Base Oils Used in Greases

Greases are not simply very viscous lubricating oils. They are in fact mixtures of lubricating oils & thickeners. The thickeners are dispersed in the oils in order to produce a stable colloidal structure or gel. Thus, a grease consists of oil constrained by minute thickener fibres. This multiphase structure gives the product a suitable consistency which enables the grease to remain in place under the conditions of use. This ensures effective lubrication, provides a sealing capability and enables the grease to position functional additives close to the working surfaces of the equipment.
Greases are manufactured by adding alkali & fatty acid to a quantity of oil. The mixture is then heated & soap is formed from the alkali & fatty acid (saponification). After the reaction, the water necessary for the soap formation is removed & the soap crystallizes. The final stages of manufacture involve mechanical working of the grease to homogenize the composition & allow blending in of additives & the remaining oil.
The fluid lubricant, or Base oil, is the major component in a grease, typically comprising 70-95% of the final product. The important properties of base oil & their effects on grease are:

Property Effect on Grease Performance
Viscosity & VI Pumpability & Low Temperature Performance
Oxidation Stability Service Life & High Temperature Application, Storage Life
Dyes Identification of oils  

Mineral oils are the most commonly used grease base stock accounting for 99% of greases produced. Napthenic oils are the most popular despite their low VI. They maintain the liquid phase at low temperatures & easily combine with soaps. Paraffinics are poorer solvents for many of the additives used in greases & with some of the soaps may generate weaker gel structure.
Synthetic oils are used for greases which are expected to operate in extreme conditions. The most commonly used are synthetic esters, phosphate esters, silicones & fluorocarbons. They are quite expensive.
Vegetable oils are used in greases intended for the food & pharmaceutical industries.