NBR nanocomposites

Journal Pre-proof Experimental and Theoretical Analyses on Mechanical Properties and Stiffness of Hybrid Graphene/Graphene Oxide reinforced EPDM/NBR nanocomposites Seyed Mohammad Reza Paran, Ghasem Naderi, Farhad Javadi, Rasoul Shemshadi, Mohammad Reza Saeb

PII:

S2352-4928(19)31118-3

DOI:

https://doi.org/10.1016/j.mtcomm.2019.100763

Reference:

MTCOMM 100763

To appear in:

Materials Today Communications

Received Date:

11 September 2019

Revised Date:

6 November 2019

Accepted Date:

6 November 2019

Please cite this article as: Paran SMR, Naderi G, Javadi F, Shemshadi R, Saeb MR, Experimental and Theoretical Analyses on Mechanical Properties and Stiffness of Hybrid Graphene/Graphene Oxide reinforced EPDM/NBR nanocomposites, Materials Today Communications (2019), doi: https://doi.org/10.1016/j.mtcomm.2019.100763

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Experimental and Theoretical Analyses on Mechanical Properties a n d S t i f f n e s s of Hybrid Graphene/Graphene Oxide reinforced EPDM/NBR nanocomposites

Seyed Mohammad Reza Paran*1; Ghasem Naderi*1; Farhad Javadi2; Rasoul Shemshadi3;

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Mohammad Reza Saeb4

Department of Polymer Processing, Iran Polymer and Petrochemical Institute, P.O. Box

14965/115, Tehran, Iran

Department of Polymer Engineering, Faculty of Graduate Studies, Tehran South Branch,

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2

Islamic Azad University, Tehran, Iran 3

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Vocational University (TVU), Guilan, Iran

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Department of Textile Engineering, Faculty of Chamran, Rasht Branch, Technical and

Department of Resin and Additives, Institute for Color Science and Technology, P.O. Box

16765-654, Tehran, Iran

*

[email protected] [email protected]

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*

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Highlights 

A novel rubber hybrid nanocomposite was produced through using graphene and Graphene Oxide



Experimental analysis of EPDM/NBR/graphene/graphene oxide nanocomposites was carried out



DMTA, morphological, mechanical and electrical properties of rubber nanocomposites were studied



Hybrid rubber nanocomposites with higher tensile strength and modulus at 300% elongation was achieved Excellent agreement between stiffness experimental data and model predictions

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

ABSTRACT

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Nevertheless, hybrid of two platy nano-size particles and theoretical evaluation of the

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mechanical properties of the resulting elastomer nanocomposite with superior properties was not addressed. Hybrid rubber nanocomposites based on ethylene propylene diene monomer

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(EPDM), acrylonitrile butadiene rubber (NBR), varying amounts of graphene nanoplatelets (GnPs) and graphene oxide nanoplatelets (GOnPs) were prepared by using a two-roll mill

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mixer. The effects of 1.5 wt% GnPs and GOnPs on the microstructure of the vulcanized EPDM/NBR/GnPs/GOnPs

compounds were precisely studied utilizing scanning electron

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microscopy (SEM) and transmission electron microscopy (TEM) micrographs. Moreover, their rheological and mechanical properties together with stiffness were discussed-both

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experimentally and theoretically- on the ground of tensile tests, dynamic mechanical thermal analysis (DMTA) and rubber processing analyzer (RPA). SEM micrographs provided from rubber nanocomposites containing 1.5 wt.% of either GnPs or GOPns revealed a rough fracture surface compared to that of the blend EPDM/NBR compounds. TEM images indicated a welldispersed fashion of nanoplatelets within the rubber matrix regardless of the type of platelet. The tensile test analyses revealed a 100 % increase in the tensile strength for the nanocomposites 2

EPDM/NBR blend filled graphene nanocomposites by Paran et al. containing both types of platelet-like nanoparticles. Theoretical analyses were in good agreement with experimental outcomes. The results of DMTA demonstrated a higher storage modulus with the introduction of nanoplatelets into the EPDM/NBR compound up to 1.59 MPa. Dielectric spectroscopy unraveled roughly 7 times higher conductivity with the incorporation of both GnPs and GOPns into the EPDM/NBR matrix. Keywords: EPDM; NBR; Graphene; Graphene oxide; nanocomposites 1. Introduction

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Elastomer nanocomposites containing platelet-like nanoparticles have been the subject of substantial research in recent years for their outstanding mechanical properties [1-3]. However, the role of rubber matrix, which can be the blend of two types of polar or nonpolar elastomers,

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in achieving high performance should undeniably be taken into account [4, 5]. Rubber blends based on ethylene propylene diene monomer (EPDM) and nitrile butadiene rubber (NBR) can

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provide the user with acceptable oil and ozone resistance [6], low-temperature flexibility [7] and good mechanical properties [8], which make these kinds of rubber compounds an

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appropriate candidate for seal applications. In recent years, many researches were focused on the production of rubber nanocomposites containing a rubber blend reinforced by a nanofiller

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in the quest of mechanical properties [9, 10]. In this regard, the use of platelet-like nanoparticles was of particular interest from both microstructural and properties viewpoints [11-13]. There

