In Situ Polymerization of Graphene, Graphite Oxide, and

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In Situ Polymerization of Graphene, Graphite Oxide, and Functionalized Graphite Oxide into Epoxy Resin and Comparison Study of On-the-Flame Behavior Yuqiang Guo, Chenlu Bao, Lei Song, Bihe Yuan, and Yuan Hu* State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China ABSTRACT: Starting from expandable graphite (EG), graphene, graphite oxide (GO), and organic phosphate functionalized graphite oxides (FGO) were prepared and incorporated into epoxy resin (EP) matrix via in situ polymerization to prepare EP based composites. The structure of the composites was characterized by transmission electron microscopy to show good dispersion without large aggregates. The thermal behavior investigated by thermogravimetric analysis indicated the EP/graphene composites show the highest onset temperature and maximum weight loss temperature compared with those added with GO and FGO. The flame retardant properties investigated by micro combustion calorimeter illustrate that both EP/graphene and EP/FGO composites perform better than EP/GO composites in flame retardant properties with a maximum reduction of 23.7% in peak-heat release rate when containing 5 wt % FGO and a maximum reduction of 43.9% at 5 wt % loading of graphene. This study represents a new approach to prepare functionalized GO with flame retardant elements to improve the flame retardancy of polymer and gives a way of application of graphene in enhancing thermal stabilities of polymer.

1. INTRODUCTION As a new member of the carbon materials family and the thinnest material in the world,1 graphene has garnered an explosion of research in many areas of science and engineering during the past few years.2 The single-atom-thick sheet of hexagonally arrayed sp2-bonded carbon atoms structure3,4 endows graphene with unique electronic, thermal, and mechanical properties,5,6 which promises a variety of potential applications such as as transistors,7 sensors, and other nanoelectronic devices.810 A new trend of research into the applications of this intriguing material is anticipated to be inspired due to its unique properties mentioned above as well as the explosiveness brought by the 2010 Nobel Prize for Physics awarded to two scientists who first isolated and explored graphene.11 Individual graphene sheet exhibits a tremendous amount of excellent properties in various fields. As prevenient literature report, graphene presents extraordinarily high values of Young’s modulus (∼1100 GPa), fracture strength (∼125 GPa), elastic modulus (∼0.25 TPa), thermal conductivity (∼5000 Wm1 K1),12 mobility of charge carriers (∼200 000 cm2 V1 s1), and specific surface area (calculated value ∼2630 m2 g1).13, The most remarkable and widely studied feature of graphene is the electrical properties, such as the quantum Hall effect, ambipolar electric field effect, and transport via relativistic Dirac fermions, arising from its semimetallic nature.15 Different from its family members such as carbon nanotube (CNT) and fullerene (C60), graphene has a two-dimensional layered structure similar to that of montmorillonite (MMT) and layered double hydroxide (LDH), both of which have attracted considerable attention in the past two decades to prepare polymer/layered inorganic nanocomposites (PLN) to enhance the thermal stabilities and flame retardancy of polymer matrix with a small (99% was supplied by Qingdao Tianhe Graphite Co., Ltd. Potassium permanganate (KMnO4), hydrochloric acid (HCl), sodium nitrate (NaNO3 3 9H2O), ethanol (C2H5OH), m-phenylenediamine(C6H8N2), hydrazine hydrate (85%), and hydrogen peroxide (H2O2) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. Epoxy resin with epoxy value about 0.42 mol/100 g was obtained from Hefei Jiangfeng Chemical Industry Co., Ltd. Phenyl dichlorophosphate (PDCP) was purchased from Deheng Chemical Co. (Shijiazhuang, China) and distilled before use. Triethylamine (TEA) was purchased from Shanghai Chemical Reagents Company of China and dried over 4 molecular sieves before use. 2.2. Preparation of Graphite Oxide, Graphene, and Functionalized Graphite Oxide. Graphite oxide (GO) was prepared by the modified Hummers’ method38 from expandable graphite (EG). Briefly, 46 mL of H2SO4 was placed in a four-neck flask, cooled in an ice bath, followed by the addition of 2 g of expandable graphite and 1 g of NaNO3. The mixture was stirred for 15 min below 5 °C after which 15 g of KMnO4 was added slowly within 10 min. After 30 min at 810 °C, the mixture was heated to 35 °C and the reaction was continued for 60 min. Then 92 mL of H2O was dropped into the mixture and the temperature of reactants rose to 98 °C. The viscous mud was diluted in 500 mL of water and the unreacted KMnO4 was reduced with 30% H2O2 until the slurry turned golden yellow. The product

