Bisphenol-S bridged penta(anilino)cyclotriphosphazene and its application in epoxy resins: Synthesis, thermal degradation, and flame retardancy

A novel bridged-cyclotriphosphazene flame retardant, named bisphenol-S bridged penta(anilino)cyclotriphosphazene (BPS-BPP), was synthesized and characterized by fourier transform infrared (FTIR), 1H NMR and 31P NMR and mass spectroscopy (MS). Then, BPS-BPP was used to flame retard epoxy resin (EP, diglycidyl ether of bisphenol-A type). The curing behavior and glass-transition temperature (Tg) of EP/BPS-BPP were investigated by differential scanning calorimetry (DSC). The thermogravimetric analysis (TGA) results showed that the incorporation of BPS-BPP improved the thermal stability of EP at high temperature region. Limiting oxygen index (LOI) value of EP sample increased to 29.7% with limited amount of 9 wt% BPS-BPP. The peak of heat release rate (PHRR), total heat release (THR), total smoke production (TSP) values of the same sample, obtained from cone calorimetry, were declined conspicuously, indicating good flame-retardant and smoke suppression efficiency of BPS-BPP. The flame-retardant mechanism of EP/BPS-BPP thermosets was investigated via analyses of gaseous products and char residues. Based on the aforementioned studies, BPS-BPP was demonstrated as an effective flame retardant for EP due to temperature-dependent and bi-phase flame retardant mechanism.