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have been also analyses to unveil the interfacial interaction and large deformation mechanical

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properties in such systems from both experimental and theoretical views [14-16]. EPDM/NBR nanocomposites containing various types of nanofillers have been studied by many researchers in order to uncover their physical and mechanical properties [17, 18]. Ghasemieh [19] studied the effect of various concentrations of nanoclay on the mechanical properties of EPDM/NBR rubber blends and reported that the introduction of nanoclay into the rubber blend could increase the compression resistance of the nanocomposites. Our previous

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. study [15] on the NBR nanocomposites based on silane modified and unmodified organoclays elucidate that hyperelastic models has some deviations from experimental data especially for thr nanocomposites containing silane modified organoclay. Ersali et al. [20] investigated the effect of two types of organoclays (Cloisite 20A and Cloisite 30B) on the properties of EPDM/NBR blend and found that the higher mechanical properties was the result of combined use of organoclays in nanocomposite preparation. Jovanovic et al. [21] reported that incorporation of nanosilica into EPDM/NBR elastomer blends varying composition would be

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the way to find the highest physical and mechanical properties in view of enhanced interaction between the rubbers and nanoparticle. Nevertheless, hybrid of two platy nano-size particles and theoretical evaluation of the mechanical properties of the resulting elastomer nanocomposite

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with superior properties was not addressed.

Graphene nanoplatelets (GnPs) and their derivatives are nanofillers that mostly utilized in

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various polymer matrices due to their extraordinary physical structure and exceptional

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properties [22, 23]. GnPs can be found in the form of a single-atom-thick-sheet as oxide with a high Young’s modulus, high thermal stability and high electrical conductivity, which make them useful for developing rubber nanocomposites [3, 24]. Mowes et al. [25] showed that the

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NBR/GnPs nanocomposites have mechanical properties different from

traditional NBR

composites containing carbon black. On the other hand, Zhange et al. [26] prepared

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EPDM/GnPs via in-situ polymerization and found that thermal stability of EPDM was

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markedly improved with addition of GnPs up to 50 °C. Li et al. [27] prepared the NBR nanocomposites containing various contents of graphene oxides (GOnPs) and found that the more the nanoparticle content the higher the wear resistance of nanocomposites. Scaffaro et al. [28] studied the synergistic effect of GnPs and carbon nanotubes (CNTs) on the mechanical properties of polylactic acid (PLA) and found simultaneous enhancement of tensile strength (up to +44%) and elongation at break (up to +36%). Gong et al. [29] investigated the synergistic

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. toughening effect of reduced graphene oxide (r-GOnPs) and double-walled carbon nanotubes (DWNTs) in combination with 10,12-pentacosadiyn-1-ol (PCDO) and discussed that the tensile strength and toughness of this kind of ternary bioinspired nanocomposites reaches 374.1±22.8 MPa and 9.2±0.8 MJ/m3, which is 2.6 and 3.3 times that of pure reduced graphene oxide film, respectively. However, NBR/EPDM blends reinforced with GnPs and GOnPs were not the subject of any report. The present study attempts to visualize the effect of individually and hybrid of two types of

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graphene-based nanoplatelets on the properties of the rubber blends based on EPDM and NBR prepared using a two-roll mixer. The effects of various concentrations of GnPs and GOnPs platelets on the morphological, mechanical, rheological and dielectric properties of EPDM/NBR

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blends were explored by using appropriate experimental analysis. The stiffness analysis of the prepared nanocomposites was also investigated using the Mori-Tanaka micromechanics theory

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[30] by considering the randomly occurred orientation of nanoplatelets into the polymer matrix.

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2. Theoretical background

The effective elastic moduli of polymer nanocomposites could be calculated by using the MoriTanaka (M-T) method [31] with hypothesis of embedding each inclusion in an infinite polymer

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matrix subjected to the applied average stress (σm) or average strain (εm) of the matrix. According to Eshelby equivalent inclusion theory [32], the average strain of the inclusions of

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the nanoplatelet form could be represented by the following relation :

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𝜀𝑓 = 𝑨𝑓 : 𝜀𝑚

(1)

where 𝑨𝑓 is the strain concentration tensor calculated by the following relation [32]: −1

−1 ): 𝑨𝑓 = [𝑰 + (𝑺: 𝑪𝑚 (𝑪𝑓 − 𝑪𝑚 )]

(2)

The parameter S is the Eshelby tensor and C is the effective stiffness tensor, defined as the below relation[33]: 5

EPDM/NBR blend filled graphene nanocomposites by Paran et al. 𝜎̅ = 𝑪: 𝜀̅

(3)

where 𝜎̅ and 𝜀̅ are effective or average stress and strain tensors, respectively. These parameters calculated by the following relations for the nanocomposites [34]: 𝜎̅ = 𝑉𝑚 𝜎𝑚 + ∑𝑛𝑖=1 𝑉𝑖 𝜎𝑖

(4)

𝜀̅ = 𝑉𝑚 𝜀𝑚 + ∑𝑛𝑖=1 𝑉𝑖 𝜀𝑖

(5)

