators can apply large
 quantities of the coating relatively quickly. For this rea-
 son, operators often use the airless spray gun to apply
 coatings to large surfaces such as buildings, the sides
 of vessels in petroleum refineries, structures such as
 bridges, etc. In  addition, operators often use this gun in
 coating facilities where the coating application must
 keep up with fast moving conveyors.

 EPA has traditionally associated this gun with transfer
 efficiency values of approximately 40 percent but con-
 siderably higher values are obtainable. For instance,
 airless spray guns  that coat large surfaces, such  as
 large electrical control panels, railcars, ships, buildings,
 etc., are usually associated with much higher transfer
 efficiency values. Alternately, operators usually do not
 use this gun to coat small targets because the high fluid
 pressure tends  to deflect small targets suspended  on
 conveyor  lines, and the  generally high fluid delivery
 rates make it difficult to achieve acceptable-looking fin-
 ishes. When using an airless spray gun to coat small
targets, therefore, the operator can expect low transfer
efficiencies, sometimes even lower than those which a
conventional spray gun could achieve.

This gun has not been approved by agencies such as

 9.4.4  Air-Assisted Airless Spray Guns

The principle of this spray gun  is very similar to that of
the airless gun  in  that high fluid  pressures force the
coating through  a small orifice in the spray gun cap.

 The gun differs from the airless spray gun in that the fluid
 pressures are only 300 to 1,000 psi. These pressures,
 however, poorly atomize the top and bottom of the fan.
 Moreover, streaks or "tails" appear at the extremities. To
 eliminate the "tails," low-pressure  air emerges from
 separate orifices in  the horns of the cap to force the
 "tails"  back into the main portion of the pattern. The
 low-pressure air, 10 to 20 psi, does not atomize  the
 coating particles, and therefore the gun differs consider-
 ably from the conventional air atomizing spray gun. The
 air-assisted airless gun is currently among the most
 popular types used in a wide range of industries. While
 it can handle relatively high fluid flow rates and therefore
 keep up with fast moving conveyor lines, it can also be
 adjusted for slow moving lines. Operators commonly
 use this gun to coat medium- and large-size targets, and
 in some cases to coat small parts, providing surprisingly
 appealing finishes.

 EPA transfer efficiency table values which appear in
 various EPA documents, such as  Control Technique
 Guidelines,   New  Source   Performance  Standards
 (NSPS), National Emission Standards for Hazardous Air
 Pollutants, are approximately 40 percent for the  air-as-
 sisted airless spray gun. Agencies such as SCAQMD have
 not included this gun on their approved list of alterna-
 tives for meeting transfer efficiency requirements. This
 is partly because the operator can increase the air pres-
 sure to the horns of the gun to a point that compromises
 transfer efficiency. Some manufacturers have designed
 equipment that limits the shaping air pressure to a maxi-
 mum of 10 psi. Some of the guns have  been approved
 as meeting the definition HVLP.

 9.4.5   Electrostatic Spray Guns

 This category of spray guns embraces a wide range of
 technologies; electrostatic guns can  use conventional
 air, airless,  air-assisted airless, and HVLP atomizing
 technologies. The paint operator obviously has a wide
 range of spray gun designs from which to choose.

 All of the electrostatic technologies  have one thing in
 common: the gun imparts an electrostatic charge to the
 coating particles as  they emerge from the spray  gun
 nozzle. The  operator must be sure to ground the  target
 well so that the charged coating particles can seek the
 grounded part and deposit themselves on the substrate.

 Operators and others commonly believe that when ap-
 plying  a coating electrostatically, the  coating  wraps
 around the target to coat not only the facing surface, but
also the reverse  side of the target. Advertisements and
vendors' literature reinforce this point.  Unfortunately,
 here lies a misconception.

Some  wrap  of course  takes place; the extent of the
wrap, however, is often overstated. If coating round or
square tubing electrostatically, the operator can expect
 almost total wrap around the entire tube because of the
 relatively small area that the coating must wrap. Alter-
 nately, when coating a medium or large flat target, the
 wrap  only extends for  approximately 1/8 to 1/4  inch
 around the reverse side. The  wrap rapidly diminishes
 toward the center of the reverse surface.

 Many parameters determine the efficiency with which
 the coating can wrap around the surface. These include:

 • Polarity of the coating

 • Voltage potential of the spray gun

 • Air velocity in the spray booth

 • Efficiency of the ground

 The operator cannot assume that the target is always
 well grounded even if it attaches to a ground strap or
 suspends from a conveyor hook. In fact, significant elec-
 trical resistance can exist between the target and the
 ground. Poor wrap leads to a lower transfer efficiency.
 The mere fact that the spray pattern tends to bend
 toward the target when the paint  particles follow the
 electrostatic field is already advantageous.

 Most regulations  that include a transfer efficiency re-
 quirement exempt electrostatic applications as being
 "deemed to comply." Although some may infer from this
 that electrostatic applications automatically provide effi-
 ciencies of 65 percent or higher, such conclusions are
 false. Electrostatic applications do not automatically pro-
 vide high transfer efficiencies, even if optimizing all the
 parameters. When compared with  non-electrostatic ap-
 plications, however, they usually show improved values.

 By using  the above information regarding spray  gun
 options along with on-line  testing, each  facility must
 determine which  pieces of equipment offer the best
 opportunities for increased transfer efficiency, and thus
 pollution prevention.

 9.5    Pollution Prevention Strategies To
       Improve  Transfer Efficiency

 This section  offers a broad range of strategies that
 facilities can use to improve transfer efficiency. Many of
 these  can be implemented immediately,  without  the
 need for any capital expenditure or management ap-
 provals. Some  strategies require  minor modifications
 either to the spray equipment or to some other aspect
 of the painting process. Only a few require a moderate
 or significant expenditure.

 9.5.1  Strategies That Require No Capital

One of the most effective strategies for improving trans-
fer efficiency  calls for the spray painter to  move closer
to the  part he or  she is painting.  A typical gun-target

distance is 8 to 12 inches. In general, as the distance
increases, transfer efficiency diminishes. As the dis-
tance decreases, however, the spray painter needs to
reduce the fluid and/or air pressure to avoid applying too
much coating to the target. This effective strategy re-
quires only that the spray painter practice a new tech-
nique in applying the coating. The technique does not
sacrifice production speeds nor does it involve important
decision-making or expenditure considerations.

Another effective technique involves reducing the fluid
flow rate. Figure 9-2 shows three different fluid flow rates
measured by disconnecting the air hose from the spray
gun. If the fluid pressure  and corresponding fluid flow
rate are high, the stream of paint emerging from the
spray gun travels a relatively long distance before bend-
ing and falling to the ground. Such a flow rate has a very
short residence time within the spray gun and requires
a large amount of energy for proper atomization.
                        Strive for the lowest fluid flow rate
                        that will do the job.

Figure 9-2. Effect of fluid flow rate on residence time in gun.

For instance,  a conventional air atomizing  spray  gun
requires a high air pressure to adequately break  up the
paint.  As the  fluid  pressure  decreases, the stream
emerging from the spray gun shortens, and less energy
is necessary to atomize it. Longer residence times lead
to more efficient atomization,  which in  turn results  in
higher transfer efficiencies.

Many spray painters may argue that lowering  the fluid
delivery  rate would slow down production speed  and
consequently raise the cost of painting.  This argument
is true for a very small  percentage of coating  facilities
which have already optimized their fluid delivery rates to
meet their production line speeds. By far, the majority of
paint  facilities do  not measure fluid delivery  rate  nor
correlate it with the production line speed. On the con-
trary, in most cases the fluid delivery rate is considerably
greater than what the job requires; the majority of spray
painters can lower their fluid pressures without impact-
ing productivity.

When  using a conventional air atomizing spray gun,
HVLP  gun,  or any of the  corresponding electrostatic
          guns,  reducing the air pressure  to accommodate the
          reduction in the fluid delivery rate results in a marked
          improvement in transfer efficiency. This translates into
          less air and waste pollution as well  as less pollution
          associated with clean-up efforts.  For the airless and  in
          some cases also for the air-assisted airless guns, using
          a smaller orifice can  achieve the same atomizing re-
          sults. Once again, this strategy requires little or no ex-
          penditure,  and in  most cases can  be implemented

          Yet another effective method for increasing transfer ef-
          ficiency optimizes the fan size to cater to th6 size of the
          part the operator is painting.  Understandably, a spray
          painter would prefer to use a wide fan when  painting
          large surfaces. The operator,  however, must appropri-
          ately reduce fan  size  when painting  smaller surfaces
          (see Figure 9-3). All too often, a spray painter uses a fan
                                   Narrow Fan
          Figure 9-3.  Effect of fan width.

          size of 6 to  8 inches to paint small- or narrow-shaped
          parts such as metal tubing or angle brackets. Adjusting
          the spray fan should not pose a major problem for spray
          painters who work on production lines that coat predomi-
          nantly long runs of one part geometry. For those whose
          targets continuously change sizes, perhaps the best and
          most practical strategy is to purchase a cap enabling the
          operator to adjust the spray fan on the fly. Because not
          all spray guns can be fitted with adjustable caps, shop-

 ping around among equipment vendors for appropriate
 spray equipment may become necessary.

 Finally, manipulating gun strokes can alter transfer effi-
 ciency. Specifically, minimizing the leading and trailing
 edges of gun strokes can significantly improve transfer
 efficiency. Figure 9-4 shows the concept of leading and
 trailing edges. On production  lines that use reciproca-
 tors, the gun usually initiates triggering seconds before
 the target passes in front of it, and ceases triggering a
 few seconds after the  target  has passed. Where high
 quality appearance and uniform film thickness are man-
 datory, leading and trailing edges are necessary to pre-
 vent fat edges. In many cases, however, operators set
 the spray guns so that they trigger sooner than is nec-
 essary, or cease triggering too long after the part has
 passed. When  painting small- or medium-sized parts,
 even a small decrease in the leading and trailing edge
 results in a significant improvement in transfer efficiency.
 Even when painting large parts, such as aircraft skins,
 this apparently small consideration can  make a large
 difference to the resulting transfer efficiency. Further, a
                    Good Practice
                   Direction of Spray
                    Poor Practice
                   Direction of Spray
Figure 9-4.  Effect of leading and trailing edges on transfer ef-
 better transfer efficiency means less waste and, thus,
 less pollution.

 The strategy of minimizing  leading and trailing edges
 also applies to using manual spray guns.  Simply, the
 spray  painter needs to  learn to reduce the distance
. between the point of triggering and the edge of the target.

 The concept of manipulating gun strokes also concerns
 the first and last  stroke of a painting operation.  For
 instance, suppose that a spray painter is applying  a
 coating to a large flat panel, and that the fan on the spray
 gun is 8 inches. To ensure a uniform film thickness of
 the coating, the spray painter must apply the first stroke
 so that only the lower half of the fan  passes over the
 panel while the upper half sprays into the air (see Figure
 9-5).  Then on the second  stroke, the spray  painter
 moves the gun down 4 inches so that the upper edge of
 the fan strikes the upper edge of the panel. For the third
 stroke, the spray painter moves the gun down another
 4 inches and repeats the process. The 50 percent over-
 lap between strokes helps to achieve a uniformly coated
 part. When the painter reaches the last stroke, only the
 upper half of the fan strikes the target, while the lower
 half sprays into the  air.  Unfortunately, the  50  percent
 overlap technique contributes to lower transfer efficien-
 cies. To minimize this, however, the spray  painter can
 use a reduced spray fan and ensure that the first stroke
 provides no more than 50 percent overlap. In too many
 cases, the spray painter  applies the first stroke so that
 only 10 or 20 percent of  the fan strikes  the  target.
 Facilities can implement this strategy immediately with-
 out the need for expenditure or management decisions.

 9.5.2   Strategies That Require Nominal
        Capital Expenditure

 Paint facilities equipped with conveyors often suspend
their parts from hooks that are spaced at 18 or 24 inch
centers. While it is appropriate to suspend medium and
large sized parts from individual hooks, it is poor practice
to do so when painting small parts or parts having a long
and  narrow shape, such as tubing or angle brackets.
The most effective method for improving transfer effi-
ciency entails suspending these parts from specially
designed racks or hooks that allow for close spacing.
Hook and rack manufacturers can provide catalogs with
a wide range of products available off-the-shelf. These
vendors also manufacture custom-designed hooks and
racks for more complex-shaped parts. Not only does
close spacing result in a significant increase in transfer
efficiency, but it speeds up the production process, mak-
ing it more efficient overall. Even though the purchase
or manufacture of special  racks may require capital
expenditures, any medium-sized paint facility should
realize the  payback within a few months. When adding
this benefit to that of minimizing pollution, the argument
to invest in this equipment seems flawless.

                                                                    Number Strokes
                                                                    With 50% Overlap
Figure 9-5.  Deliberate overspray at top of first stroke and bottom of last stroke.
When operators paint small parts on pallets, the parts
should be spaced as closely together as possible to
maximize transfer efficiency.

Operators, however, cannot always achieve close spac-
ing.  For instance,  painters can often not closely space
complex-shaped parts that require painting from various
angles without compromising finishing quality.  Also,
when using electrostatic spray guns, painters must pro-
vide sufficient spacing to allow for some wrap to take place.

Another obvious strategy for improving  transfer effi-
ciency and minimizing pollution is to apply the coatings
with the most efficient spray guns applicable  to  the
situation. Section 9.4 already discussed the benefits and
limitations  of most spray guns. Even though the pur-
chase and installation of such equipment requires capi-
tal, facilities usually realize cost paybacks within several

A strategy often overlooked concerns the velocity of air
passing through a spray booth. OSHA requires a mini-
mum air velocity of 100 to 120 feet per  minute through
spray booths in which operators use manual spray guns.
Alternately,  OSHA allows facilities using automated
electrostatic spray guns to lower their air velocities to 60
feet  per minute. Many paint facility operators inadver-
tently run their booths  at velocities well above these
guidelines  values  because they are unaware  of  the
deleterious effect this can have on transfer efficiency. On
the other hand, some situations justify the higher veloci-
ties.  When  spray  applying large volumes of polyure-
thanes  or  lead/chromate-containing  paints,  high  air
velocities minimize potential health risks to the painters.
A few facilities must quickly remove overspray from  the
booth to prevent it from settling on freshly painted sur-
faces; these cases also require high air  velocities.
High air velocities, however, are expensive. They add to
electrical costs, and companies located in cold environ-
ments must also consider additional heating costs. Most
facilities should reduce air flow rates, but not far exceed
OSHA requirements. If overspray at the lower flow rates
is high, painters  should wear air-supplied respirators.
Generally,  painters who are accustomed to  wearing
such respirators enjoy them because of the clean and
air-conditioned air that they supply. From the viewpoint
of transfer efficiency and pollution prevention, lower air
velocities through the spray booth allow the deposition
of paint particles onto parts rather than into spray booth
dry filters or water-wash curtains.

In situations requiring electrostatic spray guns, it is par-
ticularly important to lower the air velocity yet avoid
violating any OSHA regulations. At high  air velocities,
the electrostatically charged paint particles do not have
an opportunity to wrap the parts that they are intended
to coat. Instead, the strong flowing air current pulls the
particles into the booth.

Paint facilities that  comprise several spray booths, all
pulling from one air make-up system, may experience
violently turbulent air velocities that change direction
from one second to the  next. In  facilities such as these,
it  is not uncommon to  see overspray blowing in the
opposite direction from the spray booth  filter bank or
water-wash curtain. Often, an unusual amount of over-
spray deposited on spray booth ceilings and walls indi-
cates turbulent air flow through the booth.

Because correcting turbulent air flow is often difficult,
these cases may require air-conditioning  or air-ventila-
tion consultants to solve the problem. While this remedy
costs money, the advantage to having a uniform,  laminar
air flow through  a spray booth  is improved  transfer

efficiency and significantly reduced overspray and booth
maintenance. All these factors contribute to pollution

5.5.3  Strategies That Require Moderate or
        Significant Expenditure

Some paint facilities have such high VOC emissions that
their state or local air pollution agencies require them to
install abatement control equipment. The high  cost of
such  an installation often justifies looking for alternative
strategies to lower air emissions below the state's regu-
lated  threshold. If the strategies this chapter has already
covered do not lower emissions sufficiently to preclude
the use of abatement equipment, then a facility operator
may need to consider more drastic measures. Alternative
application methods such as dipping, flow coating, elec-
trocoating, or powder coating,  may resolve  the emis-
sions problems but the implementation of any of these
methods  requires  many months of planning, testing,
design, and of course implementation.

Despite the long lead time such a  process change re-
quires, and the costs associated with it, often this alter-
native is ultimately more cost effective than installing an
abatement control device.
Both choices result in the same goal—minimizing

9.6   References

1. American Society for Testing and Materials. 1995. in: Annual Book
   of ASTM Standards: Paint Related Coatings and Aromatics, vol.
   6.01. ASTM. Philadelphia.

9.7   Additional Reading

Ewert, S.A. et al. 1993. Low cost transfer efficient paint spray equip-
   ment. Metal Finishing 91(8):59.

Hund, J.P. 1994. Spray application processes. Organic Finishing
   Guidebook and Directory. Metal Finishing 92(5A):114.

Joseph, R. 1993. Commentary on determining TE and VOC emis-
   sions. Metal Finishing 91 (6):79.

Joseph, R. 1990. Spray application equipment for coatings and their
   relationship to transfer efficiency. Paper presented at the Westec
   '90 (March).

Joseph, R. 1990. Transfer efficiency test protocol development and
   validation in custom a coating facility. Final Report, South Coast
   Air Quality Management District, CA (January).

Snowden-Swan, L, and P Womer. 1993. Determining TE and VOC
   emissions. Metal Finishing 91(6):73.

                                             Chapter 10
                           Liquid Compliant Coating Technologies
 10.1  Introduction

 10.1.1  Pollution Prevention Considerations

 The purpose of this chapter is to provide facility opera-
 tors with guidelines for selecting coatings that reduce air,
 water, and/or waste pollution. Choosing the right coating
 constitutes one of the most basic decisions for an op-
 erator searching for ways to prevent pollution.

 A liquid compliant, or VOC-compliant, coating is one that
 satisfies the VOC content requirements of the relevant
 regulation. The essential criterion for compliance is that
 the as applied coating satisfies the regulatory limit. For
 instance, a user who  buys a packaged coating that just
 meets VOC content  regulations cannot add thinner to
 that coating without  rendering the  as applied coating

 Most facility operators probably already use low VOC
 coatings that meet the Reasonably Available Control
 Technology  (PACT)  limits of their state regulations,
 which is 3.5 Ib/gal in many states.  For users who still
 use VOC  coatings that exceed the PACT limits, how-
 ever,  this chapter can  hopefully  provide them  with
 means for choosing a technology  that allows  them to
 maximize  reductions in the pollution of all media.

 If coatings that go beyond PACT are  not feasible, this
 chapter still provides end-users with  suggestions for
 other process improvements that can  at least lower
 hazardous waste and water  discharges.  In addition to
 lowering and preventing  pollution, the guidelines this
 chapter presents should also lead to  improved quality
 and lower costs.

The chapter first offers the reader guidelines for prepar-
 ing to choose among the various  options for specific
 applications. It then details the advantages and limita-
tions of the specific technologies available, including a
wide spectrum of water-borne and solvent-borne coat-
 ings. All these coatings are considered  PACT, and some
may be available in  formulations that represent Best
Available Control Technology (BACT). This discussion
supplies the reader with the  necessary information re-
garding how to choose a coating  appropriate for the
application while still reducing pollution. After a brief
 introduction to emerging technologies, the chapter con-
 cludes with tips for the selection process.

 10.1.2  Decision-Making Criteria

 Decision-making criteria relevant to liquid coatings, as
 addressed in this chapter, are highlighted in Table 10-1.

 10.2  Guidelines for Choosing Best
       Management Practices

 At the start of the resin  system selection process, the
 end-user must tentatively choose between the following
 variables, keeping in mind pollution prevention, as well
 as quality and  cost:
 • Liquid versus powder  coatings

 • Water-borne versus high solids, solvent-borne coatings

 • Air/force dry versus baked coatings

 • Single-component versus two-component coatings

 This section focuses on  making these basic decisions.
 Section 10.3 helps to narrow the selection of coating still

 10.2.1  Liquid Versus Powder Coatings

 Someone  approaching  coatings for  the first time,  or
 willing to take a fresh look at the available options, must
 first decide on  whether the coatings should be in liquid
 or powder form.

 Because powder coatings are generally the lowest pol-
 luting of all coatings, they demand serious considera-
tion. Powder coatings also offer attractive cost benefits
and, in many instances, quality improvements. Powders
are generally high  performance coatings that provide
excellent  hardness, mar resistance, abrasion  resis-
tance, flexibility, elongation, UV  resistance,  and for
some resins also chemical and solvent resistance. Liq-
uid coatings, however, usually offer much more versatil-
ity in many areas.

Table 10-2 provides the most important  advantages of
liquid over powder coatings, while  Table 10-3 provides
the most important advantages of powder over liquid

 Table 10-1.  Decision-Making Criteria Regarding Liquid Compliant Coatings

 Issue                                                             Considerations
 Instead of using a liquid coating, would a powder coating be a
 viable option to coat workplaces in question?
Are the workpieces too large to fit into a baking oven?

Are the workpieces small enough to fit into an oven yet cannot
tolerate temperatures greater than 250ฐF?

Are the workpieces small enough to fit into a baking oven and do
they require qualities such as hardness, abrasion and mar
resistance, and some chemical resistance?

Are the workpieces too large to fit into an oven but require
hardness, abrasion and mar resistance, and some chemical

Are the workpieces too large to fit into an oven, but do not require
hardness, abrasion and mar resistance, and some chemical

Is the location of the painting facility one in which operators
commonly encounter low spray booth temperatures in the winter?

Is the painting facility located in an area that commonly has high
relative humidity (higher than 90%)?

Does the coating require excellent chemical and solvent
resistance, and also hardness, and abrasion and mar resistance?
Does the workpiece that requires hardness, abrasion resistance,
chemical and solvent resistance, and exterior durability also lend
itself to a dipping'application?
                                  • If after referring to Chapter 11 of this text powder coating seems
                                    appropriate, consider using this as an alternative to liquid

                                  • If powder coating appears inappropriate, use the remaining
                                    criteria to decide on the best liquid coating for the job.

                                  • If yes, you  must consider an air/force dried coating.

                                  • If yes, you  must consider an air/force dried coating.

                                  • If yes, you  may consider either baked coatings or air/force dried
                                    thermoset coatings such as epoxies and polyurethanes.

                                  • If yes, consider air/force dried thermoset single- or
                                    two-component coatings such as epoxies and polyurethanes.

                                  • If yes, consider a single component coating such as an alkyd or
                                    modified  alkyd, which would be much less expensive than a
                                    two-component coating.

                                  • If yes, solvent-borne coatings may be preferable to water-borne

                                  • If yes, solvent-borne coatings may be preferable to water-borne

                                  • If yes, consider a  solvent-borne epoxy primer followed by a
                                    single or  two-component polyurethane topcoat; new water-borne
                                    polyurethanes might also be appropriate for wood products and
                                    may soon be available for plastic and metal.

                                  • If no,  consider evaluating water-borne formulations.

                                  • If yes, consider exploring autodeposited or electrodeposited
Table 10-2.  Advantages of Liquid Over Powder Coatings

Part Versatility

Color Tinting

Application Versatility
Line Speed
More versatile for complex shaped parts.

Colors can be tinted if vendor delivers wrong
shade. Easy to color match.

Wide range of application equipment allows
flexibility in selecting appropriate equipment.
This includes spray (many different types of
spray guns), dip, flow,  and curtain.
Application equipment can keep up with very
fast-moving production line.
Often not suitable for parts with many
inaccessible areas and deep recesses.

Cannot be tinted on the job. If wrong shade, the
powder must be returned to vendor for blending.

Not suitable for applications which can easily be
dipped or flow coated. Possibly can compete
with curtain coatings, but often film build will be
too high.

May not be suitable for very large parts such as
weldments, although some large parts, such as
pipe lines, are being powder coated.

Tribo-charging guns can apply coatings at faster
line speeds than electrostatic guns, but for most
very fast moving lines, liquid coatings may still
be more cost-effective. This is especially true for
complex shaped parts.

Table 10-2.  Advantages of Liquid Over Powder Coatings (continued)

Substrate Versatility
Application Temperature
End-use Temperature
Resin Technology

Finish Versatility

Environmental Conditions
Curing Requirements
Applicability for Low Cost
Masking Requirements

Military Specifications
 Can be applied to all substrates: metals,
 plastics, wood, masonry, paper, cloth, etc.

 Ideal for coating heat-sensitive substrates.
 Greater tolerance for "finger prints," small
 blemishes, and surface texture cleanliness.

 Depending on the resin technology, can be
 applied at temperatures ranging from sub-zero
 to over 100ฐF.
 Liquid coatings can be designed for low
 temperatures (sub-zero) to high temperatures

 Preferred for cryogenic applications.

 Almost unlimited range of resin technologies
 available; resin system exists for almost any
 conceivable end use.

 Usually can be formulated in any color, gloss
 level, and with a range of texture finishes.
 Can withstand the severest of chemical
 environments, weather and atmospheric
 conditions (e.g., temperature, humidity,
 altitude), and marine conditions.
 Ideal for large machines or assemblies that
 cannot be placed in a high temperature oven.

 Ideal for fence posts, some hardware building
 supplies, farm implements, etc., which are
 coated for appearance only, but require no
 other properties. Can tolerate minimal  surface

 Because of the lower temperatures liquid
 coatings require to cure, masking is usually
 not a problem.

 Nearly all military specification coatings are
 written for liquid coatings.
 For the most part, cannot be applied to most
 plastics, wood, paper, cloth, masonry, rubber,

 Not available for substrates that cannot
 withstand at least 250ฐF, and commonly 325ฐF
 and higher.

 Need cleaner substrates and sophisticated
 phosphate pretreatment system.

 More commonly applied at ambient temperature.
 Generally is not applied at sub-zero
 temperatures, such as outdoors during winter
 months, or at high temperatures such as on
 heat stacks, etc.

 Powders generally do not withstand excessively
 high temperatures, such as on high temperature
 exhaust stacks.
Although range of resin technologies is broad, it
is not as broad as for liquids.

May require more research effort to achieve
equivalent results regarding color, gloss level,
and texture finishes.

Generally not used in chemical plants, such as
for tank linings, and are rarely used for severe
marine exposure, such as on oil rigs.  Due to a
scarcity in performance histories relative to
liquid coatings, end-user should perform
extensive tests before using powder system.

Powder coatings more sensitive to humidity and
other atmospheric conditions due to fluidized
bed of handling systems.

Require curing at temperatures in excess of
325 F. A few resins cure at a minimum of 250ฐF.

Successful applications require certain process
procedures,  such as good surface preparation,
coating thicknesses in excess  of 1.0 mil, etc.;
improved processes required would probably
raise cost of low cost item so that it would no
longer be competitive.

If the  workpiece requires extensive masking,
powder coatings may not be cost-effective.

Although some military specifications have been
written, and  more will be issued, the majority
are still for liquid systems.
Table 10-3.  Advantages of Powder Over Liquid Coatings

VOC Emissions
Essentially zero VOCs
   Usually at the RACT limits; some resin
   technologies are well below these limits, but
   are still well above zero VOC.

   Some coatings, such as UV curables, are
   available at very low (almost zero) VOC
   levels, but nave limited applications.

   Developing resin technologies will soon allow
   for zero VOC emissions; some are already
   available but for limited applications.

Table 10-3.  Advantages of Powder Over Liquid Coatings (continued)

Hazardous Waste
Water Pollution

Fire Risk

Part Suitability

Clean-up Profile
Automation Suitability
Labor Requirements
Transfer Efficiency
Essentially zero hazardous waste; although
some companies melt waste powder into solid
blocks and then discard them as hazardous
waste, volume is negligible when compared
with similar liquid coating  applications.

No water pollution due to powder coating
application; always applied in booths
containirfg dry filters or cartridges.

Less toxic to operators because no solvents
are used.
Stored in boxes; do not need to be stored in
explosion proof cabinets or storage sheds.
Lower fire hazard.
Ideal for flat parts and ones with relatively
simple geometry; more cost-effective than
liquid coatings for these items.

Relatively clean process if spray booths
operate under negative pressure.

Most of the clean-up is usually carried out
with compressed air or vacuum hoses. No
solvents are used for clean-up.
Ideal for automated processes; reciprocators
and robots can be used with relative ease.
Generally shorter learning curve for operators;
while they must know about powder gun
settings, voltage settings, etc., they do not
need to be as knowledgeable as spray
Because so many powder coating applications
have some automation involved, less labor is
usually required to apply coatings; often only
a touch-up operator is required at end of
powder coating booth.

When specially designed powder coating
spray booths are used, transfer efficiency can
easily exceed 95% because the powder can
be recycled.

Equipment vendors are now improving spray
guns to increase first pass transfer efficiency.
Disposal costs of waste liquid coatings far
exceed those for waste powders; cannot
totally eliminate hazardous waste.
Cause a water pollution problem when they
are applied in a water-wash spray booth.

Water-borne are less toxic than
solvent-borne coatings, but solvents are often
used to clean up spray application
equipment; some resins, such as
polyisocyanates, are potentially toxic.

Solvent-borne coatings are more toxic
because of the solvents they contain; some
resins, such as  polyurethanes,  may also be

Water-borne coatings do not need to be
stored in explosion proof cabinets, but often
require more storage space than do powders.

Solvent-borne coatings must be carefully
stored in accordance with OSHA regulations.

Water-borne coatings pose considerably less
fire risk than solvent-bornes, but probably
more so than powders.

Liquid coatings can  be  used for same
purposes,  but not as cost effectively.

Liquid coatings are undoubtedly more messy
and require more clean-up (e.g., more rags,
clean-up solvent).

Usually some solvents are used to clean up
residues of liquid coatings. For many
facilities, the VOC emissions and hazardous
waste from solvent clean-up operations is
considerable. New aqueous technologies
may change this trend.

Can be applied  by automated processes, but
requires more skill and effort to achieve
acceptable finishes.

Painters need more training and a more
rigorous learning curve; they must know how
to apply coatings to achieve acceptable
finishes, and about viscosity management,
dealing with two-component coatings, and
equipment clean-up; generally they need
more knowledge about pressure settings,
maintenance of pumps, spray guns, etc.

Automation is used considerably less often
than for powder coatings; liquid coating
systems usually comprise at least two coats
(primer and top coat), which requires more

Usually, transfer efficiencies are well below
60% regardless of spray gun type.

Table 10-3.  Advantages of Powder Over Liquid Coatings (continued)

Coating Profile
Electrostatic powder applications are ideal for
wire products because process transfer
efficiency remains above 95%.

Ideal for parts with cut ends and sharp edges
because one can often achieve higher film
builds in these areas.
Requires just one coat application in most
cases. A single coat of powder performs the
same job as one coat of a liquid primer
followed by a liquid top coat.
                           Generally more uniform thicknesses can be
Liquid coatings can rarely be applied as
efficiently to wire products. Electrostatic liquid
applications are relatively inefficient. If they
are spray applied, transfer efficiencies are
often well below 20%. If they are dip applied,
runs and drips often mar appearance of
coated products.

Liquid coatings can be applied with
electrostatic spray equipment that helps to  '
cover sharp ends and cut edges, but not as
effectively as powder coatings. Where good
corrosion resistance in these areas is
mandatory, powders are superior.

Rarely are liquid coatings applied as a single
coat (usually only when color rather than
corrosion resistance is required).

A Naval Air specification calls for a single
coat polyurethane, but this is applied over
aluminum surfaces where corrosion
resistance is better than for steel substrates.

Film thickness variation tends to be much
greater with liquid coatings.
For readers who think powder coating may be appropri-
ate, Chapter 11 provides more details about the powder
coating process.

10.2.2  Water-Borne Versus Solvent-Borne

If powders do not constitute a feasible option, the next
step should involve deciding between water-borne and
solvent-borne coatings. Because most states require at
the very least, RACT coatings, the discussion here on
solvent-borne coatings  only considers VOC-compliant,
high solids formulations.  (High  solids,  a loosely used
term, most commonly indicates a solvent-borne coating
with a solids volume of  52 percent or more.)

Tables 10-4 and 10-5 provide the  most important advan-
tages and limitations of each.

10.2.3  Air/Force Dry Versus Bake

Another important factor to  consider is whether to pur-
sue air/force dry coatings or select ones that bake at
elevated temperatures, above 250ฐF. Baked coatings
usually have better physical  and  chemical-resistant
properties  but they  also  have some limitations. Table
10-6 provides some  useful guidelines for each method.

Regarding the resin technologies that this chapter dis-
cusses, EPA and state regulations differentiate between
coatings that air/force dry and ones that cure by baking.

EPA defines air/force dried coatings as those that dry or
cure below 194ฐF and many rules establish special VOC
limits for this category.  In contrast, coatings  that cure
                             above  194ฐF are often regulated as "baked" coatings
                             and must follow lower VOC limits. Mostly, the limits for
                             the air/force dry category are higher than for the baked.
                             For example, Table 10-7 lists the regulations guiding the
                             coatings used in the Miscellaneous Metal Parts industries.

                             10.2.4   Single-Component Versus

                             Finally, another important  basic factor to consider in-
                             volves  whether to select a single-component or plural-
                             component  technology.  Generally,  plural-component
                             coatings  have much better physical and chemical resis-
                             tant  properties. This superiority,  however,  does  not
                             come without drawbacks.  Single-component coatings
                             are much easier and less expensive to use.  They also
                             are usually associated with a better pollution prevention
                             profile. The most important differences between the two
                             technologies are presented in Table 10-8.

                      Plural-Component Coatings

                             Because the handling of plural-component coatings is
                             more complicated and because they are associated with
                             more hazardous waste than single-component coatings,
                             plural-component coatings require a more detailed dis-
                             cussion. Usually, a plural-component coating comprises
                             two components. Occasionally, however, it comprises
                             three components, one of which may be a  thinner or

                             The largest source  of hazardous  waste generated by
                             companies  using  plural-component coatings comes
                             from batch  mixing  processes. While such  mixing is

Table 10-4.  Advantages of Water-Borne Over High Solids Solvent-Borne Coatings
                     Water-Borne Coatings
                                                      High Solids Solvent-Borne Coatings
VOC Emissions
Drying Factors
Film Thickness
Safety Profile
Dip Coating
Usually meet air pollution regulations.
Some coatings have very low VOC contents, well
below RACT limits, and help lower total emissions
from the facility.

New developments are tending toward zero VOC
formulations, but don't yet have any reliable
performance history.

Some air/force dry, single-component formulations,
particularly some emulsions, dry considerably faster
than high solids solvent-borne coatings. Others,
however,  dry slower than solvent-bomes. The
end-user  should obtain such data from the coating
vendor or should perform in-house tests to ensure
that the drying time is compatible with existing
production conditions.

Oven drying at temperatures below 194ฐF, and/or
moving air over the workpiece enhances drying.
Relatively low volume solids contents, usually
25-30%, make it possible to apply coatings at low
film builds, approximately 0.8 -1.0 mil. This can be
a major advantage of water-borne over high solids.
Water-borne coatings are generally safer to work
with than solvent-borne coatings: low fire hazard,
less of a requirement for explosion proof storage
areas, and less toxic to operators.
Water-bomes are ideal for dip coating application,
particularly if surface preparation is adequate.
Appearance defects such as orange peel, solvent
popping, and non-uniform color and gloss usually do
not pose a major problem.
These coatings often just meet the RACT limits.
They usually are not available at the typical VOCs
of some of the water-borne formulations.
New 100% solids coatings are being introduced, but
they require baking at elevated temperatures, such
as 250ฐ to 350ฐF. Moreover, they are too new to
have a performance history.

Many of the air/force dry, single-component high
solids coatings, such as alkyds and modified alkyds
take a relatively long time to dry. This is even more
noticeable with excessive film thickness.
Oven drying at temperatures below 194ฐF enhances
drying but moving air over the part offers little
benefit because solvent evaporation is not affected
by relative humidity in the air.

Unless high solids coatings have low viscosities,
most application equipment cannot atomize these
formulations well enough to provide low film builds.

At difficult to reach areas, or  when coating a
complex-shaped workpiece, excessive film
thicknesses are often unavoidable. This results in
higher than anticipated VOC  emissions, longer
drying times, longer recoatlng times, higher reject
rates due to premature damage, and increased
coating usage. Using polyurethanes, however, may
minimize these problems.

Solvents pose a fire risk.

Also, solvents can potentially cause health problems
for operators. Regardless of what type of coating is
used, water- or solvent-borne, painters must wear
the appropriate respirators and if necessary other

High solids coatings cannot be used in dip tanks
because their viscosity is too high, and runs and
sags become a major finishing problem. Moreover,
at the high film thicknesses deposited, the coatings
would take too long to dry.

Because of the generally high viscosities of these
coatings, defects such as orange peel and solvent
popping can become major factors affecting the
reject rate.

On complex-shaped workpieces, where non-uniform
film thicknesses can lead to variations in color and
gloss, customer rejects can also be a problem.
usually more cost-effective when using small quantities
of coatings,  using plural-component metering and mix-
ing equipment is better for large quantities.

Consider the following example. An operator can batch
mix components A and B by manually mixing immedi-
ately before  applying the coating. The operator must be
sure to use all the coating before its viscosity changes
                                          and  it reaches its pot-life. This may be difficult to do if
                                          the operator has mixed a large quantity.

                                          Alternatively, the operator can set special proportioning
                                          equipment to automatically  measure out each compo-
                                          nent in its prescribed ratios. This is called in-line mixing.
                                          The  equipment continuously  pumps each component
                                          separately to  a manifold where they come together in

Table 10-5.  Advantages of High Solids Solvent-Borne Coatings Over Water-Borne Coatings

                       High Solids Solvent-Borne Coatings                 Water-Borne Coatings
Application Flexibility

Surface Preparation
Appearance Defects
Viscosity Management
Solvent-borne resin technologies are available for
almost every conceivable application.
Traditionally, low solids, high VOC solvent-borne
coatings have been tolerant of improper surface
preparation. Newer high solids coatings require
cleaner surfaces. Even these, however, remain
more tolerant to surface preparation than their
water-borne counterparts.
High Solids coatings are not as sensitive to defects
such as edge pull, and cratering as are
In many cases, viscosity management is easier for
solvent-borne coatings than for water-bornes.
Grounding for electrostatic applications is usually
not a major problem. If the coating is not
sufficiently conductive, the vendor can often modify
the solvent blend so that the coating can accept an
electrostatic charge.
While water-borne coatings are able to match many
types of solvent-borne coating, they are not yet as

Coatings are sensitive to surface preparation;
therefore better cleanliness is required.

As water-borne technologies with lower, or zero
VOCs are developed, the need for better surface
preparation will probably become mandatory.

This  need for cleaner surfaces can be a major
factor for companies with marginally acceptable
pretreatment lines.

Water-bomes must be applied correctly to avoid
problems such as edge pull and cratering. This
requires good viscosity management and quality
control procedures.

Flash rusting with some formulations can be a
problem. This can be overcome  by properly
formulating the coating, and if the vendor's
requirements for surface preparation have been

Some water-borne coatings are significantly
thixotropic, and are not easy to apply by untrained

After the painters have been trained, usually by the
vendor,  this problem no longer is an issue.

For water-bornes, grounding and electrical isolation
can be a major problem, particularly in large
facilities which  pump coatings over long distances,
or pump from 55-gallon drums or totes.

New equipment technologies, however, are
available which can essentially eliminate these
problems.  The  end-user should discuss this issue
with equipment vendors.
the fluid hose leading to the spray gun. Downstream of
the manifold  is a short static mixer, which comprises a
short plastic or stainless steel tube located in the fluid
hose only a few inches or feet from the spray gun. Small
baffles in the tube thoroughly mix the components im-
mediately before they enter the spray gun. In-line mixing
allows for components A and  B to be mixed on a con-
tinuous basis. The primary advantage of this process is
that  the  viscosity  of  the  coating  remains constant
throughout the day, and the coating is  used before it can
attain its pot-life.

What constitutes a small  or large quantity? A rule of
thumb is to use a batch mixing process when mixing and
applying  less than  2 to 3  gallons of  plural-component
coating in one shift, particularly if there is a color change
between jobs. Because batch mixing requires more sol-
vent for clean-up  and generates more waste compared
with a plural-component system, it is not a good choice
for large quantities.

Plural-component metering and mixing becomes cost
effective when  using several  gallons of plural-compo-
nent  coating at any one  time, particularly if  a  color
                                    change is not required. If operators will continue the job
                                    on the following shift or the  next day, only the fluid
                                    passages that contain the mixed coating need cleaning.
                                    Fluid lines and passages that carry unmixed component
                                    A or B do not need cleaning because the coating will not
                                    cure in the absence of the other component. Companies
                                    such as automotive original equipment manufacturers
                                    (OEMs) can justify the use of  plural-component equip-
                                    ment even  when  changing colors frequently. This is
                                    because  only the short whip hose that contains  the
                                    mixed coating requires flushing. (Spray  equipment ven-
                                    dors  provide sophisticated  devices for enabling quick
                                    color changes.) Hence, for large  facilities  the cost of
                                    installing  such equipment is often  quickly offset by  the
                                    savings in waste paint and disposal of hazardous waste.
                                    Some companies cannot justify the purchase and instal-
                                    lation of plural-component metering and mixing  equip-
                                    ment and  must  practice  batch  mixing.  They can,
                                    however, dramatically cut costs of materials and waste
                                    by ensuring that painters mix only as much coating as
                                    the job on hand requires. This conservative method also

 Table 10-6.  Air/Force Dry Versus Bake

                         Air/Force Dry
 Substrate Versatility
 RACT Regulations

Appearance Defects
Curing Time

Can be applied to all substrates (e.g., metal,
plastics, wood, rubber, masonry, etc.).
Can be applied over porous materials such as
sand castings, wood, paper, etc.

Some regulations have higher VOC limits for
air/force-dry than for bake coatings.

Can dry and cure at temperatures from ambient
up to 194ฐF by EPA definition.

Solvent-borne coatings do not require an oven,
although a low temperature oven will speed up
the drying process.

Water-bome coatings would benefit from a low
temperature oven, particularly in high humidity

Offers lower energy usage.
Most single-component coatings, such as alkyds
and modified alkyds, do not exhibit superior
physical and chemical properties.

Single-component moisture-cured polyurethanes,
however, do perform comparably to
two-component polyurethanes and baked

Surface defects, such as orange peel, often do
not flow out during the drying and curing
process. Force-drying at elevated temperatures,
but below 194ฐF, can partially alleviate this.

Take longer to achieve through hardness, which
can affect production schedules.

Overspray dries on spray booth filters, spray
booth floors, walls, etc.; therefore, maintenance
is not a significant problem.
 Can only be applied to metals and substrates that can
 withstand high baking temperatures. Generally not
 suitable for heat-sensitive materials such as plastics,
 wood, rubber, hydraulic tubing, etc.

 Should not be applied over machined or other surfaces
 that are sensitive to warpage, unless taking adequate

 Can cause outgassing on sand castings and other
 porous substrates. Preheating workpiece can often
 overcome problem but adds an additional step to process.

Generally must cure at a minimum of 250ฐF. A typical
curing schedule is 10 minutes @ 350ฐF. Curing times
are inversely proportional to temperature. A cool-down
staging area is required.
Require high-temperature oven, and therefore greater
energy usage.

Often have excellent physical and chemical-resistant
properties, sometimes similar to two-component
Films tend to flow out better when in the oven,
providing smooth finishes and eliminating surface
defects such as orange peel.

After baking and cool-down, the coated parts are
usually ready for assembly or shipping.

Uncured overspray remains sticky, making it awkward
to walk on spray booth floors. Maintenance is more
costly because of difficulty handling the sticky material.
Table 10-7.  Typical RACT Limits for Miscellaneous Metal
            Parts Coatings

Most other states
Air/Force Dry
Ib/gal g/L
2.8 340
3.5 420
Ib/gal g/L
2.3 275
3.0 360
allows painters to use all the coating before it reaches
its pot-life.

In facilities that use 1 to 3 gallons per shift, painters often
find that the mixed coating reaches its pot-life before the
job is complete.  Strategies some facilities use for ex-
tending the  pot-life are:

• Mix smaller quantities
                                     •  Cool the coating

                                     •  Add more freshly mixed coating

                                     •  Add solvent (not recommended)

                                     The best of these options is to mix smaller quantities, all
                                     of which  painters can fully  use  before  the pot-life is
                                     reached. This option is associated with the least waste
                                     and the least risk. The remaining three options should
                                     be discouraged because they have too many drawbacks.

                                     Cooling the coating is a viable option because it slows
                                     the cross-linking reaction. This practice requires caution
                                     because if the coating chills below the dew point of the
                                     ambient  air,  condensation of moisture can cause gel
                                     particles of cured coating to form  inside the  coating.
                                     These cannot be easily removed, not even  by passing
                                     the coating through a fine mesh screen or filter.

Table 10-8.  Single-Component Versus Plural-Component Coatings

                           Single-Component Coatings (Such as Alkyds)
                                               Plural-Component Coatings
                                               (Such as Polyurethanes)
Hazardous Waste

Training Requirements

Mixing Requirements
Induction Times

Pot Life

Viscosity  ,

Cleaning Considerations

Physical Properties

Chemical Properties

Cost Considerations
Generally result in considerably less hazardous
waste; whatever has been mixed and cannot be
used, can be saved for another day.

Painters need not go through any specialized
training program.

Require no special mixing instructions.
Require no induction times. The coatings can be
applied immediately.

There are no pot life considerations.
Because cross-linking does not take place until
the coating has been applied, viscosity remains
constant throughout the day (at constant

Spray hoses do not need flushing out as
frequently. Large facilities that apply the same
coating each day infrequently clean the hoses.

Generally form softer and less abrasion-resistant
films. This can lead to a higher reject rate due to
early damage. As the reject rate increases, so do
costs and pollution.

Generally fewer chemical- and solvent-resistant

Generally, cost per gallon is considerably less
than for two-component coatings. Costs can vary
from a low of less than $10 per gallon to a  high
of $30 per gallon.

Maintenance costs are usually less because
coatings do not harden as quickly In fluid
passages and are easier on pumps and spray
More hazardous waste is generated, particularly
if the two components are batch mixed.

Painters must be trained to properly mix and
handle two-component coatings.

Must be precisely mixed in the proportions
vendor recommends. Failure to do so can lead
to improperly cured coatings, rejects, and
generation of more unnecessary air, water, and
waste pollution.

