Drying oil

Source: Wikipedia, the free encyclopedia.
Plant oils
Using linseed oil (a drying oil) and pigment for painting
Types
Uses
Components

A drying oil is an

polymerize) by the action of oxygen (not through the evaporation of water or other solvents). 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 Their use has declined over the past several decades, as they have been replaced by alkyd
resins and other binders.

Since oxidation is the key to curing in these oils, those that are susceptible to chemical drying are often unsuitable for cooking, and are also highly susceptible to becoming rancid through autoxidation, the process by which fatty foods develop off-flavors.[1] Rags, cloth, and paper saturated with drying oils may spontaneously combust (ignite) after a few hours as heat is released during the oxidation process.

Chemistry of the drying process

The "drying", hardening, or, more properly,

allylic radical intermediates are less stable (i.e., slower to form).[2]

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, the hydroperoxide combines with another unsaturated side chain to generate a crosslink.

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.

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.

Most drying oils rapidly increase in viscosity 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]

Role of metal catalysts

The drying process is accelerated by certain metal salts, especially derivatives of

lipophilic carboxylic acids, such as naphthenic acids to make the complexes oil-soluble. These catalysts speed up the reduction of the hydroperoxide 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 as methylethyl ketone oxime, which evaporate when the paint/oil is applied to a surface.[citation needed
]

Constituents

alpha-linolenic (middle), and oleic acids
(bottom). The order of drying rate is alpha-linolenic > linoleic > oleic acid, reflecting their degree of unsaturation.

Drying oils consist of

iodine number, which is an indicator of the number of double bonds in 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
.

Comparison to waxes and resins

Non-"drying"

paint removers do so by chemically changing them.[citation needed
]

Safety

Rags, cloth, and paper saturated with drying oils may combust 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's One Meridian Plaza, a 38-story office building, which resulted in severe structural damage, and eventually the demolition of the building.

See also

References

  1. ISBN 978-3527306732
    .
  2. doi:10.1007/BF02536034
  3. ^ a b Apps, E. A. (1958). Printing Ink Technology. London: Leonard Hill [Books] Limited. p. 14.

Further reading

  • "Autoxidation". McGraw Hill Encyclopedia. 8th ed. 1997.
  • 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 (archived 16 September 2009).
  • van den Berg, Jorit D.J. "Mobile and Stationary Phases in Traditional Aged Oil Paint". www.amolf.nl 2002. MOLART. 8 May 2006
  • Andés, Louis Edgar, Drying oils, boiled oil, and solid and liquid driers. London: Scott, Greenwood & Co., 1901.

External links

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