Synthesis of a novel bridged-cyclotriphosphazene flame retardant and its application in epoxy resin

A novel flame retardant, named bisphenol-A bridged penta(anilino)cyclotriphosphazene (BPA-BPP), was successfully synthesized. Its chemical structure was characterized by Fourier transform infrared (FTIR), 1H NMR and 31P NMR. Then, different amounts of BPA-BPP were mixed with diglycidyl ether of bisphenol-A (DGEBA) to fabricate flame retardant epoxy resin (EP). Nonisothermal differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) tests were used to study the curing kinetics and thermal degradation behaviors of flame retardant EPs. The results of limiting oxygen index (LOI), vertical burning tests (UL-94) suggested that BPA-BPP exhibited good flame-retarded efficiency on the EP loaded with low phosphorus content. Compared with the neat EP, the LOI value of EP/9%BPA-BPP increased from 21.0 vol% to 28.7 vol%. Furthermore, the peak of heat release rate (PHRR), total heat release (THR), total smoke production (TSP) of the same sample, obtained from cone calorimetry, were declined obviously, suggesting excellent flame retardancy and smoke inhibition. The morphology and chemical structures of the char layers were analyzed by SEM, Raman and FTIR. The Py-GC/MS was used to investigate the pyrolysis behavior and flame-retardant mechanism of BPA-BPP. In the process of heating, aniline, diphenylamine and NH3 were released from BPA-BPP in gaseous phase, and phosphorus-rich carbonaceous chars were left in condensed phase. BPA-BPP could promote EP to form intumencent protective char layers, enhancing flame retardancy of EP effectively.