Use of chitin, chitosan and acylated derivatives as thickener agents of vegetable oils for bio-lubricant applications

This work deals with the development of new gel-like formulations prepared from natural resources, which could be potentially applicable as environmentally friendly lubricating greases. In particular, the use of chitin, chitosan and acylated derivatives as thickener agents of vegetable oils, which may represent an alternative to the traditional metallic soaps or polyurea derivatives, was explored. Biopolymers used to obtain oleogels were chemically and thermally characterized. Oleogels thermal and rheological behaviours were studied by means of TGA and DSC tests, and linear viscoelasticity measurements, respectively. Moreover, some lubricant performance properties were evaluated. The evolution of linear viscoelasticity functions with frequency was very similar to that found for standard lubricating greases. In general, linear viscoelasticity functions increased with biopolymer concentration, whilst they decreased when acylated chitosan or soybean oil were used in the oleogel formulation. The use of acylated chitosan with a degree of acylation of around 0.3 provides oleogels with very similar rheological properties than those shown by traditional lubricating greases, as a consequence of reducing the biopolymer polarity. However, chitin and chitosan-based oleogels show higher thermal stabilities than formulations containing acylated chitosan. In general, oleogel samples studied exhibited values of the friction coefficient comparable to those found for standard lithium greases. However, most of these oleogels generally display a quite poor mechanical stability in rolling elements.