Graphene Nanohybrids: Preparation and Enhancement on

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CuO/graphene nanohybrids: preparation and its enhancement on thermal stability and smoke suppression of polypropylene yongqian shi, xiaodong qian, keqing zhou, qinbo tang, saihua jiang, bibo wang, biao wang, bin yu, Yuan Hu, and Richard K. K yuen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401535h • Publication Date (Web): 26 Aug 2013 Downloaded from http://pubs.acs.org on September 1, 2013

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Industrial & Engineering Chemistry Research

CuO/graphene nanohybrids: preparation and its enhancement on thermal stability and smoke suppression of polypropylene Yongqian Shi a,b, Xiaodong Qian a,b, Keqing Zhou a, Qinbo Tang a, Saihua Jiang a,b, Bibo Wang a, Biao Wang a, Bin Yu a,b, Yuan Hu a,b*, Richard K. K. Yuen b,c a

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road,

Hefei, Anhui 230026, P.R. China. b

USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou

Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road Suzhou, Jiangsu 215123, P.R. China. c

Department of Building and Construction, City University of Hong Kong, Tat Chee Avenue Kowloon,

Hong Kong.

Yuan Hu* Corresponding author. State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.R. China. E-mail: Tel/fax: +86-551-63601664 E-mail: [email protected]. ACS Paragon Plus Environment


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Abstract Cu/graphene nanohybrids were successfully prepared by a simultaneous chemical reduction method. Successful oxidation of Cu nanoparticles was carried out by the melt blending during preparation of the nanocomposites. Cu nanoparticles were well dispersed on graphene with a diameter of 3-7 nm. XRD and TGA results revealed successful oxidation of Cu nanoparticles during processing. TEM results indicated that PP nanocomposites decorated with CuO/graphene was better dispersed than that added with graphene because the CuO acted as spacer to keep graphene layers separated. The thermal property of the PP/G-Cu-1 and PP/G-Cu-2 was efficiently improved by around 30 oC compared with native PP. Moreover, smoke suppression property of PP/graphene, PP/G-Cu-1 and PP/G-Cu-2 nanocomposites were also improved by 28.9%, 35.8% and 47.2%, respectively. The main reasons for the improvements were due to the two factors: good mass barrier effect of graphene and easily reduced or catalytic charring effect of CuO. This work represented a novel and simple process to synthesize CuO/graphene nanohybrids to reduce the fire hazards of polypropylene, indicating further application in research and industrial areas. Keywords: CuO/graphene; smoke suppression; thermal stability; polypropylene

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1. Introduction Polypropylene (PP) is extensively used in many fields, such as housing, transportation and electrical engineering materials due to its good mechanical property and high electrical insulation.1−4 However, the inherent flammability of PP severely restricts its applications in automotive components and the aerospace industry. During the combustion process, PP generates amounts of smoke and poisonous gas,2 which is the key reason for casualties in the fire accidents. Therefore, it is necessary to reduce the smoke release of PP during combustion for its wide applications. Recently, various strategies are used to simultaneously inhibit flammability and the smokes release of polymeric materials during combustion. Accordingly, the possible mechanisms related with smoke suppression are also proposed. Polymer composites with intumescent flame retardants (IFRs) can reduce the smoke release by formation of intumescent char residues.5, 6 The combination effect of IFRs and lanthanum oxide leads to the reduction of the total smoke produce (TSP) due to the formation of compact char residues by the catalytic charring effect of La2O3.7 The smoke density rank (SDR) of PP composites with Al(OH)3/Mg(OH)2/zinc borate (ZB) is reduced, which is due to the fact that ZB can promote carbonization, reduce burning velocity, dilute smoke and generate a surface insulation effect.8 Owing to the oxidation-reduction property of the metal atoms, polymeric metal chelates or metal nanoparticles also play the same effect in the reduction of fire hazard.9, 10 Two dimension (2D) layered inorganic fillers act as inhibitor while the modifiers play a char agent role.11

