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Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.1, pp 25-36, 2009
jmmce.org Printed in the USA. All rights reserved
Role of Temperature and Carbon Nanotube Reinforcement on Epoxy
Subhranshu Sekhar Samal
Centre for Nanoscience & Nanotechnology
(A joint initiative of IGCAR, Kalpakkam & Sathyabama University, Chennai)
E-mail: firstname.lastname@example.org, Cell: +91-9962369727
This paper presents the synthesis of epoxy based Multiwalled Carbon nanotubes (MWCNTs)
reinforced composites by method of sonication. The variation in the nature of reinforcement
(Aligned & Randomly oriented MWNTS) has resulted in the improvement of mechanical
properties like flexural modulus, tensile strength and hardness. A small change in chemical
treatment of the nanotubes has a great effect in the mechanical and morphological properties
of nanocomposites due to effective load transfer mechanism and state of dispersion. The
change in properties has been verified by optical microscopy and Scanning electron
microscopy. Apart from that the prepared composites has been treated under different
temperatures (like hot water, room temperature and liquid nitrogen temperature) and the
change in mechanical as well as morphological nature has been verified by SEM of
Fractographic surface this proved the elasticity and ductility of the composites.
Key Words: MWNT/Reinforcement/Dispersion//Load transfer/SEM
The discoveries of carbon nanotubes (CNT) have initiated researches in many different areas;
one of the most intriguing applications of CNT is the polymer/CNT nanocomposites [1-5].
The high mechanical, electrical and thermal property of CNTs make them ideal candidate as
fillers in lightweight polymer composite . Due to their high specific strength and stiffness,
nanotube-reinforced polymer composites have become attractive structural materials not only
in the weight-sensitive aerospace industry, but also in the marine, armor, automobile, railway,
civil engineering structures, and sporting goods industries. Epoxy resin is the polymer matrix
used most often with reinforcing nanotubes for advanced composite applications. The resins
26 S. Sekhar Samal Vol.8, No.1
of this class have good stiffness, specific strength, dimensional stability, and chemical
resistance, and show considerable adhesion to the embedded fiber . Because micro-scale
fillers have successfully been synthesized with epoxy resin [8–11] nanoparticles, nanotubes,
and nanofibers are now being tested as filler material to produce high performance composite
structures with enhanced properties [12–14]. Scanning Electron Micrographs shows (Fig.1)
shows the Nanotube structures with diameter.
Multiwall carbon nanotubes (MWCNTs) used for the preparation of nanocomposites was
obtained from MER Corporation, USA. They are produced by arc plasma method (purity
95%, length 10-50 µm and diameters 20-70 nm). SEM morphology of the products (Fig. 1)
was carried out with a "JEOL JSM-6480 LV Scanning Microscope".
Aligned carbon nanotubes (ACNTs) used for the preparation of nanocomposites was obtained
from ARCI, Hyderabad, India. They are produced by chemical vapor deposition method
diameters 10-20 nm. SEM morphology of the products (Fig.1 c) was carried out with a
“JEOL JSM 5410 Scanning Microscope".
Figure (1a, 1b & 1c): Scanning Electron Micrographs of Multiwall carbon nanotubes
(MWCNTs) showing diameter
Epoxy polymer matrix was prepared by mixing epoxy resin (Ciba-Geigy, araldite LY-556
based on Bisphenol A) and hardener HY-951 (aliphatic primary amine) in wt. ratio 100/12.
Vol.8, No.1 Role of Temperature and Carbon Nanotube Reinforcement 27
Epoxy resin (5.3-5.4 equiv/kg) was of low processing viscosity &overall good mechanical
2.2 MWNT/Epoxy Composite Preparation
Nanocomposites were prepared by the method of sonication. To achieve better state of
dispersion first the nanotubes were treated with alcoholic medium for the deagglomeration of
the tube bundles. The treated tubes were then added to the epoxy resin and sonicated for 2 hrs
at room temperature. Then the mixture was cured under vacuum at 90
C for 10 hrs and
another set of samples were cured under refrigeration condition to get ductile samples. The
prepared samples were treated at 80
C for 6 hrs in the oven to remove the moisture contents
of the samples. The finally prepared samples were treated at different environmental
conditions for 24 hrs before the mechanical tests were performed. Few samples were treated
by hot water at 80
C and some are treated in the cryofreezer at liquid nitrogen temperature (-
The nomenclature of the samples is as given in Table.1.
