Ultrasound-Assisted Synthesis of Novel Undecylenic Estolides: Modeling and Tribological Characterization

To overcome the issues related to low-temperature characteristics and thermal degradation of fatty acid-based lubricant base stocks, chemical modification is essential. To mitigate these shortcomings, considering unsaturated undecylenic acid, the formation of estolides is one of the best transformations considering application in lubricants. Ultrasonic-assisted sulfuric acid-catalyzed synthesis of estolides of undecylenic was modeled using response surface methodology (RSM) and subsequently validated using artificial neural network (ANN) for known and unknown input variables. At optimal reaction conditions of reaction temperature of 56 °C, catalyst loading of 0.63 mol equivalent, and reaction time of 1.61 h, estolides with estolide number of 2.58, extraordinary low pour point (PP) of −52 °C, and better resistance to thermal degradation were obtained. The thermal degradation was evaluated using thermogravimetric analysis (TGA) to find improved resistance toward degradation due to the formation of estolides. Furthermore, tribological properties like wear characteristics, load carrying capacity, and oxidative stability were studied for 5% blends in SN 70 base oil. The anti-wear ability of the estolides was found to be superior to undecylenic acid, with a lower coefficient of friction, scar diameter, depth, and volume. The blend containing estolide was found to have load carrying capacity as high as 800 kgf. Moreover, owing to the double bond migration during the reaction, the oxidative stability of estolides was found to be inferior to the terminally unsaturated undecylenic acid.