GUIDE TO
CLEAN TECHNOLOGY
ORGANIC COATING
REPLACEMENTS
May 15, 1992
United States Environmental Protection Agency
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NOTICE
This Guide to Clean Technology: Organic Coating Replacements summarizes
information collected from U.S. Environmental Protection Agency programs, peer
reviewed journals, industry experts, vendor data, and other sources. The original
Quality Assurance/Quality Control (QA/QC) procedures for the reports and
projects summarized in this guide range from detailed, reviewed Quality Assur-
ance Project Plans to standard industrial practice. Publication of the guide 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 commer-
cial products constitute endorsement or recommendation for use.
This document is intended as advisory guidance in identifying new approaches
for pollution prevention through organic coating replacements. Final selection of
a technology will be shop- and process-specific and, therefore, will be done by
the individual users of organic coatings. Compliance with environmental and
occupational safety and health laws is the responsibility of each individual
business and is not the focus of this document.
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FOREWORD
Today's rapidly developing and changing technologies and industrial products
and practices frequently carry with them the increased generation of materials
that, if improperly dealt with, can threaten both public health and the environ-
ment. The U.S. Environmental Protection Agency (EPA) is charged by Congress
with protecting the Nation's land, air, and water resources. Under a mandate of
national environmental laws, the agency strives to formulate and implement
actions leading to a compatible balance between human activities and the ability
of natural systems to support and nurture life. These laws direct the U.S. EPA to
perform research to define our environmental problems, measure the impacts,
and search for solutions.
Reducing the generation of hazardous solvents at the source or recycling the
wastes on site will benefit industry by reducing disposal costs and lowering the
liabilities associated with hazardous waste disposal.
Publications in the U.S. EPA series, Guides to Pollution Prevention, provide an
overview of several industries and describe options to minimize waste in these
industries. Their focus is on the full range of operations in existing facilities.
Many of the pollution prevention techniques described are relatively easily
implemented into current operations without major process changes.
This Guide to Clean Technology: Organic Coating Replacements summarizes
new commercially available and emerging technologies that prevent and/or
reduce the production of hazardous materials during coating replacement. The
technologies described in this document and other documents in this series are
generally "next generation" clean technologies that often, but not always,
represent relatively major process changes, high levels of training, and high
capital investments compared to the technologies described in the Guides for
Pollution Prevention. The waste minimization techniques characterized in the
Guides for PoRution Prevention should be considered and implemented first.
Although some of the clean technologies described herein could be inserted into
current operations, they should be considered primarily for major plant expan-
sions or new grass roots facilities.
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CONTENTS
Section 1
Overview
Section 2 8
Available Technologies
Powder Coatings 12
High Solids Coatings 24
Water-Based Coatings 30
Ultraviolet (UV) Radiation-Cured Coatings 40
Section 3 44
Emerging Technologies
Electron Beam (EB)-Cured
Coatings 48
Radiation-Induced Thermally
Cured Coatings 49
Two-Component Reactive Liquid
Coatings 50
Water-Based Temporary
Protective Coatings 51
Vapor Permeation- or Injection-
Cured Coatings 52
Supercritical Carbon Dioxide
as Solvent 53
Section 4 54
Information Sources
References 54
Trade Associations 55
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SECTION 1
OVERVIEW
What Is Clean
Technology?
Why Use Organic
Coatings?
A dean technology is a source reduction or recycling method ap-
plied to eliminate or significantly reduce hazardous waste genera-
tion. Source reduction includes product changes and source con-
trol. Source control can be further characterized as input material
changes, technology changes, or improved operating practices.
Pollution prevention should emphasize source reduction technolo-
gies over recycling but, if source reduction technologies are not
available, recycling is a good approach to reducing waste genera-
tion. Therefore, recycling should be used where possible to mini-
mize or avoid waste treatment requirements when source reduction
options have been evaluated and/or implemented.
The clean technology must reduce the quantity, toxicity, or both of
the waste produced. !t is also essential that final product quality be
reliably controlled to acceptable standards. In addition, the cost of
applying the new technology relative to the cost of similar technolo-
gies needs to be considered.
The three major classifications of organic coatings based on end
use are:
4 Architectural coatings
* Industrial finish coatings
4 Industrial maintenance coatings.
Architectural coatings are applied on site to interior or exterior
surfaces of various buildings. They are applied for protection and
appearance, and they cure at ambient conditions.
Industrial finish coatings are applied to factory-made articles during
manufacture. 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. Alternative clean coating
materials are available for all three types of use.
Paint and other coatings are applied to surfaces to enhance corro-
sion resistance, improve appearance, or both. Examples among the
many industries that apply coatings include manufacturers of:
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Overview
4 Automobiles
* Aircraft
4 Appliances
* Wood products.
The main functions of automotive coatings are appearance, durabili-
ty, and corrosion protection. Typical automotive coatings use an
undercoat or primer to give corrosion protection and improve dura-
bility. The topcoats are formulated to give the desired color and
gloss. In some cases, the topcoat is of two or more different layers:
a low-solids polyester coat to give the color covered 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.
The main functions of aircraft coatings are to resist corrosion, fluids
and fuels, erosion, temperature extremes, weathering, and impact.
Coatings may also assist in providing protection for lighting strike.
Coatings must also give the desired appearance. Appearance may
be entirely cosmetic or, in the case of combat aircraft, serve as
camouflage. Aircraft finishes may be applied over aluminum,
titanium, composite, or other substrates.
Appliances are often referred to as "white goods" due to the tradi-
tional color of the coating applied. However, a wide variety of
colors are now applied to appeal to consumer tastes. Coatings are
applied to protect the underlying metal from water, salt, detergent,
and other common corrosive agents at temperatures in the range of
about 0°C to 100°C. The substrate is typically steel sheet.
Wood products such as furniture, siding, and doors are coated to
increase durability and improve appearance. Exterior coatings must
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 discussed above, coatings are
used in a wide variety of other industries and applications. The
coatings may provide temporary or long-term protection of wood,
metal, and plastic surfaces. This wide range of applications indi-
cates the cross-industry applicability of clean coating material
technology.
Pollution
Problem Classical organic coating materials are dilute solutions of organic
resins, organic or inorganic coloring agents, additives, and extend-
ers dissolved in an organic solvent. The organic solvent gives the
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Overview
coating fluid the necessary viscosity, surface tension, and other
properties to allow application of a smooth layer of liquid coating
solution.
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 dry film.
Environmental concerns and increasing costs of organic chemicals
and transition metals are leading to changes in the formulation of
organic coatings. Coating makers and users are seeking alternative
materials to reduce or eliminate waste of hazardous solvents and
paint residues, particularly coatings using pigments containing metal
compounds.
Typical coating solvents include methyl ethyl ketone, methyl isobutyl
ketone, toluene, and xylene. The coloring agents can be inorganic
pigments containing hazardous metals such as cadmium, chro-
mium, and lead. Mercury chemicals have been used as a paint
preservative, although this use-is declining. This guide describes
application of clean technologies for replacement coating materials
that eliminate or reduce solvent use. In many cases, the new
coating also reduces paint waste, which in turn reduces waste of
hazardous metals.
Solution Coating technology relies on covering a substrate material with an
organic film having the desired protective, mechanical, optical, ag-
ing, and adhesion properties. Conventional organic coating tech-
nology uses dilute solutions of alkyd, polyester, epoxy, polyure-
thane, acrylic, vinyl, or other resins in a volatile organic solvent.
Not too many years ago coating materials relied on organic solvents
to promote desired flow characteristics so the coating could be
applied. The solvent would then evaporate allowing the resins to
polymerize and cure to form the dry coating. Recent years have
seen expanded use of clean technologies.
Clean technologies based on physical methods now exist for materi-
als that reliably apply coatings but which contain little or no volatile
organic solvent. There are four general classes of clean technology
for organic coating materials:
4 100% dry solids materials that completely eliminate the solvent
by using a dry resin formulation
4 100% reactive liquids that use a liquid coating material but do not
rely on any volatile organic solvent
4 Water-dispersed or water-soluble polymer systems that substitute
water for some or all of the volatile organic solvent
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Overview
4 High solids polymer systems that reduce the amount of organic
solvent needed by increasing the concentration of reactive resin
in the solvent.
The clean technologies can also prevent pollution by increasing the
efficiency of use for the coating, thus reducing paint waste.
What's In
This Guide?
Other Questions
fckbout
Tnvestment Decisions
Who Should Use
This Guide?
This application guide describes clean technologies that can re-
duce waste by the use of alternative organic coating materials.
The objectives of this application guide are to help identify potential-
ly viable clean technologies to reduce waste by using alternative
organic coating materials and to provide resources for obtaining
more detailed engineering information about the technologies. We
address the following specific questions:
4 What alternative organic coating technologies are available or
emerging that reduce or eliminate pollution?
4 Under what circumstances might one or more of these alternative
coating s_ystems be applicable to your operations?
4 What pollution prevention, operating, and cost benefits could be
realized by adapting the technology?
Other aspects affecting the decision include:
* Might new pollution problems substitute for the old?
4 Are tighter and more complex process controls needed?
* Will product quality and operating rates be affected?
* Will new operating or maintenance skills be needed?
+ What are the overall capital and operating cost implications?
This guide covers several clean coating replacement systems that
are applicable under different sets of product and operating condi-
tions. If one or more are sufficiently attractive for your operations,
the next step would be to contact vendors or users of the technolo-
gy to obtain detailed engineering data and make an in-depth evalua-
tion of its potential for your plant.
This guide is intended for plant process and system design engi-
neers and for personnel responsible for process improvement and
process design. Process descriptions within this guide allow engi-
neers to evaluate options so that alternative coating materials can
be considered for existing plants and factored into the selection of
new coating materials.
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Overview
Sufficient information is presented to select one or more candidate
technologies for further analysis and in-plant testing. This guide
does not recommend any technology over any other. It presents
concise summaries of applications and operating information to sup-
port preliminary selection of clean technology candidates for testing
in specific processes. The level of detail allows identification of
possible technologies for immediate application to eliminate or
reduce waste production.
A list of keywords is provided to help you quickly scan the technolo-
gies covered.
Keywords
Clean Technology
Pollution Prevention
Source Reduction
Source Control
Recycling
Paint
Coating
Powder coating
Removal
Stripping
Depainting
Debonding
Powder Coatings
High Solids Coatings
Water-Based Coatings
Ultraviolet (UV) Radiation-
Cured Coatings
Electron Beam (EB)-Cured
Coatings
Radiation-Induced, Thermally
Cured Coatings
Two-Component Reactive
Liquid Coatings
Water-Based Temporary
Protective Coatings
Vapor Permeation- or
Injection-Cured Coatings
Supercritical CO2 as Solvent
Summary of
Benefits
The clean technologies described in this guide are divided into two
groups based on their developmental maturity commercially
available technologies and emerging technologies in advanced pilot
plant testing.
