United States
Environmental Protection
Agency•
Office of Research
and Development
Washington, DC 20460
EPA/625/R-96/003
September 1996
&EPA
Manual
Pollution Prevention in the
Paints and Coatings
Industry
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EPA/625/R-96/003
September 1996
Manual
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
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DISCLAIMER
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.
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FOREWORD
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
iii
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ABSTRACT
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
industry.
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:
r'
• 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.
iv
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Contents
Page
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
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Contents (continued)
Page
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
VI
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Contents (continued)
Page
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
t'
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
VII
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Contents (continued)
Page
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 {« 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
VIII
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Contents (continued)
Page
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
ix
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Contents (continued)
Page
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
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Contents (continued)
Page
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
XI
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Contents (continued)
Page
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
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Contents (continued)
Page
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
XIII
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Figures
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
xtv
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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
xv
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Tables
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
XVI
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Tables (continued)
Page
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
XVII
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Conversion Factors
Units of measurement used throughout this document can be converted to SI units using the
following conversion factors:
To convert...
to...
multiply by...
cubic feet
degrees Fahrenheit
feet
inches
quarts, liquid
pounds
pounds per,cubic foot
pounds per cubic foot
pints/'liquid
square inches
tons
U.S. gallons
cubic meters
degrees Celsius
meters
centimeters
to liters
kilograms
kilograms per cubic meter
kiloPascals
to liters
square inches
metric tons
liters
2.831685 x10'2
t,c=(t,F- 32)71.8
0.3048
2.54
0.946352946
0.45354237
16.0184634
6.895
0.473176473
6.4516
0.90718474
3.785
XVIII
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Acknowledgments
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
publication.
XIX
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Section 1
Overview
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Chapter 1
Introduction
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
waste.
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
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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).
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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
Workpieces
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
Spikes
Steel wire cages
Wire carts, household, grocery, made in wire
drawing plants
Conduit
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
Radiators
Wood and coal burning stoves
Door and jamb assemblies
Liquid oxygen tanks
Sheet metal hoods
Bombs and parts
Mortar fin assemblies
Rifles
Industrial gate valves
Torsion bars
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Solvent Washing,
Solvent Wiping, or
Aqueous Degreasing
Welding and Fabrication
Incoming
Raw Material
Shipping
Priming
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.
2.2.2.1 Priming and Topcoating as a Two-Stage
Process
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
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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
Tractors
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
Equipment
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
Tractors
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
Gyroscopes
Hydrophones
Nautical instruments
Laboratory balances
Laboratory hot plates
Laboratory furniture
Clothes dryer controls
Thermostats
Computer interface equipment
Differential pressure instruments
Magnetic flow meters
Speedometers
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
Industries
SIC Code
3911-3999 Electronic musical instruments
Music stands
Games, toys
Fish and bait buckets
Exercising machines
Rowing machines
Treadmills
Pen holders and parts
Artist frames
Easels
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
uncovered).
• 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.
2.2.2.2 Priming and Topcoating as a Single-Stage
Process
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 (400°F)
Incoming
Raw Material
Assembly Area
Priming Spray
Booth
Curing Oven
Propping Area
Top Coat
Spray Booth
Figure 2-2. Schematic of a process for two-stage application of a primer-topcoat system.
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Welding and Fabrication
Vapor Degreasing
Incoming
Raw Material
Metal Pretreatment Dry-Off Oven (400°F)
Priming Spray
Booth
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.
2.2.3.1 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
(Ambient)
Degrease/ Water Rinse
on Phosphate (Ambient)
(Hot)
1
2
Figure 2-4. Schematic of a three-stage Iron phosphating
process.
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 400°F.
2.2.3.2 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-
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Degrease
(Hot)
Water Rinse
(Ambient)
Iron or Zinc
Phosphate
(Hot)
Water Rinse
(Ambient)
Seal Rinse
(Ambient)
Figure 2-5. Schematic of a five-stage Iron or zinc phosphatlng process.
Degrease
(Hot)
Water Rinse
(Ambient)
Deoxidize
(120°F)
Chromate or
Nonchromate
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
Classification
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
based)
Acrylic latex
Epoxy (water based)
Alkyd and modified alkyd (water
based)
Acrylics
Alkyd and modified alkyd
Epoxy catalyzed (two component)
Poiyurethane (single or two
component)
Alkyds and modified alkyds
Acrylics
Polyester (oil free)
Autodeposited
Electrodeposited
Powder
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
Airless
High volume, low pressure (HVLP)
Electrostatic (low voltage)
Conventional air atomizing
Air-assisted airless
Airless
HVLP
Electrostatic powder application
Table 2-10. Most Common Automated Coating Processes
Dip coating
Row coating
Electrodeposition
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.
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Table 2-11. Common Spray Booth Designs
Direction of air flow
Cross draft
Down draft
Semi-down draft
Filtering mechanism
Dry filter
Water wash
Baffle
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
Biofiltration
Membrane
Condensation
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
Industry
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 300° to 400°F. 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
2.3.2.1 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 70° to 80°F,
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., 150°F) of the oven. Only after all water had been
evaporated, could the part safely enter a high-tempera-
ture zone (i.e., greater than 250°F).
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-
10
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Metal Pretreatment
Bake Oven
Dry-Off Oven Electrodeposltlon
(Optional) Tank
Bake Oven
Topcoat and/or
Clearcoat
Bake Oven
Prime
Sealers and
Sound Deadening
Sanding
Figure 2-7. Schematic of a typical process for applying a primer-topcoat system in the automotive industry.
Degrease
' < r - "- *
r
Dry-Olf Oven
Rinse
,
01 Mist Spray
Dl Rinse
*
Dl Rinse
Zinc Phosphate
(Micro-crystalline)
' -"- '
Chromate or Non
chromate Sealer
-
-
i
Rinse
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.
2.3.2.2 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.
11
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- One-component polyurethane.
- Scratch-resistant clearcoats based on silane
chemistry.
- Scratch-resistant clearcoats based on acid-epoxy
chemistry.
- 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 250° to
285°F range. In particular, the industry is becoming
increasingly interested in powder coatings (3).
2.3.2.3 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
characteristics.
Because most plastics are heat sensitive, coatings
must be air- or force-dried at relatively low temperatures
(i.e., below 180°F). Thus, coating systems that must
be baked on at temperatures above 250°F, 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.
12
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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
workpieces.
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
68:23-24.
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.
EPA/340/1-91/009.
13
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Section 2
Pretreatment Factors
15
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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
surface.
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-
16
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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'
Electrode
Reaction
Lithium
Magnesium
Aluminum
Titanium
Manganese
Zinc
Chromium
Iron
Cadmium
Cobalt
Nickel
Molybdenum
Tin
Lead
Hydrogen
Copper
Silver
Mercury
Platinum
Gold
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
Standard
Oxidation
Potential
E8 (volts), 25°C
3.05
2.37
1.66
1.63
1.18
0.763
0.74
0.440
0.403
0.277
0.250
0.2
0.136
0.126
0.000
-0.337
-0.800
-0.854
-1.2
-1.5
Iron (Fe)
Electrode
Anode
Copper (Cu)
Electrode
Cathode
> Iron Ions
Hydrogen^
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
4
Electronsl
Iron Oxide
"Rust"
Anodic Area
Cathodic Area
Aqueous Electrolyte
Figure 3-2. Mechanism of corrosion on a steel substrate (3).
