The flame retardancy and thermal stability properties of flame-retarded epoxy resins based on α-hydroxyphosphonate cyclotriphosphazene

An alpha-hydroxyphosphonate cyclotriphosphazene compound, hexa-(4-diethylphosphate-hydroxymethyl-phenoxy)-cyclotriphosphazene (HDHPCP), was synthesized and well characterized by FTIR, NMR and elemental analysis. The additive HDHPCP was blended into diglycidyl ether of bisphenol A to prepare flame-retardant epoxy resins (EP). Thermal properties, combustion behaviors and mechanical properties of the epoxy resins cured with 4,4-diaminodiphenyl methane were investigated. The results of thermogravimetric analysis revealed that HDHPCP improved char formation ability of the materials at high temperature and reduced the maximum mass loss rate. Moreover, the limiting oxygen index values of cured EP composites increased from 23.5% for pure EP to 30.7% and reached vertical burning UL-94 V-0 rating for sample with 20 mass% HDHPCP, which demonstrated that the prepared EP thermosets exhibited good flame retardancy. Microscale combustion calorimetry data displayed that peak heat release rate, total heat release and heat release capacity decreased obviously with the increasing content of HDHPCP. Furthermore, the addition of HDHPCP increased impact strengths and decreased the flexural strengths slightly of the flame-retarded EP thermosets.