A facile alternative strategy of upcycling mixed plastic waste into vitrimers

The study addresses the challenge of sustainably managing the growing accumulation of plastic waste, particularly mixed and contaminated streams that are not readily recyclable via conventional mechanical or chemical routes. Mechanical recycling suffers from contamination sensitivity and property downgrading, while chemical recycling to monomers can be energy-intensive and complicated by diverse reactivities and impurities. The authors propose an alternative upcycling strategy: controlled depolymerization of condensation polymers (e.g., PET, polyamide, polyurethane) into oligomeric polyols followed by vitrimerisation to form crosslinked yet reprocessable materials. The research question is whether partial depolymerization using glycerol and subsequent zinc-catalyzed transesterification can convert mixed plastics into vitrimers with strong mechanical performance and recyclability, reducing purification needs and enhancing compatibility across polymer types. The purpose is to demonstrate, first with PET as a model, and then with a realistic mixed plastic composition, that vitrimer networks formed from depolymerized oligomers can deliver robust, reprocessable materials compatible with circular economy goals.

Mechanical recycling is common but limited by contamination, sorting needs, and downcycling. Chemical recycling aims for monomers via depolymerization to enable closed-loop processes but faces challenges with mixed plastics and side reactions. Upcycling strategies converting polymers to oligomers can reduce purification burdens and allow functionalization. Prior work includes: carbamate exchange upcycling of post-consumer polyurethane to recyclable networks (Sheppard et al.); PBT/glycerol systems via transesterification to vitrimers (Zhou et al.); PET bottle upcycling by transesterification with polyol amines and epoxy reinforcement via reactive extrusion (Qiu et al.); and a one-shot polyester transformation to vitrimers with epoxy assistance (Kimura and Hayashi). Vitrimers, featuring covalent adaptable networks (CANs) via dynamic exchanges (transesterification, transcarbamoylation, transamination), enable reprocessability of crosslinked systems. However, most prior studies used clean/virgin plastics and did not address realistic mixed plastic waste. This work builds on the concept that controlled glycolysis to oligomeric polyols followed by vitrimerisation can homogenize different condensation polymers, potentially improving compatibility and reducing sorting/purification requirements.

Model PET vitrimerisation: PET was partially depolymerized via solvent-assisted glycolysis using glycerol as a dual-function cleaving and crosslinking reagent in N-methyl-2-pyrrolidone (NMP) solvent, catalyzed by 1-ethyl-3-methylimidazolium chloride ([EMIM]Cl). Conditions: PET 200 g dissolved in NMP 200 g at 180 °C, then glycerol and [EMIM]Cl added; reaction at 180 °C for 18 h to reach equilibrium. Three glycerol loadings were employed to target theoretical ET repeat units:OH group ratios of 5:1, 5:3, and 1:1, producing oligomeric RPET feedstocks labeled RPET 5:1, 5:3, and 1:1. The reaction mixture was quenched and precipitated in deionized water, washed twice, and dried to yield RPET (yields: 96% for 5:1, 97% for 5:3, 85% for 1:1). The ionic liquid catalyst is water-removable, aiding subsequent zinc-catalyst control. RPET characterization included DSC (melting/recrystallization behavior), GPC (molecular weight distribution), and isothermal rheology with Zn(acac)2 (5 mol% vs PET repeat units) to probe crosslinking kinetics. Vitrimer formation (vRPET): RPET powders were compounded with Zn(acac)2 (5 mol%) and processed solventlessly. For RPET 5:1 and 5:3, blends were twin-screw extruded at 220 °C, 80 rpm for 2 h, then heat-treated at 220 °C for 2 h; pellets were hot-pressed at 250 °C and 25 MPa for 30 min to form films. For RPET 1:1, the mixture was melt-reacted at 215 °C for 2 h, poured and heat-treated at 220 °C for 2 h, pelletized, and hot-pressed similarly. Thermomechanical and dynamic properties were assessed via DMA (film tension, 28–270 °C, 3 °C/min), small-amplitude oscillatory rheology (frequency sweeps at 180–270 °C), XRD (crystallinity), and tensile testing (dogbone, 5 mm/min). Reprocessability of vRPET 1:1 was tested over four hot-press cycles (250 °C, 30 min), with tensile testing after each cycle. Mixed plastic upcycling: A simulated mixed waste (100 g total) reflecting reported global waste shares was prepared: PET 49.0 g, polyamide (nylon) 35.8 g, polyurethane (PU) 15.3 g; additional unreactive polyolefin caps/labels (~1 g) were included. The mixture in NMP (120 g) was depolymerized at 180 °C for 18 h with glycerol (10.0 g) and [EMIM]Cl (1.2 g). An immiscible polyolefin layer formed and was removed by decanting. Solubilized oligomers were precipitated and washed to afford recovered mixed plastic (RMP) powder (80 g, 80% recovery). RMP was characterized by DSC (broad endotherms), FTIR (broad O–H, ester C=O, N–H and urethane/amide bands), and isothermal rheology with Zn catalyst to confirm crosslinking/gelling at elevated temperature. vRMP films were fabricated via repeated extrusion with Zn(acac)2 and thermal curing. Composite fabrication with waste glass: A colored glass bottle was ground and sieved to ≤150 µm powder. RMP, fine glass powder (80 wt% loading), and Zn(acac)2 were twin-screw extruded at 220 °C (80 rpm) with multiple passes, hot-pressed into 12 cm × 12 cm × ~1 mm tiles at 250 °C/20 MPa/5 min, and thermally cured at 250 °C for 2 h. Composite performance was evaluated via 3-point bending (support span 30 mm, crosshead 10 mm/min) and TGA (to quantify silica content). Reprocessability of the composite was tested over five hot-press cycles (250 °C, 26 MPa, cumulative 15 min), with flexural testing after selected cycles.

