Other components of these greases include unreacted fat, fatty acids and alkali, unsaponifiable matter (including glycerol and fatty alcohols), rosin or wool grease and water. Some of the other additives used in grease are oxidation inhibitors, rust and corrosion inhibitors, colour stabilizers, metal passivators, water repellents and viscosity index improvers.
In soap greases the metallic soap consists of a long-chain fatty acid neutralized by a metal such as aluminium, barium, calcium, lithium, magnesium, sodium or strontium. The fatty acids usually contain 16 to 18 carbon atoms. A common form of soap-based grease uses lithium 12-hydroxystearate as the thickener. To properly thicken the grease the soap must be in the form of fibres of suitable size dispersed throughout the lubricating oil. The crystalline fibres are usually in the size range of 1 – 100 micrometres with diameters 0.1 to 0.01 of their length. For good shear stability the fibre should have a large ratio of length to diameter, and for good oil retention the fibre should be as small as possible. Therefore, greases need a mixture of these two types of fibres. Also, there must be a balance between the solvency of the fluid and the solubility of the soap to get suitable thickening.
The clay, such as bentonite is reacted with a quaternary amine to change the clay from hydrophilic (water-loving) to hydrophobic (water-rejecting) and oleophilic (attracting oil). Effective thickening is achieved by combining the clay with a polar activator or dispersant, such as acetone, methanol or ethanol, with small amounts of water and by delaminating and reducing the platelets to a small size. This process will increase the total surface area of the dispersed clay, which immobilizes a very high percentage of oil based on the weight of clay. This will thicken the grease. Other solid additives produce thickened grease by the nature of their fi ne dispersion throughout the fl uid and by their particle-particle interactions. Solid-additive greases extend the operating temperature range over soap greases. The solid-type greases do not have a melting point, and their upper temperature limit is that of the oil being used.