Table 1 summarizes the pollution prevention, operational, and
economic benefits of organic coating replacement technologies.
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Overview
You may wish to scan this summary table to select those clean
technologies that best fit your operations and needs. Detailed
discussions of these benefits and operational aspects for each
clean technology are provided in the next two sections of this
document.
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Table 1. Summary of Benefits of the Clean Technologies for Organic Coating Replacements
Benefits
Available Technologies Emerging Technologies
Powder
Coatings
High Solids
Coatings
Water-
Based
Coatings
Ultraviolet
(UV) Radia-
tion-Cured
Coatings
Electron
Beam (EB)-
Cured
Coatings
Radiation-
Induced
Thermally
Cured Coatings
Two-Component
Reactive Liquid
Coatings
Water-Based
Temporary
Protective
Coatings
Vapor
Permeation- or
Injection-Cured
Coatings
Supercritical
Carbon Dioxide
as Solvent
Pollution Prevention:
Eliminates solvent in
coating
Reduces solvent in
coating
Reduces solvent for
cleaning
Operational:
Easy color blending
Easy color change
Can apply thick coat
Can apply thin coat
'
Economic:
Relatively low or
medium capital cost
Relatively low or
medium skill to operate
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SECTION 2
AVAILABLE TECHNOLOGIES
How to Use the
Summary Tables
Descriptive
Aspects
Operational
Aspects
Four available organic coating replacement technologies are evalu-
ated in this section:
* Powder coatings
* High solids coatings
* Water-based coatings
4 Ultraviolet (UV) radiation-cured coatings.
Tables 2 and 3 summarize descriptive and operational aspects of
these technologies. They contain evaluations or annotations de-
scribing each available clean technology and give users a compact
indication of the range of technologies covered to allow preliminary
identification of those technologies that may be applicable to specif-
ic situations. Readers are invited to refer to the summary tables
throughout this discussion to compare and contrast technologies.
Table 2 describes each available clean technology. It lists the
Pollution Prevention Benefits, Reported Applications, Opera-
tional and Product Benefits, and Hazards and Limitations of
each available clean technology.
Table 3 shows key operating characteristics for the available tech-
nologies. These characteristics serve to qualitatively rank the clean
technologies relative to each other. The rankings are estimated
from descriptions and data in the technical literature.
Process Complexity is qualitatively ranked as "high," "medium," or
"low" based on such factors as the number of process steps in-
volved and the number of material transfers needed. Process
Complexity is an indication of how easily the new technology can
be integrated into existing plant operations. A large number of
process steps or input chemicals, or multiple operations with com-
plex sequencing, are examples of characteristics that would lead to
a high complexity rating.
The Required Skill Level of equipment operators also is ranked as
"high," "medium," or "low." Required Skill Level is an indication of
the relative level of sophistication and training required by staff to
operate the new technology. A technology that requires the opera-
tor to adjust critical parameters would be rated as having a high skill
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Available Technologies
requirement. In some cases, the operator may be insulated from
the process by complex control equipment. In such cases, the
operator skill level is low but the maintenance skill level is high.
Table 3 also lists the Waste Products and Emissions from the
available clean technologies to indicate tradeoffs in potential pollut-
ants, the waste reduction potential of each, and compatibility with
existing waste recycling or treatment operations at the plant.
The Capital Cost column provides a preliminary measure of pro-
cess economics. It is a qualitative estimate of the initial cost impact
of the engineering, procurement, and installation of the process and
support equipment. Due to the diversity of data and the wide
variation in plant needs and conditions, it is not possible to give
specific cost comparisons. Cost analyses must be plant-specific to
adequately address factors such as the type and age of existing
equipment, space availability, throughput, product type, customer
specifications, and cost of capital. Where possible, sources of cost
data are referenced in the discussions of each clean technology.
The Energy Use column provides data on energy conversion
equipment required for a specific process. In addition, some gener-
al information on energy requirements is provided.
Table 3 also lists References to publications that will provide further
information for each available technology. These references are
given in full in Section 4.
The text further describes pollution prevention benefits, reported
applications, operational and product benefits, and hazards and
limitations for each available technology. Technologies in earlier
stages of development are summarized to the extent possible in
Section 3, Emerging Technologies.
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Table 2. Available Clean Technologies f
anlc Coating Replacements: Descriptive Aspects
Available
Technology
Pollution Prevention
Benefits
Reported
Applications
Operational and Product Benefits
Hazards and Limitations
Powder
Coatings
100% reactive solid
Eliminates solvent use
and exposure of work-
ers to solvents
Reduces need lor dis-
posal ol solid paint
waste
Reduces fire and ex-
plosion hazards
Applied to general appliances,
automotive, and general industrial
finishing such as:
- steel
- aluminum
- zinc and brass castings
Can apply thick coatings in one application
Lack of volatile solvent means little air flow needed
Reduced energy use for heating makeup air
No mixing or stirring needed
Cleanup requires no solvent
Allows coating with polymers that are not amenable
to solution coating techniques
Efficient material use (near 100% transfer efficiency)
Requires handling of heated parts
1 For electrostatic application systems,
- the part must be electrically conductive or be covered with
an electrically conductive primer
- parts with complex shapes are difficult to coat
1 Some difficulty in applying thin coatings
> Difficult to incorporate metal flake pigments
1 Requires specialized equipment or extra effort to make color
changes
High Solids
Coatings
Solvent-based with
high resin concentra-
tion
Reduces solvent in the
coating, but solvent
still needed for clean-
up
Same as conventional coatings;
resins available in formulations in-
clude:
- saturated polyesters
- alkyds
- acrylics
- polyurethane
- epoxy
Used on zinc-coated steel doors,
miscellaneous metal parts
Reduced solvent concentrations in the coating, thus
reducing the environmental, odor, and safety prob-
lems caused by solvents
Compatible with conventional and electrostatic ap-
plication equipment and techniques
Lower solvent loadings allow reduced air flow in
curing ovens and work spaces, thus decreasing
energy needed for heating
' Solvent use not completely eliminated
1 Shorter pot life than conventional coalings
Water-Based
Coatings
Water-based with low
solvent concentration
Eliminates or reduces
solvent use
Water used for
cleanup
Wide range of application
Coating formulations include acry-
lics; colloids; amine-solubilized,
carboxyl-terminated alkyds; and
polyesters
Used for architectural trade finish-
es, wood furniture, and damp con-
crete surfaces
Low odor levels
Easy to clean (uncured coating can be cleaned up
with water)
Reduce solvent concentrations
Existing application equipment (nonelectrostatic) can
be used with most water-based coatings
Reduced air flow in curing ovens and work spaces
decreases energy needed for heating
Coating flow properties and drying rates can change with hu-
midity affecting coating application
Sensitive to humidity, and thus require humidity control in
application and curing areas
High surface tension of water can cause poor coating flow
characteristics
Special equipment needed to allow electrostatic application
Water in the formulation can cause corrosion of coating stor-
age tanks and transfer piping, and "flash rusting" of metal
substrates under the coating
Most require careful cleaning of the substrate to ensure oil and
grease are removed
> Resins in contact with water degrade, reducing shelf life
' Susceptible to foaming due to surfactants
Ultraviolet
(UV)
Radiation-
Cured
Coatings
100% reactive liquid
Eliminates or reduces
solvent in the coating,
but solvent still need-
ed for cleanup
Wood
Some metal applications
Filler for chipboard
Used for 'wet look' finishes
Efficient material use (near 100% transfer efficiency)
Lack of volatile solvent means little air flow needed
Low-temperature processing (reduced energy use
for heating makeup air)
Rapid curing
Styrene volatility
Yellow color
High capital cost of equipment
Limited to thin coatings, particularly with pigments present
Typically best applied to flat materials
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Table 3. Available Clean Technologies for Organic Coating Replacements: Operational Aspects
Available
Technology
Type
Powder Coatings
High Solids Coatings
Water-Based Coatings
Ultraviolet (UV) Radiation-
Cured Coatings
Process
Complexity
Medium
Low
Medium
High
Required
Skill
Level
Medium
Medium
Medium
High
Waste Products and
Emissions
Uncured powder can be col-
lected lor reuse
Overspray loss similar to that
of conventional coatings
Overspray loss similar to that
of conventional coatings
Amenable to electrocoating,
which gives very high resin
use
Unreacted overspray can be
collected for reuse
Capital
Cost
High
Low
Medium
High
Energy
Use
Low
Medium
Medium
Low
References
Bowden, 1989
Crump, 1991
Fish, 1982
Hester and Nicholson, 1989
Ingleston, 1991
Maguire, 1988
Muhlenkamp, 1988
Robison, 1989
Dick, 1991
MP&C, 1988
Nelson, 1988
Paul, 1986
Pilcher, 1988
Smith, 1990
Dick, 1991
MP&C, 1988
Paul, 1986
Pilcher, 1988
Product Finishing, 1986
Richardson, 1988
Scharfenberger, 1989
Swanberg, 1990
Danneman, 1988
Dick, 1991
Keipert, 1990
Paul, 1986
Sun Chemical, 1991
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POWDER COATINGS
Pollution Prevention
Benefits
How Does
It Work?
12
Powder coating uses 100% reactive resin in dry powdered form,
thus eliminating the use of a volatile organic solvent and reducing
solvent exposure to workers and fire and explosion hazards. High
transfer efficiency and low coating waste reduce the amount of solid
paint waste requiring treatment or disposal.
Powder coating has two attractive features from both a cost and
pollution prevention standpoint:
* The total lack of solvent in the coating formulation
* The ability to apply essentially all of the coating to the substrate.
Material is applied to the surface as a dry powder and is then
melted or reacted to form the coating. There is no need for a vola-
tile solvent to provide a fluid medium. The powder is fluidized either
by air flow up through a bed or by an electrostatic airspray gun.
Because no solvent is needed to carry the coating, solvent evapora-
tion is eliminated. Further, the coating equipment can be cleaned
without the use of solvents.
For example, a conventional plant painting 12,000,000 ft2/yr with a
1.2-mil-thick coat will produce about 38 tons per year of VOC emis-
sions after a treatment system with a capture efficiency of 70%. A
powder coating system using electrostatic application of polyester-
urethane material will emit about 0.6 tons per year with no VOC
control equipment in use (Hester and Nicholson, 1989).
