P/N/Si‐Containing Schiff‐Base‐Derived Flame Retardant DPSD for Simultaneously Enhancing the Fire Safety and Mechanical Performance of Epoxy Resin

ABSTRACT Simultaneously improving the fire safety and mechanical performance of epoxy resin (EP) remains challenging because conventional additive flame retardants often require high loadings and impair network integrity. Herein, a phosphorus/nitrogen/silicon‐containing Schiff‐base‐derived flame retardant (DPSD) was synthesized and incorporated into an E‐51/DDM epoxy system. At 4 wt% DPSD, EP/DPSD‐4 achieved a UL‐94 V‐0 rating and an LOI of 30.7%, while impact strength increased by 59.5%. At 6 wt% DPSD, corresponding to only 0.33 wt% phosphorus, the LOI increased to 32.7%; PHRR and THR decreased by 37.2% and 32.3%, respectively, and PSPR and TSP decreased by 32.3% and 35.6%. Meanwhile, tensile strength increased by 40.3%. Char‐residue analyses revealed a compact, continuous, and more ordered phosphorus/silicon‐enriched protective layer, while TG‐FTIR showed suppressed evolution of combustible hydrocarbon and aromatic volatiles. These results indicate that DPSD promotes condensed‐phase charring and barrier effects, with a possible additional contribution from gas‐phase inhibition. The combined rigid aromatic units, polar groups, and flexible SiO linkages enable a favorable balance between network reinforcement and energy dissipation, providing a low‐phosphorus strategy for fire‐safe, strong, and tough epoxy thermosets.