A Chromatographic Study of Shear Stability of Poly(styrene-co-alkyl methacrylate) Viscosity Modifier for Lubricating Oils

Abstract

Shear stability of styrene, dodecyl methacrylate, and octadecyl methacrylate terpolymer of various molecular weights, as the rheology modifier of improved solubility, thermal and oxidation stability in mineral base oil, was investigated. Viscosity loss of the 5 wt% solutions caused by scission of macromolecules subjected to the high shear forces ranges between 15% and 27% for weight average molecular weights from 156000 to 252000. After the shear tests the number average molecular weights of polymer increases while weight and Z-average molecular weights decrease. As macromolecules of molecular weights above 700000 fully break the molar-mass dispersity decreases significantly, from starting at 2.34–2.79 down to below 1.8. A linear dependence between viscosity loss of terpolymer solutions and molecular weights was observed. The advantage of using the size exclusion chromatography to determine the stability of the polymer against mechanical shear directly and the viscosity loss indirectly was established.

Keywords:

• viscosity index improver
• styrene-alkyl methacrylate copolymer
• shear stability
• viscosity loss
• size exclusion chromatography