Bihe Yuan

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.

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.

Functionalized graphene oxide for fire safety applications of polymers: a combination of condensed phase flame retardant strategies

Graphene is promising for the fire safety applications of polymers, but the ease of burn out limits further developments. A novel strategy based on functionalized graphene oxide (FGO) is developed to overcome this challenge. Graphene oxide is functionalized with char-catalyzing agents and reactive compounds and incorporated into polystyrene. When FGO–polystyrene composites are degraded or burned, FGO catalyzes the char formation from polystyrene (Char A). Char A protects FGO from burning out and then FGO acts as a graphitic char (Char B). Because of the combination of Char A, Char B, the physical barrier effects of FGO, and the strong interfacial interactions of FGO and polymers, the fire safety properties of the FGO–polystyrene composites are improved, including decreased peak CO release rate (66% decrease), decreased peak CO2 release rate (54% decrease), decreased peak heat release rate (53% decrease), decreased thermal degradation rate (30% decrease), decreased total heat release (38% decrease), and increased char formation (7 times), etc. This strategy combines several condensed phase flame retardant strategies such as the nanocomposite technique, intumescent flame retardant systems and phosphorus–nitrogen synergism systems, and hence results in more significant improvements as compared with prior work.

Graphite oxide, graphene, and metal-loaded graphene for fire safety applications of polystyrene

Graphite oxide, graphene, ZrO2-loaded graphene and β-Ni(OH)2-loaded graphene (joint appellation: Gs) were prepared and incorporated into polystyrene so as to improve the fire safety properties of polystyrene. By the masterbatch-melt blending technique, Gs nanolayers were well dispersed and exfoliated in polystyrene as thin layers (thickness 0.7–2 nm). The fire safety properties were visibly improved, including an increased thermal degradation temperature (18 °C, PS/Ni–Gr-2), decreased peak heat release rate (40%, PS/Zr–Gr-2) and reduced CO concentration (54%, PS/Ni–Gr-2). The mechanism for the improved thermal stability and fire safety properties was investigated based on this study and previous works. The physical barrier effect of graphene, the interaction between graphene and polystyrene, and the synergistic effect of the metal compounds are the causes for the improvements.

Novel organic–inorganic flame retardants containing exfoliated graphene: preparation and their performance on the flame retardancy of epoxy resins

In this work, a simple and efficient approach for the preparation of organic–inorganic hybrids flame retardants (FRs-rGO), aiming at improving the flame retardant efficiency was presented. The reduced graphite oxide (rGO) was incorporated into the flame retardants matrix by in situ sol–gel process, resulting in the formation of organic–inorganic hybrids flame retardants containing exfoliated rGO. The TEM results of FRs-rGO hybrids revealed that the rGO was previously exfoliated in the phosphorus and silicon containing FRs. Subsequently, the flame retardant (FRs-rGO) was incorporated into epoxy resins (EP). The previous exfoliation of rGO in the FRs allows rGO to be intimately mixed with epoxy resins, which can be confirmed by the TEM results of FRs-rGO/EP nanocomposites. With the incorporation of 5 wt% of FRs-rGO into EP, satisfied flame retardant grade (V0) and the LOI as high as 29.5 were obtained. The char residues of the FRs-rGO/EP nanocomposites were significantly increased in air as well as nitrogen atmosphere. Moreover, the peak heat release rate (pHRR) value of FRs-rGO/EP was significantly reduced by 35%, and the glass transition temperature (Tg) of FRs-rGO/EP nanocomposites shifted to higher temperature, compared to those of neat EP. The flame retardancy strategy of FRs-rGO combines condensed phase and gas phase flame retardant strategies such as the nanocomposites technique, phosphorus–silicon synergism systems in the condensed phase and DOPO flame retardant systems in the gas phase. Moreover, the flame retardants containing exfoliated graphene (FRs-rGO) provided a novel method to prepare organic–inorganic hybrids flame retardants and the as-prepared flame retardants exhibited high flame retardant efficiency.

High-Performance Poly(ethylene oxide)/Molybdenum Disulfide Nanocomposite Films: Reinforcement of Properties Based on the Gradient Interface Effect

Herein, the molybdenum disulfide (MoS2) was simultaneously exfoliated and noncovalently functionalized by ultrasonication in a Pluronic aqueous solution and then was used to prepare the poly(ethylene oxide) (PEO) based nanocomposite films. The homogeneous dispersion of MoS2 and strong nanosheets/matrix interfacial adhesion were confirmed by representative electron microscopes. The considerable barrier action of the effective MoS2 nanosheets obviously restricted the ordering of crystal lamellae and the motion of polymer chains and then resulted in the formation of the devastated spherocrystal structure and morphological alterations in the nanocomposites, which were confirmed by polarized optical microscopy and the high value of the glass transition temperature. Importantly, MoS2 nanosheets hold great promise in reinforcing the thermal stability and mechanical property of polymer by increasing the effective volume of MoS2 nanosheets. A substantial reinforcement effect of PEO/MoS2 composite films was achieved: even at a relatively low loading level (0.9 wt %), 88.1% increase in Youngs modulus, 72.7% increase in stress-at-failure, and 62.1 °C increment of the temperature corresponding to half weight loss were obtained. These significant reinforcements can be attributed to the gradient interface region, which could effectively transfer the stress from the weak polymer chains to the robust nanosheets, thus endowing the PEO/MoS2 composite films with excellent properties.