–13

However, high loadings usually result in the

deterioration on the mechanical properties of polymeric materials. Thus, to get more effective smoke suppression, novel nano-fillers are required and the exploitation of novel nano-fillers is the focus of recent scientific work. Copper-family-elements have attracted more and more research attention in catalyst, gas sensors, anode material for lithium-ion batteries and hydrogen production14−17 due to its low cost and environmental friendliness. As a member of copper materials, cupric oxide (CuO) is more stable than the others at high ACS Paragon Plus Environment


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temperature. There are some works on the smoke suppression of polymer materials using CuO. For example, Li et al. reported that CuO could effectively reduce smoke emission by 52.0% at an addition level of 4 mol of metal atom.18, 19 However, the nano-sized particles are inclined to agglomeration in the polymer matrix, which seriously limit the efficiency of the nanofillers. Hence, it is quite crucial to reduce aggregation of the nanofillers and improve the special surface area of the nanofillers. Supportor materials, which can “carry” the nanoparticles and improve the special surface area between the nanoparticles and polymer matrix, are effective way to improve the dispersion of the nanofillers. Arising from its strictly two dimension (2D) structure, graphene exhibits unique properties including a large theoretical specific surface area20, 21 and has been served as supportor for nanoparticles to ensure good dispersion, such as CoFe2O4/graphene,22 TiO2/graphene,23 Carbon Nanotube/Graphene24 and so on. Previous work in our group reported that graphene could effectively inhibit the generation of smoke gas.25,

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Therefore, graphene can be used as a perfect supportor to ameliorate the dispersion of CuO nanoparticles in the polymer matrix. The aim of this paper was to reduce the fire hazards of PP using CuO/graphene nanohybrids from oxidation of Cu/graphene during processing. Herein, a novel and simple method was presented to synthesize Cu/graphene nanohybrids by simultaneous chemical reduction of graphite oxide (GO) and copper acetate monohydrate. The obtained Cu/graphene nanohybrids were incorporated into PP matrix through melt blending. The thermal stability and smoke release of PP nanocomposites was investigated by TGA, smoke density chamber, respectively. The thermal stability and smoke release of PP nanocomposites was investigated by TGA, smoke density chamber, respectively, and the results revealed that both the thermal stability and the fire safety of the nanocomposites were remarkably improved. The enhancement for PP/graphene could be ascribed to layer barrier effect of graphene while that for PP/G-Cu-1 and PP/G-Cu-2 was due to barrier effect of graphene and the smoke suppression effect of CuO. This work represents a potential application in research and industrial areas. ACS Paragon Plus Environment

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2. Experimental Section 2.1 Raw Materials. Polypropylene (F401) used in this study was supplied by the Yangzi Petrochemical Co. (Nanjing, China) with a melt flow rate (MFR) of 2.5 g/10 min (230 oC, 2.160 kg). Graphite powder (Spectrum Pure) was purchased from Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). Cupric acetate monohydrate (Cu(CH3COO)2.H2O) was afforded by Red Rock Reagent Factory (Tianjin, China). Concentrated sulfuric acid (98%), sodium nitrate, potassium permanganate, 30% H2O2 solution, hydrochloric acid, hydrazine hydrate (85%) and tetrahydrofuran (THF) were all reagent grades and provided by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China). De-ionized water (DI) was obtained from our laboratory itself. All reagents were used without further purification. 2.2 Preparation of Cu/graphene nanohybrids. The Schematic illustration for the fabrication of Cu/graphene nanohybrids was presented in Scheme 1. The typical process included two steps: (1) synthesis of GO powders from graphite by Hummers’ method,27 (2) simultaneous chemical reduction of the aqueous GO nanosheets and Copper acetate by hydrazine to generate Cu/graphene suspension. Briefly, 0.1 g GO was dispersed in 120 ml 1.2 mM Cu2+ solution with the assistance of ultrasonication at room temperature. Then, the solution was transferred to 250 ml three-necked flask at 98 oC and stirred for 1 h. Finally, GO coordinated with Cu2+ was reduced by adding hydrazine hydrate into the solution and further heated for 2 h under vigorous stirring. The as-prepared product was separated by centrifugation and washed using DI and anhydrous ethanol several times to remove other ions and then dried in vacuum at 40 oC. The as-synthesized product was labeled as G-Cu. 2.3 Preparation of PP and its nanocomposites. PP and its nanocomposites were prepared by incorporating Cu/graphene nanohybrids into PP matrix in a Brabender-like apparatus (LH-60, afforded by Shanghai Kechuang Plastic Machinery Co., Ltd, China) at a temperature of 180 oC for 15 min. The PP and its nanocomposites were mixed at 60 r/min with a roller blade. ACS Paragon Plus Environment