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was centrifuged, washed with dilute hydrochloric acid and hot water, and finally wet GO was obtained. In a typical procedure, wet GO was dispersed in water as solvent yielding an inhomogeneous dispersion. Yellow-brown dispersion without visible particulate was obtained by ultrasonic treatment for 30 min in a four-neck flask. Then, hydrazine hydrate (mass ratio of GO/hydrazine about 10:7) was added and the mixture was heated in an oil bath at 90 °C under a reflux condenser for 8 h.39 Graphene gradually precipitated out as black particles followed by centrifugation and washing with acetone by turns to isolate the product and obtain graphene acetone dispersion. A small piece sample of graphene was dried under a continuous air flow for the test. Distilled phenyl dichlorophosphate (PDCP, 0.03 mol, 6.33 g) was placed in a three-necked flask equipped with a mechanical stirrer, a nitrogen inlet, and a dropping funnel, and cooled in an ice-bath. Triethylamine (0.06 mol, 6.06 g) was added with stirring. Glycidol (0.02 mol, 2.32 g) in acetone (20 mL) was slowly added to the reaction vessel over 2 h. Then the mixture was stirred for 4 h and phenyl glycidol chlorophosphate (PGC) was obtained. Graphite oxide (0.5 g) in acetone (20 mL) was added to the system through a dropping funnel with stirring at ambient temperature for 12 h. The mixture was filtered and washed with water to remove triethylamine hydrochloride. Finally, functionalized graphite oxide (FGO) was obtained. 2.3. Preparation of Epoxy Resin Based Nanocomposites. The epoxy nanocomposite was prepared as follows: First, the required quantity of GO, graphene, or functionalized graphite oxide was dispersed in acetone with ultrasonic treatment, and then the GO solution was added to the epoxy oligomer with stirring for 0.5 h. The dispersion was put in a vacuum oven at 50 °C overnight to remove the solvent. When about 80% of the solvent was eliminated, m-phenylenediamine (the molar ratio of epoxy group/m-phenylenediamine was about 4:1) was added to the EP/GO system with vigorous stirring at 45 °C for 0.5 h. The mixture was poured into a stainless steel mold, dried at 60 °C for 5 h to remove the residual solvent, precured in an oven at 80 °C for 2 h, and postcured at 120 °C for 2 h to obtain the EP/GO composites. The preparation route of FGO and EP/FGO composites is shown in Scheme 1. 2.4. Characterization. X-ray diffraction (XRD) pattern was performed on the 1-mm-thick films with a Japan Rigaku D/MaxRa rotating anode X-ray diffractometer equipped with a Cu KR tube and Ni filter (λ = 0.1542 nm). X-ray photoelectron spectroscopy (XPS) was obtained using a VG ESCALB MK-II electron spectrometer. The excitation source was an Al Ka line at 1486.6 eV. Thermogravimetric analysis (TGA) were carried out on a TGA Q5000IR (TA Instruments, USA) thermo-analyzer instrument from 30 to 650 °C at a heating rate of 20 °C/min under air/ N2 flow and each sample was examined at least twice. Samples of about 5.0 mg were measured in an alumina crucible. Fourier transform infrared (FTIR) spectra were obtained using Nicolet 6700 FTIR (Nicolet Instrument Company, USA) between 500 and 4000 cm1. Raman spectroscopy (RS) measurements were performed using a SPEX-1403 laser Raman spectrometer (SPEX Co, USA) with excitation by a 514.5-nm argon laser line provided in backscattering geometry. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-100SX microscope with an acceleration voltage of 100 kV. GO, graphene, and FGO specimens were 7773

