Exfoliation approach for preparing high conductive reduced graphite oxide and its application in natural rubber composites

Materials Science and Engineering B 218 (2017) 74–83

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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Exfoliation approach for preparing high conductive reduced graphite oxide and its application in natural rubber composites Pattharaporn Wipatkrut a, Sirilux Poompradub a,b,⇑ a b

Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Center for Petroleum, Petrochemical and Advanced Material, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e

i n f o

Article history: Received 30 November 2016 Received in revised form 23 January 2017 Accepted 14 February 2017

Keywords: Graphene Natural rubber Composites Conductive filler

a b s t r a c t High conductivity reduced graphite oxide (RGO) was prepared by exfoliation of graphite waste from the metal smelting industry. To improve the surface properties of the RGO, the graphite oxide obtained based on Hummers’ method was reduced by L-ascorbic acid to give RGOV, which was then subjected to thermal reduction to obtain RGOT. The residual oxygen-containing groups in RGOV were almost completely removed by the thermal reduction and the conjugated graphene networks were restored in RGOT. The effect of the RGOT content in natural rubber (NR) on the cure, electrical and mechanical properties of the NR-RGOT (NG) composites was evaluated. The electrical conductivity of NR was increased by the inclusion of RGOT at a percolation threshold of 5 phr, with an electrical conductivity of 8.71
106 S/m. The mechanical properties, i.e., the modulus, tensile strength and hardness, of NG were comparable with those of conductive carbon black filled NR ones. Ó 2017 Elsevier B.V. All rights reserved.

1. Introduction Recently, conductive polymers have received increased attention due to their potential use in a number of applications, such as electronic devices [1–3], solar cells [4–6], supercapacitors [7–9], sensors [10,11], electromagnetic interference shielding [12,13] and biomedical applications [14,15]. This intense interest of applying polymers in electronic devices is due to their ease of processing, low cost, wide range of electrical properties and lightweight nature compared to metals. However, the limitation of polymers for most practical applications is their low conductivity. Accordingly, diverse methods have been used to induce electrical conductivity, such as mixing the polymer with a conjugated conducting polymer (polypyrrole [16] or polyacetylene [17]) or a conducting solid (metal [18,19], carbon black [11,20] or carbon nanotubes [21,22]). The difficulty in preparing polymer blends is the different polarity between the polymers, resulting in their phase incompatibility and inferior mechanical and electrical properties. Although the addition of a conductive metal solid into the polymer matrix is an alternative approach, it is rarely used because the metal component is easily oxidized. To improve the polymer conductivity, carbon black has been developed as an alternative

⇑ Corresponding author at: Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand. E-mail address: [email protected] (S. Poompradub). http://dx.doi.org/10.1016/j.mseb.2017.02.007 0921-5107/Ó 2017 Elsevier B.V. All rights reserved.

filler to metals. The carbon black is currently obtained from the incomplete combustion of hydrocarbon feedstock. However, graphene, a single-layer carbon sheet with a hexagonal packed lattice structure, has received increasing attention as a substitute for carbon black because it generally shows excellent mechanical, electrical and thermal properties [23–26]. Several methods have been used to synthesize graphene, such as chemical vapor deposition [27,28], unzipping of carbon nanotubes [29,30] and epitaxial growth on silicon carbide [31,32]. However, such methods are not suitable for polymer composites that require a large amount of graphene. For the large scale production of graphene, the chemical exfoliation of graphite is more suitable, where the graphite powder is first oxidized and then reduced using a strong reducing reagent, such as hydrazine [33,34], hydroquinone [35] and sodium borohydride [36,37]. However, under these conditions, the chemicals are highly toxic. In addition, the introduction of heteroatoms in the reduced products may prevent delocalized electrons on the carbon planes in graphene sheets [38]. Consequently, approaches that reduce the graphite oxide (GO) into a graphene sheet under mild conditions using environmentally friendly reducing agents, such as alcohols [39,40], reducing sugars [41], amino acids [42,43] or L-ascorbic acid [44] (LAA) need to be explored. From 2010 to 2015, the worldwide graphite mine production was approximately 1,000,000 metric tons [45] and the mined graphite was applied in a number of technologies, specifically refractories, foundries and metallurgy,

