r/EPA
United States
Environmental Protection
Agency
Office of Research and
Development
Washington DC 20460
Guide to
Cleaner
Technologies
Organic Coating
Replacements
EPA/625/R-94/006
September 1994
-------
EPA/625/R-94/006
September 1994
GUIDE TO CLEANER TECHNOLOGIES
ORGANIC COATING REPLACEMENTS
Office of Research and Development
United States Environmental Protection Agency
Cincinnati, OH 45268
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NOTICE
This guide has been subjected to the U.S. Environmental Protection Agency's peer and administrative review and
approved for publication. Approval does not signify that the contents necessarily reflect the views and policies
of the U.S. Environmental Protection Agency, nor does mention of trade names or commercial products constitute
endorsement or recommendation for use. This document is intended as advisory guidance only to paint and
coating applicators in developing approaches for pollution prevention. Compliance with environmental and
occupational safety and health laws is the responsibility of each individual business and is not the focus of this
document.
Users are encouraged to duplicate portions of this publication as needed to implement a waste minimization plan.
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ACKNOWLEDGMENTS
This guide was prepared under the direction and coordination of Douglas Williams of the U.S. Environmental
Protection Agency's (EPA's) Center for Environmental Research Information (CERI) and Paul Randall of the
EPA Risk Reduction Engineering Laboratory (RREL), both located in Cincinnati, Ohio. Eastern Research
Group, Inc. (ERG) of Lexington, Massachusetts, and Battelle of Columbus, Ohio, under contract to CERI,
compiled and prepared the information used in this guide.
The following individuals participated in the development and review of this document. Their assistance is kindly
appreciated.
Charles H. Darvin Richard J. Shain
Organic Control Branch King Industries
Air & Energy Engineering Research Laboratory Science Road
U.S. Environmental Protection Agency Norwalk, CT 06852
Research Triangle Park, NC 27711
Ann Goyer Thomas F. Stanczyk
Chemical Coalers Association International RECRA Environmental, Inc.
P.O. Box 54316 1 Hazelwood Drive
Cincinnati, OH 45254 Amherst, NY 14428-2298
Ron Joseph Oliver Stanley/Chuck Danick
Ron Joseph & Associates, Inc. Cargill Resin Products Division
12514 Scully Avenue Cottage Avenue & Marian Road
Saratoga, CA 95070 Carpentersville, IL 60110
Lawrence Melgary
Northern Coatings & Chemical Co.
705 Sixth Avenue
Menominee, MI 49858-0456
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CONTENTS
Page
Section One
Section Two
Section Three
Section Four
Section Five
Introduction 1
Available Technologies 15
High Solids Coatings, Solvent-Borne 15
Powder Coatings 25
Waterborne Coatings 46
Electrodeposition 63
UV/EB Radiation-Cured Coatings 67
Emerging Technologies 79
Vapor Injection Cure Coatings 80
Supercritical Carbon Dioxide as Solvent 81
Radiation-Induced Thermally-Cured Coatings 84
Pollution Prevention Strategy 85
Information Resources 89
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Section One
SECTION ONE
INTRODUCTION
What is a Cleaner
Technology?
A cleaner technology is a source reduction or recycle method applied to
eliminate or significantly reduce the amount of any hazardous substance,
pollutant, or contaminant released to the environment. The emphasis of
cleaner technologies is on process changes that can prevent pollution.
Pollution prevention occurs through source reduction, i.e., reductions in the
volume of wastes generated, and source control (input material changes,
technology changes, or improved operating practices).
Cleaner technologies include process changes that reduce the toxiciry or
environmental impact of wastes or emissions. Processes that reduce waste
toxicity by transferring pollutants from one environmental media to another
(e.g., from wastewater to sludge or from air enassions to scrubber wastes) are
not inherently cleaner and are not considered to be source reduction.
Cleaner technologies also include recycle methods, but recycling should be
considered only after source reduction alternatives have been evaluated and
implemented where technically feasible. Where they are used, recycling
techniques should occur in an environmentally safe manner.
Why Use Organic
Coatings
Organic paints and coatings serve the primary functions of surface decoration
and surface protection. Approximately 50 percent of the paints and coatings
used in the United States are for protection and decoration of new and existing
construction (architectural and industrial maintenance coatings), while 30
percent are used to protect and/or decorate industrial products (original
equipment manufacturer or OEM product finishes). The remaining 20 percent
is used for special purpose or miscellaneous applications such as traffic paint,
automotive refmishing, high-performance coatings for industrial plants and
equipment, and protection of marine structures and vessels (Bureau of the
Census, 1992). Table 1 describes the major subcategories of the OEM
product finishing and special purpose applications segments of the coatings
industry.
Architectural coatings are applied on site to interior or exterior surfaces of
residential, commercial, institutional, and industrial buildings. They are
applied for protection and appearance, and cure at ambient conditions.
Page!
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Section One
Table 1. The OEM and special purpose coatings markets.
Original equipment manufacturer (OEM) category:
Metal containers
Wood furniture and fixtures
Machinery and equipment
Automotive
Coil coatings
Wood and composition flat stock
Beverage and food cans, other metal containers
Furniture, kitchen cabinets, and other millwork (doors,
windows, trim, moldings)
Farm and construction equipment, electrical
machinery, refrigeration and heating equipment,
general industrial machinery and equipment,
computers and office equipment
Topcoats, underbody paints, and primers applied to
automobiles and light trucks; coatings for automotive
parts
Coatings applied to continuous coils of steel or
aluminum, later fabricated into products by the
household appliances, transportation, building, and
containers industries
Hardboard, plywood and particleboard fashioned into
panels
Special-purpose coatings category:
High-performance maintenance coatings
Marine coatings
Highway and traffic paint
Automotive refinishing
Coatings applied to protect metal and concrete
structures as well as tanks, pipes, and processing
equipment from the effects of corrosive environment
Applied to steel and aluminum structures exposed to
marine environments such as ships, offshore oil and
gas structures, and other structures
Coatings used for marking lanes and placing
directional signs (e.g., arrows) on the road surface
Coatings used for repair work on cars, trucks, buses,
and motorcycles
Page 2
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Section One
OEM product finishes are applied to factory-made articles as part of the
manufacturing process. Coatings may be applied onsite by the product
manufacturer or offsite by custom coalers. A wide range of coating types and
application techniques may-be used. These coatings may cure at ambient
conditions or at elevated temperatures.
Industrial maintenance coatings are field-applied high-performance coatings
formulated to resist harsh environments such as heavy abrasion, water
immersion, exposure to chemicals or solvents, and/or high temperatures.
Paints and coatings are applied to surfaces to enhance corrosion resistance,
provide one or more special properties (enhanced corrosion resistance,
weatherability, durability), and improve appearance. Among the major
industries that apply coatings are manufacturers of:
+ Automobiles
» Aircraft
> Appliances
Wood products
AutomobilesThe main function of automotive coatings are appearance,
exterior durability, and corrosion protection. Typical automotive coatings use
an undercoat or primer to give corrosion protection and improve durability.
The topcoats are formulated to give the desired color and gloss. In some
cases, a low-solids polyester basecoat is applied to give the color, followed by
an acrylic clearcoat for a high gloss finish. Automotive coatings are normally
applied on sheet steel, but body parts are increasingly being made from other
materials such as plastic, composite, or stainless steel.
AircraftThe main function of aircraft coatings are to resist the damage that
can occur from corrosion, contact with fluids and fuels, erosion, temperature
extremes, weathering, and impact. Coatings may also assist in providing
protection for lightning strike. Appearance may be entirely cosmetic or, in the
case of military aircraft, serve as camouflage. Aircraft finishes may be applied
over aluminum, titanium, composite, or other substrates.
AppliancesAppliances are often referred to as "white goods" due to the
traditional color of the coating applied. Nowadays a wide variety of colors can
be applied to suit consumer tastes. Coatings are applied to protect the
underlying metal from the effects of water, salt, detergent, and other common
household agents at temperatures in the range of about 0°C to 100°C (32°F to
212°F). The substrate is typically sheet steel.
Wood productsWood products such as furniture, siding, and doors are
coated to increase durability and improve appearance. Exterior coatings must
Page 3
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Section One
have greater weather resistance than coatings on items intended for interior
use. Color or clear coatings may be chosen depending on the type and quality
of the wood substrate and the intended end use.
In addition to the specific examples described above, coatings are used in a
wide variety of other industries and applications. Other markets for unique
coatings include: paper finishes, machinery, packaging materials (e.g.,
containers), sheet metal, coil, and truck, bus, and rail equipment.
New coatings are being formulated to meet customer needs, including
environmental criteria. Coatings markets now influence coating chemistries
and application techniques to a substantial degree. The wide range of
applications and the increasing environmental requirements of the coatings
markets indicates the cross-industry applicability of cleaner organic coating
technologies.
Pollution Problem Classical organic coating materials consist of dilute solutions of organic resins,
organic or inorganic coloring agents, additives, and extenders dissolved in an
organic solvent. The organic solvent gives the liquid coating the necessary
viscosity, surface tension, and other properties to allow application of a
smooth layer of coating.
The liquid coating is brushed, rolled, sprayed, flowed, or otherwise applied to
the surface. As the organic solvent evaporates, the organic resins polymerize
to form the desired file.
Environmental concerns and increasing costs of organic chemicals and metals
(zinc, chromium, etc.) are leading to changes in the formulation of organic
coatings. Coating formulators and users are seeking alternative materials to
reduce or eliminate use of volatile solvents and heavy metals, and generation
of paint residues and wastes.
Typical coating solvents used in coatings formulations include methyl ethyl
ketone, methyl isobutyl ketone, toluene, and xylene. A more detailed list is
shown in Table 2. Coloring agents in paints can include inorganic pigments
containing hazardous metals such as cadmium, chromium, and lead. Mercuric
chemicals have been used as a paint preservative but this use is declining.
At present, the major environmental concern of the coatings industry is the
emission of volatile organic compounds (VOCs), which react in the presence
of sunlight to create photochemical ozone or smog. VOC-containing solvents
used in the formulation of conventional liquid coatings evaporate during
application and curing. VOCs are also released during cleanup operations,
which remove paint from painting equipment, paint
Page 4
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Section One
Table 2. Representative organic solvents used in paints and coatings.
Name Boiling Range (°C)
Low Hydrogen-Bonding Solvents
Varnish Makers and Painters (VM&P) naptha 119-129
Mineral spirits 158-197
Toluene 110-111
Xylene 138-140
High flash naptha 181-201
1,1,1-Trichloroethane 73-75
Hydrogen-Bond Acceptor Solvents
Methyl ethyl ketone 80
Methyl isobutyl ketone 116
Methyl «-amyl ketone 147-153
Isophorone 215-220
Ethyl acetate 75-78
Isopropyl acetate 85-90
H-Butyl acetate 118-128
l-Methoxy-2-propyl acetate 140-150
2-Butoxyethyl acetate 186-194
1 -Nitropropane-nitroethane blend 112-133
Hydrogen-Bond Donor-Acceptor Solvents
Methyl alcohol 64-65
Ethyl alcohol 74-82
Isopropyl alcohol 80-84
n-Butyl alcohol 116-119
sec-Butyl alcohol 98-101
l-Propoxypropan-2-ol 149-153
2-Butoxyethanol 186-194
Monobutyl ether of diethylene glycol 230-235
Ethyleneglycol 196-198
Propylene glycol 185-190
Source: Wicks, et. al. (1992).
PageS
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Section One
Regulatory
Environment
lines, and spray booth surfaces. New coatings formulations and application
techniques described in this guide can reduce VOC emissions from paint
facilities.
Paint wastes are another environmental priority for the coatings industry.
These wastes include paint overspray, defective coatings removed from parts,
and wastes generated during color changeovers. Reformulated coatings and
newer coating application equipment can also reduce or eliminate some of
these wastes.
The paints and coatings industry is regulated under the Clean Water Act
(CWA), Clean Air Act Amendments (CAAA), Resource Conservation and
Recovery Act (RCRA), the Right to Know provisions of the Superfund
Amendment and Reauthorization Act (SARA), the Pollution Prevention Act
(PPA), and additional state and local authorities. Currently, the major
regulatory initiative that affects the coatings industry is the development of
Maximum Achievable Control Technology (MACT) standards under Title III
of the CAAA. Title III is a comprehensive plan for reducing emissions of
hazardous air pollutants (or HAPs). EPA has identified major source
categories of HAPs and is now developing MACT standards for these source
categories. Table 3 identifies source categories in the paints and coatings
industry targeted for MACT standards.
Table 3. Categories of major and area sources of hazardous air pollutants -
surface coating processes.
Aerospace industries
Auto and light duty truck
Metal furniture
Misc. metal parts and products
Flat wood paneling
Large appliances
Magnetic tapes
Manufacturers of paints, coatings,
and adhesives
Metal can
Metal coil
Paper and other webs
Plastic parts and products
Printing, coating, and dyeing of
fabrics
Printing/pub lishing
Shipbuilding and ship repair
Wood furniture
Source: U.S. EPA (1992).
Page 6
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Section One
In addition to these and other federal standards, many coatings facilities must
comply with local and regional regulations also designed to restrict VOC
emissions. Some of the most stringent requirements are being developed in
southern California where ozone and smog are major concerns. There, the
South Coast Air Quality Management District (SCAQMD) has mapped out
a three-tiered approach to bring VOC emissions in the district into compliance.
These regulations address both application equipment and the VOC content
of coatings. Other states with significant VOC restrictions in place or under
development include Georgia, Illinois, Indiana, Michigan, New Jersey, New
York, North Carolina, Ohio, Pennsylvania, South Carolina, Washington, and
Texas (Huberfield, 1991).
Solvent and paint waste disposal procedures and RCRA requirements increase
waste management costs, establish cradle-to-grave responsibility for wastes,
and require the waste generator to maintain a waste minimization program.
Section 313 of Title III of SARA establishes toxic chemical release reporting
requirements. Facilities in Standard Industrial Classification (SIC) codes 20
to 39 that meet company size and chemical quantity thresholds must file
reports on the discharge and recycling of chemicals. The current list of
reportable chemicals numbers over 300 and is scheduled to be expanded in the
near future. Many of these are used in the formulation of paints and coatings
or in their application.
In addition to RCRA requirements for a waste minimization program for all
hazardous wastes, the Pollution Prevention Act of 1990 establishes a hierarchy
for addressing pollution problems. The Act emphasizes prevention of
pollution at the source as the preferred alternative, with recycling and
treatment and disposal identified as less desirable options. Many states have
also embraced the pollution prevention approach and now require certain
categories of industrial facilities to prepare and submit pollution prevention
plans detailing their efforts to reduce waste and prevent pollution.
Solution Coating technology relies on covering a substrate material with an organic film
having the desired protective, mechanical, optical, aging, and adhesion
properties. Conventional organic coating technology uses dilute solutions of
alkyd, polyester, epoxy, polyurethane, acrylic, vinyl, or other resins in a
volatile organic solvent. In conventional coatings formulations, the organic
solvent performs a key function of promoting desired flow characteristics,
thereby facilitating the coating application. Once applied, the solvent
evaporates, leaving the resins and pigments behind to polymerize and form the
dry coating.
Page 7
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Section One
Cleaner technologies for the coatings industry are based on reformulations of
conventional organic coatings to reduce their VOC content, or modifications
to application or curing techniques that allow for reduced, or in some cases
zero, VOC content. Numerous cleaner technologies also reduce the generation
of paint waste by improving the efficiency of coating transfer to the substrate.
What's in this Guide? This guide describes cleaner technologies that can be used to reduce emissions
and wastes from paints and coatings application. The objectives are:
> To identify viable cleaner technologies that can reduce emissions and
waste generation through the use of modified paint and coating
formulations or application and curing techniques.
» To provide resources for obtaining more detailed engineering and
economic information about these technologies. This information can
be used by an individual facility to evaluate the potential for
integrating cleaner technologies into existing operations or planned
expansions.
The following are the main pollution prevention issues discussed in this guide.
In evaluating potential alternative processes and technologies for possible
further investigation, the reader is advised to explore these questions as
thoroughly as possible:
* What alternate coating processes are available or emerging that could
significantly reduce or eliminate the pollution and/or health hazards
associated with currently used processes?
> What advantages would alternative processes offer over those
currently used?
* What difficulties would arise and need to be overcome or controlled
if the alternative processes were used, including:
Would different or new Would the process require
pollution or health problems significantly different process
arise as a result of adopting controls?
it?
Would the product quality Would the consumer accept the
be different from present? substitute?
Page 8
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Section One
Would the process require
significantly different
procedures for handling
reject parts?
Would production
personnel need to develop
significantly different skills?
Would there be a need for
significant capital
investment?
Would production rates be
affected?
Would production costs be
increased?
Follow-up
Investigation
Procedures
This guide has been designed to provide sufficient information to users to help
in selecting one or more candidate cleaner technologies for further analysis and
in-plant testing. The guide does not recommend any single technology over
any other, since site-specific and application-specific factors often can affect
the relative attractiveness of alternatives.
The guide presents summaries of applications and operating information that
can be used to support preliminary selection of cleaner technologies for testing
in specific production settings. It is hoped that sufficient detail is provided to
allow identification of possible technologies for immediate consideration in
programs to eliminate or reduce emissions and waste generation.
This guide covers several cleaner coating replacement systems that are
applicable under different sets of product and operating conditions. If one or
more of these are sufficiently attractive for your operations, the next step
would be to contact vendors or users of the technology to obtain detailed
engineering data that will facilitate an in-depth evaluation of its potential for
your facility. Section Five of this guide provides an extensive list of trade and
technical associations that may be contacted for further information
concerning one or more of these technologies, including vendor
recommendations.
Who Should Use this
Guide?
This publication is intended for facilities in all segments of the paints and
coatings industry, including applicators of architectural coatings, finish
coatings for parts and assemblies, and maintenance coatings. Although the
guide discusses reformulation of paints and coatings to reduce pollution and
emissions, its use is intended for facilities involved in the application of
coatings rather than those that formulate, manufacture, and distribute them.
For further information concerning pollution prevention in paints and coatings
formulation, see Randall (1994).
Page 9
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Section One
The guide is intended for plant processes and system design engineers and for
personnel responsible for process improvement, process design, and
environmental compliance. Process descriptions within this guide allow
engineers to evaluate options so that alternative coating materials or
equipment can be considered for existing facilities and factored into the
selection of new coating applications.
