Green Preparation of Expandable Graphite and Its Application in

Apr 24, 2017 - Expandable graphites (EGs), including most acceptor-type graphite intercalation compounds (aGICs) and especially GICs-bisulfate, have b...
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Green Preparation of Expandable Graphite and Its Application in Flame-Resistance Polymer Elastomer Jindu Huang,†,§ Qianqiu Tang,†,§ Weibin Liao,† Gengchao Wang,*,† Wei Wei,‡ and Chunzhong Li† †

Key Laboratory for Ultrafine Materials of Ministry of Education, Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China ‡ Jiangsu Xinghua Rubber Belt Co., Ltd., Haian, Jiangsu 226600, P. R. China S Supporting Information *

ABSTRACT: Although research on expandable graphite (EG) has achieved great progress, its practical application is restricted due to high sulfur content and serious pollution during production. Herein, low-sulfur EG was prepared through a two-step intercalation method using environmentally friendly hydrogen peroxide as an oxidant. By optimizing reaction parameters and process, the as-prepared EG exhibits low sulfur content (1.85 wt %) and high exfoliation volume (282 mL g−1). Moreover, when 30 wt % EG is mixed with ethylene-vinyl acetate copolymer (EVM) matrix, the obtained EG/EVM composite exhibits a high limited oxygen index (LOI) of 30.4%, showing its excellent flame retardancy. Furthermore, for the compatibility between EG and EVM to be improved, poly(vinyl acetate) (PVAc) was grafted to EG by in situ radical polymerization. The EG-g-PVAc/EVM composite shows high flame resistant performance with improved control of fly ash. Therefore, this easily prepared EG can be broadly applied in environmentally friendly polymeric flame-resistance field.

1. INTRODUCTION Expandable graphites (EGs), including most acceptor-type graphite intercalation compounds (aGICs) and especially GICs-bisulfate, have been considered as an ideal flame retardant additive in polymer matrix due to its low cost, low smoke, antidripping melt, and halogen-free characteristics.1−8 During abrupt thermal shock or fire, with the decomposition and evaporation of intercalation guests of EG flakes pushing the adjacent graphene layers apart from each other, EG flakes in the polymer composite will expand several-hundred-fold in volume and form wormlike exfoliated graphite to isolate material from heat and air.9 Generally, GICs-bisulfate is synthesized by a chemically oxidative intercalation method using strong oxidizing agents (e.g., concentrated nitric acid, potassium permanganate, potassium dichromate, chromium trioxide, etc.) and concentrated sulfuric acid.10−18 The redox potential of the above-mentioned strong oxidizing agents is high enough to oxidize carbon atoms at plane edges and in defects,19−21 forming macrocation Cn+ for subsequent intercalation of anions and molecules.22,23 However, these conventional oxidizing agents bring severe pollution and hazards during the production and use of EG.24 Moreover, these kinds of EG release a certain amount of toxic gases like sulfur oxide16 and nitric oxide25 in fire, polluting the environment and severely harming human health. It is also found that heavy fly ash is emitted during combustion of EG/polymer composites, reducing the flame-resistance effect as a result.26,27 Furthermore, this phenomenon is also known as a “popcorn effect”, © 2017 American Chemical Society

which refers to the sudden expansion of the expandable graphite when suddenly exposed to high heat.28−30 The trend of sustainable development has set a higher demand on environmental friendliness and safety of materials. Various methods have been proposed to reduce or even eliminate the sulfur content of EG. Most of these methods adopt perchloric acid,31−34 hydrochloric acid,35 nitric acid,36−38 boric acid,39 and so forth, which may introduce new pollutants or hazardous chemicals. Recently, the electrochemical intercalation method has been claimed to be environmentally friendly as it uses dilute acid concentrations without chemical oxidizing agents.40,41 However, the installation of the electrochemical reaction is complicated, and this method also features low yield and high cost. Herein, we report a novel mild two-step intercalation method to prepare low-sulfur EG using environmentally friendly hydrogen peroxide as an oxidant, avoiding the use of highpollution oxidizing agents such as nitric acid, potassium dichromate, and so forth. First, natural flake graphite (NFG) was preoxidized by hydrogen peroxide before sulfuric acid intercalation. Second, first intercalation compounds were preoxidized again, and then concentrated phosphoric acid was added to replace some of the sulfate anions or sulfuric acid Received: Revised: Accepted: Published: 5253