- Controlled glycolysis of PET with glycerol in NMP/[EMIM]Cl produced oligomeric RPET feedstocks with tunable hydroxyl content (ratios 5:1, 5:3, 1:1) and high recovery yields (96%, 97%, 85%, respectively). GPC confirmed broad oligomer distributions; higher glycerol increased depolymerization extent. - DSC showed reduced/broadened melting transitions for RPETs; RPET 1:1 lacked recrystallization exotherms upon cooling, consistent with in situ crosslinking during heating. - Isothermal rheology at 220 °C with 5 mol% Zn(acac)2 revealed crosslinking kinetics: RPET 5:1 showed a gel-point crossover (G′=G″) at ~1056 s (~17 min); RPET 5:3 and 1:1 crosslinked faster with higher G′, indicating greater crosslink density at higher glycerol content. - vRPET films exhibited crosslinked, vitrimer-like behavior: DMA showed no melting above 250 °C but rubbery plateaus; vRPET 1:1 had higher Tg (106.4 °C) vs vRPET 5:3 (80.4 °C) and 5:1 (81.6 °C), indicating more rigid, highly crosslinked networks. - Small-angle oscillatory rheology at 250 °C showed vRPETs with G′>G″ across frequencies and crossover points at long timescales (Winter–Chambon criterion), evidencing covalent adaptable networks via transesterification exchanges. - Mechanical properties (average ± SD): vRPET 1:1 tensile strength 47.3 ± 4.0 MPa, elongation 5.3 ± 0.5%, Young’s modulus 1.06 ± 0.11 GPa; vRPET 5:3 48.0 ± 3.1 MPa, 4.4 ± 0.5%, 1.27 ± 0.06 GPa; vRPET 5:1 36.1 ± 3.7 MPa, 4.7 ± 0.8%, 1.16 ± 0.09 GPa. Strengths comparable to some common thermoplastics/thermosets. - Reprocessability: vRPET 1:1 retained tensile strength over four recycle cycles (1st–4th cycles: ~47.3, 51.1, 45.1, 43.1, 47.9 MPa reported across trials), indicating minimal degradation and efficient bond exchange. - XRD showed PET crystalline peaks absent in vRPET 1:1, confirming amorphous, crosslinked morphology. - Mixed plastic upcycling: One-pot glycolysis of PET/nylon/PU (49/35.8/15.3 wt%) yielded RMP polyols (80% recovery). FTIR confirmed polyol features (broad O–H, ester C=O 1721 cm−1, N–H and urethane/amide-associated bands). Isothermal rheology showed melting near 116 °C and gelation upon heating, with G′>G″ at 220 °C, indicating crosslinking. - vRMP exhibited homogenized behavior (DSC lacked sharp endotherms from individual components), consistent with copolymer-like networks via transesterification/esterification/urethane exchange. - vRMP–glass (80 wt% waste glass) composites achieved flexural modulus 9.83 ± 2.86 GPa and flexural strength 23.5 ± 5.3 MPa, comparable to limestone and construction bricks. TGA confirmed ≥80 wt% silica; SEM showed good filler-matrix adhesion. Flexural strength remained similar after 2 and 5 reprocessing cycles, demonstrating recyclability.

The results demonstrate that controlled partial depolymerization of condensation polymers into oligomeric polyols, followed by zinc-catalyzed transesterification-driven vitrimerisation, can convert both single-polymer (PET) and realistic mixed plastic waste streams into crosslinked yet reprocessable materials. Higher glycerol loading increases hydroxyl functionality, accelerates network formation, and raises crosslink density and Tg, yielding strong, tough vRPET with dynamic covalent networks that enable stress relaxation and remolding. The vitrimer approach homogenizes chemically similar polymer families (esters, amides, urethanes) by grafting glycerol end groups and engaging in exchange reactions, reducing immiscibility issues that plague mechanical recycling of mixed plastics. Isolation via precipitation and phase separation of unreactive polyolefins minimizes purification complexity, addressing a key limitation of monomer-focused chemical recycling. The vRPET retained mechanical performance over multiple processing cycles, and the vRMP-based composite incorporated 80 wt% waste glass, delivering structural properties and repeated reprocessability. Collectively, these findings support vitrimerisation as a practical upcycling route that preserves value, enhances compatibility, and aligns with circular economy objectives for mixed plastic waste.

Vitrimerisation of oligomeric polyols obtained by controlled glycolysis provides a facile alternative for upcycling mixed plastic waste. PET was depolymerized to oligomers and directly repolymerized via zinc-catalyzed transesterification to form vRPET with strong mechanical properties and excellent reprocessability, even from broad molecular-weight feedstocks and without additional crosslinkers. Extending the process to real mixed plastic waste (PET/nylon/PU) yielded RMP polyols that could be vitrimerised into homogeneous networks and further upcycled into high–glass-content composites with stable flexural performance over multiple reprocessing cycles. The approach reduces purification needs, separates immiscible impurities during processing, and can homogenize different condensation polymers through dynamic covalent networking. Future work could examine heavily contaminated and weathered waste streams, broaden polymer classes, optimize catalysts and processing conditions for scalability, and expand applications (e.g., composites, ionogels) based on waste-derived oligomeric feedstocks.

Characterization of the recovered mixed plastic (RMP) polyols is inherently challenging due to compositional complexity; the authors note difficulty in accurately identifying every functional group present. Discoloration was observed, attributed to thermal degradation of the imidazolium ionic liquid during depolymerization. Sample-to-sample variations affected mechanical testing error bars, and some formulations (e.g., lower-glycerol vRPET) exhibited ongoing crosslinking during initial thermal analyses, reflecting post-curing kinetics. The mixed plastic demonstration reflects a specific composition and may vary with different waste streams.