Biodiesel is defined as the monoalkyl ester derivative of long-chain fatty acids, originated from renewable
sources, such as vegetable oils or animal fats. The most common use of biodiesel is direct substitution of
fossil fuels in compression−ignition engines. The heating of vegetable oils can cause complementary
decomposition reactions, in which the results can also lead to the formation of polymeric compounds. This
work has been carried out to study the degradation process of biodiesel in different temperatures and exposure
times. The degradation process of biodiesel affected its thermogravimetric and calorimetric profiles, indicating
the formation of intermediary compounds. The spectroscopic data of degraded biodiesel suggested oxidative
polymerization, confirming thermal data. In the degraded biodiesel at 210 °C for 48 h, the formation of gum
occurred, indicating that oxidative polymerization was completed.
Phenolic compounds are widely recognized because of their antioxidant capacity. In the present work, caffeic acid was shown to minimize the formation of oxidative radicals in soybean biodiesel, according to the induction period (IP) obtained by the Rancimat method. Its efficiency was much higher than usual antioxidants, such as butylated hydroxytoluene (BHT) and tertbutylhydroquinone (TBHQ). With a decomposition temperature of 170 °C, caffeic acid retained its properties, showing excellent antioxidant activity even when subjected to accelerated oxidation tests. While contamination with metals led to a meaningful decrease of the oxidative stability of soybean biodiesel, when small amounts of caffeic acid (500 mg L À1 ) were also present in the fuel, IP values higher than 6 h were obtained, reaching the limit specified by EN 14214.
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