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resulting in a high increase in the amount of graphite waste. Accordingly, in this study, reduced graphite oxide (RGO) was prepared via an exfoliation of graphite waste from the metal smelting industry in order to avoid competition with graphite and to add value to this graphite waste. However, the RGO obtained by chemical reduction was of a low conductivity (range of 0.5–1.0 S/m) [37]. Thus, a thermal reduction process was introduced to yield a high-quality RGO sheet with a high conductivity. For practical application such as fuel hoses, conveyor belts or automotive belts, natural rubber (NR) was used as a polymer matrix to produce conductive rubber due to its excellent mechanical properties, in terms of a high strength, high elasticity and low heat build-up. The effect of the RGO content on the cure characteristic, conductivity and mechanical properties of the NR-RGO composites was studied. All the obtained results were compared to NR filled with conductive carbon black (PRINTEXÒ XE2-B) as a reference. A structureorientated model of the conductive RGO sheets prepared via the oxidation and reduction processes was proposed. The electrical conductivity model of RGO filled NR composite materials based on the Mamunya equation was also presented. 2. Experimental 2.1. Materials The NR (STR-5L) was manufactured by PI Industry Ltd., (Thailand). Graphite powder was provided by Mahamek Flow Innovation Co., Ltd., (Thailand). Sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen peroxide (H2O2), hydrochloric acid (HCl), sodium carbonate (NaCO3), tetrahydrofuran (THF) and LAA were purchased from QREC Chemical Co., Ltd. (Thailand). Active zinc oxide (ZnO) and stearic acid as an activator, N-cyclohexylbenzothiazole-2-sulfenamide (CBS) as an accelerator, and sulfur as a crosslinking agent were obtained from PI Industry Ltd., (Thailand). Conductive carbon black (PRINTEXÒ XE2-B with mean particle size of 35 nm, specific surface area of 980 m2/g, dibutyl phthalate (DBP) absorption of 420 ml/100 g, electrical conductivity by four-pint probe of 106.72 S/m) was bought from Kij Paiboon Chemical Ltd., (Thailand). All chemicals were of analytical grade and used as received. 2.2. Preparation of GO and RGO The GO was synthesized from graphite waste via Hummers method [46]. Graphite (10 g) with a mixture of H2SO4 (230 ml) and NaNO3 (5 g) was stirred and cooled in an ice-bath under 0–5 °C for 30 min. Then, KMnO4 (30 g) was added gradually with stirring under an ice-bath. The temperature of the mixture was maintained below 20 ± 2 °C for 2 h. The mixture was then allowed to rise to room temperature for 30 min. Deionized water (460 ml) was slowly added to increase the reaction temperature up to 98 °C and the mixture was maintained at this temperature for 15 min. The reaction was terminated by adding deionized water (1.4 L) followed by 30% H2O2 solution (100 ml). The solid product was filtered with Whatman paper No. 40, washed by 10% HCl to remove the sulphate, washed by deionized water for several times and dried in an air oven at 60 °C for 12 h. The obtained GO (0.25 g) was dispersed in deionized water (100 ml) and sonicated in a portable ultrasonicate cleaner (NXPC, KODO Technical Research, Korea) for 30 min in order to exfoliate the GO sheets by mechanical force. After sonication, samples were precipitated by a centrifuge (EBA 20, Hettich, UK) at 4000 rpm for 30 min and dried in air oven at 60 °C for 24 h. Exfoliated GO (2 g) was added to 1 L deionized water. The pH of solution was adjusted to 9–10 by 5% Na2CO3 solution and then LAA

