5,10-Dihydro-phenophosphazine-10-oxide(DPPA) served as a co-curing agent of 4, 4'-diaminodiphenylmethane for the curing reaction of bisphenol A diglycidyl ether (DGEBA) epoxy resin (EP). 1 H NHR spectrum tracking the reaction process of DPPA with DGEBA revealed that P-H bond of DPPA had the higher reactivity than its N-H bond for reacting with epoxy group. DPPA could promote the curing reaction of DDM with DGEBA. DPPA endowed epoxy resin with high flameretardant efficiency due to the unique combination of phosphorus and nitrogen in the phenophosphazine ring. The cured epoxy resin could pass V-0 rating of UL-94 test with limiting oxgen index (LOI) of 33.6% at only 2.5 wt% DPPA. The reduced peak heat release rate and total heat release, and increased char yield further verified the excellent flame retardancy for EP. Flame-retardant mechanism of the epoxy resin was investigated by thermogravimetry-fourier transform infrared spectrometry (TG-FTIR), scanning electron microscopy, element analysis and FTIR spectrometry. The results indicated that DPPA catalyzed epoxy resin matrix to form the rigid intumescent char and to generate the blowing-out effect.

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The flame retardancy, thermal, and mechanical properties of the cured epoxy resins are difficult to be simultaneously improved. In the present work, a novel DPPA-based curing agent, 10-[(4-hydroxyphenyl)­(4-hydroxyphenylimino)­methyl]-5,10-dihydropheno­phosphazine 10-oxide (H-DPPA), is successfully synthesized to serve as a co-curing agent of 4, 4′-diaminodiphenyl­methane (DDM) for bisphenol A diglycidyl ether (DGEBA) epoxy resin. With the aid of 3.0 wt % of H-DPPA, the cured epoxy resin, in which the phosphorus content is as low as 0.22%, passes V-0 rating of UL-94 test with limiting oxygen index (LOI) of 31.8%. The high flame retardancy of epoxy resin modified by H-DPPA originated mainly from the formation of intumescent char layer during combustion. This new type of flame retardant containing both phosphine oxide structure and nitrogen moieties provides epoxy resins with excellent integrated performances.