Pyrolysis Mechanism and Product Formation in Waste Polyvinylidene Dichloride: A Comprehensive Study on Thermodynamics, Kinetics, and Product Distribution

The environmental impact of halogenated plastics demands advanced upcycling strategies, particularly for polyvinyl dichloride (PVDC) with its disposal-related thermal sensitivity and persistent accumulation due to molecular stability and high chlorine content. This study elucidated the pyrolysis mechanisms of waste PVDC through thermogravimetry and pyrolysis gas chromatography–mass spectrometry analysis. Based on the diffusion control model (D2), two stages in decomposition were identified: primary dechlorination (65–69 wt % loss at 200–350 °C) and secondary cracking (4–9 wt % loss at 350–450 °C), with an average activation energy (Ea) of 134.7 kJ/mol. The continuous release of hydrogen chloride (HCl) is accompanied by the formation of chlorinated hydrocarbons, in which chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) account for 20–30% of the pyrolytic products. Comparison with copyrolysis of conventional waste plastics revealed critical distinctions: PVDC required higher temperatures (>700 °C) for complete dechlorination, whereas polyvinyl chloride (PVC) had a peak HCl release at 294 °C. Consequently, its high inherent chlorine content (∼70 wt %) dominates the chlorinated compounds found in the resulting pyrolytic oils, mandating rigorous downstream strategies for Cl-PAH removal. These mechanistic findings provide a vital theoretical basis for developing effective copyrolysis processes combining PVDC with other plastic wastes.