Weiyi Xing

Preparation of graphene by pressurized oxidation and multiplex reduction and its polymer nanocomposites by masterbatch-based melt blending

Graphene is prepared from graphite by pressurized oxidation and multiplex reduction. The pressurized oxidation is advantageous in easy operation and size-control, and the multiplex reduction, based on ammonia and hydrazine, produces single-atom-thick graphene (0.4–0.6 nm thick) which can be directly observed by atomic force microscopy. A masterbatch strategy, which is feasible in “soluble” thermoplastic polymers, is developed to disperse graphene into poly(lactic acid) by melt blending. The graphene is well dispersed and the obtained nanocomposites present markedly improved crystallinity, rate of crystallization, mechanical properties, electrical conductivity and fire resistance. The properties are dependent on the dispersion and loading content of graphene, showing percolation threshold at 0.08 wt%. Graphene reinforces the nanocomposites but cuts down the interactions among the polymer matrix, which leads to reduced mechanical properties. Competition of the reinforcing and the reducing causes inflexions around the percolation threshold. The roles of the heat barrier and mass barrier effects of graphene in the thermal degradation and combustion properties of the nanocomposites are discussed and clarified.

In situ polymerization of graphene nanosheets and polyurethane with enhanced mechanical and thermal properties

Polyurethane (PU) composites reinforced with graphene nanosheets (GNSs) were prepared by in situpolymerization. Graphene nanosheets, which were derived from chemically reduced graphite oxide (GO) nanosheets, were characterized by solid-state 13C MAS NMR, XRD and FTIR. A morphological study showed that, due to the formation of chemical bonds, the GNS was dispersed well in the PU matrix. With the incorporation of 2.0 wt% of GNSs, the tensile strength and storage modulus of the PU increased by 239% and 202%, respectively. The nanocomposites displayed high electrical conductivity, and good thermal stability of PU was also achieved. The facile and rapid technique presented here will provide an effective and promising method of preparing graphene-based polymer composites.

Simultaneous reduction and surface functionalization of graphene oxide with POSS for reducing fire hazards in epoxy composites

Simultaneous reduction and surface functionalization of graphene oxide (GO) was realized by simple refluxing of GO with octa-aminophenyl polyhedral oligomeric silsesquioxanes (OapPOSS) without the use of any reducing agents. The presence of OapPOSS made the hydrophilic GO hydrophobic, as evidenced by the good dispersion of the OapPOSS-reduced GO (OapPOSS-rGO) in tetrahydrofuran solvent. The structure of OapPOSS-rGO was confirmed by XPS, FTIR and TEM. A morphological study showed that, due to the good interfacial interaction between the functionalized graphene and epoxy, OapPOSS-rGO was dispersed well in the matrix. With the incorporation of 2.0 wt% of OapPOSS-rGO, the onset thermal degradation temperature of the epoxy composite was significantly increased by 43 °C. Moreover, the peak heat release rate, total heat release and CO production rate values of OapPOSS-rGO/EP were significantly reduced by 49%, 37% and 58%, respectively, compared to those of neat epoxy. This dramatically reduced fire hazards was mainly attributed to the synergestic effect of OapPOSS-rGO: the adsorption and barrier effect of reduced graphene oxide inhibited the heat and gas release and promoted the formation of graphitized carbons, while OapPOSS improved the thermal oxidative resistance of the char layer.

Enhanced thermal and flame retardant properties of flame-retardant-wrapped graphene/epoxy resin nanocomposites

Functionalized reduced graphene oxide (FRGO) wrapped with a phosphorus and nitrogen-containing flame retardant (FR) was successfully prepared via a simple one-pot method and well characterized. Subsequently, FRGO was covalently incorporated into epoxy resin (EP) to prepare flame retardant nanocomposites. The FRGO was well dispersed in the matrix and formed strong interfacial adhesion. Thermogravimetric analysis results revealed that the presence of RGO, FR or FRGO in an EP matrix led to a slight thermal destabilization effect under air and nitrogen, which increased the char yield at 700 °C and reduced the maximum mass loss rate. Furthermore, the glass transition temperature of the FRGO/EP nanocomposite with an FRGO loading of 4 wt% (FRGO/EP4) was remarkably increased by 29.6 °C, probably due to the improved crosslinking density and confinement effect of graphene sheets on the mobility of polymer networks. The evaluation of combustion behavior demonstrated that a 43.0% reduction in the peak heat release rate (PHRR) for the FRGO/EP nanocomposite containing 2 wt% FRGO and a 30.2% reduction in the total heat release (THR) for FRGO/EP4 over pure EP were achieved by the addition of FRGO. These notable reductions in fire hazards were mainly due to the synergistic effect of FRGO and the flame retardant: the wrapped flame retardant accelerated the degradation of the EP matrix, promoting the formation of additional char residues; the flame retardant improved the thermal oxidative resistance of the graphene; a high-thermal-stability char layer, consisting of graphene sheets, retarded the permeation of heat and the escape of volatile degradation products.

