IJNT
INTERNATIONAL JOURNAL OF NANOMATERIALS AND TECHNOLOGY
Volume 2, Number 1, January-June 2011, pp. 11-15, ISSN: 0976-6138
International Science Press
MWCNT/EPOXY AND CB/EPOXY NANOCOMPOSITES:
PROCESSING USING TWIN SCREW EXTRUSION AND
CHARACTERISATION FOR ELECTRICAL CONDUCTIVITY
Jeena J.K., H.N. Narasimha Murthy*, M. Krishna, Sreejith M. 1 and K.S. Rai2
1
Department of Mechanical Engineering R.V. College of Engineering, Bangalore-560059, Karnataka, India.
2
Department of Polymer Science, Mysore University, Karnataka, India.
Abstract: Research efforts are concentrated on improving the electrical conductivity of polymers and one potential
method is to disperse nanofillers such as multi-walled carbon nanotubes (MWCNT), carbon nanopowder (CB) into
them. This paper is aimed at processing of the nanocomposites based on a space grade Epoxy (AV138M, structural
adhesive) and two nanofillers namely MWCNT and CB using co-rotating twin screw extruder. Specimens with up to
31 vol % MWCNT and 45 vol % CB were fabricated and were characterised for volume resistivity and surface resistivity
as per ASTM D257 using Keithley Model 6517. Dispersion of CB and MWCNT resulted in increase in electrical
conductivity of the neat resin. The volume conductivity increased by 1.0 E-10 S/cm in both the cases of 31 vol %
MWCNT and 45 vol % CB loading. With the achieved values of volume conductivity the structural adhesive transitioned
from totally insulative (E-14 S/cm) to dissipative range (E-4 S/cm). SEM and AFM were used to examine the dispersion
of the fillers. The effect of nanofillers on the glass transition temperature was studied using DSC. Tg increased by 10o
in case of MWCNT (31 vol %) and 4oC in CB (45 vol %) loading into epoxy.
Keywords: Nanocomposites, Electrical Conductivity, Theoretical prediction, SEM, AFM.
1. INTRODUCTION
Epoxy based polymers are widely used in applications
ranging from microelectronics to aerospace. 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 nano-reinforcements.
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 force, 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.
In twin screw extrusion, the high shear stresses break
up the large agglomerates and disperse CNT throughout
the matrix and extensional flow prior to solidification
serves to further untangle the nanofillers and align them
*
Corresponding Author: hnmdatta@yahoo.com
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) varies widely, ranging from less
than 1 to over 14% [1, 3].
In the present work co-rotating twin screw extruder
has been used to obtain high shear mixing necessary to
disentangle the nanofillers and disperse them in a high
viscous epoxy matrix. The main objective of this work is
to process MWCNT/Epoxy and CB/Epoxy nanocomposites using twin screw extruder and characterize them
for electrical conductivity, glass transition temperature
and study the dispersion of nanofillers using SEM and
AFM.
2. EXPERIMENTAL
2.1 Materials
The epoxy matrix used in this study consists of modified
Bisphenol-A based two component epoxy [Structural
Adhesive-Araldite AV138M with modified Polyamine
Hardener HV998, (Huntsman, Hindustan Ciba- Geigy
Ltd.)]. The details of the nanofillers used in the
experiments are provided in Table 1.
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International Journal of Nanomaterials and Technology
Table 1
Specification of the Nanofillers
Material
Description
Properties
Suppliers (M/s)
MWCNT CVD Grown Length: 1-3 µm
Nanovatec NanoGrade: AP60- Diameter: 60-90 nm technology Division
80/90
Purity > 90%
of Chemapol Industries, Mumbai
CB
ISAF N 220
24 to 33 nm with
> 95% Purity
Philips Carbon
Black Ltd, Kolkata
2.2 Nanocomposite Preparation
For dispersing the nanofillers in high viscous epoxy
matrix, a co-rotating twin-screw extruder (OMEGA 20,
M/s STEER Engineering, Bangalore). The mixture of
nanotube and the prepolymer was fed in to 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 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/AV138M composites.
Temperature Range: 25°C to 700°C). 5 mg sample was
sealed in hermetic Aluminium 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.
3. RESULTS AND DISCUSSION
3.1 Electrical Conductivity Results
In Fig. 1 and 2, the volume and surface conductivity
results are plotted as a function of the filler volume
fraction. The volume conductivity increased by 1.0 E-10
S/cm in both the cases of 31 vol % MWCNT and 45 vol
% CB loading. The electrical properties of the nanocomposites depends on the size and shape of the filler, matrix
properties, preparation method, filler properties,
dispersion of the filler within matrix and interaction
between compounds, etc.
2.3 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
formulae:
22.9
R
σv =
(1)
t
ρs = 53.4 R
where σ v and ρ s 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
above equations.
Fig. 1: Variation of Electrical Conductivity with MWCNT
Volume Fraction
2.4 Morphological Characterization
SEM (JEOL JSM 840A, Japan) and Atomic Force
Microscope (Nanosurf Easy Scan, Sinsil, Switz) were
used to study the dispersion of the nanofillers into epoxy.
