Soaps are salts of fatty acids. In the domestic setting, sodium-based and potassium-based soaps are commonly used as natural cleaning surfactants, but for lubrication and as form-release agents, soaps derived from lithium and calcium are used, due to their higher melting points, keeping them solid or semi-solid at higher temperatures. The most useful of the non-detergent soaps are those based on lithium, as they are free of corrosive properties. The main components of lithium soaps are lithium stearate and lithium 12-hydroxystearate. In addition to soap, soap-based lubricating greases also contain hydrocarbon oils and other components.
Most lubricating greases are mixtures of an oil and a soap. The soaps are dispersed into and viscosify oils to form the stable gels that are called greases. Grease made with lithium soap ("lithium grease") adheres particularly well to metal, is non-corrosive, may be used under heavy loads, and exhibits good temperature tolerance. It has a drip temperature of 190 to 220 °C (370 to 430 °F) and it resists moisture, so it is commonly used as lubricant in household products, such as electric garage doors, as well as in automotive applications, such as CV joints. Lithium-containing greases first appeared during World War II, and were perhaps the first large-scale commercial application of lithium compounds.
Some formulations also include PTFE or other substances, such as molybdenum disulfide. For high-performance and higher-temperature applications, lithium greases have been superseded by other types of lubricants.[clarification needed]
Lithium soaps are produced in a manner similar to saponification of triglycerides. Instead of sodium hydroxide however, the fatty acids are treated with lithium hydroxide or lithium carbonate to form lithium salts of fatty acids. The lithium salts are colourless solids that melt near 200 °C (390 °F).
- Angelo Nora, Alfred Szczepanek, Gunther Koenen, "Metallic Soaps" in Ullmann’s Encyclopedia of Industrial Chemistry 2005 Wiley-VCH, Weinheim. doi:10.1002/14356007.a16_361
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