Polybenzoxazine-based imine vitrimers for carbon fibre-reinforced composites: paving the way for recyclable high-performance materials

Polybenzoxazine-based vitrimers possessing dynamic covalent linkages have emerged as a promising class of advanced materials, offering reprocessability and recyclability while maintaining the outstanding mechanical properties typical of thermosets. This study explores the influence of unsaturation in the amine precursor on the final properties of vitrimers by comparing polybenzoxazines derived from stearylamine (saturated) and oleylamine (unsaturated). The unsaturation in oleylamine positively influences the viscoelastic characteristics of the vitrimer network, resulting in better stress relaxation with a relaxation time (τ) of 50 seconds at 140 °C in p(HBA-ole-pd) compared to 122 seconds in p(HBA-ste-pd). The bond exchange dynamics are characterized by an activation energy (E_a) of 61 kJ mol⁻¹ in p(HBA-ole-pd) and 68 kJ mol⁻¹ in p(HBA-ste-pd), thereby facilitating enhanced self-healing and reprocessing capabilities. p(HBA-ole-pd) exhibits twice the number of crosslinks at 9280 mol m⁻³ in comparison with 4842 mol m⁻³ attributed to the formation of extra crosslinks via olefinic bonds present in oleylamine at elevated temperatures. The p(HBA-ole-pd) vitrimer was further integrated with carbon fabric to prepare carbon fibre-reinforced polymer (CFRP) composites. Utilizing dynamic imine in polybenzoxazine offers a method for closed-loop recyclability. The composite materials exhibited excellent mechanical and self-healing properties. The recycled p(HBA-ole-pd)-CF composite demonstrates tensile strength comparable to the pristine p(HBA-ole-pd)-CF composite. The dynamic imine bonds within the vitrimer matrix allowed efficient solubility under mild chemical conditions, ensuring complete recovery of carbon fibre for reuse in subsequent composite fabrication. This study highlights the critical role of unsaturation in tuning the vitrimer properties of thermosets for high-performance applications while advancing the sustainability of CFRPs. By leveraging the unique reactivity of imine-based vitrimer networks, we present a viable strategy for developing fully recyclable, high-strength composite materials, contributing to the broader goal of reducing composite waste and promoting a circular economy.