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After mixing, the samples were hot pressed at 185 oC under 10 MPa for 15 min into sheets with suitable thickness (100 × 100 × 3 mm3) for characterization. The detailed formation of PP nanocomposites was shown in Table 1. 2.4 Characterizations. Wide-angle X-ray diffraction patterns of the samples were recorded on an X-ray diffractometer (Rigaku Dmax/rA, Japan), using Cu-K a radiation (λ= 0.15418 nm) at 40 kV and 20 Ma with scanning range from 5 to 60o or 10 to 70o. The phase structure of the nanohybrids was observed in powdered state while PP nanocomposites are characterized using films samples. The morphology and structure of Cu/grapheme nanohybrids and PP nanocomposites were studied by transmission electron microscopy (TEM) (JEOL JEM-2100 instrument with an acceleration voltage of 100 kV). GO, graphene and G-Cu were well dispersed in absolute alcohol with ultrasonication and then dripped onto copper grids before observation. PP and its nanocomposites were observed as ultrathin sections microtomed using an Ultratome (Model MT-6000, Du Pont Company, USA) in liquid nitrogen. The ultrathin sections were transferred from liquid nitrogen to copper grids before observation. Thermogravimetric analysis (TGA) of the samples was carried out with Q5000 thermal analyzer (TA Co., USA) from 30 oC to 700 oC at a heating rate of 20 oC min-1 in air or nitrogen atmosphere (flow rate of 100 ml min-1). Smoke evolution properties were determined by using a smoke chamber conforming to NBS specifications in smoldering conditions. This experiment was performed on a specimen (75×75 mm2, 3 mm thick) in a Polymer Laboratory System FTT instrument (Fire Testing Technology, UK) following ASTM E662 specification. 3. Results and Discussion 3.1 Characterization of Cu/graphene nanohybrids. GO，graphene and Cu/graphene nanohybrids are investigated by X-ray diffraction to confirm the phase, as shown in Figure 1. It can be seen from Figure. 1(a) that GO shows a characteristic reflection peak centered at 2θ = 10.3o, corresponding to an interplanar spacing of 0.84 nm.28 From Figure 1(b), it is obvious ACS Paragon Plus Environment

