Original Article

Cure kinetics of exfoliated bio-basedepoxy/clay nanocomposites developedfrom acrylated epoxidized castor oil anddiglycidyl ether bisphenol A networks

Nagarjuna Reddy Paluvai1,2, S. Mohanty2 and S. K. Nayak1,2

AbstractA bio-based epoxy monomer was synthesized by acrylation of epoxidized castor oil (ECO). Subsequently, acrylated ECO(AECO)-toughened diglycidyl ether of bisphenol A (DGEBA) nanocomposites were prepared via sol–gel process with theaddition of organically treated montmorillonite nanoclays. In this study, the curing kinetics of anhydride-cured DGEBA/AECOmonomer with and without clays was studied by non-isothermal differential scanning calorimetry analysis. The apparent acti-vation energy obtainedbyFlynn–Wall–Ozawa method was reduced from 63 to59kJ mol�1 and 69 to61kJ mol�1, respectively,with the addition of 1 wt% clay to the DGEBA/10 wt% AECO and DGEBA/20 wt% AECO systems, respectively. The two-parameter Sestak–Berggren autocatalytic model was used to obtain the reaction orders m and n, respectively. The curvesobtained by the Malek method show good agreement with the experimental data for bio-based epoxy systems.

KeywordsAcrylated epoxidized castor oil, cure kinetics, diglycidyl ether of bisphenol A, montmorillonite clays

Introduction

Petroleum-based polymers have been attracting interest

because of their potential applications, which range from

commodity to aerospace industry. In recent years, renew-

able-based polymer products have become increasingly

important due to environmental concerns, sustainability,

and rapid depletion of petroleum-based polymers.1–4 Bio-

based polymers are derived from plants, such as cellulose,

fatty acids, starch, and sugars, which can replace the

petroleum-based polymers.5 Monomers derived from unsa-

turated fatty acid compounds such as vegetable oils are cur-

rently being given significant attention in industrial

applications due their abundance, low cost, and the pres-

ence of more functional groups.6,7 These oils are converted

into polymers through various polymerization techniques

that have been reported in earlier studies.8–14

Epoxy resins are one of the most important thermo-

setting polymers that are widely used in many industrial

applications due to their superior mechanical, thermal,

dielectric, insulating, and optical properties. The molecular

structure of the epoxy monomer can be easily modified due

to its versatile properties.4,15,16 The major disadvantage of

cured monomers is low toughness due to the presence of

aromatic groups on their backbone. To address this, the

toughness of brittle epoxy resin is increased by adding elas-

tomers and thermosetting and thermoplastic polymers to

it.4,17 However, the use of elastomers and other polymers

reduces some of the macroscopic and microscopic proper-

ties of the epoxy resin.16–20 In this regard, several attempts

have been made to develop bio-based polymers, modify

and then to improve the physical and chemical performance

of epoxy monomers, and to protect the environment from

carbon emissions.3 Herein, castor oil (CO)-based polymers

have been synthesized and used to modify the epoxy

1 Advanced Research School for Technology & Product Simulation

(ARSTPS), Central Institute of Plastics Engineering and Technology

(CIPET), Chennai, Tamil Nadu, India2 Laboratory for Advanced Research in Polymeric Materials (LARPM),

Central Institute of Plastics Engineering and Technology (CIPET),

Bhubaneswar, Odisha, India

Corresponding author:

Nagarjuna Reddy Paluvai, Advanced Research School for Technology &

Product Simulation (ARSTPS), Central Institute of Plastics Engineering

and Technology (CIPET), Chennai, Tamil Nadu, India.

Email: nag1987@gmail.com

High Performance Polymers2015, Vol. 27(8) 918–929ª The Author(s) 2015Reprints and permission:sagepub.co.uk/journalsPermissions.navDOI: 10.1177/0954008314566052hip.sagepub.com

