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

The thermal degradation of greases was studied from the viewpoint of both r heological and physico-chemical changes. Rheological investigations showed that lithium greases are weakened by thermal aging while urea greases are s trengthened. Structural, chemical, and composition modifications were the m ost 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, bu t urea networks were not degraded and even seemed to be reinforced. Chromat ography (SEC), FT-IR spectrometry, and filtration techniques were used to o btain 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 dynami c rheological tests seem to be qualified to represent the modifications occ urring in greases.