Curing Mechanisms-Convertible Coatings
Convertible coatings cure by one of several polymerization mechanisms, even when solvent evaporation is also involved. The resins used in convertible coatings undergo a chemical change as the cure progresses, so the resulting film is not readily re-dissolved in the solvent(s) used in application. These types of coatings are also known as thermoset materials. There are a number of chemical reactions that convertible coatings employ to form a protective film. Commonly chemistries used in convertible protective coatings are oxidation, chemical reaction (polymerization), hydration, fusion
Once the solvent evaporates from the film, these coatings cure by reaction with atmospheric oxygen. The main ingredient of the resin is a drying oil modified with synthetic molecules. Oxygen reacts with the oil portion of the resin, prompting a polymerization reaction known as oxidative cross-linking. This reaction can be accelerated by the addition (during manufacturing) of driers.
Chemical reaction coatings cure by polymerization reactions between at least two chemical entities. Polymerization basically means that a small molecule is transformed to a larger molecule by a variety of mechanisms. Polymerization is also referred to as cross linking. Chemical reaction curing coatings cover a vast range of chemistries such as epoxies, polyurethanes, polyureas, polyaspartics, polysiloxanes etc.
Like concrete, hydration coatings require some amount of water to complete their cure. Moisture-cured polyurethane is a hydration cured coating material. It must have some level of humidity in the surrounding air for it to cure. Another example is a solvent-based inorganic zinc coating based on an ethyl silicate resin. Upon application and solvent evaporation, water from the atmosphere reacts with the silicate to form silicic acid. The silicic acid reacts with the zinc pigment and polymerization proceeds until full cure.
Fusion is forced-heat curing. It is polymerization, but requires a particular temperature to complete the cure. Fusion-cured coatings may be single- or two-component materials. An example is fusion-bonded epoxy (FBE) commonly used to coat pipelines in the petrochemical industry.
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