28-08-2012, 11:15 AM
Surface Coating
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Description of Process
Surface coating materials are primarily organic polymers dissolved in a solvent or suspended in an
emulsion. The material may be applied to the product by brush or by spraying. In the process of
"drying", the solvent or emulsion carrier evaporates. The organic polymer is left behind to coat
the surface. Regardless of the application technique, all the volatile constituents of the finish are
released to the environment. However, the application technique can determine how much finish
must be used and the corresponding amount of air emissions per product.
Throughout most of history, the solvents and emulsion carriers have been volatile organic liquids.
Most of the compounds comprising these solvents are toxic air pollutants under WAC 173-460.
In the 1950's, the first "latex" paints were introduced to the market. Latex paints are emulsions of
organic polymers with water being the primary emulsion carrier. Volatile organic solvents may
still be included in the formulation to balance necessary flow and drying rates. However, their
proportion is greatly reduced from "solvent-based" products. Due largely to environmental
concerns, the use of water-based finishes appears to be growing rapidly in the commercial sectors.
When a finish material is rubbed or brushed onto a product, essentially all the finish material
contacts the surface. For all practical purposes, this is "100% transfer efficiency". However, it is
labor-intensive and does not give a reliably high-quality finish. It is only rarely used commercially.
Spray coating is faster than manual application, and gives a better finish more consistently. By far
the most common commercial application technique is spray coating.
Method of Determining Emissions
Estimating emissions from surface coating needs to take into account the coating material
characteristics and the coating technique. VOC speciation and quantity estimation can best be
done by a material balance of quantity and composition of coatings used.
Surface coating emissions are covered in AP-421in Section 4.2, which provides a table that
describes how to calculate the weight of VOC emissions per volume of coating depending on
what VOC composition information is available on the coating. This could be VOC by weight
percent, volume percent, or in waterborne paint, as weight percent or volume percent of total
volatiles, with or without water.
All coatings manufacturers are trying to develop lower VOC coatings, minimizing HAP
components. Emphasis on pollution prevention practices, combined with improved control
technologies should be continued.