Linseed oil , also known as flax seed oil is a clear to yellowish oil obtained from the dried ripe seeds of the flax plant ( Linum usitatissimum , Linaceae). The oil is obtained by cold pressing, sometimes followed by solvent extraction.
Linseed oil can polymerize and the reaction is exothermic, and rags soaked in it can ignite spontaneously. Due to its polymer-like properties linseed oil is used on its own or blended with other oils, resins and solvents as an impregnator and varnish in wood finishing, as a pigment binder in oil paints, as a plasticizer and hardener in putty and in the manufacture of linoleum.
It is an edible oil, but because of its strong flavor and odor is only a minor constituent of human nutrition, although it is marketed as a nutritional supplement. The use of linseed oil has declined over the past several decades with the increased use of alkyd resins, which are similar but partially synthetic materials that resist yellowing.
Chemical aspects
Linseed oil is a so-called drying oil, which means that it hardens upon exposure to air. Linseed oil is a mixture of various triglycerides that differ in terms of their fatty acid constituents. For linseed oil, these triglycerides are primarily derived from the following fatty acids:
- The saturated acids palmitic acid (about 7%) and stearic acid (3.4-4.6%),
- The monounsaturated oleic acid (18.5-22.6%),
- The doubly unsaturated linoleic acid (14.2-17%),
- The triply unsaturated omega-3 fatty acid α-linolenic acid (51.9-55.2%).
Having a high content of unsaturated esters, linseed oil is particularly susceptible to polymerization reactions upon exposure to oxygen in air. This polymerization results is the rigidification of the material, which gives the appearance of "drying."
Uses
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 hydrophobic nature of this hydrocarbon-based material is advantageous.
Paint binder
Linseed oil is a common 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 use of linseed oil was a significant step in the technology of oil painting.
Putty
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.
Wood finish
When used as a wood finish, linseed oil dries slowly and shrinks little upon hardening. Linseed oil does not cover the surface as varnish does, but soaks into the (visible and microscopic) pores, leaving a shiny but not glossy surface that shows off the grain of the wood. Wood treated with linseed oil is resistant to denting and scratches and is easily repaired, but the surface is not as hard as a modern varnish, and the wood will slowly absorb moisture if allowed to stay wet. Soft wood is protected from denting, but requires numerous applications and even more drying time than harder wood. Garden furniture treated with linseed oil may develop mildew. Oiled wood can be yellowish and can darken with age.
Linseed oil is a traditional finish for gun stocks. A very fine finish may require months to obtain. Several coats of linseed oil is the traditional protective coating for the raw willow wood of cricket bats. Linseed oil is also often used by billiards or pool cue-makers cue shafts, as a lubricant/protectant for wooden recorders, and used in place of epoxy to seal modern wooden surfboards.
Additionally, a luthier may use linseed oil when reconditioning a guitar, mandolin, or other stringed instrument's fret board. It is desirable, after cleaning a fret board, to apply a light amount of linseed oil to seal and protect it from moisture, skin oils, etc that might otherwise result in accelerated deterioration of the wood. Also, it does not "expand" the wood when it soaks into the pores.
Linoleum
Linseed oil is used to bind wood dust, cork particles, and other materials in the manufacture of the floor covering linoleum, invented and patented in 1860 by Frederick Walton. Linoleum, or "lino" for short, was a common form of domestic and industrial floor covering from the 1870s until its virtual replacement by PVC floor coverings. 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.
Nutritional supplement
Food-grade flaxseed oil is cold-pressed, obtained without solvent extraction, and marketed as edible flaxseed oil. Fresh, refrigerated and unprocessed, linseed oil is used as a nutritional supplement. It contains high levels of omega-3 fatty acids, especially alpha-linolenic acid, which may be beneficial for reducing inflammation leading to atherosclerosis, preventing heart disease and arrhythmia, and is required for normal infant development. However recent well-controlled placebo studies suggest that regular consumption of flax seed oil may not reduce the risk of stroke, heart disease, or cancer any greater than placebo. Regular flaxseed oil contains between 52 and 63% alpha linolenic acid (C18:3 n-3). Plant breeders have developed flaxseed with higher alpha linoleic acid content (70%) and very low alpha linolenic acid content (< 3%). The USFDA granted GRAS (Generally Recognized As Safe) status for high alpha linolenic flaxseed oil.
Although flax seeds themselves contain lignans, a class of phytoestrogens considered to have antioxidant and cancer-preventing properties, the extracted linseed oil does not contain the lignans found in flax seed, and therefore does not have the same antioxidant properties. In fact, flax seed oil is easily oxidized, and rapidly becomes rancid with an unpleasant odor unless refrigerated. Even when kept under cool conditions it has a shelf life of only a few weeks. Oil with an unpleasant or rancid odour should be discarded. Rancid oils contribute to the formation of free radicals and may be carcinogenic. Oxidation of flax seed oil is major commercial concern, and antioxidants may be added to prevent rancidification.
Nutrient content
Nutrition information from the Flax Council of Canada.
Per 1 Tbsp (14 g)
- Calories: 124
- Total fat: 14g
- Omega-3: 8g
- Omega-6: 2g
- Omega-9: 3g
Flax seed oil contains no significant amounts of protein, carbohydrates, or fiber.
Additional uses
- Animal feeds
- Sealants
- Caulking compounds
- Linoleum
- Earthen floors
- Adobe
- Textiles
- Bicycle maintenance as a thread fixative, rust inhibitor and lubricant
- Leather treatment
- Polishes, varnishes and oil paints
- Composition ornament for moulded decoration
- Animal care products
- Wood preservation
- Industrial Lubricant
- Particle Detectors
- Oilcloth
Boiled linseed oil
Boiled linseed oil is used as a paint binder or as a wood finish on its own. Heating the oil causes it to polymerize and oxidize, effectively making it thicker and shortening its drying time. Today most products labeled as "boiled linseed oil" are a combination of raw linseed oil, petroleum-based solvent and metallic dryers. The use of metallic dryers makes boiled linseed oil inedible. There are some products available that contain only heat-treated linseed oil, without exposure to oxygen. Heat treated linseed oil is thicker and dries very slowly. This grade of linseed oil is usually labeled as "polymerized" or "stand" oil, though some types may still be labeled as "boiled."
Spontaneous combustion
Rags soaked with linseed oil are considered a fire hazard because they provide a large surface area for oxidation of the oil, and the oil oxidizes quickly. The oxidation of linseed oil is an exothermic reaction, which accelerates as the temperature of the rags increases. When heat accumulation exceeds the rate of heat dissipation into the environment, the temperature increases and may eventually become hot enough to make the rags spontaneously combust. Linseed oil soaked rags should never be stored, least of all in an enclosed container, which allows the heat to accumulate. Instead, rags should be washed, soaked with water, or incinerated.
In 1991, One Meridian Plaza, a high rise in Philadelphia, was severely damaged and three firefighters perished in a fire initially caused by improperly disposed rags which had been used with linseed oil.
A fire that destroyed the El Rey Theater-Golden West Saloon in Albuquerque, New Mexico in February 2008 was ignited by spontaneous combustion of linseed oil soaked rags left in a plastic container.
See also
- Flax seed
- National Linseed Oil Trust
- One Meridian Plaza
References
- ^ Frank N. Jones "Alkyd Resins" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinhe
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