Interfacial Synergistic Flame Retardancy and Smoke Suppression in Siloxane-Modified Epoxy Resin/Phosphoric Acid-Treated Aramid Fiber Composites

Aramid fiber composites are of significant value in aerospace and military equipment due to their high strength, high modulus, and excellent heat resistance. However, their widespread application is limited by the large amounts of toxic smoke generated during combustion.An innovative approach was adopted by synergistically modifying aramid fibers with phosphoric acid and epoxy resin with siloxane. The flame retardancy and smoke suppression properties of the composites were systematically investigated. The phosphoric acid treatment concentration was optimized, and the interfacial bonding strength and residual char formation were analyzed.With 40wt% phosphoric acid treatment, the interfacial bonding strength of the composite increased by 50.9%, and the interlaminar shear strength reached 49.97 MPa. Thermal analysis showed that the residual char yield at 1000℃ improved to 56.8%, and the limiting oxygen index increased to 33.2%. Combustion tests revealed a 77.1% reduction in total smoke production (TSP) and a 68.1% decrease in maximum specific optical density (Ds,max). Microstructural characterization confirmed that the silicon-phosphorus synergistic effect at the resin-fiber interface promoted the formation of a dense Si-O-P-C char layer, effectively blocking heat and oxygen diffusion.This study provides a new technical approach and theoretical basis for developing high-performance flame-retardant composites.