Thermal degradation kinetics of polyvinyl chloride in presence of zinc oxide

Abstract

This work investigates the degradation kinetics and the pyrolysis behavior of Poly (vinyl chloride) (PVC) and its mixture with ZnO using thermogravimetric analysis under an inert atmosphere. The investigation was carried out due to the increased interest in the co-thermal treatment of the hazardous waste electric arc furnace dust (EAFD) which contains significant quantities of ZnO with PVC. The degradation of pure PVC was characterized by three main decomposition stages: PVC de-hydrochlorination (two overlapped stages) and subsequent polyene thermal cracking, while ZnO-PVC mixture (ZPVC) demonstrated four decomposition/volatilization stages. The Flynn-Wall-Ozawa (FWO), Kissinger-Akahira-Sunose (KAS), and Friedman models were utilized for the extraction of the kinetic parameters. The average activation energy for pure PVC de-hydrochlorination was calculated to be 119.8 ± 12.4 kJ/mol, which changed to 110.6 ± 11.2 kJ/mol when a stoichiometric quantity of ZnO was added to it. The suggested mechanism for the ZPVC de-hydrochlorination starts by chlorine abstraction on ZnO at temperatures well-below 272 °C with an activation energy comparable to that of pure PVC de-hydrochlorination (115.8 kJ/mol). The chlorination of ZnO then yields zinc oxy/hydroxide chloride phases (Zn2OCl2·2H2O/β-Zn(OH)Cl) by the reaction between ZnCl2, ZnO and emitted H2O. These phases then decompose at approximately 222 °C into ZnCl2, ZnO, and H2O with a relatively low energy barrier of 102.2 kJ/mol. Formed ZnCl2 then lowers the activation energy for the polyene thermal cracking of PVC from 218.4 ± 17.7 (PVC) to 87.3 ± 9.7 kJ/mol (ZPVC) due to the physical contribution of volatilization to the overall mass loss.