A Polyphosphazene–Zinc Complex toward Enhanced Flame Retardancy, Smoke Suppression, and Mechanical Performance of Epoxy Resin

Designing halogen-free flame retardants with balanced fire safety, thermal properties, and mechanical performance remains a significant challenge for epoxy resins (EPs). Herein, a triazole-based polyphosphazene–zinc complex (HTDA-Zn) was synthesized via metal–ligand coordination and applied as an efficient functional molecule for EP. Benefiting from synergistic interactions between Zn2+ and triazole moieties, HTDA-Zn effectively promoted char formation and increased glass transition temperature and cross-linking density, which enabled uniform dispersion in the EP matrix. With only 10 wt % HTDA-Zn, EP reached the UL-94 V-0 classification and exhibited an LOI of 28.1%. Cone calorimeter tests showed significant substantial suppression of peak heat release rate (−72%), total heat release, smoke production, and fire growth rate was achieved, demonstrating improved fire safety. These effects were attributed to a dual-mode flame-retardant mechanism, which involves gas-phase dilution by nonflammable volatiles and condensed-phase intumescent char formation by polyphosphazene/ZnO species. In parallel, the incorporation of HTDA-Zn enhanced the tensile strength, elongation at break, and impact resistance of EP, while retaining over 92% of the flexural strength compared with the neat polymer. This was ascribed to the rigid phosphazene backbone and Zn2+–triazole coordination, which enhanced interfacial adhesion and stress transfer. This work establishes an effective design concept for advanced flame-retardant EP thermosets, integrating phosphorus–nitrogen chemistry with metal coordination to simultaneously achieve fire safety, thermal properties, and mechanical reinforcement.