Synthesis and investigation of phosphorus-free ionic liquids as multifunctional lubricating additives

Ionic liquids (ILs) have been extensively studied as lubricants or lubricant additives for two decades by virtue of their unique physical and chemical properties. Here, two phosphorus-free multifunctional protic ILs were synthesized by reacting oleic acid and a dimer acid with an alkyl aromatic amine. Both were completely miscible in various base fluids like mineral oil, polyether, synthetic ester, and polyalpha olefin. Furthermore, no corrosion towards copper strips was found for these additives due to the absence of phosphorus, halogen, and sulfur in their molecular structures. Tribological tests found that they could significantly improve the tribological performance of base oil in a wide range of test conditions. Additionally, due to the presence of an alkyl diphenylamine group, they could considerably enhance the oxidative stability of the base oil. Overall, the facile preparation approach, good solubility, low corrosion, and excellent tribological behavior and antioxidation property make them suitable as multifunctional additives in various lubricants.