Preparation of zinc hydroxystannate-decorated graphene oxide nanohybrids and their synergistic reinforcement on reducing fire hazards of flexible poly (vinyl chloride)
© Gao et al. 2016
Received: 12 January 2016
Accepted: 4 April 2016
Published: 12 April 2016
A novel flame retardant, zinc hydroxystannate-decorated graphene oxide (ZHS/GO) nanohybrid, was successfully prepared and well characterized. Herein, the ZHS nanoparticles could not only enhance the flame retardancy of GO with the synergistic flame-retardant effect of ZHS but also prevent the restack of GO to improve the mechanical properties of poly (vinyl chloride) (PVC) matrix. The structure characterization showed ZHS nanoparticles were bonded onto the surface of GO nanosheets and the ZHS nanoparticles were well distributed on the surface of GO. Subsequently, resulting ZHS/GO was introduced into flexible PVC and fire hazards and mechanical properties of PVC nanocomposites were investigated. Compared to neat PVC, thermogravimetric analysis exhibited that the addition of ZHS/GO into PVC matrix led to an improvement of the charring amount and thermal stability of char residue. Moreover, the incorporation of 5 wt.% ZHS/GO imparted excellent flame retardancy to flexible PVC, as shown by increased limiting oxygen index, reduced peak heat release rate, and total heat release tested by an oxygen index meter and a cone calorimeter, respectively. In addition, the addition of ZHS/GO nanohybrids decreased the smoke products and increased the tensile strength of PVC. Above-excellent flame-retardant properties are generally attributed to the synergistic effect of GO and ZHS, containing good dispersion of ZHS/GO in PVC matrix, the physical barrier of GO, and the catalytic char function of ZHS.
Recently, graphene (GR) and graphene oxide (GO) are widely used in fabricating advanced composites to provide elevated mechanical performance, excellent electrical properties, etc, due to their outstanding physiochemical properties [1–6]. Among them, many studies also demonstrate that GR and GO are promising flame retardants, which can act as barriers reducing the heat released, insulating against the transfer of combustion gases, and increasing residual char [7–9]. However, for GR and GO, achieving an excellent flame retardancy usually needs a little high loading . Moreover, bare GR and GO possess a propensity to aggregate because of strong van der Waals attractions and π-π attraction between the nanosheets, resulting in decreasing their flame retardancy and deteriorating mechanical properties of polymer matrix. Therefore, the use of GR or GO alone as the flame retardant still remains a challenge.
As is well known, a combination of two or more components can sometimes present a synergistic effect and may impart an excellent flame retardancy to polymers . Particularly, synergistic effects have been extensively found in the family of graphene-based composites in recent years. For instance, the addition of Ce-MnO2-GR hybrid sheets can impart excellent flame-retardant properties to an epoxy matrix duo to the synergistic effect . A synergistic effect has also been observed between ZnS and GR in epoxy resin . In addition, the synergistic effects of Ni-Fe-layered double hydroxide/GR hybrids appear in epoxy resin . Surface modification of GR with layered molybdenum disulfide depicts a synergistic reinforcement on reducing fire hazards of epoxy resins . More importantly, the abovementioned modifiers, Ce-MnO2, ZnS, Ni-Fe-layered double hydroxide, and MoS2, could also inhibit the aggregation of GR and improve the dispersion of GR in polymer matrix, resulting in improved mechanical properties of polymer matrix.
In recent years, zinc hydroxystannate (ZHS) has attracted growing attention owing to significantly improving flame retardancy of poly (vinyl chloride) (PVC). Compared with other inorganic flame retardants, ZHS have outstanding properties, such as low addition, low toxicity, and high efficiency, and will become a useful replacement of the conventional inorganic fillers [15–17]. Unfortunately, ZHS as flame retardant is less competitive in terms of cost and has a poor compatibility like other inorganic fillers in PVC bringing about the deterioration of mechanical properties, which is undesirable for the fabrication of high-performance materials. To overcome this drawback, our previous studies have made efforts to synthesize a composite flame retardant with core-shell structure: nano-ZHS coated by a macromolecule flame retardant, and the nanocomposite flame retardant not only significantly improves flame-retardant and smoke suppression properties of PVC but also does no damage to mechanical properties of PVC . However, this coating technique is relatively complicated. Therefore, a succinct and effective method should be found to improve flame retardancy of ZHS to reduce the usage of ZHS.
Bearing those perspectives in mind, we use ZHS nanoparticles to modify the GO nanosheets to form ZHS/GO nanohybrids utilizing an electrostatic interaction. Herein, ZHS is expected to prevent the aggregation of GO, and GO could also prohibit the aggregation of ZHS, resulting in improving the compatibility between ZHS/GO and PVC matrix. Furthermore, the novel flame retardant, ZHS/GO, was achieved with excellent flame retardancy, using the synergistic effect between ZHS and GO.
Preparation of GO nanosheets
GO nanosheets were prepared from purified natural graphite through the method reported by Hummers and Offeman .
