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Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by non-isothermal differential scanningcalorimetry
Elnaz Esmizadeh1,2• Ghasem Naderi2 • Ali Akbar Yousefi2
• Candida Milone3
Received: 29 August 2015 / Accepted: 26 May 2016 / Published online: 18 June 2016
� Akademiai Kiado, Budapest, Hungary 2016
Abstract Chemical hybrid of nanoclay (NC)/carbon nan-
otube (CNT) was synthesized via growth of CNTs by
chemical vapor deposition. The cure kinetics of epoxy resin
in the presence of novel chemical hybrid of NC/CNT
(CNC) was studied by non-isothermal differential scanning
calorimetry. The effect of the CNC on cure kinetics was
compared with conventional nanofillers such as CNTs, NC,
and physical mixture of them (PNC). The kinetic param-
eters of the cure reaction were determined by iso-conver-
sional method. The accelerating effect of CNT, CNC, and
PNC in initial stage of cure reaction was related to the high
thermal conductivity of CNTs, while the decelerating
effect of nanofillers as the cure proceeded can be attributed
to the reduction of polymer molecules motion caused by
enhanced viscosity. The apparent activation energy (Ea) as
the function of conversion (a) was calculated by five
methods categorized into two different types: (1) conver-
sion-dependent methods: Kissinger–Akahira–Sunose
(KAS), Ozawa–Flynn–Wall (OFW), and Friedman; (2)
conversion-independent methods: Kissinger and Augis.
The accelerating effect of CNT, PNC, and CNC was
observable as the reduced Ea values in low conversion only
with KAS and OFW methods. The reverse trend of Ea
values was observed with the introduction of these
nanofillers at high conversions. The uniqueness of the CNC
was more marked in increasing Ea values of epoxy after
initial stage due to its special 3D structure of CNC. Cal-
culated data using KAS and OFW methods showed the best
agreement with the obtained experimental data.
Keywords Carbon nanotube–nanoclay hybrid � Epoxy �Chemical vapor deposition � Cure kinetics � Non-isothermal
Introduction
Epoxy is the dominant matrix material for lightweight
polymer–matrix structural nanocomposites due to its supe-
rior properties suitable for the manufacturing of composites
for structural applications in automotive, aerospace, and
marine industry [1]. Among 1D nanomaterials, carbon nan-
otubes (CNTs) are commonly used to be an supreme rein-
forcing agent for epoxy matrix due to their unique structural,
mechanical, and electrical properties [2]. Furthermore,
nanoclay (NC) platelets as 2D nanomaterials have the
potential of being low-cost alternative fillers for incorpora-
tion into epoxy matrix for commercial applications [3]. In
2005, a unique novel 3D nanostructured filler was introduced
by direct growth of CNTs on NC via chemical vapor depo-
sition (CVD) [4]. The as-prepared filler, chemical hybrid of
CNT/NC (CNC), combines the properties of both compo-
nents [5]. According to the literature, the concurrent appli-
cation of CNTs and NC both as physical and chemical hybrid
filler in epoxy provides the advantage of both fillers leading
to high-performance nanocomposites [6–8].
As known, the introduction of nanofillers to epoxy
generates potentially tremendous changes in the cure
behavior which is critical in choosing the proper set of
processing parameters [9]. Incorporation of CNTs was
& Ghasem Naderi
1 Department of Polymer Science and Technology, University
of Bonab, P.O. Box 5551761167, Bonab, Iran
2 Faculty of Polymer Processing, Iran Polymer and
Petrochemical Institute (IPPI), P.O. Box 14965/115, Tehran,
Iran
3 Dipartimento di Ingegneria Elettronica, Chimica e Ingegneria
Industriale, Universita di Messina, 98166 Messina, Italy
123
J Therm Anal Calorim (2016) 126:771–784
DOI 10.1007/s10973-016-5594-4
reported to catalyze the curing reaction of epoxies resulting in
lower initiation temperature of curing [10]. Moreover, it was
observed that cure reaction activation energy initially increases
and then decreases with the increase in the CNT content [11].
Our previous work showed that the effect of CNTs on the cure
kinetics of the epoxy depended on the temperature of isother-
mal cure [12]. Abdalla et al. [13] found that the surface modi-
fication of CNT can also affect the cure behavior of epoxy. The
effect of NC on curing properties of epoxy resins was also
reported in the literature [14, 15]. Becker et al. [16] showed that
the functionality of epoxy resin can also influence the cure
properties and exfoliation process of NC.
Even if some attempts have been accomplished to study
the effect of individual CNTs or NC on epoxy cure, no data
exist at our knowledge on how their simultaneous presence
affects the cure properties of epoxy. The effect of the novel
CNC synthesized by CVD reaction on cure behavior of
epoxy was investigated by non-isothermal DSC. The
results were compared to that of conventional nanofillers
CNT and NC and also physical hybrid of them.
Theoretical concept
For a dynamic DSC run, the total area (Stot) of the
exothermal peak, the region between the exotherm and the
baseline, is directly proportional to the total heat of the cure
reaction (DHtot). The fractional extent of conversion (a) at
any temperature (T) is expressed by Eq. 1:
a ¼ DHT
DHtot
¼ ST
Stot
ð1Þ
where DHT is the heat of reaction of partially cured samples
at the temperature T . Equation 2 can be used to describe the
curing kinetics studied by dynamic DSC analysis.
d/=dt ¼ Ze�Ea=RT f /ð Þ ð2Þ
where t is the curing time, the derivation of extent of
conversion d/=dt� �
is the rate of conversion, Z is the pre-
exponential factor,Ea is the activation energy, T is the curing
temperature, and f /ð Þ is the function of kinetic model. The
curing kinetics of epoxy resins commonly obeys auto-cat-
alyzed form Eq. 3:
f /ð Þ ¼/m � 1� /ð Þn ð3Þ
where n and m are reaction orders [17]. The knowledge of
the activation energy is necessary to determine the most
suitable kinetic model [18]. Several methods can be used to
evaluate the activation energy values at progressive
degrees of conversion as follows:
• Kissinger–Akahira–Sunose (KAS)
In KAS method, based on Eq. 4, ln bT2
� �is plotted
versus 1T
for constant conversion and heating rate.
lnbT2
� �¼ ln
AR
Ea
� �� Ea
RTð4Þ
where b is the heating rate [19].