In powder coating, a coating film is formed by applying a layer of
dry powdered resin on the surface to be covered and then melting
the powder. Either thermoplastic powders such as cellulose acetate
butyrate, polyesters, polyamides, and polyolefins or thermosetting
powders such as epoxy resins, acrylics, and polyesters can be
applied. Thermoplastic powders are usually applied-with a fluidized
bed, whereas thermosetting powders are mainly applied by elec-
trostatic spray.
A thermoplastic powder coating is one that melts and flows when
heat is applied, but continues to have the same chemical composi-
tion once it cools and solidifies. Thermoplastic powders are based
on high-molecular-weight polymers that exhibit excellent chemical
resistance, toughness, and flexibility. These resins tend to be
difficult to grind to the consistent fine particles needed for spray
application, and they have a high melt viscosity. Consequently,
Available Technologies
-------
Powder Coatings
they are used mostly in thicker film applications and are applied
mainly by the fluidized bed application technique. Typical thermo-
plastic powder coatings include:
4 Polyethylene powders
* Polypropylene powders
4 Nylon powders
* Polyvinyl chloride powders
* Thermoplastic polyester powders.
Thermosetting powder coatings are based on lower molecular
weight solid resins. These coatings melt when exposed to heat,
flow into a uniform thin layer, and chemically cross-link within
themselves or with other reactive components to form a higher
molecular weight reaction product.
The final coating has a chemical structure different from that of the
basic resin. These newly formed materials are heat stable and,
after curing, do not soften back to liquid phase when heated.
Resins used in thermosetting powders can be ground into very fine
particles necessary for spray application and for applying thin, paint-
like coatings. Because these systems can produce a surface coat-
ing that is comparable to, and competes with, liquid coatings, most
of the technological advancements in recent years have been with
thermosetting powders. Thermosetting powders are derived from
three generic types of resins, i.e., epoxy, polyester, and acrylic.
From these three basic resin types, five coating systems are
derived. Epoxy resin-based systems are the most commonly used
thermosetting powders and are available in a wide range of formula-
tions (Hester and Nicholson, 1989).
Powder coating uses conventional equipment available from a wide
variety of vendors.
Why Choose
This Technology? Applications
Powder coating is applied in thick coatings of thermoplastic
materials or medium-thickness coatings of thermosetting materials
to substrates in a single operation.
Operating Features
The part being coated must be heated either before immersion in a
fluid bed or in an oven after the powder is applied. As a result, the
part must be of a size and shape to allow immersion coating or
heating in a curing oven. In all cases, the part must be heated
13
-------
Available Technologies
above the melting temperature of the resin and thus be able to
withstand temperatures of 90°C and higher.
Basic Function. Fluidized bed systems are used to apply coatings
of thermoplastic powders to thicknesses in the range of 10 to 30
mils. In fluidized bed application, a preheated solid substrate is
immersed in a fluidized bed of thermoplastic powder.
As indicated in the schematic diagram (Figure 1), air is introduced
at the bottom of a bed of powder to fluidize the mass. The sub-
strate temperature is adjusted to be higher than the melting point of
the resin so, as particles strike the hot surface, they melt and
coalesce to form a thin, continuous film on the substrate. As the
part cools, the powder solidifies to form a coating. The fluidized
bed method was the original method for applying powder coatings
and is still favored for heavy functional coatings.
Part to be
coated
Powder coating tank
Fluidized powder
Air distribution system
Source: Battelle
Figure 1. Powder Coating in a Fluidized Bed.
14
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Powder Coatings
Electrostatic applications in either a fluidized bed (Figure 2) or by
electrostatic spray (Figure 3) can achieve coatings thicknesses in
the range of 1 to 3 mils with thermosetting resins. Electrostatic
fluidized beds are limited to an effective depth of about 2 to 3 in so
that they are best suited to coating two-dimensional parts (Muhlen-
kamp, 1988).
Part to be
coated
Powder coating tank
Charged fluidized powder
Air distribution system
Source: Battelle
Figure 2. Powder Coating in an Electrostatic Fluidized Bed.
In electrostatic applications, the dry powder is applied to the unheat-
ed substrate as film of powder held in place by electrostatic forces.
The substrate or a primer coat must be electrically conductive. The
substrate, with powder coating, is then heated in an oven to melt
and cure the coating. Additional polymerization and cross-linking
occurs in the thermoplastic material during curing.
1/>(A
15
-------
en
Electrostatic
powder gun
Air
Venturl
Powder feed tank
Fluldized powder
Air distribution system
Part to be
coated
Charged N-.'".-
fluldlzed
powder
Source: Battelle
Figure 3. Schematic for Electrostatic Spray Gun for Powder Coating.
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Powder Coatings
With spray systems,-powder is supplied to the spray gun by the
powder delivery system. This system consists of a powder storage
container or feed hopper, a pumping device that transports a stream
of powder into hoses or fed tubes. A compressed air supply is
often used as a "pump" because it aids in separating the powder
into individual particles for easier transport. The powder delivery
system is usually capable of supplying 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, number of guns to be supplied, and volume of powder
to be sprayed in a given time period. Recent improvements in
powder delivery systems, coupled with better powder chemistries
that reduce clumping of the powder, have made possible the deliv-
ery of a very consistent flow of particles to the spray gun. Agitating
or fluidizing the powder in the feed hopper also helps prevent
clogging or clumping of the powder prior to its entry into the trans-
port lines.
Electrostatic powder guns function to shape and direct the flow of
powder; control the pattern size, shape7 and density of the powder
as it is released from the gun; impart the electrostatic charge to the
powder being sprayed; and control the deposition rate and location
of powder on the target. All spray guns can be classified as either
manual (hand-held) or automatic (mounted on a mechanical control
arm) the basic principles of operation of most guns are the same;
there is an almost limitless variety in the style, size, and shape of
spray guns. The type of gun chosen for a given coating line can,
thus, be matched to the performance characteristics needed for the
products being coated.
Traditionally, the electrostatic charge was imparted to the powder
particles by a charging electrode located at the front of the spray
gun. These "corona charging" guns generate a high-voltage, low-
amperage electrostatic field between the electrode and the product
being coated. The charge on the electrode is usually negative and
can be controlled by the operator. Powder particles, passing
through the ionized electrostatic field at the tip of the electrode
become charged and are thus directed by the electrostatic field.
The particles follow the field lines and air currents to the target
workpiece and are deposited on the grounded surface of the work-
piece. One drawback to the use of this type of gun is the difficulty
of coating irregularly shaped parts that have recessed areas of
cavities (that may be affected by Faraday cages) into which the
electrostatic field cannot reach. Because the powder particles are
directed by the presence of the field, insufficient powder may be
deposited on surfaces outside the reach of the field.
17
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Available Technologies
A relatively recent innovation in electrostatic spray guns is the tribo-
electric gun. The powder particles in a triboelectric gun receive an
electrostatic charge as a result of friction which occurs when powder
particles contact a solid insulator or conductor inside the delivery
hose and gun. The resulting charge is accomplished through the
exchange of ions, or electrons, between the powder and the mate-
rial used for construction of the supply hose and gun barrel. Be-
cause there is no actual electrostatic field, the charged particles of
powder migrate toward the grounded workpiece and are free to
deposit in an even layer over the entire surface of the workplace.
With the elimination of an electrostatic field, the Faraday cage effect
can be prevented.
Other improvements that have been made to spray guns involve
variations in the spray patterns to improve the coating transfer
efficiency. Nozzles that resist clogging have been introduced.
Spray guns with variable spray patterns are also available to allow
the use of one gun on multiple parts of different configurations.
Innovations in the powder delivery system allow the powder supply
reservoir to be easily switched to another color when necessary.
However, if the overspray collection system is not also changed, the
collected powder will be is a combination of all the colors applied
between filter replacements or booth cleanings. For collected
oversprayed powder to be of greatest value, it should be free of
cross-contamination between colors. When a pellet of the wrong
color adheres to the part being powder coated, it will not blend in
with the color being used.
Numerous systems are now available that are designed to accom-
plish this segregation of colors and still allow several colors to be
applied in the same booth. Most of these systems make use of a
moveable dry filter panel or a cartridge filter that can be dedicated
to one color and can be removed easily when another color is
needed. Color changes can then be accomplished by disconnect-
ing the powder delivery system and purging the lines, cleaning the
booth with compressed air or a rubber squeegee, exchanging the
filter used for the previous color with the filter for the next color, and
connecting the powder delivery system for the new color.
Equipment manufacturers have made significant design improve-
ments in spray booths that allow color changes to be made with a
minimal downtime and allow the recovery of a high percentage of
the overspray. As with spray guns, there are a large number of
spray booth and powder recovery designs from which to choose,
depending on the exact requirement of a given finishing system
(Hester and Nicholson, 1989).
18
-------
Powder Coatings
Material and Energy Requirements. The effectiveness of powder
coating depends on obtaining a smooth, nonporous film. Formation
of a good coating free of voids, pinholes, and orange peel depends
on controlling the particle size distribution, glass transition tem-
perature, melting point, melt viscosity, and electric properties of the
powder.
A well-controlled size distribution is important to achieve good pack-
ing of the powder on the surface. Melting and melt flow properties
are important to controlling the behavior of the powder as it melts to
form the coating. The electric properties of the powder are impor-
tant when the coating is applied by electrostatic spraying.
During fluid bed coating, powder is added to replace material
carried out by the substrate. Because very little powder is lost or
degraded during coating, material utilization is near 100%. With
electrostatic spraying there is overspray of powder, much like over-
spray with liquid paint. However, unlike liquid paint overspray which
cannot be recycled, powder coating overspray can be collected and
reused. Figure 4 shows a schematic of a system for powder coat-
ing recycling. The high utilization of powder coating means waste
paint solids disposal is reduced.
Required Skill Level
Powder coating uses equipment and techniques that are very
similar to those for conventional dip coating or spray painting. The
operator skill level is, therefore, similar to that for conventional
electrostatic spray painting.
Cost
The capital cost for booths, electrostatic spray applicators, and
curing ovens to apply powder coatings is typically somewhat higher
than similar equipment for application of conventional fluid coatings.
However, the conventional solvent-based coating application system
may require equipment to control VOC emissions. VOC control
equipment will significantly increase the capital cost for the conven-
tional coating application system.
The powder coating material is more expensive than conventional
coating material when compared on the basis of volume of reactive
resin. However, the cost of the finished coating is lower for powder
coating in many cases. The higher cost of the powder coating
material is offset by the ability to effectively put essentially all of the
19
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ro
o
Powder
supply
Voltage
supply
Spray booth
Electrostatic x^O
powder gun (f
Charged
fluidized
powder
Ground
Powder
collection
Source: Battelle
Figure 4. Powder Coating Recovery System.
