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Thermosetting Mechanisms

Paints that cure by polymerization are generally one- or two-package coatings that polymerize by way of a chemical reaction and cure into a cross-linked film. Depending on composition, they may need to dry first by evaporation of solvent. Classic two-package epoxies or polyurethanes would fall into this category.

The "drying oils", counter-intuitively, cure by a crosslinking reaction even if they are not put through an oven cycle and seem to dry in air. The film formation mechanism of the simplest examples involves the first evaporation of solvents followed by a reaction with oxygen from the environment over a period of days, weeks, and even months to create a crosslinked network. Classic alkyd enamels would fall into this category. Oxidative cure coatings are catalyzed by metal complex driers such as cobalt naphthenate though cobalt octoate is more common.

Recent environmental requirements restrict the use of volatile organic compounds (VOCs), and alternative means of curing have been developed, generally for industrial purposes. UV curing paints, for example, enable formulation with very low amounts of solvent, or even none at all. This can be achieved because of the monomers and oligomers used in the coating have relatively very low molecular weight, and are therefore low enough in viscosity to enable good fluid flow without the need for additional thinner. If solvent is present in significant amounts, generally it is mostly evaporated first and then crosslinking is initiated by ultraviolet light. Similarly, powder coatings contain no solvent. Flow and cure are produced by the heating of the substrate after electrostatic application of the dry powder.