Mechanical and electrical properties of multiwall
carbon nanotube/polycarbonate composites for
electrostatic discharge and electromagnetic
interference shielding applications†
Shailaja Pande,
*
Anisha Chaudhary, Deepak Patel, Bhanu P. Singh
and Rakesh B. Mathur
Home-made multiwall carbon nanotubes (MWCNTs) were used as a reinforcing conducting filler for a
thermoplastic polymer, polycarbonate (PC) and the mechanical and electrical properties of the
composites were investigated for electrostatic discharge (ESD) and electromagnetic interference (EMI)
shielding applications. A uniformly dispersed MWCNT/PC composite system was fabricated using solvent
casting and a combination of solvent casting and compression molding techniques. The effect of
MWCNTs on the failure mechanism of the polymer under tensile loading showed a ductile to brittle
transition with increasing amount of carbon nanotubes. ESD studies showed that the composite films of
2 and 5 wt% functionalized-MWCNT/PC with respective charge decay times of 1 and 0.6 s show promise
as electrostatic dissipative materials. EMI shielding effectiveness of a five-layered system (2mm
thickness) of as-synthesized-MWCNT/PC composite films at 20 wt% loading reached 43 dB in the X-band
(8.2–12.4 GHz). The primary mechanism of shielding was absorption, suggesting possible use as an EMI
absorbing material. By using low pressure (contact pressure) compression molding the EMI shielding
properties of bulk composites (2 mm thickness) improved by about 14 dB at 10 wt% MWCNT loading.
1 Introduction
The growing demand for electrostatic discharge (ESD) protec-
tion and electromagnetic interference (EMI) shielding in the
plastics industry for the electronics sector has increased the
research on developing electrically conductive polymer
composite materials particularly using carbon-based conduct-
ing llers. Compared to conventional metal-based EMI shield-
ing materials carbon-based conductive polymer composites are
attractive due to their light weight, resistance to corrosion,
exibility and processing advantages.
1,2
Additionally, a major
advantage of using carbon nanotubes (CNTs) is that conductive
composite can be formed at low loading of CNTs due to low
percolation thresholds and with higher mechanical strength.
3
Several studies have previously discussed the EMI shielding
properties of CNT/polymer composites.
2,4–9
EMI shielding in the
range of 8.2 to 12.4 GHz (the so-called X-band) is more impor-
tant for military and commercial applications. Doppler, weather
radar, TV picture transmission, and telephone microwave relay
systems lie in X-band.
4,9
Huang et al.
4
fabricated single wall
carbon nanotube (SWCNT)/epoxy composites using long, short,
and annealed SWCNTs, with different aspect ratios and wall
integrities. Very low percolation volumes and 20–30 dB EMI
shielding effectiveness (SE) values were obtained in the X-band
for 15 wt% SWCNT loading. Liu et al.
2
obtained an EMI SE up to
17 dB in X-band for SWCNT/polyurethane (PU) composites with
20 wt% SWCNT loading. Kim et al.
5
studied the EMI shielding
properties of multiwall carbon nanotubes (MWCNT)/poly-
methylmethacrylate (PMMA) lms in the range 50 MHz to 13.5
GHz and reported up to 27 dB SE of MWCNT/PMMA composite
lms for high CNT loadings of about 40 wt%. Yang et al.
6
found
that with the addition of 1 wt% CNTs into a 10 wt% carbon
nanober/polystyrene (PS) composite, SE value of 20.3 dB was
obtained for a 1 mm thick sample. Arjmand et al.
7
studied the
EMI shielding properties of MWCNT/polycarbonate (PC)
composites and obtained EMI SE values of about 25 dB for 5 wt
% MWCNT/PC composites. However, most of these studies do
not discuss the mechanical properties of composites tested for
EMI shielding properties except our own studies on EMI
shielding properties of MWCNT/PMMA and MWCNT/PS
composites.
8,9
It is desirable that the conducting plastic should
have a proper balance of electrical and mechanical properties.