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that the diffraction peak for C (002) of GO is invisible, indicating that the face-to-face stacking of GO is broken after the chemical reduction. The very weak and broad diffraction peak at 23o in the pattern of graphene indicates re-aggregation of graphene when it is dried.29 As shown in the XRD pattern of Cu/graphene nanohybrids, the positions of diffraction peaks match well with bare Cu and graphene. The peaks at 2θ = 43.3o, 50.5o are consistent with the standard XRD data for the Cu cubic phase (JCPDS 85-1326). The small peaks at 23.5o can be attributed to the reduced graphene oxide.30, 31 Furthermore, the characteristic peak of the nanohybrids at 10.3o disappears, indicating complete chemical reduction of GO. Those results indicate that GO is successfully reduced to graphene during the redox reaction accompanied by the formation of Cu nanoparticles. The morphology and microstructure of GO, graphene and Cu/graphene nanohybrids are investigated by the TEM, as shown in Figure 2. From Figure 2(a) and 2b, it can be clearly seen that the GO and graphene sheet exhibit a typical crumpled morphology and paper-like structure with thin layers. The TEM images of Cu/graphene, as shown in Figure. 2(c), exhibit that many small dark dots are homogeneously distributed on the graphene sheets without aggregation. The well dispersed nanoparticles are generally due to carboxyl group acting as anchoring sites for the copper precursor.32 Moreover, Fig. 2(d) shows the high magnification image of Cu/graphene, in which Cu nanoparticles are more visible. The magnified TEM observations display that the diameter of the Cu nanoparticles is in the range of 3-7 nm, which contributes greatly to the contact area between the polymer matrix and the Cu nanoparticles. The thermal properties of Cu/graphene nanohybrids are tested by TGA from 30 oC to 700 oC at a heating rate of 20 oC min-1 in both air and nitrogen atmospheres. The initial degradation temperature is defined as the temperature of 5 wt% mass loss (Tonset). The temperature at maximum mass lose rate is named as Tmax. It can be found in Figure 3(a) that weight percent transcends 100% in the range of 50-200 oC, attributing to the oxidation of Cu nanoparticles. Moreover, the Tmax of the nanohybrids is lower than 150 oC, indicating that the potential oxidation of Cu nanoparticles occurred for nanohybrids preparation. Besides, ACS Paragon Plus Environment


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Cu/graphene nanohybrids perform higher degradation rate in the temperature range of 420-620 oC, demonstrating that decomposition of graphene is catalyzed by CuO. It is well known that CuO is quite stable in air atmosphere. The fact that quick transformation of Cu nanoparticles into CuO at 50-200 oC will not lead to the mass loss of G-Cu above 400 oC in air condition, revealing that the mass decrement is attributed to decomposition of graphene by the catalytic effect of CuO. Due to the existence of the oxygenated functionalities on graphene nanosheets which form hydrogen bonds with water, graphene loses about 12.6 wt% while G-Cu are reduced by around 8.0 wt%, indicating that G-Cu has the higher thermal stability than graphene due to the absence of oxygen. As a result, both graphene and Cu/graphene nanohybrids have higher thermal stability under nitrogen condition, as shown in Figure 3(b). 3.2 Structure and morphology of the PP nanocomposites. In order to attest successful oxidation of neat Cu nanoparticles during processing, XRD is performed to characterize PP nanocomposites, as shown in Figure 4. For the pristine PP, the characteristic diffraction peaks are observed at 2θ values in range of 10-30o, belonging to crystalline PP (shown in Figure 4(a)). As depicted in Fig. 4(b) and 4(c), the XRD patterns of PP nanocomposites display several main peaks at 2θ = 35, 37, 39, 50, 53 and 61o, which are consistent with the standard card of CuO (JCPDS card No. 80-1268). The formation of CuO from the oxidation of Cu is also supported by the TGA results, as shown in the inset of Fig. 3(a). The weak diffraction pattern of CuO in PP nanocomposites is ascribed to the fact that high crystalline of PP overlaps the peaks of CuO due to the low loadings of G-Cu. When larger amounts of G-Cu are incorporated, the diffraction pattern of CuO is visible, but the high loadings will lead to the aggregation of G-Cu in PP matrix. Taking those weak peaks of CuO into account, a piece of stronger evidence is brought in to further demonstrate the oxidation of Cu nanoparticles. PP nanocomposites are burned in muffle furnace at 500 oC for 1 h in air atmosphere. Figure 5 reveals that these peaks (signed as⊕) are in agreement with the results of Fig. 4. However, a sharp peak at 2θ = 26.3o corresponds to the G (002) of graphite33 due to destruction of graphene induced by the combination of thermal oxidation and CuO catalytic effect, which is ACS Paragon Plus Environment

