J Mater Sci: Mater Electron (2017) 28:5250–5261 DOI 10.1007/s10854-016-6182-x
Improvement of electrical properties and thermal conductivity of ethylene propylene diene monomer (EPDM)/barium titanate (BaTiO3) by carbon blacks and carbon fibers Jun Su1,2 · Jun Zhang1
Received: 3 August 2016 / Accepted: 5 December 2016 / Published online: 16 December 2016 © Springer Science+Business Media New York 2016
Abstract The aim of the study was to use carbon fibers and carbon blacks to improve the thermal conductivity, mechanical and dielectric properties of ethylene propylene diene monomer (EPDM)/barium titanate (BaTiO3) composites. It was found that 7.5 vol% carbon blacks, with high specific surface area, can make complex viscosity of EPDM/BaTiO3 compound to become non-sensitive to varying shear. Due to the sulfuric atom and C=C groups on surface of carbon blacks, 10 vol% carbon blacks can enhance the tensile strength and tear strength of EPDM/BaTiO3 (70/30) from 9.00 MPa and 21.06 kN m−1 to 14.32 MPa (59% increase) and 30.02 kN m−1 (43% increase). It was found that the 10 vol% spherical carbon blacks with high specific area can partially contact BaTiO3 and fill the gap between BaTiO3 particles to increase thermal conductivity and dielectric constant of EPDM/BaTiO3(70/30) from 0.323 W m−1 K−1and 7 at 5 MHz to 0.632 W m−1 K−1 (95% increase) and 746 (106 times increase) at 5 MHz, respectively. When the filler content was 10 vol%, carbon blacks and carbon fibers can decrease the volume resistivity of EPDM/BaTiO3 (70/30) from 2.23 × 1013 to 6.37 × 105 Ω m (eight order of magnitude drop) and 4.25 × 1011 Ω m (two order of magnitude drop), respectively.
* Jun Zhang
[email protected] 1
Department of Polymer Science and Engineering, College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, People’s Republic of China
2
College of Mechanics Engineering, Nanjing Institute of Industry Technology, Nanjing 210023, People’s Republic of China
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Vol:.(1234567890)
1 Introduction Recently, flexible dielectric polymer/ceramic composites with high actuated strain can transform under electric field, [1–3] which can be potentially used as pressure sensitive sensors in electronic domain [4]. In previous study, ethylene propylene diene monomer (EPDM)/barium titanate (BaTiO3) composites with high dielectric constant, high flexibility and low dielectric loss were prepared [3]. In this study, the aim was to further raise the dielectric constant and mechanical properties of EPDM/BaTiO3 composites. In addition, because the electronic devices with dielectric loss can generate heat during usage, thermal conductivity of EPDM/BaTiO3 also need to be improved to dissipate the heat generated during usage [5, 6]. The commonly used reinforcing fillers included: carbon blacks, silica, carbon fibers, multiwall carbon nanotubes, graphite and so on [7–10]. Among them, carbon blacks were the most commonly used and grouped into acetylene blacks, channel blacks, furnace blacks, lamp blacks and thermal blacks, which were produced by the incomplete combustion of coal tar, ethylene cracking tar and so on [11, 12]. In this study, a new acetylene carbon black (chezacarb AC-80) was selected.The acetylene black was a form of paracrystallinecarbon that has a high specific surface area, which were widely used as conductive and reinforcing fillers in rubber products [13, 14]. Rod-like carbon fibers, containing more than 95% carbon, were made up of layered graphite crystals stacked along the axial direction of the fibers [15–17]. Thereby, it was interesting to see whether or not the addition of spherical carbon blacks and rod-like carbon fibers into EPDM/BaTiO3 can improve tensile strength,
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dielectric constant and thermal conductivity of EPDM/ BaTiO3 composites. In order to avoid agglomeration, BaTiO3 and rod-like carbon fibers were firstly modified by coupling agent vinyltrimethoxysiloxane oligomer (SG-Si6490) [3]. Next, the carbon blacks and carbon fibers were added into EPDM/ BaTiO3 matrix by two-roll mill. The tests involved were rheological, cure, dielectric properties, volume resistivity, thermal conductivity and mechanical properties. In addition, parallel model, series model, geometric mean model and Maxwell-Eucken model were used to calculate data of thermal conductivity, which were compared with the experimental data of the thermal conductivity of EPDM composites [18, 19].
