Importance of VI Improvers for Rheological Targets in Modern Lubricants
Authors: Bhaskar Mukherjee, Mariam Sepehr, Alexander Boffa, Marta Sabater Ferret and Karine Lantuejoul
Introduction Viscosity is one of the most important properties of lubricating oils, and the key rheological parameter adopted in the design process for hydrodynamic and elasto-hydrodynamic lubrication regimes in engines. Lubricant viscosity is inversely dependent on temperature and needs to be carefully tailored to ensure consistency, flow-ability, pump-ability at low temperatures and yet have adequate minimum film thickness to prevent wear at the higher operating temperatures in engines. For example, it is desirable to have a lower viscosity at low temperatures to facilitate engine starting during cold weather, and a higher viscosity at higher ambient temperatures, to ensure minimum film thickness. Viscosity Index (VI) Improvers, are a class of additives used to tailor rheological properties in lubricants, and specifically to improve the viscosity/temperature behaviour. VI improvers are oil soluble polymers that are dissolved and make up a small percentage in a lubricant.
The polymers have a significant hydrodynamic volume in oil which impedes the flow of oil leading to an increase in viscosity. At higher temperatures, polymers exhibit increased motion and flexibility, due to decreased relaxation time, resulting inhibition of oil flow. Hence, VI improvers provide a relative increase in viscosity at higher temperatures that facilitates the formulation of multi-grade oils. Some commonly used polymers in VI Improvers are the following: Olefin Co-polymer (OCP), Hydrogenated Styrene-Isoprene (HSI), Styrene Butadiene (SBR), Polymethacrylate (PMA) and Polybutene (PIB). OCP VI Improvers are oil soluble copolymers of ethylene and propylene, which by virtue of their high thickening efficiency and relatively lower cost have a large market share in the engine oil VI Improver market1. The intent of this paper is to highlight the evolution and improvements in some OCP-VI Improvers to match up with the evolving requirements of modern engine oils in the market today.
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LUBE MAGAZINE NO.124 DECEMBER 2014
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