Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11745−11753
Novel Approach toward the Synthesis of a PhosphorusFunctionalized Polymer-Based Graphene Composite as an Efficient Flame Retardant Dattatray A. Pethsangave, Rahul V. Khose, Pravin H. Wadekar, and Surajit Some* Department of Dyestuff Technology, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
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S Supporting Information *
ABSTRACT: In this article, we report the synthesis of polymer-functionalized graphene composites as highly potent flame retardants. Functionalized polyaniline (PANI)- and polypyrrole (PPy)-supported graphene nanocomposites were synthesized by the reaction of graphene oxide, and monomers of the above-mentioned polymers, aniline and pyrrole, respectively, in the presence of phosphoric acid. These synthesized nanocomposites show excellent flame-retardant properties when coated with cotton fabric and wood. When G-fPANI and G-fPPy solutions were coated on a cloth piece which was exposed to a flame its initial shape and size were sustained by liberating a little amount of smoke. At the initial stage, the coated cloth did not catch fire for more than 620 s (10.20 min) and 380 s (6.20 min) in case of G-fPANI and G-fPPy, respectively, whereas the use of only PANI, PPy, and GO coated on blank cloths were totally burned within 14, 10, and 10 s, respectively. Blank cloth subjected to fire was totally burned within 10 s, leaving small amounts of black mass. Flame-retardant efficiency of G-fPANI- and G-fPPy-coated cloth was confirmed by detailed flame tests such as a limiting oxygen test (LOI), vertical flammability test, and exposure to high temperature (∼1500 °C). In the case of the LOI test, G-fPANI- and G-fPPy-coated cloths show high values up to 47.6 and 41.9 indicating an excellent flame-retardant property. Like cotton fiber, wood was also used to check the flame-retardant nature of prepared nanocomposites, and it showed good results. This is the first time such a novel approach has been made to prepare polymer-functionalized graphene nanocomposites as a flame retardant for fire prevention using a simple, cost-effective route in comparison to prior work. KEYWORDS: Graphene oxide, Flame retardant, Polymeric nanocomposite, Ferric chloride
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used fiber), wool, and silk, or synthetic textile fibers are highly flammable.5−7 Recently, halogen- and silicon-based flame retardants8−11 have been used as gas-phase and solid-state flame retardants, whereas boron-based flame retardants are used as char-forming and smoke suppression flame retardants.12,13 Mineral-based flame retardants such as aluminum hydroxide and magnesium hydroxide are gas-phase dilution, cooling solid state, and water-free flame retardants.14−17 However, most of these flame retardants result in the formation of toxic gases and smoke products during the burning process.18 Also, halogen-based flame retardants are the main environmental concern. Hence, it is required to synthesize innovative sustainable and effective flame retardants to equilibrate the flame retardant performance and the environmental problems caused due to the usage of conventional flame retardants. So far, many researchers have worked widely on polymeric nanocomposites with the addition of
INTRODUCTION Nowadays, polymeric materials are getting attention in several areas such as coating materials, covering vehicles, electronics, and in aircrafts. This is because they show good properties like low weight to strength ratio, physical and chemical stability, heat resistance, chemical resistance, corrosion resistance, and self-lubrication.1,2 On the other hand, compared to metal and ceramic materials, the main disadvantage of polymeric materials is their flammability, which limits their applications in some areas.3,4 Hence, it is required to improve the fire safety of polymeric materials for their applications in several useful fields since fire-related injuries and property damages are the main complications. The main role of fire-related innovation is the development of significantly effective material, which can decrease the fire exposure to protect life and property. Use of flame retardants for fabric presents less threats to human lives and the environment making it sustainable.3 Due to the growth of polymer foams, woods, and the textile industry, the amounts of textile fibers used to produce household products, industrial products, and decorative items have increased tremendously; however, these textile fibers, such as cotton (the most widely © 2019 American Chemical Society
