Enhanced flame retardancy of DGEBA epoxy resin with a novel bisphenol-A bridged cyclotriphosphazene

A novel flame retardant, namely bisphenol-A bridged penta(phenoxy)cyclotriphosphazene (A-BP), was synthesized and characterized successfully. Obtained A-BP was mixed in different proportions with diglycidyl ether of bisphenol-A (DGEBA) in order to prepare a series of flame retardant epoxy resins (EP). The differential scanning calorimeter (DSC) results indicated that the glass transition temperatures (Tg) of EPs containing A-BP had only a slight decrease in comparison to EP. Thermogravimetric analysis (TGA) showed that the incorporation of A-BP improves the thermal stability of EP at a high temperature. Moreover, an increase of LOI value (33.9%) was achieved for EP contained with 9 wt% of A-BP. Furthermore, such sample passed UL-94 V-0 rating. Additionally, according to the cone calorimeter tests, in the whole combustion, the EP/9% A-BP material showed lower heat release and smoke production than neat EP. To elucidate the flame retardant actions of A-BP for EP, comprehensive measurement including Fourier transform infrared (FTIR), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), pyrolysis gas chromatography/mass spectrometry (Py-GC/MS) and TG-FTIR techniques were adopted to analyze the char layers and gaseous products. As a result, a bi-phase flame retardant mechanism was proposed showing the efficiency of inert gas dilution and quenching of existed free radicals in gaseous phase and char formation action in a condensed phase, respectively.