Dual Role of Phosphate Ionic Liquids in Synthetic Ester Oil: from Synthetic Catalyst to Anti-Wear Additive

Two phosphate ionic liquids (ILs) were synthesized, and investigated as catalysts for the esterification of trimethylolpropane (TMP) with oleic acid (OA) to produce trimethylolpropane-trioleate (TMPTO) for the first time. Both of the two ILs showed significant catalytic activity and high selectivity to di- and tri- esters. The test results show that the ILs were fully miscible with TMPTO, and the tribological property of the ILs as antiwear additives for the steel/steel contact by an Optimol SRV-IV oscillating reciprocating friction and wear tester under high temperature. The two ILs prominently improved the friction-reducing and antiwear properties of the base oil. The morphologies of the worn steel surfaces were analyzed by scanning electron microscope and X-ray photoelectron spectrometer analysis to explore the lubrication mechanism. It is indicated that physical adsorption and tribochemical reactions occurred during the rubbing process with the generation of surface protective films, which prevented the metal-metal contact and reduced friction and wear.