Xiaowei Mu

Flame-retardant-wrapped polyphosphazene nanotubes: A novel strategy for enhancing the flame retardancy and smoke toxicity suppression of epoxy resins.

The structure of polyphosphazene nanotubes (PZS) is similar to that of carbon nanotubes (CNTs) before modification. For applications of CNTs in polymer composites, surface wrapping is an economically attractive route to achieve functionalized nanotubes. Based on this idea, functionalized polyphosphazene nanotubes (FR@PZS) wrapped with a cross-linked DOPO-based flame retardant (FR) were synthesized via one-step strategy and well characterized. Then, the obtained FR@PZS was introduced into epoxy resin (EP) to investigate flame retardancy and smoke toxicity suppression performance. Thermogravimetric analysis indicated that FR@PZS significantly enhanced the thermal stability of EP composites. Cone calorimeter results revealed that incorporation of FR@PZS obviously improved flame retardant performance of EP, for example, 46.0% decrease in peak heat release rate and 27.1% reduction in total heat release were observed in the case of epoxy composite with 3wt% FR@PZS. The evolution of toxic CO and other volatile products from the EP decomposition was significantly suppressed after the introduction of FR@PZS, Therefore, the smoke toxicity associates with burning EP was reduced. The presence of both PZS and a DOPO-based flame retardant was probably responsible for this substantial diminishment of fire hazard.

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.

Mussel-inspired functionalization of electrochemically exfoliated graphene: based on self-polymerization of dopamine and its suppression effect on the fire hazards and smoke toxicity of thermoplastic polyurethane

The suppression effect of graphene in the fire hazards and smoke toxicity of polymer composites has been seriously limited by both mass production and weak interfacial interaction. Though the electrochemical preparation provides an available approach for mass production, exfoliated graphene could not strongly bond with polar polymer chains. Herein, mussel-inspired functionalization of electrochemically exfoliated graphene was successfully processed and added into polar thermoplastic polyurethane matrix (TPU). As confirmed by SEM patterns of fracture surface, functionalized graphene possessing abundant hydroxyl could constitute a forceful chains interaction with TPU. By the incorporation of 2.0 wt % f-GNS, peak heat release rate (pHRR), total heat release (THR), specific extinction area (SEA), as well as smoke produce rate (SPR) of TPU composites were approximately decreased by 59.4%, 27.1%, 31.9%, and 26.7%, respectively. A probable mechanism of fire retardant was hypothesized: well-dispersed f-GNS constituted tortuous path and hindered the exchange process of degradation product with barrier function. Large quantities of degradation product gathered round f-GNS and reacted with flame retardant to produce the cross-linked and high-degree graphited residual char. The simple functionalization for electrochemically exfoliated graphene impels the application of graphene in the fields of flame retardant composites.

Construction of multifunctional MoSe2 hybrid towards the simultaneous improvements in fire safety and mechanical property of polymer

Organic modification of MoSe2 sheets is firstly achieved by Atherton-Todd reaction, aiming at the acquisition of multifunctional MoSe2 hybrid. Simultaneous enhancements in fire safety and mechanical property of thermalplastic polyurethane (TPU) are obtained with the presence of this hybrid. Strong interfacial interactions between the functionalized MoSe2 sheets and TPU can be obtained, making more efficient load transfer from the weak polymer chains to the robust sheets. Besides, more coherent barrier network may be formed in polymer matrix, restraining the diffusion of decomposed fragments and reducing the supply for combustion fuel. Consequently, the decreases in heat release are observed for polymer composites. Notably, the releases of toxic gases, such as HCN and CO, are also suppressed by this barrier network, resulting in the reductions in fire toxicity. This work may open a new door for the functionalization of MoSe2 sheets and evoke significant developments in its promising applications.

The effect of doped heteroatoms (nitrogen, boron, phosphorus) on inhibition thermal oxidation of reduced graphene oxide

The doping of nitrogen into reduced graphene oxide (NRGO) is achieved by reduction of graphene oxide with hydrazine hydrate and ammonium hydroxide. Boron (BRGO) or phosphorus (PRGO) doped reduced graphene oxide (RGO) is obtained by annealing of RGO (prepared by reduction with sodium borohydride) with boric acid or phosphoric acid, respectively. The successful preparation of the doped RGO is confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy. A remarkable enhancement in thermal oxidative stability of RGO is achieved by doping of these heteroatoms. The enthalpy values of BRGO and PRGO during the thermal oxidation decrease remarkably compared with that of RGO, indicating the reduced heat release by the doped heteroatoms. The mechanism for improvement in thermal oxidative resistance by doping of heteroatoms is demonstrated clearly. Doping of boron atom not only lowers the electrons density on the reactive carbon atoms and the Fermi level of carbon, but also contribute to graphitization of RGO, leading to inhibition of thermal oxidation of RGO. Phosphorus containing complexes, in the forms of metaphosphates, C–O–PO3 and C–PO3 groups, can poison the active sites on RGO and function as physical barrier for the access of oxygen. Retarding oxidation of RGO against air may be strengthened by forming more thermostable structure in NRGO, for example: pyrrolic-N (NC2), pyridinic-N (NC2) and graphitic-N (NC3). The doping of heteroatoms will provide an important strategy for broadening RGO application at the elevated temperature.