2.5 Differential Scanning Calorimetry
Glass transition temperature, Tg of the specimens were
obtained using DSC (Model-Mettler DSC-823,
Fig. 2: Variation of Electrical Resistivity with
CB Volume Fraction
In order to achieve electrical conductivity similar to
MWCNT/epoxy composite, greater amount of CB
(45vol %) was needed due to their spherical shape which
is more difficult to make interactions between particles
13
MWCNT/Epoxy and CB/Epoxy Nanocomposites: Processing using Twin Screw Extrusion ...
and particles. A high aspect ratio (330) and high
conductivity [10 4Ω–1cm–1(S/cm)] of MWCNTs enable
them to make 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.
In order to determine the critical volume
concentration or percolation threshold, the volume
conductivity data was fitted to power law in terms of
volume fraction of nanofillers. The power law is
described by the equation below:
σpc = A(v – vc)t
(3)
Where σpc the conductivity of the composite, v is the
volume fraction of nanofiller in the composite, vc 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].
Table 2
Power Law Equation Parameters
Parameter
MWCNT/AV138M
CB/AV138M
vc
0.025
0.075
t
2.71
2.49
A
2.88 × 10–3
3.10 × 10–4
3.2 Morphological Characterization
3.2.1 Scanning Electron Microscopy (SEM)
The SEM of the nanocomposite specimens are presented
in Fig. 3. The geometric parameters of the MWCNT used
in the study (length: 1 to 3 µ, dia: 60 to 90 nm) are
comparable with that shown by the scanning electron
micrographs Fig. 3(a). The formation of filler network of
MWCNT in epoxy are observed in Fig. 3(b) (12vol %
MWCNT) and 3(c) (31vol % MWCNT). The same is more
evident in Fig. 3(c) which is substantiated by the increase
in conductivity values corresponding to the extent of
filler loading. Fig. 3(d) shows the dispersion of CB in
epoxy. The presence of CB agglomerates is observed in
the micrographs corresponding to 45 vol % of the filler,
which is the reason for higher levels of loading of CB
compared to MWCNT for achieving the required levels
of conductivity.
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 fundamental
role for 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 σ versus log (ν – νc) is shown as
inset of Fig. 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, 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].
Fig. 3: SEM of MWCNT/CB/epoxy Composites. (a) MWCNT,
(b) 12vol% MWCNT/epoxy, (c) 31vol% MWCNT/Epoxy, (d)
45vol% CB/epoxy
3.2.2 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 4(b) shows greater proportion of brighter
spots indicating greater amount of MWCNT (31vol %)
compared to 12vol % in Fig. 4(a). Greater levels of
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International Journal of Nanomaterials and Technology
connectivity are observed in Fig. 4(c) and (d) which
corresponds to 45vol% of CB.
Fig. 4: AFM of MWCNT/CB/epoxy Composites. (a) 12vol%
MWCNT/epoxy, (b) 31vol% MWCNT/epoxy, (c) 12vol%
CB/epoxy, (d) 45vol% CB/epoxy
3.3 DSC
Differential Scanning Calorimeter was used to determine
the Tg, using the measurement of heat flow verses
change in temperature. Figure 5 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 100 increase was
achieved with 31vol % loading. Similar behaviour was
also observed in CB/epoxy composites. But, the increase
in Tg was very low with respect to CB loading i.e. only
40 for loading of 45 vol %. 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 case of
MWCNT is attributed to its greater thermal stability and
superior thermal properties compared to that of CB.
Fig. 5: Glass Transition Temperature (Tg) Verses the
Nanofiller Content
4. CONCLUSIONS
Co-rotating twin screw extruder as a process for
dispersing nanofillers such as MWCNT and CB into
thermosets was demonstrated. The process parameters
can be standardized based on the matrix and the filler
systems. Dispersion of the nanofillers resulted in increase
in the volume conductivity and the surface conductivity
of epoxy. The volume conductivity increased by 1.0 E10 S/cm in both the cases of 31 vol % MWCNT and 45
vol % CB loading. The increase in surface conductivity
was 1.0 E -9 S for the same levels of the loading of the
fillers. The increase in conductivity is due to the
interfacial interaction between the carbon surface and
the polymer. The measured conductivity was described
by a percolation-like power law equation with a
percolation threshold of 0.025 and 0.075 volume fractions
for MWCNT and CB respectively. DSC measurements
showed increase in Tg of the nanocomposites. Predispersion of the nanofillers in suitable solvents using
ultrasonication prior to screw extrusion will be explored
for more effective dispersion and to achieve the resulting
properties.
ACKNOWLEDGEMENTS
The authors are grateful for the financial support
provided by the Indian Space Research Organisation
(ISRO), Bangalore (Sanction No. 10/3/547 dated 10-102005, Sanction Order No 9/2/125/2005 II dated 08-032006, Principal Investigator: Dr. H N Narasimha
Murthy).
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List of Abbreviations and Symbols
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CB
:- Carbon Black
sν
:- Volume Resistivity
ρs
:- Surface Resistivity
σpc
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:- Volume Conductivity
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νc
:- Critical volume fraction
Tg
:- Glass Transition Temperature
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of
polymer