-------
Powder Coatings
reactive powder resin onto the substrate. Coating utilization effi-
ciency for conventional coatings can be as low as 40% (Hester and
Nicholson, 1989). The ability of powder coating to give a thick coat-
ing in one pass further improves the economics for powder coating
for cases where a thick coating is needed.
For coating operations with a single color, maintenance and cleanup
costs are low with powder coating. Because the powder coating is
a dry material, no liquid mixing is needed prior to coating and no
solvent is needed so cleanup can be done quickly. No waste sol-
vent is generated and the waste coating material volume is low,
reducing the cost of waste disposal.
For coating operations requiring frequent color changes, the oper-
ating costs of powder coating increase. To change colors, all of the
powder in the coating system must be removed and replaced with
the new color. The powder removal and handling time for color
changes increases the operating cost. A cost comparison of pow-
der, conventional, high solids, and water-based coatings is given in
Hester and Nicholson (1989).
Reported
Applications Powder coating is a rapidly growing area due to both cost and
pollution prevention benefits. The following five resins represent
the bulk of the approximately 250,000 tonnes of powder coating
material usage:
4 Epoxy
4 Epoxy/polyester mixture
4 Polyester
4 Polyurethane
4 Acrylic (Ingleston, 1991).
Powder coating is used commercially for a wide range of small- to
medium-sized metal parts, including lighting fixtures, equipment
cabinets, automobile wheels, outdoor furniture, and hand carts and
wagons. Some of the materials suitable for powder coatings
include:
4 Steel
4 Aluminum
4 Galvanize
4 Zinc and brass castings (Robison, 1989; Bowden, 1989).
Fluidized bed or electrostatic spray systems may be used to apply
chlorotrifluoroethylene fluoropolymer to metals. Electrostatic
spraying is used for coating thicknesses in the range of five to
21
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Available Technologies
Operational and
Product Benefits
Hazards and
Limitations
30 mils. A fluidized bed system is used if thicker coating is
required. The curing temperature is about 500°F (Maguire, 1988).
Boeing is testing a powder coating material for application to both
aluminum panels and epoxy fiberglass laminate panels. Several
surface treatments and primers were tested. Powder is applied with
an electrostatic spray gun and then oven-cured in a second step
(Crump, 1991). Both corona discharge and tribo-friction charging
systems were tested.
Coating Operation:
4 Powder coating can be applied in thick coatings in one pass,
even over sharp edges.
4 Because there is no volatile solvent, little air flow is needed
through the coating application work areas or curing ovens.
Reduced air flow requirements reduces energy use for heating
makeup air.
4 Basic resins that are not easily soluble in organic solvents can
be used.
Preparation and Cleanup:
4 Powder coating comes ready to use and therefore does not
require mixing or stirring.
4 No solvent is required for cleanup.
New Coating Types: Powder coating offers coating with polymers,
such as polyethylene, nylon, or fluorocarbons, that are not ame-
nable to solution coating techniques.
Operating Efficiency: The high material utilization, low reject rates,
generally lower energy consumption, and lack of solvent waste
reduces cost.
The application of powder coating requires handling of heated
parts because the parts must be subjected to elevated
temperature.
For electrostatic application systems, the part must be electrically
conductive or be covered with an electrically conductive primer.
For electrostatic application systems, parts with complex shapes
can leave portions of the surface uncoated unless special
application techniques are used.
Application of thin coatings requires special techniques and
equipment.
22
-------
Powder Coatings
It is difficult to incorporate the metal flake pigments that are
popular for automotive finishes.
Because powder coatings rely on large, fluidized bed reservoirs,
it is difficult to make color changes.
Color matching from batch to batch is difficult.
Tradeoffs
Summary of
Unknowns
State of
Development
Powder coating can cover substrates without needing a volatile sol-
vent to carry the film forming resins. In addition, because the pow-
der remains dry until applied and cured on the surface, no solvent is
required for equipment cleanup. In typical applications, nearly 95%
of the powder coating material is applied to the substrate. These
characteristics promote very clean operation with no loss of volatile
organic compounds (VOCs) and minimal generation of coating
waste.
Powder coating is good for applying a thick layer of thermosetting or
thermoplastic material to a wide variety of substrates. If a medium
thickness coating is needed, the part must be conductive and
should have a simple geometry.
It is not possible to achieve thin coatings with present powder
coating technology. Because curing is accomplished by heating,
the substrate must be able to withstand high temperatures.
Powder coating is routinely used in industry but at the moment has
a limited range of applications. The major area for expanding the
use of powder coatings is development of methods to apply thinner
coatings particularly to complex-shaped or nonconductive
substrates.
Powder coating has a well-established niche in the coating industry.
Both thermoplastic and thermosetting powdered resins and equip-
ment for fluidized bed, electrostatic fluidized bed, or electrostatic
spraying are available from standard vendors. Detailed information
can be obtained from the Powder Coating Institute (see Table 6 in
Section 4).
23
-------
HIGH SOLIDS COATINGS
Pollution Prevention
Benefits
How Does
It Work?
High solids coatings are an evolutionary change from current
coating formulations. The coating liquid formulation is very similar,
but the resin systems are modified to allow a higher concentration
of solids with a lower concentration of VOCs. Lower VOC levels
mean less VOC loss during curing, thus lowering fugitive solvent
sources. The lower solvent concentrations in the coating formula-
tion thus result in less solvent vapor in the air in the operating
areas. Reduced solvent concentration reduces health and safety
problems such as worker exposure to solvent fumes and fire and
explosion hazard. However, paint overspray still results in paint
waste, and solvent is still needed for equipment cleanup.
For example, a conventional plant painting 12,000,000 ft2/yr with a
1.2-mil-thick coat will release about 38 tons/yr of VOC emissions
after a treatment system with a 70% capture efficiency. A high
solids painting plant spray applying a similar coating will emit about
31 tons/yr with no VOC control equipment (Hester and Nicholson,
1989).
Some paints have been formulated with chlorinated solvents to
replace the volatile organic ozone precursors. These coatings are
sometimes referred to as "low VOC coatings," but they are not
considered clean technologies because they use 1,1,1-trichlor-
oethane or similar solvents. Low-VOC coatings formulated with
chlorinated solvents, therefore, are not discussed in this guide.
High solids coatings are being actively developed to reduce the
quantity of volatile organic solvent in coatings. Like conventional
coatings, high solids coatings consist mainly of resins, coloring
agents, extenders, and additives carried in a solvent. As the name
implies, high solids coating liquids rely on increasing the
concentration of resins in the coating formulation.
Conventional coatings typically use high-molecular-weight resins to
obtain satisfactory cured-film properties. A direct increase of solids
concentration with conventional resins would, therefore, result in an
unacceptable increase in the viscosity of the coating fluid.
Thus, high solids coating formulations use reduced-molecular-
weight resins in parallel with the increase of the solids con-
centration.
24
Available Technologies
-------
High Solids Coatings
The resulting high solids formulation is applied in much the same
way as conventional coatings using the same or similar equipment.
Thus high solids coatings must:
* Have viscosity and physical properties similar to conventional
coating materials
* Allow use of coloring additives giving the required colors and
hiding power
4 Have a reasonable curing rate
* Remain useful for an acceptable period after the coating is
removed from the original container (pot life)
* Provide a quality coating.
Many vendors of conventional coating materials now have
alternative high solids coatings, particularly urethanes, and they are
developing new formulations. For more information, contact the
trade associations described in Table 6 (Section 4).
Why Choose This
Technology Applications
Currently available high solids coatings are expressly developed to
behave as much as possible like conventional coatings. Generic
families of resins available in high solids coating formulations
include:
4 Saturated polyesters
4 Alkyds
4 Acrylics
4 Polyurethane
4 Epoxy (Paul, 1986; Pilcher, 1988)
Operating Features
Lower viscosity high solids coatings can be applied with conven-
tional equipment such as air spray, airless spray, or electrostatic
spray. The major problems with high solids coating materials occur
during application when the viscosity makes it difficult to achieve
acceptable curing times while maintaining an acceptable use time
after preparing the paint and exposing it to the air (i.e., good pot
life).
High solids coating formulations with higher viscosities can be
applied with turbine bell or rotating disk atomization spray
equipment. These new application technologies allow the use of
high-viscosity formulations while maintaining coating quality.
25
-------
Available Technologies
Basic Function. High solids coatings require the formulation to
contain lower molecular weight resins. Lower molecular weight
resins allow high solids concentration while the viscosity remains
acceptable for use in conventional application equipment. However,
the lower molecular weight resins alone will give an unacceptable
final dry film if normal curing time limits are applied. To overcome
performance limitations caused by low-molecular-weight resins,
additives are often used to increase crosslinking of the resins during
curing.
In many formulations, e.g., alkyds, polyesters, and polyurethanes,
the crosslinking additives decrease the stability of the coating fluid
and, therefore, are shipped in a separate container. The additive is
mixed into the coating formulation at the start of a painting session.
The additives typically contribute to a shorter pot life for high solid
coatings compared with conventional coatings.
In some formulations, a somewhat higher viscosity is accepted and
new types of application equipment are used. These new equip-
ment types, such as turbine atomizers, are able to effectively
atomize the more viscous high solids coatings. Disk and bell
turbine applicator systems are both used in finishing operations.
With disk or bell applicators, the coating liquid is fed into a rotating,
insulated disk or bell, where it spreads to the outer edge by centrif-
ugal force. To keep the paint from flying off the disk or bell in
coarse droplets or strings, an -100-kV electrostatic charge is
applied to the insulated disk or bell charges the paint droplets. The
charge also enables paint atomization to ensure uniformity of coat-
ing on all sides exposed to the painting operation. The turbine-
powered bells have rotational speeds of up to 50,000 rpm; disks, up
to 40,000 rpm to achieve atomization by centrifugal force. A high
electrostatic charge slightly improves the atomization and signifi-
cantly improves the transfer efficiency of the paint droplets. The
article being coated typically continues through the disk or bell
system on a continuous line into a curing oven.
A somewhat more conventional approach to high-efficiency
application to reduce overspray loss involves high-volume, low-
pressure (HVLP) application equipment. HVLP's low atomizing air
pressure requirement of between 0.1 and 10.0 psig combines with
turbine-generated high volumes of atomizing air. HVLP systems
routinely accompany commercially available paint application
equipment.
Airless Spray is another technique to improve transfer efficiency. It
does not use compressed air to atomize the liquid coating. An air-
less system utilizes hydraulic pressure to atomize the liquid coating
26
-------
High Solids Coatings
and deposit it on the work piece. Coating fluid under high pressure
is released through an orifice in the spray nozzle. The high pres-
sure separates coating fluid into small droplets, resulting in a fine
atomized spray. Since air is not used to form the spray pattern, the
term "airless" is used.