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(8.8706 g) was added to the colloidal GO solution and stirred at 95 °C for 1 h. The reaction mixture was allowed to cool down to room temperature and filtered by Whatman paper No. 40. The solid product, referred to as ‘‘RGOV”, was washed by deionized water to remove residual ions and dried in an oven at 60 °C for 24 h. In order to remove residual oxygen-containing groups in the carbon planes, the RGOV was annealed under vacuum at 750 °C for 1 h to yield ‘‘RGOT”. The abbreviation of V and T means the reduction of GO by L-ascorbic acid and thermal, respectively. 2.3. Preparation of NR-RGO and NR-conductive carbon black composites One hundred parts per hundred of rubber (phr) of NR was mixed with 4 phr ZnO, 2 phr stearic acid, 1 phr CBS, 2.5 phr sulfur and conductive fillers (RGOT or conductive carbon black at 5–25 phr) in an internal mixer (Barbender, Germany) at 50 °C and 40 rpm for 13 min. In order to achieve a good dispersion, the compounding was mixed again by a two-roll mill. The total mixing time was less than 20 min to avoid premature vulcanization from the excess heat generated during compounding. The cure characteristics of the NR compounding were determined by a Moving Die Rheometer (TECHPRO rheotech MD+, US) at 150 °C according to ASTM D5289. Subsequently, NR vulcanizates (NR-V) were performed by compression molding at 150 k/m2 at 150 °C with the optimum cure time (tc90). The composites are referred to as NC-x and NG-x for the NR composites with conductive carbon black and RGOT, respectively, where x is the amount of conductive carbon black or RGOT in phr. 2.4. Characterization of GO and RGO The obtained RGO was characterized by fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and four-point probe. The functional groups of the samples were determined by FTIR (PerkinElmer, Inc., Spectrum one, USA). The samples were evaluated using KBr pellets and examined in the attenuated total reflectance (ATR) mode. The scan was performed in the range of 500–4000 cm1 with a spectral resolution of 0.5 cm1. Elemental composition analysis and functional groups on surface of samples were measured using XPS (PerkinElmer, Inc., USA). The XPS spectra were taken at a working pressure under 107 Pa with a monochromatic AlKa radiation source. The structure of samples were characterized by XRD (Bruker, D8 advance X-ray diffractometer, Germany) with a CuKa radiation source (k = 1.54 Å). The X-ray was operated at 40 kV and 40 mA with a scanning rate of 5–50° changing at 0.1o/s. The morphology of all samples was examined by TEM (JEOL JEM-2100, Japan). The sample (0.5 mg) in THF solvent was sonicated in a bath sonicator for 5 min and then the colloidal solution was drop cast on the TEM grid. Surface area determination of samples was performed by the Brunauer-Emmett-Teller (BET) method (Autosorb-1, Quantachrome, Germany). The nitrogen (N2) adsorption isotherm was recorded at 77 K. All samples were degassed at 250 °C for 3 h under vacuum before analysis. The conductivity of sample powder was determined by fourpoint probe (RM3-AR, JENDEL, UK). The powder was compressed to from a pellet using a hydraulic press machine. The surface energy and the wettability of the composite materials were investigated by water contact angle measurement (Ramé-hart, USA) at ambient temperature. The sample particles were ground to a powder and pasted onto a glass slide with adhesive tape. The water contact angle was measured by placing a 50 ll

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deionized water droplet on the sample surface using a microsyringe. Each sample was measured at five different positions. The surface energy of each composite material ðcpf Þ was calculated by Fowkes’ [47,48] equation, as shown in Eq. (1);

cpf ¼ cp þ cf  2ðcp cf Þ0:5 ;

ð1Þ

where cp and cf are the surface energy of the polymer and filler, respectively, and cpf is the surface energy between the polymer and filler. The surface wettability properties (cos(h)) of each sample was determined using Eq. (2).

cosðhÞ ¼

cp  cpf : cf

Machine (INSTRON 3366, USA) at a crosshead speed of 500 mm/ min with a load cell of 1 kN. The sample was cut by die C as a dumbbell shape. The results are reported as the average value from five measurements. The hardness of NR composites was evaluated using a Wallace Shore A durometer (UK) following ASTM D2240. Each sample was evaluated in five locations with a sample contact time of 5 s. The morphology of the NR-V and the NC-x and NG-x composites was investigated by TEM (JEOL JEM-2100, Japan) at an accelerating voltage of 80 kV. The thin film of NR sample was prepared by ultramicrotome and placed on a copper grid.

