Linseed oil, also known asflaxseed oil orflax oil (in its edible form), is a colorless to yellowish oil obtained from the dried, ripened seeds of theflax plant (Linum usitatissimum). The oil is obtained bypressing, sometimes followed bysolvent extraction.
Owing to its polymer-forming properties, linseed oil is often blended with combinations of other oils,resins orsolvents as an impregnator, drying oil finish orvarnish inwood finishing, as apigment binder inoil paints, as aplasticizer and hardener inputty, and in the manufacture oflinoleum. Linseed oil use has declined over the past several decades with increased availability of syntheticalkyd resins—which function similarly but resist yellowing.[1]
Linseed oil is atriglyceride, like other fats. Linseed oil is distinctive for its unusually large amount ofα-linolenic acid, which oxidises in air. The fatty acids in a typical linseed oil are of the following types:[2]
Having a high content of di- and tri-unsaturatedesters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is calledautoxidation, results in the rigidification of the material.[3] To prevent premature drying, linseed oil-based products (oil paints, putty) are stored in airtight containers.
Rags soaked with linseed oil pose fire hazard because they provide a large surface area for rapidoxidation. The oxidation of linseed oil isexothermic, which may lead tospontaneous combustion.[4] In 1991,One Meridian Plaza, inPhiladelphia, was severely damaged in a fire, in which three firefighters perished, thought to be caused by rags soaked with linseed oil.[5]
Most applications of linseed oil exploit its drying properties, i.e., the initial material is liquid or at least pliable and the aged material is rigid but not brittle. The water-repelling (hydrophobic) nature of the resultinghydrocarbon-based material is advantageous.[3]
Linseed oil is the carrier used inoil paint. It can also be used as a painting medium, making oil paints more fluid, transparent and glossy. It is available in varieties such as cold-pressed, alkali-refined, sun-bleached, sun-thickened, and polymerised (stand oil). The introduction of linseed oil was a significant advance in the technology of oil painting.[citation needed]
Traditional glazingputty, consisting of a paste ofchalk powder and linseed oil, is a sealant for glass windows that hardens within a few weeks of application and can then be painted over. The durability of putty is owed to the drying properties of linseed oil.[citation needed]
When used as awood finish, linseed oil dries slowly and shrinks little upon hardening. A linseed oil finish is easily scratched and liquid water penetrates a linseed oil finish in mere minutes, and water vapour bypasses it almost completely.[6]Garden furniture treated with linseed oil may developmildew. Oiled wood may be yellowish and is likely to darken with age. Even though the oil feels dry to the touch, studies show linseed oil does not fully cure.[7]
Linseed oil is a common finish for wooden items, though very fine finish may require months to obtain. Studies show the fatty-acid structure of linseed oil has problems cross-linking and oxidizing, frequently turning black.[8]
Boiled linseed oil is used assizing in traditional oilgilding to adhere sheets of gold leaf to a substrate (parchment, canvas,Armenian bole, etc.). It has a much longer working time than water-based size and gives a firm smooth surface that is adhesive enough in the first 12–24 hours after application to cause the gold to attach firmly to the intended surface.[citation needed]
Linseed oil is used to bind wood dust, cork particles, and related materials in the manufacture of the floor coveringlinoleum. After its invention in 1860 byFrederick Walton, linoleum, or "lino" for short, was a common form of domestic and industrial floor covering from the 1870s until the 1970s, when it was largely replaced byPVC ("vinyl") floor coverings.[9] However, since the 1990s, linoleum is returning to favor, being considered more environmentally sound than PVC.[10] Linoleum has given its name to the printmaking techniquelinocut, in which a relief design is cut into the smooth surface and then inked and used to print an image. The results are similar to those obtained bywoodcut printing.[citation needed]
Raw cold-pressed linseed oil – commonly known as flax seed oil in nutritional contexts – is easily oxidized, and rapidly becomes rancid, with an unpleasantodour, unlessrefrigerated. Linseed oil is not generally recommended for use in cooking. In one study, the content ofalpha-linolenic acid (ALA) in whole flaxseeds did not decrease after heating the seeds to temperatures of up to 178 °C (352.4 °F) for one and a half hours.[11]
Linseed oil is anedible oil in demand as adietary supplement, as a source ofα-linolenic acid, anomega-3 fatty acid. In parts of Europe, it is traditionally eaten with potatoes andquark.[citation needed]
Food-grade flaxseed oil is cold-pressed, obtained without solvent extraction, in the absence of oxygen, and marketed as edible flaxseed oil. Fresh, refrigerated and unprocessed, linseed oil is used as anutritional supplement and is a traditional European ethnic food, highly regarded for its nutty flavor. Regular flaxseed oil contains between 57% and 71% polyunsaturated fats (alpha-linolenic acid,linoleic acid).[12] Plant breeders have developed flaxseed with both higher ALA (70%)[12] and very low ALA content (< 3%).[13] The USFDA granted generally recognized as safe (GRAS) status for high alpha linolenic flaxseed oil.[14]
| Typical fatty acid content | %[15] | % European[16] |
|---|---|---|
| Palmitic acid | 6.0 | 4.0–6.0 |
| Stearic acid | 2.5 | 2.0–3.0 |
| Arachidic acid | 0.5 | 0–0.5 |
| Palmitoleic acid | - | 0–0.5 |
| Oleic acid | 19.0 | 10.0–22.0 |
| Eicosenoic acid | - | 0–0.6 |
| Linoleic acid | 24.1 | 12.0–18.0 |
| Alpha-linolenic acid | 47.4 | 56.0–71.0 |
| Other | 0.5 | - |
Nutrition information from the Flax Council of Canada.[17]
Per 1 tbsp (14 g)
Flax seed oil contains no significant amounts of protein, carbohydrates or fibre.
