Arone-Containing Cyclomatrix-Type Polyphosphazene Hybrid Microspheres for Epoxy Resin: Enhancing Fire Safety and Mechanical Properties

Arone-containing cyclomatrix-type polyphosphazene hybrid microspheres (PZK) were synthesized to enhance both the fire safety and mechanical performance of epoxy resin (EP). The chemical structure and micro-morphology of PZK were characterized using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy, and X-ray photoelectron spectroscopy. Subsequently, PZK was employed in fabricating EP thermosets. Thermal properties of the PZK modified EP thermosets were comprehensively assessed through thermalgravimetric analysis (TGA) and dynamic mechanical analysis. The results indicate that PZK endowed EP with enhanced char yield and storage modulus, attributed to its exceptional thermal stability and distinctive microsphere structure. Additionally, with only a 1.5% loading of PZK, EP achieved a UL-94 V-1 rating during vertical flame testing, accompanied by an increased limiting oxygen index of 28.3%. Additional analysis with a cone calorimeter demonstrated that the addition of PZK markedly reduced the peak heat release rate, total heat release, fire growth rate, and total smoke production of EP, indicating notable efficacy in suppressing heat and smoke release. Char residue and pyrolysis products were analyzed using SEM, TG-FTIR, and pyrolysis gas chromatography-mass spectrometry, revealing that PZK effectively inhibits the release of combustible gases from EP during thermal decomposition. The expected generation of Ph-O• radicals from arone units in PZK suggested a gaseous free radical trapping mechanism. The phosphazene framework played a crucial role in forming condensed-phase char due to its exceptional thermal stability. Consequently, PZK exhibited a typical biphasic flame-retardant mechanism for EP. Notably, the study observed that the introduction of PZK simultaneously enhances the tensile strength, flexural strength, and modulus of EP, with a particularly significant improvement observed in impact strength. This research expands the scope of investigation into the preparation and flame-retardant application of cyclomatrix-type polyphosphazene hybrid microspheres in polymers.