Catalyst-free recyclable and flame-retardant epoxy resins towards sustainable polymer composites

Epoxy resins and their fiber reinforced composites are widely used for low density, high strength, easy processing and superior mechanical performance. However, their inherent flammability and cross-linked structure hinder fire safety and recyclability. This work presents a novel approach by incorporating reactive flame retardants-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and a cyclotriphosphazene derivative-along with dual dynamic covalent bonds into the epoxy network. Specifically, a hexahydroxyl cyclotriphosphazene compound (HCCP–OH) bearing imine linkages is synthesized and employed as curing accelerator for epoxy resin, in combination with DOPO and methylhexahydrophthalic anhydride (MeHHPA). The resulting thermosetting epoxy resin contains reversible imine and ester bonds, exhibits a tensile strength of 87.1 MPa, a limiting oxygen index (LOI) of 34.0%, and a UL-94 V-0 rating. Moreover, the resin can be reprocessed from powders into sheets via hot-pressing at 180 °C and 5 MPa for 1 h without any catalyst and completely degraded in diethylenetriamine at 130 °C for 6 h. Additionally, carbon fiber reinforced polymer (CFRP) composites based on this matrix demonstrate superior mechanical and flame-retardant properties. They can retain about 60% of initial flexural strength after three healing cycles, while the fabrics are recovered intact through epoxy depolymerization. The strategy offers a practical route to sustainable, fire-safe composites.