Polymer-Plastics Technology and Engineering, 49: 1207–1213, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0360-2559 print=1525-6111 online
DOI: 10.1080/03602559.2010.496413
The Processing and Characterization of MWCNT/Epoxy and
CB/Epoxy Nanocomposites Using Twin Screw Extrusion
Jeena Jose Karippal1, H. N. Narasimha Murthy1, K. S. Rai2, M. Krishna1, and
M. Sreejith1
1
Department of Mechanical Engineering, R V College of Engineering, Bangalore, Karnataka, India
Department of Polymer Science, Mysore University, Karnataka, India
2
This paper presents results of the processing of nanocomposites
based on epoxy and nanofillers, namely multiwalled carbon
nanotubes (up to 10 wt%) and carbon black (up to 15 wt%). The
twin screw extruded nanocomposites showed increases in electrical
and thermal conductivities, tensile strength, microhardness and glass
transition temperature. Electrical conductivity increased on the
order of 1011 at 10 wt% of nanotubes loading and at 15 wt% of
carbon black. Greater increases in thermal and mechanical properties were observed in cases of nanotube-dispersed composites more
so than others. SEM and AFM were used to examine the dispersion
of the fillers.
Keywords AFM; Conductivity; Mechanical properties; SEM;
Twin-screw extrusion
INTRODUCTION
Epoxy-based polymers are widely used in applications
ranging from microelectronics to the aerospace industry.
Their insulating nature can cause accumulation of electrostatic charge on their surface, causing local heating and
premature degradation to the electronic components or
space structures. They can be made electrically and
thermally conducting by the addition of various types of
nanoreinforcements.
In recent years, researchers have paid much attention on
exploiting the unique properties of MWCNT and CB[1–5].
The main issue in preparing nanocomposites is the uniform
dispersion of fillers in the polymer matrix. Because of the
high aspect ratio and van der Waals attractive forces,
MWCNTs form bundles or agglomerates. To overcome
the difficulty of dispersion, mechanical=physical methods
such as ultrasonication, high shear mixing, surfactant
addition, etc. have been used. However, successful
dispersion remains elusive.
Address correspondence to H. N. Narasimha Murthy,
Department of Mechanical Engineering, R V College of Engineering, Bangalore – 560059, Karnataka, India. E-mail: hnmdatta@
yahoo.com
In twin screw extrusion, which is a dispersion method,
the high shear stresses break up the large agglomerates
and disperse CNT throughout the matrix and extensional
flow prior to solidification, serving to further untangle
the nanofillers and align them in the direction of extension.
CNTs are loaded up to 20 wt% in polymers using screw
extruder[6,7]. Studies show that the levels of nanofiller loading required to achieve percolation threshold (appreciable
increase in electric conductivity) vary widely, ranging from
less than 1 to over 14%[1,3].
In the present work, a co-rotating twin screw extruder
was used to obtain the high shear mixing necessary to disentangle nanofillers and disperse them in a high-viscosity
epoxy matrix. The main objective of this research was to
process MWCNT=epoxy and CB=epoxy nanocomposites
using a twin screw extruder and to characterize their
electrical and thermal conductivities, mechanical properties, and glass transition temperature, and to study the
dispersion of nanofillers using SEM and AFM.
EXPERIMENTAL
Materials
The epoxy matrix used in this study consists of modified
Bisphenol-A-based, two-component epoxy [Structural
Adhesive-Araldite AV138 M with modified Polyamine
Hardener HV998, (Huntsman, Hindustan Ciba-Geigy
Ltd.)]. The details of the nanofillers used in the experiments
are provided in Table 1.
Nanocomposite Preparation
For dispersing the nanofillers in a high viscous epoxy
matrix, a co-rotating twin-screw extruder (OMEGA 20,
M=s STEER Engineering, Bangalore) was used. The mixture of nanotube and the prepolymer was fed into the
extruder. A detailed process study was conducted to arrive
at the levels of the process parameters (Speed: 150 to
200 rpm, temperature: 60 to 75 C, residence time 2 to
3 min) for extrusion. The speed was matched with that
required to process materials such as grease, which is as
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J. J. KARIPPEL ET AL.