• Kissinger
Kissinger, as a special case of KAS equation, suggests a
similar method, which relates ln bT2
P
� �with the inverse
of the peak temperature 1TP
� �of the following
expression:
lnb
T2P
� �¼ ln
AR
Ea
� �� Ea
RTP
ð5Þ
where TP is the maximum point from the dynamic DSC
analysis curve [18].
• Ozawa–Flynn–Wall (OFW)
In OFW method, based on Eq. 6, ln b is plotted versus 1T
for constant conversion and heating rate [19].
ln b ¼ �1:0516Ea
RTþ cte ð6Þ
• Friedman
In Friedman method, based on Eq. 7, ln bda=dT
� �is
plotted versus 1T
for constant conversion and heating
rate [19].
ln bd /dT
� �¼ lnAþ ln f /ð Þ � Ea
RTð7Þ
• Augis
In Augis method, Ea is obtained from the plot of
ln b=TP � T0
� �versus 1
Tfor constant conversion and
heating rate based on Eq. 8.
lnb
TP � T0
� �¼ � Ea
RTP
þ lnA ð8Þ
Experimental
Materials and methods
Organo-modified NC, Cloisite� 15A, supplied by Southern
Clay Products (USA) was used as the support for CVD
experiments. An aqueous solution of iron (III) nitrate
nonahydrate ([99 %, Fe (NO3)3�9H2O, Merck, Germany)
772 E. Esmizadeh et al.
123
was employed to prepare Fe-impregnated NC [5]. The wet
solid was dried at 100 �C and calcined in air at 500 �C for
3 h. CVD gases including methane (99.99 %), hydrogen
(99.99 %), and nitrogen (99.99 %) (Roham Gas Company,
Iran) were used as received. The catalyst precursor was
placed in a quartz boat inside a horizontal tube furnace
model P.Tube 12/38/750 (Pyro Therm Furnaces, Leicester,
UK). The catalyst was reduced for 2 h under 60 cc min-1
hydrogen flow at the 500 �C to perform reduced nanoclay
(NC). As a typical CVD [20], the reactor was heated up to
950 �C under nitrogen flow and then subjected to methane/
CVD process with a 30 cc min-1 gas flow for 1 h. After-
ward, the system was cooled under nitrogen flow and the
reaction product was represented as chemical hybrid of
CNT–NC (CNC). As-grown CNTs of the synthesized CNC
were purified to perform (CNT) which was further mixed
with NC to form physical hybrid of CNT–NC (PNC).
During purification, NC support and iron particles were
removed by refluxing the obtained CNC in a mixture of
12 % hydrochloric acid (HCl) and 12 % hydrofluoric acid
(HF) (Sigma-Aldrich, Germany), respectively [21]. In
order to check the amount of oxygen which may chemi-
sorbed on the surface of CNTs during purification,
Boehm’s titration method was employed using NaOH
(Sigma-Aldrich, Germany) [22]. During this test, the sur-
face acidity of CNT before and after purification was
evaluated based on the fact that NaOH can neutralize
acidic groups (carboxyl groups, lactones, and hydroxyl
groups) [23]. Epoxy resin utilized in this study was a
nominally multi-functional low-viscosity epoxy resin sys-
tem, Araldite LY 5052/Aradur HY 5052, supplied by
Huntsman-Switzerland. In order to study the effect of
nanofiller type on cure kinetic of epoxy resin, the samples
were prepared according to Table 1. Predetermined amount
of nanofiller (according to Table 1) were dispersed into
hardener using 30 min of ultra-sonication (60 % Ampl) in
ice bath. Epoxy was added into hardener, and then, the
mixture was sonicated in ice bath for further 15 min. The
mix ratio of epoxy/hardener (Araldite LY 5052/Aradur HY
5052) was kept 100:38 by mass [24].
Characterization
The specific surface area of supports and related catalysts
was evaluated with the A Philips X’Pert MPD (Holland)
diffractometer using a CuKa radiation source at 40 kV, and
40 mA with step size of 0.02� s-1 was employed to collect
X-ray diffraction (XRD) patterns of samples. Boehm
titration was employed in order to check the acidity of the
nanofillers. 50 mg of the sample was dispersed in 200 mL
of 0.1 M solution of NaOH in a closed conical flask and
stirred at room temperature overnight. The suspension was
filtrated by 0.2-micrometer filter paper and then titrated
against HCl to neutralize the unreacted NaOH. The amount
of needed HCl to titrate the pH value of the solution to 7.0
was used to calculate the unreacted NaOH. Morphology of
the samples was investigated using a VEGA/TESCAN
(Czech Republic) scanning electron microscope (SEM).
Table 1 Mass content, type, and surface acidity of nanofiller used in
preparation of epoxy nanocomposites
Sample
no.
