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Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates
Accepted Manuscript Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates Asish Malas, Chapal Kumar Das, Professor PII:
S1359-8368(15)00289-9
DOI:
10.1016/j.compositesb.2015.04.051
Reference:
JCOMB 3582
To appear in:
Composites Part B
Received Date: 4 January 2015 Revised Date:
27 April 2015
Accepted Date: 29 April 2015
Please cite this article as: Malas A, Das CK, Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.04.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates Asish Malas 1, Chapal Kumar Das 1* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, India. E-mail: [email protected], [email protected]
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*Corresponding Author: Chapal Kumar Das (Professor) Email: [email protected]
Keywords: A. Polymer-matrix composites (PMCs), A. Nano-structures, B. Cure behavior, B. Mechanical properties
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Abstract
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The present research work demonstrated the effect of graphene oxide (GO) on the physical, mechanical, thermo-mechanical etc., properties of neoprene (CR) and chlorosulfonated polyethylene (CSPE) vulcanizates. CR and CSPE based nanocomposites were prepared by both
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solution intercalation and melt intercalation methods. The changes obtained in the morphology, cure characteristics, mechanical, thermal, thermo-mechanical properties of the rubber nanocomposites
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have been widely investigated. X-ray diffraction analysis (XRD) and transmission electron microscopic (TEM) analysis of the samples revealed partial exfoliated structure of GO containing rubber composites. Mechanical, thermal, cure and thermo-mechanical properties of the elastomeric
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1.Introduction
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nanocomposites were improved compared to the neat rubbers.
Any mixture of dissimilar materials, commonly two, in which one of the constituent
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serves as a reinforcing agent known as composites. Reinforcing agents are usually much stronger with low densities compared to the polymer base matrix [1]. From way back, the discovery of the innovative work of the Toyota research group in the beginning of 1990s, polymer/clay nanocomposites have drawn a lot of concentration [2]. Significant improvement in the physical, mechanical, thermal, dynamic mechanical and functional properties of the polymers can be achieved by intercalating or exfoliating clay tactoids in the matrix compared to the unfilled polymers [3]. Many researchers have targeted on the development and characterization of 2
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polymer nanocomposites containing exfoliated clay platelets. This is due to the natural presence of clay and the lack of difficulty in surface functionalization of clay to increase the intergallery spacing which resulting in the formation of intercalated structure of polymer/clay composites [4-
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6]. Similarly, The same approach can be employed to another reinforcing nanomaterial, known as graphene nanoplatelets that originated after the modification of natural graphite flakes [7, 8]. Polymer graphite nanocomposites (PGNs) can be prepared by the incorporation of graphite
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nanosheets in the polymer matrix [9-11]. Analogous to natural clay platelets, graphite is a layered structured material that contains many layers (units) familiar as graphene. Graphene is a
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single layer of carbon atoms profoundly arranged in a 2-D hexagonal lattice [12]. 2-D Graphene sheets exhibit many unique physical and chemical properties along with superior mechanical, thermal and electrical properties in the exfoliated condition in the polymer matrix [13, 14]. The utilization of graphene flakes as reinforcing nanofiller for the synthesis of polymer
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nanocomposites resulting in an increase in the gas barrier and flame-retardant properties [15, 16]. As a result of the weak van der Waals force of attraction between the units of graphite, it is easy to separate (Exfoliate) graphene layers of graphite, which resulting in the formation of high
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aspect ratio (length-to-thickness ratio) reinforcing nanosheets with thicknesses in the range of 2– 10 nm [17]. To intercalate and exfoliate the graphene sheets of natural graphite, it is oxidized and
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altered to a layered solid material (Graphene oxide) with aromatic unoxidized benzene rings and aliphatic six-membered ring consists of epoxide, hydroxyl and carboxyl groups [18-20]. Due to the formation different functionalities by the chemical oxidation of the unsaturated part of graphite, the polyaromatic character of pristine graphite is diminished but the lamellar structure is preserved. Like pristine graphite, Graphene oxide (GO) consists of stacks of graphene with larger interlayer spacing. The lateral dimension of graphite nanosheets in GO is drastically
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reduced compared to the size of the pristine graphite [21]. The intergallery distance of graphite can be increased from 3.35 Ǻ (pristine graphite) to 6-10 Ǻ, depending on the interlameller water content and degree of the intercalation [22]. Some researchers had studied the effect of GO on
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the physical, mechanical and dynamic mechanical properties of different polymer matrices [2325]. Tang et al. synthesized elastomeric nanocomposites consisting of GO sheets by using butadiene–styrene–vinyl pyridine rubber (VPR) as the host through co-coagulation process and
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in situ formation of an ionic bonding interface. They observed significant improvement in the mechanical properties for the GO filled hybrid nanocomposites [26]. Hernandez et al.
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synthesized functionalized graphene sheets (FGSs) filled natural rubber nanocomposites by conventional two roll mill mixing. They noticed improvement in the mechanical and electrical properties of nanocomposites [27]. Kumar et al. fabricated conducting poly(isobutylene-coisoprene) (IIR)/reduced graphene oxide (RGO)/expanded graphite (EG) composites. They
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observed high barrier, dielectric and sensing properties for the hybrid nanocomposites [28]. Wu et al. prepared surface functionalized graphene oxide (SGO) filled natural rubber based nanocomposites and observed improvement in the mechanical as well as gas barrier properties of
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the rubber nanocomposites [29]. Lu and coworkers studied the synergistic effect of carbon fiber and GO on shape memory polymer (SMP) nanocomposites. They observed that the electrical
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properties of SMP nanocomposites are significantly improved via a synergistic effect of GO and carbon fiber [30]. J. H. Lee et al.
prepared raw graphene/chitosan and cryomilled
graphene/chitosan nanocomposites in order to study the cryomilling effect of graphene on the tensile properties of their corresponding nanocomposites. They noticed that cryomilling enhanced the dispersion, graphitic characteristics, and thermal stability of the graphene powders and also observed significant improvement in tensile properties for the cryomilled graphene
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reinforced chitosan composites compared to raw graphene/chitosan nanocomposites [31]. M. Ionita et al. synthesized polysulfone (PS)/graphene oxide (GO) composite membranes and observed improvement in the morphological, thermal and mechanical properties of the composite
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[32, 33]. Yuan et al. prepared graphene oxide/nylon 11 composites by in situ melt polycondensation. They observed superior mechanical properties including stiffness and
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toughness for the nanocomposites compared to the pure nylon 11 matrix [34].
Chloroprene rubber (CR) is exceptionally flexible synthetic rubber due to its rare combination of
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features like ozone/oil resistance, toughness, dynamic flex life, good adhesion to other materials, and excellent heat resistance [35]. Nanofiller reinforced CR based composites have been preferred most likely materials for moldings and extrudates of all types, hoses, roll covers, belting (conveyor belts), air spring bellows, cable sheathing and insulation for low-voltage cables, corrosion-resistant linings, sheeting, and footwear, hose water suits and water sealant,
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and many other applications.
