Drying oil

Last updated

A drying oil is an oil 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 components crosslink (and hence, 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, castor oil and walnut oil. Their use has declined over the past several decades, as they have been replaced by alkyd resins and other binders.

Contents

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, curing of oils is the result of autoxidation, the addition of oxygen to an organic compound and the subsequent crosslinking. This process begins with an oxygen molecule (O2) in the air inserting into carbon-hydrogen (C-H) bonds adjacent to one of the double bonds within the unsaturated fatty acid. 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 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. DryOilSteps.svg
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. 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 undergo hydrolysis, 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. 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 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 cobalt, manganese, or iron.[ citation needed ] In technical terms, these oil drying agents are coordination complexes that function as homogeneous catalysts. These salts are derived from the carboxylates 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

Representative triglyceride found in a drying oil. The triester is derived from three different unsaturated fatty acids, linoleic (top), alpha-linolenic (middle), and oleic acids (bottom). The order of drying rate is alpha-linolenic > linoleic > oleic acid, reflecting their degree of unsaturation. ModelDryingOil.png
Representative triglyceride found in a drying oil. The triester is derived from three different unsaturated fatty acids, linoleic (top), 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 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 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" 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 interconnect (crosslink) in the manner of drying oils. Thus, waxes and resins remain soluble in their original solvent whereas a cured oil paint or varnish does not. A dissolved wax or resin is recovered unchanged after the evaporation of its solvent, but liquids that can dissolve cured drying oils, such as some paint removers do so by chemically changing them.[ citation needed ]

Applications

The ability of drying oils to form strong strong but flexible films is very useful in oil paints and varnishes. In former times, walnut oil, rich in linoleic acid, was used by painters. Nowadays, linseed oil is used instead.

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

Related Research Articles

<span class="mw-page-title-main">Oil painting</span> Process of painting with pigments that are bound with a medium of drying oil

Oil painting is a painting method involving the procedure of painting with pigments with a medium of drying oil as the binder. It has been the most common technique for artistic painting on canvas, wood panel or copper for several centuries, spreading from Europe to the rest of the world. The advantages of oil for painting images include "greater flexibility, richer and denser colour, the use of layers, and a wider range from light to dark". But the process is slower, especially when one layer of paint needs to be allowed to dry before another is applied.

<span class="mw-page-title-main">Paint</span> Pigment applied over a surface that dries as a solid film

Paint is a material or mixture that, when applied to a solid material and allowed to dry, adds a film-like layer. As art, this is used to create an image or images known as a painting. Paint can be made in many colors and types. Most paints are either oil-based or water-based, and each has distinct characteristics.

<span class="mw-page-title-main">Triglyceride</span> Any ester of glycerol having all three hydroxyl groups esterified with fatty acids

A triglyceride is an ester derived from glycerol and three fatty acids. Triglycerides are the main constituents of body fat in humans and other vertebrates as well as vegetable fat. They are also present in the blood to enable the bidirectional transference of adipose fat and blood glucose from the liver and are a major component of human skin oils.

<span class="mw-page-title-main">Varnish</span> Transparent hard protective finish or film

Varnish is a clear transparent hard protective coating or film. It is not to be confused with wood stain. It usually has a yellowish shade due to the manufacturing process and materials used, but it may also be pigmented as desired. It is sold commercially in various shades.

<span class="mw-page-title-main">Lacquer</span> Liquid or powder coating material which is applied thinly to objects to form a hard finish

Lacquer is a type of hard and usually shiny coating or finish applied to materials such as wood or metal. It is most often made from resin extracted from trees and waxes and has been in use since antiquity.

<span class="mw-page-title-main">Linseed oil</span> Oil obtained from the dried, ripened seeds of the flax plant

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

<span class="mw-page-title-main">Oil paint</span> Type of slow-drying paint

Oil paint is a type of slow-drying paint that consists of particles of pigment suspended in a drying oil, commonly linseed oil. For several centuries the oil painting has been perhaps the most prestigious form in Western art, but oil paint has many practical uses, mainly because it is waterproof.

<span class="mw-page-title-main">Linoleum</span> Type of floor covering

Linoleum is a floor covering made from materials such as solidified linseed oil (linoxyn), pine resin, ground cork dust, sawdust, and mineral fillers such as calcium carbonate, most commonly on a burlap or canvas backing. Pigments are often added to the materials to create the desired color finish. Commercially, the material has been largely replaced by sheet vinyl flooring, although in the UK and Australia this is often still referred to as "lino".

<span class="mw-page-title-main">Allyl group</span> Chemical group (–CH₂–CH=CH₂)

In organic chemistry, an allyl group is a substituent with the structural formula −CH2−HC=CH2. It consists of a methylene bridge attached to a vinyl group. The name is derived from the scientific name for garlic, Allium sativum. In 1844, Theodor Wertheim isolated an allyl derivative from garlic oil and named it "Schwefelallyl". The term allyl applies to many compounds related to H2C=CH−CH2, some of which are of practical or of everyday importance, for example, allyl chloride.

In chemistry, the iodine value is the mass of iodine in grams that is consumed by 100 grams of a chemical substance. Iodine numbers are often used to determine the degree of unsaturation in fats, oils and waxes. In fatty acids, unsaturation occurs mainly as double bonds which are very reactive towards halogens, the iodine in this case. Thus, the higher the iodine value, the more unsaturations are present in the fat. It can be seen from the table that coconut oil is very saturated, which means it is good for making soap. On the other hand, linseed oil is highly unsaturated, which makes it a drying oil, well suited for making oil paints.