May require induction time of up to 30 minutes
(primarily for some epoxies).

Always have a limited  pot life, and the mixed
coating must be used within that period.

Viscosity of the mixed  coating increases with
time while the polymers cross-link.
If the coating has already been mixed, spray
hoses must be flushed before the coating has an
opportunity to gel.

Known for their superior physical properties.
Many companies have invested  in
two-component coatings specifically to reduce
reject rate.

Known for their superior chemical-resistant

Usually more expensive than single-component
products. Costs can vary from a low of $25 per
gallon to a high of well over $100 per gallon.
Some exotic-colored automotive refinishing
colors can exceed $150 per gallon.

Cost of replacement hoses, pumps, and spray
guns will increase because occasionally the
coating hardens before painters  have had a
chance to clean fluid passages.
An operator must never add one component without the
correct proportion of the other. Complete cross-linking
can  only occur when both components are present in
their stoichiometric proportions. Stoichiometric propor-
tions imply that components A and B have the same total
number of functional groups. Paint chemists formulate
coatings to allow for simple mixing ratios, such as 1:1,
2:1,4:1, etc. For instance, if only component A is added
to extend the pot-life,  the cured coating may tend  to
remain soft and cheesy, and will lose much of its chemi-
cal- and solvent-resistant properties. On the other hand,
if adding only component B, the cured coating may be
too hard and brittle, and will tend to crack and spall from
the surface.

Finally, facilities should strongly  condemn adding sol-
vent to  extend  pot-life,  even  though it  is a  popular
method. Adding solvent carries with it a great possibility
that  the VOC content of the mixture will exceed the
regulated  limit, exposing the company to  a possible
                                 Notice of Violation. Worse than this, however, the sol-
                                 vent is added to  a coating  that has already started to
                                 polymerize. While the painter may be satisfied with the
                                 finish of the applied coating, the coating may harden and
                                 cure before all of the solvent has  had an opportunity to
                                 evaporate out of the film. The entrapped solvent might
                                 gradually  migrate to the coating/substrate  interface,
                                 loosening  the  adhesive bond between  the primer and
                                 the substrate. Catastrophic coating delamination can
                                 occur, which may only become evident months or per-
                                 haps years after the finished product has been in service.

                                 10.3 Water-Borne Coatings

                                 10.3.1   Overview

                                 The term "water-borne" describes coatings in which the
                                 predominant solvent is water. Organic solvents (VOCs)
                                 are also  used but, for the most part, their concentration

 is small.  In many formulations the ratio between the
 amount of water and organic solvent is 80:20.

 The organic solvents, often referred to as co-solvents,
 enhance  the formation of the coating film, especially
 during the drying process when the water is evaporating
 from the deposited coating. As resin manufacturers de-
 velop  new resin  technologies,  they are reducing the
 amount of co-solvent required to form the film. Currently,
 new formulations exist that contain  no co-solvents, and
 consequently have zero VOC. Although this chapter will
 cover these, they do not yet have a long-term perform-
 ance history; therefore,  most end-users will probably
 consider the more conventional  water-borne coatings.

 The term "water-borne"  includes water-reducibles,
 emulsions (latexes), and dispersions. Most vendor data
 sheets do not make any distinction between the three
 types. Because the end-user does not need to know the
 differences between these types in order to select an
 appropriate coating, this chapter does not cover the

 Most VOC regulations limit the VOC  content of a coating
 in terms of pounds per gallon or grams per liter, less
 water and less exempt solvent. Because exempt sol-
 vents,  such as methylene chloride and 1,1,1 trichlo-
 roethane are being phased out, this chapter does not
 address resin technologies that rely on these solvents
 for compliance.

 When dealing with water-borne  coatings, the end-user
 must thoroughly understand the terminology most regu-
 lations use. For instance,  1.0 gallon of a water-borne
 coating contains many ingredients: the resin (or binder),
 pigments, extender  pigments,  coalescing  agents,  a
 small quantity of co-solvents,  and usually a fairly sub-
 stantial  amount of water.  The  volatile  portion of the
 coating comprises the co-solvents and water. In a gallon
 can, the co-solvents,  which are considered to be the
 VOCs, may account for  less than 1.0 pound.  In other
 words, the VOC content of the coating may only be 1.0
 pound/gallon. The VOC  regulations, however, require
that the VOC content of the coating be calculated as  if
 no water were in the coating. Depending on the coating
formulation, the VOC content, less  water, may be con-
siderably higher, such as 2.0 pounds/gallon or more.

 Figure  10-1 a illustrates what 1.0 gallon of water-borne
coating might look like if separating  the ingredients into
discrete layers.  Clearly, the amount of VOC  in the can
would be very small, especially when compared with the
amount of water. If water were removed from the can so
that  the coating only  comprised the VOC and solids
portions, and if the can were then  filled to the gallon
mark, the contents would resemble Figure 10-1b. When
EPA and state regulations  specify a VOC content less
water, they refer  to the  VOC content represented  in
Figure 10-1 b.
      : WATER
    Including water                  Excluding water

Figure 10-1.  VOCs in water-borne coatings.

In order to understand the rationale for this approach,
remember that in applying a coating, one is interested
only in the amount of solid that a substrate needs de-
posited. For instance, when applying a red enamel over
a yellow primer, a painter uses only as much coating as
will completely hide the underlying color. For many coat-
ings,  a  dry film  thickness of 1 mil  (0.001  inch)  may
suffice. It does not matter if the coating  is water-borne
or solvent-borne; the only consideration is  depositing
the specified dry film thickness of  solid coating. Assum-
ing that the composition of the solid ingredients is the
same in both coatings of Figure 10-1, a painter woulu
deposit exactly the same amount of solid coating in each
case. The only difference between the two figures is the
lack of water from the second figure. Because the gallon
can in Figure 10-1 a has less solids than that in  Figure
10-1b, a painter would use a greater volume  of the
Figure 10-1 a coating to deposit the same amount of solid
coating as he would to apply the coating in Figure  10-1b.

Note that the ratio of VOC to solid in Figure 10-1 a is the
same as the ratio of VOC to solid in Figure 10-1b. In
summary, regardless of which paint can (Figure 10-1 a
or b) a painter uses to coat a substrate, the same volume
of solids will be applied, thus emitting the same amount
of VOCs to do the job.

10.3.2   Water-Borne Air/Force Dry Alkyds,
        Acrylics, Acrylic-Epoxy Hybrids

Probably the  most common water-borne  coatings for
metals, air- or force-dry at temperatures below 194ฐF. A
wide range of coating formulations  fall into this broad
category. The most commonly available technologies

 are water-borne alkyds and modified alkyds, acrylic la-
 texes, and acrylic epoxy hybrids. Often, consumers are
 unaware of which of these technologies they are pur-
 chasing because manufacturers frequently sell the coat-
 ings as generic water-borne products. A brief description
 of the basic differences follows.

 Water-reducible, or water-thinnable, alkyds and modi-
 fied  alkyds are similar to the solvent-borne alkyds with
 which most companies are familiar. Like the solvent-
 borne coatings, they are modified polyesters but have
 high acid values and employ special chemical blocking
 agents such as carboxylic acid functionalities. When the
 alkyds are neutralized with ammonia or volatile amines,
 it is  possible to  use water as the reducing liquid. Al-
 though they may take longer to dry, the resulting coat-
 ings have gloss, flow, and leveling properties similar to
 their solvent-borne counterparts.

 The  acrylic latexes include other polymers such as vinyl
 acrylic and styrene acrylic. The resins are high molecu-
 lar weight polymers dispersed as discrete particles in
 water. Those formed by polymerizing a single monomer
 are called homopolymers, while those polymerized from
 a blend of two or more different monomers are called
 copolymers. Most of the latexes used to coat miscella-
 neous metal parts and architectural substrates are co-

 Latex coatings do not undergo a chemical change as
 they dry. The basic latex polymer and specific modifica-
 tions are responsible for the characteristics of hardness,
 flexibility, chemical resistance, abrasion resistance, and
 physical and chemical attributes. Acrylic latexes  are
 known for their good exterior durability and excellent
 resistance to ultraviolet (UV) degradation. In outdoor
 exposure, they retain their original gloss and color over
 long periods. In this regard, they are supetiorto unmodi-
 fied  alkyds, which tend  have  poorer gloss and color

 Manufacturers specify acrylic epoxy hybrids even less
 commonly than the other water-borne products.  These
 hybrids comprise two- or  three-package systems  in
which emulsified epoxies  cross-link with aqueous acryl-
 ics. Properly formulated coatings are corrosion resistant
 and  can produce finishes that have very good gloss,
 hardness, alkali, and abrasion resistance. Unlike con-
 ventional solvent-based epoxies, some mixed  water-
 borne  coatings have  pot-lives of up to 36 hours  at
 reasonable ambient temperatures.  End-users  prefer
acrylic epoxy hybrids for applications that require hard-
 ness, flexibility, and chemical resistance.  Advantages

As a generic group, water-reducible formulations, dis-
 persions, and  emulsions  are ideal for companies that
still need to comply with VOC regulations yet do not
 require their coatings have sophisticated properties. As
 a group, the water-bomes tend to have VOC contents
 well below 2.0 Ib/gal (240 g/L), less water, and some are
 even below 1.5 Ib/gal (180 g/L). Actual VOCs including
 water are usually below 1.25 Ib/gal (150 g/L), and this
 makes them an ideal choice for companies that have a
 need to dramatically reduce their VOC emissions.

 Generally, they exhibit good performance properties, but
 are probably not as durable or chemical- and solvent-re-
 sistant as  two-component polyurethanes, epoxies, or
 baked finishes. Facilities would consider them for appli-
 cations such as dipping primers and topcoats,  general
 purpose shop primers, and spray applied enamels. They
 are  suitable for coating steel, aluminum,  galvanizing,
 plastic, wood, and architectural substrates. In addition,
 they are available in a wide range of colors  and gloss

 Typical end-uses include steel roof trusses, steel  build-
 ing support structures,  farm implements (not combines
 or tractors), electrical  cabinets, boxes, frames,  fence
 posts, and similar general metal products. The electron-
 ics and  business machines industries currently use
 them to coat plastic computer housings, keyboards, and
 similar items. The architectural industry uses these coat-
 ings for interior and exterior walls, ceilings, concrete
 bridge structures, and  other commonly used masonry
 surfaces. In the industrial maintenance industry, water-
 bornes can be used to coat items  such as steel  struc-
 tures and hand rails provided that there is no exposure
 to chemical and solvent fumes or liquids. The coatings
 formulated  for architectural end-use differ from  those
 formulated  for industrial use. The latter are designed to
 provide metal parts with corrosion resistance.

 These water-borne coatings have a host of other advan-
 tages associated with their actual application. They can
 be spray-applied with standard equipment. In addition,
 they can be touched-up with self (i.e., with the very  same
 coating). And like their solvent-borne counterparts, they
 are available in a wide  range of texture finishes.

 Water-borne coatings also have safety and pollution-
 prevention  advantages. Because  of their high  water
 content  they pose a low  fire hazard.  Moreover,  they
 generally have lower toxicity because of the reduced
 concentration of organic solvents.

 Unlike solvent-borne coatings,  operators can  flush
 water-bornes from spray  hoses with tap water. The
 usual procedure for cleaning the hoses is to  flush with
water, follow with  solvent,  and follow again with water.
The  small amount of solvent is necessary  to clean out
dried or  non-water-soluble coating residues from the
 inside surfaces of the fluid hose.

-------   Limitations

 Most of the limitations associated with these  water-
 borne coatings relate to their performance. Compared
 with 2-part polyurethanes or baking water-reducibles,
 they have poorer exterior durability and poorer resis-
 tance to salt spray, humidity, chemicals, and solvents.

 In general, the coatings do not meet standards for high
 performance in industries such as heavy-duty mainte-
 nance, aerospace, appliance, and automotive. In addi-
 tion, many formulations require a greater learning curve
 with regard to viscosity  management compared with
 other compliant coatings.

 When applying the coatings in  humid or cold environ-
 ments, it is important for operators to force-dry them at
 a low oven temperature of approximately 120ฐ to  150ฐF.
 If no  oven is  available, consider blowing air  over the
 parts to promote the evaporation of water from the coat-
 ing film. Omitting this step can lead to a poor quality film,
 initially resulting in handling damage and possibly the
 early onset of corrosion and other premature failures.

 Although the lower concentrations of solvents in their
 formulations  benefit  pollution  prevention, this also
 causes these coatings to be more sensitive to substrate
 cleanliness than most solvent-borne coatings. Similarly,
 the clean-up process these coatings require  also  re-
 duces pollution, as described earlier. Unlike  solvent-
 borne coatings, however, operators must factor in a three-
 step process: water, solvent, water.

 10.3.3   Water-Borne Epoxy Water-Reducible
         Air/Force Dried Coatings

 Water-borne epoxy water-reducible air/force dried coat-
 ings have been available since the early 1980s and have
 a proven history of performance. These high perform-
 ance coatings most often are used as air/force dry coat-
 ings, where they can be cured at room temperature, or
 below 194ฐR Although many data sheets show they are
 available at 2.8 Ib/gal (340 g/L), less water, newer for-
 mulations are approximately 10 percent  lower. They
 provide a viable choice for companies wanting to signifi-
 cantly lower their VOC emissions without compromising

 Manufacturers supply these coatings as two- or  three-
 package systems. The most commonly available water-
 reducible epoxies  are formulated as primers complying
 with military specifications MIL-P-53030 (lead- and chro-
 mate-free)  and MIL-P-85582 (containing  chromates).
 Facilities can topcoat them with most other coating sys-
tems,  such as polyurethanes, particularly when requir-
 ing  good corrosion resistance. Companies that do not
 need to  comply with  military specifications can also
consider using these coatings because they are com-
patible with nonspecification topcoats. As with all high
 performance coatings, properly prepared surfaces are

 Because  epoxies tend  to  chalk when exposed to
 weather and sunlight, they usually do not serve as out-
 door topcoats. For interior exposure, however, such as
 the internal  linings of steel pipes  and vessels, pumps,
 and laboratory equipment, they can serve as both prim-
 ers and topcoats. As primers, they are commonly speci-
 fied for steel weldments, such as automotive chassis,
 cabs, truck  bodies, military hardware, steel and  alumi-
 num frames, cold rolled steel  panels and cabinets, aero-
 space components, and electronic components.

 An end-user should not implement this technology until
 after performing extensive on-line testing to ensure that
 the product  is compatible with production and perform-
 ance requirements.   Advantages

 The first advantage to note  regarding these coatings
 relates to pollution prevention. The VOC is below the
 RACT limit for all states, including California. This group
 of coatings serves as an ideal choice for a high perform-
 ance primer when emission'reductions are important.
 The VOC content is below  2.8 Ib/gal (340 g/L), less
 water, for the mixed product, and  is approximately 1.5
 Ib/gal (180 g/L), including water.

 These coatings also offer a range of choices. Primers
 are available in both chromate and non-chromate formu-
 lations. The  chromate-containing products offer improv-
 ed corrosion resistance compared with the nonchromate
 products.  The aerospace industry, for the most part,
 prefers to specify chromates, even though they are more
 toxic than the nonchromates and contribute to liquid and
 solid hazardous  waste. This preference derives from
 chromates'  improved  corrosion-resistant  properties,
 particularly with regard to filiform corrosion. The  Naval
 Air Systems  Command has written MIL-P-85582 to de-
 scribe this formulation.

 When applied to  aluminum substrates, or zinc  phos-
 phated steel, the nonchromate formulations apparently
 also perform very well, although they  are not recom-
 mended when filiform corrosion cannot be tolerated. The
 army has approved such  formulations, and has written
 MIL-P-53030 to cover them.

Water-borne epoxy water-reducible air/force dried coat-
 ings dry quickly,  even in highly  humid  environments
provided good ventilation exists. This means that recoat-
 ing with a polyurethane topcoat can take place as soon
as the water evaporates out of the film. Some facilities
 have followed intercoating time intervals of as short as
30 minutes,  although this  is not considered to be good
general  practice.  These  primers are compatible with
many types of topcoats, especially water-borne or sol-

vent-borne  polyurethanes. They are also compatible
with solvent-borne epoxy coatings.

Finally, these coatings have favorable viscosity proper-
ties. Although they have a limited pot-life, usually 6 to 8
hours or more, they retain low application viscosities for
a longer time than most low-VOC, high solids, solvent-
borne plural-component coatings. Also, because of their
low application viscosity, usually less than 20 seconds
on a Zahn #2 cup, operators can apply them with all
types of spray equipment, including conventional spray,
HVLP, and electrostatic spray guns. The low application
viscosities allow operators to apply the coating in dry film
thicknesses of 1.0 mil or less. This can be an advantage
over high solids,  solvent-borne epoxies for which such
low film  thicknesses  are  often difficult to achieve. In
addition, these coatings can be mixed with plural com-
ponent metering  and mixing equipment, but the end-
user  must  first  conduct  tests  to  confirm  that  the
viscosities prior to mixing are not so high that they cause
a materials handling problem.   Limitations

The  most important limitation  associated with water-
borne epoxy water-reducible air/force dried coatings in-
volves hazardous waste. The more corrosion-resistant
formulations contain chromates, and therefore require
disposal as hazardous waste. Some air pollution control
agencies  place severe restrictions on the emissions of
chromate-containing particulates (overspray) that spray
booth stacks emit into the air. A facility  operator can
overcome this problem by installing high  efficiency dry
filters,  but must carefully monitor them to ensure that
they do not violate the regulated limits. Chromates also
contaminate the water in water-wash spray booths.

Other limitations relate to mixing. The  coating comprises
two or three components and therefore requires mixing
prior to application. This automatically eliminates using
such coatings as  dip or flow coating  primers. Also, de-
pending on the formulation, mixing can be difficult if the
unmixed viscosities are very high. Some companies use
high powered mixers to mix components A and B. Other
companies restrict their mixing to very small quantities.
After adding water, the viscosity drops to manageable
levels.  End-users who wish to evaluate these coatings
should work with their vendors before selecting a prod-
uct. Evaluation should also take  into account the fact
that,  like all plural-component coatings, the product has
a limited pot-life.

Clean-up factors may complicate using these coatings.
Depending on the formulation, operators sometimes find
it difficult to clean equipment. In addition, removing coat-
ing from the skin (e.g., hands, face) of the operator can
be difficult.
 Finally, while the low application viscosities allow for low
 film builds, this can be a disadvantage when requiring
 higher film builds. For instance, when a  specification
 calls for a minimum film thickness of 1.5 mil, the operator
 may need to apply two coats of the water-borne epoxy
 primer in order to attain this value. The need for the
 second application is both time-consuming and costly.

 10.3.4  Polyurethane Dispersions

 Polyurethane dispersions are water-borne systems that
 can air/force dry at temperatures below 194ฐF. Essen-
 tially, they are polyurethane lacquers dispersed in water;
 therefore, as the  water  evaporates,  the  coating film
 forms. No other curing mechanisms take place. In fact,
 these coatings are completely reacted products with no
 free isocyanate groups, so after the water evaporates
 the film is as hard  as it ever will be.

 Apparently, very low VOC contents are possible. The
 technology, however, is relatively new and is still being
 tested by various  companies. While the  polyurethane
 dispersions can be useful on metal parts, much like the
 conventional two-component  polyurethanes, the  pri-
 mary focus at the present time is in the wood finishing
 industry.   Advantages

 These products are quite versatile. They can coat  met-
 als, textiles, leather, wood, glass, paper,  and rigid plastics.

 The viscosity profile of those coatings  offer several ad-
 vantages. For instance, because of their relatively low
 application viscosities, operators can  apply them  with
 the most commonly used equipment. Also,  operators
 can  modify viscosity  by  adding water. Clean-up  also
 requires water. These  coatings require very little, if any,
 solvent, and only very small quantities of coalescing aids.

 Another strength of these coatings  are the films  they
 produce. Coatings  made from polyurethane dispersions
 dry to tough films of dependable hardness and flexibility.
 Films dry to  predetermined  gloss and color  and, be-.
 cause these  films  do  not chalk, both  gloss and color
 retention are  excellent. In addition, like all lacquers, no
chemical change occurs  during drying and exposure.
Thus, the dry film retains  its original properties for  very
 long periods of time. Recoatability, such as for touch-up,
 is generally good and  like lacquers even aged coatings
can be recoated.   Limitations

The  limitations of polyurethane dispersion  products
 mostly revolve around  actual application factors. These
coatings  have problems typical of water-borne finishes:
drying time is dependent on temperature  and relative

 Regarding the film,  like most lacquers, the total non-
 volatile content is quite low (35 to 40 percent by weight).
 This  means that multiple coats may be necessary for
 any good film build-up. Also, unlike solvent-borne lac-
 quers, operators must ensure good intercoat adhesion
 because the topcoat  does  not  tend to dissolve any
 previous coats.

 Finally, surface cleanliness and freedom from any oil or
 grease both on the surface and in application equipment
 is essential for good film quality and adhesion.

 10.3.5  Water-Borne Baking Finishes—Alkyd,
         Alkyd-Modified, Acrylic, Polyester

 This group of coatings includes a wide range of prod-
 ucts. So many different combinations exist that the end-
 user  cannot assume that products available from one
 vendor are automatically similar to those formulated by
 another vendor.

 While these water-borne baking finishes are available at
 low VOCs, the technology generally is still at the RACT
 level  (for California).  For instance, formulations are
 available that satisfy 3.0 Ib/gal (360 g/L), less water," with
 some below the 2.3  Ib/gal (275 g/L) levels. The VOCs
 including water are in the 1.5 to 2.0 Ib/gal (180 to 240
 g/L) range. Compared with their air/force dried counter-
 parts, they have higher VOCs on both a "less water" and
 "including water" basis.
 What these varied coatings share is that they cure  at
 elevated temperatures, usually well above 250ฐF.  Many
 vendors recommend curing temperatures in the  range
 of 325ฐ to 350ฐF. Cross-linking occurs by formulating the
 basic resins with aminoplast resins such as melamine
 formaldehyde. Because of the high temperature curing
 requirement, these coatings are generally not appropri-
 ate for heat-sensitive substrates, such as plastics.

 Typical of other thermoset coatings, these products ex-
 hibit properties such as hardness,  mar and abrasion
 resistance,  and excellent color and gloss retention, even
 when exposed to sunlight, chemicals, detergents, and

Typical end-uses include large appliances, supermarket
 shelving, steel racks used for merchandise storage  in
stores and warehouses, metal office furniture,  metal
 laboratory and medical equipment, bicycle frames, light-
 ing fixtures, automotive and transportation applications
for components that can withstand relatively high baking
temperatures, computer main frames and metal  hard-
ware for the computer and business machines industry.  Advantages

Most of these water-borne baking finishes are available
at VOC levels that meet California's limits of 2.3 Ib/gal,
less water.  This is lower than the RACT limits of most
 other state regulations, usually 3.0 Ib/gal,  less water.
 Even with this favorable VOC profile, the performance
 properties of these coatings are often comparable with
 thermoset coatings,  such as two-component polyure-
 thanes and epoxies.

 The coatings also have several advantages relating to
 their actual application. Because they have lower volume
 solids contents (30 to 40 percent), operators can usually
 apply them at lower film builds than their solvent-based,
 high solids (greater than 60 percent) counterparts. This
 can be advantageous when film builds must be control-
 led at approximately 1.0 mil. In addition, operators can
 use standard equipment to spray-apply these products.
 And it is possible to touch-up with self.

 These coatings  are  also quite versatile. They can be
 applied on a wide range of  metal substrates, such as
 steel, galvanizing, and aluminum, all of which can tolerate
 the elevated baking temperatures. In addition, they are
 currently available in a wide range of colors, gloss lev-
 els, and textures. Moreover, these products can serve
 as primers and topcoats, and in some cases one-coat
 systems are possible, particularly if surface preparation
 includes a well deposited iron or zinc phosphate.

 Other  favorable  properties  resemble  those of  other
 water-borne coatings. For instance, they pose  a re-
 duced fire hazard and have lower toxicity than solvent-
 borne coatings.  Finally, like all water-borne coatings,
 operators require only small amounts of solvent for
 flushing out fluid spray hoses. The/perform the primary
 clean-up with tap water.   Limitations

 Many of the limitations associated with these water-
 borne baking finishes are  due to the high curing tem-
 peratures  they  require. These coatings must  usually
 cure at temperatures in the  range of  325ฐ to 350ฐF.
 Curing time is  inversely proportional to temperature.
 Facilities require high energy ovens, infrared lamps, or a
 combination in order to cure the coatings. Because of
 the high temperatures, the coatings are usually not ap-
 propriate for plastics,  wood, or other heat-sensitive sub-

 For similar reasons, facilities can rarely apply this group
 of coatings to large assembled machines that may al-
 ready  be  fitted  with rubber hoses,  hydraulic lines,
 leather, plastic upholstery, etc. Alternately, they can ap-
 ply  them to the metal components  before assembly
 takes place.

 Unless these baked  coatings go through the full cure
 cycle (i.e., for a specified time at a particular tempera-
ture), they do not attain their optimum properties. Some
formulations air dry to a dry-to-touch finish, making it
 difficult for the operator to easily determine (by sight)
whether or not the coating has been properly baked. To

 prevent uncured product from being shipped, therefore,
 facilities must include quality control tests in their daily
 production schedule.

 Another common problem of these products is outgass-
 ing and  pinholing, which occurs when coating porous
 substrates such as sand castings. This phenomenon is
 due to the expansion of air in the porous cavities  of the
 metal. To overcome the problem, facilities can first heat
 the metal to expel the air,  but this adds another step to
 the coating process.
 These coatings have other application-related complica-
 tions, as  well. Touch-up,  for example, may require a
 second bake  or  use of another coating. In addition,
 many companies  must prepare their metal surfaces with
 a minimum of  3-stage iron phosphate, although 5-stage
 iron or zinc phosphate is preferred.
 Finally, like other water-borne resin technologies, opera-
 tors may  require  a learning curve before being able to
 successfully apply these coatings. Also, like other water-
 borne coatings, problems such as edge pull and catering
 can occur, particularly if the viscosity of the coating is
 too low or the  surface is not sufficiently clean.

 10.4  Solvent-Borne Coatings

 10.4.1   Overview

 Although air pollution agencies actively promote water-
 borne coatings, all solvent-borne coatings cannot yet be
 replaced.  Some companies will  require solvent-borne
 coatings into the  21st century. Fortunately,  VOC con-
 tents are gradually decreasing, viscosities are becoming
 more manageable, and paint chemists continue work on
 developing new solvents that are not VOCs,  hazardous
 air  pollutants  (HAPs), or ozone depleting compounds
 (ODCs). These new solvents may offer a wide range of
 new opportunities.

 10.4.2  Solvent-Borne Alkyds and Modified
         Alkyds That Air or Force Dry

This group of  resin technologies has historically  been
the backbone  of the coatings industry. Prior to the  im-
plementation of the VOC  regulations,  these technolo-
gies probably accounted for well over 50 percent  of all
industrial coating usage.
Alkyd  resins  are  essentially oil-modified polyesters.
They are a combination or reaction between  an alcohol
and an organic acid. Typically, the alcohols and the  acids
they incorporate are phthalic anhydride, pentaerythritol,
maleic anhydride,  glycerine, ethylene glycol,  trimethylol
ethane, and trimethylol propane.

Vendors can join acids and alcohols in  various combi-
nations, and under very precise and controlled condi-
tions, to form a wide range of alkyd  resins. Each  resin
 or combination  has its own distinctive chemical and
 physical properties. In addition,  properties of alkyds
 such as hardness, gloss retention, color retention, sun-
 light  resistance, etc.,  can be improved by modifying
 alkyds with other resins. Typical modifications add sty-
 rene, vinyl toluene, acrylics, silicone, or other polymers.
 Any of these modified products  are  more commonly
 known as modified alkyds.

 Another way to modify the properties of alkyds is to have
 them react with oils. Depending on the ratio between the
 phthalic content and the oil content in the resin, the final
 product is known as a long-oil, medium-oil,  or short-oil
 alkyd. Long-oil alkyds are  commonly used for brushing
 enamels while medium- and short-oil  alkyds are used
 for spraying and fast-drying applications.

 Two more inevitable determinants of  course  are the
 vendor and the customer.  The coating  formulator
 chooses the appropriate resin or combination according
 to customer requirements. It is  also clear that  with so
 many possible variables in the formulation, the proper-
 ties of these coatings must differ from one vendor to the

 With the advent of the VOC regulations, coating formu-
 lators found this group of resin systems more difficult to
 reformulate into low-VOC  alternatives  than were other
 competing resin  technologies. Coatings that meet the
 PACT limits of 3.5  Ib/gal  (420  g/L) for air/force dried
 coatings, however, are readily available, as are some
 that meet the  California RACT limits of 2.8 Ib/gal (340
 g/L). They are associated with application  problems
 though that end-users must consider  before selecting
 such a coating.   Advantages

 First and foremost,  high solids compliant coatings are
 available at the RACT limit of 3.5  Ib/gal (420 g/L), and
 at this level they perform well. Alkyds and modified
 alkyds are also among the  least expensive of the VOC-
 compliant coating  systems. Compliant coatings  that
 meet California's RACT limit of 2.8 Ib/gal (340 g/L) are
 also available but are more difficult to apply.

The application of these coatings are  associated with
 several advantages. They  air-dry at ambient tempera-
ture, although some vendors recommend that their for-
 mulations be  force-dried  at approximately  150ฐF for
 better results. Because they are single-component coat-
 ings, they do not demand much of  a learning curve, nor
do they need to be mixed like plural-component coat-
ings. In addition, painters  spray-apply these products
using conventional air atomizing spray, airless, air-as-
sisted airless, HVLP, and the full range of electrostatic
spray guns. Finally,  touch-up is  easy to complete with
the coating itself.

 Facilities commonly use alkyds and modified alkyds as
 general-purpose shop primers for steel and other sub-
 strates, but not for zinc. Alkyd resins can be applied to
 most substrates including metals, wood,  masonry, etc.
 For some of these substrates painters  may  need to
 apply a  non-alkyd-based primer because alkyd resins
 tend to saponify. This is particularly true for galvanized
 and zinc-plated and zinc substrates.

 In addition, because alkyd resins can be modified  in so
 many ways, they are still among the most popular sys-
 tems to  use for general-purpose top coats. They are
 available in a wide range of colors and all gloss levels,
 and can produce a wide range of texture finishes. Fi-
 nally, they appear to be the preferred coating to use for
 many low-to-medium cost items {particularly  in cost-
 sensitive markets) or large machinery that cannot toler-
 ate high  temperature ovens.

 Regarding  the performance properties of these systems,
 they are similar to those of conventional solids alkyds.
 When high performance properties are required, how-
 ever, such as resistance to strong chemicals or solvents,
 or color and gloss retention in long-term sun exposure,
 other resin systems are usually more appropriate.    Limitations

 As stated earlier, alkyd coatings that comply with VOC
 regulations do exist. Historically, however, many compa-
 nies have  used  alkyds containing an exempt  solvent,
 particularly 1,1,1 trichloroethane (TCA), as a means of
 complying  with stringent VOC  rules.  In recent years,
 1,1,1 TCA has been listed by EPA as both an ODC and
 as a HAP. Moreover, 1,1,1 TCA will soon be phased out.
 Therefore, companies  should no longer  consider  this
 avenue for complying with RACT regulations.

 Although alkyds and modified alkyds have some advan-
 tages associated with their application, they also have
 several limitations. One of the primary limitations of the
 high solids formulations is their long ambient air-drying
 times (approximately 6 to 8 hours). This can be short-
 ened by  force drying. Some modified alkyds, however,
 do have  faster drying times but also have other limita-
 tions. For instance, some fast drying modified alkyds
 cannot be recoated within a window of time. To illustrate,
 the repair coating may not be compatible with  the first
 coat if the  painter applies  it within 2 to 10  hours after
 applying  the first coat. Compatibility is good if recoating
 occurs before 2 hours or after 10 hours. The "critical
 recoating time" varies for each formulation and depends
 on film thickness.

 In many  cases it is very difficult to achieve a  dry film
thickness of 1.0 mil. Minimum dry film thicknesses tend
to be in the range of 1.5 mil. This is particularly evident
on complex geometries, such as weldments,  assem-
 blies, etc.; therefore, by default more coating is applied
 than the target piece actually requires. Another problem
 associated with film thickness involves the inability to
 uniformly atomize many of the high solids formulations.
 This results in variations in film thickness which leads to
 inconsistent gloss and color.

 The viscosity of these coatings also often presents diffi-
 culties. They tend to exhibit higher viscosities than high
 solids polyurethanes of similar VOC content. In addition,
 some formulations require heating the coatings during
 spray application in order to adequately lower the vis-
 cosity for application.

 Performance limitations  are  also important to consider.
 These coatings tend to be relatively soft initially. Hard-
 ness  improves over a period of days to a final pencil
 hardness value of approximately HB. (Compare this with
 a pencil hardness of 3H to 6H for epoxies and polyure-
 thanes.) See Figure 10-2 for an illustration of this hard-
 ness  scale.
 In addition,  some alkyd polymers tend to have  limited
 resistance to  long-term  ultraviolet exposure. Chalking
 and color fading are prevalent. This can improve if co-
 polymerizing the  alkyd with  resins such as acrylics  or
 silicones.  The  end-user must therefore be aware that for
 good  exterior  durability  and resistance to sunlight, a
 modified alkyd will probably be necessary.

 Finally,  because of their poor alkaline resistance, facili-
 ties should  not apply these coatings over substrates
 such  as zinc and galvanizing, which tend to have an
 alkaline surface, particularly  if corrosion has formed al-
 kaline corrosion  products between the zinc and the
 organic coating.

 10.4.3   Alkyd Derivative Combinations That
         Cure by Baking

 This group of coatings includes high solids alkyds, acryl-
 ics, polyesters oil-free, melamine- and urea-formalde-
 hyde, and phenolics.  Unlike the air/force dry alkyds, this
 group of  coatings  provides excellent physical  and
 chemical  properties.  The primary  difference  is  that
 cross-linking of the resins takes place when the coating
 attains a certain minimum temperature. For most such
 coatings,  curing takes place at  temperatures  above
 250ฐF, but the curing time may  be  too long (over 30
 minutes) for most production  painting facilities. The cur-
 ing schedule is dependent on a time/temperature rela-
tionship, with curing times being inversely proportional
to the temperature. Because the curing time may be too
 long (over 30 minutes) at the lower curing temperatures
for  most facilities, those using these coatings tend to
cure the coatings for approximately 10 minutes at  350ฐF.

 High solids baking alkyds are cross-linked with stabi-
 lized aminoplast resins such as melamine- and urea-for-
 maldehyde.  These  initiate  cross-linking when  the
coating attains high temperatures (greater than 250ฐF).

         4B      3B

3H      4H
5H     6H
       Son      T
                            Epoxies, Polyurethanes
                          Baking Enamels
 Figure 10-2.  Hardness scale for solvent-borne coatings.

 These coatings have properties similar to water-borne
 alkyd-type baked coatings. As with the water-bornes,
 these solvent-borne counterparts are commonly applied
 to steel shelving, steel racks  used in stores and ware-
 houses, metal office furniture  and equipment, and large
 appliances (e.g., dishwashers, refrigerators, etc.).   Advantages

 This  wide range of  formulations is available  at VOC
 levels to meet most regulations, but usually do not drop
 below California RACT limits (which is 2.3 Ib/gal). They
 are available at 3.0 Ib/gal (360 g/L), and in some cases
 as low as 2.3 Ib/gal (275 g/L).

 As with most other compliant coatings, they are avail-
 able  in a wide range of colors  and gloss levels.  In
 addition, they  can be applied directly to  metal sub-
 strates, although they are not usually applied to heat-
 sensitive materials.

 These coatings also exhibit excellent performance prop-
 erties, such as good chemical and solvent resistance,
 hardness, mar resistance, good gloss, and good ultra-
 violet resistance (depending on the resin). They cure to
 excellent pencil hardness (2H), comparable with many
 epoxies and polyurethanes.

 Operators also experience benefits when applying these
 baking alkyd derivative combinations. Many cases re-
 quire no special application equipment. Moreover, be-
 cause of  their good adaptability to high-speed lines,
 these coatings are often applied with reciprocating elec-
 trostatic equipment, such as turbo bells and discs, on
fast-moving conveyor lines. Another application benefit
 involves film thickness. With proper controls, an opera-
tor can achieve uniform thin film thickness of approxi-
 mately 1 mil.   Limitations

 Most limitations associated  with  the  alkyd derivative
combinations involve  application  process  factors. For
instance, like all baking systems, these  require high-
temperature  ovens. They require baking at elevated
temperatures with schedules such as 45 minutes at 230ฐF
or 10 minutes at 350ฐF. These  high baking temperatures
                              preclude the coatings from being applied to plastics,
                              wood, upholstery, or other heat-sensitive substrates.

                              Another requirement associated with these coatings is
                              quality control procedures. These are necessary to ver-
                              ify that an acceptable coating has been applied. Quality
                              control  is especially important because  working with
                              them demands an operator learning curve.

                              The learning curve derives from several  factors. High
                              viscosities of some compliant formulations require spe-
                              cial  spray application  equipment. HVLP spray guns
                              need evaluation before implementing such  a coating
                              because atomization with some guns may  be somewhat
                              difficult to achieve.  Atomization improves by applying
                              the coating at fluid temperatures of 100ฐ to  110ฐF. Install-
                              ing an in-line heater can help accomplish  this.

                              Special care also is necessary during surface prepara-
                              tion because stains caused by  the spray washer clean-
                              ing process often "photograph" through the  coating
                              finish. As with many high solids coatings, operators may
                              find it difficult to achieve smooth finishes free of orange
                              peel. Another difficulty for operators is that some formu-
                              lations remain tacky at ambient temperature in addition
                              to leaving the walls and floors of spray booths tacky.

                              10.4.4   Catalyzed Epoxy Coatings

                              Catalyzed epoxy coatings constitute the counterparts to
                              the water-borne epoxy coatings, but can achieve heaver
                             film builds for many applications. For some applications,
                              such  as in  the industrial  maintenance  industry, the
                              higher film build is advantageous because  water-bornes
                             would require at least two coats to achieve the desired

                              Most commonly, these coatings are air- or force-drying,
                             two-component  materials  comprising two  separate
                             packages: component A is the epoxy resin; component
                              B  can  be  a  polyamine  (e.g., diethylene  triamine,
                             triethylene   tetramine,  or  tetraethylene  pentamine),
                             polyamide, polysulfide, or some other resin. For colored
                             finishes, component A usually contains the  pigments
                             and other additives.

                             In the case of baking epoxy coatings, which cure during
                             a high temperature bake of usually above 140ฐ to 400ฐF

 the coating manufacturer preblends the two resins and
 supplies them as a single-component package. Exam-
 ples include blends of epoxy resin with amino,  urea
 formaldehyde, or melamine formaldehyde resins.  Only
 when the applied coating attains an elevated tempera-
 ture do the two resin systems react to form the cured

 Catalyzed epoxies are beneficial when requiring resis-
 tance to many chemicals, solvents, and alkalies,  such
 as soaps and  detergents.  In addition, these coatings
 have excellent resistance to fresh water, salt water, and
 hot water. For these reasons they are a popular choice
 for protecting structures such as off-shore drilling  plat-
 forms, ships, and bridges, where  resistance to marine
 environments is critical. Facilities also use them to coat
 industrial and potable water tanks and pipelines.

 Several coating vendors supply VOC-compliant primers
 and topcoats for the general metals and plastics indus-
 tries.  Depending on the  application, VOC  contents
 range from 1.4 to 3.5 Ib/gal (168 to 420 g/L), but because
 they can be difficult to atomize, controlling film thickness
 can be problematic.  Of course,  the high film thickness
 that the low-VOC epoxies provide can be advantageous
 for some  maintenance  applications  (e.g.,  bridges,
 chemical plants). By far, the majority of these coatings
 fall at the higher, end of the range  (closer to 3.5 Ib/gal).
 Compliant epoxies  are available that meet military
 specifications such as MIL-P-23377  (primer),  MIL-P-
 53022 (ptimer), MIL-C-22750  (topcoat), and  MIL-P-
 24441 (primer and topcoat systems).

 When  a decorative,  corrosion- or chemical-resistant
 coating system is necessary, such as for bridges, chemi-
 cal refineries, or off-shore drilling equipment, companies
 usually use epoxy coatings as the primer and undercoat,
 and then apply a more UV-resistant topcoat such as an
 acrylic or polyurethane.   Advantages

 A great advantage of catalyzed epoxy coatings is that
 their VOC contents  meet the PACT limits.  In many
 cases, however, they are not as low as some of their
 water-borne counterparts. So, pollution prevention con-
 siderations suggest using water-bornes when possible.

The other advantages of these coatings relate to their
 performance. In  general, epoxy coatings are known for
their toughness, flexibility, and excellent adhesion  to a
wide range of substrates. These include most metals,
 plastics,  wood, ceramics,  masonry, glass, and more.
 Understandably, therefore, epoxies are a popular choice
as primers. Importantly, they are the preferred choice as
a primer  under polyurethanes.

 In addition, companies can obtain  improved toughness
and flexibility by reacting epoxy resins with polyamide
 resins. Unlike the  polyamines (which are more com-
 monly used in the industrial maintenance industry), they
 do not cause severe dermatitis in the operators, and
 their pot-life tends to be longer.   Limitations

 One of the most notable weaknesses of epoxy coatings
 is their relatively poor resistance to ultraviolet light. For
 instance, when exposed to sunlight many epoxy coat-
 ings tend to chalk quite readily, which causes them to
 lose gloss and  color.  Although chalking  takes  place
 primarily at the surface of the film, it does not signifi-
 cantly affect the chemical properties of the coating. In
 fact, the coatings are often so resistant that operators
 may find it difficult to strip coating from damaged, coated

 Another important concern is safety. Painting operators
 must wear proper protective  clothing and appropriate
 respirators during the mixing and  application of the
 coating. If they do not and if the unreacted amine comes
 into contact with their skin or is inhaled, the operators
 can experience severe dermatitis and other health ef-
 fects. Operators must therefore follow stringent safety

 Other procedures that facilities must follow involve haz-
 ardous waste. As with all plural-component coatings,
 facilities must dispose of any unused, mixed coating as
 hazardous waste. Although facilities can minimize the
 amount of this waste by using a plural-component me-
 tering  and mixing device, this  option is usually only
 cost-effective when using large daily quantities.
 Most limitations  of these  coatings, however, relate to
 application  process  factors.  For  instance,  epoxy-
 polyamine coatings have a relatively short pot-life and
 must be used within a short time after  mixing the two
 components. As the solids content increases, the pot-life
 usually shortens. For many formulations, a pot-life of 4
 to 6 hours or less at ambient temperature is common.
 Manufacturers' technical data sheets provide further de-
 tails. Some formulations also require an induction period
 of 20 to 30 minutes after  mixing the two components,
 before the coating can be applied. Fortunately, the
 newer formulations are more forgiving and do not re-
 quire such an induction time, but the operator must first
 confirm this, of course, with the coating vendor.
 An important limitation is that operators should not apply
 epoxies at low ambient temperatures (less than 50ฐ to
 60ฐF) because they will not cure properly. Another chal-
 lenge for operators is film thickness. As with most high
 solids coatings, it can be difficult to achieve dry films of
 less than 1.5  mil, particularly when coating complex
 Other concerns involve cleaning requirements both be-
fore and after using epoxies. Epoxies are more forgiving
than most other resin technologies to surface prepara-

 tion. They still, however, must be applied to a clean,
 well-prepared surface. And, as with all plural-component
 coatings, application  equipment requires cleaning be-
 fore the coating starts to set.

 70.4.5  Catalyzed Two-Component

 Polyurethanes are a type of coating formed by the reac-
 tion of a polyisocyanate with a polymer that contains
 hydroxyl functionality. Vendors  supply two-component
 polyurethanes in two separate containers, of which the
 first is component A and the second is component B.

 Component A can either  be clear  or pigmented and
 offers a  wide range  of colors  and gloss  levels. The
 primary resin (polyol)  is usually an acrylic, polyester, or

 The second container, component B, is the curing agent
 and contains a multifunctional,  pre-polymerized isocy-
 anate. When end-users mix components A and B ac-
 cording to the manufacturers'  prescribed  ratios, the
 polymers react to form a highly cross-linked polyurethane.

 When end-users require a two-component polyurethane
 with excellent chemical resistance, they often choose a
 polyester polyol fqr component A. When exterior dura-
 bility and sunlight resistance are of greater importance
 than chemical resistance, they more commonly opt for
 an acrylic pdlyol for component A.

 Facilities select polyurethanes for applications requiring
 a superior finish. The aerospace industry commonly
 uses them on items such as missiles, aircraft skins, and
 other aerospace components. In the transportation in-
 dustry, they appear on buses, over-the-road trucks, rail
 cars that carry chemicals and solvents, automotive re-
 finishing, as well as on some newly manufactured auto-
 mobiles. The Army, Navy, and Air Force use the coatings
 extensively on military ground support equipment such
 as tanks, personnel carriers, vehicles, etc., in which
 resistance to live chemical agents (CARC) is imperative.
 Polyurethanes are also used in the industrial mainte-
 nance, architectural, and wood furniture industries. In
 addition, they are popular for high-end consumer prod-
 ucts such as machine tools, garden lawn mowers, snow
 blowers, tractors, etc.   Advantages

One of the most compelling features of these coatings
is their VOC profiles.  Most vendors  of two-component
polyurethanes can  supply formulations at or below the
RACT limit for California, which is 2.8 Ib/gal (340 g/L).
Higher VOCs may be necessary for some  automotive
refinishing colors and clears. Even these, however, are
in compliance with  RACT limits for other states.
 In addition to a positive VOC emissions profile, two-
 component polyurethanes have attractive performance
 characteristics. These coatings are known for their ex-
 cellent physical film performance: abrasion resistance,
 toughness, and hardness up to pencil hardness of 6H.
 Moreover, of all the resin technologies available, they
 rank among the best for resistance to most solvents and
 chemicals. Finally, they exhibit excellent outdoor dura-
 bility  (primarily the aliphatic  polyurethanes),  and are
 therefore popular for most of the transportation industry.
 Adding to their popularity is the fact that they offer a
 complete range of gloss and texture levels.

 The process of applying these coatings also includes
 several advantages. First, they can be directly applied
 to steel, aluminum, plastics, composites, wood, ma-
 sonry, and other material. In most cases, however, op-
 erators apply them over an epoxy primer. Polyurethanes
 also can cure at ambient (room) and elevated tempera-
 tures. They can even be used in under sub-zero condi-
 tions, unlike epoxies.