Air Assisted Airless is a combination of airless and conventional air
spray which uses some air propellant to assist in atomizing the
coating liquid to a smaller droplet.
The HVLP technology is being debated relative to other methods
that also achieve high transfer efficiency, such as air-assisted
airless spray. Rapid developments in these high-efficiency
technologies will position some of them, such as HVLP, to dominate
paint application system manufacturing (Dick, 1991).
Material and Energy Requirements. High solids coatings are
applied in the same way as conventional coatings, so overspray
losses and solvent use for equipment cleanup are similar. Paint
loss with high solids coating can, however, be minimized because
the high solvent coatings are compatible with conventional elec-
trostatic application methods. High solids coatings typically contain
275 to 420 g VOC/I of liquid coating (2.3 to 3.5 Ib/gallon) (Pilcher,
1988). Because of the lower quantity of solvent, VOC loss during
preparation, use, and curing is reduced so air flow through curing
ovens and in operating areas may be reduced. Lower air flow
decreases energy use.
Required Skill Level
Viscosity changes in high solids coating fluids caused by solvent
addition or evaporation are not as predictable as the viscosity
behavior of conventional coating formulations. Many high solids
coatings (and some conventional coatings) are shipped as two
components. Two solutions must be mixed before the coating can
be applied, requiring an additional handling and mixing step just
prior to starting painting.
High solids coatings generally have a shorter pot life. Even though
the application equipment is similar, the above factors lead to the
need for more operator skill and attention when using high solids
coatings.
27
-------
Available Technologies
Reported
Applications
Operational and
Product Benefits
Cost
High solids coatings use conventional application equipment so the
capital cost for booths, electrostatic spray applicators, and curing
ovens are the same as for equipment to apply conventional high-
VOC coatings. In fact, many of the existing items of equipment in
the coating system could be retained in a switch to high solids coat-
ing. High solids coatings contain up to 40% solvent, so some VOC
control will still be required with many high solids formulations.
The high solids coating liquid is slightly more expensive than
conventional coating liquid per unit of reactive resin. Coating fluid
preparation, application, cleanup, and disposal costs are similar for
high solids and conventional coating. A cost comparison of high
solids, conventional, powder, and water-based coatings is given in
Hester and Nicholson (1989).
Steel door makers are using high speed turbine bell atomizers to
apply high solid coating to zinc-coated steel doors. In addition to
lower VOC emissions, the new high solids coating formulation
allows lower curing temperatures required by the heat sensitive
foam insulation cores now used in steel doors. The high solid coat-
ings meet the highest adhesion rating in ASTM D 3359 and pass a
250-hr salt spray test per ASTM B 117 (Nelson, 1988).
A U.S. Department of Energy contractor was able to identify and
qualify a low-VOC urethane material (420 g VOC/I (3.5 Ib/gal)) to
replace a conventional urethane material (520 g VOC/I (4.36 Ib/gal)
for coating miscellaneous metal parts (Smith, 1990).
High solids polyurethane materials are available meeting MIL-C-
83285B for use on aircraft and ground support equipment. These
high solids formulations contain 340 to 420 g VOC/I (2.8 to
3.5 Ib/gal). The pot life is reported to be 6 hr. The materials
are formulated for electrostatic application (MP&C, 1988).
High solids coatings have the following benefits:
* They reduce solvent concentrations in the coating, thus reducing
the environmental, odor, and safety problems caused by sol-
vents.
f They are compatible with conventional and electrostatic applica-
tion equipment and techniques.
* Lower solvent loadings allow reduced air flow in curing ovens
and work spaces, which decreases energy needed for heating.
28
-------
High Solids Coatings
Hazards and
Limitations
There are some disadvantages to using high solids coatings:
4 Although they reduce the amount of organic solvents in the coat-
ing formulation, they do not completely eliminate solvent use.
* High solids coatings have shorter pot life than conventional
coatings.
Tradeoffs
Summary of
Unknowns
State of
Development
The main tradeoff involved in selecting the high solids coating
option is the "comfort factor" versus less complete pollution preven-
tion when compared to some of the other alternatives.
High solids coatings use technology that is similar to conventional
solvent-based coatings. Many users will find that the transition to
high solids coatings meets less resistance because it allows use of
familiar equipment to apply a solvent-based coating. Both equip-
ment operators and plant management tend to prefer evolutionary
changes to radical departures in equipment and procedures.
Although high solids coatings are a valid clean technology offering a
real reduction in VOC emissions, the potential reductions are not as
great as with powder coating or 100% reactive liquid coatings.
High solids coatings are limited mainly by viscosity restrictions. The
solution viscosity increases roughly linearly with the weight fraction
of resin present in the coating formulation. Therefore, increasing
the solids content increases coating liquid viscosity. The major
unknown in high solids coating technology is what types of new
formulations will provide good coating adhesion, flexibility, and
impact resistance while maintaining coating flexibility. Methods to
optimize pot life versus curing time are also needed.
High solid content solvent-based coatings are compatible with
electrostatic spraying techniques. Many of the formulations, particu-
larly the two-component types, are compatible with conventional air
spray equipment. As a result, changing to high solids coatings is
less disruptive than changing to other technologies, such as 100%
reactive liquid, water-based, or powder coating.
Compatibility makes high solids coatings attractive as near-term
replacements for conventional solvent-based coatings. Develop-
ment of urethane formulations is the most advanced, but paints
based on other resin types are also becoming available.
29
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ATER-BASED COATINGS
Pollution Prevention
Benefits
How Does
It Work?
Water-based coatings are a diverse group of liquid coating materials
in which water supplements or replaces the organic solvent. Water,
alone or in conjunction with solvent, acts as the carrying medium for
film-forming resins, coloring agents, and other elements of the
coating formulation.
Water-based coatings have little or no solvent in the coating
formulation, and the uncured coating can be cleaned up with water.
As a result there is less solvent vapor in the air in the operating
areas and less bulk solvent storage. Reduced solvent use reduces
health and safety problems such as worker exposure to solvent
fumes or liquids and potential fire or explosion hazard.
For example, a conventional plant painting 12,000,000 ft2/yr with a
1.2-mil thick coating will release about 38 tons of VOC, even with a
treatment system running at 70% capture efficiency. A water-based
painting plant spraying a similar coating and running no VOC control
equipment will emit only about 26 tons/yr (Hester and Nicholson,
1989).
Reduced solvent concentration in the coating thus results in lower
VOC emissions during curing and can make a significant contribu-
tion to lowering fugitive solvent releases. However, paint overspray
still results in paint waste. New HVLP application equipment can
reduce overspray loss.
Water-based coatings are not directly compatible with current
generation electrostatic application equipment. Cleanup of
equipment covered with uncured water-based coatings can be done
with water, which also helps reduce solvent use.
Water-based coatings use solvents, resins, coloring agents,
extenders, and additives either dissolved or dispersed in water. A
wide range of technologies are used to achieve good coating per-
formance with water-based coatings. There are three main classes:
4 Water-soluble or water-reducible coatings
* Colloidal or water-solubilized dispersion coatings
4 Emulsion coatings.
30
Available Technologies
-------
Water-Based Coatings
These three vary significantly in physical and mechanical properties.
For example the handling characteristics and performance param-
eters for a solubilized polyester (water-soluble) with molecular
weight of 2,500 will be different from those of an acrylic latex
(emulsion) with a molecular weight of more than 1 million.
Water-soluble coatings are generally easier to apply than emulsion
coatings because they exhibit better flow, substrate wetting, and
leveling; less foaming; and easier cleanup. The emulsion coatings,
although more difficult to formulate and apply, tend to have greater
durability and resistance to chemicals (Paul, 1986).
In addition to conventional application methods, water-based
coatings are amenable to electrodeposition. Electrodeposition of
coating resembles electroplating in that the substrate is submerged
in an aqueous bath. The ionized organic coating material is
deposited on the substrate by means of the direct current flow.
Water-based latex acrylics and epoxies are available and are
routinely used. New formulations are being developed for other
applications. Many of the vendors of conventional coating materials
have alternative water-based coatings. For more specific informa-
tion, contact the trade associations listed in Table 6 (Section 4).
Why Choose
This Technology? Applications
The great diversity of water-based coating technology is a potential
strength but presents a challenge when first encountered. Because
a wide range of characteristics are available with water-based
coating technology, formulations can be prepared to fit many
specific applications. However, the coating formulation will typically
require more care in application and have a more limited range of
potential uses than similar conventional solvent-based coatings.
Thus, it is difficult to characterize or classify water-based coatings
into a few simple groups having wide ranges of application.
Two-part acrylics; colloids; amine-solubilized, carboxyl-terminated
alkyds; and polyesters are examples of some of the coating for-
mulations generally considered as water-based technologies. Each
of these groups has different properties creating challenges both for
users in defining their needs and for coating manufacturers in
providing the optimum coating to fill user needs (Pilcher, 1988).
However, the diversity of water-based coating technology allows
flexibility in tailoring the performance of the coating.
Water-soluble resins are normally prepared by one of three
methods. The most common approach is to convert the polymer
31
-------
Available Technologies
backbone to anion or cation forms by neutralizing the carboxylic or
amino groups. Other possible approaches include adding nonionic
groups such as polyols or polyesters to the resin to allow water
solubility, or using water-soluble zwitterion (organic ion with both a
positive and negative charge) copolymers.
Water-soluble formulations can include water-soluble oils, poly-
butadiene adducts, alkyds, polyesters, and acrylics. Water-soluble
coatings tend to have simpler formulations and are easier to apply
than emulsions but have lower durability and resistance to solvents.
Colloidal dispersions lie between water-soluble and emulsion
coatings with respect to both application behavior and physical
characteristics. The colloidal coatings consist of a dispersion of
very fine, partially water-soluble polymer droplets in water. Colloidal
dispersions are used mainly to coat porous materials such as paper
or leather.
Emulsion formulations (also called latex) are currently the most
commonly used of the water-based coatings. Emulsion coatings are
dispersions of polymer droplets in water. The polymer droplets are
stabilized in the aqueous medium by emuisifiers and thickeners.
Most emulsion coatings use acrylic or acrylic copolymer resins.
Emulsion coatings have lower gloss, film buildup, and pigment
loading than water-soluble coatings.
Operating Features
Water-based paints are liquid products that can be applied with
either conventional nonelectrostatic atomizers or modified electro-
static atomizers. In many cases, two-component formulations are
used. Although two-component formulations slightly increase the
complexity of coating liquid preparation, many conventional coatings
are also shipped as two separate components.