dx.doi.org/10.1021/ie200152x |Ind. Eng. Chem. Res. 2011, 50, 7772–7783

Industrial & Engineering Chemistry Research

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Scheme 1. Preparation Route of Functionalized GO and EP/FGO Composites

washed with ethanol, collected on 200 mesh copper grids, and then observed. The ultrathin sections of the EP composites membranes were microtomed using an Ultratome (MT-6000, Du Pont Company, USA) equipped with a diamond knife at room temperature, transferred to copper grids, and then observed. Atomic force microscopy (AFM) observation was investigated by DI Multimode V scanning probe microscope. The samples were dispersed in comportable solvents and dip-coated onto freshly cleaved mica surfaces before the test. Thermogravimetric analysis/infrared spectrometry (TG-IR) was carried out using a TGA Q5000 IR thermogravimetric analyzer interfaced to the Nicolet 6700 FTIR spectrophotometer (Nicolet). About 5.0 mg of the sample was put in an alumina crucible and heated from 30 to 650 °C at a heating rate of 20 °C/min (nitrogen atmosphere, flow rate of 45 mL/min). Combustion testing was investigated using GOVMARK MCC2 Micro Combustion Calorimeter (USA). The incident heat flux

was 45 kW/m2 and about 5-mg samples of the EP samples were heated to 700 °C at a heating rate of 1 °C/s and in a stream of nitrogen flowing at 80 cm3/min; each test was repeated twice.

3. RESULTS AND DISCUSSION 3.1. Structural Characterization of GO, Graphene, and FGO. 3.1.1. XRD. X-ray diffraction patterns of expandable graphite,

GO, graphene and FGO are shown in Figure 1. The (002) peak at 26.54° of the pristine expandable graphite indicates an interlayer distance of 0.34 nm. In the pattern of GO, the (002) peak shifts to 10.31°, suggesting that the interlayer spacing increases to 0.86 nm, which is in good agreement with previous results.40 After reduction, the resulting graphene sheets did not show any sharp peaks, demonstrating disorder and exfoliation. However, a broad weak peak at 23.8° appears showing some amount of reclustering. The 7774



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Table 1. Raman Peak Positions and D/G Intensity Ratio of GO, Graphene, and FGO peak of D

peak of D band

D/G intensity

band (cm1)

(cm1)

ratio

EG

1357

1583

0.55

GO

1355

1598

1.48

graphene

1352

1605

1.66

FGO

1359

1600

1.35

sample

Figure 1. XRD curves of the pristine EG, GO, graphene, and FGO.

Figure 3. FTIR spectra of PGC solution before and after the reaction with GO.

Figure 2. Evolution of the Raman spectra during the oxidation of EG, and the reduction and functionalization processes of GO.

FGO has a pattern similar to GO, so the modification process does not destroy the layered structure of GO. 3.1.2. Raman Spectroscopy. Raman spectra in Figure 2 as well as the peak positions and D/G intensity ratio listed in Table 1 reflected the significant structural changes occurring during the chemical processing from pristine EG to GO, RGO, and FGO. The Raman spectrum of the pristine EG displays a prominent G peak as the feature at 1583 cm1, corresponding to the first-order scattering of the E2 g vibration mode and a 2D band at 2725 cm1 due to the second-order two-phonon mode.41,42 A small D band adsorption at 1357 cm1 indicates the presence of defects caused inevitably by the intercalation of strong acid into the graphite layers during the EG preparation process. In the Raman spectrum of GO, the G band is broadened and shifts to 1598 cm1; the D band shifts downward to 1355 cm1 and becomes prominent, showing an increased D/G intensity ratio of 1.48 indicating the distortion of the bonds43 and destruction of symmetry possible due to the reduction in size of the in-plane sp2 domains caused by the extensive oxidation.30,41,42 In the case of graphene, an increased D/G intensity ratio of 1.66 compared

