How does oxygen cause linseed oil and tung oil to cross-link?

Linseed oil and tung oil are drying oils that contain high concentrations of unsaturated fatty acids. Their ability to form a hard, protective film relies on a complex process of oxidative cross-linking.1

When these oils are exposed to the air, atmospheric oxygen reacts with the unsaturated fatty acids to form hydroperoxide derivatives. These hydroperoxides are unstable and subsequently decompose, a process often accelerated by the presence of metallic driers like cobalt naphthenate.1

This decomposition generates free radicals, which are highly reactive species. These free radicals then react with the remaining unsaturated fatty acid chains, creating a three-dimensional, cross-linked polymer network. This transformation turns the liquid oil into a solid, durable, and waterproof film.21