INTRODUCTION
Nanostructured
materials have a special place in contemporary materials science and
engineering due to their promising properties. These materials have been found
suitable for several applications such as in structural and electrical
applications owing to their desirable mechanical [1], [2] [3] [4], optical [5],
electrical and thermal properties [6]. In this respect, materials with tailored
mechanical and electrical properties have received a lot of attention in
academia and industries due to their potential uses in sensors [7], capacitors,
and structural components [8]. The nano-filled polymers comprising conducting
fillers in different thermoplastics [9], rubbers [10], and thermosetting epoxy
resins [10][11][12][13][14] are of
special interest.
Epoxy resins (EP) are
organic compounds with a low molecular weight having more than one epoxide
group. When properly cross-linked, they have a wide range of uses due to their
superior thermal and mechanical characteristics [11]. Several curing agents,
such as amines [12], are used to harden them resulting in solid, cross-linked
structures. They have high mechanical strength and good adhesion to other
materials [13] and thus are the most widely used thermosetting resins because
of their excellent mechanical and bonding capabilities, outstanding chemical
stability, and ease of processing [14] [15] [16]. The majority of the cured
products, however, have a high cross-link density, making them susceptible to
undergo brittle fracture when subjected to external forces. Thus, epoxy
products generally exhibit low impact and environmental stress-cracking
resistance demanding extensive research works aimed at reinforcing the resins
with nanoparticles, fibres, graphene, and nanotubes. [17] [18] [19] [20] [21].
It is known that multiwalled
carbon nanotubes (MWCNTs) are used in biomedical fields [22] [23], as coatings
and films, in microelectronics, energy storage devices, for environmental
remediation, and in biotechnology [24]. In addition, MWCNT finds applications
in designing nanocomposite membranes [25], transparent electrodes [26], and
catalysis [27] and transistor [28]. Because of the extraordinary mechanical
properties of MWCNTs, they are also used as reinforcing filler in the EP
matrix. However, it has been reported that there is no noticeable increase in
the strength of epoxy nanocomposites, which has been related to the poor
adhesion between the MWCNTs and the EP matrix. The surface functionalization of
the MWCNTs via fluorination [29], silanization [30], carboxylation, and
amidation [31] [32] has been employed as a strategy to improve the filler matrix adhesion. It has
been demonstrated that polymer nanocomposites with functionalized MWCNT have
better thermal, electrical, and mechanical properties than the corresponding nanocomposites
with pristine nanotubes [33]. Furthermore, the nanocomposites containing
different functional groups [36] are applicable in the construction of
scaffolds for tissue engineering [37] [38]. It has been reported that EP/MWCNT
nanocomposites are very useful for coating applications [34] whereby the
surface wettability is especially important to determine the quality of
adhesive bonding of the coatings to the substrate [35].
In the context of the
above discussions, it is desirable to develop MWCNT-based nanocomposite
materials with optimized electrical, mechanical, thermal, and surface adhesion
properties. This paper aims at investigating the effect of the addition of a
different form of MWCNT on the thermal and surface physical properties of epoxy
thermoset.
MATERIALS AND METHODS
Materials
Pristine multiwalled
carbon nanotubes (MWCNTs, Nanocyl 7000) from Nanocyl SA were used in this
study. Epoxy Lapox L-12 (the diglycidyl ether of bisphenol A, DGEBA) and
Hardener K-6 (the aliphatic polyamines) supplied by Atul Ltd. Polymer Division,
were used for the preparation of nanocomposites. The laboratory grade Silicon
oil with a viscosity of 370-390 m Pa was used for releasing the thermosetting
resin from the mould after the solidification. Acetone and nitric acid (Fischer
Scientific, India) were used as a solvent for epoxy material and to oxidize the
MWCNT, respectively. The pristine MWCNTs used in this study have an average
diameter of 44 nm and a length of several hundred nanometres, as shown by FESEM
images given in Figure 1.
Figure 1: FESEM images
showing the bundles of pristine MWCNT (a) and highly magnified nanotubes (b))
used in this work
METHODS
Chemical
modification of MWCNTs
Pristine MWCNT is inert
and hydrophobic. It is difficult to attach metal, polymer and other kinds of
particles to their surface in a neat form. Therefore, the functionalization
with different oxygen-containing groups on the MWCNTs surface through an
oxidation process is necessary to improve their hydrophilicity and reactivity.
There exist several methods for the oxidation of carbon nanotubes such as air
oxidation, sonication, annealing, acid oxidation etc. [7][9][23][35].
In this work, the acid
oxidation method has been employed for chemical functionalization of the nanotubes.
In a typical experiment, 0.5 g of MWCNTs were first sonicated in 40 mL of
concentrated nitric acid for 15 min followed by reflux for 5 h. The mixture was
filtered under suction using membrane filter paper with a pore size of 0.2 µm.
The residue was washed with distilled water until pH value of 7 and dried in an
oven maintained at a temperature of 100 ºC for until it dried [39]. The samples
prepared in this work are listed in Table 1.