December 16, 2016 April 17, 2017 April 24, 2017 April 24, 2017 DOI: 10.1021/acs.iecr.6b04860 Ind. Eng. Chem. Res. 2017, 56, 5253−5261

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Industrial & Engineering Chemistry Research molecules. Furthermore, high flame resistant thermoplastic elastomer with improved control of fly ash was obtained by melt blending of EG-grafted poly(vinyl acetate) together with ethylene-vinyl acetate copolymer (EVM).

X-ray diffraction (XRD) data were collected by a Rigaku D/ Max 2550 VB/PC X-ray diffractometer using Cu Kα radiation with the 2θ angle from 10−50°. Raman spectra were obtained with an inVia Reflex Raman spectrometer using a 50 mW He− Ne laser at 514 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi X-ray photoelectron spectrometer equipped with a monochromatic Al Kα X-ray source. The morphologies of the samples were observed by scanning electron microscopy (SEM, Hitachi S3400) and field-emission scanning electron microscopy (FESEM, Hitachi S4800). The elemental study of raw and intercalated graphite was investigated using a Vario EL Cube elemental analyzer. The Fourier transform infrared spectroscopy (FTIR) spectra were characterized using a Nicolet 6700 spectrometer equipped with a Smart OMIN Sampler. The limited oxygen index (LOI) of the composites was performed on a JF-3B oxygen index tester according to ASTM D2863 with the size of all samples to be 150 × 6 × 3 mm3. Cone calorimeter tests were carried out with an FTT cone calorimeter according to ISO 5660. All samples, of dimensions 100 × 100 × 3 mm3, were wrapped in aluminum foil and exposed horizontally to an external heat flux of 35 kW m2. For each formulation, the test was repeated three times, and the experimental error was ±5%. The mechanical properties of the composites were measured on a CMT 2203 testing system according to ASTM D638.

2. EXPERIMENTAL SECTION 2.1. Materials. Natural flake graphite (NFG, 300 μm, ≥99.9 wt %) was purchased from Qingdao Tianheda Graphite Co., Ltd., China. Hydrogen peroxide (H2O2, 30 wt %), sulfuric acid (H2SO4, 98 wt %), and phosphoric acid (H3PO4, 85 wt %) were received from Shanghai Lingfeng Chemical Reagent Co., Ltd., China. H3PO4 was concentrated (≥99.9 wt %) by mixing with the appropriate amount of phosphorus pentoxide.38 Azobis(isobutyronitrile) (AIBN, analytical grade) was obtained from Sinopharm Chemical Reagent Co., Ltd. and purified by recrystallizing from a mixed solution of ethanol and deionized water (1:1). Vinyl acetate monomer (VAc) was supplied by Shanghai Aladdin Bio-Chen Technology Co., Ltd. Ethylenevinyl acetate copolymer (EVM, Levapren 500HV, 50 wt % vinyl acetate content, 27 ± 4 cps Mooney viscosity, ≤5 g/10 min melt flow index, 1.00 g/cm3 density) was a product of Lanxess Chemical Co., Ltd. 2.2. Synthesis of Expandable Graphite. Initially, 3 g of NFG was preoxidized by H2O2 (0.4−0.8 mL) for 60 min under mechanical stirring and ultrasonication at ambient temperature. Then, H2SO4 (8−12 mL) was added slowly to the above mixture and stirred at various temperatures (20−60 °C) and times (60−140 min). The resulting mixture was filtered, washed repeatedly with deionized water and dried at 65 °C for 12 h to obtain the first intercalation compounds (EG-1). Second preoxidation treatment was applied to 1 g EG-1 (the same as the above preoxidization process). Subsequently, 3 mL concentrated H3PO4 was added slowly and was stirred at various temperature (20−60 °C) and time (20−180 min). The resulting product was washed with deionized water, and dried, successfully achieving expandable graphite (EG-2) with low sulfur content. 2.3. Preparation of Expandable Graphite-Grafted Poly(vinyl acetate). Expandable graphite-grafted poly(vinyl acetate) (EG-g-PVAc) was prepared by in situ free radical polymerization of VAc at edges of EG-2 (shown in Figure S1), and the detailed procedure is as follows: First, 20 g of VAc was dissolved in 50 mL of butyl acetate. Then, 10 g of EG-2 was added to the solution and stirred for 30 min. Then, 0.2 g of AIBN was added slowly to the above solution. The polymerization was kept at 65 °C for 24 h under an argon atmosphere. The resulting product was washed with butyl acetate several times and dried at 50 °C under vacuum for further use. 2.4. Preparation of Flame-Resistant EG/EVM Composites. EVMs were blended with EG-1, EG-2, and EG-g-PVAc in various proportions on Thermo Haake rheomixer at 100 °C for 10 min with a rotation speed of 60 rpm. The samples of the composites were obtained by press molding at 120 °C for 8 min. The size and thickness of the samples were dependent on different test requirements. 2.5. Characterization. The exfoliation volume (EV) was determined by exfoliating the prepared EG particles (M1 gram) in a quartz beaker with scale line by heating it in a muffle furnace at 800 °C for 60 s.40 The volume (V) of exfoliated product was recorded, and the mass (M2) was obtained by analysis balance. The EV and weight loss rate (WL) were calculated by following formulas: EV = V/M1, WL = (M1 − M2)/M2

