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layered double hydroxide on epoxy resin
Accepted Manuscript Title: The flame retardancy and smoke suppression effect of a hybrid containing CuMoO4 modified reduced graphene oxide/layered double hydroxide on epoxy resin Authors: Wenzong Xu, Bingliang Zhang, Xiaoling Wang, Guisong Wang, Ding Ding PII: DOI: Reference:
S0304-3894(17)30747-1 https://doi.org/10.1016/j.jhazmat.2017.09.057 HAZMAT 18903
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
17-5-2017 22-9-2017 30-9-2017
Please cite this article as: Wenzong Xu, Bingliang Zhang, Xiaoling Wang, Guisong Wang, Ding Ding, The flame retardancy and smoke suppression effect of a hybrid containing CuMoO4 modified reduced graphene oxide/layered double hydroxide on epoxy resin, Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.09.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The flame retardancy and smoke suppression effect of a hybrid containing CuMoO4 modified reduced graphene oxide/layered double hydroxide on epoxy resin
Wenzong Xu *, Bingliang Zhang, Xiaoling Wang, Guisong Wang, Ding Ding
School of Materials Science and Chemical Engineering, Anhui Jianzhu University, 292 Ziyun Road, Hefei, Anhui 230601, People’s Republic of China
*Correspondence to: Wenzong Xu (Tel./Fax:+86-0551-63828157.
Email: [email protected])
Highlights
A novel hybrid of RGO-LDH/CuMoO4 was synthesized by co-precipitation method. RGO-LDH/CuMoO4 were well-dispersed in EP matrix according to TEM observation. RGO-LDH/CuMoO4 has better flame retardancy and smoke suppression for epoxy resin. The flame retardant mechanism of RGO-LDH/CuMoO4 was studied.
Abstract: The co-precipitation method was used to synthesize a hybrid with MgAl-layered double hydroxide loaded graphene (RGO-LDH). CuMoO4 was then introduced onto the surface of RGO-LDH to prepare a hybrid with CuMoO4 modified RGO-LDH (RGO-LDH/CuMoO4). The composition, structure and morphology of RGO-LDH/CuMoO4 were characterized by X-ray diffraction, Laser raman spectroscopy
and
Transmission
electron microscope-energy-dispersive X-ray
spectroscopy. It was found that the hybrid of RGO-LDH/CuMoO4 had been
1
successfully prepared. The effects of flame retardancy and smoke suppression of epoxy resin were studied with added RGO-LDH/CuMoO4. Results showed that the PHRR and THR of the EP composite with RGO-LDH/CuMoO4 added were decreased dramatically. The char yield, LOI and UL-94 vertical burning rating of the EP composite were increased, with improved flame ratardancy. In addition, the SPR, TSP, and Ds,max of the EP composite were decreased drastically with added RGO-LDH/CuMoO4. Its improved flame retardancy and smoke suppression performance were due mainly to the physical barrier of graphene and LDH, and the catalytic carbonization function of LDH. Meanwhile, Cu2O and MoO3 generated from RGO-LDH/CuMoO4 in the combustion process helped enhance the production of char residue and raised the compactness of the char layer.
Keywords: Graphene; Hybrid; Composites; Flame retardancy; Smoke suppression
1. Introduction Epoxy resin as a kind of very important thermosetting resin is widely used in electronic equipment, potting, flooring and adhesives, with its excellent mechanical properties, dimensional stability and chemical resistance properties [1-5]. However, epoxy resin is flammable and can generate much smoke during the combustion process in air, which causes a large limitation in its further applications [6-7]. Therefore, the development of new environmentally friendly flame retardants and smoke suppression agents is highly significant for the application of EP. In recent years, graphene as a new member of the carbon family has received 2
extensive attention from researchers due to its excellent physical, chemical and thermal properties [8-11]. According to previous reports, graphene can play a role as a physical barrier in the combustion process of polymers which may restrain the volatilization of combustible gas, so as to improve the flame retardancy of polymer [12,13]. However, there are two main problems in the application of graphene. On the one hand, graphene is liable to re-aggregate because of the existence of van der Waals forces and π-π interaction, so it is hard to be evenly dispersed in polymers. On the other hand, graphene can not effectively improve the smoke suppression performance of polymers. In order to solve these problems, many researchers focus on the surface modification of graphene. The modification methods mainly include: (1) modifying graphene with metal oxide, such as Cu2O, MoO3, Ni2O3, Sb2O3 and Co2O3 [14-16]; (2) modifying graphene with inorganic substances, such as MoS2, ferrocene and carbon nanotubes [17-19]; and (3) modifying graphene with organic species, such as melamine, DOPO, tetraethylenepentamine and siloxane [20-22]. Those results have shown that the modification of graphene can not only provide a good dispersion in polymer, but also further improve the flame retardancy and smoke suppression of polymer. However, there have been few reports about the modification of graphene with metal salts. In this study, a suitable metal salt was chosen to prepare a novel hybrid of modified graphene. The molybdenum and copper system has been widely applied in polymers due to its excellent performance of flame retardancy and smoke suppression. Zhou et al. [23] prepared carbon nanotubes that were coated with MoS2 (MoS2-CNTs) and studied
3
their effect on the flame retardancy of EP. Their results showed that, by adding 2 wt% of MoS2-CNTs, the PHRR and THR of the EP composites were decreased by 27% and 31%, respectively. Wang et al. [24] added MoO3, Sb2O3 and FeOOH into EVA and studied the effect of different smoke suppression agents on the smoke suppression of the composites. Their study found that the smoke suppression could be significantly improved with the addition of 5 wt% MoO3 into EVA, and the maximum smoke density was decreased by 38%. That was due mainly to the MoO3 generated in the combustion process of MoS2 which could promote the formation of a char layer, so as to achieve improved smoke suppression. Moreover, Chen et al. [25] studied the effect of Cu2O on the flame retardancy and smoke suppression of EP containing microencapsulated ammonium polyphosphate (MAPP), and their results showed that the addition of Cu2O not only further improved the flame retardancy and smoke suppression of composite materials, but also reduced the production of CO and CO2. This was mainly attributed to the fact that Cu2O could promote the formation of a char layer in the burning process of the polymer and enhance the compactness of the char layer, thereby improving the flame retardancy and smoke suppression of the composites. Thus, copper molybdate (CuMoO4) was selected by us to modify graphene because it contains two elements of molybdenum and copper. In addition, LDH was used as intermediate which could connect graphene and CuMoO4, respectively, because LDH has good flame retardancy and smoke suppression and the positive charge located on its surface can adsorb negative ions [26-27]. In this work, the hybrid of RGO-LDH was prepared through the co-precipitation
4
method. Then, CuMoO4 was loaded onto the surface of RGO-LDH to prepare a new hybrid of RGO-LDH/CuMoO4, as shown in Scheme 1. RGO-LDH/CuMoO4 was added into epoxy resin and its effect on the thermal properties, flame retardancy and smoke suppression of the EP composites were investigated. In addition, the mechanism of the improved flame retardancy and smoke suppression of the EP composites was investigated by analyzing the char residue. 2. Experimental 2.1. Materials Graphite powder (spectral purity), H2SO4 (98%), NaNO3, KMnO4, HCl (37%), H2O2 (30%), H2N2 (80%), ethyl alcohol, NaOH, Al(NO3)3•9H2O, Na2MoO4•2H2O, Cu(NO3)2•3H2O and Mg(NO3)2•6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Epoxy resin was purchased from Wuxi Qianguang Chemical
Materials
Company
(Jiangshu,
China).
