How do drying oils harden into protective films?

Drying oils, such as linseed oil or tung oil, harden into protective films through a process of oxidative cross-linking. These oils contain high concentrations of unsaturated fatty acid triglycerides, which react with atmospheric oxygen when spread into a thin film.1

The chemical process involves several key stages:
* Hydroperoxide Formation: Oxygen adds to the unsaturated fatty acids, creating hydroperoxide derivatives.1
* Free Radical Generation: These hydroperoxides decompose into free radicals, a process often accelerated by metallic catalysts known as driers (e.g., cobalt or zirconium naphthenate).12
* Cross-linking: The resulting free radicals react with remaining unsaturated fatty acids to build a complex, three-dimensional network polymer.1

This transformation turns the liquid oil into a hard, tough, and elastic solid. While this reaction occurs naturally, the use of driers is essential in industrial applications to ensure the coating cures thoroughly and within a reasonable timeframe.342