17
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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
rate.
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-
lyte.
• 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
electrons.
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
18
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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 (>250°F). 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
(dyne/cm)
Acetone
n-Butyl alcohol
Ethyl acetate
Glycol
Mercury
Methylene chloride
Toluene
Xylene
Water
24
20-26
20-26
48
465
27
27-29
28-30
73
Table 3-3. Approximate Surface Tension of Metallic Elements
in Inert Gas (5)
Surface Tension
(dyne/cm)
Chromium
Iron
Manganese
Molybdenum
Nickel
Titanium
Zinc
Copper
1,500-1,600
1,700-1,800
1,100
1,915-2,250
1,700-1,800
1,500-1,600
750-800
1 ,200-1 ,300
19
-------
(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.
3.3.3.1 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.
3.3.3.2 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
Scale
Rust Lifts Scale
Figure 3-5. Cross-sectional view of surface spalling caused by
scale.
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.
3.3.3.3 Welding By-Products
Adhesion also can be undermined by weld slag and
spatters in the area of a welded seam. Because the
20
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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-
Berkeley.
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
Technology.
Weld Spatter
Figure 3-6. Cross-sectional view of compromising effect of weld slag and spatters on a coating.
21
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Table 3-4. Surface Tensions of Coating Ingredients Versus Plastic Substrates (6)
Solvents dyne/cm Resins dyne/cm
Substrates
Polyester resin solution
34-38
dyne/cm
Water
Butyl cellosolve
Isopropyl alcohol
Propylene glycol
methyl ether (PM)
Dipropylene glycol
methyl ether (DPM)
N-methyl-pyrrolidon
(NMP)
72
28
22
28
31
30
Acrylic latec
Acrylic resin solution
Polyurethane emulsion
Polyurethane resin solution
PV Ac latec
Melanine resin
30-38
32-38
32-36
28-34
30-35
42-58
Plastics:
PVC (nonplasticized)
PVC (plasticized)
PP
Polyester SMC, BMC
PTFE
ABS
34-44
25-35
28-30
22-30
19-20
30-38
Coatings:
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
22
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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
Issue
Considerations
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
outdoors?
Such coatings can be effective in
preventing corrosion; however, they can
be difficult to remove prior to fabrication or
priming.
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
cleaner.
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
system.
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
23
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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
storage.
• 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
generation.
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
detergent.
24
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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
inventory.
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.
25
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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
Considerations
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
degreaser.
• 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
26
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Table 5-1. Decision-Making Criteria Regarding Surface Degreasing Process Efficiency and Alternatives to Conventional
Solvent-Based Methods
Issue
Considerations
Are the workplaces that need to be
cleaned large (e.g., assembled
machinery)?
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
vendor).
• 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
residue).
• 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
air.
• 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
facilities.
• 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.
27
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Table 5-1. Decision-Making Criteria Regarding Surface Degreaslng Process Efficiency and Alternatives to Conventional
Solvent-Based Methods (continued)
Issue
Considerations
For the workplaces that need to be
cleaned, is the production rate high
enough to justify using a continuous
system?
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 230° to 400°F) 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
wiping.
• Dissolvirig/washing by application of a chemical
solvent.
• Washing/dissolving by application of an aqueous so-
lution.
• Displacing/washing by application of a detergent (i.e.,
applying surface-active materials that displace the
grime).
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.
28
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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-
tions:
• 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
electronics.
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
5.3.1.1 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.
29
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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.
5.3.1.2 Process Basics and Best Management
Practices
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
tank.
• Racking parts for thorough drainage.
• Storing both fresh and used solvent in closed con-
tainers.
30
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Diffusion and
Convection
Exhaust -, Wq|
Retractable
Cover
G&
Waste
Solvent
- 7- ^T \ i
/ , Air
Of 0
o » o
o t | _o
"""" "" " i "" ^"™" . ." ^ ^~
• ( ' . \ ' '• 1
' i • i 1 . . I • .
'.',(••"' •!•/•/
. / (J; vapor ;!.-'•
t
Liquid
C
Condenser
Coils
Drag-Out
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.
5.3.1.3 Operational Strategies Involving the Use
of Conventional Vapor-Solvent
Degreasers
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)
Compound
Methylene chloride
1,1,1 Trichloroethane
Perchloroethylene
Trichloroethylene
CFC-113
Formula
CH2CI2
CH3CCI3
CI2C=CCI2
CCI2=CHCI
C2Ci3F3
Boiling
Point (°F)
104
165
250
188
180
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).
31
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• 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
use.
Additional possible interim strategies include:
• Use HFCs for vapor degreasing and drying until De-
cember 31, 1999, after which the HFCs must be
replaced.
• 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)
5.3.2.1 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
process.
32
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Table 5-3. Typical Organic Solvents Used in Degreasing
Operations
Solvent Group
Examples
tions off site to commercial operations. These vendors
typically recycle the spent solution and sell the recycled
solvent at low cost.
Alcohols
Ketones
Ester solvents
Aliphatic solvents
Aromatic solvents
Chlorinated solvents
Fluorinated solvents
Isopropanol, methanol, ethanol,
isobutanol
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-
plosions.
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.
5.3.2.2 Process Basics and Best Management
Practices
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
33
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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-
tages.
• To control drag-out, workpieces that have porous
substrates, which tend to entrap solvent, should be
degreased by aqueous or semi-aqueous methods.
5.3.2.3 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
ODCs.
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
5.4.1.1 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
34
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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
used
Can be used in batch or
continuous operations
Well suited to cleaning
processes that will be
followed by a phosphate
coating
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
systems)
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
clean
Operator may need to experiment
with various degreasing chemicals if
stubborn deposits are not easily
removed
Poor rinsing can contribute to paint
failures
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.)
5.4.1.2 Process Basics and Best Management
Practices
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
35
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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 230°F (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
geometries.
• 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.
36
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Best management practices for enhancing process effi-
ciency in the aqueous degreasing operation include the
following:
• 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
somewhat.)
- Compare costs of powdered and liquid degreasing
formulations.
• For process efficiency:
- Purchase degreaser and phosphate formulations
from the same vendor to avoid compatibility
problems.
5.4.1.3 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 260°
to 400°F. 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
5.4.2.1 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
37
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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-
greasers.
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
crevices.
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 90°F.
Flammability can be minimized, however, by formulating
or using them in their emulsion form. Additionally, certain
formulations can cause plastics and elastomers to swell.
5.4.2.2 Process Basics and Best Management
Practices
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)
Constituent
Comment
Terpenes Derived from citrus and pine oils; can be
formulated into emulsions; new formulations
raise flashpoint to >144°F 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 >200°F;
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 >200°F
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 199°F
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
temperature.
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
failures:
• 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
180°F), 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
38
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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
accomplished.
• Deionized rinse water was used to avoid contamina-
tion by dissolved salts in the municipal tap water.
• The deionized water was heated to approximately
180°F 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
generator.
• 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
primed.)
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
39
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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.
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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
operations.
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
41
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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?
or
Is the workpiece a low-cost
product for a price-sensitive
market such that adding the
cost of pretreatment would
undercut sales? '
r
Will the topcoated workpiece be
exposed to aggressive corrosive
environments?
or
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.
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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
crane?
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
formulation?
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
configurations).
• 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
required.
