3.4. Thermal properties
The effect of possible hydrogenation and dimerization/polymerization reactions can influence the thermal properties, including melting point and thermal degradation temperatures. Results from DSC and TGA are shown in Fig. A2, in the supplementary data section. Untreated soybean oil has a melting point of -28±0.4°C, while PHO is solid at room temperature with a melting point of 32.3±0.3°C. HVACP treated samples showed a melting point of -27.6±0.3, -23.5±0.4 and -4.9±0.3°C for liquid, gel, and solid fractions treated for 6h, respectively (Fig. 6). HVACP treated samples increased the melting point but not at the level of a PHO, because of the high content of PUFA (36.6%) compared to a PHO that has a PUFA content of 20.4%. Moreover, elaidic acid in PHO behaves like a saturated fatty acid because the trans isomers create a linear hydrocarbon chain, increasing the melting point. HVACP treated samples have a lower content of trans fatty acids, and consequently, they have a lower melting point. The reason for the increase and broadening in melting points observed in Fig. A2a comparing to the untreated oil might be due to the possible amorphous dimers/polymers formed, as suggested in Fig. 4.
Additionally, a reduction of melting enthalpy was also observed in treated samples, from 59.1±3.3 J/g to 9.7±3 J/g for the untreated and treated 6h-solid fractions, respectively. A reduction of the endothermic peak is linked with less amount of crystals or a reduction of crystals size.
Thermal degradation involves scission, side group elimination, and depolymerization phenomenon (Crompton, 2012). Samples were exposed to a temperature up to 600°C, where 100% of sample degradation was achieved (Fig. A2b). The onset temperature of untreated soybean oil was 357±1.5°C. This temperature increased with treated samples, reaching 369±1.2°C for the 6h-solid fraction. Therefore, a 12-14°C shift was observed in treated samples through the whole thermal degradation process, which might be attributed to the formation of dimers/polymers proposed (Fig. 4). In contrast, PHO showed an onset temperature of 345 °C. Hence, it was characterized by a thermal degradation shifted to a lower temperature, which means that saturated triglycerides require less energy to be degraded compared to untreated soybean oil that has more double bonds and requires more energy to degrade.