3,3′-dichloro-4,4′-
diaminodiphenylmethane (MOCA) was purchased from Guangzhou Fufei Chemical Company (Guangdong, China). 2.2. Preparation of RGO-LDH Graphene oxide (GO) was synthesized from graphite powder through the Hummers method [28]. 0.32g GO was dispersed into 150 ml deionized water solution containing 0.20 M NaOH and 0.05 M Na2CO3 to obtain an even dispersion of GO through ultrasonic for 1h. 0.005 mol Mg(NO3)2•6H2O and 0.005 mol Al(NO3)3•9H2O were dissolved in 200 ml deionized water, and then dropped into the dispersion of GO with strong stirring. The pH of the above solution was adjusted to 10 ± 5 by using 0.5 M
5
NaOH solution. This solution was heated to 60 °C for 6h. Then 0.75 ml N2H2•H2O was added into it and it was kept under 100 °C for 2h. Finally, the product was isolated by using centrifuge, washed with ethyl alcohol and deionized water until pH 7, and dried at 50 °C. In addition, RGO and MgAl-LDH were prepared by using the similar conditions. 2.3. Preparation of RGO-LDH/CuMoO4 1g RGO-LDH was dispersed into 150 ml deionized water with appropriate ultrasonication. 0.24g Na2MoO4 was dissolved in 150 ml deionized water and added into the dispersion of GO, with rapid stirring at 60 °C for 10h. Then, 0.24g Cu(NO3)2 was dissolved in 150 ml deionized water and added into the above mixed solution, which was kept at 60 °C for 2h with strong stirring. After the reaction, the product was isolated by using centrifuge, washed with ethyl alcohol and deionized water, and dried at 50 °C. 2.4. Preparation of EP composites EP composites were prepared by using a simple blending method under ultrasonic and stirring. For example, RGO-LDH/CuMoO4 was added into appropriate acetone solution to obtain an evenly dispersed solution. The above solution was sufficiently mixed with epoxy by strong stirring at 60 °C under ultrasonic, forming an even mixture. A computational amount of curing agent (molten MOCA) was added into this mixture and stirred fully, and then poured into a teflon mold to let it sit overnight. Finally, the mixture was cured at 110 °C for 2h and at 150 °C for 2h in the oven to complete the preparation of the EP composite. In addition, EP composites with added
6
RGO, MgAl-LDH and RGO-LDH were prepared under the same conditions, respectively (the specific formulations are displayed in Table 1). 2.5. Characterization X-ray diffraction (XRD) measurements were conducted by using a Japanese Rigaku X-ray diffractometer (Cu-Kα tube and Ni filter, λ= 0.1542 nm). Raman spectroscopy measurements were conducted at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., U.S.) and the measurements were performed by using a back-scattering geometry with the 514.5 nm excitation wavelength. Transmission electron microscope-energy-dispersive X-ray spectroscopy (TEM-EDS) images were obtained using a Japanese JEOL JEM-2100F instrument with a 200 keV accelerated voltage. The thermogravimetric analysis (TGA) was undertaken with a STA 409PC apparatus (NETZSCH, Germany) from room temperature to 700 °C under an air flow rate of 20 °C·min-1. Differential scanning calorimetry (DSC) was performed on a Q20 instrument (TA, U.S.) from 20 to 150 °C at a linear heating rate of 20 °C·min−1. The cone calorimeter combustion test was performed with a JCZ-2 cone calorimeter (Jiangning Analytic Instrument Company, China) in accordance with the ISO5660 standard procedures. Specimens with the size of 100 ×100 × 4 mm3 were exposed under a heat flux of 50 kW·m-2. The limited oxygen index (LOI) value was determined by using an HC-2 oxygen index meter (Jiangning Analytic Instrument Company, China) in accordance with the standard procedures of ASTM D2863-2012 and the specimens were 100 × 10 × 3 mm3 in size. The UL 94 vertical burning test was conducted on a CZF-3 instrument (Jiangning Analytic Instrument Company,
7
China) according to the UL 94 vertical burning test standard, and the size of all the samples was 125×13×3 mm3. Smoke density tests were conducted on a JSC-2 smoke density test instrument (China) in accordance with ISO 5659-2 and specimens with the size of 75×75×3 mm3 were irradiated horizontally under a heat flux of 25 kW·m-2. Tensile properties were tested on a REGER 3010 test machine (China) according to ASTM D638, and the size of all the samples was 165×13×3.2 mm3. Flexural strength was also tested on a REGER 3010 test machine according to GB/T 9341, and the size of all the samples was 80×10×4 mm3. Impact strength was tested on a XJJUH-50Q (China) according to ASTM D256, and the size of all the samples was 64×12.7×3.2 mm3. All data of mechanical properties were the average values of five samples. X-ray Photoelectron Spectroscopy (XPS) spectra were collected with an Escalab 250 spectrometer (Thermo Scientific Ltd., U.S.) using an Al Kα excitation radiation (hν=1486.6 eV). 3. Results and discussion 3.1. Characterization of as-prepared samples XRD is an effective method for characterizing the structure of layer or sheet nano-materials. Fig.1 is the XRD pattern of GO, RGO, MgAl-LDH, RGO-LDH, RGO-LDH/CuMoO4. As shown in the figure: the (002) peak of GO at 2θ = 9.8°; it can be calculated that the interlayer spacing is 0.89 nm through the Bragg equation; compared with graphite (the interlayer spacing of graphite is 0.32 nm), the interlayer spacing of GO is increased due to the oxygen functional groups that were introduced [29]. Compared with GO, the (002) peak of RGO moves to 2θ=25.1°; the interlayer
8
spacing is 0.35 nm. It indicates that the oxygen functional groups on the surface of GO have been removed after reduction. In addition, as shown in the pattern of MgAl-LDH, the (003), (006), (009), (015), (018), (110) and (113) diffraction peaks of MgAl-LDH can be observed at 2 θ = 11.6 °, 23.2 °, 34.7 °, 38.9 °, 45.8 °, 60.6 °, 62.1 °, respectively. The interlayer spacing of MgAl-LDH is 0.76 nm, indicating that CO32- exists in the interlayer of LDH [30]. It can be observed that the XRD pattern of RGO-LDH is almost the same as neat LDH except that the intensity of the diffraction peaks is smaller than neat LDH, and no characteristic peaks of RGO can be observed. The results suggest that restacking of graphene is effectively prevented with LDH loaded on the surface of RGO [31]. The standard diffraction peaks of CuMoO4 are shown in the XRD pattern [32]. In addition, compared with the XRD patterns of CuMoO4 and RGO-LDH, the diffraction peaks of both CuMoO4 and RGO-LDH can be observed in the pattern of RGO-LDH/CuMoO4, indicating that CuMoO4 is loaded on the surface of RGO-LDH successfully. Raman spectroscopy is used to characterize the structure of graphite materials. The Raman spectra of RGO, RGO-LDH and RGO-LDH/CuMoO4 are shown in Fig.2. The spectra of all the samples have two characteristic peaks at 1359 cm-1 and 1599 cm-1, corresponding to D and G bands, respectively. Among them, the D band is assigned to the lattice damage of graphene and amorphous carbon; the G band is indicative of the sp2 carbon atom vibration model [33]. In addition, as shown in the spectrum of RGO-LDH, an obvious peak can be observed at 1049 cm-1, attributed to the vibration peak of CO32- existing in the LDH space [30]. In the spectrum of RGO-LDH/CuMoO4,
9
compared with RGO-LDH, not only can a vibration peak of CO32- be observed, but a new characteristic peak can also be found at 931 cm-1, corresponding to the symmetric stretching vibration peak of Mo=O [34]. The Raman results indicated that the hybrid of RGO-LDH/CuMoO4 has been successfully prepared. TEM is used to directly observe the structure and morphology of GO, RGO, RGO-LDH and RGO-LDH/CuMoO4, and EDS is used to analyze the element composition of RGO-LDH/CuMoO4. As shown in Fig.3 (a), GO has the typical and thin two-dimensional layer structure. Fig.3 (b) indicates that RGO has many folds and restacking in some areas on the sheet, and these phenomena are due to the presence of van der Waals forces. Fig.3 (c) indicates that a lot of nano-sheets appear on the surface of graphene, showing that LDH has been successfully loaded onto graphene. The morphology of RGO-LDH/CuMoO4 in Fig.3 (d) is similar to the morphology of RGO-LDH. Fig.3 (e) is a partially enlarged image of the red rectangle in Fig.3 (d), and it shows that the surface of LDH becomes rough with lots of particles, indicating that CuMoO4 is well dispersed on the surface of RGO-LDH. Meanwhile, as seen in Fig.3 (f), the elements of C, O, Cu, Mg, Al and Mo can be found from the EDS of RGO-LDH/CuMoO4.
