Shear stability of polymers used as viscosity modifiers in lubricating oils

Viscosity modifiers (VM), also often called as viscosity index improvers (V II), are polymers used as additives in lubricant compositions for minimizin g the variations in viscosity of the lubricant as a function of temperature . The degradation stability towards mechanical shearing (shear stability) o f three types of polymers, namely polymethacrylate (PMA), olefin copolymer (OCP) and hydrogenated styrene-isoprene copolymer (SIP) which are generally used as VMs in lubricants, has been investigated at varied polymer concent rations (0.5 to 4.0% wt/wt) in five high viscosity index (HVI) and one medi um viscosity index (MVI) category of mineral oils using a Bosch injector sh ear rig and ASTM D-3945 method. The effects of mineral oil (base stock) vis cosity and the composition of the base stock as well as that of polymer con centration on shear stability Index (SSI) were primarily studied. The effec t of polymer concentration on SSI was distinctly observed and its magnitude was dependent on polymer chemistry and the nature of base oil. The studies were extended to determine shear stability of these polymers in terms of h igh temperature, high-shear (HTHS) viscosities and it was,found that there is no correlation between SSI values based on kinematic viscosities and tho se based on HTHS viscosities. Studies were also briefly extended towards an understanding of the relationship between thickening ability of the polyme rs and their shear stability.