Sample Identification Nature of the Sample
0 Plane/Neat Epoxy (Brittle)
1 Epoxy/CNT Composite (Brittle)
D0 Plane/Neat Epoxy (Ductile)
D1 Epoxy/CNT Composite (Ductile)
H0 Plane Epoxy Hot water treated (Brittle)
H1 Epoxy/CNT Composite Hot water treated
DH0 Plane/Neat Epoxy Hot water treated
DH1 Epoxy/CNT Composite Hot water treated
C0 Plane Epoxy Cryofreezer treated (Brittle)
C1 Epoxy/CNT Composite Cryofreezer treated
DC0 Plane/Neat Epoxy Cryofreezer treated
DC1 Epoxy/CNT Composite ductile Cryofreezer
28 S. Sekhar Samal Vol.8, No.1
2.4 Flexural & Tensile Modulus Test
2.4.1 Flexural test & tensile test
Four types of flexural test samples were fabricated: they were pure epoxy beam & its ductile
one and the nanotube composite beam & its ductile one. The beams were placed into different
temperature environments for 24 hours prior the test, these were: (a) room temperature (20
C), (b) warm water (80
C) and (c) liquid nitrogen (-180
C). From each sample, five
rectangular specimens were taken for three-point bend test as per ASTM D790
(width=2.7cm, thickness=0.7cm, span=11.2cm, length=12cm). Flexural tests were carried out
at ambient temperature using Instron-1195 keeping the cross-head speed 2 mm/min. Flexural
modulus of each sample was determined from the average value of five specimens.
The samples were made in the form of dog bone shape as shown (Fig.2) and the testings were
carried out carried out with INSTRON-1195.
Tensile tests were carried out at room temperature with a constant cross-head rate of 2
mm/min. The choice of this quite low loading rate is to consider the brittle character of
composites .Ten specimens were tested for each sample and the average value was obtained
from the data of these measured specimens (Fig.2). The thickness of samples is 3 mm ± 0.5
Figure 2: Dog Bone Shape sample for tensile modulus measurement.
2.4.2 Flexural measurements
Flexural modulus of pure resin, RCNT composite & ACNT composite are shown in Fig.7a.
Both the composite samples are showing greater modulus than pure resin sample that is
attributed to the high mechanical strength of Composite sample.
Vol.8, No.1 Role of Temperature and Carbon Nanotube Reinforcement 29
2.5 Fracture Surface Topography Characterization
Scanning electron microscope (JEOL-JSM-6480 LV) was used to conduct the fracture
surface topography characterization. The cured samples were fractured and the fracture
surfaces were coated with a thin platinum layer to study the morphology.
3. RESULTS AND DISCUSSION
3.1 Flexural Modulus, Tensile modulus & Young’s Modulus Measurement
Flexural modulus of pure brittle resin (sample-0) & its respective CNT composite (sample-1)
and ductile resin (sample-D0) & its CNT composite samples (D1) are found to be 24.52,
136.86, 44.15 &76.38 respectively. Increase in flexural modulus is more pronounced in
samples treated in hot water for both brittle (H0-49 & H1-148.8) and ductile (DH0-64 &
DH1-210) samples in comparison to samples treated in cryofreezer ( C0-33,C1-140.2, DC0-
36, DC1-159.3) as shown in the Figure(3&4).
