The two components of a lubricant are the base oil and additives; a grease adds one more component – the thickener. “Thickener” is an umbrella…
The two components of a lubricant are the base oil and additives; a grease adds one more component - the thickener. "Thickener" is an umbrella term for a range of molecules, polymers or particles that are partially insoluble in lubricating oil that give the grease a semi-solid consistency. There are many types of chemical compounds that can thicken grease.
Simple soaps are the most commonly used grease thickeners. Simple soap is made from the reaction of an organic acid (long chain or fatty carboxylic acids) and a metal base to make a salt (acid + base = salt + water). This reaction is known as saponification. Simple soaps tend to be based on salts o lithium and calcium and less often on sodium, aluminum and barium. Simple soap thickeners include calcium stearate or lithium 12-hydroxystearate.
Complex soaps are similar to simple soaps in that they use a single metallic hydroxide. A fatty acid and a short-chain complexing acid are combined to make complex-thickened grease. To create complex thickeners, the acid mixture is combined with a metallic hydroxyide. The most well-known lithium complex grease in North America is made from lithium hydroxide (12 HSA) and azelaic acids. Because of their superior high-temperature properties, these thickener types typically perform better than simple soaps.
Non-soap products can also be used to thicken grease. Polyurea, clay and fluoropolymers are common non-soap grease thickeners.
Polyurea can be used to refer to diureas, tetraureas, urea-urethane and other related chemicals. A common polyurea thickener is a reaction product of a mono- and/or diamines with a di-isocyanate. The characteristics of the thickener can vary greatly and are determined by the ratios of the ingredients. Polyurea thickeners are not made from metallic elements and therefore they are non ash-forming. They also tend to be oxidatively resistant due to the lack of metals which catalyse oxidation.
The minerals hectorite and bentonite are both organophilic clay thickeners. These minerals are first purified of any non-clay materials, then ground to the required particle size. Finally, they are chemically treated to help them bond with organic chemicals. To make grease, clay particles are dispersed in fluid lubricant. To stabilize the thickener structure, clay particles must be activated using a polar substance. Clay thickened greases are not produced by any chemical reaction. Clay thickeners do not have a defined melting point so they are often used in high-temperature greases.
Fluoropolymer powders such as PTFE (polytetrafluoroethylene) can also be used to thicken lubricating fluids to form grease. These premium niche products are highly resistant to chemicals, oxygen, water, and other substances. These greases are designed to withstand high temperatures and provide extended service life for demanding applications.
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