Self-assembly of Ni–Fe layered double hydroxide/graphene hybrids for reducing fire hazard in epoxy composites

Ni–Fe layered double hydroxide/graphene hybrids were synthesized by a one-pot in situ solvothermal route. X-ray diffraction and X-ray photoelectron spectroscopy analyses showed that the formation of Ni–Fe layered double hydroxide (Ni–Fe LDH) and the reduction of graphene oxide occurred simultaneously during the one-pot solvothermal process. TGA results showed that the incorporation of Ni–Fe LDH significantly improved the thermal stability of the graphene. Subsequently, Ni–Fe LDH/graphene hybrids were introduced into epoxy resins for reducing their fire hazard. With the incorporation of 2.0 wt% of Ni–Fe LDH/graphene, the onset thermal degradation temperature of the epoxy composite was significantly increased by 25 °C compared to that of pure epoxy. Also, the addition of Ni–Fe LDH/graphene hybrids imparted excellent flame retardant properties to the epoxy matrix, evidenced by the dramatically reduced peak heat release rate and total heat release values obtained from a micro combustion calorimeter and cone calorimeter. This dramatically reduced fire hazard was mainly attributed to the synergistic effects of Ni–Fe LDH/graphene hybrids: the adsorption and barrier effect of graphene slowed down the thermal degradation of the polymer matrix, inhibited the heat and flammable gas release and promoted the formation of graphitized carbons, while Ni–Fe LDH improved the thermal oxidative resistance of the char layer.

Functionalization of graphene with grafted polyphosphamide for flame retardant epoxy composites: synthesis, flammability and mechanism

A polyphosphamide (PPA) was synthesized and covalently grafted onto the surface of graphene nanosheets (GNSs) to obtain a novel flame retardant, PPA-g-GNS, and subsequently PPA-g-GNS was incorporated into epoxy resins (EPs) to enhance the fire resistance. The chemical structures and morphology of the precursors and target product were confirmed using 1H-NMR spectroscopy, Fourier transform infrared spectroscopy and atomic force microscopy. The tensile results showed that the mechanical strength and modulus of the PPA-g-GNS/EP composite were higher than those of pure EP and PPA/EP, owing to the outstanding reinforced effect of graphene. The evaluation of the thermal properties demonstrated that the addition of PPA or PPA-g-GNS to epoxy had a thermal destabilization effect below 400 °C, but led to a higher char yield at higher temperatures. Furthermore, the PPA-g-GNS/EP composite exhibited superior fire resistant performance, such as higher LOI values and reduced PHRR and FIGRA values, compared to pure EP and PPA/EP, which was probably attributed to the higher char yield during combustion. A possible flame retardant mechanism was speculated according to the direct pyrolysis-mass spectrometry results: the phosphate species degraded from PPA catalyzed the decomposition of the PPA-g-GNS/EP composites to generate various pyrolysis products; the pyrolysis products were absorbed and propagated on the graphene which served as a template of micro-char, and thus a continuous and compact char layer was formed; such a char layer provided effective shields to protect the underlying polymers against flame.

Cobalt oxide/graphene composite for highly efficient CO oxidation and its application in reducing the fire hazards of aliphatic polyesters

Tricobalt tetraoxide-functionalized graphene composites (Co3O4/graphene) were prepared to reduce the fire hazards of aliphatic polyesters. Characterization of the Co3O4/graphene by X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscope (TEM) and atomic force microscopy (AFM) confirmed the chemical structure. The incorporation of Co3O4/graphene into both poly(butylene succinate) (PBS) and polylactide (PLA) improved the initial degradation temperature, and slowed down the thermal decomposition process. The heat release rate of PBS-Co3O4/graphene and PLA-Co3O4/graphene composites were reduced by 31% and 40%, respectively, compared with that of the pure PBS and PLA. Moreover, the addition of Co3O4/graphene significantly decreased the gaseous products, including hydrocarbons, carbonyl compounds and carbon monoxide, which is attributed to the combined properties of the barrier effect and high catalytic activity for CO oxidation of Co3O4/graphene.