In this paper, the importance of mechanical properties has
been highlighted because typically, the behavior of composites
changes upon addition of CNTs. For a thermoplastic, such as
Physics and Engineering of Carbon, Division of Materials Physics and Engineering,
CSIR-National Physical Laboratory, New Delhi – 110012, India. E-mail:
shailajapande@yahoo.com; Fax: +91-11-45609310; Tel: +91-11-45608460
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra47387b
Cite this: RSC Adv.,2014,4, 13839
Received 6th December 2013
Accepted 28th February 2014
DOI: 10.1039/c3ra47387b
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PC, which has been used in the present studies, there is a
change from ductile to brittle failure mechanism during tensile
loading as the CNT content increases. This behavior is impor-
tant to consider if CNT/PC composites are to be used for EMI
shielding and ESD applications. There are no studies on actual
electrostatic dissipative character of CNT/PC composites.
Studies on mechanical properties of CNT/PC composites
show that while modulus mostly increases with CNT loading the
level of reinforcement with respect to tensile/exural strength
depends upon several factors such as the type (as-received/
surface-modied; randomly oriented/aligned) and amount of
CNT, the state of nanotube dispersion and polymer/nanotube
interfacial bonding.
10–14
In their studies on MWCNT/PC
composites using as-received or surface-modied MWCNTs
Eitan et al.
11
found that elastic modulus and yield strength
increased with increasing MWCNT content at 2 and 5 wt% CNT
loading levels and the dispersion and load transfer efficiency
improved by using surface-modied CNTs. Studies have
revealed that extrusion during melt-processing may result in a
considerable reduction or shortening of CNTs and supposedly
weaken the effect of CNTs as a reinforcing agent or as
conductive ller.
12
Those studies revealed that tensile strength
decreased from 60.64 MPa in neat PC to 25.55 MPa in 8 wt%
MWCNT/PC though the elastic modulus increased by 78% from
2350 MPa to 4183 MPa. In those studies the tensile strength of
PC composite with 1 wt% MWCNT was 4.5% higher than that of
pure PC but when the content of MWCNT exceeded 3 wt% the
tensile strengths of PC decreased further.
12
Studies on aligned
CNT/PC composites showed that both modulus and yield
strength of nanocomposite bers increased and ductility
decreased as MWCNT content increased.
13
Kim and Jo
15
used a
compatibilizer to improve dispersion of MWCNT in PC and
obtained superior mechanical properties of MWCNT/PC
composites. More studies may therefore help to add new
insights to develop nanotube/polymer composite systems with
effective mechanical properties and realize use of such types of
composites for structural/electrical applications.
The focus of this paper is effect of MWCNTs on the
mechanical and electrical properties of MWCNT/PC composites
for ESD and EMI shielding applications. Among the thermo-
plastic group of polymers, polycarbonates have attracted a great
deal of attention due to their combination of properties such as
high impact resistance, excellent toughness, very good dimen-
sional stability, transparency and thermal stability.
15,16
Unlike
PMMA and PS, which are commodity plastics, PC is an engi-
neering thermoplastic widely used in commercial and military/
defence sectors as shatter-proof or bullet-resistant windows,
lightweight eyeglass lenses, motorcycle windshields, windows
and vision blocks for armored vehicles and boats, transparent
armor for military, face shields, protective helmets, safety
goggles, aircra windows, ghter jets canopies, laptop casings
and screens, CDs and DVDs, glazing etc. The scope of applica-
tions of PC can be further increased by incorporating CNTs into
PC to not only enhance mechanical properties but also develop
conductivity in an otherwise non-conducting matrix for newer
applications such as ESD and EMI shielding materials A
modication in their properties to suit specic requirements is
an interesting proposition. A uniform dispersion of CNTs in the
polymer matrix is of primary importance to translate the
properties of CNTs into the polymer matrix. Different process-
ing methods have been studied to improve nanotube dispersion
in polymer matrix and enhance interfacial bonding between
nanotubes and polymer.
17–23
We used a combination of ultra-
sonication and magnetic stirring to disperse MWCNTs in PC
and fabricate a uniformly dispersed MWCNT/PC composite
system using solvent casting and a combination of solvent
casting and compression molding techniques. The change in
material properties of composite in terms of electrical and
mechanical reinforcement or sacrice upon CNT addition was
analyzed in order to have a composite for EMI shielding and
ESD applications with known and suitable material properties.