 Because of the relatively low viscosities that the low-VOC
 polyurethanes exhibit, operators can spray-apply them
 with standard equipment. This includes conventional air
 atomizing, airless, air-assisted airless,  HVLP, and elec-
 trostatic  equipment. Unlike most other solvent-borne
 high solids coatings, the automotive refinishing industry,
 which  demands  good-looking finishes for customer
 acceptance,  is currently  using  high  solids  polyure-
 thanes. Moreover, these coatings are available in a wide
 range of solid and metallic colors with quick turnaround.
 This includes the availability  of on-site intermixing of
 colors, predominantly for  automotive  refinishing. An-
 other benefit for operators is that self touch-up  is possi-
 ble with these coatings.   Limitations

 Many of the limitations of two-component polyurethanes
 reflect those  of other plural-component coatings. For
 instance, two-component systems require mixing in pre-
 scribed  proportions. Plus, as with all two-component
 systems, they have a  limited pot-life. For some high
 solids polyurethanes, this can be less than 4 hours. In
 addition, like many high  solids coatings, it can be difficult
to achieve a uniform film thickness on complex shaped
 parts. This problem, however, is  not  as  severe with
 polyurethanes as with other high solids resin technologies.

 Cleaning requirements  also resemble those of some
other systems. Equipment requires cleaning before the
coating begins to set. Finally,  like most other coatings,
operators must apply  two-component polyurethanes
over clean, pretreated surfaces.

Two-component polyurethanes also have more unique
limitations. These coatings are expensive relative to
competing technologies, but their enhanced properties

 usually offset this cost. In addition, paint operators must
 use appropriate  respirators because polyisocyanates
 sensitize a small percentage of the population  that
 comes into contact with them. End-users must consult
 their coating  vendors for more detailed information.

 10.4.6  Moisture Curing Polyurethanes

 Moisture curing  polyurethanes  have  an  interesting
 mechanism. When  a polyhydroxy resin pre-reacts  with
 a polyisocyanate, but not completely, some unreacted
 isocyanate groups remain. The coating then cures in the
 presence of moisture from the air. Such materials are
 called moisture curing polyurethanes. Vendors supply
 this type of coating in one package, the second compo-
 nent being atmospheric moisture.

 Although many would prefer single-component polyure-
 thanes to two-component products, few companies cur-
 rently  sell moisture curing  polyurethanes.  This  is
 because their manufacture is considerably more com-
 plex than two-component products.  The complicating
 issue  is that  manufacturers  must eliminate moisture
 from all ingredients they use.

 Currently, the major supply of these coatings goes to
 military bases and military contractors who use camou-
 flage moisture curing polyurethanes as the exterior coat-
 ing for army  tahks, personnel carriers, cranes, jeeps,
 and similar material.
         tf   Advantages

 Moisture curing polyurethanes are desirable for several
 reasons. They possess no pot-life limitations because
 they are single-component products. They do, however,
 retain all of the performance advantages of two-compo-
 nent polyurethane coatings.  Moreover,  they  achieve
 chemical-resistant properties  more quickly than some
 two-component polyurethanes.

 Although availability is scarcer than other types of coat-
 ings, the army has written specification MIL-C-53039
 around the camouflage moisture cure polyurethane,  and
 VOC-compliant coatings  are  available.  Commercial
 coatings are also  available in a limited range of colors,
 but end-users may need to shop extensively to find  a
 coating satisfying  their needs.   Limitations

 Unlike  two-component polyurethanes, currently only  a
few companies supply MIL-spec approved camouflage
coatings, and this is also true for commercial colors. Like
the two-component  coatings, however, the operators
 must wear appropriate clothing and take similar health
and safety precautions.

Regarding  the application process,  moisture curing
polyurethanes are very sensitive to moisture contamina-
 tion and therefore require special effort to keep moist air
 from the  packaged or stored coating. In addition, the
 fluid hose leading to the spray gun and the headspace
 above the coating in the pressure pot or reservoir must
 remain free of moisture. Many companies use a nitrogen
 blanket or a desiccant to keep the headspace dry. Dry-
 ing time also  is affected by moisture in air. In very dry
 climates,  the drying time may be longer than usual.

 10.5  Specialized Coatings

 10.5.1   Overview

 This section  discusses several specialized coatings.
 These are:

 • Autodeposition

 • Electrodeposition

 • Radiation Cured Coatings

 • Supercritical CO2

 • Vapor Injection Cure (VIC)

 Each of these technologies has a narrow window of
 applications.  For some end-users, one of these tech-
 nologies will  be the ideal choice. They are, however,
 unlikely to make a significant penetration into the total
 coatings market.

 Regarding the VOC emissions profile of these systems,
 with the exception of UV Curables and some autodepo-
 sited coatings, none of the others technologies is likely
 to soon have VOC contents that approach zero.

 Autodeposition and electrodeposition, however, do have
 favorable  pollution minimization profiles. They have ma-
 jor advantages concerning VOC emissions as well as
 the disposal of  hazardous waste and water pollution.
 Both technologies have low VOC emissions and when
 properly   operated,   generate  essentially  no  liquid
 wastes. Also,  because of the sophistication of these
 processes, water pollution is minimal.

 UV Curables, Supercritical CO2, and Vapor Injection are
 all spray  application  processes. If an  operator can
 achieve low transfer efficiencies, hazardous waste will
 be much the same as for any other liquid coating proc-
 ess this chapter describes. If any of these five technolo-
 gies are applied in  a water-wash spray booth, water
 pollution generation will also be the  same as for the
 other liquid coating technologies.

 10.5.2  Autodeposition

 Predominantly  large  coating  users,  whose annual
throughput of  metal is at least 1,000,000 square feet,
would find this process cost-effective. Moreover, such
companies are usually aware of this technology be-
cause they are sufficiently large to have on staff materi-

 als or coating engineers with access to the major coating
 technologies. Autodeposition  is generally not a viable
 option  for  small  or  medium-sized  coating  users.
 Parker+Amchem  Corporation  of  Madison  Heights,
 Michigan, is the sole source for this technology.

 During the autodeposition process, a resin in the form
 of a  latex  is electrochemically deposited on steel sur-
 faces. Unlike electrodeposition, however, the deposition
 does not require an electric current.

 The process is currently  limited to steel, but the steel
 does not require pretreatment with a phosphate coating
 like iron or zinc phosphate. While the process can elimi-
 nate  phosphating, it still requires superior cleaning that
 may comprise several stages.

 The process includes at least the following:

 • Alkaline  spray clean (1  minute)

 • Alkaline  immersion (2 minutes)

 • Plant water rinse (spray or dip)

 • Deionized water rinse (spray; 5 to 10 seconds)

 Operators then immerse  the steel  part in the coating
 tank at 68ฐF for approximately 60 to 90 seconds, after
 which it stands in 'air for a  brief period to  allow the
 coating reaction to continue.  Thereafter, at  least two
 more rinse stages follow. The first rinse involves immer-
 sion in tap water and the second is a non-chromate seal
 or a deionized water rinse.

 Depending on  the resin system used, the  steel then
 enters either a  two-zone  or a single-zone oven. The
 curing temperature may either be 284ฐ to 356ฐF or 210ฐ
 to 230ฐF, depending on the resin.

 The coating consists of a pigmented water-dispersible
 (latex) resin, hydrofluoric acid, hydrogen peroxide, and
 deionized water. No solvents  are used in the coating
 process. The coating has a very low-volume solids per-
 centage of approximately 3 to 10 percent, and because
 of the hydrofluoric acid, the pH is in the range of 2.6
 to 3.5.

 The coating  can act as a primer and topcoat in one
 application,  providing  excellent  salt spray  resistance.
 Alternatively, it  can serve  as a primer  that companies
 can overcoat with a wide range of coatings such as
 alkyds, epoxies, polyurethanes, etc.

 Coating thickness is a function of bath solids, viscosity,
density, and  temperature.  As  the immersion time in-
creases, the coating thickness increases. Because of
this process, the coating can deposit on  all surfaces that
come into contact with the solution. It can deposit in
 holes, crevices, and otherwise inaccessible areas.
 Although autodeposited coatings have limitations, they
 have a place in industry. They are currently used primar-
 ily for under-the-hood automotive applications, including:

 • Leaf and helical springs

 • Axle housings

 • Lamp  housings

 • Engine mounts    Advantages

 The primary advantage of the autodeposited coating is
 that VOC emissions are extremely low and, depending
 on  resin,  may  even  be zero. Also,  according  to
 Parker+Amchem, the coating is non-toxic and a very
 dilute solution can  be disposed of easily. In addition,
 these coatings generate very little hazardous waste and
 pose little or no fire hazard. The  very high transfer
 efficiencies (greater than 98 percent) that the efficient
 deposition process  allows also contributes to pollution

 In addition to its favorable pollution  prevention charac-
 teristics,  the coating conveys performance advantages.
 It is associated with excellent corrosion resistance and
 can also  have excellent flexibility and impact resistance.
 Its hardness is beneficial for many applications, meas-
 uring at a pencil hardness of between 2H and 5H.

 In addition, operators can achieve a uniform coating film
 thickness (0.6 to 1.0 mil), which contributes to its uniform
 appearance. This process coats all cut edges and high-
 energy areas, which makes it ideal for fasteners. In this
 regard, it is even  more efficient than electrostatic liquid
 spray painting applications

 Basically, autodeposition avoids runs, sags, or similar
 defects associated  with other organic liquid coatings,
 with the  exception of electrodeposited coatings. As a
 primer, the  performance of autodeposited coatings is
 apparently comparable  with that of powder coatings,
 electrodeposited coatings, and polyurethanes.

Applying  the process  is also associated with  benefits.
Operators can immerse assemblies comprising steel,
 plastics, and rubber  in the various stages of the process
without affecting the non-metallic, heat-sensitive com-
ponents.  Only the steel will be coated. In addition, al-
though the  process requires thorough degreasing  of
steel, it does not require phosphating. Nor is an external
electric current necessary in order to deposit the coat-
ing. This  is the primary difference between autodeposi-
tion and electrodeposition.

Autodeposition offers an excellent method for applying
uniform coating inside tubular steel and otherwise inac-
cessible areas.  In fact, if immersing a nut-and-bolt as-
sembly in  an autodeposition  tank,  the  process will
properly coat the  internal threading surfaces  between

 the nut and the bolt. This phenomenon is not possible
 with electrodeposition. Another useful feature of autode-
 position is that operators can topcoat it with most or-
 ganic liquid coating systems.

 It also  does not always  require high  temperatures for
 curing.  A low-temperature cure  at 200ฐ  to  250ฐF
 achieves fully cured properties immediately. With some
 resins curing temperature may be higher, between 284ฐ
 to 356ฐF. Infrared drying, however, is possible. Convec-
 tion ovens are not necessarily required.   Limitations

 A major limitation of autodeposition  is that it is only
 suitable for steel substrates (cold or hot rolled). It is not
 appropriate for aluminum, zinc, plastics,  rubber, etc.
 Even if coating steel, however, autodeposition  is in-
 tended  only for large production shops with high steel
 throughput. The process would not contribute efficiently
 to low  volume coating  facilities or those that coat a
 multitude of component configurations.

 Surface cleanliness with this process is critical. Excel-
 lent  degreasing may be necessary. The system may
 include up to seven separate cleaning or rinsing stages,
 most of which  use immersion.  Largely because of all
 these stages, autodeposition requires significant space
 allocation when compared with  unsophisticated  liquid
 spray coating lines.
 Other  labor  intensive drawbacks  also  exist.  For in-
 stance, hanging parts is important  to achieve reliably
 uniform appearance on all parts. In addition, autodepo-
 sition requires frequent bath monitoring.
 Finally,  choices are minimal when ordering  materials.
 Currently, Parker+Amchem Corporation constitutes the
 only provider of these coatings. Also, most colors avail-
 able are black and greys.

 10.5.3   Electrodeposition

 As with autodeposition, predominantly large coating us-
 ers whose annual throughput  of  metal  is  at  least
 2,000,000 square feet, would find electrodeposition cost
 effective. Again, such companies are usually well aware
 of this technology. Similar to autodeposition, this proc-
 ess deposits the coating electrochemically onto the met-
 al surface. Electrodeposition  requires, however,  an
 implied  DC current to carry out the process.

 Metal parts pass through  a  multistage cleaning and
treating process. Unlike  autodeposition, however, thor-
 ough cleaning precedes  a multistage zinc or iron phos-
 phate process,  which might include a  chromate or
chromic acid seal rinse and at least one deionized water

The  next step then immerses the  metal parts  in the
process coating tank containing the coating (5 to 20
 percent solids dispersed in water). The workpieces are
 connected to  a DC  power supply and, depending on
 whether the process is anodic or cathodic, they will be
 charged either positively  (anode)  or negatively (cath-
 ode). This creates a  strong electric field in the tank.

 The electric field causes the coating with an opposite
 electrical  charge to deposit on the metal surfaces. As
 coating deposits uniformly, it covers, and thus, begins to
 isolate the parts from the electric field.  This process
 diminishes the strength of the electric field, which, in
 turn, slows down the coating process. When coating has
 totally covered the workpiece, no  charged part is left
 exposed. This reduces the electric field around the work-
 piece to zero,  and no more coating can deposit.

 From the  coating tank the workpieces pass through at
 least one deionized  water rinse tank that washes off
 excess unreacted coating. They then travel to a baking
 oven that cures the coating at 275ฐ to  375ฐF for 15 to
 30  minutes. The excess  rinse water/coating that the
 rinse tank recovers passes through an ultrafiltration unit
 that concentrates coating while recycling the water for

 From an  environmental perspective, electrodeposited
 coatings have approximately the same VOC content as
 conventional baking  water-borne coatings. Hazardous
 waste disposal  and  the  discharge  of contaminated
 water, however,  are considerably less.  Because of the
 environmental benefits  of electrodeposited coatings,
 EPA favors this technology over most other water-borne
 liquid coating technologies.
Typical applications include:

 • Truck beds

 • Engine  blocks

 • Water coolers

 • Microwave ovens

 • Dryer drums

 • Compressors

 • Furnace parts
 • Housings for the automotive industry

• Shelving

• Washers

• Air conditioners

• File cabinets

• Switch boxes

• Refrigerators

• Transmission housings

• Light fixtures

 • Farm machinery

 • Fasteners   Advantages

 Electrodeposition has excellent pollution prevention ad-
 vantages.  Because of its recycling ability, the process
 can achieve very high transfer efficiencies, greater than
 98 percent. It also uses a low concentration of coating
 dissolved in water (5 to 20 percent solid dispersion  in
 water); therefore, minimal  solvent emissions generate
 from the tank. In fact, electrodeposition is associated
 with low hazardous waste  and, in most cases, no dis-
 charge of contaminated water. In addition, because the
 coatings are water-borne, the process poses a low fire

 Electrodeposition also has many performance strengths.
 Electrodeposited coatings can be applied to steel, gal-
 vanized steel, and aluminum. With all these, excellent
 uniform finishes are possible without runs, sags, etc.
 Also, because the process requires an electric field to
 promote deposition, it can achieve'excellent uniform film
 thicknesses (approximately 1.0 mil). All sharp edges and
 cut ends become  coated because the electric charge
 focuses at these points.
 In addition, electrodeposition imparts excellent hard-
 ness (F-24) and good flexibility. The  coating film also
 provides excellent corrosion and chemical resistance.
 Because of the high quality of coating and its inherent
 hardness and abrasion resistance, reject rates are low.
 Another attractive  feature is the extremely high gloss
 these coatings can provide. Because of this, automotive
 finishes are quite possible. Some coatings even act as
 both primer and topcoat in a one-coat finish.

 Electrodeposition also has  several advantages associ-
 ated with  the  application process. Primarily,  the auto-
 mated nature of this process entails low labor require-
 ments. Another attractive feature  is that with primers
 applied by  electrodeposition, operators  can top-coat
 without sanding.

 Finally, with electrodeposited coatings, choices are not
 limited. They are available in epoxy/urethane hybrids
 and other hybrids. The coatings also are available in  a
 wide range of colors, although operators would apply
 large runs of only one color at a time.   Limitations

Although electrodeposition has some benefits regarding
 its application, most of its limitations are also applica-
tion-related. Compared with other spray applied coat-
 ings, electrodeposition is a sophisticated coating process.
 It is generally not a viable  process for small and me-
dium-sized companies that either do not have sufficient
throughput of material to justify the process, or manu-
 facture workpieces of too many sizes and shapes. Fa-
 cilities must invest very  high capital expenditure for
 cleaning  and  pretreatment  systems, coating tanks,
 oven, etc. For  large facilities, the cost-effectiveness of
 the operation can offset these expenses.

 The entire  process has many requirements. First,  it
 requires large floor space. It also requires proper system
 design to  ensure that all hidden and inaccessible areas
 are coated.  The coating process itself is very sensitive
 to cleanliness of  the substrate. Then, the coating re-
 quires baking for 15 to  30 minutes at 275ฐ to 375ฐF.

 Finally, facilities cannot use  electrodeposition to  coat
 plastics or other electrically non-conductive substrates.
 It also is not appropriate for multicolor finishing require-
 ments; generally, the process works best when a com-
 pany uses only one or two colors in its product line. This
 is because a company must usually dedicate a separate
 tank to a  single color. Floor space and cost limitations
 may prohibit a  company, therefore,  from having many

 10.5.4  Radiation Cured Coatings

 These unique coatings cure when they are exposed to
 specific wavelengths of ultraviolet (UV) or electron beam
 (EB) radiation.  Like the other specialized coatings, ra-
 diation cured coatings constitute the ideal choice for a
 very narrow niche  of the overall coatings market.  This
 manual includes them because their VOC emissions are
 very low, even approaching zero for some formulations.
 The coatings have low  VOC emissions because curing
 takes place without the need for solvents to evaporate.

 Because UV irradiation is low energy, the polymers of
 UV curable coatings contain special photo-initiators to
 promote cross-linking. The chemistry of the photo-initia-
 tors can be controlled  through the  concentration  and
 type of formulation. EB coatings, on the  other hand,
 require a high energy source so that the polymers can
 cross-link  without the need for photo-initiators.

 The primary resins used in UV and EB curable coatings
 are multifunctional acrylates, acrylated oligomers,  and
 monofunctional  diluent monomers. As individual, unre-
 acted resins, diluent monomers are considered VOCs if
 they are allowed to evaporate. As the curing process
 takes place, however, they participate in the cross-link-
. ing  reactions and form  part of the  solid coating  film.
 Thus, while they qualify as VOCs in the unreacted state,
 they are not VOCs during the curing process. EPA has
 recognized that most of these reactive diluents are not
 emitted into the air during the coating process.

 While EB  coatings receive energy  from an  electric
 heated filament or cathode, low pressure mercury arc
 lamps generate  the energy to cure the UV curable coat-
 ings. In order to ensure a consistent film cure, the mer-
 cury arc lamps must sit within a few inches of the coated

 substrate. This is why the substrate must have a very
 simple geometry, such  as a flat or uniformly  round
 shape. For instance, UV curable coatings are applied
 primarily to flat metal stock,  and serve  as  the clear
 coating on coated screen or printed metal signs. Clear
 coatings are used as overprint varnishes  on  beverage
 cans,  aerosol  cans, lipstick  containers,  and  similar
 items. Adding colored pigments to the formulation  re-
 tards curing and  extends curing times; therefore, most
 of the coatings being used are clear.

 The rest of this discussion on radiation cured coatings
 focuses exclusively on UV curables  because of their
 predominance  in the paints and coatings  industry. EB
 coatings are usually used in applications  such as the
 manufacture of printing inks.

 With few exceptions, facilities do not use radiation cured
 coatings extensively on general metal  parts unless they
 have a very simple geometry. The wood furniture indus-
 try is beginning to try these coatings but application is
 still limited. As researchers continue to develop curing
 ovens (including the lamps) that are more forgiving to
 three-dimensional applications, these coatings will un-
 doubtedly find numerous other applications.  Advantages

 Radiation cured boatings have several pollution preven-
 tion benefits. Coatings are available with  zero or very
 low VOC contents. Vapors from the process (e.g., from
 photo-initiator, surfactant, burn-off, etc.) are easily ex-
 hausted with no  measurable air quality damage.  Ex-
 haust  of  irradiated  cooling  air  also assists  heat
 management and ozone disposal.

 The unique curing process, of course, conveys many
 advantages.  First, extremely short curing  times, often
 less than 5 seconds are possible. This feature makes
 radiation cured coatings ideal for fast moving production
 lines (i.e., conveyor speeds of several hundred feet per
 minute). In fact, almost unlimited production speeds are
 possible when using efficient UV radiation at watt levels
 of 1,000 to 1,200 watts per square inch.

 As stated, UV curing usually relies on medium-pressure
 mercury vapor lamps. Lamps emitting energy levels of
 several hundred watts/inch are available. At least one is
 also available  that  emits an energy level  of 1,000
 watts/inch. Another available energy source even cures
 photo-initiated chemistries instantly.

 The distance from the substrate to the UV source be-
 comes less of a consideration when sufficient UV energy
 is available. High UV energy can be applied to most
 substrates without heat damage.

 Curing efficiency often relies on  focusing the energy
towards the substrate by means of reflectors. Reflectors
 can be elliptical, parabolic, or planar. They.must have
 good thermal stability.

 The performance of these coatings is versatile. By ad-
 justing the formulation, an operator can modify viscosity,
 hardness,  abrasion  resistance,  adhesion,  flexibility,
 gloss, solvent resistance, and color. A key performance
 feature is their excellent adhesion to many substrates.

 Facilities commonly use these coatings on flat-stock or
 uniformly round products. Examples include paper web,
 large decals, wood  panelling, fiberboard, aluminum sid-
 ing for interior or exterior exposure, coated coil products,
 cosmetic bottles, lipstick dispensers, compact discs, etc.
 The coatings can be applied to many plastics although
 checking the application is necessary to verify that the
 plastic has not embrittled.

 Radiation cured coatings are readily available in clear
 finishes, and are now being explored for wood furniture.
 European furniture manufacturers  have  been  using
 them for several years. Conversion is underway in  the
 United States.  Limitations

 Safety is a major concern with radiation cured coatings.
 Vapors  from the coating application, process can be
 hazardous, and  the system design  must minimize  op-
 erator exposure. Operators should wear respirators with
 organic vapor cartridges that have  been approved by
 the  National  Institute for Occupational  Safety and
 Health  (NIOSH). Operator protection considerations
 must account for:

 • Eyes

 • Lungs

 • Skin (which  one can wash with citrus based cleaner)

 These coatings also are quite limited in their applicabil-
 ity. They are not yet  applicable to all shapes, and will not
 be  until an  energy source can irradiate  all  surfaces
 equally with the correct intensity of energy. The technol-
 ogy is not suitable for substrates with inaccessible ar-
 eas, blind holes, crevices, and other areas not in direct
 exposure to the energy source. In addition, operators
 are limited as to coating thicknesses. Thicknesses of 0.1
to 0.5 mil are common.  Thicker films may be more
 difficult to cure within a short duration.

Also problematic is the specific equipment and process
 requirements these systems must have. They require
 special ovens and energy sources. The distance of the
 energy source to the coated part must be within speci-
fied tolerances. The  lamps are sophisticated. The reflec-
tor  must be protected from heat  and other process
vapors. At the  same time, the set-up of the lamps must
optimize the energy distribution in the coating. Opera-
tors must be careful not to unnecessarily heat up, and

 especially not to overheat, the substrate. If any deterio-
 ration occurs, however, it is likely due to monomers in
 the chemistry formulation, not overexposure to UV.

 70.5.5   Vapor Injection Cure

 Vapor Injection Cure (VICฎ) is a patented process that
 mixes a conventional two-component polyurethane prior
 to use. While an operator is applying the coating, a
 tertiary amine catalyst, dimethylethanol amine, is intro-
 duced into the atomizing or air shaping chamber of the
 spray gun. The catalyst acts as an accelerator for the
 polyurethane reaction.

 This  specialized process is not a  pollution prevention
 technology in that it does not affect overall air, water, or
 waste emissions. End-users may be motivated to imple-
 ment this system primarily because it accelerates the
 curing process of an already available low-VOC polyure-
 thane coating, allowing operators access to the work-
 pieces sooner. It, therefore, has cost benefits. Possibly,
 the coating setting faster may reduce handling damage,
 which in turn  lowers the reject rate. Repainting fewer
 parts would reduce air, water, and waste pollution.

 Apparently, this process can accommodate most types
 of guns, including conventional  air spray, air-assisted
 airless, conventional electrostatic, and rotating atomiz-
 ing discs. The cqating can probably also be applied by
 an HVLP gun.

 The tertiary amine is generated in a separate heated
 steel  or aluminum  vessel. Compressed  air from the
 supply line feeds into the vessel where it picks up the
 tertiary amine vapor. The air/amine mixture then feeds
 to the air inlet of the spray gun. In order to prevent amine
 vapor from condensing in the air hose, the hose requires
 insulation or heat tracing.   Advantages

 Many of the advantages of VIC are associated  with
 speed. VIC allows rapid curing of two-component poly-
 urethanes without shortening  the  pot-life of  the  pre-
 mixed coating. Masking of sections for two-tone finishes
 can take place sooner. Sanding of primer can also take
 place sooner. Another feature speeding the process is
 that several guns can operate from one amine catalyst
 generator. In general,  all these features combine to
 reduce shipping time of a coated product.

 Despite the speed involved,  VIC does not affect the
 physical and chemical-resistant properties of the poly-
 urethane.  In addition, the process prevents or reduces
 outgassing or air bubbles from porosities in casting.   Limitations

 Most  importantly, because amine vapor is a VOC, it
does  add  to the VOC content of the two-component
 polyurethane. This can increase the VOC content of the
 applied coating by approximately 0.5 Ib/gal. To ensure
 that the applied coating does not exceed the RACT limit,
 the coating vendor must formulate the two-component
 polyurethane so that the VOC content of the mixture of
 components A and B is at least 0.5 Ib/gal less than the
 RACT limit. Health and safety concerns also may need
 addressing, but these should not differ from those of any
 other two-component polyurethane.

 Other limitations involve either additional steps or costs.
 For instance, some electrostatic spray gun components
 may need modifying if they are sensitive to the amine
 catalyst. Generally, just a gasket change  is necessary.
 Operators also must monitor and control the air/amine
 ratio. In most cases, operators must heat trace the air
 hose to the gun in order to prevent  condensation of the
 amine catalyst. These and other VIC issues contribute
 to slightly increasing the cost of the coating system.

 10.5.6  Supercritical C&i lor Paints and
 The Union Carbide Company has introduced their  Uni-
 carbฎ System which is designed to  use liquified carbon
 dioxide (CO2) as a solvent for coatings.
 Because  of the excellent solubility characteristics of
 CO2, the company claims that manufacturers  can  add
 less smog-forming solvents to conventional or high  sol-
 ids coatings. Liquified CO2 can make up the balance.
 While a system feeds the high solids coating to the spray
 gun, liquified CO2 feeds to a chamber where it intimately
 mixes with the coating. The coating viscosity drops  to a
 manageable level and excellent atomization takes place.   Advantages

 The biggest advantage the Unicarb system offers is  that
 it can reportedly reduce VOC emissions by as much as
 50 to 80 percent (1). Many companies, but especially
 companies struggling to comply with increasingly strin-
 gent VOC regulations, should find this compliance  op-
tion very  attractive.  In addition, because companies
would substitute CO2 for conventional  solvents, they
would also  realize substantially  lower  solvent waste
costs without compromising quality.

Companies can also greatly improve transfer efficiency.
One company's application process saw a 30 percent
 increase  in transfer efficiency (2).  This improved  effi-
ciency contributed to a higher coating deposition rate
and better ability to achieve the desired film thickness.
Better transfer efficiency, of course,  also translates  into
lower costs because less coating is used.

Other advantages include (1):

• CO2 is much less toxic than organic solvents  and  has
  a much better health-effects profile.

 •  CO2 has a significantly better safety profile because
   it is nonflammable and mostly inert.

 •  CO2 is a low-cost product.

 •  The process uses recycled CO2 and, therefore, does
   not contribute to the "greenhouse effect" (2).

 The technology can also be used for applications other
 than conventional coatings, such as 100 percent solids
 lubricants. In this case, the CO2, which is under high
 pressure,  works to atomize the lubricant rather than
 lower the viscosity of the product (3).   Limitations

 The capital expense associated with switching from a
 conventional system to Unicarb is relatively high. This is
 because expenses include a new array of special equip-
 ment to apply the system. Companies would also need
 to purchase coatings  that have been  specially formu-
 lated for this process (4).

 Adding to initial expenses and difficulties is the learning
 curve. Whenever a company begins using a new tech-
 nology, it can expect slower turnaround times and sev-
 eral glitches. Supercritical  CO2  technology is  no
 exception. In  fact,  a  wood furniture manufacturer  in
 Pennsylvania found that the nonconventional "high-tech
 appearance of the system can be intimidating" (4). Com-
 panies must ensure their operators know how to control
 CO2 temperature and pressure (two vital variables in the
 process) and how to start up and shut down the system.

 In addition, companies should test various workpieces
 with this system to ensure quality finishes;  it may not be
 appropriate for  every workpiece.  For example, the
 Pennsylvania manufacturer immediately realized  high-
 quality finishes on chairs and vertical surfaces but expe-
 rienced  small bubbles  (solvent trapping) on horizontal
 surfaces, such as tables (4). With  patience and  good
 testing procedures, however, companies may be able to
 resolve problems like this.

 While a very attractive compliance option, supercritical
 CO2 technology is not for everybody, partly because the
 coatings must be formulated specially for this system.
 One cannot take a conventional commercially available
 coating and simply spray with supercritical CO2.

 Currently, only one vendor, Nordson Corporation of Am-
 herst,  Ohio, makes  the coating  application equipment,
 while the license for the CO2 technology is held by Union
 Carbide of Danbury, Connecticut. Possibly,  therefore,
companies located  in the smaller towns and cities might
find it  more .difficult to get on-site customer service.

 10.6  Emerging Technologies

The term "emerging technology" does not necessarily
 mean, as many believe, a new and innovative technol-
 ogy providing some form of breakthrough. It also does
 not usually mean, as many in regulatory circles infer, a
 breakthrough to significantly reduce one or more forms
 of pollution.
 In  fact, many of the  newer,  specialized technologies
 such as radiation cured coatings, Supercritical CO2, and
 Vapor Injection Cure, do provide some benefits but are
 not the panacea that the industry is waiting for. More-
 over, they are unlikely to ever make a major dent in the
 overall coatings market.

 The technologies making the greatest strides towards
 zero VOCs (and to a limited extent, also zero hazardous
 waste) are the water-borne and high solids coatings that
 this  chapter  has already  extensively   discussed.
 Changes occur gradually, and often comprise substitu-
 tions of only one ingredient at a time. For instance, a
 flow modifier of lower VOC content might be developed
 for polyurethanes, thus slightly lowering the overall VOC
 content of a formulation. Unless end-users keep up with
 current literature in the journals or attend special confer-
 ences, they are unlikely to  know about these types of
 discrete developments.
 Over a period of years, coating vendors gradually intro-
 duce modified formulations to their customers, and VOC
 reductions take place  on an evolutionary  basis. For
 instance, during the 1980s, formulating a two-compo-
 nent polyurethane with a VOC  of 3.5 Ib/gal was very
 difficult. By the early 1990s, many such coatings were
 readily available at VOCs of less than 2.8 Ib/gal.
 Perhaps the  greatest advances are proceeding in the
 water-borne field. By 1994, manufacturers had devel-
 oped water-borne single- and two-component polyure-
 thanes with low VOC contents, less than 2.8 Ib/gal, less
 water.  Several research projects are underway to de-
 velop new cross-linking agents and emulsifiers for other
 resin systems in order to further reduce VOC levels.

 The high solids arena has also seen major develop-
 ments. By the end of 1993, a few coating manufacturers
 had already  begun conducting  preliminary production
 trials on 100  percent solids baking coatings. They are
 gathering more test data before offering these coatings
 to the industry at large.

 Importantly, many of the new developments are taking
 place in generic technologies that the industry is cur-
 rently using, such as water-bornes and high solids. For
the most part, then, end-users will be able to implement
the new formulations without making major modifica-
tions to their existing processes. This is a great advan-
tage. For instance, switching from a liquid coating to a
 powder coating requires a complete change in manufac-
turing philosophy. A major portion of a  coating facility
would  require modification  before implementing pow-
ders. Alternatively, changing from a conventional 1994
water-borne coating to a zero VOC water-borne may

 require only minor process changes. As these technolo-
 gies become more  readily available, local vendors will
 introduce them to end-users.
 10.7 Selecting the Best Technology for
       Specific Applications

 After studying all the technologies that this chapter pre-
 sents, the end-user should follow the steps noted below
 to narrow the best choice for his or her facility:

 • Eliminate those  technologies that  obviously do not

 • Create a list of those technologies that look feasible.

 • Review the lists of advantages and limitations again
   to determine whether any of the technologies on the
   list can be eliminated.

 • Call coating  vendors to solicit  samples  of  the tech-
   nologies that remain.

 • Arrange to conduct laboratory-type tests that will fur-
  ther differentiate between  the most likely and  least
   likely options.

 • Obtain  larger samples of the technologies  that
  passed the laboratory  phase,  and commence with
  limited production tests.

 In some cases, the end-user will not have the proper
 equipment (e.g., a  high temperature oven,  dip tank,
 powder coating spray gun) to conduct these tests. Usu-
 ally, coating and equipment vendors can make arrange-
 ments to conduct the production  tests at a third-party
 location. Often, the coating or equipment vendors even
 have in-house applications laboratories where  these
 tests can be conducted.

 At this point, the end-user should have  sufficient infor-
 mation, both technical and financial, to  make the final
 selection.  Arrangements can proceed in implementing
 the selected technology.

 10.8  References

 1. No Author. 1991. A New Pollution Prevention Technology. Wood
  and Wood Products. Vance Publishing Corporation, Lincolnshire,
  IL, p.56. (October)
 2. Baumert, D.F., 1995. Supercritical fluid sprays: New horizons. The
  Coatings Agenda America 1995/1996, p. 150.

3. Personal  communication with Union Carbide,  39 Old Ridgebury
  Road, N1, Danbury, CT. 06817-0001.
4. Christiansen, R. 1991. Pennsylvania House scores a finishing first.
  Wood and Wood Products. Vance Publishing Corporation, Lincoln-
  shire, IL, p. 53. (October)
 10.9  Additional Reading


 Hill, L. 1987. Mechanical properties of coatings. Federation of Socie-
    ties for Coatings Technologies, Philadelphia, PA.

 Joseph, R.  1994. Paints and coatings training program. Saratoga,
    CA: Ron Joseph & Associates.

 Joseph, R.M.  1993. Environmental paints & coatings training pro-
    gram. Saratoga, CA: Ron Joseph & Associates.

 McBane, B.N.  1987. Automotive coatings. Federation of Societies for
    Coatings Technologies.  Philadelphia, PA.

 Prane, J. 1986. Introduction to  polymers and resins. Federation of
    Societies for Coatings Technologies, Philadelphia, PA.

 Schoff, C.K. 1991. Rheology. Federation of Societies for Coatings
    Technologies. Philadelphia, PA.

 U.S. EPA. 1991. Report on compliance coatings for the miscellaneous
    metal parts industry. Stationary Source Compliance Division.

 High Solids, Solvent-Borne Coatings

 Joseph,  R. 1995.  High solids, low-VOC solvent-borne coatings. In:
    Metal Finishing Organic Guide Book and  Directory, vol. 93 (No.
   4A). New York, NY:  Elsevier Science Publishers.

 Water-Borne Coatings

 Joseph, R. 1995. Low-VOC, water-borne coatings. In: Metal Finishing
   Organic Guide Book and Directory, vol. 93 (No. 4A). New York,
   NY: Elsevier Science Publishers.

 Konieczynski, R. 1995.  Converting to water-bomes. In: Metal Finish-
   ing Organic Guide Book and Directory, vol. 93 (No. 4A). New York,
   NY: Elsevier Science Publishers.


 Hall, W.S. 1985. Autodeposition: One step pretreatment and coating.
   Available from Parker+Amchem Coatings,  Madison Heights, Ml.

Jones, T.C. 1990. Autodeposition: Tough coatings and no VOCs. The
   Finishing Line 6(3):1. Society of Manufacturing Engineers, Dear-
   bom, Ml.

Jones, T. 1995. Autodeposition. In: Metal Finishing Organic  Guide.
   Book and Directory, vol. 93 (No. 4A). New York, NY: Elsevier
   Science Publishers.

Parker+Amchem. No date. Autodeposition in  a Nutshell. Madison
   Heights, Ml. Trade literature.

Stockbower, E.A. 1994. Autodeposition of organic films: Current de-
   velopments. Presented  at Finstrat 1984 Conference, Anaheim,
   CA, November.


Austin, H. 1994. Electrocoat basics: Chemistry. Electrocoat 1994 Con-
   ference, March 23-25. Sponsored by Products Finishing maga-
   zine, Clough Pike, OH.

Brewer, G.E.F.  1995. Electrodeposition of organic coatings. In: Metal
   Rnishing Organic Guide  Book and Directory, vol. 93 (No. 4A). New
   York,  NY: Elsevier Science Publishers.

Kraft, K.  1994.  Electrocoat system design. Electrocoat 1994 Confer-
   ence, March 23-25. Sponsored by Products Finishing magazine,
   Clough Pike, OH.

McPheron, D. 1994. Electrocoat basics: Equipment. Electrocoat 1994
   Conference, March  23-25.  Sponsored  by Products Finishing
   magazine, Clough Pike, OH.

Vapor Injection Cure

Cassil, L. 1995. Vapor injection curing. In: Metal  Finishing Organic
   Guide Book and Directory, vol. 93 (No. 4A). New York,  NY: El-
   sevier Science Publishers.

Radiation Cured Coatings

Constanza, J.R. et al. 1986. Radiation cured coatings. Federation of
   Societies for Coatings Technologies, Philadelphia, PA.

Kallendorf, C.J. 1992. Radiation curing  primer I: Inks, coatings and
   adhesives. Radtech International of North America,  Northbrook,
   IL. Trade literature.
Klein, A.  1991. Developments in low energy BEAM processors and
   processes. Presented at the 20th Conference on  Radiation and
   Radioisotopes, Tokyo, Japan, November 21.

Meskan,  D.A. No date. Developments in electron beam processing:
   Higher capacity, better performance. RPC Industries, Hayward,
   CA. Trade literature.

Van Iseghem, L.C. 1993. UV/EB curing: Technology, applications and
   new developments. Paints & Coatings (June) p. 34.

Supercritical CO2

Argyropoulos, J.N. et al.  1994.  Polymer chemistry and phase rela-
   tionships of supercritical fluid sprayed coatings. Water-Bome and
   Higher Solids Conference, New Orleans,  February 9-11.  Spon-
   sored  by the University of Southern Mississippi, Polymer Science,
   and the Southern Society for Coatings Technology.

                                             Chapter 11
                                         Powder Coatings
 11.1  Introduction

 11.1.1  Pollution Prevention Considerations
 Of all the coating technologies on the market, powder
 coatings are particularly popular for their  low volatile
 organic compounds (VOCs) content. For many applica-
 tions, powders offer cost advantages over either sol-
 vent- or  water-borne  liquid  technologies.  Moreover,
 powder coatings provide many pollution prevention
 benefits. The act of applying powder coatings does not
 contribute to air, water, or hazardous waste pollution.
 A powder coating facility does, however, generate some
 pollution, primarily from two associated processes. The
 first  and most important involves surface preparation.
 Like liquid coatings, operators apply powders over well-
 degreased surfaces, which receive an iron or zinc phos-
 phate. Chapters 5 through 8 covered these processes
 in detail, including best management practices.
The  second important pollution-generating process is
the stripping of powder coating from  hooks and  reject
parts. Chapter 14 will cover this subject. Then, Chapter
 15 discusses spray booths, including pollution preven-
tion strategies.
Several books and manuals on powder coating exist. In
addition, those seeking to leam more about the subject
may choose from numerous  conferences and  work-
shops each year. Anyone considering powder coatings
should  remember that  they  have inherent  pollution
prevention characteristics versus conventional  tech-

 11.1.2  Decision-Making Criteria

Decision-making criteria relevant to powder coating, as
addressed in this chapter, are highlighted in Table 11-1.

11.2 Suitability for Specific Applications

 11.2.1  Suitable Applications

Powder coating can benefit many applications. In addi-
tion,  the list of parts for which it is being used, as well
as the list of industries that use powders, continue to
grow. Powder coatings  are ideal for metal parts that
 have relatively simple geometries and surfaces that are
 all reasonably accessible. Because of this, powders are
 currently being used for automotive under-the-hood ap-
 plications. These products include oil filters, air filters,
 shock absorbers, coil springs, lamp housings, and more.
 The architectural products industry is using powders to
 coat interior and exterior aluminum extrusions, air con-
 ditioning equipment, aluminum windows and doors, win-
 dow and  door screening, etc.  In the  miscellaneous
 metals industries, the use of powder coating is quickly
 moving toward having equal status with liquid coatings.

 In addition to all these successful applications, the auto-
 motive original  equipment manufacturers (OEMs) are
 currently evaluating using powders as base coats and
 clear coats.

 Almost every industry that finishes metal products has
 at least some companies that use powders. Powders,
 however, are not for everyone. The section below elabo-
 rates on this.

 11.2.2  Unsuitable Applications

 Many applications are unsuitable for powder coatings
 because they either:

 • Are not appropriate for the particular surface.

 • Cannot  provide corrosion protection equivalent  to
  high-performance liquid coatings,

 • Are not cost competitive with liquids.

One factor in the powder coating process that largely
contributes to its application limitations is the heat that
powders require for curing. For instance, certain metal
alloys may lose critical metallurgical properties when
cured at elevated temperatures for a  long period. Pow-
der coatings also are unsuitable for large parts  that
cannot enter a high temperature oven. (Some low tem-
perature epoxies that cure at approximately 250ฐF are
available). Other materials inappropriate for the powder
coating process include thermoplastic or heat-sensitive
plastics, wood, upholstery, rubber tubing, etc.

Color may also be a complicating factor when using
powders. For example, powder coatings are  not  well
suited for short runs of multiple colors. In addition, cus-

 Table 11-1   Decision-Making Criteria Regarding Powder Coating

 Issue                                  Considerations
 Is powder coating a suitable
 option for the workpiece?
Is workpiece small enough to be
suspended from a conveyor?
Does the coating operation
comprise long runs of the same

Does the workpiece comprise
many faraday cages?
Which powder coating material
should a facility select?
•  If workpiece is small enough to fit into a commercial oven that would operate at
   325ฐ to 400ฐF, consider powder coatings; if not, additional research would need to
   determine if powder coatings can be applied practically and cost effectively.

•  If workpiece comprises heat-sensitive material, such as wood, plastic, upholstery,  hydraulic
   tubing, electronic equipment, accurately machined parts, it might not be practical or
   cost-effective to  powder coat because of potential damage; if workpiece does not comprise
   such materials, consider powder coatings.

•  If workpiece is a fully assembled machine that contains any flammable material, such as
   gasoline, powder coatings can probably not be considered.

•  If workpiece can be well-cleaned and treated with an iron or zinc phosphate, consider
   powder coatings; if not, additional research might be required  and the end-user should seek
   advice from vendors or consultants.

•  If geometry of workpiece is relatively simple (such as flat surfaces), consider powder
   coatings; if geometry is relatively complex (difficult-to-reach areas, many brackets, channels,
   etc.), additional research might be required and the end-user should seek advice from
   vendors or consultants.

•  If coated workpiece will be exposed to aggressive corrosive environments, such as a
   petroleum refinery, severely corrosive marine atmosphere, etc., additional research might be
   required and the end-user should seek advice from vendors or consultants; if not, consider
   powder coatings.

•  If workpiece requires extensive masking prior to coating application, additional research
   might be required because powders might not be practical or cost-effective; if workpiece
   does not require extensive masking, consider powder coatings.

•  If the workpiece  requires coating with more than one color, as in two-tone products,
   additional research might be required and the end-user should seek advice from vendors or
   consultants; if  the workpiece requires coating with just one color, consider powder coatings.

•  If coated  workpiece will be post-formed, machined, or worked  on, powder coatings might be
   an excellent choice because they are malleable and can tolerate handling often with
   minimum, if any, damage.

•  If coating operation uses many colors, predominantly in small quantities, additional research
   might be  required because powder may not be cost-effective under these conditions.

•  If yes, consider an enclosed spray booth that reclaims the powder.

•  If no, operators may need to apply the coating  in a large walk-in booth, and powder
   reclamation might be impractical.

•  If yes, facility might be able to  automate the application by means of reciprocators or
   stationary powder guns, thereby minimizing the cost of labor.

•  If not, employing manual operators to apply the coating might  be more cost-effective.

•  If yes, operators  might be able to effectively coat the relatively inaccessible areas or acute
   angles by means of a tribo-charging powder gun.

•  If no, either a corona- or tribo-charging gun are possibilities.

•  A facility should consult with a  powder coating vendor before selecting a resin technology.
torn colors may not be easily available  in quantities of
less than 1,000 Ib (although some vendors do specialize
in small batches).

Finally, other types of applications that are  usually un-
suitable for powder coatings  are:

•  On parts that cannot tolerate warpage.

•  On parts that require thin films (less than 1  mil).

•  For porous castings in  which air blisters  would mar
   the final coated  finish.
                        11.3  The Powder Coating Process

                        Generally, powder technology, as a group, is the fastest
                        growing coating technology in the organic coatings mar-
                        ket. In some industry sectors, it is rapidly competing with
                        and penetrating the liquid coatings applications market.
                        The primary reason  for this  success  is  its favorable
                        environmental  profile. Unlike  liquid coatings, powders
                        essentially do not contribute to air, water, or hazardous
                        waste pollution.

                        As the name implies, powder coatings are organic coat-
                        ings that are supplied in dry powder form. Unlike liquid

 coatings, each discrete  powder particle contains the
 entire coating formulation, namely the resins, pigments,
 fillers, and modifiers. A powder coating contains no sol-
 vents. The powder particles are extremely finely divided
 and resemble talcum powder.

