Preparation and Properties of Two Intrinsic Flame-Retardant Bismaleimide Resins with Enhanced Thermal and Smoke Suppression Performance

Bismaleimide (BMI) resins are widely used in high-performance applications due to their excellent thermal stability and mechanical properties; however, their inherent flammability presents significant safety concerns. This study reports the synthesis of two diamines containing DOPO groups, using DOPO as a phosphorus source, to prepare two intrinsic flame-retardant bismaleimides (D-M and 2D-M) with varying phosphorus contents. These compounds were incorporated into a conventional BMI system consisting of diallyl bisphenol A (DBA) and bisphenol A-based bismaleimide (BDM). The results demonstrate that the introduction of D-M and 2D-M significantly enhances flame-retardant performance through a nitrogen–phosphorus synergistic effect, effectively reducing thermal hazards and smoke generation. Compared to the baseline BMI resin, the total heat release (THR) of BMI/D-M-30% and BMI/2D-M-30% decreased by 40.2% and 19.3%, while total smoke production (TSP) dropped by 9.2 m2/m2 and 13.3 m2/m2. Additionally, the average effective heat of combustion (Av-EHC) decreased by 36.3% and 7.3%, respectively. The residual carbon exhibited increased density and graphitization, confirming the synergistic interaction between condensed-phase char formation and gas-phase flame inhibition. This study offers an efficient strategy for developing BMI resins with superior flame-retardant properties through nitrogen–phosphorus synergy.