(100f) Chemical Recycling of Polycaprolactone Via Reactive Melt Processing

An astounding ~300 million tons of plastic waste are dumped into our environment or landfills every year, making it one of the biggest crises in history. Among a broad range of potential solutions, chemical recycling is an exciting option which involves the depolymerization of end-of-life plastics into monomers or intermediates, which can then be purified by conventional means and used to synthesize virgin polymers. In principle, this mode of recycling can be used for multiple cycles without any deterioration of properties in the resynthesized polymer and can also be used for streams containing multiple plastic waste types. However, chemical recycling comes with a unique set of challenges: first, many chemical recycling processes involve the use of harsh solvents, thus introducing a new environmental hazard on the path to solving another. Secondly, and most importantly, chemical recycling processes are technologically and economically challenging to scale up, thereby slowing implementation in industry. In this work, we designed a modified twin-screw compounder which demonstrated successful chemical recycling of polyesters to their respective monomers. Inspired by industrial use of devolatilization extrusion set-ups, the modified compounder used herein simultaneously heated and mixed the polymer with a catalyst, allowing evolved monomer vapors to be removed by a vacuum system and collected in a cold trap. To explore the performance of this setup, we used commercially available polycaprolactone (PCL) for our experiments. Reactive distillation experiments were initially conducted in a heated round bottom flask with a distillation head to determine the optimum temperature and catalyst loading necessary for successful ring-closing depolymerization (RCD) . Recovered monomers were collected and assessed for yield and purity by ¹H nuclear magnetic resonance spectroscopy (NMR) and gas chromatography-mass spectroscopy (GC-MS). Compared to analogous distillation experiments, the compounder depolymerization experiments resulted in similar values of purity and yield. The results of this study establish the modified compounder as an easily scalable approach for polyester depolymerization.