• chemical recycling
• complex plastic
• Deep Eutectic Solvents
• depolymerization
• PA66
• PET

This thesis provides a comprehensive exploration of innovative chemical recycling methodologies for managing post-consumer plastic waste, focusing on the depolymerization of polyethylene terephthalate (PET), polyamide 6,6 (PA 66), and PET-based blends like polycotton and PET/polyethylene (PE) multilayer packaging. The findings highlight the potential of novel solvent systems, such as Lewis-Brønsted Deep Eutectic Solvents (LBDESs) and Low Temperature Transition Mixtures (LTTMs), to achieve efficient monomer recovery and address the limitations for mixed polymer recycling.
LBDESs and LTTMs were initially characterized by their thermal, physical, and chemical proprieties. Significant discrepancy from the theoretical solid-liquid phase diagrams and huge difference in transitions point (melting or glass temperature, respectively) were revealed, confirming the chemical nature of the solvents. Post-consumer PET depolymerization was extensively investigated, reveling that LBDESs such as FeCl3⋅6H2O/methanesulfonic acid (system A1_1), FeCl3⋅6H2O/p-toluenesulfonic acid (system B1_1) and FeCl3⋅6H2O/acetic acid (system C1_1) were highly effective. Complete depolymerization to terephthalic acid (TA) was achieved within 1 hour at 100 °C using system A1_1, with TA yields exceeding 91%. System C1_1, despite initially being lower in efficiency (21% PET conversion, 4% TA yield in 30 minutes at 100 °C), showed significant improvement through extended post-reaction alkaline treatment, achieving quantitative depolymerization with 1−1.5 hours, depending on PET source and thickness. The addition of CaCl2 as an additive improved solvent performance, increasing PET conversion and reducing reaction times from 1.5 to 1 hour for mineral bottle flakes. Microwave-assisted reactions further enhanced the process, reducing the reaction time to 10 minutes at 180 °C while maintaining quantitative TA recovery. Furthermore, system A1_1 was demonstrated to process up to four cycles of PET without any loss of efficiency. Finally, environmental assessments positioned the LBDES-based method as highly competitive in terms of sustainability metrics compared to other chemical recycling techniques.
The application of LTTMs, specifically FeCl3⋅6H2O/polyacrylic acid mixtures, further expanded the scope of PET depolymerization. Quantitative PET conversion was achieved within 3 hours at 100 °C, with TA yields reaching 90% under aqueous HCl work-up conditions. Dilution of the LTTMs before the reaction significantly reduced acid consumption, enabling the use of water during the work-up process, thereby enhancing the sustainability of the whole process. Kinetic analyses confirmed that depolymerization primarily proceeds through surface erosion, as evidenced by the formation of oligomeric intermediates. These were subsequently converted into monomers during the work-up procedures. In Chapter 4, PA 66 depolymerization was optimized by varying reaction time, temperature, and polymer loading. System A1_1 and C1_1 proved effective, achieving complete depolymerization within 5 hours at 180 °C. Adipic acid (AA) and hexamethylene diammoniumdichloride (HMDA∙2HCl) were recovered with yield exceeding 83 and 85%, respectively, through optimized work-up procedures. The process was successfully extended to post-consumer PA 66, demonstrating the system’s versatility. Environmental parameters were also evaluated, confirming the sustainability of the process.
The challenges of mixed-material recycling were addressed through selective depolymerization of PET blends. For polycotton (40% PET/60% cotton), LBDESs enabled PET depolymerization while preserving a large part of the cellulose fraction. System B1_1 achieved nearly complete depolymerization below 110 °C, balancing TA yield (> 90%) and cellulose recovery (up to 69%). System C1_1 offered a less corrosive alternative, with extended reaction times yielding similar results. The preserved cellulose fraction was valorized into cellulose nanocrystals, as confirmed by X-ray diffraction and scanning electron microscopy (SEM).
For PET/PE multilayer packaging, characterization through FTIR and elemental analysis confirmed the polymeric composition, with PET and PE contents varying across the tray sections. Selective PET depolymerization of the body section achieved a TA yield higher than 90% within 1 hour at 100 °C using system C1_1. However, the film section, with higher PE content, showed limited PET conversion due to the encapsulation effect of PE. Further optimization is required to achieve complete PET removal and enable the recovery of a pure PE fraction suitable for mechanical recycling.