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monomers due to their low-cost preparation, easy process-

ing, and ability to make partially or fully sustainable mate-

rials. CO is one of the most abundant vegetable oils, also

renewable, biodegradable, biocompatible, and is of low

cost. It contains unsaturated fatty acid compounds (i.e, rici-

noleic acid (12-hydroxy-cis-9-octadecenoic acid)) that

have been used to develop eco-friendly polymers via in situ

polymerization processes.21–23 However, one of the main

disadvantages of bio-based polymers obtained from CO is

the poor mechanical properties they exhibit. Thus, the

novel bio-based acrylated epoxidized CO (AECO) resins

have been obtained by the acrylation of ECO resin to

improve their thermomechanical properties and water resis-

tance due to the presence of pendant acrylic double bonds,

which readily participate in the free radical reaction.24,25

AECO has been considered as a partial replacement of digly-

cidyl ether of bisphenol A (DGEBA) epoxy monomer in

developing industrial components. For example, several

studies have reported on the acrylated epoxidized soybean

oil (AESO) system for making greener composites.26–28

Further, the AECO-toughened DGEBA epoxy systems

are reinforced with nanoparticles to enhance their mechan-

ical and thermal properties. Nanocomposites are formed by

mixing different organic or inorganic fillers such as silica

particles, aluminum nitride nanoparticles, organoclay, tita-

nium dioxide, and mesoscopic into the polymer matrix to

improve their properties.29–33 Herein, organically treated

montmorillonite (OMMT) clays were used to prepare the

acrylated epoxy nanocomposites to enhance their polymer

properties. Kardar et al.34 used nano-alumina particles to

improve the physical–mechanical properties of epoxy acry-

late composites. Thielemans et al.35 have prepared AESO

nanocomposites and observed a 30% enhancement in their

mechanical properties. It is well known that the properties

of cured epoxy monomers depend on the curing conditions

such as the temperature and the time.36 Differential scan-

ning calorimetry (DSC) technique was used to study the

kinetics of cure reaction under non-isothermal modes.37–41

A variety of kinetic models had been proposed to inves-

tigate kinetic parameters, such as Kissinger,42,43 Flynn–

Wall–Ozawa,44 Kamal,45 autocatalytic,46 and Borchardt

Daniels47 methods. Sestak–Berggren model applied to a

two-parameter autocatalytic model to give emphasis to the

cure kinetics of epoxy systems by non-isothermal DSC.48,49

The objective of this study is to understand the effect of

OMMT clays on the cure mechanism and kinetics of AECO-

toughened DGEBA epoxy systems. The chemical structures

and functional groups of CO, ECO, and AECO systems were

examined by Fourier transform infrared (FTIR) and nuclear

magnetic resonance (NMR) spectroscopies. DSC technique

was used to investigate non-isothermal cure kinetics of bio-

based systems. The cure kinetic parameters were obtained by

applying iso-conversional Kissinger and Flynn–Wall–Ozawa

methods and a two-parameter Sestak–Berggren autocatalytic

model.

Experimental

Materials

Anhydrous sodium sulfate, acrylic acid, diethyl ether, glacial

acetic acid, hydrogen peroxide (30 wt.%), hydroquinone,

sodium carbonate, sulfuric acid (98%), triethylamine, and

other analytical grade products used were procured from

Fisher Scientific Company (Hyderabad, Telangana, India).

CO was procured from SD Fine-Chem Limited (Mumbai,

Maharashtra, India). DGEBA epoxy resin (viscosity: 9–

12 Pa s and epoxide equivalent: 182–192 g eq�1) was sup-

plied by M/s Allied agencies Ltd., Hyderabad, Telangana,

India. OMMT Cloisite 30B contains bis(2-hydroxyethyl)

methyl tallow ammonium ions from Southern Clay Products

(Gonzales, Texas, USA) and were used as filler. Methylhex-

ahydrophthalic anhydride (MHHPA), benzyltriethylammo-

nium chloride (TEBAC), and 3-aminopropyltriethoxy

silane were purchased from Sigma-Aldrich (Bangalore, Kar-

nataka, India).

Synthesis of ECO

Typically, ECO was synthesized by the following procedure:

100 g of CO, 26 g of glacial acetic acid, and 29 g of H2SO4

were charged to a three-necked round flask. The mixture was

rapidly stirred for 30–40 min at 55�C. The epoxidation reac-

tion was carried out using 80 g of H2O2 (30 wt%) slowly

added dropwise using a funnel to the mixture for 3 h at

55–60�C. After the addition of all reagents, the obtained

mixture was further stirred at 55�C for 7 h. The reacted mix-

ture was then extracted using diethyl ether in a separating

funnel and followed by washing two times with 10 wt% of

sodium carbonate (Na2CO3) solution. The distilled water was

used to remove the free acids. The collected organic phase

was kept in a vacuum oven at 70�C overnight to remove

diethyl ether and to obtain a light reddish viscous liquid

of ECO (yield: 97 g, average epoxy equivalent weight ¼278 g eq�1, oxirane oxygen of about 6.19%).3,8,50,51

Synthesis of AECO

A mixture of ECO (50 g) and acrylic acid (12 g) containing

0.5 wt% hydroquinone was charged into a 250-ml round-

bottomed flask and stirred at 70�C for 30 min. Then,

triethylamine (2 g) was added to the mixture and stirred for

another 1 h at 45–60�C. An additional amount of acrylic

acid (8 g) was added to the mixture and then kept at

60�C for another 1 h. The mixture was brought to room

temperature and then the unreacted acrylic acid was

removed by dissolving the product in diethyl ether and

washed several times with 10% sodium carbonate solution.