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In the above relations, 𝑉𝑚 and Vi represent the volume fraction of the matrix and nanoplatelets, respectively. The stress and strain tensors in the dispersed phase were represented as σi and εi, respectively. The parameter n is the number of nanoparticles with different physical shape or

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elastic properties. Consequently, the effective stiffness tensor for nanocomposite can be presented as the following equation emphasizing the alignment of the nanoplatelets[34]:

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𝑪 = (𝑉𝑚 𝑪𝑚 + ∑𝒏𝒊=𝟏 𝑉𝑖 𝑪𝑖 : 𝑨𝑖 ): (𝑉𝑚 𝑰 + ∑𝒏𝒊=𝟏 𝑉𝑖 𝑨𝑖 )−1

(6)

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where 𝑨𝑖 is the strain concentration tensor for each inclusions. The overall elastic stiffness of the nanocomposites containing randomly oriented nanoplatelets could be calculated by the following relation [35]:

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𝑪 = (𝑉𝑚 𝑪𝑚 + ∑𝒏𝒊=𝟏 𝑉𝑖 〈𝑪𝑖 : 𝑨𝑖 (𝜃, 𝜑)〉𝑪𝑖 : 𝑨𝑖 ): (𝑉𝑚 𝑰 + ∑𝒏𝒊=𝟏 𝑉𝑖 〈𝑨𝑖 (𝜃, 𝜑)〉)−1 (7)

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Where the angular brackets 〈𝑪𝑖 : 𝑨𝑖 (𝜃, 𝜑)〉 and 〈𝑨𝑖 (𝜃, 𝜑)〉 represents the average of the effective

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stiffness and strain concentration tensors for randomly orientated nanoplatelets. The effective bulk (K) and shear (G) moduli of the nanocomposites demonstrating the isotropic behavior of inclusions can be represented using Hill’s elastic parameters as [36]: 𝐾 = 𝑘𝑚 + 𝐺 = 𝐺𝑚 +

𝑉𝑓 (𝛿𝑓 −3𝑘𝑚 𝛼𝑓 ) 3(𝑉𝑚 +𝑉𝑓 𝛼𝑓 ) 𝑉𝑓 (𝜂𝑓 −2𝐺𝑚 𝛽𝑓 ) 2(𝑉𝑚 +𝑉𝑓 𝛽𝑓 )

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(8) (9)

EPDM/NBR blend filled graphene nanocomposites by Paran et al. The dimensionless parameters used in equations (8) and (9) are defined as [36]: 𝛼𝑓 = 𝛽𝑓 = 𝛿𝑓 = 𝜂𝑓 =

3𝑘𝑚 +2𝑛𝑓 −2𝑙𝑓

(10)

3𝑛𝑓

4𝐺𝑚 +7𝑛𝑓 +2𝑙𝑓 15𝑛𝑓

+

2𝐺𝑚

(11)

5𝑝𝑓

3𝑘𝑚 (𝑛𝑓 +2𝑙𝑓 )+4(𝑘𝑓 +𝑛𝑓 −𝑙𝑓2 )

(12)

3𝑛𝑓 2

(𝑘𝑓 + 6𝑚𝑓 + 8𝐺𝑚 − 15

𝑙𝑓2 +2𝐺𝑚 𝑙𝑓 𝑛𝑓

)

(13)

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where l, m, n, k and p are the Hill’s elastic parameters, so that l, m and p denotes the shear modulus under uniaxial tension in the x3 direction, in the (x1,x2) plane and in the (x1,x3) or (x2,x3) plane, respectively. The parameter n is defined as the modulus under uniaxial tension in the x3

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direction. The parameters k and G are defined as the bulk modulus in the (x1, x2) plane and shear modulus in the (x1,x3) plane, respectively. The subscriptions m and f used in the

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aforementioned equations represent the matrix and disperse phases in the nanocomposites. Therefore, the equation (3) could be re-written in terms of Hill’s elastic moduli as follows [37]: 𝑙 𝑙 𝑛 0 0 0

0 0 0 𝑝 0 0

0 0 𝜀11 0 0 𝜀22 0 0 𝜀33 0 0 2𝜀23 𝑝 0 2𝜀13 0 𝑚] [2𝜀12 ]

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𝑘−𝑚 𝑘+𝑚 𝑙 0 0 0

(14)

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𝜎11 𝑘+𝑚 𝜎22 𝑘−𝑚 𝜎33 𝑙 𝜎23 = 0 𝜎13 0 [𝜎12 ] [ 0

The Young’s modulus of the prepared nanocomposites can accordingly been obtained by using

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the Hill’s elastic moduli of the polymer matrix and nanoplatelets considering the fraction volume of each phases and some physical assumptions in the elastic behavior of polymer and nanoplatelets. The Young’s modulus of the nanocomposites can be calculated by the following relation [38]: 𝐸=

9𝐾𝐺

(15)

3𝐾+𝐺

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. 3. Experimental 3.1. Materials Ethylene propylene diene monomer (EPDM), KEP-270, containing 57 wt.% ethylene monomer with viscosity of 71 (ML (1+4), 100 °C) was supplied by the Kumho Petrochemical Co. (Korea). Nitrile butadiene rubber (NBR), N3345, containing 33% acrylonitrile with viscosity of 45 (ML (1+4), 100 °C) was purchased from Enichem Co. (Italy). Graphene nanoplatelets (GnPs) with density of 2.25 g/cm3 and surface area of 300-750 cm2/g was provided