 The powder coating process entails two basic steps:

 •  Applying the coating onto a pretreated part

 •  Curing the coated part in an oven

 11.3.1   Applying the Coating

 Operators  can use one  of three primary methods to
 apply the powder coating:

 •  Electrostatic attraction  by corona charge

 •  Electrostatic attraction  using tribo-charging guns

 •  Fluidized bed

 In both of the  electrostatic  methods,  the parts to be
 coated  are suspended from an electrically grounded

 The first method listed is  the most common. To charge
 the powder, the operator uses a gun that contains  a
 high-voltage electrode. Upon pulling the gun trigger, the
 high electrical potential around the electrode ionizes
 the surrounding air, causing a corona. As powder parti-
 cles leave the gun and pass through the charged air,
 the electrostatic charges transfer to the powder parti-
 cles, which then become attracted to the grounded part.
 The individual particles essentially "adhere" loosely to
 the metal substrate; at this stage, the only mechanism
 particles use to adhere to the substrate, or to each other,
 is  electrostatic attraction.

 The second application method also uses a gun, but one
 that comprises internal passages made of plastic, usu-
 ally nylon. As the powder particles rub  over the plastic,
 they receive an electrostatic charge, much like the phe-
 nomenon that occurs when running a comb  through
 one's hair on a dry day. Once again, when the particles
 leave the gun, they seek the grounded  part and loosely
 adhere to it by electrostatic attraction.

 In  the fluidized bed approach, the powder is contained
 in  a tank. The bottom of  the tank comprises a porous
 plate. Low pressure air passes through the plate causing
 the powder to become suspended in the air as a cloud.
 In  fact, this cloud  is known as a fluidized bed. The part
 to  be coated must be preheated to a temperature usually
 in  excess of 400ฐF,  and is immediately immersed into
 the fluidized bed. Upon contact, the powder particles
 melt and remain on the heated substrate. The higher the
 part temperature or the longer the  part remains in the
fluidized bed, the heavier the film build. This assumes
that the temperature of the part does not  drop to below
the melting point of the powder.
 Note that none  of these methods involve solvents or
 generate hazardous waste. Also, clean-up efforts are
 minimal, benefiting both pollution prevention and time
 and material resources.

 Regardless of which method operators use to apply the
 coating, the coated part must then enter an oven. In the
 oven, the powder melts, fuses, and cures into a hard,
 chemical- and abrasion-resistant coating.
 11.3.2  Curing the Coated Part

 Curing of the powder entails heating the powder-coated
 part in a convection oven at a temperature of between
 325ฐF and 400ฐF (163ฐ to 204ฐC) for approximately 8 to
 20 minutes. Developments are underway to lower the
 curing temperatures  to 250ฐF (121ฐC). Two variables
 that affect the curing period are time and temperature.
 For instance, the lower  the  curing temperature, the
 longer the curing time, and vice versa. Another variable
 affecting the curing period is the mass of the part.

 An alternate method for curing  the powder uses an
 infrared oven, which  heats only those surfaces that are
 exposed directly to the infrared rays (i.e., the coating).
 The advantage of infrared curing is that the entire work-
 piece does not have to reach the curing temperature in
 order for curing to take  place. Because of this fact,
 infrared curing can provide cost savings.

 When the powder coating is oven cured, some vapors—
 approximately 0.5 to 5 percent  by weight  of powder
 coating—are emitted  into the atmosphere. These com-
 prise mainly water and some organics. The organics are
 not solvents,  but rather plasticizers  or resins emitted at
 the high baking temperatures. To  a large  extent, the
 emitted vapors that have  high boiling points condense
 on the oven walls as they pass through it. It is question-
 able whether they are truly VOCs as defined  by EPA. In
 fact, most air pollution regulatory agencies assume that
 the emissions from powder coating operations are es-
 sentially zero; therefore, operators  are usually not re-
 quired to measure or  record their emissions.  In  addition
 to advancing pollution prevention, this is a major eco-
 nomic benefit.

 As soon as the part leaves the oven and cools to ambi-
 ent temperature, it can be handled, worked on, and

The fully cured coating is extremely hard and abrasion-
 resistant, and exhibits excellent physical properties. De-
 pending on the resin system,  the coating can  also be
 resistant to chemicals, solvents, sunlight, and  most of
the other chemical properties  that are associated with
 high-performance liquid coatings.

 11.4  Costs Associated With Powder

 For a facility operator considering either switching to or
 adding powder  coating capabilities, cost analysis of
 equipment and other requirements should encompass
 the following areas:

 •  General and environmentally related costs

 •  Costs of materials

 •  Pretreatment costs

 •  Costs associated with actual coating process

 •  Costs associated with heating/curing

 Most importantly, regarding pollution prevention oppor-
 tunities in powder coating facilities, major cost savings
 are probable in the area of environmental compliance.
 For instance, one company recently calculated an an-
 nual cost of hazardous waste disposal  for  its liquid
 coatings to be in the order of $30,000. By converting to
 powders, that cost would essentially drop to zero. Simi-
 larly, the conversion would dramatically reduce costs
 associated  with  obtaining  air  permits, administering
 emissions inventories, etc.

 Converting to  a  powder coating application, however,
 would  require that the operators learn how to use these
 coatings. Any training costs, however, should  be offset
 by the  fact that applying powder coatings is less compli-
 cated than applying liquid coatings. For expertise, how-
 ever, the facility operator may decide to hire a relatively
 experienced supervisor to oversee the operation.

 Regarding materials, when coating the same number of
 workpieces, the costs for liquids and powders are some-
 what comparable. Liquid coatings are purchased by the
 gallon, while powders are purchased by the pound. The
 rule of thumb in  the industry is to equate 3 pounds of
 powder to 1 gallon of liquid coating. Powders  range in
 cost from $2.50 to approximately $6.00 per pound, de-
 pending on the  resin type, color, texture, etc. Exotic
 powders are more expensive. Alternately, costs for liquid
 coatings can vary from $10.00 per gallon for some of the
 low-end resin formulations to $90.00 per gallon for poly-
 urethanes. Costs for some polyurethanes in exotic col-
 ors can even exceed $150 per gallon. While it may be
 difficult to make  a cost comparison based on the  per
 pound  versus per gallon measures, the industry's rule
 of  thumb is to assume that for the majority of scenarios,
the cost solely of the coating  materials  are  approxi-
 mately the same.

The cost of  equipment, of course, depends on the de-
gree of sophistication of the facility. Most pretreatment
 requirements, however, are universal. The facility  will
need a pretreatment system at least comparable to that
which  a high-performance  liquid  coating system  re-
 quires. If the facility already has a 3- or 5-stage iron or
 zinc phosphating system for steel parts or a conversion
 coating system for aluminum parts, no new pretreatment
 equipment is necessary. Alternately, a facility lacking
 such equipment would have to install it. The costs to
 install a pretreatment system vary according to size and
 throughput of material, but for most painting facilities the
 range is usually $50,000 to $120,000. Of course, a
 facility operator intending to install a pretreatment sys-
 tem in order to apply powders would also need to do so
 for liquid coating application. In the past,  many liquid
 coating painting facilities could avoid the installation of
 a sophisticated pretreatment system because the high-
 VOC liquid coatings were somewhat tolerant to surface
 contamination. With the introduction of high solids and
 water-borne coatings, however, this is no  longer true;
 therefore, with current technologies,  liquid or powder, a
 sophisticated pretreatment system is necessary.

 Unlike the pretreatment phase, the actual powder coat-
 ing process is associated with many options. Parts can
 be  coated on conveyors or racks and, provided that
 liquid and powder coatings are not applied on the same
 line, it may be possible to use existing conveyor or rack

 Spray guns and associated application and electronic
 control equipment can cost from $5,000 to $100,000 per
 facility. Portable  units  are available for companies that
 use only small quantities of  powder or that coat on an
 intermittent basis.

 If a facility uses a few standard colors in reasonably
 large quantities, the operator can feasibly purchase one
 dedicated powder coating booth for each color. When
 the facility is using a specific color, that particular booth
 is rolled in-line with the conveyor system.

 In addition, a device is necessary to  filter the overspray
 powder from the exhaust air. Cartridge filters, or cy-
 clone/bag houses may be appropriate for this purpose.

 If a facility operator intends to use both liquid and pow-
 der coatings, then the operator may consider having two
 separate dedicated coating lines: one for liquid and one
for  powder. This, however,  might require more facility
floor space.

 If, on the other  hand, the facility operator intends to
 replace its liquid coatings with powder coatings, then a
 major retrofitting may be necessary.  This would require
a shut-down period for the powder coating equipment to
 be installed.

The final step of the powder coating process involves
heating and curing the coated piece. This step requires
ovens, but because solvents do not evaporate from the
coatings, only low air replenishment is required. The
oven, which can be a convection,  infrared, or similar
type, must be capable of raising the coating temperature

 to approximately 325ฐF  to 400ฐF (163ฐC to 204ฐC).
 Some cases may require higher oven temperature, par-
 ticularly when using special high-performance coatings
 such as nylon.

 Generally,  however,  energy  costs are  considerably
 higher for powders because these coatings cure at ele-
 vated temperatures. These costs, though, must be bal-
 anced  against  savings  realized  from  emitting  no

 11.4.1   Profiles of Economic Impact of
         Switching to Powders

 Case  histories of the economic  and environmental
 benefits of powder versus liquid abound in the literature.
 Monthly or bi-monthly journals such as Powder Coating
 Journal, Metal Finishing,  Products  Finishing, Industrial
 Paint and Powder (formerly Industrial Finishing), regu-
 larly publish case histories demonstrating the  advan-
 tages of the dry versus the wet technologies. Each year
 in the United States, at least two national conferences
 are devoted to powder coatings.  Examples  of typical
 cases are presented below.

 First, in most cases, a clear economic advantage exists
 for converting from liquid to powder coatings (1). The
 Return on Investment (ROI) results in a short payback
 period. Liberto reports that American Yard  Products,
 producers of walk-behind and riding mowers, enjoyed
 substantial cost savings.  The company's total powder
 conversion investment was $2,150,000. When compar-
 ing this cost with the company's annual operating cost
 reduction from powders,  which was $2,354,870, it is
 clear that the benefits in this case were dramatic. They
 were also quick; the  projected payoff period was only
 about 11 months (1).

 The next profile is of the Self-Serve Fixture Company of
 Texas, which manufactures a line of shelving and fix-
 tures for the self-serve retail market. It switched to pow-
 der in January 1991  and within the first year saved
 approximately $100,000 solely on material usage. In  its
 previous  liquid painting  operation  the company  esti-
 mated a transfer efficiency of 40 percent, which implies
 that 60 percent was wasted. Changing to powder led to
 a significant improvement; it  realized 85  percent effi-
 ciency for colors that could be reclaimed and recycled,
 and 55 to 60 percent for custom colors for which it was
 not economical to reclaim the powder. The  improved
 quality of the finish contributed to a 20-percent increase
 in sales in 1992, and the president  of the  company
 projected another 15 percent for 1993 (2).

 American Desk, a leading manufacturer of business and
 institutional furniture,  had been  using high solids, sol-
vent-borne coatings. In 1993, the company partially con-
verted to powder due to high environmental costs, waste
disposal, and high solvent throughput. Their new pow-
 der coating system comprised a 5-stage washer, 6-min-
 ute dry-off oven, 2 powder coating booths, 28 powder
 guns, and a 25-minute bake oven. Bailey reports that
 because of the increased line speed and improved parts
 hanging technique, the powder coating system was able
 to increase productivity by 50 to 100 percent. Moreover,
 the powder  coating line, which operates two shifts per
 day, produces more than  the  previous liquid painting
 system that  required three shifts (3).

 Maytag-Galesburg, manufacturer of refrigeration prod-
 ucts, converted to powder  in 1992 as a voluntary effort
 to comply with EPA's "33/50"  initiative. This initiative
 called  on large companies to  voluntarily reduce their
 emissions of 17 listed toxic chemicals by 50 percent
 before the end of 1995. According to Schrantz, this
 change allowed the company to increase its production
 capacity. At  least one benefit was that the cost of reject
 or repair parts due to frequent handling during manufac-
 ture and assembly was dramatically reduced because
 of the increased durability of the polyester powder coat-
 ing (2).

 11.5  Advantages and Limitations of
       Powder Coatings

 11.5.1  Advantages

 Facilities considering using powder coatings  have a
 comfortable  range from which to choose their coatings.
 Powder coatings are available in several resin formula-
 tions:  acrylic,   polyurethane,  epoxy,  polyester, ep-
 oxy/polyester hybrids, TGIC, nylon, etc. This technology
 also  offers a reasonable range of colored, clear, and
 textured coatings. In addition, depending on the resin
 system, powders are available in various gloss levels.

 These  coatings are associated with other advantages,
 too. They  have excellent physical performance proper-
 ties, and many powders have excellent machinability as
 well. Powders  also are associated with excellent salt
 spray resistance. Partly because of these attractive fea-
 tures, military agencies are starting to accept powder
 coating as replacement for liquids.

The largest  advantages to  a powder coating process
 derive from  its application  benefits and, especially, its
pollution prevention benefits.

 Regarding application, powder coatings allow operators

• Coat all sharp edges and cut ends.

• Provide thin  to heavy film builds in one application
  (they usually require no primer).

• Apply coating to hot or cold parts.

Powder coatings also prove very economical for long
runs of a few colors. They are  especially adaptable to

 robotic or  reciprocating application and, generally re-
 quire less skill than application of liquid coatings.

 Finally, another feature of applying powder coatings that
 should interest many facilities  involves  masking.  In
 some cases, masking is not required because uncured
 powder can be brushed off critical surfaces before the
 coated part enters the curing oven.

 The pollution prevention opportunities associated with
 powder coatings, however, probably offer the greatest
 advantages. These opportunities  relate to high transfer
 efficiencies, the cleanliness of the powder coating proc-
 ess, and the lack of hazardous waste.

 Operators  can achieve very high transfer efficiencies
 with powder  coatings. They can even  attain transfer
 efficiencies of greater than 95 percent if powder over-
 spray is collected and recycled.

 In addition, powder coating is a relatively clean process,
 particularly if facilities operate spray booths under nega-
 tive pressure. Regarding clean-up, to a large extent an
 operator can clean the spray booth using compressed
 air. Solvents may only be required during the final stage
 of cleaning.
 Perhaps the most attractive advantages to powder coat-
 ings relate to their hazardous waste profile. Liquid coat-
 ings are applied in dry filter or water-wash spray booths,
 and either the filters or the wastewater require disposal
 as hazardous waste. Powder coatings, on the other
 hand, are Always applied in dry filter booths. The filters,
 however, generally do  not require discarding. Instead,
 cartridge filters in modern powder coating  booths are
 designed so that operators can reclaim the powders that
 collect in the filters. The filters are good  for hundreds if
 not thousands of  pounds  of powder. Facilities can
 eventually dispose of the filters either as solid hazard-
 ous  waste (if the entrapped powder contains heavy
 metals) or can discard them  in a landfill.

 In addition, waste powder that might fall to  the  floor
 outside the booth, can be swept up into a small  pile,
 placed into an oven where it melts  into a solid block, and
 be discarded either as solid hazardous waste or in a
 landfill. The economic benefits from the environmental
 considerations are sufficient reason for many facilities to
 convert from liquids to powders.

This pollution prevention profile of powder coating trans-
 lates specifically into:

 •  Emissions of almost zero VOC content (0.5 to 5 per-
   cent by weight).

 •  Minimal generation of hazardous waste (if any).

 11.5.2   Limitations

As with all  systems, powder coatings also have limita-
tions. Some of these relate to heat requirements.
 Most decorative resin systems require temperatures of
 325ฐF to 450ฐF  (163ฐC to 204ฐC) for curing. Some
 functional  resins require temperatures in excess  of
 500ฐF. Apparently, however, some epoxies are available
 that only require 250ฐF  (121ฐC). Because of these
 needs, powder coatings are associated with high energy

 Regarding personnel,  although an earlier section de-
 scribed the powder coating process as uncomplicated
 to perform, it does require a skilled operator to set up
 guns for each run,  and check for quality before parts
 enter the oven. Also, powder  coating often requires
 manual touch-up  by an operator who stands at the end
 of an automated booth. Other quality-oriented tasks in-
 volve ensuring that the metal surfaces for coating have
 been well-cleaned and treated, as well as seeing that
 the oven remains clean so that dust and other contami-
 nants will not blow onto the coated parts during coating.

 When considering the coating  process  itself,  electro-
 static equipment  makes it difficult to achieve high film
 thicknesses (greater than 5 mil), unless the part is pre-
 heated prior to the coating application. Of course, most
 cases do not require 5 mil. In addition, in fluidized bed
 applications, operators cannot easily control film thick-
 ness due to differing heat contents of the metal assem-
 bly (i.e., light gauge metal fixed  to casting).

 Another example of challenges associated with the pow-
 der coating process involves the difficulties associated
 with coating "faraday cages" unless using alternative
 techniques. A farady cage is the area inside an acute
 angle that  is  shielded  from the electrostatic field. For
 instance, if the inside of a box is to be powder coated,
 it might be difficult to  deposit powder onto the inside
 corners of the box. The  inside surfaces of a channel
 bracket or the area between the fins  of a radiator all
 represent faraday cages. Recent years have seen new
 methods to overcome some of these problems. In  par-
 ticular, the tribo-charging gun appears to successfully
 coat many of these surfaces. Moreover, powder coat-
 ings can be more difficult to  repair after curing when
 compared to liquid coatings.

 Finally, capital equipment outlay is generally greater for
 powder coating than for conventional coatings ($5,000
to $100,000). Small or portable  systems are available,
 however, which are less expensive. Because each case
 is different, the costs for converting to powder can be
comparable to those for liquid coatings. Costs seem to
 rise when a facility operator intends to use an automated

 11.6  References

 1. Liberto, N.P. etal. 1990. Appliance Manufacturer. Business News
   Publishing Company, (March).

 2. Schrantz, J. 1993. Powder saves $100,000 in first year. Industrial
    Finishing  69(4):22-25 (April).

 3. Bailey, J.M. 1994. Back to school with powder. Industrial Paint
    and Powder 70(9):14-16 (September).

11.7 Additional  Reading

Collins, B. 1988. From wet to dry: Can ft be done? The Finishing Line
Dawson, S., and V. Reddy,  eds. 1990. Powder coating applications,
   1st ed. Dearborn, Ml: Society of Manufacturing Engineers.

Liberto, N.P., ed. 1994. Powder coating; The complete finisher's hand-
   book, 1st ed. Powder Coating Institute, Alexandria, VA.

Hart, J.  1988. Serving New England's powder coating needs. Prod-
   ucts  Rnishing 52:60-64.
Joseph, R. 1994.  Paints and coatings training program. Saratoga,
    CA: Ron Joseph & Associates, Inc.

Maguire, G. 1988. Fluoropolymer powder coatings on the move.
    Products Finishing 52:66-69.

Miller, E., ed. 1985. User's guide to powder coating, 1st ed. Dearborn,
    Ml: Society of Manufacturing Engineers.

Muhlenkamp, M. 1988. High performance powder coatings for alumi-
    num. Products Finishing 52:53-58.

The Powder Coating Institute, 1800 Diagonal Road, ste. 370, Alex-
   andria, VA. [Serves the powder coating industry and can provide
   a wide range of information to prospective powder coating users.]

U.S. EPA. 1989. Powder coatings technology update. EPA/450/3-

                                             Chapter 12
                       Viscosity Management for Pollution Prevention
 12.1  Introduction

 12.1.1  Pollution Prevention Considerations

 This chapter discusses one of the most important prop-
 erties of a coating, namely viscosity. Viscosity is a meas-
 ure of the degree to which a fluid resists flow under an
 applied force.

 Controlling viscosity has an indirect yet important effect
 on  pollution prevention. Best management practices
 that control viscosity do not, in and of themselves, di-
 minish air,  water, and waste pollution. With  proper vis-
 cosity management, however, operators can achieve
 more acceptable finishes,  dramatically reducing  the
 number of  reworks and rejects. Repainting fewer work-
 pieces reduces all forms of pollution.

 Coating manufacturers often attempt to formulate prod-
 ucts that can be used as packaged. Sometimes, how-
 ever, the spray painter must make adjustments, such as
 diluting the coating,  in order to  obtain an acceptable
 finish.  Unfortunately, most spray painters do not fully
 understand their options, and hence rejects and reworks
 abound, particularly with high solids coatings.

 The purpose of this chapter is to provide a better under-
 standing of the available techniques for beneficially al-
 tering the viscosity of a coating.

 Decision-making criteria relevant to viscosity manage-
 ment are not specifically called out in a table because
 the recommendations discussed throughout this chapter
 should be followed by all facilities.

 12.2  Description of Viscosity

A thorough understanding of the concept of viscosity as
well as the parameters that affect it can be very  useful
 in applying coatings  efficiently and minimizing rejects
 and reworks.

 Consider a basic example.  Water has a low viscosity
compared with cold syrup. Upon  heating, however, the
syrup's viscosity drops and it flows more easily. This, of
course, is a simplistic example of viscosity. Because the
subject is more complex a few definitions may be helpful.
Absolute dynamic viscosity is the force per unit area that
resists the flow of two parallel fluid layers past one
another when their differential velocity is 1 cm/sec per
centimeter separation (1). Figure 12-1 illustrates a liquid
lying between two parallel plates (2). Suppose that the
              AX -VAt
            Ax = Distance Travelled by Top Plate
             F = Force Exerted on Top Plate
             v = Velocity of Travel
            At = Duration of Travel
             A = Area of Top Plate

Figure 12-1.  The concept of viscosity (2).

lower plate is fixed, while the upper plate can move to
the right at a velocity (v) under the action of an externally
applied force. With this movement, the liquid between
the two plates would distort as shown. One of the pa-
rameters illustrated here is  shear stress. By definition
shear stress is as follows:
                 Shear Stress = —
(Eq. 12-1)
where A is the area of the top plate and F represents
the force exerted on the top plate.

Another important parameter is shear strain, which is:

                 Shear Strain = 4*        (Eq. 12-2)

where i is the distance between the two plates, and Ax
is the distance that the upper plate has moved.

Figure 12-1 also shows how the velocity of the fluid
changes from zero at the lower plate to v at the upper.

Therefore, over a period (At) the fluid at the upper plate
moves a distance Ax =  vAx.

The coefficient of viscosity (n.) is defined as the ratio
between the shear stress and the rate of change of the
shear strain:

Viscosity is usually expressed in terms of poise or cen-
tipoise (cp), where:

                  1 poise = 1 00 cp

The units of absolute viscosity, poise or centipoise, are

Finally,  a measure of kinematic viscosity is given in
stokes, where:
                  Stokes =
Although most people will never have a need to perform
the calculations presented in the preceding definitions,
the calculations do illustrate a couple important points.
First, they make  clear the many variables  that affect
viscosity.  Second, they lay the groundwork for under-
standing much more commonly used definitions regard-
ing viscosity. These common definitions follow.

A  Newtonian liquid is any liquid for  which the  shear
stress is proportional to the  shear  rate.  If the ratio of
shear stress to shear rate is small and the effect on
viscosity is not constant, the liquid is non-Newtonian (2).

For instance, when measuring the viscosity of water,
which is a Newtonian liquid, the  viscosity remains con-
stant regardless of how fast it is  stirred.

A near-Newtonian liquid is one for which the variation of
viscosity with  shear  rate is  small  and  the  effect on
viscosity of mechanical disturbances, such as stirring, is

A non-Newtonian  liquid is any liquid that does not satisfy
the requirements for a Newtonian liquid.  Such liquids
have plastic flow, pseudo-plastic flow, or dilatant flow.
For each of these, the shear rate is not proportional to
the shear stress.

For plastic flow, the liquid must overcome or exceed a
yield  stress before flow will take place. No yield value
exists for  pseudo-plastic flow and the curve of the plot
of shear stress versus shear rate is non-linear, with the
shear rate increasing  faster than the shear stress. For
fluids exhibiting dilatant flow, the viscosity increases as
the shear rate increases. The curve of the plot of shear
stress versus  shear rate is  non-linear, with the  shear
stress increasing  faster than the  shear rate.
 Finally, the consistency of thixotropic materials depends
 on the duration of shear as well as on the rate of shear.
 To better understand this property, one can imagine how
 the viscosity of an acrylic latex paint changes as it is
 being stirred. For instance, when stirring the paint very
 slowly with a stick or paddle, its viscosity is relatively
 high. As stirring becomes faster and more vigorous, the
 viscosity drops. When stirring ceases  altogether, the
 viscosity increases again, although it may not increase
 to its original value. Figure 12-2 demonstrates the rela-
 tionship between viscosity and shear rate for a thixot-
 ropic fluid.
          Shear Rate
Shear Rate
Figure 12-2. Thixotropy.

A coating with thixotropic properties may have a rela-
tively high viscosity while being pumped from a pressure
pot to the spray gun. As the coating is forced through
the very small orifice of  the gun, its viscosity drops
appreciably and remains relatively low while the parti-
cles travel from the gun to the target. As  they settle on
the target, such as a vertical panel, the viscosity rapidly
builds up again, thus minimizing the possibility for the
coating to run or sag.

12.3 Measuring Viscosity

Viscosity is one of the most important coating properties
in determining if the coating can be  applied to an accept-
able finish. This is why measuring viscosity is so important.

The most commonly used viscometers for measuring paint
on  a production line are gravity type cups such as the
series of Zahn cups (#1, #2, #3, and #4) and the Ford cup
(#2, #3, #4). All require little skill and can be used by paint
operators who have been shown how to use them.

12.3.1   Zahn Cup

The Zahn Cup is made of stainless steel and resembles
a cup as shown in Figure  12-3. It has a small orifice at

Figure 12-3.  Zahn cups.
Figure 12-4. Ford viscosity cups (photo courtesy of Pacific Sci-
           entific Catalog publisher).

in the Ford cup is usually large enough so that one clean
break takes place regardless of viscosity.

Although rf>aint operators use the  Ford cup less fre-
quently than the Zahn cup, it is the standard method for
measuring viscosity in a paint manufacturing laboratory.
It has the advantage that no  coating flows down the
walls of  the  outside of the cup to interfere with the
viscosity measurements. Facilities that  use the paints
and coatings usually prefer the Zahn cup because it is
less expensive and  simpler to use (i.e., the operator
need not transfer coating from the container or pressure
pot to the cup).

If the coating is to be applied at an elevated tempera-
ture, it is preferable to measure the viscosity at the same
temperature. This, however, may be impractical in many
facilities. An alternative to measuring the viscosity at the
application temperature is to measure it at ambient tem-
perature, and then determine what viscosity is required
under ambient conditions to yield the desired application
viscosity at application temperature.

12.3.3   Brookfield Viscometer

A major disadvantage of the gravity type viscometers is that
they do not reflect the true viscosity of non-Newtonian
and  thixotropic coatings.  Because many water-borne
and some solvent-borne products fall into this category,
the gravity type viscometers are inappropriate.

A Brookfield  viscometer can determine the apparent
viscosity and the shear thinning and thixotropic  proper-
ties of non-Newtonian fluids in the shear rate range of
0.1 to 50 per second"1 (4).

Three methods exist for characterizing the rheological
properties of the coating. The first consists of determin-
ing the apparent viscosity of a coating by measuring the
torque on a spindle rotating at  a constant speed. Unfor-
tunately, this method only measures the viscosity at one
rotational speed  so one cannot fully  understand the
non-Newtonian nature of the coating.

The second and  third methods consist of determining
the shear  thinning  and thixotropic (time-dependent)
rheological properties of the coating by measuring vis-
cosity at a series of rotational speeds of the spindle. The
agitation of the coating immediately before measuring
the viscosity is closely controlled. Measurements show
the correlation between the drop in viscosity with in-
creasing rotational speed, and also the increase or re-
covery in viscosity when lowering the rotational speed.
When the shear rate is high, the behavior of the coating
under true  application conditions provides more accu-
rate information as to how the  coating will behave after
it has been applied.

When measuring  only the apparent viscosity, the opera-
tor uses a constant rotational velocity, usually 200 rpm.
Brookfield  viscometers contain  a  spindle or paddle
designed to rotate at this constant  speed (see Figure
12-5). The instrument then measures the energy  re-
quired to maintain this constant  shear  rate, and since
the viscosity of thixotropic coatings is directly dependent
Figure 12-5.  Brookfield viscometer (photo courtesy of Pacific
           Scientific Catalog publisher).

 on the shear rate, the instrument is appropriate for this
 type of measurement.

 As with all viscosity measurements, the operator should
 measure viscosity at the application temperature. If this
 is not possible, however, the viscosity can be measured
 at a standard ambient temperature and  viscosity can
 then be extrapolated to the operating temperature.

 Unlike the gravity type viscometers,  the  Brookfield in-
 struments do require a certain degree of skill to achieve
 repeatable results. These  instruments are well  worth
 using, however, when the uniformity of film appearance
 is critical, such as in the automotive industry.

 12.4  Guidelines for Best Management
       Practices (BMPs)

 As stated earlier, properly managing viscosity  prevents
 pollution  by limiting the number of rejects and  reworks.
 Fewer rejects and reworks means less materials, less
 waste, and less pollution, as well as less cost. BMPs for
 controlling viscosity include the following:

 • Measuring  viscosity.

 • Maintaining constant temperature.

 • Drawing a viscosity/temperature chart.

 • Specifying an acceptable range of viscosities.

 • Developing  alternatives  for  achieving acceptable

 • Using heat to reduce viscosity.

 • Minimizing  waste disposal by choosing appropriate
  mixing  procedure.
 • Recognizing  thixotropic  properties of  water-borne

 12.4.1   Measuring Viscosity and Temperature

 Measurement of viscosity by the paint operator should
 not be optional because this  coating property helps
determine whether the operator can achieve an accept-
able finish. In measuring viscosity, the operator should
also measure the temperature of the coating.

To ensure  constant viscosity throughout the  working
day, the spray booth and the coating in the fluid  hose
leading to the spray gun should remain at a constant
temperature. This can be accomplished in one of two

• A facility may opt to use an air make-up unit to control
  the inlet air to the spray booth, but must consider the
  cost of heating  the air. Large spray booths, particu-
  larly downdraft ones, have high air throughputs (usu-
  ally well in  excess of  20,000 cfm) so the  cost of
  energy is high. Many paint facilities,  particularly those
   in cold climates, already have air make-up units in-

 • The facility can heat the coating  to a constant tem-
   perature, usually above ambient.  Even facilities with
   air make-up units on their spray  booths can benefit
   because the coating must often travel to  the booth
   from a mixing room that may be quite a distance from
   the booth. The coating that reaches the booth from
   the  uncontrolled mixing  room may be cold  in the
   morning and warm in the afternoon.

 An operator should draw a viscosity/temperature chart
 before qualifying or using any coating for the first time.
 Measuring the viscosity of the coating at progressively
 higher temperatures  accomplishes  this. The operator
 must ensure, however, that solvent does not evaporate
 from the coating while it heats.

 The procedure for preparing a viscosity/temperature
 chart using a Zahn cup follows:

 1. Mix the coating thoroughly before sampling.

 2. Fill  a quart can with the coating and measure the
   temperature, and then determine its  viscosity with
   the appropriate  Zahn cup. (Clean the cup before

 3. Take the lid from the  can and punch a small hole
   through the center. Insert an impeller  or paddle
   through the hole and replace the lid on the container.
   Then place the can in a  larger container of warm
   water. Thoroughly stir the  coating and measure the
   temperature.  After the  temperature  rises by an
   appropriate amount,  such as  5ฐF,  measure  the
   viscosity again.  Continue in this fashion,  always
   adding warmer water to the outer container, until
   several points can be plotted on a chart.

 4. If the ambient temperature is too warm, add ice to
   the  outer container to cool down  the  coating below

 The viscosity/temperature chart is very useful because
 it allows the spray painter to  interpolate  or extrapolate
 the appropriate viscosity when mixing the coating at the
 beginning of the shift. For instance,  if the paint is cold
 when starting in the morning,  instead of adding solvent
 to lower the  viscosity, the spray painter can set  the
 in-line heater to the temperature that yields  the most
 appropriate application viscosity.

 12.4.2   Specifying a Viscosity Range

The coating facility should specify an acceptable narrow
 range of viscosities that are compatible with the spray
equipment. Thereafter, quality control tests on incoming
 material should ensure that the coating vendor supplies
the same viscosity from batch to batch. If end-users do
 not perform such tests, they can expect batch to batch

 viscosity differences. This may not make a noticeable
 difference for manual spray gun applications but it will
 affect the  finishes  produced  by automated  guns,
 whether on  reciprocators or on robots. Viscosity control
 is critical  when applying metallic coatings; otherwise,
 apparent  color differences can  lead to  rejects or re-

 12.4.3  Developing Acceptable Alternatives

 If the coating is VOC compliant but the spray gun cannot
 achieve an  acceptable  finish, several alternatives are
 available. One alternative might be to experiment with
 different spray guns. For instance, if currently using an
 HVLP gun the end-user should experiment with HVLP
 guns from other vendors.  If this does not solve the
 problem, trying different spray gun types, such as air-as-
 sisted airless or electrostatic, may help.

 Also, if the coating is a single-component formulation,
 such as an air/force dry coating, or one that bakes at an
 elevated temperature, the end-user should experiment
 with a closed loop, recirculating, in-line paint heater. The
 advantage of heat is that it not only lowers the viscosity
 of the coaling but also tends to produce a more uniform

 12.4.4   Using Heat To Reduce Viscosity

 Although tlpis topic is closely associated with the prac-
 tice just described, its importance cannot be overstated.
 The use of heat rather than solvents to reduce viscosity
 is one of  the most effective  strategies for minimizing
 solvent emissions into the air. Facilities should consider
 in-line paint  heaters for both water-borne and solvent-
 borne, single-component coatings. The end-user must
 discuss with the paint manufacturer the efficacy of using
 heat to adjust viscosity of the coating. The manufacturer
 can  determine whether heat will be beneficial. Paint
 heaters are discussed in more detail in Section 12.7.3.

 12.4.5  Minimizing Waste Disposal

 To minimize the disposal of waste from  mixed plural-
 component coatings, a facility should carefully consider
 the manner  in which the coating is mixed. Chapter 10
 provides several guidelines but a summary follows.   Batch  Mixing

 For small batches, usually less than 1 gallon, it probably
 is best to premix the components in batches rather than
 to install a proportioner and mixing device. When using
 many colors in small quantities, usually less than 1
 gallon, it also probably makes more sense to premix the
 components. When  selecting the premix option, the
 spray painter should mix only as much coating  as the
job requires.
 Pot life is the time that elapses after a plural-component
 coating has been mixed, until its viscosity is so high that
 the operator can no longer achieve an acceptable finish.
 If the coating has  a short  pot life, the spray  painter
 should mix only as much coating as can be applied
 before reaching the pot life. If the pot life  needs to be
 longer in order to avoid wasting valuable material, the
 operator can cool the coating but should not chill it to a
 temperature that will cause condensation of moisture
 from the air to settle on the surface of the mixed coating.

 Pot life should not be extended by adding solvent to the
 mixed coating.  Not only may this  cause the coating to
 exceed the regulated VOC  limit,  but the solvent  may
 remain entrapped in the applied  coating  and lead to
 paint failures several months or years after the coating
 has been applied.  In-Line Mixing

 When a facility uses relatively  large volumes of plural-
 component  coatings, such  as epoxies and polyure-
 thanes, it might  be beneficial to  install proportioning
 equipment. Such equipment is designed to continuously
 measure the exact ratios of  the components being fed
 to the spray gun. For instance, an epoxy might be mixed
 in the ratio of 4 parts component A, 1 part component B,
 and 1/2 part thinner.
 A small stainless steel or plastic static mixer is inserted
 into the fluid hose only a foot or so upstream from the
 spray gun. A static mixer is  nothing more than a short
 tube,  approximately 8 inches long and with a diameter
 of about 3/4 of an inch. On the inside of the tube are a
 series of baffles  that force  the coating to repeatedly
 change direction as it passes through the tube.  As the
 unmixed components enter the static mixer, the  baffles
 cause extensive turbulence of the  components, so that
 when they emerge from the mixing tube, they have been
 thoroughly mixed.

 This strategy is probably the  most effective for minimiz-
 ing air, waste, and water pollution, and for cutting the
 costs associated with the disposal of the waste material.
 The cost to install a proportioner and mixing device will
 be offset by the savings. Afacility can expect a cost-pay-
 back within a few months, depending on the quantities
 of coatings used.
 Proportioners are ideal  when  using relatively large
 quantities, usually larger than 1 gallon, of a single color
 on  a  regular basis. They can  be  justified even when
 using many colors, but the quantity of each color must
 be large enough to warrant the use of the equipment.

 12.4.6  Recognizing Thixotropic Properties

When using water-borne coatings, the  spray painter
should try to apply them at the highest viscosity that will
give an acceptable finish. The spray painter should have

 little need to reduce the coating with water. Because
 many water-borne coatings are thixotropic, they can be
 spray applied at higher viscosities than most solvent-borne
 coatings; therefore, the spray painter should not assume
 that the application  viscosity for water-borne coatings
 should be the same  as for solvent-borne coatings.

 12.5  Managing Viscosity Differences for
       Different Coatings

 When reducing (thinning) coatings, it is important to
 recognize the different viscosity trends for solvent-bome
 and water-borne coatings. Figure 12-6 shows the  vis-
 cosity trend when reducing two different resin technolo-
 gies with the same solvent. The reduction in viscosity is
 somewhat predictable; that is, if adding solvent to each
 coating in small but equal increments, the viscosity re-
 duction will follow a uniform curve.  Because  of  this
 predictability many spray paint operators thin their coat-
 ings instinctively, without either measuring exactly how
 much solvent they add or determining the final coating
 viscosity by means of a suitable viscometer.

 Conventional low solids, solvent-borne coatings have
 traditionally been spray applied at viscosities of 18 to 25
 seconds and measured on  a Zahn #2 viscosity cup.
 Alternately, the newer higher solids formulations need to
 be sprayed at viscosities as high as 35 seconds, or even
 higher, which require measurement on a Zahn #3 cup.

 With  water-borne  coatings,  additional  complexities
 arise. Some formulations behave similarly to solvent-
 borne coatings  in that viscosity reduction follows a uni-
 form curve, as shown in Figure 12-7, Water-Borne Paint
 #2. Although the curve may have a similar shape as that
 for the solvent-borne paint, the entire curve is shifted to
 higher viscosities.

 A misconception exists among spray painters and oth-
 ers that all coatings  must be applied  at approximately
the same viscosity. Thus, when changing from  a high
 solids, solvent-borne coating to a water-borne, many
 painters immediately want to  reduce the paint with water
to bring  down  its viscosity  to so-called manageable
 levels. This, however,  is not always  appropriate.  For
 example, Figure 12-7 shows the preferred application
 viscosity for a solvent-borne paint as determined by the
 spray painter. It is possible that Water-Borne Paint #2
would need so  much diluting water to bring  down  the
viscosity to that of the solvent-bome application viscos-
 ity, that the thinned paint would be transparent and  run
down vertical surfaces.

Viscosity  management becomes  more complicated
when the viscosity/reduction  curve is not uniform, as is
the case for Water-Borne Paint #1 in Figure 12-7. In-
itially, the viscosity of the coating is relatively high and
 remains high even with the addition of small increments
of water. As more water is added, the viscosity drops
                  Reduction with Solvent #1
                  at Constant Temperature

Figure 12-6.  Effect of solvent reduction on viscosity.
                         Water-Bome Paint #1
                                 Water-Borne Paint #2
     Preferred Application Viscosity (Solvent-Borne)
                                  Solvent-Borne Paint

Figure 12-7.  Effect of reduction on viscosity for water-borne

precipitously before leveling out at even higher dilution
concentrations. If a spray painter were to apply Water-
Borne  Paint #1 at the "preferred application viscosity,"
the over-diluted coating would be transparent and would
simply flow down  vertical surfaces. The spray painter
might not be aware that it may be possible to apply

Water-Borne Paint #1 at its high package viscosity with-
out  any need to thin with water. Thus,  with current
coating technologies, the concept of a preferred appli-
cation viscosity for all coatings does not exist.

Unfortunately, vendor literature is not always sufficiently
clear on how much dilution a coating can tolerate. Most
spray painters who are new to the application of water-
borne coatings tend to want to over-dilute rather than
under-dilute these formulations.

The most  effective  method for determining  optimum
dilution is to start by spray-applying the coating to the
substrate at the package  viscosity.  If the results  are
unacceptable, the spray painter can dilute the coating
with water in small, measured increments. At the end of
each dilution,  the spray painter should stir the coating
well and spray-apply it. The optimum viscosity is  the
highest viscosity at which the coating can be applied to
achieve the desired dry film thickness as well as  the
absence of defects such as crate ring, pin holing, runs,
and sags.

12.6  Problems Associated With Viscosity

This section illustrates why it is critical to measure and
control viscosity. In the absence of proper viscosity con-
trol,  numerous types of film defects can  occur, often
resulting in reworks and rejects. Not only is this harmful
to the environment by adding to air,  water, and waste
pollution, but  it adds unnecessarily to the cost of  the
finished product.

12.6.1  Effect of Film Thickness Variations on
         Color, Gloss, and Drying Time

Generally, spray guns can more easily atomize low vis-
cosity coatings than  high viscosity formulations. In  the
case of most high solids solvent-borne coatings, which
tend to have relatively high viscosities, spray gun atomi-
zation becomes more difficult. This is why it is not  un-
common for film thickness variations  to occur when
spray applying a high solids solvent-borne coating to a
workpiece.  Such variations are accentuated when  the
workpiece has a complex geometry, as is the case with
many weldments and assemblies.

An observer can notice real and apparent color differ-
ences attributable to the film thickness of any coating
applied to adjacent areas of a workpiece.

If the coating  demonstrates some degree  of transpar-
ency, then the color of the substrate may shine through
in  those areas where the coating film build is relatively
light. If an adjacent area has a slightly heavier film build,
the coating may totally obliterate the substrate and  the
observer notices a  color difference between the two
adjacent areas.
 Similarly, when two adjacent areas exhibit differences in
 film thickness, the gloss of the coating appears different.
 Generally, the thicker the film, the higher the gloss. If
 gloss differences between adjacent areas are too pro-
 nounced, they can be a cause for rejects.

 A person's perception of color is influenced by the gloss
 of the finish. For instance, if a spray painter applies a
 black coating to a panel so that one  area has a high
 gloss while  the adjacent area has a matt or lusterless
 finish, an apparent color  difference ensues, depending
 on  the angle at which the observer views these areas.
 When the observer stands in a position such that the
 gloss of adjacent areas  cannot be seen, the color of
 these areas is identical.  On the other hand, when the
 observer stands at an angle that illuminates the gloss
 differences,  the higher gloss area tends to look a deeper
 and richer black, while the adjacent area looks dark grey
 or charcoal. Similar apparent color differences  occur
 with other colors, but sensitivity to gloss varies for each

 Film thickness variations also cause drying time differ-
 ences between adjacent  areas. Not only do the thicker
 films take longer to dry and cure, but other defects such
 as pin holing, cratering, solvent entrapment, and corro-
 sion are more likely to occur.

 12.6.2   Effect of Viscosity Differences on
         Metallic Colors

The application of metallic pigmented coatings is par-
ticularly sensitive to viscosity differences. Spray painters
who operate in industries  such as automotive and auto-
motive refinishing must know how to manage viscosity
to avoid color differences in metallic-pigmented coatings.

The luster that metallic pigments can achieve depends
to a great extent on the orientation of the pigments on
the top surface of the coating. Because most metallics
are flat platelets, the manner in which  they reflect light
depends on  their orientation relative to the observer. If
the coating has a high viscosity, the pigments will orient
themselves  differently than if the  coating has a lower
viscosity. Even minor viscosity differences can affect the
appearance  of metallic colors, and for industries that
require tight  color tolerances, such defects are among
the most common causes for reworks and rejects.

 12.6.3   Effects of Too Low a Viscosity

When the viscosity is too low, other  problems occur
resulting in rejects and reworks. For instance:

• The film  thickness may be insufficient  to provide
  proper hiding of the substrate.

• Transparency may occur particularly with pastel colors.

• Runs  and  sags are difficult to avoid.

 • Corrosion may take place prematurely.

 Each coating defect can result in reworks, rejects, and
 consequently more pollution and higher costs.

 12.7  Strategies That Optimize Factors
       Affecting Viscosity

 12.7.1  Effect of Plural-Component, In-Line

 One of the most common sources of liquid hazardous
 waste in a paint facility results  from surplus premixed
 plural-component coatings, such as epoxies and poly-
 urethanes, that can no  longer be used. A facility can
 often reduce the volume of hazardous waste from this
 source alone, sometimes by as much as 90 percent, by
 installing a  proportioner and in-line mixer. As  stated
 earlier, this equipment can measure the correct propor-
 tions  of the unmixed components and then mix them
 immediately prior to their entering the spray gun.

 The proportioner accurately measures or meters the two
 or three components only as they are about to be used
 (see Figure 12-8). Because the distance from the mani-
 fold to the spray gun is usually no more than a few feet,
 only a small amount of mixed material requires disposal
 at the end of the working shift.

With plural-component metering and mixing equipment
the  viscosity remains constant throughout the day, and
pot  life is no longer a concern. When selecting a plural-
component system, a facility must carefully establish the
accuracy of the measuring or metering mechanism.
Some  vendors have designed mechanical devices that
proportion the components, and others  use positive
pressure pumps.  In addition, some have alarms that
sound if one of the ball valves becomes blocked either
with resin or with dirt, and other design options are

All of the large spray equipment manufacturing compa-
nies sell at least one type of proportioning and mixing
device, and each  provides various options. Ease of
maintenance is a critical characteristic; it is important to
select  equipment requiring little  maintenance and hav-
ing few moving parts.

Muir (5) has written extensively on the selection of plural
component proportioners.

 12.7.2 Effect of Dilutant (Reducer or Thinner)
        on Viscosity

Although pollution prevention efforts  attempt to use
strategies other than solvents in managing viscosity,
solvent use is often unavoidable.

Each  organic solvent  affects the viscosity of a given
resin  system differently. Some  solvents may be very
 effective in dissolving the resin, while others may be
 marginal, ineffective, or even harmful.

 Most coating formulations contain a blend of true sol-
 vents  and diluents, the combination of which are in-
 tended to  provide  the  desired  coating  application

 A true solvent is defined as: "A substance capable of
 dissolving  another substance (solute)  to  form  a uni-
 formly dispersed mixture (solution) at the molecular or
 ionic size level. Solvents are either polar (high dielectric
 constant) or nonpolar (low dielectric constant)."  Water,
 the most common of all solvents, is strongly polar (di-
 electric constant 81), but hydrocarbon solvents are non-
 polar.  Aromatic  hydrocarbon  solvents have  higher
 solvent powers than aliphatics (alcohols). Other organic
 solvent groups are esters, ketones, amines, and nitrated
 and chlorinated hydrocarbons (6).

