A Phosphorus–Silicon Synergistic Reactive Flame Retardant: Simultaneously Enhancing the Flame Retardancy and Mechanical Properties of Epoxy Resin

To overcome the mechanical property compromises typically caused by reactive flame retardants while imparting excellent flame retardancy to epoxy resin (EP), this work designed a multifunctional phosphorus–silicon synergistic reactive flame retardant (PTSi). The aim was to achieve simultaneous enhancement of both flame retardancy and mechanical properties. The curing kinetics of the EP/PTSi system were studied using nonisothermal differential scanning calorimetry. Results revealed that adding only 1 wt % PTSi significantly reduced the activation energy of the curing process from 52.37 to 47.96 kJ/mol, thus accelerating the curing reaction. Furthermore, with a mere 2 wt % PTSi loading (PTSi-2), the cured product exhibited a notable increase in limiting oxygen index (LOI) from 22.6% to 28.5%, achieving UL-94 V-0 certification. Compared to pure EP, PTSi-2 reduced total heat release (THR) and total smoke production (TSP) by 25.3% and 43.1%, respectively, thereby enhancing fire safety. The high amino group content in PTSi promoted the uniform dispersion of phosphorus and silicon elements, while mitigating rigidity loss through increased cross-linking density, resulting in an 11.2% improvement in tensile strength for PTSi-1. Additionally, the incorporation of flexible Si–O segments led to an 86.3% improvement in impact strength. This study provides a promising strategy for developing reactive flame retardants that synergistically enhance both flame resistance and mechanical performance.