Drying oil

Drying oil

Overview
A drying oil is an oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

 that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (not through the evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 of water or other solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s). Drying oils are a key component of oil paint
Oil paint
Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and varnish may be added to increase the glossiness of the...

 and some varnish
Varnish
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...

es. Some commonly used drying oils include linseed (flax seed) oil
Linseed oil
Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...

, tung oil
Tung oil
Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree . As a drying oil, tung oil hardens upon exposure to air. The resulting coating is transparent plastic-like and is exploited in most of its applications, which include wood finishing and the...

, poppy seed oil
Poppyseed oil
Poppyseed oil is an edible oil from poppy seeds . The oil has culinary and pharmaceutical uses, as well as long established uses in the making of paints, varnishes, and soaps.Poppy seeds yield 45–50% oil...

, perilla oil
Perilla oil
Perilla oil is obtained from the seeds of herbs of the genus Perilla, usually from the species Perilla frutescens. The seeds contain 35 to 45 percent oil which is obtained by pressing. Perilla oil is used along with synthetic resins in the production of varnishes...

, and walnut oil
Walnut oil
Walnut oil is oil extracted from English walnuts . It is about 50% linoleic acid, an essential omega-6 fatty acid. Walnut oil is also a good source of omega-3 fatty acids which are also essential to human nutrition....

. Their use has declined over the past several decades, as they have been replaced by alkyd
Alkyd
An alkyd is a polyester modified by the addition of fatty acids and other components. They are derived from polyols and a dicarboxylic acid or carboxylic acid anhydride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and...

 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
Rancidification
Rancidification is the chemical decomposition of fats, oils and other lipids . When these processes occur in food, undesirable odors and flavors can result. In some cases, however, the flavors can be desirable . In processed meats, these flavors are collectively known as "warmed over flavor"...

 due to autoxidation, the process by which fatty foods develop off-flavors.
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Encyclopedia
A drying oil is an oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

 that hardens to a tough, solid film after a period of exposure to air. The oil hardens through a chemical reaction in which the components crosslink by the action of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 (not through the evaporation
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....

 of water or other solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...

s). Drying oils are a key component of oil paint
Oil paint
Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and varnish may be added to increase the glossiness of the...

 and some varnish
Varnish
Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...

es. Some commonly used drying oils include linseed (flax seed) oil
Linseed oil
Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...

, tung oil
Tung oil
Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree . As a drying oil, tung oil hardens upon exposure to air. The resulting coating is transparent plastic-like and is exploited in most of its applications, which include wood finishing and the...

, poppy seed oil
Poppyseed oil
Poppyseed oil is an edible oil from poppy seeds . The oil has culinary and pharmaceutical uses, as well as long established uses in the making of paints, varnishes, and soaps.Poppy seeds yield 45–50% oil...

, perilla oil
Perilla oil
Perilla oil is obtained from the seeds of herbs of the genus Perilla, usually from the species Perilla frutescens. The seeds contain 35 to 45 percent oil which is obtained by pressing. Perilla oil is used along with synthetic resins in the production of varnishes...

, and walnut oil
Walnut oil
Walnut oil is oil extracted from English walnuts . It is about 50% linoleic acid, an essential omega-6 fatty acid. Walnut oil is also a good source of omega-3 fatty acids which are also essential to human nutrition....

. Their use has declined over the past several decades, as they have been replaced by alkyd
Alkyd
An alkyd is a polyester modified by the addition of fatty acids and other components. They are derived from polyols and a dicarboxylic acid or carboxylic acid anhydride. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and...

 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
Rancidification
Rancidification is the chemical decomposition of fats, oils and other lipids . When these processes occur in food, undesirable odors and flavors can result. In some cases, however, the flavors can be desirable . In processed meats, these flavors are collectively known as "warmed over flavor"...

 due to autoxidation, the process by which fatty foods develop off-flavors. Rags, cloth, and paper saturated with drying oils may combust spontaneously
Spontaneous combustion
Spontaneous combustion is the self-ignition of a mass, for example, a pile of oily rags. Allegedly, humans can also ignite and burn without an obvious cause; this phenomenon is known as spontaneous human combustion....