TABLE 1
Details of nanofillers used
Material
Description
Properties
Suppliers (M=s)
MWCNT
CVD Grown Grade:
AP60–80=90
Nanovatec Nanotechnology Division
of Chemapol Industries, Mumbai
CB
ISAF N 220
Length: 1–3 mm
Ave Diameter: 45 nm
Purity > 90%
24 to 33 nm with > 95% Purity
viscous as the epoxy used in this study (50,000 to 100,000
mPaS). Mixing was done in two passes to ensure good dispersion. The MWCNT=prepolymer melt was collected
from the extruder; the curing agent (Hardener: HV998)
was added and was allowed to cure under room temperature for one week. The same procedure was followed for
preparing CB=AV138 M composites.
Electrical Resistivity Measurement
Volume and surface resistance of the samples were measured according to ASTM D257 using Keithley 6517A
model 8009 Resistivity Test Fixture. The specimens were
copper plated for better contact after subjecting to
humidity conditioning at 95%RH and 37 C.
The corresponding resistivity is calculated using the
formulas:
22:9
R
t
ð1Þ
qs ¼ 53:4 R
ð2Þ
rv ¼
where rv and qs are the volume and surface resistivities,
respectively, R is the corresponding resistance in ohms,
22.9 and 53.4 are constants for the apparatus. The volume
and surface conductivities are calculated from the
resistivity values obtained from the preceding equations.
Thermal Properties
Thermal conductivity was measured using Thermal
Conductivity Instrument (TCI) – 2022 SX211 as per
ASTM E 1530 under 105 torr vacuum environment with
measurement accuracy of 3%. The 50 mm dia and
10 mm thick specimens were used for this study. To ensure
good contact between the test samples and the flex meter,
the surface finish of the sample was improved by working
with a fine emery paper. The experimental results of the
thermal conductivity in the temperature range 50 C to
150 C are presented in Figure 3.
Morphological Characterization
SEM (JEOL JSM 840A, Japan) and Atomic Force
Microscope (Nanosurf Easy Scan, Sinsil, Switzerland) were
used to study the dispersion of the nanofillers into epoxy.
Philips Carbon Black Ltd., Kolkota
Differential Scanning Calorimetry
Glass transition temperatures, Tg, of the specimens were
obtained using DSC (Model Mettler DSC-823, Temperature Range: 25 C to 500 C). A 5 mg sample was sealed hermetically in an aluminum crucible. To obtain the curing
heat flow pattern of the composite, a dynamic scanning
experiment was conducted from room temperature to
150 C at 20 C per minute.
Mechanical Properties
UTS. A tensile test was performed on the specimens as
per ASTM D 3039 at a strain rate of 5 mm per minute
using a universal testing machine (M=s Kalpak, Pune).
The specimen dimensions were 208 mm 12.7 mm 3 mm.
Hardness. Hardness of the specimens was measured
as per ASTM E 384 using micro-hardness tester (M=s
Metatech, Pune). The specimens were polished with
sandpaper for getting smooth surfaces for indentation.
The indenter used for the test was a 136 square-based
Vicker’s diamond pyramid. Ten measurements at different spots were averaged for each sample. A force of
100 gm was applied for 15 seconds for getting the indentation. The unit and magnitude of the hardness are
defined by Vicker’s Hardness, Hv and determined by
the following equation[15]:
Hv ¼
1:854L
d2
where L is the load applied in grams and d is the diagonal length of diamond impression of the indentation
in mm.
RESULTS AND DISCUSSION
Electrical Conductivity Results
In Figures 1 and 2, the volume and surface conductivity
results are plotted as a function of the filler concentration.
The volume conductivity increased by 1.0 E-11 S=cm
in both the cases of 10 wt% MWCNT and 15 wt% CB
loading. The electrical properties of the nanocomposites
depends on the size and shape of the filler, matrix properties, preparation method, filler properties, dispersion
PROCESSING OF EPOXY NANOCOMPOSITES
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FIG. 1. Variation of electrical conductivity of nanocomposite with
MWCNT weight fraction.
of the filler within matrix and interaction between compounds, etc.
