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EPA Self-Audit and Inspection
Guide
CURING METHODS Curing is the final stage of the organic finishing process. Curing has two stages. The first is the removal of the solvent or diluent through evaporation so that the coating is no longer wet to the touch. This is often called drying. The second stage is the actual curing, during which the resins or binders in the coating material are undergoing a chemical reaction. The reaction causes crosslinking between the resin molecules and renders the coating film relatively inert to the environment. In the curing of powder coating materials, because no solvent or diluent is there to be removed, only the second stage occurs. Regardless of the type of technology used, curing equipment generates thermal energy that is absorbed by the coating and part. The curing stage elevates the workpiece and coating to a particular temperature and holds that temperature for a set period of time. The combination of temperature and time serves to evaporate solvents and set the coating. Temperature and exposure times are carefully monitored to ensure proper curing and di-ying. Extended baking or exposure to heat sources may impair the coating characteristics. If ambient air conditions permit, curing of low-solvent coatings can be completed in open areas. No heat is generated or supplied to the area, but air circulation may be enhanced by blowers and fans. Open air curing is often done during the warmer summer months. The type of curing method employed is often dictated by the coatings materials used. Air dried coatings are defined by the EPA as those that cure at room temperatures, while those that cure at temperatures up to 194 F are classified as forced-air dried. Baked coatings require a curing stage at temperatures above 250 F. Others coatings are classified as radiation curable. The curing occurs when the part and coating are exposed to infrared, ultraviolet, microwave, or other radiation. Two common curing technologies are convection ovens and
infrared radiation systems. Both provide consistent
curing of many different coatings. This chapter
describes each of these systems and their environmental
issues.
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