The product was washed with saturated sodium chloride

solution and dried over magnesium sulfate. The collected

organic phase of AECO was kept in a vacuum oven at

70�C for 4 h to remove diethyl ether.9,28,29

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Sample preparation

Bio-based epoxy nanocomposites were prepared by adding

1 wt% clay to the DGEBA/AECO systems. Prior to the addi-

tion of clays, the different weight ratios of 10, 20, 30, and 40

wt% AECO-toughened DGEBA epoxy systems were pre-

pared by mechanical mixing for 2 h followed by sonication

for 30 min. OMMT clays were dried in an oven at 120�C for

4 h. High-pressure mixing method was chosen to disperse

the clays into DGEBA/AECO systems.52 OMMT clays were

dispersed in acetone and mixed at 1400 r min�1 for 20–30

min until a wet slurry was obtained. The slurry was com-

bined with a mixture of DGEBA/AECO at room tempera-

ture with a mechanical stirring for about 4 h to obtain a

homogeneous liquid. Then the mixture was sonicated for

about 2 h with pulse mode (15 s on/15 s off) to achieve a tem-

perature of 70�C to remove the acetone. After that, a stoi-

chiometric ratio of MHHPA and TEBAC were added to

the sonicated mixture and continuously stirred for 10 min

at room temperature using a mechanical stirrer. The opti-

mum weight of the uncured sample was placed in the DSC

pan and dynamically cured from room temperature to

200�C at various heating rates to obtain the characteristics

of the exothermic peak. The prepared samples for this inves-

tigation were named as E1–E4 (DGEBA/10–40% AECO)

and EC1–EC4 (DGEBA/10–40% AECO/1% OMMT).

Characterizations

FTIR spectra of CO, ECO, and AECO systems were

recorded using a Perkin-Elmer 1720X (Norwalk, California,

USA) spectrometer. Each sample was placed between two

potassium bromide disks, and the spectrum was obtained

by co-adding 64 consecutive scans with a resolution of 4

cm�1 within the range of 400–4000 cm�1 using a DTGS

detector.

The proton NMR (1H NMR) spectra of CO, ECO, and

AECO were recorded on a Bruker Avance III 500 MHz

(AV 500) NMR Spectrometer (Billerica, Massachusetts,

USA) operating at 400 MHz. Tetramethylsilane was used

as the internal standard and deuterated chloroform (CDCl3)

as the solvent.

Dynamic DSC analysis of samples was carried out using

a differential scanning calorimeter (Q 50, TA Instruments,

New Castle, Delaware, USA) as per ASTM E 1868, sam-

ples of �10 mg were secured from 0�C to 200�C at differ-

ent heating rates (5, 10, 15, and 20�C min�1) under nitrogen

atmosphere.

Kinetic models

The cure kinetics has been studied using DSC analysis.

The basic assumption in DSC analysis of cure kinetics is

that the area under the exothermic curve is proportional

to the conversion or degree of the cure (�) and can be

expressed as53:

�ðtÞ ¼ HðtÞ�HT

ð1Þ

where �HT is the total heat of cure reaction and H(t) is the

partial heat of cure reaction at time t. The rate of conversion

(d�/dt) at a given temperature (T) is usually obtained by54:

d�

dt¼ q

d�

dt¼ dHðtÞ

�HTdtð2Þ

where q ¼ dT/dt is the heating rate and dH(t)/dt is the

instantaneous heat flow. The cure reaction is carried out

at a constant heating rate and then the �rate of the kinetic

process can be expressed as55:

d�

dt¼ KðTÞ � f ð�Þ ð3Þ

where K(T) is the temperature-dependent reaction rate con-

stant, T is the absolute temperature, and f(�) is a dependent

kinetic model function. The reaction rate constant is usu-

ally assumed to be of Arrhenius form56:

KðTÞ ¼ A exp�Ea

RT

� �ð4Þ

where the parameters A is the pre-exponential factor, R

is the gas constant (8.31 J mol�1 K�1), and Ea is the app-

arent activation energy. It is calculated using various

iso-conversional methods such as Kissinger and Flynn–

Wall–Ozawa method. According to the Kissinger analysis,

the equation (5) is expressed as:

� lnq

T2max

� �¼ Ea

RTmax

� lnAR

Ea

� �ð5Þ

where, Tmax is the peak temperature and A is the pre-

exponential factor. The activation energy could be calculated

from the plot of an approximately linear relationship of�ln(q/

Tmax2) versus 1/Tmax with the correlation of experimental

data. The Flynn–Wall–Ozawa model was also used to calcu-

late the Ea throughout the entire reaction. According to the

Flynn–Wall–Ozawa analysis, equation (6) is expressed as:

log q ¼ logAEa

g �ð ÞR� 2:315� 0:4567Ea

RTmax

� �ð6Þ

where g(�) is a conversion-dependent function, and other

parameters are the same as described earlier. To determine

the corresponding activation energy at each degree of cure,

a plot of log q versus 1000/Tmax was drawn. The resulting

slope obtained from linear fitting plot indicates the activa-

tion energy. For non-isothermal DSC analysis of thermo-

setting resin cure, f(�) can be obtained using

autocatalytic models. Sestak–Berggren proposed that two

independent reaction orders can be considered: m and n

as shown in equation (7).48

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d�

dt¼ A exp

�Ea

RT

� �ð1� �Þn�m ð7Þ

On the other hand, Malek proposed two special func-

tions, y(�) and z(�), and can be expressed as57:

yð�Þ ¼ d�

dtexpðxÞ ð8Þ

zð�Þ ¼ �ðxÞ d�dt

T

qð9Þ

where x is reduced activation energy (Ea/RT) and �(x) is

the expression related to the temperature integral, which

can be approximated using the equation of Senum and

Yang58:

� xð Þ ¼ x3 þ 18x2 þ 88xþ 96

x4 þ 20x3 þ 120x2 þ 240xþ 120ð10Þ

The values of both y(�) and z(�) functions normalized

within the interval from 0 to 1. The kinetic parameters such

as n, m, and A can be obtained from the curves of normal-

ized y(�) and z(�) versus the degree of cure (�). The curves

of y(�) and z(�) exhibit maxima at �M and �1p , respec-

tively. The mean of �M is used to calculate the kinetic para-

meters. The ratio of reaction order, p ¼ m/n as pointed out

by Malek, can be replaced by:

p ¼ m

n¼ �Mm

1� �Mm

ð11Þ

where �Mm is the mean of �M.

Results and discussions1H NMR spectroscopy

The synthesis reactions of CO, ECO, and AECO systems

are shown in Figure 1. 1H NMR spectroscopy analysis was

performed to confirm the chemical structures in CO, ECO,

and AECO, depicted in Figure 2. The characteristic signals

appeared at 5.3 to 5.7 ppm of CO associated with C¼C

bonds which got converted into an oxirane ring at 2.8 to

3.2 ppm in case of ECO.31–34 The peaks obtained in AECO

from 5.7 to 6.2 ppm indicate the presence of C¼C of the

acrylate ester.24,51,52

FTIR spectroscopy

Figure 3 shows the infrared spectra of the CO, ECO, and

AECO systems. The spectrum of ECO shows OH stretching

at 3400 cm�1, CH2-asymmetric stretching at 2927 cm�1, sym-

metric stretching at 2852 cm�1, asymmetric C¼O ester at 1744

cm�1, epoxy group at 1244 and 844 cm�1, asymmetric C–O–C

Figure 1. The possible synthesis mechanism of ECO and AECO systems. ECO: epoxidized castor oil; AECO: acrylated epoxidizedcastor oil.

Figure 2. 1H NMR spectra of the CO, ECO, and AECO sys-tems. 1H NMR: proton nuclear magnetic resonance; CO: castoroil; ECO: epoxidized castor oil; AECO: acrylated epoxidizedcastor oil.