To achieve electrical conductivity similar to MWCNT=
epoxy composite, greater amount of CB (15 wt%) was
needed due to their spherical shape which is more difficult
to make interactions between particles and particles. A
high aspect ratio (100) and high conductivity [104S=cm]
of MWCNTs enable them to make a better reinforcing
effect in epoxy compared to CB.
Systems composed of an insulating material and a
conductive filler experience an insulator-conductor transition at the electrical percolation threshold. The electrical
percolation threshold is the minimal volume fraction of fillers so that a continuing conductive network exists in the
composite. Above this volume fraction, the electrical conductivity of the composite is relatively high. Below the electrical percolation threshold, the compound essentially
behaves as an insulator.
FIG. 2. Variation of electrical conductivity of nanocomposite with CB
weight fraction.
FIG. 3. Thermal conductivity of the composites with different weight %
of MWNT and CB at different temperatures.
To determine the critical volume concentration or
percolation threshold, the volume conductivity data was
fitted to the power law in terms of the volume fraction of
nanofillers. The power law is described by the following
equation:
rpc ¼ Aðn n c Þt
ð3Þ
where rpc the conductivity of the composite, n is the volume fraction of nanofiller in the composite, n c is the critical
volume fraction (volume fraction at percolation) A and t
are fitted constants. Theoretical predictions of the critical
exponent, t ranges from 1.5 to 2.0, while experimental
values between 1.3 and 3.1 have been reported[1,10–12].
The value of percolation threshold is very sensitive to
the polymer type and aspect ratio of the reinforcement.
In nanocomposites the interfacial area which is inversely
proportional to the filler diameter will play a fundamental
role in electrical contact and also for mechanical properties
through load transfer between the filler and the matrix[13].
The volume conductivity data of MWCNT=epoxy and
CB=epoxy has been analyzed using the power law model.
The plot of log r versus log (n–n c) is shown as inset of
Figures 1 and 2. Theoretically the value of A should
approach the conductivity of the nanofillers. The experimental values of A (Table 2) were found to be lower than
the expected values for both MWCNT and CB composites.
This may be due to lack of physical contact between the
nanofillers caused by the contact resistance between the
adjacent nanofillers, which decreases the effective conductivity of the nanofillers. Moreover, in these composites,
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J. J. KARIPPEL ET AL.
TABLE 2
Power law equation parameters
Parameter
nc
t
A
MWCNT=AV138 M
CB=AV138 M
0.025
2.71
2.88 103
0.075
2.49
3.10 104
conducting fillers are separated by the insulating polymers
that act as a potential barrier, so that it is likely that
the electrical conductivity is limited by hopping and=or
tunneling of the charge carriers between conductive
fillers[14].
Thermal Conductivity Results
Both MWNT=epoxy and CB=Epoxy specimens showed
increase in thermal conductivity (K) with increase in temperature in the 50 to 150 C and the filler loading. In
the measured temperature range, the K values of the samples increased linearly. Between 50 and þ50 C the
increase is linear and stable. Beyond þ50 C, a sudden
increase in the measured value of K was observed, which
may be due to some thermal activity of the samples tested.
The maximum increase in K for MWNT=epoxy was 60% at
150 C in case of 7 wt% MWNT nanocomposites compared
to that of neat resin. The corresponding value for CB=
epoxy was 25%.
FIG. 4.
Morphological Characterization
Scanning Electron Microscopy (SEM). The SEM of
the nanocomposite specimens are presented in Figure 4.
The geometric parameters of the MWCNT used in the
study (length: 1 to 3 m, average dia: 45 nm) are comparable
with that shown by the scanning electron micrographs
Figure 4a. The formation of filler network of MWCNT
in epoxy is observed in Figure 4b (5 wt% MWCNT) and
4(c) (10 wt% MWCNT). The same is more evident in
Figure 4c, which is substantiated by the increase in conductivity values corresponding to the extent of filler loading.
Figure 4d shows the dispersion of CB in epoxy. The presence of CB agglomerates is observed in the micrographs
corresponding to 15wt% of the filler, which is the reason
for higher levels of loading of CB compared to MWCNT
for achieving the required levels of conductivity.