Many coatings facilities are under pressure from federal, state, and local
regulators to reduce emission levels of VOCs and other pollutants. Small- to
medium-sized shops are often at a disadvantage when it comes to making
decisions concerning environmental compliance. This guide should be
especially useful to these companies evaluating opportunities for pollution
prevention.
Organization of this This guide describes available cleaner technologies for paints and coatings
Guide applications. Section Two discusses technologies for pollution prevention that
are well established and that have been implemented in a wide variety of
settings. Section Three addresses newer or more specialized technologies.
Section Four deals with strategies for understanding and implementing
pollution prevention technologies. Section Five provides list of information
sources for further guidance. In addition, each section contains its own list of
references relative to individual cleaner technologies.
Keyword List Table 4 presents keywords that enable the reader to scan the list of
technologies and identify those that are generally available and those that are
less widely used. Some but not all of the emerging technologies may still in
development or pilot stages.
Table 4. Keyword list - cleaner technologies for organic coatings.
General
Keywords
Cleaner technology
Pollution prevention
Source reduction
Source control
Recycling
Available
Technologies
High solids, solvent-borne
Powder coatings
Waterborne coatings
Ultraviolet- or electron
beam-cured coatings
Technologies Under
Development
Vapor injection cure
coatings
Supercritical carbon
dioxide as solvent
Radiation-induced
thermally-cured coatings
Page JO
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Section One
The distinction between "available" and "emerging" technologies made in
this guide is based upon the relative state of development of each group. It
is not intended to reflect judgements concerning the ultimate potential for
any one technology over any other.
Summary of Benefits The cleaner technologies described in this guide are categorized as either
"available" or "emerging", depending on their level of development and extent
of adoption within the industry. Available technologies include commercially
available processes that have been adopted by numerous coatings applicators
and are perhaps being used for more than one application. Emerging
technologies are less developed and are limited in commercial application.
Table 5 summarizes the pollution prevention, operational, and economic
benefits of these coating alternatives. You may wish to scan this summary
table to identify the cleaner technology options that best fit the operations and
needs of your facility. Detailed discussions of the benefits and operational
aspects for each cleaner technology are provided in Sections Three and Four.
Page 11
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Section One
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Section One
Notes to Table 5 1. High solids formulations do reduce solvent in the coating when compared
with low solids formulations. In many cases, however, high solids coatings
represent the baseline for regulatory limits, and low solids no longer comply.
For this reason, high solids are considered the baseline for solvent content, and
consequently do not have a reduced solvent content even though they qualify
as a cleaner technology compared to traditional low solids coatings.
2. The ability to form thin coatings is a function of the spray equipment, and
of the formulation. Polyurethanes, for instance, will spray on as thin coats
while polyesters can only be applied in thick coats.
3. Color changing equipment for powder coating is available for relatively
quick and easy changes. The maximum color change frequency that is
manageable from an operational standpoint is in the order of a few per day.
More frequent changes than this will hinder operations excessively.
4. Generally, it is difficult to apply thin coats with powder. Dependant on
the application, however, some formulations allow thin coats, particularly
polyester powders.
5. For medium to small facilities, it is easy and relatively inexpensive to start
a powder coating operation.
6. Recent waterborne technologies will completely eliminate solvent.
Waterborne formulations currently available still contain some solvent,
however, often up to 2.8 Ibs/gal VOC less water.
7. The thickness of the coating is dependant on the voltage potential.
Thicker coats can be achieved with higher voltages but practical limits exist.
8. Some UV/EB coatings eliminate solvent. All UV/EB coatings contain
reactive diluents, however, so it cannot be assumed that VOC content is zero.
9. Ease of changing colors is not a problem, but availability of colors is
limited. Clearcoat is the most common coating cured with UV/EB radiation.
10. Thick pigmented coats take a longer time to cure under UV radiation. EB
curing is quicker because the electron beam punches through pigmented
coatings more easily than UV.
11. UV lamps are relatively cheap. Thermal ovens are not required, so overall
capital cost is relatively low. EB systems, on the other hand, are costly
because of the expensive electron beam generator.
Page 13
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Section One
REFERENCES Bureau of the Census, 1992. Current Industrial Reports. Series M28F, U.S.
Department of Commerce.
U.S. EPA. 1992. Initial list of categories of sources under Section 112(c)(l)
of the Clean Air Act Amendments of 1990. Federal Register. July 16. pp.
31576-31592.
Huberfield, D. 1991. VOCregs vary widely. Industrial Finishing. March.
pp. 32-32.
Randall, Paul M. 1994. Pollution prevention opportunities in the manufacture
of paint and coatings. In Proceedings: Pollution Prevention Conference on
Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organics Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Page 14
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Section Two
SECTION TWO
AVAILABLE TECHNOLOGIES
Introduction
This chapter describes cleaner technologies available for the organic coating
industry that can reduce emissions of VOCs. In reducing VOCs, these
technologies may also reduce generation of hazardous wastes and decrease
worker exposures to hazardous air emissions.
HIGH SOLIDS COATINGS. SOLVENT-BORNE
Pollution Prevention
Benefits
How Does it Work?
High solids, solvent-borne coating systems were developed to reduce
emissions of volatile organic compounds (VOCs) released during curing.
High solids coatings have been reformulated to meet regulated levels of VOCs
while retaining the essential character of the low solids coating formulation.
High solids coatings typically contain 275 to 420 g VOC/1 of liquid coating
(2.3 to 3.5 Ib/gallon) (Pilcher, 1988). High solids coatings currently available
are generally similar to low solids coatings in their application, curing, and
final film properties, though there are important differences. The major
difference is the higher viscosity of the high solids formulation, which often
leads to increased film thicknesses.
A standard definition of high solids does not really exist in the coatings
industry. "High solids" coatings are generally considered to contain more than
80 percent solids, while the term "higher solids" refers to coatings containing
less than 80 percent solids but more than the 30 to 40 percent contained in low
solids coatings (Munn, 1991).
High solids paints have not made the inroads that other systems such as
powder coatings have in replacing conventional coatings. Particular problems
include high viscosity, viscosity changes due to temperature variation, and
storage stability. Other issues are control of film thickness and the drying
characteristics of the film.
Creating a high solids formulation is not as easy as simply reducing the
solvent concentration. A reduction in solvent concentration without other
changes lead to an unacceptably high level of viscosity. Because polymer
binders (resins) used in coatings have traditionally been of moderate to high
molecular weight, the molecular weight of the polymer must be lowered to
retain acceptable viscosity. Lowering the molecular weight of the polymer is
problematic, as unmodified low molecular weight polymers produce an
unacceptable final dry film when normal curing times are applied. To
overcome performance limitations caused by these polymers, additives often
are used to increase cross-linking during curing. However, with chemical
Page 15
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Section Two
modification, polymers can retain good coating properties and lowered
viscosity (Storey, 1987).
Types of High Solids High solids solvent-home coatings fit into three general categories:
Coating Systems
> air/force-dry
» baking
» two-component
Resin systems generally belong to one category, although some resins cross
over between categories.
Air/Force-Dry Coatings
Air/force dry coatings cure by exposure to moisture or oxygen. Alkyd resins
are most common in air-dry coatings. Modified alkyds are also popular, while
styrene, siliconized and acrylic resins are less common. Air-dry alkyds are
termed oxidizing or auto-oxidizing because they cure in air without baking or
the addition of a catalyst (Wicks, et. al., 1992).
These coatings cure at low temperatures, below 180°F. Low temperature
ovens can be used to speed curing by evaporating the solvent more quickly.
Air-dry high solids coatings usually have longer drying, tack-free, and
hardness curing times than their low solids counterparts. These properties can
alter production possibilities if the applicator needs to wait longer before
handling parts. For instance, if the coating remains soft for a longer time than
previously, the coating may become scratched or damaged during handling
operations. This may necessitate rework which adds to the cost and increases
the amount of pollution generated.
The recent development of new resins has resulted in a range of fast-drying
high solids air-dry acrylics suitable for general metal finishing. These resins
are inexpensive, offer excellent flow and drying properties, good hardness,
durability, and color and gloss stability, and do not suffer from air entrapment
or sagging. Early air-dry coatings contained highly volatile solvents, causing
the surface of the coating to dry first and trapping solvent underneath. The
result was pinholing or solvent-popping in the finished film (Ballway, 1992).
These high-solids acrylics are suitable for various metal finishing applications,
and have both indoor and outdoor uses.
Bake Coatings
Bake coatings predominantly use acrylic and polyester resins, although some
alkyds and modified alkyds are also used. These resin systems cure at high
temperatures to form a crosslinked film. Crosslinking agents such as
Page 16
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Section Two
melamine-formaldehyde (MF) or blocked isocyanates are commonly used. MF
coatings are usually one pack systems, catalyzed by a strong acid such asp-
toluenesulfonic acid (Storey, 1987). Latent or blocked catalysts are used for
fast cure and good pot life with melamine-formaldehyde crosslinked coatings.
High solids resins based on alkyds or polyesters have low molecular weights,
and contain hydroxyl groups that can be crosslinked with melamine-
formaldehyde resins.
Blocked isocyanate urethane resins are often used as high solids binders for
baking systems because of their outstanding performance properties and broad
formulation latitude. Urethanes based on blocked isocyanates require an
elevated temperature to cure. The polyisocyanates have surprisingly low
viscosities, which is an asset in high solids coatings. Aliphatic polyisocyanate
cross-linking agents are recommended for superior weathering properties,
especially their resistance to yellowing (Storey, 1987).
Temperatures in the range of 350°F to 400°F are needed to cure baking
enamels, requiring the use of high-temperature ovens.
Two-Component Coatings
The name "two-component" refers to the presence of two separate coating
solutions which are mixed together just before use. "Two-component" is also
known as "two-pack" and "2K" (from the German word, Komponent).
Two-component systems cure by a crosslinking reaction between the two
components: reactive resins, or a resin and a catalyst. Epoxies and
polyurethanes are the most common two-component coating systems.
Polyisocyanates serve as the crosslinking agents for polyurethanes (Wicks, et.
al., 1992).
Two-pack polyol cured urethane resins are often used as high solids binders
because of the excellent properties of the finished film and the low energy
needed for curing. Urethanes based on two-component systems cure at lower
temperatures than baking polyurethanes.
Two-component polyol urethane coatings are suitable for metal finishing
applications where outstanding film properties are required. These coatings
are also suitable for the automotive and machine tool industries because of
their excellent resistance to solvents, lubricants, cutting oils and other
chemicals. Urethane clearcoats for automobile finishes, for example, provide
hard wearing films with exceptional chemical and abrasion resistance.
Page 17
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Section Two
Urethane coating systems do pose some health and safety concerns. For
example, isocyanates are toxic and can affect the respiratory system. Damage
from low-level exposure is usually reversible, however, sensitization can
occur. Sensitization can be temporary or permanent. Following chronic
exposure to urethane coating systems, permanent lung damage has been
observed (Rees, 1992). Stability and pot life of two-component urethanes can
also be problematic.
Epoxy resin systems are the oldest form of high solids coating. Two-
component, thick film, solventless resin formulations for specialty applications
have been available for many years.
In a two-component reactive liquid coating system, two low-viscosity liquids
are mixed prior to entering the application system. One liquid contains
reactive resins, while the other contains an activator or catalyst that promotes
polymerization of the resins. Conventional, airless, or electrostatic spray
equipment can be modified to accommodate new coating materials such as
two-component epoxies, polyurethanes, and polyesters. The two components
are fed into the spray gun through separate metering devices. Flow control
valves and cleaning valves are built into the spray unit to prevent the
components from coming into contact with each other before release.
Two-component systems enable coatings to be applied without the use of a
volatile organic solvent. Some solvents might be used to clean up any
unreacted liquids.
High temperature ovens are not required for curing two-component coatings.
Reactive Diluents
A class of compounds known as "reactive diluents" can replace some organic
solvents with low molecular weight resins designed to react, crosslink, and
form an integral part of the coating. The most important asset of reactive
diluents is that they are VOCs when tested individually per EPA Method 24,
but are not VOCs when the mixed coating is allowed to crosslink before
subjecting it to EPA Method 24.
Operating Features As with conventional formulations, high solids coatings can be applied using
numerous methods including:
> Brush or roller
> Pouring or flow-coating/curtain coating
> Dipping
> Spraying by low-pressure equipment
> Spraying by high-pressure/air-assisted equipment
* Spraying by airless equipment
Page 18
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Section Two
> Spraying by electrostatic equipment
» Spraying by high volume, low pressure (HVLP) equipment
> Application by turbine bell or rotating disk
Spray Application Systems
Depending on the formulation, high solids coatings may be of similar or
greater viscosity than low solids coatings. Traditionally, high viscosity makes
coatings difficult to atomize and achieve a uniform film thickness. Today,
emerging formulations are tending towards lower viscosity and therefore easier
spraying. These new formulations may be based on new resin systems, or
additives which modify viscosity and rheology for easier spraying.
Lower viscosity high solids coatings can be readily applied with.all types of
equipment such as air spray, airless spray, air-assisted airless, HVLP, or
electrostatic spray. High-solids coatings that have high viscosity (or that have
been exposed to cold temperatures thereby raising the viscosity) are more
problematic to apply with spray apparatus. Spray equipment should be tested
with the new coating to see if a suitable finish is obtained. If necessary, fluid
tips can be exchanged or new spray guns purchased.
Spray application problems stemming from high viscosity are often solved by
use of an in-line paint heater to reduce viscosity. The heater raises the fluid
temperature thereby lowering the viscosity. An alternative is to use a
temperature-controlled spray booth and set the temperature for reduced
viscosity while still maintaining operator comfort.
High-volume, low pressure (HVLP) and electrostatic spraying equipment are
approaches to high-efficiency application that reduce overspray loss and raise
transfer efficiency. HVLP uses low atomizing air pressures of less than 10.0
psi along with high volumes of atomizing air to apply paint with less velocity
than standard air spray guns, reducing losses from coating overspray.
Electrostatic guns charge the coating and then deposit it on parts which are
grounded.
Airless and air-assisted airless spray systems are used to apply high solids
coatings. These systems use hydraulic pressure to atomize the coating into
small droplets, resulting in a fine spray.
Other Application Equipment
High solids coating formulations with higher viscosities can be applied with
electrostatic turbine bell or rotating disk atomization spray equipment. Disk
and bell turbine applicator systems are primarily used in production line
applications. With disk or bell applicators, the coating is fed into a rotating
Page 19
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Section Two
insulated disk or bell. Centrifugal force causes the coating to spread to the
outer edge of the bell or disk. Turbine powered bells have rotational speeds
of up to 50,000 RPM while disks can achieve speeds of up to 40,000 RPM.
The disk or bell has approximately 100 kV of electrostatic charge to ensure
uniformity of the coating by improving atomization and transfer efficiency of
the paint droplets.
Surface Preparation
Surface preparation techniques need to be more thorough when using high
solids coatings. Low solids coatings contain substantial quantities of organic
solvents which allow a certain amount of self-cleansing (or greater wetting) of
the substrate. Grease and other contaminants are wetted or dissolved by the
solvent, resulting in a cleaner surface on which the coating adheres. High
solids coatings do not have as great a self-cleansing ability, therefore surface
preparation must remove more of the grease and contaminants when using
these coatings. Cleaning with organic solvents would defeat the purpose of
low-VOC, high solids coatings. Luckily, other measures exist, including
aqueous cleaning systems, abrasive blasting and other surface preparation like
phosphating treatments. See for example U.S. EPA (1993), Mounts (1993)
and Wang and Merchant (1993) for alternatives to organic solvent cleaning.
Required Skill Level
Although the application equipment is similar, more operator skill and
attention is needed when using high solids coatings, mainly because of
problems of higher viscosity. Substantial air temperature changes will alter
coating viscosity and change film thickness unless the applicator can make
adjustments. Film thickness control is difficult, however, particularly on
complex-shaped parts. On complex shapes, thickness variation of between
approximately 1 and 7 mils is possible because of differing build up of coating
due to changing spray angles. A high solids coating with low viscosity and
good rheology is easier to apply in a uniform thickness.
Applications Products Finishing
High solids VOC-compliant coatings have been used to replace low solids
formulations in lining drum interiors at Russell-Stanley Corp., of New Jersey.
Coatings for steel drums need to have good chemical resistance. Historically
this resistance derived from high molecular weight resins, however these
formulations required significant solvent use to lower viscosity. The VOC-
compliant formulation still uses high molecular weight resins for chemical
resistance, but sprayability is maintained by means of heating the coating to
reduce viscosity. Heating equipment for lowering viscosity was found to be
Page 20
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Section Two
cheaper than alternatives such as exotic formulations or afterburners to
incinerate VOCs (MP&C, 1990).
High-speed turbine bell atomizers are being used to apply high solid coatings
to zinc-coated steel doors. In addition to lower VOC emissions, the new high
solids coating formulations use lower curing temperatures which are required
by the heat sensitive foam insulation cores now used in steel doors. High solid
coatings meet the highest adhesion rating in ASTM D 3359, and pass a 250-hr
salt spray test per ASTM B 117 (Nelson, 1988).
High solids polyurethane materials that meet MIL-C832858 are available for
use on aircraft and ground support equipment. These high solids formulations,
which are formulated for electrostatic application (MP&C, 1988), contain 340
g to 420 g VOC/1 (2.8 Ib/gal to 3.5 Ib/gal). The pot life is reported to be 6 hr.
High solids coatings are also used for aluminum extrusions, office furniture,
appliances, business machines, containers and many other OEM applications.
Automotive Applications
High solids coatings are also used as automotive primers, topcoats, basecoats,
and clearcoats. Two-component polyurethane coatings are increasingly being
used for clear topcoats on automobiles. In comparison to conventional acrylic-
melamine and alkyd-melamine systems, two-component systems offer many
benefits, including low solvent emissions; high gloss and body; flexibility; and
weather, chemical, and stone chip resistance. In addition, two-component
polyurethane coatings cure at lower temperatures than baking systems,
reducing energy costs.
Cost Since high solids coatings use application equipment similar to low solids
solvent-borne coatings, the capital cost for booths, electrostatic spray
applicators, and curing ovens are approximately the same. In fact, many
existing application systems can be used with minor or no modification for
high solids coatings. For low temperature applications or high viscosity high
solids formulations, paint heaters may be required. VOC control equipment
may be required if the high solids coating emits greater quantities of VOC than
regulations allow.