Received: April 9, 2019 Revised: May 20, 2019 Published: June 11, 2019 11745
DOI: 10.1021/acssuschemeng.9b01975 ACS Sustainable Chem. Eng. 2019, 7, 11745−11753
Research Article
ACS Sustainable Chemistry & Engineering nanosized inorganic fillers. In this area, the main focus of the researchers was on nanocomposites reinforcement, barrier properties, electro-optical properties, biological properties, and flame-retardant properties.19−24 Previous researchers, like Gilman,19 Wilkie and Jang,25 Alexandridis et al.,26 Camino et al.,27 Camino et al.,28 and Hu et al.,29 have applied polymericbased nanocomposites for advancement of flame-retardant properties by combining small amounts of nanoadditives, such as montmorillonite (MMT), carbon nanotubes (CNT), graphene, layered double hydroxides (LDHs), and polyhedral oligomeric silsesquioxane (POSS) into the polymer matrix. Many research articles reported a 2D nanomaterial-reinforced polymer composite showing flame-retardant properties such as Hbn/PTFE composites,30 multifunctional MoS2 hybrid materials,31 carbon fiber/acrylonitrile-butadienestyrene composites,32 conductive boron nitride/polyimide composites,33 thermal conductivities for PPS dielectric nanocomposites,34 and multifunctional boron nitride nanosheets.35 Graphene is coming up as a very interesting support material for flame retardation systems. It is a perfect fit for the skeleton of such objectives. It is thermally stable36,37 and has certain unique physical and chemical properties38 due to its inflexible twodimensional (2D) arrangement, and thus has probable applications in nanomaterial electronics,39 energy-storing materials,40 polymer-fused materials,41 and sensors.42 In addition, graphene is the most thermodynamically stable allotrope of carbon.36,37 Therefore, many researchers reported the synthesis of phosphorus-doped graphene oxide (PGO),43 GO in ABS,44 GOTP,45 CNF-GO-BA-SEP,46 IFR-PAM/ GO,47 PP/APP/TRGO,48 r-GO film,49 DES-GO,50,51 and graphene phosphoric acid (GPA) as flame retardants.52 So phosphorus had been functionalized to various FR materials like PU sponges, polycables, and clothes.53 The phosphorusbased flame retardants can dehydrate cellulose and enhance the formation of char.46,54 The flame-retardant efficiency of phosphorus compounds can be improved by either having a nitrogen-based external synergist55,56 or having a nitrogen linked directly to the phosphorus atom (i.e., phosphoramidates).57 Pandya et al.58 experimentally proved the better flame-retarding properties of P−N-containing materials in comparison to only P-containing materials on cotton fabric. In higher temperature cellulose, the presence of P and N together increases the char formation reactions giving extra stability to the cotton fabric. Many researchers reported deposition− precipitation and dip-coating methods by using a Fe catalyst on a C-fiber textile material,59 even though the requirement remains for the development of a new, easy to implement method that is ecofriendly and cost effective with improved quality of performance. Hence, it presents an innovative and positive opportunity to deal with the utilization of graphene in fire-retardant systems. We report herein for the first time, the development of a simple, safer solvent, fire place prevention, ecofriendly, and easy procedure to efficiently synthesize a polymer-functionalized graphene nanocomposite (G-fPANI and G-fPPy) to serve as a highly potent flame-retardant material. GO contains abundant oxygen-containing functional groups and has a high surface area, which activate the formation of hydrogen bonding, π−π interactions, and electrostatic interactions with an active counterpart which has a useful role in the nanocomposite. We have applied functionalized polymers on the surface of graphene to prepare active nanocomposites as an efficient flame retardant. We designed these novel function-
alized polymer-supported graphene nanocomposites (G-fPANI and G-fPPy) by applying aniline/(G-fPANI and G-fPPy) and pyrrole and GO in the presence of phosphoric acid to keep the acidic medium and ferric chloride as a polymerizing agent Scheme 1. Synthesis of Polymer-Functionalized Graphene Composite Flame Retardant (G-fPANI and G-fPpy)
(Scheme 1). The respective mixtures were stirred for 2 h at 70 C. The resulting nanocomposites contained a high amount of nitrogen and phosphorus functional groups on the activated carbon surface. The as-made materials were used in flameretardant experiments.
