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A drying oil is an oil which hardens to a tough, solid film after a period of exposure to air. The term "drying" is actually somewhat of a misnomer, since the oil does not harden through the evaporation of water, but through a chemical reaction in which oxygen is absorbed from the environment (autoxidation). Drying oils are a key component of oil paint and many varnishes. Some commonly used drying oils include linseed oil, tung oil, poppy seed oil and walnut oil.
Drying oil composition
Drying oils are characterized by high levels of polyunsaturated fatty acids. One common measure of the siccative (drying) property of oils is iodine number. Oils with an iodine number greater than 130 are considered drying, those with an iodine number of 115-130 are semi-drying, and those with an iodine number of less than 115 are non-drying. While a variety of drying oils are used in industry, linseed oil remains the preferred medium for artists' paints.
Drying process
The drying of oils is the result of a reaction, called autoxidation, which is chemically equivalent to slow, flameless combustion. In this process, oxygen attacks the hydrocarbon chain, touching off a series of addition 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. 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.
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. However, chemical changes in the paint film continue.
As time passes, the polymer chains begin to cross-link. Adjacent molecules form covalent bonds, forming a molecular network called the that extends throughout the oil. In this network, known as the stationary phase, molecules are no longer free to slide past each other, or to move apart. The result is 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.
References
- “Autoxidation.” McGraw Hill Encyclopedia. 8th ed. 1997.
- Flanders, Peggy J. “How Oils Dry.” www.peggyflanders.com. 5 May 2006
- Friedman, Ann, et. al. “Painting.” www.worldbookonline.com. 2006. 46 Stetson St. #5 Brookline, MA. 10 May, 2006
- “History of Oil Paint.” www.cyberlipid.org. 5 May 2006
- Mecklenburg, Marion. "Autoxidation of Oil" 13 Jan 2006. www.thepaintershandbook.org. Mark David Gottsegen. 11 June 2006
- van den Berg, Jorit D.J. “Mobile and Stationary Phases in Traditional Aged Oil Paint.” www.amolf.nl 2002. MOLART. 8 May 2006
See also
"Drying oil"