Stduy On The Preparation And Pyrolysis Behavior Of High Strength And Toughness Polybenzoxazine/Polyhexahydrotriazine,and Combustion Behavior Of Its Composites

In order to solve the problems of high brittleness and difficulty in film formation of polybenzoxazine.An IPN（interpenetrating cross-linked network）structure was prepared using polybenzoxazine（PDMB）and polyhexahydrotriazine（PHT）via a simple,controllable sequential curing method.And PDMB/PHT IPN was used to prepare glass fiber reinforced composites.The relationship between the structure and performance of IPN was explored by analyzing the thermal properties,mechanical properties,and flame retardancy.Then,the pyrolysis mechanism and the key structures leading to flame retardancy were carried out.The main research content is as follows:（1）The benzoxazine monomer（DMB）was synthesized from 4,4’-diamindiphenyl-methane,phenol and formaldehyde.Then DMB was blended with 4,4’-（1,3-phenylenedioxy）-dianiline and formaldehyde to prepare PDMB/PHT IPN films.When the molar ratio of DMB is 0.8,the elongation at break and tensile strength reach 12.40%and 118.67 MPa,respectively,which effectively solved the drawbacks of brittleness and inability to form films.The glass transition temperature reaches 238°C.The independent cross-linking and self-curing reaction sequence of PDMB and PHT can synergistically enhance the mechanical and thermal properties.（2）PDMB/PHT-0.8 was used as a resin matrix to prepare glass fiber reinforced composites PDMB/PHT-GF.The combustion behaviors of the glass fiber reinforced composites was studied using cone calorimetry testing,ultimate oxygen index testing and vertical combustion testing.The strengthening effect on gas-phase flame retardancy of the PHT structure was verified by analyzing the flame retardant mechanism.The results show that PDMB-based composite has a heat release rate of 76.27 k W/m² and total heat release of and 1.5 MJ/m²,and reaches a limit oxygen index of 63.0%.The flame retardancy of the PHT modified composite is further improved,with the heat release rate reducing to 61.55 k W/m²and total heat release rate reducing to 1.15 MJ/m²,at a decreasing rate of 19.3%and 23.3%,respectively.Meanwhile,the ultimate oxygen index increases to 86.6%.（3）The pyrolysis behaviors and mechanisms of PDMB/PHT IPN system were studied by TGA-FTIR and Py-GC/MS.The TGA-FTIR results show that the char yield of PDMB/PHT IPN reaches 42.6%under nitrogen atmosphere,which is close to PDMB（43.1%）,indicating that the modification of PHT can effectively delay the release of pyrolysis products of PDMB.The release intensity of ammonia in the system is significantly higher than that of PDMB,promoting the isolation of O₂ and the flame retardancy of PDMB/PHT IPN.The Py-GC/MS results show the modification of PHT brings new pyrolysis products such as 4,4’-diaminobenzophenone and a benzofuran structure containing imidazole,which will promote the carbonization of PDMB at high temperatures.The two test results mentioned above indicate that the modification of PHT is expected to enhance the synergistic effect on gas phase flame retardancy and condensed phase flame retardancy of benzoxazine-based materials.