 A diluent is defined as: "A volatile liquid which, while not
 a solvent for the non-volatile constituent of a coating or
 printing ink, may yet be used in conjunction with a true
 solvent, without  causing precipitation. An ingredient
 used to reduce the concentration of an active material
 to achieve a desirable or beneficial effect" (7).

 Figure 12-9 illustrates how different solvents and dilu-
 ents can affect the viscosity of one resin.

 Some high solids, solvent-borne coatings are packaged
 with volatile organic compounds (VOCs) contents lower
 than the regulated limits, thus allowing the end-user to
 add a  small quantity of reducer for viscosity manage-
 ment. Since many spray painters experience difficulty
 when applying high solids, solvent-bome coatings, they
 often prefer to add reducers that eliminate film defects
 such as that known as orange peel. When the coating
 can tolerate only a small quantity of solvent, the spray
 painters must be able to select  a solvent or blend that
 can perform the reduction quickly. They usually prefer
 the solvent with the highest solubility parameters.  Unfor-
tunately, such solvents often evaporate rapidly resulting
 in a relatively dry coating application. The best recourse
 is for the operator to work closely with the coating manu-
 facturer who can identify the most effective solvent or
 solvent blend without degrading the coating application

 12.7.3  Effect of Temperature on Viscosity

One of the most effective methods for reducing viscosity
 is to raise the temperature of the coating  (see Figure

The effect of temperature differs from one  resin  to an-
other.  For instance,  a  high solids alkyd,  air-drying
enamel might have a relatively flat viscosity/temperature
curve, whereas a high solids, baking enamel may have

                                                          Catalyst Supply
                                                           and Pump
                      Solvent Supply
                       and Pump
                     Base Supply and Pump  Portioning Pump

Figure 12-8.  Plural-component proportioning system (Illustration courtesy of Graco Catalog publisher).
              Reduction with Solvents and Diluents
                  at Constant Temperature
Figure 12-9. Effect of solvents and diluents on viscosity.

a curve that is much steeper. Advantages and disadvan-
tages exist in both situations.

In the case of a flatter curve (see Coating A of Figure
12-10),  small fluctuations in  temperature  during  the
working day are unlikely to markedly affect the viscosity
and application properties. If there is a significant differ-
ence,  however, between the early morning and late
afternoon temperatures, the spray painter would notice
the change.

A reasonably flat viscosity/temperature curve is advan-
tageous to a paint facility that has no temperature con-
trols on the spray booth air and does not want to invest
in an in-line fluid heating system.

The disadvantage of a fjat viscosity/temperature curve
is that heating the coating by means of an in-line heating
system does not offer much benefit in terms of viscosity

Contrast Coating A in Figure 12-10  with a high solids,
baking enamel, such as Coating B.  Here, the viscosity
drops rapidly with even small temperature increases.
The most important advantage  of such a  resin is that
heating the coating to a reasonable temperature, such
as  100ฐF to 120ฐF, allows the operator to  spray the
coating at a reasonably low viscosity. In fact,  it is possi-
ble that solely heating the paint eliminates any need for
additional  solvent  reduction. Therefore,  heating the
coating to a reasonable temperature can be a very
effective strategy for lowering VOC emissions.

The disadvantage  of a  steep  viscosity/temperature
curve is that small temperature fluctuations can make
noticeable difference on the application of  the coating.
Facilities that struggle to maintain coating  quality may
find that the primary cause for coating finish differences
is the major and minor temperature fluctuations that take
place during a normal working day.


Figure 12-10.   Effect of temperature on viscosity.

Control of coating temperature  for single-component
coatings is usually cost-effective  because less solvent,
if any, is necessary for viscosity reduction, and coating
defects and rejects are minimized.
ne Heating
Paint heaters are available in various designs. In some,
the coating comes into contact with a heating element.
In others, heat transfer takes place  between a heated
fluid, such as water or an oil, and the coating. In at least
one other design, the heated fluid travels through the
outer annulus of a coaxial fluid hose, while the coating
travels through the inner core.

Although the first design might be the least  expensive,
its most  important disadvantage is  that if the  coating
does not constantly circulate through the fluid hose, hot
spots can occur where the coating remains in  contact
with the heating element for more than a few seconds.

In order to ensure constant  temperature throughout the
day, regardless of whether or not the spray gun is being
triggered, the equipment should be fitted with a return
loop so that heated coating that flows to the gun has an
opportunity to flow back to  the heater upon release of
the trigger. Moreover, the loop should go back only to
the inlet to the heater rather than all the way  back to the
pressure pot or coating reservoir. No need exists to heat
the coating in the hose between the reservoir and the
heater, nor does the coating  in the reservoir itself require
heating as this  unnecessarily consumes energy and
results in solvent losses from the open portion of the
system. To minimize the volume of coating  that needs
 heating, the in-line heater can be located close to the
 spray gun, on the wall of the spray booth. This way, the
 only coating that requires heating is the volume in the
 fluid hose between the heater and the spray gun, and in
 the return hose.

 Facilities that require absolute  consistency  in color,
 gloss, and film thickness should insulate the fluid hose
 between the heater and the spray gun. This is because,
 as Figure 12-10 shows, even slight fluctuations of tem-
 perature can cause noticeable  viscosity  differences,
 particularly with high solids baking coatings.

 Many end-users try to save money by purchasing dead-
 end  heating systems. This means that the fluid hose
 from the heater to the spray gun does not return back to
 the heater. While this may save a few dollars in initial
 capital expense, every time the spray gun is left untrig-
 gered, the temperature in the hose from the heater to
 the spray gun drops resulting in a corresponding viscos-
 ity increase. Then, when the operator pulls the trigger,
 the coating in the fluid line between the heater and the
 spray gun has a higher viscosity than the coating that
 emerges from the heater. This results in uneven finishes
 and other defects,  which of course leads to rejects and
 waste. The cost to convert a dead-end system into a
 recirculating one is expected to be minimal.  In-line Heating of Plural-Component
          Coatings With Metering and Mixing

 As was discussed  earlier, the use of in-line heaters for
 premixed plural-component coatings is not  recom-
 mended because this leads to very short pot lives. When
 using plural-component proportioners and mixers, how-
 ever, in-line heaters are beneficial because the coating
 is mixed only a few seconds prior to application. Thus,
 it is  possible to lower the viscosity of the individual
 components, even  if the mixed coating would otherwise
 have a high viscosity.

 Once again, heating a mixed plural-component coating
 dramatically shortens its pot life. As a result,  the fluid
 hose from the mixer to the spray gun, and the gun itself
 must be flushed clean before the coating has an oppor-
 tunity to gel! If the  operator does not follow this proce-
 dure, the fluid hose and spray gun  may need to be

 12.7A   Effect of Batch Mixing of
         Plural-Component Coatings

 Chapter 10 included a detailed explanation of pot life,
which  results when  plural-component  coatings  are
 mixed together.  Figure  12-11  illustrates the viscosity
 increase that takes place soon  after mixing occurs and
cross-linking commences.

 For a single-component coating, provided that the tem-
 perature remains constant throughout the day, the vis-
 cosity also remains constant. On the other hand, for a
 plural-component coating, the viscosity gradually rises
 within the first few  hours after mixing. Thereafter, it
 increases rapidly until the coating gels. The time that
 elapses after the coating has been mixed and until the
             Plural-Component Coating

Figure 12-11.  Effectปof viscosity on single- and plural-compo-
            nent coatings.

spray paint^ can no longer apply the coating to achieve
an acceptable finish, is known as the pot life. For some
plural-component coatings, the pot life  can be as long
as 8 to 16 hours, while for some of the more reactive
systems, it can be  less than  2 hours. A few new high
solids, two-component technologies possess pot lives
as short as a few seconds. These require special mixing

Bear in mind that even when controlling the spray room
temperature, the coating temperature can rise due to the
exothermic chemical reaction  that takes place.

While Figure 12-11 assumes that the coating remains at
a constant temperature, Figure 12-12  illustrates how
rapidly pot life can accelerate  when the temperature
increases. Clearly,  two  counteracting processes take
place when the temperature of a plural-component coat-
ing increases. First, higher temperatures tend to lower
the viscosity of resin systems, and this is beneficial
when high viscosity coatings  require spray application
to achieve acceptable finishes. Second, an increase in
temperature accelerates cross-linking, which in turn
shortens the pot life.

A facility may find  itself wondering which  of the two
mechanisms it should be  more concerned with. If the
viscosity of the coating is  allowed to increase well be-
yond the pot life, the coating would gel and plug the fluid
                       Plural-Component Coating
               High Temperature

Figure 12-12.  Effect of temperature on pot-life of plural-com-
            ponent coatings.

line and spray gun. Frequently, the cost and effort re-
quired to clean out the fluid passages is higher than the
cost to simply replace the equipment. This is why when
using plural-component coatings, the general rule is to
maintain as low as  practicable a coating temperature.
Usually, this is ambient, but in cases where  the spray
booth warms up during the working day, it is not uncom-
mon to wrap the reservoir with an insulating  blanket to
prevent a corresponding increase in coating temperature.

12.7.5  Methods for Increasing the Pot-Life of
         Batch-Mixed Plural-Component

The most effective method for increasing the pot-life is
to maintain the mixed coating at a cool temperature, but
not so  cold as to allow condensation of moisture to take
place.  The mixed coating should not be placed in a
refrigerator because moist air in the  head space above
the level of the mixed coating may condense and cause
gel particles to form within the body of the coating.

If the container or reservoir has no head space  and the
coating is filled to the top, then placing the mixed coating
into a refrigerator can prolong its pot-life. Before opening
the container again, however, allow the temperature  of
the coating to increase to approximately ambient condi-
tions to prevent  condensation of the outside air from
settling on the surface of the coating.

Another method to increase pot-life involves constantly
agitating the mixed coating, but at a slow speed rather
than too vigorously. The coating should not be agitated
by bubbling compressed air through it because moisture

in the air can react with the coating's curing agent, once
again promoting gelation.

The curing agents of many plural-component coatings,
particularly polyisocyanates, are sensitive to moisture. If
the reaction between the moisture and the curing agent
is allowed to take place, the viscosity rapidly increases
until the polymer gels. To prevent this, replace the air in
the head space above the curing agent with either a
nitrogen blanket or  pass  the air in the  head space
through a desiccant.

12.8 References
1. Sward, G.  1972. Paint testing manual, 13th ed., p. 181. Philadel-
   phia,  PA: American Society for Testing Materials.
2. Serway, R., and J. Faughn. 1992. College physics, 3rd ed. Saun-
   ders  Golden Sunburst Series. Orlando,  FL: Saunders College

3. Gardco. 1990. Gardco Catalog. Paul N. Gardner Co. Trade literature.

4. American  Society of Testing and Materials. 1995.  Rheological
   properties of non-newtonian materials by rotational (Brookfleld)
   viscometer. ASTM D2196-81. ASTM, Philadelphia, PA.

5. Muir, G. 1994. Plural component proportioners. In: Metal Finishing
   Organic Guide Book and Directory, p. 217. New York, NY: Elsevier
   Science Publishers.

6. Lewis Sr.,  R.J. ed. Hawley's condensed chemical dictionary, 12th
   ed. 1993.  New  York, NY: Van Nostrand Reinhold Publishing.

7. American Society of Testing and Materials. 1986. Compilation of
   ASTM standard definitions, 6th ed. ASTM, Philadelphia, PA.

                                              Chapter 13
                     Minimizing Solvent Usage for Equipment Clean-Up
 13.1  Introduction
 13.1.1  Pollution Prevention Considerations

 Solvents are used in various parts of a coating facility,

 •  Vapor degreasing

 •  Solvent wiping of substrates prior to painting

 •  Reducing paints and coatings to adjust viscosity

 •  Clean-up of spray equipment

 Earlier chapters covered in depth the strategies for mini-
 mizing solvent use through  different pretreatment fac-
 tors as well as application process factors. This chapter
 focuses primarily on minimizing the use of solvents for
 the clean-up of spray application  equipment, and for
 other miscellaneous purposes.

 Most facilities that use solvent-borne coatings find that
 their liquid  hazardous waste comprises mainly the sol-
 vents they  use to clean the fluid hoses, pressure pots,
 and spray guns. The solids content of liquid hazardous
 waste from a paints and coatings facility is often as low
 as 10 to 15 percent. This means that 85 to 90 percent
 of the liquid hazardous waste is a mixture of solvents.

 Most state  volatile organic compounds (VOCs) regula-
tions require that solvents used for equipment clean-up
 must be stored in closed containers. In addition, the
 regulations mandate that facilities clean the spray guns
within an  enclosed  container. The purpose  of these
provisions  is to minimize solvent  evaporation during
equipment  clean-up. As  a consequence, the industry
generates large volumes of spent solvent-paint mixtures
that are usually stored in  55-gallon drums.

Facilities can incorporate strategies for utilizing and
minimizing waste solvent. The strategies that this chap-
ter discusses are:

• Recycle solvents

• Minimize  emissions  of  hazardous  air  pollutants

• Follow regulatory provisions
 13.1.2  Decision-Making Criteria

 Decision-making criteria relevant to minimizing solvent
 usage for clean-up,  as addressed in this chapter, are
 highlighted in Table 13-1.

 13.2  Solvent Recycling

 A facility may use a solvent distillation unit to boil off the
 solvents, and then condense them in a clean 5-gallon
 pail  or 55-gallon drum. Figure  13-1 shows a  typical
 distillation unit.

 The distillation units that paint facilities  use are usually
 explosion-proof. They consist of a large permanent con-
 tainer  with a tight-fitting cover. Heating coils surround
 the outside  of the container. At  the end of a shift, or
 whenever appropriate,  the painter pours or pumps the
 mixture of waste solvent and paint into the container,
 closes the cover, and turns on the heating element. As
 the temperature rises, the most volatile solvents start to
 evaporate off into a long condensation coil. A refrigera-
 tion unit cools the coil, and as the solvents pass through
 the coil they condense  into clean liquid solvent. A hose
 at the end of the coil transfers the condensed solvent
 into a 5-gallon pail or 55-gallon drum. As the temperature
 in the unit continues to climb, less volatile solvents start
 to evaporate and subsequently condense. This process
 continues  until approximately 85 to 90 percent  of the
 waste solvent/paint mixture evaporates and condenses.

 The sludge remaining at the bottom of the unit is  a very
 high concentration of the paint solids. Typically, this only
 accounts for 10 to 15 percent of the original volume, and
this,  together with the polyethylene bag that contains it,
 is disposed of as solid hazardous waste.

The facility can  re-use the clean collected solvent as  a
clean-up solvent. Because the  solvent mixture might
contain a different blend of solvents from that used in
the formulation of the coatings, it is not common to use
the  condensed  solvent as a reducer for the  coatings.
One other approach is to sell the waste solvent to a solvent
blender or a facility that can use the solvents as fuel.

Solvent distillation units are available in all sizes, from
less than 5 gallons to 55 gallons. Names and addresses

 Table 13-1.  Decision-Making Criteria Regarding Minimizing Solvent Usage for Equipment Clean-Up

 Issue                            Considerations
 Does the facility send large
 volumes of solvent-borne paints
 as well as reducing and
 clean-up solvents out as waste?
Are any of the solvents used in
the facility HAPs or ODCs?

Does the  facility's potential to
emit solvents cause it to exceed
a threshold for Title III and/or
Title V?

Has the facility checked to see if
the solvents it is using have
high-boiling points?

Do the operators clean the
spray guns and fluid hoses by
atomizing solvent through the
spray guns?
• If yes, consider installing a solvent distillation machine that would allow for the on-site recovery of
 solvents; first, however, fully investigate the hazardous waste regulations that concern on-site
 solvent recovery and recycling.

> If distilling on-site is not possible, consult with a solvent recycling company to see if it is worth
 having the company perform your recycling.

> If yes, consider finding alternatives because most solvent companies now offer solvent blends
 that have excellent solvency but contain neither HAPs nor ODCs; for some applications, you may
 find solvents that are not VOCs, HAPs, nor ODCs.

> If yes, find alternatives that would allow the facility not to exceed this threshold; the cost benefit to
 the company is usually well worth the effort.
> If not, ensure this is done because It may be possible to substitute some of the more volatile
 (lower boiling) solvents with ones that have higher boiling points and evaporate more slowly.

> If yes, check local regulations because this is now an air pollution violation in many states.

> Consider flushing the hoses and guns by spraying a stream of solvent (not atomized) directly into
 a 55-gallon hazardous waste drum, and immediately replacing the lid.
Figure 13-1.  Typical solvent distillation unit (Illustration cour-
            tesy of Siva, a division of Flair Environmental).

of vendors appear in the annual buyers' guides that the
major coatings journals publish annually.

Joseph (1) has described  the permitting requirements
and alternatives for dealing with solvent recyclers. Be-
cause the issue is complex, readers should delve further
into the matter with their local state agencies or legal advi-
sors before making a decision to install such equipment.

In the absence of a solvent distillation unit, a facility can
reduce the cost of hazardous waste disposal by segre-
gating  the wastes. Water-borne paints, and  any other
                         water-borne  products, should  not be  mixed  with  the
                         solvent'wastes.  Papers,  masking tape, waste cups,
                         rags, etc., should also be segregated and not dumped
                         into the solvent waste drums. While the overall volume
                         of waste  remains the same, by segregating, the facility
                         can minimize the volume of waste it needs to send to a
                         hazardous waste  disposal  site.  Some  of the other
                         wastes might be able to go to a landfill. The potential costs
                         reductions for such segregation are well worth the effort.

                         13.3  Minimizing Emissions of Hazardous
                                Air Pollutants

                         Title III of the Clean Air Act Amendments of 1990  (40
                         CFR Part 63) lists solvents considered to be hazardous
                         air pollutants (HAPs).  The following list includes  the
                         most common  HAP solvents found in paints and coat-
                         ings formulations, as well as in  clean-up solvents:

                         •  Methylethylketone (MEK).

                         •  Methylisobutylketone (MIBK).

                         •  Toluene.

                         •  Xylene(s).

                         •  1,1,1 Trichloroethane (also an ozone depleting com-
                            pound, or ODC).

                         •  Methylene chloride.

                         Title III lists many other solvents and chemicals used in
                         paints and coatings formulations but they generally  ap-
                         pear in smaller quantities. To determine whether a coat-
                         ing formulation contains one of these HAPs or ODC's,
                         refer to the Material Safety Data Sheet (MSDS) that is
                         submitted with every delivery of paints and solvents.

 While paints and coatings facilities can still use these
 solvents in the foreseeable future, stringent air quality
 regulations encourage facilities to find substitutes. If
 substitutes are not possible, facilities must try to estab-
 lish measures for minimizing their emissions into the air.

 Both Title III "Hazardous Air Pollutant? and Title V "Per-
 mit Operating  Rule" (40 CFR Part 70) require a major
 facility to comply with their regulations. A major facility
 is one that has the potential to emit more than  10 tons
 per year (tpy) of any one HAP, or more than 25  tpy of a
 combination of HAPs, and some states may decide to
 lower these thresholds. Title V also considers a facility
 to  be major if its emissions  of VOCs  are more than
 100 (tpy).

 The definition of potential to emit is complex, so a facility
 should  seek clarification from  its local air pollution
 agency. Even a facility that does not use large quantities
 of paints and solvents may have a potential to emit over
 the threshold levels.  Because the definition of  "major"
 from above covers only some of the requirements from
 Titles III and V,  each company  should refer to both
 regulations to  determine all  of the criteria that might
 affect it. Both Basset  (2) and The Air Pollution Consult-
 ant (3) provide excellent sources for understanding the
 implications of  these regulations.

 13.3.1   Strategies To Minimize
         HAP Emissions    Using Substitutes

 Facilities that want to minimize their HAP emissions can
 ask their coating  vendors and solvent suppliers to use
 substitutes where such exist. Substitute solvents, how-
 ever, may affect the viscosity, drying time, and flow-out
 characteristics of  the coatings. Substitute solvents used
 for clean-up of equipment may not be as efficient as the
 original solvent blend. Therefore, tests must ensure that
 the compromises being made are acceptable to the
 paint facility. If  compromises are necessary, the facility
 should balance them  against the difficulty of having to
 comply with strict and possibly cumbersome Title III or
 Title V regulations.   High-Boiling Solvents

 High-boiling solvents evaporate slower than those with a
 lower boiling point. Thus, if an operator is cleaning spray
 equipment, he has a greater opportunity to capture dirty
 solvent before it evaporates. Therefore,  in formulating a
 blend for clean-up  purposes, a facility should consider one
 or more of the solvents listed in Table 13-2.   Example Blend: Ashland Chemicals

Ashland Chemicals has provided one blend of clean-up
solvent that has  worked well for  alkyds, epoxy, and
 Table 13-2.  High-Boiling Solvents (4, 5)
 Solvent Type
                                 Boiling Range
 Hydrocarbon Solvents:
 HI flash VM&P Naphtha         260 • 288        126 - 142
 VM&P Naphtha                244-287        118-140
 Mineral spirits                 307 - 389        153-198
 Odorless mineral spirits          354-388        179-198
 Stoddard solvent               308 - 388        154-197
 Aromatic Hydrocarbons:
 Toluene*                     230-232        110-111
 Ethyl benzene                 275 - 277        135-136
 Isobutyl alcohol                223 - 229        106-109
 n-Butyl alcohol                243-245        117-119
 Methylisobutylketone            237 - 244        114 - 117
 Methylisoamylketone            287 - 297        141-148
 a On the EPA 33/50 list as a hazardous air pollutant.

 polyurethane coatings, including chemical agent resis-
 tant coatings (CARC). The blend's formulation follows:
 VM&P naphtha (40%), methanol (20%), acetone (15%),
 n-butyl acetate UG (15%), and isobutyl alcohol (10%).

 Note that none of the solvents in the blend is either an
 HAP or an  ODC. In addition,  this formulation  is not
 unique or proprietary to Ashland and can be formulated
 by any solvent distributer.

 13.4 Regulatory Provisions

 State regulations regarding paints and coatings exist in
 the interest  of preventing pollution. These  regulations
 act as necessary and useful guides for facilities seeking
 to minimize the environmental  impact of their solvent
 usage for clean-up purposes. Surface coating regula-
 tions in several states have provisions similar to those
 in California. The following example comes from Califor-
 nia's South Coast Air Quality  Management  District rule
for the painting of metal parts and products.

 13.4.1  South Coast Rule 1107, (b)(3-7)

A person shall not use VOC-containing materials for the
clean-up of equipment used in  coating operations unless:

 • The VOC  is collected in a container which  is  closed
  when not  in use and is  properly disposed of, such
  that the VOC is  not emitted into  the atmosphere; or

 •  The spray equipment is disassembled and cleaned
   in a solvent vat, and the vat is closed when  not in
   use; or
 •  The clean-up materials contain no more than 200 g/L
   of VOC per liter of material.

 13.5  Process for Cleaning Spray Guns
       and Fluid Hoses
 When considering the  clean-up  of  equipment, one
 should bear in mind that all organic solvents have a VOC
 content well above 200 g/L, but the regulation implies
 that the organic solvent can be mixed with water or with
 an exempt solvent. Because the most common exempt
 solvent is 1,1,1 trichloroethane, which is both an HAP
 and an ODC, the end-user should use caution in using
 this option. Because of its status as an ODC, use of this
 solvent is gradually being phased out.
    Dirty t
Figure 13-2.  Typical spray gun cleaner (illustration courtesy of
           Siva, a division of Flair Environmental).

Spray gun cleaners are available in many different de-
signs but, in essence, they perform much like cold clean-
ing tanks. Figure 13-2 shows a typical spray gun cleaner.

A gun cleaner essentially comprises an enclosed sol-
vent tank. A door or lid allows access inside. The opera-
tor attaches the fluid hose  of the spray gun to a fluid
hose within the tank. Upon  closing the door or lid, sol-
vent pumps through the fluid hose of the  gun. The
operator can remove the clean gun after a few seconds.
When not in  use, the door or lid of the  cleaner must
remain closed.

When using an air atomizing or HVLP spray gun, a com-
mon method for flushing coating from the fluid hose of the
gun back into the container or reservoir is as follows:

• Turn down the fluid pressure from the reservoir but
  keep the valve open.
 • Set the air pressure to the gun at approximately 40
   psi or more.

 • Hold  a cloth  tightly in position in front of the gun air
   cap, and pull the gun trigger.

 • The air, which cannot escape from the cap, enters
   the fluid hose and forces the coating in the hose all
   the way back to the reservoir.

 • After the coating returns to the reservoir, use a small
   quantity of solvent to clean the inside of the hose.

 This technique  is very effective in dramatically reducing
 the quantity of solvent required. The following cautionary
 note, however,  must be read!

    Note: Under no circumstances must the technique for
    back-flushing coating to the  reservoir be used when
    air-assisted  or airless spray guns are being used.

    Airless guns do not have an air hose. If the spray
    painter holds his hand in front of the gun orifice and
    then pulls the trigger, the coating can be injected
    through the painter's skin. This will result in serious
    injury and hospitalization.
    Even though air-assisted airless guns have an air
    hose, the fluid  pressure for these guns can also
    cause harm to the painter. As a result, the technique
    for back-flushing the  coating into the pressure  pot
    also must not be carried  out with this gun.
 When cleaning an  air-assisted airless gun, the com-
 pressed air regulator should be closed to prevent unnec-
 essary atomization when  the trigger is pulled. The fluid
 orifice of both the air-assisted  airless and  the airless
 spray guns should be removed before pulling the trigger.
 The operator, however, must be cautious when remov-
 ing the  orifice  for the  reasons described in the note
 above.  The  operator  should  point  the gun  into a
 grounded 55-gallon solvent waste drum and then pull
 the trigger to flush solvent through both the fluid hose
 and the fluid passage of the spray gun.

 For maintenance of pressure  pots, many companies
 provide  pressure pots  with a polyethylene inner liner.
This is  advantageous  because the paint comes into
contact with the liner rather than with the steel or stain-
 less steel body. Cleaning the liner requires only a small
 quantity  of solvent.  After pouring it into the liner and
 swirling  it around for a few seconds, the  operator can
discard  the dirty solvent into  a 55-gallon  hazardous
waste drum. The inner liner can then be reused.

Some operators choose to allow the paint that sticks to
the side of the liner  to dry out, which causes it to flake
off with ease. If the solid paint is shown to be hazardous
per EPA guidelines,  it will be disposed of as solid haz-
ardous waste. If dry paint is shown to be non-hazardous,
it might be discarded to a landfill. Again, the liner can be

Regarding conventional air-atomizing, HVLP, and air-
atomizing electrostatic  guns, special spray gun hose
cleaners are available from spray equipment manufac-
turers. These force a mixture of air and solvent turbu-
lently through the fluid hose. The turbulence is  effective
in efficiently flushing the coating from the hose, whereas
only a small quantity of solvent is required. Some equip-
ment vendors, however, have withdrawn their products
from the market because they felt that the turbulence
atomized the solvent which could not then be collected
for reuse. The end-user must experiment to determine
whether or  not such a  device would minimize solvent
emissions into the air.
13.6  References
1. Joseph, R. 1995. Dealing with solvent distillation of waste paint
   filters. Metal Finishing 93:44.

2. Basset, S., ed. 1995. Complying with Clean Air Act regulations: Issues
   and techniques. New York, NY:  Elsevier Science Publishers.

3. The Air Pollution Consultant. (Bimonthly). New York, NY: Elsevier
   Science Publishers.

4. Ashland Chemical Co. No date. Solvent Property Chart. Colum-
   bus, OH. Trade literature.

5. Lide, D., ed. 1991/92. CRC handbook of physics and chemistry,
   72nd ed. Boca Raton, FL: CRC Press.

                                             Chapter 14
                 Paint Stripping: Alternatives to Solvent-Based Methods
14.1  Introduction

14.1.1  Pollution Prevention Considerations

Paint stripping is a process stage common to paints and
coatings operations. Although efficiently run operations
attempt to minimize the need for  paint stripping, the
need can still arise for workpieces either because the
applied coating is defective, the job specifications have
changed, or the original coating has aged. Occasionally,
process equipment (e.g., racks, vessels, booths, and
grates) also must undergo paint stripping to remove the
buildup of overspray.

Historically, operations relied extensively on chlorinated
solvents (e.g., primarily containing methylene chloride)
to remove coatings because formulations were inexpen-
sive to use and  their effectiveness well established.
Reliance on such approaches has become more expen-
sive, however, due to the cost of managing wastewater
contaminated with toxic chemicals  and controlling the
release of volatile organic compounds (VOCs).

For a  number  of years, operators—especially in the
automotive and heavy equipment industries—also have
been using aqueous paint stripping products where ap-
propriate. These  formulations  generate less pollution
because they are  based  on a relatively small amount of
organic solvent, but they are effective on a narrower
range  of coatings. The relatively new semi-aqueous
products, formulated with water and a nonchlorinated
solvent, await more extensive  use  in industrial opera-
tions to demonstrate apparent advantages (e.g., pollu-
tion reduction and effectiveness on resistant coatings).

In the  meantime, an array of  alternative approaches
involving "cleaner" technologies are gaining wider use
in paint stripping operations. These methods are consid-
ered cleaner because they rely on physical mechanisms
of action for coating removal rather than chemical sol-
vents. As a result, when used in appropriate industrial
applications, these approaches can help operators mini-
mize pollution generation, and thereby hold down asso-
ciated process costs.

Although these newer approaches offer important pollu-
tion prevention opportunities, the broad application of
any single method is unlikely. That is, rather than one
coating removal technology replacing solvent strippers
in all applications, operators will need to assess tech-
nologies on a process-specific basis. The appropriate-
ness of a technology for a particular facility will depend
on factors that include the type of coatings to be re-
moved and the nature of the workpieces' substrate.

Alternative technologies are discussed in this chapter in
the  context of  pollution prevention and process effi-
ciency considerations.

14.1.2  Decision-Making Criteria

Decision-making criteria relevant to the use of alterna-
tive paint stripping approaches, as addressed in this
chapter, are highlighted in Table 14-1.

14.2 Process  Basics

Paint stripping operations generally are conducted when
a  previously  applied coating on a substrate must be
removed. Usually paint is stripped from workpieces in
preparation for recoating. In some cases, however, met-
al workpieces and parts undergo surface polishing in-
stead of  painting (e.g., polished aluminum used for
some components in aircraft); thus, when appropriate,
one pollution prevention approach is to avoid the need
to apply a coating in the first place.

Paint stripping is a stage in  most paints and coatings
processes—even  at facilities where best management
practices are closely adhered to throughout the opera-
tion. This process step may be necessary for any of the
following reasons:

•  Defects are detected in the finished piece.

•  Specifications change after finishing (e.g., color, per-
   formance requirements).

•  A workpiece's original coating has aged.

•  Paint has built up on production line equipment from
   overspray (e.g.,  conveyor hooks  and racks, spray
   booth grates).

The decision whether to rework or scrap workpieces calls
for assessing the value of the particular piece in regard to

Table 14-1.  Decision-Making Criteria Regarding Paint Stripping Operations

Issue                                                Considerations
Are workpieces currently being stripped using a methylene
chloride product?
Are the workpieces high-cost items with high-performance
specifications (e.g., aircraft components), requiring that critical
tolerances not be altered by processing?
Must paint coatings be stripped from workpieces selectively (e.g.,
remove only the topcoat) or removed from delicate substrates?
Are the workpieces assemblies that include machined surfaces
and moving parts, and thus cannot be subjected to extensive
contact with water or to a small media that can become
entrapped by components?
Are the items to be stripped process-related equipment (hooks,
grates, baskets) coated with overspray?
• If so, consider the appropriateness of switching to "cleaner" paint
  stripping technologies that generate less pollution.

• If so and the workpieces cannot be subjected to extensive contact
  with water, consider plastic media blasting, wheat starch blasting, or
  carbon dioxide pellet cryogenic blasting.

• If so and the workpieces can be subjected to contact with water,
  consider high- or medium pressure water blasting or sodium
  bicarbonate wet blasting.
  If so, consider wheat starch blasting, sodium bicarbonate wet blasting,
  high-pressure water blasting, or carbon dioxide pellet cryogenic blasting.

  If so, consider carbon dioxide pellet cryogenic blasting.
  If so, consider medium-pressure water blasting, burnoff, molten salt
  bath stripping, or liquid nitrogen cryogenic blasting.
the number of pieces in the lot and the cost of reprocess-
ing. For all but the simplest and cheapest items, reworking
usually proves more cost effective than disposal.

As discussed in this chapter, paint stripping can  be
conducted  by various  means. The conventional  ap-
proach involves the application of a chemical solvent.
Traditional formulations are based on methylene chlo-
ride (60 to  65 percent), which penetrates the coating
causing it to swell and separate from the substrate. This
approach, however, generates organic vapors, which
raise concerns about threats to worker health and about
damage to  the ozone layer of the atmosphere, as well
as considerable sludge and wastewater laden with sol-
vent. Aqueous and semi-aqueous paint stripping formu-
lations, with smaller percentages of  chemical content,
also are available.  Although, these  less-concentrated
chemical  formulations  minimize  pollution concerns,
drawbacks  can include high cost, limited applicability,
and slower  and less-thorough performance.

As a result, industry and government have been con-
ducting extensive research into  the development of
paint stripping methods whose performance relies less
on solvents. Alternative approaches under development
or already in use involve on one or more of the  following '
general mechanisms of action:

• Impaction. Breaking up the coating by subjecting the
  workpiece surface to a flow  of grit material (media)
  delivered at high  velocity.

• Abrasion. Wearing away the coating by scouring the
  workpiece surface with a rough material; some media
  delivered at high  velocity have a scouring effect.

• Thermodynamics. Oxidizing,  pyrolizing, and/or vapor-
  izing the coating by subjecting the workpiece to heat.

• Cryogenics. Releasing the bond between the coating
  and the substrate by subjecting the workpiece to ex-
       treme cold, making the coating friable and inducing
       differential contraction.

     One abrasion approach, media blasting, is also used to
     clean corrosion and other contaminants from uncoated
     metal  workpieces  before  applying  a  primer-topcoat
     system, as discussed in Chapter 8 (on abrasive blast

     The  various  alternative approaches  discussed in this
     chapter are considered "cleaner" in terms of pollution
     generation because their performance  is  based  on
     physical mechanisms rather than solvents. These ap-
     proaches  also have their drawbacks. The information
     provided  is intended  as a brief introduction to  each
     technology; for more  detailed  information, see  EPA's
     Guide to  Cleaner Technologies: Organic  Coating Re-
     moval and EPA's  Reducing Risk in Paint Stripping: Pro-
     ceedings  of an International Conference (References 1
     and 2, respectively).

     Before adopting an alternative paint stripping approach,
     the facility operator  must fully consider the associated
     tradeoffs in respect to the specific paints and coatings
     operation. Factors to consider include:

     • Workpiece characteristics (e.g., size, substrate)

     • Coating composition

     • Surface specifications for the  stripped substrate

     • Processing rate

     • Facility space and process compatibility considerations
     1 The material presented in this chapter draws extensively from both
      of these EPA documents. Information is also available on the In-
      ternet; see, for example, the U.S. department of Defense's library
      home  page (http://clean.rt'.org/larry/nav_in.html)  or  EPA's Envi-
      rosense home page (http://es.inel.gov). Detailed information on par-
      ticular approaches also may be available from industry groups and
      trade associations.

 •  Pollution/waste generation

 •  Costs (i.e., capitol and operating)

 14.3  Solvent-Based, Aqueous, and
       Semi-aqueous Methods

 As described  in this section,  three conventional ap-
 proaches to paint stripping involve the use of chemical
 solvents in varying amounts. Given the increasing cost
 and regulatory constraints associated with traditional
 solvent-based approaches, more facility operators are
 assessing expanded application of aqueous methods
 and examining the potential advantages of semi-aque-
 ous products. This discussion briefly considers these
 three methods of coating removal. The section that fol-
 lows reviews a selection of more recently developed
 approaches that place particular emphasis on pollution
 prevention—the so-called cleaner technologies.

 14.3.1  Solvent-Based Methods

 Most paint stripping is conducted by immersing or spray-
 ing workpieces with an organic solvent-based formula-
 tion. The solvent penetrates the coating and undermines
 its bond with the  substrate, as indicated by wrinkling,
 bubbling, and blistering on the surface of the piece. The
 softened coating and solvent sludge are then wiped,
 scraped, or flushed away from the substrate.  Often a
 workpiece must undergo this process step several times
 before the coating is completely removed. After coating
 removal, the piece usually undergoes a water rinse.

 In general, solvent is only sprayed on workpieces if they
 are too large for immersion or if they are assemblies with
 sophisticated components that could be damaged by
 extensive contact with the solvent. If only very specific
 areas of an assembly need to be reworked, then the
 solvent may be wiped onto the appropriate surfaces.
Additionally, if only a small number of pieces need to be
 reworked, spraying might present a more cost-effective
approach than installing an immersion stripping line.

The most widely used paint stripping products are for-
 mulated with methylene chloride (also known as dichlo-
 romethane [DCM]). Although these chlorinated solvents
are effective, versatile, and relatively economical, their
use results in the release of VOCs, which are becoming
the focus of increasing regulation under the Clean Air
Act Amendments.  In particular, EPA has identified paint
stripping operations as a hazardous air pollutant (HAP)
source category. As a result,  such operations might
eventually be required to implement Maximum Achiev-
able Control Technology.

Additionally, solvent-based methods generate  sludge
and wastewater that contain toxic chemicals. Disposal
procedures required under the Resource Conservation
and Recovery Act (RCRA) and recordkeeping  require-
 ments under Section 313 of Title III can increase the cost
 of managing such wastes.

 Nonchlorinated solvents represent another broad cate-
 gory of paint stripping products. These solvents, which
 are based on such diverse chemicals as N-methyl py-
 rollidone, various glycols or glycol esters, and dimethyl
 sulfoxide,  are  used  almost exclusively in  immersion
 paint stripping operations. Although these solvents allow
 facility operators to avoid  concerns about  VOCs and
 minimize the generation of sludge with toxic constitu-
 ents, nonhalogenated products tend to be considerably
 more expensive than methylene chloride formulations.
 Additionally,  immersion baths of  nonhalogenated sol-
 vents must be heated (from 140ฐ to 250ฐF) to speed up
 their performance capabilities, which adds to opera-
 tional costs. Even when heated, however, nonhalogen-
 ated solvents  have  a somewhat selective chemical
 action and thus tend to be used in a narrower range of
 applications than methylene chloride solvents.

 14.3.2  Aqueous Methods

 Stripping paint with aqueous products is a  well-estab-
 lished methods for use in industrial operations process-
 ing  metal workpieces. Although aqueous products are
 water based,  formulations generally include  some
 amount  of an organic solvent. The most widely  used
 aqueous strippers have a caustic component. A typical
 formulation might include water, 10 to 20 percent sodium
 hydroxide, up to 20 percent organic solvent,  substantial
 amounts of surfactants (which are caustic, stable, sur-
 face-active agents), and a chelating agent.

 Caustic aqueous strippers are  primarily used in immer-
 sion processes. Immersion baths are heated  (from 180-
. to 240ฐF) to  accelerate the performance of the active
 agents in such formulations. In most operations, immer-
 sion is followed by a water rinse step.

 Historically this  type of aqueous paint stripper was
 widely used  in the automotive and heavy  equipment
 industries. The use of aqueous products in these indus-
 tries has declined over the years, however, as more
 resistant coatings have been introduced. Aqueous strip-
 pers are  still used in many operations that process home
 appliances and  are used generally to clean  process
 equipment. Formulations that include sulfuric or chromic
 acid also are  in use, but for more selective applications.

 As with nonchlorinated solvents, because aqueous for-
 mulations must  be heated  to  enhance their perform-
 ance, using  them can add to operating costs. Also,
 although they  minimize pollution  generation issues,
 aqueous products are effective on a  limited range of
 coatings  and can be  used  only on ferrous  metal and
 magnesium substrates.

 14.3.3  Semi-aqueous Methods

 Semi-aqueous products, which include water and a non-
 chlorinated solvent in roughly equal amounts, are rela-
 tively new and thus not yet in wide use. Such products
 are attracting considerable interest, however, based on
 indications that they are effective for stripping even the
 most resistant aircraft and aerospace paints.  Another
 attraction is that they can be used in both spray and
 immersion process lines. Also, sludge and wastewater
 generated by this  approach are considered  relatively
 easy to manage because they are generally free of toxic
 components (e.g., chrome, phenol).

 Drawbacks of this approach  include the higher cost of
 semi-aqueous products and the longer time required to
 achieve desirable performance.

 14.4  "Cleaner" Technologies: Alternatives
       to Conventional Methods

 This section briefly describes a selection of alternative
 paint stripping approaches and lists their respective ad-
 vantages and potential drawbacks. Approaches are pre-
 sented according to  their mechanism of action. It is
 unlikely that any one of these approaches will offer a
 broadly applicable means of stripping coatings in indus-
 trial  processes. Nonetheless, facility operators should
 consider such cleaner technologies when developing a
 strategy for minimizing pollution generation. A number
 of newer approaches not covered in this document also
 show promise for reducing process-related pollution in
 paint stripping operations. Certain of these emerging
 technologies in particular are promising and thus bear
 watching, including laser heating, flashlamp heating,
 and ice crystal blasting. (For information on these meth-
 ods, see Refs. 1 and 2.)

 14.4.1   Impaction Methods   Plastic Media Blasting

 Plastic media blasting (PMB) is an impaction  method
that is capable of removing a coating without damaging
the substrate of  a delicate workpiece. The approach
 involves projecting  plastic media at a workpiece's sur-
face either pneumatically with a hose-and-nozzle sys-
tem (usually in manual operations) or centrifugally from
 rotating wheels (in automated operations within a cabi-
 net). After the coating has been removed, the workpiece
 is vacuumed or subjected to high-pressure airto remove
 residual plastic dust. Because PMB is a completely dry
process that relies on a nontoxic media to remove coat-
ings, no wastewater or VOCs are generated.

 In most applications, the plastic media are collected and
cleaned,  using an air cyclone or vibrating screens, and
then reused several times before being discarded. De-
pending on the particular coating being removed, how-
ever, debris cleaned from the media may contain haz-
ardous metals or unreacted resins that require special
handling. In general, spent media are not recyclable or
biodegradable, although research is being conducted on
beneficial approaches to managing spent media.

The PMB approach has been widely used in both the
military and commercial  sectors.  While PMB is suffi-
ciently sensitive to selectively remove individual coating
layers, with larger and harder media this approach also
can be used to remove such resistant finishes as poly-
urethane and epoxy coatings. The PMB method is ap-
plicable for metal substrates as well as plastic surfaces.
PMB is used in the aerospace industry to remove coat-
ings without damaging sensitive underlying substrates
(e.g., the aluminum skins of aircraft).

Key advantages of the PMB approach include:

• Minimizes pollution generation. Avoids generation of
  wastewater and VOCs.

• Recyclability. If the correct plastic media is selected,
  they can be recycled up to 30 times.

• High throughput. Can be effective at a higher coating
  removal rate than is possible using some solvents.

• Broad applicability.  For example, it can be used on
  steel, aluminum, plastic, fiberglass, glass, printed cir-
  cuit boards,  and aluminum clad  materials.

• Sensitivity. Avoids damaging substrates or altering
  the dimensions of critical components; can remove
  individual coatings.

• Limited masking required. Less than for other con-
  ventional stripping processes, such as chemical strip-
  ping or sand blasting.

Principal limitations of the PMB approach include:

• Conventional sand or grit blasting can be faster.

• Less effective than other methods for cleaning proc-
  ess equipment with a heavy buildup of coatings.

• Less effective than other methods for removing rust
  and corrosion from metals.

• Larger and  harder  media can  damage plastic and
  composite substrates.

• Contaminants remaining in the  recycled media can
  damage substrates.

• Capital and startup costs can be higher than for con-
  ventional abrasive blasting.  Wheat Starch Blasting

Wheat starch blasting is an impaction  method that in-
volves use of generally the same techniques and proc-
ess  equipment  as   PMB.  The  principal distinction
between these two methods is the blast media: Wheat

 starch is even more gentle than  plastic. Additionally,
 because wheat starch is 100 percent carbohydrate, the
 spent media  is biodegradable. Using aerobic waste
 management processes, the media can be digested into
 a liquid that can then be separated from the coating
 debris. Also, wheat starch is a renewable agricultural
 resource that, for certain applications, can be used ef-
 fectively in place of petroleum-based media.

 Although wheat starch is relatively soft, it can be recy-
 cled several times before the particles become too small
 to be effective.  As the media  breaks  down, dust-like
 particles must be stripped from coarser particles in the
 recycling process.

 Wheat starch blasting is of interest primarily for its gentle
 stripping action. Thus, application and testing  of this
 method have been focused  on sensitive  substrates,
 such as thin aluminum (e.g., in the aircraft industry) and
 fiberglass and certain plastics (e.g., in the automotive

 Key advantages of the wheat starch blasting approach

 •  Sensitivity.  Avoids  damaging  substrates;  recom-
   mended particularly for substrates such as aluminum,
   soft alloys, anodized surfaces, and composites.

 •  Selectivity. Individual coatings can be removed (e.g.,
   only |he topcoat).
 •  Minimizes pollution generation. Avoids generation of
  VOCs and excess wastewater.

 •  Recyclability. If the correct plastic media is selected,
  they can be recycled up to 30 times.

 •  Moderate throughput. Can be effective at a moderate
  coating removal rate.

 • Low-cost, biodegradable media.  Wheat starch is an
  inexpensive, renewable resource; spent  media can
  be  biodegraded from sludge.

 Principal limitations  of the wheat  starch blasting ap-
 proach include:

 • Stripping action can be slow, depending on coating
• Media are sensitive to  moisture and can require the
  addition of an air drying system in humid environ-

• Removal of the media dust and paint chips requires
  a somewhat extensive  media recovery system.

• Dust generation can present  an  explosion potential
  unless precautions are taken.

• May not be  appropriate for workpieces that are as-
  semblies because media particles  can become en-
 • Less effective than other methods for cleaning proc-
   ess equipment because of the heavy buildup of coat-

 • Less effective than other methods for removing rust
   and corrosion from metals.

 • Contaminants remaining in the recycled media can
   damage substrates.   High- and Medium-Pressure Water

 Water blasting is a well-established impaction method
 for high-throughput surface cleaning that has emerging
 applications for coating removal processes. This blast-
 ing approach involves subjecting workpieces to jets of
 water delivered at sufficient pressure from rotating noz-
 zles to strip surface material without the benefit of an
 abrasive media. For high-pressure blasting operations,
 water is pumped at a rate ranging from 15,000 to 30,000
 psi. Medium-pressure blasting is performed with water
 jets operating in the range of 3,000 to 15,000 psi.