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in accordance with the results of Fig. 3(a). All of these results suggest that successful oxidation of Cu nanoparticles during processing. TEM is a widely used technique to evaluate the dispersion state of nano-additives in a polymeric matrix. Fig.6 presents the dispersion of graphene and CuO/graphene in PP matrix. Most of the graphene nanolayers are poorly dispersed in the PP matrix (Fig. 6(a)) while CuO/graphene nanohybrids in PP matrix display well dispersion. The CuO nanoparticles can act as spacer which can intercalate the graphene nanosheets and weaken the hydrogen bonds among the graphene layers. This usually results in good dispersion of graphene in the polymer, which is similar to the distribution of intercalated montmorillonite (MMT) in polymer matrix due to its 2D structure with analogous functional groups to that of MMT. The results accompanied with the conclusion of Fig. 2(c) demonstrate better dispersion of CuO/graphene in PP matrix than graphene and the dispersion of the nanofillers are crucial for the performance of the nanocomposites. 3.3 Thermal properties of the PP nanocomposites. TGA and DTG profiles for PP and its nanocomposites as a function of temperature in air atmospheres are shown in Fig. 7 and the related data are recorded in Table 2. As it can be observed from Fig. 7(a), the incorporation of CuO/graphene nanohybrids into PP remarkably improves the thermal stability of PP at high temperature. Moreover, the temperature for the maximum DTG peaks (Tmax) also shifts to higher temperatures for copper oxide-containing nanocomposites (Fig. 7(b)). All of these improvements can be attributed to the combination of crosslinking effect of CuO nanoparticles and good mass barrier effects of graphene without regard to secondary catalytic effect of CuO in graphene.18, 25, 26 At the same time, well dispersion of CuO nanoparticles in PP matrix is also the key to the properties performance of the nanocomposites, which is suggested by TEM of the nanocomposites in Fig. 6. However, compared with neat PP, the PP/graphene nanocomposites exhibit lower degradation temperatures at Tmax because of the high heat conductivity of graphene, which is very common in the graphene based nanocomposites.34 The Tonset of PP nanocomposites containing copper oxide is also lower than that of native PP, attributing to the catalytic ACS Paragon Plus Environment


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effect of CuO in PP.35, 36 As for the Tmax as shown in Table 2, it is found that the Tmax of PP nanocomposites with CuO/graphene are markedly increased by 30 oC compared with that of pure PP. Moreover, similar with the Tonset of PP/graphene, the Tmax of PP/G-Cu-2 is lower than PP/G-Cu-1, which is ascribed to incorporation of high heat conductivity of graphene and catalytic effect of CuO in PP. 3.4 The smoke behavior of the PP nanocomposites. The smoke behavior of the materials, which caused most of the casualties during fire, is investigated by the smoke density chamber. The curves of smoke gas release of local PP and its nanocomposites as a function of time are presented in Fig. 8. It can be found that smoke density for all the PP nanocomposites is much less than that of pure polymeric material. Smoke density for PP/graphene is reduced by 28.9% compared with that of PP, as shown in Table 3. This improvement in smoke suppression can be attributed to the so-called “tortuous path” effect of graphene, which hinders the escape of volatile degradation products.25, 26, 34, 37

The proposed mechanisms of the physical barrier effect are depicted in scheme 2. The low loading of

graphene cannot work as physical barrier effect. However, the barrier effect becomes obvious with the increased content of graphene. Smoke release of PP nanocomposites is remarkably decreased with the incorporation of CuO/graphene nanohybrids into the PP matrix. Compared with virgin PP, the smoke density of PP/G-Cu-1 and PP/G-Cu-2 is decreased by 35.8% and 47.2%, respectively, which may be due to the mass barrier effect of graphene and smoke suppression effect of CuO. It has been reported that CuO can work as a smoke suppressant in polymers by previous literatures.18 Based on the above fact, we proposes the possibility of smoke suppression of CuO in PP matrix. Fortunately, the experiment results are in accordance with our anticipation that CuO can act as the smoke suppression agent in the PP matrix. Moreover, the well dispersed CuO nanoparticles have a great contact area due to the “carrying effect” of graphene, resulting in the higher smoke suppression compared with CuO. In general, the improved thermal stability and high-efficiency smoke suppression of the nanocomposites can be attributed to the combination of mass barrier effect of graphene, catalytic effect of ACS Paragon Plus Environment

