Chemolytic depolymerisation of PET: a review

Abstract

Developing new, alternative ways to recycle plastics is an important and pressing challenge for industry and academy, given its significant impact on CO₂ emissions as well as improvement of resource-efficiency and reduction of landfilling. Currently, re-use and mechanical recycling are the most employed routes to exploit post-consumer plastics; however, chemical recycling, which involves the depolymerization of polymer chains to reclaim the original monomers or intermediate oligomers, is gaining much attention. Chemical recycling allows for the transformation of various types of plastic waste from single-polymer and mixed waste streams back into their original components. By creating a new, secondary virgin-quality raw material, chemical recycling can help close the loop and reduce the consumption of fossil resources. The energy barrier for depolymerization is normally high; thus, these reactions tend to require high temperatures and/or pressures; therefore, diminishing the reaction temperature and pressure through incorporation of catalysts has led to a whole field of study, looking for catalysts that promote solvolysis under mild reaction conditions. In this review, we assess the different depolymerization conditions for polyethylene terephthalate (PET) in the literature, according to three proposed green chemistry metrics that allow us to compare the energy economy (ε coefficient), the environmental factor (E) and the combined effect of both (ξ). These green chemistry metrics parameters allowed us to make a numerical comparison of different studies and to determine their relative feasibility, which can assist in finding better routes for viable implementation of chemolytic depolymerization for present and future studies in the field of chemical depolymerization of PET and other polymer materials.