Oxidation stability and tribological behavior of vegetable oil hydraulic fluids

The new generation of vegetable oil hydraulic fluids has a price advantage and provides field longevity comparable to synthetic esters at moderate temperature. This article investigates the oxidation stability and tribological behavior of fully formulated vegetable oil hydraulic fluids. Differences in origin (canola, soybean, and sunflower oil) and the degree of unsatitration are studied. Modified ASTM D943 (Dry TOST) was found reliable to evaluate the impact on oxidative stability of various additive packages in vegetable oils. Tribological testing utilized laboratory hydraulic pumps such as the Vickers V 104C, Vickers 20VQ, and Vickers PFB-5 under a variety of temperatures, pressures, pump outputs, and speeds. The conditions of aging fluids were monitored by evaluation of TAN, viscosity, wear rate, and depletion of antioxidants. Voltammetric technique was applied to determine the concentration of antioxidant in systems. Based on analysis of antioxidant consumption, the main degenerative process in vegetable oils under demanding pump operations is similar to pyrolytic degradation, rather than oxidation. By selecting the appropriate synergistic additive packages, a remarkable pump longevity can be obtained for each vegetable base stock studied.