Preparation of ZHS/GO nanohybrids
In a typical procedure, 0.27 g of zinc sulfate heptahydrate and 10.29 mg of as-prepared GO were added to 100 ml of distilled water, followed by sonication for half an hour. Subsequently, 0.28 g of sodium stannate tetrahydrate was dissolved in 20 ml distilled water and added into the above solution, and then the reaction system was maintained at 5 °C for 5 h. Thereafter, 0.27 g of zinc sulfate heptahydrate was added into the above reactant and kept stirring for another 1 h. The final precipitate was collected by filtration and washed several times with distilled water to remove the remaining impurities. In the next step, as-prepared products were dried in an air atmosphere at 80 °C for 12 h. For comparison, bare ZHS nanoparticles were also prepared using the same route without the addition of GO.
Preparation of ZHS/GO/PVC composite
Preparation of PVC nanocomposite containing 5 wt.% ZHS/GO has similar processing condition with the procedure of DOPO-VTS-ZHS/PVC reported in our literature , while DOPO-VTS-ZHS was instead by ZHS/GO in this study. Meantime, for comparison, pristine PVC and PVC nanocomposites with 5 wt.% contents ZHS or GO were also prepared under the same processing conditions.
Apparatus and experimental method
The morphology and microstructure of as-prepared samples were documented by an X’ Pert Pro MPD X-ray powder diffractometer (XRD), a JEM-2010 transmission electron microscopy (TEM), and an AVATAR360 Fourier transform infrared (FTIR) spectrometer, respectively. Dispersion state of additives in PVC matrix was observed on a scanning electron microscope (SEM) (Nova Nano SEM 450 instrument with an acceleration voltage of 5 kV). Thermogravity analysis (TGA) were conducted on a DSC6200 thermal analyzer at the scanning rate of 10 °C/min. Flame retardancy were carried out by a JF-3 oxygen index meter and a FTT Cone calorimeter, respectively. The tensile strength of the nanocomposites was measured according to the Chinese standard method (GB T1040-92) with a WDW-10D electronic universal testing instrument at the cross head speed of 20 mm/min at 23 °C. Dynamic mechanical analysis (DMA) was performed using a DMA 861e instrument (Metter Toledo Instruments Inc., Swiss Confederation) at a fixed frequency of 10 Hz and a temperature range from 30 to 150 °C at a linear heating rate of 5 °C/min.
Results and discussion
Structure characterization of ZHS/GO
Morphology of PVC and its nanocomposites
Thermal degradation of PVC and its nanocomposites
Summary of the parameters of thermal and mechanical properties
Td, max1 (°C)
Td, max2 (°C)
Char yield at 650 °C
Tensile strength (MPa)
24.5 ± 0.1
11.9 ± 0.4
27.2 ± 0.1
13.4 ± 0.2
25.9 ± 0.1
13.2 ± 0.2
28.5 ± 0.1
14.3 ± 0.3
Fire hazard evaluated by LOI and cone calorimetry
Cone date of PVC, ZHS/PVC, GO/PVC, and ZHS/GO/PVC
The mechanical properties of PVC and its nanocomposites
Dynamic mechanical analysis (DMA) test was performed to determine the reinforcement of ZHS/GO on the dynamic mechanical properties of the PVC nanocomposites. Figure 11 shows the storage modulus of neat PVC and its nanocomposites. It can be seen that the storage modulus of either ZHS/PVC or GO/PVC is increased compared to that of neat PVC within the entire temperature range, which is attributed to the reinforcement effect of additives. Similar with the tensile strength, the storage modulus of ZHS/GO/PVC nanocomposite further increased compared to ZHS/PVC and GO/PVC, which is also attributed to the synergistic reinforcement effect of ZHS/GO nanohybrids in polymer matrix.
ZHS/GO nanocomposites were successfully synthesized. The morphological characterization showed that the synthetic ZHS/GO nanocomposites exhibited some independent and separate ZHS nanoparticles are well distributed on the surface of GO. TGA revealed that the ZHS/GO nanocomposites could enhance the residue yield compared with pure GO. The incorporation of 5 wt.% ZHS/GO nanocomposites into PVC led to the improvement of Td,max2, char residue, DTG peak value, LOI, and mechanical properties compared to those of pure PVC. Furthermore, the pHRR and THR values for ZHS/GO/PVC were significantly reduced by 50 and 59.7 %, respectively, compared to those of pure PVC. Moreover, the amount of organic volatiles released during the combustion of PVC was significantly reduced after incorporating ZHS/GO. The improved flame retardancy was obtained through the synergistic effect between ZHS nanoparticles and GO nanosheets, resulting from the physical barrier effect acted by GO in combination with the effect of ZHS.
This work is supported by National Basic Research Program of China, Grant No. 2015CB674703, Science and Technology Research Program of Henan Educational Committee, Grant No. 16A430001 and National Natural Science Foundation of China, Grant No. 21371050.
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