Atomic Force Microscopy (AFM). The compositional
mapping with AFM is often used for observation of multiple phases. The phase contrast is related to the inherent
properties of the two phases. The brighter areas in the
image can be attributed to the greater force experienced
by the cantilever tip when in contact with the filler (tubular
or spherical). The matrix is in amorphous state and hence
appears dark in the images. Figure 5b shows greater
proportion of brighter spots indicating greater amount
of MWCNT (10 wt%) compared to 5wt% in Figure 5a.
Greater levels of connectivity are observed in Figures 5c
and 5d, which corresponds to 15wt % of CB.
SEM of MWCNT=CB=epoxy composites (a) MWCNT, (b) 5 wt% MWCNT=epoxy, (c) 10 wt% MWCNT=epoxy, and (d) 15 wt% CB=epoxy.
PROCESSING OF EPOXY NANOCOMPOSITES
1211
FIG. 5. AFM of MWCNT=CB=epoxy composites (a) 5 wt% MWCNT=epoxy, (b) 10 wt% MWCNT=epoxy, (c) 5 wt% CB=epoxy, and (d) 15 wt%
CB=epoxy.
DSC
A differential scanning calorimeter was used to determine the Tg, using the measurement of heat flow verses
change in temperature. Figure 6 shows the variation of
glass transition temperature with increase in nanofiller
content in the composite. The Tg increased with increase
in MWCNT, and a maximum of 10 increase was achieved
with 10 wt% loading. Similar behavior was also observed in
CB=epoxy composites. But, the increase in Tg was very low
with respect to CB loading, i.e., only 4 for loading of
15 wt%. The addition of nanofillers enhanced the thermostability of the composites, which may be due to the
reduction in the mobility of the epoxy polymer chains
around the nanofillers by strong interfacial interactions.
Greater increase in Tg, in the case of MWCNT, is attributed to its greater thermal stability and superior thermal
properties compared to that of CB.
Mechanical Property Results
UTS Results. Tensile tests were performed to evaluate
the reinforcing effect of nanofillers on the mechanical
properties of the nanocomposites. Figure 7 shows the tensile strengths of the nanocomposites with increase in nanofiller content. The results reveal that the nanofillers
modified the properties of epoxy resin. MWCNT=epoxy
nanocomposites exhibited much better performances than
CB=epoxy. The MWCNT=epoxy nanocomposites showed
a 46% increase in tensile strength compared to 12% in the
case of CB=epoxy nanocomposites at 10 wt% filler loading.
At 15 wt% of CB loading, the CB=epoxy specimens showed
15% increase in tensile strength.
FIG. 6. Variation of glass transition temperature (Tg) with the nanofiller
content.
Hardness Results. In Figure 8, the hardness of the nanocomposites with increase in nanofiller weight percentages is
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J. J. KARIPPEL ET AL.
FIG. 7. Tensile strengths of the composite with increase in nanofiller
content.
increase in the volume conductivity and the surface conductivity of the epoxy. The volume conductivity increased
by 1.0 E-11 S=cm at the loading levels of 10 wt% MWCNT
and 15 wt% CB loading. The increase in surface conductivity was 1.0 E-9 S for the same levels of filler loading.
The increase in conductivity is due to the interfacial
interaction between the carbon surface and the polymer.
The measured conductivity was described by a percolationlike power law equation with a percolation threshold of
0.025 and 0.075 volume fractions for MWCNT and CB,
respectively. Dispersion of nanofillers resulted in increase
in thermal conductivity. DSC measurements showed
increases in Tg of the nanocomposites. Greater improvements in thermal conductivity, UTS and hardness were
observed in MWCNT=epoxy specimens than that of the
CB=epoxy nanocomposites.
ACKNOWLEDGMENT
The authors are grateful for the financial support provided by the Indian Space Research Organisation (ISRO),
Bangalore (Sanction No. 10=3=547 dated 10-10-2005,
Sanction Order No 9=2=125=2005 II dated 08-03-2006,
Principal Investigator: Dr. H. N. Narasimha Murthy).