Chlorosulfonated polyethylene (Hypalon) exhibits some outstanding properties such as ozone
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resistance chemical resistance, fire-retarding due to the presence of chlorine atom and also electric insulator. CSPE can be generally used to manufacture the conveyor belt lining, cable,
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conductor insulation, and building materials, and others; but till the important properties such as mechanical properties, thermal stability, and damping can be further increased to boost its application in commodities with some distinctive requirements [36]. In the present research work, GO filled (3 wt%, 6 wt% and 9 wt%) rubber nanocomposites based on neoprene rubber (CR) and chlorosulfonated polyethylene (CSPE/Hypalon) were synthesized by two step methods (Solution mixing and compounding on
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an open two roll mill). The changes obtained in the physical, mechanical, dynamic mechanical, thermal etc., properties of the rubber matrices have been widely analyzed in this study. The GO/CR and GO/hypalon nanocomposites exhibited significant improvement in the mechanical,
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thermo-mechanical and thermal properties compared to their respective controls. 2. Experimental
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2.1 Materials
Polychloroprene rubber (Neoprene-W) was supplied from Bayer, Germany (Mooney
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viscosity ML (1+4) at 100 0C = 48 and Mw is about 349,000 g/mol). Chlorosulfonated polyethylene (Hypalon 40) was supplied from Du Pont, USA (Mooney viscosity ML (1+4) at 100 0C = 56 and Mw is about 188,000 g/mol). Graphite Fine Powder (Extra Pure) was obtained from Loba Chemie Pvt. Ltd., Mumbai (India). Concentrated sulfuric acid (H2SO4), phosphoric acid (H3PO4), hydrochloric acid (HCl) and nitric acid (HNO3) were obtained from Merck, India
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Ltd. Potassium permanganate (KMnO4), ethanol (C2H5OH), tetrahydrofuran (THF) and hydrogen peroxide were bought from Sigma-Aldrich, India. Compounding additives like zinc oxide (ZnO) and magnesium oxide (MgO) were purchased from Merck, India. Ethylene thiourea
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(NA-22) was procured from Du Pont.
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2.2 Synthesis of Graphene Oxide by Improved Method A 9:1 mixed acid of concentrated H2SO4/H3PO4 (360:40 mL) was incorporated to a
mixture of graphite flakes (3.0 g, 1 wt equiv) and KMnO4 (18.0 g, 6 wt equiv), generating a minor exotherm to 35-40 °C. Then, the reaction mixture was heated upto 50 °C and stirred for 12 h. Afterthat, the reaction mixture was cooled to room temperature and poured onto ice water (~400 mL) with 30% H2O2 (10 mL). The solution mixture was then centrifuged (4000 rpm for 4 h), and the supernatant liquid was decanted away. Then, the rest of the solid material was washed 6
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subsequently with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol (2 times). The solid material collected in the centrifuge tubes was dried overnight at room temperature in a
2.3 Preparation of the rubber nanocomposites
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vacuum oven [37].
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Graphene oxide (GO) containing CR and CSPE based nanocomposites were fabricated by using both solution and melt interaction. Initially, CR and CSPE was dissolved seperately in
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THF (rubber to solvent ratio was 1:3 weight/ volume). Required amount of GO was then incorporated into the rubber solution under continuous stirring and followed by ultrasonication for 30 min. Afterthat, the final mixture was then poured over a Petri dish and kept in the air for the full evaporation of solvent at room temperature under the fume hood cabinet. Solution
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intercalated different rubber/GO composites (thin films) were then mixed with the bulk CR and CSPE matrices in separate batches by melt blending (Direct mixing) in a laboratory scale open two roll mixing mill (the speed ratio of the rotor was 1:1.4) at rt. Compositions of the GO loaded
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CR and CSPE based nanocomposites are represented in the Table 1. C-3G (100 g CR/3 g GO), C-6G (100 g CR/6 g GO), C-9G (100 g CR/9 g GO), Hy-3G (100 g Hy/3 g GO), Hy-6G (100 g
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Hy/6 g GO) and Hy-9G (100 g Hy/9 g GO) are the abbreviations of the prepared elastomeric nanocomposites based on chloroprene (neoprene) and chlorosulfonated polyethylene vulcanizates.
3. Characterization techniques Curing studies of the GO loaded CR and CSPE based nanocomposites were carried out in Monsanto Rheometer R- 100 testing instrument at 160 oC with 3o arc for 60 min.
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Wide angle X-ray diffraction (WAXD) analysis of Graphite, GO and GO containing rubber nanocomposites was done in Rigaku Miniflex Diffractometer with Cu-Kα radiation at a generator voltage of 40 kV. The scanning rate was 3°/min between 70-700, chart speed
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10mm/2θ, current 20mA and wavelength 0.154 nm at room temperature. The intergallery space of graphite and GO were achieved from the Bragg’s equations nλ= 2d sin θ. The state of dispersion of GO in the different rubber matrices was also confirmed from the WAXD analysis.
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The morphology of the GO filled elastomeric nanocomposites was observed through high resolution transmission electron microscope (HR-TEM, JEOL 2100). For HR-TEM analysis,
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ultra thin cross sections of the rubber nanocomposites were cut by the help of Leica Ultra Cut UCT Ultra microtome instrument equipped with a diamond knife.
Viscoelastic properties of the GO containing rubber nanocomposites were investigated by using a TA instrument DMA 2980 model in tension mode at a constant frequency of 1 Hz, a strain of
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0.1%, in the temperature range of -80°C to + 80°C and at a heating rate of 3°C/min. Mechanical features of the GO loaded rubber nanocomposites were measured in a Universal tensile testing machine (Hounsfield H 10KS) at room temperature. Tensile strength and tear
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strength were measured according to specifications ASTM D412 - 06ae2 and ASTM D624-00 (2012). 5 samples of each rubber nanocomposites were tested and the average values (mean)
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were represented in the respective table. The tensile tests were done on dumb-bell shaped samples and length of the tensile testing sample was 25mm, thickness was 1.5 mm and the speed of jaw separation was 500mm/minute with a load cell of 10 kN. The hardness of the rubber nanocomposite was measured by Shore A hardness tester as per ASTM D2240-05(2010) standard.
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In order get the wear resistance, DIN abrasion test of GO filled rubber nanocomposites was carried out in a DIN abrasion tester [ASTM D5963-04 (2010)]. The cryo-fractured surface of the nanocomposites was scanned in VEGA TESCAN// LSU
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(Scanning Electron Microscope) instrument. For the SEM analysis, the samples were gold coated prior to analysis.
To investigate the thermal stability of the nanocomposites, thermo gravimetric analysis (TGA) of
30°C -700°C (heating rate was 10°C/min).
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the samples was carried out in DuPont TGA- 2100 thermal analyzer in the temperature range
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FT-IR analysis of Graphite and GO was carried out in the range of 800cm-1 to 2000 cm-1 in Thermonicolet /Nexus 870 FT-IR spectrometer.
Raman spectra of Graphite and GO were acquired in back scattering geometry by utilizing a micro-Raman setup connected with Ar+-Kr+ laser (Model 2018-RM, Newport, USA) as the
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excitation light source, a spectrometer (model T64000, JY, France) and a Peltier cooled CCD detector. The raman spectroscopic analysis of the samples was carried out at an excitation wave
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length of 785 nm.
