BIO-BASED LUBRICANTS FROM FARM-BASED RAW MATERIALS
Location: Bio-oils Research Unit
Title: Temperature dependence of the oxidative stability of corn oil and polyalphaolefin in the presence of sulfides
Submitted to: Thermochimica Acta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 8, 2010
Publication Date: January 18, 2011
Citation: Bantchev, G.B., Biresaw, G., Mohamed, A., Moser, J.K. 2011. Temperature dependence of the oxidative stability of corn oil and polyalphaolefin in the presence of sulfides. Thermochimica Acta. 513(1-2):94-99.
Interpretive Summary: The needs for energy independence and reduced carbon footprint require a search for renewable materials. Currently, lubricants are produced mainly from imported petroleum, and efforts are underway to replace it with vegetable oil-based materials. However, to achieve this goal, oxidative stability and other critical properties of vegetable oils have to be significantly improved. Various methods, such as chemical modification and blending with additives are practiced to achieve this goal. In the work described here, sulfur-containing materials have been investigated as additives to enhance the oxidative stability of corn oil and synthetic oil. One of the investigated materials was corn oil, chemically modified with sulfide groups. The investigation showed that the antioxidant properties were dependent on temperature. It was observed that these additives were effective on corn oil at low temperatures, but were effective on the synthetic oil at all temperatures. Such findings are helpful to researchers working on improving the oxidative stability of lubricants formulated from vegetable oils and will result in increased utilization of vegetable oils in lubricant applications.
The effect of sulfide-modified corn oil (SMCO) and ditertiary dodecyl pentasulfide (PS) additives on the oxidative stability of corn (CO) and polyalphaolefin (PAO) oils was investigated using pressurized differential scanning calorimetry in dynamic (DDSC) and isothermal (IDSC) modes. DDSC showed a weak prooxidant effect of SMCO and PS in CO, and an antioxidant behavior in PAO. Analysis of the IDSC data showed that, in both oils, the sulfides behaved as anti-oxidants at low temperatures but, as pro-oxidants, at high temperatures. To explain this observation, we propose the concept of cross-over temperatures, below which the sulfides act as antioxidants and above which act as prooxidants. The cross-over temperature was found to be a function of the degree of unsaturation of the oil. For the highly unsaturated CO, the cross-over temperature was 100-135°C, while for the fully saturated PAO it was above 200°C. The cross-over temperature concept can also explain the oxidation behaviors of sulfide additives in the literature, which appear to provide contradictory results (pro- vs. anti-oxidant) depending on the experimental temperatures. According to the cross-over temperature concept, accelerated oxidation test methods, which are usually performed at very high temperatures, are poor predictors of the low-temperature performance of sulfide-containing antioxidants.