Depolymerization of Polyester Polymers from the Oxidation of Soybean Biodiesel

Aging of soybean biodiesel at an elevated temperature (90 °C) with aeration, simulating diesel engine fuel system conditions, yielded higher molecular weight materials and increased kinematic viscosity. A considerable proportion of the incorporated oxygen was identified as new ester bonds, beyond those in the biodiesel fatty acid methyl esters (FAMEs). The purpose of this study was to characterize the high molecular weight components of oxidized biodiesel fuel and understand the contribution of esters to the increased viscosity through experiments to depolymerize ester and ether bonds. Methods used to characterize high molecular weight materials included acid and base transesterification of oxidized biodiesel with methanol; hydrolysis with hydroiodic acid (HI); and GC-MS, gel permeation chromatography, FTIR, and 13C NMR of the aged and transesterified materials. Depolymerization via transesterification with NaOCH3 or CH3OH/HCl, specifically targeting ester bonds, resulted in a marked reduction in the molecular weight distribution and viscosity of the aged biodiesel. Depolymerization with HI, targeting both ester and ether bonds, showed no additional reduction in molecular weight or kinematic viscosity, suggesting that ether bonds play a negligible role. FTIR spectra showed a shift in carbonyl stretching vibrations after depolymerization treatments consistent with ester involvement. With GC-MS, dicarboxylic acids were tentatively identified in the depolymerized products that were not present in the untreated aged biodiesel, suggesting a role as cross-linking agents in forming polyester products. Finally, 13C NMR showed data consistent with biodiesel having undergone significant oxidation yielding new carboxylic acid and ester groups and a decrease in unsaturation. No evidence of tertiary carbons was found in 13C 135° DEPT NMR spectra that would be characteristic of products of free radical vinyl polymerization, Diels–Alder reactions, or aldol condensation reactions. Thus, the data presented here suggest that esters are responsible for the majority of the polymerization reactions in this, and presumably other, aged/oxidized biodiesel fuel