2 Experimental 2.1 Materials Ethylene propylene diene monomer (EPDM J-4045), barium titanate (BaTiO3), carbon fiber (CFEU-MC-6mm) and carbon black (AC-80) were purchased from Jilin Petrochem., Songbao electrically functional materials Co., Ltd. in Foshan, TOHO chemical Co., Ltd. and chezacarb, respectivly. The density of EPDM was 0.9 g cm−3. Stearic acid (SA), zinc oxide, dicumyl peroxide (DCP), 2-mercapto benzimidazole (MB) were of industrial grade. The physical properties of BaTiO3, carbon blacks and carbon fibers were listed in Table 1. The Si, K, Na, Ca, Mg and Fe impurities in carbon fibers are 0.02, 0.04, 0.06, 0.08, 0.07 and 0.03 wt%, respectively. The structure of coupling agent vinyltrimethoxysiloxane oligomer (SG-Si6490), purchased by Nanjing Shuguang Chemical Group Co. Ltd, was shown in Fig. 1
Fig. 1 Molecular structure of SG-Si6490
2.2 Sample preparation 2.2.1 Surface modification of fillers The coupling agent SG-Si6490 was utilized to modify the surface of the BaTiO3 and carbon fibers. Because there were functional groups on surface of carbon blacks, the carbon blacks were not treated by SG-Si6490. The content of SG-Si6490 was 1 wt% of BaTiO3 or carbon fibers amount. The surface treatment process was prepared according to the previous literature [20]. The modified BaTiO3 and carbon fibers were then extracted by diethyl ether through soxhlet extractor [3]. 2.2.2 Compounding of EPDM/BaTiO3 mixes According to ISO2393, EPDM, modified carbon fibers and BaTiO3, carbon blacks and ingredients were mixed by a two roll mixing mill, which was purchased from Shanghai Rubber Machinery Works. The formulation was listed in Table 2. 2.3 Testing procedures 2.3.1 Fourier transform infrared spectroscopy
Table 1 Physical properties of BaTiO3, carbon black and carbon fibers Samples
BaTiO3
Carbon black
Carbon fiber
Density (g cm ) Average size (μm)
6.0 1.8 μm
2.0 0.05 μm
pH value shape SBET (m2 g−1) Sulphur on surface (%)
7.0 Spherical 4 0
7.5 Spherical 800 0.8
1.8 6 mm (length of carbon fibers) 6.4 Rodlike 30 0
−3
Fourier transform infrared (FT-IR) spectra of carbon black, unmodified carbon fibers and modified carbon fibers after extraction were detected by Nicolet NEXUS 670. The carbon fibers, carbon blacks were individually mixed with potassium bromide and then pressed into thin plates for test. The scan range was set from 400 to 4000 cm−1. The test resolution was 4 cm−1. 2.3.2 Curing characterizations Curing properties of EPDM gums were carried out at 170 °C by Rheometer MDR 2000, purchased from Wuxi Liyuan Electronic & Chemical Equipment Co., Ltd.
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5252 Table 2 Formulation of EPDM composites (phr)
J Mater Sci: Mater Electron (2017) 28:5250–5261 Sample (volume fraction)
EPDM
Modified BaTiO3
Modified CF
CB
ZnO
SA
DCP
MB
EPDM/BT (70/30) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10)
100 100 100 100 100 100 100 100 100
285 285 285 285 285 285 285 285 285
– 7 14 21 28 – – – –
– – – – – 8 16 24 32
5 5 5 5 5 5 5 5 5
1 1 1 1 1 1 1 1 1
4 4 4 4 4 4 4 4 4
1 1 1 1 1 1 1 1 1
BT, CF and CB is the abbreviation of BaTiO3; CF and CB, respectivly
2.3.3 Crosslink density measurement
2.3.7 Thermal conductivity of EPDM composites
Crosslink density of EPDM specimens was tested according to solvent swell method and was calculated by Flory–Rehner Equation [21].
According to ISO 22007-2-2008, thermal conductivity of EPDM composites was measured by thermal constants analyzing TSP 2500, provided by Hot Disk. The models for thermal conductivity included parallel model, series model, geometric mean model and Maxwell-Eucken model [18, 19].
2.3.4 Scanning electron microscopy (SEM) Scanning electron microscope (SM-5900) purchased from JEOL, was applied to observe the fractured surface of EPDM composites.