The
result
further
indicates
that
the
hybrid
of
RGO-LDH/CuMoO4 has been successfully prepared. It is well known that the dispersion of inorganic nano-fillers in polymer is an extremely important factor affecting the performance of polymer composites. The morphology of EP3 and EP4 ultrathin sections are observed by TEM, in order to investigate the dispersion of RGO-LDH and RGO-LDH/CuMoO4 in EP. As shown in
10
Fig.4, there is no obvious agglomeration and the size of RGO-LDH and RGO-LDH/CuMoO4 in EP ranges from 200 to 400 nm. The results of TEM show that RGO-LDH and RGO-LDH/CuMoO4 have a good dispersion in EP. 3.2. Thermal behavior of EP composites The thermal decomposition process of EP and EP composites is analyzed by thermogravimetric analysis. The TGA and DTG curves of EP and EP composites under the air condition are shown in Fig.5. The temperature at which the sample quality loss is 5 wt% is defined as the Tonset (The initial decomposition temperature); the maximum temperature at which the sample reaches the fastest thermal decomposition rate is defined as the Tmax. It can be seen from Fig.5 (a) and (b) that the Tonset and Tmax of EP are 396.3 °C and 402.5 °C, respectively. Compared with neat EP, the Tonset and Tmax of all EP composites are decreased in various degrees, due mainly to the high thermal conductivity of graphene and the catalytic effect of LDH and CuMoO4 [35-37]. The char yield of neat EP is 0.1% at 700 °C. Compared with neat EP, the char yields of all EP composites are increased in various degrees at 700 °C and the specifics are displayed in Table 2. Among them, the char yield of EP4 is the highest against that of other samples, reaching 4.7%, indicating that CuMoO4 has a role in promoting the formation of char residue. Furthermore, the segmental motion of polymer chains could be evaluated by the glass transition temperature (Tg). In this work, the differential scanning calorimetry (DSC) is carried out to measure the Tg of EP and EP composites, and the DSC curves
11
of EP and EP composites are shown in Fig.6 The results reveal that the Tg value of neat EP is 114.8 °C. Compared to neat EP, the Tg values of the EP composites are all increased in various degrees, which is approximately 10 °C higher than that of neat EP. This may be ascribed to the fact that the addition of flame retardants restricted the movement of polymer segments in some degree. 3.3. Flammability of EP composites The cone calorimeter test can be used to study the fire safety performance of polymers by simulating the scene of a fire, and the PHRR and THR values are two important parameters to evaluate the properties of flame retardancy. The heat release rate (HRR) and THR curves of neat EP and EP composites are shown in Fig.7 (a) and (b), respectively. As can be seen from the HRR curve, neat EP burns severely when it is exposed to the heat source, and its PHRR and THR values could reach 1159 kW• m-2 and 56.1 MJ•m-2, respectively. Compared with neat EP, the PHRR and THR of EP1 are decreased by 21.3% and 9.8%, respectively. It is due mainly to the physical barrier effects of the LDH layer and metal oxide formed in the combustion process, inhibiting the volatilization of flammable gas and isolating oxygen. At the same time, LDH can absorb heat in the decomposition process and produce water vapor, which can lower the surface temperature of the matrix, resulting in decreased heat release of composites [26]. The PHRR and THR of EP2 are decreased by 31.1% and 7.0% in comparison with EP, respectively. This is ascribed to the physical barrier effect of graphene layers. In addition, compared with EP, the PHRR and THR values of EP3 are decreased by 37.9% and 13.6%, respectively. Compared with EP, it is worth
12
noting that the PHRR and THR values of EP4 are decreased higher than those of EP3, reaching 47.6% and 28.5%, respectively, showing that the flame retardancy of the EP composites can be further improved with the introduction of CuMoO4. Fig.7 (c) provides the mass loss curves of neat EP and EP composites in the process of combustion, and the specific data are displayed in Table 3. According to Fig.7 (c), the mass of neat EP is 5.6% after burning for 6 min. In addition, the char yield of all composites is obviously improved in comparison with EP. Especially, compared with all other samples, the char yield of EP4 is the highest, which is consistent with the results of TG. Fig.8 shows the digital images of the char residue of neat EP and EP composites after the cone calorimeter test. It can be seen from Fig.8 (a) that the char residue of neat EP is badly damaged and has large holes. After adding different flame retardants, the holes of the char residue of the EP composites are decreased obviously, especially in Fig.8 (e), where it can be seen that the surface of the char residue of EP4 becomes dense and the holes are almost invisible. The reasons for this are as follows: on the one hand, this is attributed to the physical barrier effect of RGO and LDH; on the other hand, the decomposition of CuMoO4 can produce metal oxides--MoO3, Cu2O and so on. These metal oxides could promote the formation of char by Friedel-Crafts alkylation and reductive coupling reaction, and the compactness of char residue is improved [38-39]. The LOI and UL94 vertical burning tests are used to further evaluate the flame retardancy of RGO-LDH/CuMoO4 in EP, and the results are listed in Table 3. It can be observed that the LOI value of neat EP is 20.3%, indicating it is flammable in air.
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When different flame retardants are added into EP, the LOI value of the EP composites are all improved in different degrees, and that of EP4 is improved most. The UL94 vertical burning test results show that neat EP has no rating, and EP1, EP2 and EP3 have no rating, either. However, EP4 with 2wt% RGO-LDH/CuMoO4 added can reach V-1 rating (representative images taken during the combustion process of EP and EP4 are shown in Fig.9). All those show that RGO-LDH/ CuMoO4 has better flame retardancy. 3.4. Smoke suppression of EP composites The smoke production rate (SPR) and total smoke production (TSP) are two important parameters in the cone calorimeter test, and can be used to evaluate the smoke generation of polymers during the process of combustion. Fig.10 presents the SPR (Fig.10 (a)) and TSP (Fig.10 (b)) curves of neat EP and EP composites, and the specific data are displayed in Table 3. It can be observed from Fig.10 (a, b), compared with neat EP, the SPR and TSP of EP1 composite are decreased by 12.9% and 18.4%, respectively, which is attributed mainly to the physical barrier effect of LDH, and the adsorption effect of the metal oxide generated from decomposed LDH with a larger surface area that inhibits the diffusion of smoke effectively [40]. The SPR and TSP of EP2 composite are decreased by 7.1%, 4.6%, respectively, compared with EP. This is ascribed to the physical barrier effect of the RGO sheets. Moreover, the SPR and TSP of EP3 are decreased by 23.5% and 25.3%, respectively. Fortunately, compared with all other samples, the SPR and TSP of EP4 is the lowest, decreased by 35.3% and 38.0%, respectively. The results show that RGO-LDH/CuMoO4 has a better effect of
14
smoke suppression. The smoke density test is used to further study the effect of RGO-LDH/CuMoO4 on the smoke suppression of EP. Fig.11 gives the smoke density curves of neat EP and EP composites. It can be seen that the maximum smoke density (Ds, max) of neat EP is 798. When added with different flame retardants, the smoke density of the EP composites is decreased in different degrees, reaching 37.6%, 25.6%, 42.4% and 52%, respectively. Among them, EP4 has the amplitude of decrease, showing that the smoke suppression can be further improved with the introduction of CuMoO4. 3.5. Mechanical properties of EP composites For practical applications, the mechanical properties of epoxy composites are also important. In this work, the mechanical properties of different flame retardants added into EP are studied. The data of tensile, flexural and impact properties of EP and EP composites are listed in Table 4. It can be seen from Table 4 that the tensile strength, strain at break, flexural strength and impact strength of the EP composites are decreased to some extent in comparsion with neat EP, and those of EP2 are decreased most obviously. This may be attributed to the strong van der Waals forces and π-π attraction between nanosheets of graphene, so graphene is easy to re-aggregate and hard to be evenly dispersed in EP [16,41]. However, it is worth noting that the tensile strength, strain at break, flexural strength and impact strength of EP3 and EP4 are increased in comparsion with EP2. This may be due to the fact that agglomeration of graphene is suppressed after LDH or LDH/CuMoO4 is loaded onto the surface of graphene, and therefore the dispersion of RGO-LDH and RGO-LDH/CuMoO4 in EP
15
are improved. 3.6. Char residue analysis of EP composites In order to study the mechanism of flame retardants, the residual char after the cone calorimeter tests of EP and EP composites is analyzed. Fig.12 is the Raman spectra of the char residue of EP, EP3, and EP4. As shown in Fig.12, the Raman spectra of the three samples show two strong absorption peaks at 1586 and 1354 cm-1, which correspond to G and D peaks, respectively. The area intensity ratio of the D to G bands (ID/IG) could be used to assess the degree of graphitization in the residual char. In general, the higher the value of ID/IG, the lower the graphitization degree of char residue [42]. The more content of graphite carbon, the more compactness of the char layer. The compact char layer can effectively inhibit the diffusion of flammable gas and further degradation of materials, enhancing the flame retardant properties of the materials. The ID/IG value of neat EP is 2.6, and the ID/IG value of EP3 is obviously decreased after the addition of 2 wt% RGO-LDH, reaching 2.4. This is because graphene can improve the content of graphite carbon, and the metal oxide generated from the decomposition of LDH also plays a role of promoting char formation. Furthermore, the ID/IG value of EP4 is further decreased with the addition of 2wt% RGO-LDH/CuMoO4, reduced to 2.2. The results show that CuMoO4 could improve the role of promoting the formation of graphitized carbon of graphene, and thus improving the flame retardancy and smoke suppression of the composites more effectively. The char layers of EP, EP3 and EP4 obtained from the cone calorimeter test are
16
further analyzed by the XPS. Fig.13 shows the C1s spectra of three samples respectively, and the detailed data are displayed in Table 5. As shown in the C1s spectra, there are three peaks appearing at 284.7, 285.8 and 287.5 eV, attributed to the carbon atoms of C-C, C-H, C-O and C=O, respectively [43]. The Cox/Ca is presented to study the thermal oxidation resistance of char residue, and Cox and Ca represents the content of the carbon atoms of C-O, C=O and C-C, C-H, respectively. The lower ratio of Cox/Ca, the better thermal oxidation resistance of the char residue [44]. As can be seen from Fig.13 and Table 5, the Cox/Ca value of EP4 is the lowest, indicating that RGO-LDH/CuMoO4 can improve the thermal oxidation resistance of the char layer effectively, which also further proves that RGO-LDH/CuMoO4 has better flame retardancy and smoke suppression. Fig.14 is the Mo 3d and Cu 2p spectra of EP4 char residue. It can be seen from Fig.14 (a) that the two peaks at 233.1 eV and 236.2 eV correspond to the Mo 3d3/2 binding energy of Mo4+ (MoO2) and Mo6+ (MoO3), respectively [45]. In addition, as shown in Fig.14 (b), the two peaks at 933.2 eV and 953.8 eV are attributed to the Cu2O binding energy [46]. Results show that MoO3 and Cu2O generated from RGO-LDH/CuMoO4 could enhance the production of char residue and raise the compactness of the char layer, so as to further improve the flame retardancy and smoke suppression of the composites. Fig.15 exhibits the XRD patterns of EP4 char residue obtained from the cone calorimeter test and the characteristic peaks of MoO3, MoO2 and Cu2O can be found in [14, 47-48]. Among them, MoO3 would be restored to MoO2 in the process of
17
decomposition. Meanwhile, MoO2 would be oxidized to MoO3. The results further evidence the generation of MoO3 and Cu2O from RGO-LDH/CuMoO4 during the combustion process. In view of the above, the mechanism of flame retardancy and smoke suppressing of the EP composite with RGO-LDH/CuMoO4 added is illustrated in Scheme 2. The reason for the improved flame retardancy and smoke suppression of the EP composite, on the one hand, is attributed to the physical barrier function of RGO and LDH, and the catalytic carbonization effect of LDH. On the other hand, MoO3 and Cu2O generated from CuMoO4 during the combustion process could enhance the production of char residue and raise the compactness of the char layer by the function of the Friedel−Crafts alkylation and reductive coupling [37,49], respectively, so as to further improve the flame retardancy and smoke suppression of composites. 4. Conclusions In this work, a novel hybrid of RGO-LDH/CuMoO4 was successfully prepared and applied into epoxy resin. TEM results indicated that RGO-LDH/CuMoO4 was well dispersed in EP. The TG and DSC results showed that the char yield at 700 °C and Tg were increased obviously, respectively, compared with neat EP. The results in respect to the mechanical properties showed that the tensile strength, strain at break, flexural strength and impact strength of EP4 were decreased to some extent in comparsion with neat EP, but increased in comparsion with EP2. The results of the cone calorimeter test indicated that the PHRR, THR, SPR and TSP of EP4 were the lowest in all the composites. At the same time, LOI test results indicated that the LOI value
18
of EP4 was the highest, compared with other composites. And the UL94 vertical burning test of EP4 reached the V-1 rating. In addition, the smoke density test results showed that the Ds, max of EP4 was decreased more significantly than that of EP3. The reduced fire harmfulness of the composites was attributed to the physical barrier effect of RGO and LDH and the catalytic carbonization effect of LDH. Furthermore, the results of residual char analysis showed that the introduction of CuMoO4 improved the graphitization degree and oxidation resistance of the residual char. It was due mainly to the MoO3 and Cu2O generated from RGO-LDH/CuMoO4 in the combustion process which could promote the formation of char and improve the compactness of the char layer. Compact char could isolate heat and oxygen, inhibiting the volatilization of flammable gas, and preventing further burning of the matrix, so as to efficiently improve the flame retardancy and smoke suppression of the composites. Acknowledgements The authorsare grateful to the Anhui Provincial Natural Science Foundation (17080 85ME113) and National Key Technology R&D Program (2013BAJ01B05) for their financial support.