Figure 4. FTIR spectra of graphite oxide, graphene, and FGO.

to GO reflects a decrease in the average size of the sp2 domains upon reduction of GO30 but more numerous in number.41 In addition, the vibration frequency of the G band increases to 1605 cm1, slightly higher than that in graphite oxide, which can be attributed to the influence of defects and isolated double bonds.44,45 The Raman spectra of FGO is similar to that of GO, with a slight increase of vibration frequency of D and G bond and a decrease of D/G intensity ratio (D/G:1.35) which maybe caused by reaction of PGC with GO. 7775



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Figure 5. C1s XPS spectra of graphite oxide (a), graphene (b) and FGO (c), and P2p XPS spectra of FGO (d).

3.1.3. IR and XPS. The IR spectra of the samples from the flask were obtained to track the reaction of PGC with GO as shown in Figure 3. The peak at 570 cm1, which can be assigned to PCl, disappears after reaction with GO, indicating consumption of PCl group from PGC by the OH group from GO. Figure 4 shows the IR spectra of pristine EG, GO, graphene, and FGO. EG pattern shows some small peaks corresponding to the intercalated acid and oxidant. The GO pattern shows characteristic peaks labeled in the figure as others report.46 After the hydrazine reduction, all the oxygen-containing groups become very weak and a new peak around 1560 cm1 attributed to the stretching vibration of CdC from an aromatic group appears. In FGO, the peaks around 12501100 cm1 attributed to the aromatic OH become very weak indicating some of the OH groups on GO sheet are consumed during the reaction with PGC. The new emerged band at 1272 cm1 corresponds to the OdP group and the peak at 1044 cm1 is assigned to POC, both deriving from of the phosphorus functionality. The bands at 1388 cm1 and between 2975 and 2868 cm1 are due to the CH stretching vibrations of glycidol functionality. All these results confirmed the existence of PGC grafted on graphite oxide sheets. The C1s X-ray photoelectron spectroscopy (XPS) spectra of GO, graphene, and FGO are shown in Figure 5a, b, and c, respectively. The P2p XPS spectra of FGO is listed in Figure 5d. In the spectrum of GO, the peak at 284.8 eV is attributed to CC

(sp2 carbon), and the peaks between 286 and 289 eV are typically assigned to oxygen containing functionalities such as epoxide, hydroxyl, and carboxyl.47,48 It can be seen in Figure 5b that the reduction process largely suppressed the binding energy and intensity higher than 286 eV, leaving only a small shoulder around 288 eV, while the peak corresponding to CC (284.8 eV) becomes dominant, which confirms the successful reduction of GO. The functionalization process influences a lot as can be seen in Figure 5c. A new peak at 285.5 eV holding the percentage of 31% of the C1s peak is attributed to the COP group caused by the phosphorus functionality, and the P2p XPS spectra of FGO in Figure 4d also confirms the presence of OdPOC group, highly consistent with the IR results. 3.1.4. TG. Thermogravimetric analysis of GO, graphene, and FGO was carried out in N2 atmosphere and is shown in Figure 8a. Graphite oxide is not thermally stable, and its pyrolysis can be divided into three stages: the first stage, below 100 °C, is caused by evaporation of water; the second and the major mass loss occurring at ∼200 °C corresponds to the loss of the labile oxygen-containing groups such as hydroxyl, carboxyl etc. yielding CO, CO2, and steam;30 the third stage, 250800 °C, is due to the combustion of the residual char and the char yield obtained at 800 °C is 45.9 wt %. For graphene, the removal of the thermally labile oxygen functional groups by chemical reduction causing a decrease in defect density improves the thermal stability of graphene sheets to a large extent, which is consistent with the 7776



