Submitted to: Tribology Letter
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/16/2015
Publication Date: 9/9/2015
Citation: Biresaw, G., Bantchev, G.B. 2015. Tribological properties of limonene bisphosphonates. Tribology Letters. 60(11). doi: 10.1007/s11249-015-0578-2.
Interpretive Summary: Limonene is a natural product widely found in many plants as a constituent of “essential oils.” It is commercially obtained as a byproduct of the citrus industry from the processing of fruits such as oranges, lemons, limes, tangerines, mandarins, and grapefruits. Commercial limonene can be used in various industrial applications without further chemical modification. However, value-added products can be obtained from limonene through chemical modification. In this work, limonene was chemically modified by reacting it with various structured phosphites to develop value-added products for use in biolubricant formulations. The bisphosphonate products were carefully identified using a combination of various analytical techniques such as gas chromatography-mass spectroscopy, infrared spectroscopy, and nuclear magnetic resonance. The bisphosphonates were investigated for their physical properties and displayed higher density and viscosity than unmodified limonene. They also displayed improved oxidation stability, slightly lower viscosity index, and lower solubility in petroleum-based and biobased base oils than limonene. Tribological characterization of the neat bisphosphonates showed improved extreme pressure weld point, improved anti-wear coefficient of friction (COF), and wear scar diameter (WSD). The bisphosphonates were also investigated as additives in petroleum-based and biobased base oils and displayed improved COF and WSD at low concentrations. The result demonstrates that biobased lubricant ingredients with considerably improved tribological properties can be developed by chemical modification of limonene and other natural oils. The results of this work will be of great interest to scientists and engineers who are engaged in the development of biobased lubricant formulations for a variety of applications. The bisphosphonate products from this work have the potential for use as ingredients in biobased hydraulic, engine, metalworking, and grease formulations.
Technical 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 diadduct 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 (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 of the unreacted limonene. They also displayed improved oxidation stability but lower viscosity index and solubility in polyalpha olefin (PAO6) and high oleic sunflower oil (HOSuO) base oils. Tribological characterization of the neat modified oils on a 4-ball tribometer showed improved extreme pressure weld point by all three diadducts, and improved anti-wear coefficient of friction (COF) and wear scar diameter (WSD) by the n-butyl diadduct 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 (PO3) into the limonene structure, and complete absence of unsaturation in the modified products.