Direct polymer-to-polymer upcycling of mixed PET/PBT wastes into biodegradable plastics: an environmental...

Upcycling of post-consumer poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT) typically relies on depolymerization. However, depolymerization can be energy-intensive and operationally complex, undermining the benefits of upcycling. In this study, we designed a direct polymer-to-polymer copolymerization process that converts mixed PET/PBT waste into biodegradable copolyesters. We evaluated the effects of poly(tetramethylene glycol) (PTMG) loading (20-40 wt%) and the inclusion of a glass fiber removal step on process performance. To this end, we conducted an environmental techno-economic assessment of the proposed process to quantify its environmental and economic viability. Increasing PTMG loading generally increased both cost ($324-539/ton-product) and greenhouse-gas emissions (from -1.24 to 1.03 kg CO₂ eq./kg-product), indicating that PTMG loading is a primary lever governing the tradeoff between economic and climate performance. Moreover, glass fiber removal increased the minimum selling price from $1335-1588/ton to $1717-1807/ton owing to solvent handling and recovery. However, glass fiber removal enables fiber-free specifications when required, and this quality-cost tradeoff can be mitigated by minimizing solvent losses and recovery duty. Applying a carbon price of $22.04/ton to monetize greenhouse gas reductions lowered the break-even price by $34-155/ton across cases. Overall, PTMG loading and glass fiber management emerged as explicit and controllable levers, enabling flexible, grade- and specification-dependent operation within a single plant.