Dual Nature of Noncatalytic Hydrothermal Dechlorination Kinetics of PVCs: Insights from Thermal Dehydrochlorination

Noncatalytic hydrothermal dechlorination kinetics of polyvinyl chloride (PVC) have traditionally been regarded as a single-step process, which fails to capture the dynamic nature of the mechanism. This study reveals that dechlorination is a dual process, encompassing a slow chemical-reaction-controlled stage followed by a rapid mass-transfer-controlled stage, observed in PVC with degrees of polymerization (DP) of 480, 1050, and 3000. In the initial stage, nucleophilic substitution plays a significant role in the reaction mechanism, while the latter stage is primarily driven by an elimination mechanism. The onset of mass transfer limitations is marked by a discontinuity in the first-order reaction plots within a dechlorination degree range of 50 %–56 %. The activation energy (EA) for PVC480, PVC1050, and PVC3000 is 213.5, 219.8, and 225.0 kJ/mol, respectively, during the chemical-reaction-controlled stage, and 244.9, 203.3, and 202.2 kJ/mol, respectively, in the mass-transfer-controlled stage. When particle aggregation occurs, thermal dehydrochlorination parallels hydrothermal dechlorination, as both processes exhibit two-stage kinetics, are prone to mass-transfer limitations at the latter stages, and the reaction rate is influenced by the propagation of the zipper mechanism along the chain length in nonisomerizing PVCs. The average EA for the thermal dehydrochlorination of PVC with DPs of 480, 1050, and 3000 are 93.8, 106.0, and 121.0 kJ/mol, respectively. The corresponding intensities of autocatalysis are − 82.8, −49.5, and –30.6 kJ/mol per unit of conversion, respectively. This study provides a detailed analysis of hydrothermal dechlorination kinetics, aligning with the dynamic nature of the dechlorination process and contributing to a more comprehensive understanding of PVC hydrothermal degradation chemistry.