Tribological Properties of Limonene Bisphosphonates

Abstract

Limonene was chemically modified by reacting it with dialkyl phosphites of varying alkyl structures under inert atmosphere in the presence of free radical initiators. The reaction gave a mixture of mono- and di-adduct products and was optimized to produce only the di-adduct product limonene bisphosphonate by forcing both limonene double bonds to react completely. The product mixture was carefully characterized using a combination of gas chromatography–mass spectroscopy, infrared spectroscopy, and nuclear magnetic resonance spectroscopy (1H, 13C, 31P). The bisphosphonates were investigated for their physical and tribological properties. The alkyl bisphosphonates displayed density and viscosity that was a function of the alkyl structure (methyl, ethyl, n-butyl) and much higher than the values for the unreacted limonene. They also displayed improved oxidation stability but lower viscosity index and solubility in polyalphaolefin (PAO6) and high-oleic sunflower oil (HOSuO) base oils. Tribological characterization of the neat modified oils on a four-ball tribometer showed improved extreme-pressure weld point by all three di-adducts and improved anti-wear coefficient of friction (COF) and wear scar diameter (WSD) by the n-butyl di-adduct only. The limonene bisphosphonates also displayed improved COF and WSD as additives in PAO6 and HOSuO base oils at low concentrations. The effects of chemical modifications on physical and tribological properties can be explained in terms of increased polarity of the modified products, insertion of heavy atoms (from PO3) into the limonene structure, and complete absence of unsaturation in the modified products.