Technical Papers

Selection of Viscosity Index Improvers for Formulation of Modern Lubricants

Abstract:

In formulation of multi-grade lubricants Viscosity Index Improvers (VII's) are used in order to compensate the base oil's unwanted loss of viscosity by their inherent property to increase the hydrodynamic volume with increasing temperature. Thus, they provide a substantial viscosity lift at high temperatures compared to the unadditized fluid, i.e. the Viscosity Index of the lubricant is improved, and failures of equipment due to wear in a boundary lubrication regime can be prevented. On the other hand, VI Improvers may lose their thickening efficiency during service due to shear in gear contacts, bearings, hydraulic pumps et cetera. Shear stability correlates with the life time of the lubricant and is therefore important criterion for the selection of VI Improvers. Shear stability of transmission fluids is usually determined in the Taper Roller Bearing test (KRL), which is used to predict the viscosity loss due to shear in the vehicle. Excessive shear loss and transmission failure might be the consequence of using a non-appropriate method like the less severe Bosch injector pump for VII selection. Ultrasonic shear tests produce misleading results, when comparing VI Improvers of different chemistry.  

The major trends of machine design today are higher efficiency, higher load capacity, reduced size and longer oil drain intervals. Smaller oil sumps and higher specific loads lead to increasing operating temperatures. With selection of appropriate VI Improvers lubricants with increased efficiency, reduced operating temperature and longer oil lifetimes can be formulated.  

The paper shows examples of VI Improvers like PAMA poly(alkyl­meth­acrylates), PIB poly(isobutylene) and OCP poly(ethylene-co-propylene). It explains the methods used by OEMs and formulators for determination of properties like shear stability, efficiency and operating temperature.