Flame-Retardant Epoxy Resin with Excellent Mechanical Properties through an EPOSS/PAPP Synergistic System

Epoxy resin (EP) shows great promise in various fields, while its poor flame retardancy limits further application. Herein, an epoxy-containing polyhedral oligomeric silsesquioxane (EPOSS) was designed and combined with piperazine pyrophosphate (PAPP) to improve the comprehensive performance of the cured E-44/DDS (E-44 epoxy resin was cured with 4,4′-diaminodiphenyl sulfone) system. Upon incorporating 4.0 wt % PAPP and 1.0 wt % EPOSS into E-44/DDS (EP-3), the char residues are approximately twice that of EP-0, and the glass transition temperature reaches 184.5 °C. The bending strength of EP-3 reaches 102.3 MPa, showing excellent mechanical performance. Furthermore, the EP-3 exhibits outstanding flame retardancy; the limiting oxygen index (LOI) value reaches 33.6%; the UL-94 achieves the V-0 level; and the peak heat release (p-HRR), total heat release (THR), and total smoke release (TSR) are decreased by 60.0%, 21.2%, and 44.4%, respectively. The EPOSS/PAPP synergistic system exerts an effective flame-retardant effect by forming a dense char layer, capturing active free radicals, and diluting combustible gases. This strategy provides a feasible approach for preparing EP composites with excellent comprehensive properties at a low additive content, meeting the demands for high-performance materials.