Migration Mechanism of Chlorine during Hydrothermal Treatment of Rigid PVC Plastics

Rigid PVC plastics (R-PVC) contain large amounts of chlorine, and improper disposal can adversely affect the environment. Nevertheless, there is still a lack of sufficient studies on hydrothermal treatment (HTT) for the efficient dechlorination of R-PVC. To investigate the migration mechanism of chlorine during the HTT of R-PVC, R-PVC is treated with HTT at temperatures ranging from 220 °C to 300 °C for 30 min to 90 min. Hydrochar is characterized via Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy. The results revealed that the hydrothermal temperature is the key factor that affects the dechlorination of R-PVC. Dramatic dechlorination occurs at temperatures ranging from 240 °C to 260 °C, and the dechlorination efficiency increases with the increase in the hydrothermal temperature. The main mechanism for the dechlorination of R-PVC involves the nucleophilic substitution of chlorine by -OH. CaCO3 can absorb HCl released by R-PVC and hinder the autocatalytic degradation of R-PVC; hence, the dechlorination behavior of R-PVC is different from that of pure PVC resins. Based on these results, a possible degradation process for R-PVC is proposed. This study suggests that HTT technology can be utilized to convert organochlorines in R-PVC to calcium chloride, achieving the simultaneous dechlorination of R-PVC and utilization of products.