 This blasting approach generally avoids the generation
 of VOCs and  other air quality issues  associated with
 some wet  blasting media. For some operations, how-
 ever, workpieces first undergo a presoak with alcohol or
 a similar inorganic solvent. Water used in blasting op-
 erations can be recycled after if has been processed to
 remove debris.

 In  the  automotive  industry, medium-pressure water
 blasting is  used for stripping overspray coatings from
 part support hooks  used in water wall spray paint
 booths. Also, a German airline has used this approach
with presoaking to strip  aged coatings from  planes.
 High-pressure water blasting is being developed by the
 U.S. Air Force for paint stripping operations on large
aircraft.  (Ultra  high-pressure  water  blasting—from
30,000 to 50,000 psi—reportedly has been used selec-
tively to remove resistant coatings in  the automotive,
aircraft, ship building, and nuclear industries [2}.)

Key advantages of the water blasting approach include:

• High  throughput.  Can yield a high  rate  of  coating

• Minimizes pollution generation. Avoids generation of
  VOCs, dust,  and spent media; wastewater  can  be
  treated in a conventional treatment system.

• Recyclability. Water can be recycled after filtering out

• Low cost. Medium-pressure operations can have low
  capital and operating costs.

• Broad size applicability. No workpiece size restric-
  tions  unless  blasting cabinets are  used; the process

   can be used indoors or outdoors and in mobile op-
 Principal limitations of the water blasting approach in-

 • Capital costs can be high if  sophisticated systems
   are used (i.e., high-pressure systems would typically
   involve the use of robots).

 • High  volumes of water are necessary.

 • A filtration system is required to recycle blasting water.

 • Pressurized water jets can present a hazard to workers.

 14.4.2  Abrasion Method   Sodium Bicarbonate Wet Blasting

 Sodium bicarbonate wet blasting is an abrasion method
 that is similar in many respects to wheat starch blasting.
 The principal distinction is that the especially fine media
 used  for this method (baking soda) scours the surface,
 rather than breaking up the coating by impaction. As with
 wheat starch blasting, this method is sufficiently gentle
 to remove coatings without damaging the substrate.
 The media is delivered to the workplace from a nozzle
 generally at low pressure with a wet blast system (i.e.,
 in water at from 20 to 70 psi, although the system can
 deliver the media at up to 500  psi). As with other wet
 blasting approaches, the use of water avoids possible
 damage t
 Burnoff is widely used to strip thick overspray buildup
 from a variety of process equipment used in paints and
 coatings operations. Burnoff technology may be useful
 for removing coatings from workpieces in certain opera-
 tions, but limitations apply. For example, metals with a
 melting point below 900-F generally are not appropriate
 for this approach.

 Key advantages of the burnoff approach include:

 •  Performance. Effective for rapid removal of heavy,
   resistant coating deposits.

 •  Minimizes pollution generation. Avoids generation of
   VOCs and excess wastewater.

 •  Applicable for a wide range of part sizes. Applicable
   to all shapes; limitations relate only to the size of the
   burnoff unit.

 Principal limitations of the burnoff approach include:

 •  Temperatures are too high for parts made of plastics,
   composites,  or  metals with  relatively  low  melting
   points (e.g., zinc-bearing materials).

 •  Coatings  that contain chlorinated compounds can
   emit hydrochloric acid; when  part surfaces cool, hy-
   drochloric acid together with  atmospheric moisture
   can cause severe corrosion.

 •  Products of incomplete combustion containing heavy
   metals or other compounds may be generated,  re-
   quiring disposal as a  hazardous waste.

 •  Resulting  gases can present risk of fire.
 •  Abatement equipment (e.g., scrubbers or filters) may
   be required for offgas treatment.   Molten Salt Bath Stripping

 Molten salt bath stripping is a process that, like burnoff,
 is  currently  used primarily for fast removal of heavy
 coating deposits from process equipment used in paints
 and coatings operations. The molten salt process in-
volves immersing parts (either in baskets or suspended
from hooks) into a heated bath (from 550ฐ to 900ฐF)
containing inorganic salts (e.g., sodium carbonate). The
 salt functions  as a heat transfer medium, subjecting
 immersed parts to uniformly high temperatures  that re-
sult in chemical oxidation of the coating. Carbon and
 hydrogen in  the coating are oxidized to carbon dioxide
and water. The exothermic reaction that occurs in the
 molten salt bath minimizes the loss of heat that might
otherwise result from the immersion of cool parts.  In
general, metals from the coating pigments are retained
in  the molten salt bath, entering the offgas only in small

After the reaction has ceased, parts are removed from
the bath and allowed to cool. A thin coating of salt will
have formed on part surfaces and  must be removed.
This is accomplished by rinsing the parts in a tap water
bath at ambient temperature.

Along with sludge containing  primarily metal salts, the
process generates offgases and wastewater from the
rinsing step. Thus, operators must make provisions for
sludge disposal  and include  offgas abatement equip-
ment and wastewater treatment in their process line.

Molten salt bath  stripping is used primarily for supports
and fixtures used in coating application lines. The ap-
proach is relatively fast (bath dwell times range from
seconds to minutes) and particularly effective on heavy,
resistant coatings. It can be used to remove a variety of
organic coatings, including nylon, polyester, and epoxies.

Key advantages  of the burnoff approach include:

• Performance.  Effective for  rapid removal  of  heavy,
  resistant coating deposits.

• Pollution prevention. No VOCs (or odors) are gener-

• Applicable for a wide range of part sizes. Applicable
  to all shapes; limitations relate only to the size of the

• Not time-critical. If the metal substrate can  withstand
  immersion in the molten bath, the substrate  will not
  be harmed by overexposure (e.g., applies to most
  steels and to aluminum).

• Long bath life.  Sludge must be removed, but the bath
  itself does not need to be dumped  and replenished.

• Minimal treatment required for waste rinse  water. Af-
  ter making minor pH adjustments with a mineral acid,
  rinse water can be discharged; alternatively, because
  of its high pH, the water can be used beneficially to
  neutralize wastewater from  other acidic operations
  (e.g., from an acid pickling or phosphating  process).

Principal limitations of the molten salt bath  approach

• Generated sludge  must be disposed of and rinse
  wastewater treated.

• Abatement equipment (e.g., scrubbers or filters) is
  required for offgas treatment.

• Temperatures are too high for parts made of plastics,
  composites,  or metals  with  relatively  low  melting
  points (e.g., some die-cast alloys).

• Not appropriate for parts with sealed tubing because
  internal  pressure buildup can cause tube or weld
  failures  and pose a threat of explosion.

• Operator safety measures and  equipment must be
  included in the process (e.g., a fume hood must be
  installed to remove smoke generated by the process).

 14.4.4  Cryogenic Methods   Carbon Dioxide Pellet Blasting

 Carbon  dioxide (CO2)  pellet blasting is a cryogenic
 method  capable of removing coatings from specified
 areas of a workpiece while minimizing the amount of
 residue  left on the piece's surface. The approach in-
 volves projecting dry ice pellets at a workpiece's surface
 (at from 75 to 1,000 ft/sec) from  a nozzle. (A centrifugal
 projection system  is in development.)

 The equipment for this technology includes a system for
 converting refrigerated  liquid CO2 into the pelletized
 blasting  media. The media remove coatings by a com-
 bination of impact, embrittlement, thermal  contraction,
 and gas expansion. After the pellets strike the workpiece
 surface, they revert to a gaseous state, both enhancing
 coating  removal  and  avoiding  significant   residue
 buildup.  After blasting, workpieces are subjected to jets
 of air to  remove coating fragments.

 Because the  approach can  strip coatings selectively
 (i.e., specific areas of a workpiece as well as individual
 coating layers), it  has broad application for industries
 processing sophisticated parts and components. Appli-
 cations include the aerospace, automotive,  electronics,
 and food  processing industries.  For  example,  this
 method can be used on surfaces near moving parts and
 on sensitive electronic pieces.

 Key advantages of the CO2 pellet cryogenic blasting
 approach include:

 •  Selectivity/sensitivity. Can be used on specific areas
   of a workpiece and to remove individual  coatings.

 •  Process .efficiency. Minimizes  residue on workpiece
   surfaces. Also, the need for masking is either elimi-
   nated  or reduced to a minimum.

 •  Pollution prevention. Generates only small amounts
   of solid waste; also avoids handling of spent media
   and wastewater.

 •  Broad applicability. Can be used on a variety of sub-
   strates (e.g., steel, aluminum,  printed circuit boards,
  fiberglass, plastics).

 •  Minimized hazards. Uses a nonflammable, noncoh-
  ductive blast media.
Principal limitations of the CO2 pellet cryogenic blasting
approach include:
• Media cannot be recycled.
• Process equipment is relatively expensive.
• Throughput can be slow for workpieces with resistant
• Condensation can occur on the workpiece surface.
 • Safety equipment must be included in the process
   (e.g., a ventilation system for CO2 gas).   Liquid Nitrogen Blasting

 Liquid nitrogen cryogenic blasting is a variation of the
 PMB method that involves  chilling the workpiece  to
 embrittle  the coating before subjecting it to  impaction
 with a plastic media. The piece is sprayed with liquid
 nitrogen as it rotates on a spindle within a cabinet, and
 then  is blasted with  the  impaction media, which are
 projected into the cabinet by throw wheels.

 After chilling the coating  (to about -320ฐF),  the liquid
 nitrogen warms to ambient  temperatures  and evapo-
 rates into a gaseous form. This harmless  gas can be
 vented  to the  atmosphere,  leaving the media to be
 collected, separated from coating debris, and recycled.

 The liquid nitrogen cryogenic blasting approach is used
 primarily to remove coating buildup from certain types
 of process equipment used in paints and coatings op-
 erations (e.g., paint hangers, coating racks, floor grat-
 ings).  Operations in the  automotive and  appliance
 industries have used this method with success.

 Key advantages of the liquid nitrogen cryogenic blasting
 approach include:

 •  Minimizes pollution generation. Avoids generation  of
   wastewater and VOCs; because the process is dry,
   no water is used.

 •  Recyclability. If the correct plastic media is selected,
   they can be recycled numerous times.

 •  High throughput. Can be effective at a relatively high
   coating removal rate.

 •  Low operating costs. Compressed air and  electricity
   requirements are low.

 Principal  limitations  of  the  liquid  nitrogen cryogenic
 blasting approach include:

 •  Capital and startup costs can be high.

 •  Not appropriate for thin  coatings and  less effective
  on epoxies and urethanes.

 • The stripping cabinet  restricts the size of parts that
  can be  processed.

 14.5 References
 1.  U.S. Environmental Protection Agency.  1993. Guide to Cleaner
  Technologies: Organic Coating Removal. EPA/625/R-93/015. Of-
  fice of Research and Development, Cincinnati, OH (November).
2.  U.S. Environmental Protection Agency.  1991. Reducing Risk in
  Paint Stripping: Proceeding of an International Conference. Wash-
  ington, DC, February 12-13. NTIS PB91-224-303. Office of Toxic
  Substances, Washington, DC.

                                              Chapter 15
                             Minimizing Pollution in Spray Booths
 15.1  Introduction
 15.1,1  Pollution Prevention Considerations

 Almost every paint facility that applies coatings by spray
 has at least one spray booth  on the premises. This
 chapter describes the most common booths.  By ma-
 nipulating inefficient spray booth  parameters, facilities
 can minimize rejects and  reworks,  thus  lowering all
 forms of pollution.

 Efficient operation of  nearly all spray booths requires
 that they  capture  and retain paint particulates  using
 either dry filters or water. Thereafter, facilities must dis-
 pose of the medium (filters or water) together with the
 overspray. In large facilities, disposal can be a  major
 problem because  waste is so  voluminous. Moreover,
 disposing  pf these large quantities can be costly.

 Unfortunately, most companies purchase their dry filters
 on the basis of price rather than efficiency and holding
 capacity. This chapter offers guidelines for selecting the
 most appropriate filters. Alternately, water-wash  spray
 booths require chemicals to detackify or "kill" the paint
 overspray. Because  selecting  the most  appropriate
 chemical(s) is more complex than simply choosing a
 highly alkaline hydroxide, the chapter also offers advice
 for making this choice properly.

 Beyond these most basic decisions,  recognizing and
 altering the other factors that contribute to rejects and
 reworks allows facilities to minimize pollution and  maxi-
 mize efficiency. Transfer efficiency of a spray application
 is very sensitive to booth conditions, particularly air flow.
 Moreover, many coating defects result from poor booth
 design, poor booth maintenance, improper air flow, high
 booth humidity,  and other factors.  Rework of large ma-
chines can require  major repaints, which result in the
 unnecessary use of coatings. This of course  leads to
 more air, water, and waste pollution,  as well as higher
overall finishing costs.

The  primary  purpose  of this chapter is to provide a
background concerning  spray booths,  and to outline
strategies  for minimizing reworks that result from spray
booth parameters.  As most previous  chapters  have
 explained, a reduction in rework automatically lowers all
 forms of pollution and improves the bottom line.

 15.1.2  Decision-Making Criteria

 Decision-making criteria relevant to minimizing pollution
 in spray booths, as addressed  in this chapter, are high-
 lighted in Table 15-1.

 15.2  Definition and Function of Spray Booths

 A spray booth is an enclosure that directs overspray and
 solvent emissions from painting operations  away from
 the paint operator and toward  an entrainment section.
 Note that a spray booth is an abatement  device for
 particulates. It  is not an abatement device  for volatile
 organic compounds (VOCs). One can assume that all
 conventional spray booths emit all of the coatings' VOCs
 through the stack of the booth  or from the booth open-
 ings. The spray booth primarily exists to protect the spray
 painters and other employees from exposure to potentially
 toxic vapors and particulates.

 High concentrations of flammable solvent vapors always
 constitute a fire  hazard, particularly in facilities with
 welding and other spark-producing operations. Thus,
 another function of the spray booth is to  prevent fires
 within a facility.  Without spray booths, the risk of collect-
 ing a high concentration of flammable vapors in a facility
 is high. Booths quickly and efficiently exhaust the vapors
 to the atmosphere where the outside air dilutes them so
they no longer cause concern for a fire.

 Some researchers have tried to  determine whether VOC
 emissions can be concentrated by recycling the VOC-
laden air back  to the  booth and then  bleeding only a
 portion of it off to the outside. One of the industry's
 primary concerns is that this process can expose spray
painters to high concentrations of VOCs, although this
can be mitigated by providing the painters with fully air
conditioned suits so that they breath only outside clean
air. In fact, experimental work is now taking place at a
Marine Corps  base in Barstow, California, to further
explore this concept. Ayer (1) has already shown that
recycling the air and bleeding off only a small fraction,
which a thermal oxidizer or carbon  adsorber will then
abate, is both cost-effective and environmentally sound.

 Table 15-1.  Decision-Making Criteria Regarding Minimizing Pollution in Spray Booths

 Issue                                            Considerations
 Are the workplaces generally small and are they
 suspended from a conveyor or rack, or are they on
 Are the workplaces large and long, such as trucks,
 and must the paint operator paint predominantly
 from the sides?

 Are the workpieces large and long and must the
 paint operator paint from the sides, the top, and
 possibly the bottom?

 Can defects in the paint, such as settling of dust,
 be tolerated (e.g., paint operator is applying a
 primer, or a primer/topcoat system for workpieces
 that do not have high visibility)?
 Is the facility located in a cold or hot climate where
 spray booth air can go below 50ฐF or above 80ฐF,
 and must the coated finish be consistent, and have
 a high quality appearance?

 Is the facility located in a humid climate where the
 air in the spray booth can reach a relative humidity
 in the 90 percent range?

 Is coating usage generally less than 2 gallons/day
 per square foot of filtering surface area?
 • If yes and if the paint operator will always stand facing the filtering area, then using
   a cross-draft booth is probably best.

 • If yes but the paint operator must walk around the workpieces in order to spray
   from all sides, consider redesigning the conveyor or rack to one that rotates the
   workpieces, allowing the  paint operator to always face the filtering area; then a
   cross-draft booth would work.

 • If no, or if redesign of the conveyor or rack is not an option, you may need to
   consider a down or semi-down draft booth, even though it may cost more than a
   cross-draft booth.

 • If yes, consider using a cross-draft booth,  but ensure that air flows parallel to the
   floor toward the  filters.
   If yes, consider a down or semi-down draft booth.
   If yes, consider a two- or three-sided open booth.

   If yes, but overspray from the operation would enter the factory work area,
   affecting other workers and depositing on machinery, consider a totally enclosed
   spray booth.

   If no because the coating  must be free of dust and dirt, consider using a totally
   enclosed booth that draws air from either the factory area or, for even cleaner
   finishes, from clean outside air via an air make-up unit.

   If yes, consider installing temperature controls as part of an air-make-up system.
   If yes and if the paint operators are applying water-borne coatings, polyurethanes,
   moisture-sensitive coatings, or fast-evaporating solvent-borne paints, then consider
   dehumidifying the incoming air, preferably to 50 to 55 percent, although such low
   humidity levels might prove cost-prohibitive.

   If yes, you should probably use a dry filter spray booth.

   Even if coating usage is considerably higher, calculate the cost-effectiveness of
   using a dry filter versus water-wash booth since water-wash booths are associated
   with so many costs (i.e., dry filter booths require disposal of spent filters whereas
   water-wash booths require disposal of wet paint sludge as hazardous waste,
   buying necessary chemicals, occasional disposal of water in trough, etc.).
Is coating usage higher than 2 gallons/day per
square foot of filtering surface area?

Is the facility operator currently using inexpensive
paper or cardboard filters and finding the cost to
dispose of these filters to be a major problem?
Is the facility operator currently using water-wash
spray booths and finding the disposal of wet
sludge to be a problem?
• If yes, you may need to consider a water-wash booth but must first calculate its
  cost-effectiveness versus a dry filter booth.

• If yes, consider using a filter medium  with a higher holding capacity. Although filter
  cost would be higher, cost savings from lower disposal  costs would be significant;
  therefore, perform a cost-effectiveness analysis, consulting vendors for the wide
  range of available media.

• If considering switching to expanded polystyrene filters, experiment with them first
  to evaluate their cost-effectiveness. Brushing off dry overspray allows re-use  of
  these filters but disposal may involve  dissolving them in solvent waste and
  disposing of them as liquid hazardous waste.

• If yes, consider reviewing the chemicals currently in use because generally,
  chemicals are available that detackify the sludge, allowing for disposal of relatively
  dry sludge. A cost-analysis can determine if the resulting reduction in hazardous
  waste disposal costs justifies using the newer, more expensive chemicals.
15.3  Spray Booth Design

Spray booths come in all types of styles and configura-

•  Large or small
                 •  Open or enclosed

                 •  Bench type, walk-in,  or drive-through

                 •  Cross-draft,  down draft, or semi-down draft

                 •  Dry filter, water-wash, or baffle

The rest of the chapter provides guidelines for determin-
ing what type of booth is probably the most appropriate
for a particular application.

Facilities select the size of the booth based on the size
of the largest workpieces they must coat. If very few
large workpieces need coating in relation to the number
of smaller parts, it may be more economical to install two
booths: an inexpensive booth for the large pieces, and a
more sophisticated booth  for the remainder of the work.

Although a spray booth is  generally thought of  as an
enclosure, the booth  need not be totally enclosed. For
instance, when painting very  large workpieces, an op-
erator's booth may comprise only one side, namely the
exhaust plenum that draws the solvents and particulates
away from the operator (see Figure 15-1 a).

It  is atso not uncommon to  install two spray booths
opposite one another (see Figure 15-1b). This set-up
allows very large workpieces to  be transported in be-
tween the two booths, either  via a conveyor that runs
between the booths  or a forklift truck. Often neither
booth has a ceiling, and they draw air from the surround-
ing factory itself.
Exhaust plenum

                                            Exhaust duct
                                 Spray booth floor
                                 (a) Single-sided booth
          < r\-\Afi,y^
           N	•* " " 7  •' -'
                               (c) Three-sided walk-in
                                 or drive-in booth
                                 with open front
                               Spray booths can also  be small enough  to fit onto a
                               laboratory bench. It is not unusual to see a spray booth
                               that is 5 feet wide and only 4 or 5 feet high.

                               Spray booths with three sides have the exhaust plenum
                               along with two additional sides which simply prevent the
                               solvents and overspray from migrating into other parts of the
                               operations facility. Moreover,  these sides promote more
                               efficient air flow through the booth (see Figure 15-1c).

                               Totally enclosed booths comprise one or two sides with
                               the exhaust plenum(s). One of the other sides usually
                               contains the doors that can be opened to allow operators
                               to drive the workpieces into the booth (see Figure 15-1d).

                               15.3.1   Cross-Draft

                               In a cross-draft spray booth the air moves from behind
                               the operator toward the  dry filter or water curtain (see
                               Figure 15-2). The air travels parallel to the floor.

                               This type of booth is ideal when parts are suspended
                               from racks or a conveyor, and the spray painter applies
                               the coating essentially from only one direction. If, how-
                               ever, both sides of the part require coating, two options
                               are available:
                                                                   (b) Two booths
                                                                   facing each other
                                                                 w,  -,~/~N
                                                                 ;    U *
Figure 15-1.  Spray booth design concepts.
                                                             (d) Totally enclosed drive-in
                                                              booth with doors in front

                                            Filter or
Figure 15-2.  Cross-draft spray booth.

• The spray painter can rotate the part manually or, if
  a conveyor is used, it  can contain  a spindle" that
  automatically  rotates the part.

• The facility operator can install a second cross-draft
  booth that sits side-by-side with the first (see Figure

Facilities that do not require high-quality coating finishes
can draw the incoming air from the factory space around
the booth. Facilities in cold climates or those that require
high-quality, defect-free finishes can install an air make-
up  unit on the roof  of either the booth or the factory
building, and draw clean outside air into the booth.

Cross-draft  booths  are usually less expensive  than
down draft  or semi-down  draft booths. Vendors  can
provide detailed cost comparisons based on customer

15.3.2  Down Draft

Down draft spray booths move the air from the ceiling
of the  booth vertically downward  toward the  exhaust
plenum in the floor.
These types of booths have several strengths:

•  They remove the particulates by blowing the polluted
   air downward from the painter's face, minimizing the
   potential for inhalation.

•  When coating a large machine, they pull  the over-
   spray in the  shortest direction, downward,  thus pre-
   venting  overspray from  collecting  on the  freshly
   painted sides of the machine.

•  They allow more than one spray painter to coat the
   workpiece at the same time; overspray does not blow
   from  one operator toward the  face of another (see
   Figure 15-4).

•  They have the potential to provide the highest quality

The down draft booth is preferred when the paint opera-
tor walks around the part.  This method is particularly
popular  when  painting large  machines  or vehicles
(which cannot be rotated) that sit on a floor or grating.
In fact,  most facilities that paint large workpieces such
as weldments, assembled machines, vehicles, etc., use
a down draft spray booth.

These booths usually  cost  more than the cross-draft
booths because they require a pit below the floor of the
booth. The facility operator can either have the pit dug
from the floor of the factory or elevate the booth so that
the pit sits on the floor. In the latter design, three or four
steps lead from the floor into the booth. The advantage
of the first design is that operators can drive large work-
pieces into the booth, either on their own power or by a
forklift truck. The primary disadvantage is that the pit
must  be dug below the factory floor. The advantage of
the elevated booth is that it is less expensive, but this is
offset by the inability to drive workpieces into the booth.
Instead, either a conveyor or a hoist crane  is necessary
to perform this function.

Some down draft spray booths do not have  a ceiling,
and draw incoming  air from the  surrounding factory
area.  Most booths have a ceiling, however,  and draw air
either from the factory area or from the outside. The
Figure 15-3.  Side-by-side cross-draft booths.

                                                      15 feet deep, then for a cross-draft booth the minimum
                                                      air flow would be:
                                                          8,000 cfm
                    = 100 (fpm) x 10 feet x 8 feet
 Figure 15-4.  Down draft spray booth.

 need for a heated or unheated air make-up unit depends
 on climatic conditions and the need for a high quality,
 defect-free finish. For instance,  it is always  advanta-
 geous to maintain the booth temperature at between 65ฐ
 and  80ฐF, but very few companies feel they can afford
 the cost of controlling the temperature of high volume
 air flow rates. Facilities that produce high-quality fin-
 ishes and are located in very cold climates (in winter)
 and/or very hot climates (in summer), however,  often
 have little choice but to provide temperature controls.
 Similarly, iris usually beneficial to control the relative
 humidity in the booth at less than 50 percent, particularly
 when using water-borne coatings, polyurethanes, and
 other moisture-sensitive resins. This is because humid-
 ity can affect  the drying time of water-borne coatings,
 while it can  cause blemishes in polyurethane coatings.
 The  costs associated with controlling humidity can be
 prohibitive. Despite these high costs, companies that
 must produce high-quality finishes free from defects do
 indeed need to invest in air conditioning controls.

 While the capital cost of a down draft booth usually
 exceeds that of a cross-draft booth, the operating ex-
 pense is almost always considerably higher,  primarily
 because of higher air flow requirements.

 For example,  Occupational Safety and Health Admini-
stration  (OSHA) requires that the  minimum air velocity
through a spray  booth exceeds 100 feet  per minute
(fpm) in the direction of the exhaust  plenum or filter
bank, and primarily away from the face of the operator.
The following equation expresses the volumetric air flow:

   Volumetric flow  = Velocity (fpm) x Cross-
   (cfm)             sectional area (square feet)
                     of the filter opening

Consider two  spray  booths, each of identical interior
dimensions.  If the booth is 10 feet high x 8 feet wide x
 If the booth is a down draft design, and the entire floor
 opening draws air, then the minimum air flow would be:

    12,000 cfm     = 100 (fpm) x 8 feet x 15 feet

 In most cases, a down draft booth draws more air than
 a cross-draft booth, and the energy requirements in-
 crease proportionally. If the booth requires heat to warm
 the air during the winter months, the energy require-
 ments are accentuated.

 In addition, because the floor opening is usually signifi-
 cantly larger for a down draft booth than for a cross-draft
 design, the cost of replacing and disposing of spent dry
 filters, water, and chemicals are considerably higher.

 These are all  factors a facility operator must consider
 before selecting a down  draft booth.

 15.3.3  Semi-down Draft

 Semi-down  draft booths offer  two different  designs.
 First,  the booth can move the air from the ceiling at the
 front of the booth  toward  the floor at the back of the
 booth where the exhaust is located. Air movement is in
 a diagonal direction. Alternatively, the air can move from
 the center of the ceiling  down toward one of two level
 exhaust plenums located  along the  side walls of the
 booth. Figure 15-5 illustrates these two types of semi-
 down draft designs.

 Semi-down draft booths offer a compromise  between
 the cross-draft and down draft configurations  and pro-
 vide many of the advantages of the other designs.

 Companies choose between cross, down, and semi-
 down draft booths based on the type of workpieces they
 must paint. When paint operators  must walk around a
 large workpiece, the choice is usually between a down
 or semi-down draft booth. The latter is less expensive
 because it does not require a pit below the floor for the
 exhaust plenum.

 15.4   Dry Filter Spray Booths

 Because the choice between dry filter, water-wash, and
 baffle spray booths encompasses many issues, sepa-
 rate sections discuss each of these types of booths.

 Vickers (2) estimates that 80 percent or more of spray
 booths used in paint facilities are of the dry filter type. In
 recent years, many facilities have converted water-wash
 booths to dry filter because of their lower maintenance
costs and the often significantly lower hazardous waste
costs. The cost to  actually purchase  dry filters ranges

Figure 15-5.  Semi-down draft spray booths.

from $1 to $5 per filter, depending on a filter's efficiency,
holding capacity, and other characteristics.

15.4.1  Advantages

The advantages of dry filter spray booths are plentiful
and varied, ranging in areas from effectiveness to pollu-
tion prevention. For instance, regarding effectiveness,
dry filters effectively remove up to 95 to 99 percent of
particulates. High efficiency filters can reliably retain 99
percent of particulates. As a rule-of-thumb they are ideal
for low paint loading, i.e., approximately 2 to 5 gallon
coating usage per square foot of filter area per day.

They are also quite versatile. Facilities can use dry filters
in booths of all designs (small, large, cross-draft, down
draft, and semi-down draft). In addition, a wide selection
of available dry filter media can satisfy many end-users.
Filters  can accommodate companies that  require the
highest paint finish quality without constraints on the
cost of the  filters,  as well as  those having very low
appearance requirements and wishing to purchase the
least expensive.
 Unlike water-wash spray booths, facilities can operate
 dry filter booths even when using a range of coating
 technologies (e.g., polyurethanes, epoxies, alkyds, etc.)
 on the same day. Some exceptions, however, do exist:

 • If using nitrocellulose  paints,  auto-ignition (fire)  is
   possible if non-compatible coating is also deposited
   onto  some filters. Thus, do  not  apply nitrocellulose
   coatings and those of other resin technologies in the
   same booth.

 • Some filters are not suitable for water-borne coatings.
   Thus, if using solvent-borne and water-borne  coat-
   ings in the same booth, select  filters that are  com-
   patible with both.

 Finally,  dry filter booths are relatively inexpensive when
 compared with water-wash booths. This is partially be-
 cause of low maintenance and partially because of low
 waste disposal requirements. Maintenance essentially
 only requires periodic replacement of  the filter media.
 The cost of  waste disposal  can be negligible. Some
 companies dispose of their dry filters as follows:

 • They leave filters in the open to allow all solvents to
   flash  off.

 • If using baking enamels, they place filters in a baking
   oven  to allow the paint overspray to fully cure.

 • They conduct a toxicity characteristic leaching proce-
   dure (TCLP) test (usually only the first time this pro-
   cedure  is carried  out) to confirm that the filters do
   pass the test (i.e., that they are not hazardous).  If the
   filters  pass  the  test, they  are  disposed of  in  a
   dumpster. If they fail the tests (i.e.,  if they are haz-
   ardous), they are sent out as solid hazardous waste.

This TCLP test strategy alone can dramatically lower the
generation of hazardous waste. At best, the filters do not
constitute waste at all, and at worst, companies dispose
of them as solid hazardous waste, for which the disposal
costs are considerably less than for liquid hazardous
waste. If, however, a company would like to follow the
testing guidelines outlined above, it must take the fol-
lowing precautions:

•  Ensure  that the state acknowledges that dry filters
  containing  cured paint that have passed the TCLP
  test can legally be disposed of as garbage.

•  Ensure  that the spent filters are  tested for TCLP.

•  If the  contaminated filters pass the TCLP test, but at
  some time in the future the coatings change, then the
  TCLP test  must be conducted again to confirm that
  the new coatings also pass the test.

• The precaution to recheck for TCLP applies also  if the
  new coating is a low-VOC water-borne. Do not assume
  that water-borne coatings are non-hazardous. Some

   contain  heavy  metals and other ingredients  that
   would cause the filters to fail the test.

 15.4.2  Disadvantages

 While dry filters are ideal for some paint facilities, they
 do have limitations. For instance, they are generally not
 appropriate for large coating usage, i.e., greater than 5
 gallons per square foot of filter area per day. They also
 have disadvantages that affect their pollution and safety
 profiles, as well as their cost.

 Unless using continuous roll-up filters,  which are pro-
 hibitively expensive  for most  companies,  air flow
 through the booth diminishes as the filters load up with
 overspray. This can be a major drawback if air flow plays
 an important role in the finishing operation. (See Case
 Study #3 in Chapter 16.) In some facilities, an enclosed
 finishing room comprises several spray  booths. In most
 cases, the booths do not each have an air make-up unit,
 but all draw air from only one inlet duct. As each dry filter
 section becomes  loaded with overspray, the air flow
 within the  finishing room constantly changes, causing
 quality problems. Turning the blowers on and off during
 the working shift aggravates the situation. In extreme
 cases,  the air flow can become so turbulent that it
 continuously changes direction (e.g., moving toward the
 filter bank  for a few seconds, and then  reversing itself
 for the next few seconds). Such turbulent air can carry
 overspray from one booth onto freshly painted surfaces
 in the other booth(s). This cross-contamination can lead
 to very expensive reworks and  rejects, and  ultimately
 results  in  unnecessary pollution  and costs. Another
 pollution-related consideration is that dry  filters do not
 remove VOCs.

 Regarding safety, dry filters  are a potential fire hazard,
 particularly if dry overspray is allowed to build up. Over-
 spray of coatings, such as nitrocellulose lacquers, can
 cause spontaneous combustion. Fortunately, nitrocellu-
 lose  coatings are phasing out and states that enforce
 low VOC limits on coatings have all but outlawed them.
 Because of their risk of fire, installation of a  sprinkler
 system  is a requirement.

 Finally, storage of  unused filters  requires space. Facili-
 ties with large spray booths may find this problematic.
 In addition, used, spent filters are bulky and also occupy
 much space. This  potentially increases the cost of dis-

 15.4.3   Selecting Dry Filter Media

 When selecting the proper dry filter, facilities need to
take  into account several filter characteristics. Among
these are:

 •  Efficiency

 •  Resistance
 • Holding capacity

 • Incineration profile

 • Biodegradability

 • Landfill option profile

 • Flammability

 • Suitability for various coatings

 Efficiency is the ability of the filter to remove particulates
 before they can enter the exhaust stack.  Selecting a
 filter that has a high retention efficiency, at least 96 to
 99 percent, is  important.  Note, however, that the effi-
 ciency only affects the amount of particulates, or PM10,
 that escapes into the air. Some state  regulations place
 daily limits on the amount of PM10 that facilities can emit,
 and here the retention efficiency of the filters  can be
 crucial to compliance.

 If the efficiency is relatively low, less  than  96 percent,
 escaping particulates can possibly settle outdoors, even
 on vehicles in the employee parking lot. Companies that
 have  experienced these problems have found it  well
 worth the expense to purchase higher efficiency filters.
 Also,  note that the retention efficiency of the filters has
 no bearing on the amount of hazardous waste that may
 require disposal.

 In addition, high efficiency filters reduce the loading of
 overspray on surfaces inside the spray booth exhaust
 duct,  and particularly on the fan impeller. This,  in turn,
 reduces the frequency with which the  interior section of
 the booth requires cleaning. If a  significant amount of
 overspray escapes into the spray booth stack, it  can
 increase the energy required to drive the impeller. High
 efficiency filters minimize this problem.

 Resistance of a filter refers to the pressure differential
 that ensues when the high velocity air passes across the
 filter bank. Facilities should select a filter with low airflow
 resistance. While this strategy lowers  the energy costs
 required to run the booth, it is unlikely to have an impact
 on pollution.

 Holding capacity is the amount of overspray that a filter
 can hold or retain during its service life. Selecting a filter
 medium with a high  loading capacity  is wise. This  re-
 duces the frequency for replacing the  filters, and  re-
 duces the volume of waste that may require disposal as
 hazardous solid waste.

 Facilities should check on whether they can incinerate
the filters and that the filters meet all incinerator stand-
 ards. This gives the end-user the option to incinerate the
filters  rather than to dispose of them as  a solid hazardous
or non-hazardous waste. The end-user should first de-
termine whether incineration is an acceptable procedure
within the state. Some states may not allow  incineration

 by the end-user, although a certified waste incineration
 company may be able to perform this function.
 Facilities also may find it advantageous to use filters that
 are biodegradable. Regardless of this,  facilities should
 check to ensure that the filters meet all landfill standards.
 Another important factor is flammability. Check that the
 filters meet the requirements of the National Fire Protec-
 tion Bulletin #33 and that Underwriter's Laboratories has
 approved the filters as Class 2.
 Finally,  some water-borne coatings can complicate the
 choice of dry filter used. Some filters, particularly if made
 of paper, may not be suitable for water-borne coatings.
 The literature contains very little concerning the selec-
 tion of dry filters for capturing paint particulates. Vickers
 (2)  provides  some interesting information about the
 materials  manufacturers use to make  spray booth fil-
 ters. Howery (3) has presented an excellent paper ad-
 dressing the properties of different dry filter media (see
 Table 15-2).
 Table 15-2 shows that when filtering high solids baking
 enamel, filters such as  the standard filter have a low
 retention efficiency of  96.5 to 97.5 percent and a low
 holding capacity of only 2.8 pounds per filter pad. Com-
 pare this with the high efficiency filter that has a retention
 efficiency  of 98.5 to 99 percent and a corresponding
 holding capacity of 5.4 pounds per filter pad. The stand-
 ard filter is a low-cost paper or cardboard filter, while the
 high efficiency filter comprises several layers of material,
 including Kraft paper and several layers  of fiberglass
 matting, each with progressively smaller porosities.
 Tables  15-3 and 15-4  provide the worksheets for hypo-
 thetical  paint facilities  using 65 and 30 percent  transfer
 Table 15-2.   Efficiency and Holding Capacity of Dry Filters" (3)
 Description            Efficiency        Holding Capacity

Standard filter
High-capacity filter
High-efficiency filter

Standard filter
High-capacity filter
High-efficiency filter
High Solids Baking
Enamel Average
Efficiency Range
96.5 - 97.5
94.0 - 96.0
98.5 - 99.5
Water-Borne Bake
Enamel Average
Efficiency Range
93.0 - 94.0
97.0 - 98.0
Holding Capacity1"
(inches, water column)
2.8 Ibs @ 0.10
6.5 Ibs @ 0.10
5.4 Ibs @ 0.50
Holding Capacity6
(inches, water column)
4.8 Ibs @ 0.50
8.7 Ibs @ 0.50
4.0 Ibs @ 0.50
 efficiency as the basis. Both values seem reasonable for
 a typical paint facility. The 30 percent value represents
 the average small-to-medium parts facility, while the 65
 percent  value  represents medium-to-large size  parts
 and machines. This worksheet  model allows one  to

 Table 15-3.  Cost of Waste With 65 Percent Transfer Efficiency

 Table of Assumptions (Vary Filter Holding Capacity
 and Cost of Filter)
a Performance figures were obtained using representative current in-
 dustry coatings in an air-atomizing gun with two pads in tandem at
 a face velocity of 200 fpm.
 Test paint was very fluid and slow drying, resulting in excessive
 run-off on standard and high-capacity filters, with little resistance
 Surface area to be coated
 VOC of coating
 Density of VOC portion
 % Volume solids
 Weight per gal (WPG)
 % Weight solids (Calculated)
 Cost of coating
 Dry film thickness
 Transfer efficiency
 Size of filters
 Number of filters across
 Number of filters down
 Total number filters affected
 Holding capacity of filters
 Percent of overspray going into filters
 Percent efficiency of the filters
 Percentage PM10 in the overspray
 Cost of filters
 Number of filters which can be
 disposed of in 55-gal drum
 Cost to dispose of 55-gal drum
 Days of operation
 Labor required to replace filters
 Labor rate
 Total liquid gallons required
 Total liquid coating used
 Total solid coating used
 Density of solid coating (Calculated)
 Weight of solid coating used
 Weight of total solid overspray
 Weight of solid overspray in filters
 Number of filters to be disposed of
 Number of filter changes per year
 Number of 55-gallon drums to be
 disposed of
 Cost of hazardous waste disposal
 Cost of filters
 Labor hours to change filters
 Labor cost to change filters
 Number of wasted gallons
 Cost of wasted paint
 Cost of waste + filters + labor
 Cost of wasted paint
Total cost of waste
 3,500.00 ff/day
 3.5 Ib/gal
 7.36 Ib/gal
 9.8 Ib/gal
 $20.00 $/gal
 1.5 mil
 20" x 20"
 3 Ib/filter
 40 filters/drum

 $300.00 $/drum
 251 days/yr
 0.5 hours
 $15.00 $/hour

 9.60 gal/day
 2,409.94 gal/yr
 1,263.91 solid gal/yr
 12.01 Ib/gal
 15,182.61 Ib solid/yr
 5,313.91 Ib solid/yr
 3,188.35 Ib solid/yr
 1,062.78 filters/yr
 33.21 filter changes/yr
 26.57 drums/yr

 $7,970.87 $/yr
 $1,062.78 $/yr
 16.61 hours/yr
$249.09 $/yr
843.48  gal/yr
$16,869.56 $/yr

$9,282.74 $/yr
$16,869.56 $/yr
$26,152.30 $/yr

 Table 15-4.  Cost of Waste With 30 Percent Transfer Efficiency

 Table of Assumptions (Vary Filter Holding Capacity
 and Cost of Filter)
 Surface area to be coated
 VOC of coating
 Density of VOC portion
 % Volume solids
 Weight per gal (WPG)
 % Weight solids (Calculated)
 Cost of coating
 Dry film thickness
 Transfer efficiency
 Size of filters
 Number of filters across
 Number of filters down
 Total number filters affected
 Holding capacity of filters
 Percent of overspray going into filters
 Percent efficiency of the filters
 Percentage PMio in the overspray
 Cost of filters
 Number of filters which can be
 disposed of in 55-gal drum
 Cost to dispose of 55-gal drum
 Days of operation
 Labor required to replace filters
 Labor rate
 Calculations (
 Total liquid gallons required
 Total liquid coating used
 Total solid coating used
 Density of solid coating (Calculated) ~
 Weight of solid coating used
 Weight of total solid overspray
 Weight of solid overspray in filters
 Number of filters to be disposed of
 Number of filter changes per year
 Number of 55-gallon drums to be
 disposed of
 Cost of hazardous waste disposal
 Cost of filters
 Labor hours to change filters
 Labor cost to change filters
 Number of wasted gallons
 Cost of wasted paint

 Cost of waste + filters + labor
 Cost of wasted paint
Total cost of waste
 3,500.00 ft2/day
 3.5 Ib/gal
 7.36 Ib/gal
 9.8 Ib/gal
 $20.00 $/gal
 1.5 mil
 20' x 20"
 3 Ib/filter
40 filters/drum

$300.00 $/drum
251 days/yr
0.5 hours
$15.00 $/hour

20.80 gal/day
5,221.53 gal/yr
2,738!47 solid gal/yr
12.01 Ib/gal
32,895.65 Ib solid/yr
23,026.95 Ib solid/yr
13,816.17 Ib solid/yr
4,605.39 filters/yr
143.92 filter changesyr
115.13 drums/yr

$34,540.43 $/yr
$4,605.39 $/yr
71.96 hours/yr
$1.079.39 $/yr
3,655.07 gal/yr

$40,225.21 $/yr
$113,326.65 $/yr
calculate the total costs of hazardous waste from a dry
filter spray booth.  The first half of each of the tables
provides the  assumptions used, and the second half
provides the calculated results.
 Figure 15-6 charts the cost of filter disposal for different
 filters of increasing  holding capacity. In producing the
 chart, the same two transfer efficiency values were as-
 sumed, 30 and 65 percent. The chart assumes a con-
 stant dry filter retention efficiency, but demonstrates how
 the cost savings increase when using filters with higher
 holding capacities. (The more expensive the clean, new
 filter is, the higher its holding capacity.)

 Figure 15-6 clearly shows that the greatest pollution and
 cost reductions occur when the initial transfer efficiency
 is  low and  small  improvements are made. At higher
 transfer efficiencies, the benefits are  less pronounced.
 The next most important parameter is the filter's holding
 capacity. Even though the cost of the filter increases with
 greater holding capacity, so do significant cost savings
 and pollution reduction.

 Another issue that may require attention when assess-
 ing filter booths is PM10 (particulate matter, the size of
 which  is less than  10 microns).  Industrial  hygienists
 have established that particulates of such small dimen-
 sions  often  remain suspended in  air for long  periods,
 allowing workers to breathe them. Due to gravity, larger
 particles tend to settle to the ground. Many states are
 now including conditions in spray booth permits that limit
 PM10.  For instance,  California requires Best Available
 Control Technology  (BACT) when a  new  or modified
 permit requests an increase of 2.0 Ibs/day PM10.  Both
 transfer efficiency and filter efficiency play dominant
 roles in determining  whether or not the PM10 threshold
 will be exceeded.  In addition, as transfer efficiency in-
 creases, the  need is less to pay for high retention effi-
 ciency filters.

 Filters are available that are made from expanded poly-
 styrene. The advantage to these filters is that facilities
 can reuse them after carefully brushing overspray off the
 surface with a bristle brush. Hence, the same filters can
 function several times until they break or have otherwise
 degraded. The manufacturers also promote the idea that
 when  a facility is ready to scrap a polystyrene filter, to
 immerse it into a 55-gallon drum of existing solvent/paint
 waste. Because of the strong solvents paint facilities
 use, the large filters quickly dissolve into small volumes
 of liquid, which must also be handled as liquid  hazard-
 ous waste. Some companies argue that converting solid
 hazardous waste to liquid hazardous waste is counter-
 productive. A cost  analysis will determine whether the
 conversion from solid to liquid is  cost effective. Other
 companies argue that solvent and paint wastes exist
anyway, and the small volume of added polystyrene is
 negligible when compared with the existing liquid waste.
 Finally, there are those who say that converting the solid
filters into liquid hazardous waste is actually  treating a
 hazardous waste,  and  that this is a  violation of the
 Resource Conservation and  Recovery Act (RCRA)

    $70,000  T
    $60,000  - -
    $50,000  - -
 g  $40,000  --
 J5  $30,000
    $20,000  - -
                                                                                   Transfer Efficiency = 30%

                                                                                   Transfer Efficiency = 65%
                                                       4                      6
                                         Filter Holding Capacity (Ibffilter)
Figure 15-6. Cost gf filter disposal based on holding capacity.

regulations. The journal Metal Finishing further detailed
this complex issue in its May 1995 issue (4).
This is a controversial topic. Each company must decide
its best option based on its own policy, the facts surround-
ing its individual situation, and its own state's regulations.

15.5  Water-Wash Spray Booths

The most important alternative to dry filter spray booths
are water-wash booths. Instead of collecting overspray
in the filter bank, a constant stream of  water  in an
entrainment section scrubs the overspray from the air
that the booth exhausts. Some water-wash booths are
designed with a water curtain, but this is not a prereq-
uisite. Most commonly, cross-draft and semi-down draft
booths have water curtains while down draft booths do not.

Water  flowing down the curtain collects  much of the
overspray, but the scrubbing action in the entrainment
section is more  important. In the entrainment section,
fixed baffles force the exhaust air to constantly change
direction and, as this occurs, the water scrubs the par-
ticulates from the air or they simply fall into the  water
Even after the overspray enters the water,  it remains
sticky  and can  plug  up holes,  nozzles, pipes, and
pumps. In addition, it can form a deposit on the  water
curtain, slowly building  up a layer that eventually im-
pedes  the smooth water flow down the curtain's face.
With time, the water becomes contaminated with bacte-
ria and requires disposal.