Table
1: The list of samples used in the present work with their codes and
description
Preparation of
EP/MWCNT nanocomposites
The pristine and
acid-treated MWCNTs were incorporated into the EP matrix via physical mixing
assisted by ultrasonication. For this purpose, 2.5 g of Lapox – L12 was
dissolved in 2.5 mL of acetone taken in a 50 mL beaker, followed by 15 min of
ultrasonication. The mixture was agitated for 1 h at 60 ºC with a magnetic
stirrer to evaporate the residual solvent. The resulting mixture was allowed to
cool down to room temperature, and the amine hardener was added (volume ratio
of epoxy to hardener being 10:1). Finally, the mixture was cast into a plastic
mould and pre-cured at 25 ºC for 24 h followed by post-curing at 70 ºC for 5 h.
Characterization
Techniques
Fourier Transform
Infrared (FTIR) spectra of the materials were recorded with an IR Prestige 21
spectrophotometer equipped with a ZnSe ATR crystal over a wavenumber range
between 4000 cm-1 and 600 cm-1 with a resolution of 4 cm-1.
X-ray Diffraction (XRD)
of the samples was carried out using a diffractometer equipped with the
radiation source of Cu Kα with a wavelength of 0.15406 nm. The specimens were
scanned over the diffraction angle 2θ values between 5° and 80°.
Thermogravimetric
Analysis (TGA) of the specimens was performed by SDT 0600, TA instruments over
a temperature range of 30 °C to 700 °C at the heating rate of 10 °C /min.
Contact Angle
Measurements of the nanocomposites were conducted in the FTA 100 series (First
Ten Angstrom, Portsmouth, Virginia, 23704, USA) using distilled water. The
measurements were carried out on samples of dimensions 1 cm x 1 cm x 1 mm which
were subjected to the tests at 25 °C in closed chambers, maintaining the volume
of the sessile drop at 5 µl in all cases using a microsyringe.
RESULTS AND DISCUSSION
Structural
Characterization of Nanocomposites
The FTIR spectra of
neat EP, EP/pMWCNT(0.05), and EP/mMWCNT(0.05) are shown in Figure 2. For
simplifying the discussion, we present only the FTIR results obtained from the
nanocomposite comprising a low amount of MWCNTs.
The FTIR spectra reveal
information regarding the functional groups as well as the variations in
mechanical and thermal properties of various samples with their molecular
structures [40]. The spectrum in Figure 2 shows that the addition of 0.05 wt.-%
of nanotubes does not significantly alter the chemical structure of the EP
matrix. The peaks centred at 2960 cm-1, 2830 cm-1, and 1521 cm-1 represent the
benzene ring [41]. The EP groups present in all the samples are further denoted
by the bands located at 824 cm-1, 915 cm-1, and 1220 cm-1. It can be noticed
that the peak centred at 2960 cm-1 is eliminated in the case of both types of
nanocomposites. In addition, the peak centred at 915 cm-1 has a slight position
shift towards a higher wavenumber in the case of the EP/MWCNT nanocomposites
which may be due to the uniform dispersion of MWCNT on the EP matrix [42].
Figure 2: FTIR spectra
of neat EP, EP/pMWCNT(0.05) and EP/mMWCNT( 0.05) nanocomposites
The broad band was
observed in the range of 3132 cm-1- 3783 cm-1 indicating the presence of
hydroxyl groups attached due to acid treatment on the surface of the samples
which is responsible for creating hydrogen bonds with other nucleophilic
groups. The intensity of these bands is found to increase in the nanocomposites
comprising mMWCNT, which may be due to the interaction of the epoxy groups of
the resin with the -COOH groups introduced onto the surface of the nanotubes
through acid treatment, thereby enabling the formation of hydrogen bonds
between the –COOH group and EP resin resulting in cross-linked structures [43]
as well as hydrogen bonds between the unreacted carboxylic groups and –OH
groups within the mixture [44].
XRD was performed to
extract an overview of the microstructure evolution of nanocomposites with
various nanotube loadings. The XRD patterns of neat EP, EP/pMWCNT(0.5), and
EP/mMWCNT(0.5) nanocomposites are displayed in Figure 3.
Figure 3: XRD patterns
of neat EP, EP/pMWCNT(0.5) and Ep/mMWCNT(0.5) nanocomposites
XRD patterns of the
materials present some broad peaks centred on 2θ values of around 19º and 43º,
which can be attributed primarily to the amorphous structure of the epoxy resin
[42]. The nanocomposites comprising both pristine and acid-treated filler also
show similarly positioned broad bands which imply a quite uniform dispersion of
the nanotubes in the epoxy network [43]. The XRD patterns of the nanocomposites
further attest that the incorporation of MWCNT
in epoxy has not exercised any influence to modify the inherent epoxy
resin matrix morphology on the microscopic scale [45]. The decreased peak
intensity observed in both the nanocomposites may be correlated to the
exfoliation of nanotubes bundles resulting from the diffusion of epoxy chains
between each bundle of nanotubes space of carbon nanotubes filled
nanocomposites [46]. In both nanocomposites, the characteristic peaks of the
nanotubes were not observed, indicating the homogeneous dispersion of the
nanotubes in the polymer matrix [47].
