Impact of natural sources-derived antioxidants on the oxidative stability and rheological properties of castor oil based-lubricating greases

Abstract

It is well known that spontaneous oxidation is one of the main drawbacks for the industrial use of eco-friendly vegetable oil-based lubricants. Herein a quick and easy synthesis of different molecules belonging to stilbene family is reported, which together with a number of representative natural sources-derived products with antioxidant (AO) capacity have been tested as additives for improving the oxidative stability of a selected bio-lubricating grease formulation. Thus, different gel-like dispersions based on N-acylated chitosan in castor oil also including these additives have been prepared. Gel-like dispersions were characterized through thermogravimetric analysis (TGA), oxidation onset temperature (OOT) and small-amplitude oscillatory shear (SAOS) rheological measurements. These preliminary results demonstrate that, in general, these antioxidants delay N-acylated chitosan-based gel-like dispersions oxidation. However, the AO activity is more relevant in the case of propyl gallate and ascorbic acid palmitate and not so important in the family of stilbene derivatives. On the other hand, the addition of any antioxidant does not qualitatively affect the rheological response but unexpectedly may significantly modify the values of the SAOS functions. Remarkably, the delay of oxidation reactions occurs by exclusively employing natural resources-derived components, being these formulations environmentally acceptable as a whole and potentially employed in specific industrial applications.