Oil/Thickener Interactions and Rheology of Lubricating Greases

Abstract

The present communication concerns the effect of base oil/thickener interaction on the flow properties of lubricating greases. The flow properties of paraffinic and naphthenic lithium greases in the temperature range from −40 to 60°C have been characterized by using the traditional flow pressure test and advanced rheological measurements including the determinations of apparent viscosity, yield stress, grease rheology intersection stress, and critical stress. The rheology of greases appears to be typical of viscoelastic materials possessing a certain degree of internal structure and memory effect (thixotropy), and there exists an obvious correlation between the pumpability and the rheological characteristics of greases at all temperatures. At high temperatures (above 0°C), the flow properties of paraffinic and naphthenic greases are rather similar. However, the pumpability of naphthenic greases is clearly superior to that of paraffinic greases at temperatures between −20 and −40°C. As evidenced by the flow pressure measurements, among the naphthenic greases studied, the grease made of a highly refined naphthenic oil with the lowest aromatic content has the best pumpability at extremely low temperatures (−40°C). The observed differences in pumpability and rheological properties of greases are attributed to differing solvencies of the base oils, which have a significant impact on the structure of the thickener and, as a result, on the rheology and lubricating efficiency of grease. The rheological studies have been complemented by in situ characterization of the thickener structure in grease by using the cryo-TEM technique in combination with a specially developed thickener versus a background contrasting method. In this way, both the characteristic fiber dimensions and the aggregate structure have been determined. The soap fibers were found to have a worm-like shape with the average diameter of around 30 nm and the length of around 1 μm. The only apparent difference between paraffinic and naphthenic greases was that, in the former, the fibers tend to be more irregular in shape. Furthermore, it has been demonstrated that severe oxidation of grease in accelerated ageing tests causes a significant aggregation of fibers. Simultaneously with fiber compaction, some fibers get broken and the average fiber diameter increases from around 30 nm to around 50 nm. The described changes in the fiber morphology can be explained by the adsorption of oil oxidation products to the surface of fibers or by soap oxidation and hydrolysis, which are suspected to be the primary degradative processes lying behind the grease ageing in applications.