Thermal Stability of Abietic Acid and Its Oxidation Products

The thermal oxidation characteristics of abietic acid were investigated through tracing the oxidation process in custom-designed mini closed pressure vessel test under isothermal and step temperature conditions. Peroxide generation and the peroxide value of abietic acid oxidation process were measured using thin-layer chromatographic analysis and iodimetry. The primary oxidation product—peroxide—was separated by column chromatography with further structure characterization, and its thermal decomposition characteristics were assessed via a differential scanning calorimeter (DSC). The oxidation was mainly initiated through the radical generation from hydrogen abstraction on the unsaturated conjugated double bonds of abietic acid above 343 K. The main hydroperoxide, 7-hydroperoxy-13-abiet-8(14)-enoic acid, was found in abietic acid with a high peroxide value. The exothermic onset temperature (T0) and decomposition heat (QDSC) of this peroxide were 353.94 K and 545.37 J·g–1, respectively. Finally, a second-stage oxidation process of abietic acid was first investigated when the temperature reached the abietic acid melt point (448 K) with complex oxidation products forming, such as dehydroabietic acid, palustric acid, 7-oxodehydroabietic acid, neoabietic acid, 7-methoxy-tetradehydroabietic acid, 12-deoxyroyleanone acid, and 12-methoxy-abietic acid.