Design, Synthesis and Tribological Properties of Fatty Acid Diethanolamide Borate Esters with Different Carbon Chain Lengths

Abstract

With the increasing attention to environmental issues and the improvement of environmental regulations, traditional lubricant additives are facing huge challenges, while biodegradable green lubricant additives are facing new opportunities. Organic borate esters, as lubricating additives, have core competitiveness over traditional lubricating additives containing S, P, and Cl. Borate esters not only overcome the poor corrosion resistance of traditional additives, but also have excellent load-bearing capacity and extreme pressure performance. In addition, introducing fatty acid diethanolamide containing active groups such as hydroxyl and amide bonds into borate esters enhances the adsorption capacity. In this article, three borate esters with different carbon chain length were prepared, namely stearic acid diethanolamide borate ester with a chain length of 18, tetradecanoic acid diethanolamide borate ester with a chain length of 14, and octanoic acid diethanolamide borate ester with a chain length of 8. The as-prepared borate ester, especially C₁₈ONB, exhibits excellent tribological properties as lubricant additives in poly (a-olefin) (PAO6), significantly improving the friction reducing and antiwear properties of the base oil. This is due to the weak polarity of C₁₈ONB with long carbon chains, which exhibits good solubility in PAO6 with weaker polarity and forms thick multilayer viscoelastic adsorption film on the friction surface. In addition, the adsorption film undergoes tribochemical reactions during the rubbing process, generating a tribofilm containing excellent lubricants such as B₂O₃ and BN, which further plays a good role in reducing friction and antiwear.