- How does paint dry?
- What is the chemical process?
From "Introduction to Paint Chemistry and principles of paint technology" by John Bentley:
In the can of paint the mixture of substances must remain stable for long periods, so the ingredients must not react with one another chemically. Yet the paint must dry on the coated surface. Under some conditions some paints do not dry, which suggests that the drying process does not merely consist of evaporation of the liquid. In fact, there are three broad mechanisms of drying, two of which involve chemical reaction, while one does not.
Drying without chemical reaction
In this case the paint does dry solely by evaporation of liquids. The polymer is fully formed in the can and, when free of solvent, is relatively hard and not sticky. During the drying process there is no chemical change in the polymer...Nitrocellulose lacquers and decorative emulsion paints dry by this process.
Drying by chemical reaction
Drying by Chemical Reaction Between Paint and Air
...oxygen reacts with drying oils and other unsaturated compounds to produce free-radicals and bring about polymerization. Also, the reaction between water and isocyanates can cause condensation polymerization. Both reactions can give a cross-linked film. In both cases the principle is the same: the air is used as a chemical reactant and is kept apart from the reactive ingredients in the paint by the tightly fitting lid on the can. If it is not, it may cause the formation of a skin, which seals off the paint below from further reaction with the air.
Drying by Chemical Reaction Between Ingredients in the Paint
Obviously the paint must remain chemically stable. The reactants must not react until the paint has been applied, yet they must all be in the paint. This paradox is resolved, either by separating the reactive ingredients in two or more containers and mixing just before use, or by choosing ingredients which only react at higher temperatures or when exposed to radiation.
Tldr: paint may dry through evaporation, or chemical reactions (these chemical reactions may be driven with heat or radiation)