The Quest for Sustainable Composites: Developing High-Performance, Flame-Retardant, and Recyclable Carbon Fiber-Reinforced Epoxy Vitrimer Composites

Developing high-performance carbon fiber-reinforced polymer (CFRP) composites that are simultaneously recyclable and flame retardant and have the prepregs long shelf life is a significant challenge in sustainable composites manufacturing. To address this, we synthesized a multifunctional secondary amine curing agent, called T1PI, derived from vanillin. T1PI was synthesized via a Schiff base reaction involving the six-vanillin functionalized cyclophosphazene and 2-(piperazin-1-yl) ethanamine. By utilizing T1PI to cure bisphenol A diglycidyl ether (DGEBA), we fabricated a carbon fiber-reinforced epoxy vitrimer composite (T1PI/DGEBA) with a tensile strength of 490 MPa and a moderate glass transition temperature (Tg) of 139 °C, which are beyond the composite performance requirements for a wide range of applications such as automotive and sports goods. While the curing process of T1PI/DGEBA starts at lower temperatures compared to a commercial amine–epoxy resin system, its curing rate constant at ambient temperature is 11 times lower. This slower curing rate results in a longer shelf life for prepregs made of T1PI/DGEBA compared to a commercial epoxy-diamine system. Uniform incorporation of dynamic Schiff base bonds enabled the epoxy vitrimer to be both thermoformable and chemically recyclable. The use of nondestructive techniques allowed for the recovery of carbon fibers from the CFRP under mild chemical conditions. The T1P/DGEBA CFRP also showed exceptional flame retardancy, attributed to its high N/P content within the network structure. Overall, T1PI exhibits simple production, low curing temperature, and a long shelf life, making it a promising candidate for an efficient curing agent for the fabrication of sustainable CFRP composites.