How Polymers Behave as Viscosity Index Improvers in Lubricating Oils

Abstract

One of the requirements of engine lubricating oil is that it must have a low enough viscosity at low temperatures to assist in cold starting and a high enough viscosity at high temperatures to main-tain its load-bearing characteristics. Viscosity Index (VI) is one approach used widely in the lubri-cating field to assess the variation of viscosity with temperature. The VI of both mineral and syn-thetic base oils can be improved by the addition of polymeric viscosity modifiers (VMs). VI im-provement by VMs is widely attributed to the polymer coil size expanding with increasing tem-perature. However, there is very little physical data supporting this generally accepted mechan-ism. To address this issue, intrinsic viscosity measurements and Small-Angle Neutron Scattering (SANS) have been used to study the variation of polymer coil size with changing temperature and concentration in a selection of solvents. The results will show that coil size expansion with tem-perature is not necessary to achieve significant elevation of viscosity index.