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,
43
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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)
Part
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
42.5
12.7
7.9
0.6
52.9
12.7
6.9
0.31
400
700
900
8,500
330
660
4,950
8,900
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
44
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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 120° to 140°F,
in keeping with vendor recommendations. An exception
applies when using low-temperature phosphate materi-
als, for which the phosphating solution is maintained at
90°F. 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
exactly.
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
phosphating.
• 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
45
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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.
6.3.1.1 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
process.)
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.
46
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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., 112° to 130°F) 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
47
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Table 6-5. Corrosion Resistance of Zinc Phosphate Coatings
on Steel and Electrogalvanized Steel (1)
Constant
Temperature Water
Salt Spray Condensate
Coating
Steel
Untreated
Zinc phosphate
(unaccelerated)8
(DIN 50021 SS)
0.1
6
(DIN 50017)
0.1
40
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
accelerated)
24
800
700
24
150
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
Phosphating
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
48
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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
Consecutive
Pollutant or Maximum for Monitoring Days Shall
Pollutant Property Any 1 Day (mg/U) Not Exceed (mg/L)
Cyanide, total
Copper '
Nickel
Chromium*
Zinc
Lead
Cadmium
Total metals
1.9
4.5
4.1
7.0
4.2
0.6
1.2
10.5
1.0
2.7
2.6
4.0
2.6
0.4
0.7
6.8
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)
0.69
2.77
3.38
0.69
3.98
0.43
2.61
1.20
2.13
Monthly Average
Shall Not Exceed
(mg/L)
0.26
1.71
2.07
0.43
2.38
0.24
1.48
0.65
Source:, Metal Finishing Point Source Category, 40 CFR Section
433.15.
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
Phosphating
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
soils.
• 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.
49
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• 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).
50
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• 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
undermined.
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).
I—©-
Load Station
Metal Pretreatment
Dry-Off Oven (400'F)
Unload Station
Priming Spray
Booth
Curing
Oven
Curing or
Baking Oven
Top Coat
Spray Booth
Prepping Area
Figure 6-3. Schematic of a conveyorized paints and coatings operation.
51
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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.
Acknowledgment
The author wishes to acknowledge Joe Schrantz, former
editor of Industrial Finishing Journal, for his contribution
to the information in this chapter.
52
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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
waste.
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.
53
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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.
54
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7.2.1 Rinsing Basics and Best Management
Practices
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
limited.
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
55
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equipment and training as well as advice about testing
frequency.
7.2.1.1 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
(Optional)
I
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.
7.2.1.2 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.
56
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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
baths.
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-
57
-------
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
I
Rinse Stage #1
c Ib/gal
y gpm, a Ib/gal
I
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
10
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)
(99-4.3)
% Reduction = „. ; * 100 = 95.7%
99
(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.
58
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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
(gal/min)
Dilution
Ratio
100:1
1,000:1
2,000:1
5,000:1
10,000:1
20,000:1
Stage 1
Rinse
99
999
1,999
4,999
9,999
19,999
Stage 2
Rinse
9.5
31.1
44.2
70.2
99.5
140.9
Stages
Rinse
4.3
9.6
12.2
16.7
21.2
26.8
Stage 4
Rinse
2.9
5.3
6.4
8.1
9.7
11.6
Stage 5
Rinse
2.3
3.7
4.3
5.3
6.0
7.0
Dilution
Ratio
100:1
1,000:1
2,000:1
5,000:1
10,000:1
20,000:1
Stage 1
Rinse
198
1,999
3,999
9,999
19,999
39,999
Stage 2
Rinse
19
62.2
88.4
140.4
199
281.8
Flow Rates
(gal/min)
Stage 3
Rinse
8.5
19.3
24.5 .
33.5
42.4
53.6
Stage 4
Rinse
5.7
10.6
12.8
16.2
19.5
23.3
Stage 5
Rinse
4.5
7.5
8.7
10.5
12.2
14.1
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
Dilution
Ratio
100:1
1,000:1
2,000:1
5,000:1
10,000:1
20,000:1
Stage 1 > 2
90
97
98
99 ,
99
{.99
Flow-Rate
P
Stage 2 > 3
95.7
99.0
99.4
99.7
99.8
99.9
Reduction
'<•)
Stage 3 > 4
97.1
99.5
99.7
99.8
99.9
99.9
Flow Rates
Stage 4 > 5
97.7
99.6
99.8
99.9
99.9
100.0
Dilution
Ratio
100:1
1,000:1
2,000:1
5,000:1
10,000:1
20,000:1
Stage 1
Rinse
50
500
1,000
2,500
5,000
10,000
Stage 2
Rinse
4.8
15.6
22.1
35.1
49.8
70.5
(gal/min)
Stage 3
Rinse
2.2
4.8
6.1
8.4
10.6
13.4
Stage 4
Rinse
1.5
2.7
3.2
4.1
4.9
5.8
Stages
Rinse
1.2
1.9
2.2
2.7
3.0
3.5
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
system.
59
-------
20,000 ,-
-=. 15,000 -
3
c
be
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.
150
Q.
S 100
I
E
I
§ 50
if
H 2nd Rinse Stage
Q 3rd Rinse Stage
• 4th Rinse Stage
S 5th Rinse Stage
III
ill
1
1
ill
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.
Deionized*
Sptay Rinse'
(Optional),
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.
7.3.2.1 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."
60
-------
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?
7.3.2.2 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
minimization.
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
system.
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.
61
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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
(June).
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
19-24).
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
(December).
62
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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-
mized.
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
degreasing.
63
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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
blasting?
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
64
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Clean Air
Air + Particulates
Ambient Air
Enters Through Doors
Compressed Air
for Ventilation
Cyclone
Separator
(for Fine Mesh Sizes)
Air to Blast Hose
Nozzle (90-100 psi)
Abrasive Blast Room
Dust Collector
Abrasive
Hopper
Waste Drum
Sieves Eliminating
the Fines (Optional)
Recycled Usable Abrasives
Abrasive
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
Sand
Garnet
Aluminum oxide
Steel grit
Chilled cast iron
1
6-8
10-15
>200
50-100
65
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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
valleys.
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
8
10
12
14
16
18
20
25
30
35
40
45
50
80
120
200
2.8
2.4
2.0
1.7
1.4
1.2
1.0
0.9
0.7
0.6
0.5
0.4
0.4
0.3
0.2
0.125
0.075
0.1
0.1
0.1
0.1
0.1
0
0
0
0
0
0
0
0
0
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).
8.2.4.1 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.
66
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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
Criteria
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-
plications.
• 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-
eration.
• Cost: Cost comparisons should include consideration
of all process factors, including the cost of rejected/
67
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Table 8-4. Guide for Selected Abrasive Media
Finishing
Cleaning/Removal
Peening
Surface profiling (Etch)
Working speed
Recyclability
Probability of metal removal
Hardness, MOH scale
(Rockwell Re)
Bulk density (Ib/cu ft)
Mesh sizes
Typical blast pressures (psi)
Shape
Source: Industry literature.