3. RESULTS AND DISCUSSION 3.1. Optimization of Preparation Conditions. High exfoliation volume (EV) of expandable graphite is the key for improving the level of flame retardancy.42 For high-performance expandable graphite to be obtained, the preparation parameters and conditions of EG-1 were optimized by orthogonal method.32 According to standard 34 orthogonal experimental design, the factors (temperature and time of H2SO4 intercalation, dosage of H2SO4 and H2O2) and levels are listed in Table S1, which can be arranged into nine reactions (Table S2). The treatment results are shown in Figure 1. The height of the column represents the average value of EV of three products in each level under one factor. The bigger range indicates the greater influence of the factor to EV. By the calculations, the ranges of four factors (temperature, time, and dosage of H2SO4 and H2O2) are 40, 26, 9, and 8, respectively. Obviously, the value of EV is contributed mainly by

Figure 1. Column graph of orthogonal design results of EG-1. The levels of each factor are as follows: 30, 40, and 50 °C for temperature; 40, 60, and 80 min for intercalation time; 8, 10, and 12 mL for dosage of H2SO4, and 0.4, 0.6, and 0.8 mL for dosage of H2O2. 5254

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Figure 2. Exfoliation volume and weight loss as functions of the temperature and intercalation time for expandable graphite by (a, b) first and (c, d) second step intercalation.

Figure 3. (a) XRD patterns and (b) Raman spectra of natural flake graphite (NFG), EG-1, and EG-2.

Similarly, synthesis conditions of EG-2 have been optimized by using optimal EG-1 as raw material as shown in Figure 2c and d. It is interesting that EG-2 obtains considerably higher EV (up to 282 mL g−1) than that of EG-1 under the interaction temperature of 40 °C and time of 100 min. It is noteworthy that, different from EG-1, the WL and EV of EG-2 seem negatively correlated in the temperature range of 20−40 °C and intercalation time range of 20−100 min, which will be studied below. 3.2. Structure and Morphology Characterization of EG-1 and EG-2. XRD and Raman analysis were conducted to investigate the oxidation and intercalation effects of EG-1 and EG-2. For the untreated graphite, the sharp peak at 26.6° corresponds to the 002 crystal phase of pure graphite (Figure 3a). In comparison, EG-1 and EG-2 exhibit broader dominant diffraction peaks at lower angle (2θ = 26.0°), which is due to the expansion of c-axis of graphite and the increasing lattice defects such as stacking disorder43 resulting from the addition of oxygen-containing functional groups and intercalation of anions and molecules into the graphite. The disordered structure can be verified by the Raman spectra as well (Figure 3b). As we known, the D-band is associated with disorder-

