Isothermal thermogravimetric method using a fast scanning calorimeter and its application in the isothermal oxidation of nanogram-weight polypropylene

Abstract

Conventional thermogravimetry (TG) is a common method for studying thermal degradation and/or thermal oxidation of polymers. However, the temperature range for isothermal TG studies is limited to regions close to the melting temperature, where thermal degradation and/or thermal oxidation proceed slowly. Higher temperatures cannot be reached because thermal degradation and/or thermal oxidation start upon heating, even using the highest possible heating rate of the conventional TG. The method of fast scanning calorimetry (FSC) to obtain a quasi-real-time isothermal TG curve via a nanogram-weight sample (nano-TG method) was reported. During the 50 cycles of FSC measurements of polypropylene (PP) between room temperature and 300 °C, regardless of air flow, all FSC traces were consistent with each other (i.e., thermal oxidation of PP was fully suppressed during the fast scanning). On the other hand, when the sample was annealed for a few seconds at 300 °C during the cyclic FSC runs, the apparent heat capacity (which can be converted into relative weight) decreased with increasing total annealing time. The decrease in apparent heat capacity is explained by the progression of thermal oxidation. The nano-TG method was applied to determine the thermal oxidation kinetics of PP using the Avrami-Erofeev model. The non-isothermal TG curve calculated from the kinetic parameters of the nano-TG analysis was in good agreement with the experimental data obtained from the conventional TG measurements for the early stage of weight loss. The present method is applicable for the thermal degradation/oxidation analysis of microplastics in the environment or small impurity analysis of polymers.