Stainless
Cut Wire
Yes
Yes
No
Yes
Med
High
Med-Hi
6-7.5
280
20-62
50-90
Angular
Table 8-5. Sample Specification Sheet for Steel
Product 7 8 10
S780 0.85
min
S660 •
S550
I'
S460
S390
S330
S280
S230
S170
S110
S70
Screen number 7 8 10
Screen size mm 2-8 2-4 2
Screen size Inches 0-1 0-1 0-1
12 14
97%
min
85% 97%
min min
85%
min
5% max
Steel
Shot
Yes
Yes
Yes
No
Med
Very-Hi
Very Lo
6-7.5
280
8-40
50-90
Spherical
Shot (2)
16 18
97.5%
min
85% 96%
min min
5% max 85%
min
12 14
1.7 1.4
0.1 0.1
5% max
5% max
16 18
1.2 1
0 0
Steel Grit
Yes
Yes
No
Yes
Med-Hi
Very-Hi
Med
6-7.5
230
10-325
50-90
Angular
20 25 30
96%
min
85% 96%
min min
85% 96%
min min
10% 85%
max min
10%
max
20 25 30
0.9 0.7 0.6
000
Aluminum
Oxide
Yes
Yes
No
Yes
High
Med-Hi
Med-Hi
8-9
125
12-325
20-90
Angular
35 40
S
97%
min
85%
min
10%
max
35 40
0.5 0.4
0 0
Silicon
Carbide
Yes
Yes
No
Yes
Very-Hi
Med-Lo
Med-Hi
9
95
36-220
20-90
Angular
45 50 80
97%
min
80.5 90%
min min
10% 80.5
max min
45 50 80
0.4 0.3 0.2
0 0 7.0e-03
Garnet
Yes
Yes
No
Yes
High
Med
Med
8
130
16-325
30-80
Angular
120 200
90%
min
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:
68
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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%
G.16
G.18
G.25
G.40
G.50
G.80
G.120
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)
0.9
0.75 0.85
0.75
16 18 20
1.2 1 0.9
0.05 0.04 0.03
Cleaning Finishes
Brush-Off
SSPC-SP7
NACE No. 4
SA-1
0.85
0.7
25 30 35
0.7 0.6 0.5
0.03 0.02 0.02
Commercial
SSPC-SP6
NACE No. 3
SA-2
3rd Quality
0.8
0.7 0.8
0.65 0.75
0.65
40 45 50 80
0.4 0.4 0.3 0.2
0.02 0.01 0.01 0.01
Near-White Metal
SSPC-SP10
NACE No. 2
SA-2V2
v 2nd Quality
0.75
0.6 70%
120 200 325
0.1 0.075 0.045
0.0049 0.0029
White Metal
SSPC-SP5
NACE No. 1
SA-3
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
$300.
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.
69
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• 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
cleaning.
• 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
Engineers.
• 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
Examples)
8.4.1 Abrasive Blasting Preceded by
Degreasing
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.
8.4.1.1 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
70
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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
Phosphating
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.
8.4.2.1 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-
tions.
10. Griese, II, J. 1988. Using recyclable steel grit for portable field
applications. J. Coatings and Linings (October), pp. 35-37.
71
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Section 3
Application Process Factors
73
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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
or
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
Efficiency
Benefits associated with improving transfer efficiency
include:
• 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.
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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
efficiency?
How should the transfer
efficiency of the process be
measured?
• 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
method.
• If workpieces are small with complex geometry but one cannot calculate surface area, a special
protocol may need to be designed.
v
• 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
Factors
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-
75
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Table 9-2. Effect of Transfer Efficiency on VOC Emissions
Transfer
Efficiency (%)
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
90 '
95
100 V
VOC of Coating
Dry Film Thickness
Emissions of VOC
In Ib VOC/gal
Solids Applied
133.5
66.7
44.5
33.4
26.7
22.2
19.1
16.7
14.8
13.3
12.1
11.1
10.3
9.5
8.9
8.3
7.9
7.4
7.0
6.7
3.5lb\gal
1.0 mil
Emissions of
VOC In Ib
VOC/1,000 ft2 of
Coated Surface
83.2
41.6
27.7
20.8
16.6
13.9
11.9
10.4
9.2
8.3
7.6
6.9
6.4
5.9
5.5
5.2
4.9
4.6
4.4
4.2
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.
76
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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
Waste
Costs With
TE = 30%
$29,649.18
Waste
Costs With
TE = 45%
$15,530.52
Savings Due
toTE
Improvement
$14,118.66
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
Efficiency
9.3.1 Defining Parameters Before
Commencing the Transfer Efficiency
Test
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
established:
• 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-
ditions.
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.
77
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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.
9.3.2.1 Measuring the Weight of Coating During
Application
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
conditions.
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
9.3.2.2 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 230°F 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
78
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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
9.3.2.3 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,
79
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limit the air atomizing pressure to 10 psi to ensure the
realization of transfer efficiency benefits from low air
pressure.
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 110°F, 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
option.
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
SCAQMD.
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.
80
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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
Expenditure
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
81
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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.
Long
Short
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
immediately.
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-
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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
.eading
Edge
Trailing
Edge
Direction of Spray
Poor Practice
Leading
Edge
Trailing
Edge
Direction of Spray
Figure 9-4. Effect of leading and trailing edges on transfer ef-
ficiency.
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.
83
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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
months.
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
84
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efficiency and significantly reduced overspray and booth
maintenance. All these factors contribute to pollution
prevention.
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
pollution.
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.
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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
noncompliant.
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
further.
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
coatings.
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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 250°F?
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
resistance?
Are the workpieces too large to fit into an oven, but do not require
hardness, abrasion and mar resistance, and some chemical
resistance?
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
coating.
• 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
coatings.
• If yes, solvent-borne coatings may be preferable to water-borne
coatings.
• 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
coatings.
Table 10-2. Advantages of Liquid Over Powder Coatings
Liquid
Powder
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.
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Table 10-2. Advantages of Liquid Over Powder Coatings (continued)
Liquid
Powder
Substrate Versatility
Application Temperature
Versatility
End-use Temperature
Versatility
Resin Technology
Finish Versatility
Environmental Conditions
Curing Requirements
Applicability for Low Cost
Items
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 100°F.
Liquid coatings can be designed for low
temperatures (sub-zero) to high temperatures
(over1,500°F).
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
preparation.
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,
etc.
Not available for substrates that cannot
withstand at least 250°F, and commonly 325°F
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 250°F.
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
Powder
Liquid
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.
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Table 10-3. Advantages of Powder Over Liquid Coatings (continued)
Powder
Liquid
Hazardous Waste
Water Pollution
Toxicity
Storage
Fire Risk
Part Suitability
Clean-up Profile
Automation Suitability
Learning/Training
Requirements
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
painters.
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
toxic.
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
painters.
Usually, transfer efficiencies are well below
60% regardless of spray gun type.
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Table 10-3. Advantages of Powder Over Liquid Coatings (continued)
Powder
Liquid
Electrostatic
Applications
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
achieved.
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
Coatings
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 250°F. 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 194°F and many rules establish special VOC
limits for this category. In contrast, coatings that cure
above 194°F 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
Plural-Component
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.
10.2.4.1 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
chemical.
The largest source of hazardous waste generated by
companies using plural-component coatings comes
from batch mixing processes. While such mixing is
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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
Appearance
Defects
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 194°F, 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 250° to 350°F. 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 194°F 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
apparel.
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
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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
Electrostatic
Application
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
water-bornes.
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
versatile.
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
met.
Some water-borne coatings are significantly
thixotropic, and are not easy to apply by untrained
painters.
