Tribological properties of biobased ester phosphonates

Authors: Biresaw, Girma; Bantchev, Grigor

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/13/2013
Publication Date: 5/25/2013
Citation: Biresaw, G., Bantchev, G.B. 2013. Tribological properties of biobased ester phosphonates. Journal of the American Oil Chemists' Society. 90(6):891-902.

Interpretive Summary: Application of biobased oils in lubrication requires overcoming a number of inherent weaknesses in vegetable and other natural oils. Among the problems frequently encountered are: unacceptably poor oxidation stability and cold flow properties. One of the methods employed to improve the properties of natural oils is by chemical modification. In the work described here, chemical modification was used to improve the properties of methyl oleate (MeOl), a biobased oil derived from fats and vegetable oils. MeOl, in addition to its unacceptably low viscosity, also has poor oxidation stability and high pour point and cloud point temperatures. However, use of chemical modification resulted in oil with higher density and viscosity than the parent MeOl. In addition, the chemical modification resulted in improved oxidation stability and cold flow properties. The friction and wear test also showed that the chemically modified MeOl had lower coefficient of friction and wear scar diameter than the parent MeOl. This work demonstrates that viable lubricant ingredients can be developed from vegetable oils and their derivatives through chemical modification. 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.

Technical Abstract: Three phosphonate derivatives of methyl oleate (MeOl) were chemically synthesized in a radical chain reaction and their physical and tribological properties investigated. The three phosphonates differed from each other in the structure of the alkoxy groups attached to the phosphorous, which were as follows: methoxy, ethoxy and n-butoxy. Phosphonylation eliminated the unsaturation in MeOl and also introduced various carbon chain length branches. The phosphonate oils had higher density and viscosity than MeOl, which was attributed to the contribution of the heavier phosphorous atom in their structures, and to their higher molecular weights, respectively. The phosphonates also displayed improved oxidation stability and cold flow properties, which were attributed to the elimination of the double bond and the introduction of branching in their molecular structures, respectively. Tribological investigations were conducted using 4-ball anti-wear (AW) and extreme pressure (EP) methods. The phosphonates displayed lower AW coefficient of friction and wear scar diameter than MeOl. The improved AW results were attributed to the higher viscosity of the phosphonates, since AW is conducted in the mixed film regime. The phosphonates were found to have no effect on EP performance as neat oils or as additives in vegetable and petroleum based oils. It was proposed that this lack of EP characteristics could be the high dissociation energy of C-P bonds in the phosphonates, compared to, for example, S-P bonds in ZDDP. According to this proposal, under the tribochemical conditions of the 4-ball EP test, the temperature at or near weld point was insufficient to break the C-P bond in the phosphonates and make it available for in situ tribochemical reaction with the metal surface to produce the low friction lubricant films.