temperature and time in the process of intercalation. However, dosage of H2O2 and H2SO4 have limited effects on EV. On the basis of these data, further studies have been carried out with single factor (temperature: 20−60 °C; time: 60−140 min) experiments under certain dosages of H2SO4 (10 mL) and H2O2 (0.8 mL) (Figure 2a and b). The optimum conditions (80 min, 40 °C) are obtained, and the maximum EV is up to 240 mL g−1. Moreover, it can be seen that the weight loss (WL) is positively correlated with EV in first step intercalation, which is because, during the exfoliation process of EG, gases are released resulting from the volatilization and decomposition of hydrogen sulfate anions and sulfuric acid molecules and the released gases make the volume of EG larger at the same time. In the temperature range of 40−60 °C and intercalation time range of 80−140 min, overoxidation occurs following deintercalation of inserts; thus, EV decreases. Furthermore, compared with intercalation temperature, EV does not change much with longer intercalation time. Thus, it can be said that EV is more sensitive to temperature, which is due to the fact that there is a higher tendency for volatilization of intercalates and decomposition of H2O2 under high temperature, further confirming the results of the orthogonal experiment. 5255

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Moreover, the overlapping C 1s peaks of both were resolved by curve fitting (Figure 5b, d, and f). The peak area at 286.2 eV (green peak) is related to the amount of oxidized carbon (i.e., C−OH, C−O−C, and CO bonds) at the edge of the sheets.10,45 It can be calculated that the peak area ratio of the oxidized carbon to conjugated CC bond structure (284.5 eV, blue peak) for EG-2 is 11.6%, which is more than that of EG-1 (8.2%) and nearly twice that of NFG (6.3%). Thus, the XPS results further demonstrate the process of twice oxidation. 3.4. Formation Mechanism of Expandable Graphite. For the process of two-step oxidative intercalation to be determined, a scheme of exchange-and-diffusion intercalation was proposed. As shown in Figure 6, after the second preoxidation for EG-1, more original graphite galleries are enlarged resulting from the addition of oxygen-containing functional groups at plane edges and defects. Moreover, during the second intercalation process, exchange between sulfur compounds and phosphoric compounds occurs due to a concentration difference effect. Furthermore, the mixture acid solution diffuses into new graphite grids and remains stable. After being washed and dried, EG-2 with low sulfur content was prepared. From the section morphologies of EG-1 and EG-2, it is obvious that EG-2 owns more detached sheets than that of EG-1. This macroscopic delamination damage may have been caused by large-scale oxidation, which indicates further disordering of graphite, leading to more insertion. 3.5. Flame-Resistant and Mechanical Properties of Composites. The flammability properties of flame-resistant EG-1/EVM and EG-2/EVM composites were analyzed by limited oxygen index (LOI). From Figure 7, it can be seen that, with the increase of EG content, the LOI values of EG-1/EVM and EG-2/EVM increase from 23.1 to 29.5% and 23.3 to 30.4%, respectively, which suggests that EG-2 enhances flame resistance for EVM. The reason for the improvement can be explained as follows. On the one hand, phosphorus guests between graphite layers of EG-2 will carbonize the polymeric matrix rapidly in fire because of its strong dehydration property. On the other hand, the char layers from polymer carbonization form strong adhesion with wormlike exfoliated graphite, which prevents the underlying material from heat and mass transfer with the outside fire. Thus, the flame retardancy of EG-2/EVM will be improved. However, because of the weak cohesiveness between inorganic graphite and organic polymeric matrix, rich fly ash will rise sharply during combustion, limiting the application of EG. Furthermore, the addition of EG may sharply deteriorate the mechanical properties of matrix resin materials because of their poor compatibility.46,47 Therefore, we attempt to graft organic poly(vinyl acetate) (PVAc) to EG-2 by in situ free radical polymerization to improve its compatibility and cohesiveness with EVM matrix. From the FTIR spectra of EG-2 and EG-g-PVAc in Figure S4, the characteristic peaks at 1228 and 1733 cm−1 correspond to the C−O and CO in the ester group of PVAc. Even FTIR cannot prove the successful grafting, it is thought that the grafting is the objective and it is likely that it happened. It can be seen from the FE-SEM images of EG-2/EVM (Figure 8a) that EG particles are curly and distinct in matrix, and there is a huge gap (Figure 8b, marked with a red arrow) in the phase interfaces, indicating incompatibility between EG particles and EVM matrix. In comparison, EG-g-PVAc in EVM (Figure 8c) becomes smooth and homogeneous and exhibits tight cohesiveness (Figure 8d, marked with blue arrows) with

