Cavity expansion and nanohybrid copper phytate@halloysite coated with polyphosphazene for reducing smoke release and fire hazard of epoxy resin

Halloysite (Hal) nanotubes have become a good nano-carrier of flame retardant systems. In the present work, cavity expansion of Hal was achieved by etching with sulfuric acid to improve its loading capacity. Then, copper phytate was synthesized and encapsulated in the Hal to obtain Phyt-Cu@Hal. Finally, a sulfonyldiphenol-containing polyphosphazene derivative (PZS) was polymerized and wrapped on Phyt-Cu@Hal external surface to form a hierarchical nanohybrid structure (Phyt-Cu@Hal@PZS). Herein, Phyt-Cu encapsulated in Hal acts as catalyzing and char-forming agents while the wrapped PZS acts as an interfacial compatibilizer which can improve the dispersion and flame retardancy of Phyt-Cu@Hal@PZS in the EP matrix. The flame retardancy and smoke suppression of the corresponding EP nanocomposites were significantly enhanced due to the synergistic effect of Phyt-Cu@Hal@PZS. After adding 5 wt% Phyt-Cu@Hal@PZS, the peak heat release rate and smoke release rate were reduced maximally by 34.3% and 42.1%, respectively. Moreover, the char residue was also greatly enhanced which is 97.3 times higher than that of neat EP. The flame retardant mechanism of Phyt-Cu@Hal@PZS was assumed that the Phyt-Cu immobilized within the Hal could distinctly inhibit the flammable pyrolysis products; the external PZS layer promotes the char formation of EP matrix; the nanoscale combination of Phyt-Cu, Hal and PZS could further enhance the catalyzing carbonization to produce a physical barrier effect.