Long-term lubrication by grease is an important problem with respect to effective use of natural resources and extending the life of machines. The solution to this problem is extension of the grease life. Heat and shear result in deterioration of grease by one of two deterioration types. The first type is where the grease becomes soft and flows out of the lubrication area, while the second is where the grease solidifies and can no longer supply oil to the lubrication area. Long-term lubrication grease therefore must have high heat resistance and shear stability.
Calcium sulfonate complex greases have higher performance compared to metal soap greases, especially with respect to heat resistance, load carrying capacity, water resistance and shear stability. They can be used above 160 degrees C. These characteristics have led to extensive use of calcium sulfonate complex greases in the iron and steel industry, construction machinery, the paper industry and automobiles.
The objective of this presentation is to introduce the favorable properties of calcium sulfonate complex greases and to describe the relationship between their properties and structures.
Grease properties are affected by the structure of the calcium sulfonate complex greases. We recently developed a new method for analyzing grease thickeners by direct observation using small angle x-ray scattering. In our experience, the primary structure of a calcium sulfonate complex grease consists of 21nm diameter spheres. We assume that a calcium sulfonate complex grease is a reverse micelle consisting of a calcium carbonate core surrounded by alkylbenzene sulfonate. We assume that the high heat resistance and shear stability of calcium sulfonate complex greases reflect the core calcium carbonate structure. Calcium carbonate, as an inorganic salt, has higher heat-resistance and harder particles than organic salts.
The higher structure of calcium sulfonate complex greases was analyzed to calculate the pair-distance distribution function, which is calculated by integrating the function of the distance over infinity determined from SAXS results. The shape of the higher structure is ellipsoidal, with a maximum length of about 140nm. The higher order structure arises from aggregation of the primary structures. We assume that the higher order structure has a sea-island structure, with the sea made by base oil and the island made by the higher-order structures. We assume that if the sea-island structure is broken by heat or shear, the primary structure readily aggregates, resulting in rapid reformation of the sea-island structure.
Calcium sulfonate complex greases can be used at high temperature, at high load conditions, and under high shear. Their superior properties can be attributed to their primary structure consisting of calcium carbonate, and their higher-order structure that readily aggregates if this structure is broken.