induced scattering resulting from imperfections or loss of hexagonal symmetry of disordered graphite. The G-band corresponds to an E2g mode of graphite and is related to vibration of sp2-bonded carbon atoms in a two-dimensional hexagonal lattice.44 Therefore, the peak intensity ratio of Dand G-bands (ID/IG) was applied to evaluate the regularity of the graphite structure.45 As calculated, the ID/IG ratios of NFG, EG-1, and EG-2 are 0.076, 0.115, and 0.279, respectively, which suggests that the graphitic crystalline structure has been further damaged with increasing defect by two-step intercalation. Furthermore, the SEM images (Figure 4) of exfoliated EG-1 and EG-2 show an expanded wormlike structure. For

Figure 4. SEM images of (a, b) exfoliated EG-1 and (c, d) exfoliated EG-2 at magnifications of 300× and 1000×.

comparison, NFG and its high temperature treatment product exhibit a densely packed lamellar structure (shown in Figure S2). It suggests that only a sudden heating process will not lead to the expansion of graphite. Instead, the wormlike structure of EG comes from decomposition of intercalated sulfur compounds under high temperature. Moreover, compared with exfoliated EG-1 (Figure 4a and b), exfoliated EG-2 (Figure 4c and d) obtains more and tighter cellular structure, which verifies its increase of inserted layers and deintercalation of sulfur compounds. For the change in sulfur content of EG-2 at different intercalation times and temperatures to be confirmed, elemental analysis results (Figure S3) were obtained using an element analyzer. The results show that the sulfur content decreases with increasing intercalation time and temperature; thus, the WL will decrease correspondingly, which reasonably explains the negative correlation between WL and EV of EG-2. 3.3. Chemical Composition Analysis of EG-1 and EG-2. For the intercalated element content of EG-1 and EG-2 to be further quantified, XPS and Element Analysis were adopted. The XPS spectra of EG-1 (Figure 5c) and EG-2 (Figure 5e) show obviously stronger O 1s peaks (∼532 eV) than that of NFG (Figure 5a), indicating the introduction of a large amount of oxyacid. In the case of EG-2, after two-step oxidative intercalation, appearance of a P 2p peak (∼135 eV) and weakening of the S 2p peak (∼169 eV) were observed from the enlarged views, showing the introduction of a P element and decrease in the content of the S element for EG-2. Furthermore, the element analysis also shows that the sulfur content of EG-2 (1.85 wt %) is lower than that of EG-1 (3.74 wt %) as shown in Table 1. 5256

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Figure 5. XPS spectra with survey scan (a) NFG, (c) EG-1, and (e) EG-2 and C 1s core level XPS spectra for (b) NFG, (d) EG-1, and (f) EG-2.