High solids coatings are slightly more expensive than conventional coatings
per unit of reactive resin. Preparation, application, cleanup, and disposal costs
are similar for high solids and low solids coatings. A detailed comparison of
the costs of high solids, conventional, powder, and water-based coatings can
be found in Hester and Nicholson (1989).
Page 21
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Section Two
Benefits Benefits of high solids coatings are:
They contain a lower concentration of solvents than conventional
coatings reducing environmental, odor, safety, and health problems.
The manufacture and curing of high solids coatings requires less
energy than conventional coatings, reducing energy costs.
High solids coatings are easier to store than conventional coatings
because of lower solvent concentrations, reducing storage and
handling costs.
High solids coatings produce films with greater thickness than
conventional coatings, allowing increased line speeds and reduced
number of coats.
High solids coatings are compatible with application equipment and
techniques used in conventional coating systems.
Limitations The disadvantages of high solids coatings include:
High solids coatings have a tendency toward excessive flow. Coatings
with higher solids content require lower viscosity resins creating a
more serious problem of excessive flow. When applied to a vertical
surface, high solids coatings also have a tendency to sag. Many
additives are available that control flow and prevent sagging in
conventional formulations. The effectiveness of these additives with
high solids coatings, however, has not been demonstrated. The use
of flow control additives can also result in additional problems,
particularly gloss reduction.
High solids formulations produce films with increased thickness,
which can blister during the baking process.
Since high solids coatings use low molecular-weight resins, it is
possible for these resins to become volatile at elevated curing
temperatures, resulting in reduced binder content, poor film
formation, and greater VOC emission.
The overspray of high solids paints tends to create a sticky mass,
whereas conventional coatings have a dry, powdery overspray. As a
result, spray booths become clogged, creating severe collection and
disposal problems.
Page 22
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Section Two
Tradeoffs
» The viscosity of high solids formulations is extremely sensitive to
changes in temperature. Temperature variations in the workplace can
cause problems in spray application. Thermostatic heating of the
coatings in the pot or in-line might be necessary for easier application.
* High solids coatings take a longer time to cure than conventional
coatings increasing the opportunity for dirt pickup on the film. Work
areas, therefore, must be kept clean; some form of air filtration is
recommended.
» Although high solids coatings use less organic solvents, they do not
completely eliminate solvents.
> Two-component high solids coatings have shorter pot lives than
single-component coatings.
+ Incinerators or carbon adsorber pollution control equipment might be
needed when applying high solids coatings to meet VOC regulations.
High solids coatings use application systems that are similar to those used
with conventional solvent-based coatings, easing the transition. Both
equipment operators and plant management prefer simple transitions rather
than radical changes in equipment and procedures.
Although high solids coatings offer reductions in VOC emissions, these
reductions are not as great as those gained with powder coatings, many
waterborne coatings or various other coating technologies.
REFERENCES
Agostinho, M. 1985. Proceedings of the Waterborne Higher Solids
Symposium, 12:89
Ballway, Bill. 1992. What's new in high solids coatings. Metal Finishing.
March, p. 21.
Farmer, Richard N. 1992. Low-temperature-cure baking enamels provide
high-solids alternative. Modern Paint and Coatings. June, p. 36.
Ferrarini, Dr. James. 1990. Reduced-viscosity isocyanates: high performance
with low VOC. Modern Paint and Coatings. March, p. 36.
Goldberg, Daniel, and Robert F. Eaton. 1992. Caprolactone polyols as
reactive diluents for high-solids. Modern Paint and Coatings. November, p.
36.
Page 23
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Section Two
Hester, Charles L. and Rebecca L Nicholson, 1989. Powder Coating
Technology Update. EPA-45/3-89-33. October.
Luthra, Sanjay, and Dr. Richard R. Roesler. 1994. High-build aliphatic
polyurethane topcoats. Modern Paint and Coatings. February, p. 20.
Mayer, W. P. 1990. High performance, high solids coatings using solution
vinyl resins. Journal of the Oil Chemists and Colourists Association.
1990(4): 159.
Mounts, Michael L. 1993. Converting from vapor degreasing to the optimum
alternative. Metal Finishing. August, p. 15.
MP&C. 1988. Low-VOC coatings applications for military usage present
major formulation challenge. Modern Paint and Coatings. 78(9):35
MP&C. 1990. Steel drum interiors use VOC-compliant coating. 1990.
Modern Paint and Coatings. November, p. 32.
Munn, R. H. E. 1991. Towards a greener coatings industry: resin
developments. Journal of the Oil Chemists and Colourists Association.
1991(2):46.
Nelson, Larry. 1988. Door Market Opens to Low-VOC Coatings." Products
Finishing. 53(3): 48-53. December.
Pilcher, Paul. 1988. Chemical Coatings in the Eighties: Trials, Tribulations,
and Triumphs. Modern Paint and Coatings. 78(6): 34-36.
Rees, S. W. 1992. Non-isocyanate two-pack coatings. Journal of the Oil
Chemists and Colourists Association. 1992(3): 102
Smith, Mark D. 1990. Low VOC Coating Alternatives." In: First Annual
International Workshop on Solvent Substitution, p. 237-244. . Phoenix,
Arizona, December 4-7.
Storey, Robson F. 1987. High Solids Coatings. In: Wilson, Alan D., John W.
Nicholson and Havard J. Prosser, eds. Surface Coatings - 1. New York, NY:
Elsevier Applied Science Publishers Ltd.
Wang, Victor and Abid N. Merchant. 1993. Metal-cleaning alternatives for
the 1990s. Metal Finishing. April, p. 13.
Weinmann, Daniel J. 1994. Low-VOC epoxy/polyamide maintenance
coatings. Modern Paint and Coatings. February, p. 24.
Page 24
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Section Two
Wicks, Zeno W., Frank N. Jones, and S. Peter Pappas. 1992. Organic
coatings: science and technology. (Subtitled: volume 1: film formation,
components, and appearance.) New York, NY: John Wiley & Sons, Inc.
POWDER COATINGS
Pollution Prevention
Benefits
How Does it Work?
Powder coatings are attractive from a pollution prevention standpoint because:
» No solvents are used in the coating formulation.
* Essentially all of the coating is applied to the substrate (high transfer
efficiency).
» There is little or no hazardous waste to dispose of.
Powder coating technology uses dry resin powders for coating substrates with
thermoplastic or thermoset films. The coating is formed after a layer of
powder is applied with a powder coating spray gun or fluidized bed tank to the
substrate and heated, thereby melting the powder. Automotive, appliance
finishing, outdoor furniture manufacturing, architectural and building
industries all use powder coatings. A major driving force in the growth of
powder coatings is attributed to increasingly stringent environmental
regulations (Major, 1992).
Powder coatings usually are applied in a single coat. The thickness of the coat
is typically greater than that used with a solvent-based finish. Powder coatings
use resins in dry powdered form without volatile organic solvents. Volatile
solvents are not needed because clean, dry compressed air acts as the solvent
or fluidization agent for the coating. No VOCs are released because solvents
are eliminated from the entire process. Furthermore, the coating equipment
can be cleaned with compressed air, eliminating the use of solvent in cleanup.
Lower solvent levels reduce worker exposure and fire and explosion hazards.
Because VOCs are eliminated, expensive VOC destruction equipment
(incinerators or carbon adsorbers) is not required.
Hester and Nicholson (1989) present the following example to show the
potential VOC reductions tohievable with powder coatings. A large
conventional coating facility covers 12 million ft2/yr of substrates with 1.2
mil-thick coats. The plant uses a VOC treatment system with a 70 percent
capture efficiency. Emissions of VOC are approximately 38 tons/yr. By
comparison, a powder coating facility using electrostatic application of
polyester-urethane resins will emit only 0.6 tons/yr of VOCs, and avoids the
need for emission control equipment.
Page 25
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Section Two
Operating Features Many different resins are available for powder coating. The two most general
categories are thermoplastic and thermoset resins. Table 6 compares the
properties of various thermoplastic and thermoset resins.
Thermoplastic resins
Thermoplastic resins form a coating but do not undergo a change in molecular
structure. These resins can be re-melted after they have been applied.
Thermoplastic resins are used mainly in functional coatings such as thick,
protective coatings on dishwasher trays (Lehr, 1991).
Examples of thermoplastic resins used in powder coating are:
» Polyethylene
» Polypropylene
* Nylon
> Polyvinylchloride
* Thermoplastic polyester
These thermoplastic resins are designed for functional and protective uses, not
as a replacement of thin film coatings from solvent-borne paints.
Thermoset resins
Thermoset resins crosslink to form a permanent film that withstands heat and
cannot be remelted. These resins are ground into very fine powders that can
be applied with a spray gun for thin film coatings. They are used for
decorative, protective, coatings in architecture, on appliances, furniture, and
elsewhere. Because thermoset systems can produce a surface coating that is
comparable to liquid coatings, most of the technological advancements in
recent years have been focused on these resins (Lehr, 1991).
There are five basic families of thermoset resins:
Epoxies
> Hybrids
* Urethane polyesters
> Acrylics
> Triglycidyl isocyanurate (TGIC) polyesters
Page 26
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Section Two
Table 6. Summary of powder coating resin properties.
Resin Type
Epoxy-
Urethane Urethane
TGIC
Coating Property
Hardness
Flexibility
Resistance to
overbaking
Exterior
durability
Corrosion
protection
Chemical
resistance
Thin coat
Colors available
Clears available
Textures
available
Epoxies
excellent
excellent
fair
poor
excellent
excellent
no
all
yes
yes
Hybrids
excellent
excellent
very
good
poor
excellent
excellent
no
all
no
yes
Polyesters
very good
very good
very good
very good
very good
very good
yes
all
yes
yes
Polyesters
excellent
excellent
excellent
excellent
excellent
very good
no
all
yes
yes
Acrylics
very
good
fair
good
very
good
fair
very
good
no
all
yes
no
Source: Lehr (1991)
EpoxiesEpoxies have always been the staple of the powder coating
industry. These materials cure at temperatures below 300°F, many around
260°F. Mechanical surface properties are excellent; their pencil hardness can
reach 7H, Impact resistance is approximately 160 inch-pounds. These resins
also can be bent around a 1/4 inch mandrel with no loss of adhesion.
Corrosion resistance and chemical resistance is excellent with epoxy materials.
Epoxies, however, have poor UV resistance and consequently are best suited
to indoor applications (Lehr, 1991).
HybridsHybrid materials are combinations of epoxy and polyester resins
designed for a good mix of characteristics, although their UV resistance is still
poor. The presence of polyester resins helps to slow down or reduce yellowing
of the film that can be caused by overbaking.
Page 27
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Section Two
UrethanesUrethane polyesters are suitable for outdoor use because of
superior exterior durability. Urethane polyesters have very good surface
properties (hardness, flexibility, corrosion protection, etc.) although not quite
as good as epoxies. Urethanes can be applied in thin coats; coats of 1 mil to
2 mils (25 microns to 50 microns) usually are recommended. Most other
powder coat resins need to be applied in heavier coats, forming thicker films.
AcrylicsAcrylics have very good to fair surface properties, and are
becoming more common in the U.S. Acrylic's good weatherability makes them
suitable for exterior use.
TGICTGIC polyesters incorporate the cross-linking agent triglycidyl
isocyanurate. TGIC resins produce films with excellent surface properties
such as hardness, flexibility, exterior durability, and corrosion and overtaking
protection. Coating thickness of 3 mils to 5 mils (75 microns to 125 microns)
is recommended.
Exposure to trimellitic anhydride (TMA), a monomer used in polyester resins,
has been reported to cause allergic reactions. Hybrid resins can help to reduce
these reactions. TGIC resins, for example, have been used alongside TMA-
based powders in ratio of 70:30 to reduce the potential for allergic reactions
(Reich, 1993). TGIC-based resins, however, have been under attack. In
Europe, reduced occupational exposure limits (OEL) were recommended for
TGIC powders as a result of in vivo mutagenicity tests. In response, several
chemical companies have launched alternative hardeners, including
caprolactam-blocked isophorone diisocyanate (IPDI) adducts (Loutz, et. al,
1993).
As an alternative to blocked isocyanate cross-linking agents, melamine resins
mounted on polymer support materials can be used for curing solid polyester
resins. Advantages of these resins include lower curing temperatures, lower
hardener content, and lower volatile emissions.
The physical and chemical properties of the powder have to be carefully
controlled. The effectiveness of powder coating depends on obtaining a
smooth, nonporous film. Formation of a good coating free of voids, pinholes,
and orange peel distortions depends on controlling the particle size
distribution, glass transition temperature, melting point, melt viscosity, and
electrostatic properties. Well-controlled size distribution is important in
achieving good powder-packing on the surface.
Application Methods
Application systems for powder coatings include:
Page 28
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Section Two
> Electrostatic spraying
> Tribocharge spraying
> Fluidized bed
These application techniques offer much higher transfer efficiency (TE) than
wet-spray liquid coating methods for two reasons. An electrostatic charge
causes an attractive force between the powder coating material and the
substrate, resulting in higher TE than non-electrostatic solvent-borne or
waterborne spraying. Second, wasted powder can be readily reused. Powder
coating systems are designed to reclaim powder that has not formed part of the
coating.
Electrostatic sprayingThe most commonly used powder coating method
is electrostatic spraying. Dry powder is applied to an unheated substrate and
held in place by electrostatic force. The substrate or primer coat must be
electrically conductive. A transformer supplies high voltage (typically 100
kV) low-amperage current to an electrode in the spray gun nozzle. The current
ionizes the surrounding air, transferring a negative charge to the powder
particles as they pass through the corona of ionized air. The substrate to be
coated is grounded, allowing powder particles to follow electric field lines and
air currents from the gun to the substrate (see Figure 1 )(Lehr, 1991; Loutz, et.
al., 1993).
Compared to conventional air spraying of wet coatings, electrostatic spraying
achieves greater coverage of the substrate because the powder tends to "wrap"
around corners and coat surfaces that are not "line-of-sight" with the spray
gun. This results in less overspray and consequently a higher transfer
efficiency.
The following shortcomings of electrostatic powder coating led to the
development of tribocharging:
(1) The thickness of the coating can be reduced on areas where the
electric field is interrupted. This phenomenon is caused by the
Faraday cage effect, which occurs when a hollow or other complex
geometrical shape resembling a cage is found in the substrate. This
shape distorts the electric field lines, causing uneven powder
distribution.
(2) Air ions can become trapped in the coating and build up a strong
electrical field, resulting in surface imperfections in the coating such
as reduced thickness, orange peel-like distortions, and cratering
(Loutz, et. al., 1993).
Page 29
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Section Two
Figure 1
Electrostatic Spray Gun for Powder Coating
Charged powder
particles
Charging electrode
Powder supply
1
Ground
High-voltage
supply
r;-:.: ': }i'i:)':.'.:-..-'??-vV..'.*!. ..
Part to be
coated
Powder
wrap-around
Source: Eastern Research Group, Inc.
Page 30
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Section Two
TribochargingThe basic principle of tribocharging relies on friction
between the powder and the spray gun. The action of the powder flowing
through the barrel of the gun generates a frictional charge on the powder. The
charged powder is carried by air stream to the substrate where it deposits and
sticks due to electrostatic attraction. Because there is no high-voltage system
generating a field between the spray gun and the substrate, the electric field is
substantially smaller and the powder tends to follow air currents rather than
field lines. The smaller electric field results in a much reduced Faraday cage
effect. Consequently, tribo guns produce smoother finishes, allow deposition
of thicker films, and provide better coverage of intricately-shaped objects.
The frictional charge is generated because the powder and the gun have
different dielectric constants. The tribocharging annular chamber is
constructed of polytetrafluoroethylene (PTFE). Because PTFE has a low
dielectric constant, a positive charge will be imparted to most powders. Some
powders have low dielectric constants (e.g., mixtures of polyester and TGIC)
and do not pick up electrical charges readily. Therefore, attempts have been
made to modify the powder composition. Small quantities of additives such
as amines or quaternary ammonium phosphate salts increase the ability of the
powder to accept positive charges. Additives, however, can modify the
reactivity of the powder and also create a non-uniform composition. Uneven
composition can lead to powder segregation and create problems with
recycling. Steps are underway to produce a polyesteramide that has increased
ability to accept frictional charge, reducing the need for additives (Loutz, et.
al., 1993).
Tribocharging is less complex than traditional electrostatic powder coating
systems because it does not use high-voltage transformers for applying the
charge on the powder. Tribocharging guns do wear out faster than regular
guns because of the abrasion of the powder on the PTFE surfaces. Because
of the absence of electric field lines and a reduced dependence on leakage to
ground of free ions, tribo guns are more suitable for painting nonconductive
surfaces.
Fluidized bedsFluidized beds provide another way of coating powder,
similar in action to a dip tank (see Figure 2). Powder rests in a tank or hopper,
which is fitted with a porous bottom plate. Low pressure dry air is circulated
through the bed, causing the powder to attain a lofted, fluid-like state. The
workpiece is preheated, usually to greater than 500°F then dipped into the
tank. Powder melts on contact with the part forming the coating. This method
allows fairly complete, uniform coverage of complex-shaped parts.
Fluidized bed systems are primarily used to apply coatings of thermoplastic
powder to thicknesses in the range of 10 to 30 mils. The substrate is heated
Page 31
-------
Section Two
Figure 2
Fluidized Bed for Powder Coating
Part to be coated
/Ml-.Vfmo.1 .' <.*'.- i*^f..v-.;
Fluidized
powder
Air-permeable
membrane
Source: Eastern Research Group, Inc.
Page 32
-------
Section Two
to a higher temperature than the melting point of the resin so, as particles
strike the hot surface, they melt and coalesce to form a thick, continuous film
on the substrate. During fluidized bed coating, powder is added to replace
material that has formed a coating on the substrate. Because very little powder
is lost or degraded during coating, powder utilization is near 100 percent. The
fluidized bed method is the original method used for applying powder
coatings, and is still the method of choice for heavy functional coatings (Lehr,
1991).
A modification of this system is the electrostatic fluidized bed (see Figure 3).