ð2Þ 3. Results and discussion

2.5. Characterization of NR composites

3.1. Characterization of RGO

The formation of crosslink density of NR samples can be calculated using the Flory-Rehner equation [49], as shown in Eq. (3);

Representative FTIR spectra of the graphite, GO and the two RGOs are shown in Fig. 1. The spectrum of graphite was featureless in the fingerprint region, while the spectrum of GO displayed a peak at 1581 cm1 attributed to the asymmetric stretch of sp2hybridized C@C bands and the skeletal vibration of the graphene sheets [54]. The broad absorption peak at 3443 cm1 originated from the CAOH stretching vibration of carboxyl groups. The peak at 1730 cm1 was attributed to the C@O stretching of carbonyl or carboxylic groups, while stretching vibration peaks of CAO (epoxides) and CAO (hydroxyl) were observed at 1225 and 1047 cm1, respectively [55–57]. These oxygen-containing groups were introduced into the graphite sheets during the oxidation process. The FTIR spectrum of RGOV, obtained by the reduction of GO by LAA as a reducing agent, showed that the peak intensities of the hydroxyl, epoxy and carboxyl groups were markedly reduced compared to those of GO. However, a small peak at 1730 cm1 (C@O stretching vibrations) was still observed. This might be attributed to the selectivity of LAA, which more easily reduced the epoxide and hydroxyl groups than carbonyl groups [58]. To completely remove the oxygen-containing groups in RGOV, thermal reduction was required in this study. It was found that the peak intensities of C@O (1730 cm1) in RGOT was significantly decreased, because the increased temperature made the oxygen-containing groups attached on the carbon planes decompose into CO or CO2 [59]. Accordingly, a large proportion of oxygen functionalities were effectively removed from the GO surface after chemical and thermal reduction. The binding energy of graphite, GO and the two RGOs, as analyzed by XPS, is presented in Fig. 2. Graphite was found to have a

gc ¼

lnð1  V r Þ  V r  vV 2r
 ; 2V V s V r1=3  f f

ð3Þ

where gc is the crosslink density of the rubber (mol/cm3), Vs is the molar volume of the toluene (106.2 cm3/mol), v is the polymersolvent interaction parameter (v = 0.3795), f is the functionality of the crosslinks for the sulfur curing system (f = 4) and Vr is the volume fraction of rubber in the swollen gel, which was calculated based on Eq. (4) [50,51];




 W  qf f 
; Vr ¼
Wf W1 þ W 2qW 1 q  q W1

qr

r

f

ð4Þ

s

where W1 and W2 are the weights of the sample before and after toluene immersion, Wf is the weight of the filler in the rubber vulcanizates (g), qr is the density of the rubber vulcanizates (g/cm3), qf is the density of the filler particles (g/cm3) and qs is the density of toluene (0.862 g/cm3) [50,51]. The rubber-to-filler interaction was examined by a bound rubber measurement reported by the gravimetric method [52]. The rubber compound (1 g) was immersed in 80 ml toluene for 7 d at ambient temperature. The rubber gel was then harvested from the solvent by filtration and dried in an oven at 80 °C for 1 d, whereupon the percentage of bound rubber was calculated from Eq. (5);

h i m W fg  W m fmp fþ h i RB ¼
100; m W m pmp

ð5Þ

fþ

where Wfg is the weight of the filler-rubber gel, W is the weight of the test sample and mf and mp are the weight of the filler and polymer, respectively, in the rubber compound. The electrical conductivity (r) was measured by a two probe setup connected with a multichannel potentiostat. The sample was placed in a fixture designed in our laboratory where the sample was kept under consistent pressure. The composite sample was placed between copper electrodes. The electrical resistance (R) was obtained by dividing voltage drop by the current applied, while the electrical conductivity (r) was obtained from Eq. (6) [53];

r¼

t ; R
A

ð6Þ

where R is the electrical resistance (O), t is the sample thickness (m) and A is the area of the sample (m2). The tensile properties of NR-V, NC-x and NG-x composites were measured according to ASTM D412 using a Universal Testing

Fig. 1. Representative FTIR spectra of (a) graphite, (b) GO, (c) RGOV and (d) RGOT.