Sample
code
Nanofiller
Content by
mass/%
Type Surface acidity/
mmol g-1
1 Control 0 – –
2 0.2 NC 0.2 NCa 1.3423
3 0.2 CNT 0.2 CNTb 1.8883
4 0.2 PNC 0.2 PNCc 1.6153
5 0.2 CNC 0.2 CNC 1.6998
a Nanoclay support just before CVD reactionb Carbon nanotubes after purification of CNCc Mixture of NC and CNT (50/50)
Fig. 1 a SEM and b TEM of synthesized chemical hybrid of CNT–
NC (CNC)
Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by… 773
123
Energy-dispersive X-ray (EDX) spectroscopy coupled with
SEM was used to detect NC particles. The morphology of
CNCs was investigated using Philips EM 208 (Germany)
transmission electron microscope (TEM) under an accel-
erated voltage of 100 kV. Thermogravimetric (TG) and
differential thermal (DTA) analyses were carried out by a
PerkinElmer Pyris instrument (USA) using a ramp rate of
10 �C min-1. Raman spectra were recorded on a Micro-
Raman system RM 1000 RENISHAW using a 50-mW laser
excitation line at 785 nm equipped with Leica DMLM
microscope and a Peltier-cooled CCD detector. Differential
scanning calorimetry (DSC) measurements were taken on a
Netzsch DSC 200 F3 (Netzsch, Germany). Cure behavior of
nanofiller-loaded epoxy/hardener system was investigated
non-isothermally at various heating rates, 5, 10, 15, and
20 �C min-1 with 15 mg of mixture in DSC pan.
Results and discussion
Chemical hybrid of CNT–clay (CNC)
In order to check that the growth of CNTs was successfully
achieved over NC in CNC hybrid, the morphological study
was accomplished by SEM and TEM (Fig. 1). From
Fig. 1a, it was clearly seen that the filamentous structures
(will be proven to be CNTs below) are successfully pro-
duced over Fe-loaded clay as catalyst, consistent with our
previous results observed on Na?-exchanged clay [25]. The
highly entangled structure of filamentous products in
bundles can be related to the presence of defects in CNT
structure [25]. Dimension measurements using SEM
micrographs (not shown here) showed that the CNTs are of
the average outer diameter of 25–50 nm and the average
length over 10 microns. The observed hollow nature of the
as-grown filamentous products on NC confirms the suc-
cessful formation of CNTs in CNC (Fig. 1b). The presence
of encapsulated Fe nanoparticles within the CNTs’ chan-
nels shows the crucial role of Fe3C for CNT growth [26].
The formation of a unique 3D nanostructure of CNC can be
revealed by TEM (Fig. 1b), in which a 2D clay platelet has
several 1D CNTs attached to it.
Figure 2 presents the XRD pattern of the NC on dif-
ferent stages of catalyst’s preparation, synthesized CNC. A
2h peak was observed in the diffractogram of pristine NC
(Cloisite� 15A) (Fig. 2a) indicating the layered structure of
montmorillonite. The XRD pattern of pristine support,
1 2 3 4 5 6 7 8 9 10(g)
(f)(e)
(d)(c)
(b)(a)
2θ/°
Inte
nsity
/a.u
.
Fig. 2 XRD spectra of CVD support in various steps of catalyst
preparation: a Cloisite 15A (15A), b calcinated Fe-loaded 15A,
c reduced Fe-loaded 15A, d CNC and different epoxy nanocompos-
ites: e 0.2 NC, f 0.2 PNC, g 0.2 CNC
(a) (b)
1000 2000 3000 4000 0
10
20
0
20
40
60
80
100
30
40
200 400 600 800 1000
200 400 600 800 1000
Ram
an in
tens
ity/a
.u.
ΤGΑ
DTGTpeak = 619 °C
Temperature/°C
Der
iviti
ve m
ass
lo
ss/%
min
–1M
ass
loss
/%
Raman shift/cm–1
O
Δ
Fig. 3 a Raman spectrum of synthesized chemical hybrid of CNT–NC (CNC): O–D-band: 1340–1350 cm-1, D–G-band: 1570–1610 cm-1,
D–G0-band: 1570–1610 cm-1 and b TG/DTG of CNC
774 E. Esmizadeh et al.
123
calcinated and reduced NC and CNC hybrid revealed that
calcination of the Fe-loaded NC and further CVD reaction
has a severe effect on its (001) reflection. The disappear-
ance of basal reflection at 2h = 2.8� corresponded to
001-dspacing in XRD patterns in the calcinated support with
respect to the parent NC suggested a strong delamination of
the structure caused by the degradation of quaternary
ammonium salt modifiers during calcinations [27]. Then,
the layers of the NC were further delaminated in CNC
hybrid (Fig. 2d) due to the growth of CNTs on the
platelets.
Raman spectroscopy and TG/DTG were further con-
ducted to obtain more details about the type of the CNTs
synthesized in CNC, as shown in Fig. 3. Figure 3a
demonstrates that a typical Raman spectrum of multi-
walled CNTs (MWCNTs) with three modes referred to as
D-, G-, and G0-bands was observed for synthesized CNC.
The D-band correlated with the lattice disorder and defects
in the sidewall structure of CNTs whereas the G-band
showed the in-plane vibrations of the graphene sheet in
crystalline graphitic carbons. The G0-band as the overtone
of the D-band was defect-independent [25]. Raman anal-
ysis revealed MWCNTs grown rather than single-walled
CNTs (SWCNTs) in synthesized CNC, because SWCNTs
typically show stronger intensity of G-band compared to
that of the D-band [20].
TG/DTG was also employed as a confirmation for the
formation of MWCNTs using the combustion temperature
(Fig. 3b). It was evidenced from TG results that there was
no significant mass loss below 400 �C, indicative of little/
no amorphous carbon existed in the synthesized CNTs. The
TG curve of CNC presented a single mass drop between
400 and 700 �C with approximate mass loss of *50 %.
