Aqueous buffer solution-induced crystallization competes with enzymatic depolymerization of pre-treated post-consumer poly (ethylene terephthalate) waste

The complex relationship between kinetics and substrate morphology during enzymatic depolymerization of melt processed poly(ethylene terephthalate) (PET) is explored and the effects of competing transformations are analyzed. Extruded PET substrates from post-consumer recycled PET (RPET) bottles flakes subjected to enzymatic depolymerization are examined to reveal increases in crystallinity from ∼10% post-extrusion to > 30% after 3 days of depolymerization as well as increases in glass transition temperature (Tg) from ∼66 °C to > 80 °C. Further investigation into this behavior shows that post-extrusion RPET substrates do not exhibit changes in crystallinity when subjected to dry annealing at 65 °C in an oven over 7 days, but they do experience annealing in depolymerization buffer solution with no enzymes at 65 °C within 3 days. This difference may be attributed to plasticization of PET in the presence of water, also known as solvent induced crystallization. The impact of this plasticized annealing behavior is demonstrated by subjecting RPET substrates to increasing enzyme-to-substrate loads. As enzyme load increases, overall conversion of substrates increases despite crystallinity also increasing to similar levels regardless of the initial enzyme loading. The competition between depolymerization and crystallization suggests that the rate at which PET substrates are depolymerized at the operational temperature is integral to achieving high conversion to monomeric product. This, in turn, also suggests that savings from lowered enzyme loadings may be more detrimental than helpful in pursuing the most cost-effective recycling systems.