Mesoporous Zinc Ferrite Microsphere-Decorated Graphene Oxide as

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Mesoporous zinc ferrate microsphere-decorated graphene oxide as a flame retardant additive: preparation, characterization, and flame retardance evaluation Cuizhen Yang, Zhiwei Li, Laigui Yu, Xiaohong Li, and Zhijun Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 20, 2017

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Mesoporous zinc ferrate microsphere-decorated graphene oxide as a flame retardant additive: preparation, characterization, and flame retardance evaluation Cuizhen Yanga,b, Zhiwei Lia,b,*, Laigui Yua,b, Xiaohong Lia,b, Zhijun Zhanga,b a. National & Local Joint Engineering Research Center for Applied Technology of Hybrid Nanomaterials, Henan University, Kaifeng 475004, People’s Republic of China b. Collaborative Innovation Center of Nano Functional Materials and Applications of Henan Province, Henan University, Kaifeng 475004, People’s Republic of China

Corresponding Author *E-mail: [email protected]; Tel: +86-371-23881358

Keywords：：Mesoporous; Zinc ferrate; Graphene oxide; Flame retardant Abstract A mesoporous zinc ferrate decorated graphene oxide (MZF-GO) nanohybrid was prepared through a one-step solvothermal method and well characterized, with the aim of reducing fire hazards of epoxy resin (EP). Results show that the LOI value of the EP filled with 3 wt.% MZF-GO is 27.2, much higher than that of the EP (22.1). Simultaneously, in comparison with the EP, the peak heat release rate, total smoke release, and the peak CO productive rate of the MZF-GO/EP nanocomposite are reduced by 39.57%, 32.56%, and 58.80%, respectively. This is attributed to the synergistic flame-retardant effect between GO and MZF. Namely, GO can act as the physical barrier to block the release of combustible gases and the transfer of heat energy and oxygen, while MZF can catalyze the cross-linking of macromolecules and promote the char formation of EP and absorb inflammable gas and heat.

1. Introduction Environmental concern is stimulating the scientific research toward the design and proposal of new engineered materials for construction, packaging, furniture, and automotive industries involving thermoset polymers.1-3 As one of the most important thermoset polymers, epoxy resin (denoted as EP) is often used as a coating, adhesive, 2 ACS Paragon Plus Environment

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laminate, semiconductor encapsulate and the matrix for advanced fiber-reinforced composites, due to its outstanding mechanical stiffness and toughness, good solvent and chemical resistance, and superior adhesion.4-6 However, EP is highly flammable, which restricts its applications.7, 8 Such a drawback, fortunately, can be overcome by introducing nano-fillers used as adjuvants to other flame retardants to improve the flame retardancy of pristine polymers, owing to that nano-fillers have a better flame retardancy at a relatively low loading than conventional fillers have.9, 10 Among various nano-fillers, graphene oxide (denoted as GO) has been extensively investigated as a flame retardant additive because GO can act as the barrier to reduce heat release, insulate against the transfer of combustion gases, and increase residual char.11-13 However, there are strong van der Waals forces and π-π attraction between GO nanosheets, which makes it difficult for GO to be well dispersed in polymer matrices.14-16 Moreover, bare GO used as the flame retardant needs a high loading to achieve good flame retardant effect.5, 17 Therefore, it usually needs to encapsulate GO with molecules containing desired functional groups in order to improve its dispersion and control its interfacial structure as a reinforcing nano-filler.18-21 Recently, many efforts have been made to fabricate polymer-matrix composites filled with various GO-based additives, which afford excellent flame retardant properties with the assistance of the synergistic fire-retardant effect among multiple components.22-25 For example, Liao et al. reported that there exists a synergism between DOPO and graphene sheets on the flame retardance of epoxy composites.18 In our previous study, we also found that zinc hydroxystannate (denoted as ZHS) nanoparticles-modified GO nanosheets can effectively improve the flame retardance of poly(vinyl chloride).26 However, the rare reserve of tin element in the earth’s crust is unfavorable for its wide application.27 In the present research, therefore, we intend to design mesoporous zinc ferrate (ZnFe2O4; denoted as MZF) to modify GO nanosheets based on the following three considerations. First, Fe element has an abundant reserve in the earth’s crust and is of a low cost. Second, MZF is a composite oxide composed of zinc and iron. Apart from the reports that ZnO was a promising catalyst in CO oxidation, it has been observed that metal oxide can serve as a catalyst for the cross-linking of macromolecules and promote the char formation, leading to increased char yield and consequently suppresses the release amount of flammable volatile products during the combustion of polymers.28,29 Therefore, it is reasonable to believe that MZF contained 3 ACS Paragon Plus Environment