3 Results and discussion
2.3.5 Mechanical properties
3.1 FT‑IR analysis
The mechanical properties of samples were tested by electromechanical universal testing machine (CMT 5254), purchased from Shengzhen SANS Testing Machine Co., Ltd. The testing standard was based on ISO37 and ISO34.
FT-IR spectra was used to detect whether or not the surface modification can endow functional groups to the surface of carbon fibers. In previous study, the coupling agent SG-Si6490 was chemically adhered to the BaTiO3 by surface modification [3]. The same modification process was applied to modify the surface of carbon fibers in this study. In Fig. 2, there were FT-IR spectra of coupling
2.3.6 Electric properties 2.3.6.1 Volume and surface resistivity According to IEC 60093, the volume and surface resistivity of composites were tested by high-insulation resistance meter, provided by Shanghai Precision & Scientific Instrument Co., Ltd. 2.3.6.2 Dielectric constant and dielectric loss According to IEC 60250, the dielectric constant and dielectric loss tangent were characterized by precision impedance analyzer 4294 A, provided by Agilent. The testing range was from 1 kHz to 60 MHz.
Fig. 2 FT-IR spectra of carbon blacks and carbon fibers
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agent SG-Si6490, carbon blacks, untreated carbon fibers and modified carbon fibers after extraction. On curves of SG-Si6490,there appeared the absorption peaks at 2945, 2839, and 1600 cm−1, assigned to asymmetric stretching vibration of methylene, symmetric stretching vibration of methylene, and stretching vibration of C=C groups, respectively [22]. In curves of untreated carbon fibers, there were not absorption peaks around 2800–2900 cm−1. By comparison, the modified carbon fiber after extraction absorbed at 2945 and 2839 cm− 1, suggesting that there were functional groups physically or chemically on the surface of modified carbon fibers after extraction. During the soxhlet extraction, the physically adhered functional groups can be washed away from the surface of carbon fibers by diethyl ether [3]. So the absorption bands of modified carbon fibers at 2945, 2839 and 1600 cm−1 after extraction can suggest the chemical adherence of SG-Si6490 to the surface of carbon fibers [3]. The absorption peaks around 2920 and 2854 cm−1 were ascribed to the asymmetric stretching vibration of methylene and symmetric stretching vibration of methylene, respectively. This is because that there are chemisorbed oxygen complexes, such as quinonic, lactonic, and phenolic groups on surface of carbon blacks during the manufacture.
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3.2 Rheological properties of EPDM/BaTiO3/ carbon blacks and EPDM/BaTiO3/carbon fibers compounds The aim of the rheological test was to investigate the effect of carbon blacks and carbon fibers on the complex viscosity and the fluidity of EPDM compounds before vulcanization process. In Fig. 3, it was shown that the complex viscosity of EPDM/BaTiO3/carbon blacks and EPDM/BaTiO3/carbon fibers compounds had the trend to increase with the increase of carbon blacks (or carbon fibers) content and decrease with the increase of shear rate. Although carbon fibers had rod-like shape, and carbon blacks had spheric shape, the complex viscosity of EPDM/BaTiO3/carbon fibers (70/30/10) compound at 1.7 Hz was around 1.0 × 105 Pa·s, which was lower than that of EPDM/BaTiO3/carbon blacks (70/30/10) compound (around 1.0 × 106 Pa·s). In Table 3, at the same filler content, the EPDM/ BaTiO3/carbon blacks have higher bound rubber content than EPDM/BaTiO3/carbon fibers. This was because that the specific surface area of carbon blacks was 800 m2 g−1, which was higher than that of carbon fibers (only 30 m2 g−1). In this way, the carbon blacks can physically absorb more EPDM chains than carbon fibers, leading to the rise of bound rubber content and complex viscosity at the same filler content.