To prevent these unfortunate occurrences, the water
needs treatment with one or more chemicals designed
to detackify the overspray particles  (i.e.,  remove the
stickiness). Properly selecting the chemical(s) allows for
long-term recycling of  the water in the booth  and re-
duces the frequency of the dumps. Paint facilities that
implement regular and thorough maintenance programs
run their booths for up to one year  and more before
exchanging the water in the trough.

15.5.1   Advantages

Water-wash spray booths are ideal when using large
quantities of coatings, usually more than 5  gallons per
day per  square  foot of face area. These  booths are
available in any type of booth design  (i.e., small, large,
open, closed, cross-draft, down draft,  semi-down draft).
Water-wash booths can effectively and efficiently  re-
move particulates. Efficiency of approximately  99 per-
cent is possible.

Unlike dry filter spray booths, the air velocity through the
booth remains constant, provided that operators prop-
erly maintain the watertrough. This helps to manage the
overspray.  In addition,  facilities  may choose  to use
chemicals (deflocculants) that either sink, float, or dis-
perse the paint overspray.

Another benefit of such  booths is that they provide
essentially low fire risk.

 75.5.2  Disadvantages

Like dry filter spray booths, water-wash spray booths do
not remove  VOC's, except for  a small concentration
dissolved in the water. Other issues also relate to their

For instance, they  are  more expensive to install and
operate than are dry filter booths. Also, although they
provide low fire  risk, like dry filter booths,  they require
installation of a sprinkler  system. Facilities, therefore,
cannot realize cost savings  in this feature. In addition,
because of the water trough and entrainment area, they
usually require  slightly  more space than  a dry filter

Maintaining water-wash spray booths also has  some
drawbacks.  Operators must remove paint  sludge from
the water and dispose of it in order to prevent plugging
of fluid passages. In addition, after running the booth for
several months, the water will eventually require disposal.

Finally, water-wash spray  booths require regular  moni-
toring for:

• Level of water in trough

• Concentration of chemicals to  detackify  paint

• Foaming

• Rancidity

75.5.3  Selecting the Appropriate Chemicals

The  correct choice of chemical  deflocculant and de-
foamer is essential to the efficient operation of a water-
wash booth. Some chemicals are available as solids or
liquids.  The  more expensive chemicals detackify the
paint sludge and dewater it, thus reducing the volume of
sludge requiring disposal.

Available  chemicals  can sink, float, or disperse  paint
sludge. Choosing between them depends entirely on the
design of the booth. For instance, if the booth draws
water from the bottom of the trough and  circulates it to
the top of the water curtain, one would  not want a
deflocculant that causes the overspray to sink. In such
a booth, paint sludge at the bottom of the trough would
find its way into the pump and piping, eventually block-
ing these passages. In this situation, a deflocculant that
allows the overspray to  float or  disperse in the water
is best.

If, on the other hand, the  booth draws water from the top
of the trough and pumps water to the curtain, the pre-
ferred choice would be a deflocculant that sinks or dis-
perses the paint sludge.
 The most effective method for selecting a spray booth
 deflocculant involves sending a one  quart sample  of
 each coating to a chemical vendor, together with details
 of the  booth design. The vendor can carry out tests to
 determine which chemical or combination of chemicals
 would  remove the stickiness quickly and efficiently.

 Depending on the type of coating they are using, opera-
 tors may need to also use a defoamer. This  prevents
 foam from building up at the water/curtain interface and
 allows the booth to continue functioning normally. If too
 much foam builds up, it can affect the pressure differen-
 tials that are necessary for the proper function of the

 Note that a chemical effective for one type of coating
 resin may not be effective for  another. The  type  of
 coatings being spray-applied, therefore, dictates the se-
 lection of chemicals.

 A problem can arise when applying more than one type
 of coating in the same booth. For instance, if operators
 apply high solids solvent-borne polyurethanes as well as
 water-borne alkyds in the same water-wash booth, pos-
 sibly no single chemical, or even combination of chemi-
 cals would effectively perform. In such a case, operators
 may need  to segregate  the painting  by applying the
 solvent-borne  polyurethanes in  one  booth  and the
 water-borne alkyds in another. While this may seem
 unreasonable, it may constitute the best solution to the

 75.5.4  Methods for Treating Water From
         Water-Wash Booths

 One of the most important sources of hazardous waste
 from a water-wash spray booth  is the-water trough
 where the paint sludge collects.

 Facilities can use several mechanisms to prolong the
 useful life of the water itself and minimize the  disposal
 of the paint waste.

 First, if the paint sludge sinks, operators can shovel out
 the booth, dropping the sludge into a  55-gallon drum.
 Invariably, however, the sludge is wet and contains  a
 high percentage of water. Its disposal, therefore, gener-
 ates an unnecessarily high volume of waste.

Alternately, if the sludge floats to the top, a weir placed
 at the top of the water level can collect it. Operators can
then scoop it into  a 55-gallon drum. Or, the sludge can
feed into a centrifuge or a perforated drum that sepa-
 rates the sludge from the water. Provided that operators
treat the water with a biocide to prevent rancidity, they
can return it to the water trough for further use.

The most effective method for removing the sludge and
 minimizing the amount of water the sludge carries, how-
ever, is to use a polymer  deflocculant  that not only
suspends the sludge as fine particles in the water, but

 also chemically dewaters it. This causes the sludge to
 no longer be sticky. It feels like wet sea sand. Filtering
 and removing this from the water is relatively easy using
 a drum filter, centrifuge, or hydrocyclone.

 Finally, at least one company provides a process that
 takes the dry sludge (which has been dried in an oven),
 bakes and pulverizes it, and then sells the inert, non-
 hazardous waste for use as a raw material in the cement

 15.6  Baffle Booths

 A baffle spray booth is a less common alternative to both
 dry filter and water-wash booths. In a baffle spray booth,
 the face of the booth has steel baffles that run the height
 of the booth  and are several inches wide. The baffles
 usually overlap each other, forcing the air that passes
 through the booth to change direction in order to reach
 the back of the booth.  When the air does reach the
 entrainment section at the back, the  paint particulates it
 was carrying fall into a trough. Paint  operators can then
 collect the paint from the trough for reuse.

 These booths are much  less frequently used than either
 dry filter  or water-wash booths.  This is because unless
 a company is reclaiming the paint, this booth offers no
 advantage. In addition, not all paints can be reclaimed.
 Although the recycling  opportunities associated with
 baffle booths present strong pollution prevention bene-
 fits, most companies cannot use reclaimed paint and so
 cannot take advantage of these benefits.

 15.7  Best Management Practices To
       Minimize Coating Defects in the
       Spray Booth

This section  provides suggestions  for minimizing the
defects that  result in reworks and  rejects. The  most
frequent  coating defects that relate directly to the func-
tioning of a spray booth  include:

 • Poor wrap  when using electrostatic paints.

 • Dust and dirt in the finish.

 • Water  spots in the finish.

 • Haziness (blushing) that detracts from the gloss.

• Dry overspray on the  finish.

• Non-uniform coating finish with gloss patches, orange
  peel, voids, etc.

Most of these defects often  cause operators to perform
rework or in some cases to altogether reject the work-
pieces they have coated. This of course leads to addi-
tional pollution  and waste.  Avoiding the defects then
reduces unnecessary work and pollution.
 15.7.1  Poor Wrap

 This defect can derive from many possible  reasons.
 Reasons that relate directly to the operation of the spray
 booth, however, are the lack of a proper ground and too
 high or turbulent an air flow through the booth.

 To prevent poor wrap when using electrostatic paint, a
 facility operator must ensure that the spray booth has a
 proper ground. Changing the  air flow might require as-
 sistance from an air ventilation expert.

 15.7.2  Dust and Dirt in the Finish

 This is probably one of the most frequent causes for
 reworks and rejects. Often, a fully assembled  machine
 may require repainting because of  dirt contamination.
 Unless the coating itself contains dirt the vendor did not
 strain or filter out, the problem usually results from poor
 spray  booth operation. Facilities  should  take several
 measures and precautions to  avoid this problem:

 • Ensure that sanding  or other dirty operations do not
  take place immediately outside the booth, as the
  booth blowers would draw in the dust.

 • Ensure that the air  filters  at the air intakes of the
  booth are not dirty or have  too large a  mesh size.

 • Ensure that the booth is operating under  negative
  instead of positive pressure. In a closed booth, an air
  make-up  system should provide the incoming air,
  which should more than compensate for the air the
  booth exhausts.

 • When an air make-up system draws fresh outside air
  into the booth, ensure that its intake stack is not too
  close to  the exhaust ducts from sanding and other
  dirty operations.

 • Keep booth walls, floor, and ceiling free of loose, dry
  overspray or booth blowers may pry particles loose,
  allowing them to fall  onto freshly painted surfaces.

 • Select the correct booth design.

 Regarding the last bulleted item, as earlier sections of
this chapter suggest, selecting the right booth design is
essential. For instance, when coating large workpieces,
 use a down draft booth. Using  a cross-draft booth would
cause overspray to pass the sides of the freshly painted
workpiece.  If, however,  using a cross-draft booth is un-
avoidable, minimize the problem by starting the painting
operation at the back  of the workpiece  and  moving
forward to the filter bank.

 15.7.3   Water Spots in the Finish

When using a water-wash booth, operators must prop-
erly clean the nozzles above the water curtain.  Omitting
this step creates  the possibility for water droplets to
settle on the painted finish.

 75.7.4   Haziness That Detracts From the

 This problem can occur under high humidity conditions
 when moisture vapor condenses on freshly painted sur-
 faces and causes blushing. Clearly, this is more likely to
 happen with a water-wash booth than a dry filter one. In
 order to avoid this problem, remove the workpiece from
 the booth shortly after painting. If freshly painted  sur-
 faces  remain in the spray booth  overnight when the
 blowers are not on, humidity will quickly build up in the
 booth, increasing the probability of moisture condensa-
 tion depositing on the cold metal surfaces.

 If facilities cannot easily resolve this problem, they may
 need to convert the water-wash booth to dry filter.

 15.7.5   Dry Overspray on the Finish
 The most common reason for this defect is  that the
 solvent is too fast. As  the solvent flashes off during
 coating application, the overspray loses its wetness and
 cannot easily incorporate onto the freshly painted  sur-
 faces. While this problem is independent of the spray
 booth, a high air velocity in the booth will aggravate the
 situation.  This, again, suggests that the airflow through
 the booth requires monitoring and controlling.
 Another possible, reason for dry overspray on the finish
 arises when more than one dry filter spray booth is being
 used. If the air flow within the larger spray room (incor-
 porating the booths) is not uniform, overspray from one
 booth can settle on the freshly painted surfaces in an-
 other booth. This problem points to the need for proper
 air flow between all booths within the larger room. One
 solution is to provide each booth with its own air make-
 up unit.

 15.7.6  Non-uniform  Coating Finish  With Gloss
         Patches, Orange Peel,  Voids, etc.
 Numerous causes exist for such defects, but those that
 are solely due to the spray booth are often associated
 with poor lighting. A vast number of spray  booths  are
 either poorly lit or have overspray almost totally conceal-
 ing  the glass  panels that cover the lights. Providing  a
 good looking finish is virtually impossible in such inade-
quately lit and poorly maintained booths.

 Facilities should provide lighting not only from the ceiling
 but  also from the sides of the booths. Most automotive,
drive-in booths possess side lighting but very few three-
sided, cross-draft booths have the same luxury. Invest-
 ing in adequate lighting and regular cleaning of the cover
 plates will have a quick pay-back period in the form of
 better looking finishes  and fewer  touch-ups and  re-

 Aside from lighting, other suggestions for avoiding or
 resolving problems of non-uniform coating finishes follow:

 • When using water-wash spray booths, strongly dis-
   courage spray  painters from dropping paper cups,
   gloves, and other garbage into the water trough. They
   also must not empty leftover paint from quart or gal-
   lon cans into the trough. The chemicals cannot de-
   tackify such a large mass at one  time, resulting  in a
   sticky mess of paint that can plug fluid passages later.

 • Ensure that sanding  dusts  cannot enter the spray
   booth. Before bringing a workpiece that has been
   scuff sanded into a spray booth, wipe down the entire
   surface with tack rags or wash it down with aqueous
   detergents. Sanding dusts that  remain can contami-
   nate freshly painted surfaces.

 • As stated earlier, if using electrostatic spray guns,
   properly ground the booth and/or conveyor.  Do not
   assume  that they are grounded;  only an ohmmeter
   can confirm grounding.

 • Ensure you select the proper dry filter media for a
   dry filter booth.  Selection guidelines appear in Sec-
   tion 15.4.3.

 15.8  References
 1. Ayer, J. 1995. Recirculation ventilation in paint spray booths: New
   insights. Metal Finishing 93:20.
 2. Vickers, T.W. 1995. Selecting your best route to proper overspray
   collection. In: Metal Finishing Organic Guide Book and Directory,
   vol. 93 (No. 4A), p. 236. New York, NY: Elsevier Science Publish-
 3. Howery, J. 1984. How much do you  know about spray booth
   exhaust? Products Finishing 48:5 (May).
 4. Joseph, R. 1995. Dealing with solvent distillation of waste paint
   filters. Metal Finishing 93:44.
15.9 Additional Reading
Joseph, R. 1993. Pollution prevention in a paints and coatings facility.
   Ron Joseph & Associates, Saratoga, CA.

DeVilbiss. No date. Spray booth basics. DeVilbiss Spray Booth Prod-
   ucts, Atlanta, GA 30336. Product literature.
Thomas, B. 1995. Spray booths. In: Metal Finishing Organic Guide
   Book and Directory, vol. 93 (No. 4A),  p. 213. New York, NY:
   Elsevier Science Publishers.

   Section 4
Problem Solving

                                             Chapter 16
             Problem Solving: Case Studies of Some Typical Paint Facilities
 16.1  Introduction

 This publication has tried to demonstrate how simple
 and often inexpensive strategies can resolve production
 and pollution problems. This chapter  presents several
 realistic paint facility scenarios that reflect typical day-
 to-day production problems. Suggested solutions follow
 each presented problem.  In almost all cases, the solu-
 tion to a technical problem leads to improved quality of the
 finished product, reduced costs, and pollution prevention.
 The three scenarios this chapter discusses are:
 • Case Study #1: Flaking paint on tool boxes.
 • Case Study #2: High reject rate and volatile organic
  compounds  (VfOC)  emissions from aluminum lamp
 • Case Study #3:  Start-up  problems for automotive
  component manufacturer.
 It should become clear to the reader that by  improving
 the painting processes for the sake of efficiency and the
 quality of the  finished product, pollution minimization
 becomes an automatic consequence.

 16.2 Case Study #1:  Flaking Paint on Tool

 16.2.1   Background of Problems

 Company A manufactures tool boxes that it sells to large
 retail stores. Customers have complained that the coat-
 ings on some, but not all of the boxes, have flaked off in
 small chips. Several thousand boxes, good  and bad,
 have been  returned for refinishing. Knowing  about the
 usual coating process the company uses, of course, is
The boxes are made of cold rolled steel. Surface prepa-
 ration comprises a three-stage spray washing process
 plus a drying stage. These stages  are:
• Stage #1: Degrease and iron phosphate
• Stage #2: Tap-water rinse (intermittent overflow)
• Stage #3: Tap-water rinse

• Stage #4: Dry-off oven at 230ฐF
 Operators load the parts onto a conveyor immediately
 before they enter the first stage of the washer. After
 running through the washing and dry-off process, the
 conveyor then passes through the priming spray booth
 and finishing spray booth. The facility uses a fast-drying
 alkyd primer and topcoat system. The conveyor then
 loops the parts back to the spray washer where they are
 off-loaded before they can go through the  washer a
 second time. The parts are sufficiently dry to be off-loaded.

 Before going ahead with the refinishing process, Com-
 pany A needs to identify which boxes are good from
 those which are likely to fail. By conducting an adhesion
 test, such as the Tape Adhesion Test described in Ameri-
 can Society of Testing  and Materials D3359, Method B,
 the quality control department can distinguish between
 the good boxes and those that will likely fail. In this test,
 a quality control operator applies a short piece (approxi-
 mately 3 inches) of masking tape or, preferably, alumi-
 num duct tape to the coating. After about 90 seconds,
 the tape is  quickly removed by pulling back 180ฐ.  If
 coating does not peel away with the tape, the coating is
 good.  If  pieces  of coating lift  off onto the  tape, the
 coating has  poor adhesion, and the quality control op-
 erator considers  it a failure.

 The issues the company must address then are:

 • After identifying the bad boxes, how should Company
  A strip the coatings?

 • What are the likely causes of the inconsistent problem?

 • What strategies should the company follow to im-
  prove the quality of the finish and to prevent similar
  occurrences in the future?

 • Do any pollution minimization opportunities present

 16.2.2   Possible Solutions

To identify and nullify the problem, Company A should
first assess the whole process  it will use to strip and
 recoat the boxes.

The stripping method must be fairly rapid because thou-
sands of boxes  require  stripping.  Also, the  company
must keep the cost to  a minimum  because the  boxes

 themselves are .inexpensive. If the stripping process is
 too costly, it no longer pays to refinish them.

 Chemical stripping is a possibility but it requires several
 steps. Moreover, if any chemicals remain on the surface
 or if water ingresses between spot-welded plates, paint
 failures can re-occur. Air, water, and waste pollution also
 are major negating factors.

 Mechanical stripping in the form of abrasive blasting
 using  grit, aluminum oxide, or another such abrasive
 probably offers the greatest advantages. While the proc-
 ess should be relatively fast, the abrasive cannot be too
 coarse or it will deform the boxes, and misalignment of
 the lids is a serious concern. The company might seri-
 ously consider plastic bead blasting because the beads
 are unlikely to damage the metal surfaces. Moreover,
 the beads can be  recycled and  only the paint  chips
 require disposal. If the waste passes the TCLP tests, the
 company can dispose of it as non-hazardous solid waste.

 Although a three-stage washer is not ideal for treating a
 cold rolled steel substrate, it is usually adequate for the
 intended purpose, namely to coat the tool boxes. The
 process  requires careful control, however, to  ensure
 good paint adhesion.  Operators must perform quality
 control checks on the temperature of the first stage (per
 the chemical manufacturer's recommendations), on the
 pH and free acid of the bath, and on the contact time
 between the steel and the chemical.

 Poor rinsing practices can also contribute to the failures,
 specifically with a three-stage process. In this scenario,
 Stage #2  overflows on an intermittent basis. This sug-
 gests that contamination from Stage #1 can build up to
 an unspecified level before the  water is diluted with
 make-up tap water. Company A, therefore, should pro-
 vide for continuous overflow. In addition, the company's
 manufacturing engineers should  study the hanging of
 parts from the conveyor to minimize drag-out from Stage
 #1 to Stage #2.

 Stage #3 is also a tap-water rinse.  This can pose  a
 problem if the tap water contains a relatively high con-
 centration of dissolved  solids. In  an ideal process, the
 second rinse tank includes a sealer  that consolidates
 the phosphate film. Moreover,  deionized water works
 better than tap water. In reality, however, it  may not be
 cost-effective nor necessary for Company A to incur the
 expense of a deionized water generator. Tool boxes are
 not the type of commodity that warrants a sophisticated
finish. At the very least, though, Stage #3 should be a
 nonchromate seal rinse,  for which the added  cost is
 minimal. (A chromate seal rinse often offers better pro-
tection, but poses an environmental problem.)

The next stage uses the dry-off oven. In this scenario,
the dry-off oven temperature is too low. For rapid drying
of the wet parts, the oven temperature should be greater
 than 300ฐF. The low temperature allows parts to flash
 rust even before they have left the oven.

 A quick drying alkyd system has the advantage of pre-
 cluding the need for a paint baking or curing oven. The
 compromise Company A makes when selecting such a
 coating, however, is that the coating film  is often hard
 and brittle. Slower  drying alkyds tend to have better
 flexibility and adhesion properties, but to  keep up with
 production speeds the company might need to install a
 force-dry oven. The company should, therefore, re-ex-
 amine its selection of coating.

 Although each step in the  process has been reviewed,
 one final important factor remains: the conveyor system
 itself is associated with problems. The conveyor passes
 through  the spray washer, dry-off oven,  and the two
 paint spray booths.  A common problem resulting from
 this configuration is that the spray painter in either of the
 spray booths has the ability to start and stop the line at
 will. This occurs during coffee and lunch breaks, or when
 the spray painter needs to perform  another function,
 such as filling the pressure pot with fresh coating. When
 the conveyor stops, several of the parts still in the spray
 washer may be between stages.  If the stoppage lasts
 for more than a few minutes, the parts might start to flash
 rust or may receive too heavy a phosphate  coating.
 Parts that have proceeded through the rinse stages but
 have stopped short of the dry-off oven will be left wet for
 too long. Quick drying is critical to prevent  flash rusting.
 This intermittent action would also  explain why only
 some, and not all, tool boxes fail. To avoid these prob-
 lems, Company A should consider one of two options.
 The company should prohibit anyone from stopping the
 conveyor until all of the parts  in the spray washer go
 through the oven. Alternately, Company A should break
 the conveyor into two separate conveyors: one solely
 dedicated to the spray washer and dry-off oven, the
 other dedicated  to  the  spray  booths. Operators can
 either transfer the parts from one conveyor to the other
 manually, or the company can  install a power-and-free
 conveyor to pass through the spray booths.

 A power-and-free conveyor has two or more segments,
 each separated from the other. For instance, the first
 segment might be a short length of continuous  loop
 conveyor that forms a closed circuit. The  second  seg-
 ment may comprise  racks that  receive parts from Con-
 veyor #1 and transfer them laterally. Finally, Conveyor
 #3 might pick up the parts from Conveyor #2, and trans-
fer the parts along another continuous loop conveyor. All
three conveyors can be moving at different speeds and
 in different directions (see  Figure 16-1). The  transfer
 from one conveyor to the next is usually automated.

 16.2.3   Pollution Prevention Opportunities

This case study has suggested several strategies that
would not only solve the immediate  problem, but also

Continuous Conveyor
 Moving at Speed "a"
   Horizontal Racks
                                   Conveyor #1
                                   Conveyor #2
                                   Conveyor #3
             Continuous Conveyor
             Moving at Speed "b"
Figure 16-1.  Example of power-and-free conveyor.

lead to better management practices. If the company is
only interested in solving the immediate problem, then it
would need to experiment to identify the specific cause
of the  failures. The company could inexpensively im-
prove so many existing practices, however, that it would
definitely benefit by overhauling the entire process. This
type of overhaul would improve the company's product,
reduce the number of rejects and refinishes, and  ulti-
mately lead to dramatic  pollution prevention (mostly in
the form of fewer rejects and refinishes).
In solving its catastrophic production problem, Company
A can  substantially reduce  its  air, water, and waste
pollution. The described options that offer the best op-
portunity to improve the process and reduce pollution
• Use  mechanical  stripping via plastic bead blasting.
• Enforce stricter quality control in three-stage washer.
• Provide for continuous overflow in rinse stages.

• Minimize drag-out between washer stages.
• Use  deionized water or at least a nonchromate seal

• Increase baking  temperature in dry-off  oven.
• Switch to slower drying alkyd or install force-dry oven.

• Prohibit operators from turning off conveyor in mid-
  process or divide conveyor into two.

16.3 Case Study #2: High Reject Rate and
      VOC Emissions From Aluminum
      Lamp Housings

16.3.1   Background of Problems

Company B manufactures long  aluminum lamp hous-
ings (or covers) for fluorescent  lamps. They are long
 12-inch diameter tubes cut in half longitudinally. Some
 housings can be as long as 16 feet. Architects specify
 these products for shopping complexes, banks lobbies,
 insurance companies, hotels, and other high-profile
 buildings and institutions.

 Because of the  housings' high visibility to the public,
 Company B uses a two-component  high solids, low-
 VOC polyurethane.  The polyurethane is pre-mixed at
 the beginning of  a job and operators mix sufficient coat-
 ing for one shift's work. Generally, they do not mix more
 coating than the job requires. Because some of the
 orders are large, the paint shop often uses one color for
 an entire  shift.  Sometimes,  however, coating several
 small jobs on the same day requires more  than one
 color. Spray painters use airless spray guns because the
 lamp housings are  long and the spray painters must
 keep up with the fast production speed of 15 feet/minute.

 Quality control engineers reject approximately 10 per-
 cent of all housings because of color and gloss patches
 (differences), which are clearly visible when viewing the
 finished products from a distance.  The rejected hous-
 ings return to the finishing shop where operators scuff-
 sand them to a  uniform finish. Operators remove the
 sanding dust with tack rags, and then wipe the sanded
 finish with a strong solvent to soften  the cured finish and
 allow for the application of a fresh coat of polyurethane.
 In most cases, engineers approve the refinished hous-
 ings for sale. A few require refinishing a second time. In
 addition, some customers have returned housings sev-
 eral months or even 18 months after original manufac-
 ture because the coating peeled off in sheets.

 These problems  have caused two major consequences
 for Company B.  The cost of the added  coatings, wipe
 solvents, hazardous waste disposal, and labor required
 to refinish the reject and returned housings has had a
 disastrous effect on  the profitability of the company. In
 addition, VOC emissions are several tons over the per-
 mitted annual cap. The company must resolve the fol-
 lowing  issues in order to solve its problems:

 • What are the probable causes for the color and gloss

 • What strategies can minimize or even eliminate the
  rejects and returns?

 • What pollution minimization strategies can get the
  company back into compliance with its air quality permit
  and further reduce the final cost of the housings?

 16.3.2  Possible Solutions

Given the  problems Company B  faces, its search for
solutions should concentrate on three  different areas:

• Spray gun options

• Viscosity management

 • Coating selection

 Each of these factors contributes to one or all the prob-
 lems the company faces.   Spray Gun Options

 The primary cause of the color and gloss patches (dif-
 ferences) is that the housings receive an uneven coat-
 ing. Film thickness measurements of the coating would
 probably show that a significant variation exists over the
 length of the housing. Two primary factors may be re-
 sponsible for this:

 • The airless spray gun is inappropriate for this job.

 • For  the rejected lamp housings, the viscosity of the
   high solids, two-component polyurethane is probably
   too  high.

 This section discusses the spray gun possibilities. The
 next will cover viscosity.

 An airless spray gun is appropriate for jobs in which the
 spray  painter must move quickly and apply the coating
 in one application. This gun, however,  may  not be ap-
 propriate when a high visibility product needs to have a
 uniform film thickness. This is particularly  true when
 applying high solids, low VOC coatings because  the
 spray painter does not have sufficient control of the gun.
 If the coating had a lower solids content, the airless gun
 might  be able to apply a  high-quality finish, but under
 such circLffnstances the painter  might have  difficulty
 preventing runs and sags. High-quality finishes require
 excellent atomization. The more appropriate guns would
 be conventional air atomizing, HVLP, and electrostatic.

 The conventional air atomizing gun would almost cer-
 tainly give the desired finish but transfer efficiency tends
 to be  lower than for other gun  options. The poorer
 efficiency  dramatically increases the coating,  solvent,
 and hazardous waste costs, and significantly increases
 VOC emissions into the air. This gun, therefore, would
 not prove to be the best option.

 Some  HVLP guns would also provide the desired finish.
 Company B, however, may need to shop around and
 experiment with different HVLP guns before making a
 final selection. This is because not all guns will  give the
 desired results. If the company can make an  up-front
 investment, the HVLP guns that would probably give the
 best finishes are those that use a turbine to generate the
atomizing air. Only on-site  testing can demonstrate jus-
tification of this extra expense.

 Electrostatic guns would also probably satisfy the com-
pany's requirements. In addition, airless or air-assisted
airless electrostatic guns  allow  for faster application
 If using electrostatic guns,  the company must ensure a
proper ground each  time  a housing is coated. When
 properly used, electrostatic guns can give the highest
 transfer efficiencies compared with all other guns. The
 additional  capital costs the company would incur to
 purchase such equipment can have a rapid payback.

 Because of the simple geometry of the housings and the
 fast production line speeds, the company might also
 want to investigate the use of high-voltage electrostatic
 bells. These might prove to be an ideal choice because
 they lend themselves to automation and can achieve
 transfer efficiencies of greater than 90 percent.   Viscosity  Management

 Although high solids  polyurethanes have relatively low
 viscosities compared with  other coating resins of the
 same solids content,  they nevertheless  require proper
 atomization in order to avoid color and gloss patches. In
 this scenario, operators pre-mix the coatings at the be-
 ginning of the  shift. This implies that as the day pro-
 gresses,  the  viscosity  slowly  builds  up  until  .it
 approaches its pot  life. Although, in this case, the coat-
 ing  does  not  seem to  actually  reach its  pot life, the
 viscosity of the pre-mixed coating definitely increases.

 Rather than pre-mix coating for large jobs, the company
 should consider the efficacy of plural-component meter-
 ing and mixing equipment. The primary advantages are
 that the viscosity of the coating would remain constant
 all day, and at the end of the shift, operators would only
 need to clean the unmixed coating in the fluid line be-
 tween the mixing  manifold and the spray gun.  This
 equipment would drastically reduce hazardous waste
 from pre-mixed coating. The most important disadvan-
 tages are that  the  equipment is relatively expensive,
 $5,000 to $15,000  depending on the system, and it is
 only feasible to install such equipment for relatively large
 jobs that use well over 1 gallon per day.

 For smaller quantities, pre-mixing is more economical.
 Even if the company decides to invest in plural-compo-
 nent equipment, the spray painters would still need to
 pre-mix coatings for the smaller jobs.

 Another inexpensive method for managing viscosity is
 to mix smaller quantities. Instead of pre-mixing the entire
 day's coating requirements, the  operators should mix
 smaller quantities, perhaps one batch before lunch and
 another after. This would minimize the viscosity differen-
tial that  occurs while the mixed coating is waiting to be

An additional strategy is to keep the pre-mixed coating
cool,  but not below the dew point  as this will  cause
condensation to diffuse into the coating and produce
visible white gel particles.

The  problem of the coating peeling off in sheets  also
probably relates to viscosity management. This problem
suggests that the spray painters added  solvent to the

 pre-mixed polyurethane to prolong its pot life. This is a
 very poor practice because  the coating can entrap the
 extra solvent while it is cross-linking. Entrapped solvent
 often remains in the coating for months and years, and
 in many cases can lead to  coating delamination. This
 probably occurred in this scenario.   Coating Selection

 If the problem of color and gloss differences continues
 to occur, the company may  need to search for another
 coating, or ask its vendor to consider reformulating the
 product to improve  its application properties.  Not all
 polyurethanes.are alike, and converting to a different
 formulation could possibly solve the.problem.

 Another consideration for Company B is switching to one
 of the new low-VOC water-borne polyurethanes that are
 now becoming available. The company may benefit from
 sampling a few formulations and testing them for their
 application and  physical properties. Usually,  water-
 borne coatings have a lower solids content than solvent-
 borne high solids coatings;  therefore, this might solve
 the current problem. The  company's total annual VOC
• emissions would also probably drop by at least 20 percent.

 Alternately, the most effective strategy by far for mini-
 mizing  pollution is to convert from liquid polyurethanes
 to powder coating's. The  lamp housings Company B
 manufactures are ideally  suited to these coatings be-
 cause of the^r fairly simple shape. After such a conver-
 sion, the company would emit essentially no VOCs, and
 it can melt the excess and unused powder coating into
 solid blocks. If the blocks pass the TCLP tests, the
 company can dispose of them as non-hazardous waste.

 In addition to pollution  prevention, switching to powder
 coatings would offer other benefits as well. The simple
 shapes of the  housings lend themselves to an auto-
 mated coating application, a factor that makes powder
 coatings an even more  attractive  option. Moreover,
 the color and gloss patches experienced with the liq-
 uid  high solids coatings are less likely to occur with
 powders because they tend to produce  more uniform
 film thicknesses.

 Of course,  in order to convert  from liquid to powder
 coatings, Company B would  need to reassess its entire
 coating facility. One cannot simply swap one coating for
 another. The existing spray  booths would require re-
 placement with special powder coating booths that are
 designed to capture and recycle the oversprayed pow-
 der. The guns and ancillary equipment would also need
 replacement. In addition, the  oven  must have the capa-
 bility to cure the powders for  8 to 20 minutes at 325ฐ to
 400ฐF. The existing surface preparation process may be
 adequate, but the company would  need to confirm this.

 To make such a major  conversion requires capital and
 time. Most large companies wait for a scheduled shut-
 down period before switching the equipment. In addi-
 tion, the company must train its operators to  use pow-
 ders, and must write and implement a new set of quality
 control procedures. For this scenario, it seems likely that
 Company B would solve its current problems and re-
 ceive a payback on its investment. It can also expect,
 however, new problems unique to powders, although
 these too would be resolved with time.

 16.3.3   Pollution Prevention Opportunities

 In rectifying its problems,  this company can  automat-
 ically enjoy the benefits of reduced pollution and accom-
 panying cost reductions.

 If the company decides to stay with  liquid  coatings,
 choosing a high  transfer  efficiency spray  gun would
 result in considerably less overspray on the spray booth
 filters, and on its floor and walls.  Not  only does this
 immediately translate into  maintenance labor savings,
 but the company would need to discard fewer filters and,
 thus, also purchase fewer. As was discussed in Chapter
 9, coating usage and emissions decrease when transfer
 efficiency increases.  If transfer  efficiency is generally
 low, e.g., 30 to 40 percent, a  small improvement in
 application efficiency can result in a significant reduction
 in all forms of pollution as well as in  costs. Hidden
 benefits  would be improved  labor  conditions, better
 pride in the  finished  product, and  improved cus-
 tomer/vendor relations and credibility.

 If Company B finds that its situation lends itself to using
 plural-component metering and mixing equipment, it can
 realize great pollution prevention   and  cost  benefits.
 Companies that have implemented an in-line  metering
 and mixing system for plural-component coatings have
 reported  significant savings  in hazardous  waste dis-
 posal. Because the cost to  dispose of a 55-gallon drum
 of liquid  hazardous waste  can be  as high as $500 to
 $600, the equipment change obviously can quickly gen-
 erate a payback.

 If Company B were to replace liquid coatings with pow-
 ders, it would essentially eliminate all forms of its pollu-
 tion. This cost reduction alone might justify the capital
 outlay it would require to tear down the old system and
 install the new one.

 16.4 Case Study #3: Start-Up Problems
      for Automotive Component

 16.4.1   Background of Problems

 Company C manufactures components for the auto-
 motive industry, and its paint finishing shop is a brand
 new facility. Numerous start-up problems are preventing
the company from getting  its finished components to
the market, and environmental problems are already

 surfacing. Before introducing the problems, a brief back-
 ground of the facility's process follows.

 Several types of substrates require coating by this com-
 pany. The facility cleans and treats components made
 of cold rolled steel in a 7-stage zinc phosphate  line
 designed to give a heavy zinc phosphate coating of 400
 mg/ft2 to prevent corrosion. Alternately, operators treat
 aluminum parts with a chromate conversion coating. All
 metal  substrates then receive one  coat of an  epoxy
 basecoat. Operators then apply a clear coat of two-com-
 ponent polyurethane over the basecoat.

 All plastic housings are scuff-sanded and washed with
 a mixture of methyl ethyl ketone (MEK) and isopropanol
 before  being primed  with  an  epoxy sanding  primer.
 Thereafter, operators sand the primer to a smooth finish
 with 240-grit to 400-grit paper to  hide all swirl marks in
 the plastic molding. The plastic parts then receive a
 conductive primer.

 All of the spray booths in  the facility are of the  cross-
 draft, dry filter type. Operators change the filters as soon
 as the magnahelic gauges on  the sides  of the booths
 show a pressure differential of  0.5  inches. Because
 coating usage differs for each booth, the filters are not
 all changed at the same time.

 One large finishing  room  contains  all  of  the  spray
 booths. One very large air make-up unit supplies all of
 the air to the booths. Strict procedures prevent  un-
 authorized employees from entering the finishing room.
 Those who do enter must wear cotton booties, lint-free
 coveralls, and lint-free disposable caps to cover their
 hair. In the booths, electrostatic turbo bells held  by
 robots or reciprocators actually apply the coatings.

 The problems that need addressing are:

 • Heavy sludge from the zinc phosphate pretreatment
  tank requires disposal of relatively  large volumes of

 • Large volumes of rinse water from the spray washer
  require treatment before being discharged to the pub-
  lically owned treatment works  (POTW). In addition,
  the POTW is complaining because the facility was
  never designed to handle so much water, and is ask-
  ing Company C to urgently address this problem.

• The  finished steel components and aluminum parts
  do not have the same gloss, even though they use
  the same basecoat and clear coat.  For some colors,
  the gloss difference  also appears to the observer as
  a color difference. This is a major problem when steel
  and  aluminum parts  are adjacent to each other on
  the same assembly.

• Most finishes are marred with  dust and dirt despite
  the fact that all personnel wear clean lint-free clothing
   and the company has made every effort to keep dust
   out of the finishing room.

 •  Dry overspray mars some of the clear coats. Viewing
   the parts under a microscope suggests that the over-
   spray may come from the basecoat booths.

 •  Although the company uses electrostatic turbo bells,
   the transfer efficiency is too low for such guns. Con-
   sequently, VOC emissions already approach the per-
   mitted cap and are almost double what was originally
   estimated when the permits were applied for. A con-
   sequence of the poorer than expected transfer effi-
   ciency  is that  the  filters  become clogged more
   frequently and require disposal.  Because the com-
   pany has determined that the overspray from at least
   one of the coatings fails the TCLP test, the  policy is
   to dispose of all the spent filters as solid hazardous
   waste. In a  typical month, the company disposes of
   ten 55-gallon drums of spent filters, and this signifi-
   cantly increases the total cost of running the paint

 16.4.2  Possible Solutions

 Zinc phosphates do produce sludge, and this is one of
 the disadvantages that  end-users must  accept when
 they specify a zinc phosphate system.  If the sludge
 build-up is higher than expected, then end-users may
 need to evaluate several operating parameters. For in-
 stance, causes for the high sludge build-up may include:

 •  The concentration of the bath may be too high

 •  The temperature may be too high

 •  The tank may be over-agitated

 •  Parts may be in the phosphating stage for too long

 •  The pH may be incorrect

 Company C should call its chemical supplier and ask a
technical representative to  troubleshoot  the problem.
The  supplier will probably find at least one of the pa-
 rameters out of  specification. This implies that better
 process controls may need to be in place, and operators
 may need to monitor the parameters more frequently.

The  large volumes of water the facility treats and dis-
charges may be warranted, or they may be excessive.
 In  order to assess this, the  company must address
several questions:

• Are the fabricated metal surfaces more contaminated
  than they  should be? If so, how can the company
  minimize their contamination loading?

• Is  the drag-out from the process tanks to the rinse
  tanks too high?

• Has the rinse water overflow rate been correctly cal-

 •  Is the company maintaining the concentration of the
   rinse tanks at unrealistically low levels?

 •  Are the rinse tanks designed to achieve optimum
   counterflow characteristics?

 •  Why can't the company treat and recycle the water
   in a closed loop system?

 The gloss differences between the steel and aluminum
 parts may result from a difference in the surface finish
 of the two metals or from the zinc phosphate coating.

 If  the two metals have different surface finishes, the
 company  may need to specify a different finish, or if that
 is not possible, operators can apply a sanding primer to
 the rougher  of the two metals.  This option,  however,
 would add significantly to the cost of the finishing proc-
 ess.  Not  only  does it require an extra coat,  but all
 components  would require sanding to a smooth finish
 before applying the basecoat.

 Some zinc phosphates produce macro-crystals that ab-
 sorb the coating, thus giving the appearance of lower
 gloss. The automotive industry tends to purchase zinc
 phosphates that produce micro-crystals. For such sys-
 tems, one would  not expect to see a noticeable gloss
 difference  between the steel and aluminum coated parts.

 Keeping dust and dirt out of a  painting facility is ex-
 tremely difficult. In this scenario, the company has ap-
 parently taken precautions to keep the employees from
 bringing contaminants  into the facility; therefore, other
 possible causes may be:

 • The spray room may be operating under negative

 • The air  make-up unit may be pulling in dust-laden air.

 • The coatings may contain dirt that was not previously
  filtered out.

 If the spray room is at times  operating under negative
 pressure,  the vacuum can pull dust and  dirt from adja-
 cent areas of the factory.

The air make-up unit,  which has obviously been de-
 signed to pull a large volume of air into the spray room,
 may be pulling in dust-laden air from the exhaust stacks
 of other operations (specifically the sanding operation).
 If the intake  filters to  the  air make-up  unit have too
coarse a mesh, or if the pressure differential across the
filters is too high, the dust and dirt could easily enter the
 spray room.

The possibility also exists, although it is less likely, that
the coatings themselves contain dirt that was not filtered
out prior to  use.  End-users can easily  check this by
taking a spatula and dipping it into the pressure pot. As
the paint runs off the end, one can spot small dirt parti-
cles in the wet coating. If these particles are present,
 using simple filtering techniques can usually solve the

 The problem of overspray from one spray booth affect-
 ing the finishes  in another booth  may  at first  seem
 baffling.  Bear in  mind,  however, that all of the  spray
 booths are dry filter booths. Thus, as each filter pad
 collects overspray, the pressure differential across the
 filter bank increases, and the air flow into the exhaust
 stack decreases accordingly. Since one air make-up unit
 feeds all the booths, the air flow within the larger finish-
 ing room is constantly changing. Some filters become
 more plugged with overspray than others, pressure dif-
 ferentials constantly change, and air movement is never
 constant. As soon as  operators change the contami-
 nated filters in one spray booth, this booth suddenly
 draws its maximum  capacity of air, which may  entail
 drawing air from  another spray booth with clogged fil-
 ters. In addition, sometimes one or more booths may be
 idle for a few hours of the day. Then, as operators turn
 on the blowers,  the booth  suddenly  draws air, once
 again changing  the dynamics of  the  entire finishing

 The most effective method for eliminating this problem
 is to provide a separate air make-up system for each
 booth. This would ensure that the air make-up is always
 sufficient to supply the needed volume of air. If too much
 of a positive pressure develops, however, the air would
 once again start affecting other booths.

 The problem of improper ventilation is not easy to solve,
 and the company may need to hire ventilation consult-
 ants to rethink the system.

 The final issue for this company, transfer efficiency, is
 one of the  most important parameters that affects VOC
 emissions into the air, as well as the volume of hazard-
 ous waste  generated. Because electrostatic turbo bells
 are known for their high efficiencies, the company must
 look at other factors that may be causing the problem.
 These include:

 •  Inconsistent air flow

 •  Improper grounding

 The company has already established that the air flow
 in the finishing room is not laminar and changes direc-
tion from one moment to the next. The efficiency of turbo
 bells is extremely sensitive to airflow in the booth. Even
when air flow is (correctly) toward the filter bank, if the
velocity is  too high, the air carries the paint particles
 away from  the parts being painted and into the filters.

Alternately, if the parts are not properly grounded, or if
the paint does not have the proper polarity, the  turbo
 bells cannot apply the coating electrostatically.

A.few quick experiments can determine why the  turbo
bells are not achieving the desired  transfer efficiency.

Since this is a major factor affecting VOC emissions and
hazardous waste, the company should investigate this
problem to the fullest so that it can achieve the maxi-
mum efficiency.

16.4.3  Pollution Prevention Opportunities

Pollution  prevention  opportunities  for Gompany  C
abound. First, if it could reduce the  amount of solid
sludge waste from the zinc phosphate treatment, which
is in itself a pollution prevention measure, the company
would also be extending the life of the bath. This would
necessarily reduce the total volume of water that Com-
pany C needs to treat and dispose of occasionally.

If the company installs a closed loop system (i.e., by
treating all the effluent water from the treatment system
and then recycling it), its cost for city water would drop.
No  guarantee exists, however, that this would lead to an
overall cost reduction, particularly if the city tap water is
inexpensive and the cost for in-house treatment is high.

The pollution prevention and  cost benefits associated
with overcoming the poor spray booth conditions and the
inappropriate choice of spray guns are very similar to
those provided in Case Study #2.

If Company C can solve the critical problem of turbulent
air  flow in the spray booths, total air emissions would
immediately drop, as would the generation of unneces-
sary hazardous waste, both in the form of used dry filters
and waste paint.

Clearly,  the company has little choice but to resolve its
problems if it wants to remain competitive  and stay  in
business. By solving its production problems, it will auto-
matically enjoy many unexpected cost benefits, and it will
dramatically improve the environment of the community.
16.5 Conclusion

This chapter has presented three typical scenarios. All
of them relate to day-to-day production problems  in a
coating facility, and to a large extent the problems have
little to do directly with the environment. The suggested
solutions, however, show that once a company imple-
ments better management practices, the rate of reworks
and rejects diminishes, as do the parameters that affect
the quantity of coatings and solvents used. It is a win-win
situation for all parties:

• The company enjoys fewer environmental/regulatory
  problems, more efficient processes, greater produc-
  tivity,  greater competitiveness in  the market,  and
  lower finishing costs.

• The customer gets a higher quality product.

• We all enjoy dramatically reduced air, water, and waste

A reader who wants to keep updated with current coat-
ing and  equipment technologies  can access  marty
monthly technical journals that are often  available free
of charge. Some address the scientific community  and
are very technical. Alternately, other journals are solely
pragmatic and target finishing engineers,  paint supervi-
sors, and painters who are looking for any hints that will
make their jobs easier.

Readers  can often find in  the  literature solutions to
problems  such as  those this chapter has discussed.
Failing that, the reader has access to chemical, coating,
and equipment vendors. When the problem is too com-
plex or crosses many different fields, consultants can be
retained.  The end-user can usually find one or more
avenues  to  resolve problems. As  a consequence to
solving these production problems, the end-user will be
contributing to pollution prevention.


                                    Appendix A
       Selected List of Suppliers of Aqueous and Semi-aqueous Degreaser
                           Formulations and Equipment*
                               AQUEOUS EQUIPMENT SUPPLIERS
                                      LARGE UNITS
Spray Washer
ESTECH C-15154
Final Phase Industrial
Parts Cleaners
Aqueous Parts Cleaner
New Pac, USA
P. 0. Box 1461
Palatine, IL 60078
Equipment Systems Technology
P. 0. Box 550
Findlay, Ohio 45840
Final Phase
23540 Pinewood
Warren, MI 48091
N. 5th St., at Ford Blvd.
Hamilton, OH 45011
Inline, Overhead Monorail
Heavy Duty Monorail
(C-15154) or Conveyorized
Conveyorized Monorail -or
Drum Aqueous Cleaners
Inline Monorail,
Conveyorized Automated, or
Batch. Complete Line of
Equipment, Small to Large
Cleans &
1 Reproduced with permission from the Waste Reduction Resource Center for the South East, Raleigh, NC.