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by XG Sciences (USA). Graphene oxide nanoplatelets (GOnPs) with density of 1 g/cm3 and surface area of 500-1200 cm2/g was prepared by US Research Nanomaterials (USA). The EPDM/NBR nanocomposites were cured by using N-cyclohexyl-2-benzothiazole sulfonamide

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(CBS) and sulphur (S) purchased from Bayer (M) Ltd. (Germany). Other ingredients such as zinc oxide (ZnO) and Stearic acid (St. Ac.) as accelerator and activators were similarly obtained

3.2. Sample preparation

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from Bayer (M) Ltd. (Germany) and utilized as-received.

According to Table 1, the rubber nanocomposites were prepared in open two-roll mill mixing

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operated at 50 °C under rotors speed ratio 1:1.2 for 20 min according to Table 1. First, the EPDM was masticated in two-roll mixing mill for 7 min, and then NBR was added to the

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EPDM and mixing was continued for 6 further min. The graphene nanoplatelets in the form of

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GnPs or GOnPs and combination of both of them, were added to the rubber compounds and mixed for 4 min with the polymer matrix. Finally, the curing ingredients were added to the rubber nanocomposite and mixed for 3 min. According to the optimum cure time obtained from Monsanto Rheometer, the rubber nanocomposites were vulcanized in a compression molding machine at 160 °C and time needed to reach the required optimum cure.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. 3.3. Characterization The morphology of rubber nanocomposites was imaged on a Vega II XMU scanning electron microscope (SEM), Czech Republic with magnification of 1000 x for sample codes of EN-0 and EN-1.5C,X. The samples were cryogenically fractured and coated with gold powders by sputtering technique prior to SEM analyses. The nanostructure of the cryogenically microtomed (with a diamond knife at -100 °C) fracture surface of the samples containing GnPs and GOnPs nanoplatelets were observed using a Philips CM-200 transmission electron

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microscope (TEM), Netherlands, with an accelerating voltage of 100 kV and magnification of 200 kx. The SEM and TEM micrographs enables analysis of the texture and interface regions of the fracture surfaces, respectively.

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The cure characteristics of the prepared EPDM/NBR nanocomposites containing various concentrations of GnPs and GOnPs nanoplatelets were studied by using a Monsanto Rheometer

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R-100 testing instrument operated at 160°C with 3° arc at a period of 15 min in accordance

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with ASTM D2084.

Dumbbell-shaped specimens for tensile tests were cut from the molded slabs. Tensile test was done according to the ASTM D412 by a Universal tensile testing machine, Instron 6025 model

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operated at room temperature at an extension speed of 500 mm/min with an initial gauge length of 25 mm. Resilience test was carried out in accordance with ASTM D2632 using a vertical

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rebound apparatus includes means for suspending a plunger at a given height above the

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specimen, and for releasing it followed by measuring the subsequent rebound height. The values of tensile strength, modulus, elongation at break and resilience were recorded directly from the digital display at the end of each test. The hardness of the rubber nanocomposites was measured, as per the ASTM D2240 testing method by using a Durometer Hardness Tester, TA instruments (USA). The sheets with effective thickness of 6 mm were used for measuring the hardness.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. The phase structure of the EPDM/NBR/graphene nanocomposites was investigated by dynamic mechanical thermal analysis (DMTA) using Triton Technology Tritec 2000DMA (UK). The storage modulus and damping factor of the rubber nanocomposites were also studied in bending mode at a constant heating rate of 5 ºC/min and a frequency of 1 Hz in a strain of 0.02 mm from -100 ºC to 100 ºC. Rheological measurements made on EPDM/NBR/graphene nanocomposites were done in oscillation mode using a MCR300 strain controlled rheometer from Anton Paar. The

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experiments were conducted in parallel plate geometry with a diameter of 25 mm under a nitrogen atmosphere at 80 ºC. A frequency sweep test was conducted on the samples from 0.01 to 600 rad/s.

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Swelling test of the rubber nanocomposites in standard oil no. 1 was carried out in accordance with ASTM D5964. The samples were immersed in standard oil for 168 h, after taking the

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determination of swelling ratio.

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initial dry weights. The swollen weights of the samples have been recorded for the

Dielectric properties of the prepared nanocomposites were monitored using a LCR-8000G analyzer from Good Will Instrument Co. (Taiwan) at frequency range between 20 to 1 MHz.