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
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Table 10-6. Air/Force Dry Versus Bake
Air/Force Dry
Bake
Substrate Versatility
RACT Regulations
Heating
Requirements
Physical/Chemical
Properties
Appearance Defects
Curing Time
Clean-up
Requirements
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 194°F 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
environments.
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
coatings.
Surface defects, such as orange peel, often do
not flow out during the drying and curing
process. Force-drying at elevated temperatures,
but below 194°F, 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
precautions.
Can cause outgassing on sand castings and other
porous substrates. Preheating workpiece can often
overcome problem but adds an additional step to process.
Same
Generally must cure at a minimum of 250°F. A typical
curing schedule is 10 minutes @ 350°F. 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
polyurethanes.
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
California
Most other states
Air/Force Dry
Ib/gal g/L
2.8 340
3.5 420
Bake
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.
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Table 10-8. Single-Component Versus Plural-Component Coatings
Single-Component Coatings (Such as Alkyds)
Plural-Component Coatings
(Such as Polyurethanes)
Hazardous Waste
Considerations
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
temperature).
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
properties.
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
guns.
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
properties.
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
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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
distinctions.
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.
(a)
: 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 194°F. A
wide range of coating formulations fall into this broad
category. The most commonly available technologies
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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-
polymers.
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
retention.
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.
10.3.2.1 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
levels.
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.
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10.3.2.2 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 120° to 150°F.
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 194°R 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
performance.
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
mandatory.
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.
10.3.3.1 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-
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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.
10.3.3.2 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 194°F. 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.
10.3.4.1 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.
10.3.4.2 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
humidity.
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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.
f'
What these varied coatings share is that they cure at
elevated temperatures, usually well above 250°F. Many
vendors recommend curing temperatures in the range
of 325° to 350°F. 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
solvents.
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.
10.3.5.1 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.
10.3.5.2 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 325° to 350°F.
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-
strates.
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
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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
next.
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.
10.4.2.1 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 150°F 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.
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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.
10.4.2.2 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
250°F, 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 350°F.
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 250°F).
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4B 3B
Very
Very
Soft;
unusable
2B
HB
2H
3H 4H
5H 6H
Son T
Alkyds
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.).
10.4.3.1 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.
10.4.3.2 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 230°F
or 10 minutes at 350°F. 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 100° to 110°F. 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
thicknesses.
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 140° to 400°F
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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
finish.
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.
10.4.4.1 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.
10.4.4.2 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
parts.
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
procedures.
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 50° to
60°F) 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
shapes.
Other concerns involve cleaning requirements both be-
fore and after using epoxies. Epoxies are more forgiving
than most other resin technologies to surface prepara-
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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
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
polyether.
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.
10.4.5.1 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.
10.4.5.2 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
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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
10.4.6.1 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.
10.4.6.2 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-
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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 68°F 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 284° to 356°F or 210°
to 230°F, 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
10.5.2.1 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
prevention.
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
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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 200° to 250°F
achieves fully cured properties immediately. With some
resins curing temperature may be higher, between 284°
to 356°F. Infrared drying, however, is possible. Convec-
tion ovens are not necessarily required.
10.5.2.2 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
rinse.
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 275° to 375°F 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
reuse.
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
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• Farm machinery
• Fasteners
10.5.3.1 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
hazard.
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.
10.5.3.2 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 275° to 375°F.
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
tanks.
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
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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.
10.5.4.1 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.
10.5.4.2 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
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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 (VIC®) 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.
10.5.5.1 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.
10.5.5.2 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
Coatings
The Union Carbide Company has introduced their Uni-
carb® 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.
10.5.6.1 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.
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• 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).
10.5.6.2 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
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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
apply.
• 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
General
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.
EPA/340/1-91/009.
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.
Autodeposition
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.
Electrodeposition
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.
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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.
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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-
nologies.
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 250°F 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-
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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
workpiece?
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
325° to 400°F, 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
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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
conveyor.
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 400°F, 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
325°F and 400°F (163° to 204°C) for approximately 8 to
20 minutes. Developments are underway to lower the
curing temperatures to 250°F (121°C). 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
shipped.
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.
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11.4 Costs Associated With Powder
Coating
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
equipment.
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
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to approximately 325°F to 400°F (163°C to 204°C).
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
solvents.
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
to:
• 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
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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
325°F to 450°F (163°C to 204°C) for curing. Some
functional resins require temperatures in excess of
500°F. Apparently, however, some epoxies are available
that only require 250°F (121°C). Because of these
needs, powder coatings are associated with high energy
usage.
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
system.
11.6 References
1. Liberto, N.P. etal. 1990. Appliance Manufacturer. Business News
Publishing Company, (March).
119
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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
4(4):3-5.
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-
89/33.
120
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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)
i
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.
121
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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:
Fi
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
gm/(cm)(second).
Finally, a measure of kinematic viscosity is given in
stokes, where:
Stokes =
density
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
negligible.
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.
£
53
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
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Handle
Orifice
Stream
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).
124
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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
finishes.
• Using heat to reduce viscosity.
• Minimizing waste disposal by choosing appropriate
mixing procedure.
• Recognizing thixotropic properties of water-borne
solvents.
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
ways:
• 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-
stalled.
• 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
reusing.)
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 5°F, 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
ambient.
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
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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-
works.
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
finish.
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.
12.4.5.1 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.
12.4.5.2 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
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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
-A;
Reduction
Figure 12-7. Effect of reduction on viscosity for water-borne
coatings.
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
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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
Mismanagement
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
color.
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.
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• 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
Mixing
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
available.
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
properties.
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
properties.
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
12-10).
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
129
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Mixer/Manifold
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
reduction.
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 100°F to 120°F, 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.
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Temperature
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.
12.7.3.1
Vn-Li
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.
12.7.3.2 In-line Heating of Plural-Component
Coatings With Metering and Mixing
Equipment
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
discarded.
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.
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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
Hours
Figure 12-11. Effect»of 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
equipment.
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
Pot-Life
Low
Temperature
Hours
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
Coatings
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
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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
Publishing.
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.
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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,
including:
• 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
(HAPs)
• 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
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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.
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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
13.3.1.1 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.
13.3.1.2 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.
13.3.1.3 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
Alcohols:
Isobutyl alcohol 223 - 229 106-109
n-Butyl alcohol 243-245 117-119
Ketones:
Methylisobutylketone 237 - 244 114 - 117
(MIBK)
Methylisoamylketone 287 - 297 141-148
(MIAK)
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
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• 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
Solvent
Drum
Clean
Solvent
Drum
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
re-used.
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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.
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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
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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
cleaning).
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.
140
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• 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 140° to 250°F) 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 240°F) 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.
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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
14.4.1.1 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.
14.4.1.2 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
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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
industry).
Key advantages of the wheat starch blasting approach
include:
• 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
hardness.
• Media are sensitive to moisture and can require the
addition of an air drying system in humid environ-
ments.
• 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-
trapped.
• Less effective than other methods for cleaning proc-
ess equipment because of the heavy buildup of coat-
ings.
• Less effective than other methods for removing rust
and corrosion from metals.
• Contaminants remaining in the recycled media can
damage substrates.
14.4.1.3 High- and Medium-Pressure Water
Blasting
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
removal.
• 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
debris.
• 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
143
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can be used indoors or outdoors and in mobile op-
erations.
Principal limitations of the water blasting approach in-
clude:
• 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
14.4.2.1 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.
14.4.3.2 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 550° to 900°F)
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
amounts.
