Towards a better understanding of the cosolvent effect on the low-temperature glycolysis of Polyethylene Terephthalate (PET)

As the demand of polyethylene terephthalate (PET) increases worldwide along with the waste generated from its use, it is urgent to develop cost-efficient and sustainable recycling processes, such as depolymerization that yields monomers with virgin-like qualities. However, most of these processes require harsh conditions and their true mechanisms are poorly understood, leading to marginal gains in energy efficiency and yield. In spite of the lack of solubility of PET, we demonstrate that the swelling and plastification of PET in a good solvent allows a better mass transport of ethylene glycol and the catalyst into the polymer during glycolysis. Based on these insights, we report a process in which PET is depolymerized into bis(2-hydroxyethyl)terephthalate (BHET) with a yield of 88 % at 65 °C within 1 h in the presence of 1,3-Dioxolane as green cosolvent carrier. The improved mass transport allows to perform the depolymerization of PET even below the Tg of the polymer. Indeed, by kinetic modelling we demonstrated that a heterogeneous depolymerization process could be easily transformed into an homogeneous process by an appropriate solvent selection. The environmental impact of the proposed process, including solvent recovery, is compared to the solvent-free counterpart and the results demonstrates that by using 1,3-Dioxolane, the carbon footprint of the newly developed glycolysis process can be reduced up to 20 % due to the increased energy efficiency. This process enables a viable recycling strategy of PET into its repetitive unit, contributing to the development of competitive chemical recycling solutions to reduce PET-derived pollution.