What role do driers such as cobalt naphthenate play in oil-based coatings?

In oil-based and alkyd coatings, driers such as cobalt naphthenate function as essential catalysts that accelerate the film-formation process.1

These coatings rely on drying oils—which contain unsaturated fatty acids—to harden through oxidative cross-linking. When the coating is exposed to air, atmospheric oxygen reacts with the fatty acids to form organic hydroperoxides. Driers like cobalt naphthenate catalyze the decomposition of these hydroperoxides into free radicals. These free radicals then initiate the chain reactions necessary to cross-link the remaining unsaturated fatty acids, allowing the coating to cure thoroughly and harden within a reasonable timeframe.21

While lead-based compounds were historically used for this purpose, they have been replaced by safer organometallic alternatives like cobalt and zirconium naphthenate due to the toxicity of lead.1