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zinc oxide and iron oxide should play an important role in the elimination of volatile organic compounds and toxic gases; third, in particular, the mesoporous structure of MZF may play an important role in the combustion process. Namely, the mesopore size is much larger than the width of EP (about 0.5 nm), which means that EP can be easily penetrated into the mesochannels of MZF under capillary force to form interpenetrating networks, thereby enhancing thermal stability of EP. In addition, the mesopore structure is helpful to absorb inflammable gas and heat. Bearing those perspectives in mind, in this research we utilize a one-step hydrothermal method to fabricate MZF-GO nanocomposites. We further incorporate the as-fabricated MZF-GO nanohybrid as a flame-retardant additive into EP matrix in order to acquire reduced fire hazard in comparison with ZHS-GO. This article reports the effect of MZF-GO on the thermal stability, fire resistance, and smoke suppression properties of EP-matrix composites. And the mechanism the flame retardancy in MZF-GO/EP during combustion has also been discussed.

2. Experimental section 2.1 Materials Natural flake graphite was obtained from Qingdao Guyu Graphite Co., Ltd (Qingdao, China). EP was purchased from Feicheng Deyuan Chemicals Company Limited (Shandong, China). Analytical grade zinc chloride (ZnCl2), iron (III) chloride hexahydrate (FeCl3.6H2O), ethylene glycol (denoted as EG), polyethylene glycol (denoted as PEG), and sodium acetate (NaAc) were provided by Tianjin Kermel Chemical Reagent Company (Tianjin, China). Ethanol was purchased from Anhui Ante Food Company., Limited (Anhui, China). All reagents were used without further purification. 2.2 Preparation of GO, MZF-GO and MZF-GO/EP GO was synthesized from natural flake graphite by a modified Hummers method.30, 31

MZF-GO hybrid was prepared with a one-step solvothermal method. Briefly, 300 mg of GO, 2.43 g (2 mmol) of FeCl3.6H2O, and 0.617 g (1 mmol) of ZnCl2 were dispersed in 150 mL of EG and ultrasonically stirred for 2 h. Into the resultant mixed 4 ACS Paragon Plus Environment

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solution were then added 16.2 g of NaAc and 4.5 g of PEG, followed by additional 30 min of stirring. The resultant mixture was transferred to a Teflon-lined stainless steel autoclave and heated at 200 ℃ for 10 h. Upon completion of reaction, the black product was collected by filtration and fully washed with deionized water and ethanol, followed by being dried at 60 ℃ in a vacuum oven. The schematic diagram for fabricating the target product is shown in Figure 1. For a comparative study, ZHS-GO was prepared through the method reported in our previous work.26 MZF-GO/EP nanocomposites were prepared as follows: Briefly, a proper amount of the as-fabricated MZF-GO powder was mixed with EP (the optimal filler loading was determined to be 3% (mass fraction) based on a series of screening tests) on a planetary vacuum mixer to form a dispersion. The EP curing agent (H 593) was added in to the resultant dispersion at a resin-to-curing agent mass ratio of 4:1. The asobtained mixture was poured into a Teflon mold and cured at 30 oC for 0.5 h to afford MZF-GO/EP nanocomposites. ZHS-GO/EP composite and EP sample were also prepared in the same manners for comparative studies. 2.3 Characterization X-ray powder diffraction (XRD) patterns were collected with an X'Pert Pro diffractometer (Cu-Kα radiation; λ = 0.15418nm, voltage: 40 kV, current: 40 mA). Transmission electron microscopy (TEM) images were obtained with a JEM-2010 transmission electron microscope. A drop of the to-be-tested sample was placed onto a copper grid covered by carbon film and dried completely at ambient temperature to provide the sample for TEM analysis. Fourier transform infrared (FT-IR) spectra were measured with an Avatar 360 FT-IR spectrometer (background correction was conducted with reference to blank KBr pellet). The dispersion state of the nano-filler in EP matrix was observed with a NovaNano SEM 450 scanning electron microscope (SEM) and a JEOL JEM-ARM200F transmission electron microscope. The ultrathin sections (200 nm) were obtained by a Leica EM UC6. The surface element composition of the as-prepared products was analyzed with an energy dispersive spectrometer (EDS) attachment of the SEM. A RM-1000 confocal microscopic Raman spectrometer (excitation source: 632.8 nm laser) was performed to record the Raman scattering spectra. A porosity analyzer (Quadrasorb SI, USA) was employed to determine the pore diameter (DBJH) of the nano-filler according to full-automatic 5 ACS Paragon Plus Environment