4. Results and discussion
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4.1 FT-IR and Raman spectroscopic analyses FT-IR spectra of graphite and GO are represented in the Figure 1a. It can be seen from
the figure 1a that one peak was raised at 1625 cm-1 (GO), which was due to the vibration of the adsorbed water contents and simultaneously the structural vibrations of unoxidized graphitic spheres [38-40]. The band at 1746 cm-1 was designated to the C=O stretching vibration of
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carboxylic acid groups (-COOH) present on the surface of GO. The peak raised at 1054 cm-1 was because of the C-O-C stretching vibration [41]. Figure 1b shows the Raman spectra of graphite and GO. It can be seen from the curves that the
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conventional G band of graphite was raised near 1580 cm-1 [42, 43]. For graphene oxide (GO), a D band (1357 cm-1) alongwith G band (1584 cm-1) were raised, which indicates the prominent
hydroxyl, epoxide, and carboxyl groups [44-47]. 4.2 XRD analysis
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growth of sp3 character due to the presence of oxygen-containing functional groups, such as the
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XRD patterns of graphite, graphene oxide (GO) and GO containing rubber nanocomposites based on halogen containing elastomers (CR and CSPE) are shown in the Figure 2 (a-d). It can be seen (Figure 2a-b) that after the modification of graphite to GO, the main peak (001) of GO was raised at 2θ = 10.72° corresponding to the d-spacing of 0.818 nm. The d-
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spacing of graphite flakes (0.335 nm) was significantly increased in the GO. Figure 2c shows that after the incorporation of GO in the CSPE matrix, the (001) peak of GO was totally diminished for the hypalon based rubber nanocomposites which confirms that the GO platelets
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was delaminated in the rubber matrix. The other peaks raised (broad peak and sharp peaks) were due to the amorphous polymer and compounding additives respectively. It can also be seen from
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the figure 2d that the main peak (001) of GO was more or less diminished for the CR based nanocomposites but the (001) peak intensity gradually increases with the increase in GO loading. (100) peak of CR was merged with the (001) peak of GO. Chieng and cowokers prepared graphene nanoplatelets (xGnP) reinforced poly(lactic acid) (PLA)/epoxidized palm oil (EPO) blend. They observed disappearance of peak for the nanocomposites and concluded it as a result of exfoliation and random distribution of the platelets within the polymer matrices at low loading
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of xGnP [48]. Qi and coworkers observed no characteristic peak of GO in the XRD patterns of polyvinyl alcohol (PVA)/GO nanocomposites. They concluded it as a good dispersion of GO in the PVA matrix [49].
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XRD analysis of the rubber nanocomposites shows only a partial evidence of the dispersion of nanofiller and inexistence of peak analogous to d-spacing does not always imply the delamination of the nanofiller in polymer matrix. XRD is incapable to diagnose regular
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stacking surpassing 8.8 nm [50]. Hence, to observe the dispersion of nanofiller in the elastomeric nanocomposites, microscopic study is necessary [51]. For better observation regarding the
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morphology of GO containing rubber matrices, HR-TEM analysis of the samples were carried out and discussed later. 4.3 HR-TEM analysis
HR-TEM images of the GO loaded (3 wt%, 6 wt% and 9 wt%) halogen containing
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elastomers are shown in the Figure 3 (a-f). The black marks in the images direct the graphene oxide nanosheets. It can be seen from the images that GO is fully exfoliated in both the halogen containing elastomeric matrices which resulting in a significant increase in the overall physical
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and mechanical properties of the nanocomposites. As the halogen containing elastomers are polar in nature, so the halogen groups and other polar functionalities may better interact with the
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functional groups of GO. Because of superior interactions and interfacial adhesion between the rubber matrices and GO, mechanical and dynamic mechanical features of the rubber compounds were enhanced.
4.4 SEM analysis
SEM images of GO and the cryofractured surfaces of the GO containing halogen containing elastomers based nanocomposites are displayed in the Figure 4 and 5. It can be seen
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from the images that the GO filled rubber nanocomposites show more uneven and tortuous pathway of the fracture surface, and tear paths were broadened compared to the fractured surface of unfilled halogen containing elastomers. Layered nanosized GO uniformly dispersed in rubber
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matrices and changed the fracture lines relying upon their location in the elastomer matrix. More rough and the tortuous fractured surface of the nanocomposites resulted in the enhancement in
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the general mechanical features of the nanocomposites.
4.5 TGA analysis
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Thermo gravimetric curves of the different rubber nanocomposites filled with graphene oxide (GO) are shown in the Figure 6 (a-c). Figure 6a shows that GO exhibits a gradual weight loss in the temperature range up to 100 °C which was due to the removal of physiosorbed water. After that, a sharp weight loss at about 130 °C can be featured by the decomposition of different
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oxygen containing functional groups in the GO layer [52]. From the Figure 6 (b-c), it can be seen that the onset decomposition temperature of the different GO loaded nanocomposites was lowered compared to the unfilled rubber which was because of the early degradation of oxygen
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functionalities on the surface of GO. The maximum decomposition temperature of the different rubber composites was higher compared to the neat rubber. GO nanosheets were dispersed
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uniformly in the different rubber matrices and create a mass transfer obstruction to the volatile matters during the thermal degradation of the nanocomposites as well as GO acts as an efficient heat absorber which indeed significantly increase the thermal stability of the nanocomposites. 4.6 Cure characteristics
The cure behavior of the GO filled halogen containing elastomers based nanocomposites are depicted in the Table 2. Maximum torque (MH) which can be dealt with the composite
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modulus [53] and difference in torque value (∆S) of the different rubber nanocomposites were increased in comparison with the neat rubber. MH and ∆S values for the different nanocomposites were gradually increased with increasing the loading of GO. Significant
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exfoliation of GO in the different rubber matrices increases the stiffness of the matrices as well as increases the degree of crosslinking of the rubber vulcanizates. Scorch time and cure time of different rubber nanocomposites reduced significantly. The broad surface area of the GO and the
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existance of polar functionalities on the outer surface of GO facilitate the curing reaction.
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4.7 DMTA analysis
Viscoelastic behaviors like temperature dependent storage modului (E´) and loss tangent (Tan δ) of the GO filled halogen containing elastomers are represented in the Figure 7(a-b) and 8 (a-b). Figure 7a shows that the storage moduli of the Hy/GO elastomeric nanocomposites were
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increased with increasing the loading of GO. GO nanosheets were exfoliated in the elastomer matrix and increases the stiffness of the elastomeric nanocomposites by restricting the mobility of the rubber chain (chemical and physical adsorption of the rubber chains on the nanofiller
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surface) [54]. No significant change was observed in the glass transition temperatures (Tg) of the different GO loaded CSPE (hypalon) based nanocomposites.