In a rst study of its kind, the electrostatic dissipative properties
of MWCNT/PC composites were investigated. Composite lms
of 2 and 5 wt% functionalized-MWCNT/PC (f-MWCNT/PC)
showed potential for ESD applications. Also, for the rst time,
low pressure (contact pressure) compression molding tech-
nique as against full pressure molding was used as a method to
obtain composites with improved EMI shielding properties. The
EMI SE of low pressure compression-molded samples was
measured and compared with the EMI SE of full pressure
compression molded samples and the corresponding shielding
mechanisms were analyzed. It has been previously reported that
EMI shielding properties of composites decrease aer
compression molding.
9,24
No studies have been reported on the
use of contact pressure molding as a method to improve EMI
shielding properties of composites though Molenberg et al.
25
have reported foamed nanotube/polymer composites for
improved EMI shielding properties.
2 Experimental
2.1 Synthesis and functionalization of MWCNTs
MWCNTs were synthesized by chemical vapour deposition
(CVD) using toluene as a carbon source and ferrocene as iron
catalyst precursor at about 750
C and atmospheric pressure.
8
As-synthesized-MWCNT (a-MWCNT) had a uniform diameter in
the range of 60–70 nm, lengths in the range of 50–100 mm and
purity about 90%.
8
a-MWCNT were reuxed with 60% (v/v) nitric
acid (HNO
3)
for 35 h to obtain acid-functionalized MWCNTs (f-
MWCNT).
16,26
The treated material was washed several times
with distilled water till washings were neutral to pH paper and
dried in an oven at 100
C for 12 h before use.
2.2 Fabrication of MWCNT/PC composites
MWCNT/PC composite lms were prepared by solvent casting
technique.
16
MWCNT were rst ultrasonically dispersed in
tetrahydrofuran (THF) for 2 h to obtain a stable suspension of
CNTs in THF. The suspensions were then mixed with solutions
of PC in THF to obtain a series of mixtures of MWCNT/PC
containing different weight percent (wt%) of MWCNT varying
from about 0.1 to 23 wt% of a-MWCNT and about 0.1 to 5 wt%
of f-MWCNT in PC. Under similar processing conditions
dispersion of >5 wt% of f-MWCNT in PC was difficult due to
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high viscosities of the mixtures resulting from increased surface
area of f-MWCNTs. The mixtures were magnetically stirred for
24 h to obtain a uniform dispersion of CNTs in PC. Thin poly-
mer composite lms were cast from this solution by pouring the
solutions into a Teon spray coated Petri dish (diameter 4
00
) and
allowing the solvent to evaporate over 3–5 days followed by
drying in oven. Blank PC was also cast by the same technique.
The composite lms (thickness about 0.25–0.3 mm) of MWCNT/
PC were removed from the Petri dish and the samples were cut
to desired size for various measurements. To prepare molded
bulk composites the solvent casted lms were broken down into
pieces and stacked in a specially fabricated die mold (60 mm
20 mm 2 mm) and compression molded at 170
C at contact
pressure (10 kg cm
2
) or full pressure of 50 kg cm
2
. The
resulting composites of MWCNT/PC had a thickness of about
1.8–2.0 mm.
2.3 Characterization
The morphologies of MWCNT/PC composite lms were investi-
gated by SEM (Leo 440S, UK). TGA studies of polymer composites
were performed on Mettler Toledo TGA/SDTA 851E thermal
analysis system in air from room temperature to 900
Cata
heating rate of 10
Cmin
1
. Electrical conductivity of the
composite lms was measured by 4-point contact method.
8
The
polymer composite lm was cut into rectangular strips of size 70
mm in length and 10 mm in width. Current was supplied using
Keithley 224 programmable current source and the voltage drop
was measured using Keithley 197 A auto ranging micro volt
DMM. EMI SE measurements of the MWCNT/PC composites
were carried out on an Agilent E8362B Vector Network Analyzer in
the frequency range of 8.2 to 12.4 GHz (X-band).