Here the powder receives a charge from air which flows through a high voltage
charging system, while the object, which is grounded, is lowered or suspended
over the tank. Variations on this principle allow wire mesh or other endless-
type products to be coated. Electrostatic fluidized beds are limited to an
effective depth of about 2 to 3 inches so that they are best suited to coating
two-dimensional parts (Muhlenkamp, 1988).
Curing Powder coatings must be "heat-cured" or melted on to the substrate. For
thermoplastic resins, the substrate can be heated prior to coating so that the
resin melts directly on application. Thermoset resins are normally cured in
either heat convective or infra red ovens, or a combination of the two. The
substrate must be able to withstand temperatures of 260 ° F or higher. Delicate
substrates like certain thermoplastics or wood cannot be cured in ovens. The
substrate must also be of a size and shape to allow immersion coating or
heating in a curing oven. This prevents general indoor or outdoor application
where heating options are not available.
Certain thermoplastic powder coatings can be applied by a flame-spraying
method. This technology, developed by Plastic Flamecoat Systems Inc. in
Texas, uses a propane and compressed air flame in the spray gun to melt the
powder as it is propelled towards the substrate. The molten powder hits the
substrate and flows into a smooth pinhole-free coating (Major, 1992).
Thermoplastic powder coalings are used in military applications such air force
weapon systems and aircraft. The ability to field repair these coatings using
spray guns is valued in these applications (Ellicks, 1994).
Gas-fired ovens have considerable economic advantages over other energy
sources, however, they can produce nitrous oxides which come in contact with
the coating as it cures. Some grades of powder are more susceptible to
yellowing under these conditions than others. Grilesta has developed several
powders (grades P 7307.3 and P 7309.3) that can be used in gas-fired ovens
without yellowing (Reich, 1993).
Page 33
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Section Two
Figure 3
Electrostatic Fluidized Bed for Powder Coating
High-voltage
supply
Part to be coated
Charging electrodes
Charged
powder cloud
Air-permeable
membrane
Source: Eastern Research Group, Inc.
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Other Issues
Pretreatment of the part to be coated needs to be quite thorough. Because
powder contains no organic solvents no cleansing action is available as the
coating is applied to the part, therefore the substrate must be very clean, free
of grease and other contaminants. Before powder is applied, the surface must
be totally clean, dry and enhanced. This last term describes the condition of
the surface which is attained after an acid wash or rinse. Typical pretreatment
methods include sophisticated solvent cleaning systems, abrasive blasting or
cleaning, and aqueous chemical cleaning. They are sold as complete systems
and add cost to the overall powder operation (see Lehr (1991) for further
details).
If only partial coating of a part is required, the part needs to be masked to
prevent powder from adhering to the entire surface. Large numbers of
manufacturers need to mask parts and this can be a major problem, depending
on the part and the degree of masking required.
Because powder coatings rely on large, fluidized bed reservoirs, it is more
difficult to make color changes than with liquid coatings. Swapping frequently
between a large variety of colors is time-consuming and problems with cross-
contamination of color can occur. Powder is more suited to operations where
color change is infrequent.
Powder Coating Equipment
With powder spraying equipment, powder is supplied to the spray gun by the
powder delivery system. This system consists of a powder storage container
or feed hopper with a pumping device that transports a stream of powder to
the gun through hoses or feed tubes. A supply of compressed air often is used
as a "pump" because the air separates the powder into individual particles for
easier transport.
All spray guns can be classified as either manual (hand-held) or automatic
(mounted on a mechanical control arm), however, the basic principles of
operation are the same. Spray guns are available in a variety of styles, sizes,
and shapes. The type of gun used can be selected to achieve whatever
performance characteristics are needed for the products being coated.
Improvements have been made to spray guns to improve the coating transfer
efficiency. Many of these changes involve variations in spray patterns.
Nozzles that resist clogging have been introduced. Spray guns with variable
spray patterns are also available for using one gun on multiple parts of
different configurations.
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The powder delivery system can supply powder to one or several guns, often
located many feet from the powder supply. Delivery systems are available in
many different sizes depending on the application, the number of guns to be
supplied, and the volume of powder to be sprayed during a given time period.
Recent improvements in powder delivery systems, coupled with improvements
in powder chemistries that can reduce clumping, have made it possible to
deliver a very consistent flow of particles to the spray gun. Agitating or
fluidizing the powder in the feed hopper also helps to prevent clogging or
clumping before the powder enters the transport lines.
Innovations in powder delivery systems allow the powder supply reservoir to
be switched easily to another color powder when necessary. If the overspray
collection system is also not changed, however, the collected powder will
include all of the colors applied between filter replacements or booth cleaning.
For collected oversprayed powder to have the greatest value, it should be free
of cross-contamination between colors.
Numerous systems now are available for segregating colors, and that allow
several colors to be applied in the same booth. Most of these systems use a
moveable dry filter panel or cartridge filter that is dedicated for one color and
that can be removed easily when another color is needed. Color changes are
accomplished by:
(1) disconnecting the powder delivery system and purging the lines;
(2) cleaning the booth with compressed air or a rubber squeegee;
(3) exchanging the filter used with the filter for the next color; and
(4) connecting the powder delivery system for the new color.
Equipment manufacturers have made significant improvements in design of
spray booths, enabling color changes to be made with a minimal downtime and
recovery of a high percentage of the overspray.
Electrostatic and tribocharged spraying both result in overspray of powder.
However, unlike the overspray from most solvent coatings, powder overspray
can be collected and reused. Figure 4 shows a schematic of a recycling system
for powder coating. Recycling systems coupled with the inherently high
transfer efficiency of powder coating results in reduced waste paint disposal,
lowering both costs and environmental impacts. As with spray guns, a large
number of spray booth and powder recovery designs are available to choose
from, depending on the exact requirements of a given finishing system (Hester
and Nicholson, 1989).
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Figure 4
Powder Coating Recycling System
Section Two
Electrostatic
powder gun
Powder supply
Spray booth
Cyclone
Powder
collection
Source: Eastern Research Group, Inc.
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Required Skill Level
Powder coating equipment and techniques are easier than those used for
conventional dip coating or spray painting. The operator needs less skill than
a comparable liquid spray painter. However, the operator does need
experience with powder to determine if the deposited coating will result in a
good film before the coating enters the oven.
Developments in Powder Coating
Powder coatings traditionally have had poor weathering properties, especially
the common epoxies. Certain acrylic powders give better ageing and UV
weathering resistance. One of the better systems for high UV resistance uses
carboxylated polyesters cured with TGIC. In an accelerated weathering
experiment, this system showed UV resistance similar to an acrylic resin cured
with TGIC hardener. Values for gloss retention are as high as 60 to 70 percent
after 4 years exposure in Florida (Loutz et al., 1993)
Curing by IR radiation induces very rapid development of the crosslinked film,
enhancing line speed, however viscosity of the film increases quickly,
hindering transport of water during the curing process. Water evolved as a
byproduct from the crosslinking reaction can be trapped in the film leading to
problems of severe pinholing and gloss reduction (Loutz et al., 1993).
Based on epoxy/polyester hybrids, thin-layer coatings are now available in the
range 1-1.2 mils (25 to 30 microns) for colors with good hiding power.
Currently these are only suitable for indoor applications because the epoxies
degrade on exposure to outdoor weathering (Major, 1992).
Thin coats may be desirable where protection from corrosion and other
environmental factors is not so important. The reduced thickness directly
reduces the amount of powder needed and consequently the cost of the coating.
For thin coat application, the powder particle size is approximately 25
microns, and must have a narrow size distribution (Loutz et al., 1993).
When thin coats are applied, impurities in the powder can give rise to visible
surface defects such as cratering. For this reason, the resin is carefully filtered
to remove traces of gel, unreacted monomers and other non-soluble materials.
Thin coatings show surface imperfections more readily than thick coatings.
Small amounts of thermoplastic resin can be added to the formulation and this
will act as a permanent plasticizer and reduce the melt viscosity, giving better
flow to the finished coating (Loutz et al., 1993).
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Growing concerns over solvent and solid waste emissions from solvent-borne
coil coating operations have led to use of powder for coil coating. Powder
formulation technology has advanced and current powders are now highly
formable and stain resistant, allowing their use in coating coil stock
manufactured for domestic appliances such as refrigerators, washing machines
and microwave ovens (Loutz et al. 1993).
Blank coating by powder offers similar benefits to coil coating. Flat metal
blanks are cut and cleaned before powder coating, then formed into the part.
One advantage is high speed of operation; blanks are cut, cleaned, coated and
cured in as little as two minutes. Other benefits include uniform film
thickness, high transfer efficiency and a compact finishing operation (Major,
1992).
Low-gloss coatings are now available with good mechanical surface properties
and appearance. Gloss values range from 1 percent or less with epoxies to
approximately 5 percent for weather-resistant polyesters (Major, 1992).
Textured powder finishes range from fine textures with low gloss to rough
textures suitable for hiding an uneven surface on the substrate. Textured
powder coatings have shown large improvements in mechanical and
processing abilities compared to those of several years ago (Major, 1992).
Metallic powder coatings incorporate metal flakes which are blended with the
powder before being sprayed onto the substrate. Aluminum extrusions are
commonly coated and efforts are concentrated on matching anodized parts. A
clear topcoat over the metallic base improves exterior durability of the coating
(Major, 1992).
In-mold powder coatings allow manufactures of certain molded plastic
products to coat with powder during the molding operation. Powder is sprayed
into the mold before the molding compound is added; the powder then melts
and cures, chemically bonding to the molding compound and producing a
finish with excellent surface properties such as chip and impact resistance.
Suitable substrates are sheet molding compounds and bulk molding
compounds, used to produce automotive body panels and other items (Major,
1992).
Polyester and acrylic powder coats have been developed for outstanding
weather resistance. Powder coatings can meet all the requirements of AAMA
603 and 605 specifications, except the Florida five-year exposure test which
is still underway (Major, 1992).
Powder coatings with very high reactivity have been developed to cure at
250°F; these allow higher line speeds (increased productivity) and greater use
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of heat-sensitive substrates. Shell Development Company in Houston is
working on low-temperature cure epoxies that have adequate physical and
chemical properties to allow exterior use. (Major, 1992) Adding a selective
catalyst, such as Crylcoat 164, to a conventional polyester allows curing at
reduced temperatures of approximately 260°F to 280°F. Two major
drawbacks are a risk of premature reaction during the extrusion process and
poor surface appearance because of partial crosslinking before complete fusion
of the powder (Loutz, etal. 1993).
American Cyanamid Co. produces a solid amino cross-linking resin for
powder coatings with the tradename Powderlink 1174 (tetramethoxymethyl
glycoluril). This resin can be used with either hydroxyl function polyesters or
acrylics to produce highly durable, light-stable coatings with good mechanical
properties. The cross-linking resin has low toxicity, low environmental
impact, and both performance and economic advantages. An internally
catalyzed polyester, designed for use with 1174, is commercially available.
The polyester has wide cure responses along with good film appearance and
other properties (MP&C, 1992).
Applications Applications of powder coating are growing rapidly because of numerous
benefits, including lower cost, higher quality, and increased pollution
prevention opportunities.
Materials suitable for powder coatings include (Robison, 1989; Bowden,
1989):
Steel
> Aluminum
> Galvanized steel
Magnesium
* Aluminum, magnesium, zinc and brass castings
Plated products
Product Finishing
Powder coatings are used commercially for a wide range of small- to
medium-sized metal parts, including lighting fixtures, equipment cabinets,
outdoor furniture, heat exchangers, microwave antennas, shelving, and hand
carts and wagons. Radiation Systems, Inc., based in Virginia, coats
microwave antennas with polyester powder, forming a coating that is resistant
to physical damage, solar radiation and environmental effects (P&SF, 1992).
Powder coatings are firmly established in industries that manufacture metal
furniture, lawn and garden equipment, store shelving, exercise equipment and
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aluminum extrusions. Job-shop electroplaters have adopted powder coatings
to meet customer specifications and environmental regulations (CMR, 1993).
Clear coatings, especially topcoats, are increasing in importance. The
automotive industry use clearcoats on wheels, trim and other fittings. Bicycle
manufacturers also use powder clearcoat when performance and quality count
more than small cost increases (Reich, 1993).
Automotive components coated by powder include vacuum booster housings,
doorhandles, steering columns, oil filters, aluminum wheels, shock absorbers,
and antennae, among others. Toyota, General Motors and Chrysler use
powder coatings for door frames, and, in some instances, for lower body anti-
chip coatings. Powder coating is used as an effective primer surfacer, and for
blackout finish. Current activity is focused on applications for light trucks and
sport-utility vans. General Motors, for example, applies primer surfacer and
blackout finishing to Chevrolet S-lOs and Blazers (Cole, 1993).
Industrial and Architectural Finishing
Powder coatings are used to protect many parts of buildings, both exterior and
interior, residential and commercial. Polyester powders are applied in
controlled factory settings to aluminum and galvanized steel profiles and sheet
products for use in windows, doors, curtain walls and exterior cladding.
Fusion-bonded epoxy powder coatings are used for protection of steel
reinforcing bar and mesh in concrete structures. Polyurethane or polyester
powders finish many ancillary components in the building industry, such as
downpipes, lampposts, fencing, railing, street furniture and other metal parts.
Epoxy and epoxy/polyester hybrid powders are not suitable for exterior use
and therefore coat interior components including air-conditioners, light
fittings, partitioning and radiators (P&II, 1991; P&SF, 1993).
Polyester powder coats can be applied to galvanized steel, producing strong,
corrosion-resistant and visually pleasing architectural components, however,
problems with "pinholing" and poor adhesion have undermined powder's
reputation in this area. Pinholing is unique to galvanized steel and although
unsightly, it does not impair corrosion resistance. Pretreatment of the metal
surface is necessary to reduce pinholing. A vigorous cleaning regime
involving a soak clean, an etching cleaner, a chromate solution and various
cold and de-mineralized water rinses is recommended (Metallurgia, 1991).
Industrial powder coatings can provide a corrosion-resistant finish without
pretreatment such as priming or even sandblasting [according to Manchester
Industrial Coatings Ltd], Powder coatings applied to substrates that had not
been pretreated were, upon testing, found to be resistant to conditions
encountered in the North Sea and eastern Asia (AMM, 1991). Powder coating
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without any pretreatment should be considered quite risky. Proper testing
needs to be applied to the product in question if this method is to be
considered.
Cost The capital cost for booths, electrostatic spray applicators, and curing ovens
is typically higher than similar equipment used for applying conventional fluid
coatings. Powder coating systems, however, do not require control equipment
to lower VOC emissions.
Powder coating materials are typically more expensive than conventional
coating materials on a volume basis. In many cases, however, the cost of
producing a finished coating is lower, thereby offsetting the higher cost of the
powder.
Because powder coatings can provide a coating of the required thickness in
one pass, the economics for powder coatings improves in cases where a thick
coating is needed.
For powder coating operations using a single color, maintenance and cleanup
costs are low. The operating costs increase for powder coating systems that
require frequent color changes. Solvents are not needed; cleanup can be
accomplished quickly using only compressed air. No waste solvents are
generated and the waste coating material volume is low, reducing disposal
costs.
Benefits The benefits of powder coating systems versus liquid coating systems are:
* Powder does not contain solvents, therefore powder is VOC-
compliant. Compressed air, rather than solvent, can be used for
cleanup.
» Thick powder coatings can be applied in one pass, even over sharp
edges.
> Powder coatings have higher operating efficiencies than conventional
coatings. High transfer efficiency results in high material utilization
rates.
» Powder coatings require less energy to cure than baking systems.
» Because no volatile solvent is used, little air flow is needed in work
areas or near curing ovens. The airflow used to contain the powder
in the booth can be safely recovered, eliminating the need for make up
air. Energy use for heating makeup air declines when air flow
requirements are reduced.
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Resins that are not soluble in organic solvents can be used. Powder
coatings can coat substrates using polymers such as polyethylene,
nylon, or fluorocarbons that are not amenable to solution coating
techniques.
Powder coatings come ready to use and, therefore, do not require
mixing or stirring.
Limitations Limitations of powder coatings may include the following:
> The application of powder coatings requires handling of heated parts
(unlike air-dry systems) because the parts must be subjected to
elevated temperatures in processing. A cool-down zone is normally
required.
> Because powder coatings rely on large, fluidized bed reservoirs, it is
more difficult to make color changes than with liquid coatings.
Swapping often between a large variety of colors is time-consuming
and problems with cross-contamination of color can occur.
» Color matching from batch to batch is difficult.
» Shading or tinting cannot be done by the end user.
» It is difficult to incorporate metal flake pigments that are popular in
some automotive finishes in powder coatings. Aluminum flakes have
potential for explosion if ignited, although new developments in
encapsulating flakes in resin may solve this problem.
* For electrostatic application systems, the parts must be electrically
conductive or they must be covered with an electrically conductive
primer.
> For electrostatic application systems, parts with complex shapes
might be unevenly coated unless special application techniques are
used.
State of Development Powder coating has a well-established niche in the coating industry. Both
thermoplastic and thermoset powdered resins are available for use with
fluidized bed, electrostatic fluidized bed, electrostatic spray guns or
tribocharging spray guns. Detailed information can be obtained from the
Powder Coating Institute (see Section 4).
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REFERENCES AMM. 1991. New powder coating needs no priming. American Metal
Market. December 18, p. 4.
AP&CJ. 1992. Powder coatings developments described. American Paint
& Coatings Journal. 76(54):43
Bailey, Jane. 1992. Powder coating: an environmental perspective.
Industrial Finishing. 68(9):60
Brown, Larry W. 1994. Aerospace applications for powder coating at Hughes
Aircraft Company. In Proceedings: Pollution Prevention Conference on
Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organics Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
CMR 1993. Powder coatings. Chemical Marketing Reporter. October 25,
p. 22.
Cole, Gordon E., Jr. 1993. Automotive "takes a powder." Industrial Paint
& Powder. 69(12):24.
Crump, David L. 1991. Powder Coating Technology at Boeing. In: Sixth
Annual Aerospace Hazardous Waste Minimization Conference. Boeing
Company, Seattle, Washington, June 25-27.
CW. 1993. Powder coatings sparkle. Chemical Week. November 10, p. 77.
Ellicks, David F. 1994. Environmental Compliant Thermoplastic Powder
Coating. In Proceedings: Pollution Prevention Conference on Law-
andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organics
Control Branch. Research Triangle Park, NC. EPA-600/R-94-022.
February, 1994.
