Functionalization Of Graphene And Its Epoxy Composites, And Study On Their Flame Retardant Properties And Mechanism

In order to overcome the low flame retardant efficiency and the deterioration effect on the thermal stability and mechanical behaviors caused by the conventional flame retardants, some functionalized graphene materials with flame retardant properties have been synthesized and applied into preparing high-performance epoxy nanocomposites, on the basis of the latest research progress on the flame retardant epoxy and graphene/polymer composites. Based on the molecular design, the phosphorus-, nitrogen-and silicon-containing compounds were grafted onto the surface of graphene to obtain functionalized graphene with flame retardant properties; moreover, functionalized graphene materials with the catalysis of char formation were also prepared. The thermal degradation and flammability behaviors of functionalized graphene/epoxy nanocomposites were investigated, and the flame retardant mechanism was clarified. The main research work of this dissertation was illustrated as follows:1. Silane-functionalized graphene nanosheets (f-GNSs) were synthesized by the dehydration reaction between3-aminopropyl triethoxysilane (APTS) and graphene. Subsequently, f-GNSs were covalently incorporated into epoxy to obtain the nanocomposites. The results from the solid-state29Si NMR spectra and FTIR spectra indicated that epoxy resins were chemically linked into graphene through APTS as the "bridge". The addition of f-GNSs improved the thermal stability and tensile strength of epoxy matrix, and the reinforcement can be attributed to two aspects:the silane chain grafted at the GNS surface can prevent stacking and aggregation of GNSs, thus improved the dispersion state of the graphene sheets in EP matrix; the amine functional groups of f-GNSs were expected to form the chemical bonding to epoxy resins, consequently, the interfacial interaction between different components became stronger.2. Based on the molecular design, the phosphorus-and nitrogen-containing compound was synthesized and grafted onto the surface of graphene to obtain polyphosphoramide-g-graphene (PPA-g-GNS). A series of epoxy nanocomposites with different content of PPA-g-GNS were prepared, and the influence of PPA-g-GNS on the LOI, HRR and the structure of char of epoxy nanocomposites was studied. The results demonstrated that incorporating PPA-g-GNS could enhance the LOI and tensile strength, reduce the PHRR and THR values. This is believed to be attributed to the PPA-g-GNS catalyzed the degradation of epoxy to form an intumescent char layer on the surface of inner polymer matrix, which inhibited the heat and mass transfer between solid and gas phases, delayed the degradation of inner polymer matrix, and thus retarded the combustion.3. Octa-aminophenyl polyhedral oligomeric silsesquioxanes (OapPOSS) functionalized graphene (OapPOSS-rGO) was synthesized by the reaction between OapPOSS and GO, and then the epoxy nanocomposites with the same content of OapPOSS, GO and OapPOSS-rGO were prepared. The XPS and FTIR data suggested that GO were partly reduced by OapPOSS. From TEM images, it can be observed that OapPOSS-rGO was well dispersed in the epoxy matrix. TGA results indicated that OapPOSS-rGO/EP exhibited better thermal stability compared to GO/EP and OapPOSS/EP. The parameters obtained from Cone showed that the addition of OapPOSS-rGO significantly decreased the PHRR, THR, COPR and FIGRA of epoxy nanocomposites, suggesting the reduced fire hazard of the materials. This dramatically reduced fire hazards was mainly attributed to the synergestic effect of OapPOSS-rGO:graphene promoted the formation of graphitized and compact char layer, while OapPOSS improved the thermal oxidative resistance of the char layer.4. Ni-Fe layered double hydroxide/graphene (Ni-Fe LDH/GNS) hybrid material was successfully synthesized by a co-precipitation route, and then incorporated into epoxy resins. The influence of Ni-Fe LDH/GNS on the thermal stability, HRR and the structure of char of epoxy nanocomposite were investigated, and the flame retardant mechanism was clarified from both gas and condensed phase. The parameters obtained from Cone showed that the addition of Ni-Fe LDH/GNS significantly decreased the PHRR, THR, AMLR and FIGRA of epoxy nanocomposites, suggesting the reduced fire hazard of the materials. TG-IR results showed that, the main decomposition products of Ni-Fe LDH/GNS/EP were water, CO2, CO, hydrocarbons, acetone, and aromatic compounds, which are similar to those of EP; however, the release of the flammable and toxic products for Ni-Fe LDH/GNS/EP is much lower than that for EP. This dramatically reduced fire hazards was mainly attributed to the synergestic effect of Ni-Fe LDH/GNS:graphene promoted the formation of graphitized and compact char layer, while Ni-Fe LDH improved the thermal oxidative resistance of the char layer. The char layer with high thermal oxidative resistance and compact structure could effectively inhibit the energy and mass transfer between the flame and matrix, delayed the degradation of inner polymer, and thus retarded the combustion.5. Metal oxide decorated graphene hybrid materials, Co3O4-graphene and SnO2-graphene, were synthesized by co-precipitation method and applied to flame retardant epoxy resins. Incorporation of2wt%metal oxide/graphene hybrids leads to a37and27℃increment in the onset degradation temperature of SnO2-graphene/EP and Co3O4-graphene/EP, respectively, compared to that of pure EP. The barrier effect of graphene is believed to be the crucial factor for improving the thermal properties. Meanwhile, the addition of Co3O4-graphene and SnO2-graphene could reduce the release amount of flammable gases, carbon monoxide and smoke density, which is probably attributed to the synergism between the catalysis effect of metal oxide and the adsorption effect of graphene.