Fatty-Acid-Constituted Halogen-Free Ionic Liquids as Renewable, Environmentally Friendly, and High-Performance Lubricant Additives

Stearic, oleic, and linoleic acids, which have a variable degree of unsaturation (n = 0, 1, and 2, respectively), were selected as model fatty acids to synthesize halogen-free, renewable, and environmentally friendly ionic liquids. The preparation of fatty acid ionic liquids was confirmed by H-1 and C-13 NMR and FTIR analyses. The fatty acid ionic liquids as additives to the polyol ester lube base oil exhibited remarkably improved friction and antiwear properties for steel tribopair. The fatty acid anions exhibiting inherent negative charge promptly interact with the engineering surfaces compared with the polyol ester and vegetable oils. Therefore; the addition of 2% fatty acid ionic liquid to the polyol ester, which is composed of fatty acid, showed remarkable improvement in both the friction-reducing (28-60%) and the antiwear (20-28%) properties under the boundary lubrication regime. The magnitudes of friction and wear reduction are largely controlled by the degree of unsaturation in the fatty acid anion. Further, copper strip corrosion tests revealed the noncorrosiveness of the fatty acid ionic liquids. A facile, scalable, and economic approach to synthesize the environmentally friendly ionic liquids containing renewable fatty acid anions promises immense potential for lubrication applications.