Grafilin, David M. Fluoropolymer coatings for architectural, automotive and
general industrial applications. In Proceedings: Pollution Prevention
Conference on Low- and No-VOC Coating Technologies. U.S.
Environmental Protection Agency. Air and Energy Engineering Research
Laboratory. Organics Control Branch. Research Triangle Park, NC. EPA-
600/R-94-022. February, 1994.
Harrison, Alan. 1993. Perfect partners: powder in perspective. European
Polymers Paint Colour Journal. 183(4330):295
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Holder, Albert. 1994. Advantages of Powder Coating. In Proceedings:
Pollution Prevention Conference on Low- and No-VOC Coating
Technologies. U.S. Environmental Protection Agency. Air and Energy
Engineering Research Laboratory. Organic Control Branch. Research
Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Ingleston, Roy. 1991. Powder Coatings: Current Trends, Future
Developments. Product Finishing. 6-7. August.
Keebler, Jack. 1991. Paint it green: powder coating and sludge recycling may
clean up environmental problems. Automotive News. May 27, p. 51.
Lehr, William D. 1991. Powder coating systems. New York, NY: McGraw-
Hill, Inc.
Loutz, J. M., D. Maetens, M. Baudour, L. Mouens. 1993. New developments
in powder coatings, Part 1 and Part 2. European Paint Polymers Journal.
Decembers,p. 584.
Major, Michael J. 1992. Innovation and regulations aid powder coatings.
Modern Paint and Coatings. 82(13):6
Metallurgia. 1991. New guidelines for powder coating of steels. 1991.
November, p. 443.
Mounts, Michael L. 1993. Converting from vapor degreasing to the optimum
alternative. Metal Finishing. August, p. 15.
MP&C. 1991. Powder coatings share program with water-bomes and higher-
solids. Modern Paint and Coatings. May, p. 53.
Muhlenkamp, Mac. 1988. The Technology of Powder. Modern Paint and
Coatings. 78(1): 52-68. November.
Osmond, M. 1993. Architectural powder coatings: a review of new advances
in exterior durable systems. Surface Coatings International. 1993(10):402
P&II. 1993. U.S. powder coatings study. Paint & Ink International.
September, p. 43.
P&R. 1991. Thin coatings, deep profit. Paint & Resin. 61(4): 12
P&SF. 1992. Powder coatings play major role in microwave antenna
manufacture. Plating and Surface Finishing. October, p. 8.
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P&SF, 1993. For aluminum window/door fabricator: powder coating is finish
of choice. Plating and Surface Finishing. July, p. 8.
Plas, Daniel J. 1992. Powder, the comeback coating. Materials Engineering.
109(9):23
Reich, Albert. 1993. Market trends in powder coatings. European Polymers
Paint Colour Journal. June 9, p. 297.
Russell, Bob. 1991. BATNEECs to boost powder usage. (Best available
techniques not entailing excessive cost.) Finishing. 15(6):38
Seymour, R. B. 1991. Progress in powder coatings. Journal of the Oil
Chemists and Colourists Association. 1991(5): 164
Sheasby, Andy, and Matthew Osmond. 1991. Powder coatings for the
building sector. Paint & Ink International. February, p. 2.
P&II. 1991. Tintas Coral's new anti-corrosion powder coating. Paint & Ink
International. May, p. 40.
Wang, Victor and Abid N. Merchant. 1993. Metal-cleaning alternatives for
the 1990s. Metal Finishing. April, p. 13.
WATERBORNE COATINGS
Pollution Prevention Waterborne coatings substitute water for a portion of the solvent used as the
Benefits resin carrier in typical organic coatings formulations. In addition to reducing
VOC emissions during formulation and application, waterbome coatings pose
a reduced risk of fire, are more easily cleaned up (creating less hazardous
residues), and result in reduced worker exposure to organic vapors.
How Does it Work? Waterbome coatings are defined as "coatings which are formulated to contain
a substantial amount of water in the volatiles" (Nicholson, 1988). This
definition appears fairly vague but it is based on practical considerations. A
coating's physical properties are largely determined by the type of solvent
carrier. Waterborne coatings contain water as a primary solvent, although
substantial quantities of organic solvents can be present.
This definition encompasses a variety of waterbome paints, including
emulsions (or latexes) in which only water is used as a solvent as well as
water-reducible coatings that incorporate a mix of water and organic solvents.
Emulsions are dispersions of resin, pigments, biocides, and other additives in
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water. Water-reducible coatings are solutions of resins dissolved in solvents;
these solvents might be water or a mix of water and organic solvents. Organic
solvents are used to aid the solubility of the resins and other additives.
Waterbome coatings have experienced much growth and development in
recent years because of concerns about solvent emissions in the workplace and
to the atmosphere; waterbome coatings have the potential for zero emissions
of VOCs, although many formulations still use some organic solvents.
Knowledge of how EPA or state regulations define VOC content is very
important. For solvent-borne coatings, the regulations are straight-forward:
VOC content in pounds per gallon is measured by the standard test ASTM D
2369-81. This test weighs a portion of coating material, then evaporates off
the VOCs and reweighs it; calculating the difference gives the weight of VOC
per gallon. With waterbome coatings, most regulations require a VOC content
of less than, say, 3.5 Ib/gal, less water. This means that to measure the VOC
content, the water would first have to be removed, then the standard test
procedure should produce a result of 3.5 Ib/gal or less in the remaining
solution.
Many industries use waterborne coating systems, including building and
architecture, automotive, metal finishing, industrial corrosion-protection, and
wood finishing. Common perceptions exist in industry that waterborne
coatings have inferior properties when compared with their solvent-borne
counterparts. For instance, it is commonly thought that all waterbomes take
longer to dry than solvent-borne coatings. This is untrue, as many emulsion
coatings, especially architectural finishes, dry faster and can be recoated
sooner than solvent-bomes. Cured waterbome films are also believed by some
to be more sensitive to moisture. Some waterborne automotive topcoats had
these problems in the 1980s, but many other waterbome systems cure to
moisture-impervious coatings.
Waterborne coatings pose less risk of fire and are easier to clean up and
dispose of than solvent-based coatings. Because the cost of water is less than
that of organic solvents, waterborne coatings generally present an economic
advantage. The costs of storage also are lower because waterborne coatings
are non-hazardous and do not require storage in flame-proof enclosures. Costs
associated with installing ventilation systems in the workplace also are lower.
Waterborne Resin Systems
Waterbome coatings were historically formulated with polymers of high
molecular weight and high glass transition temperature to produce a film with
good chemical resistance, toughness, and durability without the need for
crosslinking in the film. The glass transition temperature is an important
concept; it is defined as the temperature at which there is an increase in
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thermal expansion coefficient (Wicks, et. al., 1992). Often the coating is
brittle below this temperature and flexible above it, but this is not always the
case. High molecular-weight polymers require solvents to reduce viscosity to
acceptable levels and to enable the polymer to soften and flow for good film
formation.
Today, formulations with lower molecular-weight resins and lower viscosity
are more common. These formulations provide a reaction mechanism for
cross-linking between the polymers to achieve desired film properties. A
crosslinked polymer generally produces a film with enhanced physical and
chemical properties compared to a non-crosslinked film. Cross-linking can be
achieved by curing one-component systems at ambient or elevated
temperatures, or by mixing two reactive components (two resins or resin plus
catalyst) in two-component systems.
Almost all types of resins are now available in a waterborne version, including
vinyls, two-component acrylics, epoxies, polyesters, styrene-butadiene,
amine-solubilized, carboxyl-terminated alkyds, and urethanes. Each of these
resins has different properties that challenge users to define their needs and
coating manufacturers to provide the optimum coating to fill those needs
(Pilcher, 1988).
Types of Waterborne Coatings
The three classes of waterborne coatings are:
* Water-soluble or water-reducible coatings
* Colloidal or water-solubilized dispersion coatings
* Latex or emulsion coatings
Water-Soluble CoatingsResins for water-soluble coatings can be
solubilized in pure water or in water-solvent mixes. These coatings are termed
"water-soluble" or "water-reducible" because the resin is dissolved primarily
in water, and addition of water reduces the viscosity of the coating. Polymers
that solubilize in coatings containing only water must be hydrophilic, that is,
they must be attracted to and wetted by water. The presence of polar groups
on the resin molecule produces hydrophilic polymers.
Water-soluble formulations include water-soluble oils, polybutadiene adducts,
alkyds, polyesters, and acrylics. Water-soluble coatings tend to have simpler
formulations than emulsions and are easier to apply but have lower durability
and lower resistance to solvents (Paul, 1986).
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Colloidal CoatingsA colloidal or water-solubilized dispersion coating is an
intermediate between water-soluble and emulsion coatings, combining resin
systems from each.
The application and physical characteristics of colloidal dispersions lie in
between water-soluble and emulsion coatings. Colloidal dispersion or coatings
are composed of very fine, partially water-soluble resin droplets dispersed in
water. Colloidal dispersions are used mainly to coat porous materials such as
paper or leather.
Emulsion or Latex CoatingsAn emulsion coating contains resin dispersed
as a solid in water. In this case, since the resin is not dissolved it is not
required to be hydrophilic, nor is organic solvent required. The droplets are
stabilized in an aqueous medium by emulsifiers and thickeners. The most
common resin used for emulsion coatings are of the vinyl type, derived from
the monomer vinyl acetate. When mixed with other monomers, the coating
polymerizes into a film with the desired properties.
Acrylic resins are an alternative binder. These resins are derived from the
monomer acrylic acid, which is also the parent monomer for the methacrylates.
Acrylic latexes are generally more durable than vinyl acetate copolymers, and
they have higher gloss. The two types of monomers, such as a vinyl and
acrylic, can be combined to produce films with alternative surface properties.
Emulsion coatings are complex mixtures. Among the ingredients that might
be present in the formulation are polymer particles, surfactants, pigments and
extenders, thickeners, coalescing solvents, preservatives, and corrosion
inhibitors, among others. These ingredients comprise approximately half of
the paint formulation; the other half is water. Titanium dioxide is the most
common pigment, providing white color and opacity. Since the pigment must
be dispersed in the water along with the resin, two dispersing agents are
required, one for the resin and one for the pigment. Thickeners or protective
colloids must be added to raise the viscosity to a level that is acceptable for
application. Preservatives or biocides are added to prevent microorganisms
from degrading the additives.
Emulsion coatings are widely used in the building industry, as a decorative and
protective coating for domestic houses and other architectural applications.
Emulsions dry quickly and additional coats can be applied within a few hours.
Tools and containers can be cleaned with water.
The physical and mechanical properties of water-soluble, emulsion and
colloidal coatings vary significantly. For example, the handling considerations
and performance parameters for a water-soluble with a polymer of molecular
weight 2,500 will be different from those of an acrylic latex with a polymer of
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molecular weight greater than 1 million. Coating systems need to be chosen
that will best satisfy all requirements of the application.
The diversity of water-based coating technology is a strength but it also is a
challenge. Because water-based coatings provide a range of characteristics,
formulations can be prepared to fit many different applications. The
waterbome formulations, however, require more careful preparation of
substrate and application of coating than conventional solvent-based coatings.
Application Methods
Waterbome coatings can be applied by:
» Brush or roller
* Dip coating
> Flow coating
* Air spray
* Airless spray
> Air-assisted airless spray
HVLP spray
* Electrostatic spray
All application systems work with waterbome coatings. Waterbornes are very
viscous and thixotropic, so spray gun systems must be able to spray the higher
viscosity coating. Thixotropy is the extent of shear thinning (degree of
liquification) as a result of shear forces in the solution. As with high solids
coatings, spray guns may need to be modified (fluid tips replaced, etc.) or new
guns purchased. Experimentation can be required to find the best gun for the
particular coating that is to be applied. Correct spray viscosity can be
achieved by adding water to reduce the viscosity of coating, however, it is
better to change spray guns or fluid tips because too much thinning with water
will alter the flow and other properties of the coating, potentially causing
problems. The applicator should test the guns with the coating material to see
if a problem exists, then try different guns and different waterborne coating
systems. Spray guns must have stainless steel components where contact with
water is an issue.
Some systems such as high-pressure airless and air-assisted airless can be
problematic when spraying waterbome coatings. Air bubbles can be generated
in the gun and get carried by the coating material to the substrate, becoming
entrapped in the surface of the coating. Testing of different spray guns and
coating materials should lead to a solution for this problem.
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Because waterbome coatings are viscous and thixotropic, they might not be
compatible with existing pumps and piping designed for solvent-borne
coatings, so new pumping systems may need to be considered.
Electrostatic SprayingElectrostatic application can be used to raise the
transfer efficiency (TE), thereby reducing overspray. Special equipment and
techniques are needed for electrostatic application of water-based coatings
because of the electrical conductivity of aqueous solutions.
The coating picks up an electrostatic charge in the spray gun nozzle and is
attracted to the grounded substrate, resulting in raised TE. Because water
becomes easily charged and conducts the charge from the spray gun back to
the source of the coating (e.g., container, 55 gal drum etc.), the source must
also be isolated from ground. The pressure pod or drum must be kept away
from the operator, or a cage must be built to isolate the system and protect the
operator. The hose to the spray gun is rubber and must be long enough to
reach the distant coating source. At excessive distances, charge can bleed off
through the hose to ground.
Four options exist for applying water-based coatings using an electrostatic
system (Scharfenberger, 1989):
» Isolate the storage and supply system from electrical grounds to
prevent leakage from the application atomizer.
* Use an external charging system that is attached to, but electrically
isolated from, the application atomizer.
* Electrically isolate the coating liquid storage and supply system from
the application atomizer to prevent current leakage through the
coating supply system.
> Place the electrostatic charge on the substrate and ground the
application atomizer.
For large waterborne spray systems, a patented solution is available.
Developed by Nordson and called the Isoflo system, the system provides
solutions to the problem of isolating and caging the coating source, especially
if the volume is large and pumps and equipment are large or complicated.
This system electrically isolates paint in the spray gun from the paint source,
allowing coatings reservoirs and pumping systems to be grounded rather than
isolated. Therefore, cages around the coating containers and pumps are not
necessary.
In addition to conventional application methods, water-based coatings also are
amenable to electrodeposition. Electrodeposition of water-based coatings
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resembles electroplating where a substrate is submerged in an aqueous bath.
The coating material is deposited on the substrate by direct current flow.
Electrodeposition is described in Section 2, pages 58 to 61.
Other Issues
Environmental factors such as humidity must be controlled when applying
waterborne coatings in order to achieve the best film formation. Too much
moisture can prevent curing; too dry an atmosphere can cause very rapid
curing, resulting in poor film formation. Humidity can be controlled in spray
booths with a microprocessor-controlled water-spray system.
For product finishing, coatings need to dry or cure at elevated temperatures to
assure complete cure in a reasonable period of time. Ovens are used for
baking waterborne coatings and these have different requirements from ovens
used in solvent-borne baking. Lower temperatures will be used and ovens may
need to be relined with stainless steel. A stainless flue is especially important
to conduct moisture away from the oven.
Dry-filter spray booths will need new or modified filters to cope with
waterborne systems. Water-wash spray booths require new chemicals in the
water to help dissolve waterborne solids.
Pretreatment or cleaning of the substrate is vitally important with waterborne
coatings, for similar reasons to those of powder coating and high solids.
Waterbomes contain little or zero organic solvents which can wet grease
effectively. Water has a high surface tension and grease spots or other
contaminants will cause defects in the film unless they are removed prior to
coating. Aqueous degreasing systems and abrasive blasting are pretreatment
techniques that can clean surfaces in preparation for coating. See for example
U.S. EPA (1993), Mounts (1993), Wang and Merchant (1993) and Lehr
(1991).
Required Skill Level
Although the application equipment is similar, greater operator skill and
attention is needed for application of water-based coatings.
Applications
Water-based coatings are used primarily as architectural coatings and
industrial finish coatings because these paints are easy to apply and adhere to
damp surfaces, dry rapidly, and lack solvent odor. More than 70 percent of
architectural coatings are water-based paints. Water-based architectural
coatings include:
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Section Two
» Industrial protective coatings
* Wall primers and sealants
> Interior flat and semigloss wall paints
> Interior and exterior trim finishes
» Exterior house paints
Water-based coatings have not been readily accepted in the industrial sector.
Stricter regulations, however, are increasing demand for both primer and
topcoat industrial finishes.
ArchitecturalWater-based epoxy coatings provide excellent adhesion when
applied to green or damp concrete. Odor levels during curing of water-based
epoxy coatings is low, and the cured coating surface is easy to clean.
Water-based epoxy coatings, therefore, are suitable for sanitary areas, such as
hospitals or food processing plants (Richardson, 1988).
Development of exterior waterborne gloss enamels has progressed in recent
years, although these enamels cannot match solvent-borne enamels in all areas.
A recent study (1989) found that waterborne exterior enamels are inferior with
respect to gloss, flow, brushability, and opacity. Waterborne enamels,
however, are superior in gloss retention, chalking and adhesion (Hayward,
1990).
Products FinishingWood products traditionally have been coated with
solvent-bome nitrocellulose lacquers, such as clear coatings used on furniture.
These lacquers are fast drying and easy to apply with an excellent appearance
and hardness, but they have high solvent contents. A waterborne, low solvent
nitrocellulose-acrylic latex (NC-A latex) is available. This product, designated
CTG D-857 and produced by Aqualon Inc., of Delaware, contains no organic
solvent carrier, although small quantities of plasticizers, coalescing solvents
and other resins are added to aid in film formation. VOC levels are 2.3 Ib/gal
or lower. NC-A latex coatings have better clarity, resistance to alcohol, and
strippability than acrylic-based latexes. NC-A formulations have lower gloss
when applied than acrylic-based latexes, but gloss levels increase after rubbing
and polishing (Haag, 1992).
Plastic products have traditionally been difficult to coat with latexes. Since the
surface tension of water is higher than the surface tension of most plastics,
wetting of the surface and hence film formation is poor. Reducing the surface
tension of the coating with surfactants can help, but does not guarantee wetting
or adhesion. Polymer chemists have found that by matching surface energy
profiles, the coating polymer and plastic substrate can be improved.
Waterborne systems are available that meet appearance and resistance
requirements of the automotive industry. The computer and business machine
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Section Two
industries in Silicon Valley, California, have been using waterbornes on
plastics since the early 1980s.
