Preparation Of Graphene-based Hybrids And Their Polypropylene Nanocomposites And Investigation On Thermal Stability And Combustion Properties

As a new member of carbon family, graphene has attracted considerable attention in the field of graphene/polymer nanocomposites (GPNs), due to its excellent electronic, electrical, thermal and mechanical properties. GPNs have become one of the most promising applications of graphene. Significant efforts have been made to develop GPNs with good dispersion and improved properties. However, some important factors like dispersion and interfacial interactions between graphene and polymer matrices still restrict the achievement of better performance for GPNs, especially nonpolar polyolefin. Polypropylene (PP), one of the largely consumed plastic, is widely used in daily life and industry. However, its inherent inflammability has restricted its application in some fields. Therefore, it is imperative to improve its flame retardancy. The application of nanotechnology in flame retardant polymer has been regarded as one of the most important advancements in this research field. The potential application of graphene as a flame retardant nanomaterial has been explored. However, its flame retardant performance is not marked as compared with conventional layered materials, such as montmorillonite. The relatively low flame retardant efficiency for graphene may be ascribed to inferior dispersion, poor resistance to oxidation and simple flame retardant mechanism.During the thermal decomposition or combustion, the defects and vacancies are formed on the graphene nanosheets from the thermal removal of oxygen functional groups, resulting in weaken the barrier effect of graphene. In this dissertation, non-covalent and covalent functionalization approaches are adopted to modify surface feature of graphene, and then to improve the flame retardancy of PP. The graphitizable modifiers including melamine (MA) and polyaniline (PANI) are in situ decomposed to carbon based protective layer coating on graphene nanosheets, which strengthen its barrier performance. Covalent functionalization of graphene with polymer can decrease the cohesive energy between graphene and render uniform dispersion. Enhanced radical trapping ability of graphene by the absorbed phosphomolybdic acid (PMoA) is expected to endow PP nanocomposites with improved thermal oxidative stability and flame retardancy. Phosphorus and nitrogen containing flame retardant and transition metal nanomaterials with catalytic carbonization function are decorated on the graphene to endow it with different flame retardant mechanisms. Graphene is combined with intumescent flame retardant (IFR) and the flammability and combustion behavior at different fire scenarios are investigated. The research work in this dissertation is composed of the following parts.1. MA and PMoA are employed to non-covalently functionalize graphene for preparing PP nanocomposites with enhanced flame retardant properties. The strong interactions including π-π interactions, hydrogen bonding and electrostatic attraction, enables the tight absorption of MA on graphene oxide (GO) nanosheets. It can be observed that the functionalized graphene oxide (FGO) is homogeneously dispersed in PP matrix with intercalation and exfoliation microstructure. The FGO/PP nanocomposite exhibits higher thermal stability and flame retardant property than those of the GO counterpart. In comparison to neat PP, a 29.2% decrease in peak heat release rate (PHRR) is obtained by the incorporation of 2 wt% FGO. The same loading of GO just slightly reduce PHRR value. The sublimation of MA is inhibited by GO and the intercalated MA is condensed to graphitic carbon nitride (g-C3N4), due to the barrier effect of GO. The in situ formed g-C3N4 coating on graphene nanosheets can negate the negative influence of the defects on graphene formed during the combustion. Thus, the g-C3N4 nanosheets provide protective layer on graphene and enhance its barrier effect. The heat release rate and the escape of volatile degradation products are reduced in the FGO-based nanocomposites. PMoA is immobilized on the graphene nanosheets via electrostatic interactions. The incorporation of the graphene nanohybrid into PP matrix not only obviously increases flame retardancy and thermal oxidative stability, but also enhances stiffness and heat deflection temperature. Compared to neat sample, the onset decomposition temperature and the temperature at maximum weight loss rate of the nanocomposite increase by as much as 44 ℃ and 34 ℃, respectively, at just 1 wt% loading of the nanohybrid. The nanohybrid exhibits more marked reinforcing effects than the graphene. The heat release of the nanocomposites during the combustion is considerably reduced compared to neat PP. The enhancements in thermal oxidative stability and flame retardant properties of PP nanocomposites are predominantly ascribed to the barrier effect of graphene and enhanced radical capturing property of the nanohybrid.2. Using p-phenylenediamine (PPD) and cyanuric chloride as modifiers, the grafting of maleic anhydride grafted polypropylene (MAPP) onto graphene nanosheets is achieved by the reaction