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specific surface. Thermogravimetry analysis (TGA) was conducted with a DSC 6200 thermal analyzer, with which about 3.0 mg of the to-be-tested sample was tested in ambient air and nitrogen flow from room temperature to 600 °C at a heating rate of 10 °C/min. An HC-2 oxygen index meter (Jiangning Analysis Instrument Company, Nanjing, China) was performed to measure the limiting oxygen index (LOI) values of the specimens (dimensions: 100 × 6.5 × 3 mm3) according to ASTMD 2863. Cone calorimetry (Fire Testing Technology, UK) test was conducted following the procedures in ISO5660, with which the specimen with the dimensions of 100 × 100 × 3 mm3 was exposed to 35 kW·m-2 heat flux.

3. Results and discussion 3.1 Structure characterization of GO and MZF-GO nanohybrid Figure 2 shows the XRD patterns of GO and MZF-GO. The sharp peak of GO at 2θ = 11.11° corresponds to its (001) reflection. MZF-GO nanohybrid shows peaks at 2θ = 18.3°, 30.1°, 35.3°, 43.0°, 53.5°, 56.3° and 62.4°, and they correspond to the (111), (220), (311), (400), (422), (511) and (440) reflections of cubic spinel crystal MZF. However, no typical patterns of graphene (002) or GO (001) are observed for MZFGO nanohybrid. According to previous report,32 this is ascribed to the fact that the regular layer stacking of GO were destroyed which leads to the disappearance of the diffraction peak of graphene (002). Figure 3 shows the FTIR spectra of the as-synthesized GO and MZF-GO hybrid. GO shows the C-O stretching vibrations of -COOH group at 1720 cm-1, O-H deformation vibrations of -COOH group at 1620 cm-1, O-H deformation vibrations of tertiary C-OH at 1396 cm-1, and C-O stretching vibrations of epoxy group at 1050 cm1

. As to MZF-GO nanohybrid, the absorption peak around 1570 cm-1 is assigned to the

stretching vibrations of the unoxidized carbon backbone,33 and the two strong absorption peaks at lower frequency (around 550 cm-1 and 415 cm-1) are assigned to the stretching vibrations of the Zn-O bonds and the Fe-O bonds, respectively.34, 35 Besides, some of the characteristic peaks of GO disappear after the hydrothermal reaction, because some oxygen-containing functional groups of GO are reduced during the hydrothermal reaction.36 This can be further confirmed by corresponding Raman spectroscopic analysis. As shown in Figure 4, both the G- and D- bands of MZF-GO nanohybrid shift to lower values as compared with those of GO (the G-band 6 ACS Paragon Plus Environment