Emulsion formulations are thixotropic and may, therefore, not be
compatible with existing pumps and piping. Uncured water-based
coatings can be removed with water, so solvent use is reduced both
in the formulation itself and in secondary uses associated with
cleanup at the end of the coating operation.
Basic Function. Water-based coatings are one- or two-component
liquids that are applied in essential the same manner as conven-
tional solvent coatings. The thixotropic viscosity behavior of
emulsion coatings may require the use of special pumping and
application equipment.
32
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Water-Based Coatings
Also, the electrical conductivity of aqueous solutions means that
special equipment and techniques are needed for electrostatic
application of water-based coatings. Electrostatic coating appli-
cation is widely used to reduce overspray losses. Cutting the
amount of coating that does not reach the substrate helps prevent
pollution and reduce costs.
There is also a clean technology advantage to electrostatic appli-
cation because reduced overspray cuts solvent releases and coat-
ing waste production. However, aqueous solutions have higher
electrical conductivity than organic solvent solutions, so it is more
difficult to maintain voltage in an electrostatic application system
using water-based coatings. Four options exist for electrostatic
application with water-based coatings:
t Isolate the coating liquid storage and supply system from any
electrical ground to prevent current leakage from the application
atomizer to ground through the coating supply system.
* Use an external charging system attached to but electrically iso-
lated 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 (Scharfenberger, 1989).
Although the electrical conductivity of water-based coating solutions
presents problems for conventional electrostatic spray application, it
opens up the possibility of a new application approach. Coating
resins can be applied to electrically conductive substrates by
electrodeposition. In electrodeposition, ionized resins are attracted
to the substrate by electrical charge as shown in Figure 5.
Film-forming cations can be obtained as organic substituted
ammonium macro-ions such as RNH3+ or R3NH+. When an appro-
priate film-forming resin, R, is used, an adherent deposit will form
on the substrate held at cathodic potential. Anodic film-forming
resins can be prepared from coating resins that have carboxylic
groups. Resins with molecular weights in the range of 2,000 to
20,000 typically are used for electrodeposition from water-based
coating solutions.
Electrodeposition is used most in applying automotive primers
because of its high ability to provide protection from corrosion with
very thin, evenly spread films. Electrodeposition gives remarkably
uniform films no matter what the surface. Recesses, tapped holes,
and sharp edges exhibit uniform coating. By "forcing" a dense film
against the substrate during coating, electrodeposition provides
33
-------
Available Technologies
excellent adhesion and resistance to corrosion. The fully automated
systems that are available give higher than 95% deposition effi-
ciency. Because electrocoatings use water as the main solvent, fire
hazards and air pollution are minimized.
Spray
rinse
Resin attracted
to part
Part to be
coated
Resin
solution
Elect rodeposition
coating tank
Source: Battelle
Figure 5. Electrodeposition Coating.
The electrodeposition technology does, however, require special
equipment and procedures resulting in high installation and other
costs. Electrodeposition requires new coating tanks, extra-clean
34
-------
Water-Based Coatings
application and curing areas, and infrared radiation to speed up the
final setting of the coating surface. Operators must learn to formu-
late and precisely control the coatings. Electrodeposited coatings
are highly sensitive to contaminants. To produce a high gloss
finish, the first coating must include a conductive pigment. The
surface of metal substrates can dissolve into the coating and cause
discoloration. Thus, only very large numbers of similar parts justify
the costs of installing electrodeposition equipment and providing the
required training (Dick, 1991).
Material and Energy Requirements. Water-based coating for-
mulations contain fewer VOCs than conventional coatings, and
uncured coating waste can be cleaned up with water. Because of
the lower solvent use, the VOC loss during coating preparation, use,
curing, and cleanup is reduced so that the air flow through curing
ovens and in operating areas may be lower. Lower air flow
decreases energy use.
Required Skill Level
Special equipment and procedures are needed for electrostatic
application of water-based coatings. Some water-based coatings
(and some conventional coatings) are shipped as two components.
Two solutions must be mixed before the coating can be applied,
requiring an additional handling and mixing step just prior to starting
painting.
Water-based coatings typically require more care in surface clean-
ing and preparation than do conventional coatings. Even though
the application equipment is similar, the above factors lead to the
need for more operator skill and attention for application of water-
based coatings.
Cost
The capital cost for electrostatic spray applicators for water-based
coatings will typically be higher than similar equipment for appli-
cation of conventional fluid coatings because of the electrical con-
ductivity of water-based coatings. Water-based coatings typically
contain some solvent but are less likely to require VOC control
equipment. If VOC control is needed, the lower concentration of
solvent in the coating should reduce the volume of carbon absorber
needed, thus reducing capital and operating cost.
The water-based coating liquid is more expensive than conventional
coating liquid per unit of reactive resin. Coating fluid preparation,
35
-------
Available Technologies
Reported
Applications
application, cleanup, and disposal costs are simitar for water-based
and conventional coating. A cost comparison of conventional,
powder, high solids, and water-based coatings is given in Hester
and Nicholson (1989).
Water-based coatings are primarily used as architectural coatings
and industrial finish coatings. Because water-based paints are easy
to apply, adhere to damp surfaces, dry rapidly, are easy to clean
with soap and water, and lack solvent odor, more than 70% of
architectural coatings are water-based paints. Coatings include:
4 Wall primers, sealants
4 Interior flat/semigloss wall paints
4 Interior and exterior trim finishes
4 Exterior house paints
Water-based coatings have not been so readily accepted in the
industrial sector. But tightening regulations are making compliance
more economical through use of these coatings, and they are gain-
ing a foothold in both primer and topcoat industrial finish applica-
tions where these attributes count:
4 Can be used with existing solvent-based coatings application
equipment
4 Can be thinned using only water
4 Have low flammability
4 Offer minimized toxicity, environmental pollutants, and odor
4 Application equipment can be cleaned using only water (Dick,
1991).
Resins include:
4 Acrylics
4 Epoxy esters
4 Alkyds
4 Polyesters.
About 10% of U.S. coil lines, which have aluminum as the primary
substrate use acrylic-based, waterborne coatings because of the
flexible and adhesive properties of these coatings. Acrylics also
offer exterior durability and resistance to yellowing. Epoxy esters
also reliably adhere to the substrate and are resistant to corrosion
and detergents. Polyesters, on the other hand, lack detergent resis-
tance but provide good exterior durability. Alkyds balance lower
performance with a lower cost (Dick, 1991).
36
-------
Water-Based Coatings
Water-based epoxy coatings are particularly useful for application
on green or damp concrete. The water-based formulations provide
excellent adhesion to the concrete substrate. Water-based epoxy
coatings give low odor levels during curing, and the cured coating
surface is easy to clean. These characteristics make water-based
epoxy coatings suitable for areas requiring a high level of hygiene,
such as hospitals or food processing plants (Richardson, 1988).
A low-VOC water-based flexible epoxy primer is available as a two-
component formulation. Unlike most water-based formulations, no
water is present in either of the two components as shipped. The
components, which are supplied in a 3 to 1 volume ratio, are mixed
just before application to prepare a catalyzed resin mixture.
Water is added to the mixture to reduce the viscosity to allow
application. At application, the formulation contains about 340 g
VOC/I (2.8 Ib/gal) (MP&C, 1988).
An aerospace company compared a series of water-reducible
coatings to current aerospace topcoats. The base case coatings
selected were a conventional solvent-based, two-component polyur-
ethane meeting Military Specification MIL-C-83286 and a high solid
polyurethane meeting MIL-C-85285. The VOC concentrations of the
conventional and high solids coatings were 600 g/l (5 Ib/gal) and
420 g/l (3.5 Ib/gal), respectively.
Both polyurethane and polyester resin formulations were tested with
and without crosslinking agents. The crosslinkers tested were
carbodiimide, polyaziridine, di-functional amine, or tertiary amino
alkylylamine. The VOC concentration in the water-based formula-
tions ranged from 65 g/l to 345 g/l (0.54 Ib/gal to 2.9 Ib/gal).
Skydrol resistance testing by ASTM D 1308 showed severe soften-
ing for all the water-based formulations and the high solids base
case. However, the overall performance of the cross-linked water-
based coatings was found to be nearly as good as the high solids
solvent-based polyurethane (Swanberg, 1990).
Application methods for water-based coatings include:
4 Dip coating
4 Flow coating
4 Air spray
4 Airless spray
4 Electrostatic spray (Dick, 1991).
37
-------
Available Technologies
Operational and
Product Benefits
Hazards and
Limitations
There are several operational and environmental benefits of water-
based coatings:
4 Because water-based coatings reduce the solvent concentrations
in the coating, they reduce the environmental, odor, and safety
problems caused by solvents.
4 Existing application equipment (nonelectrostatic) can be used
with most water-based coatings.
4 Reduced air flow in curing ovens and work spaces decreases
energy needed for heating.
Water-based coatings have several drawbacks:
4 They are sensitive to humidity, so effective application normally
requires humidity control in the application and curing areas.
4 The high surface tension of water can cause poor coating flow
characteristics.
4 Special equipment is needed to allow electrostatic application.
4 Water in the formulation can cause corrosion of the coating
storage tanks and transfer piping.
4 Water in the formulation can cause 'Hash rusting" of metal sub-
strates under the coating.
4 Most waterborne coatings require careful cleaning of the sub-
strate to ensure oil and grease are removed.
4 Resins in contact with water degrade, reducing shelf life for
water-based coating formulations.
4 Water-based emulsion coatings are susceptible to foaming due
to surfactants used to stabilize the emulsion.
Tradeoffs
Summary of
Unknowns
Water-based coatings offer a tradeoff of "comfort factor" versus
pollution prevention similar to high solids coatings. However, the
factors involved are different. The "comfort factor" with water-based
coatings is lower because they do not use the familiar solvent tech-
nology and are not compatible with existing electrostatic application
equipment. However, the potential for solvent reduction is higher.
At least some of the water-based coatings eliminate solvent from
the formulation. Also, cleanup of uncured water-based coatings can
be done with water, which eliminates solvent use in cleanup. Thus
the solvent reduction potential is higher with water-based coatings
compared to high solids coatings.
Water-based coatings have proven compatible with porous surfaces
such as concrete, paper, and leather. Compatibility with metal
surfaces is less well established. Unless the pH of the coating
38
-------
Water-Based Coatings
formulation is high or a corrosion inhibitor is included in the
formulation, the water can corrode the substrate. Also, water-based
coatings typically require a cleaner surface than solvent-based coat-
ings. Future work is needed to develop low-cost cleaning methods.
