| Plant oils |
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 Using linseed oil (a drying oil) and pigment for painting |
| Types |
| Uses |
| Components |
Adrying oil is anoil that hardens to a tough, solid film after a period of exposure to air, at room temperature. The oil hardens through a chemical reaction in which the componentscrosslink (and hencepolymerize) by the action ofoxygen (not through theevaporation of water or othersolvents). Drying oils are a key component ofoil paint and somevarnishes. Some commonly used drying oils includelinseed oil,tung oil,poppy seed oil,perilla oil,castor oil andwalnut oil. The use of natural drying oils has declined over the past several decades, as they have been replaced byalkyd resins and other binders.
Since oxidation is the key to curing these oils, those that are susceptible to chemical drying are often unsuitable for cooking and are also highly susceptible to becomingrancid through autoxidation, the process by which fatty foods develop off-flavors.[1] Rags, cloth, and paper saturated with drying oils mayspontaneously combust (ignite) after a few hours as heat is released during the oxidation process.
The "drying", hardening, or, more properly,curing of oils is the result ofautoxidation, the addition of oxygen to an organic compound, and the subsequent crosslinking. This process begins with an oxygen molecule (O2) in the air inserted into carbon-hydrogen (C-H)bonds adjacent to one of the double bonds within the unsaturated fatty acid. The resultinghydroperoxides are susceptible to crosslinking reactions. Bonds form between neighboring fatty acid chains, resulting in apolymer network, often visible by the formation of a skin-like film on samples. This polymerization results in stable films that, while somewhat elastic, do not flow or deform readily. Diene-containing fatty acid derivatives, such as those derived from linoleic acid, are especially prone to this reaction because they generatepentadienyl radicals. Monounsaturated fatty acids, such as oleic acid, are slower to undergo drying because theallylic radical intermediates are less stable (i.e., slower to form).[2]
The early stages of the drying process can be monitored by weight changes in an oil film. The film becomes heavier as it absorbs oxygen.Linseed oil, for instance, increases in weight by 17 percent.[3] As oxygen uptake ceases, the weight of the film declines as volatile compounds evaporate. As the oil ages, further transitions occur. A large number of the original ester bonds in the oil molecules undergohydrolysis, releasing individual fatty acids. In the case of paints, some portion of these free fatty acids (FFAs) 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. Themobile phase may play a role in plasticizing paint films, preventing them from becoming too brittle. Carboxyl groups in the polymers of the stationary phase ionize, becoming negatively charged and form complexes with metalcations 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.
Most drying oils rapidly increase inviscosity after heating in the absence of air. If the oil is subjected to raised temperatures for a long time, it will become a rubbery oil-insoluble substance.[3]
The drying process is accelerated by certain metal salts, especially derivatives ofcobalt,manganese, oriron.[citation needed] In technical terms, theseoil drying agents arecoordination complexes that function ashomogeneous catalysts. These salts are derived from the carboxylates oflipophiliccarboxylic acids, such asnaphthenic acids to make the complexes oil-soluble. These catalysts speed up the reduction of thehydroperoxide intermediates. 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. Premature action of the drying agents causes skinning of the paint, this undesirable process is suppressed by the addition of antiskinning agents such asmethylethyl ketone oxime, which evaporate when the paint/oil is applied to a surface.[citation needed]
Drying oils consist ofglycerol triesters of fatty acids. These esters are characterized by high levels ofpolyunsaturatedfatty acids, especiallyalpha-linolenic acid. One common measure of the "siccative" (drying) property of oils isiodine number, which is an indicator of the number ofdouble bonds in the oil. Oils with an iodine number greater than 130 are considered drying, those with an iodine number of 115–130 aresemi-drying, and those with an iodine number of less than 115 arenon-drying.
Non-"drying"waxes, such as hard-filmcarnauba or paste wax, andresins, such asdammar,copal, andshellac, consist of long, spaghetti-like strands of hydrocarbon molecules, which interlace and compact but do not interconnect (crosslink) in the manner of drying oils. Thus, waxes and resins remainsoluble in their original solvent whereas a cured oil paint or varnish does not. A dissolved wax or resin is recovered unchanged after theevaporation of its solvent, but liquids that can dissolve cured drying oils, such as somepaint removers do so by chemically changing them.[citation needed]
The ability of drying oils to form strong but flexible films is very useful inoil paints andvarnishes. In former times,walnut oil, rich inlinoleic acid, was used by painters. Nowadays,linseed oil is used instead.
Rags, cloth, and paper saturated with drying oils maycombust spontaneously (ignite) due to heat released during the curing process. This hazard is greater when oil-soaked materials are folded, bunched, or piled together, which allows heat to accumulate and accelerate the reaction. Precautions include: wetting rags with water and spreading them away from direct sunlight; keeping them in air-tight fireproof metal containers; immersing them in water inside air-tight metal containers designed for such applications; or storing them immersed in solvents in suitable closed containers.[citation needed]
Leaving linseed-oil-soaked rags in a pile after refinishing woodwork was the cause of a 1991 fire in Philadelphia'sOne Meridian Plaza, a 38-story office building, which resulted in severe structural damage, and eventually the demolition of the building.
