What are compositions of lubricating grease?
The most common composition of a lubricating grease is a Lithium-12-hydroxy sterate thickened mineral oil with anti-oxidant(s), anti-corrosion and extreme pressure (EP) improving agent(s) added. Several other metal cations are used; Calcium, Aluminium, Sodium and Barium being the more common metals.
Whilst 12-hydroxy stearic acid, in its refined fatty acid form or as the triglyceride (hydrogenated castor oil), is the most commonly used anion, several other anions are used. Tallow, lard, stearic acid, oleic acid and many other fatty acids or fats are used either as the major anion or as modifiers for the desired soap structure. When combining different types of anions, normally diacids like acetic, adipic or azelaic acid, with the same metal cation, so-called complex greases are formed. By the use of different types of metal cations, but with one major type of anion, so called mixed base greases are formed.
Mineral oils are the most common base fluid for lubricating greases, even though synthetic fluids like polyalphaolefines and synthetic and natural esters are gaining importance every year. Other types of synthetic fluids seem to play a minor role and are mostly used for very special applications.
The use of esters from natural sources is increasing due to environmental concern encouraging the use of raw material from renewable sources. Refined rape seed, sunflower and corn oils have already been introduced in the market and more types are expected in the future.
Additives are often, although not always, used in lubricating greases. Typically, antioxidants, corrosion inhibitors, anti-wear and EP agents are added. Solid lubricants (usually graphite or Molybdenum disulphide), metal deactivators and polymers are also examples of additives used in lubricating greases.