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-
ated.
• Applicable for a wide range of part sizes. Applicable
to all shapes; limitations relate only to the size of the
bath.
• 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
include:
• 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).
145
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14.4.4 Cryogenic Methods
14.4.4.1 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
coatings.
• Condensation can occur on the workpiece surface.
• Safety equipment must be included in the process
(e.g., a ventilation system for CO2 gas).
14.4.4.2 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 -320°F), 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.
146
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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.
147
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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
pallets?
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 50°F or above 80°F,
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-
tions:
• 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
148
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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
(2-Sided)
w, -,~/~N
; U *
Figure 15-1. Spray booth design concepts.
(d) Totally enclosed drive-in
booth with doors in front
149
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Filter or
Water
Curtain
Workpiece
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
15-3).
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
requirements.
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
finishes.
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
Operator
Conveyor
Figure 15-3. Side-by-side cross-draft booths.
150
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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 65°
and 80°F, 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
151
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I
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
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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-
posal.
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
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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
91.5-92.5
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
increase.
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
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
Summary
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
52.45%
9.8 Ib/gal
64.29%
$20.00 $/gal
1.5 mil
65%
20" x 20"
8
4
32
3 Ib/filter
60%
99%
99%
$1.00/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
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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
Summary
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
52.45%
9.8 Ib/gal
64.29%
$20.00 $/gal
1.5 mil
30%
20' x 20"
8
4
32
3 Ib/filter
60%
99%
$1.00/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
$73,101.44$/yr
$40,225.21 $/yr
$73,101.44$/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)
155
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$70,000 T
$60,000 - -
$50,000 - -
g $40,000 --
J5 $30,000
$20,000 - -
$10,000
$0
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
trough.
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.
156
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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
practicality.
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
booth.
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
booth.
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
problem.
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
157
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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
industry.
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.
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75.7.4 Haziness That Detracts From the
Gloss
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-
works.
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-
ers.
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.
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Section 4
Problem Solving
161
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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
housings.
• 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
Boxes
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
essential.
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 230°F
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 180°. 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
themselves?
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
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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 300°F. 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
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Continuous Conveyor
Moving at Speed "a"
Horizontal Racks
Conveyor #1
Conveyor #2
Conveyor #3
Continuous Conveyor
Moving at Speed "b"
J
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
include:
• 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
rinse.
• 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
patches?
• 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
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• Coating selection
Each of these factors contributes to one or all the prob-
lems the company faces.
16.3.2.1 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
speeds.
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.
16.3.2.2 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
used.
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
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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.
16.3.2.3 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 325° to
400°F. 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
Manufacturer
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
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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
waste.
• 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
shop.
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-
culated?
167
-------
• 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
pressure.
• 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
problem.
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
room.
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.
168
-------
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
pollution.
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.
169
-------
Appendixes
171
-------
Appendix A
Selected List of Suppliers of Aqueous and Semi-aqueous Degreaser
Formulations and Equipment*
AQUEOUS EQUIPMENT SUPPLIERS
LARGE UNITS
NAME
Spray Washer
ESTECH C-15154
C-15158
i
Final Phase Industrial
Parts Cleaners
Aqueous Parts Cleaner
COMPANY
New Pac, USA
P. 0. Box 1461
Palatine, IL 60078
312-541-3961
Equipment Systems Technology
P. 0. Box 550
Findlay, Ohio 45840
419-424-4239
Final Phase
23540 Pinewood
Warren, MI 48091
Ransohoff
N. 5th St., at Ford Blvd.
Hamilton, OH 45011
513-863-5813
TYPE
Inline, Overhead Monorail
Heavy Duty Monorail
(C-15154) or Conveyorized
(C-15158)
Conveyorized Monorail -or
Drum Aqueous Cleaners
Inline Monorail,
Conveyorized Automated, or
Batch. Complete Line of
Equipment, Small to Large
COMMENT
Constructed
of
Composite
Non-
Corroding
Materials
Cleans &
Phosphates
Aqueous
Cleaners
Existing
Equipment
Modification
Services
Available
Controlled
Spray
Impingement
System.
Complete
Design
Services
Available
1 Reproduced with permission from the Waste Reduction Resource Center for the South East, Raleigh, NC.
172
-------
AQUEOUS/SEMI AQUEOUS
CLEANERS
CLEANER
Bio Act EC7
Simple Green
Daraclaan 220,
282, 283
Quaker 624 GD
Turco 3878
6753
6778
4215 -NC-LT
SUPPLIER
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
404-691-8646
800-232-6100
Quaker Chemical Co
Elm « Lee Streets
Conshohocken, PA 19428
215-832-4000
Atochem - NA
3 Parkway
Philadelphia, PA
215-587-7000
TYPE
Terpene t Esters
Terpene
Alkaline With or
without Glycol
Ethers
Alkaline
Emulsion
w/agitation (3878)
Non-Chromated
Alkaline (6778)
USE
Electronics &
Parts Cleaners
Metal Cleaning
Metal Cleaning &
Electronics Parts
Cleaning
Immersion
Ultrasonic
Replace Vapor
Degreasing
POTENT I'M.
PROBLEM
Flammability
Flammability
Treatability
Corrosivity
Silicates
Immediate
Rinse Kay Be
Required
Corrosivity
Silicates
Chromates
from 3878
LF-NC Hon-
Chromate
Form
CLEANER
Coors Bio-T
Ridolene 1025
TD 1414-F-B
3HA-HF
Kwik Dri 66
Actrel 333BL,
3349L, 3360L,
1160L
Bxxate 800
SUPPLIER
Spectro-Chemical Lab
Division
Coors Porcelain Co.
600 Ninth Street
Golden, CO 80401
303-277-4254
Parker Arochen
32100 Stephenson Hwy
Madison Heights, MI
48071
800-222-2600 Ext. 286
DO
Arsol
Ashland Chemical, Inc.
Industrial Chemicals
P. 0. Box 2219
Columbus, OH 43216
614-869-3627
Exxon Chemical
P. O. BOX 5200
Bay town, TX 77522
713-425-2115
Exxon. Chemical
P. 0. Box 5200
Bay town, TX 77522
713-425-2115
TVPE
Terpene
Alkaline (HaOH)
Petroleum Solvent
Terpene
Hydrocarbon
Aliphatic
Irocarbon
. -troleum
Distillate
Hydrocarbon
Hydrocarbon
USE
Metal Cleaning
Vapor Degreaser
Parts Cleaning &
Paint Prep
Lacquer Stripper
Paint Thinner
Drawing Oil,
Cutting Oil,
Grease
Drawing' Oil
POTENTIAL
PROBLEM
Flammability
Safety
Flash Point
Flash Point
Flash Point
VOC's
Flammability
VOC's
Flammability
173
-------
CLEANER
Rust corrosion
Remover
CT-3/CT4
CT1/2
XUS11269.01
XUS11268
XUS-11267
r
Action Bioclean
SUPPLIER
Chen-Tech International
Hid America Chem Corp.
4701 Spring Road
Cleveland, Ohio 44131
216-749-0100
Do
Dow Chemicals & Metals
2020 Dow Center
Midland, MI 48674
517-636-3029
Dow Chemicals t Metals
2020 Dow Center
Midland, MI 48674
517-636-3029
Dow Chemicals £ Metals
2020 Dow Center
Midland, MI 48674
517-636-3029
Action Products, Inc.
