What is the role of Viscosity Index Improvers in Lubricants?

Viscosity Index Improvers began their commercial debut around the 1950s to accommodate the new developments in automotive oils, which were then adapting multigrade viscosities. However, they were used even before (back in the 1930s) when workers in crude distillation realized that small amounts of rubber improved the VI of the oil but also increased sludge formation.

Today, VIIs are still primarily used as engine lubricants. They can also be found in automatic transmission fluids, multipurpose tractor transmission fluids, power steering fluids, shock absorber fluids, hydraulic fluids, manual transmission fluids, rear axle lubricants, industrial gear oils, turbine engine oils, and aircraft piston engine oils. (Mortier, Fox, & Orszulik, 2010)

Essentially, VIIs try to maintain the oil’s viscosity at varying temperatures. They try to ensure that the oil does not experience a loss of viscosity, which can occur due to high temperature or shear. VIIs can be considered polymers, which are tightly wound coils. When temperature or shear is applied to these coils, they unravel (lose their viscosity). Depending on the amount of shear, they may never recover their original shape (or viscosity).

As seen in Figure 1 below, Mortier, Fox, & Orszulik (2010) describe the change in the shape of the VIIs as a result of high temperature or shear. They can coil and uncoil depending on the shear stress, but if the bonds are broken, they will not reform their original coil and lose their intended viscosity.

Figure 1: Mechanical Polymer Degradation (excerpted from (Mortier, Fox, & Orszulik, 2010)
Figure 1: Mechanical Polymer Degradation (excerpted from (Mortier, Fox, & Orszulik, 2010)

Interestingly enough, it must be noted that some VIIs provide lubricants with additional functions of Pour point depression and dispersancy. This is highly dependent on their composition.