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shifts from 1595 cm-1 to 1591 cm-1, and the D- band shifts from 1351 cm-1 to 1339 cm-1), which is attributed to the reduction of GO during the hydrothermal process.37 The other low-frequency phonon modes are due to metal ion involved in octahedral groups (BO6); and these modes correspond to the symmetric and antisymmetric bending of oxygen atom in the MO bond of octahedral groups.38, 39 The morphology, microstructure and surface element composition of the MZF-GO nanohybrid were analyzed by FESEM-EDS and TEM. Figure 5 shows the representative FESEM image and corresponding EDS as well as TEM/HRTEM images of MZF-GO nanohybrid. It can be seen that MZF microspheres are well integrated with silk-like GO nanosheets, which indicates that the solvothermal approach is well applicable to synthesizing regular MZF-GO nanohybrid. Furthermore, the EDS analysis indicates that the surface of as-prepared MZF-GO nanohybrid consists of Zn, Fe, C, and O. Here the C element mainly comes from the basal plane of GO nanosheets, while O element mainly originates from MZF and the residual O-containing functional groups of GO. The low magnification TEM morphology of MZF-GO nanohybrid is consistent with corresponding SEM image. Namely, the flake-like GO nanosheets are sparely coated with MZF microspheres; and the MZF microspheres exhibit a uniform average diameter of about 20 nm (Figure 5c). And the ambiguity of pores on the surface of MZF can be seen from the magnified TEM image (inset of Figure 5c). Moreover, the high-resolution TEM image (HRTEM) of the selected area of the MZF microspheres indicates a well-defined crystallinity with a lattice spacing of 0.48 nm (Figure 5d), which corresponds to the (111) crystal plane of MZF. The pores of the as-prepared MZF-GO nanohybrid were further studied by nitrogen adsorption-desorption; and the pore size distributions of the MZF-GO nanohybrid were calculated by the BJH method from the adsorption branch. Figure 6 shows the N2 adsorption-desorption isotherms and pore size distribution curves of MZF-GO nanohybrid. It can be seen that the as-prepared MZF-GO nanohybrid shows type-IV isotherms with a broad H2-type hysteresis loop (Figure 6a), a feature of mesoporous materials with interconnected regular and ink bottle-like pores. Moreover, the MZFGO nanohybrid mostly has a pore size of 3.9 nm. The relative specific surface area of MZF-GO nanohybrid is 19.33 m2·g-1, and its mesopore volume is 4.66×10-2 cm3·g,



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which demonstrates that the MZF-GO nanohybrid is a potential porous material with strong adsorbing capability. 3.2 Dispersion state of MZF-GO nano-filler in EP matrix The performance of polymer-matrix composites filled with nano-filler like MZFGO highly depends on the dispersion state of the nano-filler in the polymer matrix. Firstly, SEM analysis was conducted to evaluate the dispersion state of the nano-filler in the EP matrix and the SEM results of all samples were shown in Figure 7. As to EP (Figure 7a), it exhibits a smooth and mirror-like fracture surface, corresponding to its brittle failure. As to GO/EP composite (Figure 7b), the GO nanosheets are agglomerated obviously in the EP matrix, because of the high surface area and strong van der Waals forces of the GO nanosheets. The surface of ZHS-GO/EP is coarser than that of EP and shows signs of aggregation of particles (Figure 7c). As to MZFGO/EP nanocomposite, there is no clear agglomeration of fire retardant additive on the surface, which is because the introduction of MZF prevents the exfoliated GO nanosheets from direct stacking. In order to further prove the dispersion of EP nanocomposites, TEM analysis was also conducted and the corresponding TEM results were shown in figure 8. It can be seen that most of GO nanosheets form aggregation (figure 8a), while ZHS-GO and MZF-GO show a better dispersion in EP. In particular, the MZF-GO exhibits the best dispersion which is almost the same as TEM results of the MZF-GO powder (figure 5c). The TEM results are consistent with the above SEM results. 3.3 Thermal stability and flammability of EP-matrix composites The thermal stability of EP and its nanocomposites was evaluated by TGA and DTG in air atmosphere and nitrogen atmosphere. As shown in Figure 9a, EP and the as-prepared nanocomposites undergo similar thermal degradation process in air atmosphere. Namely, EP undergoes two stage of thermal degradation: the decomposition of the macromolecular chains in the temperature range of 350–480 °C and the thermal oxidation of char residue beyond 500 °C. Here T-5% is defined as the temperature at which 5% of weight loss occurs, and Tmax is defined as the temperature at which the maximum decomposition occurs. Corresponding thermal analysis data are listed in Table 1. The onset degradation temperature of GO/EP composite is slightly lower than that of EP. The reasons are as follows. Firstly, the degree of 8 ACS Paragon Plus Environment

