Abstract
The kinetic behavior of such plastics as polystyrene (PS), polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), etc., was discussed in supercritical acetone, n-hexane and methanol. The degradation of PS in supercritical acetone, n-hexane and water as well as the thermal pyrolysis was found to follow the first-order kinetics at the initial stage of the reaction. From the Arrhenius plots, the activation energies were evaluated to be 131.6 kJ/mol for the degradation in supercritical n-hexane, 156.8 kJ/mol in supercritical water, and 224 kJ/mol for the thermal pyrolysis. It has been found that the degradation in supercritical fluids can take place at lower temperatures than the thermal pyrolysis. A kinetic model to describe the depolymerization of PET in supercritical methanol was proposed, where the scission of one ester linkage in PET by a methanol molecule results in the formation of one carboxymethyl group and one hydroxyl group. The values of the forward reaction rate constant at different temperatures were determined by comparing the observed time dependence of carboxymethyl group concentration with that calculated by the proposed model. The activation energy was evaluated to be 49.9 kJ/mol, a value close to a literature value (55.7 kJ/mol) for the depolymerization in subcritical water. The reaction enthalpy was evaluated to be +106.1 kJ/mol (endothermic reaction), whereas that for the same depolymerization in subcritical water had been reported to be -99.9 kJ/mol (exothermic reaction).
