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: twiriya@me.psu.ac.th

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

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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

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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

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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

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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.

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(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

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