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Electrical and Mechanical Properties of Ternary Rubber Composites
for Electronic Sensors
Benjaporn Nooklay 1 1# 2
, Pitsanu Bunnaul , Kanadit Chetpattananondh , 3 3*
Pruittikorn Smithmaitrie and Wiriya Thongruang
1Department of Mining and Materials Engineering,
2Department of Electrical Engineering
3Department of Mechanical Engineering, Faculty of Engineering,
Prince of Songkla University, Koh Hong, Hat Yai, Songkhla 90112, Thailand #NANOTEC Center of Excellence at Prince of Songkla University,
Hai Yai, Songkhla 90112, Thailand *Corresponding Author Email: [email protected]
Keywords: Natural rubber, conductive rubber, nanofillers, electrical conductivity, carbon
nanotubes
Abstract
Electrical and mechanical properties of the electrical conductive composites made of natural
rubber filled with carbon black and multiwall carbon nanotubes were studied. The AC electrical
conductivity was measured in the frequency range of 0.001-1MHz. The threshold concentration
of the binary composite of carbon black filled the rubber was found at the carbon black content
of 10 phr. The nanotube was added as the third composition of binary composites for enhancing
the conductivity. It was found that both compressive strength and compression set were
increased with the increase of carbon black contents. The ternary composite containing 50 phr
carbon black and 7 phr nanotube was significantly increased with applied pressure. From the
results, it could be considered that the ternary composites can be selectively used as the
electronic sensors, at some concentration.
Introduction
Electrical conductive polymers are ubiquitous in technological applications because a
generalized processing method to mix the polymer with conductive fillers is easy and low cost
[1]. Compared with metallic conductor, conductive polymer composites have the advantages in
ease of shaping, low density, flexibility, ability to absorb mechanical shock and wide range of
electrical conductivities as well as corrosion resistance [2, 3]. In recent years, carbon black (CB)
filled conductive rubbers have widely applied mainly as an electromagnetic interference (EMI)
shielding, electrostatic charge dissipation, vapor sensors, power cable, magnetic media parts and
pressure sensors [4-6]. Carbon black particles have much greater tendency to form a conductive
network due to their chain like aggregate structures compared with other conducting additives
such as metal powder. While stress is applied to the conductive composite, the elastic polymer
matrix deforms and forces conductive particles getting closer, leading to the increase of
conductive paths. Consequently, the understanding of elastic properties of the composites is very
important in order to design pressure sensors.
Carbon nanotubes (CNTs) are interesting nanofillers because of their high electrical
conductivity, modulus of elasticity and high dispersion in the matrix [7]. For this reason, CNTs
were filled in polymer matrices and applied for various industrial applications due to their
interesting characteristics of size stability, lightweight, high electrical conductivity and
mechanical strength [8].
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In this work, multiwall carbon nanotubes (MWNTs) and CB were used to form natural
rubber (NR) composites. The objective is to develop the conductive rubber for using as force and
pressure sensors. The mechanical properties of the composites such as tensile strength,
elongation at break compressive strength, compression set and electrical properties were
examined.
Experimental
Materials
Natural rubber (NR; STR5L) was used as a polymer matrix supplied by Chalong Latex Industry
Co., Ltd (Thailand). Carbon black (Vulcan XC-72) was chosen as electrical conductive
nanofiller supplied by Cabot India Ltd. The MWNTs were used as the second nanofiller supplied
by Chengdu Organic Chemicals Co., Ltd Chinese Academy of Sciences. The physical
characteristics of the CB and the MWNTs were presented in Table I. and II. Chemicals used for
rubber vulcanization are; zinc oxide (ZnO) and stearic acid, dimercaptobenzothiazole (MBT),
tetra-methyl thiuram disulphide (TMTD) and sulphur. These chemicals materials were obtained
from Kitpyboon Limited Parnership and Polymer Innovation Co., Ltd.
Table I. Physical characteristics of carbon black (Vulcan XC-72)
Properties Carbon black
Density (g/cm3)
Iodine absorption value (mg/g)
DBP*
absorption value (ml/100 g)
Average particle diameter (nm)
0.312
273
184
29 *Dibutyl phthalate.
Table II. Physical characteristics of MWNTs
Properties Multi wall nanotubes
Purity (%)
Outside diameter (nm)
Inside diameter (nm)
Length (µm)
Special surface area, SSA (m2/g)
Bulk density (g /cm3)
Volume electric resistivity (Ω.cm)
Product method
> 95
8-15
3-5
10-50
> 233
0.07
0.11
CVD
Sample preparation
The NR and CB were mixed in a kneader internal mixer (YFM Dispersion mixers 3 L) at 70-
80°C with an estimate mixing time about 1-1.5 hours depending on the content of carbon black.