The residual mass, above 700 �C (Fig. 3b), was mainly the
indicative of the NC remained after combustion of
MWCNTs [28]. Therefore, the yield of MWCNT of
–1.0–0.3
–0.2
–0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 50 100 150 200 2500 50 100 150 200 250
= 5= 10
β
β β
β = 15= 20
Control Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
–0.5
0.0
0.5
1.0
1.5
Temperature/°CTemperature/°C
Hea
t flo
w/W
g–1
Exo
(a) (b)
Hea
t flo
w/W
g–1
Exo
Fig. 4 Dynamic DSC curves. a Control sample at different heating rates of 5, 10, 15, and 20 �C min-1. b Heat flow of epoxy and its
nanocomposites at b = 5 �C min-1
Table 2 Non-isothermal cure data for the epoxy nanocomposite
samples
Sample b/�C min-1 Tonset/�C Tpeak/�C Tend/�C DHtot/J g-1
Control 5 58.6 101.6 149.8 512.9
10 69.0 116.4 166.1 510.1
15 76.0 126.0 173.5 497.7
20 82.2 132.2 180.0 504.2
Average 506.2
0.2 CNT 5 56.9 100.3 151.1 497.0
10 68.9 115.5 166.8 493.1
15 73.2 124.8 173.7 491.8
20 81.5 126.7 176.3 492.4
Average 493.5
0.2 NC 5 59.2 102.1 151.8 502.7
10 71.6 116.7 165.0 490.6
15 76.5 126.3 172.7 484.2
20 83.1 132.8 179.7 479.8
Average 489.3
0.2 PNC 5 56.8 100.1 159.0 510.7
10 65.2 114.3 169.0 492.2
15 74.5 121.7 173.1 479.1
20 81.3 129.9 178.9 479.5
Average 490.3
0.2 CNC 5 57.0 100.0 150.9 511.8
10 62.9 113.6 166.3 496.3
15 75.3 122.6 172.0 483.0
20 81.6 129.7 177.9 493.2
Average 496.0
Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by… 775
123
synthesized CNC was estimated to be *50 %. The maxi-
mum exothermic peak (Tpeak) of DTG curve occurred at
619 �C related to the combustion of MWCNTs. High
combustion temperature (above 600 �C) of the carbon
nanostructures grown over NC catalyst revealed that they
were MWCNT rather than SWCNTs which exhibit less
thermal stability (below 600 �C) [29].
The quantities of acidic groups may further affect the
cure mechanism of epoxy [23]. During purification, CNTs
were refluxed with HCl and HF acid to remove catalyst and
support particles, which may also increase the amount of
oxygen chemisorbed onto CNT surface. This will cause
more oxygenous functional groups, such as hydroxyl
groups, carboxyl groups, and lactones, formed on the sur-
face of CNTs (mainly hydroxyl * �90 %) [22, 23]. To
quantitatively calculate the functional groups on the sur-
face of nanofiller, the Boehm’s titration method was used.
This method based on that NaOH neutralizes carboxyl
groups, hydroxyl groups, and lactones [23].
The quantities of these groups on the surface of nano-
fillers calculated from the difference in initial NaOH and
unreacted one (after Boehm titration) are given in Table 1.
The acidity of all CNT and PNC was higher than other
types, which was directly related to the purification process
using acids. The probable effect of acidity of the surface of
nanofillers on curing process of the epoxy will be discussed
in the following.
Cure kinetics of epoxy nanocomposites
The effect of NC, CNT and their hybrids on the cure of the
epoxy resin was analyzed by non-isothermal DSC experi-
ments. The cure behavior of the neat LY5052/HY5052
system in four heating rates (b = 5, 10, 15, and
20 �C min-1) was given in Fig. 4a. Similar trend was
observed for epoxy nanocomposites (not shown here). Cure
behavior of the epoxy with the inclusion of 0.2 mass% of
NC, CNT, CNC, and PNC at heating rates b = 5 is shown
in Fig. 4b. The single peak noticed in heat flow curves
revealed that the curing had occurred uniformly in epoxy
and its nanocomposites [29]. The total area under the
exothermic peak, based on the extrapolated baseline at the
end of the reaction, was calculated as the total heat of
reaction (DHtot).
The initial and final cure temperature ðTonset and TendÞ,and the maximum exothermal peak temperature ðTpeakÞ and
DHtot of various systems at the different heating rates (b)
are presented in Table 2. From the table, it was observed
that in all epoxy systems increasing the heating rate
increased Tonset and Tpeak. According to Table 2, in the
Fig. 5 Morphology of epoxy nanocomposites a EDX map of 0.2 NC, b SEM micrograph of 0.2 CNT, c EDX map of 0.2 PNC, d SEM
micrograph of 0.2 PNC, e EDX map of 0.2 CNC, f SEM micrograph of 0.2 CNC
776 E. Esmizadeh et al.
123
presence of CNT, NC, and their hybrid, the heat of the
epoxy curing reaction (DHtot) was lower than in the pristine
epoxy (control sample). The decrease in DHtot could be
directly related to the proportional reduction of epoxy
concentration in the composite [12]. It can be also related
to the increased viscosity caused by the presence of
nanofiller which hindered the mobility of the reactive
species and resulted in decreased enthalpy. Furthermore,
the presence of nanofiller can decrease the degree of cure.
Introducing a very small amount of NC (0.2 mass%)
caused a small decrease in Tonset and Tpeak of 0.2 NC
comparing to control sample. This was probably due to the
physical hindrance of the NC to the mobility of epoxy
monomers which delayed the cure process [23]. In the case
of well-dispersed nanocomposites, introduction of nano-
filler into epoxy normally results in the viscosity built-up
behavior of epoxy nanocomposites, as a result of the fric-
tional interactions [30].