 (ignite) in a few hours, due to accumulation of heat released during the oxidation process.

Chemistry of the drying process


The "drying", hardening, or, more properly, curing
Curing (chemistry)
Curing is a term in polymer chemistry and process engineering that refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation, electron beam or heat...

 of oils is the result of autoxidation
Autoxidation
Autoxidation is any oxidation that occurs in open air or in presence of oxygen and/or UV radiation and forms peroxides and hydroperoxides. A classic example of autoxidation is that of simple ethers like diethyl ether, whose peroxides can be dangerously explosive. It can be considered to be a slow,...

, the addition of oxygen to an organic compound and the subsequent crosslinking. In this process, O2 inserts into C-H bonds
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...

 adjacent to double bonds within the unsaturated fatty acids. The resulting hydroperoxides are susceptible to crosslinking reactions. Bonds form between neighboring fatty acid chains, resulting in a polymer network, often visible by 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 generate pentadienyl radicals. Monounsaturated fatty acids, such as oleic acid, are slower to undergo drying because the allyl
Allyl
An allyl group is a substituent with the structural formula H2C=CH-CH2R, where R is the connection to the rest of the molecule. It is made up of a methylene , attached to a vinyl group . The name is derived from the Latin word for garlic, Allium sativum. Theodor Wertheim isolated an allyl...

ic radical intermediates are less stable (i.e., slower to form).


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
Linseed oil
Linseed oil, also known as flaxseed oil, is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant . The oil is obtained by cold pressing, sometimes followed by solvent extraction...

, for instance, increases in weight by 17 percent. 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 undergo hydrolysis
Hydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...

, releasing individual fatty acids. 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. The mobile 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 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.

Most drying oils rapidly increase in viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher 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.

Role of metal catalysts


The drying process is accelerated by certain metal salts, especially derivatives of cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

, manganese
Manganese
Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature , and in many minerals...

, or iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

. In technical terms, these oil drying agent
Oil drying agent
An oil drying agent is a coordination compound that accelerates the hardening of drying oils through chemical crosslinking. The catalysts affect the autoxidation of the oils with air...

s are coordination complexes that function as homogeneous catalysts. These salts are derived from the carboxylates of lipophilic
Lipophilic
Lipophilicity, , refers to the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. These non-polar solvents are themselves lipophilic — the axiom that like dissolves like generally holds true...

 carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s, such as naphthenic acid
Naphthenic acid
Naphthenic acid is the name for an unspecific mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units. The main fraction are carboxylic acids with a carbon backbone of 9 to 20 carbons. The naphtha fraction of the crude oil...

s 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
Methylethyl ketone oxime
Methylethyl ketone oxime is the organic compound with the formula C2H5CCH3. This colourless liquid is the oxime derivative of methyl ethyl ketone. MEKO, as it is called in the paint industry, is used to suppress "skinning" of paints: the formation of a skin on paint before it is used...

, which evaporate when the paint/oil is applied to a surface.

Constitution of drying oils


Drying oils consist of glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

 triester
Ester
Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s of fatty acids. These esters are characterized by high levels of polyunsaturated
Polyunsaturated fat
In nutrition, polyunsaturated fat, or polyunsaturated fatty acid, are fatty acids in which more than one carbon–carbon double bond exists within the representative molecule. That is, the molecule has two or more points on its structure capable of supporting hydrogen atoms not currently part of the...

 fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...

s, especially alpha-linolenic acid
Alpha-linolenic acid
α-Linolenic acid is an organic compound found in many common vegetable oils. In terms of its structure, it is named all-cis-9,12,15-octadecatrienoic acid. In physiological literature, it is given the name 18:3 ....

. One common measure of the "siccative" (drying) property of oils is iodine number
Iodine number
The iodine value in chemistry is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. An iodine solution is yellow/brown in color and any chemical group in the substance that reacts with iodine will make the color disappear at a precise concentration...

, which is an indicator of the number of double bond
Double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...

s 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
Semi-drying oil
A semi-drying oil is an oil which partially hardens when it is exposed to air. This is as opposed to a to drying oil, which hardens completely, or a non-drying oil, which does not harden at all. Oils with an iodine number of 115-130 are considered semi-drying.-Examples of semi-drying oils:*Corn...

