Oil drying agent

An oil drying agent, also known as siccative, is a coordination compound that accelerates (catalyzes) the hardening of drying oils, often as they are used in oil-based paints. This so-called "drying" (actually a chemical reaction that produces an organic plastic) occurs through free-radical chemical crosslinking of the oils. The catalysts promote this free-radical autoxidation of the oils with air.

Typical oil drying agents are derived from ions of cobalt, manganese, and iron, prepared as "salts" of lipophilic carboxylic acids such as naphthenic acids, in order to give them a soap-like chemical formula and make them oil-soluble.

Varieties of drying agents

Oil drying agents are often called salts, but they are probably non-ionic coordination complexes formed with coordinate covalent bonds, akin to basic zinc acetate. These catalysts were traditionally hydrocarbon carboxylate chelates of lead, but due to lead's toxicity, cobalt and other driers such as zirconium, zinc, calcium, and iron have become popular. Most driers are colorless but cobalt driers are a deep blue purple color and iron driers are reddish orange. These colored driers are therefore compatible only with certain darker pigmented paints where their color will be lost.

Japan drier is a common lay term and generic product name for any oil drying agent that can be mixed with drying oils such as boiled linseed oil and alkyd resin paints to speed up "drying". The name refers to "japanning", a term for the use of drying oils as an imitation or substitution for urushiol based Japanese lacquer.

Antiskinning

Premature hardening of paint, called "skinning" because it forms a skin on the surface of the paint, can be inhibited by the addition of volatile ligands that bind to the oil drying agents, then lose their action as they themselves slowly volatilize and leave the drying agent to work. One such inhibitor is methylethyl ketone oxime (MEKO), which is called an "antiskinning agent".