Trans-Phosphonamidate Exchange Enables Epoxy Covalent Adaptable Networks with Intrinsic Fire Safety

This work presents the demonstration of phosphonamidate-derived covalent adaptable networks (CANs), offering a promising strategy to overcome the traditional trade-off between sustainability and fire safety in thermoset materials. We introduce phosphonamidate-based curing agents that integrate epoxy curing, intrinsic flame retardancy, and dynamic bond exchange within a single molecular design. These curing agents enable the formation of epoxy-based CANs that can be reprocessed multiple times without loss of performance, enabled by an efficient trans-phosphonamidate exchange mechanism at the P-O bond. The phosphonamidate structure further promotes faster bond exchange relative to other phosphorylated networks, facilitating dynamic network rearrangement. Beyond recyclability, the intrinsic phosphorus-nitrogen framework provides excellent fire resistance, achieving UL-94 V-0 ratings at only 2 wt % phosphorus loading while significantly reducing peak heat release. Importantly, the flame-retardant performance remains stable after repeated recycling, demonstrating the robustness of this approach. Thermal and mechanical analyses reveal tunable glass-transition temperatures, high thermal stability, and durable performance over five recycling cycles. To demonstrate application potential, carbon-fiber-reinforced composites were fabricated, confirming that these CAN systems can be processed into high-performance structural materials. Overall, phosphonamidate hardeners provide a versatile platform for thermoset resins and composites requiring recyclability, fire safety, and structural performance.