Rheological behavior of greases: Part II -- effects of thermal aging, correlation with physico-chemical changes

摘要

The thermal degradation of greases was studied from the viewpoint of both rheological and physico-chemical changes. Rheological investigations showed that lithium greases are weakened by thermal aging while urea greases are strengthened. Structural, chemical, and composition modifications were the most likely changes in the grease samples. The occurrence of structural changes was confirmed by Transmission Electron Microscopy (TEM), experiments. Lithium networks were degraded by aging, bias urea networks were not degraded and even seemed to be reinforced. Chromatography (SEC), FT-IR spectrometry, and filtration techniques were wed to obtain information on the composition and on the chemical aspects. The main phenomenon observed in these tests concerns the lithium greases, which show a relatively important loss of thickener. Finally, this work showed that parameters obtained in continuous and dynamic rheological tests seem to be qualified to represent the modifications occurring in greases.