Screening of common synthetic polymers for depolymerization by subcritical hydrothermal liquefaction

Hydrothermal liquefaction could potentially utilize mixed plastic wastes for sustainable biocrude production, however the fate of plastics under HTL is largely unexplored for the same reaction conditions. In this study, we evaluate how synthetic waste polymers can be depolymerized to bio-crude or platform chemicals using HTL at typical conditions expected in future commercial applications with and without alkali catalyst (potassium hydroxide). We evaluate different characteristics for HTL processing of poly-acrylonitrile-butadiene-styrene (ABS), Bisphenol-A Epoxy-resin, high-density polyethylene (HDPE), low density PE (LDPE), polyamide 6 (PA6), polyamide 66 (PA66), polyethylene terephthalate (PET), polycarbonate (PC), polypropylene (PP), polystyrene (PS) and polyurethane (PUR) at 350 °C and 20 min residence time. Polyolefins and PS showed little depolymerization due to lack of reactive sites for hydrolysis. HTL of PC and Epoxy yielded predominantly bisphenol-A in oil fraction and phenols in aqueous phase. PA6 and PA66 yielded one of its monomers caprolactam and a range of platform chemicals in the aqueous phase. PET produces both original monomers. PUR yields a complex oil containing similar molecules to its monomers and longer hydrocarbons. Our results show how HTL can depolymerize several different synthetic polymers and highlights which of those are the most attractive or are unsuitable for subcritical processing.