Thermocatalytic degradation of common polymers: A microscale combustion calorimetry study

Abstract

This paper presents the first application of the microscale combustion calorimetry (MCC) to study catalyzed pyrolysis of the practical polymers comprising most of currently produced plastic wastes. The polymers considered include olefines (HDPE, LDPE, and PP), PS, PMMA, PET, PC, and PVC. Each of the polymers is mixed with one of the two catalysts, ZSM-5 or MgO, and the measurements are undertaken for the catalyst mass fractions of 0.1, 0.2, and 0.3 at several heating rates ranging from 15 to 60 K/min. Strong catalytic effect of ZSM-5 has been demonstrated for the polyolefin polymers (HDPE, LDPE, and PP). Such effect manifests itself in a significant reduction of characteristic pyrolysis temperatures and apparent activation energy; the latter is shown to decrease greatly (by a factor of about two) when the catalyst is added. As the catalyst mass fraction increases up to 0.1 (0.2 for LDPE + ZSM-5 mixture), the saturation is attained, and further addition of the catalyst has no effect. The heat of combustion of pyrolysis volatiles does not reveal a definitive dependence on the catalyst mass fraction, while in some cases the width of pyrolysis temperature interval as well as the char yield are affected. Decomposition of the remaining polymers in presence of ZSM-5 or MgO is either catalyzed rather weakly or even inhibited such as that in PS + ZSM-5 mixture. Among the polymers examined, PC turns out to be the only polymer that is considerably catalyzed by MgO. For the mixtures in which significant catalytic effect was detected, the formal kinetic models have been derived from the microscale measurements and shown to replicate the MCC data at different heating rates. These polymers, both individually and mixed with the catalyst, are often characterized by the non-monotonic conversion function which cannot be approximated by the n-th order reaction model.