The dispersion of CNT and NC within the epoxy matrix
was characterized by SEM and EDX, respectively (Fig. 5).
The red points observed in Fig. 5a, c, e were related to the
main element of NC, silicone (Si) [31]. According to these
images, 0.2 NC, 0.2 PNC, and 0.2 CNC developed a
homogenous dispersion of NC, since very few silicate
layers gathered in tactoids could be observed. The obtained
result was in accord with not observing any 2h peak in
XRD results as demonstrated in the previous section.
It could be seen from the SEM image of the 0.2 CNT
sample (Fig. 5b) that the CNTs were randomly dispersed in
epoxy matrix. Randomly dispersed CNTs are observed for
0.2 PNC and 0.2 CNC nanocomposite in Fig. 5d, f,
respectively. In addition, almost no aggregates were
observed for clay particles in EDX image of 0.2 PNC and
0.2 CNC nanocomposites (Fig. 5c, e). Strong interfacial
adhesion between CNT and the epoxy matrix was con-
firmed with the observation that most CNTs were broken
upon failure rather than just pulled out [32]. In the case of
0.2 CNC, the CNTs attached to the clay sheets are obvi-
ously seen in Fig. 5e. It resulted in the formation of a
unique 3D structure in which a 2D NC has several 1D
CNTs attached to it.
Shift of Tonset and Tpeak to lower temperatures (Table 2)
was observed in the presence of CNT, PNC, and CNC,
illuminated that 0.2 CNT, 0.2 PNC, and 0.2 PNC were
obviously faster in reaching the exothermic peak than neat
epoxy. Twofold effect of various nanofillers on cure reac-
tion of epoxy was reported in the literature: (1) facilitating
the cure reaction due to their high thermal conductivity
(conductivity effect) and also acting as the catalyst owing
their catalytic groups (catalytic effect) [12], (2) retarding
the cure reaction because of their steric hindrance or vis-
cosity-increasing effect (viscosity effect) [33].
The noticeable accelerating effect in the initial stage of
cure observed for CNT, PNC, and CNC nanofillers sug-
gested that the observed phenomenon was related to the
CNTs in these nanofillers. The main reason of accelerating
effect of CNTs can be the high thermal conductivity of
CNTs. Epoxy resin acted as a thermal insulator due to its
low thermal conductivity (0.2–0.5 W mK-1). Therefore, in
DSC experiment, it took relatively a longer time for the
heat to transfer from the outside environment to the center
of epoxy sample. In contrast, multi-walled CNTs exert a
generally high thermal conductivity value of
650–830 W mK-1 [34]. Homogenous dispersion of CNTs
in the epoxy matrix facilitated cure reaction with the for-
mation of thermal network ‘‘thermal pathway’’ within the
sample.
Furthermore, the catalytic effect of purified CNT present
in 0.2 CNT and 0.2 PNC samples attributed to the forma-
tion of hydroxyl groups on the surface of CNT during
purification process observed in Boehm titration results
(Table 2). The catalytic effect of hydroxyl-containing
materials on the cure reaction of epoxy systems cured with
amines was previously reported in the literature [35]. OH–
group on the surface of CNT could exert a catalytic effect
for peroxide ring opening of epoxy [36, 37]. The proposed
mechanism includes (1) transfer of hydrogen from the
hydroxyl to the epoxy, (2) hydrogen bonding between the
epoxy and hydroxyl in the transition state, and (3) hydro-
gen bonding of both the epoxy and the amine [35, 38].
Figure 6 shows the evolution of the degree of cure
versus time for epoxy nanocomposite systems at two dif-
ferent heating rates, b = 5 and 15 �C min-1. The form of
the curves reported in was a typical of the cure reaction of
thermosetting polymers with the so-called autocatalytic
behavior, that is, with a maximum reaction rate at nonzero
times often observed in epoxy systems [17]. As known
before in our previous work, the LY5052–HY5052 system
possesses autocatalytic kinetics [12]. Cure reaction of
epoxies by amines is known to be autocatalytic, because
the OH groups formed during the reaction facilitate the ring
opening of epoxy groups. Illustration of the various reac-
tions that may occur during the cure of epoxy is given in
Scheme 1: (i) etherification via homopolymerization of
epoxide groups; (ii-a) the reaction of the epoxide groups
with the primary amine to form secondary amine; (ii-b) the
reaction of the epoxide groups with the secondary amine to
form tertiary amine, and (iii) the reaction of epoxide groups
with hydroxyl groups (etherification via the secondary
amine, being catalyzed by the tertiary amine) [39]. In
Fig. 6, S-shaped curves were obtained for all materials
studied regardless of type and content of nanofiller, and
confirmed that the autocatalytic curing kinetics remained
unchanged even with the addition of nanofiller. The
Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by… 777
123
accelerating effect of CNT, CNC, and PNC along with
decelerating effect of NC was obvious in the initial stage of
epoxy cure. The closest zoom scale of the initial stage is
illustrated in the inserts of Fig. 6.
The plots required to calculate the activation energy (Ea)
of 0.2 CNC nanocomposite with different models are
shown in Fig. 7: (a) ln b�T2
� �versus 1=T for KAS model;
(b) ln b�T2
P
� �versus 1=TP
for Kissinger model; (c) ln bð Þ
versus 1=T for OFW model; (d) ln bd að Þ=dt� �
versus 1=T for
Friedman model; and (e) lnðbðTP � T0Þ�1Þ versus 1=TPfor
Augis model. Similar plots (not shown here) were obtained
for the other epoxy nanocomposites. Making fitted linear
regression lines, then activation energy (Ea) values were
obtained for each value of degree of cure (a) using the
models’ equation mentioned before.
