Predicting EHD film thickness of lubricant polymer solutions

It is well-known that the elastohydrodynamic (EHD) film thicknesses formed by solutions of medium to high molecular weight polymers in lubricating oils are less than those predicted from EHD theory based on the low shear rate viscosities of the polymer solutions. This is usually ascribed to shear thinning of polymer solutions in the high shear stress conditions in the inlet of the EHD contact. This paper describes a systematic study of the EHD film-forming properties of polymer solutions in which a range of well-characterized low dispersivity polymers is employed to investigate the influence of polymer type, molecular weight and concentration in base oil on EHD film thickness. It is shown that the disparity between measured film thickness and that predicted from EHD theory using low shear rate viscosity increases progressively as both polymer concentration and polymer molecular weight are raised. Analysis of the results suggests, however, that conventional EHD film thickness equations can be employed to predict the film-forming behavior of polymer solutions if viscosity values are used which are derived from intrinsic viscosities on the assumption of dilute polymer solution behavior, i.e., negligible polymer-polymer molecular interaction in solution.