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high intensity C 1s peak at 284.6 eV that corresponds to graphitic materials with only a small amount of oxygen atoms on the graphite surface [60]. The surface of GO was mainly composed of carbon and oxygen (O 1s peak at 531.0 eV) due to the introduction of oxygen-containing groups by the oxidation process. To investigate the distribution of the functional groups on the GO surface, a narrow scan of the C 1s spectra by XPS was examined (Fig. 3). The C 1s spectra of GO fitted into five peak components. The dominating peak represented the sp2-hybridized carbon at 284.6 eV [61]. The other four peaks represented the oxygen-containing carbons of the carbon-hydroxyl groups (CAOH) at 285.8 eV, epoxy/ether groups (CAO) at 286.6 eV, carbonyl groups (C@O) at 288.4 eV and carboxylate carbon groups (OAC@O) at 289.8 eV [36,62]. After reduction, the O 1s peak intensities of both RGOV and RGOT were decreased owing to the removal of the oxygen-containing groups during the reduction processes. However, some oxygen functional groups were still observed, such as the CAOH and C@O peaks at 285.8 and 288.8–288.9 eV, respectively. The atomic composition and oxygen/carbon (O/C) elemental percentage proportions of graphite, GO and the two RGOs are summarized in Table 1. The% oxygen atoms was clearly increased after the oxidation process (by over 8.1- and 10-fold for the% O atoms and O/C ratio, respectively), while the O/C ratios of RGOV and RGOT were markedly (2.16- and 11.5-fold) decreased to 0.139 and 0.026, respectively. However, the O/C ratio of RGOV was higher than that of RGOT and comparable with graphite. According to the obtained results, it was concluded that the thermal reduction is an important process to remove the oxygen components on the carbon planes in RGO. In this study, the thermal reduction was performed at 750 °C under vacuum furnace, since under this condition a high efficiency for removing oxygen-containing groups was obtained. This was a lower temperature than previous reports that performed the thermal reduction at more than 1000 °C under an argon inert gas atmosphere [63,64]. Representative XRD patterns of graphite, GO and the two RGOs are shown in Fig. 4, while the possible structure-orientated model of graphite before oxidation and the RGOV and RGOT after reduction are presented in Fig. 5. The XRD pattern of graphite showed the high intensity peak of the (002) reflection with an interlayer spacing of 3.41 Å (2h = 26.0°) [65]. This was attributed to the ordered arrangement of carbon planes. Generally, graphite is composed of stack layers of carbon planes in which each plane is orderly arranged with an interlayer spacing between the planes

Fig. 3. Representative narrow scan of C 1s XPS spectra of (a) GO, (b) RGOV and (c) RGOT.

Fig. 2. Representative wide scan of the XPS spectra of (a) graphite, (b) GO, (c) RGOV and (d) RGOT.

of 0.34 nm (Fig. 5). After oxidation, the interlayer spacing of GO was markedly expanded to 7.24 Å with a small and broad peak of (002) reflection at 2h = 11.25° [66]. The large interlayer spacing has been attributed to the formation of oxygen-containing groups after oxidation that disturbed the van der Waal forces between the carbon planes. In order to destroy the van der Waal forces, sonication was used before the reduction process (Fig. 5). After reduction, RGOV and RGOT exhibited a markedly reduced intensity of the (002) peak at 2h = 25.77° with an interlayer spacing of 3.50 Å

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Table 1 The XPS data and electrical conductivity of graphite, GO, RGOV and RGOT. Sample

Graphite GO RGOV RGOT

Composition

Conductivity (S/m)

C 1s (% atom)

O 1s (% atom)

O/C ratio

97.16 76.89 87.79 97.44

2.84 23.11 12.21 2.56

0.029 0.300 0.139 0.026

46.45 10.05 91.11 121.47

Remark: Conductivity of conductive carbon black of 106.72 S/m.

Fig. 4. Representative XRD patterns of (a) graphite, (b) GO, (c) RGOV and (d) RGOT.