9
SE of two layers
of bulk composite and various layers of composite lm was
measured using sample specimen size of 21.32 mm 10.66 mm
to t the wave guide sample holder. John Chubb Instrument (JCI
155 v5) charge decay test unit was used for ESD studies for the
measurement of static decay time of f-MWCNT/PC samples at
room temperature.
27
The static decay time was measured by
applying a positive as well as negative high corona voltage of 5000
V on the surface of material to be tested and recording the decay
time at 10% cut-off. A fast response electrostatic eld meter
observes the voltage received on the surface of sample and
measurements were to observe how quickly the voltage falls as
the charge is dissipated from the lm. The basic arrangement for
measuring the corona charge transferred to the test sample
during corona charge decay measurements is given in ESI section
SP1 (Fig. S1†). Mechanical properties of MWCNT/PC composites
were determined using Instron machine model 4411. For the
measurement of Young's modulus and tensile strength the
composite lm samples were cut into dog bone shape using a
specially fabricated die-punch. The gauge length and width of the
test sample (ASTM D638) were 30 and 6 mm respectively. The
cross-head speed was maintained at 0.5 mm min
1
.Fourtove
samples were tested for each composite type. For the measure-
ment of exural modulus and exural strength (ASTM D790) of
the composite bars the span to depth ratio was about 25 and
cross-head speed was maintained at 0.5 mm min
1
.
3 Results and discussion
3.1 Electrical conductivity of MWCNT/PC composite lms
The electrical conductivity of MWCNT/PC composites prepared
by solvent casting is shown in Fig. 1. Conductivity increases in a
classical percolating way with increasing concentration of
MWCNT. Conductivity increased by almost 12 orders of magni-
tude with increase in MWCNT loading from 0.1 to 23 wt%. The
percolation at 2–3 wt% suggests the formation of a conductive
pathway of CNTs at this loading that transforms the insulating
PC matrix to a conductive plastic. Conductivities as high as 3 S
cm
1
were achieved with 10 wt% loading of MWCNT in PC. Other
studies have also reported similar increase in conductivity with
percolation threshold varying between 1 and 5 wt%.
12,14,28,29
3.2 Tensile properties of a-MWCNT/PC and f-MWCNT/PC
composite lms
The stress–strain curves of a-MWCNT/PC and f-MWCNT/PC
composites are shown in Fig. 2(a) and (b) respectively. As seen
from Fig. 2(a), as a-MWCNT content increases the elongation at
break decreases and there is a transition from the characteristic
ductile fracture behavior of pure PC matrix to a brittle failure
mechanism. The change from ductile to brittle behavioral mode
is also evident from gures of actual failed composite samples
of a-MWCNT/PC aer tensile tests with increasing CNT loading
as shown in Fig. 2(a) from le to right. This type of a qualitative
change in the stress–strain behavior with the composite
becoming less ductile as CNT concentration increases has also
been reported by other workers.
12,14,30
The reduced ductility of
CNT/PC composites by addition of CNTs has also been reported
for oriented melt-spun bers of CNT/PC composites
10,13
and for
CNT/PC composites studied at cryogenic temperatures.
31,32
Interestingly the 2 wt% (Fig. 2(a)) and 3 wt% samples did not
show a pure ductile or a pure brittle behavior rather a behavior
indicative of strain hardening, which suggests that 2 to 3 wt%
loading level is probably the transition zone between the two
types of behaviors i.e. ductile behavior below 2 wt% and brittle
behavior above 3 wt%. The stress–strain curves of f-MWCNT/PC
Fig. 1 Electrical conductivity of a-MWCNT/PC composites as a
function of CNT loading.
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(Fig. 2(b)) showed ductile behavior up to 2 wt% and strain
hardening at 5 wt% loading level. Fig. 2(b) also shows 5 wt%
f-MWCNT/PC composite before and aer tensile test.
Fig. 3(a) shows the general scatter of data for tensile strength
(ultimate tensile strength (UTS)) and elastic modulus of pure PC
and the composite samples of a-MWCNT/PC. The trend in
elastic modulus with CNT loading is consistent with other
studies.