Fig. 3 Rheological properties of EPDM compounds: a complex viscosity of EPDM/ BaTiO3/carbon blacks compound; b storage modulus of EPDM/BaTiO3/carbon blacks compound; c complex viscosity of EPDM/BaTiO3/carbon fibers compound; d storage modulus of EPDM/BaTiO3/carbon fibers compound
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slightly with the increase of shear rate. This was because, at 7.5 vol%, the interfacial adhesion of carbon blacks with EPDM became strong enough, which can prevent random coils from being unwrapped with the increase of shear rate [24]. However, the addition of rod-like carbon fibers did not have such phenomenon. Because the bound rubber content test shows that the carbon blacks can absorb more EPDM than carbon fibers, making the EPDM/BaTiO3/carbon blacks more viscous. Storage modulus (Gʹ) was the ability of materials to store the energy of the elastic deformation. The storage modulus of EPDM/BaTiO3/carbon blacks and EPDM/ BaTiO3/carbon fibers increased with the increase of shear rate. What’s more, the storage modulus of EPDM/ BaTiO3/carbon blacks was higher than that of EPDM/ BaTiO3/carbon fibers at the same filler content. This was due to the fact that the carbon blacks with higher specific surface area can absorb more EPDM chains than carbon fibers. In this way, it was easier for EPDM/BaTiO3/carbon blacks compound to form networks to store elastic energy than EPDM/BaTiO3/carbon fibers compound [3].
Table 3 Bound rubber content of EPDM composites Sample (volume fraction)
Bound rubber content (%)
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
32 34 36 38 40 33 34 35 36
Generally, the random coils of EPDM macromolecular chains can be unwrapped with the increase of shear rate [23]. It was found that when the carbon blacks content reached 7.5 vol%, the complex viscosity decreased
Fig. 4 Cure properties of EPDM compounds: a EPDM/ BaTiO3/carbon blacks compound; b EPDM/BaTiO3/carbon fibers compounds
Table 4 Cure characteristics of EPDM compounds
Sample (volume fraction)
ML Nm
tML min
MH Nm
tMH min
ts2 min
t90 min
Cure rate N m min− 1
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
0.22 0.32 0.43 0.58 1.01 0.17 0.23 0.30 0.35
0.23 0.21 0.18 0.15 0.11 0.25 0.22 0.19 0.16
2.95 3.40 3.98 4.24 4.75 2.52 2.92 3.27 3.50
12.87 12.43 11.69 10.82 10.17 13.28 12.76 11.87 11.51
0.85 0.81 0.74 0.63 0.58 0.95 0.84 0.76 0.69
6.80 6.47 6.12 5.75 5.27 7.01 6.85 6.41 5.94
0.21 0.25 0.31 0.34 0.37 0.18 0.21 0.24 0.27
ML—minimum torque; tML—time to minimum torque; MH—maximum torque; tMH—time to maximum torque; ts2—scorch time; t90—optimum cure time; Cure rate: (MH−ML)/(tMH−tML)
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Table 5 Crosslink density of EPDM composites Sample (volume fraction)
Crosslink density (mol cm−3)
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
0.000493 0.000690 0.000772 0.000854 0.000929 0.000417 0.000485 0.000561 0.000624
3.3 Cure properties and crosslink density of EPDM/ BaTiO3/carbon blacks and EPDM/BaTiO3/carbon fibers composites The cure data and crosslink density of EPDM/BaTiO3/carbon fibers and EPDM/BaTiO3/carbon blacks compounds were shown in Fig. 4 and Tables 4 and 5, respectively. In Fig. 4, the curves of EPDM compounds firstly decreased and then sharply ascended. The decrease of curves can be explained by the softening behavior of materials caused by heat, while the latter increase of curves were attributed to the formation of chemically crosslinked points [25]. In Tables 4 and 5, the maximum toque (MH), cure rate and crosslink density followed the order: EPDM/ BT/CF (70/30/2.5) < EPDM/BT/CF (70/30/5) < EPDM/ BT (70/30) < EPDM/BT/CF (70/30/7.5) < EPDM/BT/CF (70/30/10). The C=C groups in SG-Si6490 can promote the vulcanization of EPDM by generating extra chemical crosslink points [3, 4, 6] But it was found that the SG-Si6490 treated carbon fibers can retard the vulcanization when the content of carbon fibers was lower than 5 vol%. This was attributed to the fact that the pH level of carbon fibers was 6.4, due to the surface treatment by nitric acid during manufacturing [26]. The acidic carbon fibers can make the DCP decompose into ionics rather than radicals. Moreover, the ionics can not initiate the vulcanization and can not generate chemical bonds to increase the crosslink density of EPDM/ BaTiO3/carbon fibers [27]. So the C=C groups in SG-Si6490 and acidic carbon fiber were two competing factors affecting the vulcanization process. When the content of carbon fibers was lower than 5 vol%, the acidic carbon fibers played dominant role on the vulcanization, retarding the cure process. By comparison, when the content of carbon fibers was higher than 5 vol%, the effect of C=C groups in SG-Si6490