The different values of the slope of fitted linear
regression lines in KAS, OFW, and Friedman plots
(Fig. 7a, c, d) show the conversion dependence of activa-
tion energy. In contrast, the unique fitted linear regression
line obtained for Kissinger and Augis model (Fig. 7b, e)
demonstrated that the calculated activation energy was
independent of conversion.
The changes in Ea values of the epoxy nanocomposites
as a function of a in different epoxy nanocomposite system
are illustrated in Fig. 8. The Ea values were calculated by
three different methods, i.e., KAS, OFW, and Friedman
methods, as expressed in Eq. 4–6, respectively. The Ea
values of the control sample gradually decreased after
a = 0.6 as the degree of the conversion increased. A
similar trend was previously reported for epoxy resins [40].
This result could be attributed to the fact that the raised
amount of OH groups increased during the cure as shown
05.0 7.5 10.0
5
10
30
20
10
03 4 5 6
0 5 10 15 20 25 30 35 0 3 6 9 12 15
Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
0
20
40
60
80
100
0
20
40
60
80
100= 5 °C min–1 = 15 °C min–1
Time/min Time/min
Deg
ree
of c
ure/
%
Deg
ree
of c
ure/
%
(a) (b)β β
Fig. 6 Evolution of degree of cure (a) as a function of time for epoxy nanocomposite system in different heating rates, a b = 5 �C min-1 and
b b = 15 �C min-1
nCH2
CH2
O
R1
OH
R1
OH
CH2NH R2 CH2 R3CH
O
NH
R2
CH2 CH
R1
O CH2 CH
OH
R3CH
CH CH2 NH R2
R2
CH2 CH
O
R3R1 CH
OH
CH2 N CH2 CH
OH
R3
CH R1 R1 CH
OH
CH2 NH R2H2NR2
o
CH R1 [CH2 CHR1 O]n(i)
(ii–a)
(ii–b)
(iii)
Scheme 1 Schematic
illustration of the various
reactions that may occur during
the cure of epoxy: i epoxy
homopolymerization, ii epoxy–
amine reaction: (a) secondary
amine and (b) tertiary amine,
iii epoxy–OH reaction
(etherification)
778 E. Esmizadeh et al.
123
–8.8 0.2 CNC–KAS
0.2 CNC–OFW
0.2 CNC–Augis
0.2 CNC–Friedman
0.2 CNC–Kissinger
–9.2
–9.6
–10.0
–10.4
3.2
2.8
2.4
2.0
1.6
2.8
3.2
3.6
2.4
2.0
1.6
1.2
0.0024 0.0027 0.0030
α = 0.1α = 0.2α = 0.3α = 0.4α = 0.5α = 0.6α = 0.7α = 0.8α = 0.9
0.0024 0.0027 0.00300.0024 0.0027 0.0030
0.0025 0.0026 0.0027
0.0025 0.0026 0.0027
–8.8
–9.2
–9.6
–10.0
–10.4
–0.8
–1.2
–1.6
–2.4
–2.0
1/T/K–1
1/T/K–1 1/T/K–1
1/TP/K–1
1/TP/K–1
Ln( β
/T2 )
/K2
min
–1
Ln( β
/TP
2 )/K
2m
in–1
ln( β
)/K
min
–1
ln( β
(TP–T
0)–1
)/m
in–1
Ln( β
d( α
)/dt
)/K
min
–2
(a) (b)
(c) (d)
(e)
α = 0.1α = 0.2α = 0.3α = 0.4α = 0.5α = 0.6α = 0.7α = 0.8α = 0.9
α = 0.1α = 0.2α = 0.3α = 0.4α = 0.5α = 0.6α = 0.7α = 0.8α = 0.9
α = 0.1α = 0.2α = 0.3α = 0.4α = 0.5α = 0.6α = 0.7α = 0.8α = 0.9
α = 0.1α = 0.2α = 0.3α = 0.4α = 0.5α = 0.6α = 0.7α = 0.8α = 0.9
Fig. 7 Plots required to calculate activation energy (Ea) for 0.2 CNC sample according to different equations a KAS, b Kissinger, c OFW,
d Friedman, e Augis
Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by… 779
123
(a) KAS
(c) Friedman
(b) OFW
Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
Control0.2 NC0.2 CNT0.2 PNC0.2 CNC
64,000
60,000
56,000
52,000
48,000E/ J
mol
–1
44,000
40,000
64,000
60,000
56,000
52,000
48,000
44,000
40,000
64,000
60,000
56,000
52,000
48,000
44,000
40,000
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
α
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0α
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
α
E/ J
mol
–1
E/ J
mol
–1
α
α
α
Fig. 8 Activation energy (Ea) as a function of degree of cure (a) for the non-isothermal cure experiments obtained by different models a KAS,
b OFW, c Friedman
Table 3 Parameters obtained by polynomial fitting of Ea versus a for prepared epoxy nanocomposites
Sample Models Polynomial parameters (Ea = B0 ? B1 9 a ? B2 9 a2 ? B3 9 a3 ? B4 9 a4)