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Figure 8 (a-b) showed that GO loaded neoprene (CR) rubber compounds showed a drastic enhancement in the storage modulus with increasing the loading of GO. The Tg value of the different nanocomposites was increased compared to the unfilled CR. 4.8 Mechanical properties
Mechanical features like tensile strength, tear stress, modulus, elongation, abrasion resistance and hardness of the different GO filled rubber nanocomposites are tabulated in the
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Table 3. Hy-3G (Hypalon/3 wt% GO), Hy-6G (Hypalon/6 wt% GO) and Hy-9G (Hypalon/9 wt% GO) nanocomposites show 124%, 228% and 344% increase in tensile stress; 67%, 111% and 211% increase in modulus (300% elongation); 152%, 248% and 236% increase in the tear
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resistance compared to the unfilled rubber, respectively. C-3G (CR/3 wt% GO), C-6G (CR/6 wt% GO) and C-9G (CR/9 wt% GO) nanocomposites exhibit 133%, 250% and 411% increase in tensile strength; 85%, 142% and 214% increase in modulus; 61%, 222% and 494% increase in
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tear strength compared to the unfilled elastomer, respectively. Exfoliated GO nanosheets imparts better reinforcing effect to the various rubber vulcanizates which resulting in an improvement in
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the overall mechanical features of the elastomer nanocomposites. At 9 wt% GO loading, the nanocomposites show increased elasticity compared to the 6 wt% GO loaded nanocomposites which may be attributed in part to the plasticizing effect of the functional groups present on the surface of GO, which is responsible for pendant chain formation in the matrix, as well as
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responsible for conformational effects on the polymer at the GO-matrix interface [55]. Inclusion of reinforcing nanofiller particulate (GO) into the elastomer matrices improves the stiffness and strength of the elastomer vulcanizates [56]. The wear resistance of the GO
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loaded different rubber nanocomposites was improved which was due to the better filler (high aspect ratio) rubber interfacial adhesion. Wear resistance of the GO filled rubber nanocomposites
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also relies upon the effective hardness of the nanocomposites. In the presence of GO nanosheets, the potential hardness of the different rubber nanocomposites is increased as well as the wear resistance also increased. 5. Conclusions
GO containing rubber nanocomposites based upon CSPE (Hypalon) and polychloroprene rubber were synthesized by two step methods (solution mixing along with direct mixing on a
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laboratory scale open two roll mill). We observed the significant exfoliation of GO in the different elastomer matrices from the XRD and HR-TEM analyses of the nanocomposites. As a result of the large surface area of GO, GO containing rubber nanocomposites exhibited reduced
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scorch time and cure time in comparison with the unfilled elastomers. Overall mechanical properties of the rubber nanocomposites were increased with increasing the loading of the GO. Storage moduli of the elastomer composites were significantly increased with increasing the
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loading of GO. Uniformly dispersed layered GO in the various elastomer matrices increases the thermal stability of the nanocomposites. It can be seen from the cryo-fractured surface of the
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different nanocomposites that depending upon the orientation of GO in the rubber matrices, the crack lines were changed their direction (SEM analysis). The interactions between the functional groups of GO and chloride/sulfonyl chloride groups present on the side chain of the elastomers help GO to exfoliate in the different rubber matrices as well as interfacial adhesion between GO
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and matrices also increased. Because of better dispersion, GO imparts significant reforcing effect to the different halogen containing elastomers that resulted in a superior morphological,
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mechanical, dynamic mechanical and thermal properties of the rubber nanocomposites.
Acknowledgement: Author (Asish Malas) is very grateful to the Council of Scientific and
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Industrial Research (CSIR), New Delhi, India for their financial assistance.
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Figure Captions
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Figure 1: (a) FT-IR and (b) Raman spectra of graphite and GO.
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Figure 2: XRD patterns of (a) Graphite, (b) GO, and (c) Hypalon based and (d) CR based nanocomposites. Figure 3: HR-TEM images of (a) Hy-3G, (b) Hy-6G, (c) Hy-9G, (d) C-3G, (e) C-6G and (f) C9G.
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Figure 4: SEM images of (a) Unfilled Hypalon, (b) Hy-3G, (c) Hy-6G, (d) Hy-9G (e) GO. Figure 5: SEM images of (a) Unfilled CR, (b) C-3G, (c) C-6G and (d) C-9G. Figure 6: TGA curves of (a) Graphite and GO,(b) Hypalon based and (c) CR based nanocomposites. Figure 7: Storage modului (a), Tan δ (b), magnified Tan δ (c) curves of the hypalon based nanocomposites. Figure 8: Storage modului (a) and Tan δ (b) curves of the CR based nanocomposites.
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Table 1: Compositions of the GO filled rubber nanocomposites
Sample code
CSPE (Hy)/CR
ZnO
MgO
NA-22
GO
100
3
2
Hy-3G
100
3
2
Hy-6G
100
3
2
Hy-9G
100
3
CR
100
3
C-3G
100
C-6G
100
C-9G
100
-
1.6
3
1.6
6
1.6
9
2
1.6
-
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3
2
1.6
3
3
2
1.6
6
3
2
1.6
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Hy
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Content [Weight (g)]
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Table 2: Cure characteristics of the different rubber nanocomposites Torque Difference (dN.m) 6.6 31.8 11.7 13.3 18.9 43.7 39.4 40.4
Scorch time (min.) 18.2±0.4 0.7±0.07 16.5±0.2 15.4±0.3 15.2±0.4 0.6±0.05 0.6±0.07 0.5±0.06
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Cure time (min.) 26.6±0.5 9.7±0.3 22.7±0.5 19.5±0.4 19.1±0.4 7.0±0.1 6.5±0.2 6.1±0.2
Cure rate index 12.5±0.7 11.1±0.6 16.2±1 24.2±1.3 25.3±1.4 15.6±0.8 17.1±1 17.8±1
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Max. Torque (dN.m) 25.7±0.6 48.9±0.7 31.2±0.5 34.2±0.6 38.7±0.4 55.3±0.7 56.0±0.5 58.2±0.5
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Hy CR Hy-3G Hy-6G Hy-9G C-3G C-6G C-9G
Min. Torque (dN.m) 19.1±0.3 17.1±0.1 19.5±0.3 20.9±0.2 19.8±0.4 11.6±0.2 16.6±0.2 17.8±0.3
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Sample Code
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Table 3: Mechanical properties of the GO containing rubber nanocomposites
Shore A Hardness
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Designation Tensile % 300% Tear DIN strength Elongation modulus strength abrasion (MPa) at break (MPa) (N/mm) Mass loss (%) Hy 2.5±0.2 820±11 0.9±0.1 2.5±0.2 40±2 CR 1.8±0.1 770±10 0.7±0.2 1.8±0.1 35±3 Hy-3G 5.6±0.2 723±9 1.5±0.1 6.3±0.5 32±1 Hy-6G 8.2±0.3 655±10 1.9±0.2 8.7±0.3 34±3 Hy-9G 11.1±0.2 721±15 2.8±0.3 8.4±0.2 27±2 C-3G 4.2±0.1 625±12 1.3±0.1 2.9±0.1 31±3 C-6G 6.3±0.2 575±11 1.7±0.2 5.8±0.3 26±1 C-9G 9.2±0.2 645±10 2.2±0.2 10.7±0.4 23±2
40±2 52±3 43±3 44±2 49±1 55±3 57±2 61±2
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S1359-8368(15)00289-9
DOI:
10.1016/j.compositesb.2015.04.051
Reference:
JCOMB 3582
To appear in:
Composites Part B
Received Date: 4 January 2015 Revised Date:
27 April 2015
Accepted Date: 29 April 2015
Please cite this article as: Malas A, Das CK, Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates, Composites Part B (2015), doi: 10.1016/j.compositesb.2015.04.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of graphene oxide on the physical, mechanical and thermo-mechanical properties of neoprene and chlorosulfonated polyethylene vulcanizates Asish Malas 1, Chapal Kumar Das 1* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, India. E-mail: [email protected], [email protected]
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*Corresponding Author: Chapal Kumar Das (Professor) Email: [email protected]
Keywords: A. Polymer-matrix composites (PMCs), A. Nano-structures, B. Cure behavior, B. Mechanical properties
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Abstract
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The present research work demonstrated the effect of graphene oxide (GO) on the physical, mechanical, thermo-mechanical etc., properties of neoprene (CR) and chlorosulfonated polyethylene (CSPE) vulcanizates. CR and CSPE based nanocomposites were prepared by both
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solution intercalation and melt intercalation methods. The changes obtained in the morphology, cure characteristics, mechanical, thermal, thermo-mechanical properties of the rubber nanocomposites
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have been widely investigated. X-ray diffraction analysis (XRD) and transmission electron microscopic (TEM) analysis of the samples revealed partial exfoliated structure of GO containing rubber composites. Mechanical, thermal, cure and thermo-mechanical properties of the elastomeric
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1.Introduction
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nanocomposites were improved compared to the neat rubbers.
