Recyclable and Efficient Fire-Retardant Amino-Ester Networks Enabled by Phosphonate Salts

Thermosets are particularly attractive materials due to their outstanding mechanical strength, thermal stability, and stability in harsh environments. Yet, these advantages come at a cost: conventional thermosets are challenging to recycle and often lack sufficient flammability properties, limiting their sustainability and safety. This work reports flame-retardant amino-ester covalent adaptable networks (AE-CANs) prepared via aza-Michael addition/retro-addition chemistry, incorporating tailor-made phosphonate salts as nonreactive additives. A scalable, high-yield synthesis yielded three distinct phosphonate salts, enabling tunable phosphorus content in AE networks. Remarkably, UL-94 V-0 ratings were achieved at phosphorus loadings as low as 1.2 wt %, accompanied by limiting oxygen index values up to 28%. Cone calorimetry confirmed drastic reductions in peak heat release rate (up to 40%) and total heat release (up to 63%), with fire performance retained after ten recycling cycles. The networks combined dynamic adaptability with stability, undergoing multiple thermomechanical recycling steps without loss of mechanical or thermal properties. Furthermore, AE-based carbon-fiber composites exhibited a 9-fold increase in flexural stiffness compared to neat networks. They can be chemically depolymerized in cyclohexylamine, allowing for the complete recovery of intact carbon fibers. This work introduces phosphonate salt as a versatile strategy to simultaneously achieve recyclability, fire safety, and high mechanical performance in thermoset resins and composites, providing a sustainable pathway toward next-generation structural materials.