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crosslinking of the EP is possible reduced, due to the addition of the GO. Secondly, there may be a little heat conductivity of GO, which accelerates the spread of the heat in the material.22, 40 In addition, some functional groups, such as hydroxyl and epoxy group, were first got away the surface of the GO during the thermal decomposition process.41 Therefore, the onset degradation temperature of GO/EP composite is slightly lower than that of EP. Simultaneously, the T-5% values of ZHS-GO/EP and MZF-GO/EP nanocomposites are also slightly lower than that of EP, this is because these two samples both contain the GO. Just as the above analysis, GO can lead to a lower onset degradation. Besides, the Tmax of ZHS-GO/EP and MZF-GO/EP nanocomposites is higher than that of EP and GO/EP composite, which is possibly attributed to the synergistic action between graphene and ZHS or MZF (GO exhibits a physical barrier effect while ZHS or MZF exhibits a catalytic effect). As far as the char yield is concerned, the incorporation of MZF-GO in the EP matrix significantly enhances the formation of char residue at 600 °C, which is also attributed to the synergistic effect between GO and MZF. Namely, GO exhibits a physical barrier effect, while ZHS or MZF shows a catalytic effect. Thermogravimetric analysis under air atmosphere show thermal degradation of EP and its composites surface during combustion. But the air content of combustion surface is different from that of combustion interior, thermal degradation of surface materials also differs from that of internal materials. In order to estimate thermal behavior for the internal materials of EP and its composites, we also conducted thermal behavior of EP and its composites under N2 atmosphere. As can be seen in Figure 9(b, d), after incorporating the GO into EP, the T-5% show increase compared with EP, due to barrier effect of GO. The MZF-GO/EP composites have increased 23 °C in comparison to EP. Furthermore, char yields for MZF-GO/EP are higher than EP and other composites, indicating that the barrier effect of GO and the catalytic effect of MZF would promote catalytic carbonization of EP and inhibit the mass loss. These results further prove the synergistic effect between GO and MZF. To investigate the effect of GO, ZHS-GO and MZF-GO on the flame retardance of the EP-matrix composites, we measured the LOI values. As shown in Figure 10(a), the LOI value of GO/EP composite is 22.3%, and it is similar to that of EP (22.1%). The LOI values of MZF/EP, ZHS-GO/EP and MZF-GO/EP nanocomposite are 26.2%, 24.2% and 27.2%, respectively, which indicates that there does exist a 9 ACS Paragon Plus Environment


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synergistic flame-retarding effect between GO and MZF. This is because the mesoporous structure of MZF can absorb pyrolysis products and MZF can catalyze EP into char, while GO exhibits the physical barrier effect, and such a synergistic effect contributes to resist droplet dripping, delay the evaporation of polymeric segments and protect the EP below char residue. Such a supposition is supported by corresponding digital photographs of the EP-matrix composites shown in Figure 10(b). Namely, among the tested EP-matrix composites, MZF-GO/EP nanocomposite has the maximum char residue and does not undergo any droplet dripping. Cone calorimetry is widely used to evaluate the flammability of materials so as to forecast their fire behavior in real-world fire.42 Figure 11 shows the heat release rate (HRR) curves of EP and EP-matrix composites. Several important parameters obtained from the cone calorimetry data, such as peak heat release rate (pHRR), total heat release (THR), total smoke release (TSR), total heat release (THR), average mass loss rate (AMLR), average of effective heat of combustion (av-EHC), average CO yield (av-COY), time to ignition (TTI), and the char yields at flame out are listed in Table 2. The av-EHC is the ratio of average of heat release rate to the average mass loss rate from the cone calorimetry test. As presented in Table 2, the av-EHC of EP and its composites are almost unchanged. As for the TTI, in some studies, the nanoparticles/polymer systems, such as the various carbon nanomaterials/polymer, show an earlier TTI, this is complex reason and is attributed to the heat absorption and of thermal conductivity of different samples.43,

44

In this study, the earlier TTI of

GO/EP and MZF-GO/EP composites were also observed. While TTI of the ZHSGO/EP are the same as that of EP. The earlier TTI is helpful to form protective charring. Besides, the HRR, TSR, THR of EP-matrix composites have obvious changes. As showed in Figure 11, EP has a pHRR value of 1045.1 kW/m2 (Figure 11(a)). The incorporation of GO in EP matrix reduces the pHRR value to 921.9kW/m2. Similarly, the pHRR values of ZHS-GO/EP and MZF-GO/EP nanocomposites are reduced to 871.8 kW/m2 and 631.5 kW/m2, respectively, corresponding to a decrease by 16.57% and 39.57% as compared with that of EP. These results thank to form protective residue layers.45 Among them, the MZF-GO/EP exhibits the best results in comparison with the control samples. This simultaneously demonstrates that there exists a synergistic flame-retardant effect between GO and MZF. Such a synergistic effect contributes to promoting the char formation on the 10 ACS Paragon Plus Environment

