Poly(vinyl chloride) (PVC) is a thermoplastic polymer that is used in numerous applications (e.g. construction, medical, electrical, automobile, packaging), being the third most widely produced plastic after polyolefins (i.e., polypropylene and polyethylene). PVC’s production volume was 44.3 million metric tons (MMT) and is expected to reach 60.0 MMT by 2025. Yet, the recycling of PVC is a non-trivial task because of its tendency to undergo dehydrochlorination through side chain elimination upon exposure to heat. In turn, in mixed plastic streams the produced HCl contributes to the formation of highly toxic and corrosive Cl-containing compounds, thereby making the mechanical recycling of PVC unfeasible. On the other hand, a chemical recycling approach, which involves a “low-temperature” and “non-catalytic” dehydrochlorination step followed by catalytic pyrolysis, has been highlighted as a promising alternative. Further improvements to the latter approach can be made by gaining in-depth understanding into the molecular- and atomic-scale processes occurring during the thermal degradation of PVC. To this end, we employ density functional theory (DFT) and investigate the mechanism by which HCl is eliminated from pristine and defective PVC. In line with experimental observation, we find that defects, such as short branches linked to the primary backbone and internal allylic groups, are the most likely points for the initiation of HCl removal. Moreover, we highlight the autocatalytic role of HCl during dehydrochlorination and explore the potential of other chemical compounds (e.g., H<sub>2</sub>O, hydrogen halides and other) for catalyzing this process. Finally, we will be discussing the kinetics and thermodynamics of reaction pathways, which are relevant to the PVC pyrolysis and lead to other compounds such as aromatics, non-condensable gases and crosslinked products. This work contributes to the fundamental understanding of molecular scale phenomena that take place during PVC pyrolysis and may serve as a guide toward the optimization of this process.

Mechanistic insights into the pyrolysis of poly(vinyl chloride)

ACS Spring 2022

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