19
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[33] Hong NN, Pan Y, Zhan J, Wang BB, Zhou KQ, Song L, et al. Fabrication of graphene/Ni-Ce mixed oxide with excellent performance for reducing fire hazard of polypropylene. RSC Adv 2013; 3 (37): 16440-48. [34] Ciocan CE, Dumitriu E, Cacciaguerra T, Fajula F, Hulea V. New approach for synthesis of Mo-containing LDH based catalysts. Catal Today 2012; 198 (1): 239-45. [35] Chen LF, Zhao PF, Xie HQ, Yu W. Thermal properties of epoxy resin based thermal interfacial materials by filling Ag nanoparticle-decorated graphene nanosheets. Compos Sci Technol 2016; 125: 17-21. [36] Zhou KQ, Gao R, Qian XD. Self-assembly of exfoliated molybdenum disulfide (MoS2) nanosheets and layered double hydroxide (LDH): Towards reducing fire hazards of epoxy. J Hazard Mater 2017; 338: 343-55. [37] Wang B, Zhou KQ, Wang BB, Gui Z, Hu Y. Synthesis and characterization of CuMoO4/Zn-Al layered double hydroxide hybrids and their application as a reinforcement in polypropylene. Ind Eng Chem Res 2014; 53 (31): 12355-62. [38] Wescott LD, Starnes WH, Mujsce AM, Linxwiler PA. Mechanistic studies on the role of copper- and molybdenum-containing species as flame and smoke suppressants for poly(vinyl chloride). J Anal Appl Pyrol 1985; 8: 163-72. [39] Chen MJ, Lin YC, Wang XN, Zhong L, Li QL, Liu ZG. Influence of cuprous oxide on enhancing the flame retardancy and smoke suppression of epoxy resins containing microencapsulated ammonium polyphosphate. Ind Eng Chem Res 2015; 54:12705-13.
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26
Sonochemical synthesis of Cu2O nanocubes for enhanced chemiluminescence applications. Ultrason Sonochem 2016; 29: 388-93. [47] Ressler T, Jentoft RE, Wienold J, Gunter MM, Timpe O. In situ XAS and XRD studies on the formation of Mo suboxides during reduction of MoO3. J Phys Chem B 2000; 104 (27): 6360-70. [48] Bhosale MA, Bhanage BM. A simple approach for sonochemical synthesis of Cu2O nanoparticles with high catalytic properties. Adv Powder Technol 2016; 27 (1): 238-44. [49] Tu HB, Wang JQ. An XPS investigation of thermal degradation and charring processes for PVC and PVC/Cu2O systems in the condensed phase-II. Polym Degrad Stab 1996; 54 (2-3): 195-203.
27
Schemes, Figures and Tables captions Fig. 1. XRD spectra of as-prepared samples. Fig. 2. Raman spectra of as-prepared samples. Fig. 3. TEM images of (a) GO, (b) RGO , (c) RGO-LDH, (d) RGO-LDH/CuMoO4, (e) Partially enlarged image of RGO-LDH/CuMoO4. and (f) EDS analysis of RGO-LDH/CuMoO4. Fig. 4. TEM images of EP3 (a) and EP4 (b) composites. Fig. 5. TGA (a) and DTG (b) curves of neat EP and EP composites. Fig. 6. DSC curves of neat EP and EP composites. Fig. 7. HRR (a), THR (b) and mass (c) curves of neat EP and EP composites. Fig. 8. Digital images of the char residue (a) EP, (b) EP1, (c) EP2, (d) EP3 and (e) EP4 Fig. 9. Combustion processes of neat EP and EP4 during the UL94 vertical burning test at different time Fig. 10. SPR (a) and TSP (b) curves of neat EP and EP composites. Fig. 11. Smoke density curves of neat EP and EP composites. Fig. 12. Raman spectra of char residue of EP, EP3 and EP4 composites. Fig. 13. C1s spectra of char residue of EP composites: (a) EP (b) EP3 (c) EP4. Fig. 14. Mo 3d and Cu 2p spectra of char residue of EP4. Fig. 15. XRD spectra of char residue of EP4.
28
Scheme. 1. Illustration of the CuMoO4 modification of RGO-LDH. Scheme. 2. Illustration for the flame-retardant mechanism for the effect of the RGO-LDH/P on EP.
Table 1 Formulas of neat EP and EP composites. Table 2 TG data of neat EP and EP composites. Table 3 The data from cone calorimeter, LOI, smoke density and UL94 vertical burning test of neat EP and EP composites. Table 4 Mechanical properties of neat EP and EP composites. Table 5 Results of C1s XPS of char residue of EP composites.
29
Fig. 1. XRD spectra of as-prepared samples.
30
Fig. 2. Raman spectra of as-prepared samples.
31
Fig. 3. TEM images of (a) GO, (b) RGO , (c) RGO-LDH, (d) RGO-LDH/CuMoO4, (e) Partially enlarged image of RGO-LDH/CuMoO4. and (f) EDS analysis of RGO-LDH/CuMoO4.
32
Fig. 4. TEM images of EP3 (a) and EP4 (b) composites.
33
Fig. 5. TGA (a) and DTG (b) curves of neat EP and EP composites.
34
Fig. 6. DSC curves of neat EP and EP composites.
35
Fig. 7. HRR (a), THR (b) and mass (c) curves of neat EP and EP composites.
36
Fig. 8. Digital images of the char residue (a) EP, (b) EP1, (c) EP2, (d) EP3 and (e) EP4
37
Fig. 9. Combustion processes of neat EP and EP4 during the UL94 vertical burning test at different time
38
Fig. 10. SPR (a) and TSP (b) curves of neat EP and EP composites.
39
Fig. 11. Smoke density curves of neat EP and EP composites.
40
Fig. 12. Raman spectra of char residue of EP, EP3 and EP4 composites.
41
Fig. 13. C1s spectra of char residue of EP composites.
42
Fig. 14. Mo 3d and Cu 2p spectra of char residue of EP4.
43
Fig. 15. XRD spectra of char residue of EP4.
44
Scheme. 1. Illustration of the CuMoO4 modification of RGO-LDH.
Scheme. 2. Illustration for the flame-retardant mechanism for the effect of the RGO-LDH/CuMoO4 on EP.
45
Table 1 Formulas of neat EP and EP composites.
Sample
EP
LDH
RGO
RGO-LDH
RGO-LDH/
（wt%）
（wt%）
（wt%）
（wt%）
CuMoO4 （wt%）
EP
100
0
0
0
0
EP1
98
2
0
0
0
EP2
98
0
2
0
0
EP3
98
0
0
2
0
EP4
98
0
0
0
2
46
Table 2 TG data of neat EP and EP composites. Sample
T5% (°C)
Tmax (°C)
EP
387.2
402.5
0.1
EP1
350.3
364.1
3.0
EP2
372.8
397.1
0.6
EP3
358.1
394.8
3.8
EP4
342.4
366.3
4.7
47
Char yield (%)
Table 3 The data from cone calorimeter, LOI, smoke density and UL94 vertical burning test of neat EP and EP composites. PHRR
THR
MASS
SPR
TSP
Sample
LOI
UL94
(%)
rating
Ds,max (kW· m-2) (MJ· m-2)
(%)
(m2·s-1)
(m2)
EP
1159
56.1
5.6
0.85
43.4
798
20.3
NR
EP1
912
50.6
9.8
0.74
41.4
498
23.2
NR
EP2
798
52.2
8.6
0.79
35.4
595
22
NR
EP3
720
48.5
14.8
0.65
32.4
460
23.5
NR
EP4
607
40.1
15.2
0.55
26.9
383
24.2
V-1
48
Table 4 Mechanical properties of neat EP and EP composites.
