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Linseed oil

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Flax, flax seeds, linseed oil, linseed cake

Linseed oil, also known as flaxseed oil or flax oil (in its edible form), is a colourless to yellowish oil obtained from the dried, ripened seeds of the flax plant (Linum usitatissimum). The oil is obtained by pressing, sometimes followed by solvent extraction.

Owing to its polymer-forming properties, linseed oil is often blended with combinations of other oils, resins or solvents as an impregnator, drying oil finish or varnish in wood finishing, as a pigment binder in oil paints, as a plasticizer and hardener in putty, and in the manufacture of linoleum. Linseed oil use has declined over the past several decades with increased availability of synthetic alkyd resins—which function similarly but resist yellowing.[1]

Linseed oil is an edible oil in demand as a dietary supplement, as a source of α-Linolenic acid, an omega-3 fatty acid. In parts of Europe, it is traditionally eaten with potatoes and quark.

Structure and composition

Representative triglyceride found in a linseed oil, a triester (triglyceride) derived of linoleic acid, alpha-linolenic acid, and oleic acid.

Linseed oil is a triglyceride, like other fats. Linseed oil is distinctive for its unusually large amount of α-linolenic acid, which oxidises in air. The fatty acids in a typical linseed oil are of the following types:[2]

Drying properties

Main article: drying oil

Having a high content of di- and tri-unsaturated esters, linseed oil is susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization, which is called "drying", results in the rigidification of the material. To prevent premature drying, linseed oil-based products (oil paints, putty) are stored in airtight containers.

Rags soaked with linseed oil stored pose fire hazard because they provide a large surface area for rapid oxidation. The oxidation of linseed oil is exothermic, which may lead to spontaneous combustion.[3] In 1991, One Meridian Plaza, a high rise in Philadelphia, was severely damaged in a fire, in which three firefighters perished, thought to be caused by rags soaked with linseed oil.[4]

Applications

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 resulting hydrocarbon-based material is advantageous.[citation needed]

Paint binder

"Your country needs flax .." U.S. WWII poster soliciting linseed oil for use in paint
A can of French linseed oil.

Linseed oil is the carrier used in oil 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]

Putty

Traditional glazing putty, consisting of a paste of chalk 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]

Wood finish

When used as a wood 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.[5] Garden furniture treated with linseed oil may develop mildew. 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.[6]

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.[7] It is no longer encouraged to use linseed oil to recondition and regenerate finishes because of the cross-linking oxidization that occurs.

Gilding

Boiled linseed oil is used as sizing in traditional oil gilding 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]

Linoleum

Linseed oil is used to bind wood dust, cork particles, and related materials in the manufacture of the floor covering linoleum. After its invention in 1860 by Frederick 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 by PVC ('vinyl') floor coverings.[8] However, since the 1990s, linoleum is returning to favor, being considered more environmentally sound than PVC.[9] Linoleum has given its name to the printmaking technique linocut, 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 by woodcut printing.[citation needed]

Nutritional supplement and food

Raw cold-pressed linseed oil – commonly known as flax seed oil in nutritional contexts – is easily oxidized, and rapidly becomes rancid, with an unpleasant odour, unless refrigerated. Linseed oil is not generally recommended for use in cooking. Alpha linolenic acid (ALA) while bound to flaxseed ALA can withstand temperatures up to 175 °C (350 °F) for two hours.[10]

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 a nutritional 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).[11] Plant breeders have developed flaxseed with both higher ALA (70%)[11] and very low ALA content (< 3%).[12] The USFDA granted generally recognized as safe (GRAS) status for high alpha linolenic flaxseed oil.[13]

Nutrient content

Typical fatty acid content % [14] % European[15]
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.[16]

Per 1 tbsp (14 g)

Flax seed oil contains no significant amounts of protein, carbohydrates or fibre.

