Cyclotriphosphazene based epoxy vitrimer with excellent recyclability and flame retardancy

The widespread application of epoxy resins in thermosetting materials is limited by inadequate flame retardancy and recyclability. This study reports the synthesis of a bio-based, hexafunctional star-shaped epoxy cyclotriphosphazene (NEP) via nucleophilic substitution and oxidation of hexachlorocyclotriphosphazene and eugenol. An intrinsically flame-retardant epoxy vitrimer (EV/NEP) was subsequently prepared, exhibiting superior mechanical properties, heat resistance, and thermal stability. Incorporation of 9 mol % NEP significantly enhanced flame retardancy, increasing the limiting oxygen index (LOI) to 26.2 % while reducing peak heat release rate (PHRR) and total smoke release (TSR) by 24.9 % and 35.4 %, respectively. Flame retardancy mechanisms were elucidated in both solid and gas phases. Recyclability was confirmed through solvent recovery and thermal compression experiments. Dynamic ester bonds conferred excellent shape memory properties to EV/NEP. The combination of flame retardancy and recyclability makes this epoxy vitrimer a promising candidate for aerospace and automotive applications.