Sample EP
Tensile properties Strength Strain at break (MPa) (%) 70.5 5.5
Modulus (GPa) 2.7
Flexural property Strength (MPa) 108.6
Impact property Strength (KJ/m2) 7.7
EP1
67.9
4.4
3.0
98.1
7.4
EP2
57.9
2.6
3.3
70.2
5.3
EP3
61.8
3.6
3.1
87.7
6.5
EP4
65.2
4.0
3.0
89.8
6.9
49
Table 5 Results of C1s XPS of char residue of EP composites. C-C area
C-O area
C=O area
Sample
Cox/Ca (%)
(%)
(%)
EP
62.0
21.6
16.4
0.61
EP3
66.4
17.9
15.6
0.51
EP4
72.2
16.6
11.2
0.39
50
S0304-3894(17)30747-1 https://doi.org/10.1016/j.jhazmat.2017.09.057 HAZMAT 18903
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
17-5-2017 22-9-2017 30-9-2017
Please cite this article as: Wenzong Xu, Bingliang Zhang, Xiaoling Wang, Guisong Wang, Ding Ding, The flame retardancy and smoke suppression effect of a hybrid containing CuMoO4 modified reduced graphene oxide/layered double hydroxide on epoxy resin, Journal of Hazardous Materials https://doi.org/10.1016/j.jhazmat.2017.09.057 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
The flame retardancy and smoke suppression effect of a hybrid containing CuMoO4 modified reduced graphene oxide/layered double hydroxide on epoxy resin
Wenzong Xu *, Bingliang Zhang, Xiaoling Wang, Guisong Wang, Ding Ding
School of Materials Science and Chemical Engineering, Anhui Jianzhu University, 292 Ziyun Road, Hefei, Anhui 230601, People’s Republic of China
*Correspondence to: Wenzong Xu (Tel./Fax:+86-0551-63828157.
Email: [email protected])
Highlights
A novel hybrid of RGO-LDH/CuMoO4 was synthesized by co-precipitation method. RGO-LDH/CuMoO4 were well-dispersed in EP matrix according to TEM observation. RGO-LDH/CuMoO4 has better flame retardancy and smoke suppression for epoxy resin. The flame retardant mechanism of RGO-LDH/CuMoO4 was studied.
Abstract: The co-precipitation method was used to synthesize a hybrid with MgAl-layered double hydroxide loaded graphene (RGO-LDH). CuMoO4 was then introduced onto the surface of RGO-LDH to prepare a hybrid with CuMoO4 modified RGO-LDH (RGO-LDH/CuMoO4). The composition, structure and morphology of RGO-LDH/CuMoO4 were characterized by X-ray diffraction, Laser raman spectroscopy
and
Transmission
electron microscope-energy-dispersive X-ray
spectroscopy. It was found that the hybrid of RGO-LDH/CuMoO4 had been
1
successfully prepared. The effects of flame retardancy and smoke suppression of epoxy resin were studied with added RGO-LDH/CuMoO4. Results showed that the PHRR and THR of the EP composite with RGO-LDH/CuMoO4 added were decreased dramatically. The char yield, LOI and UL-94 vertical burning rating of the EP composite were increased, with improved flame ratardancy. In addition, the SPR, TSP, and Ds,max of the EP composite were decreased drastically with added RGO-LDH/CuMoO4. Its improved flame retardancy and smoke suppression performance were due mainly to the physical barrier of graphene and LDH, and the catalytic carbonization function of LDH. Meanwhile, Cu2O and MoO3 generated from RGO-LDH/CuMoO4 in the combustion process helped enhance the production of char residue and raised the compactness of the char layer.
Keywords: Graphene; Hybrid; Composites; Flame retardancy; Smoke suppression
1. Introduction Epoxy resin as a kind of very important thermosetting resin is widely used in electronic equipment, potting, flooring and adhesives, with its excellent mechanical properties, dimensional stability and chemical resistance properties [1-5]. However, epoxy resin is flammable and can generate much smoke during the combustion process in air, which causes a large limitation in its further applications [6-7]. Therefore, the development of new environmentally friendly flame retardants and smoke suppression agents is highly significant for the application of EP. In recent years, graphene as a new member of the carbon family has received 2
extensive attention from researchers due to its excellent physical, chemical and thermal properties [8-11]. According to previous reports, graphene can play a role as a physical barrier in the combustion process of polymers which may restrain the volatilization of combustible gas, so as to improve the flame retardancy of polymer [12,13]. However, there are two main problems in the application of graphene. On the one hand, graphene is liable to re-aggregate because of the existence of van der Waals forces and π-π interaction, so it is hard to be evenly dispersed in polymers. On the other hand, graphene can not effectively improve the smoke suppression performance of polymers. In order to solve these problems, many researchers focus on the surface modification of graphene. The modification methods mainly include: (1) modifying graphene with metal oxide, such as Cu2O, MoO3, Ni2O3, Sb2O3 and Co2O3 [14-16]; (2) modifying graphene with inorganic substances, such as MoS2, ferrocene and carbon nanotubes [17-19]; and (3) modifying graphene with organic species, such as melamine, DOPO, tetraethylenepentamine and siloxane [20-22]. Those results have shown that the modification of graphene can not only provide a good dispersion in polymer, but also further improve the flame retardancy and smoke suppression of polymer. However, there have been few reports about the modification of graphene with metal salts. In this study, a suitable metal salt was chosen to prepare a novel hybrid of modified graphene. The molybdenum and copper system has been widely applied in polymers due to its excellent performance of flame retardancy and smoke suppression. Zhou et al. [23] prepared carbon nanotubes that were coated with MoS2 (MoS2-CNTs) and studied
3
their effect on the flame retardancy of EP. Their results showed that, by adding 2 wt% of MoS2-CNTs, the PHRR and THR of the EP composites were decreased by 27% and 31%, respectively. Wang et al. [24] added MoO3, Sb2O3 and FeOOH into EVA and studied the effect of different smoke suppression agents on the smoke suppression of the composites. Their study found that the smoke suppression could be significantly improved with the addition of 5 wt% MoO3 into EVA, and the maximum smoke density was decreased by 38%. That was due mainly to the MoO3 generated in the combustion process of MoS2 which could promote the formation of a char layer, so as to achieve improved smoke suppression. Moreover, Chen et al. [25] studied the effect of Cu2O on the flame retardancy and smoke suppression of EP containing microencapsulated ammonium polyphosphate (MAPP), and their results showed that the addition of Cu2O not only further improved the flame retardancy and smoke suppression of composite materials, but also reduced the production of CO and CO2. This was mainly attributed to the fact that Cu2O could promote the formation of a char layer in the burning process of the polymer and enhance the compactness of the char layer, thereby improving the flame retardancy and smoke suppression of the composites. Thus, copper molybdate (CuMoO4) was selected by us to modify graphene because it contains two elements of molybdenum and copper. In addition, LDH was used as intermediate which could connect graphene and CuMoO4, respectively, because LDH has good flame retardancy and smoke suppression and the positive charge located on its surface can adsorb negative ions [26-27]. In this work, the hybrid of RGO-LDH was prepared through the co-precipitation
4
method. Then, CuMoO4 was loaded onto the surface of RGO-LDH to prepare a new hybrid of RGO-LDH/CuMoO4, as shown in Scheme 1. RGO-LDH/CuMoO4 was added into epoxy resin and its effect on the thermal properties, flame retardancy and smoke suppression of the EP composites were investigated. In addition, the mechanism of the improved flame retardancy and smoke suppression of the EP composites was investigated by analyzing the char residue. 2. Experimental 2.1. Materials Graphite powder (spectral purity), H2SO4 (98%), NaNO3, KMnO4, HCl (37%), H2O2 (30%), H2N2 (80%), ethyl alcohol, NaOH, Al(NO3)3•9H2O, Na2MoO4•2H2O, Cu(NO3)2•3H2O and Mg(NO3)2•6H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Epoxy resin was purchased from Wuxi Qianguang Chemical
Materials
Company
(Jiangshu,
China).
