Viscosity modification of high‐oleic sunflower and castor oils with acid oils‐derived estolides for lubricant applications

Abstract

The use of estolides as additives to increase the viscosity of vegetable oils for lubricant applications was investigated. For this purpose, high‐oleic sunflower (HOSO) and castor (CO) oils were blended with estolides derived from acid oils resulting from vegetable oil refining process (HOSO and olive pomace acid oils). These blends were characterized from both viscous and tribological point of views. Significant increments in viscosity were generally found when added estolides were synthetized using sulphuric acid as catalyst, compared to p‐toluensulphonic acid catalyst. Estolides prepared from these two by‐products gave rise to similar viscosity modification in HOSO, yielding maximum viscosity increments of 322 and 362%, at 10°C, respectively. Smaller viscosity modifications were obtained for CO/estolide blends, with maximum increments in kinematic viscosity of 84 and 152%, respectively. Generally, the addition of estolides to HOSO yielded a significant increase in the temperature dependence of viscosity. The addition of acid oils‐derived estolides slightly improved CO thermal dependence of viscosity for lubricant applications. Although wear was significantly reduced by adding these estolides to any of these vegetable oils, the addition of acid oils‐derived estolides to CO increased the friction coefficient at low rotational speeds, extending the boundary lubrication regime to higher Sommerfeld number values. On the contrary, a single Stribeck master curve was obtained for HOSO/estolide blends.