Viscous, thermal and tribological characterization of oleic and ricinoleic acids-derived estolides and their blends with vegetable oils

This work deals with the viscous, thermal and tribological characterization of a variety of estolides, obtained from both oleic and ricinoleic acids, using different acid-catalysed synthesis protocols, and their blends with vegetable (high-oleic sunflower, HOSO, and castor, CO) oils. Estolides with molecular weights between 4.4 and 6.9 times higher than the originating fatty acids were obtained. Polymerization degree was larger when using the sulphuric acid-catalysed synthesis protocol. Estolides obtained from oleic acid displayed higher freezing temperatures than the fatty acid, whereas the crystallization process was delayed in estolides obtained from ricinoleic acid, yielding improved low-temperature properties. Ricinoleic acid-derived estolides showed much higher viscosity values than those prepared from the oleic acid, with values of kinematic viscosity up to around 6700 mm2/s. In general, viscosities were related to estolide molecular weight. Significant increments in HOSO and CO viscosities were found when they were blended with estolides, especially those prepared from the ricinoleic acid using the sulphuric and p-toluensulphonic acids-catalyzed methods. Relative increments in kinematic viscosities up to 1500% and 700% were obtained for HOSO and CO, respectively. HOSO's viscosity-temperature dependence was significantly improved when it was blended with different estolides, whereas CO/oleic acid-derived estolides blends showed a more moderate improvement of CO thermal dependence. The sulphuric acid-catalysed method influences friction and wear in the ball-on-plates contact lubricated with estolides. The addition of the different estolides to HOSO or CO does not modify their frictional behavior, resulting in just one single Stribeck curve for all samples, and significantly reduces wear.