3,3′-dichloro-4,4′-
diaminodiphenylmethane (MOCA) was purchased from Guangzhou Fufei Chemical Company (Guangdong, China). 2.2. Preparation of RGO-LDH Graphene oxide (GO) was synthesized from graphite powder through the Hummers method [28]. 0.32g GO was dispersed into 150 ml deionized water solution containing 0.20 M NaOH and 0.05 M Na2CO3 to obtain an even dispersion of GO through ultrasonic for 1h. 0.005 mol Mg(NO3)2•6H2O and 0.005 mol Al(NO3)3•9H2O were dissolved in 200 ml deionized water, and then dropped into the dispersion of GO with strong stirring. The pH of the above solution was adjusted to 10 ± 5 by using 0.5 M
5
NaOH solution. This solution was heated to 60 °C for 6h. Then 0.75 ml N2H2•H2O was added into it and it was kept under 100 °C for 2h. Finally, the product was isolated by using centrifuge, washed with ethyl alcohol and deionized water until pH 7, and dried at 50 °C. In addition, RGO and MgAl-LDH were prepared by using the similar conditions. 2.3. Preparation of RGO-LDH/CuMoO4 1g RGO-LDH was dispersed into 150 ml deionized water with appropriate ultrasonication. 0.24g Na2MoO4 was dissolved in 150 ml deionized water and added into the dispersion of GO, with rapid stirring at 60 °C for 10h. Then, 0.24g Cu(NO3)2 was dissolved in 150 ml deionized water and added into the above mixed solution, which was kept at 60 °C for 2h with strong stirring. After the reaction, the product was isolated by using centrifuge, washed with ethyl alcohol and deionized water, and dried at 50 °C. 2.4. Preparation of EP composites EP composites were prepared by using a simple blending method under ultrasonic and stirring. For example, RGO-LDH/CuMoO4 was added into appropriate acetone solution to obtain an evenly dispersed solution. The above solution was sufficiently mixed with epoxy by strong stirring at 60 °C under ultrasonic, forming an even mixture. A computational amount of curing agent (molten MOCA) was added into this mixture and stirred fully, and then poured into a teflon mold to let it sit overnight. Finally, the mixture was cured at 110 °C for 2h and at 150 °C for 2h in the oven to complete the preparation of the EP composite. In addition, EP composites with added
6
RGO, MgAl-LDH and RGO-LDH were prepared under the same conditions, respectively (the specific formulations are displayed in Table 1). 2.5. Characterization X-ray diffraction (XRD) measurements were conducted by using a Japanese Rigaku X-ray diffractometer (Cu-Kα tube and Ni filter, λ= 0.1542 nm). Raman spectroscopy measurements were conducted at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., U.S.) and the measurements were performed by using a back-scattering geometry with the 514.5 nm excitation wavelength. Transmission electron microscope-energy-dispersive X-ray spectroscopy (TEM-EDS) images were obtained using a Japanese JEOL JEM-2100F instrument with a 200 keV accelerated voltage. The thermogravimetric analysis (TGA) was undertaken with a STA 409PC apparatus (NETZSCH, Germany) from room temperature to 700 °C under an air flow rate of 20 °C·min-1. Differential scanning calorimetry (DSC) was performed on a Q20 instrument (TA, U.S.) from 20 to 150 °C at a linear heating rate of 20 °C·min−1. The cone calorimeter combustion test was performed with a JCZ-2 cone calorimeter (Jiangning Analytic Instrument Company, China) in accordance with the ISO5660 standard procedures. Specimens with the size of 100 ×100 × 4 mm3 were exposed under a heat flux of 50 kW·m-2. The limited oxygen index (LOI) value was determined by using an HC-2 oxygen index meter (Jiangning Analytic Instrument Company, China) in accordance with the standard procedures of ASTM D2863-2012 and the specimens were 100 × 10 × 3 mm3 in size. The UL 94 vertical burning test was conducted on a CZF-3 instrument (Jiangning Analytic Instrument Company,
7
China) according to the UL 94 vertical burning test standard, and the size of all the samples was 125×13×3 mm3. Smoke density tests were conducted on a JSC-2 smoke density test instrument (China) in accordance with ISO 5659-2 and specimens with the size of 75×75×3 mm3 were irradiated horizontally under a heat flux of 25 kW·m-2. Tensile properties were tested on a REGER 3010 test machine (China) according to ASTM D638, and the size of all the samples was 165×13×3.2 mm3. Flexural strength was also tested on a REGER 3010 test machine according to GB/T 9341, and the size of all the samples was 80×10×4 mm3. Impact strength was tested on a XJJUH-50Q (China) according to ASTM D256, and the size of all the samples was 64×12.7×3.2 mm3. All data of mechanical properties were the average values of five samples. X-ray Photoelectron Spectroscopy (XPS) spectra were collected with an Escalab 250 spectrometer (Thermo Scientific Ltd., U.S.) using an Al Kα excitation radiation (hν=1486.6 eV). 3. Results and discussion 3.1. Characterization of as-prepared samples XRD is an effective method for characterizing the structure of layer or sheet nano-materials. Fig.1 is the XRD pattern of GO, RGO, MgAl-LDH, RGO-LDH, RGO-LDH/CuMoO4. As shown in the figure: the (002) peak of GO at 2θ = 9.8°; it can be calculated that the interlayer spacing is 0.89 nm through the Bragg equation; compared with graphite (the interlayer spacing of graphite is 0.32 nm), the interlayer spacing of GO is increased due to the oxygen functional groups that were introduced [29]. Compared with GO, the (002) peak of RGO moves to 2θ=25.1°; the interlayer
8
spacing is 0.35 nm. It indicates that the oxygen functional groups on the surface of GO have been removed after reduction. In addition, as shown in the pattern of MgAl-LDH, the (003), (006), (009), (015), (018), (110) and (113) diffraction peaks of MgAl-LDH can be observed at 2 θ = 11.6 °, 23.2 °, 34.7 °, 38.9 °, 45.8 °, 60.6 °, 62.1 °, respectively. The interlayer spacing of MgAl-LDH is 0.76 nm, indicating that CO32- exists in the interlayer of LDH [30]. It can be observed that the XRD pattern of RGO-LDH is almost the same as neat LDH except that the intensity of the diffraction peaks is smaller than neat LDH, and no characteristic peaks of RGO can be observed. The results suggest that restacking of graphene is effectively prevented with LDH loaded on the surface of RGO [31]. The standard diffraction peaks of CuMoO4 are shown in the XRD pattern [32]. In addition, compared with the XRD patterns of CuMoO4 and RGO-LDH, the diffraction peaks of both CuMoO4 and RGO-LDH can be observed in the pattern of RGO-LDH/CuMoO4, indicating that CuMoO4 is loaded on the surface of RGO-LDH successfully. Raman spectroscopy is used to characterize the structure of graphite materials. The Raman spectra of RGO, RGO-LDH and RGO-LDH/CuMoO4 are shown in Fig.2. The spectra of all the samples have two characteristic peaks at 1359 cm-1 and 1599 cm-1, corresponding to D and G bands, respectively. Among them, the D band is assigned to the lattice damage of graphene and amorphous carbon; the G band is indicative of the sp2 carbon atom vibration model [33]. In addition, as shown in the spectrum of RGO-LDH, an obvious peak can be observed at 1049 cm-1, attributed to the vibration peak of CO32- existing in the LDH space [30]. In the spectrum of RGO-LDH/CuMoO4,
9
compared with RGO-LDH, not only can a vibration peak of CO32- be observed, but a new characteristic peak can also be found at 931 cm-1, corresponding to the symmetric stretching vibration peak of Mo=O [34]. The Raman results indicated that the hybrid of RGO-LDH/CuMoO4 has been successfully prepared. TEM is used to directly observe the structure and morphology of GO, RGO, RGO-LDH and RGO-LDH/CuMoO4, and EDS is used to analyze the element composition of RGO-LDH/CuMoO4. As shown in Fig.3 (a), GO has the typical and thin two-dimensional layer structure. Fig.3 (b) indicates that RGO has many folds and restacking in some areas on the sheet, and these phenomena are due to the presence of van der Waals forces. Fig.3 (c) indicates that a lot of nano-sheets appear on the surface of graphene, showing that LDH has been successfully loaded onto graphene. The morphology of RGO-LDH/CuMoO4 in Fig.3 (d) is similar to the morphology of RGO-LDH. Fig.3 (e) is a partially enlarged image of the red rectangle in Fig.3 (d), and it shows that the surface of LDH becomes rough with lots of particles, indicating that CuMoO4 is well dispersed on the surface of RGO-LDH. Meanwhile, as seen in Fig.3 (f), the elements of C, O, Cu, Mg, Al and Mo can be found from the EDS of RGO-LDH/CuMoO4.