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CuO and well dispersion of CuO nanoparticles in PP matrix. Moreover, the incorporation of CuO can improve the dispersion of graphene while the improved dispersion of graphene can promote the contact area between the CuO and the polymer matrix, Thus, due to mutual promoted effects, the properties of both graphene and CuO are enhanced. The method, using graphene as supportors for nanoparticles to ensure good dispersion, provides an effective way to enhance the fire safety of the polymer materials. Conclusions Cu/graphene nanohybrids were successfully prepared by simultaneous chemical reduction method. Successful oxidation of Cu nanoparticles was carried out by the melt blending during preparation of the nanocomposites. The thermal property of the PP/G-Cu-1 and PP/G-Cu-2 was efficiently enhanced by around 30 oC compared with native PP. Moreover, smoke suppression property of PP/graphene, PP/G-Cu-1 and PP/G-Cu-2 nanocomposites were also improved by 28.9%, 35.8% and 47.2%, respectively. The enhancement for PP/graphene was ascribed to layer barrier effect of graphene while that for PP/G-Cu-1 and PP/G-Cu-2 was due to barrier effect of graphene and the smoke suppression effect of CuO. The combination of graphene and metal oxides could contribute a promising strategy to the enhancement on the smoke suppression of polymers. Acknowledgements This work was supported by the National Basic Research Program of China (973 Program) (2012 CB719701), the National Natural Science Foundation of China (No.51036007) and the Opening Project of State K-ey Laboratory of Fire Science of USTC (No. HZ2011-KF05).



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(20) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A. Graphene: The New Two-Dimensional Nanomaterial. Angew. Chem. Int. Ed. 2009, 48, 7752 − 7777. (21) Yu, B.; Wang, X.; Xing, W. Y.; Yang, H. Y.; Song, L.; Hu, Y. UV-Curable Functionalized Graphene Oxide/Polyurethane Acrylate Nanocomposite Coatings with Enhanced Thermal Stability and Mechanical Properties. Ind. Eng. Chem. Res. 2012, 51, 14629 − 14636. (22) Fu, Y. S.; Chen, H. Q.; Sun, X. Q.; Wang, X. Combination of cobalt ferrite and graphene: High-performance and recyclable visible-light photocatalysis. Appl. Catal. B-Environ. 2012, 111-112, 280 − 287. (23) Liu, S. W.; Liu, C.; Wang, W. G.; Cheng, B.; Yu, J. G. Unique photocatalytic oxidation reactivity and selectivity of TiO2-graphene nanocomposites. Nanoscale 2012, 4, 3193 − 3200. (24) Fan, Z. J.; Yan, J.; Zhi, L. J.; Zhang, Q.; Wei, T.; Feng, J.; Zhang, M. L.; Qian, W. Z.; Wei, F. A Three-Dimensional Carbon Nanotube/Graphene Sandwich and Its Application as Electrode in Supercapacitors. Adv. Mater. 2010, 22, 3723 − 3728. (25) Wang, X.; Song, L.; Yang, H. Y. Cobalt oxide/graphene composite for highly efficient CO oxidation and its application in reducing the fire hazards of aliphatic polyesters. J. Mater. Chem. 2012, 22, 3426 − 3431. (26) Bao, C. L.; Guo, Y. Q.; Yuan, B. H.; Hu, Y.; Song, L. Functionalized graphene oxide for fire safety applications of polymers: a combination of condensed phase flame retardant strategies. J. Mater. Chem. 2012, 22, 23057 − 23063. (27) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339 − 1339. (28) Hontoria-Lucas, C.; Lopez-Peinado, A. J.; Lo´pez-Gonza´lez, J. D.; Rojas-Cervantes, M.; Martin-Aranda, R. M. Study of oxygen-containing groups in a series of graphite oxides: physical and chemical characterization. Carbon 1995, 33, 1585 – 1592. ACS Paragon Plus Environment