EVM continuous phase without an obvious interface. This is because the existence of PVAc functions as a bridge between the inorganic part of EG and organic part of EVM. Because of its chemical structure and homogeneous morphology, EG-g-PVAc is an ideal candidate for flameresistant materials, especially thermoplastic elastomers. The digital photo shows that the ignition of EG-g-PVAc/EVM

Table 1. Element Analysis of NFG, EG-1, and EG-2 sample

C (wt %)

S (wt %)

NFG EG-1 EG-2

97.8 74.2 76.2

nil 3.74 1.85

Figure 6. Schematic illustration of the green preparation process of expandable graphite. 5257

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Figure 7. LOI as a function of EG content for EG-1/EVM and EG-2/ EVM composites.

(Figure 9c) was more stable with lower smoke and less fly ash compared with that of EG-2/EVM (Figure 9a). The residual expanded char layers of EG-2/EVM (Figure 9b) were fluffy and easily flake away from the substrate. In contrast, the carbon residues of EG-g-PVAc/EVM (Figure 9d) were compact and adhesive on the surface. The LOI of EG-g-PVAc/EVM is 28.7% compared to 27.9% of EG-2/EVM when the filler is 20 wt % in both composites (shown in Table S3), showing the enhanced flame resistance of EG via the grafting process. In addition, mechanical performances were measured by tensile testing. The stress−strain curves (Figure S5) show that the EG-g-PVAc/ EVM composite acquires better mechanical properties than those of EG-2/EVM. This may be due to the fact that the huge gaps between large EG particles and EVM may function as stress defect points during the stretching process, leading to sharply reduced tensile strength and breaking elongation. In comparison, surface modification of EG by PVAc narrows or even eliminates the gap, thus delaying the occurance of breakage.

Figure 9. Digital photos of ignition and residue of (a, b) EG-2/EVM and (c, d) EG-g-PVAc/EVM.

The cone calorimeter test, an effective bench-scale apparatus to simulate real fire scenarios, has been widely used to evaluate the flammability characteristics of polymeric materials.48 The curves of heat release rate (HRR) and total smoke release (TSR) curves at a heat flux of 35 kW m−2 are given in Figure 10. The related time to ignition (TTI), peak HRR (PHRR), total heat release (THR), and average specific extinction area (SEA) are also recorded in Table 2. The TTI of EG-g-PVAc/ EVM decreased to 33 from 46 s of pure EVM, which may be due to the high thermal conductivity of EG.49 However, the flame-resistant EG-g-PVAc/EVM composite showed a dramatic decline of the HRR and TSR curves with prolongation of the

Figure 8. FE-SEM images of the low temperature brittle fracture face of (a, b) EG-2/EVM and (c, d) EG-g-PVAc/EVM at magnifications of 100× and 1000×. 5258

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Figure 10. (a) Heat release rate (HRR) and (b) total smoke release (TSR) curves of pure EVM and flame-resistant EG-g-PVAc/EVM composite.

Table 2. Cone Test Results of Pure EVM and FlameResistant EG-g-PVAc/EVM Composite sample EVM EG-gPVAc/ EVM

TTI (s)

PHRR (kW m−2)

THR (MJ m−2)

TSR (m2 m−2)

average SEA (m2 kg−1)

FPI

46 33

790.8 124.5

107.8 68.8

1958.0 107.2

518.9 33.7

0.058 0.265

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flame-resistant and mechanical properties of EVM, EG2/EVM, and EG-g-PVAc/EVM (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 64253527. E-mail: [email protected]. ORCID

Gengchao Wang: 0000-0002-5421-0164 Author Contributions §

J.H. and Q.T. contributed equally to this work.

combustion time, and the PHRR and TSR values were reduced by 84.3 and 94.5%, respectively. In addition, the increased fire performance index (FPI, defined as the proportion of TTI and PHRR) of EG-g-PVAc/EVM suggests its great safety in a fire hazard. The modified EG may tend to form compact and continuous graphite worms in fire, which can act as a physical barrier to prevent heat transfer from heat source and smoke from substrate.42,50 As a result, the flame retardancy of EVM with EG-g-PVAc is significantly enhanced, which is also confirmed by the decrease of THR and average SEA.

