The Thixotropic Properties of Lithium Soap Greases With Various Thickener Concentrations and Base Oil Viscosities

The apparent viscosity reduction of lubricating greases composed of lithium stearate and polyalphaolefin (PAO) was examined as a function of thickener concentration and base oil viscosity. The relationship between thixotropy and the structural change was then investigated. Hysteresis loop measurement is one of the most common ways to measure thixotropy. We improved this technique in order to measure continual thixotropic change. This enabled us to compare thixotropy among samples quantitatively. The relationship between the reduction in viscosity and the structural change within a gel network of samples was considered, and continual structural breakdown caused by shearing was determined by calculating the energy differences among the flow curves. Furthermore, a flow curve observed during an increase shear rate was fitted to the Herschel-Bulkley equation. The structural change during an increasing shear rate was represented by coefficients of the equation. The obtained data were analyzed carefully, and the relationship between the grease components and the shear strength of the structure was investigated. It was found that the base oil viscosity and the thickener contents determined the shear strength of the structure units, and that the thixotropy changed along with the shear history of grease.