After cooling, the mixture was formulated with chemicals listed in Table III., following with the
efficient vulcanization (EV). MWNTs were added on a two-roll mill in this step. The total
mixing time used for the latter stage of mixing is 30 minutes. Vulcanization of rubber was taken
place on a compression molding machine under a pressure of 3,000 psi and at a temperature of
150°C which the time period was obtained from the Moving Die Rheometer (MDR 2000). The
vulcanized rubbers were allowed to mature at room temperature for 24 hours before testing.
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Table III. Mixture* formation
Ingredient NR,
STR5L
Zinc
oxide
Stearic
acid CB MWNTs MBT TMTD Sulphur
V10M0 100 5 3 10 0 1.5 2.5 0.5
V20M0 100 5 3 20 0 1.5 2.5 0.5
V30M0 100 5 3 30 0 1.5 2.5 0.5
V40M0 100 5 3 40 0 1.5 2.5 0.5
V50M0 100 5 3 50 0 1.5 2.5 0.5
V30M3 100 5 3 30 3 1.5 2.5 0.5
V40M3 100 5 3 40 3 1.5 2.5 0.5
V50M3 100 5 3 50 3 1.5 2.5 0.5
V30M7 100 5 3 30 7 1.5 2.5 0.5
V40M7 100 5 3 40 7 1.5 2.5 0.5
V50M7 100 5 3 50 7 1.5 2.5 0.5 *All the ingredients are in phr (weight per hundred weight of rubber).
VXXMY; Refers to carbon black and Multiwall carbon nanotubes content, respectively
Testing
Composites specimens were cut into a circle disc with a diameter of 12.7 mm and a thickness of
2 mm for the electrical measurement.
The electrical conductivity of composites was measured by placing a sample between a
couple of copper-wired electrodes connecting to the programmable automatic RCL meter (Fluke
PM-6306). The relationship between electrical and mechanical properties was studied using the
universal testing machine (Instron, 8872) (see Figure 1). The measuring frequency was varied
directly on the RCL meter. The electrical conductivity data reported here was obtained from the
average of three samples.
The mechanical properties such as tensile strength, hardness and compression set were
investigated to optimize the electro-mechanical properties of the composite, which is suitable for
pressure sensor application. Tensile strength was measured using the tensile testing machine
(5655 series) according to ISO 37 (type 1). Hardness, Compression set and compressive stress
were tested according to ASTM D2240, ASTM D395 method B and ASTM D575, respectively.
Result and discussions
Effect of CB and MWNTs loading on the electrical conductivity
The relationship of electrical conductivity of NR/CB binary composites with frequency and the
percolation threshold concentration of CB and MWNTs were studied. At this threshold
concentration, electrons can jump possibly across the gap between conductive particles and
potentially get out of the aggregate to form conductive paths [9] and therefore, the electrical
conductivity of the composites increases rapidly as shown in Figure 2 (a). However, the
electrical conductivity data of NR/MWNTs composites has no sigh of this threshold behavior.
This is might due to the MWNTs content was not enough to form conductive path at this range
of concentration as shown in Figure 2 (b). It was also observed that the variation of frequency
1041
has no effect to the electrical conductivity of NR/CB composites at CB content higher than 40
phr. At these high contents of CB the conductivity of rubber composites are controlled by
conductivity of fillers and connections between conductive particles [2, 10-11].
In addition, it was observed that the addition of MWNTs in NR/CB composites at CB
contents of 30-40 phr slightly affect the conductivity of the composites. At CB contents of 30-40
phr distance between conductive particles is large so electrons can not pass to surrounding
particles although MWNTs filled. Frequency has no effect to the electrical conductivity at high
content of CB of ternary composites as shown in Figure 3 (a).
Figure 1. Experimental set-up for measurement of electrical conductivity
-10
-9
-8
-7
-6
-5
-4
-3
-2
0 10 20 30 40 50 60
Carbon black loading (phr)
Lo
g σ
(Ω
.cm
)-1
1000kHz
100kHz
10kHz
1kHz
-10
-9
-8
-7
-6
-5
-4
-3
0 2 4 6 8 10
Carbon nanotube loading (phr)
Lo
g σ
(Ω
.cm
)-1
1000kHz
100kHz
10kHz
1kHz
(a) (b)
Figure 2. The variation of electrical conductivity of binary conductive rubber to content of
(a) carbon black (b) carbon nanotubes and frequency
1042
Effect of compressive strain on electrical conductivity
The electrical conductivity of NR/CB/MWNTs composites was increased with applied strain or
pressure as shown in Figure 3 (b). The applied pressure forces to move polymer phase, which
affects the network structure of the conductive fillers [12]. The change of electrical conductivity
with pressure can be explained by considering two phenomena; the formation of additional
conductive networks and the breakdown of existing conductive networks. The formation of this
continuous conducting path occurs not only by direct contact between conductive particles
dispersed in the matrix, but also with a few nanometers of the interparticle distance which
electrons can easily jump across the gap [12]. The breakdown of existing conductive network,
however, occurs in composites with low contents of conductive fillers due to the excessive
deformation of the matrix. Hence, by applying pressure to the composite at low filler
concentration, at the first stage, the gap is narrow leading to increases of electrical conductivity.
