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.

It is difficult to realize flame retardancy of epoxy without suffering much detriment in thermal stability. To solve the problem, a super-efficient phosphorus-nitrogen-containing reactive-type flame retardant, 10-(hydroxy(4-hydroxyphenyl)methyl)-5,10-dihydrophenophosphazinine-10-oxide (HB-DPPA) is synthesized and characterized. When it is used as a co-curing agent of 4,4′-methylenedianiline (DDM) for curing diglycidyl ether of bisphenol A (DGEBA), the cured epoxy achieves UL-94 V-0 rating with the limiting oxygen index of 29.3%. In this case, the phosphorus content in the system is exceptionally low (0.18 wt %). To the best of our knowledge, it currently has the highest efficiency among similar epoxy systems. Such excellent flame retardancy originates from the exclusive chemical structure of the phenophosphazine moiety, in which the phosphorus element is stabilized by the two adjacent aromatic rings. The action in the condensed phase is enhanced and followed by pressurization of the pyrolytic gases that induces the blowing-out effect during combustion. The cone calorimeter result reveals the formation of a unique intumescent char structure with five discernible layers. Owing to the super-efficient flame retardancy and the rigid molecular structure of HB-DPPA, the flame-retardant epoxy acquires high thermal stability and its initial decomposition temperature only decreases by 4.6 °C as compared with the unmodified one.

Flame retardants endow epoxy resins (EP) with flame retardance, however, their introduction often lead to the decrease in the glass transition temperature (Tg), which is an important property for EP. Therefore, it is significant to design and synthesize a high‐efficiency flame retardant that enhances flame retardance and Tg of EP. To achieve this goal, a hyperbranched polyamide oligomer containing DOPO (HPD) was successfully synthesized by A4 + B2 polymerization and its structure was confirmed by FTIR, 1H NMR, and 30P NMR spectra and GPC. HPD was used as an additive flame retardant in epoxy resin, and its effect on the flame retardance and thermal properties of epoxy resin was studied. The epoxy resin with 7.5 wt% HPD reached UL‐94V‐0 rating and a higher LOI value of 29.6%, and its Tg was 176.2°C, which is higher than the pure epoxy resin. Moreover, the results of cone calorimetry testing (CCT) and TG‐FTIR analysis suggested that the gas‐phase and condensed‐phase flame‐retarded roles of HPD delayed the time to ignition, and reduced the value of relevant combustion parameters, including the peak of heat release rate, average of heat release rate, total heat release, smoke production rate, and total smoke production. The analysis results of Py‐GC/MS test of HPD further confirmed that HPD was able to play the flame‐retarded role in the gaseous and condensed phases.

In order to give epoxy resin good flame retardance, a novel bio-based flame retardant based on 2-aminopyrimidine (referred to as VAD) was synthesized from renewable vanillin as one of the starting materials. Its structure was confirmed by NMR and mass spectra. The epoxy resins containing VAD were prepared by utilizing 4,4-diaminodiphenylmethane (DDM) as a co-curing agent, and their flame-retardant, mechanical and thermal properties and corresponding mechanisms were studied.VAD accelerated the cross-linking reaction of DDM and E51 (diglycidyl ether of

The flame-retarded epoxy resin with improved thermal properties based on environmentally friendly flame retardants is vital for industrial application. Hereby, a novel reactive-type halogen-free flame retardant, 10-(3-(4-hydroxy phenyl)-3,4-dihydro-2H-benzo[e] [1,3] oxazin-4-yl)-5H-phenophosphazinine 10-oxide (DHA-B) was synthesized via a two-step reaction route. Its structure was characterized using 1 H, 13 C, and 31 P NMR and HRMS spectra. For 4,4′-diaminodipheny ethane (DDM) and diglycidyl ether of bisphenol A (DGEBA)-cured systems, the epoxy resin with only 2 wt% loading of DHA-B passed V-0 rating of UL-94 test. Significantly, its glass transition temperature (T g ) and initial decomposition temperature (T 5% ) were as high as 169.6°C and 359.6°C, respectively, which were even higher than those of the corresponding original epoxy resin. Besides, DHA-B decreased the combustion intensity during combustion. The analysis of residues after combustion suggested that DHA-B played an important role in the condensed phase.