Industrial ApplicationsWaterbome coatings for coil coating operations are
commercially available. These coatings are based on polyester or acrylic
resins that are cross-linked with a water-soluble melamine derivative during
baking. The complex copolymers that result produce films that can stand up
to the rigors of the coiling manufacturing process and later finishing
operations (Nicholson, 1988). About 10 percent of U.S. aluminum coil lines
use acrylic-based waterborne coatings. Acrylics offer good adhesion, good
exterior durability, are resistant to yellowing and are flexible. Epoxy esters
also offer good adhesion and are resistant to corrosion and detergents but
degrade on exposure to UV. Polyesters, on the other hand, are not resistant to
detergents but provide good exterior durability. Alkyds have lower
performance but also a lower cost.
A low-VOC water-based epoxy primer is available as a two-component
system. Unlike most water-based formulations, water is not present in either
of the two components. The components are supplied in a 3:1 volume ratio
and mixed prior to application. Water is added to the mixture to reduce
viscosity. After mixing, the formulation contains about 340 g VOC/1 (2.8
Ib/gal) (MP&C, 1988).
New Developments A series of coatings developed by ICI Mond Division Laboratories are now
available for corrosion protection applications on structural steel. Called
Haloflex, the coatings incorporate copolymers of vinylidene chloride, vinyl
chloride, and alkyl acrylate or methacrylate with a small amount of acrylic
acid. The Haloflex resins form a coating film with low permeability to water
and oxygen. The formulation also is low in surfactants, which reduce water
sensitivity that is inherent in conventional latex coatings.
Research has now found that modified styrene-free acrylic binders in latex
formulations work better than styrene acrylic dispersion paints for structural
steel protection. This technology is not currently available, but might become
available in the near future (van der Kolk et al., 1993).
A series of water-soluble epoxies were developed for corrosion protection of
steelwork. These coatings can be applied by brush, spray, or dip tank, and can
be dried at ambient temperatures to produce an anticorrosive glossy film
(Wilson et. al.,eds, 1988).
Water-soluble epoxies also have been developed for coating the interior of
metal food and beverage cans. These epoxies have been cross-linked by
Page 54
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Section Two
baking with amino resins such as melamine-formaldehyde (Wilson et. al., eds,
1988).
Waterbome coating systems for automotive basecoats are gaining ground,
although the application technology for these coatings is complex. Drying, for
example, involves infrared (IR) heating and high-velocity heated air. (Modern
Paints and Coatings, 1989,79, 38).
A new technology for water-based coatings has been developed that is a great
benefit to the wood and furniture finishing industry. "Core-shell" technology
provides for a rapid development of coating hardness, improving the early and
ultimate print and block resistance. These properties allow parts to be handled
and stacked sooner after coating.
Ecopaint: a Complete System
The Ecopaint waterborne paint system was developed in Europe by a
consortium of manufacturers to meet strict legislative controls on VOCs. The
Ecopaint system comprises a fully water-soluble baking enamel, a water-wash
spray booth (Figure 5), and an ultrafiltration unit (Figure 6). The spray booth
is a water-wash design that traps overspray and collects paint particles in the
water. The waste water/particle mixture then passes through an ultrafiltration
unit that separates the water from the particles. In principle, all of the
overspray and the waste paint from the cleanup of spray guns can be collected
and filtered for reuse. Excess coating material from cleanup of the storage
containers (cans) also can be recovered.
The Ecopaint system is suitable for coating a wide variety of products,
including automotive parts, workshop furniture, steel shelving, steel pipelines,
and other parts requiring a stove enamel finish. The coatings offer a range of
gloss levels, textured and structural finishes, and a wide range of color.
Surface film properties generally are high enough to meet most normal
industrial requirements. Ecopaint coatings are marketed in Europe under the
trade name Unicolour, and also are available in the United Kingdom and
Japan. These coatings will soon be available in the U.S. (JOCCA, 1993).
Ecopaint baking enamels are based on fully water-soluble resins. They contain
less than 5 percent VOCs, with some containing only 3 percent. If required,
the coatings can be formulated with virtually no organic solvents; only a small
amount is needed to modify film properties. Ecopaint coatings can be applied
with most spray methods, including conventional air spray, HVLP, airless, and
electrostatic systems, including high speed rotary disks and bells. Flash off
times are short and conventional baking temperatures are employed so existing
equipment can be used.
Page 55
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Section Two
Figure 5
Water-Wash Spray Booth
Paint
Emission
E
Gun
Water cleaning
Object
I
Overspray
1\
Recyclable paint
/Spray
V Booth
Water
Source: Eastern Research Group, Inc.
Page 56
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Section Two
Figure 6
Ultrafiltration Unit for Concentrating Waterborne Enamel
Membrane
Paint-water mixture
Permeate
Concentrate Circulating tank
Out
Ultrafiltration
Unit
Source: Eastern Research Group, Inc.
Page 57
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Section Two
Water-based Temporary Protective Coatings
Consumer products, particularly automobiles, are shipped from the factory to
the consumer through an uncontrolled and potentially harsh environment. A
protective coating helps to maintain the quality of the factory finish.
Temporary protective coatings typically have been solvent-based. These
coatings release solvents into the atmosphere on curing, and often require
solvents for removal. Temporary coating materials using a water-based acrylic
copolymer system can produce a tough transparent film that protects the
coating for up to one year. The film can be removed with an aqueous alkali
wash solution. Removal of a water-based temporary protective coating takes
10 minutes for an automobile compared to 20 minutes for a wax coating
(Product Finishing, 1986).
Water-based coatings also have been tested as masking layers to protect
specific areas of metal substrate during chemical milling and etching (Toepke,
1991).
Cost Water-based coatings are more expensive than conventional coatings per unit
of reactive resin. The costs of coating fluid preparation, application, cleanup,
and disposal are similar for water-based and conventional coatings. A cost
comparison of conventional, powder, high solids, and water-based coatings is
presented in Hester and Nicholson (1989).
The capital cost for electrostatic spray systems for water-based coatings
typically will be higher than application equipment for solvent-borne coatings
because of the electrical conductivity problem. Although water-based coatings
typically contain some solvents, they are less likely to require VOC control
equipment. High levels of VOC in the waterbome formulation may require
carbon absorber equipment or VOC incineration equipment; both are
expensive.
Benefits Because water-based coatings uses less or no organic solvents, problems such
as environmental, odor, and safety and health concerns are reduced. The
benefits of water-based coatings are:
» Compliance with VOC regulations (though not automatic -
compliance depends on actual VOC levels and limitations).
> Less exposure to harmful organic vapors in the workplace. Less
need for ventilation systems to ensure safety and/or meet OSHA
requirements.
* Lower risk of fire from ignition of organic vapors.
Page 58
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Section Two
Do not cause skin irritation from solvent contact
Good to excellent surface properties including excellent gloss, rub
resistance, anti-sealing effect and non-yellowing film.
Cost savings, depending on the application.
Clean up and disposal is simpler than with solvents and solvent-borne
coatings. Water is primarily used to wash up.
Existing equipment (nonelectrostatic) can be used for application of
most water-based coatings, although stainless steel inserts are
required.
Limitations The drawbacks of water-based coatings are:
Some waterborne coatings still contain organic solvents (VOCs),
though usually less than high solids formulations.
The flash off time may be longer than for solvent-based coatings.
This depends on the formulation and environmental conditions.
The film has a tendency to be sensitive to water with an increased
potential for degradation, though not after full cure.
More energy can be required to force-dry or bake waterborne coatings
than solvent formulations because of the high latent heat caused by
water evaporation; energy requirements can be as much as four times
greater for water-based coatings.
Waterborne coatings are sensitive to humidity, requiring humidity
control in the application and curing areas. Low humidity can cause
those coatings to dry extremely fast, resulting in craters in the final
film. High humidity can cause very slow drying times, resulting in
sagging.
The quality of the final film is dependent upon surface cleanliness; the
high surface tension of water prevents wetting of some surfaces,
especially when grease or other contaminants are present. The high
surface tension of water also can cause poor coating flow
characteristics.
High gloss levels are often difficult to achieve.
Page 59
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Section Two
> Efflorescence or a growth of crystals can occur on certain substrates.
Plasterboard is commonly affected when water-soluble salts like
sodium sulfate leach out of the coating, disrupting the integrity of the
final film.
» Water in the coating formulation can cause "flash rusting" of metal
substrates. For this reason, most industrial waterbomes are
formulated with inhibitors.
> Toxic biocides often are added to kill microorganisms that attack
additives such as fatty emulsifiers or defoamers.
* Emulsion coatings do not penetrate porous substrates, such as wood,
very well; this assists "good holdout" but can be a disadvantage if
lack of penetration prevents good adhesion on old, chalky surfaces.
» Some types of resins degrade in water, reducing shelf life
formulations containing these resins.
> Water-based latex coatings are susceptible to foaming because
surfactants often are used to stabilize the latex.
> Water in the formulation also can cause corrosion of storage tanks
and transfer piping.
» Special equipment is needed for electrostatic application.
Tradeoffs Many water-based formulations are compatible with conventional
nonelectrostatic spray equipment but require special provisions for
electrostatic application. As a result, a change to water-based coatings can be
somewhat less disruptive than a change to other technologies such as powder
coating.
REFERENCES Adams, Larry. 1991. Finishing materials: must compliance mean an inferior
product? Wood & Wood Products. 96(13):89
Baracani, Al. 1990. A coat of many colors; coating technology promises to
move into the 1990s with water-based coatings and new drying/curing
developments. American Printer. 205(6):38
Giinsel, R. 1993. Water-based coatings and the environment. Surface
Coatings International. 1993(9):364
Page 60
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Section Two
Haag, Harold F. 1992. Low-VOC waterborne coatings for wood based on
nitrocellulose-acrylic latex. Journal of Coatings Technology. 64(814):19
Hayward, George R. 1990. Problems with waterborne coatings. In:
Waterborne coatings: surface coatings-3. Eds. Wilson, Alan D., John W.
Nicholson and Havard J. Prosser. New York, NY: Elsevier Science Publishers
Ltd. p. 295.
Huang, Eddy, Larry Watkins and Robert C. McCrillis. Development of ultra-
low VOC wood furniture coatings. In Proceedings: Pollution Prevention
Conference on Low- and No-VOC Coating Technologies. U.S.
Environmental Protection Agency. Air and Energy Engineering Research
Laboratory. Organics Control Branch. Research Triangle Park, NC. EPA-
600/R-94-022. February, 1994.
Jacobs, Patricia B. and David C. McClurg. 1994. Water-reducible
polyurethane coatings for aerospace applications. In Proceedings: Pollution
Prevention Conference on Low- and No-VOC Coating Technologies. U.S.
Environmental Protection Agency. Air and Energy Engineering Research
Laboratory. Organics Control Branch. Research Triangle Park, NC. EPA-
600/R-94-022. February, 1994.
Jaffari, Mark D. 1994. Waterbome maskant. In Proceedings: Pollution
Prevention Conference on Low- andNo-VOC Coating Technologies. U.S.
Environmental Protection Agency. Air and Energy Engineering Research
Laboratory. Organics Control Branch. Research Triangle Park, NC. EPA-
600/R-94-022. February, 1994.
JOCCA, 1993. Ecopaint: a completely recyclable waterborne paint system.
Journal of the Oil Chemists and Colourists Association. 1993(3): 116
Lehr, William D. 1991. Powder coating systems. New York, NY: McGraw-
Hill, Inc.
Marwick, Wm. F. 1994. Waterborne lacquers for aluminum foil. In
Proceedings: Pollution Prevention Conference on Low- and No-VOC
Coating Technologies. U.S. Environmental Protection Agency. Air and
Energy Engineering Research Laboratory. Organics Control Branch.
Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
MIN, 1991. Coating costs reduced for auto parts. Metals Industry News.
8(2):2
Mounts, Michael L 1993. Converting from vapor degreasing to the optimum
alternative. Metal Finishing. August, p. 15.
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MP&C, 1991. Polyurethane dispersions for waterborne basecoats. Modern
Paint and Coatings. July, p. 44.
MP&C, 1992. 19th Waterborne symposium: record attendance; full program.
Modern Paint and Coatings. May, p. 50.
Paul, Swaraj. 1986. Surface Coatings Science and Technology. John Wiley
& Sons, New York, NY.
PF. 1986. ICI Wins Pollution Abatement Award for Low Solvent Emission
Paints. Product Finishing. 39(s): 23. May.
Raghavan, Vaikunt, and Wayne H. Lewis. 1991. Epoxy waterborne primer:
low-temp cure and zero VOCs. Modern Paint and Coatings. July, p. 46.
Reitter, Chuck. 1993. A window with a view of the future: waterborne
symposium offers high-tech sneak preview. American Paint & Coatings
Journal. 77(41):36
Richardson, Frank B. 1988. Waterborne Epoxy Coatings: Past, Present and
Future. Modern Paint and Coatings 78(4): 84-88. April.
Roman, Nick. 1991. Advances in waterborne coatings. Modern Paint and
Coatings. November, p. 34.
Ryder, Peter C. and Peter I. Hope. 1994. New environmentally acceptable
metal coating systems. In Proceedings: Pollution Prevention Conference on
Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organic Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Stewart, Regina M., George E. Heinig, and Francis L. Keohan. 1992.
Waterborne anticorrosive coatings offer low VOC. Modern Paint and
Coatings. September, p. 39.
Toepke, Sheldon. 1991. Water Base Chemical Mill Maskant. In: Sixth Annual
Aerospace Hazardous Waste Minimization Conference. Boeing Company,
Seattle, Washington, June 25-27.
Tuckerman, Richard and David W. Maurer. 1994. The development of
practical zero-VOC decorative paints. In Proceedings: Pollution Prevention
Conference on Low- and No-VOC Coating Technologies. U.S.
Environmental Protection Agency. Air and Energy Engineering Research
Laboratory. Organic Control Branch. Research Triangle Park, NC. EPA-
600/R-94-022. February, 1994.
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van der Kolk, C. E. M, R. Kruijt, and E. A. de Rouville. 1993. Water-based
acrylic dispersion paints for the protection of structural steel; a technology in
full development. Journal of the Oil Chemists and Colourists Association.
1993(7):280
Wang, Victor and Abid N. Merchant. 1993. Metal-cleaning alternatives for
the 1990s. Metal Finishing. April, p. 13.
Waterbome acrylic copolymer exceeds California standards. 1991. Modern
Paint and Coatings. March, p. 34.
Wicks, Zeno W., Frank N. Jones, and S. Peter Pappas. 1992. Organic
coatings: science and technology. (Subtitled: volume 1: film formation,
components, and appearance.) New York, NY: John Wiley & Sons, Inc.
Wilson, Alan D., John W. Nicholson and Havard J. Prosser. 1988. Surface
Coatings Volume 2. Elsevier Applied Science.
Winchester, Charles M. 1991. Waterbome nitrocellulose wood lacquers with
lower VOC. Journal of Coatings Technology. 63(803):47
ELECTRODEPOSITION
Pollution Prevention
Benefits
Electrodeposition uses waterborne coatings with reduced levels of VOCs. The
reduced VOC content of the coatings, combined with the superior transfer
efficiency of the process, results in reduced VOC emissions. In fact, closed
loop operation (which would eliminate VOC emissions completely) is
possible with electrodeposition. In addition, because of the higher transfer
efficiency there is less waste generated from the coating operation in
comparison with conventional coatings operations.
How Does it Work?
Electrodeposition of paints, also known as electrocoating (or E-coat), has
existed since the 1920s when the first processes were patented.
Electrodeposition is a waterborne coating technology with excellent pollution
prevention potential because of the very low organic solvent content and very
high transfer efficiency. The automotive industry is the largest user of
electrocoat, priming car bodies for corrosion-protection and subsequent
painting. Electrocoating was commonly used in industry in the 1960s with
anodic electrodeposition (AED) but, by the mid-1980s, almost all of industry
was using cathodic electrodeposition (CED) because of superior film
properties.
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Section Two
In electrodeposition, the substrate is immersed in a tank filled with coating
resins and pigments dissolved or dispersed in water. The substrate must act
as an electrode, and, therefore, must be metallic; in practice, steel is the most
common material coated. The substrate is connected to a power supply that
provides a voltage between the substrate and another electrode which is also
immersed in the electrocoating tank (see Figure 7). The electric potential
between the electrodes causes a current flow that results in electrodeposition
of charged resins and pigments onto the substrate. When the coating process
is complete, the substrate is removed from the tank and rinsed; elevated
temperatures then are used to cure the coating.
Electrodeposition technology employs a process known as electrolysis that
involves both electrophoresis and deposition. Electrophoresis describes the
movement of the charged coating particles in solution toward the substrate,
deposition occurring when resins and pigments are deposited on the substrate.
Aqueous systems are necessary for electrocoating because the solvent needs
to have a high dielectric constant to charge the resin particles. Water, unlike
most organic solvents, has a high dielectric constant.
Operating Features Resin Systems
Electrodeposition resins must be able to pick up a charge in solution.
Therefore, resins must contain cationic or anionic molecular groups, depending
on the polarity of the electrodeposition system. For AED systems, resins
generally contain free or neutralized carboxylic acid groups. For CED
systems, film-forming cations can be obtained as organic substituted
ammonium macro-ions such as RNH3- or R3NH+ (R denotes the resin).
Resins with molecular weights in the range of 2,000 to 20,000 are typically
used for electrodeposition of water-based coatings.
Many different resins are available for electrodeposition. All resins have three
common properties: (1) they can be rendered soluble; (2) made elastomeric;
(3) and crosslinked. Amino-containing resins include acrylic and methacrylic
esters, styrene, vinyl ethers and vinyl esters with unsaturated monomers that
contain secondary or tertiary amino groups. Epoxy-based resins are most
commonly used for electrocoating because they have excellent corrosion
resistance. Epoxy groups containing copolymers (e.g., glycidyl methacrylate)
can react with amines to form the amino groups necessary for cationic
behavior.
Electrocoats contain pigments and extenders along with'resins. These
materials must be deposited on the substrate at a similar rate or the
proportions of pigment/extender to resin in the bath will change over time,
causing uneven formation. Careful formulation of the coating prevents these
Page 64
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Section Two
Figure 7
Electrodeposition
Resin
solution
Resin attracted
to part
Part to be
coated
Electrodeposition
coating tank
Spray
rinse
Rinse tank
Source: Eastern Research Group, Inc.
