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Overview:
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The Clean Air Act (CAA) requires the U.S.
Environmental Protection Agency (U.S. EPA) to establish standards that
limit hazardous air pollutants (HAPs) and volatile organic compound (VOC)
emissions from various sources. There are 188 CAA-regulated HAPs, which
include many of the solvents commonly used in surface coating. Many of
these solvents are VOCs. EPA is currently engaged in the development of
regulations for a number of categories of industrial surface coating operations
and composite operations. Collectively known as the Coatings and Composites
Coordinated Rule (CCCR), the regulations under development are national
emission standards for hazardous air pollutants (NESHAPs) under Section
112 of the CAA and for some of the categories, national VOC rules or control
technique guidelines (CTG) under Section 183(e) of the Act. The Agency
has already promulgated regulations covering the surface coating activities
of aerospace facilities and shipbuilding operations. EPA is expected to
issue final NESHAPs or Section 183(e) rules for the remaining categories
included in the CCCR by May 2002. Examples of remaining categories include
automobiles and light-duty trucks, plastic parts, and miscellaneous metal
parts.
These new standards will apply to facilities
engaged in the surface coating of synthetic polymer components; facilities
engaged in the coating of a wide range of metal products that vary in
size, shape, application method, and use; and vehicle assembly operations
that apply primers, guide coat/primer surfacers, top coats, and other
related coatings. New surface coating technologies have lower VOCs and
HAPs content than the conventional coating systems because of the reduced
solvent concentration in the formulation, and due to improved coating
chemistries. Six of the technologies are briefly described below, including
reference to a Joint Service Pollution Prevention Technical Library data
sheet, if applicable.
Powder Coating:
Powder coating paint systems, also referred to as "dry painting,"
apply finely ground thermosetting or thermoplastic powders, usually by
electrostatic means, followed by curing to melt and fuse the powder into
a continuous coating. Powder coating has no VOC content, and the HAPs
content is considerably reduced because of the absence of solvents in
the coatings. The presence of HAPs in the coating is dependent on the
specific formulation of the coating.
Joint Service Pollution Prevention Technical
Library Data sheet: Section 4-5, Powder Coating Painting System.
High-Solids Coatings:
High solids coatings are one- or two-component systems based on acrylic,
alkyd, epoxy, polyester, or urethane resins and can be cured at ambient
temperatures or in high temperature bake ovens. High-solids coatings and
primers contain higher concentrations (40 to nearly 100 percent) of non-volatile
organics than conventional paints (which typically contain 8 to 30 percent
solids) because the high solids formulations use low-molecular weight
resins and require less solvent to attain the viscosity needed for the
application. Depending on the coating formulation, the VOC concentrations
of high-solids coatings typically range from 0 to 60 percent. HAPs from
the application of high-solids coatings tend to be less than with conventional
coatings because of the reduced solvent concentrations. Emissions of non-solvent
HAPs, such as isocyanatyes and chromates, are specific to the coating.
Waterborne Coatings
Waterborne coatings differ from conventional coatings in that water is
substituted for certain organic solvents in the coating and is used as
the dispersal medium for synthetic resins and pigments in the coatings.
These types of coatings may still contain between 5 to 20 percent organic
solvents for wetting, viscosity control, and pigment dispersion. The organic
solvents in the paint allow the particles of resin to coalesce, as the
water evaporates, to form a continuous coating. The HAPs content tends
to be lower because of the reduced solvent concentrations. Concentrations
of metals and other HAPs are specific to the type of paint used.
Joint Service Pollution Prevention Technical Library Data sheet: Section
4-7, Waterborne Paint.
UNICOAT
UNICOAT was originally developed at the Naval Warfare Center (NAWC) Warminster.
It now refers to a self-priming topcoat for aircraft that replaces the
traditional primer/topcoat systems. UNICOAT is a polyurethane that provides
the adhesion and corrosion resistance of a primer and the chemical resistance,
durability, and flexibility of the original topcoat. It is lead-free,
chromate-free, and a blend of non-toxic, organic, and inorganic zinc compounds;
however the polyurethane chemistry is based on isocyanates which, according
to the Naval Air Warfare Center Patuxent, has a "free" monomer
(usually 0.5% by mass or less) that is a HAP, which may be released during
mixing and spraying. UNICOAT complies with the VOC levels of 420 grams
per liter (g/L), thereby reducing VOC emissions and hazardous waste generation
by 50 to 70 percent.
Joint Service Pollution Prevention Technical Library Data sheet: Section
4-6, UNICOAT Paint Technology.
Radiation Curable Coatings
Radiation-Curable Coatings are especially formulated to cure quickly by
exposure to ultraviolet light (UV), electron beam (EB), infrared (IR),
or microwave radiation. IR is sometimes included in the radiation-curable
category, but in reality, IR is an improved method of transferring thermal
energy to a coated part. IR can be used with any coating material to accelerate
the curing process where a conventional oven is typically used. Radiation-Curable
Coatings have a higher solids content and consist of a low-molecular weight
resin, a reactive solvent containing unsaturated groups, and a photoinitiator
(only contained in UV-curable coatings). Radiation-Curable Coatings are
usually clear, but can be pigmented and tend to exhibit good resistance
to abrasion, heat staining, and weathering. According to the Concurrent
Technologies Corporation, radiation curable coatings are available in
zero-VOC. The HAPs content of these coatings is dependent on the composition
of the specific coating, but should be less than that found in conventional
coatings.
Supercritical Carbon Dioxide (CO2) Spraying
Supercritical CO2 Spraying is a paint-spraying process that substitutes
CO2, above its critical temperature and pressure, for approximately 20-60
percent of the solvents that are used in conventional coating formulations.
Supercritical CO2 reduces paint viscosity and produces a vigorous atomization
and a quality finish. VOC content and emissions may be reduced up to 60
percent, and HAPs emissions can lowered by as much as 90 percent.
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