Greases are primarily classified by their thickeners, the most common being metallic soaps. Others include bentonite clay, silica gel, polyurea and inorganic thickeners.
Soap‑based greases are produced from three main ingredients. One is the fatty material (animal or vegetable), which is usually 4 to 15 percent of the total, called the acid. The next is the base or alkali, which is the opposite of an acid. Bases used in making greases include calcium, aluminum, sodium, barium and lithium, with 1 to 3 percent normally needed. The third portion is the fluid, which can be selected from mineral oils, various types of synthetics, polyglycols or a never‑ending combination of fluids.
A more complex structure can be formed by using a complexing salt. This converts the thickener to a soap‑salt complex, hence the term complex grease. Complex greases offer a higher working temperature of about 38 degrees C (100 degrees F) than normal soap‑thickened products.
Complex greases were developed to improve the heat resistance of soap greases, with the most popular being lithium, aluminum, calcium and barium. These products exhibit a higher dropping point (about 260 degrees C or 500 degrees F), with the exception of barium, which is about 218 degrees C (425 degrees F). The dropping point is the temperature at which the product turns from a semi‑solid to a liquid. In other words, it is the point at which the lubricant may begin to separate from the thickener.
Complex greases offer a higher working temperature than normal soap types and generally provide outstanding oxidation resistance, although this is not true in all cases. They are also increasing in popularity, with lithium and aluminum being the front-runners.
Inorganic thickeners such as clay and silica consist of spheres and platelets. They thicken fluids with their large surface area. These products produce a very smooth non-melting grease that can be made to perform very well when careful consideration is given to product application.
Polyurea is a type of non‑soap thickener that is formed from urea derivatives. It is not a true polymer but rather a different chemical that has a thickening structure similar to soap. Polyurea greases are very stable, have a high dropping point and give outstanding service life.