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surface of MZF-GO/EP composite, thereby slowing down the heat and mass transfer between gases and condensed phases and preventing the underlying material from further combustion. Usually, smoke and toxic gases are the real killers in fires, which means that reducing smoke and toxic gases (especially carbon monoxide) is beneficial to fire rescue. The smoke production rate (SPR) curves for various EP-matrix composites are shown in Figure 12(a). MZF-GO/EP nanocomposite has the lowest SPR value among all the tested composites. Besides, the incorporation of GO, ZHS-GO, or MZF-GO reduces the TSR value of the composites; and in particular, MZF-GO nanohybrid exhibits the best smoke suppression for EP and reduces the TSR by nearly 32.56% (Figure 12(b)). This demonstrates that the MZF-GO nanohybrid with char barrier effect can retard the diffusion of flammable gases, thereby suppressing the formation of smoke and toxic gases during the combustion of the EP-matrix composites46. The CO release curves for EP and EP-matrix composites obtained from cone calorimetry data are shown in Figure 12(c). It can be seen that MZF-GO/EP exhibits a decrease of 58.80% of the peak CO production rate compared to EP, which implies that iron oxide and zinc oxide contribute to the oxidation of CO into CO2 which can suppress the release of CO.47,48 Besides, the MZF microspheres with mesoporous structure is helpful to absorb CO. 3.4 Flame retardant mechanism It is imperative to investigate the properties and structure of char layers in order to acquire more insights into the flame-retardant mechanisms of flame retardant additives in the condensed phase. Figure 13 shows the digital photographs and SEM images of the residual chars after cone calorimetry tests of EP and its nanocomposites. As can be seen from Figure 13a, EP undergoes complete burning and nearly does not form any char, while GO/EP and ZHS-GO/EP composites provide a small amount of char. As to MZF-GO/EP nanocomposite, the amount of the char residue is 18.5%, which well corresponds to relevant TGA data. Moreover, EP generates a small amount of fragile and flaky char residue (Figure 13e). After 3 wt.% of GO or ZHSGO is added into EP, the resultant char residues seem to be rough and contain a large amount of cracks and holes (Figure 13f and Figure 13g). The MZF-GO/EP nanocomposite, however, provides a highly compact and complete char layer (Figure 11 ACS Paragon Plus Environment


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13h). The continuous and compact char surfaces are good barriers to protect the underlying polymers and inhibit the exchange of degraded products, combustible gases and oxygen. As a result, MZF-GO/EP nanocomposite exhibits the best flameretardant performance among the tested EP-matrix composites. Figure 14 shows the Raman spectra of the residues of EP and EP-matrix composites after cone calorimetry tests. The four kinds of char residues exhibit similar shape of Raman spectra: the two peaks around 1603 cm−1 and 1358 cm−1. The peak at 1603 cm−1, the G band, corresponds to the first order scattering of the E2g mode of hexagonal graphite; and the one at 1358 cm−1, the D band, arises from the carbon atoms in disordered graphite46. The graphitization degree of the char residue can be estimated by ID/IG, where ID and IG are the integrated intensities of the D and G bands, respectively. Approximately, the lower the ID/IG ratio, the better structure of the char layer is. As shown in Figure 14, the ID/IG of the char residue of EP is 3.11, and that of the char residue of MZF-GO/EP significantly declines to 2.23. Particularly, the char residue of MZF-GO/EP nanocomposite has the minimum ID/IG among the tested EPmatrix composites, which indicates that the char residue of the MZF-GO/EP nanocomposite exhibits a high degree of graphitization and an increased thermal stability as well. Figure 15 shows the XRD pattern of the char residue of MZF-GO/EP nanocomposite. It can be seen that, aside from zinc ferrate, the char residue of MZFGO/EP nanocomposite also contains iron oxide. This can be well understood when one notices that the decomposition of zinc ferrate generates iron oxide and zinc oxide. The resultant Fe species can effectively catalyze the carbonization of EP matrix, thereby adding to the formation of the char residue of EP-matrix composites.49 In the meantime, the unreacted MZF microspheres can prevent the melting of EP matrix and absorb combustible gases, thereby improving the thermal stability and delaying the combustion process to some extent. Combining the TGA data with the cone calorimetry data, we can reasonably suppose that some epoxy chains participate in the carbonization process of EP under the barrier effect of GO and the catalytic function of MZF, which leads to the formation of compact and complete chars and decreases the flammability of EP-matrix nanocomposites.


