Influence of Soap Concentration and Oil Viscosity on the Rheology and Microstructure of Lubricating Greases

The effects that both soap concentration and base oil viscosity exert on the rheology of lubricating greases and its relationship with grease microstructure are discussed in this work. With this aim, different lubricating grease formulations were manufactured by modifying the concentration of lithium 12-hydroxystearate and the viscosity of the base oil, according to an RSM statistical design. These lubricating greases were rheologically characterized through small-amplitude oscillatory shear (SAOS) and viscous flow measurements. In addition to these, scanning electronic microscopy (SEM) observations and mechanical stability tests were also carried out. It has been found that the structural skeleton (size and shape of the disperse phase particles) was highly influenced by the base oil viscosity. In this sense, the values of the viscoelastic functions in the linear viscoelastic region and the mechanical stability of the lubricating greases increase as the viscosity of the base oil decreases. An opposite tendency was observed during viscous flow tests at high shear rates, when the grease microstructure was mostly destroyed. On the other hand, the microstructural network of these greases becomes stronger as soap concentration increases. These results have been explained taking into account the balance between the solvency of the thickener in the base oil and the level of entanglements formed by soap fibers, which influence the lubricating grease network.