Synthesis of a phosphorus/nitrogen-containing compound based on maleimide and cyclotriphosphazene and its flame-retardant mechanism on epoxy resin

In this paper, hexa(4-maleimido-phenoxyl)-cyclotriphosphazene (HMCP) was successfully synthesized by nucleophilic substitution reaction between N-(4-hydroxyphenyl) maleimide (HPM) and hexachlorocyclotriphosphazene (HCCP). The chemical structure of HMCP was characterized by Fourier transform infrared spectroscopy (FTIR), 1H and 31P nuclear magnetic resonance (NMR) and elemental analysis (EA). HMCP was then blended with diglycidyl ether of bisphenol-A (DGEBA) to prepare flame retardant epoxy resins. The flame-retardant properties of the epoxy resins were evaluated using limited oxygen index (LOI) measurement, UL94 test and cone calorimeter test. The fire properties indicated that HMCP exhibited high flame-retardant efficiency on the epoxy resin loaded with low phosphorus content. For example, the EP/HMCP-1.25 sample had a LOI value of 36.5%, and achieved UL94 V-0 rating. Compared with the neat EP sample, the peak of heat release rate (pk-HRR) and average of heat release rate (av-HRR) of the EP/HMCP-1.25 sample were decreased by 71% and 32.2%, respectively. The flame-retardant effect of HMCP in condensed and gaseous phase was studied respectively using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), TGA, chemical composition and morphology analysis of the char residues. HMCP decomposed to form phosphorus-containing compounds which promoted the carbonization of EP matrix in condensed phase, and release PO2 and phenoxyl free radicals with quenching effect to gaseous phase. The char layers with high oxidation resistance exhibited intumescent structures with more sealed surfaces, which served as protective char layers. The combined action of barrier effect and quenching effect endowed epoxy resin with excellent flame retardancy.