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Figure 16 schematically shows the flame-retardant mechanism of MZF-GO nanohybrid as the flame-retardant additive of EP. During the early stage of combustion, MZF functions to catalyze the carbonization of the degraded products of EP. In the meantime, GO acts as a physical barrier to block the release of combustible gases and the transfer of heat energy to EP bulk. With the development of combustion process, zinc ferrate is partly decomposed into iron oxide and zinc metal. The iron oxide can catalyze the cross-linking of macromolecules and promote char formation. Furthermore, the MZF microspheres can absorb inflammable gases. As a result, the MZF-GO/EP nanocomposite exhibits improved flame retardance.

4. Conclusions In summary, mesoporous MZF-GO nanohybrid is prepared via a one-step hydrothermal method. The composition and structure of the as-synthesized MZF-GO nanohybrid are confirmed, and its effect on the thermal stability and flammability of EP-matrix composites are investigated. Results show that the MZF-GO nanohybrid as the flame-retardant additive can significantly increase the amount of char residues, improve the flame-retardant behavior during the combustion process of EP-matrix composites. This is attributed to the synergistic flame-retardant effect of MZF and GO. Namely, GO acts as a physical barrier to block the release of combustible gases and the transfer of heat energy and oxygen, while MZF can catalyze the cross-linking of macromolecules and promote the char formation of EP and absorb inflammable gas and heat. As a result, the fire hazard of the EP-matrix nanocomposite is considerably reduced.

Acknowledgements The authors appreciate the financial support from the Ministry of Science and Technology of China (973 Program; grant No. 2015CB654703), the Scientific Innovation Talent of Henan Province (grant No. 164200510005), and the Program for Innovative Research Team from the University of Henan Province (grant No. 17IRTSTHN004).



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Table and Figure Captions Table 1 Thermal analysis data of EP and EP-matrix composites under air atmosphere and under nitrogen atmosphere. Table 2 Cone calorimetry data of EP, GO/EP, ZHS-GO/EP, and MZF-GO/EP composites. Figure 1 Schematic illustration for preparing MZF-GO. Figure 2 XRD patterns of GO and MZF-GO nanohybrid. Figure 3 FTIR spectra of GO and MZF-GO nanohybrid. Figure 4 Raman spectra of GO and MZF-GO. Figure 5 FESEM image and corresponding EDS spectrum (a and b) as well as TEM/HRTEM images of MZF-GO nanohybrid (c and d) Figure 6 Nitrogen adsorption/desorption isotherms (a) and pore size distribution (b) of MZF-GO nanohybrid. Figure 7 SEM photographs of freeze-fractured surface of (a) EP, (b) GO/EP, (c) ZHS-GO/EP, and (d) MZF-GO/EP. Figure 8 TEM pictures of GO/EP(a), ZHS-GO/EP(b), and MZF-GO/EP(c) Figure 9 TGA and DTA curves of EP, GO/EP, ZHS-GO/EP, and MZF-GO/EP under air atmosphere (a and c) and under nitrogen atmosphere (b and d). Figure 10 LOI values of EP-matrix composites (a) and digital photographs of EPmatrix nanocomposites undergoing LOI test (b). Figure 11 HRR (a) and THR (b) curves of EP and EP-matrix nanocomposites obtained from cone calorimetry test. Figure 12 Smoke production rate (a), total smoke release curves (b) and CO release curves (c) of EP and its nanocomposites obtained from cone calorimetry tests. Figure 13 Digital photos and SEM images of residual char surfaces of EP (a, e), GO/EP (b, f), ZHS-GO/EP (c, g) and MZF-GO/EP (d, h) obtained from cone calorimetry tests. Figure 14 Raman spectra of the residues of EP and EP-matrix composites after cone calorimetry test. Figure 15 XRD patterns of MZF-GO/EP residual chars. Figure 16 Schematic diagram showing the flame-retardant mechanism of MZF-GO nanohybrid in EP-matrix composite. 17 ACS Paragon Plus Environment