11,12,14
The reinforcement effect of CNTs on elastic
modulus was evident up to 20 wt% loading level. The average
elastic modulus increased by 43% (1650 MPa) at 20 wt% and by
29% (1480 MPa) at 10 wt% loading over the neat polymer
(1150 MPa). Increase in elastic modulus with increase in CNT
loading suggests that CNTs are uniformly dispersed and inu-
ence or restrict the polymer mobility by imparting stiffness to
the polymer matrix.
The tensile strengths do not vary much with CNT loading.
Rather the average tensile strengths lie in a plateau. King et al.
also found no effect of CNT loading on tensile strength of
composites.
14
Pure PC samples and composite samples with
<5 wt% a-MWCNT generally showed a larger variation in strain
to failure in stress– strain curves compared to CNT-reinforced
PC samples at and above 5 wt% loading, and this reected in
their UTS values. The UTS values of all samples tested for 5 and
10 wt% a-MWCNT/PC composites were more consistent at
about 17 to 18 MPa suggesting that the samples are dominated
by CNT reinforcement. The UTS decreased to 15 MPa at 20 wt%.
Pure PC has extensive variation in strain to failure in general.
31
When CNT content is low there are not enough CNTs to
dominate the composite properties. As there is a gradual tran-
sition between two behavioral modes from ductile to brittle with
increasing CNT concentration there are sufficient CNTs to affect
and dominate the composite properties which is reected as a
consistency in the data scatter at MWCNT loadings $5 wt%.
The UTS values did not give much information about the role
of CNTs in the composites. Since the fracture behavior changed
from a ductile fracture to a brittle fracture with increasing CNT
content the role of CNTs at higher concentrations $5 wt% could
be understood better by comparing stress values at a given
strain for pure PC and the MWCNT/PC composites (Fig. 2(a)). At
low strains before the yield point of PC, composites exhibited
higher stress values (14 to 15 MPa for 5, 10 and 20 wt%
a-MWCNT/PC composites) compared to pure PC (11 to 12 MPa).
At strain levels of about 1.3% to 1.8% (aer the yield point of
PC) a comparison of tensile stresses showed that the compos-
ites exhibited higher tensile stresses (16 to 18 MPa for 5 and
10 wt% a-MWCNT/PC composites) compared to pure PC (about
14 to 15 MPa). Importantly, the 20 wt% composite sample also
exhibited stress values of about 15 MPa comparable to that of
Fig. 2 Tensile stress–strain curves of (a) a-MWCNT/PC composites and failed composite samples of (i) and (ii) 0.5 wt% MWCNT/PC, (iii) 5 wt%
MWCNT/PC, (iv) 20 wt% MWCNT/PC; (b) f-MWCNT/PC composites and composite sample of 5 wt% f-MWCNT/PC (i) before and (ii) after failure.
Fig. 3 Tensile strength and elastic modulus of (a) a-MWCNT/PC composites; (b) f-MWCNT/PC composites.
13842
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pure PC at similar strain levels. This comparison may be
important for applications demanding a composite part to be
stiff and not undergo yielding or strain beyond a certain level
and yet maintaining high-strength properties. A good level of
stiffness is oen desired but it comes at the expense of ductility
which was shown in case of 10 and 20 wt% a-MWCNT/PC
composites. Due to the CNT-reinforced material properties of
PC in 10 and 20 wt% a-MWCNT/PC composites in terms of
modulus and electrical conductivity and maintaining strength
comparable to the polymer these composite systems were
further studied for EMI shielding applications which have been
discussed later.
Fig. 3(b) shows the effect of f-MWCNT reinforcement on the
elastic modulus and tensile strength of the f-MWCNT/PC
composites. The data scatter for tensile strength of f-MWCNT/
PC composites was less and the dominating effect of f-MWCNT
was evident from 0.6 wt% loading level. The average elastic
modulus increased by only 5% at 2 wt% loading and was found
to decrease at 0.6 and 5 wt% loading level. The average tensile
strength increased by about 35% at 0.6 and 5 wt% loading level
and by about 12% at 2 wt% loading level over the neat polymer.
The increase in tensile strength of composites suggests that the
load transfer from the matrix to the nanotubes is improved by
using surface-modied CNTs. The main load transfer mecha-
nism in a-MWCNT/PC composites is van der Waals forces
between MWCNT and PC.
31
Acid-treatment of CNTs results in
introduction of –COOH groups on the surface of CNTs.