Any mixture of dissimilar materials, commonly two, in which one of the constituent
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serves as a reinforcing agent known as composites. Reinforcing agents are usually much stronger with low densities compared to the polymer base matrix [1]. From way back, the discovery of the innovative work of the Toyota research group in the beginning of 1990s, polymer/clay nanocomposites have drawn a lot of concentration [2]. Significant improvement in the physical, mechanical, thermal, dynamic mechanical and functional properties of the polymers can be achieved by intercalating or exfoliating clay tactoids in the matrix compared to the unfilled polymers [3]. Many researchers have targeted on the development and characterization of 2
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polymer nanocomposites containing exfoliated clay platelets. This is due to the natural presence of clay and the lack of difficulty in surface functionalization of clay to increase the intergallery spacing which resulting in the formation of intercalated structure of polymer/clay composites [4-
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6]. Similarly, The same approach can be employed to another reinforcing nanomaterial, known as graphene nanoplatelets that originated after the modification of natural graphite flakes [7, 8]. Polymer graphite nanocomposites (PGNs) can be prepared by the incorporation of graphite
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nanosheets in the polymer matrix [9-11]. Analogous to natural clay platelets, graphite is a layered structured material that contains many layers (units) familiar as graphene. Graphene is a
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single layer of carbon atoms profoundly arranged in a 2-D hexagonal lattice [12]. 2-D Graphene sheets exhibit many unique physical and chemical properties along with superior mechanical, thermal and electrical properties in the exfoliated condition in the polymer matrix [13, 14]. The utilization of graphene flakes as reinforcing nanofiller for the synthesis of polymer
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nanocomposites resulting in an increase in the gas barrier and flame-retardant properties [15, 16]. As a result of the weak van der Waals force of attraction between the units of graphite, it is easy to separate (Exfoliate) graphene layers of graphite, which resulting in the formation of high
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aspect ratio (length-to-thickness ratio) reinforcing nanosheets with thicknesses in the range of 2– 10 nm [17]. To intercalate and exfoliate the graphene sheets of natural graphite, it is oxidized and
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altered to a layered solid material (Graphene oxide) with aromatic unoxidized benzene rings and aliphatic six-membered ring consists of epoxide, hydroxyl and carboxyl groups [18-20]. Due to the formation different functionalities by the chemical oxidation of the unsaturated part of graphite, the polyaromatic character of pristine graphite is diminished but the lamellar structure is preserved. Like pristine graphite, Graphene oxide (GO) consists of stacks of graphene with larger interlayer spacing. The lateral dimension of graphite nanosheets in GO is drastically
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reduced compared to the size of the pristine graphite [21]. The intergallery distance of graphite can be increased from 3.35 Ǻ (pristine graphite) to 6-10 Ǻ, depending on the interlameller water content and degree of the intercalation [22]. Some researchers had studied the effect of GO on
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the physical, mechanical and dynamic mechanical properties of different polymer matrices [2325]. Tang et al. synthesized elastomeric nanocomposites consisting of GO sheets by using butadiene–styrene–vinyl pyridine rubber (VPR) as the host through co-coagulation process and
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in situ formation of an ionic bonding interface. They observed significant improvement in the mechanical properties for the GO filled hybrid nanocomposites [26]. Hernandez et al.
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synthesized functionalized graphene sheets (FGSs) filled natural rubber nanocomposites by conventional two roll mill mixing. They noticed improvement in the mechanical and electrical properties of nanocomposites [27]. Kumar et al. fabricated conducting poly(isobutylene-coisoprene) (IIR)/reduced graphene oxide (RGO)/expanded graphite (EG) composites. They
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observed high barrier, dielectric and sensing properties for the hybrid nanocomposites [28]. Wu et al. prepared surface functionalized graphene oxide (SGO) filled natural rubber based nanocomposites and observed improvement in the mechanical as well as gas barrier properties of
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the rubber nanocomposites [29]. Lu and coworkers studied the synergistic effect of carbon fiber and GO on shape memory polymer (SMP) nanocomposites. They observed that the electrical
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properties of SMP nanocomposites are significantly improved via a synergistic effect of GO and carbon fiber [30]. J. H. Lee et al.
prepared raw graphene/chitosan and cryomilled
graphene/chitosan nanocomposites in order to study the cryomilling effect of graphene on the tensile properties of their corresponding nanocomposites. They noticed that cryomilling enhanced the dispersion, graphitic characteristics, and thermal stability of the graphene powders and also observed significant improvement in tensile properties for the cryomilled graphene
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reinforced chitosan composites compared to raw graphene/chitosan nanocomposites [31]. M. Ionita et al. synthesized polysulfone (PS)/graphene oxide (GO) composite membranes and observed improvement in the morphological, thermal and mechanical properties of the composite
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[32, 33]. Yuan et al. prepared graphene oxide/nylon 11 composites by in situ melt polycondensation. They observed superior mechanical properties including stiffness and
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toughness for the nanocomposites compared to the pure nylon 11 matrix [34].
Chloroprene rubber (CR) is exceptionally flexible synthetic rubber due to its rare combination of
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features like ozone/oil resistance, toughness, dynamic flex life, good adhesion to other materials, and excellent heat resistance [35]. Nanofiller reinforced CR based composites have been preferred most likely materials for moldings and extrudates of all types, hoses, roll covers, belting (conveyor belts), air spring bellows, cable sheathing and insulation for low-voltage cables, corrosion-resistant linings, sheeting, and footwear, hose water suits and water sealant,
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and many other applications.