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EXPERIMENTAL SECTION
The characterizations of the detailed testing method are shown in the Supporting Information. Materials and Method. Aniline, pyrrole, ferric chloride, phosphoric acid, graphite powder, potassium permanganate, sulfuric acid, and hydrogen peroxide were all purchased from Sigma-Aldrich and used without further purification. Preparation of Graphene Oxides (GO). GO was prepared from natural graphite powder by using the Modified Hummer’s Method.43 Synthesis of Graphene-Functionalized Polyaniline and Graphene-Functionalized Polypyrrole (G-fPANI and G-fPPy). Here, 100 mg of graphene oxide was dispersed in 1 mg/mL in 100 mL deionized water. Then, 1 mL of phosphoric acid, 1 mL of aniline, and 1 mL of pyrrole were added and were stirred for 10 min. Then, 3 g of ferric chloride was added which acts as a polymerizing agent, and after that, this mixture was stirred for 2 h at 70 °C to get the resulting nanocomposite which was used in further experimentation. After preparing a nanocomposite, we washed the sample with DI water to remove the unreacted phosphoric acid from the nanocomposite. Preparation of G-fPANI/Cloth, G-fPPy/Cloth, and GO/Cloth. The cloth was cut into 5 cm × 10 cm pieces. These cloth specimens were dip coated in G-fPANI, G-fPPy, and GO solutions, dried in air for 5 h, and dried under reduced pressure at 50 °C overnight prior to the flame-retardant testing. Preparation of G-fPANI/Wood, G-fPPy/Wood, and GO/ Wood. The wood was cut into 5 cm × 4 cm pieces. These wood specimens were dip coated in G-fPANI, G-fPPy, and GO solutions, dried in air for 5 h, and dried under reduced pressure at 50 °C overnight prior to the flame-retardant testing.
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RESULTS AND DISCUSSION The as-made materials were characterized by X-ray photoelectronic spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), Xray diffraction (XRD), and scanning electron microscopy (SEM). XPS analysis was used to study the surface electronic 11746
DOI: 10.1021/acssuschemeng.9b01975 ACS Sustainable Chem. Eng. 2019, 7, 11745−11753
Research Article
ACS Sustainable Chemistry & Engineering state, and atomic composition of GO (Figure 1a and Table S1) showed a high amount of oxygen, with the atomic percentage
Figure 2. (a) XPS spectra of G-fPpy C 1s. (b) XPS spectra of G-fPpy N 1s. (c) XPS spectra of G-fPpy O 1s. (d) XPS spectra of G-fPpy P 2p. (e) TGA spectra of GO, G-fPANI, and G-fPpy. (f) XRD spectra of GO, G-fPANI, and G-fPpy.
C (284.8 eV), C−O/C−N/C−P (286.3 eV), and CO (288.0 eV), exhibiting the different functionalities in comparison to GO, which indicates a polymer-functionalized graphene composite. N 1s (Figure 2b) spectra showed three peaks at a small quinoid-imine component −NH (397.3 eV), benzenoid-amine component −NH− (398.8 eV), and positively charged nitrogen −NH+ (400.2 eV), respectively.62,63 Peaks at 531.5 and 532.9 eV in the O 1s (Figure 2c) XPS spectrum of the G-fPPy nanocomposite correspond to CO, PO, P−O, and C−O, respectively. The high resolution XPS spectrum of P 2p (Figure 2d) and P 2s (Figure S3) for the G-fPPy nanocomposite exhibited broad peaks found at 133.5 and 191.8 eV, respectively.43 XPS survey spectra of G-fPANI/cloth and G-fPPy/cloth char residues are shown in Figure S4. FT-IR spectra of GO, G-fPANI, G-fPPy, PANI, and PPy are shown in (Figure 1f and Figure S5). In the case of GO, peaks were observed at 3424, 1728, 1623, 1412, and 1053 cm−1, respectively.50,51 The peak visible at 3424 cm−1 indicates the broad vibration of an O−H band. The peaks at 1728 and 1623 cm−1 confirmed the presence of the CO stretching of acid and CC stretching vibration of aromatic carbon atom. The peaks at 1412 and 1053 cm−1 confirmed the presence of C−H bends and C−O stretching, respectively.60,61 In the case of only PANI (Figure S5) observed peaks appeared at 799, 1096, 1309, 1484, 1559, and 3254 cm −1, respectively.64 The peaks appearing at 799 and 1096 cm−1 indicate the C−H aromatic out of plane deformation and C−H aromatic in plane deformation. The peaks appearing at 1309 and 1484 cm−1 indicated the presence of secondary aromatic amine C−N stretching and a CC bond of benzenoid ring stretching, respectively. The peaks appearing at 1559 and 3254 cm −1 indicate the presence of a
Figure 1. (a) XPS survey spectra of GO, G-fPANI, and G-fPpy. (b) XPS spectra of G-fPANI C 1s. (c) XPS spectra of G-fPANI N 1s. (d) XPS spectra of G-fPANI O 1s. (e) XPS spectra of G-fPANI P 2p. (f) IR spectra of GO, G-fPANI, and G-fPpy.
