Preparation of a cyclotriphosphazene microsphere bearing a phosphaphenanthrene structure towards fire-safety and mechanical enhancement for epoxy and its aramid fiber composite

The mismatch of flame retardancy and mechanical properties is still a challenge for developing high-performance epoxy resin (EP) and its aramid fiber (AF) composite. In this study, a multifunctional cyclotriphosphazene microsphere EHP modified by a phosphaphenanthrene structure was prepared to solve the issue by taking advantage of multiple flame-retardant actions and interfacial interaction. After introducing EHP into epoxy resin, the thermoset exhibited good thermal stability, excellent flame retardancy and enhanced mechanical properties. Especially for sample EP/3%EHP with only 0.39 wt% P-loading, the limiting oxygen index (LOI) value increased from 25.5% to 33.5% accompanied by the V-0 rating in the UL-94 test; besides, the heat release and smoke release behaviors were also inhibited. Additionally, the tensile and impact strengths of the same epoxy sample increased by 11 MPa and 64% relative to pure epoxy resin. Moreover, compared with sample AF/EP, the incorporation of EHP improved both the burning behavior and mechanical properties of the composite, reflecting as reduced burning intensity, increased tensile strength and interlaminar shear strength. The flame-retardant actions, involving flame inhibition and charring and barrier effects, were evaluated quantitatively. Meanwhile, the strengthening and toughening effect benefitted from good compatibility and increased interface bonding.