Thermorheological behaviour of a lithium lubricating grease

Thermal-induced changes in the viscous and viscoelastic responses of lubricating greases have been investigated through different rheological techniques in a temperature range of 0–175 °C. Small-amplitude oscillatory shear and viscous flow measurements were carried out on a model conventional lithium lubricating grease prepared by inducing the in situ saponification reaction between 12-hydroxystearic acid and hydrated lithium hydroxide. The linear viscoelasticity functions dramatically decrease above 110 °C, but not below this critical temperature, which determines the maximum recommended operating temperature in relation to its durability and resistance under working conditions. Two different regions, below and above this critical temperature, in the plateau modulus versus temperature plot have been detected. From this thermal dependence, a much larger thermal susceptibility of the lubricating grease at temperatures above 110 °C is apparent. The thermo-mechanical reversibility of this material has been studied by applying different combined stress–temperature protocols. Regarding the viscous flow, a minimum in the shear stress versus shear rate plots appeared at temperatures above 60 °C, more pronounced as temperature increases, resulting from material instabilities. The experimental results obtained have been explained on the basis of the thermo-mechanical degradation of the lubricating grease microstructure.