After that, conductive network is destroyed due to aggregates of carbon black are separated. In
contrast, composite at high filler loading shows the opposite effect to the above with applied
pressure. This is due to the gaps between conducting particle agglomerates are very small
resulting to the further increase of the conductivity [4].
The electrical conductivity of the NR composite at CB content of 50 phr and MWNTs of
7 phr increases with applied pressure. This is because long tube of MWNTs filler potentially
bridge between the carbon black aggregates. This sample also has good sensitivity with applied
pressure as shown in Figure 3 (b). Therefore sample V50M7 was chosen to study the additional
properties for pressure sensor applications.
1.0E-08
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1 10 100 1000
Frequency (Hz))
σ (
Ω.c
m)-1
V30M0 V40M0 V50M0
V30M3 V40M3 V50M3
V30M7 V40M7 V50M7
0.000
0.005
0.010
0.015
0.020
0.025
0 5 10 15 20 25
Compressive strain %
σ (
Ω.c
m)-1
V30M0 V40M0 V50M0
V30M3 V40M3 V50M3
V30M7 V40M7 V50M7
(a) (b)
Figure 3. The variation of the electrical conductivity of conductive rubbers
(a) with frequency (b) with compressive strain
1043
0
5
10
15
20
25
30
35
40
30 40 50
Carbon black loading (phr)
Ten
sile
str
en
gth
(M
Pa)
MWNT 0 phr
MWNT 3 phr
MWNT 7 phr
0
100
200
300
400
500
600
30 40 50
Carbon black loading (phr)
Elo
ngat
ion a
t bre
ak (
%)
MWNT 0 phr
MWNT 3 phr
MWNT 7 phr
(a) (b)
0
10
20
30
40
50
60
70
80
90
100
110
30 40 50
Carbon black loading (phr)
Hard
nes
s (S
ho
re A
)
MWNT 0 phr
MWNT 3 phr
MWNT 7 phr
0
1
2
3
4
5
6
7
8
0 5 10 15 20 25 30 35 40 45 50
Compressives strain (%)
Co
mp
ress
ive s
tress
(M
Pa)
V30M3 V40M3 V50M3
V30M7 V40M7 V50M7
(c) (d)
0
5
10
15
20
25
30
30 40 50
Carbon black loading (phr)
Co
mp
ress
ion
set
(%)
MWNT 0 phr
MWNT 3 phr
MWNT 7 phr
0
5
10
15
20
25
30
30 40 50
Carbon black loading (phr)
Co
mp
ress
ion
set
(%)
MWNT 0 phr
MWNT 3 phr
MWNT 7 phr
(e) (f)
Figure 4. Mechanical properties of ternary composites (a) Tensile strength
(b) Elongation at break (c) Hardness (d) Compressive stress & compressive strains
(e) Compression set at room temperature (f) Compression set aging temperature at
70°C with variation of carbon black and carbon nanotubes loading
1044
Effect of carbon black and multiwall carbon nanotube loading on mechanical properties
Mechanical properties of the composites with variation of CB and MWNTs loading as shown in
Figure 4. Tensile strength and elongation at break of rubber composites tend to slightly decrease
with the addition of MWNTs (a and b). This is due to non-uniform dispersion of the fillers in
rubber matrix resulting to the decrease of mechanical properties [13]. Hardness was increased
with increasing of the filler. This is due to characteristic properties of the filler (c) [14].
The compression set data was obtained for sensor application. These properties show the
elastic behavior of composites. The compression set was slightly increased with increasing filler
content both at 25°C and 70°C (e and f).This is due to the effect of carbon black agglomeration
collapse in composites. At 25°C, the compression set is less than 15% for all content. For aging
temperature of 70°C, the compression set is higher than at room temperature. This results from
higher collapse of CB agglomerates complied with more relaxation of the rubber molecule at
high temperature. So this conductive rubber is practically used at room temperature. Similarly to
hardness, compressive stress increases with increasing filler loading (d).
