| Due to its excellent mechanical properties and good chemical resistance, cured epoxy resins have been widely used in modern industrial area. However, flammability is one of the major disadvantages of epoxy resin, which limited its utilization in some fields that require high flame resistance. The organophosphorus flame retardants, which are proved to have great flame retardant effect on the cured epoxy resins, favor good alternative for halogenated flame retardant. However, the introduction of the phosphoric weak bonds of P-C and P-O may bring negative effects on the mechanical and thermal properties of cured epoxy resins. Therefore, the high-performance intrinsic halogen-free flame-retarded cured epoxy resins are prepared using an organophosphorus flame retardant with high efficiency to reduce the loading of phosphorus, thus decreasing the negative effects from the presence of phosphorus-containing groups, and by incorporating reinforcement materials to improve mechanical properties of cured epoxy resins simultaneously.9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO) is one of the most commonly used organophosphorus compounds. Due to its active P-H bond, DOPO-derivatives can be preparated via chemical reaction, and is expected to assist cured epoxy resins for achieving an excellent flame retardant grade at the low phosphorus loading. Based on the above background, the researcher focuses on the utilization of DOPO-based flame retardant in this paper. The halogen-free flame-retarded cured epoxy resins with comprehensive performance were prepared using the DOPO-based derivative as highly effective intrinsic flame retardant. Moreover, the mechanical properties were improved by the utilization of reinforcement materials, including crown ether(CE) and polyhedral oligomer silsesquioxane(POSS).(1) Flame retardant 4,4’-[methylenebis[4,1-phenyleneimino[(6-oxide-6H-dibenzene[c,e] [1,2]oxaphosphorin-6-yl)methylene]]]diphenol(abbreviation is D-bp) was preparated using 4,4’-diaminodiphenylmethane(DDM), 4-hydroxybenzaldhehyde and DOPO. D-bp was used as co-curing agent for DDM to improve the flame retardancy of cured D-bp/DDM/diglycidyl ether of bisphenol A(DGEBA). Furthermore, cured D-bp/18-crown-6(CE6)/DDM/DGEBA and D-bp/Octa(glycidoxypropyl) polyhedral oligomeric silsesquioxane(POSS-EP)/DDM/DGEBA were prepared via the preparated CE6/DDM host-guest inclusion complex was introduced into the molucular chain of cured epoxy resins and POSS-EP was used to partially substitute the epoxy pre-polymer DGEBA, respectively.(2) The curing behavior of DDM/DGEBA system with the addition of D-bp, CE6 and POSS-EP was studied by non-isothermal DSC method. It is found that introduction of D-bp is able to reduce the apparent activation energy and thus promote the curing reaction. Incorporation of CE6 or POSS-EP would not affect the curing reaction.(3) D-bp endowed cured epoxy resins with excellent flame retardancy at the low loading of phosphorus. The cured D-bp/DDM/DGEBA passed V-0 rating of UL-94 vertical burning test with the limiting oxygen index(LOI) of 39.7 % when the phosphorus content was 0.50 wt%. Cured D-bp/CE6/DDM/DGEBA achieved V-0 rating of UL-94 vertical burning test when the phosphorus content was 0.75 wt%. The flame retardant efficiency of D-bp was improved by the incorporation of POSS-EP, in which cured D-bp/POSS-EP/DDM/DGEBA passed V-0 rating of UL 94 vertical burning test when the phosphorus content was only 0.25 wt%. As for the cured D-bp/DDM/DGEBA whose UL 94 classification passed V-0 rating, peak of heat release rate(P-HRR), total heat release(THR), and effective heat of combustion(EHC) were 634 k W/m2, 145 MJ/m2 and 23.6 MJ/kg, declining 52.4 %, 40.5 % and 32.6 %, respectively, compared with the neat epoxy resin. P-HRR, THR and EHC of flame-retarded cured D-bp/POSS-EP/DDM/DGEBA were 516 k W/m2, 163 MJ/m2 and 23.9 MJ/kg, declining 45.1%, 28.2 % and 24.4 %, respectively, compared with the neat epoxy resin.The flame retardant mechanism of D-bp was exerted via advance thermal degradation that generates phosphorus-containing compounds to promote the dehydration and formation of char layer, and via the construction of a stable barrier of nitrogen-containing compounds to cover the surface of underlying matrix. Moreover, the compactness of barrier was enhanced with the introduction of POSS-EP, thus realizing the phosphorus-silicon synergistic flame retardation effect.(4) For the cured epoxy resin whose UL 94 classification passed V-0 rating, the glass transition temperature(Tg) of cured D-bp/DDM/DGEBA was 157.5 °C, decreasing by 7.0 °C compared with the neat epoxy resin, while the tensile strength and flexural strength were 60.9±2.8 MPa and 94.7±2.3 MPa, decreasing by 9.2 MPa and 10.9 MPa, respectively. The mechanical properties of cured D-bp/CE6/DDM/DGEBA were significantly improved by the incorporation of CE6, in which the tensile strength, flexural strength and fracture energy of the flame-retarded thermoset were 84.8±3.2 MPa, 139.5±5.6 MPa and 2.09 MJ/m3, increasing 13.7 MPa, 33.9 MPa and 0.63 MJ/m3 compared with the neat one, respectively. The comprehensive performance of cured D-bp/POSS-EP/DDM/DGEBA were improved via incorporation of POSS-EP and the Tg reached out the value of 180.1 °C, increasing 15.6 °C compared with the neat epoxy resin. Moreover, the tensile strength, flexural strength and fracture energy were 81.9±1.2 MPa, 125.7±2.5 MPa and 2.20 MJ/m3, increasing 11.4 MPa, 20.1 MPa and 0.74 MJ/m3, respectively. |
