Study On Intrinsic Halogen-free Flame Retarded Cured Epoxy Resins Based On 5,10-dihydro-phenophosphazine-10-oxide(DPPA) And Its Derivatives

Epoxy resins have been one of the most widely used engineering materials in modern industrial area due to their excellent integrated properties and versatility in formulation. However, there is a big security danger in their application process and great limitation of their application in many areas involving in electric and electronic industries due to their high flammability. The research and development of phosphorus-containing halogen-free flame retarded epoxy resins have received a lot of attention in recent years. Although the incorporation of phosphorus-containing flame retardants can endow epoxy resins with good flame retardancy, large additive amount of phosphorus-containing flame retardants is necessary to make epoxy resins achieve the required flame retardant grade, which may seriously deteriorate the heat resistance and mechanical properties. Therefore, new kinds of phosphorus-containing compounds with higher flame retardancy are urgent to be investigated, in order to reducing the negative effects on the properties of epoxy resins by incorporation of flame retardants and preparing halogen-free flame retarded epoxy resins with good integrated properties.5,10-Dihydro-phenophosphazine-10-oxide(DPPA) has phosphorus heterocyclic structure, high phosphorus content and P-H bond, which is similar with the commonly used flame retardant, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO). Moreover, its molecule contains the high nitrogen content and N-H bond. So the flame retardant effect of DPPA should be better than that of DOPO due to the synergistic effect between phosphorus and nitrogen. It is expected that DPPA can provide epoxy resins with good flame retardant effect at the lower loading, and the negative effects on thermal and mechanical properties of epoxy resins by incorporation of flame retardants are reduced. Base on the above research background, our research focused on DPPA and its derivatives, and intrinsic halogen-free flame retarded cured epoxy resins based on DPPA and its derivatives which were incorporated into epoxy resins via the curing reaction. The flame retardant, thermal and mechanical properties and flame retardant mechanism of cured epoxy resins were studied. Our study found that the phenophosphazine ring was a more high-efficiency flame retardant group. Derivatives of DPPA may become another important flame retardant category similar to derivatives of DOPO. The study has opened a new direction for the research of halogen-free flame retarded epoxy resins.(1) Flame retardant DPPA was synthesized from phosphorus trichloride and diphenylamine. 1H NHR spectrum tracking the reaction process of DPPA with bisphenol A type epoxy resins(E-51) revealed that both its P-H and N-H bonds could react with epoxy group, and its P-H bond had the higher reactivity than its N-H bond. It was found that P-H bond in DPPA molecule had high addition activity with C=N bond in Schiff base and could be used for synthesizing two new DPPA derivatives, H-DPPA and HD-DPPA for short. H-DPPA was synthesized via a one-pot procedure using 4-hydroxybenzaldhehyde, 4-aminophenol and DPPA as reactants. HD-DPPA was synthesized via a one-pot procedure using 4-hydroxybenzaldhehyde, 4, 4’-diaminodiphenylmethane(DDM) and DPPA. The structure of DPPA, H-DPPA and HD-DPPA was characterized by 1H NMR, 13 C NMR, 31 P NMR, FTIR and HRESI-MS. Their thermal stability was investigated by TGA, and the gas and condensed phase products from their thermoxidative degradation were analyzed by TGA-FTIR and FTIR. The onset degradation temperature of H-DPPA was 39.3 °C higher than 195.0 °C of DPPA, while the onset degradation temperature of HD-DPPA was 25.3 °C higher than H-DPPA. All of DPPA, H-DPPA and HD-DPPA had the ability to play a flame retardant role in both gas and condensed phases.(2) DPPA, H-DPPA and HD-DPPA were used as a co-curing agent of DDM, respectively. Taking the active proton number in flame retardants and DDM into consideration, including P-H, N-H and O-H, the co-curing agents and E-51 were mixed in a 1:1 equivalent ratio. DPPA, H-DPPA and HD-DPPA based intrinsic halogen-free flame retarded cured epoxy resins were prepared via the thermal curing process. The influence on the curing reaction of E-51/DDM system by introduction of DPPA, H-DPPA and HD-DPPA were studied by non-isothermal DSC method. The results showed that their introduction could promote the curing reaction of E-51/DDM system, and had no impact on its main reaction.(3) DPPA, H-DPPA and HD-DPPA could endow cured epoxy resins with excellent flame retardancy at the low loading. When cured epoxy resins reached V-0 rating of UL-94 test, the required amount for DPPA, H-DPPA and HD-DPPA was 2.5 wt%, 3.0 wt%, 2.5 wt%, respectively, where the phosphorus contents were as low as 0.36 wt%、0.22 wt%、0.19%, respectively, and the limiting oxygen index(LOI) of DPPA, H-DPPA and HD-DPPA based cured epoxy resins was as high as 33.6%、31.8%、31.3%, respectively. Moreover, the incorporation of flame retardants could reduce the peak heat release rate and total heat release, and obviously increase the char yield of cured epoxy resin. Flame retarded cured epoxy resins based on DPPA and its derivatives had the same flame retardant mechanism that flame retardants could catalyze epoxy matrix to form the intumescent char layer and to take the blowing-out effect during the combustion. The flame retardant effect of cured epoxy resins based on DPPA and its derivatives were all better than that of cured epoxy resins based on DOPO and its similar derivatives. The results showed that the phenophosphazine ring was a more high-efficiency flame retardant group.(4) A intensive blowing-out effect was found during the combustion of flame retarded cured epoxy resins based on DPPA, H-DPPA and HD-DPPA, respectively. The results indicated that the blowing-out effect universally existed in phenophosphazine-containing cured epoxy resins. It could be deduced that the formation process of blowing-out effect based on the analysis of gas and condensed phases. During the ignition of samples of flame retarded cured epoxy resins, the phosphate compounds derived from flame retardants catalyzed epoxy resin matrix to quickly form a char layer on the surface. The matrix under the char layer degraded to produce lots of gaseous products. When the accumulation of gases achieved to some degree, the strong airflows generated, which mainly consisted of non-flame gases. They cut through the char layer and blew out the fire. As a result, gas pores and bumps on the surface of char layer and the cavity below the char layer were left.(5) The tensile strength was 79.3±0.9 MPa, 85.9±1.2 MPa, 81.0±1.0 MPa for the cured epoxy resins with 2.5 wt% DPPA, 3.0 wt% H-DPPA and 2.5 wt% HD-DPPA, respectively. Their impact strength was 44.0±3.4 kJ/m2, 36.6±3.1 kJ/m2, 50.2±2.2 kJ/m2, respectively. Their glass transition temperature(Tg)(from DMA) was 160.0 °C, 162.0 °C, 162.3 °C, respectively. Their 5% weight loss temperature(T5%) under N2 was 355.4 °C, 367.4 °C, 364.3 °C, respectively. Compared with cured epoxy resin of E-51/DDM, the tensile strength of flame retarded cured epoxy resins increased by 3.4%、12.0%、5.6%, respectively and impact strength increased by 23.2%、2.4%、40.6%, respectively. While, its Tg only decreased by 3.9%、2.6%、2.4%, respectively, and its T5% under N2 only decreased by 3.3%、1.9%、0.8%, respectively. Therefore, there existed a little negative effect on the properties of cured epoxy resins by the incorporation of flame retardants, and the flame retarded cured epoxy resins possessed good integrated properties and had a good application prospect.