Submitted to: Industrial Crops and Products
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2003
Publication Date: 11/10/2003
Citation: CERMAK, S.C., ISBELL, T. 2003. IMPROVED OXIDATIVE STABILITY OF ESTOLIDE ESTERS. INDUSTRIAL CROPS AND PRODUCTS. 18:223-230. Interpretive Summary: A functional fluid is a material that is usually an oil that acts as a lubricant or hydraulic fluid. Functional fluids must undergo certain tests in order to demonstrate themselves as commercially useful fluids; one such test is the rotating bomb oxidation test (RBOT). The RBOT will test the fluids ability to resist oxidation, ie reaction with oxygen. This is very important because as the material is oxidized it becomes less stable, and other important characteristics in the material has also started to change become less useful. The most common way to correct this problem is to mix in an additive package which helps stabilize this problem. These additive packages are usually expensive and toxic to the environment. Thus there is a serious need for functional fluids that have oxidative stability already built into the fluid which will help minimize the use of these additive packages. Our lab is working to create these new cost-effective vegetable-based functional fluids that are more oxidatively stable than materials commercially available. These solutions will make functional fluids more competitive and environmentally safe in the future.
Technical Abstract: 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 minutes. Two estolides, coconut-oleic estolide 2-ethylhexyl ester and oleic estolide 2-ethylhexyl ester, 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 was 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 mins with 3.5% oxidative stability package. Both estolides were formulated to meet commercial crank case requirements (~200 min) with as little as 1.0 and 1.5% oxidative stability package respectively. The viscosity index ranged from 179 to 190 for the oleic estolide 2-ethylhexyl ester while the coconut-oleic estolide 2-ethylhexyl ester had slightly lower viscosity indices ranging from 161-174. These two estolide esters have displayed far superior oxidative stability, 504 and 426 mins, are of reasonable cost, and were more suitable as a base stock for biodegradable lubricants and functional fluids than current commercial, vegetable-based materials.