2401 W. First Street
Tempe, Arizona 85281
602-894-0100
TYPE
Mineral Acids/
Glycol Ethers
Acid
Hydrocarbon
Surfactants With
Corrosion
Inhibitors
Semi Aqueous
Glycol/
Hydrocarbons
Cold Cleaner
w/Hydrocarbons
Water
Biodegradable
USE
Remove Oxidation
Rust. Requires
Pretreat with CT.l
Precleaning Multi-
Substitutes
Light Oils/Grease
Light Oils, Metal
Films
Oils, Grease
Metals Parts Wash
POTENTIAL
PROBLEM
Safety
Preclean
CTl, Rinse
CT2 , Dry
Flammability
Safety
Corrosion of
Some Metals
Odor - Must
Be
Incinerated
for Disposal
Toxicity,
VOC'S
Treatment
?
CLEANER
Teile Reinigung
Smlttel 09
SH-528
R. B. Degrease
BioClean
Citrex
Citra Safe
Axarel 38/52
RHA 6 RA Flux
Remove & Cleaner
SUPPLIER
RAASM USA
P. O. BOX 150146
Nashville, TK 37215
615-255-7434
Lubrichem, Inc.
P. O. Box 30665
Raleigh, NC 27622
919-839-1211
Environmental
Technology
Sanford, FL 32771
407-321-7910
Kester
515 E. Touhy Ave.
Des Plaines, IL 60018-
2675
Inland Technology
2612 Pacific Hwy, E.
Tacoma, WA 98424
206-922-8932
Dupont chemicals
Chestnut Run Plaza
P. 0. Box 80711
Wilmington, DE 19880-
0711
Mid America Chemical
Cleveland, OH 44131
216-744-0100
TYPE
Alkaline
Alkaline
KAOH pH13
Sulphanate
Alkaline
Terpene
Hydrocarbon
Alkaline &
Surfactants
USE
Steam, Pressure
Cleaning
Metal Cleaner
Metal Cleaning
Printed Circuit
Boards
Methylene Chloride
1,1,1 Vapor
degreasing
38- Electronics
52-Grease Metal
Cleaner
Circuit Boards
POTENTIAL
PROBLEM
Safety
Aluminum
Alloys
Safety
Foaming
Safety
Flammability
Flash Point
Treatment
174
-------
CLEANER
P P Degreaser
Arconate TM 1000
Gil lite 0650
Hurricane
Cleaning
Compounds
Aquaease
EZE 267D
SUPPLIER
PT Technologies, Inc.
106 4th Ave. , South
Safety Harbor, FL
34695
813-726-4644
Arco Chemical
3B01 West Chester Pike
Hewtown Square, PA
19073
1-800-321-7000
Man-Gill Chemical
2300 St. Clair Ave.
Cleveland, OH 44117
1-800-627-6422
Hidbrook Products
2080 Brooklyn Road
BOX 867
Jackson, Mich 49204
517-787-3481
Hubbard-Hall, Inc
P. 0. Box 790
Water bury, CT 06725-
0790
203-756-5521
EZE Products, Inc.
P. 0. Box 5744
Greenville, SC 29606
803-879-7100
TYPE
Low Aliphatic
Hydrocarbon/
Terpene
Propylene
Carbonate
Alkaline
Alkaline
Alkaline, Terpenes
and/or
Hydrocarbons
Alkaline
USE
Substitute for
1,1, '1 Cable &
Metal Cleaner
Replace Methylene
Chloride
Metal Cleaning
Metal Cleaning-
Vapor Degreasing
Alternative
Cleaners
Steel Parts
Dip Tank
POTENTIAL
PROBLEM
Combustible
Safety
Requirement
Safety
Safety
Process
Specific
Safety
CLEANER
Brulin
815 GD
815 GR
Alka - 2000
(1) DOT 111/113
(2) Omni Clean
H. D.
Glidsafe Family
Rentry Solvent
Blends
SUPPLIER
Brulin Corporation
Calgon Vestal Labs.
7501 Page Avenue
St. Louis, MO 63133
800-648-9005
Delta - Omega
Technologies, Inc.
P. 0. BOX 81518
Lafayette, LA 70598-
1518
318-237-5091
GLIDCO Organics
P. O. BOX 389
Jacksonville, FL 32201
904-768-5800
800-231-6728
Enviroaolve, Inc.
1840 Southside
Boulevard
Jacksonville, FL 32216
904-724-1990
TYPE
Alkaline
Potassium
Hydroxide
(l) Proprietory
"Surfactants
System"
(2) "Hater Based"
Proprietary
Terpene Blends
Terpenes With
Additives
USE
Metal Cleaning
Ferrous Metals
Cleaning pnlvl
(1) Metal Cleaning
(2) Heavy Oil
Buildup
All Surfaces
Ink Removal, Hand
Wiping, Emulsion
Cleaning
Tailored To Meet
Cleaning Needs
POTENTIAL
PROBLEM
Mild
Corrosivity
Silicates
High pH
Safety and
Handling
(1) None
Listed 'In
MSDS. High
Concentra-
tions could
cause
Aquatic
Toxcity
(2) None
Listed
Flammability
Treatment
Disposal
Haste
Disposal
Safety
175
-------
CLEANER
Oxsol Solvents ™
Family
(1) Parts Prep
(2) Micropure
(1) lonox FC, HC,
MC, LC
(2) Aquanox 8SA 4
101
3D SUPREME
Precision Clean
SUPPLIER
OXYCHEM
Occidental Tower
5005 LBJ Freeway
Dallas, TX 75244
800-752-5151
international Specialty
Products
1361 Alps Road
Wayne, NJ 07470
800-622-4423
XYZEN Corporation
413 Harding Industrial
Drive
Nashville, TN 37211
615-831-0888
800-845-5524
3D Inc.
2053 Plaza Drive
Benton Harbor, MI
49022-2211
616-925-5644
800-272-5326
LPS Laboratories, Inc.
4647 Hugh Howell Road
Tucker, GA 30085-5052
800-241-8334
TYPE
Halogenated
Aromatic
Derivative Of
Toulene
K-Methyl
Pyrrol idone Plus
Additives
(1) Alcohol £
Surfactants t
Sponifiers
(2) Alcohol
Alkaline,
Water Blend
Alkaline With Rust
Inhibitor C Anti
Foaming Agent
Contains Glycol
Ether
Alkaline
USE
Formulated To Meet
Specific Cleaning
Needs
(1) Parts
(2) Circuit Board
Cleaning
Electronics
Precision Parts
"Any Washable
Surface M
Metals t Plastics
POTENTIAL
PROBLEM
Varies With
Formulation
Check MSDS
With Company
VOCs Drying •
Step Usually
Required
Flammability
Treatability
Aquatic
Toxicity.
Health (?)
Treatment t
Disposal.
Safety
BATCH PARTS CLEANERS
MAKE
Safety Clean
Action Bio-Clean
Jet Cleaner
Turbulator Cleaning
Tanks
P-30B "Spray clean"
Hydro Pulse
COMPANY
Safety Xleen Corp.
Box 1419
Elgin, IL 60120
Action Products, Inc. •
2401 W. 1st Street
Tempe, AZ 85281
602-894-0100
Autop North America
P. O. Box 150146
Nashville, TN 37215
615-255-7434
Atochem
Turco Products, Inc.
