Oxidative Stability of Biodiesel by Dynamic Mode Pressurized-Differential Scanning Calorimetry (P-DSC)

Abstract

Biodiesel, an alternative diesel fuel made from transesterification of vegetable oils or animal fats, is composed of saturated and unsaturated long-chain fatty acid alkyl esters. During long-term storage, oxidation caused by contact with ambient air presents legitimate concerns for monitoring fuel quality. Extended oxidative degradation can affect kinematic viscosity, cetane number, and acid value of the fuel. This work investigates the suitability of dynamic mode (positive air purge) pressurized-differential scanning calorimetry (P-DSC) as a means for evaluating the oxidation reaction during non-isothermal heating scans. Methyl oleate, methyl linoleate, and soybean oil fatty acid methyl esters (FAME) were analyzed by P-DSC and the results compared with those from thermogravimetric analyses (TGA), conventional DSC, and static mode (zero purge gas flow) P-DSC scans. Results from TGA showed that ambient air pressure was too low to allow measurable oxidation during analyses. Although some degree of oxidation was detected for DSC and static mode P-DSC heating scans, results demonstrated that the highest degree of oxidation occurred during dynamic mode P-DSC scans. For DSC and P-DSC analyses, oxidation onset temperature (OT) increased with relative oxidative stability, with the highest values being observed for methyl oleate. Treating soybean oil FAME with antioxidants increased their relative oxidative stability, resulting in an increase in OT. Statistical comparison of response factors (RF) relative to methyl oleate obtained from non-isothermal heating scans with those obtained from OSI analyses showed the highest degree of correlation (P = 0.79) with respect to dynamic mode P-DSC.