Effects of carbon nanomaterials modified hexaphenoxycyclotriphosphazene on the properties of polymethyl methacrylate

Oxidized carbon nanotubes (CNTs), fullerene (C60), and graphene oxide (GO) prepared via the Hummers method were reacted with hexaphenoxycyclotriphosphazene (HPCTP) to form N/P/C ternary flame retardants. Flame-retardant polymethyl methacrylate (PMMA) composites were then prepared via an in situ polymerization method. The results of material performance research showed that C60@HPCTP exhibited the most effective flame-retardant performance‌, evidenced by a 47.4% increase of the limiting oxygen index (LOI) and a 54.0% reduction in the peak heat release rate (pHRR) relative to pure PMMA, while the LOI increases for CNTs@HPCTP and GO@HPCTP were 40.6% and 41.1%, and the pHRR reductions were 43.0% and 48.4%, respectively. Post-combustion residues of the PMMA composites were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The SEM images revealed a continuous, compact char layer formed via the catalytic carbonization effect of HPCTP and the nanofillers (CNTs/GO/C60), which act as a physical barrier to the heat and mass transfer. Raman spectra further confirmed enhanced graphitization (AD/AG ratio reduced by up to 26.2% compared to pure PMMA), indicating improved thermal stability of the char. The synergistic flame-retardant effect in the condensed phase arises from the protective coating generated by phosphorus in HPCTP, the crosslinked network formed by carbon nanomaterials, and the carbonaceous char layer resulting from their co-catalyzed dehydration of PMMA, while in the gas phase, synergistic mechanisms involve the dilution effects of nitrogen as well as the radical quenching effect by phosphorus species and carbon nanomaterials. The synergistic effect of nitrogen, phosphorus, and carbon effectively inhibits the occurrence and development of combustion, and significantly improves the flame retardancy of PMMA.