Thickeners and viscosity index improvers are polymeric, and are added to lubricants to reduce the degree of change in viscosity seen at high and low temperatures.
Mineral oil lubricants become less effective at high temperatures because heat reduces their viscosity and film-forming ability. The traditional solution to this problem was to make seasonal oil changes in some applications. With the advent of viscosity improvers, that’s no longer necessary or desirable.
When viscosity improvers are added to low viscosity oils, they effectively thicken the oil as temperature increases. This means that the lubricating effect of mineral oils can be extended across a wider temperature range.
When creating a viscosity improver, a balance between the thickening efficiency and shear stability of the polymer is important. Higher molecular weight polymers make better thickeners, but tend to have less resistance to mechanical shear. Lower molecular weight polymers are more shear resistant, but do not improve viscosity as effectively at higher temperatures and have to be used in larger quantities.
Polymer additives can also undergo thermal and oxidative degradation, unzipping back to smaller monomers, which reduces their effect. The highest possible degree of thermal and oxidative stability is desirable in addition to the features above.
Afton’s family of viscosity improvers is primarily used in multigrade engine oils, gear oils, automatic transmission fluids, power steering fluids, greases, and some hydraulic fluids.