The escalating accumulation of plastic waste presents a significant environmental challenge. Current disposal methods, such as incineration and landfilling, are unsustainable. Mechanical recycling, while common, is limited by contamination issues and often results in downcycling. Chemical recycling, aiming for monomer recovery, is hampered by the diverse reactivities of plastics and the complexities of purification. Upcycling offers an alternative, focusing on repurposing plastic waste into new materials, rather than solely recovering original polymers. Chemical deconstruction to oligomers, particularly suitable for condensation polymers like PET, polyamide, and polyurethane, reduces purification needs and allows for functionalization. Vitrimers, crosslinked yet reprocessable polymers with dynamic covalent bonds, represent a promising upcycling strategy. They achieve recyclability through reversible bond exchange reactions, such as transesterification. Previous studies have demonstrated vitrimerization of individual plastics, but challenges remain in upcycling mixed plastic systems. This research uses PET as a model to develop a vitrimerization process applicable to mixed plastics, leveraging glycerol as both a cleaving and crosslinking agent to create a strong, recyclable material.

The paper extensively reviews existing literature on plastic waste management, highlighting the limitations of mechanical and chemical recycling methods. It discusses the growing interest in upcycling plastic waste, particularly the chemical deconstruction approach that produces oligomers instead of monomers. Several studies are cited that successfully upcycle various types of plastics using vitrimerization and transesterification, but these studies often focus on individual, clean plastics rather than the more challenging mixed plastic waste streams. The authors highlight the potential of using glycerol in these processes due to its dual function as both a cleaving and crosslinking agent. Previous research utilizing glycerol-based transesterification for the depolymerization and subsequent vitrimerization of PET and other similar polymers is reviewed, emphasizing the potential but also the limitations of these methods regarding mechanical strength and processing challenges in mixed plastic systems.

The study uses PET as a model polymer. Controlled depolymerization is achieved through glycolysis using glycerol as a cleaving agent and 1-Ethyl-3-methylimidazolium chloride ([EMIM]Cl) as a catalyst in NMP solvent. Three different oligomeric PET feedstocks (RPET 5:1, RPET 5:3, RPET 1:1) are synthesized by varying the glycerol-to-PET ratio. Recovered PET (RPET) oligomers are obtained by precipitation in water. The RPET is then mixed with zinc acetylacetonate as a catalyst and processed via melt processing (extrusion or melt mixing) to create crosslinked vitrimers (vRPET). Rheological and dynamic mechanical analyses (DMA) are employed to characterize the crosslinking and dynamic behavior of vRPET, while tensile testing assesses the mechanical properties and impact of repeated processing. To demonstrate applicability to mixed plastic waste, a simulated mixture of PET, polyamide, and polyurethane is depolymerized using the same glycerol-based glycolysis method, resulting in Recovered Mixed Plastics (RMP). A composite is created by extruding RMP with waste glass powder as a filler. Three-point bending tests are used to evaluate the composite's mechanical properties, and recyclability is assessed through repeated reprocessing cycles.

Controlled depolymerization of PET with glycerol produces branched oligomers with a wide molecular weight distribution. Rheological analysis shows that the RPET samples exhibit crosslinking behavior, with the rate of crosslinking increasing with glycerol content. The vRPET vitrimers display good mechanical properties comparable to some commercial thermoplastics and thermosets, showing little degradation in strength after multiple reprocessing cycles. The method is successfully applied to a simulated mixed plastic waste stream. The resulting RMP shows good crosslinking behavior and the resulting vRMP/glass composite exhibits good mechanical strength and recyclability. DSC and XRD analyses reveal significant differences in thermal and crystalline properties between virgin PET and vRPET, suggesting effective crosslinking and amorphization in the vitrimer samples. The glass composite exhibits good adhesion between the binder and filler. The three point flexural bend test shows no major degradation in the vRMP composite after several reprocessing cycles.

The study successfully demonstrates a facile and efficient method for upcycling mixed plastic waste into mechanically strong and reprocessable vitrimers. The use of glycerol as both a cleaving and crosslinking agent simplifies the process by eliminating the need for additional crosslinkers and extensive purification steps. The results show that the vitrimerization process can effectively homogenize different plastic species into a single, recyclable material, overcoming a major challenge in conventional recycling of mixed plastics. The excellent reprocessability of the produced vitrimers is crucial for a circular economy, minimizing waste and maximizing material reuse. The successful incorporation of waste glass into the vRMP composite further enhances the sustainability of the process by reducing waste streams.

This research presents a viable upcycling strategy for mixed plastic waste through vitrimerization. Glycerol's dual role simplifies the process, yielding strong, reprocessable materials with minimal purification. The approach is adaptable to diverse plastic types, including polyamides and polyurethanes, creating opportunities for addressing the complex challenge of mixed plastic waste recycling. Future research could explore the optimization of the process for different waste plastic compositions and explore potential applications of the resulting vitrimers beyond the composites demonstrated in this study.

The study employs a simulated composition of mixed plastic waste, which may not fully represent the complex variability of real-world waste streams. Further investigation is needed to assess the method's effectiveness with highly contaminated or aged plastics. Long-term durability and potential environmental impacts of the vitrimers also warrant further investigation. The characterization of RMP was limited due to the complex composition of the mixed plastics.