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

摘要

In this paper, hexa(4-maleimido-phenoxyl)-cyclotriphosphazene (HMCP) was successfully synthesized by nucleophilic substitution reaction between N-(4-hydroxyphenyl) maleimide (HPM) and hexa-chlorocyclotriphosphazene (HCCP). The chemical structure of HMCP was characterized by Fourier transform infrared spectroscopy (FTIR), ~1H and ~(31)P 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 PO_2 and phenoxyl free radicals with quenching effect to gaseous phase. The char layers with high oxidation resistance exhibited intu-mescent 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.