Page 65
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Section Two
problems. Pigments undergo electrophoresis because they adsorb resin
molecules on their surface, effectively forming a cation or anion out of the
uncharged pigment particle. The cathodic process (CED) is more successful
than AED because of superior corrosion resistance and throwing power at low
film thicknesses. Most automotive coating plants today use CED.
Application Methods
Substantial amounts of equipment are necessary for applying electrocoats
resulting in high installation costs. A dip tank is required that contains the
anode (or cathode for AED). A power supply and rectifier also are required;
for large automobile body coating tanks, power supplies of 200V to 500V and
up to 1500A are necessary. Other required equipment includes water rinse
facilities, extra-clean application and curing areas, and ultrafiltration units and
baking ovens. Coating large numbers of similar parts is the only way to justify
the costs of installing electrodeposition equipment and providing operators
with the required training.
Applications
Electrodeposition systems are used most commonly for applying automotive
primers because of their high ability to provide very thin, evenly spread films
for corrosion protection regardless of the shape of the substrate. Uniform
coating can be achieved on substrates with recesses, tapped holes, and sharp
edges. By "forcing" a dense film against a substrate, electrodeposition
provides excellent adhesion and resistance to corrosion. Small metal parts can
be coated in a dip tank. Other parts, such as auto bodies, auto wheels,
appliances and other industrial products with high volume runs may be
attached to a conveyor and coated in a line process. Electrodeposition can be
applied to galvanized steel surfaces, as well as aluminum and other metals.
Benefits
The electrodeposition process is successful in industrial applications for a
number of reasons:
> Good edge protection and uniform coating thickness.
» High coating utilization (greater than 95 percent).
> An application process that is easy to automate and control.
> Low levels of organic solvents and pollution because it uses
waterborne coatings.
> A closed loop process is possible, reducing the potential for
pollutants to escape.
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Limitations * Electrodeposition systems have high capital expense
+ Electrodeposited coatings are highly sensitive to contaminants.
* To produce a high gloss finish, the coating must contain a conductive
pigment.
» The metal substrate can dissolve into the coating, causing
discoloration in anodic deposition.
REFERENCES Gupta, S. C., and M C. Shukla. 1992. Cathodic electrodepositable coating
compositions based on epoxy resins. Journal of the Oil Chemists and
Colourists Association. 1992(9):369.
Ryder, Peter C. and Peter I. Hope. 1994. New environmentally acceptable
metal coating systems. In Proceedings: Pollution Prevention Conference on
Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organic Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
ULTRAVIOLET (UV) RADIATION- AND
ELECTRON BEAM (EB)-CURED COATINGS
Pollution Prevention Radiation curing relies on ultraviolet (UV) radiation or electron beam (EB)
Benefits technology to cure solvent-free coatings formulations. Depending on the
formulation and resin type, some VOCs may be emitted from the resins,
although these emissions are quite low. Some UV- and EB-cured coating
systems emit virtually no VOCs. Organic solvents are still needed for cleanup
of uncured coating material, unless the system is based on waterborne finishes.
How Does it Work? Radiation-cured coatings use radiation from ultraviolet light or electron beam
sources to cure solvent-free coating systems. These processes produce high-
performance protective and decorative finishes for various product finishes.
Radiation curing can avoid the use of solvents entirely, although solvent dilu-
tion might be required for some spray applications.
Radiation curing has been adopted by the wood finishing industry (e.g.,
flatstock fillers in particle boards, hardwood flooring) because of the short
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cure times and the high quality films that are produced without baking. The
graphic arts industry uses UV curing of various printing inks and coatings on
paper, cardboard and other substrates.
Radiation-cured coatings usually are sprayed on the object and then subjected
to radiation from either ultraviolet (UV) lamps or an electron beam (EB)
generator (see Figure 8: UV curing system, and Figure 9: EB generator). The
radiation creates free radicals in the coating, initiating crosslinking (or
polymerization) of the film. The curing process takes place almost
instantaneously when radiation is applied, rather than the minutes, hours, or
even days that conventional coatings take.
UV/EB-cured coatings can use 100 percent reactive liquids, eliminating
solvent use altogether. However, certain resins can volatilize and become
VOCs, so zero VOC depends on the formulation. UV/EB-cured coatings
consist of:
> An oligomer or prepolymer containing double-bond unsaturation.
> A reactive solvent (e.g., monomers with varying degrees of
unsaturation).
> A photoinitiator to absorb the UV/EB radiation.
> Pigments/dyes and other additives
Radiation Chemistry
The first type of radiation-cured coatings to become available used free-
radicals in the polymerization process. Free-radicals are highly reactive
molecules containing an unpaired electron. They are produced when
photoinitiator molecules undergo photochemical reactions on exposure to UV
light or EB radiation. Free-radicals react with activated double bonds from
acrylate groups, activating a chain reaction that causes polymerization.
During the 1980s, a second type of photochemical reaction known as cationic
polymerization emerged for use with radiation-curing. This process uses salts
of complex organic molecules to initiate cationic chain polymerization in
resins and monomers containing expoxides (oxirane rings). UV radiation is
the most efficient method of creating the cationic intermediates; EB radiation
can be used but is inefficient and expensive. Acrylic alkene double bonds and
the oxirane ring can be activated directly by UV radiation without use of a
photoinitiator, but this method is much less efficient.
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Figure 8
Ultraviolet Curing System
Air
Conveyor
motion
Air ventilation
Ultraviolet rays
UV-curing paint Woo(j pane)s
on conveyer belt
Source: Eastern Research Group, Inc.
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Figure 9
Electron Beam Generator
lead
1
High voltage
power supply
Source: Eastern Research Group, Inc.
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Operating Features Resin Systems
Resins used in conventional solvent-based coatings can be chemically
modified for use in radiation-cured chemistries. Resin types include epoxide,
polyester, polyurethane, polyether, and others that are modified by introducing
acrylate functionality, typically by reacting acrylic acid with alcohol groups or
hydroxyethyl acrylate with acid groups.
The general physical and chemical characteristics of the resins are retained
after modification. Crosslinking of the polymers in the coatings yields
excellent chemical and physical resistance. Polyester acrylates exhibit
excellent properties in wood and paper coatings. Urethane acrylates can
produce films that are tough and abrasion resistant or soft and flexible. The
additional chemical reactions that are needed to produce a functional radiation-
curable resin, however, add to the cost, hindering the acceptance of UV/EB
coating systems (Holman, 1992; Sawyer, 1991). Cationic systems that use
vinyl ethers, epoxides, and polyols require less chemical modification than
acrylates, therefore these resins should be cheaper, although they are more
limited in their applications than acrylates.
Radiation-cured systems using acrylates in waterborne formulations such as
water-soluble coatings or aqueous emulsions are available. Wood and
chipboard water-based coatings have successfully used acrylates, although
gloss and coating resistance is lower than with solvent formulations. High
coating specification requirements limits the application of waterborne
products in other industries.
Application Methods
Coating material is applied to the substrate by spraying, and is subsequently
cured with UV- or EB-radiation. Because curing takes place so quickly, it is
advisable to allow a sufficient amount of time between application and curing
for the coating to flow-out and achieve maximum gloss. If this is not possible,
other precautions and equipment should be considered for use in achieving the
desired gloss level (Sun Chemical, 1991). During this flow-out time,
emissions of VOCs could conceivably take place.
Radiation Sources
The radiation source most commonly used in industry is the medium-pressure
mercury-electrode arc lamp. These lamps, together with high voltage power
supplies, are compact and inexpensive and have lifetimes of thousands of
hours. The lamps can be retrofitted easily to existing production lines, but
they require an extraction system to remove excess heat and ozone that is
generated by UV action on oxygen in the air. A disadvantage of the lamps is
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a prolonged warm-up period that prevents on/off operation; lamps that are
modified by doping with metal ions to change spectral characteristics also
have a shorter working life.
An alternative type of lamp produces radiation through microwave excitation
of the mercury vapor. These lamps are more expensive, but they have the
advantage of instant startup/shutdown capabilities. Further spectral
modification of the lamps by metal ions does not reduce their working life.
Electron beam generators are expensive, complex and large. These factors
inhibit their more widespread use in radiation curing. In addition, oxygen has
an inhibiting effect on free-radical polymerization that is initiated by EB, thus
an inert atmosphere of nitrogen, with oxygen concentrations of less than 100
ppm is required if adequate curing is to be achieved with EB generators
(Holman, 1992). The high capital cost of EB curing equipment has limited the
acceptance of EB-cured coatings (Paul, 1986).
Radiation-curing technologies have lower energy requirements for curing
compared to conventional solvent or waterbome coating systems. The heat
energy required to evaporate solvents or induce thermal reactions in
conventional systems is orders of magnitude higher than the energy used in
UV/EB systems. Curing a thermoset acrylic resin with conventional
technology, for instance, requires 24 times the energy needed for curing clear
lacquer with UV, and 12 times the energy needed for curing a pigmented
coating with EB (O'Hara, 1989).
Production Issues
Radiation curing occurs on line-of-sight as UV/EB radiation cannot travel
around comers of three-dimensional substrates. Consequently, UV/EB
systems are most suitable for flat components such as wood panels and
materials found in the graphic arts industries. Recently, 3-D radiation sources
have begun to cure substrates with more complex surfaces.
Because radiation-curing is a fast and relatively cool process, inks and
coatings can be cured on heat-sensitive substrates such as paper and wood.
Coating color and opacity affect the curing rate. Darker and more opaque inks
block UV radiation and require longer exposure times for adequate curing.
Likewise, thicker films and multiple films cure more slowly than thin or single
films.
EB curing is not affected by coating color or opacity; electrons penetrate
pigmented coatings effectively to cure coatings in short exposure times. The
high energy of EB curing provides the highest margin of safety in applications
where extractables or low odors are essential. High energy also ensures
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adequate conversion from oligomer to polymer so that very thick films and
laminating adhesives also can be cured.
Applications UV curing is used in these industrial finishing areas:
» Wood finishing
» Metal decorative coatings
» Automotive coatings
> Wire coatings
» Packaging coatings
» Floor finishing
UV curing has been investigated in the U.S. to replace thermally cured
coatings for aluminum and galvanized steel cans; UV-cured coatings have
hardness and salt spray resistance that last 200 hr to 500 hr. UV-cured
coatings provide highly cross-linked 10-mil-thick films on both bare and
insulated wire that are strong, yet flexible.
At least one company in Japan uses EB curing for metal coil stock. EB has
also seen limited use in high-volume printing operations. UV curing also offers
a low-cost, high throughput alternative for finishing automotive hubcaps and
wheel rims.
Liquid acrylic and liquid polyurethane-acrylic UV-cured coatings surpass
press varnish and water-based coatings in quality and film lamination.
Likewise, UV-cured coatings have found a market niche in high-gloss vinyl
floor coverings, surpassing the conventional urethane coatings in ease of
application, and in abrasion, solvent, and stain resistance.
UV coatings formulated from polyester styrene resins have been used as filler
for chipboard. Although commercially available, the polyester-styrene system
has not been applied widely because of styrene's volatility, and the yellow
color of the coating that is produced.
Waterborne UV/EB Coating
Radiation-cured waterbome urethanes are available for the wood finishing
industry. Waterbornes range from systems that contain small amounts of
water for viscosity-reduction purposes to fully water-soluble coatings or
latexes. Water-based latexes offer the greatest opportunity for pollution
prevention because acrylate diluent monomers are usually not necessary for
viscosity reduction in this type of coating. The benefits of waterbomes
include:
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» Reduced acrylate content (which lowers skin irritancy and odor) and
reduces the levels of film shrinkage on cure.
> Increased viscosity control.
> Easy cleanup of equipment and spills with water.
» Possibility of very low film thicknesses with low solids formulations
» Additives such as matting agents are easily added to water-based
systems.
> Reduced flammability
Waterborne radiation-cured systems do have some drawbacks, however,
including:
> A water flash-off step before curing that often requires ovens or other
dryers, higher energy use, and longer application/cure times (thin
coatings on wood can result in absorption of the water allowing
immediate radiation-curing).
* Certain wood substrates will show a grain-raising effect.
> Reduced coating performance with some substrates or coating
formulations
Waterborne UV systems have been evaluated by several authors (Mahon and
Nason, 1992; Stenson, 1990) with promising results reported for wood
finishing applications. Mahon and Nason identified five outstanding UV-
cured sealers and topcoats that meet certain performance criteria. Resistance
to cold cracking was the major potential problem reported with these coatings.
A radiation-curable urethane polymer was evaluated and tested on oak panels
in accordance with the National Kitchen Cabinet Association test procedures.
Most of the coating properties rated excellent, with only flow appearance and
stain resistance to mustard rating average.
Benefits UV- and EB-cured coatings have a number of benefits:
» Eliminates or reduces solvent use; virtually no VOC emissions.
+ High reactivity, very rapid curing.
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High productivity from rapid curing and instant startup and
shutdown.
Low-temperature processing, which allows for the use of heat
sensitive substrates such as plastic.
Long shelf life of coating materials.
Stable pot life because most coatings are single-component systems.
Relatively low capital investments in equipment.
Good film properties and performance, such as hardness; and
improved solvent, stain and abrasion resistance.
Higher non-volatile content that results in higher gloss, better build,
and lower shrinkage.
Lower energy use because of high efficiency UV/EB systems when
compared to thermal ovens.
Equipment requires less space than curing ovens
Limitations UV/EB technology has several drawbacks:
Higher cost coating formulations because of expensive raw materials
and smaller volume.
Line-of-sight curing is limited to flat or cylindrical materials that can
be directly exposed to the radiation. Radiation systems for 3-D
substrates are being developed to overcome this limitation.
The presence of pigments reduces penetration by UV light, limiting
use in high-build applications.
Polymers for radiation curing are highly reactive and can cause skin
irritation and sensitization.
UV/EB curing is not always suitable for porous materials.
EB systems generally require an inert environment because
atmospheric oxygen prevents curing of resins.
EB curing requires equipment with high capital costs.
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* Many systems have relatively high viscosity which causes processing
and appearance problems.
» No FDA approval for radiation-cured coatings in direct contact with
food.
* Generally more expensive on a per pound basis than solvent-borne
products.
Future use of UV/EB coatings depends on development of the following:
» More highly developed UV equipment,
> New products/markets for radiation processing technologies.
» New 100 percent reactive monomers and oligomers that are nontoxic
and low in viscosity.
» New monomers, oligomers, and polymers that better adhere to metal
substrates.
> Lower cost materials
REFERENCES Berejka, Anthony. 1992. Strategies for switching to UV/EB-cure coatings.
Modern Paint and Coatings. April, p. 64.
Donhowe, Erik T. 1994. UV pollution prevention technology in can
manufacturing. In Proceedings: Pollution Prevention Conference on Low-
andNo-VOC Coating Technologies. U.S. Environmental Protection Agency.
Air and Energy Engineering Research Laboratory. Organic Control Branch.
Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Holman, Dr. R 1992. Solutions without solvents and the radcure alternative.
Journal of the Oil Chemists and Colourists Association. 1992(12):469
Laird, Edwin C. 1994. Water based and UV-cured coatings for plastics. In
Proceedings: Pollution Prevention Conference on Low- and No-VOC
Coating Technologies. U.S. Environmental Protection Agency. Air and
Energy Engineering Research Laboratory. Organic Control Branch. Research
Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Mahon, William F., and Dale L. Nason. 1992. Testing UV-cure coatings
systems for wood. Modern Paint and Coatings. June. p. 44.
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Oanneman, Jeffrey. 1988. UV Process Provides Rapid Cure for Compliant
Wood Finishes. Modern Paint and Coatings. 78(2): 28-29. February.
O'Hara, K. 1989. UV and Electron Beams: Energy Efficient Methods of
Curing. Polymer Paint Colour Journal. January, p 11.
Peaff, George. 1993. Taking the cure. Chemical Marketing Reporter,
244(17): 18
Pelling, D. 1991. Infra red and ultra violet curing methods. Journal of the
Oil Chemists and Colourists Association. 1991(8):302
Sawyer, Richard. 1991. Tapping the value of UV-curables. ModernPaint
and Coatings. June, p. 34.
Stenson, Dr. Paul H. 1990. Radiation-curable waterborne urethanes for the
wood industry. Modern Paint and Coatings. June, p. 44.
Stowe, Richard W. 1994. Radiation curing technology: Ultraviolet and
electron beam processing. In Proceedings: Pollution Prevention Conference
on Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organic Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
Webster, G. 1991. Radiation curing: where does it fit in? Journal of the Oil
Chemists and Colourists Association. 1991(1):7
Wilkins, G. J. 1992. Vinyl ethers in cationic and/or free radical induced UV-
curable formulations. Journal of the Oil Chemists and Colourists
Association. 1992(3): 105
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Section Three
SECTION THREE
EMERGING TECHNOLOGIES
Introduction
Coatings Literature
This section on emerging technologies describes coatings systems that are
newly available. "Emerging" in this context refers to innovative technologies
that have just recently become commercially available and accepted for use in
the marketplace. These technologies are less well known than conventional
alternatives.
Emerging technologies should not be confused with technologies presently in
research laboratories, or those being reported in scientific papers; such
technologies are not yet available for industry consumption. These
technologies could be termed "developing technologies." Knowledge of
developing technologies is important for coatings applicators concerned with
improving cost or quality competitiveness, technical performance,
environmental improvements, etc. when these technologies become
commercially available.
Developing and emerging technologies are not confined to innovative
technologies alone. Waterborne coatings are an example of a well established
organic coating which is, however, one of today's most dynamically
developing technologies. High performance waterborne coatings that are under
development or emerging in the market are now able to perform the functions
of traditional solvent-borne coatings (such as automotive topcoats). High
solids coatings also are under development that will greatly reduce VOC levels
while maintaining good performance. The coatings user should not ignore
coating types (e.g., waterborne) based on past experiences with earlier
versions of the technology. The high rate of technological change results in
products that can fill a new performance niche in a very short space of time.
To monitor developments in technologies for organic coatings, applicators
need to read current literature, i.e., trade magazines or scientific journals. Good
reference journals and magazines include:
Journal of Coatings Technology
Surface Coatings International
American Paint and Coatings Journal
Modern Paint and Coatings
Metal Finishing
In addition, proceedings from scientific conferences can provide access to
information about the latest in research. An example is the proceedings of the
Pollution Prevention Conference on Low- and No-VOC Coatings
Technologies (EPA-600/R-94-022, February, 1994) that was sponsored by
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the U.S. Environmental Protection Agency.