16,20,26
In f-MWCNT/PC composites the ller matrix interface is
strengthened due to additional hydrogen-bonding interactions
between acid-functionalized CNTs and the PC matrix.
The lower values of elastic modulus of f-MWCNT/PC
composites suggest that the shortened f-MWCNTs are less
effective than longer a-MWCNTs in restricting mobility of
polymer chains. To maximize composite strength and stiffness,
long tubes are required.
18
Since long tubes are difficult to
disperse and moreover the requirements of load transfer effi-
ciency demand that nanotube surfaces be compatible with the
host matrix, chemical modication of CNTs through surface
functionalization is desirable.
18,20,33
However, acid-functionali-
zation results in shortening the length of CNTs.
20,16
The results of tensile strength and elastic modulus show that
the role of both CNT-matrix adhesion and CNT lengths is
maximum at 0.6 wt% and 5 wt% f-MWCNT loading level and
minimum at 2 wt% f-MWCNT loading. It may be stressed that
these conclusions are based on the average results obtained
from testing the limited number of samples shown for
f-MWCNT/PC composites for each sample type. At 2 wt%
f-MWCNT content there are sufficient CNTs to reduce the
ductility but not enough to increase the strength (Fig. 2(b)).
Despite having similar values of tensile strength and elastic
modulus a study of the stress–strain curves of 0.6 and 5 wt%
f-MWCNT/PC composites showed their different mechanical
characteristics. The stress–strain curve of 5 wt% f-MWCNT/PC
(Fig. 2(b)) composite showed that aer initial slight yielding the
presence of CNTs restricted further movement of the plastic
phase (strain hardening) and had a strengthening effect by
better sustaining the applied load and resulting in a brittle
failure. The stress–strain curve of 0.6 wt% f-MWCNT/PC (Fig. S2
in ESI† section SP2) showed yielding and plastic deformation
resulting in a ductile failure (with necking and cold drawing)
and rapid increase in stress at which failure occurred. In a study
by Kim and Jo
15
the use of a small amount of compatibilizer
P3HT-g-PCL improved tensile strength, Young's modulus and
elongation at break with an optimum at 0.5 wt% loading level.
In the absence of the compatibilizer the mechanical properties
of the composites containing 0.1 to 1 wt% MWCNT decreased
compared to PC. Man et al.
30
studied tensile mechanical prop-
erties of acid-treated MWCNT-lled PC composites and found
that coincidently a tough-to-brittle transition took place when
the interface achieved the state of percolation at 2 wt% loading.
Elastic modulus increased constantly with MWCNT loading up
to 10 wt%. However they compared the yield stresses only up to
2 wt% loading level due to the tough to brittle transition
occurring at 2 wt% MWCNT loading. They found that the yield
stress increased by 18.2% in 2 wt% MWCNT/PC composites.
3.3 Dispersion and CNT/polymer matrix characteristics
Fig. 4(a–c) show SEM images of cross-section of the fracture
surfaces of 10 wt% a-MWCNT/PC molded composite (Fig. 4(a)
and (b)) and 20 wt% a-MWCNT/PC composite lm (Fig. 4(c))
and. As seen from the gures CNTs are uniformly dispersed.
The good dispersion ensures maximum surface area for CNT/
polymer interaction. A coating of PC visible on individual tubes
suggests good wetting of CNTs by PC and is also a good indi-
cator of strong interaction between CNTs and the polymer. The
network of CNTs as visible in the images ensures good
connectivity throughout the polymer matrix. These samples
generally showed a rough but uniformly dispersed surface
which may also be indicative of the brittle fracture behavior.
The f-MWCNT/PC composite lm samples had a good
surface nish compared to a-MWCNT/PC composites at the
same loading level suggesting excellent dispersion characteris-
tics. Fig. 4(d) shows the SEM image of 2 wt% f-MWCNT/PC
Fig. 4 SEM image of (a and b) cross-section of fracture surface of
10 wt% a-MWCNT/PC molded composite at low and high magnifi-
cation respectively; (c) 20 wt% a-MWCNT/PC composite film; (d) 2 wt
% f-MWCNT/PC composite film.
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