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Table 1 T-5% (℃ ℃)

Samples EP GO/EP ZHS-GO/EP MZF-GO/EP

Residue at 600 ℃(%)

Tmax (℃ ℃)

N2

Air

N2

Air

N2

Air

88.9 144.7 129.7 112.5

214 192.5 205 207.6

365.9 371.2 365.4 351.2

369.7 362.5 378.7 373.7

6.8 10.6 13.5 21.2

0.3 0.7 1.8 3.2

Table 2 Samples EP GO/EP ZHS-GO/EP MZF-GO/EP

TTI (S) 85 35 85 65

pHRR (kW.m-2) 1045.1 921.9 871.8 631.5

THR (MJ.m-2) 115.6 110.5 108.0 89.4

TSR (m2.m-2) 4363.2 4141.1 3691.9 2942.6

AMLR (g.s-1) 0.13 0.17 0.12 0.07

av-EHC (MJ/kg) 21.51 21.24 22.13 21.65

Figure 1


av-COY (kg/kg) 0.053 0.050 0.053 0.054

Char yield (%) 0.2 1.6 13.2 18.5

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GO MZF-GO

Intensity (a.u.)

PDF#22-1012>Franklinite-ZnFe2O4

10

20

30

40

50

60

70

80

2 Theta (degree) Figure 2

(a)

4000

3500

3000

2500

2000

1500 -1

Wavenumbers (cm )

Figure 3


1050

1395

550 415

(b)

1726 1620

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1000

500


18000

D-band G-band

16000

-1

1595 cm

-1

1351 cm

14000

In tensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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12000 10000 -1

8000 6000

-1

1339 cm

1591 cm

GO

4000 2000

MZF-GO

0 500

1000

1500

Raman shift (cm-1)

Figure 4

Figure 5


2000

30

(a)

25 20 15 10 5 adsorption isotherm desorption isotherm

0 0.0

0.5

1.0

Relative Pressure (P/P0)

0.15

(b)

BJH-Plot

0.12

dVp/drp

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Adsorbed volume /cm3(STP)g-1

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0.09 0.06 0.03 0.00 0

5

10

15

20

25

rp (nm)

Figure 6


30

35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7

a

b

c

Figure 8


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EP GO/EP ZHS-GO/EP MZF-GO/EP

100

80

60 40 20 0


100

Weight(%)

Weight(%)

80

60 40 20

a 100

0 200

300

400

500

600

700

800

b 100

200

Temperature(oC)

300

400

500

600

700

Temperature（ ℃）

2

0.0000

Derivation Weight(%/min）

Derivation Weight（ %/min）

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.0005 -0.0010 -0.0015


-0.0020 -0.0025

c 100

200

300

400

500

600

700

0 -2 -4 -6 -8 -10 -12 -14 -16 -18

800


d 100

200

Temperature (oC)

300

400

500

Temperature(℃ ）

Figure 9

Figure 10


600

700


1200 EP GO/EP ZHS-GO/EP MZF-GO/EP

(a)

2 HRR (kw/m )

1000 800 600 400 200 0 0

100

200

300

400

500

Time (s)

（ b）

120 100

THR (kw/m2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60 40


20 0 0

100

200

300

400

Time (s)

Figure 11


500

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0.35 EP GO/EP ZHS-GO/EP MZF-GO/EP

(a)

0.30

2

SPR (m s)

0.25 0.20 0.15 0.10 0.05 0.00 0

100

200

300

400

500

Time (s)

5000

(b) TSR (m2/m2)

4000

3000

2000 EP GO/EP ZHS-GO/EP MZF-GO/EP

1000

0 0

100

200

300

400

500

Time (s) EP GO/EP ZHS-GO/EP MZF-GO/EP

0.16

(c)

0.14 0.12

COP(g/s)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10 0.08 0.06 0.04 0.02 0.00 0

100

200

300

400

Time (s) cn

Figure 12



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 13

Figure 14


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Intendity (a.u)

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PDFNO.22-1012>Franklinite-ZnFe2O4 PDFNO.24-0072>Hematite-Fe2O3

20

40

60

2 Theta (degree)

Figure 15

Figure 16


80

Mesoporous Zinc Ferrite Microsphere-Decorated Graphene Oxide as

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