Comparative Theoretical and Experimental Study of Linear and Star Polymer Additive Degradation in Kurt Orbahn Tests

摘要

The process of molecule scission for a polymer additive dissolved in a lubricant is analyzed quantitatively based on the probabilistic kinetic equation(s) for the density of the polymer molecular weight distribution of linear polymeric molecules/arms or star polymeric molecules. The considered polymer molecule scission process is caused by the temperature and the shear strain rate in the lubricant. The expressions for the probability of scission and the density of the conditional probability of scission are derived. It is shown that in an isolated system the total polymer weight per unit mass of the lubricant is conserved in time along the lubricant flow streamlines while the total number of polymer molecules/arms of star polymeric molecules per unit of the lubricant mass is a non-decreasing function of time. It is shown that the numerically calculated polymer molecular weight distributions and viscosity loss are in excellent agreement with the corresponding test data. The loss of lubricant viscosity depends on the nature of base stock, polymer additive, and the degradation regime. Based on the experimental data and the calculated results the viscosity loss varies from about 10% to 27% for two lubricants formulated with olefin copolymer viscosity modifiers of different average molecular weights.