Screening of common synthetic polymers for depolymerization by subcritical hydrothermal liquefaction

TL;DR

This study explores the depolymerization of various synthetic polymers using subcritical hydrothermal liquefaction, identifying which plastics can be effectively processed into bio-crude or chemicals under specific conditions.

Contribution

It provides a comparative analysis of how different waste plastics respond to HTL, highlighting the potential and limitations for sustainable chemical recovery.

Findings

Polyolefins and PS show limited depolymerization due to lack of reactive sites.

PC and epoxy produce bisphenol-A and phenols in oil and aqueous phases.

PA6, PA66, PET, PUR yield monomers and platform chemicals.

Abstract

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 {\deg}C and 20 minutes 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 depolymerizes several different synthetic polymers and highlights which of those are the most attractive or are unsuitable for subcritical processing.