[67]. It is important to note that the formation of RGOV and then RGOT from the reduction of GO and RGOV, respectively, was successfully obtained. However, the broad nature of (002) in RGOV and RGOT indicated the poor ordering of the graphene sheets along the stacking direction, as shown in Fig. 5 [68]. Additionally, the weak peak and a shoulder observed at 2h at 42.5° and 44° may correspond to the hexagonal structure of carbon [69], while a shoulder

was observed at 2h around 18°, indicating that some portion of the graphite oxide layer was not fully intercalated [66]. The morphology of the graphite and the two RGOs, as analyzed by TEM, is shown in Fig. 6. Graphite displayed a flaky type morphology with many stacks of layers that overlapped with each other (indicated by the opaque area). In contrast, RGOV showed a semitransparent view of a few thin layers of sheets. The overlapping RGO sheets could be due to the aggregation of carbon planes after the chemical reduction process. Low solubility of RGO sheets in water during the chemical reduction, caused the strong p-p stacking tendency between RGO sheets and led to the formation of agglomerates [70]. The thermal reduction of RGOV degraded the oxygen-containing functional groups to CO and CO2 that were released into the spaces between the RGO sheets. The resulting increased pressure between the stacked layers then caused the exfoliation of the carbon planes [59]. Thus, RGOT exhibited a transparent single layer RGO sheet with some folds and wrinkles with a specific surface area of 201.35 m2/g. The electrical conductivity of the graphite, GO, RGOV and RGOT is summarized in Table 1. Graphite consists of layered planes of carbon atoms with two dimensional lattice bonds that are vertical and horizontal. The hexagonal carbon rings in the horizontal plane provide the delocalized electrons, allowing an easy conduction within the planes, while the conduction is much lower perpendicular to the planes [71]. The electrical conductivity of a bulk powder is generally lower than that of the individual particles, since the interface between the particles offers extra resistance to charge transport [72]. As a result, the electrical conductivity of graphite in this study was 46.45 S/m. The presence of the oxygen-

Fig. 5. The structure-orientated model of RGO sheets obtained by oxidation and reduction processes.

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Fig. 6. Representative TEM micrographs of (a) graphite (40,000 x magnification, scale bar = 100 nm) (b) RGOV (40,000 x magnification, scale bar = 200 nm) and (c) RGOT (40,000 x magnification, scale bar = 100 nm).

containing groups on GO destroyed the p-p electronic conjugation of graphite, resulting in a large decrease (4.63-fold) in the electrical conductivity of the GO. However, the electrical conductivity could be restored by the reduction of GO, with the RGOV and RGOT having a markedly improved electrical conductivity, some 9.0- and 12.0-fold more than in GO, respectively, and 1.96- and 2.62-fold more than the graphite, respectively. The conductivity of RGOT was 1.3- and 1.14-fold higher than that of RGOV and the commercial conductive carbon black (PRINTEXÒ XE2-B, 106.72 S/m), respectively. Accordingly, the thermal reduction improved the electrical properties of the RGO materials to a greater extent than the chemical reduction. 3.2. Cure characteristics of NR composites The cure characteristics of the NR-V and the NC-x and NG-x composites are summarized in Table 2. The scorch time (ts2) and cure time (tc90) of the NC-x and NG-x composites were shorter than those of the NR-V and the ts2 values of the composite materials gradually decreased with increasing filler loadings, up to 2.3- and 1.6-fold for NC-25 and NG-25, respectively. This could be due to the increased friction between the particles as the quantity of filler increased, resulting in an increased level of heat generation in the system during mixing that accelerated the vulcanization [73]. At the same filler loading, the ts2 values of the NG-x composite materials were higher than those of NC-x ones, which may be due to the