, and those with an iodine number of less than 115 are non-drying
Non-drying oil
A non-drying oil is an oil which does not harden when it is exposed to air. This is as opposed to a to drying oil, which hardens completely, or a semi-drying oil, which partially hardens...

.

Comparison to waxes and resins


Non-"drying" wax
Wax
thumb|right|[[Cetyl palmitate]], a typical wax ester.Wax refers to a class of chemical compounds that are plastic near ambient temperatures. Characteristically, they melt above 45 °C to give a low viscosity liquid. Waxes are insoluble in water but soluble in organic, nonpolar solvents...

es, such as hard-film carnauba
Carnauba wax
Carnauba , also called Brazil wax and palm wax, is a wax of the leaves of the palm Copernicia prunifera, a plant native to and grown only in the northeastern Brazilian states of Piauí, Ceará, and Rio Grande do Norte. It is known as "queen of waxes" and usually comes in the form of hard yellow-brown...

 or paste wax, and resin
Resin
Resin in the most specific use of the term is a hydrocarbon secretion of many plants, particularly coniferous trees. Resins are valued for their chemical properties and associated uses, such as the production of varnishes, adhesives, and food glazing agents; as an important source of raw materials...

s, such as dammar
Dammar gum
Dammar gum is obtained from the Dipterocarpaceae family of trees in India and East Asia, principally those of the genera Shorea, Balanocarpus or Hopea. Most is produced by tapping trees; however, some is collected in fossilized form from the ground. The gum varies in colour from clear to pale...

, copal, and shellac
Shellac
Shellac is a resin secreted by the female lac bug, on trees in the forests of India and Thailand. It is processed and sold as dry flakes , which are dissolved in ethyl alcohol to make liquid shellac, which is used as a brush-on colorant, food glaze and wood finish...

, 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
Spontaneous combustion
Spontaneous combustion is the self-ignition of a mass, for example, a pile of oily rags. Allegedly, humans can also ignite and burn without an obvious cause; this phenomenon is known as spontaneous human combustion....

 (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; 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.

Although spontaneous combustion has been said to occur rags soaked in paint thinner
Paint thinner
A paint thinner is a solvent used to thin oil-based paints or clean up after their use, although all such solvents have other uses. Commercially, "paint thinner" is usually a name for mineral spirits.Products used as paint thinners include:*Mineral spirits...

s, this is not true unless a drying oil is used as a thinner. However, classical paint thinners do not include drying oils. The danger is due to rags soaked in oil based paints, as it is due to the drying oils in the paints (or varnishes), not thinners per se unless they have been mixed with drying oils such as linseed.

See also

  • “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
  • 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

  • Oil paint
    Oil paint
    Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. The viscosity of the paint may be modified by the addition of a solvent such as turpentine or white spirit, and varnish may be added to increase the glossiness of the...

  • Semi-drying oil
    Semi-drying oil
    A semi-drying oil is an oil which partially hardens when it is exposed to air. This is as opposed to a to drying oil, which hardens completely, or a non-drying oil, which does not harden at all. Oils with an iodine number of 115-130 are considered semi-drying.-Examples of semi-drying oils:*Corn...

  • Blown oil
    Blown oil
    A blown oil is a drying oil which has been polymerized through a manufacturing process. Oils are "blown" through oxidation and are sold for use in industrial applications, etc. Some common types of blown oil include linseed oil, rapeseed oil, castor oil and soybean oil....

  • Wood finishing
    Wood finishing
    Wood finishing refers to the process of embellishing and/or protecting the surface of a wooden material. The process starts with surface preparation, either by sanding by hand , scraping, or planing. Imperfections or nail holes on the surface may be filled using wood putty or pores may be filled...

  • Varnish
    Varnish
    Varnish is a transparent, hard, protective finish or film primarily used in wood finishing but also for other materials. Varnish is traditionally a combination of a drying oil, a resin, and a thinner or solvent. Varnish finishes are usually glossy but may be designed to produce satin or semi-gloss...


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