Morphological characteristics
The dispersion of carbon black and carbon nanotube in rubber matrix was characterized by using
Scanning Electron Microscopy (SEM). At CB loading of 30 and 50 phr and MWNTs of 3 and 7,
phr it was shown that CB are homogenously dispersed in the rubber matrix (see Figure 5).
However, at CB content of 50 phr (b, d and f), the micrograph shows high packing of CB
aggregates than the low CB content of 30 phr (a, c and e). However, this micrograph can not see
MWNTs due to their very small size. This dispersion phenomenon affects to electrical
conductivity of the composite.
Conclusions
1. Frequency affects the electrical conductivity of composites at low filler concentration.
2. Electrical conductivity increases significantly with increasing applied pressure at high
loading fillers. The ternary composite containing 50 phr CB and 7 phr MWNTs is
suitable for using as a sensor due to its good conductive signal.
3. Mechanical properties of ternary composites decrease with increasing of filler content but
the electrical conductivity is in the opposite way.
4. Temperature of the conductive rubber is practically used at 25°C.
Acknowledgements
The authors are pleased to acknowledge NANOTEC Center of Excellence at Prince of Songkla
University and Graduate School at Prince of Songkla University for their financial support.
1045
(a) (b)
(c) (d)
(e) (f)
Figure 5. SEM image of carbon black in natural rubber (a) V30M0 (b) V50M0
(c) V30M3 (d) V50M3 (e) V30M7 (f) V50M7
1046
References
1. N.M. Renukappa, Siddaramaiah and R.D. Sudhakar Samuel, “Styrene butadiene
rubber/aluminum powder composites-mechanical, morphological and electrical behaviors,” J
Mater Sci: Mater Electron, (2006), DOI 10.1007/s10854-006-9077-4
2. Waleed E. Mahmoud, A.M.Y. EI-Lawindy, M.H. EI Eraki and H.H. Hassan, “Butadiene
acrylonitrile rubber loaded fast extrusion furnace black as a compressive strain and pressure
sensors,” Sensors and Actuators, A 136 (2007), 229-233
3. Wei Zhang, Abbas A. Dehghani-Sanij, Richard S. and Blackburn, “Carbon based conductive
polymer composites,” J Mater Sci, 42 (2007), 3408-3418
4. Premamoy Ghosh and Amit Chakrabarti, “Conducting carbon black filled EPDM
vulcanizates: assessment of dependence of physical and mechanical properties and
conducting character on variation of filler loading,” European Polymer Journal, 36 (2000),
1043-1054
5. Y. Ishigure, S. Iijima, h. Ito, T. Ota, H. Unuma, M. Takahashi and Y. Hikichi, “Electrical and
elastic properties of conductor-polymer composites,” Journal of Materials science, 34 (1999),
2979-2985
6. A.E. Job, F.A. Oliveira, N. Alves, J.A. Giacometti and L.H.C. Mattoso, “Conductive
composites of natural rubber and carbon black for pressure sensors,” Synyhetic Metals, 135-
136 (2003), 99-100
7. A. Fakhur’l-Razi, M.A. Atieh, N. Girun., T.G. Chuah, M. El-Sadig and D.R.A. Biak, “Effect
of multi-wall carbon nanotubes on the mechanical properties of natural rubber,” Composite
Stuctures, 75 (2006), 496-500
8. C.S. Choi, S.J. Park and H.J. Choi, “Carbon nanotube/polyaniline nanocomposites and their
electrorheological characteristics under an applied electric field,” Current Applied Physics
(2006)
9. Medalia, A. I., Rubber Chemistry and Technology, New York, U.S.A. (1986) 448-440
10. Avrom, I. Medalia, “Electrical Conduction in Carbon Black Composites,” Rubber Chemistry
and Technology (1985), 432-454.
11. K.P. Sau, T.K. Chaki and D. Khastgir, “Conductive rubber composites from different blends
of ethylerne-propylene-diene rubber and nitrile rubber”, Jornal of materials science 32 (1997)
5717-5724
12. N.C. Das, T.K. Chaki and D. Khastgir, “Effect of Processing Parameters, applied pressure
and temperature on the electrical resistivity of rubber-based conductive composites,” Carbon,
40 (2002), 807 – 816
13. Patcharaphun, S.1, Pinsanor, V.1, Junpoonsup, S.1, and Sombatsompop, N.2., “Effects of
silica, calcium carbonate, and SiO2/CaCO3 blends on some properties of cellular NR
compounds,” Songklanakarin J. Sci. Technol., 25(1)(2003), 75-90Yue, D., Liu, Y., and Shen,
Z., “Study on preparation and properties of carbon nanotubes/rubber composites,” Journal
Mater Science, 41 (2006), 2441-2544
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