B0 B1 B2 B3 B4
Control KAS 50,084.13 -8988.74 40,708.36 -54,496.57 14,600.11
OFW 63,216.97 -84,847.24 251,551.01 -299,449.77 117,648.86
Friedman 53,289.92 -41,154.18 180,798.82 -306,618.41 157,636.62
0.2 CNT KAS 39,479.12 56,403.79 -104,588.07 120,686.18 -63,919.49
OFW 47,302.12 17,279.97 15,248.43 -29,778.43 3706.81
Friedman 51,360.28 -5108.17 106,093.45 -208,904.73 107,769.52
0.2 NC KAS 52,256.35 -12,531.50 47,464.65 -62,383.27 16,944.40
OFW 56,665.9 -22,928.07 82,859.94 -108,192.51 38,772.53
Friedman 54,853.65 -37,938.32 153,304.03 -255,717.51 127,174.76
0.2 PNC KAS 48,104.08 8882.73 7203.86 -12,472.80 -5949.85
OFW 61,045.57 -56,050.28 166,090.57 -178,057.37 57,934.09
Friedman 55,914.50 -42,820.19 179,174.98 -265,785.69 118,450.49
0.2 CNC KAS 47,742.71 14,998.21 -5282.57 14,802.97 -25,248.51
OFW 54,772.64 4048.38 -12,483.99 56,781.81 -52,300.75
Friedman 53,156.90 4187.06 -1124.00 48,037.90 -65,318.70
780 E. Esmizadeh et al.
123
in Scheme 1-ii. It could also be related to complex chem-
ical reaction at the near end of conversion by mass transfer
processes such as viscous relaxation and vitrification. Then,
the monomer molecules immobilized in their positions in
the glassy state resulted in the virtual cessation of poly-
merization leading to the decrease in the activation energy
with increasing temperature. However, an increase in the
Ea values with increasing a was also reported for epoxy
resin [41].
Figure 8 indicates that the NC increases Ea value in all
ranges of a which was in accordance with the delayed cure
of 0.2 NC nanocomposite observed before (Fig. 6). In other
words, the presence of exfoliated NC required more energy
to start the cross-linking in cure process, which increased
the activation energy [18].
On the other hand, other epoxy nanocomposites, 0.2
CNT, 0.2 PNC, and 0.2 CNC, showed a very different kind
of behavior. The Ea curve in other epoxy nanocomposites,
with an exception of 0.2NC, reached a maximum around
a = 0.6–0.7 and then decreased as a increased.
The reduction in Ea at low a values (0.1and 0.2) was
be clearly observed with addition of CNT, CNC or PNC
to the epoxy system for KAS and OFW curves (Fig. 8a,
b). The results confirmed the advanced cure reaction in
initial stages of epoxy with the addition of CNT, CNC,
or PNC, as could be seen by the reduction in Tonset and
Tpeak for these samples in Table 2. The observation
explained by inherent high thermal conductivity of CNT
could act as a network to accelerate the heat distribution
into the epoxy (conductivity effect). In addition, the
catalytic effect of OH groups of purified CNT could
further increase the initial cure reaction in 0.2 CNT and
0.2 PNC samples. Besides this, though, it was also
important to bear in mind that the presence of nanofillers
resulted in a physical impediment to the cross-linking
reaction. The increased Ea values after initial stage
a[ 0.2 could be related to the restricted mobility of
polymer chains caused by nanofillers (viscosity effect).
Among all nanocomposite samples, the samples with
0.2 mass% CNC show the highest Ea values at high avalues. This result provided the evidence that the CNC
could be very much effective in hindering the cure
reaction at high a values with enhancing the viscosity of
the epoxy comparing to PNC. The results could be
related to the strong interfacial interaction of synthesized
CNC with epoxy matrix owing to the unique 3D
Table 4 Activation energy of prepared epoxy nanocomposites
Sample Activation energy/J mol-1
KAS (average) Kissinger OFW (average) Friedman (average) Augis
Control 48,706.34 48,534.45 52,527.48 47,832.68 47,659.88
0.2 NC 49,871.62 50,327.81 53,643.23 48,560.83 49,513.58
0.2 CNT 50,826.15 52,122.04 54,527.92 53,759.23 57,710.94
0.2 PNC 51,184.43 52,479.45 54,864.78 51,621.35 54,346.57
0.2 CNC 53,027.87 54,941.27 56,607.68 54,574.91 55,679.30
Table 5 Cure kinetic parameters of the epoxy nanocomposite system
resulting from different models
Sample Models Autocatalytic model:
d/=dt ¼ Ze�Ea=RT /m 1� /ð Þn
Z m n m ? n
Control KAS 33,018.83 0.289 1.764 2.053
Kissinger 31,237.53 0.291 1.761 2.053
OFW 113,338.44 0.247 1.821 2.068
Friedman 24,908.80 0.299 1.751 2.050
Augis 23,558.28 0.301 1.748 2.049
0.2 CNT KAS 51,284.64 0.134 1.707 1.842
Kissinger 77,348.00 0.115 1.720 1.836
OFW 165,857.36 0.078 1.744 1.823
Friedman 129,984.23 0.090 1.737 1.827
Augis 455,006.55 0.030 1.775 1.806
0.2 NC KAS 47,011.61 0.289 1.798 2.087
Kissinger 54,448.08 0.284 1.805 2.089
OFW 158,349.78 0.247 1.856 2.103
Friedman 30,827.98 0.304 1.778 2.082
Augis 41,894.012 0.293 1.793 2.086
0.2 PNC KAS 70,447.82 0.256 1.819 2.076
Kissinger 108,509.58 0.240 1.838 2.079
OFW 230,773.31 0.214 1.872 2.086
Friedman 32,597.04 0.336 1.737 2.073
Augis 206,782.06 0.217 1.867 2.085
0.2 CNC KAS 115,252.97 0.239 1.832 2.072
Kissinger 245,917.99 0.213 1.870 2.084
OFW 367,155.32 0.200 1.889 2.090
Friedman 218,432.22 0.218 1.864 2.082
Augis 312,245.71 0.205 1.881 2.087
Investigation of curing kinetics of epoxy resin/novel nanoclay–carbon nanotube hybrids by… 781
123
structure of them as well as shown before by SEM. Ea
values obtained by the OFW method were greater than
those of the KAS and Friedman methods. This observa-
tion was consistent with the reported results for mela-
mine–formaldehyde resins [18].
