Polybenzoxazine Resins with Polyphosphazene Microspheres: Synthesis, Flame Retardancy, Mechanisms, and Applications

[Image: see text] Polyphosphazene microspheres were fabricated by ultrasonic-assisted precipitation polymerization using 4,4′-(hexafluoroisopropylidene)diphenol, 4,4′-sulfonyldiphenol, 4,4-(9-fluorenylidene)diphenol, and phenolphthalein to obtain poly[4,4′-(hexafluoroisopropylidene)diphenol]phosphazene (PZAF), poly(4,4′- sulfonyldiphenol)phosphazene (PZS), poly[4,4'-(9-fluorenylidene)diphenol]phosphazene, and poly(phenolphthalein)phosphazene (PZPT) and were incorporated into polybenzoxazines (PBa) to obtain corresponding PZAF/PBa, PZS/PBa, fluorenyl polyphosphazene (PZFP)/PBa, and PZPT/PBa composites. Addition of 5 wt % of PZAF, PZS, PZFP, and PZPT microspheres improved the thermal stability and fire retardancy of PBa resin significantly. Notably, addition of PBa with 5% PZAF led to a 62.5% decrease in the peak heat release rate and 49.3% reduction in total heat release. The role of microspheres in the gas-phase flame-retardancy mechanism in the PBa matrix was studied. Dynamic mechanical analysis results demonstrated that the T(g) of PBa flame-retardant composites was still around 210 °C compared to 221 °C of pure PBa. Hence, the synthesized PBa composites had potential applications as high flame-retardancy materials.