<span class="mw-page-title-main">Hot-melt adhesive</span> Glue applied by heating

Hot-melt adhesive (HMA), also known as hot glue, is a form of thermoplastic adhesive that is commonly sold as solid cylindrical sticks of various diameters designed to be applied using a hot glue gun. The gun uses a continuous-duty heating element to melt the plastic glue, which the user pushes through the gun either with a mechanical trigger mechanism on the gun, or with direct finger pressure. The glue squeezed out of the heated nozzle is initially hot enough to burn and even blister skin. The glue is sticky when hot, and solidifies in a few seconds to one minute. Hot-melt adhesives can also be applied by dipping or spraying, and are popular with hobbyists and crafters both for affixing and as an inexpensive alternative to resin casting.

Autoxidation refers to oxidations brought about by reactions with oxygen at normal temperatures, without the intervention of flame or electric spark. The term is usually used to describe the gradual degradation of organic compounds in air at ambient temperatures. Many common phenomena can be attributed to autoxidation, such as food going rancid, the 'drying' of varnishes and paints, and the perishing of rubber. It is also an important concept in both industrial chemistry and biology. Autoxidation is therefore a fairly broad term and can encompass examples of photooxygenation and catalytic oxidation.

<span class="mw-page-title-main">Alkyd</span> Polyester resin modified by the addition of fatty acids and other components

An alkyd is a polyester resin modified by the addition of fatty acids and other components. Alkyds are derived from polyols and organic acids including dicarboxylic acids or carboxylic acid anhydride and triglyceride oils. The term alkyd is a modification of the original name "alcid", reflecting the fact that they are derived from alcohol and organic acids. The inclusion of a fatty acid confers a tendency to form flexible coatings. Alkyds are used in paints, varnishes and in moulds for casting. They are the dominant resin or binder in most commercial oil-based coatings. Approximately 200,000 tons of alkyd resins are produced each year. The original alkyds were compounds of glycerol and phthalic acid sold under the name Glyptal. These were sold as substitutes for the darker-colored copal resins, thus creating alkyd varnishes that were much paler in colour. From these, the alkyds that are known today were developed.

<span class="mw-page-title-main">Photopolymer</span>

A photopolymer or light-activated resin is a polymer that changes its properties when exposed to light, often in the ultraviolet or visible region of the electromagnetic spectrum. These changes are often manifested structurally, for example hardening of the material occurs as a result of cross-linking when exposed to light. An example is shown below depicting a mixture of monomers, oligomers, and photoinitiators that conform into a hardened polymeric material through a process called curing.

<span class="mw-page-title-main">Perilla oil</span> Vegetable oil derived from perilla seeds

Perilla oil is an edible vegetable oil derived from perilla seeds. Having a distinct nutty aroma and taste, the oil pressed from the toasted perilla seeds is used as a flavor enhancer, condiment, and cooking oil in Korean cuisine. The oil pressed from untoasted perilla seeds is used for non-culinary purposes.

Curing is a chemical process employed in polymer chemistry and process engineering that produces the toughening or hardening of a polymer material by cross-linking of polymer chains. Even if it is strongly associated with the production of thermosetting polymers, the term "curing" can be used for all the processes where a solid product is obtained from a liquid solution, such as with PVC plastisols.

<span class="mw-page-title-main">Danish oil</span> Wood finishing oil

Danish oil is a wood finishing oil, often made of tung oil or polymerized linseed oil. Because there is no defined formulation, its composition varies among manufacturers.

An oil drying agent, also known as siccative, is a coordination compound that accelerates (catalyzes) the hardening of drying oils, often as they are used in oil-based paints. This so-called "drying" occurs through free-radical chemical crosslinking of the oils. The catalysts promote this free-radical autoxidation of the oils with air.

Detection of peroxide gives the initial evidence of rancidity in unsaturated fats and oils. Other methods are available, but peroxide value is the most widely used. It gives a measure of the extent to which an oil sample has undergone primary oxidation; extent of secondary oxidation may be determined from p-anisidine test.

<span class="mw-page-title-main">Tung oil</span> Chemical compound

Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree. Tung oil hardens upon exposure to air, and the resulting coating is transparent and has a deep, almost wet look. Used mostly for finishing and protecting wood, after numerous coats, the finish can even look plastic-like. Related drying oils include linseed, safflower, poppy, and soybean oils. Raw tung oil tends to dry to a fine, wrinkled finish ; this property was used to make wrinkle finishes, usually by adding excess cobalt drier. To prevent wrinkling, the oil is heated to gas-proof it.

References

  1. Ulrich Poth (2002). "Drying Oils and Related Products". Ullmann's Encyclopedia of Industrial Chemistry . Weinheim: Wiley-VCH. doi:10.1002/14356007.a09_055. ISBN   978-3527306732.
  2. Ned A. Porter, Sarah E. Caldwell, Karen A. Mills "Mechanisms of free radical oxidation of unsaturated lipids" Lipids 1995, volume 30, Pages 277-290. doi : 10.1007/BF02536034
  3. 1 2 Apps, E. A. (1958). Printing Ink Technology. London: Leonard Hill [Books] Limited. p. 14.

Further reading