Chlorosulfonated polyethylene (Hypalon) exhibits some outstanding properties such as ozone
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resistance chemical resistance, fire-retarding due to the presence of chlorine atom and also electric insulator. CSPE can be generally used to manufacture the conveyor belt lining, cable,
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conductor insulation, and building materials, and others; but till the important properties such as mechanical properties, thermal stability, and damping can be further increased to boost its application in commodities with some distinctive requirements [36]. In the present research work, GO filled (3 wt%, 6 wt% and 9 wt%) rubber nanocomposites based on neoprene rubber (CR) and chlorosulfonated polyethylene (CSPE/Hypalon) were synthesized by two step methods (Solution mixing and compounding on
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an open two roll mill). The changes obtained in the physical, mechanical, dynamic mechanical, thermal etc., properties of the rubber matrices have been widely analyzed in this study. The GO/CR and GO/hypalon nanocomposites exhibited significant improvement in the mechanical,
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thermo-mechanical and thermal properties compared to their respective controls. 2. Experimental
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2.1 Materials
Polychloroprene rubber (Neoprene-W) was supplied from Bayer, Germany (Mooney
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viscosity ML (1+4) at 100 0C = 48 and Mw is about 349,000 g/mol). Chlorosulfonated polyethylene (Hypalon 40) was supplied from Du Pont, USA (Mooney viscosity ML (1+4) at 100 0C = 56 and Mw is about 188,000 g/mol). Graphite Fine Powder (Extra Pure) was obtained from Loba Chemie Pvt. Ltd., Mumbai (India). Concentrated sulfuric acid (H2SO4), phosphoric acid (H3PO4), hydrochloric acid (HCl) and nitric acid (HNO3) were obtained from Merck, India
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Ltd. Potassium permanganate (KMnO4), ethanol (C2H5OH), tetrahydrofuran (THF) and hydrogen peroxide were bought from Sigma-Aldrich, India. Compounding additives like zinc oxide (ZnO) and magnesium oxide (MgO) were purchased from Merck, India. Ethylene thiourea
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2.2 Synthesis of Graphene Oxide by Improved Method A 9:1 mixed acid of concentrated H2SO4/H3PO4 (360:40 mL) was incorporated to a
mixture of graphite flakes (3.0 g, 1 wt equiv) and KMnO4 (18.0 g, 6 wt equiv), generating a minor exotherm to 35-40 °C. Then, the reaction mixture was heated upto 50 °C and stirred for 12 h. Afterthat, the reaction mixture was cooled to room temperature and poured onto ice water (~400 mL) with 30% H2O2 (10 mL). The solution mixture was then centrifuged (4000 rpm for 4 h), and the supernatant liquid was decanted away. Then, the rest of the solid material was washed 6
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subsequently with 200 mL of water, 200 mL of 30% HCl, and 200 mL of ethanol (2 times). The solid material collected in the centrifuge tubes was dried overnight at room temperature in a
2.3 Preparation of the rubber nanocomposites
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vacuum oven [37].
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Graphene oxide (GO) containing CR and CSPE based nanocomposites were fabricated by using both solution and melt interaction. Initially, CR and CSPE was dissolved seperately in
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THF (rubber to solvent ratio was 1:3 weight/ volume). Required amount of GO was then incorporated into the rubber solution under continuous stirring and followed by ultrasonication for 30 min. Afterthat, the final mixture was then poured over a Petri dish and kept in the air for the full evaporation of solvent at room temperature under the fume hood cabinet. Solution
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intercalated different rubber/GO composites (thin films) were then mixed with the bulk CR and CSPE matrices in separate batches by melt blending (Direct mixing) in a laboratory scale open two roll mixing mill (the speed ratio of the rotor was 1:1.4) at rt. Compositions of the GO loaded
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CR and CSPE based nanocomposites are represented in the Table 1. C-3G (100 g CR/3 g GO), C-6G (100 g CR/6 g GO), C-9G (100 g CR/9 g GO), Hy-3G (100 g Hy/3 g GO), Hy-6G (100 g
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Hy/6 g GO) and Hy-9G (100 g Hy/9 g GO) are the abbreviations of the prepared elastomeric nanocomposites based on chloroprene (neoprene) and chlorosulfonated polyethylene vulcanizates.
3. Characterization techniques Curing studies of the GO loaded CR and CSPE based nanocomposites were carried out in Monsanto Rheometer R- 100 testing instrument at 160 oC with 3o arc for 60 min.
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Wide angle X-ray diffraction (WAXD) analysis of Graphite, GO and GO containing rubber nanocomposites was done in Rigaku Miniflex Diffractometer with Cu-Kα radiation at a generator voltage of 40 kV. The scanning rate was 3°/min between 70-700, chart speed
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10mm/2θ, current 20mA and wavelength 0.154 nm at room temperature. The intergallery space of graphite and GO were achieved from the Bragg’s equations nλ= 2d sin θ. The state of dispersion of GO in the different rubber matrices was also confirmed from the WAXD analysis.
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The morphology of the GO filled elastomeric nanocomposites was observed through high resolution transmission electron microscope (HR-TEM, JEOL 2100). For HR-TEM analysis,
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ultra thin cross sections of the rubber nanocomposites were cut by the help of Leica Ultra Cut UCT Ultra microtome instrument equipped with a diamond knife.
Viscoelastic properties of the GO containing rubber nanocomposites were investigated by using a TA instrument DMA 2980 model in tension mode at a constant frequency of 1 Hz, a strain of
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0.1%, in the temperature range of -80°C to + 80°C and at a heating rate of 3°C/min. Mechanical features of the GO loaded rubber nanocomposites were measured in a Universal tensile testing machine (Hounsfield H 10KS) at room temperature. Tensile strength and tear
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strength were measured according to specifications ASTM D412 - 06ae2 and ASTM D624-00 (2012). 5 samples of each rubber nanocomposites were tested and the average values (mean)
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were represented in the respective table. The tensile tests were done on dumb-bell shaped samples and length of the tensile testing sample was 25mm, thickness was 1.5 mm and the speed of jaw separation was 500mm/minute with a load cell of 10 kN. The hardness of the rubber nanocomposite was measured by Shore A hardness tester as per ASTM D2240-05(2010) standard.
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In order get the wear resistance, DIN abrasion test of GO filled rubber nanocomposites was carried out in a DIN abrasion tester [ASTM D5963-04 (2010)]. The cryo-fractured surface of the nanocomposites was scanned in VEGA TESCAN// LSU
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(Scanning Electron Microscope) instrument. For the SEM analysis, the samples were gold coated prior to analysis.
To investigate the thermal stability of the nanocomposites, thermo gravimetric analysis (TGA) of
30°C -700°C (heating rate was 10°C/min).
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FT-IR analysis of Graphite and GO was carried out in the range of 800cm-1 to 2000 cm-1 in Thermonicolet /Nexus 870 FT-IR spectrometer.
Raman spectra of Graphite and GO were acquired in back scattering geometry by utilizing a micro-Raman setup connected with Ar+-Kr+ laser (Model 2018-RM, Newport, USA) as the
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excitation light source, a spectrometer (model T64000, JY, France) and a Peltier cooled CCD detector. The raman spectroscopic analysis of the samples was carried out at an excitation wave
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length of 785 nm.