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Figure 6. TEM (left) and AFM (right) images of GO, graphene, and FGO showing large single sheets.

thermal behavior of graphene reported elsewhere.30 As can be seen from Figure 8a, no significant mass loss of graphene is detected until 800 °C; char residue at 800 °C is 83.5 wt % which is much more than that of GO. For FGO, the PGC group on the edge of GO sheets improves the thermal stability of GO during the whole thermal decomposition process with an increase of char residue of 50.8 wt %. The enhanced thermal stability can be attributed to the phosphorus-containing group of PGC which is considered as the flame retardant element. 3.1.5. TEM and AFM. TEM evaluation of GO, graphene, and FGO dispersed in ethanol after drying is shown in Figure 6 (left), and relatively large (tens of micrometers) and uniform platelets except for a few wrinkles and protrusions can be observed from the image. GO and graphene images reveal the presence of large single sheets.49 Rather than retaining a stacked structure, the material is exfoliated into monolayer or few-layered stacks, so graphite oxide colloid can be named as graphene oxide here.14 The peak in the XRD result of GO is caused by the restacking of graphene oxide sheet during the drying process. The lateral size of the sheets is about 15 μm and some sheets have wrinkles and folds on one edge. For the functionalized GO, the layer structure of graphite oxide is not destroyed which is consistent with the XRD results, but some cracks, perhaps caused by the chemical reaction between GO and PGC, producing defects are observed. AFM observations (Figure 6, right) were applied to find single sheets of chemically converted graphene as shown in Figure 6a (GO), b (graphene), and c (FGO). The AFM topography shows typical images of GO and graphene with the corresponding line

profile showing the average thickness of ∼1.2 nm for the graphene oxide platelets and 1 nm for the graphene sheets. It should be noted that FGO still preserves the layer structure of the GO with the average thickness of about ∼1.2 nm. 3.2. Structural Characterization of EP-Based Composites. 3.2.1. TEM. TEM images of EP composites with 1 wt % addition amount of GO, graphene, and FGO are shown in Figure 7a, b, and c, respectively, to reveal the dispersion. For the EP/GO composites, the dispersion of GO is generally uniform, but some bundles and stacks can be observed, which maybe due to the van der Waals force and hydrogen bond interaction between GO sheets caused by the oxygen-containing groups, hindering the dispersion of GO into the polymer matrix. Differently, the dispersions of graphene and FGO are generally uniform throughout the matrix with intercalated-exfoliated and hair-like structure without aggregates, and single or ultrathin sheets whose thickness is less than several nanometers can be observed, indicating good compatibility between graphene (or FGO) and EP to achieve nanoscale dispersion. The elimination of polar functional groups and the single or few layers structure weakens the interactions between graphene sheets, which improve the dispersion of graphene. As for the EP/FGO composites, the enhancement of interface interactions between FGO sheets and the EP matrix facilitate the dispersion of FGO sheets. From the morphology of the nanocomposites, the layered structure of graphite fillers is supposed to act as a barrier preventing the transfer of combustion gases to the flame zone and energy feedback, thus better thermal stability may be obtained. 7777



Figure 7. TEM micrograph showing the dispersion of graphite fillers in EP composites with GO (graphene or FGO) content of 1 wt %. (The scale bar is 200 nm).