different structure and specific surface area for each filler type. The different torques (DM = MH  ML) of the NC-x and NG-x composite materials increased with increasing filler loading levels since the rigid particles reduced the mobility of the rubber chain segments. In addition, the increased DM values may also reflect the increased crosslink density, as seen in Table 2, and material stiffness. Accordingly, the RGOT obtained in this study did not prevent the vulcanization of the NR. 3.3. Electrical properties and morphology of NC-x and NG-x composites The electrical conductivity of the composite materials versus the volume fraction of filler is shown in Fig. 7. The electrical conductivity of the NC-x and NG-x composites at a volume fraction of filler of <0.01 (<5 phr) was comparable with that of NR-V due to the dilution effect. Thereafter, at higher volume fractions of conductive filler, the filler particles began to connect with each other and formed conducting paths and so improved the conductivity, although this tended to asymptote above a volume fraction of 0.05 (20–25 phr). These results were supported by the TEM analysis (Fig. 8), where both the NC-5 and NG-5 composites exhibited networks of aggregated or agglomerated filler networks with a decreased gap between the filler aggregates or agglomerates, known as the percolation threshold. The electrons could, therefore, easily jump though the continuous filler networks. Interestingly,

Table 2 Cure characteristics of various NR compounds. Sample code

NR-V NC-5 NC-10 NC-15 NC-20 NC-25 NG-5 NG-10 NG-15 NG-20 NG-25 a b c d e f g

gc (
104)f

Cure characteristic tS2a

MLb

MHc

(min)

(dN.m)

(dN.m)

DM (dN.m)

tc90 (min)

7.76 4.59 3.95 3.10 2.83 1.99 6.36 5.32 4.74 4.29 3.89

0.74 1.05 1.65 3.28 5.47 7.81 0.77 0.71 0.73 0.96 0.97

6.59 8.59 11.29 14.54 20.04 23.74 8.11 8.77 9.41 10.39 11.72

5.84 7.54 9.64 11.26 14.58 15.92 7.34 8.06 8.68 9.43 10.74

10.22 7.91 7.67 7.32 8.51 8.09 8.95 7.90 7.29 7.22 7.32

Scorch time. Minimum torque. Maximum torque. Torque difference (MH  ML). Curing time. Crosslink density. Bound rubber content.

d

e

(mol/cm3)

BRCg (%)

1.89 2.27 2.47 2.59 2.71 4.13 3.32 2.42 2.45 2.47 2.49

31.54 77.18 84.99 86.30 95.04 102.54 77.78 85.29 88.43 89.55 92.69

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black in the composites could trap some rubber within their aggregates/agglomerates to form occluded rubber, which explains why the bound rubber content of the NC-x composites was higher than that of the NG-x ones. The theoretical electrical conductivity, based on that Mamunya model [74,75], as shown in Eqs. (7) and (8), were evaluated and then compared to the experimentally determined electrical conductivity results.

log r ¼ log h : for ; < ;c ;

log r ¼ log rc þ ðlog rF

Fig. 7. The electrical conductivity of NR vulcanizates filled with RGOT (NG) and conductive carbon black particles (NC).

the NG-5 composite showed an equivalent percolation threshold to the NC-5 one. However, the conductivity of the NC-x composites was slightly higher than that of the corresponding NG-x ones. As seen in TEM images, the conductive carbon black particles tended to be more aggregated/agglomerated due to the low wettability between the filler and rubbery matrix (cos(h) of NC = 0.73 and cos(h) of NG = 0.89), the higher BET specific surface area and high bound rubber content (Table 2) compared to RGOT. The gap between the secondary agglomeration of fillers and/or formation of interconnected agglomerates was decreased, leading to an increased electrical conductivity of the composite materials. Accordingly, the filler-to-filler interaction of conductive carbon black was stronger than that of RGOT in the respective composites. Additionally, the aggregates/agglomerates of conductive carbon

cpf Þ;c  ð0:11þ0:030:7 ð;;c Þ ;  ;c  log rc Þ : for ; > ;c F  ;c

ð7Þ

ð8Þ

where rh is the electrical conductivity of the polymer matrix, rC is the electrical conductivity at the percolation threshold, rF is the composite electrical conductivity at the maximum packing volume fraction, F is the maximum packing volume fraction, ; is the volume fraction of filler, ;C is volume fraction at percolation threshold and cpf is a interfacial surface tension. The comparison of the experimental results with the predicted results from the Mamunya model for the NC-x and NG-x composites are shown in Fig. 9. The curve fitting was in a good agreement with the experimental data for the both types of filler, and so the principal factor influencing the electrical conductivity was the filler volume fraction. In addition, the electrical conductivity of a composite material is known to depend on the electrical conductivity of the filler, filler aspect ratio and surface energies [76]. A filler with a high electrical conductivity can enhance the electrical conductivity of the composite material at a low percolation threshold [74]. An increased filler aspect ratio makes it easier to form a conductive network, resulting in a decreased percolation threshold [77]. Finally, the smaller differences between the surface energy of the filler and rubber lead to a decreased electrical conductivity of the composite material [78].