Thermal Behavior
of Nanocomposites
Thermogravimetric
analysis is a highly valuable tool for determining the overall quality of the
material. The neat EP, EP/pMWCNT, and EP/mMWCNT nanocomposite samples were
subjected to thermogravimetric analysis (TGA) to study the impact of adding
pristine and functionalized MWCNTs on their thermal degradation behaviour.
Figure 4a shows the TGA
thermogram, and Figure 4b shows the DTG plot of EP, EP/pMWCNT, and EP/mMWCNT.
In all the cases, there is no weight loss below 315 ºC, and drastic weight loss (about 90 %) occurs
at a temperature ranging from 315 ºC to 450 ºC with a single DTG curve at 361
ºC (EP), 370 ºC (EP/pMWCNT(0.2) and 374
ºC (EP/mMWCNT(0.2)), which showed
one-step degradation mechanism. The nature of the TGA and DTG curves indicates
that the presence of MWCNT does not affect the breakdown of the EP matrix. The
residual masses of the EP, EP/pMWCNT(0.2), and EP/mMWCNT(0.2) composites are
around 10, 9, and 7 wt.-%, respectively [48].
On careful examination,
it is found that, as compared to neat EP, the EP/mMWCNT(0.2) nanocomposites
showed greater thermal stability. The well-dispersed nanotubes and strong
interfacial adhesion between the nanotubes and the EP matrix contribute to the
nanocomposite's increased thermal stability [49].
Figure 4: TGA (a) and
DTA (b) plots of neat EP along with that of EP/pMWCNT(0.2) and EP/mMWCNT(0.2)
nanocomposites
Surface Wetting
Behavior of Nanocomposites
The water contact angle
indicates the wettability of the solid substrate surface correlates with the
wettability of the solid. The contact angle is important wherever the intensity
of the phase contact between liquid and solid substances needs to be assisted,
such as in coating, painting, cleaning, printing, bonding, dispersing etc. The
wettability of the nanocomposites was studied by measuring the contact angle
with the help of the sessile drop technique using a built-in camera system
operated through software [49]. The values of contact angles (θ) made by water
droplets on various sample surfaces are indexed in Table 2. Figure 5 shows
further the photographs illustrating the surface wetting properties of the
nanocomposites using the sessile drop technique are presented in Figure 5. It
can be observed that the contact angles of the EP/pMWCNT(0.2) and
EP/mMWCNT(0.2) nanocomposite are greater than that of the neat EP, indicating
that these nanocomposites are more hydrophobic than the hydrophilic neat EP
matrix.
Figure 5: The sessile
drop geometric shapes on (a) neat EP, (b) EP/pMWCNT(0.05), (c) EP/pMWCNT(0.2),
(d) EP/mMWCNT(0.05) and (e) EP/mMWCNT(0.2) nanocomposites
Table 2. Contact angles
of EP and that of EP/pMWCNT(0.2) and EP/mMWCNT(0.2) nanocomposites measured
using sessile drop technique
The arrangement of the
MWCNT nanostructure in the epoxy matrix may have an impact on the surface
behaviour, and the heterogeneity effects may further be responsible for an
increase in contact angle values [50]. The increase in contact angle values may
be attributed to the surface roughness of the samples caused by the
incorporation of the nanotubes. The further increase in the contact angles in
the materials comprising the mMWCNT may be interpreted as an effect of the
presence of oxygen-containing functional groups in the nanofiller [51].
CONCLUSION
On adding the MWCNTs to
the EP resin, the constituents are found to be well cross-linked to each other,
the phenomenon being more pronounced in the cases with acid-treated nanotubes,
implying the latter's potential prospect of enhancing the composites' physical
properties for structural applications. From the XRD analysis, it can be
concluded that the incorporation of MWCNT in epoxy resin has no impact on the
primary structure of the resin.
The thermal analysis of
the materials indicate that the addition of the nanofiller does not affect the
degradation behaviour of the matrix polymer over the investigated composition
range while a slight improvement in the thermal stability is visible for the
EP/mMWCNT nanocomposite. The hydrophilicity has been successfully introduced to
the epoxy by addition of the mMWCNT.
The details on the
correlation of the nanocomposites' detailed morphological features (as observed
by electron microscopy) with their structural and physical properties shall be
discussed in a separate publication.
CONFLICT OF INTEREST
All authors declare
that there is not any kind of conflict of interest.
ACKNOWLEDGEMENT
ST gratefully
acknowledges the Nepal Academy of Science and Technology (NAST), Lalitpur, Nepal,
the Polymer Service GmbH Merseburg (PSM), Germany, and Mahatma Gandhi
University, Kerala, India for providing scholarships for carrying out the
research works. We gratefully acknowledge St. Thomas College in Pala, India for
providing the XRD results and cordially thank the Japanese Ministry of Science
and Technology for supporting the research stay of ST and JG at Meijo
University, Nagoya, Japan in the frame of "Sakura Science Program
2020".