Bio Act EC7
Simple Green
Daraclaan 220,
282, 283
Quaker 624 GD
Turco 3878
4215 -NC-LT
Petrofirn, Inc.
Specialty Chemicals
5400 First Coast Hwy.
Fernandina, Ft, 32304
Simple Green
P. 0. Box 880135
El Paso, TX 88588-0135
W. R. Grace
55 Hayden Ave.
Lexington, MA 62173
Quaker Chemical Co
Elm ซ Lee Streets
Conshohocken, PA 19428
Atochem - NA
3 Parkway
Philadelphia, PA
Terpene t Esters
Alkaline With or
without Glycol
w/agitation (3878)
Alkaline (6778)
Electronics &
Parts Cleaners
Metal Cleaning
Metal Cleaning &
Electronics Parts
Replace Vapor
Rinse Kay Be
from 3878
LF-NC Hon-
Coors Bio-T
Ridolene 1025
TD 1414-F-B
Kwik Dri 66
Actrel 333BL,
3349L, 3360L,
Bxxate 800
Spectro-Chemical Lab
Coors Porcelain Co.
600 Ninth Street
Golden, CO 80401
Parker Arochen
32100 Stephenson Hwy
Madison Heights, MI
800-222-2600 Ext. 286
Ashland Chemical, Inc.
Industrial Chemicals
P. 0. Box 2219
Columbus, OH 43216
Exxon Chemical
P. O. BOX 5200
Bay town, TX 77522
Exxon. Chemical
P. 0. Box 5200
Bay town, TX 77522
Alkaline (HaOH)
Petroleum Solvent
. -troleum
Metal Cleaning
Vapor Degreaser
Parts Cleaning &
Paint Prep
Lacquer Stripper
Paint Thinner
Drawing Oil,
Cutting Oil,
Drawing' Oil
Flash Point
Flash Point
Flash Point

Rust corrosion
Action Bioclean
Chen-Tech International
Hid America Chem Corp.
4701 Spring Road
Cleveland, Ohio 44131
Dow Chemicals & Metals
2020 Dow Center
Midland, MI 48674
Dow Chemicals t Metals
2020 Dow Center
Midland, MI 48674
Dow Chemicals ฃ Metals
2020 Dow Center
Midland, MI 48674
Action Products, Inc.
2401 W. First Street
Tempe, Arizona 85281
Mineral Acids/
Glycol Ethers
Surfactants With
Semi Aqueous
Cold Cleaner
Remove Oxidation
Rust. Requires
Pretreat with CT.l
Precleaning Multi-
Light Oils/Grease
Light Oils, Metal
Oils, Grease
Metals Parts Wash
CTl, Rinse
CT2 , Dry
Corrosion of
Some Metals
Odor - Must
for Disposal
Teile Reinigung
Smlttel 09
R. B. Degrease
Citra Safe
Axarel 38/52
RHA 6 RA Flux
Remove & Cleaner
P. O. BOX 150146
Nashville, TK 37215
Lubrichem, Inc.
P. O. Box 30665
Raleigh, NC 27622
Sanford, FL 32771
515 E. Touhy Ave.
Des Plaines, IL 60018-
Inland Technology
2612 Pacific Hwy, E.
Tacoma, WA 98424
Dupont chemicals
Chestnut Run Plaza
P. 0. Box 80711
Wilmington, DE 19880-
Mid America Chemical
Cleveland, OH 44131
Alkaline &
Steam, Pressure
Metal Cleaner
Metal Cleaning
Printed Circuit
Methylene Chloride
1,1,1 Vapor
38- Electronics
52-Grease Metal
Circuit Boards
Flash Point

P P Degreaser
Arconate TM 1000
Gil lite 0650
EZE 267D
PT Technologies, Inc.
106 4th Ave. , South
Safety Harbor, FL
Arco Chemical
3B01 West Chester Pike
Hewtown Square, PA
Man-Gill Chemical
2300 St. Clair Ave.
Cleveland, OH 44117
Hidbrook Products
2080 Brooklyn Road
BOX 867
Jackson, Mich 49204
Hubbard-Hall, Inc
P. 0. Box 790
Water bury, CT 06725-
EZE Products, Inc.
P. 0. Box 5744
Greenville, SC 29606
Low Aliphatic
Alkaline, Terpenes
Substitute for
1,1, '1 Cable &
Metal Cleaner
Replace Methylene
Metal Cleaning
Metal Cleaning-
Vapor Degreasing
Steel Parts
Dip Tank
815 GD
815 GR
Alka - 2000
(1) DOT 111/113
(2) Omni Clean
H. D.
Glidsafe Family
Rentry Solvent
Brulin Corporation
Calgon Vestal Labs.
7501 Page Avenue
St. Louis, MO 63133
Delta - Omega
Technologies, Inc.
P. 0. BOX 81518
Lafayette, LA 70598-
GLIDCO Organics
P. O. BOX 389
Jacksonville, FL 32201
Enviroaolve, Inc.
1840 Southside
Jacksonville, FL 32216
(l) Proprietory
(2) "Hater Based"
Terpene Blends
Terpenes With
Metal Cleaning
Ferrous Metals
Cleaning pnlvl
(1) Metal Cleaning
(2) Heavy Oil
All Surfaces
Ink Removal, Hand
Wiping, Emulsion
Tailored To Meet
Cleaning Needs
High pH
Safety and
(1) None
Listed 'In
MSDS. High
tions could
(2) None

Oxsol Solvents ™
(1) Parts Prep
(2) Micropure
(1) lonox FC, HC,
(2) Aquanox 8SA 4
Precision Clean
Occidental Tower
5005 LBJ Freeway
Dallas, TX 75244
international Specialty
1361 Alps Road
Wayne, NJ 07470
XYZEN Corporation
413 Harding Industrial
Nashville, TN 37211
3D Inc.
2053 Plaza Drive
Benton Harbor, MI
LPS Laboratories, Inc.
4647 Hugh Howell Road
Tucker, GA 30085-5052
Derivative Of
Pyrrol idone Plus
(1) Alcohol ฃ
Surfactants t
(2) Alcohol
Water Blend
Alkaline With Rust
Inhibitor C Anti
Foaming Agent
Contains Glycol
Formulated To Meet
Specific Cleaning
(1) Parts
(2) Circuit Board
Precision Parts
"Any Washable
Surface M
Metals t Plastics
Varies With
Check MSDS
With Company
VOCs Drying •
Step Usually
Health (?)
Treatment t
Safety Clean
Action Bio-Clean
Jet Cleaner
Turbulator Cleaning
P-30B "Spray clean"
Hydro Pulse
Safety Xleen Corp.
Box 1419
Elgin, IL 60120
Action Products, Inc. •
2401 W. 1st Street
Tempe, AZ 85281
Autop North America
P. O. Box 150146
Nashville, TN 37215
Turco Products, Inc.
7300 Bolsa Ave.
Westminster, CA 92684-3600
Peterson Machine Tool
5425 Antioch Drive
Shawnee Mission, KS 66202
COFF Corp.
P. O. Box 1607
Seminole, OK 74868
Shop Parts Cleaners
Small Parts Hashers
Automated Batch
Cleaning Small Parts
Agitated Aqueous Tank
High Pressure Spray
Cabinet With Turntable
Hot Hater Parts Hasher
Solvents and/or
Aqueous Process
Cleaning Cycles
Engine t Shop
Parts Cleaner
No Cleaners

Jet Washing
Polychem Alternative
Immersion Washers
Microdroplet Module
Cleaning Process
MAGNUS Equipment
Aqua-Quick, Modal
600, Model 6300, 'Model
6307, etc.
Better Engineering, Mfg.
7101 Belair Road
Baltimore, MD 21206
U. S. Polychemical Corp.
Rout* 45, P. 0. Box 268
Spring Valley, NY 10997
Bowden Industries
1004 Oster Drive, NW
Hunts vi Vie, AL 35816
Digital Equipment Corp.
Maynard, MA
Artisan (Vendor)
Man-Gill Chemical
23000 St. Clair Ave
Cleveland, OH 44117
Electronic Controls Design
4287-A SE International Way
Milwaukee, OR 97222-8825
Cabinet With Turntable
Fixed Jet Spray
Aqueous 6 Semi Aqueous
Batch t Continuous
Cleaners Including
Multiple unit Inline
Automated Masher
Conveyor or Monorail
Aqueous Inline Multiple
Unit Precision Cleaner
Aqueous Metal Cleaning
Batch t Inline
Alcohol - Water (Batch)
Closed System
Custom Design and
Standard Units
Family of
Different Sized
Units. Hill
Modify Existing
Oil Skimmers,
Multiple Rinse
. Components .
Standard Units t
Custom Design
Surface Mount
Cleaning Aqueous
With SaponiCiers
Uses stoelting
CBW224 Circuit
Board Washer
Replace Vapor
Flash Point
Precision &
5 Station Automated
Jet Edge
Precision Cleaners
Advanced Vapor
Proceco Typhoon
Equipment Systems Technology
P. O. Box 550
Findlay, OH 45640
Advanced Deburring t
Hwy. 70 East, P. 0. Box 1004
Statesville, NC 28677
Jet .Edge Inc.
825 Rhode Island Ave. So.
Minneapolis, MN 55426
150 Great Oaks Blvd.
San Jose, CA 95119-1367
Petrofirm, Inc.
5400 First coast Highway
Fernandian Beach, FL 32034
Proceco, Inc.
1020 East 8th Street
Jacksonville, FL 32206
Rotary Drum with '/wo
Wash/Rinse/Dry Batch or
Aqueous, Inline Multi
Station Cleaning t
Surface Preparation
System or Cabinet Units
High Pressure Water Jet
Inline t Batch
Closed System
vapor Degreasing With
Perfluocarbon Rinse
Heavy Duty Conveyor and
Parts Washers
Drum or Power
Spray Models
36,000 - 60,000
psi Cutting and
Acqueous w/wo
Semi Acqqurous
(Terpane) New
Design or

                                          Appendix B
              How To Calculate the Flow Rate of Rinse Water Required To
                             Achieve a Specified Dilution Ratio
 The three-stage counter-flow rinsing schematic in Fig-
 ure B-1 (which also appears in Chapter 7), is based on
 the following definitions and assumptions:


  a = concentration of chemical in Bath #3 (Ib/gal)
  b = concentration of chemical in Bath #2 (Ib/gal)
  c = concentration of chemical in Bath #1 (Ib/gal)
  d = concentration of chemical in Process Bath
  x = flow rate of counter-flow rinse (gal/min)
  y = flow rate of drag-in (gal/min)


 • All baths^are operating at their equilibrium concentra-

 • The  flow rate of all counter-flow rinses (x) (gal/min)
  are the same.

 • The flow rate of the drag-in (y) (gal/min) are the same
  for each stage.

 • The  concentration  of contaminant  in  the makeup
  water to Bath #3 is zero.

 Based  on these definitions and assumptions, the flow
 rate and dilution ratio are calculated as follows:
Concentration of Solution in Bath #3

                 y (gaMnirt) * b (Ib/gal) + 0
                  y (gal/fain) + x (gal/min)
Note: Because the incoming rinse water is clean, the
concentration of process chemical in the water is zero.



Therefore, the dilution ratio is:
                    a    y
 Concentration of Solution in Bath #2

  L. /ซ./  .>  Y(9&tfain)* c (lb/gal)+ x(gaIMn) * a
  jy (Jo/QQl) 3= i.'  i..—-.....  i  —I—.
                 y (gal/min) + x (gaMnin)

                      yc + xa
                   b = >
The dilution ratio b/a is:
                   b  yc+xa
                   a ~ a(y+ x)
But from Equation #1 :
                 y+x   yc + xa
                   y  ~a(y+x)

            a(y+x)(y+x) = y(yc + xa)

            a(y + 2xy+ x) = yzc + xya
                                                                 + 2

            ay + 2xya + ax= y  c + xya

              y2c=ay+xya + ax2
The dilution ratio c/a is:

                c   ^
Concentration for Bath #3

                  c =	

Dividing both sides by a:

                     c   yd+xb
                     a ~ a(y+ x)
 Therefore, from Equation #2:
                     x  x2   yd+ xb
                   + y+y2~a(y+x)
         (y + X) + (y + X)  + (y +
 Substituting from Equation 1 for b/a:
                   y   y   y2

Write the equation for the dilution ratio d/a:

             d      _    Ox2   x3  x2
            y—=y+2x+2— + -J-T- — ~x

                   ,           23

                  a~  +y   /+7
                               2   'Xf      (Eq.B-3)
                       yj  (^yj   [y
The equation can be expanded for multiple counter-flow
rinse stages,
                                K = concentration of the chemical in the process
                                    tank (Ib/gal).
                                a = concentration of the chemical in the final rinse
                                x = counter-flow rate (gal/min).
                                y = drag-in rate (gal/min).
                                n = number of rinse tanks.

                               For the simple case in which the drag-out (y) = 1 gal/min:

                                 For Bath #3 b/a = 1 + x

                                 For Bath #2 c/a = 1 + x + x2

                                 For Bath #3 d/a = 1 + x + x2 + x3

                               Note that the ratio x/y recurs in each term. Therefore,
                               when the drag-in y = 2 gal/min, the counter-flow rate (x)
                               must be twice as large; if y = 3, x will be three times as
                               high compared with the counter-flow rate corresponding
                               to a  1 gal/min drag-in rate.

                               Conversely, if the drag-in is well controlled and can be
                               reduced to a fraction of 1 gal/min, the counter-flow rate
                               (x) will be correspondingly lower.
      y gpm, d Ib/gal
y gpm, c Ib/gat
y gpm, b Ib/gal
y gpm, a Ib/gal
                             y gpm, b Ib/gal
  y gpm, c Ib/gal

   To WWT
                                     y gpm, a Ib/gal
                                             Process Flow
                                    Counter Current Rinse Flow
a = concentration of chemical in Bath #3 (Ib/gal)
b = concentration of chemical in Bath #2 (Ib/gal)
c = concentration of chemical in Bath #1 (Ib/gal)
d = concentration of chemical in process bath (Ib/gal)
x = flow rate of counter-flow rinse (gal/min)
y = flow rate of drag-in (gal/min)

Figure B-1.  Schematic of counter-flow rinsing.

                                     Appendix C
                Spreadsheet Model To Estimate Transfer Efficiency
Table C-1.  Table of Assumptions
Table C-2.  Calculation of Costs (TE = 30%)
Surface area to be coated
VOC of coating
Density of VOC portion
% Volume solids
Weight per gal (WPG)
% Weight solids (Calculated)
Cost of coating
Dry film thickness
Transfer efficiency
Size of filters 20"x20"
No. of filters across
No. of filters down
Total no. of filters affected
Holding capacity of filters
Percent of overspray going into
Percent efficiency of the filters
Percent PM10 in the overspray
Cost of filters
Number of filters which can be
disposed of in 55-gal drum
Cost to dispose of 55-gal drum
Days of operation
Labor required to replace filters
Labor rate



Total liquid gallons required
Total liquid coating used
Total solid coating used
Density of solid coating
Weight of solid coating used
Weight of total solid overspray
Weight of solid overspray in filters
Number of filters to be disposed of
Number of filter changes per year

Number of 55-gallon drums to be
disposed of
Cost of hazardous waste disposal
Cost of filters
Labor hours to change filters
Labor cost to change filters
Number of wasted gallons
Cost of wasted paint
Cost of waste + filters + labor
Cost of wasted paint
Total cost of waste





solid gals/yr
Ibs solid/yr
Ibs solid/yr
Ibs solid/yr




Table C-3.  Calculation of Costs (TE = 45%)
Total liquid gallons required          13.87       gals/day
Total liquid coating used             3,481.02     gals/year
Total solid coating used             1,825.64     solid gals/yr
Density of solid coating (Calculated)  12.01       Ibs/gal
Weight of solid coating used         21,931.90   Ibs solid/yr
Weight of total solid overspray       12,062.54   Ibs solid/yr
Weight of solid overspray in filters    7,237.53     Ibs solid/yr
Number of filters to be disposed of   1,206.25     filters/yr
Number of filter changes per year    60.31       filter changes/yr
Number of 55-gallon drums to be     30.16       drums/yr
disposed of
Cost of hazardous waste disposal    $9,046.91    $/yr
Cost of filters                       $6,031.27   $/yr
Labor hours to change filters         30.16       hours/yr
Labor cost to change filters          $452.35     $/yr
Number of wasted gallons           1,914.56     gals/yr
Cost of wasted paint                $38,291.23  $/yr
Cost of waste + filters + labor        $15,530.52  $/yr
Cost of wasted paint                $38,291.23  $/yr
Total cost of waste                  $53,821.76  $/yr
 Table C-4.   Formulas Used To Perform Calculations
 A                          B    C                  D
 Table of Assumptions
 Surface area to be coated
 VOC of coating
 Density of VOC portion
 % Volume solids
 Weight per gal (WPG)
 % Weight solids (Calculated)
 Cost of coating
 Dry film thickness
 Transfer efficiency
 Size of filters 20" x 20"
 No. of filters across
 No. of filters down
 Total no. of filters affected
 Holding capacity of filters
 Percent of overspray going
 into filters
 Percent efficiency of the filters
 Percent PMio in the overspray
 Cost of filters
 Number of filters which can
 be disposed of in 55-gal
 Cost to dispose of 55-gal
 Days of operation
 Labor required  to replace
 Labor rate
 Calculation of Costs (TE = 30%)
 Total liquid gallons required

 Total liquid coating used
 Total solid coating  used
 Density of solid coating
 Weight of solid  coating used
 Weight of total solid
 Weight of solid  overspray in
 Number of filters to be
 disposed of
 Number of filter changes per
 Number of 55-gallon drums
 to be disposed of
 Cost of hazardous waste
 Cost of filters
 Labor hours to change filters
 Labor cost to change filters
 Number of. wasted gallons
 Cost of wasted  paint
 Cost of waste + filters +
Cost of wasted paint
Total cost of waste
3,500.00           ft2/day
3.5                Ibs/gal
7.36               Ibs/gal
= (1-C5/C6))*100   %
9.8                Ibs/gal
64.29              %
$20               $/gal
1.5                mils
30                 %
= C14*C15
6                  Ibs/filter
60                 %




                                                                                                  = C4*C11*100*    gals/day
                                                                                                  = C30*C24        gals/year
                                                                                                  = C31 *C7/100      solid gals/yr
                                                                                                  = C9*C8/C7
                                                                                                  = C32*C33        Ibs solid/yr
                                                                                                  = C34*(1-C12/100)  Ibs solid/yr

                                                                                                  = C35*C18/100     Ibs solid/yr

                                                                                                  = C36/C17         filters/yr

                                                                                                  = C37/C16         filter
                                                                                                  = C37/C22         drums/yr

                                                                                                  = C39*C23        $/yr

                                                                                                  = C37*C21         $/yr
                                                                                                  = C38*C25        hours/yr
                                                                                                  = C42*C26         $/yr
                                                                                                  = C31*(1-C12/100)  gals/yr
                                                                                                  = C44*C10        $/yr
                                                                                                  = C40 + C41 +    $/yr
                                                                                                  = C45            $/yr
                                                                                                  = SUM (C48:C49)  $/yr

 Inclusive references in bold type indicate a general
 discussion of the entry topic.
abatement equipment, process emissions
      automotive industry, use in, 12
      for coating application, 85
      in custom coating operations, 13
      for paint stripping  operations, 145
      for plastics, 13
      types of, 9-10
      in vapor degreasing operations, 32
abrasive blast cleaning,  63-71. See also media, abrasive
   blast cleaning
      efficiency calculation, 64-65
      managing wastewater, 71
      media, 63, 65-66
      performance standards, 68-69
      processes, 64-65, 70-71
      purpose of, 8, 63
      screen sizes for media recovery systems, 66
      waste reduction, 66
      zinc-rich primer, as preparation for, 70
accelerating agent for phosphating, 45, 46, 47
acid etch. See phosphating, wash primers
acrylic-epoxy hybrid coatings, pros and cons, 95-97. See
   also water-borne coatings
acrylic latex coatings, pros and cons, 95-97. See also
   water-borne coatings
adhesion, 16-22
      abrasive blast cleaning, improved by, 64
      coating mixture, undermined by, 94
      phosphating, improved by, 41-52
      testing of,  162
adhesive forces
      in powder coating  application, 116
      in surface wetting, 19
aerospace industry, paint stripping methods used in, 142,
   143, 146
      of coating reservoir, to extend pot-life, 132-133
      of immersion bath, 36, 45, 55
air-assisted airless spray guns
      cleaning of, 137
      described, 80-81
air atomizing spray guns,
      appropriateness of, 162
      described, 79-80
 air drying. See also water-borne coatings
      vs. oven drying, 90, 93
      RACT limits for coatings, 93
      temperature, 90, 95
 airless spray guns
      appropriateness of, 165
      cleaning of, 137
      described, 80
 alkyd coatings. See also solvent-borne coatings;
   water-borne coatings
      solvent-borne formulations, pros and cons, 100-101
      water-borne formulations, pros and cons, 95-97
      degreasing of, 35,39-40
      phosphating of, 44, 48-49
      corrosion, role in, 16-17
      phosphating, role in, 45-46, 47
 appliance industry
      paint stripping  methods for, 141, 146
      pretreatment of workpieces for, 44, 46
      rinsing operations used in, 56
 application equipment, 9-10
      cleaning of, 96, 99, 134-138,  143, 145, 146
      costs, 117
 application of coatings, efficiency of. See transfer efficiency
 aqueous degreasing. See also degreasing
      as alternative to solvent-based methods, 39-40
      formulations, 34, 35
      products and equipment, 170-175
      pros and cons, 35
      with steam cleaning, 36
      with phosphating, 36
      process variations, 37
 aqueous paint stripping. See also stripping
      drawbacks,  140
      process factors, 141-142
 architectural products industry, powder coatings used in, 114
 autodeposition. See liquid coatings
      of burnoff paint stripping operations, 144
      of coating operations, 9, 51, 119, 83-84
      of conveyor loading, 55
      of liquid coating mixing, 91-92, 129
      of phosphate chemical  addition, 45, 48
      of rinse water flow control, 59
automotive industry
      coating operations in, 10-12
      metallic paint, viscosity for use in, 128

      paint stripping methods for, 141, 143, 146
      plural-component liquid coatings, use in, 92
      powder coatings, use in, 114
      rinsing operations, use in, 56, 61
      water-borne coatings,  use in, 10
      zinc phosphating, use in, 47,167

 baffle spray booths, 158
 baking finishes. See also powder coatings; solvent-borne
   coatings; water-borne coatings
      solvent-borne formulations, pros and cons, 101-102
      water-borne formulations, pros and cons, 99-100
 batch mixing of coatings, appropriateness of, 91, 92-93,
   126, 165
 best management practices
      for abrasive blast cleaning, 69-70
      for aqueous degreasing, 37
      for equipment cleaning, 134-138
      for liquid coatings selection, 86
      for liquid-solvent degreasing, 33-34
      for phosphating, 45
      for rinsing, 55-56
      for semi-aqueous degreasing, 38
      for spray booth operations, 158-159
      for vapor degreasing, 30
      for viscosity management, 125-127, 129-133,162
 blast cleaning. See abrasive blast cleaning
 Brookfield viscometer, 124-125
 burnoff of paint coatings, 144-145

 CAAA. See Clean Air Act Amendments
 cabinet, abrasive blast cleaning, 65
 carbon dioxide pellet blasting, 146
      corrosion, role in, 17
      phosphating, role in, 45-46, 47
 cathodic protection, 18
 CFCs. See also methyl chloroform
      degreasing generally, use in, 29
      vapor degreasing, use in, 30
 chiller coils, use in vapor degreasing, 30, 31
      of coatings to extend pot-life, 126, 132-133
      of workpiece for paint stripping, 146
 chlorinated solvents in paint stripping formulations, 140-141.
   See also solvents
 chlorofluorocarbon-113. See  CFCs
 chromate-based sealing rinse formulations, 60-61
 chromate oxide
      alternatives, nonchromate, 44
      for phosphating aluminum, 8, 44
      in water-borne epoxy coatings, 97, 98
 Clean Air Act Amendments (CAAA)
      chemicals in coatings, regulation of, 135
      degreasing, relevance to, 29
      major source facilities, 32,136
      paint stripping chemicals, regulation of, 141
cleaning of application equipment, 134-138
      cleaning formulation, 136
      fluid hoses, 138
      by paint stripping, 141, 143, 145, 146
      pressure pots, 137
      regulation of solvent use in, 135-136
      spray guns, 137-138
      transfer efficiency, relevance for,  76
 cleaning of workpieces, 28-29. See also degreasing;
   abrasive blast cleaning
      coverage of, 74-85
      liquid, 85-113
      types of, 9
 cohesive forces, in surface wetting, 19
 coil coating on raw materials, 24
 cold cleaning. See liquid-solvent degreasing
 compliant coatings. See liquid coatings
 component parts
      protective coatings on, 24-25
      storage of, 25
 computers, use in inventory control, 25. See also automation
      in mixed coatings, 93, 132
      on substrate, 16
      in vapor degreasing, 30
 contaminants, surface, 19-21
      abrasive blast cleaning of, 64
      on plastics, 21
      rinsing of, 53-59
      in tap water, 55
      testing for removal of, 50
      types of, 26-27
 conversion coating. See phosphating
      automated loading, 55
      continuous operation, 163-164
 copper, corrosion of,  17
 corrosion, galvanic
      abrasive  blast cleaning, removal by, 66
      caused by poor wetting, 19
      mechanisms of, 16-18
      phosphating for resistance to, 41 -52
      protection of raw materials against, 23-25
      sealing rinses, resistance provided by, 59
      zinc phosphates, resistance provided by, 48
      zinc vs. iron phosphating, resistance provided by, 45,
 counter-flow rinsing
      dilution ratio for, 57
      process calculations for, 58, 176-177
      process flow, 58
      process rates for, 59, 60
      purpose of, 57
      water usage in, 58
      of baked on solvent-borne liquid coatings, 101-102
      of powder coatings, 114,116, 117-118,119
      for transfer efficiency assessment, 78-79
      of water-borne liquid coatings, 99-100

degreasing, 26-40
      abrasive blast cleaning after, 8-9, 70
      approach selection, 27-28
      aqueous methods, 34-38
      formulation characteristics, 49-50
      immersion, 28

      of plastic workpieces, 12
      before priming, 5
      products and equipment, 170-175
      regulations, relevant, 29
      rinsing after, 56
      semi-aqueous methods, 37-38
      solvent-based methods, 29-34
      spray, 28
      testing for thoroughness of, 50
 deionized water
      automotive industry, use in, 10, 12
      characteristics of, 55
      degreasing operations, use in, 37, 39, 50
      phosphating operations, use in, 46
      rinsing, use in,  55-57, 62
      sealing, use in, 59-61
      defined, 129
      effect on coating viscosity, 127-128,129,130
 dilution of coatings, 127-128, 129, 130
 dilution ratio
      calculation of flow rate for, 176-177
      in counter-flow  rinsing, 57
 distillation of  solvent,  134-135
 drag-in/drag-out, avoidance of
      in counter-flow  rinsing, 57-59
      in degreasing, 30, 33, 35-36
      by rinsing generally, 53-59
 dry filter spray booths, 151-156
 dry-off ovens     '
      vs. air drying, 90, 93
      degreasing operations, use in, 37
      phos^hating operations, use in, 8
      PACT limits for coatings, 93
      of water-borne baking coatings, 99-100

 electrodeposition.  See liquid coatings
 electrolytic reactions.  See corrosion
 electromotive force series, 17
 electrostatic attraction
      for powder coating application, 116, 119
      transfer efficiency for, 77, 165, 167-169
      velocity as application factor, 84
      wrapping effect  in, 81, 84,158
 electrostatic spray guns
      appropriateness of, 162
      cleaning of, 137-138
      described, 81
 epoxy coatings. See also solvent-borne coatings;
   water-borne coatings
      solvent-borne formulations, pros and cons, 102-103
      stripping methods for, 142, 145
      water-borne formulations, pros and cons, 97-98

ferrous hydroxide.  See corrosion; flash rusting
filtering  of paint, need for, 168
filters, dry, for spray booths
      cost of, 152
      disposal of, 152, 153-154,156, 167
      efficiency of, 154
      particulate control, use for, 152
      polystyrene,  155
      selection of, 153-156
      waste-related costs, 154-156
 flash rusting
      blast profile, related to, 66-67
      after degreasing, 39
      after phosphating, 8
      rinsing, related to, 55
 fluidized bed
      for paint stripping, use of, 144
      for powder coating application, use of, 116
 fluorinated hydrocarbons, use in degreasing, 29
 Ford cup, 123-124
 freeboard ratio
      for vapor degreasing, 30
 fugitive emissions, control of
      in clean-up operations, 135-136
      in vapor degreasing, 30

 grease. See contaminants
 grime. See contaminants

 hazardous air pollutants (HAPs)
      from degreasing generally, 29
      from liquid-solvent degreasing, 33
      from liquid vs. powder coatings, 119, 166
      in paint stripping formulations, 141
      from semi-aqueous degreasing, 37
      from solvents generally, 135
      from vapor degreasing, 30
 hazardous waste
      disposal costs of, 166
      liquid vs. solid, 134
      separation from nonhazardous waste, 135,137
      in spray booth filters, 152,154-156
      in wash-water spray booth troughs, 157-158
 HCFCs, use in degreasing, 29
 heating of coatings reservoir
      pot-life and viscosity, tradeoff between, 132
      systems for, 131
      viscosity, for adjustment of, 125, 126, 129-130
 high volume, low pressure (HVLP) spray gun
      appropriate use of,  162
      cleaning methods, 137-138
      described, 79-80
 humidity control in spray booths, 151. See also severe
 hydrofluorocarbons (MFCs)
      degreasing, use in, 29
      in development, 32

 immersion  baths
      agitation of, 36, 45, 55
      counter-flow rinsing, use of, 57-59
      degreasing, use in,  33
      draining step after, 55, 61
      dwell time, 55, 145
      paint stripping, use  in, 141,145
      phosphating, use in, 46
      tank design considerations, 35, 50
impingement. See spray application
infrared rays, use of for curing powder coatings,  116
in-line mixing, 91-92, 126,129, 130,  166

 inventory control, 25
      corrosion, role in, 17-18
      phosphating, role in, 45-46
 iron, corrosion of,  17
 iron phosphating, 45-47. See also phosphating
      application parameters for, 44-45
      conversion coating process for, 43
      deposition related to blast profile, 71
      formulations, 46
      processes for, 8, 46-47
      sludge from, 45

 liquid coatings, 86-113. See also solvent-borne coatings;
   water-borne coatings
      application rate, 82
      autodeposition of, pros and cons, 105-107
      costs vs. powder coatings, 117,118
      drying of, 90
      effects of inappropriate mixture, 94
      electrodeposition of, pros and cons, 107-108
      mixing of, 91-92
      vs. powder coatings, 86-90
      radiation curing of, 108-109
      selection of,  112
      single- vs. plural-component, 90-94, 131-132
      supercritical  C02, use for application of, 110-111
      technology developments, 111-112
      thinning of, 127-128, 129-130
      vapor injection curing of, 110
 liquid nitrogen blasting, 146. See also plastic media blasting
 liquid-solvent degreasing, 32-34. See also degreasing
      draining of workpieces, 33, 55
      process costs, 33
      solvents used in, 33
      typical process, 33

 marine environments. See severe environments
 Maximum Achievable Control Technology (MACT) for
   degreasing operations, 29
 media, abrasive blast cleaning
      contamination of media, 70-71
      as determinative of phosphate deposition, 71
      recycling of, 65-66
      selection of, 67-68
      steel grit, specifications for, 69
      steel shot, specifications for, 68
      types of, 63,  67
media, paint stripping
      biodegradability of, 143
      carbon dioxide pellets, 146
      plastic, 142, 146
      recycling of, 142, 143,146
      sodium bicarbonate, 144
      wheat starch, 143
methyl chloroform.  See 1,1,1 trichloroethane
molten salt bath stripping, 145
Montreal Protocol,  relevance to degreasing operations, 29

noble  metals, oxidation of, 17
nonchlorinated solvents in paint stripping formulations, 141,
      sealing rinse formulations, 61
      water-borne epoxy coatings, 97

 Occupational Safety and Health Administration (OSHA)
    regulations, relevance to degreasing operations, 29
 ODCs. See ozone-depleting compounds; CFCs; 1,1,1
 oil, protective, 24. See also contaminants
 1,1,1 trichloroethane
      degreasing generally, use in, 29
      equipment cleaning operations, use in, 135, 137
      in solvent-borne coatings, 101
      substitution of, 39
      vapor degreasing, use in, 30
 OSHA (Occupational Safety and Health Administration)
    regulations, relevance to degreasing operations, 29
 ovens.  See curing; dry-off ovens
      collection with water curtain, 156
      on filters, problem of, 153, 166-169
      process equipment, removal from, 139,143, 144,145,
      spray booths, control in, 147, 159
      transfer efficiency, related to, 76,  84
 oxidation potential of metals, 17
 oxides. See scale
 ozone-depleting compounds (ODCs). See also CFCs;
   methyl chloroform
      degreasing, use in, 29
      equipment cleaning, use in, 135,  136, 137

 parts. See component parts; workpieces
 perchloroethylene (perc)
      regulation of, 31
      vapor degreasing, use in, 30
 perfluorinated carbon compounds (PFCs) in development
   for degreasing, 32
 phosphating, 41-52. See also iron phosphating; zinc
      abrasive blast cleaning before, 71
      of aluminum workpieces, 8, 9
      application parameters for, 44-45
      cost constraints on, 44
      deposition related to peening, 71
      formulation selection, 49
      heated rinse water, use of, 55
      heated solution, use of, 45
      processes, 8-9
      rinsing stage after, 56-57
      sealing of deposition, 59-61
      of steel workpieces, 41-52
      wash primers, use for, 48
      waste minimization, 48-49
pickling. See phosphating
      adhesion of coating to, 21
      degreasing of, 33,35
      paint stripping of, 142,143, 144, 146
      pretreatment of vs. metal workpieces, 12
plastic media blasting, 142. See also liquid nitrogen blasting
plural-component liquid coatings, 90-94

      heating of, 131
      mixing of, 126, 129, 130, 131-132, 165
      vs. single-component liquid coatings, 90, 94, 131-132
      stoichiometric proportions of ingredients, 94
 polymer sealing formulations, 61
 polyurethanes. See also solvent-borne coatings;
   water-borne coatings
      appropriate use of, 165-166
      humidity in spray booths, relevance of for perform-
         ance, 151
      solvent-borne, 104-105
      stripping methods for, 142, 145
      water-borne dispersions, pros and cons, 98-99
 pot-life of mixed coatings
      for acrylic-epoxy hybrids, 96
      defined, 132
      extension  of generally, 93-94, 131-132
      in-line mixing for extension of, 126, 129
      for water-borne epoxy coatings, 98
 powder coatings, 114-120
      application methods, 116
      application rate, 82
      appropriate use of, 114-115, 166
      conversion costs, 118, 166
      costs of generally, 117, 119
      curing of, 114, 116, 117-118
      vs. liquid coatings, 114
      process, 115-116
      pros and cons, 118-119
      types of, 118
      wetting of, 19
 pressure pot life. See pot-life
      degreasing of substrates, 26-40
      phosphating of metal, 41 -52
      system cost, 117
      application of, 4-5
      blast profile of substrate, relation to, 66
      mechanisms of adhesion, 18
      on products without a topcoat, 4
      water-borne epoxies, use of, 97
      on weld seam, 21
      zinc-rich, 70
 primer-topcoat systems. See also liquid coatings; powder
      appliance industry, used  in, 44
      application processes, 5-7,11
      automotive industry,  used in, 10-12
      compatibility with protective coating on substrate, 24
      products with, 6
proportioning equipment, 91-92, 126,129, 130, 166
protective coatings on vendor-supplied materials
      avoiding need for with inventory control,  25
      compatibility with primer-topcoat system, 24
      removal of, 24
pyrolysis, use of, for paint stripping, 144

quality control
      abrasive blast cleaning, absence of for, 67, 70-71
      abrasive blast cleaning, performance standards for,
      for ferrous metals, cleaning of, 51
      premature coating failure, for avoidance of, 162-164
      rinse water monitoring, 59
      titrations for rinse water, 55

 radiation curing of liquid coatings, 108-109
 Reasonably Available Control Technology (RACT) limits
      pot-life extension, relevance for, 94
      for solvent-borne coatings, 100-105
      state imposed, for coatings, 86, 93
      for water-borne epoxy coatings, 97, 99
      of abrasive blast cleaning media, 65-66
      of degreasing solvent, 33
      of paint from baffle booths, 158
      of paint stripping media, 142, 143, 146
      of paint stripping water, 143
      of polystyrene spray booth filters, 155
      of sealing rinse water, 61
      of solvent, 134-135
      of spray booth wash water, 156
      of spray booth wash-water sludge, 158
 reducers, 127-128, 129, 130
 removal of coatings. See stripping
 reservoir life. See pot-life
 right-first-time processing, 16, 53, 63
 rinsing, 53-59
      bath dwell time, 55
      counter-flow approach, 57-59
      after degreasing, 49-50, 56
      minimizing water usage, 57-59
      after paint stripping,  141, 145
      after phosphating, 56-57
      process variations, 50, 56-57
      pros and cons of spraying, 50-51
      with sealing of phosphate coating, 59-61
      spraying method, 55
      temperature of water, 55
      testing of bath, 55
      water quality, 55
      workpiece geometries, 55
 rust. See corrosion; flash rusting

 sacrificial protection, 18
 saponification, 56
      abrasive blast cleaning, removal by, 66
      on aluminum, 20
      on iron, 20
      rust converter, use of, 24
sealing, 59-61. See also rinsing
      chromate-based, 60-61
      managing wastewater, 60-61
      mechanism of, 59
      nonchromate, 61
      typical process, 60
semi-aqueous degreasing. See also degreasing
      formulations, 38
      products and equipment, 170-175
      pros and cons, 37-38
semi-aqueous paint stripping, drawbacks and process
   factors, 140, 142. See also stripping

 severe environments
      coating durability in, 47, 49
      corrosion in, 20, 66-67
      storage of raw materials in, 24
      zinc-rich primers, use of, 70
 single- vs. plural-component liquid coatings, 93, 94
 SIPs (State Implementation Plans), relevance to
   degreasing, 29
      from aqueous degreasing, 35, 36, 50
      dewatering of, 157-158
      disposal of, 158
      generation relative to transfer efficiency, 76
      from iron vs. zinc phosphating, 45, 48
      from molten salt bath stripping, 145
      from phosphating, excess heavy metals in, 44, 48, 49
      from solvent-based paint stripping, 141,142
      from solvent recycling, 135
      from spray booth filters, 154-155
      from spray booth wash water,  157
 sodium bicarbonate wet blasting, 144
 solvent-based paint stripping. See also stripping
      drawbacks, 140
      formulation, 140
      process factors, 141
 solvent-borne coatings, 100-105. See also liquid coatings
      alkyd, pros and cons, 100-101
      baked on, pros and cons, 101-102
      epoxy, pros and cons, 102-104
      hardness scale, 102
      polyurethane, 104-105
      spray viscosities, 127
      vs. water-borne coatings, 91, 92
      defined, 129
      drying of during coating application, 159
      entrapment of in coatings, 165-166
      equipment cleaning, use in, 134-138
      high boiling points, with, 136
      paint stripping, use in, 139-142
      recycling of, 134-135
      thinning coatings, use for, 127, 129, 130,165-166
solvent wiping. See also liquid-solvent degreasing
      for degreasing, 33
      disposal of rags, 34
      toluene and xylene, use of, 34
South Coast Air Quality Management District (SCAQMD),
   California, 79, 80, 81,136-137
spray booths, 147-159
      cleaning of, 134-138
      defined, 147
      dry filter type, pros and cons, 152-152
      enclosure, extent of, 149
      configurations in general, 10, 148-151
      custom coating operations, use in, 13
      lighting in, 159
      process management, 158-159
      temperature and humidity in, 151
      types of, 151-158
      ventilation considerations, 149-151,152, 153, 158,159
      water-wash type, pros and cons, 156-157
spray application
      for aqueous degreasing, 35, 36
      of custom coatings, 13
      efficiency techniques, 81-85,131
      equipment cleaning, 134-138
      gun types, 9, 79-81, 119
      for iron phosphating, 46
      for liquid solvent degreasing, 32-33
      of paint stripping formulation, 141
      for rinsing, 50-51, 55
      for semi-aqueous degreasing, 38,
      techniques,  149-150, 158,159
      transfer efficiency factors for, 75-76, 79-81,162
      of water after sealing rinse,  61
 State Implementation Plans (SIPs), relevance to
   degreasing, 29
      autodeposition of coatings on, 106
      corrosion of, 17, 55
      galvanized, 24, 48
      phosphate coating on, 41-52
      stainless, 24
      surface tension of, 19
      flash rusting of materials in,  66, 70
      inventory control, 25
      sealing rinses on materials in, use of, 59
      of vendor-supplied materials, 24
 stripping of coatings, 139-146
      alternative methods, pros and cons, 140-141, 142,
         143, 143-144, 145,  146
      approaches, 139-140
      formulations for, 140
      mechanism of, 140, 141
      need for generally, 139
      solvent use for,  139
      temperature of formulations, 141
 supercritical COz, use of for liquid  coating application,
 surface preparation
      after priming, 7
      steps in, 7-10
      for substrate, 64, 66
      of welded seam, 21
 surface tension
      of plastics, 21-22
      for semi-aqueous degreasers,  38
      of water, 20
      in wetting, 18-19
      approach for removal from workpiece, 56
      in phosphating formulations,  46
      testing  for removal of, 50
      wetting, role  in, 19

 tap water, municipal
      characteristics of, 55
      coating adhesion, as factor in,  57
      flushing application equipment, use for,  96
      rinsing, use in, 55, 56, 61, 145
Tape Adhesion Test, 162
TCLP (Toxicity Characteristic Leaching Procedure) testing of
   spray booth filters,  152-153, 167

 thickness, coating
      relative to viscosity, 128, 164-166
 thinning of liquid coatings, 127-128, 129,130
 33/50 program,  118
 Title III  Hazardous Air Pollutants. See Clean Air Act
   Amendments; hazardous air pollutants (HAPs)
 Title V Permit Rule. See Clean Air Act Amendments
 towel-wipe test
      after degreasing, 50
 Toxicity Characteristic Leaching Procedure (TCLP) testing of
   spray booth filters, 152-153,  167
 transfer efficiency, 74-85
      assessment approaches,  77-79
      benefits of improving, 74-77
      calculation of, 74, 76, 78,  79, 178-179
      cost of assessment, 78
      defined, 74
      filter selection for spray booth, factor in,  154, 155
      improvement approaches, 81-85
      for powder coatings, 119
      spray gun type, relative to, 79-81,165

 ultrafiltration of phosphating wastewater, 49

 vapor injection curing. See liquid coatings
 vapor-solvent degreasing, 29-32. See also degreasing
      alternative solvents, 31-32
      types of solvents and boiling points, 31
      typical process, 31
 vendor-supplied materials, 23-25
 ventilation of solvent degreasing vapors, 30, 34
 ventilation of spray booths, 149-151
      calculation of, 151
      minimum requirements, 151
      problems with, 153, 158, 159, 168
 viscosity, of liquid coatings, 121-133
      adjusted by thinning, 127-128, 129-130
      calculation of, 121-122
      control techniques, 125-127, 129-133
      defined, 121-122
      measurement technologies, 122-125
      of mixed coatings, 92, 132-133, 165
      for plural- vs. single-component, 131-132
      vs. pot-life with heating, 132
      problems associated with, 128-129,164-166
      of solvent- vs. water-borne coatings, 127
      of thixotropic coatings, 122, 127
      of water-borne coatings, 98
volatile organic compounds (VOCs)
      limits on for drying coatings, 90, 93
      in liquid coatings generally, 90, 91
      paint stripping, emitted from, 141,  142-146
      in powder coatings, 114, 116,  119
      in primers, 5
      in solvent-borne coatings,  100-105
      spray booths, management with, 147, 166-169
      state monitoring of, 29
      state regulation of, 134,136-137
      transfer efficiency, relative to,  76, 77,167-169
    . in wash primers, 48                       ....;...
      in water-borne coatings, 94-100, 166
 volume method of assessing, transfer efficiency, 79

      from abrasive blast cleaning, 63, 71
      from degreasing, 36
      from liquid vs. powder coatings, 119
      from phosphating, treatment of, 49, 167-168
      from rinsing, minimizing, 57-59, 61-62
      from seal rinsing, chromate-based, 60-61
      from solvent-based paint stripping, 141
      in wash-water spray booth troughs, 157-158
 water blasting, high- and medium-pressure, 143-144
 water-borne coatings, 94-100.  See also liquid coatings
      acrylic-epoxy hybrids, pros and cons, 95-97
      acrylic latex, pros and cons, 95-97
      alkydrpros and cons, 95-97
      baked on, pros and cons, 99-100
      epoxy, pros and cons, 97-98
      humidity, control in spray booths when  using, 151
      organic solvent  (co-solvent) in, 95
      polyurethane dispersion, pros and cons, 98-99
      vs. solvent-borne coatings, 90, 91, 92
      thixotropic property of, 127
      types of,  95-98
      VOC content of, 94-100
 water break-free test,  use of after degreasing, 50
 water treatment in  wash-water spray booths
      methods, 157-158
      selection of chemical, 157
 water-wash spray booths, 156-158
 weight method  for assessing transfer efficiency, 78-79
 weld slag and spatter, role of in corrosion, 21
 wetting, surface, role of in adhesion, 18-19
 wheat starch blasting, 142-143
 workpieces                              —
      of aluminum, ฃ,  9
      geometry relative to processing, 55,119, 162
      of plastic vs.  metal, 12
      size of relative to processing, 51, 63,  141
      size of relative to transfer efficiency, 79, 84
      size of relative to spray booth selection, 149, 158
      of steel, 41-52

Zahn cups, 122-123, 127
zinc phosphating, 47-48. See also phosphating
      conversion coating process,  43
      corrosion resistance provided by, 48
      mechanism of, 47'
      as pretreatment, 8, 167
      process flow for, 11        •
      sludge from,  47, 48, 167
      spray application vs. immersion, 48
      titanium salt in rinse water, use of, 56
zinc, sacrificial use of
      on Golden Gate Bridge, 18
      on raw materials, 24
                                                                     AUS. GOVERNMENT PRINTING OFFICE.-1998-7JO-101/00004

United States
Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
Official Business
Penalty for Private Use, $300