(272b) Monomer recovery optimization from hydrothermal processing of condensation polymers at subcritical and supercritical conditions

The objective of this work is to find the optimum hydrothermal conditions that maximize monomer recovery from condensation polymers, especially polyesters, polyamides, and polycarbonates, to promote circular economy for plastics via chemical recycling. The development of separation and purification methods is also included in this study as a crucial requirement to ensure the future of hydrothermal processing as a feasible plastic-to-monomer depolymerization technology. This way, a green alternative to the current trend of turning plastic waste into chemicals and fuels is proposed.

Since 1970, mechanical recycling of plastic waste mono-streams has been promoted and generally seen as the best recycling practice. However, high quality plastic waste is required to maximize the environmental benefits of this technology. Apart from the physical and chemical degradation caused by environmental factors, mechanical recycling leads to molecular rearrangements triggered by thermo-mechanical and thermo-oxidative degradation. Hydrothermal processing can be a game changer in the plastic recycling value chain, able to tackle both contamination and degradation of plastic waste and providing unparallel monomer recovery yields. Condensation polymers, containing highly polar functional groups with strong tendency to undergo ionic cleavage, will follow selective depolymerization into monomers under hydrothermal conditions. Unselective free radical decomposition reactions will be hinder by tuning process parameters in order to maximize monomer production.

In this work, the optimum critical process parameter values for polyethylene terephthalate (PET), polyamide (PA), polyurethane (PU) and polycarbonate (PC) will be defined by conducting batch experiments in micro tubular reactors following design of experiments. Exceptional cases of non-polycondensates undergoing selective plastic-to-monomer depolymerization like polystyrene (PS) and poly (methyl methacrylate) (PMMA) will be included. The treatment of polymer blends will be also studied and separation methods to extract highly pure monomer fractions from the HTP products will be presented. Finally, the results will be analyzed from both chemical and techno-economic point of view to establish the baseline for future studies.