State of
Development Many of the water-based formulations are compatible with con-
ventional nonelectrostatic spray equipment but require special
provisions for electrostatic application. As a result, a change to
water-based coatings can be somewhat less disruptive to existing
coating operations than a change to other technologies such as
powder coating. Compatibility gives water-based coatings the
potential to replace conventional solvent-based coatings. But the
high cost of clean application areas, environmental control, and
infrared flashoff may limit the use of water-based coatings in small
industrial finish coating shops.
39
-------
K
LTRAVIOLET (UV)
ADIATION-CURED COATINGS
Pollution Prevention
Benefits
How Does
It Work?
Why Choose
This Technology?
UV-cured coatings allow the use of 100% reactive liquids, eliminat-
ing solvent use, and near 100% transfer efficiency, reducing paint
waste. UV curing provides an alternative in cases where baking
has been used to remove the solvent component from convention-
ally processed organic coatings.
UV curing uses high-intensity UV light to initiate the free radical
crosslinking of acrylate oligomers and prepolymers. Of the avail-
able chemistries, this is the most frequently used. The UV light
changes the ink or coating from wet to dry product. This fast,
relatively cool process enables inks and coatings to be cured on
heat-sensitive substrates. The crosslinking of the inks and coatings,
when properly cured, yields high chemical and physical resistance
(Sun Chemical, 1991).
UV-cured coatings consist of:
4 An oligomer or a prepolymer containing double-bond
unsaturation
4 A reactive solvent, e.g., monomers with varying degrees of
unsaturation
* A sensitizer to absorb the UV radiation-initiating polymerization
4 Coloring agents (Sun Chemical, 1991).
UV-cured coatings are available from a variety of commercial ven-
dors. For more specific information, contact the trade associations
listed in Table 6 (Section 4).
Applications
UV curing is most used in these industrial finishing areas:
* Wood finishing
4 Metal decorative coatings
4 Automotive coatings
4 Wire coatings
4 Packaging coatings
4 Floor finishing.
40
Available Technologies
-------
Ultraviolet (UV) Radiation-Cured Coatings
UV curing often replaces conventional thermal treatments for coat-
ing flat wood surfaces because of its low cost and ease of applica-
tion. Because the UV radiation must reach the entire coated
surface, UV curing systems are most easily used with flat material.
However, curing systems are being modified to allow three-
dimensional coating, or coating of all surfaces, at one time. UV
curing has been investigated for the U.S. metal-can industry and
often replaces thermally cured coatings for aluminum and galva-
nized steel tubing because of its hardness and salt spray resistance
lasting 200 to 500 hr. The technology offers a low-cost, high-
throughput alternative for finishing automotive hubcaps and wheel
rims. Used on both bare and insulated wire, UV-cured coatings pro-
vide highly cross-linked, strong, yet flexible 10-mil-thick films.
Liquid acrylic and liquid polyurethane-acrylic UV-cured coatings
surpass press varnish and water-based coatings in quality and
resemble film laminations. Likewise, tough UV-cured coatings have
found a market niche in high-gloss vinyl floor coverings. They
surpass the conventional urethane polymers in ease of application
and in abrasion, solvent, and stain resistance (Dick, 1991).
Operating Features
UV curing actually takes place in two instantaneous steps using first
the UV light energy and then the UV thermal energy. First, the
photoinitiator absorbs the UV light energy and converts it to free
radicals through several chemical mechanisms. Then the UV lamps
provide thermal energy to make the free radicals attack the acrylic
double bonds and make the ink or coating polymerize. Because
curing takes place so quickly, it is advised to allow time between
application and curing to allow the ink or coating to level out and to
achieve maximum gloss.
Some radicals "live on" in the film after exposure to high-intensity
UV light, causing postcuring if the crosslink conversion is weak.
Postcuring cannot achieve the desired properties if the initial curing
has been inadequate.
Ink color and opacity to UV light affect the curing rate of inks.
Darker and more opaque inks block the light and require longer
exposure time for adequate curing. Likewise, thicker films and
multiple films cure more slowly than thin or single films of inks and
coatings (Sun Chemical, 1991).
Solvent is not needed in most UV curing uses, although some dilu-
tion may be needed for spray applications. Solvent is used for the
cleanup of uncured coating. The coating is applied as a fluid and
41
-------
Available Technologies
exposed to the UV light, which initiates formation of a cross-linked
polymer coating (see Figure 6).
100% Reactive single
liquid paint system
Ultraviolet
light curing
Overspray can be
collected and reused
Source: Battelle
Figure 6. 100% Solids Ultraviolet-Light-Curable Liquid Paint System.
Reported
Applications
UV coating formulated from polyester resin in styrene has seen
commercial application as filler for chipboard. The polyester-styrene
system has not been widely applied because of styrene's volatility,
the yellow color of the coating, and the high capital cost of UV
coating equipment.
UV-cured coatings are being developed based on acrylate-modified
resins such as polyesters, epoxides, and urethanes. UV-cured
42
-------
Ultraviolet (UV) Radiation-Cured Coatings
Operational and
Product Benefits
Hazards and
Limitations
coatings are typically limited to wood substrates, but application to
metals has been studied (Paul, 1986). UV coating techniques are
used to coat flat sheet stock and to apply "wet look' finishes to
assembled furniture.
A variety of Dual Cure urethane/acrylate and epoxy/acrylate UV-
cured coating systems are being developed (Keipert, 1990).
UV-cured coatings have a number of operational, cost, and
environmental benefits:
4 Eliminates or reduces solvent use
* High transfer efficiency
4 Low-temperature processing
4 Rapid curing
4 Equipment requires less space than curing ovens.
UV technology has several drawbacks:
4 It requires new equipment with high capital cost.
4 The presence of pigments limits penetration of UV.
Tradeoffs
Unknowns and
Future Developments
UV curing offers good pollution prevention potential but requires
new operating procedures and expensive new equipment.
More widespread future use depends on the development of the
following:
4 More highly developed UV equipment
4 New products/markets for radiation processing technologies
4 New 100% reactive monomers and oligomers that are nontoxic
and of low viscosity
4 New monomers, oligomers, and polymers that adhere to metal
substrates
4 New electronic and optronic resist materials
4 Lower costs for materials
4 Further study of coating service life (Dick, 1991).
43
-------
^ECTION 3
EMERGING TECHNOLOGIES
How to Use the
Summary Tables
Descriptive
Aspects
Operational
Aspects
Six emerging coating replacement materials are evaluated in this
section, namely
t Electron beam (EB)-cured coatings
4 Radiation-induced thermally cured coatings
4 Two-component reactive liquid coatings
4 Water-based temporary protective coatings
4 Vapor permeation- or injection-cured coatings
4 Supercritical carbon dioxide as solvent.
Tables 4 and 5 summarize descriptive and operational aspects of
these technologies. They contain evaluations or annotations
describing each emerging clean technology and give a compact
indication of the range of technologies covered to allow preliminary
identification of those that may be applicable to specific situations.
Readers are invited to refer to the summary tables throughout this
discussion to compare and contrast technologies.
Table 4 describes each emerging technology. It lists the Pollution
Prevention Benefits, Reported Applications, Operational and
Product Benefits, and Hazards and Limitations of each.
Table 5 shows the key operating characteristics for the emerging
technologies. These characteristics serve to qualitatively rank the
clean technologies relative to each other. The rankings are
estimated from descriptions and data in the literature.
Process Complexity is qualitatively ranked as "high," "medium," or
"low" based on such factors as the number of process steps in-
volved and the number of material transfers needed. Process
Complexity is an indication of how easily the new technology can
be integrated into existing plant operations. A large number of
process steps or input chemicals, or multiple operations with
complex sequencing, are examples of characteristics that would
lead to a high complexity rating.
44
-------
Table 4. Emerging Clean Technologies for Organic Coating Replacements: Descriptive Aspects
Technology Type
Pollution Prevention Benefits
Reported
Applications
Operational and Product Benefits
Hazards and
Limitations
Electron Beam (EB)-Cured
Coatings
100% reactive liquid
Eliminates or reduces solvent in the coating,
but solvent still needed lor cleanup
Used on
-paper
- wood
- plastic
Efficient material use (near 100% transfer efficiency)
Lack of volatile solvent means little air flow needed
Low-temperature processing (reduced energy use
for heating makeup air)
Rapid curing
1 Typically best applied
to flat material
Radiation-Induced Thermally
Cured Coalings
100% reactive liquid
Eliminates or reduces solvent in the coating
May be water-reducible lor application and
cleanup depending on coating
Pilot testing for curing powder
or waterborne coatings
Low-temperature processing
1 Best when used with
robotic system
Typically best applied
to flat material
Two-Component Reactive Liquid
Coatings
100% reactive liquid
Eliminates or reduces solvent in the coating,
but solvent still needed for cleanup
Undergoing initial feasibility
testing
Low-temperature processing
Uncured resin may be
harmful
Water-Based Temporary
Protective Coalings
Water-based formulation eliminates or reduc-
es solvent use
Water used for cleanup
Automobiles in shipment
Machined metal during ship-
ping or between manufacturing
steps
Metal masking during milling
and etching
Produces tough, transparent film
Safe, quick removal
Coaling is temporary
Vapor-Permeation or
Injection-Cured Coatings
100% reactive liquid
Eliminates or reduces solvent in the coating,
but solvent still needed lor cleanup
Pilot testing of permeation
curing by amine vapor
Low-temperature processing
Further testing
required
Supercritical C02 as Solvent
Reduces solvent in coating formulation
Replacement for conventional
spraying systems
Compatible with conventional and electrostatic
application equipment and techniques
Lower solvent loadings allow reduced air flow in
curing ovens and work spaces, thus decreasing
energy needed for heating
Solvent use not
completely eliminated
Requires new C02
handling equipment
en
-------
-tx
CD
Table 5. Emerging Clean Technologies for Organic Coating Replacements: Operational Aspects
Technology
Type
Electron Beam (EB)-Cured
Coalings
Radiation-Induced Thermally
Cured Coatings
Two-Component
Reactive Liquid Coatings
Water-Based Temporary
Protective Coatings
Vapor-Permeation or
Injection-Cured Coatings
Supercritical C02 as Solvent
Process
Complexity
High
High
High
Medium
High
Medium
Required
Skill
Level
High
High
High
Medium
High
Medium
Waste Products and
Emissions
Unreacted overspray can be collec-
ted lor reuse
Unreacted overspray can be collec-
ted for reuse when used with
powder coatings
Overspray loss similar to that ol
conventional coating
Overspray loss similar to that of
conventional coating
Unreacted overspray can be collec-
ted lor reuse
Overspray loss similar to that of
conventional coating
Capital
Cost
Very high
High
High
Medium
High
High
Energy
Use
Low
Low
Low
Medium
Low
Medium
References
Dick, 1991
Paul, 1986
Sun Chemical, 1991
Paul, 1986
Poullos, 1991
No data available
Product Finishing, 1986
Toepke, 1991
Richer, 1988
Nordson, 1991
-------
Emerging Technologies
The Required Skill Level of equipment operators also is ranked as
"high," "medium," or "low." Required Skill Level is an indication of
the level of sophistication and training required by staff to operate
the new technology. A technology that requires the operator to
adjust critical parameters would be rated as having a high skill
requirement. In some cases, the operator may be insulated from
the process by complex control equipment. In such cases, the
operator skill level is low but the maintenance skill level is high.