between amino groups in the grafted PPD and maleic anhydride groups. Because of the presence of the polymeric compatibilizer, homogeneous dispersion of FGO and strong interfacial interactions are achieved. The effects of FGO on the crystallization behavior, thermal stability, flame retardancy, mechanical and thermomechanical properties are investigated. A significant enhancement in thermal stability of the nanocomposites is obtained at low FGO loadings. The mechanisms for improving thermal stability are fully demonstrated. However, the elongation at break of nanocomposites is decreased, and tensile strength shows no change with increasing loading of FGO. The detailed mechanism for the decreased elongation at break and the unchanged tensile strength of nanocomposites is proposed according to the study on the mobility of polymer chain and lamellae. In addition, the flame retardant property of the nanocomposites is improved to a certain extent.3. To enhance barrier effect and dispersion of graphene simultaneously, graphitizable PANI nanofibers are used to modify graphene surface using "grafting from" approach. The grafted PANI can not only enhance the dispersion, but also improve the electrical conductivity of the FGO, due to the conductive characteristics of PANI. The PANI-RGO can reduce obviously the flammability of PP, including the reductions in PHRR and total heat release (THR). The smoke release of the nanocomposites during the combustion in cone tests is also decreased, indicating the enhanced fire safety by the incorporation of PANI-RGO. Because of its conjugated structure, PANI is readily formed into graphitic materials by way of the graphitization process. Carbon nanofibers are in situ formed on the graphene nanosheets, thus, the barrier effect of the graphene is reinforced. Therefore, the heat release rate of the nanocomposites is greatly decreased and the smoke emission is also retarded. Furthermore, high electrical conductivity of the nanocomposites is obtained with the addition of PANI-RGO, due to the enhanced dispersion and the high conductivity of PANI.4. A phosphorus and nitrogen containing flame retardant, polyphosphazene, is covalently grafted onto the surfaces of GO via the "grafting from" strategy. Then, Ni(OH)2 are loaded on the nanosheets through the strong interactions between Ni2+ and the amino groups in the phosphazene flame retardant. This work is to take advantages of high flame retardant efficiency for this flame retardant and transition metal with catalytic carbonization effect. Because of the organic modification and collaborative dispersion effects between FGO and Ni(OH)2, the functionalized graphene exhibits uniform distribution and homogeneous dispersion in the matrix. A significant improvement in char yield is achieved by the addition of FGO. The PHRR and THR are greatly reduced by the FGO, especially FGO2, which is modified with phosphazene flame retardant and Ni(OH)2. The strategy of FGO2 combines several flame retardant mechanisms:physical barrier of graphene, phosphorus-nitrogen synergy and catalytic carbonization effect, resulting in significant improvement in flame retardancy.5. Graphene is expected to act as a synergistic agent with IFR to further reduce the flammability of PP. The flammability tests and fire behavior are investigated under different fire scenarios. In the small flame tests, the incorporation of graphene results in the gradually decreased limiting oxygen index values and deteriorated UL-94 rating. However, in the combustion tests under forced-flaming scenario, the loading of graphene (0.5 and 1 wt%) can further reduce the PHRR and improve fire safety. The higher graphene content (2 wt%) in the IFR system results in the worse flame retardancy than that of neat IFR/PP. Furthermore, the smoke density of the flame retardant PP nanocomposites is markedly decreased by the incorporation of graphene. The thermal decomposition products at condensed and gaseous phase are analyzed by real time Fourier transform infrared spectra and thermogravimetric analysis-infrared spectrometry techniques, respectively. The chemical composition and microstructure of char residues from the combustion in cone tests are assessed. Due to the excellent barrier effect of graphene, the release of pyrolysis gas products including flammable and noninflammable gases is greatly inhibited. The graphene is observed to enhance the compactness and graphitization degree of the final char. Owing to the barrier function of graphene, more phosphorus and nitrogen are available for char formation. Graphene can considerably increase the melt viscosity of IFR/PP system. The low loading of graphene is observed to enhance the intumescence of the char, and the swelling of char is inhibited, when higher content graphene is added. The mechanisms for these abnormal flame retardant performances are concluded. When optimum loading of graphene is added, compactness, mechanical properties of the char and melt viscosity of the matrix are appropriately enhanced, resulting in the enhancements in swelling of char and flame retardancy. The superabundant graphene is harmful to reduce heat release rate, due to excessively strengthened char.