dominated on the vulcanization, promoting the cure process. Because the C=C groups on filler surface can increase the crosslink density more remarkably, when the volume of CF exceeds 5 vol%. In terms of carbon blacks, the MH, cure rate and crosslink density all boosted with the increased content of carbon blacks. The reason was ascribed to the C=C groups and sulfuric atoms on surface of carbon blacks. It was discussed above that the C=C groups can form extra chemical crosslink points and promote the vulcanization process of EPDM [3]. What’s more, the sulfur atoms on carbon blacks surface was 0.8%. The sulfuric atoms can participate into vulcanization and generate extra S–O bonds to increase the crosslink density [20]. In addition, the scorch time (ts2) and optimum cure time (t90) of EPDM/BaTiO3/carbon blacks can be shortened with the increasing amount of carbon blacks, meaning the presence of sulfuric atoms and C=C on surface of carbon blacks can increase the cure efficiency [3]. Although the ultimate cure state (MH) of EPDM/ BT/CF (70/30/10) was higher than that of EPDM/BT/ CB (70/30/2.5), the crosslink density of the former was lower than that of the latter. This was because that the MH of EPDM determined by the combined effect of complex viscosity and chemical crosslink density. According to Fig. 3, the complex viscosity of EPDM/BaTiO3/CF (70/30/10) was 8.4 × 104 Pa·s, which was higher than that of EPDM/BaTiO3/CB (70/30/2.5) (7.6 × 104 Pa·s). So, the MH was mainly determined by complex viscosity. 3.4 Dispersion of carbon blacks and carbon fibers in EPDM composites The dispersion of carbon blacks and carbon fibers in EPDM/BaTiO3composites were shown in Figs. 5 and 6. It was observed that the surface modified BaTiO3, carbon blacks and carbon fibers were dispersed well in EPDM matrix, indicating the coupling agent SG-Si6490 can decrease the agglomeration of fillers (BaTiO3 and carbon fibers) in EPDM matrix to some extent. Moreover, it was observed that the particles of carbon blacks was smaller than BaTiO3. The size of carbon black was 0.05 μm, which was much smaller than that of BaTiO3(1.8 μm). Therefore, carbon blacks were not obviously observed in SEM images in Fig. 5. According to SEM images, it was indicated that the presence of rod-like carbon fibers were embedded in EPDM matrix and can connect surrounding BaTiO3 particles to some extent.
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Fig. 5 SEM micrographs of EPDM/BaTiO3/carbon blacks vulcanizates: a EPDM/BaTiO3/ carbon blacks (70/30/2.5) (×1000); b EPDM/BaTiO3/ carbon blacks (70/30/2.5) (×10,000); c EPDM/BaTiO3/ carbon blacks (70/30/5) (×1000); d EPDM/BaTiO3/carbon blacks (70/30/5) (×10,000); e EPDM/BaTiO3/carbon blacks (70/30/10) (×1000); f EPDM/BaTiO3/carbon blacks (70/30/10) (×10,000)
3.5 Effect of carbon blacks and carbon fiber on mechanical properties of EPDM/BaTiO3 composites Table 6 listed the data of mechanical properties of EPDM/ BaTiO3/carbon blacks and EPDM/BaTiO3/carbon fibers composites. With the increase of carbon blacks, the hardness, 100% modulus, tensile and tear strength of EPDM/ BaTiO3/carbon blacks composites gradually increased. This was ascribed to the fact that the presence of sulfuric atoms and C=C groups on surface of carbon blacks can increase the chemical crosslink density, enhancing the mechanical properties of EPDM/BaTiO3/carbon blacks composites [20, 28]. In terms of carbon fibers reinforced EPDM/BaTiO3 (70/30), the trend of mechanical properties was correlated with that of crosslink density. The crosslink density was correlated with both pH level and C=C groups of carbon
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fibers based on above discussion. It was found that with the increase of carbon fibers, the tensile strength firstly decrease and then increase. In this way, the 10 vol% carbon fibers can increase the tensile and tear strength of EPDM/BaTiO3 (70/30) from 9.00 MPa and 21.06 kN m−1 to 10.09 MPa (12% increase) and 27.61 kN m−1 (31% increase), respectively [3]. By comparison, the addition of only 2.5 vol% carbon blacks can enhance the tensile strength and tear strength of EPDM/BaTiO3 (70/30) from 9.00 MPa and 21.06 kN m−1 to 11.47 MPa (27% increase) and 27.42 kN m−1 (30% increase). Because the crosslink density of EPDM/ BaTiO3/carbon blacks (70/30/2.5) was higher than that EPDM/BaTiO3/carbon fibers (70/30/10). When the content of carbon blacks increase to 10 vol%, the tensile and tear strength of EPDM/BaTiO3/carbon blacks can further increase to 14.32 MPa (59% increase) and 30.02 kN m−1 (43% increase).
