The "drying", hardening, or, more properly, curing of oils is the result of an exothermic reaction in the form of autoxidation and is chemically equivalent to slow, flameless combustion. In this process, oxygen oxidizes the hydrocarbon chain, touching off a series of chemical reactions. As a result, the oil polymerizes, forming long, chain-like molecules. Following the autoxidation stage, the oil polymers cross-link: bonds form between neighboring molecules, resulting in a vast polymer network. Conceptually, this network equates to a fusing of individual, randomly interlocking, strands into a cohesive mass or, in the case of varnishes and paints, into a solid film. Over time, this network may undergo further change. Certain functional groups in the networks become ionized, and the network transitions from a system held together by nonpolar covalent bonds to one governed by the ionic forces between these functional groups and the metal ions present in the pigment.
The drying process is highly accelerated by certain metal ions, such as cobalt, manganese, or iron salts, that act as catalysts. These oil drying agents are often salts and complexes of these metals with lipophilic carboxylic acids such as naphthenic acids to make the ions oil-soluble.
Vegetable oils consist of glycerol esters of fatty acids, long hydrocarbon chains with a terminal carboxyl group. In oil autoxidation, oxygen attacks a hydrocarbon chain, often at the site of an allylic hydrogen (a hydrogen on a carbon atom adjacent to a double bond). This produces a free radical, a substance with an unpaired electron which makes it highly reactive. A series of addition reactions ensues. Each step produces additional free radicals, which then engage in further polymerization. The process finally terminates when free radicals collide, combining their unpaired electrons to form a new bond. The polymerization stage occurs over a period of days to weeks, and renders the film dry to the touch.
Chemical changes in the paint film continue as time passes; the polymer chains begin to cross-link. Adjacent molecules form covalent bonds resulting in a molecular network, called the stationary phase, that extends throughout the oil. Molecules are no longer free to slide past each other or to move apart. In terms of paint or varnish, the stationary phase is the equivalent to a stable film which, while somewhat elastic, does not flow or deform under the pull of gravity.
During the drying process, a number of compounds are produced that do not contribute to the polymer network. These include unstable hydroperoxides (ROOH), the major by-product of the reaction of oxygen with unsaturated fatty acids. The hydroperoxides quickly decompose, forming carbon dioxide and water, as well as a variety of aldehydes, acids, and hydrocarbons. Many of these compounds are volatile, and in an unpigmented oil, they would be quickly lost to the environment. However, in paints, such volatiles may react with lead, zinc, copper or iron compounds in the pigment, and remain in the paint film as coordination complexes or salts. A large number of the original ester bonds in the oil molecules undergo hydrolysis, releasing individual fatty acids. Some portion of the free fatty acids react with metals in the pigment, producing metal carboxylates. Together, the various non-cross-linking substances associated with the polymer network constitute the mobile phases. Unlike the molecules that are part of the network itself, they are capable of moving and diffusing within the film, and can be removed using heat or a solvent. The mobile phase may play a role in plasticizing the paint film, preventing it from becoming too brittle.
One simple technique for monitoring the early stages of the drying process is to measure weight change in an oil film over time. Initially, the film becomes heavier, as it absorbs large amounts of oxygen. Then oxygen uptake ceases, and the weight of the film declines as volatile compounds are lost to the environment.
As the oil ages, a further transition occurs. Carboxyl groups in the polymers of the stationary phase lose a hydrogen ion, becoming negatively charged, and form complexes with metal cations present in the pigment. The original network, with its nonpolar, covalent bonds is replaced by an ionomeric structure, held together by ionic interactions. At present, the structure of these ionomeric networks is not well understood.
Prior to polymerization or curing, drying oils consist of medium length hydrocarbon chain molecules that are joined at one end by a triglyceride and in shape are partially hooked or kinked. By contrast, non-"drying" waxes, such as hard-film carnauba or paste wax, and resins, such as dammar, copal, and shellac, consist of long, spaghetti-like strands of hydrocarbon molecules which interlace and compact but do not form covalent bonds in the manner of drying oils. Thus, waxes and resins are re-dissoluble whereas a cured oil varnish or paint is not.