3.3. Thermal Stability and Flame-Retardant Properties of EP and EP/G Composites. 3.3.1. TG. The TG and DTG curves

of EP and EP composites in N2 atmosphere are shown in Figure 8bd to investigate the differences in the degradation process of the composites. The onset degradation temperature (Td), which is defined as temperature at the 5 wt % weight loss (T5%) is obtained from the TG curves; the temperature of the maximum weight loss rate (Tmax) of samples is obtained from the DTG curves; the char yield of the composites at 650 °C is characterized by subtracting the char residue of graphite samples. All are listed in Table 2. The presence of graphite fillers influences the degradation of EP. There is one stage of weight loss corresponding to a strong

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DTG peak at 382 °C (Tmax) during the degradation process of pure EP. EP/GO composites decompose a little earlier compared with EP, and the T5% decreases gradually as the GO content increases. Tmax of the EP/GO composites is quite similar to EP, moreover, the values of the peaks from DTG pattern decrease dramatically and the char yield increases gradually as the GO content increases, indicating that the addition of GO can improve the thermal stability of EP/GO composites, which can be attributed to the absorption of free-radicals generated during polymer decomposition by the carbon surface. The T5% of EP/ graphene composites increases and the thermal stability is enhanced at low addition, but the EP/5 wt % graphene sample decomposes earlier than EP due to the high addition amount of graphene which limits the formation of network structures during the curing process similar to that of EP/GO composites, but more char residue of graphene is obtained due to the enhancement of thermal stability of graphene and larger specific area. For EP/FGO composites, all the samples show thermal degradation behavior similar to EP/GO composites but higher T5% is obtained than that of EP/GO and EP/RGO composites at 5 wt % addition, which may be due to the phosphoruscontaining group chemically bonded to the graphite sheet which performs dehydration effect during the thermal degradation process. These indicate that the incorporation of GO, graphene, and FGO can improve the degradation stability of the EP composites at higher temperature and promote the formation of char residue. 3.3.2. DSC. DSC test gives direct identification of the influence of the various forms of graphite on the glass transition temperature of EP, and the Tg data are listed in Table 2. The Tg of the EP composites shifts to lower temperature compared with EP, which indicates that the graphite acts as a plasticizer increasing the flexibility of chain segments of the EP matrix. With the addition amount of 1 wt %, the Tg data of EP/GO and EP/FGO are lower than that of EP/graphene, but when the amount increases to 5 wt %, EP/graphene has the lowest Tg compared to EP/GO and EP/ FGO, 17.6 °C lower than that of neat EP, which may be due to the compatibility of the additives in EP matrix. Epoxy groups on the GO and FGO sheets can be chemically bonded to the EP resin which improves the compatibility but restricts their mobility. Instead, graphene with few epoxy groups has better mobility in the EP resin and is a better plasticizer. At low content, the compatibility is dominant, so Tg of EP/graphene composite changes little (0.9 °C) for the 1 wt % graphene sample, but the decrement is 9.4 and 8.4 °C for EP/GO and EP/FGO, respectively. At high content, mobility is the key factor affecting the change of Tg, so a 17.6 °C reduction of Tg is obtained for EP/5 wt % graphene composite, which is more than that of EP/GO and EP/ FGO. 3.3.3. MCC. The micro combustion calorimeter (MCC) is a new, rapid, and effective test that uses thermal analysis methods to investigate the combustion properties of polymer materials on the bench scale. HRR (heat release rate) is a very important parameter as it expresses the intensity of a fire, which in turn can be used to predict the combustion behavior of a material in a real fire. The flammability properties of the EP composites are characterized by MCC, and the HRR, THR, and Tmax data are summarized in Figure 9. It is a distinct trend that the THR (total heat released), PHRR (peak heat release rate) and Tmax (maxium heat release rate temperature) of all the EP composites decrease as the content of graphite fillers increase compared to the neat EP resin. EP/Graphene 7778



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Figure 8. TGA curves of GO, graphene, and FGO in N2 atmosphere (a); TGA and DTG curves of EP composites in N2 atmosphere (b, c, d).