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4. Results and discussion 4.1. Cure characteristics

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The cure plots of the EPDM/NBR nanocomposites containing various loadings of GnPs and

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GOnPs nanoplatelets are shown in Figure 1. The relevant cure parameters of the nanocomposites are extracted from the figure and summarized in Table 2. It is apparent that the higher amount of GnPs caused a fall in both scorch time and cure time compared to the neat blend of EPDM/NBR due to the effect of GnPs on the thermal conductivity of sample [39]. According to Table 2, the maximum torque value and torque difference were both increased with the GnPs loading, which can be attributed to the effect of nanoplatelets on the extent of

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. crosslinking and reinforcing effect towards elastomer blend[30]. By contrast, GOnPs loading at the same level more efficiently decreased the cure time and scorch time as well as increased the maximum torque values much considerably compared to the GnPs. However, the nanocomposites containing both GnPs and GOnPs nanoflakes revealed a lower scorch time and cure time with respect to the ones containing only one kind of the nanoplatelets. Table 2 shows that the values of maximum torque and torque difference increased in the nanocomposites containing both types of the nanoplatelets. It may be due to the synergistic

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effect of the nanoplatelets in the EPDM/NBR nanocomposites which led to the enhanced reinforcing effect and crosslinking density in the rubber nanocomposites [40].

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4.2. Morphological investigations

The SEM micrographs of tensile fracture surfaces of the EPDM/NBR blend (EN-0) and

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EPDM/NBR nanocomposites containing GnPs and GOnPs nanoplatelets (EN-1.5C,X) are

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depicted in Figure 2. As can be seen, the EPDM/NBR nanocomposite (Fig. 2b) revealed a rough texture at the fracture surface suggesting an enhanced interfacial interaction between the nanoplatelets and the blend of rubbers compared to that of EPDM/NBR unfilled rubber blend

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(Fig. 2a) [41]. A more detailed analyses on fracture surface was performed by the TEM images of cryogenically microtomed fracture surfaces of the EPDM/NBR nanocomposites containing

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various amounts of GnPs and GOnPs (Figure 3). As it is visible in Figure 3, the bright region

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indicates the EPDM phase, whereas the NBR phase appeared as the dark region due to the differences between the densities of the two rubber phases. Furthermore, the nanoplatelets can be seen as the dense dark domains in comparison with the rubber phases indicated with arrow lines in Figure 3. We can see that the nanoplatelets are dispersed in both EPDM and NBR phases, while there are some agglomerations of the nanoplatelets in the rubber phase. Figure 3c shows that in case of nanocomposites containing hybrid nanoplatelets the dispersion state is

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. remarkably better than that one’s of nanocomposites containing one type of nanoplatelets. It may be due to the GOnPs-GnPs and GOnPs-matrix interactions [42].

4.3. Mechanical properties Mechanical properties of the prepared EPDM/NBR nanocomposites containing various amounts of GnPs and GOnPs are shown in Figure 4 and Figure 5. The stress-strain curves of

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rubber nanocomposites in Figure 4 indicate that the introduction of both types of nanoplatelets leads to higher tensile properties in the nanocomposites with sample code EN-1.5C,X due to the effect of rubber-nanoplatelets physical interactions [43] and the concentrations of

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nanoplatelets because of stiffening effect of nanoplatelets [44]. It is clear that the nanocomposites containing both types of nanoplatelets exhibited higher mechanical properties

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compared to the other rubber nanocomposites. It seems that the EPDM/NBR nanocomposite

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containing both types of nanoplatelets have more interactions with the rubber matrix which leads to the higher mechanical properties [45]. The effects of GnPs and GOnPs on the various parameters related to the mechanical properties of the EPDM/NBR nanocomposites are

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demonstrated in Figure 5. One can see from Figure 5(a) that the tensile strength of the rubber matrix was enhanced up to 103% by the introduction of proper loadings of GnPs and GOnPs.

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However, there is a reduction in the elongation at break at higher concentrations of

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nanoplatelets in Figure 5(b) due to the induction of some restrictions in the chain mobility of rubber matrix by the incorporation of nanoplatelets[46]. It is obvious from Figure 5(c), that the introduction of GnPs and GOnPs cause a rise in the modulus by 300% elongation of the EPDM/NBR nanocomposites, which can be attributed to the dispersion state of nanoplatelets and their interactions with the rubber matrix [47]. The results of hardness presented in Figure 5(d), show no obvious change with higher loadings of GnPs and GOnPs. Therefore, the

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EPDM/NBR blend filled graphene nanocomposites by Paran et al. hardness of nanocomposites was strongly dependent on the nanoparticle concentration, which was very low in the EPDM/NBR nanocomposites [48]. Cylindrical shaped specimens for resilience tests were directly compression molded. Variations of resilience with the concentration of nanoplatelets are depicted in Figure 5(e). The results indicated a decrease in the resilience with increasing the nanoplatelets amount due to the physical interactions between the nanofiller and rubber matrix which restrict the mobility of rubber chains [49].