Preparation of UV-curable functionalized phosphazene-containing nanotube/polyurethane acrylate nanocomposite coatings with enhanced thermal and mechanical properties

Poly(cyclotriphosphazene-co-4,4′-sulfonyldiphenol) (PZS) nanotubes with active hydroxyl groups were fabricated via an in situ template method under mild conditions, and then modified by acryloyl chloride to obtain the acryloyl-group functionalized PZS (f-PZS) nanotubes. The structure of the PZS nanotubes was characterized by Fourier transform infrared spectroscopy and the morphology was investigated by scanning electron microscopy and transmission electron microscopy. The f-PZS/polyurethane acrylate (f-PZS/PUA) nanocomposite coatings were prepared by UV radiation technology to covalently introduce f-PZS nanotubes into a PUA matrix. Dynamic mechanical analysis and tensile tests were performed to characterize the mechanical properties of the f-PZS/PUA nanocomposite coatings. The optimal reinforcing effect for the PUA matrix was observed when the content of f-PZS nanotubes was 3.0 wt%. The thermal stability of the PUA nanocomposites was studied by thermo gravimetric analysis. It indicates that the onset thermal degradation temperature of the f-PZS/PUA nanocomposites with 1.0 wt% f-PZS nanotubes is increased by 36.3 °C. These remarkable property reinforcements are attributed to the covalent functionalization of PZS nanotubes, which can effectively improve the interfacial interaction between the f-PZS nanotubes and the PUA matrix.

Novel Melamine/o-Phthalaldehyde Covalent Organic Frameworks Nanosheets: Enhancement Flame Retardant and Mechanical Performances of Thermoplastic Polyurethanes

Covalent organic frameworks (COFs) nanosheets prepared from condensation reaction between melamine and o-phthalaldehyde are first prepared through ball milling and then incorporated into thermoplastic polyurethanes (TPU) by solution mixing. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) spectrometer are applied to characterize COFs nanosheets. It is observed apparently from TEM image that COFs nanosheets are obtained. Successful preparation of COFs nanosheets is proved further by vanishment of typical diffraction peak of COFs at around 23.5° in COFs nanosheets XRD pattern, appearance of quadrant and semicircle stretching of the s-triazine ring at 1568 and 1469 cm-1 in FTIR spectra and N═C bond at 389.5 eV in N1s high-resolution XPS spectra of COFs nanosheets. The thermal property, combustion behavior and mechanical performance of TPU naoncomposites are also investigated. Incorporation of COFs nanosheets into TPU contributes to char forming of TPU under nitrogen atmosphere and 14.3% decrease of peak heat release rate of TPU. Besides, the elongation at break, Youngs modulus, and fracture strength of TPU nanocomposites increase sharply compared with that of neat one.

Novel incorporation of mesoporous NiCo2O4 into thermoplastic polyurethane for enhancing its fire safety

In this work, two kinds of NiCo2O4 particles are synthesized and their structures are confirmed from X-ray diffraction patterns and Fourier transform infrared spectra. The excellent dispersion of the particles in thermoplastic polyurethane (TPU) is confirmed by transmission electron microscopy. Char residue yield of TPU is enhanced obviously after the incorporation of NiCo2O4, suggesting a catalyzing carbonization effect. Toxic gases released from decomposition of the polymer, such as HCN, are fatal to humans, so it is of great significance to evaluate the toxic gases from TPU composites by thermogravimetric analysis/infrared spectrometry. The release of HCN is inhibited effectively by NiCo2O4, and this can be attributed to the gas adsorbing and barrier effects of the mesoporous particles. In addition, the amount of flammable gases is also reduced markedly. Moreover, the peak heat release rate and total heat release of TPU are decreased appreciably by NiCo2O4 with a higher specific surface area.

Manganese phytate dotted polyaniline shell enwrapped carbon nanotube: Towards the reinforcements in fire safety and mechanical property of polymer

High fire hazard of epoxy resin (EP) has been an unavoidable obstruction on its wide application. Here, a manganese phytate dotted polyaniline shell enwrapped carbon nanotube (MPCNT) is facilely constructed and employed as flame retardant for EP. By adding 4.0 wt% MPCNT, the peak heat release rate, total heat release values, peak CO yields and total CO yields are decreased by 27.2, 12.3, 44.8, and 23.3%, respectively. The decreased absorbance intensity of toxic aromatic volatiles is also observed. Then, a tripartite cooperative flame retardant mechanism (a continuous barrier network, catalytic charring function of phytate, and catalytic activity of MnP/C system) is proposed. Furthermore, the storage modulus of EP composites with 2.0 and 4.0 wt% MPCNT are increased by 23.0 and 25.8% at 40 °C, respectively. Thus, the simultaneous reinforcements in fire safety and mechanical performance of EP are successfully achieved. This work may represent a significant step forward in the facile construction of functionalized carbon materials for achieving their whole potentials in polymer-matrix composite.