Lithium soap, often loosely referred to as "lithium grease" or "white lithium", is a soap that is a lithium derivative. Lithium soaps are primarily used as components of certain lubricant greases.
Contents
Soaps are salts of fatty acids. In the domestic setting, sodium-based and potassium-based soaps are commonly used as natural cleaning surfactants. The most useful of the non-detergent soaps are those based on lithium, as they are free of corrosive properties. For lubrication, and as form-release agents, soaps derived from lithium are used due to their higher melting points. 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.
Lithium grease
Most lubricating greases are mixtures of an oil and a soap. The soap is dispersed into the oil to form a stable, viscous gel that is called a grease. 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 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 For high-performance and higher-temperature applications, lithium greases have been superseded by other types of lubricants.
Production
Lithium soaps are produced in a manner similar to saponification of triglycerides. However, instead of sodium hydroxide, 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).