Molecular and material property variations during the ideal degradation and mechanical recycling of PET

Abstract

Poly(ethylene terephthalate) (PET) is an important polyester utilized for a wide variety of applications such as in the manufacturing of bottles, fibers and engineering compositions. Its chemical composition depends on the use of main monomers (e.g. terephthalic acid and ethylene glycol) and comonomers (e.g. diethylene glycol and isophthalic acid) at low concentrations, defining several reaction pathways upon its degradation or (mechanical) recycling. The present work provides a detailed overview of these molecular pathways, differentiating among thermal, thermo-mechanical, thermo-oxidative, photo-oxidative, hydrolytic and enzymatic degradation reactions. Considering the lowest contaminant amount, under ideal (mechanical) recycling (lab) conditions, a wide range of functional group variations has already been revealed, specifically during consecutive polyester processing cycles. Moreover, as a key novelty, how molecular variations influence material behavior is explained, considering rheological, thermal and mechanical properties. Supported by basic life cycle analysis, it is highlighted that our future improved assessment of the mechanical recycling potential of PET must better link the molecular and material scales. Only this linkage will open the door to a well-balanced polyester waste strategy, including (i) the evaluation of the most suitable recycling technology at the industrial scale, dealing with the mitigation of contaminants, and (ii) its further adoption and design in the context of overall virgin and recycling market variation.