Figure 3: Flexural modulus of Neat epoxy & Composite samples treated at different
conditions (Brittle Samples)
Because at high temperature the filler can constrain the mobility of polymer chains as well
as their relaxation spectra , which can change the glass transition temperature [25, 26]
and modulus of the matrix. All the composite samples are showing greater modulus than
pure resin samples. This may be due to the high mechanical strength of CNT. The plane
ductile sample after submerged into liquid nitrogen (DC0) revealed reduction in flexural
modulus in comparison to room temperature treated sample (D0). The cause of reduction is
may be due to the structural non-homogeneity and/or existence of a weak bonding interface
between the nanotubes and surrounding matrix. The results of the refrigerated samples are
showing variable behaviour. Ductile samples, which were, treated in hot water (HD0 &
HD1), showed the best result. And those kept in cryofreezer became more brittle and hence
30 S. Sekhar Samal Vol.8, No.1
less modulus. This may be due to contraction of the matrix, which increased the clamping
stress to the nanotube surface, and thus increased the frictional force between the nanotubes
and the matrix.
The general tendency observed from the result is that both the tensile strength and the
fracture strain changed with variation of the thermal conditioning while Young’s modulus
(Elastic Modulus) reduced at the same time. The tensile strength exhibited a maximum value
of 69.7 MPa (DH1) (Fig.5) and the flexural modulus reached the highest value of 210 MPa
(DH1) too (Fig.4).
Figure 4: Flexural modulus of Neat epoxy & Composite samples treated at different
conditions (Ductile Samples)
Figure 5: Tensile modulus of samples (1, D1, DC0, DC1, DH1)
Vol.8, No.1 Role of Temperature and Carbon Nanotube Reinforcement 31
The above mentioned relative improvement of epoxy resin in strength by adding CNTs can
be explained by the high specific mechanical property and specific surface area of the
MWCNTs . Here, MWCNTs play the role of an enhancing framework under the
treatment of hot water. The Young’s modulus reduced simultaneously with thermal
conditioning of the samples followed the reverse ordered followed by flexural modulus and
Elastic modulus (Fig.6).
Figure 6: Elastic modulus of different samples (0, H0, C0, DH0 and DC0)
An explanation for the behavior can be found by considering the contraction and expansion
of the matrix under high temperature and low temperature.
The flexural modulus was found to be 136.86 MPa in case of epoxy/RCNT composite which
is about six times than the flexural modulus of plane epoxy sample (24.52 MPa). Increase in
modulus is more pronounced in epoxy/ACNT composite i. e. 837.42 MPa which is more than
six times that of epoxy/RCNT composite and thirty four times that of epoxy sample. This may
be due to efficient load transfer from matrix to aligned CNT in Axial direction. Local
stiffening due to nanotubes results in improved load transfer at the fibre/matrix interface. It
had been reported that the increase in elastic modulus between the random and aligned
nanocomposite is a consequence of the nanotube orientation, not polymer chain orientation.
A considerable enhancement of hardness is observed by the nanocomposites in comparison to
pure resin sample (Fig.7 b). Pure resin samples showed hardness of 12 MPa. Epoxy/RCNT
had hardness value of 18 MPa which is 50% more and epoxy/ACNT had 49 MPa that is about
four times that of epoxy sample. High strength and long nanotube reinforcements may result
in forming a network structure that improves the hardness of the composites.
32 S. Sekhar Samal Vol.8, No.1
Fig.7 a Flexural Modulus profile
Fig.7 b Hardness profile
3.2 Fracture surface
Neat epoxy resin (Fig.8a) exhibits a relatively smooth fracture surface and the higher
magnification SEM picture (Fig.8b) indicates a typical fractography feature of brittle fracture
behavior, thus accounting for the low fracture toughness of the unfilled epoxy.
Figure 8 (a&b): Fracture surface of Epoxy resin at different magnifications
Vol.8, No.1 Role of Temperature and Carbon Nanotube Reinforcement 33
The distance between two cleavage steps (Fig.9a) is about 23-32 µm and the cleavage plane
between them is flat and featureless. The fracture surfaces of the nanocomposites show
considerably different fractographic features. The failure surfaces of the nanocomposites are
rougher with the CNTs added into the epoxy matrix (Fig.9b).