C/O ratio being 2.04. G-fPANI and G-fPPy (Figure 1a and Table S1) exhibited a low C/O ratio of ∼ 0.95 and 1.00, respectively, in comparison to that of GO, which corresponds to an increased oxygen content in these materials. The C 1s spectra of graphene oxide showed a very good degree of oxidation with three components which corresponds to the carbon atom in different functional groups, i.e., C−C at 284.4 eV, carbon in CC/C−O bonds (286.4 eV), and carboxylate carbon CO (OH) (288.4 eV) (Figure S1 and Table S1). However, the high resolution C 1s spectra of G-fPANI (Figure 1b) exhibited different functionalities in comparison to GO, which indicates a polymer-functionalized graphene composite. G-fPANI composite characteristic peaks were observed at C− C (284.4 eV), C−O/C−N/C−P (285.7 eV), and C = O (289.2 eV).50,51 IThe N 1s XPS spectrum (Figure 1c) could be three peaks observed at a small quinoid-imine component −NH (398.2 eV), benzenoid-amine component −NH− (399.8 eV), and positively charged nitrogen −NH+ (401.5 eV), respectively (Figure1c).60,61 Peaks at 532.8 eV in the O 1s XPS spectrum (Figure 1d) of the G-fPANI nanocomposite correspond to CO, PO, P−O, and C−O, respectively. The high resolution XPS spectrum of P 2p (Figure 1e) and P 2s (Figure S2) for the G-fPANI nanocomposite showed broad peaks at 134 and 191.6 eV, respectively.50,51 High resolution C 1s (Figure 2a) XPS spectra of G-fPPy (Figure 2a) showed C− 11747
DOI: 10.1021/acssuschemeng.9b01975 ACS Sustainable Chem. Eng. 2019, 7, 11745−11753
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ACS Sustainable Chemistry & Engineering
nanocomposites, and this result indicates the formation of polymer-functionalized graphene nanocomposites. Scanning electron microscopy (SEM) was useful to study the overall surface morphology of the as-prepared materials. In the case of the GO image, we observed a thin and wrinkled sheet.50,51 The as-made G-fPANI and G-fPPy exhibited a different morphology in comparison to only GO (Figure 3a−d). SEM images of
CC double bond of a quinoid ring and N−H stretching, which confirmed the presence of a secondary amine in PANI.64 In the case of PPy (Figure S5), the observed peaks appearing at 811, 920, 1315, 1487, 1554, and 1685 cm−1 confirm the presence attributed to C−H wagging and a C−N bond, CC bond, and CN stretching, respectively. The small peak appearing at 3522 cm−1 indicates the presence of N−H stretching.65 In the case of G-fPANI and G-fPPy, the same peaks appeared at 3430, 2954, 1631, 1129, 1018, and 884 cm−1 confirming the presence of N−H stretching, C−H stretching, aromatic CC bond, C−P bond, C−O stretching bond, and C−H aromatic out of plane, respectively. Only two peaks are differentiated in the case of G-fPPy, showing peaks at 902 and 831 cm−1 indicating the presence of C−H wagging and aromatic out of plane. FT-IR spectra of G-fPANI/cloth and GfPPy/cloth char residues are shown in Figure S6. Thermogravimetric analyses (TGA) of GO, G-fPANI, G-fPPy, PANI, and PPy (Figure 2e and Figure S5) were used to find the weight loss of them. All samples were heated under an air atmosphere from room temperature to 800 °C at a heating rate of 20 °C min−1. In the case of the as-prepared GO material, it can be observed that major weight loss was between 100 and 200 °C, indicating the release of CO2 and vapor from the most labile functional groups during pyrolysis. At a temperature 800 °C, the total weight loss was 80%, which was due to the thermal decomposition of the carbon skeleton.60,61 In the case of G-fPANI as shown in Figure 2e, it can be observed that the initial weight loss was about 21% at ∼202 °C and second step from 284 to 500 °C, which is due to the thermal decomposition of graphene and PANI.65,66 In the case of the G-fPPy nanocomposite, it can be observed that the initial weight loss was 28% at ∼147 °C, which is due to elimination of a water molecule and moisture (Figure 2e). The second step is from 357 to 600 °C, which is due to the decomposition of the nanocomposite. Only PANI (Figure S7) showed a two-step weight loss. The initial weight loss was ∼28% at ∼145 °C, which is due to decomposition of the aromatic chain fragment, and the second step of weight loss was 43% at 351 °C, corresponding to the decomposition of the PANI backbone.65,66 Only PPy (Figure S7) showed an initial weight loss of about 11% at ∼106 °C, which is due to the removal of a water molecule and unreacted monomer elimination. In the second step, weight loss was 17% at ∼206 °C, which is due to decomposition of composite material.65 Thermal stabilities of GO-, G-fPANI-, and G-fPPy-coated cloths and blank cloth have also been measured (Figure S8). DTA and DSC data analysis of cloth, GO, G-fPPy, and G-fPANI are summarized in Figures S9 and S10, respectively. Weight loss in % at different temperatures, different TGA parameters, and comparison of peak temperatures and heat release rates are ummarized in Tables S2, S3, and S4, respectively. Thermogravimetric analysis of G-fPANI/cloth and G-fPPy/cloth char residues are shown in Figure S11. The XRD spectra of GO, G-fPANI, G-fPPy, PANI, and PPy are shown in Figure 2f and Figure S12. In the spectra, the strong peak appearing at 2θ = 10.31° for GO represents the interlayer distance as 7.25 Å, which indicates that the interlayer space increased via the full oxidation of graphite layers into GO.60,61 After formation of the polymerfunctionalized graphene nanocomposites, changes appeared in the 2θ peak positions of G-fPANI (22.6, 27.0°) and G-fPPy (20.0, 28.1°) (Figure 2f) in comparison with GO (10.31°) (Figure 2f), PANI (25.0°),64 and PPy (24.3°)65 (Figure S12). The GO peak disappeared in these G-fPANI and G-fPPy
Figure 3. SEM Images of G-fPANI/cloth (a) before fire and (b) after fire retardant experiment, respectively. Magnified SEM Images of GfPANI on cloth (c) before and (d) after the experiment. Corresponding EDX element mapping (e) after the experiment of carbon (red), nitrogen (green), oxygen (dark green), and phosphorus (cyan) and (f) before carbon (purple), nitrogen (gray), oxygen (red), and phosphorus (light green).
G-fPANI- and G-fPPy-coated cloths were observed before and after flame retarding test (Figure 3a−d and Figure S13, respectively). The images of the coated cloth before the flame test reveals the presence of the G-fPANI and G-fPPy cloth surfaces (Figure 3a and c and Figure S13). After the flame test, G-fPANI- and G-fPPy-coated cloth retained the textile fiber morphology and structure, which were preserved by the formation of the shielding layer of G-fPANI and G-fPPy on the cloth surface (Figure 3b and Figure S13).64,65 Elemental mapping was nitrogen (blue), oxygen (gray), and phosphorus (red) atoms, uniformly performed to exhibit the polymerfunctionalized graphene composite distribution of carbon, oxygen, nitrogen, and phosphorus atoms on G-fPANI and GfPPy surfaces. Following carbon (green), the distribution on the cloth surface was confirmed from SEM. SEM images of GO-coated cloth before and after the flame test are shown in Figure S14. The possible mechanism of insertion of phosphorus and nitrogen atoms in the polymer-functionalized graphene composite surface is demonstrated in Figure S15a− d.52 When G-fPANI- and G-fPPy-coated fabrics were subjected to fire, intramolecular and intermolecular reactions52,54 occur to produce a charred layer on the surface of the fabrics. Water 11748
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Table 1. (Top) Limiting Oxygen Index (LOI) of GO-, PANI-, G-fPANI-Coated Cloths and Blank Cloth. (Bottom) Limiting Oxygen Index (LOI) of GO-, Ppy-, G-fPpy-Coated Cloths and Blank Cloth Sample no. 1 2 3 4 5 Mean LOI
LOI (%) of blank cloth LOI (%) of GO-coated cloth LOI (%) of PANI-coated cloth
LOI (%) of PANI-PA
LOI (%) of G-fPANI-coated cloth
Sample no.