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. 4.4. Stiffness analyses

The effect of concentrations of GnPs and GOnPs on the elastic modulus of EPDM/NBR blend

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rubber was investigated by using the Mori-Tanaka micromechanics method considering the isotropic mechanical behavior for polymer matrix and transversely isotropic mechanical

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behavior of both nanoplatelets. Our previous work was done for dynamically vulcanized

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thermoplastic elastomer vulcanizates based on polyethylene and reclaimed rubber nanocomposites containing various GnPs loadings through using Christensen and Lo model [50] which is defined for spherical inclusions. Whereas, the assumption of transversely

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isotropic mechanical behavior is more precise for nanoplatelets. The Hill’s moduli of polymer matrix can be calculated by the following equations [51]:

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𝑘𝑚 = 𝑙𝑚 =

𝐸𝑚

2(1+𝜐𝑚 )(1−2𝜐𝑚 ) 𝜐𝑚 𝐸𝑚 (1+𝜐𝑚 )(1−2𝜐𝑚 )

𝑚𝑚 = 𝑝𝑚 = 𝑛𝑚 =

𝐸𝑚 2(1+𝜐𝑚 )

(1−𝜐𝑚 )𝐸𝑚 (1+𝜐𝑚 )(1−2𝜐𝑚 )

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(16) (17) (18) (19)

EPDM/NBR blend filled graphene nanocomposites by Paran et al. where 𝐸𝑚 is the Young’s modulus of the EPDM/NBR matrix, which is set to take value of 1.62 MPa, the 𝜐𝑚 is the Poisson’s ratio, which is set to be 0.45 for the polymer matrix [52]. The GnPs and GOnPs are considered as transversely isotropic materials with an axis of material symmetry due to their physical structure [53]. The stiffness analyses of the nanoplatelets with assumption of a single layer structure can independent elastic parameters consisting of

be represented by five

in-plain Young’s modulus (E1), in- plain

Poisson’s ratio (ν12), out-of-plain Young’s modulus (E3), out-of-plain shear modulus (G13) and

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out-of-plain Poisson’s ratio (ν13). The stiffness tensor for nanoplatelets can be represented in terms of Hill’s elastic coefficients [54]: 𝐶 = (2𝑘, 𝑙, 𝑛, 2𝑚, 2𝑝)

(20)

-p

where 𝑙 = 2𝑘𝜐13 2 𝑛 = 𝐸3 + 4𝑘𝜐13 𝑙2

𝑚 = 𝑥 (𝑘 − )

lP

𝑛

re

(21)

𝑝 = 𝐺13

𝐸1 𝐸32 (1+𝑥) 2 2 (𝐸 𝑥−𝐸 −2𝑥)] 4[𝐸3 +𝐸1 𝜐13 3 3

na

𝑘=

(22)

(23) (24) (25)

The dimensionless parameter x is defined in equation (26):

ur

𝑥=

1−𝜐12

(26)

1+𝜐12

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Lee et al. [55] reported that the five independent elastic parameters of nanoplatelets can approximately be set as E1=1000 GPa; ν12=0.4; E3=100000 GPa; G13=100000 GPa; ν13=0.004 Therefore, the investigated stiffness tensor for nanoplatelets is: 𝐶 = (1667, 6.67, 100000,714.23,200000) (27) The effect of various concentrations of GnPs and GOnPs on the modulus at 300% elongation of EPDM/NBR matrix demonstrated in Figure 6. According to analyses, the enhancement of 14

EPDM/NBR blend filled graphene nanocomposites by Paran et al. modulus due to nanoplatelets was overestimated by the M-T theory; so that for cases in which nanoplatelets were finely dispersed into the matrix random orientation assumption was acceptable. On the other hand, the experimental data suggest that the gained enhancement at all concentrations was limited to the lower modulus in comparison to the M-T model owing to the weakening effect of dispersion state of nanoplatelets into the rubber matrix [56]. Therefore, the higher interfacial interactions fueled by appropriate dispersion of nanoplatelets into the EPDM/NBR matrix was responsible for improved modulus, where fewer differences with M-

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T model prediction was detected in Figure 6 at 1.5 phr loadings of the hybrid nanoplatelets.

-p

4.5. DMTA measurements

Figure 7 shows the results of the DMTA measurements for various EPDM/NBR

re

nanocomposites containing GnPs and GOnPs nanoplatelets. Figure 7 shows that there are two

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steps/peaks observed in the storage modulus and tan δ due to the presence of two distinct rubber phases in the nanocomposites. As depicted in Figure 7(a), the storage modulus of the rubber nanocomposites is enhanced with the nanoplatelets concentrations due to the stiffening effect

na

of GnPs and GOnPs nanoplatelets and their great physical interactions with the rubber matrix [14]. The results of tan δ in Figure 7(b) suggests that the incorporation of nanoplatelets into

ur

the rubber matrix leads to an increase in the glass-rubber transition temperature (Tg) in both

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EPDM and NBR phases. However, there is some reduction in the tan δ parameter indicating a more elastic behavior for the EPDM/NBR nanocomposites due to the restricted mobility of the polymer chains induced by the interaction between the rubber matrix and nanoplatelets [50].

15

EPDM/NBR blend filled graphene nanocomposites by Paran et al. 4.6. Rheological analysis The viscoelastic behavior of EPDM/NBR nanocomposites containing various loadings of GnPs and GOnPs are compared in Figure 8. The storage modulus of polymers with respect to the applied frequency is highly related to the segmental motion of polymer chains, which depends on the applied strain at low frequencies without

delay and any loss of energy [57].