4. Results and discussion
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4.1 FT-IR and Raman spectroscopic analyses FT-IR spectra of graphite and GO are represented in the Figure 1a. It can be seen from
the figure 1a that one peak was raised at 1625 cm-1 (GO), which was due to the vibration of the adsorbed water contents and simultaneously the structural vibrations of unoxidized graphitic spheres [38-40]. The band at 1746 cm-1 was designated to the C=O stretching vibration of
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carboxylic acid groups (-COOH) present on the surface of GO. The peak raised at 1054 cm-1 was because of the C-O-C stretching vibration [41]. Figure 1b shows the Raman spectra of graphite and GO. It can be seen from the curves that the
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conventional G band of graphite was raised near 1580 cm-1 [42, 43]. For graphene oxide (GO), a D band (1357 cm-1) alongwith G band (1584 cm-1) were raised, which indicates the prominent
hydroxyl, epoxide, and carboxyl groups [44-47]. 4.2 XRD analysis
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growth of sp3 character due to the presence of oxygen-containing functional groups, such as the
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XRD patterns of graphite, graphene oxide (GO) and GO containing rubber nanocomposites based on halogen containing elastomers (CR and CSPE) are shown in the Figure 2 (a-d). It can be seen (Figure 2a-b) that after the modification of graphite to GO, the main peak (001) of GO was raised at 2θ = 10.72° corresponding to the d-spacing of 0.818 nm. The d-
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spacing of graphite flakes (0.335 nm) was significantly increased in the GO. Figure 2c shows that after the incorporation of GO in the CSPE matrix, the (001) peak of GO was totally diminished for the hypalon based rubber nanocomposites which confirms that the GO platelets
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was delaminated in the rubber matrix. The other peaks raised (broad peak and sharp peaks) were due to the amorphous polymer and compounding additives respectively. It can also be seen from
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the figure 2d that the main peak (001) of GO was more or less diminished for the CR based nanocomposites but the (001) peak intensity gradually increases with the increase in GO loading. (100) peak of CR was merged with the (001) peak of GO. Chieng and cowokers prepared graphene nanoplatelets (xGnP) reinforced poly(lactic acid) (PLA)/epoxidized palm oil (EPO) blend. They observed disappearance of peak for the nanocomposites and concluded it as a result of exfoliation and random distribution of the platelets within the polymer matrices at low loading
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of xGnP [48]. Qi and coworkers observed no characteristic peak of GO in the XRD patterns of polyvinyl alcohol (PVA)/GO nanocomposites. They concluded it as a good dispersion of GO in the PVA matrix [49].
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XRD analysis of the rubber nanocomposites shows only a partial evidence of the dispersion of nanofiller and inexistence of peak analogous to d-spacing does not always imply the delamination of the nanofiller in polymer matrix. XRD is incapable to diagnose regular
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stacking surpassing 8.8 nm [50]. Hence, to observe the dispersion of nanofiller in the elastomeric nanocomposites, microscopic study is necessary [51]. For better observation regarding the
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morphology of GO containing rubber matrices, HR-TEM analysis of the samples were carried out and discussed later. 4.3 HR-TEM analysis
HR-TEM images of the GO loaded (3 wt%, 6 wt% and 9 wt%) halogen containing
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elastomers are shown in the Figure 3 (a-f). The black marks in the images direct the graphene oxide nanosheets. It can be seen from the images that GO is fully exfoliated in both the halogen containing elastomeric matrices which resulting in a significant increase in the overall physical
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and mechanical properties of the nanocomposites. As the halogen containing elastomers are polar in nature, so the halogen groups and other polar functionalities may better interact with the
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functional groups of GO. Because of superior interactions and interfacial adhesion between the rubber matrices and GO, mechanical and dynamic mechanical features of the rubber compounds were enhanced.
4.4 SEM analysis
SEM images of GO and the cryofractured surfaces of the GO containing halogen containing elastomers based nanocomposites are displayed in the Figure 4 and 5. It can be seen
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from the images that the GO filled rubber nanocomposites show more uneven and tortuous pathway of the fracture surface, and tear paths were broadened compared to the fractured surface of unfilled halogen containing elastomers. Layered nanosized GO uniformly dispersed in rubber
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matrices and changed the fracture lines relying upon their location in the elastomer matrix. More rough and the tortuous fractured surface of the nanocomposites resulted in the enhancement in
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the general mechanical features of the nanocomposites.
4.5 TGA analysis
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Thermo gravimetric curves of the different rubber nanocomposites filled with graphene oxide (GO) are shown in the Figure 6 (a-c). Figure 6a shows that GO exhibits a gradual weight loss in the temperature range up to 100 °C which was due to the removal of physiosorbed water. After that, a sharp weight loss at about 130 °C can be featured by the decomposition of different
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oxygen containing functional groups in the GO layer [52]. From the Figure 6 (b-c), it can be seen that the onset decomposition temperature of the different GO loaded nanocomposites was lowered compared to the unfilled rubber which was because of the early degradation of oxygen
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functionalities on the surface of GO. The maximum decomposition temperature of the different rubber composites was higher compared to the neat rubber. GO nanosheets were dispersed
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uniformly in the different rubber matrices and create a mass transfer obstruction to the volatile matters during the thermal degradation of the nanocomposites as well as GO acts as an efficient heat absorber which indeed significantly increase the thermal stability of the nanocomposites. 4.6 Cure characteristics
The cure behavior of the GO filled halogen containing elastomers based nanocomposites are depicted in the Table 2. Maximum torque (MH) which can be dealt with the composite
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modulus [53] and difference in torque value (∆S) of the different rubber nanocomposites were increased in comparison with the neat rubber. MH and ∆S values for the different nanocomposites were gradually increased with increasing the loading of GO. Significant
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exfoliation of GO in the different rubber matrices increases the stiffness of the matrices as well as increases the degree of crosslinking of the rubber vulcanizates. Scorch time and cure time of different rubber nanocomposites reduced significantly. The broad surface area of the GO and the
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existance of polar functionalities on the outer surface of GO facilitate the curing reaction.
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4.7 DMTA analysis
Viscoelastic behaviors like temperature dependent storage modului (E´) and loss tangent (Tan δ) of the GO filled halogen containing elastomers are represented in the Figure 7(a-b) and 8 (a-b). Figure 7a shows that the storage moduli of the Hy/GO elastomeric nanocomposites were
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increased with increasing the loading of GO. GO nanosheets were exfoliated in the elastomer matrix and increases the stiffness of the elastomeric nanocomposites by restricting the mobility of the rubber chain (chemical and physical adsorption of the rubber chains on the nanofiller
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surface) [54]. No significant change was observed in the glass transition temperatures (Tg) of the different GO loaded CSPE (hypalon) based nanocomposites.