18.5 17.5 17.0 17.7 17.8 17.7 LOI (%) of blank cloth
18.3 18.8 18.4 17.8 19.0 18.4 LOI (%) of GO-coated cloth
23.8 23.0 24.5 23.5 22.9 23.5 LOI (%) of Ppy-coated cloth
27.5 27.9 26.8 26.1 27.3 27.1 LOI (%) of PPy-PA-coated cloth
46.5 47.6 48.6 47.5 47.9 47.6 LOI (%) of G-fPPy-coated cloth
1 2 3 4 5 Mean LOI
18.5 17.5 17.0 17.7 17.8 17.7
18.3 18.8 18.4 17.8 19.0 18.4
22.7 23.5 22.7 21.8 22.8 22.7
25.1 25.5 24.8 24.6 25.3 25.0
41.5 42.0 40.9 41.9 43.5 41.9
extinguishing properties as compared to the PANI-coated cloth and Ppy-coated cloth. In case of PANI-coated cloth (Figure 4d-f and Video 5 in the SI) and PPy-coated cloth (Figure 4g−i and Video 6 in the SI) caught fire within ∼7 and 5 s, respectively, and completely burned within 14 s with very little self-extinguishing properties. On the other hand, the GOcoated cloth experiment caught fire within 5 s (Figure 4a− c and Video 7 in the SI) as control experiments. On the basis of lighting time of the flame-retardant experiment, G-fPANI and G-fPPy are more useful applicants as flame retardants in comparison to only GO and other reported materials (Table S7).43−52 We have determined the washing stability of the GfPANI/cloth and G-fPPy/cloth before and after washing with % weight loss shown in Table S8, and a snapshot of the flame retardant after washing is shown in Figure S19.69,70 Also, similar results were observed in a bunsen burner flame test caried out high temperature (∼1500 °C). Figure S20 shows a high flame (Video 10 and 11 in the SI). Details about the dipcoating process (time, concentration, and volume) are in Table S9. Flame retardancy of G-fPANI/cloth and G-PPy/cloth is compared with already reported results using other grapheneand phosphorus-based materials (Table S10). In Table S11, we summarize the effect of loading G-fPANI and G-fPPy coatings for flame retardant properties. Blank wood, G-fPANI-coated wood (G-fPANI/wood), and G-fPPy-coated wood (G-fPPy/ wood), respectively, were tested with the same time in air with an ethanol flame. The blank wood sample also captured fire with in 5 s and was completely burned within 10 s and converted into a small black mass. However, the G-fPANIcoated wood and G-fPPy-coated wood initially emitted little smoke without catching fire up to 64 s (1.4 min) (Figure 5a−c and Video 8 in the SI) and 50 s (Figure 5d−f and Video 9 in the SI), respectively, maintaining almost its initial form. GfPANI and G-fPPy nanocomposites are better applicants as flame-retardant materials compared to the only GO and others. We have conducted a vertical flammability test with good results, and details regarding flammability data are reported in Table S5. Snapshots of the vertical flammability test of blank cloth, GO/cloth, PANI/cloth, PANI-PA/cloth, Ppy/cloth, Ppy-PA/cloth, G-fPANI/cloth, and G-fPpy/cloth are shown in Figure 6. The vertical flammability test suggests that different parameters were measured as per the ASTMD 641309 standard test method68−70 for flame resistance of cloth
produces and the charred barrier does not allow air, and hence, oxygen pass inside the material protecting the fiber inside.54 After the structural confirmation of G-fPANI, G-fPpy, and GO, the material was dispersed in DI water (1 mg/mL; Figure S16), the as-prepared G-fPANI and G-fPPy were used for further experiments of the flame-retardant test. In SEM images of G-fPANI and G-fPPy, at low magnification, we observed that the cloth is well coated (Figure S17a and c, whereas at higher magnification, we observed that the threads are well coated by the G-fPANI and G-fPPy (Figure S17b and d) flame-retardant materials. In the SEM morphology of GO, we observed a wrinkled sheet-like structure (Figure S21a). After functionalization of GO with phosphorus-based PANI and PPy (Figure S21b and c), we observed different morphologies in comparison to GO. For the preparation of the flame retardation sample, G-fPANI and G-fPPy ink was used to coat a piece of cloth and dried under a vacuum oven at 60 °C overnight, while the control cloth sample was not coated with nanocomposites (right image in Figure 3). Flammability Section. Standard method limiting oxygen index (LOI) test was conducted to confirm the thermal stability of GO-, PANI-, PPy-, G-fPANI-, and G-fPPy-coated cloths and blank cloth. The LOI values of GO-, PANI-, PPy-, G-fPANI-, and G-fPPy-coated cloths and blank cloth are shown in Table 1.67 The snapshot of the vertical flammability test is shown in Figure 6, and detailed flammability data with excellent results are summarized in Table S5.68,69 The percentage loading of G-fPANI and G-fPPy to the mass of the cloth was approximately 4.096% and 4.225%, respectively (Table S6a and b). The tensile strength of G-fPANI- and GfPPy-coated cloth loading was 1%−6% in comparison to blank cloth (Figure S18a and b). Blank cloth and G-fPANI-coated cloth sample captured fire with in 5 s and were completely burned within 10 s and converted into small black mass. Consequently, the G-fPANI-coated cloth and G-fPPy-coated cloth initially emitted little smoke without catching fire up to > 620 s (10.20 min) (Figure 4s−u and Video 1 in the SI) and >380 s (6.20 min) (Figure 4p−r and Video 2 in the SI), respectively, maintaining almost their initial shape and size. However, in the case of PANI-PA-coated cloth (Figure 4j−l and Video 3 in the SI) and Ppy-PA-coated cloth (Figure 4m−o and Video 4 in the SI) caught fire within 30 and 26 s, respectively, and completely burned within 60 s with good self11749
DOI: 10.1021/acssuschemeng.9b01975 ACS Sustainable Chem. Eng. 2019, 7, 11745−11753
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ACS Sustainable Chemistry & Engineering
Figure 5. Snapshots of the flame-retardant test of G-fPANI-coated wood (left) with and blank wood (right) with respect to time from (a−c) 0−64 s. Snapshots of the flame-retardant test of the G-fPpycoated wood (left) with and blank wood (right) with respect to time from (d−f) 0−50 s.
Figure 4. Snapshots of the flame-retardant test of the GO-coated cloth (left) with and blank cloth (right) with respect to time from (a− c) 0−10 s. Snapshots of the flame-retardant test of the PANI-coated cloth (left) with and blank cloth (right) with respect to time (d−f) 0− 14 s. Snapshot of the flame-retardant test of PPy-coated cloth (left) with and blank cloth (right) with respect to time from (g−i) 0−10 s. Snapshot of the flame-retardant test of PANI-PA-coated cloth (left) with and blank cloth (right) with respect to time from (j−l) 0−60s. Snapshot of the flame-retardant test of PPy-PA-coated cloth (left) with and blank cloth (right) with respect to time from (m−o) 0−52 s. Snapshot of the flame retardant test of G-fPPy-coated cloth (left) with and blank cloth (right) with respect to time from (p−r) 0−380 s. Snapshot of the flame-retardant test of G-fPANI-coated cloth (left) with and blank cloth (right) with respect to time from (s−u) 0−620 s.
Figure 6. Snapshot of the vertically flammability test: (a) blank cloth, (b) GO/cloth, (c) PANI/cloth, (d)PANI-PA/cloth, (e) G-fPANI/ cloth, (f) PPy/cloth, (g) PPy-PA/cloth, and (h) G-fPPy/cloth.
PA/cloth burning times were 6, 5, 6, and 5 s, respectively, and the GO/cloth sample burning with flame time was 12 s. Hence, G-fPANI/cloth and G-fPpy/cloth show no flame, and they are good materials compared to other material. Burning with afterglow times after flame stop of G-fPANI/cloth and GfPpy/cloth are 1 and 2 s. PANI-PA/cloth, Ppy-PA/cloth, PANI/cloth, and Ppy/cloth is 8, 7, 9, and 8 s, respectively, and the remaining other two have no afterglow flame stop. Char lengths of G-fPANI/cloth and G-fPpy/cloth sample are 1 and 2 cm, respectively. Char lengths of PANI-PA/cloth, Ppy-PA/ cloth, PANI/cloth, and Ppy/cloth are 17, 19, 28, and 30 cm, respectively. GO/cloth and only cloth have no char length, so G-fPANI/cloth and G-fPpy/cloth are good, compared to
material. The sample size was 30 cm × 7.6 cm, and flame height was 3.8 cm. The cut edge of the cloth on the bottom was exposed to a controlled flame for 12 s. The occurrence of flashing over the surface, burning with flame time, burning with afterglow after flame stop, and char length were measured. The vertical flammability test with excellent results and details of flammability data are reported in Table S5. These results indicate that untreated cloth burned within 18 s, and the GfPANI/cloth and G-fPpy/cloth showed no flame, burning with flame time 2 s. PANI/cloth, PANI-PA/cloth, Ppy/cloth, Ppy11750
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ACS Sustainable Chemistry & Engineering others because only 1 and 2 cm char lengths, respectively. The limiting oxygen index (LOI) (Table 1) test was done according to the standard method ASTM D2863-09.66,67 The limiting oxygen index method has been carried out using a test cloth sample which was 13 cm in length and 6 cm in width.66,67 The cloth samples were suspended vertically and ignited using a burner. According to LOI, the a value of more than 26 indicates that the material has a good fire retardant property. As-made G-fPANI- and G-fPpy-coated cloth show comparatively high LOI values (47.6) and (41.9), respectively, which are an excellent fire retardant in comparison to other control samples.