Nevertheless, at large applied frequencies the entanglements of polymer chains act as temporary crosslinks due to their inability to follow the applied strain [58]. Figure 8 indicates

ro of

that the storage modulus of the EPDM/NBR nanocomposites significantly increases at higher concentrations of nanoplatelets, especially in case of the nanocomposites containing hybrid nanofillers, due to the uniform dispersion state of nanoplatelets in the rubber matrix [59]. As

-p

inferred from data in Figure 8, the trends in the complex viscosity of EPDM/NBR nanocomposites show a shear thinning behavior [60] which has been intensified at higher

an almost solid-like network was formed that changed the behavior of the

lP

GOnPs

re

nanoplatelet loading. The results of complex viscosity revealed that the higher GnPs and

nanocomposites [61], especially for EPDM/NBR nanocomposites containing hybrid

ur

4.7. Oil Swelling

na

nanoplatelets.

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The effect of the introduction of GnPs and GOnPs into the EPDM/NBR rubber blend on the swelling behavior of the resulting nanocomposites in the ASTM oil no.1 and ASTM oil no.3 are investigated in Figure 9. As can be observed in Figure 9(a) and Figure 9(b), the higher concentration of nanoplatelets in the rubber matrix leads to a considerable fall in the oil uptake in the EPDM/NBR nanocomposites. Therefore, reinforcement of rubber matrix limits the extensibility for rubber chains induced by swelling [62]. Figure 9 indicates that the

16

EPDM/NBR blend filled graphene nanocomposites by Paran et al. introduction of both GnPs and GOnPs into the EPDM/NBR matrix enhances the swelling behavior in ASTMS oil no.1 and ASTM oil no.3, roughly by26 and 35%, respectively.

4.8. Dielectric properties Figure 10 shows the variation of dielectric parameters of the EPDM/NBR and its nanocomposites hybrid nanoplatelets as a function of applied frequencies. As in Figure 10(a), the dielectric constant or real part of permittivity (έ) was increased by the introduction of both

ro of

GnPs and GOnPs into the EPDM/NBR rubber compound. It is clear in Figure 10(a) that the dielectric constant has no obvious changes in the range of 100-10000 Hz. At higher frequencies, it was decreased due to the less ability of the orientation of dipolar polarization

-p

in the matrix, and at the interface of filler-matrix region [63]. Figure 10(b) showed a higher loss dielectric constant with the incorporation of nanofiller due to the polarization at the

re

interface region and electrically conductive nature of the used nanoparticles [64]. It is also clear

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from Figure 10(b) that the loss dielectric constant the samples increases with the higher applied frequencies due to the higher chain entanglements’ relaxation time of the matrix in comparison with the applied frequencies [65]. Figure 10(c) shows that the complex dielectric parameters

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of the nanocomposites are higher than that of the neat EPDM/NBR compound, which roots in high electrical conductivity of GnPs and GOnPs [66]. It should be emphasized that improved

ur

dispersion in the case of hybrid systems observed in Figure 3(c) is behind the corresponding

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behavior. In agreement with this observation, Figure 10(d) revealed that the introduction of both GnPs and GOnPs into the EPDM/NBR matrix leads to higher loss factor at various applied frequencies due to the higher conductivity of nanofiller with respect to the polymer matrix [67] and dipolar polarization at the interface region between the nanofillers and polymer matrix [68].

17

EPDM/NBR blend filled graphene nanocomposites by Paran et al. 4.9. Electrical conductivity Figure 11 represents the effect of hybrid nanofillers loadings on the electrical conductivity of EPDM/NBR compounds. The conductivity of the nanocomposite increased with the incorporation of nanoparticles into the rubber matrix due to the higher electrical conductivity of the carbon-based nanoplatelets [69]. The trends observed for electrical conductivity with respect to the applied frequency for all prepared samples in Figure 11 approved a higher electrical conductivity at higher applied frequency. Thus, the dissipation mechanism for

ro of

electrical conductivity does not act properly at higher frequencies, which results in higher electrical conductivity of the EPDM/NBR rubber compound and its nanocomposites [70].

-p

5. Conclusion

The rubber nanocomposites based on ethylene propylene rubber (EPDM) and nitrile butadiene

re

rubber (NBR) containing various concentrations of GnPs and GOnPs hybrid nanoplatelets were

lP

prepared using a two-roll mill mixer. The results of cure curves indicated that the introduction of GnPs and GOnPs into the EPDM/NBR matrix effectively governed the cure behavior of rubber blend. The nanoplatelets were responsible for a rise in the maximum torque up to 69

na

N.m and reduction in the scorch time to 2 min leading to a higher cure rate. Moreover, the results confirmed that the GOnPs has more efficiency in decreasing the cure time and scorch

ur

time; meanwhile, in increasing the maximum torque values compared to the GnPs. SEM

Jo

micrographs of tensile fracture surfaces of the EPDM/NBR nanocomposites showed a rough texture for fracture surface indicating some interactions between the rubber matrix and nanoplatelets. TEM images of the prepared nanocomposites revealed that the nanoplatelets finely dispersed in the nanocomposites when two types of nanoplatelets were coincidently used. The results of the measurements of mechanical properties show that the introduction of GnPs and GOnPs into the EPDM/NBR matrix leads to a higher tensile strength and modulus