Table 2. TG and DSC Data of EP Composites in N2 sample no.

sample ID

T5% (°C)

Tmax (°C) 214

char residue (%)

char yield from EP (%)

Tg (°C)

GO

GO

112.5

GNS

GNS

245

48.9

FGO

FGO

158

209

EP

EP

363

381

EP1-1

EP/0.3% GO

355

380

14.6

14.5

EP1-3

EP/1.0% GO

352

377

17

16.7

137.1

EP1-5

EP/5.0% GO

254

379

17

15.3

135.4

EP2-1 EP2-3

EP/0.3% GNS EP/1.0% GNS

368 362

379 378

14.4 16.6

14.2 15.9

145.6

EP2-5

EP/5.0% GNS

349

380

19.4

16.0

128.9

EP3-1

EP/0.3% FGO

355

372

14.1

14.0

EP3-3

EP/1.0% FGO

326

373

16.2

15.8

138.1

EP3-5

EP/5.0% FGO

342

373

18.3

16.4

132.0

84.6

composites have the lowest PHRR with a maximum reduction of 43.9% at 5% loading. The reduction trend of THR of EP/FGO composites is more remarkable than that of EP/graphene composites, and reaches the lowest value with a reduction of 23.7% at 5% loading. Tmax of EP/GO composites decrease with the increase of GO content, which may be due to the unstable oxygen-containing groups on the GO sheets, but no obvious changes are seen for the

54.4 13.4

146.5

Tmax of EP/graphene and EP/FGO composites. The EP/graphene and EP/FGO composites show better flame retardant properties than that of EP/GO composites, and the mechanism can be described as follows: first, the unstable oxygen-containing groups on the GO sheet and the poor dispersion of GO in the EP matrix make the GO filler less effective; second, as for graphene, the thermal stability of graphene is increased due to the elimination 7779



Figure 9. THR and peak-HRR data of EP/G composites from MCC test in N2 atmosphere.

of oxygen-containing groups on the GO sheets and the dispersion of graphene is improved; third, as for FGO, the flame retardant element phosphorus chemically bonded on the FGO sheet enhanced the flame retardant effect of GO, and the dispersion of FGO is also better than that of EP/GO. The THR and PHRR are apparently reduced by the addition of GO, graphene, and FGO, implying that the incorporation of GO, graphene, and FGO in EP improves the flame retardancy of EP. The flame retardant effect of both graphene and FGO is much better than that of GO.

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3.3.4. TG-FTIR Analysis of EP and EP/G Composites. The TG-IR technique is used to identify the gaseous products formed during thermal degradation; this can help to understand the thermal degradation mechanism.50 The 3D TG-FTIR spectra of the gases formed in the thermal degradation of EP and EP composites per milligram are shown in Figure 10. The evolved gas analysis for EP, EP/5% GO, EP/5% graphene, and EP/5% FGO at maximum decomposition exhibited characteristic bands of H2O and/or phenol (3656 cm1), CO2 (2360 cm1), hydrocarbon (CH3 and CH2) groups (29702880 and 10401300 cm1), compounds containing aromatic rings (1604, 1507 cm1) and aromatic CH: 30153060 cm1.51,52 The gaseous products of pure EP resin mainly evolve between 20 and 30 min, generating a series of sharp peaks in the 3D spectra. As to the EP composites, the absorbance intensity decrease a lot compared to neat EP, indicating less volatile evolution. The maximum decomposition rate shifts to earlier time and some small peaks at 713 min are also observed for EP/5 wt % GO which is in accordance with the TG results (Figure 8); this may be attributed to the release of oxygencontaining groups on the GO layers. The samples of EP/5 wt % graphene and EP/5 wt % FGO are similar to EP and a series of sharp peaks appear at 1825 min, but the absorbance intensity decreases a lot all along the degradation range. It can be deduced that GO, graphene, and FGO may catalyze the thermal decomposition of EP. However, the intensities of the pyrolysis products for EP/GO, EP/graphene, and EP/FGO are lower than those of EP, especially EP/FGO. Consequently, the release of combustible gases and the weight loss are reduced by the presence of the various form of graphite. 3.3.5. Flame Retardancy and Thermal Degradation Mechanism. Studies on TG and TG-IR curves of the composites demonstrate that below the onset degradation temperature of EP, the graphite samples have already begun to degrade because of the oxygen-containing groups on the surface of carbon layers. These groups can accelerate the degradation of EP, but the released H2O also absorbs part of the heat to reduce the total heat released, which is observed in the MCC. The dispersion of the graphite materials in the matrix can act as a barrier and effectively slow the heat released and hinder the transfer of combustion gases to the flame zone and energy feedback, which is similar to the process by which MMT and LDH perform.17,18 In addition, the carbon layer formed during the degradation of the graphite sample can hinder transfer of heat. Consequently, the peak HRR and THR are reduced. The various graphite materials increase the viscosity and hinder the thermal movement of polymer chains, as has been previously reported in a study on the flame retardancy of CNT.53 The large specific surface of the graphite may act as a radical trap for the radicals generated during the combustion and degradation process, but this needs further study. There are some differences in the thermal degradation and flame retardancy performance among the three systems. GO contains many oxygen groups which begin to be released at 200 °C, promoting the degradation of EP at low temperature. Compared to GO, graphene has fewer oxygen-containing groups and the graphite sheets are restored to a certain extent, so more char residue is obtained from EP/graphene to enhance the thermal stabilities at high temperature, thus the peak of the DTG curve from the EP/graphene composite is diminished and a large reduction in HRR and THR values is obtained. And from the TG-IR figure of EP/graphene composites, little peaks before 350 °C are observed and the maximum decomposition peaks are 7780



