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AMONG THE MOST CRITICAL AND OFTEN MIS-understood properties of a coating are cure and the measurement of cure. If ASTM standard technology  is examined, one will find no listing for the word “cure.” Cure is something that can have different meanings for different people. In the coating and paint field of study, cure is usually thought of as the chemical reaction between functional groups that results in cross-linking of a system by one or more of a variety of schemes including polymerization of monomers, rearrangement, condensation or elimination, reaction with adventitious moisture or oxygen, as well as others. However, the word can be taken to mean much more and should include the drying of a lacquer as when thermoplastic coatings are applied, the solidification and/or crystallization of polyolefins or other hot-melt applied coatings, the drying of latexes or emulsions that can form films through the high-pressure compacting forces of surface tension, as well as of other ways of forming a coherent, useful thin film with good mechanical, electrical, or other properties. This chapter will mainly be concerned with cross-linked systems and cure will be taken to mean reactions that result in a thermoset or network-formed coating , unless otherwise indicated. The desired and often optimum chemical and physical properties of a coating are dependent on proper curing conditions—time, temperature, and humidity. For example, if a thermosetable, acrylic polymer-based lacquer is applied to a substrate and solvent is allowed to evaporate under ambient conditions, the resulting film might have cracks, low gloss, and be hard and brittle. Yet, the same system could result in a coherent, useful film if the solvent were evaporated under controlled temperatures for specific times. The film might still be brittle and have poor chemical resistance, but such factors may not be important in certain instances. However, strength properties will be markedly improved as a cross-linking agent is added, and the system is allowed to react under proper temperature conditions for an appropriate length of time. The resulting coating will be glossy, hard, tough, and chemical resistant. The time and temperature relationship and the interaction of these parameters has been reported by Neag and Prime  in the case of powder coating cure.
Miranda, Thomas J.
Senior Consultant, Consolidated Research Inc., Granger, IN