Formulation of lubricating greases from renewable basestocks and thickener agents: A rheological approach

This work deals with the development and rheological characterization of a variety of green lubricating grease formulations based on different combinations of vegetable oil-derived basestocks and biopolymers obtained from renewable resources. Castor oil, high-oleic sunflower acid oil (HOS-AO) and a ricinoleic acid-derived estolide were tested as basestocks, whereas chitosan and Kraft cellulose pulp, apart from traditional lithium soap, were selected as thickener agents. Rheological behaviour was characterized by means of small-amplitude oscillatory shear (SAOS) and viscous flow measurements. In addition to these, some standard mechanical tests were carried out in order to evaluate the suitability of these formulations for lubricant applications. The rheological response depends on the type of vegetable oil-derived basestocks and thickener agent. Higher values of the SAOS functions were obtained for chitosan-based formulations. High viscosity and consistency were particularly achieved by combining the estolide sample and lithium hydroxide. On the contrary, soft greases and generally higher flow indexes were found in cellulose pulp-based formulations. The effect of the different basestocks on the rheological response was explained attending to the influence exerted by oil viscosity and basestock-thickener polar interactions, as well as the chemical reaction between the basestock carboxylic moieties and lithium hydroxide to form different thickener structures.