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ester at 1165 cm�1, respectively. The acrylation of ECO was

further confirmed by the appearance of FTIR peaks at 1639,

1462, and 1400 cm�1 corresponding to acrylate groups.59–62

DSC analysis

To identify the curing process of E1–E4 and EC1–EC4 sys-

tems, DSC analysis (with 10�C min�1 heating rate) was

undertaken and the corresponding thermograms are shown

in Figures 4 and 5. The data presented in Table 1 indicate

the heat of reaction, which decreased with the increasing

AECO content within the DGEBA resin. This phenomenon

is primarily due to the decrease of rate of reaction with the

addition of AECO to the DGEBA resin, which decreases in

the cross-linking density. As shown in Figure 4, the cure

reaction of exothermic peaks of E1–E4 increased with the

addition of AECO to the tune of 10 to 40 wt% to the

DGEBA system. It is evident that Tend decreased from

159 to 175�C, which indicated the fact that the rigid aro-

matic groups of DGEBA were replaced by the flexible

groups of AECO resin. Then plasticization increased with

the lack of chemical interactions between the DGEBA and

higher concentrations of AECO resin. As shown in Table 1,

the heat of reaction (�H) obtained for the E1, E2, E3, and

E4 were 524, 440, 412, and 324 J g�1, respectively. These

values suggest that the mixing of AECO into DGEBA

needs more heat for the curing process. Figures 4 and 5

show that the curing reaction of EC1–EC4 had lower

exothermic (low �H) values than that of E1–E4 sys-

tems.63,64 The enthalpies of the DGEBA/AECO/OMMT

samples were decreased which is attributed to the catalytic

Figure 3. FTIR spectra of the CO, ECO and AECO systems. FTIR:Fourier transform infrared; CO: castor oil; ECO: epoxidized castoroil; AECO: acrylated epoxidized castor oil.

Figure 4. DSC thermograms of DGEBA/AECO blends. DSC:differential scanning calorimetry; DGEBA: diglycidyl ether ofbisphenol A; AECO: acrylated epoxidized castor oil.

Figure 5. DSC thermograms of DGEBA/AECO/OMMTnanocomposites. DSC: differential scanning calorimetry; DGEBA:diglycidyl ether of bisphenol A; AECO: acrylated epoxidizedcastor oil; OMMT: organically modified montmorillonite.

Table 1. Data obtained from DSC curves.

Samples Tmax (�C) Tend (�C) �H (J g�1)

E1 95 159 524E2 89 160 440E3 93 170 412E4 100 175 324EC1 93 190 437EC2 80 170 340EC3 88 145 310EC4 87 130 220

DSC: differential scanning calorimetry; Tmax: maximum peak temperature;�H: enthalpy.

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effect and dispersion of OMMT silicate layers of clay in the

DGEBA/AECO systems. This behavior can be further

explained by the fact that the presence of alkyl ammonium

ions in the OMMT clay acted as a catalyst for the DGEBA/

AECO system, thus contributing to an increase in the rate

of reaction with faster curing at lower temperatures. This

resulted in a shift of the exothermic peaks to the lower tem-

perature as manifested in our findings. However, when

compared with unfilled DGEBA/AECO systems, the max-

imum peak temperature (Tmax) was slightly decreased with

the addition of 1% OMMT to the DGEBA/AECO samples.

This behavior is possibly attributed to the increase in the

cross-linking density, which results in strong interactions

due to formation of covalent bond between the clay and

DGEBA/AECO system. As observed from Table 1, the

E2 system shows improved performance with regards to

decrease in the curing temperature in presence of 1%OMMT nanoclay from 89�C to 80�C in EC2. Camille

et al.65 reported that the montmorillonite is responsible for

the decrease of Tmax and heat of reaction of the nanocom-

posite as compared to the DGEBA system. Similar facts

have been substantiated in our findings. Further, the �H

values decreased with the addition of 30% and 40% AECO

to the DGEBA due to the plasticizing effect.

The cure kinetics of E1, E2, EC1, and EC2 systems were

studied using non-isothermal differential scanning (DSC)

