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

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