fPPy dispersed in DI water; SEM images of different magnification and location; G-fPANI flame retardant material; G-fPPy flame retardant material; flammability test result of vertical flammability; result of G-fPANI/ cloth and G-fPPy/cloth before and after flame retardant experiment; tensile strength of G-fPANI/cloth and GfPPy/cloth; comparative data of our material with reported materials; washing stability of G-fPANI/cloth and G-fPPy/cloth before and after washing % weight loss; dip coating process (time, volume, and concentration); flame retardancy achievement with reported results using other graphene- and phosphorus-based materials; different loading of G-fPANI and G-fPPy coating on flame retardant property; snapshots of the flame retardant test after wash; snapshots of flame retardant test of G-fPANI-coated cloth; G-fPPy-coated cloth and blank cloth with respect to time (Bunsen burner flame test around 1500 °C); SEM images of GO after modification of G-fPANI, and G-fPPy. (PDF) Video 1. (AVI) Video 2. (AVI) Video 3. (AVI) Video 4. (AVI) Video 5. (AVI) Video 6. (AVI) Video 7. (AVI) Video 8. (AVI) Video 9. (AVI) Video 10. (AVI) Video 11. (AVI)
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CONCLUSION In conclusion, we have developed polymer-functionalized graphene composites, which are an effective flame retardant. The recent process demonstrated that this is simple and ecofriendly for efficient polymer functionalization on graphene surface. The as-prepared G-fPANI- and G-fPPy-coated cloths have shown highly efficient flame-retardant properties under the flame-retardant experiment. Initially, G-fPANI- and GfPPy-coated cloth emitted little smoke at the beginning without catching fire for more than 620 s (10.20 min) and 380 s (6.20 min), respectively and maintaining its initial structure with slight contraction, whereas the “blank cloth” caught fire within 5 s. The coated cloth has shown the highest efficiency in the flame-retardant experiments even at high temperature (∼1500 °C). These nanocomposites were also successfully demonstrated on a wood surface. The polymerfunctionalized graphene composites were synthesized by using a simple, environmentally, safer solvent, fire place prevention and cost-effective method for many useful applications other than flame retardation.
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AUTHOR INFORMATION
Corresponding Author
ASSOCIATED CONTENT
*E-mail:
[email protected].
S Supporting Information *
ORCID
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01975.
Surajit Some: 0000-0003-0775-4676 Author Contributions
D.A.P and S.S wrote the manuscript. D.A.P. performed the experiments. D.A.P., R.V.K., and P.H.W. prepared the graphene oxide and nanocomposite. D.A.P. and R.V.K. analyzed all data. S.S. supervised the work.
Characterization, EDX, EA, and XPS data of GO, GfPANI, and G-fPPy of (at. %); XPS spectra of GO C 1s; XPS spectra of G-fPANI P 2s; XPS spectra of G-fPPy P 2s; XPS survey spectra of G-fPANI/cloth and G-fPPy/ cloth char residue; FT-IR spectra of PANI and Ppy; FTIR spectra of G-fPANI/cloth and G-fPPy/cloth char residue. Thermogravimetric analysis of PANI and PPy; thermogravimetric analysis of GO-, G-fPANI-, and GfPPy-coated cloth and blank cloth; DTA analysis of cloth, GO, G-fPPy, and G-fPANI; DSC analysis of cloth, GO, G-fPPy, and G-fPANI; weight loss in % at different temperatures; different TGA parameters of blank cloth, GO/cloth, G-fPPy, and G-fPANI; comparison of peak temperature and heat release rate; thermogravimetric analysis of G-fPANI/cloth and G-fPPy/cloth char residue; XRD spectra data of PANI and PPy; SEM images of G-fPPy/cloth before fire and after fire retardant experiment; magnified SEM Images of GfPPy on cloth before and after the experiment; corresponding EDX element mapping; SEM images of GO/cloth before and after fire test; mechanism of GfPANI by using GO and fPANI; mechanism of G-fPPy by using GO and fPPy; images of GO, G-fPANI, and G-
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the BRNS fund (34/14/14/2015/ BRNS), DST fund (YSS/2015/000788), and CSIR fund (22(0748/17/14/EMR-II). D.A.P. is thankful to UGC-SAP for his fellowship.
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