Table 5 also lists the Waste Products and Emissions from the
emerging clean technologies. It indicates tradeoffs in potential
pollutants, the waste reduction potential of each, and compatibility
with existing waste recycling or treatment operations at the plant.
The Capital Cost column provides a preliminary measure of pro-
cess economics. It is a qualitative estimate of the initial cost impact
of the engineering, procurement, and installation of the process and
support equipment. Due to the diversity of cost data and the wide
variation in plant needs and conditions, it is not possible to give
specific cost comparisons. Cost analysis must be plant-specific to
adequately address factors such as the type and age of existing
equipment, space availability, throughput, product type, customer
specifications, and cost of capital. Where possible, sources of cost
data are referenced in the discussions of each clean coating
material.
The Energy Use column provides data on energy conversion
equipment required for a specific purpose. In addition, some
general information on energy requirements is provided.
The last column in Table 5 lists References to publications that will
provide further information for each emerging coating material.
These references are given in full in Section 4.
The text further describes the operating pollution prevention
benefits, reported applications, operational and product benefits,
and hazards and limitations. Technologies in later stages of
development are discussed in Section 2, Available Technologies.
47
-------
LECTRON BEAM (EB)-
CURED COATINGS
In a process similar to that of UV curing, EB curing uses the high
energy of accelerated electron to directly cause the crosslinking of
inks and coatings. Also as with UV curing, acrylate oligomers and
prepolymers are used, although other chemistries are available for
EB curing.
The high energy of EB curing gives the highest margin of safety for
applications where extractables or low odors are essential. This
high energy ensures adequate conversion from oligomer to polymer.
Thus, unlike UV curing, very thick films and laminating adhesives
can be EB-cured. Furthermore, the EB process is not sensitive to
either ink color or the opacity of the film or paper.
EB curing is a cold curing process. Thus, heat-sensitive substrates
can be printed and cured without causing deformation.
The curing chamber must be inerted, i.e., pressurized or filled with
nitrogen or carbon dioxide, during EB curing. Air, i.e., oxygen,
significantly retards EB curing.
EB curing takes place instantaneously. Thus, as with UV curing for
high gloss applications, time should be allotted between coating and
curing to let the coating or ink flow out and level. If this is not
possible, other precautions and equipment should be considered to
achieve the desired gloss level (Sun Chemical, 1991).
EB coatings are used mainly for paper, wood, and plastic sub-
strates. They often replace conventional thermal treatments for
coating flat surfaces. At least one company in Japan used EB
curing for metal coil stock. EB has seen limited use in high-volume
printing operations. A more frequent use is to finish automotive
hubcaps and wheel rims (Dick, 1991). The high capital cost of EB
curing equipment has limited the acceptance of EB-cured coatings
(Paul, 1986).
48 Emerging Technologies
-------
RADIATION-INDUCED THERMALLY
CURED COATINGS
Laser heating, particularly when applied by a robotic system, allows
accurate heat input to rapidly cure thermoplastic or water-based
coatings. The laser fusion system was originally designed to cure
fluorocarbon thermoplastics such as polytetrafluoroethylene. The
curing of other powder and water-based coatings is being tested.
Infrared, microwave, laser, or radio-frequency radiation can be used
to heat a fluid coating to induce curing by thermal mechanisms.
The curing reaction is essentially similar to conventional curing in a
convection oven, except that heat is supplied by the incident
radiation (Paul, 1986; Poullos, 1991).
Emerging Technologies 49
-------
iWO-COMPONENT REACTIVE
IQUID COATINGS
In a two-component reactive liquid coating system, two low-viscosity
liquids are mixed just before entering the application system as
shown in Figure 7. One liquid contains reactive resins, and 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 two reactive components from
coming in contact with each other until just before spray release.
Two-component application allows coating without the use of a
volatile organic solvent in the coating formulation. Some solvent
may be used to clean up any unreacted liquids.
Part
A - Epoxy resin
Part
B - Curing agent
or catalyst
Room temperature cure
or heat with conventional
oven or Infrared lamps
Paint overspray would
have to be collected and
disposed of. These paints
are permanently reactive
after mixing and cannot
be reused
Source: Battelle
Figure 7. 100% Solids Chemical Cure Two-Component Liquid Paint System.
50
Emerging Technologies
-------
WATER-BASED TEMPORARY
PROTECTIVE COATINGS
Consumer products, particularly automobiles, are shipped from
factory to consumer through an uncontrolled and potentially harsh
environment. A protective coating helps to maintain the quality of
the factory finish. Temporary coatings are also applied to protect
machined metal surfaces from corrosion during shipping or between
manufacturing steps.
The normal practice is to apply a solvent-based coating. Solvent in
the coating evaporates into the atmosphere as the coating cures.
Solvent may also be needed for removal of the temporary coating
when the product arrives at the point of sale or use.
Temporary coating materials using a water-based acrylic copolymer
system are available to produce a tough transparent film. The film
can protect the paint for up to 1 year but can be removed by
washing with an aqueous alkali solution.
The reported removal time for the water-based temporary protective
coating is 10 minutes for an automobile compared to 20 minutes for
removal of a wax coating (Product Finishing, 1986).
Water-based coatings have also been tested as masking layers to
protect areas of metal substrate during chemical milling and etching
(Toepke, 1991).
Emerging Technologies 51
-------
PERMEATION- OR
NJECTION-CURED COATINGS
Solvent use is avoided in vapor cure coating formulations by
applying a reactive resin as a liquid and then inducing curing by
exposing the liquid to a vapor containing a compound that initiates
polymerization. Some solvent may be used to clean up unreacted
resin. One example is polyol-isocyanate coatings cured by tertiary
amine vapor injection (Pilcher, 1988).
52 Emerging Technologies
-------
SUPERCRITICAL CARBON DIOXIDE
AS SOLVENT
Supercritical CO2 fluid can be used to replace organic solvent in
conventional coating formulations. This new technology can sub-
stitute up to 80% of the organic solvent required in many spray
applications. The CO2 solvent is compatible with classical high-
molecular-weight resins and existing painting facilities and proce-
dures. Thus, it enables finishers to continue to use their existing
resin formulations while substantially reducing VOC emissions.
Application of supercritical solvent coatings requires investment in
new paint mixing, handling, and spraying equipment.
Supercritical CO2 proportioning and supply units are available from
at least one commercial supplier. The unit mixes coating concen-
trates and CO2 to give a coating fluid with the required viscosity.
The supercritical CO2 solvent is mixed with coating concentrate and
supplied to a specially designed spray gun. The coating/solvent mix
is applied in the same way as conventional paint. The CO2 oper-
ating pressure ranges from 1200 to 2000 psi, and the airspray
supply pressure is about 70 psi (Nordson, 1991).
Emerging Technologies
53
-------
SECTION 4
INFORMATION SOURCES
References
Bowden, Champ. 1989. "The Future is Clear." Products Finishing.
53(10), July.
Crump, David L. 1991. "Powder Coating Technology at Boeing."
In: Sixth Annual Aerospace Hazardous Waste Minimization Con-
ference. Boeing Company, Seattle, Washington, June 25-27.
Danneman, Jeffrey. 1988. "UV Process Provides Rapid Cure for
Compliant Wood Finishes." Modern Paint and Coatings. 78(2):
28-29. February.
Dick, Richard J. 1991. Personal Communication. Battelle,
Columbus, Ohio, December 5.
Fish, John G. 1982. "Organic Polymer Coatings." In: Rointan F.
Bushah (eci.), Deposition Technologies for Films and Coatings.
Noyes Publications, Park Ridge, New Jersey, pp. 490-511.
Hester, Charles I. and Rebecca L Nicholson, 1989. "Powder
Coating Technology Update." EPA-45/3-89-33. October.
Ingleston, Roy. 1991. "Powder Coatings: Current Trends, Future
Developments." Product Finishing. 6-7. August.
Keipert, Steven. 1990. "Dual Cure Photocatalyst Systems." In:
First Annual International Workshop on Solvent Substitution, pp.
245-249, Phoenix, Arizona, December 4-7.
Maguire, George. 1988. "Fluoropolymer Powder Coatings on the
Move." Products Finishing. 53(1), October.
MP&C (Modern Paint and Coatings). 1988. "Low-VOC Coatings
Applications for Military Usage Present Major Formulation Chal-
lenge." Modern Paint and Coatings. 78(9): 35. September.
Muhlenkamp, Mack. 1988. 'The Technology of Powder." Modem
Paint and Coatings. 78(1): 52-68. November.
Nelson, Larry. 1988. "Door Market Opens to Low-VOC Coatings."
Products Finishing. 53(3): 48-53. December.
54
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Information Sources
Nordson. 1991. UNICARB System Supply Units. Amherst,
Ohio.
Paul, Swaraj. 1986. Surface Coatings Science and Technology.
John Wiley & Sons, New York, New York.
Pilcher, Paul. 1988. "Chemical Coatings in the Eighties: Trials,
Tribulations, and Triumphs." Modern Paint and Coatings. 78(6):
34-36. June.
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Trade
Associations Table 6 shows the trade associations and the technology areas they
cover. Readers are invited to contact these trade associations and
request their assistance in identifying one or more companies that
could provide the desired technological capabilities.
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Information Sources
Table 6. Trade Associations and Technology Areas
Trade Association
Association for Finishing Processes of the
Society of Manufacturing Engineers
Federated Societies for Coating Technology
National Paint & Coatings Association
Powder Coating Institute
Radtech International
Technology Areas Covered
Industrial finishing operations
Decorative and protective coatings
Paints and chemical coatings, related raw ma-
terials, and equipment
Powder coating materials and equipment
Radiation-curable coatings
Contact
P.O. Box 930
One SME Dr.
Dearborn, Ml 48121
tel. (313) 271-1500
492 Norristown Road
Bluebell, PA 19422
tel. (215) 940-0777
1500 Rhode Island Ave. N.W.
Washington, DC 20005
tel. (202) 462-6272
1800 Diagonal Rd., Suite 370
Alexandria, VA 22314
tel. (703) 684-1770
60 Revere Drive
Suite 500
Northbrook, IL 60062
tel. (708) 480-9576
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