Rheological and Tribological Characterization of a New Acylated Chitosan–Based Biodegradable Lubricating Grease: A Comparative Study with Traditional Lithium and Calcium Greases

Abstract

This work compares the thermal, rheological, and tribological properties of a new gel-like biodegradable formulation, prepared using an acylated chitosan thickener and castor oil, with properties exhibited by two conventional greases thickened with lithium and calcium soaps, respectively, taken as benchmarks. Thermogravimetric (TGA), rheological (small-amplitude oscillatory shear [SAOS], rheodestruction, and viscous flow) and tribological (friction and wear analysis) tests, as well as roll-stability measurements were carried out to characterize the three grease samples. In addition, infrared spectroscopy and differential scanning calorimetry (DSC) were used to chemically characterize the acylated chitosan thickener agent. From a thermogravimetric point of view, the new formulation displayed better thermal resistance than the calcium and lithium lubricating greases. The evolution of the linear viscoelasticity functions with frequency and viscosity values in the shear rate and temperature ranges studied were similar to those obtained with the commercial lubricating greases. However, the linear viscoelasticity functions of the biodegradable formulation were slightly more affected by temperature. The mechanical stability behavior and recovery of the rheological functions found in the biodegradable formulation were also better than that exhibited by the calcium-based grease. However, the friction coefficient measured at low rotational speed is slightly higher than that obtained with the benchmarks, with similar or lower values obtained at a high rotational speed. Resulting wear marks obtained after the frictional tests using the acylated chitosan–based grease were larger than those obtained with the commercial greases.