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Figure 8 (a-b) showed that GO loaded neoprene (CR) rubber compounds showed a drastic enhancement in the storage modulus with increasing the loading of GO. The Tg value of the different nanocomposites was increased compared to the unfilled CR. 4.8 Mechanical properties
Mechanical features like tensile strength, tear stress, modulus, elongation, abrasion resistance and hardness of the different GO filled rubber nanocomposites are tabulated in the
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Table 3. Hy-3G (Hypalon/3 wt% GO), Hy-6G (Hypalon/6 wt% GO) and Hy-9G (Hypalon/9 wt% GO) nanocomposites show 124%, 228% and 344% increase in tensile stress; 67%, 111% and 211% increase in modulus (300% elongation); 152%, 248% and 236% increase in the tear
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resistance compared to the unfilled rubber, respectively. C-3G (CR/3 wt% GO), C-6G (CR/6 wt% GO) and C-9G (CR/9 wt% GO) nanocomposites exhibit 133%, 250% and 411% increase in tensile strength; 85%, 142% and 214% increase in modulus; 61%, 222% and 494% increase in
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tear strength compared to the unfilled elastomer, respectively. Exfoliated GO nanosheets imparts better reinforcing effect to the various rubber vulcanizates which resulting in an improvement in
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the overall mechanical features of the elastomer nanocomposites. At 9 wt% GO loading, the nanocomposites show increased elasticity compared to the 6 wt% GO loaded nanocomposites which may be attributed in part to the plasticizing effect of the functional groups present on the surface of GO, which is responsible for pendant chain formation in the matrix, as well as
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responsible for conformational effects on the polymer at the GO-matrix interface [55]. Inclusion of reinforcing nanofiller particulate (GO) into the elastomer matrices improves the stiffness and strength of the elastomer vulcanizates [56]. The wear resistance of the GO
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loaded different rubber nanocomposites was improved which was due to the better filler (high aspect ratio) rubber interfacial adhesion. Wear resistance of the GO filled rubber nanocomposites
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also relies upon the effective hardness of the nanocomposites. In the presence of GO nanosheets, the potential hardness of the different rubber nanocomposites is increased as well as the wear resistance also increased. 5. Conclusions
GO containing rubber nanocomposites based upon CSPE (Hypalon) and polychloroprene rubber were synthesized by two step methods (solution mixing along with direct mixing on a
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laboratory scale open two roll mill). We observed the significant exfoliation of GO in the different elastomer matrices from the XRD and HR-TEM analyses of the nanocomposites. As a result of the large surface area of GO, GO containing rubber nanocomposites exhibited reduced
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scorch time and cure time in comparison with the unfilled elastomers. Overall mechanical properties of the rubber nanocomposites were increased with increasing the loading of the GO. Storage moduli of the elastomer composites were significantly increased with increasing the
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loading of GO. Uniformly dispersed layered GO in the various elastomer matrices increases the thermal stability of the nanocomposites. It can be seen from the cryo-fractured surface of the
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different nanocomposites that depending upon the orientation of GO in the rubber matrices, the crack lines were changed their direction (SEM analysis). The interactions between the functional groups of GO and chloride/sulfonyl chloride groups present on the side chain of the elastomers help GO to exfoliate in the different rubber matrices as well as interfacial adhesion between GO
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and matrices also increased. Because of better dispersion, GO imparts significant reforcing effect to the different halogen containing elastomers that resulted in a superior morphological,
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mechanical, dynamic mechanical and thermal properties of the rubber nanocomposites.
Acknowledgement: Author (Asish Malas) is very grateful to the Council of Scientific and
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Industrial Research (CSIR), New Delhi, India for their financial assistance.
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Figure Captions
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Figure 1: (a) FT-IR and (b) Raman spectra of graphite and GO.
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Figure 2: XRD patterns of (a) Graphite, (b) GO, and (c) Hypalon based and (d) CR based nanocomposites. Figure 3: HR-TEM images of (a) Hy-3G, (b) Hy-6G, (c) Hy-9G, (d) C-3G, (e) C-6G and (f) C9G.
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Figure 4: SEM images of (a) Unfilled Hypalon, (b) Hy-3G, (c) Hy-6G, (d) Hy-9G (e) GO. Figure 5: SEM images of (a) Unfilled CR, (b) C-3G, (c) C-6G and (d) C-9G. Figure 6: TGA curves of (a) Graphite and GO,(b) Hypalon based and (c) CR based nanocomposites. Figure 7: Storage modului (a), Tan δ (b), magnified Tan δ (c) curves of the hypalon based nanocomposites. Figure 8: Storage modului (a) and Tan δ (b) curves of the CR based nanocomposites.
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Table 1: Compositions of the GO filled rubber nanocomposites
Sample code
CSPE (Hy)/CR
ZnO
MgO
NA-22
GO
100
3
2
Hy-3G
100
3
2
Hy-6G
100
3
2
Hy-9G
100
3
CR
100
3
C-3G
100
C-6G
100
C-9G
100
-
1.6
3
1.6
6
1.6
9
2
1.6
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3
2
1.6
3
3
2
1.6
6
3
2
1.6
9
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Content [Weight (g)]
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Table 2: Cure characteristics of the different rubber nanocomposites Torque Difference (dN.m) 6.6 31.8 11.7 13.3 18.9 43.7 39.4 40.4
Scorch time (min.) 18.2±0.4 0.7±0.07 16.5±0.2 15.4±0.3 15.2±0.4 0.6±0.05 0.6±0.07 0.5±0.06
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Cure time (min.) 26.6±0.5 9.7±0.3 22.7±0.5 19.5±0.4 19.1±0.4 7.0±0.1 6.5±0.2 6.1±0.2
Cure rate index 12.5±0.7 11.1±0.6 16.2±1 24.2±1.3 25.3±1.4 15.6±0.8 17.1±1 17.8±1
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Max. Torque (dN.m) 25.7±0.6 48.9±0.7 31.2±0.5 34.2±0.6 38.7±0.4 55.3±0.7 56.0±0.5 58.2±0.5
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Hy CR Hy-3G Hy-6G Hy-9G C-3G C-6G C-9G
Min. Torque (dN.m) 19.1±0.3 17.1±0.1 19.5±0.3 20.9±0.2 19.8±0.4 11.6±0.2 16.6±0.2 17.8±0.3
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Table 3: Mechanical properties of the GO containing rubber nanocomposites
Shore A Hardness
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Designation Tensile % 300% Tear DIN strength Elongation modulus strength abrasion (MPa) at break (MPa) (N/mm) Mass loss (%) Hy 2.5±0.2 820±11 0.9±0.1 2.5±0.2 40±2 CR 1.8±0.1 770±10 0.7±0.2 1.8±0.1 35±3 Hy-3G 5.6±0.2 723±9 1.5±0.1 6.3±0.5 32±1 Hy-6G 8.2±0.3 655±10 1.9±0.2 8.7±0.3 34±3 Hy-9G 11.1±0.2 721±15 2.8±0.3 8.4±0.2 27±2 C-3G 4.2±0.1 625±12 1.3±0.1 2.9±0.1 31±3 C-6G 6.3±0.2 575±11 1.7±0.2 5.8±0.3 26±1 C-9G 9.2±0.2 645±10 2.2±0.2 10.7±0.4 23±2
40±2 52±3 43±3 44±2 49±1 55±3 57±2 61±2
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AC C
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TE D
M AN U
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AC C
EP
TE D
M AN U
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RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT