Lipoate ester multifunctional lubricant additives

Authors: Biresaw, Girma; Compton, David - Dave; Evans, Kervin; Bantchev, Grigor

Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2015
Publication Date: 3/22/2016
Publication URL: http://handle.nal.usda.gov/10113/62393
Citation: Biresaw, G., Compton, D., Evans, K., Bantchev, G.B. 2016. Lipoate ester multifunctional lubricant additives. Industrial and Engineering Chemistry Research. 55(1):373-383.

Interpretive Summary: Lipoic acid is a naturally occurring cellular co-factor found in animals and plants. It has a cyclic disulfide structure which makes it desirable as an extreme pressure (EP) additive in lubricants. Unfortunately, lipoic acid is a highly polar white powder insoluble in lubricant base oils. One way of overcoming this problem is to chemically modify it so it becomes compatible with biobased and petroleum based lubricant base oils. In this work, seven lipoate esters were synthesized by solvent-free esterification of lipoic acid with different structures of alcohols. The esters were obtained in good yield and positively identified using 1H NMR. Four of the seven lipoate esters displayed good solubility in high oleic sunflower (HOSuO) and polyalphaolefin base oils. The neat lipoate esters displayed high oxidation stability and viscosity index of 355-567, which is much higher than typically observed for most petroleum or vegetable base oils. Lipoate esters in HOSuO base oil displayed multi-functional additive properties with good viscosity index improver, good antioxidant, and good EP additive properties. The result demonstrates that biobased lubricant ingredients with considerably improved tribological properties can be developed by chemical modification of natural products. The results of this work will be of great interest to scientists and engineers engaged in biobased lubricant development. The lipoate esters from this work have the required properties to be used as multi-functional biobased additives in formulations for hydraulic, engine and metalworking lubricants and in grease formulations. However, in light of the current high price for lipoic acid, more research is needed to make them cost-competitive relative to additives currently in the market.

Technical Abstract: Seven lipoate esters were synthesized by esterification of lipoic acid with different structures of alcohols in the presence of a solid acid catalyst and without solvent. The esters were obtained in good yield, characterized using 1H NMR and GPC; and their physical properties investigated. Four of the seven lipoate esters, with good solubility in high oleic sunflower oil (HOSuO) and polyalphaolefin (PAO6) base oils, were further investigated for their lubricant additive property. Additive properties were investigated in HOSuO base oil by varying the additive concentrations in the range 0–20 (w/w %). Properties investigated include: density; kinematic and dynamic viscosity; viscosity index (VI); oxidation stability from onset and peak oxidation temperatures on a PDSC; 4-ball anti-wear (AW); and 4-ball extreme pressure (EP). Neat lipoate esters displayed high oxidation stability and high VI. As additives to HOSuO base oil, lipoate esters displayed good viscosity index improver (VII), good antioxidant (AO), good EP, but poor AW properties. The VI and VII properties were attributed to the preence of low concentrations of polymeric byproducts in the ester product mixtures. This work demonstrates that lipoate esters can be developed into effective mulfi-functional biobased lubricant additives with excellent VII, AO, and EP properties.