Sustainable fire-safe epoxy composites enabled by reactive polyphosphonates

The closed-loop recycling of thermoset plastics remains a critical challenge, especially as demand grows for materials that combine high performance with fire safety. This study presents a new class of inherently flame-retardant epoxy thermosets (ETs) incorporating reactive polyphosphonates as co-monomers. The resulting amine-cured ETs achieve UL94 V-0 ratings at just 2.5 wt% phosphorus loading, alongside excellent mechanical and thermal performance (T g = 140–175 °C, tensile strength = 100–110 MPa, modulus = 3.8 GPa). Remarkably, these ETs enable repeated thermomechanical reprocessing, up to ten cycles, without significant loss of performance. When cured with acid anhydrides, the polyphosphonate-based ETs undergo complete chemical depolymerization via alcoholysis, in stark contrast to conventional amine-cured systems. This recyclability extends to carbon-fiber reinforced composites (CFRCs), allowing both carbon fiber recovery and matrix regeneration. These results highlight a promising route toward sustainable, high-performance thermosets with intrinsic flame retardancy and closed-loop recyclability. • A novel class of vitrimeric epoxy thermosets (ETs) was developed using polyphosphonates as multifunctional hardeners, enabling recyclability and flame retardance. • These ETs achieve UL-94 V-0 flame retardant ratings at only 2.5 wt% P loading and maintain high mechanical properties (T g > 140 °C, modulus ~3.8 GPa). • ETs exhibit excellent thermomechanical recyclability, allowing up to ten hot-pressing cycles with negligible degradation in flame-retardant performance. • Chemical recycling via alcoholysis enables recovery of both the epoxy matrix and embedded carbon fibers, supporting closed-loop composite reuse. • Unlike traditional thermosets, these dynamic networks retain or enhance functional performance despite internal chemical reconfiguration.