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Fig. 6 SEM micrographs of EPDM/BaTiO3/carbon fibers vulcanizates: a EPDM/BaTiO3/ carbon fibers (70/30/2.5) (×1000); b EPDM/BaTiO3/ carbon fibers (70/30/2.5) (×10,000); c EPDM/BaTiO3/ carbon fibers (70/30/5) (×1000); d.EPDM/BaTiO3/carbon fibers (70/30/5) (×10,000); e EPDM/ BaTiO3/carbon fibers (70/30/10) (×1000); f EPDM/BaTiO3/carbon fibers (70/30/10) (×10,000)
Table 6 Mechanical properties of EPDM composites Sample
Hardness Shore A
Modulus at 100% (MPa)
Tensile strength (MPa)
Elongation at break (%)
Tear strength (kN m−1)
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
74 76 80 82 86 72 73 76 78
4.21 ± 0.08 9.31 ± 0.23 12.40 ± 0.47 12.91 ± 0.19 14.02 ± 0.18 4.01 ± 0.19 7.39 ± 0.76 8.40 ± 0.09 8.63 ± 0.37
9.00 ± 0.52 11.47 ± 0.74 12.5 ± 1.50 13.43 ± 0.49 14.32 ± 0.43 4.81 ± 0.19 7.20 ± 0.85 8.56 ± 0.93 10.09 ± 0.69
196 ± 9 146 ± 12 121 ± 7 109 ± 7 99 ± 6 223 ± 27 157 ± 9 126 ± 9 103 ± 8
21.06 ± 2.48 27.42 ± 1.68 29.28 ± 0.96 28.29 ± 1.33 30.02 ± 0.46 15.24 ± 1.65 22.74 ± 0.67 26.18 ± 2.61 27.61 ± 1.07
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J Mater Sci: Mater Electron (2017) 28:5250–5261 Samples
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
Thermal conductivity (W m−1 K−1) Experimental data
Calculated data Parallel model
Series model
Geometric mean model
MaxwellEucken model
0.323 0.452 0.546 0.582 0.632 0.485 0.509 0.565 0.595
0.729 2.524 4.019 7.218 32.323 2.579 4.133 7.395 32.571
0.201 0.072 0.071 0.071 0.071 0.201 0.201 0.201 0.201
0.321 0.329 0.334 0.34 0.346 0.346 0.371 0.398 0.426
1.027 1.065 1.135 1.21 1.29 1.099 1.208 1.329 1.464
3.6 Effect of carbon blacks and carbon fiber on thermal conductivity of EPDM composites The experimental and calculated data of thermal properties of EPDM/BaTiO3 composites were shown in Table 7. It was apparent that the incorporation of 10 vol% carbon blacks and carbon fibers can raise the thermal conductivity of EPDM/BaTiO3(70/30) from 0.323 to 0.632 W m−1 K−1 (95% increase) and 0.595 W m−1 K−1 (84% increase), respectively. Although carbon fibers have the rod-like shape, the thermal conductivity of EPDM/BaTiO3/carbon fiber was slightly lower than that of EPDM/BaTiO3/carbon blacks. Several models were used to calculate the thermal conductivity. The assumption of parallel model was the perfect contact between fillers in composites, while assumption of series model was no contact between fillers [18, 19]. So the fact that the experimental data fell in between two modeled values can demonstrate the BaTiO3 partially contacting Fig. 7 Dispersion models of carbon blacks and carbon fibers in EPDM/BaTiO3: a EPDM/BaTiO3 (70/30); b EPDM/BaTiO3/carbon fibers (70/30/2.5); c EPDM/BaTiO3/ carbon fibers (70/30/10); d EPDM/BaTiO3/carbon blacks (70/30/2.5); e EPDM/BaTiO3/ carbon blacks (70/30/10); f Carbon blacks
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with carbon blacks (or carbon fibers). In addition, it was found that the data of geometric mean model was close to the experimental data of EPDM/BaTiO3/carbon blacks and EPDM/BaTiO3/carbon fibers composites. By comparison the data of Maxwell-Eucken model was higher than experimental data of EPDM/BaTiO3 composites. This was because that the geometric mean model was only used at lower filler loading. Based on the SEM images and the discussion above, a model was proposed and presented in Fig. 7. It was shown that the carbon fibers can bridge surrounding BaTiO3 particles, and carbon blacks can fill the gap between BaTiO3 and partially contact with the BaTiO3, both of which can form inorganic passages in EPDM to facilitate the transfer of heat [29]. Because of the porous structure of carbon blacks, there were micro-holes in carbon blacks which can increase the specific surface area. Such unique structure can absorb more EPDM chains and increase the contact area with EPDM chains. By comparison, the specific