Comparison to other vegetable oils

Properties of vegetable oils[17][18]
The nutritional values are expressed as percent (%) by mass of total fat.
Type Processing
treatment[19]		 Saturated
fatty acids		 Monounsaturated
fatty acids		 Polyunsaturated
fatty acids		 Smoke point
Total[17] Oleic
acid
(ω−9)		 Total[17] α-Linolenic
acid
(ω−3)		 Linoleic
acid
(ω−6)		 ω−6:3
ratio
Avocado[20] 11.6 70.6 52–66
[21]		 13.5 1 12.5 12.5:1 250 °C (482 °F)[22]
Brazil nut[23] 24.8 32.7 31.3 42.0 0.1 41.9 419:1 208 °C (406 °F)[24]
Canola[25] 7.4 63.3 61.8 28.1 9.1 18.6 2:1 204 °C (400 °F)[26]
Coconut[27] 82.5 6.3 6 1.7 0.019 1.68 88:1 175 °C (347 °F)[24]
Corn[28] 12.9 27.6 27.3 54.7 1 58 58:1 232 °C (450 °F)[26]
Cottonseed[29] 25.9 17.8 19 51.9 1 54 54:1 216 °C (420 °F)[26]
Cottonseed[30] hydrogenated 93.6 1.5 0.6 0.2 0.3 1.5:1
Flaxseed/linseed[31] 9.0 18.4 18 67.8 53 13 0.2:1 107 °C (225 °F)
Grape seed   10.4 14.8 14.3   74.9 0.15 74.7 very high 216 °C (421 °F)[32]
Hemp seed[33] 7.0 9.0 9.0 82.0 22.0 54.0 2.5:1 166 °C (330 °F)[34]
High-oleic safflower oil[35] 7.5 75.2 75.2 12.8 0 12.8 very high 212 °C (414 °F)[24]
Olive (extra virgin)[36] 13.8 73.0 71.3 10.5 0.7 9.8 14:1 193 °C (380 °F)[24]
Palm[37] 49.3 37.0 40 9.3 0.2 9.1 45.5:1 235 °C (455 °F)
Palm[38] hydrogenated 88.2 5.7 0
Peanut[39] 16.2 57.1 55.4 19.9 0.318 19.6 61.6:1 232 °C (450 °F)[26]
Rice bran oil 25 38.4 38.4 36.6 2.2 34.4[40] 15.6:1 232 °C (450 °F)[41]
Sesame[42] 14.2 39.7 39.3 41.7 0.3 41.3 138:1
Soybean[43] 15.6 22.8 22.6 57.7 7 51 7.3:1 238 °C (460 °F)[26]
Soybean[44] partially hydrogenated 14.9 43.0 42.5 37.6 2.6 34.9 13.4:1
Sunflower[45] 8.99 63.4 62.9 20.7 0.16 20.5 128:1 227 °C (440 °F)[26]
Walnut oil[46] unrefined 9.1 22.8 22.2 63.3 10.4 52.9 5:1 160 °C (320 °F)[47]

Additional uses

Modified linseed oils

Stand oil

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 conjugated dienes, which then undergo Diels-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.[50]

Boiled linseed oil

Boiled linseed oil is a combination of raw linseed oil, stand oil (see above), and metallic oil drying agents (catalysts to accelerate drying).[50] In the Medieval era, linseed oil was boiled with lead oxide (litharge) to give a product called boiled linseed oil.[51][page needed] The lead oxide forms lead "soaps" (lead oxide is alkaline) which promotes hardening (polymerisation) of linseed oil by reaction with atmospheric oxygen. Heating shortens its drying time.[citation needed]

Raw linseed oil

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 a drying oil.[52] Raw linseed is sometimes used for oiling cricket bats to increase surface friction for better ball control.[53] It was also used to treat leather flat belt drives to reduce slipping.[citation needed]

See also

References

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  2. ^ Vereshchagin, A. G.; Novitskaya, Galina V. (1965). "The triglyceride composition of linseed oil". Journal of the American Oil Chemists' Society. 42 (11): 970–974. doi:10.1007/BF02632457. PMID 5898097. S2CID 29785363.
  3. ^ Ettling, Bruce V.; Adams, Mark F. (1971). "Spontaneous combustion of linseed oil in sawdust". Fire Technology. 7 (3): 225. doi:10.1007/BF02590415. S2CID 109500727.
  4. ^ Routley, J. Gordon; Jennings, Charles; Chubb, Mark (February 1991), "Highrise Office Building Fire One Meridian Plaza Philadelphia, Pennsylvania" (PDF), Report USFA-TR-049, Federal Emergency Management Agency
  5. ^ Flexner, Bob. Understanding Wood Finishing. Reader's Digest Association, Inc., 2005, p. 75.
  6. ^ Lazzari, Massimo; Chiantore, Oscar (1999). "Drying and oxidative degradation of linseed oil" (PDF). Polymer Degradation and Stability. 65 (2): 303–313. doi:10.1016/S0141-3910(99)00020-8.
  7. ^ Kerschner, Richard L.; Ravenel, Nancie (2013). "Here We Go 'Round Again: Cleaning Linseed Oil from Carousel Animalsatthe Shelburne Museum". Journal of the American Institute for Conservation. 45 (3): 201–210. doi:10.1179/019713606806112450. S2CID 191561961.
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  11. ^ a b Morris, Diane H. (2007). "Chapter 1: Description and Composition of Flax; In: Flax – A Health and Nutrition Primer". Flax Council of Canada. Retrieved 16 September 2019.
  12. ^ Thompson, Lilian U.; Cunnane, Stephen C., eds. (2003). Flaxseed in human nutrition (2nd ed.). AOCS Press. pp. 8–11. ISBN 978-1-893997-38-7.
  13. ^ "U.S. FDA/CFSAN Agency Response Letter GRAS Notice No. GRN 00256". U.S. FDA/CFSAN. Retrieved 2013-01-29.
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  17. ^ a b c "US National Nutrient Database, Release 28". United States Department of Agriculture. May 2016. All values in this table are from this database unless otherwise cited or when italicized as the simple arithmetic sum of other component columns.
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  40. ^ Orthoefer, Frank T. (2020). "Rice Bran Oil". Bailey's Industrial Oil and Fat Products. pp. 1–25. doi:10.1002/047167849X.bio015.pub2. ISBN 978-0-471-38460-1.
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  51. ^ Merrifield, Mary P. (2012). Medieval and Renaissance Treatises on the Arts of Painting: Original Texts. Dover Publications, Inc. ISBN 978-0486142241.
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  53. ^ "Caring for your Bat". Gunn & Moore.

Further reading

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