Drying oil

Plant oils
linseed oil
Types
Vegetable fats	(list)
Macerated	(list)
Uses
Drying oil - Oil paint
Cooking oil
Fuel - Biodiesel
Components
Saturated fat
Monounsaturated fat
Polyunsaturated fat
Trans fat

A drying oil is an oil that hardens to a tough, solid film after a period of exposure to air. The term "drying" is actually a misnomer - the oil does not harden through the evaporation of water or other solvents, but through a chemical reaction in which the components crosslink by the action of oxygen. Drying oils are a key component of oil paint and some varnishes. Some commonly used drying oils include linseed oil, tung oil, poppy seed oil, perilla oil, and walnut oil. Their use has declined over the past several decades as they are replaced by alkyd resins and then other binders.^[1]

Chemistry of the drying process

The "drying", hardening, or, more properly, curing of oils is the result of autoxidation, the addition of oxygen to an organic compound. In this process, O₂ inserts into C-H bonds adjacent to double bonds within the unsaturated fatty acids. The resulting hydroperoxides are susceptible to crosslinking reactions. The formation bonds form between neighboring fatty acid chains, results in a polymer network. The resulting stationary phase is the equivalent to a stable film which, while somewhat elastic, does not flow or deform under the pull of gravity.

Simplified chemical reactions associated with cobalt-catalyzed drying process. In the first step, the diene undergoes autoxidation to give a hydroperoxide, in the second step a cobalt drying agent converts the hydroperoxide to a hydroxyl radical, which in the final step in this sequence combines with another unsaturated side chain to generate a carbon-based radical that is capable of further polymerization.

The drying process is accelerated by certain homogeneous catalysts, especially derivatives of cobalt, manganese, or iron. These oil drying agents are coordination complexes derived from the carboxylates of lipophilic carboxylic acids, such as naphthenic acids to make the complexes oil-soluble. The species reduce the hydroperoxides as shown in the scheme. A series of addition reactions ensues. Each step produces additional free radicals, which then engage in further crosslinking. The process finally terminates when pairs of free radicals combine. The polymerization occurs over a period of days to years, and renders the film dry to the touch.

The early stages of the drying process can be monitored by weight changes in an oil film over time. Initially, the film becomes heavier as it absorbs oxygen. As oxygen uptake ceases, the weight of the film declines as volatile compounds are lost to the environment. As the oil ages , a further transitions occur. A large number of the original ester bonds in the oil molecules undergo hydrolysis, releasing individual fatty acids. Some portion of these free fatty acids (ffa's) 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.Carboxyl groups in the polymers of the stationary phase ionize, 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. The structure of these ionomeric networks is not well understood.

Constitution of drying oils

Representative triglyceride found in a drying oil. The triester is derived from three different unsaturated fatty acids, alpha-linoleic (top), linolenic (middle), and oleic acids (bottom).

Drying oils consist of glycerol triesters of fatty acids. These esters are characterized by high levels of polyunsaturated fatty acids, especially alpha-linolenic acid. One common measure of the "siccative" (drying) property of oils is iodine number, which is an indicator of the number of double bonds in the fatty acid components of the oil. 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.

Comparision to waxes and resins

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.

Safety

Rags, cloth, and paper saturated with drying oils may combust spontaneously (catch on fire) due to heat released during the curing process. This hazard is more serious when where oil-soaked materials are folded, bunched, compressed, or piled together, which allows the heat to accumulate and accelerate the reaction. Precautions include: wetting the rags with water and spreading them away from direct sunlight; closing them off completely in water inside air-tight metal containers designed for such applications; or storing them immersed in solvents in suitable closed containers.

References

1. ^ Ulrich Poth, "Drying Oils and Related Products" in Ullmann's Encyclopedia of Industrial Chemistry Wiley-VCH, Weinheim, 2002. doi:10.1002/14356007.a09_055

See also

• “Autoxidation.” McGraw Hill Encyclopedia. 8th ed. 1997.
• Flanders, Peggy J. “How Oils Dry.” www.peggyflanders.com. 5 May 2006 <http://www.peggyflanders.com/Information/how_oils_dry.htm>
• Friedman, Ann, et al. “Painting.” www.worldbookonline.com. 2006. 46 Stetson St. #5 Brookline, MA. 10 May, 2006 <http://www.worldbookonline.com/wb/Article?id=ar410780>
• “History of Oil Paint.” www.cyberlipid.org. 5 May 2006 <http://www.cyberlipid.org/perox/oxid0011.htm>
• van den Berg, Jorit D.J. “Mobile and Stationary Phases in Traditional Aged Oil Paint.” www.amolf.nl 2002. MOLART. 8 May 2006 <http://www.nwo.nl/nwohome.nsf/pages/NWOP_62WDAG/$file/molart%20eindverslag.pdf>

See also

• Oil paint
• Semi-drying oil
• Blown oil
• Wood finishing
• Varnish

External links

• Tung and Linseed Oils by Steven D. Russel