| Type | Processing treatment[19] | Saturated fatty acids | Monounsaturated fatty acids | Polyunsaturated fatty acids | Smoke point | ||||
|---|---|---|---|---|---|---|---|---|---|
| Total[18] | Oleic acid (ω−9) | Total[18] | α-Linolenic acid (ω−3) | Linoleic acid (ω−6) | ω−6:3 ratio | ||||
| Avocado[20] | 11.6 | 70.6 | 67.9 | 13.5 | 1 | 12.5 | 12.5:1 | 250 °C (482 °F)[21] | |
| Brazil nut[22] | 24.8 | 32.7 | 31.3 | 42.0 | 0.1 | 41.9 | 419:1 | 208 °C (406 °F)[23] | |
| Canola[24] | 7.4 | 63.3 | 61.8 | 28.1 | 9.1 | 18.6 | 2:1 | 204 °C (400 °F)[25] | |
| Coconut[26] | 82.5 | 6.3 | 6 | 1.7 | 0.019 | 1.68 | 88:1 | 175 °C (347 °F)[23] | |
| Corn[27] | 12.9 | 27.6 | 27.3 | 54.7 | 1 | 58 | 58:1 | 232 °C (450 °F)[25] | |
| Cottonseed[28] | 25.9 | 17.8 | 19 | 51.9 | 1 | 54 | 54:1 | 216 °C (420 °F)[25] | |
| Cottonseed[29] | hydrogenated | 93.6 | 1.5 | 0.6 | 0.2 | 0.3 | 1.5:1 | ||
| Flaxseed/linseed[30] | 9.0 | 18.4 | 18 | 67.8 | 53 | 13 | 0.2:1 | 107 °C (225 °F) | |
| Grape seed[31] | 9.6 | 16.1 | 15.8 | 69.9 | 0.10 | 69.6 | very high | 216 °C (421 °F) | |
| Hemp seed[32] | 7.0 | 9.0 | 9.0 | 82.0 | 22.0 | 54.0 | 2.5:1 | 166 °C (330 °F)[33] | |
| High-oleic safflower oil[34] | 7.5 | 75.2 | 75.2 | 12.8 | 0 | 12.8 | very high | 212 °C (414 °F)[23] | |
| Olive (extra virgin)[35] | 13.8 | 73.0 | 71.3 | 10.5 | 0.7 | 9.8 | 14:1 | 193 °C (380 °F)[23] | |
| Palm[36] | 49.3 | 37.0 | 40 | 9.3 | 0.2 | 9.1 | 45.5:1 | 235 °C (455 °F) | |
| Palm[37] | hydrogenated | 88.2 | 5.7 | 0 | |||||
| Peanut[38] | 16.2 | 57.1 | 55.4 | 19.9 | 0.318 | 19.6 | 61.6:1 | 232 °C (450 °F)[25] | |
| Rice bran oil | 25 | 38.4 | 38.4 | 36.6 | 2.2 | 34.4[39] | 15.6:1 | 232 °C (450 °F)[40] | |
| Sesame[41] | 14.2 | 39.7 | 39.3 | 41.7 | 0.3 | 41.3 | 138:1 | ||
| Soybean[42] | 15.6 | 22.8 | 22.6 | 57.7 | 7 | 51 | 7.3:1 | 238 °C (460 °F)[25] | |
| Soybean[43] | partially hydrogenated | 14.9 | 43.0 | 42.5 | 37.6 | 2.6 | 34.9 | 13.4:1 | |
| Sunflower[44] | 8.99 | 63.4 | 62.9 | 20.7 | 0.16 | 20.5 | 128:1 | 227 °C (440 °F)[25] | |
| Walnut oil[45] | unrefined | 9.1 | 22.8 | 22.2 | 63.3 | 10.4 | 52.9 | 5:1 | 160 °C (320 °F)[46] |
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Stand oil is generated by heating linseed oil near 300 °C for a few days in the complete absence of air. Under these conditions, the polyunsaturated fatty esters convert to conjugateddienes, which then undergoDiels-Alder reactions, leading to crosslinking. The product, which is highly viscous, gives highly uniform coatings that "dry" to more elastic coatings than linseed oil itself.Soybean oil can be treated similarly, but converts more slowly. On the other hand,tung oil converts very quickly, being complete in minutes at 260 °C. Coatings prepared from stand oils are less prone to yellowing than are coatings derived from the parent oils.[49]
Boiled linseed oil is a combination of raw linseed oil, stand oil (see above), and metallicoil drying agents (catalysts to accelerate drying).[49] In theMedieval era, linseed oil was boiled withlead oxide (litharge) to give a product called boiled linseed oil.[50][page needed] The lead oxide forms lead "soaps" (lead oxide isalkaline) that promote hardening (polymerisation) of linseed oil by reaction with atmospheric oxygen. Heating shortens its drying time.[citation needed]
Raw linseed oil is the base oil, unprocessed and without driers or thinners. It is mostly used as a feedstock for making a boiled oil. It does not cure sufficiently well or quickly to be regarded as adrying oil.[51] Raw linseed is sometimes used foroiling cricket bats to increase surface friction for better ball control.[52] It was also used to treatleather flat belt drives to reduce slipping.[citation needed]