Notes

4. CONCLUSIONS In this paper, a simple two-step intercalation method to prepare EG has been put forward. With conditions optimized, the prepared product obtains high exfoliation volume (282 mL g−1) and low sulfur content (1.85 wt %) with a certain amount of phosphorus between graphite layers. Analysis of the morphology, structure, and elements has validated the two-step exchange-and-diffusion intercalation process. LOI of the EG2/EVM composite can reach 30.4% when the filler content is 30 wt %. By grafting PVAc via in situ radical polymerization, EG had stronger interfacial adhesion with polymer matrix; the EG-g-PVAc/EVM composite exhibits enhanced flame retardancy and mechanical properties, and the fly ash has been effectively controlled during combustion. In summary, this investigation provides an easy and environmentally friendly preparation method for modified low-sulfur expandable graphite, which can be broadly applied in green polymeric fire resistance in the foreseeable future.

(1) Zheng, Z. H.; Liu, Y.; Zhang, L.; Wang, H. Y. Synergistic effect of expandable graphite and intumescent flame retardants on the flame retardancy and thermal stability of polypropylene. J. Mater. Sci. 2016, 51, 5857. (2) Yang, S.; Wang, J.; Huo, S. Q.; Wang, M.; Wang, J. P.; Zhang, B. Synergistic flame-retardant effect of expandable graphite and phosphorus-containing compounds for epoxy resin: Strong bonding of different carbon residues. Polym. Degrad. Stab. 2016, 128, 89. (3) Guo, Y. Q.; Bao, C. L.; Song, L.; Yuan, B. H.; Hu, Y. In situ polymerization of graphene, graphite oxide, and functionalized graphite oxide into epoxy resin and comparison study of on-theflame behavior. Ind. Eng. Chem. Res. 2011, 50, 7772. (4) Wang, B. B.; Hu, S.; Zhao, K. M.; Lu, H. D.; Song, L.; Hu, Y. Preparation of polyurethane microencapsulated expandable graphite, and its application in ethylene vinyl acetate copolymer containing silica-gel microencapsulated ammonium polyphosphate. Ind. Eng. Chem. Res. 2011, 50, 11476. (5) Xi, W.; Qian, L. J.; Huang, Z. G.; Cao, Y. F.; Li, L. J. Continuous flame-retardant actions of two phosphate esters with expandable graphite in rigid polyurethane foams. Polym. Degrad. Stab. 2016, 130, 97. (6) Chen, L.; Wang, Y. Z. A review on flame retardant technology in China. Part I: development of flame retardants. Polym. Adv. Technol. 2010, 21, 1. (7) Ye, L.; Meng, X. Y.; Ji, X.; Li, Z. M.; Tang, J. H. Synthesis and characterization of expandable graphite−poly (methyl methacrylate) composite particles and their application to flame retardation of rigid polyurethane foams. Polym. Degrad. Stab. 2009, 94, 971. (8) Chattopadhyay, D. K.; Webster, D. C. Thermal stability and flame retardancy of polyurethanes. Prog. Polym. Sci. 2009, 34, 1068.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We greatly appreciate the financial support of the National Natural Science Foundation of China (51673064), Shanghai Municipality Research Project (15520720500), and the Special Fund of Jiangsu Province for Scientific and Technological Achievements Transformation (BA2016119).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b04860. Schematic diagram of EG-g-PVAc, orthogonal table of samples, micromorphology of NFG and its high temperature treatment product, sulfur content of EG-2 in different reaction conditions, FTIR spectra, and the 5259

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DOI: 10.1021/acs.iecr.6b04860 Ind. Eng. Chem. Res. 2017, 56, 5253−5261

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DOI: 10.1021/acs.iecr.6b04860 Ind. Eng. Chem. Res. 2017, 56, 5253−5261

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Industrial & Engineering Chemistry Research coupling agent on thermal properties, flammability and mechanical properties of polypropylene/thermoplastic polyurethanes composites filled with expanded graphite. J. Therm. Anal. Calorim. 2014, 115, 689.

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DOI: 10.1021/acs.iecr.6b04860 Ind. Eng. Chem. Res. 2017, 56, 5253−5261