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Fig. 8 Dielectric constant and loss of EPDM composites: a dielectric constant of EPDM/ BaTiO3/carbon blacks composites; b dielectric constant of EPDM/BaTiO3/carbon fibers composites; c dielectric loss of EPDM/BaTiO3/carbon blacks composites; d dielectric loss of EPDM/BaTiO3/carbon fibers composites
surface area of carbon fibers were lower than that of carbon blacks. In this way, the carbon blacks can improve the thermal conductivity more remarkably than carbon fibers. 3.7 Effect of carbon blacks and carbon fiber on dielectric properties of EPDM/ BaTiO3composites Dielectric constant was used to characterize the capability of materials to store energy under cyclic electric excitation [30]. Figure 8 showed the curves of dielectric constant and loss of EPDM/BaTiO3/carbon blacks and EPDM/BaTiO3/ carbon fibers composites. Generally, the dielectric constant and loss were correlated with interfacial polarization and polarity of fillers. At low frequency, the high dielectric constant is mainly caused by interfacial polarization. With the increase of frequency, interfacial polarization can not respond to the change of frequency, leading to the decrease of dielectric constant at high frequency. It was apparent that the increase of carbon blacks and carbon fibers can increase the dielectric constant and loss of EPDM/BaTiO3composites. When the content was 7.5 vol%, the carbon blacks and carbon fibers can increase the dielectric constant of EPDM/BaTiO3 (70/30) from 7 to 329 (46 times increase) and 10 (42% increase) at 5 MHz, respectively [23, 31]. Moreover, when the content increased to 10 vol%, the dielectric constant of EPDM/BaTiO3/carbon blacks (70/30/10) and EPDM/ BaTiO3/carbon fibers (70/30/10) can further increase to
746 (106 times increase) and 11 at 5 MHz, respectively. The basic element of carbon fibers and carbon blacks were carbon. Because of the larger surface area of CB, the interfacial polarization between CB and EPDM is higher than the interfacial polarization between CF and EPDM. The higher interfacial polarization can contribute to the higher dielectric constant of EPDM/BaTiO3/CB. So the carbon blacks can increase the dielectric constant greatly. With the increase of test frequency, the dielectric constant and loss dropped dramatically. When the test frequency increased from 5 to 60 MHz, the dielectric constant and dielectric loss of EPDM/BaTiO3/carbon blacks (70/30/10) decreased from 746 to 265 to 62 and 60, respectively. At 60 MHz, the dielectric constant and loss was so close, so the electric energy was almost dissipated in the form of heat. Such composites may have potential use in electro-magenetic interference applications [32]. By comparison, the dielectric loss of EPDM/BaTiO3/carbon fibers were lower than 0.1 at 60 MHz. Moreover, the addition of 2.5 and 5 vol% carbon blacks can increase the dielectric constant of EPDM/ BaTiO3 (70/30) from 7 to 19 (171% increase) and 29 (314% increase) at 60 MHz, respectively. In the meantime, the dielectric loss increased from 0.003 to 2.2 and 4.4, respectively. It seemed that when the content of carbon blacks were lower than 5 vol%, the dielectric constant of materials can be increased with relatively lower dielectric loss, which can be used in electric devices [33].