Figure 9: Cleavage plane of (a) pure epoxy (b) composite sample
The higher magnification SEM picture shows that the size of the cleavage plane decreased to
14-18 µm after the infusion of the CNTs. The decreased cleavage plane and the increased
surface roughness imply that the path of the crack tip is distorted because of the carbon
nanotubes, making crack propagation more difficult. Figure 9 shows the SEM images of the
fracture surfaces of CNT/epoxy composite. The fracture surface of the CNT composite is
very rough, which indicates that its failure was the result of a ductile deformation (Fig. 10a).
However, the smooth fracture surface of the CNT composite suggests that it had a brittle
failure mode (Fig. 10 b).
Figure 10: SEM images of the fracture surface of carbon nanotubes/epoxy composites. (a)
Composites show a ductile deformation & (b) Composite with a brittle crack surface.
A large particle, an agglomeration of several carbon nanotubes (Fig.11 a, b, c) was observed
in the fracture surface. But in Fig.11 (d), the agglomeration is less which results in the better
34 S. Sekhar Samal Vol.8, No.1
Figure 11(a, b, c & d): Agglomeration of several carbon nanotubes shown by black area in
At a low stress level, the agglomerated particle increased the stiffness of the material, but at a
high stress level, the stress concentration caused by the agglomerated particle initiated a
crack, which made the sample fail quickly. Figure.12 (a) shows original traces of nanotubes
in the composites. The fracture process did not follow the nanotube pullout pattern as in Fig.
12 (b), cracks propagated along the plane of the nanotube mesh.
Higher magnification showed a crack interacting with the nanotube reinforcement. RCNT
matrix pullout was observed along with extension and bridging of RCNTs across the crack.
In epoxy/ACNT composites, the cracks were spanned by the nanotubes causing enhanced
resistance to the crack propagation process. The bridging of the nanotubes as a mechanism of
inhibiting the crack initiation in polymer and ceramic based nanocomposites has been well
illustrated in literature[20-24].
Figure 12 (a) Traces of nanotubes in the composite near crack, (b) crack propagation along
the nanotube mesh, (c) Traces of stress concentrators in the composite near crack, (d) Show
bridging by aligned nanotubes in the composites during deformation.
This agrees with the previous result submitted by the researchers that the all nanotubes,
aligned close to the load direction in nanotube polymer composites were always pulled out at
failure. This was a result of a poor interfacial bonding between the nanotubes and matrix.
Vol.8, No.1 Role of Temperature and Carbon Nanotube Reinforcement 35
Therefore, the nanotubes inside the composites could not fully take up the load on the
nanotube’s longitudinal direction, which resulted in the decrease of flexural strength of the
nanotube composite beams.
Addition of nanotubes enhanced the flexural & tensile properties because all the composite
samples are showing better result than pure resin samples. Flexural modulus is maximum in
case of ductile composite samples treated in hot water (DH1) at 80
C. These nanocomposites
appeared tough while sub-ambient ductile samples (cryofreezer treated) are showing
brittleness. The fracture surfaces of nanotube/polymer composites after flexural tests show
different failure mechanisms for composites pre-treated under different conditions. The
fracture process of composite beam appeared to indicate that failure was the result of
agglomeration due to which crack initiation occurs.
In addition, aligned nanotube composites resulted in significantly improved flexural modulus
and hardness indicating that there is efficient load transfer between the polymer matrix and
the nanotube reinforcement along axial direction. Reduction in flexural modulus and hardness
value in epoxy/ RCNT composites was due to formation of agglomerates of nanotubes inside
polymer matrix that reduced reinforcing effects of the CNTs by acting as flaws in the resin.
Investigation of fracture surface in nanocomposite revealed that narrower crack-tips
underneath the advancing cracks were more efficiently bridged by the nanotubes in
epoxy/ACNT resulting in an increased resistance against crack propagation.
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