The
result
further
indicates
that
the
hybrid
of
RGO-LDH/CuMoO4 has been successfully prepared. It is well known that the dispersion of inorganic nano-fillers in polymer is an extremely important factor affecting the performance of polymer composites. The morphology of EP3 and EP4 ultrathin sections are observed by TEM, in order to investigate the dispersion of RGO-LDH and RGO-LDH/CuMoO4 in EP. As shown in
10
Fig.4, there is no obvious agglomeration and the size of RGO-LDH and RGO-LDH/CuMoO4 in EP ranges from 200 to 400 nm. The results of TEM show that RGO-LDH and RGO-LDH/CuMoO4 have a good dispersion in EP. 3.2. Thermal behavior of EP composites The thermal decomposition process of EP and EP composites is analyzed by thermogravimetric analysis. The TGA and DTG curves of EP and EP composites under the air condition are shown in Fig.5. The temperature at which the sample quality loss is 5 wt% is defined as the Tonset (The initial decomposition temperature); the maximum temperature at which the sample reaches the fastest thermal decomposition rate is defined as the Tmax. It can be seen from Fig.5 (a) and (b) that the Tonset and Tmax of EP are 396.3 °C and 402.5 °C, respectively. Compared with neat EP, the Tonset and Tmax of all EP composites are decreased in various degrees, due mainly to the high thermal conductivity of graphene and the catalytic effect of LDH and CuMoO4 [35-37]. The char yield of neat EP is 0.1% at 700 °C. Compared with neat EP, the char yields of all EP composites are increased in various degrees at 700 °C and the specifics are displayed in Table 2. Among them, the char yield of EP4 is the highest against that of other samples, reaching 4.7%, indicating that CuMoO4 has a role in promoting the formation of char residue. Furthermore, the segmental motion of polymer chains could be evaluated by the glass transition temperature (Tg). In this work, the differential scanning calorimetry (DSC) is carried out to measure the Tg of EP and EP composites, and the DSC curves
11
of EP and EP composites are shown in Fig.6 The results reveal that the Tg value of neat EP is 114.8 °C. Compared to neat EP, the Tg values of the EP composites are all increased in various degrees, which is approximately 10 °C higher than that of neat EP. This may be ascribed to the fact that the addition of flame retardants restricted the movement of polymer segments in some degree. 3.3. Flammability of EP composites The cone calorimeter test can be used to study the fire safety performance of polymers by simulating the scene of a fire, and the PHRR and THR values are two important parameters to evaluate the properties of flame retardancy. The heat release rate (HRR) and THR curves of neat EP and EP composites are shown in Fig.7 (a) and (b), respectively. As can be seen from the HRR curve, neat EP burns severely when it is exposed to the heat source, and its PHRR and THR values could reach 1159 kW• m-2 and 56.1 MJ•m-2, respectively. Compared with neat EP, the PHRR and THR of EP1 are decreased by 21.3% and 9.8%, respectively. It is due mainly to the physical barrier effects of the LDH layer and metal oxide formed in the combustion process, inhibiting the volatilization of flammable gas and isolating oxygen. At the same time, LDH can absorb heat in the decomposition process and produce water vapor, which can lower the surface temperature of the matrix, resulting in decreased heat release of composites [26]. The PHRR and THR of EP2 are decreased by 31.1% and 7.0% in comparison with EP, respectively. This is ascribed to the physical barrier effect of graphene layers. In addition, compared with EP, the PHRR and THR values of EP3 are decreased by 37.9% and 13.6%, respectively. Compared with EP, it is worth
12
noting that the PHRR and THR values of EP4 are decreased higher than those of EP3, reaching 47.6% and 28.5%, respectively, showing that the flame retardancy of the EP composites can be further improved with the introduction of CuMoO4. Fig.7 (c) provides the mass loss curves of neat EP and EP composites in the process of combustion, and the specific data are displayed in Table 3. According to Fig.7 (c), the mass of neat EP is 5.6% after burning for 6 min. In addition, the char yield of all composites is obviously improved in comparison with EP. Especially, compared with all other samples, the char yield of EP4 is the highest, which is consistent with the results of TG. Fig.8 shows the digital images of the char residue of neat EP and EP composites after the cone calorimeter test. It can be seen from Fig.8 (a) that the char residue of neat EP is badly damaged and has large holes. After adding different flame retardants, the holes of the char residue of the EP composites are decreased obviously, especially in Fig.8 (e), where it can be seen that the surface of the char residue of EP4 becomes dense and the holes are almost invisible. The reasons for this are as follows: on the one hand, this is attributed to the physical barrier effect of RGO and LDH; on the other hand, the decomposition of CuMoO4 can produce metal oxides--MoO3, Cu2O and so on. These metal oxides could promote the formation of char by Friedel-Crafts alkylation and reductive coupling reaction, and the compactness of char residue is improved [38-39]. The LOI and UL94 vertical burning tests are used to further evaluate the flame retardancy of RGO-LDH/CuMoO4 in EP, and the results are listed in Table 3. It can be observed that the LOI value of neat EP is 20.3%, indicating it is flammable in air.
13
When different flame retardants are added into EP, the LOI value of the EP composites are all improved in different degrees, and that of EP4 is improved most. The UL94 vertical burning test results show that neat EP has no rating, and EP1, EP2 and EP3 have no rating, either. However, EP4 with 2wt% RGO-LDH/CuMoO4 added can reach V-1 rating (representative images taken during the combustion process of EP and EP4 are shown in Fig.9). All those show that RGO-LDH/ CuMoO4 has better flame retardancy. 3.4. Smoke suppression of EP composites The smoke production rate (SPR) and total smoke production (TSP) are two important parameters in the cone calorimeter test, and can be used to evaluate the smoke generation of polymers during the process of combustion. Fig.10 presents the SPR (Fig.10 (a)) and TSP (Fig.10 (b)) curves of neat EP and EP composites, and the specific data are displayed in Table 3. It can be observed from Fig.10 (a, b), compared with neat EP, the SPR and TSP of EP1 composite are decreased by 12.9% and 18.4%, respectively, which is attributed mainly to the physical barrier effect of LDH, and the adsorption effect of the metal oxide generated from decomposed LDH with a larger surface area that inhibits the diffusion of smoke effectively [40]. The SPR and TSP of EP2 composite are decreased by 7.1%, 4.6%, respectively, compared with EP. This is ascribed to the physical barrier effect of the RGO sheets. Moreover, the SPR and TSP of EP3 are decreased by 23.5% and 25.3%, respectively. Fortunately, compared with all other samples, the SPR and TSP of EP4 is the lowest, decreased by 35.3% and 38.0%, respectively. The results show that RGO-LDH/CuMoO4 has a better effect of
14
smoke suppression. The smoke density test is used to further study the effect of RGO-LDH/CuMoO4 on the smoke suppression of EP. Fig.11 gives the smoke density curves of neat EP and EP composites. It can be seen that the maximum smoke density (Ds, max) of neat EP is 798. When added with different flame retardants, the smoke density of the EP composites is decreased in different degrees, reaching 37.6%, 25.6%, 42.4% and 52%, respectively. Among them, EP4 has the amplitude of decrease, showing that the smoke suppression can be further improved with the introduction of CuMoO4. 3.5. Mechanical properties of EP composites For practical applications, the mechanical properties of epoxy composites are also important. In this work, the mechanical properties of different flame retardants added into EP are studied. The data of tensile, flexural and impact properties of EP and EP composites are listed in Table 4. It can be seen from Table 4 that the tensile strength, strain at break, flexural strength and impact strength of the EP composites are decreased to some extent in comparsion with neat EP, and those of EP2 are decreased most obviously. This may be attributed to the strong van der Waals forces and π-π attraction between nanosheets of graphene, so graphene is easy to re-aggregate and hard to be evenly dispersed in EP [16,41]. However, it is worth noting that the tensile strength, strain at break, flexural strength and impact strength of EP3 and EP4 are increased in comparsion with EP2. This may be due to the fact that agglomeration of graphene is suppressed after LDH or LDH/CuMoO4 is loaded onto the surface of graphene, and therefore the dispersion of RGO-LDH and RGO-LDH/CuMoO4 in EP
15