Fig. 8. Representative TEM micrographs of NR vulcanizates (44,000 x magnification, scale bar = 200 nm) filled with (a) conductive carbon black particles at 5 phr and (b) RGOT at 5 phr. Proposed conductive networks (c) NR-filled with conductive carbon black and (d) NR-filled with RGOT.

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3.4. Mechanical properties of the NC-x and NG-x composites The mechanical properties of NC-x and NG-x composite materials are displayed in Fig. 10. The inclusion of the rigid filler particles improved the mechanical properties, in terms of the modulus, tensile strength and hardness of the composite materials, compared to those of the NR-V. The tensile modulus (M100) values increased with increasing filler loadings due to the reinforcing effect of the filler. The reinforcement effect of the conductive carbon black filler in the NC-x composites was higher than that of RGOT in the NG-x ones. The reinforcement of composite materials has previously been shown to depend on several factors, such as the particle size, specific surface area, surface-activity of the filler particles and polymer type [79]. In addition, the tensile strength (TB) of the NC-x and NG-x composite materials increased with increasing filler loadings but at >10 phr the TB values gradually decreased. Thus, the incorporation of filler into the NR matrix improved the material hardness but reduced the elasticity of the resulting NC-x or NG-x composites. The elongation at break (EB) decreased slightly with increasing filler loadings for both the NC-x and NG-x composites. However, the mechanical properties of the NC-x composites were higher than that of the NG-x ones due to the better reinforcement ability of the conductive carbon black.

4. Conclusion

Fig. 9. The experimental (Exp) and theoretical (Model) data for the electrical conductivity of the (a) NC-x and (b) NG-x composites.

Graphite waste from the metal smelting industry was exfoliated thought oxidation, and then chemical and thermal reduction to produce RGO sheets of a high quality. L-ascorbic acid was used as an environmentally friendly reducing agent, but the FTIR and XPS results indicated that the obtained RGOV still had some oxygen functional groups on the surface, such as CAOH and C@O. Thus, the subsequent thermal reduction of RGOV was required to eliminate most of these residual oxygen-containing groups in the carbon planes. The O/C ratio of the obtained RGOT after thermal reduction was strongly decreased (5.35-fold) compared to that of RGOV. The morphology of RGOT, based on TEM analysis, was a single layer sheet. The conductivity of RGOT was remarkably improved, presumably due to the recovery of the sp2 conjugation

Fig. 10. The mechanical properties of the NC-x and NG-x composites, showing the (a) 100% modulus, (b) tensile strength, (c) elongation at break and (d) hardness.

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on the carbon planes compared to RGOV or graphite. It is interesting to note that the conductivity of the NG-5 vulcanizate (8.71
106 S/m) was similar to that with the NR-commercial conductive carbon black (8.15
106 S/m) composite at the same filler loading (5 phr). The mechanical properties of the NG-x vulcanizates were improved with increasing filler loading levels. The present procedure is a promising strategy to obtain conductive rubber composites for use as conductive elements in a rubbery matrix. Accordingly, the improved properties of RGOT filled NR composite materials (NG-x) could be widely used for energy storage devices, sensors, and many other applications.

Acknowledgements The authors gratefully acknowledge the funding support from Thailand Research Fund (TRF) and National Research Council of Thailand (NRCT) (RDG5650080); the Thailand Research Fund (IRG5780001); Special Task Force for Activating Research (STAR), Ratchadaphiseksomphot Endowment Fund, Ratchadaphiseksomphot Endowment under the Outstanding Research Performance Program (GF_58_08_23_01), Chulalongkorn University and the L’Oréal’s ‘‘For Women in Science” Fellowship program (2015).

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