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

G.Naderi@ippi.ac.ir

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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