Surface Coating of Miscellaneous Metal Parts
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Emissions, Formulations, and Preparation
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Coating Emissions
 The majority of emissions that occur during surface coating are
volatile organic compounds (VOC) and hazardous air pollutants
(HAP) that evaporate from the solvents contained in the coatings.
Individual HAPs associated with surface coating operations include
ethylene glycol, methyl ethyl ketone (2-butanone), toluene,
glycol ethers, methyl isobutyl ketone (hexone), and xylenes
(includes o, m, and p).
The most common solvents are organic compounds such as ketones,
esters, aromatics, and alcohols. To obtain or maintain certain
application characteristics, solvents are also added to coatings
immediately before use. Other ingredients of the coatings, such
as metals and particulates, may also be emitted during coating
operations.
Coating Formulations
 Conventional coatings contain at least 30% solvents to permit
easy handling and application. They typically contain 70 to
85% solvents by volume. These solvents may be of one component
or of a mixture of volatile ethers, acetates, aromatics, aliphatic
hydrocarbons, and/or water. Coatings with 30% of solvent or
less are called low solvent or "high solids" coatings.
 Waterborne coatings, which have recently gained substantial use,
are of several types: water emulsion, water soluble and colloidal
dispersion, and electrocoat. Common ratios of water to solvent
organics in emulsion and dispersion coatings are 80:20 and 70:30.
Two-part catalyzed coatings to be dried, powder coatings, hot
melts, and radiation cured (ultraviolet and electron beam) coatings
contain essentially no VOCs/HAPs, although some monomers and other
lower molecular weight organics may volatilize.
Description of Coatings Processes
The coating of metal substrates can be broken up into four major
steps: surface preparation, coating application/curing process,
curing and equipment cleaning.
Surface Preparation
 Proper surface preparation is essential in ensuring the success
of a particular coating. In fact, as high as 80% or more of
all coating adhesion failures can be directly attributed to
improper surface preparation. Typically, metal surfaces are
cleaned with a detergent or aqueous alkaline solution in order
to remove dirt, oil, grease and other contaminants. In some
cases, organic degreasers or solvent wipes may be used to remove
heavy machine grease and milling agents.
As part of surface preparation, a conversion coating might be
applied to improve adhesion, corrosion resistance, and thermal
compatibility. The processes used most often for the application
of conversion coatings on metal are phosphating (using iron or
zinc) and chromating.
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