Copolymerisation of polyesters using reactive extrusion
The high recycling rate of post-consumer polyethylene terephthalate (PET) through cost-effective mechanical process faces challenges such as undesirable colour waste and material degradation after repeated cycles, which limit its long-term effectiveness. Meanwhile, bioplastics such as polybutylene adipate-co-terephthalate (PBAT) offer sustainable alternatives but are constrained by their high costs and reliance on a specific degradation environment. This thesis explores a novel alternative recycling strategy, copolymerizing PET waste with polycaprolactone (PCL) through transesterification to create an economical, sustainable, and degradable copolymer over PBAT. Ti(OBu)4 emerged as an ideal catalyst for inducing rapid transesterification through the insertion-coordination mechanism, forming homogenous copolymers with short PET segments in a reaction time of two minutes. The copolymer's PET segments can be effectively reduced to statistical copolymers by increasing catalyst loading up to the reaction equilibrium predictable by the homopolymer blending ratios. Although it is possible to randomize the statistical copolymer using larger catalyst loading, this approach to further reduce the PET segment is highly inefficient, as explained by the reaction probability hypothesis. A mathematical model using empirical data successfully predicted the minimum catalyst required to yield a copolymer with two PET repeating units (poly[(ET)2-co-(CL)2]) for blending compositions of over 50wt.% PET. Furthermore, the copolymer's microstructure primarily governs its tensile strength and stiffness, making poly[(ET)2-co-(CL)2] with these properties comparable to PBAT. However, the high extent of transesterification necessary for shortening PET block lengths results in lower molecular mass and ductility, about five times lower than commercial PBAT. Upscaling this process in reactive extrusion revealed that a screw configuration constructed with at least 50% kneading blocks is necessary to achieve critical processing parameters, including a minimum residence time of four minutes and an average degree of fill of 0.5. In particular, the degree of fill is critical in successful copolymerization as the increase in the shear force is necessary to overcome the gravitational force, ensuring high mixing efficiency in processing a low-viscosity melt. Notably, copolymer synthesized from virgin and post-consumer PET has no statistical differences, demonstrating this process's feasibility as a recycling strategy. While the degradation test under a simulated marine environment has shown minimal mineralization, the copolymer has shown significant physical deterioration and reduction in molecular mass compared to PBAT. The drastic reduction in mechanical properties post-degradation was attributed to chain scission, predominately at the polymers' amorphous regions and microcracks formation due to water penetration and osmotic pressure. These findings integrate chemistry and engineering principles to advance scalable recycling strategies, providing a framework for developing degradable copolymers and expanding transesterification applications in continuous processes.
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