New insight into the relationship between molecular effects and the rheological behaviour of polymer-thickened lubricants
Résumé
The present work focuses on the rheological study of a simplified automotive lubricant (base oil + polymer additive) at high temperature, high pressure and high shear stress. Various rheological experiments have been performed especially on a high pressure falling body viscometer and on a high pressure Couette rheometer. From the experimental results, a complete model describes the lubricant viscosity variation as a function of temperature (25-150°C), pressure (0.1-800MPa) and shear stress (0.1Pa-10^5Pa). In order to fully understand the rheological response of lubricants under such operating conditions and thus in an engine, it is necessary to investigate the mechanisms occurring at the molecular scale. Using an Einstein law as a first approximation, it becomes possible to study the variation of the polymer radius of gyration as a function of temperature and pressure. Since the polymer solubility is directly dependent on the radius of gyration, this method is an easy way to access the solubility of a polymer in oil under extreme conditions. Rheology is closely correlated to solubility when it comes to lubricants; consequently, this approach will bring new answers about their behaviour.