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Table 8 Surface and volume resistivity of EPDM composites Volume fraction of BaTiO3
Volume resistivity (Ω m)
Surface resistivity (Ω)
EPDM/BT (70/30) EPDM/BT/CB (70/30/2.5) EPDM/BT/CB (70/30/5) EPDM/BT/CB (70/30/7.5) EPDM/BT/CB (70/30/10) EPDM/BT/CF (70/30/2.5) EPDM/BT/CF (70/30/5) EPDM/BT/CF (70/30/7.5) EPDM/BT/CF (70/30/10)
2.23 × 1013 2.01 × 1012 1.70 × 107 5.31 × 106 6.37 × 105 3.19 × 1012 8.49 × 1011 7.43 × 1011 4.25 × 1011
1.35 × 1010 5.39 × 1010 1.31 × 106 4.62 × 105 2.31 × 104 9.24 × 1010 6.16 × 1010 8.47 × 1010 7.70 × 1010
3.8 Effect of carbon blacks and carbon fiber on volume and surface resistivity of EPDM/BaTiO3composites Surface and volume resistivity were the resistance of electric current to transfer in materials [3]. Table 8 showed the data of surface and volume resistivity of EPDM/BaTiO3/ carbon blacks and EPDM/BaTiO3/carbon fibers composites. It was obvious that the increase of carbon blacks and carbon fibers can decrease the volume resistivity of EPDM/ BaTiO3 composites [20, 34]. At 10 vol% content, carbon blacks and carbon fibers can decrease the volume resistivity of EPDM/BaTiO3 (70/30) from 2.23 × 1013 to 6.37 × 105 Ω m (eight order of magnitude drop) and 4.25 × 1011 Ω m (two order of magnitude drop), respectively. According to SEM images and thermal conductivity analysis, the rod-like carbon fibers can bridge the surrounding BaTiO3 to form more inorganic passages, facilitating the passage of electric carriers in EPDM/ BaTiO3 composites [35]. However, carbon blacks with high specific surface area can partially fill the gap between BaTiO3 particles to facilitate the transfer of electrons. In addition, it was shown that only 5 vol% carbon blacks can decrease the volume resistivity of EPDM/BaTiO3 (70/30) composites from 2.23 × 1013 to 1.70 × 107 Ω m (six order of magnitude drop), which was below 1.0 × 109 Ω m and can be regarded as semi-conductive materials [36]. Thus, the 5 vol% of carbon blacks can be regarded as the percolation threshold of EPDM/BaTiO3/carbon blacks. This also testified that the high dielectric constant and loss of EPDM/BaTiO3/carbon blacks was caused by the high mobility of electric carriers in composites.
4 Conclusion In this work, carbon blacks and carbon fibers were used to improve the thermal conductivity, mechanical and dielectric properties of EPDM/BaTiO3 (70/30) composites. It
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was found that 7.5 vol% carbon blacks with high specific surface area, can cause the complex viscosity to become non-sensitive to varying shear, while rod-like carbon fibers, with low specific surface area cannot have such phenomenon. Because of the sulfuric atoms and C=C groups on surface of carbon blacks, 10 vol% carbon blacks can improve can enhance the tensile strength and tear strength of EPDM/BaTiO3 (70/30) from 9.00 MPa and 21.06 kN m−1 to 14.32 MPa (59% increase) and 30.02 kN m−1 (43% increase). By comparison, due to the combined effect of pH level and C=C groups of carbon fibers, the tensile strength firstly decreased and then increased slightly with the increase of carbon fibers. It was found that the spherical carbon blacks can fill in the gap among surrounding BaTiO3 particles and partially contact BaTiO3 particles to increase thermal conductivity of EPDM/BaTiO3 (70/30) from 0.323 to 0.632 W m−1 K−1 (95% increase). In terms of carbon fibers, the rod-like carbon fibers can bridge surrounding BaTiO3 particles to increase thermal conductivity of EPDM/BaTiO3 (70/30) from 0.323 to 0.595 W m−1 K−1 (84% increase). Due to high specific surface area, the 10 vol% carbon blacks can improve the dielectric constant from 7 to 746 (106 times increase) at 5 MHz, which was higher than rodlike carbon fibers (only 11 at 5 MHz). When the filler content was 10 vol%, carbon blacks and carbon fibers can decrease the volume resistivity of EPDM/ BaTiO3 (70/30) from 2.23 × 1013 to 6.37 × 105 Ω m (eight order of magnitude drop) and 4.25 × 1011 Ω m (two order of magnitude drop), respectively. Acknowledgements The authors gratefully acknowledge the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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