What is the thickener in lubricating grease?
The thickener in a lubricating grease is the component that sets grease apart from fluid lubricants. Thickeners are molecules, polymers, or particles that are partially soluble in lubricating fluid; they arrange themselves in such a way that they impart a semi-solid consistency to the grease. Many different types of chemical compounds can be used to thicken grease.
Simple soaps are the most common grease thickeners. A simple soap is the reaction product of an organic acid (long-chain or fatty carboxylic acid) and an alkali metal to form an organic salt. Thus, simple soap is an acid-base reaction product. This reaction has a special name: saponification. Simple soaps are most commonly based on salts of lithium and calcium, and less commonly on salts of sodium, aluminum, and barium. Examples of simple soap thickeners include lithium 12-hydroxystearate and calcium stearate.
Complex soaps are also used widely as grease thickeners. The term “complex” refers to the combination of a simple soap and a complexing agent. For example, a lithium complex thickener typically contains lithium 12-hydroxystearate (simple soap) and a salt of a shorter chain difunctional carboxylic acid, boric acid, or an aromatic acid (complexing agent). Complex thickeners are usually based on lithium, calcium, or aluminum compounds. In some cases, dissimilar thickener types are combined in a grease. This type of thickener system can be referred to as a hybrid thickener or in some cases as a complex thickener.
Grease can also be thickened with non-soap materials. Common non-soap thickeners include polyurea, organophilic clay, fumed silica, fluoropolymers, and others.
Polyurea is a generic term that includes include diurea, tetraurea, urea-urethane, and many related chemistries. A typical polyurea thickener is the reaction product of a di-isocyanate with mono and/or diamines. The ratios of the ingredients determine the characteristics of the thickener. It should be noted that because polyurea thickeners do not contain any metallic elements, they are ashless and tend to be oxidatively stable.
Organophilic clay thickeners include the minerals bentonite and hectorite. These minerals are purified to remove any non-clay material, ground to the desired particle size distribution, and then chemically treated to make the particles organophilic (more compatible with organic chemicals). Clay particles are then dispersed in a fluid lubricant to form grease. Clay particles must be activated with a polar material to stabilize the thickener structure. No chemical reaction takes place in the production of clay thickened greases. Clay thickeners have no defined melting point, so they have been used historically in high-temperature greases.
Fumed silica powder is used in relatively few grease products. Like clay, silica particles are dispersed in lubricating fluid. These greases also have no defined melting point and can be used in high-temperature applications. Fumed silica is used to thicken only a limited number of specialty grease products.
Fluoropolymer powders such as PTFE (polytetrafluoroethylene) can be used to thicken lubricating fluids to form grease. These greases are premium niche products with very good resistance to chemicals, oxygen, and water. They are formulated to withstand wide temperature ranges and often provide an extended service life in demanding applications.