Improved oxidative stability of estolide esters

Abstract

Some concerns have been raised regarding the oxidative stability of vegetable-based fluids. Thus, a wide range of commercial and vegetable-based materials were evaluated for their oxidative stability by the rotating bomb oxidative test (RBOT). RBOT values ranged from 13 to 552 min. Two estolides, coconut-oleic estolide 2-ethylhexyl ester (coco) and oleic estolide 2-ethylhexyl ester (oleic), were evaluated for their oxidative stability by RBOT. As in the case with all vegetable oils, an oxidative stability package must be utilized to help decrease their rate of oxidation. A series of formulations were conducted in which the two estolides had an oxidative stability package added prior to the RBOT. In both cases, dramatic increases in the oxidative stability were observed. The coconut-oleic estolide 2-ethylhexyl ester gave the best RBOT values, 504 min with 3.5% oxidative stability package. Both estolides were formulated to meet commercial crankcase requirements (∼200 min) with as little as 1.0 and 1.5% oxidative stability package. The viscosity index ranged from 179 to 190 for the oleic estolide 2-ethylhexyl ester, whereas the coconut-oleic estolide 2-ethylhexyl ester had slightly lower viscosity indices ranging from 161 to 174. These two estolide esters have displayed far superior oxidative stability, 504 (coco) and 426 (oleic) min, are of reasonable cost, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial, vegetable-based materials.