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Figure 10. 3D TG-FTIR spectrum of gas phase in the thermal degradation of EP composites.

reduced. Phosphorus is often used as acid source in intumescent flame retardant systems, and the phosphorus-containing groups covalent bonded to the surface of FGO accompanied by the amine groups from the curing agent can interact to enhance the flame retardancy of graphite oxide. The onset degradation temperature of EP/graphene and EP/FGO is much higher than that of EP/GO, and the 3D TG-IR spectrum, unlike EP/GO, shows few peaks before the maximum degradation temperature and the peak absorbance intensity at the maximum decomposition temperature decreases a lot compared to neat EP. In addition, PGC covalent bonded to GO provides extra epoxy groups, which can react with the curing agent during the curing process, thus FGO is chemically bonded to EP, which improves the dispersion of FGO in EP matrix. This provides a way to modify GO with phosphorus and active functional groups to improve the dispersion of graphene into polymer matrix and enhance the flame retardancy of polymer.

reduction of the THR and HRR, which indicates a good flame retardant effect of graphite sample in EP composites. The TG-IR indicates graphite sample can promote combustion at low temperature but reduce the gas released at high temperature, which may be due to the layered effect thus enhancing the flame retardancy and promoting the char formation. From this study, we confirm that the graphite samples we prepared can both improve the thermal stability and flame retardancy of epoxybased composites, especially graphene and functionalized graphite oxide. Our work provides a way to modify GO with flame retardant elements to enhance its flame retardancy, and more work is needed to investigate the flame retardant performance and mechanism of graphene in other polymers.

’ AUTHOR INFORMATION Corresponding Author

*Tel/fax: þ86-551-3601664. E-mail: [email protected].

4. CONCLUSION GO, graphene, and FGO were incorporated into epoxy resin followed by in situ thermocuring to prepare a series of epoxy resin/G composites containing 0.3 wt %, 1 wt %, and 5 wt % of additives. Graphite-containing epoxy resin composites were prepared and their stability and flammability properties were investigated. The TG results show that the graphite samples addition can promote the char residue and the DTG figure shows a decline of the DTG peaks. The MCC data reveal a considerable

’ ACKNOWLEDGMENT This work was financially supported by the joint fund of National Science Foundation of China (NSFC) and Civil Aviation Administration of China (CAAC) ( 61079015), and the Program for Education combined with production and research of Guangdong province and Education Department of Chinese government (2009A090100010, 2009A090100029). 7781



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