Cusano, Ilaria (2025) Optimizing reactive extrusion of recycled PET through rheology. [Tesi di dottorato]

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Abstract

The mechanical recycling of polyethylene terephthalate (PET) is fundamentally limited by the molecular degradation that occurs during reprocessing, which reduces melt strength and restricts the reuse of recycled PET in high-performance applications. This thesis investigates how reactive extrusion, supported by advanced rheological analysis, can restore and redesign PET’s molecular architecture. By combining linear and nonlinear rheology with complementary thermal, morphological, and spectroscopic techniques, the work establishes an integrated framework for monitoring chain extension, detecting branching, and identifying degradation mechanisms with high sensitivity. The study elucidates how different chain extenders, bifunctional and multifunctional, interact with PET to generate linear, long-chain-branched, or network-like structures. Time-resolved rheology reveals that pyromellitic dianhydride (PMDA) promotes progressive branching, while phenoxyethanol-based oligomers (PBO) induce rapid chain extension; when used together, they exhibit strong synergistic behavior. Optimized formulations lead to increases in zero-frequency viscosity and relaxation times by orders of magnitude, confirming the formation of highly branched architectures. These results demonstrate the capacity of rheology to serve as a kinetic probe, capturing molecular evolution during reactive extrusion with a precision unattainable by conventional characterization. Nonlinear rheology further highlights the structural effects of reactive extrusion. Extensional tests show pronounced strain hardening, while Large Amplitude Oscillatory Shear (LAOS) reveals enhanced higher-harmonic intensities characteristic of heterogeneous, branched networks. These features are essential for processes such as foaming, where melt elasticity governs cell stability and expansion. The investigation extends to PET–PLA blends modified through solid-state polymerization, providing insight into phase morphology, interfacial relaxation, and thermal or thermo-oxidative degradation. SEM and DSC confirm phase separation with limited miscibility, while rheology detects additional relaxation mechanisms related to PLA droplets. Under oxidative conditions, both linear and nonlinear tests reveal accelerated degradation, a trend supported by time-resolved Raman spectroscopy, which evidences PLA-induced chain scission even in mildly inert environments. Overall, this work demonstrates that rheology functions as a powerful design and diagnostic tool for optimizing PET recycling. The combined use of reactive extrusion and linear and nonlinear rheology provides actionable guidelines for tailoring molecular architecture, improving melt processability, and enabling the production of high-value materials from recycled PET, in support of a more circular and sustainable plastics economy.

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