State-of-the-art of industrial PET mechanical recycling: technologies, impact of contamination and guidelines for decision-making

Abstract

Poly(ethylene terephthalate) (PET) waste streams hold high societal and commercial value, with complementary mechanical and chemical recycling technologies enabling a circular implementation for many generations and life cycles, provided the multi-scale characteristics of polyester synthesis and recycling are properly quantified. The present contribution highlights the challenges faced by the PET mechanical recycling industry, connecting (i) variations at the molecular scale (e.g. degradation reactions), co-defining the material and ultimately the application properties, with (ii) variables at the plant scale (e.g. pre-treatment efficiencies). It is explained why both a polymer reaction engineering (PRE) analysis for the key processing (e.g. extrusion and solid state modification) units and a life cycle assessment (LCA) analysis at the process level (e.g. energy calculations) from the field of environmental engineering science (EES), acknowledging changes in contamination (e.g. non-intentionally added substances (NIAS)) levels and PET supply, are necessary. For each unit in the production plant the main influencing factors are discussed, highlighting how the overall performance is affected by the performance of each unit, from collection to relaunch of the PET product on the market. It is also elaborated how model-based design and data analysis can support the overall process and energy optimization. General guidelines are formulated, facilitating the combined molecular and process scale driven assessment of the feasibility of mechanical recycling technology. This in turn allows the initialization of a more fundamentally based framework for decision-making regarding preferred recycling technologies, including both the PET mechanical and chemical recycling routes.