REFERENCES
FIG. 8. Vicker’s Hardness of the composite with increase in nanofiller
content.
plotted. It can be seen that the addition of nanofillers into
epoxy increased the hardness for both CB=epoxy and
MCWNT=epoxy nanocomposites. The continuous increase
of the nanotube content resulted in increasing the number
of high-strength reinforcements inside the composites, thus
increasing their hardness property. Specimens with MWCNT
showed 36% increase in hardness compared to 20% with CB
at 10 wt% loading. This may be related to the difference in the
properties of fillers. The high aspect ratio and the network
formation of MWCNTs in the nanocomposites resulted in
better interfacial bonding that, thus, causes greater enhancement in hardness compared to CB=epoxy composites.
CONCLUSIONS
A co-rotating twin screw extruder was successfully
adopted for dispersing nanofillers such as MWCNT and
CB into epoxy. Dispersion of the nanofillers resulted in
1. Ounaies, Z.; Park, C.; Wise, K.E.; Siochi, E.J.; Harrison, J.S. Electrical properties of single wall carbon nanotube reinforced polyimide
composites. Comp. Sci. Tech. 2003, 63, 1637–1646.
2. Gojny, F.H.; Malte, H.G.W.; Fiedler, B.; Ian, A.K.; Bauhofer, W.;
Windle, A.; Schulte, K. Evaluation and identification of electrical
and thermal conduction mechanisms in carbon nanotube=epoxy
composites. Polymer 2006, 47 (6), 2036–2045.
3. Novak, I.; Krupa, C. Analysis of correlation between percolation
concentration and elongation at break in filled electroconductive
epoxy-based adhesives. Euro. Poly. J. 2003, 39 (3), 585–592.
4. Yang, K.; Gu, M.; Guo, Y.; Pan, X.; Mu, G. Effects of carbon
nanotube functionalization on the mechanical and thermal properties
of epoxy composites. Carbon 2009, 47, 1723–1737.
5. Yeh, M.K.; Hsieh, T.-H.; Tai, N.-H. Fabrication and mechanical
properties of multi-walled carbon nanotubes=epoxy nanocomposites.
Mat. Sci. Eng. A 2008, 483–484, 289–292.
6. Nahass, P.R.; Friend, S.O.; Hausslein, R.W. Conductive silicone and
methods for preparing same. U.S. Patent 5591382, 1994.
7. Thostenson, E.T. Conductive thermosets by extrusion. U.S. Patent
20070176319, 2007.
8. Lau, K.-T.; Lu, M.; Liao, K. Improved mechanical properties of
coiled CNTs reinforced epoxy nanocomposites. Comp. Pt. A: Appl.
Sci. Manuf. 2006, 37 (10), 1837–1840.
9. Song, Y.S.; Youn, J.R. Influence of dispersion states of CNTs on physical
properties of epoxy nanocomposites. Carbon 2005, 43 (7), 1378–1385.
10. Yu-Hsuan, L.; Zhiyong, L.; Philippe, G. Investigation of the dispersion process of SWNT=SC-15 epoxy resin nanocomposites. Mater.
Sci. Eng. A 2004, 385.
11. Thostenson, E.T.; Chou, T.-W. Aligned multi-walled carbon
nanotube-reinforced composites: Processing and mechanical characterization. J. Phys. D: Appl. Phys. 2002, 35 (16), L77–L80.
12. Weber, M.; Kamal, M.R. Estimation of the volume resistivity of electrically conductive composites. Poly. Comp. 1997, 18 (6), 711–725.
PROCESSING OF EPOXY NANOCOMPOSITES
13. Srivastava, N.K.; Mehra, R.M. Study of Electrical Properties of
Polystyrene=Foliated Graphite Composite. Dept of Electronic Science,
University of Delhi South Campus: New Delhi, India.
14. Battacharya, S.; Sachdev, V.K.; Tandon, R.P. Electrical properties of
graphite filled polymer composites. 2nd National Conference
1213
Mathematical Techniques: Emerging Paradigms for Electronics and
IT Industries, September 26–28, 2008.
15. Lau, K.-T.; Shi, S.-Q.; Zhou, L.-M.; Cheng, H.-M. Micro-hardness
and flexural properties of randomly-oriented carbon nanotube
composites. J. Comp. Mater. 2003, 37 (4), 365–376.
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