Flame retardant and anti-dripping properties of polylactic acid/poly(bis(phenoxy)phosphazene)/expandable graphite composite and its flame retardant mechanism

Flame retardant polylactic acid (PLA) composites with poly(bis(phenoxy)-phosphazene) and expandable graphite are prepared by melt blending. Limiting oxygen index, UL-94 vertical burning test, cone calorimeter and thermogravimetric analysis are applied to characterize the flame retardant properties and thermal stability of PLA composites. This flame retardant system shows improved thermal stability, flame retardancy, synergy effect and anti-dripping performance. Raman spectroscopy, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy are employed to investigate the chemical structure and composition of the residual char of flame retardant PLA composites after cone calorimeter tests. The residual char is composed of graphite and phosphorus-containing materials. The gaseous phase mechanism of the flame retardant system is investigated with thermogravimetry/Fourier transform infrared spectroscopy and mass spectrometry. The radical species, such as C6H5OP˙, C6H5O˙ and PO2˙, are detected in the gaseous products of PLA composites. Thus, the flame retardant system exhibits both condensed and gas phase flame retardant action in the PLA composites.

Enhanced flame retardancy of polypropylene by melamine-modified graphene oxide

Graphene oxide (GO) is modified by melamine (MA) via the strong π–π interactions, hydrogen bonding, and electrostatic attraction. PP composites are prepared by melt compounding method, and GO/functionalized graphene oxide (FGO) is in situ thermally reduced during the processing. The results of scanning electron microscopy and transmission electron microscopy indicate that FGO nanosheets are homogeneously dispersed in polymer matrix with intercalation and exfoliation microstructure. The FGO/PP nanocomposite exhibits higher thermal stability and flame retardant property than those of the GO counterpart. During the thermal decomposition, the intercalated MA is condensed to graphitic carbon nitride (g-C3N4) in the confined micro-zone created by GO nanosheets. This in situ formed g-C3N4 provides a protective layer to graphene and enhances its barrier effect. The heat release rate and the escape of volatile degradation products are reduced in the FGO-based nanocomposites.

Solid acid-reduced graphene oxide nanohybrid for enhancing thermal stability, mechanical property and flame retardancy of polypropylene

Reduced graphene oxide (RGO) is functionalized with a solid acid, phosphomolybdic acid (PMoA), via electrostatic interactions. RGO and PMoA in this nanohybrid (PMoA–RGO) exhibit strong interactions and the surface characteristic of the graphene nanosheets is modified. RGO and PMoA–RGO are blended with polypropylene (PP) and maleic anhydride grafted polypropylene via a master batch-based melt mixing method. Thermal stability, mechanical and flame retardancy properties of the nanocomposites are investigated. This nanohybrid greatly improves the stiffness and thermal-oxidative stability of PP. Compared to the neat sample, the onset decomposition temperature (Tonset) and the temperature at the maximum weight loss rate (Tmax) of the nanocomposite increase by as much as 44 °C and 34 °C, respectively, at just 1 wt% loading of PMoA–RGO. Remarkable enhancements of the storage modulus in the glassy region and heat deflection temperature are obtained in PMoA–RGO/PP nanocomposites. The nanohybrid exhibits more marked reinforcing effects than the RGO. The heat release of the nanocomposites during the combustion is considerably reduced compared to neat PP. The improved thermal-oxidative stability and flame retardant properties of PP nanocomposites are mainly attributed to the barrier effect of graphene, in tandem with the enhanced radical trapping property of the nanohybrid.

Click-chemistry approach for graphene modification: effective reinforcement of UV-curable functionalized graphene/polyurethane acrylate nanocomposites

This work presents an effective method of preparing graphene reinforced UV-curable materials. Octamercaptopropyl polyhedral oligomeric silsesquioxane (OMP-POSS) functionalized reduced graphene oxide (FRGO) was prepared through a thiol–ene click approach and the FRGO/polyurethane acrylate (PUA) nanocomposite was fabricated by UV irradiation technology. The results of transmission electron microscopy and X-ray photoelectron spectroscopy indicated that OMP-POSS was successfully grafted onto the surface of graphene nanosheets. Thermal and mechanical properties were investigated by thermogravimetric analysis and dynamic mechanical analysis, respectively. The initial degradation temperature, storage modulus at −65 °C and glass transition temperature of FRGO/PUA1.0 nanocomposite were remarkably improved by 12 °C, 57.8% and 10 °C, respectively, compared to those of neat PUA. Those significant enhancements were attributed to the good dispersion of the nanofiller and the strong interfacial interactions between FRGO and PUA matrix.