The values of Ea as a function of a obtained using
Friedman method are plotted in Fig. 8c. As it can be seen
in the figure, the accelerated cure reaction in the initial
stage of cure could not be observed using Friedman
method. Therefore, the values of Ea seemed more reliable
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0.0014
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0.0014ExperimentalKASKissingerOFWFriedmanAugis
ExperimentalKASKissingerOFWFriedmanAugis
ExperimentalKASKissingerOFWFriedmanAugis
ExperimentalKASKissingerOFWFriedmanAugis
ExperimentalKASKissingerOFWFriedmanAugis
0.0012
0.0010
0.0008
0.0006
0.0004
0.0002
0.0000
0.0014 (a) Control
(c) 0.2 CNT
0.0012
0.0010
0.0008
0.0006
0.0004
d α/d
t /%
s–1
d α/d
t /%
s–1
d α/d
t /%
s–1
d α/d
t /%
s–1
d α/d
t /%
s–1
0.0002
0.00000.0 0.2 0.4 0.6
α0.8 1.0 0.0 0.2 0.4 0.6
α0.8 1.0
0.0 0.2 0.4 0.6
α0.8 1.00.0 0.2 0.4 0.6
α0.8 1.0
0.0 0.2 0.4 0.6
α0.8 1.0
(e) 0.2 CNC
(d) 0.2 PNC
(b) 0.2 NC
Fig. 9 Comparison of experimental data with the kinetic method results (b = 5 �C min-1 heating rate), a control, b 0.2 NC, c 0.2 CNT,
d 0.2 PNC, and e 0.2 CNC
782 E. Esmizadeh et al.
123
for OFW and KAS methods than Friedman method as
reported by Jubsilp et al. [42].
The average Ea values for OFW, KAS, and Friedman
methods and the E value obtained from the slope of the
Kissinger and Augis plots are given in Table 3. The acti-
vation energy calculated for epoxy nanocomposite systems
was higher than that of the pristine epoxy using all kinetic
methods. This result indicated that the presence of nano-
filler inhibited the total cure reaction.
Trying different multiple regression equations to fit
the data in Fig. 8, we found fourth-order polynomial
could have enough goodness of fit. The coefficients of
the best-fitted fourth-order polynomial are given in
Table 4. The obtained polynomials were employed to
find the autocatalytic kinetics where the studied epoxy
system follows according to our previous study [12].
Table 5 summarizes the cure kinetic parameters of the
autocatalytic model obtained by Eq. 2. As shown in
Table 5, the values of m ? n obtained by these five
methods were similar (m ? n * 2), which demonstrated
that the cure reaction was complex as proved by the
variation in Ea during the cure reaction. Meanwhile, the
calculated m ? n values indicated that the presence of
nanofiller, except CNT, could enhance the overall order
of the cure reaction.
To demonstrate the applicability of these five kinetic
methods, the results were compared with the experimental
data of 5 �C min-1 heating rate, as shown in Fig. 9. Gen-
erally speaking, the results with conversion-dependent Ea
methods (KAS, OFW, and Friedman) were in better
agreement with the experimental data than conversion-in-
dependent Ea methods (Kissinger and Augis). The similar
behavior was reported for epoxy/nano SiC system before
[43]. Furthermore, as far as the kinetic method results with
conversion-dependent Ea methods was concerned, differ-
ences between model predictions and experimental data
were observed to be very small in the case of KAS and
OFW methods.
The Friedman method results diverged obviously from
the experimental results at higher conversions. Meanwhile,
the KAS and OFW kinetic method results were in good
agreement with the experimental data in the whole tested
conversions range.
Conclusions
The cure behavior of epoxy was investigated using non-
isothermal DSC in the presence of CNT, NC, and their physical
and chemical hybrids (PNC and CNC, respectively). Two
categories of kinetic methods were employed to calculate the
activation energy of the cure reaction: (1) conversion-
dependent including KAS, OFW, and Friedman methods, and
(2) conversion-independent including Kissinger and Augis
methods. The following items could be concluded in this study:
• The final cure characteristics of epoxy nanocomposites
were controlled by the competition of accelerating effect
(conductivity effect of CNTs and catalytic effect of OH
groups of purified CNTs) and decelerating effect of
nanofillers due to increased viscosity (viscosity effect).
• The accelerating effect of CNTs in samples 0.2 CNT,
0.2 PNC, and 0.2 CNC was predominant in the initial
stage of cure reaction.
• According to the conversion-dependent methods, acti-
vation energy (Ea) was proved to decrease gradually at
high conversions (a[ 0.6).
• Compared with neat epoxy, the Ea values of the 0.2 NC
composite are improved owing to the restricted motion
of polymer chains (viscosity effect).
• In KAS and OFW methods, theEa values for 0.2 CNT, 0.2
PNC, and 0.2 CNC are lower than that of control sample at
low conversions. The trend is reverse at high conversions.
• Among the hybrid nanofillers (CNC and PNC), the
uniqueness of the CNC was more marked in increasing
Ea values of epoxy after initial stage.
• Friedman method was not able to illustrate the accel-
erating effect of CNTs in initial stage of cure.
• Conversion-dependent methods were in better agree-
ment with the experimental data than conversion-
independent methods.
• KAS and OFW were proved to be more reliable than
Friedman due to their negligible diverge from exper-
imental results.
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