18

EPDM/NBR blend filled graphene nanocomposites by Paran et al. for the nanocomposites about 100% and 200%, respectively. However, there is a decrease in the elongation and resilience with the higher concentration of nanoplatelets. The DMTA measurements suggested a higher storage modulus up to 1.6 MPa with the higher nanoplatelets loading, whereas the damping factor showed about 12% decrease with the introduction of GnPs and GOnPs into the EPDM/NBR matrix. The analysis of rheological measurements for the prepared nanocomposites showed that the storage modulus increases with the nanoplatelets concentration at low frequencies. Furthermore, the complex viscosity increases with the higher

ro of

loadings of GnPs and GOnPs that leads to formation of a solid-like network with the corresponding behavior seen for the EPDM/NBR nanocomposites. The oil swelling tests in ASTM oil no.1 and ASTM oil no.3 exhibited a considerable fall in oil uptake in the

-p

EPDM/NBR nanocomposites with increasing the amount of nanoplatelets. The dielectric spectroscopy of the EPDM/NBR nanocomposites containing both GnPs and GOnPs indicated

re

a higher electrical conductivity of the nanocomposites in the range of applied frequencies.

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Overall, it was deduced that simultaneous introduction of GnPs and GOnPs into the EPDM/NBR rubber blend ends in development of a rubber nanocomposite with enhanced mechanical, dynamic mechanical and rheological behavior.

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Acknowledgement

The authors of this paper would like to thank Zahra Farrokhi from the Faculty of Literature and

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Human Science, Science and Research Branch, Islamic Azad University, Tehran, Iran for

Jo

editing the English translation version.

19

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

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Figure 1. Cure curves of the EPDM/NBR nanocomposites containing various loadings of GnPs

Jo

ur

na

and GOnPs

25

EPDM/NBR blend filled graphene nanocomposites by Paran et al. Figure 2. SEM photomicrograph of the fracture surfaces taken from tensile testing of the (a)

Jo

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na

lP

re

-p

ro of

EPDM/NBR blend (EN-0) and (b) EPDM/NBR nanocomposite (EN-1.5C,X).

26

Jo

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na

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re

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

27

EPDM/NBR blend filled graphene nanocomposites by Paran et al. Figure 3. TEM photomicrographs of the EPDM/NBR nanocomposites containing (a) 3 phr

na

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ro of

GnPs (b) 3 phr GOnPs; and (c) 1.5 phr GnPs + 1.5 phr GOnPs.

rubber blends.

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Figure 4. The effect of GnPs and GOnPs on the stress-strain behavior of the EPDM/NBR

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Figure 5. The effect of the GnPs and GOnPs on the mechanical properties of the EPDM/NBR rubber blends: (a) tensile strength; (b) elongation at break; (c) modulus at 300% elongation; (d) hardness (e); resilience

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Figure 6. Comparison between theoretical stiffness and experimental data for modulus at

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300% elongation of various EPDM/NBR nanocomposites containing GnPs and GOnPs.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Figure 7. The results of DMTA measurements for various EPDM/NBR nanocomposites containing GnPs and GOnPs.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Figure 8. The effect of GnPs and GOnPs loading on the variation of storage modulus (G') and complex viscosity (η*) of EPDM/NBR nanocomposites.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

Figure 9. Swelling behavior of the EPDM/NBR nanocomposites containing various loading of GnPs and GOnPs in (a) ASTM oil no.1 and (b) ASTM oil no.3.

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Figure 10. Dielectric spectroscopy of the EN-0 and EN-1.5XC samples: (a) real part of

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permittivity (έ); (b) dielectric loss (ε"); (c) complex permittivity; and (d) loss tangent (tan δ).

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Figure 11. The effect of GnPs and GOnPs loading on the conductivity of EPDM/NBR

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nanocomposites.

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EPDM/NBR blend filled graphene nanocomposites by Paran et al.

. Table 1. Formulation of the nanocomposites prepared in this study. Ingredients(phr)

EN-0

EPDM

70

70

70

70

70

70

NBR

30

30

30

30

30

30

Zinc Oxide

5

5

5

5

5

5

Stearic acid

1.5

1.5

1.5

1.5

1.5

1.5

CBS

0.8

0.8

0.8

0.8

0.8

0.8

Sulfur

2

2

2

Graphene oxide (GOnPs)

0

0

1.5

Graphene (GnPs)

0

1.5

0

EN-3X

EN-3C

EN-1.5C,X

ro of

EN-1.5C

2

2

2

0

3

1.5

3

0

1.5

re

-p

EN-1.5X

Table 2. Cure characteristics of the EPDM/NBR/nanoplatelets nanocomposites. 𝑀ℎ (lbf)

𝑀𝑙 (lbf)

(𝑀ℎ-𝑀𝑙 ) (lbf)

lP

Sample

𝑡𝑠2 (min)

𝑡𝑐90 (min)

11.20

1.36

9.84

2.51

6.22

EN-1.5X

11.33

1.47

9.86

2.35

5.40

EN-1.5C

11.40

1.48

9.92

2.30

5.37

EN-3X

11.44

1.48

9.96

2.27

5.33

EN-3C

11.47

1.50

9.97

2.17

5.21

12.3

1.50

10.8

2.10

5

na

EN-0

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EN-1.5C,X

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