7300 Bolsa Ave.
Westminster, CA 92684-3600
714-890-3600
Peterson Machine Tool
5425 Antioch Drive
Shawnee Mission, KS 66202
1-800-255-6308
COFF Corp.
P. O. Box 1607
Seminole, OK 74868
1-800-654-4633
TYPE
Shop Parts Cleaners
Small Parts Hashers
Automated Batch
Cleaning Small Parts
Agitated Aqueous Tank
Cleaner.
High Pressure Spray
Cabinet With Turntable
Hot Hater Parts Hasher
COMMENT
Solvents and/or
Petroleum
Distillates
Aqueous
Aqueous Process
Programmed
Cleaning Cycles
Engine t Shop
Parts Cleaner
No Cleaners
176
-------
AQUEOUS EQUIPMENT, SUPPLIERS
SMALL TO MEDIUM
NAME
Jet Washing
Polychem Alternative
2000
Immersion Washers
Microdroplet Module
Cleaning Process
MAGNUS Equipment
Aqua-Quick, Modal
600, Model 6300, 'Model
6307, etc.
COMPANY
Better Engineering, Mfg.
7101 Belair Road
Baltimore, MD 21206
1-800-638-3380
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
1-800-553-3637
Digital Equipment Corp.
Maynard, MA
207-636-3939
Artisan (Vendor)
C17-893-6800
Man-Gill Chemical
23000 St. Clair Ave
Cleveland, OH 44117
1-800-627-6422
Electronic Controls Design
4287-A SE International Way
Milwaukee, OR 97222-8825
800-323-4548
TYPE
Cabinet With Turntable
Fixed Jet Spray
Aqueous 6 Semi Aqueous
Batch t Continuous
Cleaners Including
Ultrasonic
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
COMMENT
Custom Design and
Standard Units
Family of
Different Sized
Units. Hill
Modify Existing
Units
Oil Skimmers,
Filtration
Multiple Rinse
. Components .
Standard Units t
Custom Design
Surface Mount
Cleaning Aqueous
With SaponiCiers
Uses stoelting
CBW224 Circuit
Board Washer
Replace Vapor
Degreasing
Flash Point
Precision &
Electronics
NAME
ES TECH
5 Station Automated
Cleaner
Jet Edge
Precision Cleaners
Advanced Vapor
Degreasing
Proceco Typhoon
COMPANY
Equipment Systems Technology
P. O. Box 550
Findlay, OH 45640
419-424-4239
Advanced Deburring t
Finishing
Hwy. 70 East, P. 0. Box 1004
Statesville, NC 28677
800-553-7060
Jet .Edge Inc.
825 Rhode Island Ave. So.
Minneapolis, MN 55426
612-545-1477
800-538-3343
ATCOR
150 Great Oaks Blvd.
San Jose, CA 95119-1367
408-629-6080
800-827-6080
Petrofirm, Inc.
5400 First coast Highway
Fernandian Beach, FL 32034
904-261-8288
Proceco, Inc.
1020 East 8th Street
Jacksonville, FL 32206
904-355-2888
TYPE
Rotary Drum with '/wo
Ultrasonics
Conveyorized
Wash/Rinse/Dry Batch or
Continuous
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
COMMENT
Aqueous
Drum or Power
Spray Models
36,000 - 60,000
psi Cutting and
Cleaning
Acqueous w/wo
Ultrasonics
Semi Acqqurous
(Terpane) New
Design or
Retrofit.
Multiple
Processes
177
-------
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:
Definitions
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)
Assumptions'
• All baths^are operating at their equilibrium concentra-
tions.
• 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
a(lb/gal)='
y (gal/fain) + x (gal/min)
Note: Because the incoming rinse water is clean, the
concentration of process chemical in the water is zero.
or
a*-£-
y+x
Therefore, the dilution ratio is:
b_y+x
a y
or
(Eq.
Concentration of Solution in Bath #2
L. /«./ .> 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 = >
y+x
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
yd+xb
c =
y+x
Dividing both sides by a:
178
-------
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,
where:
K = concentration of the chemical in the process
tank (Ib/gal).
a = concentration of the chemical in the final rinse
tank.
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
I
y gpm, c Ib/gal
To WWT
y gpm, a Ib/gal
I
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.
179
-------
Appendix C
Spreadsheet Model To Estimate Transfer Efficiency
Table C-1. Table of Assumptions
Table C-2. Calculation of Costs (TE = 30%)
A
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
filters
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
B C
3,500.00
3.5
7.36
52.45
9.8
64.29
$20.00
1.5
30
OK
5
4
20
6
60
99
99
$5.00
40
$300.00
251
0.5
$15.00
D
ffrday
Ibs/gal
Ibs/gal
%
Ibs/gal
%
$/gal
mils
%
Ibs/filter
%
%
%
$/filter
filters/drum
$/drum
days/yr
hours
$/hour
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
Summary
Cost of waste + filters + labor
Cost of wasted paint
Total cost of waste
20.80
5,221.53
2,738.47
12.01
32,897.84
23,028.49
13,817.09
2,302.85
115.14
57.57
$17,271.37
$11,514.24
57.57
$863.57
3,655.07
$73,101.44
$29,649.18
$73,101.44
$102,750.62
gals/day
gals/year
solid gals/yr
Ibs/gal
Ibs solid/yr
Ibs solid/yr
Ibs solid/yr
filters/yr
filter
changes/yr
drums/yr
$/yr
$/yr
hours/yr
$/yr
gals/yr
$/yr
$/yr
$/yr
$/yr
180
-------
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
Summary
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
drum
Cost to dispose of 55-gal
drum
Days of operation
Labor required to replace
filters
Labor rate
Calculation of Costs (TE = 30%)
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
Summary
Cost of waste + filters +
labor
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 %
OK
5
4
= C14*C15
6 Ibs/filter
60 %
99
99
5
40
300
251
0.5
15
$/filter
filters/drum
$/drum
days/yr
hours
$/hour
= C4*C11*100* gals/day
100/(1604*C7*C12)
= 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
changes/yr
= 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
C43
= C45 $/yr
= SUM (C48:C49) $/yr
181
-------
Index
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
agitation
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
aluminum
degreasing of, 35,39-40
phosphating of, 44, 48-49
anodes
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
automation
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
183
-------
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
cathodes
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
chilling
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
coatings
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
condensation
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
conveyors
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,
47
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
curing
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
184
-------
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
diluent
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
environments
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
185
-------
inventory control, 25
ions
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,
142
nonchromate
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
trichloroethane
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
overspray
collection with water curtain, 156
on filters, problem of, 153, 166-169
process equipment, removal from, 139,143, 144,145,
146
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
phosphating
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
plastic
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
186
-------
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
pretreatment
degreasing of substrates, 26-40
phosphating of metal, 41 -52
system cost, 117
primers
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
coatings
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,
68-69
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
recycling
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
scale
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
187
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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
sludge
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
solvents
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
steel
autodeposition of coatings on, 106
corrosion of, 17, 55
galvanized, 24, 48
phosphate coating on, 41-52
stainless, 24
surface tension of, 19
storage
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,
110-111
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
surfactants
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
188
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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
wastewater
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, £, 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
189
AUS. GOVERNMENT PRINTING OFFICE.-1998-7JO-101/00004
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United States
Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
Official Business
Penalty for Private Use, $300
EPA/625/R-96/003
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