Functionalized graphene oxide/phosphoramide oligomer hybrids flame retardant prepared via in situ polymerization for improving the fire safety of polypropylene

A novel strategy based on functionalized graphene oxide (FGO)/phosphoramide oligomer flame retardant was developed to overcome the challenges of the dispersion of graphene sheets in polymer matrix and the ease of the burn-out of graphene under air atmosphere. Graphene oxide (GO) was modified by 4,4-diaminodiphenyl methane (DDM) and then in situ incorporated into phosphoramide oligomer, resulting in a nanocomposite flame retardant (FRs-FGO) containing exfoliated graphene. Subsequently, the flame retardant (FRs-FGO) was incorporated into polypropylene (PP) and simultaneously compatilized with PP-grafted maleic anhydride. TEM results showed that the FGO was dispersed more uniformly in PP than the bare GO because of the strong interfacial interaction and previous exfoliation of FGO in FRs before blending. The thermal properties investigated by thermogravimetric analysis (TGA) indicated that the addition of FRs-FGO into PP resulted in a significant improvement of thermal stability at elevated temperature with higher char yields. Moreover, the crystallization and fire safety properties of PP composites were also improved by the incorporation of FRs-FGO, including increased crystallization temperature (11.4 °C increase), reduced peak heat release rate (66.9% reduction) and decreased total heat release (24.4% decrease), and decreased fire growth rate index (73.0% decrease). The cone results indicated the simple blending of GO with FRs and exhibited less improvement in fire safety properties than FRs-FGO, which resulted from the improved dispersion and thermal stability of FGO sheets. The flame retardant mechanism was because of the shielding effect of FGO and char layers, which could reduce the release of combustible gases and inhibit the mass and heat transfer between the gas phase and condensed phase.

In situ synthesis of a MoS2/CoOOH hybrid by a facile wet chemical method and the catalytic oxidation of CO in epoxy resin during decomposition

In this work, a new MoS2/CoOOH hybrid material was successfully synthesized by a facile wet chemical method, and its structure was confirmed by X-ray diffraction and Raman spectroscopy. A morphological study showed that, due to the different sizes of the two components, the resulting MoS2/CoOOH hybrid displayed a disordered structure in which large MoS2 sheets had many independent and separate CoOOH nanoplatelets on the surface. The catalytic oxidation effect of MoS2/CoOOH hybrids on the thermal decomposition of epoxy resin was studied by thermogravimetric analysis-infrared spectrometry. It was found that the amount of organic volatiles of epoxy resin significantly decreased and non-flammable CO2 was generated after incorporating MoS2/CoOOH hybrids, which implied the reduced toxicity of the volatiles and obvious smoke suppression. Meanwhile, the incorporation of MoS2/CoOOH hybrids also resulted in a remarkable increase in the char residue of the epoxy composite, indicating the efficient catalytic carbonization of MoS2/CoOOH hybrids. Based on the X-ray diffraction and Fourier transform infrared results of the char residue, the possible mechanism of the reduced fire hazards and high char formation of the epoxy composites was proposed as the combination of the adsorption and synergistic catalytic effect of the MoS2/CoOOH catalyst, which would provide promising applications in the development of fire safety polymer materials.

Thermal exfoliation of hexagonal boron nitride for effective enhancements on thermal stability, flame retardancy and smoke suppression of epoxy resin nanocomposites via sol–gel process

The structure of hexagonal boron nitride (h-BN) is similar to that of graphite before functionalization and exfoliation. For applications in polymer nanocomposites, chemical exfoliation is a more economically attractive route to few-layer h-BN nanosheets. A thermal oxidation process of h-BN powder could achieve large scale exfoliation and hydroxylated functionalization, as described in prior literature. In this report, hydroxylated h-BN (BNO) was prepared by heating h-BN under air, and then covalently incorporated into epoxy resin modified with (3-isocyanatopropyl)triethoxysilane to prepare epoxy resin (EP) nanocomposites by sol–gel process. The structure and morphology of BNO were well characterized. BNO was dispersed in the EP matrix with the form of mainly exfoliated and intercalated structures, and formed strong interfacial interaction with the matrix. Thermogravimetric analysis results revealed that BNO significantly improved thermal stability and thermal oxidative resistance of EP nanocomposites at high temperature. The char yield and the temperature at 50 wt% mass loss were increased and the maximum mass loss rate was remarkably reduced. Moreover, the addition of 3 wt% BNO led to extremely high Tg of EP nanocomposite, 42.7 °C higher than that of pure EP, due to improved crosslinking density and confinement effect of BNO sheets on the mobility of polymer networks. Cone calorimeter test results indicated that fire safety properties of EP nanocomposites were also enhanced by the addition of BNO, such as 53.1% reduction in peak heat release rate and 32.6% decrease in total heat release, and decreased release of smoke and toxic gases. The mechanism for enhanced fire retardancy is that thermally stable condensed barrier consisting of h-BN sheets and silicon dioxide for heat and mass transfer protects the matrix from further combustion.