On the Thermal Degradation of Lubricant Grease: Experiments

This study assesses the thermal aging of lubricant grease via five rheological, mechanical, and thermal grease properties—yield stress, storage modulus, bleed capacity, specific heat capacity, and thermal strain coefficient. Three different grease types are studied: a polyurea-thickened grease with fully synthetic ester base oil, and two lithium-thickened greases, one with a mineral base oil and the other with a semi-synthetic base oil. The properties are measured at intervals as the grease samples aged for 30 days in a vacuum oven at 130∘C. The evaporation of base oil and volatiles, which affects the grease properties, is also measured. Results show inconsistent changes in the properties throughout the aging process, with the most significant transformations occurring in the first 10 days—during which most of the evaporation occurred. In addition to evaporation, other mechanisms appear active. Various levels of discolorations are observed. About 0.5thickened grease is lost to evaporation, and about 5thickened greases evaporates. Results from a separate thermogravimetric test show that the fully synthetic ester base oil bled from the polyurea grease has the highest latent heat of evaporation of 337.9 kJ/kg, while the mineral and semi-synthetic base oils bled from the lithium greases have comparable latent heats of evaporation of 154 kJ/kg and 137.7 kJ/kg, respectively.