Alternative chemical recycling approaches for traditional and biodegradable plastics: two case studies

To reduce the amount of plastic that ends up in the environment, which is nowadays a critical global challenge, and recover value from wastes, the transition from the traditional linear economy to a circular economy represents a key solution. However, energy-efficient, eco-friendly, and cost-effective recycling technologies for low-grade plastics are still lacking, as virgin fossil-based plastics remain cheaper to produce. This gap has driven efforts to find methods that address existing plastic waste to achieve new valuable polymer materials that overcome recyclability challenges . In this study, waste expanded polystyrene (wEPS) and a widespread biodegradable polyester, namely polybutylene adipate terephthalate (PBAT), were selected as case-study materials. Consequently, depending on their chemical nature, two different chemical recycling approaches were explored to obtain building-blocks that, in turn, can be used to produce high-molecular-weight polymers and/or polymer-based materials. For wEPS, a one-pot process involving its dissolution in styrene monomer followed by a polymerization step carried out in suspension to build-up new PS chains was explored. The method is advantageous because it enables the upcycling of wEPS by embedding it directly in the polymerizing system, thereby eliminating the need for a subsequent polymer-solvent separation step . The results showed that the wEPS amount (20 and 50 wt.%) and other reaction parameters, such as polyvinyl alcohol amounts, initiator/styrene ratio, temperature, time, and stirring speed significantly affect percentage yield, glass transition temperature, molecular weight and rheological properties of the final PS-based materials. Chemical recycling of PBAT was instead investigated by exploiting the ring-chain equilibria, which are typical of polycondensate systems. Strainless macrocyclic oligomers (MCOs), which can be easily re-converted to high-molecular-weight polymers by entropically driven ring-opening polymerization (ED-ROP) reactions, were obtained by means of cyclodepolymerization (CDP) reactions . Green catalysts (Ti- and Zn-based) and solvents (2-MeTHF) were tested and compared with n-Bu2SnO and CHCl3, as typical catalyst-solvent system. Gel permeation chromatography (GPC) results indicate that n-Bu2SnO catalyst remains the most efficient catalyst for the synthesis of PBAT-MCOs while, Ti(OR)4 (R=2-ethylhexyl) and ZnO are poorly effective in both explored solvents. Further investigations on CDP to improve the yields in MCOs are ongoing.