Recently published books can provide insight into the direction of new
coatings research. Other sources include trade associations that can direct the
applicator to both literature and companies that are researching or developing
new coatings technologies. A list of trade associations appears in section five
of this guide.
Emerging Technologies This section describes three emerging cleaner technologies for paints and
coatings application:
» Vapor permeation or injection-cured coatings
» Supercritical carbon dioxide as solvent
* Radiation-induced, thermally-cured coatings
VAPOR INJECTION CURE COATINGS
Description Vapor injection curing (VIC) is a newly commercialized process that uses an
amine vapor catalyst for rapid coating polymerization (Railway, 1993;
Blundell and Bryan, 1991; Cassil, 1994). Two-component urethane coatings
contain a blocked accelerator that is activated during coating application with
an amine vapor catalyst. The amine vapor is made by an amine generator in
a predetermined concentration and dispersed in an air stream channel in the
spray gun. The coating material and catalyst are mixed as they leave the spray
gun. This technology is a "high solids" coating system because the coating
still uses solvent in the formulation. However, ease of use and production
efficiency arising from the rapid cure times provide reasons to use this two-
component technology rather than low solids or high solids air or bake coating
systems.
VIC can produce a variety of finishes with outstanding urethane performance
characteristics, including excellent chemical, solvent, and stain resistance; high
humidity and water resistance; high mar and abrasion resistance; and excellent
color and gloss retention. Pencil hardness can be achieved in 15 minutes to 45
minutes, with no baking. Dry times can be slashed from 8 hours to 1 hour,
without affecting pot life.
The coatings can be used on a broad range of substrates, including plastic,
steel, aluminum, wood, and castings. Heat sensitive parts such as
thermoplastics and thermosets are ideally suited to the low temperature cure
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of used with VIC. Other advantages of VIC include increased productivity
(resulting from faster handling), decreased operating costs (caused by lowered
energy consumption), decreased space for paint curing area, reduced rejects
(caused by uncured paint), and reduced recoating and tape time for multiple
coats and colors.
VIC is compatible with most conventional, air-assisted airless, electrostatic,
and HVLP spray equipment. Electrostatic equipment might need to be
modified to accommodate the amine generator. Some types of spray guns
might have rubber or plastic seals that degrade when exposed to the amine
(dimethylethanolamine). Air-assisted airless spray guns have been used for
some time and provide excellent results. The amine catalyst generator is made
of aluminum for light weight and mobility, and uses dried and filtered air at 90
psi to 120 psi. Capacity is limited to two spray guns. Some solvent might be
required to clean up unreacted resin (Pilcher, 1988).
REFERENCES Ballway, Bill. 1993. Vapor injection technology for no-bake curing of
urethanes. Metal Finishing, January, p. 62.
Blundell, D., and H. H. Bryan. 1991. Vapor injection cure of two pack
polyurethane. Journal of the Oil Chemists and Colourists Association.
1991(3):98
Cassil, Linda. 1994. New technology speeds curing of urethanes. Metal
Finishing. May. pp. 33-35.
SUPERCRITICAL CARBON DIOXIDE AS SOLVENT
Description Supercritical C02 fluid can be used to replace organic solvents in conventional
coating formulations. Union Carbide has developed a system, marketed under
the trade name UNICARB, which replaces organic solvents for many liquid
coatings. Supercritical carbon dioxide is C02 gas that has been heated above
its critical temperature of 88 °F and then compressed to approximately 1100
psi until its density approaches that of a liquid. This fluid is similar in
character to organic solvents and can be used to replace solvents in paint
formulations, reducing VOC levels by up to 80 percent. The CO2 solvent is
compatible with high molecular weight resins and existing painting facilities
and procedures, enabling finishers to use solvent-borne resin formulations
while substantially reducing VOC emissions.
Application of Supercritical C02 solvent coatings requires investment in new
equipment for paint mixing, handling, and spraying. Supercritical C02
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proportioning and supply units are available from at least one commercial
supplier. The unit mixes coating concentrates and CO2 to produce a coating
with the required viscosity (see Figure 10). The coating then is supplied to a
specially designed spray gun. Coating/solvent mixes are applied in the same
way as conventional paint (Nordson, 1991).
In 1991, five coating formulators were licensed to develop, manufacture, and
market UNICARB systems. These formulators are Akzo (automotive
components, furniture), BASF (automotive), Guardsman (furniture), Lilly
(furniture, plastics, heavy equipment), and PPG Industries (automotive, heavy
equipment) (MP&C, 1991).
REFERENCES Busby, D. C., C. W. Glancy, K. L. Hoy, C. Lee, and K. A. Nielsen. 1991.
Supercritical fluid spray application technology: a pollution prevention
technology for the future. Journal of the Oil Chemists and Colourists
Association. 1991(10):362
Miller, Wayne Paul and Tom Morrison. 1994. Supercritical fluid spray
application of low pollution coatings for plastic substrates. In Proceedings:
Pollution Prevention Conference on Low- and No-VOC Coating
Technologies. U.S. Environmental Protection Agency. Air and Energy
Engineering Research Laboratory. Organics Control Branch. Research
Triangle Park, NC. EPA-600/R-94-022. February, 1994.
MP&C. 1991. Supercritical CO2 as a solvent: update on Union Carbide's
process. Modern Paint and Coatings. June, p. 56.
Nordson. 1991. UNICARB System Supply Units. Amherst, Ohio. Product
information.
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Section Three
Figure 10
Supercritical Carbon Dioxide Spray Apparatus
Coating
Material
Mixing
Valve
Spray Gun
CO,
Source: Eastern Research Group, Inc.
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Section Three
RADIATION-INDUCED THERMALLY-CURED COATINGS
Description
REFERENCES
Infrared, microwave, laser, or radio-frequency radiation can be used to heat a
fluid coating and induce curing by thermal mechanisms. The curing reaction
is essentially similar to conventional curing in a convection oven, except that
heat is supplied by radiation (Paul, 1986; Poullos, 1991).
Laser heating applied by a robotic system produces accurate heat input for
rapidly curing thermoplastic or water-based coatings. The laser fusion system
originally was designed to cure fluorocarbon thermoplastics such as
polytetrafluoroethylene. Curing of other powder and water-based coatings is
currently being tested.
Paul, Swaraj. 1986. Surface Coatings Science and Technology. John Wiley
& Sons, New York, New York.
Poullos, Mark. 1991. Laser Curing of Finishing Systems. Pigment and Resin
Technology. 20(3): 6. March.
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Section Four
SECTION FOUR
POLLUTION PREVENTION STRATEGY
Introduction The organic coatings industry is affected primarily by guidelines or regulations
governing volatile organic compound (VOC) content in liquid coatings. VOC
regulations are being developed at the federal level by the U.S. EPA for a
number of categories of sources under the 1990 CAAA. In addition,
regulatory authorities in several states have developed or are developing their
own VOC standards. California has long been a leader in regulations to limit
solvent or VOC content in organic coatings, and other states have since
developed or are developing regulations.
Current VOC standards for coatings vary depending on the particular industry
in question. Prior to 1970, the VOC content of most paint was well above 600
g VOC/1 (5 Ib/gal). Current major industrial paints now are limited to
approximately 420 g VOC/1 (3.5 Ib/gal) of VOC. Stricter legislation in the
future will reduce these levels further.
VOC reduction strategies can be pursued by either the coatings manufacturer
or the coating user. Manufacturer strategies differ from user strategies.
Manufacturers can work on reformulating existing coatings to reduce VOCs;
this is commonly done with high solids coatings. Coatings manufacturers also
can research or develop new coatings technologies which are inherently lower
VOC or zero VOC. Powder coating technology is an example of a zero VOC
coatings system. Various waterborne systems may also approach zero VOC
or near-zero VOC content. Manufacturers are investigating the possibilities
for low or zero VOC technologies to replace present solvent-based coating
systems. Their technical and commercial potential is assessed and decisions
are made on whether to produce that coating system for sale and where (see,
for example, Randall, 1994).
Organic coating users are able to influence pollution prevention through their
choice of coating technology and in-house practices. Industries that currently
use coatings with high levels of solvents should investigate the possibilities
that exist with low solvent, low VOC coatings because of current and future
environmental regulations, liability issues, sales potential (consumer demand),
advertising possibilities and environmental friendliness. This Cleaner
Technologies Guide is directed toward coating users, rather than
manufacturers.
Because VOC limits vary from industry to industry, and since the regulations
are often in a state of flux, strict compliance methodologies cannot be provided
for each industry. Instead, a general strategy is outlined which allows the
industry to examine the issues and formulate a plan to move towards cleaner
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Section Four
pollution prevention technologies. The strategy is presented as a list of
questions which the user would answer.
Strategy Questions that a coating user might ask include:
1. Examine current product and associated coating.
* Is the performance of the current coating satisfactory?
» Are there other needs not being met?
» What types of performance from the coating are desired?
What is the product/substrate made of?
Will the present coating technology suffice or is change necessary or
desirable?
2. Examine state and federal regulations.
> What are the current regulations concerning coatings?
> Are there exemptions or certain requirements for your facility?
> What are likely or possible future regulations concerning coatings?
3. Examine alternative coating technologies.
* What coating technologies fit the above performance and regulatory
requirements?
Will the technology meet expected future environmental regulations?
Will application equipment need to be changed?
What type of drying and curing? Will an oven need to be installed?
What will happen to production line speed? Will the new coating
slow it down?
How will the size of the labor force change?
What learning curve will the staff face?
What type of surface preparation techniques are necessary? Are
existing techniques satisfactory?
Are capital costs for equipment high or low?
Are operating costs higher or lower?
4. Other factors to consider in changeover:
> What is the impact of a changeover on immediate operations?
> What is the impact on the long-term efficiency of operations?
» What is impact on occupational health and safety?
What is the potential for reduced likelihood of liability suits?
5. Search for information.
» Where to find information on these technologies?
- vendors
- trade associations
- state regulators/technical assistance centers
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Section Four
- federal regulators (EPA, OSHA)
- environmental information clearinghouses
- environmental groups
Particular Technologies A number of coating technologies are already known to have very low or zero
VOC emissions and these can be recommended to a coating user for further
research or possibly immediate use if the technology suits their needs.
Powder Coating
Powder coating has the twin benefits of zero VOC emissions and high transfer
efficiency, while still producing a variety of films with excellent properties.
Restrictions on the use of powder come mainly from the need to heat the
substrate to high temperatures (500 °F) to melt the powder, its applicability to
metallic substrates only, and the small sizes of parts that can be placed in
baking ovens.
Waterborne Coatings
Many emulsion coatings are formulated with extremely little or zero solvent.
Water-reducible coatings usually contain some solvent but less than high
solids coatings. Some water reducible coatings allow overspray to be
recovered and recycled, effectively raising the transfer efficiency.
Electrodeposition technology is not only zero VOC, but has a very high
transfer efficiency as well, making greater use of the coating.
High Transfer Efficiency Spraying
This technology can be readily applied to most, if not all, spray shops using
liquid or powder coatings. Powder spraying by electrostatic or tribocharging
is already inherently efficient, however improvements may be made by
organizing and orienting the parts for greater coverage. The transfer efficiency
of liquid coatings can be increased greatly if the current spray system is a high
pressure air spraying apparatus (see, for example, Ewert et al. (1993); van
Bieman and Oldenburger (1993)). Electrostatic spraying apparatus is
available for both solvent-borne and waterborne coatings. Airless and air-
assisted airless systems tend to have higher transfer efficiency than high
pressure air. High volume, low pressure (HVLP) spray apparatus increases
transfer efficiency by reducing the velocity of the coating so that less coating
"bounces" off the substrate. Rotary bells and disks, combined with
electrostatic charge allow efficient atomization and transfer of coatings,
especially high solids. Transfer efficiency can also be raised by reclaiming
coating material that has not successfully adhered to the substrate. Powder
reclamation systems are available commercially, and recently systems like
"Ecopaint" with water wash spray booths and ultrafiltration units reclaim
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overspray of waterbome coatings.
REFERENCES Ewert, Stephen A., Steven R. Felstein and Thomas Martinez. 1993. Low-cost
transfer-efficient paint spray equipment. Metal Finishing. August, pp. 59-
64.
Randall, Paid M. 1994. Pollution prevention opportunities in the manufacture
of paint and coatings. In Proceedings: Pollution Prevention Conference on
Low- andNo-VOC Coating Technologies. U.S. Environmental Protection
Agency. Air and Energy Engineering Research Laboratory. Organics Control
Branch. Research Triangle Park, NC. EPA-600/R-94-022. February, 1994.
van Bieman, W.F. and J.R, Oldenburger. 1993. More efficient spraying
techniques. Journal of the Oil Chemists' and Colourists' Association.
August, p. 318.
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Section Five
SECTION FIVE
INFORMATION SOURCES
Trade Associations
The following is a list of trade, professional, and standard-setting organizations that can provide technical and
other support on various issues related to cleaner technologies for the organic coating industry. Readers are
invited to contact these associations and request their assistance and to help identify one or more companies
that could provide the desired technological capabilities.
American Institute of Chemical
Engineers (AIChE)
345 E. 47th St..
New York, NY 10017
212/705-7338
212/752-3297 FAX
American National Standards
Institute (ANSI)
11 West 42nd St., 13th Floor
New York, NY 10036
212/642-4900
212/398-0023 FAX
Architectural Spray Coalers
Association (ASCA)
230 W. Wells, Ste. 311
Milwaukee, WI 53203
414/273-3430
Association of Industrial
Metallizers, Coalers and
Laminators (AIMCAL)
21 IN. Union SI., Ste. 100,
Alexandria, VA 22314
703/684-4868
703/684-4873 FAX
American Society for
Nondestructive Testing (ASNT)
1711 Arlington Lane
P.O. Box 28518
Columbus, OH 43228-0518
614/274-6003
800/222-2768
614/274-6899 FAX
American Chemical Society
(ACS)
11-55 16thSt.,N.W
Washington, DC 20036
202/872-4600
202/872-6067 FAX
Association of Metal Sprayers
(AMS)
5 Keals Rd.
Slratford upon Avon
Warwickshire CV37 7JL,
England
(789)299661
European Confederation of
Painl, Printing Ink and Artists'
Colours Manufacturers
Associations (CEPE)
4, ave. E. Van Nieuwenhuyse
B-l 160 Brussels, Belgium
(2) 6767480
American Society for Testing
Materials (ASTM)
1916 Race St.
Philadelphia, PA 19103-1187
215/299-5400
215/977-9679 FAX
American Society for Quality
Control (ASQC)
310 W. Wisconsin Ave.
Milwaukee, Wl 53203
414/272-8575
414/272-1734 FAX
Association of Finishing
Processes of the Society of
Manufacturing Engineers
P.O. Box 930
One SME Dr.
Dearborn, MI 48121
313/271-1500
European Technical Association
for Protective Coatings
(ETAPC)
Rijenlanddreef 19, bus 5
B-2170 Merksem, Belgium
(3) 6463373
Page 89
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Section Five
European Coil Coating
Association
47, rue Montoyer
B-1040 Brussels, Belgium
(2)5136052
Halogenated Solvents Industry
Alliance (HSIA)
2001LSt.,N.W.,Ste. 506
Washington, DC 20036
202/775-2790
202/223-7225 FAX
National Coil Coalers
Association (NCCA)
401 N. Michigan Ave., Chicago,
IL 60611 -4267
312/644-6610
312/321-6869 FAX
National Association of Pipe
Coating Applicators (NAPCA)
Commercial Natl. Bank Bldg.,
8th Fl. 333 Texas St.,
Shreveport,LA71101-3673
318/227-2769
318/222-0482 FAX
Powder Coating Institute (PCI)
1800 Diagonal Rd., Ste. 370
Alexandria, VA 22314
703/684-1770
Federation of Societies for
Coatings Technology (FSCT)
492 Norristown Rd.
Blue Bell, PA 19422
215/940-0777
215/940-0292 FAX
International Committee to
Coordinate Activities of
Technical Groups in the Coatings
Industry (ICCATCI)
34, chemin du Halage
F-95540 Mery-sur-Oise, France
(1)48675224
National Paints & Coatings
Association (NPCA)
1500 Rhode Island Ave., NW
Washington, DC 2000
202/462-6272
National Spray Equipment
Manufacturers Association
(NSEMA)
550 Randall Rd.
Elyria,OH44035
216/366-6808
216/892-2018 FAX
Radtech International
60 Revere Drive
Suite 500
Northbrook, IL 60062
708/480-9576
Federation of the Associations of
Technicians of the Paint, Varnish,
Enamel and Printing Ink
Industries of Continental Europe
(FATIPEC)
28, rue St. Dominique
F-75007 Paris, France
(1)48675224
National Association of Metal
Finishers (NAMF)
401 N. Michigan Ave.
Chicago, IL 606114267
312/644-6610
312/321-6869 FAX
National Paint Distributors
(NPD)
701 Lee St., Ste. 1020
DesPlaines,IL60016
708/297-6400
Paint, Body and Equipment
Association (PBEA)
c/o Martin Fromm and Assoc.
9140WardPky.
Kansas City, MO 64114
816/444-3500
816/444-0330 FAX
Roof Coatings Manufacturers
Association (RCMA)
6000 Executive Blvd., Ste. 201
Rockville, MD 20852-3803
301/230-2501
301/881-6572 FAX
Page 90
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Section Five
Society of Automotive Engineers Society of Manufacturing Society of Plastics Engineers
(SAE) Engineers (SME) (SPE)
400 Commonwealth Dr. One SME Dr., P.O. Box 930 MFairfieldDr.
Warrendale, PA 15096 Dearborn, MI 48121 Brookfield, CT 06804-0403
412/772-7129 313/271-1500 203/775-0471
412/776-2103 FAX 313/271-2861 FAX 203/775-84 90 FAX
Steel Structures Painting Council Transocean Marine Paint
(SSPC) Association (IMPA)
4400 5th Ave. Prins Hendrikkade 14
Pittsburgh, PA 15213-2683 NL-3071 KB Rotterdam,
412/268-3327 Netherlands
412/268-7048 FAX (10)4134477
'U.S. Government Printing Office: 1994 550-001/00201 Page 91
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