Hydrothermal recycling of step and chain growth mixed plastics under sub- and supercritical conditions: chemical synergies, and antagonisms

The automotive industry consumes 7 % of global plastics, with polymers accounting for nearly half the volume of the vehicle. End-of-life automotive plastics are heterogeneous mixtures of polymers, fillers, and additives that current sorting systems cannot handle, preventing their otherwise feasible conversion into monomers or chemical building blocks. To address this challenge, we treated hydrothermally a representative unsorted automotive plastic waste stream of polyethylene terephthalate (PET), polyamide 6 (PA6), polyurethane (PU), polypropylene (PP) and polyvinyl chloride (PVC) under sub/supercritical water at 250 to 400 °C. Between 250 and 300 °C, PET converts to terephthalic acid (TPA) with 80-95 % yield, recovered as a solid, and PA6 depolymerizes to caprolactam and aminohexanoic acid. Water cleaves PU into amines and polyols, PVC dechlorinates, while PP remains intact after subcritical water treatment. PA6 hinders PET hydrolysis, while PU and PVC improve TPA's yield, and PP has no effect. Starting at 300 °C, PA6 and PU monomers recombine and degrade, but PU at least quadruplicates its yield in nitrogenous aromatics. Above the critical point, TPA fragments to a range of oxygenated aromatics, PA6 monomers disappear, PP and PVC cracks into C₈-C₁₄ branched aliphatic, and PU yield increase only slightly (10 % compared to 300 °C). We demonstrate that no single condition valorises all polymers simultaneously. Instead, process design must prioritize the most valuable products, for instance via staged heating: subcritical conditions recover PA6 and PU monomers as liquids, PET hydrolyses optimally at 300 °C as a solid, and PP converts to fuels above supercritical conditions.