Oil drying agent
An oil drying agent is a coordination compound that accelerates (catalyzes) the hardening of drying oils through chemical crosslinking. The catalysts affect the autoxidation of the oils with air. Typical oil drying agents are derived from cobalt, manganese, and iron and lipophilic carboxylic acids such as naphthenic acids to make them oil-soluble.
Varieties of drying agents
Oil drying agents are often called salts, but they are probably non-ionic coordination complexes akin to basic zinc acetate. These catalysts were traditionally carboxylates of lead, but cobalt and other driers such as zirconium, zinc, calcium, and iron have become popular. Most driers are colorless but cobalt is a deep blue purple color and iron driers are reddish orange. Thus the colored driers are compatible only with certain paints.
Premature hardening of paint, called skinning, can be inhibited by the addition of volatile ligands that bind to the oil drying agents. One such inhibitor is methylethyl ketone oxime (MEKO), which is called an "antiskinning agent."
- Ulrich Poth, "Drying Oils and Related Products" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a09_055
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