Shear stability of the VM is one of the important criteria that determine its suitability in a lubricant formulation. Several kinds of polymers, such as poly(methyl methacrylate) (PMMA), olefin copolymer (OCP) and hydrogenated styrene-diene copolymer (SDP) are generally used as VMs in lubricating oil compositions. to minimize the temperature dependency of viscosity) are called as viscosity index improvers (VII) or viscosity modifiers (VM) and corresponding VII – treated lubricants are categorized as multigrade oils as opposed to monograde oils which contain no VI improvers. Polymers employed as additives in lubricants to enhance the VI (i.e. A higher VI value signifies a lesser effect of temperature on viscosity. In the fields of lubricants, the degree of susceptibility of viscosity of a fluid to temperature variations is quantitatively expressed by an empirical term known as viscosity index (VI). The hydrodynamic volume of the polymer in the fluid increases with increase in temperature thereby resulting in an increase in viscosity, which greatly compensates the opposing effect of commonly observed reduction in viscosity of a fluid with rise in temperature. Introduction Polymer containing fluids exhibit less reduction in viscosity with increase in temperature than the corresponding base fluids containing no polymer.
