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

Abstract

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.

Rheology of oleogels based on sorbitan and glyceryl monostearates and vegetable oils for lubricating applications

“…Only at the highest frequencies, lower values of the loss tangent can be observed for oleogels prepared with low-viscosity oils, especially in the case of those containing soybean oil, as a consequence of the displacement of the minimum in G" to higher frequencies. Similar results were previously found with standard lithium soapbased lubricating greases manufactured with paraffinic oils of different viscosities (Delgado et al, 2006a). Larger gel strength, associated to enhanced elastic properties, was found in greases prepared with low-viscosity paraffinic oils.…”

“…an apparent minimum in G" and a power-law relationship for G' with slopes of around 0.1-0.2. This behavior is typical of entangled polymers (Ferry, 1980) and gel-like disperse systems (Mewis and Spaull, 1976;Franco et al, 1995;Gallegos et al, 2004;Ruiz-Márquez et al, 2010), including standard lubricating greases (Delgado et al, 2006a;Martín-Alfonso et al, 2007). This minimum in G" is shifted to high frequencies for the lowest GMS concentration, yielding slightly lower values of the loss tangent (tan δ ϭ G"/G') ( Figure 2d).…”

Effect of Thickener Particle Geometry and Concentration on the Grease EHL Film Thickness at Medium Speeds

“…A higher degree of physical entanglements was observed for Li/SS over Li/M, and CaS/M showed large spaces between the thickeners in comparison with CaS/MS (not shown). This suggests that there is an influence on the fiber interaction by the thickener concentration and base oil viscosity as is suggested in [28,29]; however, the effect on the particle shape and size is neglectable.…”

The rheological and tribological properties of calcium sulfonate complex greases

“…The thickener forms an entanglement network, which traps the oil and confers the appropriate rheological and tribological properties to the grease. Fatty acid soaps made of lithium, calcium, sodium, aluminum, and barium are most commonly used as thickeners [1,2]. In recent years, a multipurpose grease known as calcium sulfonate complex grease, has attracted much attention due to its good performances at high and low temperatures and its resistance to clipping and rubbing friction.…”