are improved. 3.6. Char residue analysis of EP composites In order to study the mechanism of flame retardants, the residual char after the cone calorimeter tests of EP and EP composites is analyzed. Fig.12 is the Raman spectra of the char residue of EP, EP3, and EP4. As shown in Fig.12, the Raman spectra of the three samples show two strong absorption peaks at 1586 and 1354 cm-1, which correspond to G and D peaks, respectively. The area intensity ratio of the D to G bands (ID/IG) could be used to assess the degree of graphitization in the residual char. In general, the higher the value of ID/IG, the lower the graphitization degree of char residue [42]. The more content of graphite carbon, the more compactness of the char layer. The compact char layer can effectively inhibit the diffusion of flammable gas and further degradation of materials, enhancing the flame retardant properties of the materials. The ID/IG value of neat EP is 2.6, and the ID/IG value of EP3 is obviously decreased after the addition of 2 wt% RGO-LDH, reaching 2.4. This is because graphene can improve the content of graphite carbon, and the metal oxide generated from the decomposition of LDH also plays a role of promoting char formation. Furthermore, the ID/IG value of EP4 is further decreased with the addition of 2wt% RGO-LDH/CuMoO4, reduced to 2.2. The results show that CuMoO4 could improve the role of promoting the formation of graphitized carbon of graphene, and thus improving the flame retardancy and smoke suppression of the composites more effectively. The char layers of EP, EP3 and EP4 obtained from the cone calorimeter test are
16
further analyzed by the XPS. Fig.13 shows the C1s spectra of three samples respectively, and the detailed data are displayed in Table 5. As shown in the C1s spectra, there are three peaks appearing at 284.7, 285.8 and 287.5 eV, attributed to the carbon atoms of C-C, C-H, C-O and C=O, respectively [43]. The Cox/Ca is presented to study the thermal oxidation resistance of char residue, and Cox and Ca represents the content of the carbon atoms of C-O, C=O and C-C, C-H, respectively. The lower ratio of Cox/Ca, the better thermal oxidation resistance of the char residue [44]. As can be seen from Fig.13 and Table 5, the Cox/Ca value of EP4 is the lowest, indicating that RGO-LDH/CuMoO4 can improve the thermal oxidation resistance of the char layer effectively, which also further proves that RGO-LDH/CuMoO4 has better flame retardancy and smoke suppression. Fig.14 is the Mo 3d and Cu 2p spectra of EP4 char residue. It can be seen from Fig.14 (a) that the two peaks at 233.1 eV and 236.2 eV correspond to the Mo 3d3/2 binding energy of Mo4+ (MoO2) and Mo6+ (MoO3), respectively [45]. In addition, as shown in Fig.14 (b), the two peaks at 933.2 eV and 953.8 eV are attributed to the Cu2O binding energy [46]. Results show that MoO3 and Cu2O generated from RGO-LDH/CuMoO4 could enhance the production of char residue and raise the compactness of the char layer, so as to further improve the flame retardancy and smoke suppression of the composites. Fig.15 exhibits the XRD patterns of EP4 char residue obtained from the cone calorimeter test and the characteristic peaks of MoO3, MoO2 and Cu2O can be found in [14, 47-48]. Among them, MoO3 would be restored to MoO2 in the process of
17
decomposition. Meanwhile, MoO2 would be oxidized to MoO3. The results further evidence the generation of MoO3 and Cu2O from RGO-LDH/CuMoO4 during the combustion process. In view of the above, the mechanism of flame retardancy and smoke suppressing of the EP composite with RGO-LDH/CuMoO4 added is illustrated in Scheme 2. The reason for the improved flame retardancy and smoke suppression of the EP composite, on the one hand, is attributed to the physical barrier function of RGO and LDH, and the catalytic carbonization effect of LDH. On the other hand, MoO3 and Cu2O generated from CuMoO4 during the combustion process could enhance the production of char residue and raise the compactness of the char layer by the function of the Friedel−Crafts alkylation and reductive coupling [37,49], respectively, so as to further improve the flame retardancy and smoke suppression of composites. 4. Conclusions In this work, a novel hybrid of RGO-LDH/CuMoO4 was successfully prepared and applied into epoxy resin. TEM results indicated that RGO-LDH/CuMoO4 was well dispersed in EP. The TG and DSC results showed that the char yield at 700 °C and Tg were increased obviously, respectively, compared with neat EP. The results in respect to the mechanical properties showed that the tensile strength, strain at break, flexural strength and impact strength of EP4 were decreased to some extent in comparsion with neat EP, but increased in comparsion with EP2. The results of the cone calorimeter test indicated that the PHRR, THR, SPR and TSP of EP4 were the lowest in all the composites. At the same time, LOI test results indicated that the LOI value
18
of EP4 was the highest, compared with other composites. And the UL94 vertical burning test of EP4 reached the V-1 rating. In addition, the smoke density test results showed that the Ds, max of EP4 was decreased more significantly than that of EP3. The reduced fire harmfulness of the composites was attributed to the physical barrier effect of RGO and LDH and the catalytic carbonization effect of LDH. Furthermore, the results of residual char analysis showed that the introduction of CuMoO4 improved the graphitization degree and oxidation resistance of the residual char. It was due mainly to the MoO3 and Cu2O generated from RGO-LDH/CuMoO4 in the combustion process which could promote the formation of char and improve the compactness of the char layer. Compact char could isolate heat and oxygen, inhibiting the volatilization of flammable gas, and preventing further burning of the matrix, so as to efficiently improve the flame retardancy and smoke suppression of the composites. Acknowledgements The authorsare grateful to the Anhui Provincial Natural Science Foundation (17080 85ME113) and National Key Technology R&D Program (2013BAJ01B05) for their financial support.
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Schemes, Figures and Tables captions Fig. 1. XRD spectra of as-prepared samples. Fig. 2. Raman spectra of as-prepared samples. Fig. 3. TEM images of (a) GO, (b) RGO , (c) RGO-LDH, (d) RGO-LDH/CuMoO4, (e) Partially enlarged image of RGO-LDH/CuMoO4. and (f) EDS analysis of RGO-LDH/CuMoO4. Fig. 4. TEM images of EP3 (a) and EP4 (b) composites. Fig. 5. TGA (a) and DTG (b) curves of neat EP and EP composites. Fig. 6. DSC curves of neat EP and EP composites. Fig. 7. HRR (a), THR (b) and mass (c) curves of neat EP and EP composites. Fig. 8. Digital images of the char residue (a) EP, (b) EP1, (c) EP2, (d) EP3 and (e) EP4 Fig. 9. Combustion processes of neat EP and EP4 during the UL94 vertical burning test at different time Fig. 10. SPR (a) and TSP (b) curves of neat EP and EP composites. Fig. 11. Smoke density curves of neat EP and EP composites. Fig. 12. Raman spectra of char residue of EP, EP3 and EP4 composites. Fig. 13. C1s spectra of char residue of EP composites: (a) EP (b) EP3 (c) EP4. Fig. 14. Mo 3d and Cu 2p spectra of char residue of EP4. Fig. 15. XRD spectra of char residue of EP4.
28
Scheme. 1. Illustration of the CuMoO4 modification of RGO-LDH. Scheme. 2. Illustration for the flame-retardant mechanism for the effect of the RGO-LDH/P on EP.
Table 1 Formulas of neat EP and EP composites. Table 2 TG data of neat EP and EP composites. Table 3 The data from cone calorimeter, LOI, smoke density and UL94 vertical burning test of neat EP and EP composites. Table 4 Mechanical properties of neat EP and EP composites. Table 5 Results of C1s XPS of char residue of EP composites.
29
Fig. 1. XRD spectra of as-prepared samples.
30
Fig. 2. Raman spectra of as-prepared samples.
31
Fig. 3. TEM images of (a) GO, (b) RGO , (c) RGO-LDH, (d) RGO-LDH/CuMoO4, (e) Partially enlarged image of RGO-LDH/CuMoO4. and (f) EDS analysis of RGO-LDH/CuMoO4.
32
Fig. 4. TEM images of EP3 (a) and EP4 (b) composites.
33
Fig. 5. TGA (a) and DTG (b) curves of neat EP and EP composites.
34
Fig. 6. DSC curves of neat EP and EP composites.
35
Fig. 7. HRR (a), THR (b) and mass (c) curves of neat EP and EP composites.
36
Fig. 8. Digital images of the char residue (a) EP, (b) EP1, (c) EP2, (d) EP3 and (e) EP4
37
Fig. 9. Combustion processes of neat EP and EP4 during the UL94 vertical burning test at different time
38
Fig. 10. SPR (a) and TSP (b) curves of neat EP and EP composites.
39
Fig. 11. Smoke density curves of neat EP and EP composites.
40
Fig. 12. Raman spectra of char residue of EP, EP3 and EP4 composites.
41
Fig. 13. C1s spectra of char residue of EP composites.
42
Fig. 14. Mo 3d and Cu 2p spectra of char residue of EP4.
43
Fig. 15. XRD spectra of char residue of EP4.
44
Scheme. 1. Illustration of the CuMoO4 modification of RGO-LDH.
Scheme. 2. Illustration for the flame-retardant mechanism for the effect of the RGO-LDH/CuMoO4 on EP.
45
Table 1 Formulas of neat EP and EP composites.
Sample
EP
LDH
RGO
RGO-LDH
RGO-LDH/
（wt%）
（wt%）
（wt%）
（wt%）
CuMoO4 （wt%）
EP
100
0
0
0
0
EP1
98
2
0
0
0
EP2
98
0
2
0
0
EP3
98
0
0
2
0
EP4
98
0
0
0
2
46
Table 2 TG data of neat EP and EP composites. Sample
T5% (°C)
Tmax (°C)
EP
387.2
402.5
0.1
EP1
350.3
364.1
3.0
EP2
372.8
397.1
0.6
EP3
358.1
394.8
3.8
EP4
342.4
366.3
4.7
47
Char yield (%)
Table 3 The data from cone calorimeter, LOI, smoke density and UL94 vertical burning test of neat EP and EP composites. PHRR
THR
MASS
SPR
TSP
Sample
LOI
UL94
(%)
rating
Ds,max (kW· m-2) (MJ· m-2)
(%)
(m2·s-1)
(m2)
EP
1159
56.1
5.6
0.85
43.4
798
20.3
NR
EP1
912
50.6
9.8
0.74
41.4
498
23.2
NR
EP2
798
52.2
8.6
0.79
35.4
595
22
NR
EP3
720
48.5
14.8
0.65
32.4
460
23.5
NR
EP4
607
40.1
15.2
0.55
26.9
383
24.2
V-1
48
Table 4 Mechanical properties of neat EP and EP composites.
Sample EP
Tensile properties Strength Strain at break (MPa) (%) 70.5 5.5
Modulus (GPa) 2.7
Flexural property Strength (MPa) 108.6
Impact property Strength (KJ/m2) 7.7
EP1
67.9
4.4
3.0
98.1
7.4
EP2
57.9
2.6
3.3
70.2
5.3
EP3
61.8
3.6
3.1
87.7
6.5
EP4
65.2
4.0
3.0
89.8
6.9
49
Table 5 Results of C1s XPS of char residue of EP composites. C-C area
C-O area
C=O area
Sample
Cox/Ca (%)
(%)
(%)
EP
62.0
21.6
16.4
0.61
EP3
66.4
17.9
15.6
0.51
EP4
72.2
16.6
11.2
0.39
50