Analysis of oxidative stability of methyl soyate by pressurized-differential scanning calorimetry

Biodiesel, an alternative diesel fuel that may be derived from soybean oil, is composed of long-chain saturated and unsaturated fatty acid alkyl eater s. Extensive oxidative degradation can compromise viscosity, acid value, pe roxide value, and other parameters in the current ASTM biodiesel fuel guide line. This work examines pressurized-differential scanning calorimetry (P-D SC) for analysis of oxidative stability of biodiesel. Heating scans were co nducted under static air-atmosphere with 5 degreesC/min ramping. Increasing pressure from 1000 to 5000 kPa did not significantly affect results and 20 00 kPa was selected for the remainder of this study. Curves were analyzed f or oxidation temperature (OT) of methyl soyate samples from four separate s ources. With respect to unmodified methyl soyate, addition of antioxidants increased OT from 108.4-127.2 degreesC to 237.2-163.4 degreesC. Mixtures wi th added tert.-butylhydroquinone consistently gave higher OT values than th ose with +/-alpha -tocopherol; therefore, P-DSC may be useful for screening antioxidants. Although P-DSC results were consistent with corresponding oi l stability index (OSI) data measured at 50 degreesC, no correlation for pr edicting OSI directly from OT results was evident, with the possible except ion of unmodified methyl soyate. This work establishes P-DSC as an analytic al tool in evaluating the oxidative stability of biodiesel with and without antioxidants.