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Table captions Table 1. The detailed formula of PP nanocomposites. Table 2. Related TGA, DTG data of PP and its nanocomposites in air atmosphere. Table 3. Related smoke density data of PP and its nanocomposites. Scheme captions Scheme 1. Schematic illustration of Cu/graphene nanohybrids and PP nanocomposites formation. Scheme 2. The possible mechanisms of the physical barrier effect with different content of graphene. The content of graphene is increased gradually from a to c. Figure captions Fig. 1. X-ray diffraction pattern of (a) GO, (b) Graphene and (c) Cu/graphene nanohybrids. Fig. 2. TEM images of (a) GO, (b) Graphene, (c) and (d) Cu/graphene nanohybrids. Red framework represents high magnification section of Cu/graphene. Blue lines are signed as edge of graphene sheets. Fig. 3. TGA of graphene and Cu/graphene nanohybrids in (a) air and (b) nitrogen conditions. Fig. 4. XRD spectra of (a) PP, (b) PP/G-Cu-1 and (c) PP/G-Cu-2 after processing. Fig. 5. XRD spectra of PP nanocomposites after combustion at 500 oC×1 h in air atmospheres. Fig. 6. TEM photographs of (a) PP/Graphene and (b) PP/G-Cu-2; (c) and (d) presents the higher magnification of (a) and (b), respectively. Fig. 7. TGA (a) and DTG (b) curves of PP and its nanocomposites in air atmosphere. Fig. 8. Curves of smoke gas release of PP and its nanocomposites during combustion.



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Table 1. The detailed formula of PP nanocomposites. Sample

PP

Graphene

G-Cu

Loading level

PP

50.00 g

—

—

—

PP-Graphene

49.50 g

0.50 g

—

1.0 wt%

PP-G-Cu-1

49.75 g

—

0.25 g

0.5 wt%

PP-G-Cu-2

49.50 g

—

0.50 g

1.0 wt%


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Table 2. Related TGA, DTG data of PP and its nanocomposites in air atmosphere.

Tonset (oC)

Tmax (oC)

PP

270.0

325.4

PP-Graphene

257.8

311.3

PP-G-Cu-1

266.6

355.8

PP-G-Cu-2

257.7

357.4

Sample



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Table 3. Related smoke density data of PP and its nanocomposites.

Sample

Smoke density in 1200 s (Ds)

PP

503.3

PP-Graphene

357.6

PP-G-Cu-1

322.9

PP-G-Cu-2

265.9


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Scheme 1. Schematic illustration of Cu/graphene nanohybrids and PP nanocomposites formation.



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Scheme 2. The possible mechanisms of the physical barrier effect with different content of graphene. The content of graphene is increased gradually from a to c.


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Fig. 1. X-ray diffraction pattern of (a) GO, (b) Graphene and (c) Cu/graphene nanohybrids.



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Fig. 2. TEM images of (a) GO, (b) Graphene, (c) and (d) Cu/graphene nanohybrids. Red framework represents high magnification section of Cu/graphene. Blue lines are signed as edge of graphene sheets.


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Fig. 3. TGA of graphene and Cu/graphene nanohybrids in (a) air and (b) nitrogen conditions.



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Fig. 4. XRD spectra of (a) PP, (b) PP/G-Cu-1 and (c) PP/G-Cu-2 after processing.


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Fig. 5. XRD spectra of PP nanocomposites after combustion at 500 oC×1 h in air atmospheres.



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Fig. 6. TEM photographs of (a) PP/Graphene and (b) PP/G-Cu-2; (c) and (d) presents the higher magnification of (a) and (b), respectively.


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Fig. 7. TGA (a) and DTG (b) curves of PP and its nanocomposites in air atmosphere.



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Fig. 8. Curves of smoke gas release of PP and its nanocomposites during combustion.


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