Fiber-Reinforced PET Composites: Characterization of a Sustainable and Circular Processing Route

In this study, a fully circular processing method based on solid-state polymerization at the composite level (in situ SSP), tackling viscosity issues in thermoplastic composites’ impregnation, was demonstrated on carbon fiber (CF)-poly(ethylene terephthalate) (PET) composites. Initially, the effects of attained intrinsic viscosity (IV) on the transverse tensile properties were evaluated, showing an ultimate strength saturation at ∼69 MPa for PET IV = 0.82 dL/g, reaching failure strains of ∼1.18%. The latter improved further at higher IVs. Moreover, PET from used bottles, processed via the in situ SSP method, showed properties identical to those of virgin grade, with performance reaching/outperforming that of typical epoxy-based systems. Then, composites’ recycling via glycolysis was studied on the aforementioned CF-PET, along with glass fiber-PET ones. The results showed that glycolysis advances unhindered by the presence of fibers, removing ≥99% of the polymer, which was recovered as pure, crystalline bis(2-hydroxyethyl) terephthalate (BHET), i.e., the starting material for PET synthesis. During the process, fiber alignment and length are intact, enabling the constituent materials’ usage in applications equally demanding as those of the starting material. Polymer residue is a function of parameters such as depolymerization time, constituent polymer crystallinity, and IV, while some polymer residue on fibers may anyhow replace the need for sizing, contributing further to process’s circularity.