calorimeter at various heating rates (q) at 5, 10, 15, and

20�C min�1 (Table 2). The curing of bio-based systems

exhibited only one exothermic peak during the non-

isothermal DSC analysis. The maximum peak temperature

(Tmax) even increased with the increase of heating rates

(Figures 6 to 9, Table 2), which is due to the addition of

AECO. The obtained peak63–65 curing temperature of

EC2 (70�C) was much lower than that of E2 (79�C) at a

heating rate 5�C min�1, which indicates that the reactivity

of EC2 was higher than that of E2.66 The activation energy

of the curing reaction was determined by both Kissinger

and Flynn–Wall–Ozawa methods using equations (5) and

(6). The activation energy for cure kinetics can be calcu-

lated from the slope of the plot from Figure 10 and values

obtained for E1, E2, EC1, and EC2 were 59, 66, 53, and 57

kJ mol�1, respectively. Figure 11 shows that the resulting

slope obtained from linear fitting plot indicates the

activation energy. The activation energy for cure kinetics

was 63, 69, 59, and 61 kJ mol�1 obtained for E1, E2,

EC1, and EC2 systems, respectively. An increase in the

AECO content to DGEBA resulted in increased activation

energy due to the decrease of the viscosity and increase of

the number of epoxy groups. The activation energy of the

E2 system was higher than that of the E1 system. This is

due to the presence of more flexible (aliphatic) groups of

AECO added to the aromatic groups of DGEBA epoxy sys-

tem, which greatly decreases the reactivity of the system

during the curing process. Higher activation energy was

obtained for the lower reactivity systems. The reactivity

of the DGEBA/AECO/OMMT system was higher than that

of the DGEBA/AECO systems, due to the strong bonds

Table 2. Data obtained from non-isothermal DSC curves atheating rates of 5, 10, 15, and 20�C min�1.

Heating rate (�C min�1) Ea (kJ mol�1)

Samples 5 Tp 10 Tp 15 Tp 20 Tp Ea,Kissnger Ea,Ozawa

E1 84 95 102 108 59 63E2 79 89 95 100 66 69EC1 81 93 101 106 53 59EC2 69 80 87 92 57 61

DSC: differential scanning calorimetry; Ea: activation energy.

Figure 6. DSC thermograms of 90% DGEBA/10% AECO atdifferent heating rates. DSC: differential scanning calorimetry;DGEBA: diglycidyl ether of bisphenol A; AECO: acrylatedepoxidized castor oil.

Figure 7. DSC thermograms of 80% DGEBA/20% AECO atdifferent heating rates. DSC: differential scanning calorimetry;DGEBA: diglycidyl ether of bisphenol A; AECO: acrylatedepoxidized castor oil.

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formed between DGEBA/AECO and OMMT clay. The

results revealed that the presence of organoclay has lowered

the curing temperature to a greater extent, significantly low-

ering the activation energy and increasing the reactivity of the

bio-based epoxy system.67 The activation energies obtained

for all bio-based systems from the Flynn–Wall–Ozawa model

were found to be higher than that of the kissinger Model. Sim-

ilar results have been reported by several authors.68,69

The degree of cure or conversion for 10 and 20 wt%AECO toughened DGEBA epoxy nanocomposites have

been calculated46,70 using equation (1). The degree of cure

versus temperature curves of both EC1 and EC2 systems

shows an ‘S’ shape; which indicates that the cure reaction

was autocatalytic (Figures 12 and 13).69

The estimated Ea values from Flynn–Wall–Ozawa model

were used to calculate the y(�) and z(�) functions using

equations (8) and (9), respectively. Figures 14 and 15 and

16 and 17 represent the variation of y(�) and z(�) values with

the degree of cure for 10 and 20 wt% of AECO-toughened

DGEBA nanocomposites, respectively. As noted, the y(�)

curve exhibits maximum value at�M, and that of z(�) exhibits

maximum value at�1P , as shown in Table 3. The results are in

good agreement with those reported by several authors36,71

that the values of �1P lower than 0.632 indicate that the stud-

ied cure reaction can be described using the two-parameter

autocatalytic kinetic model Sestak–Berggren. Using equation

(7), the value of the kinetic exponent n can be obtained from

the slope of linear dependence In½qeX d�dt� versus ln[�P(1 –

�)]. The estimated n value was used to calculate the kinetic

parameter m from equation (11). The kinetic parameters n,

m, and ln A at different heating rates are listed in Table 4.

Figure 11. Plots to determine activation energies by the Flynn–Wall–Ozawa model DGEBA/AECO and its nanocomposites.DGEBA: diglycidyl ether of bisphenol A; AECO: acrylated epoxi-dized castor oil.

Figure 9. DSC thermograms of 80% DGEBA/20% AECO/1%OMMT at different heating rates. DSC: differential scanningcalorimetry; DGEBA: diglycidyl ether of bisphenol A; AECO:acrylated epoxidized castor oil; OMMT: organically modifiedmontmorillonite.

Figure 8. DSC thermograms of 90% DGEBA/10% AECO/1%OMMT at different heating rates. DSC: differential scanningcalorimetry; DGEBA: diglycidyl ether of bisphenol A; AECO:acrylated epoxidized castor oil; OMMT: organically modifiedmontmorillonite.

Figure 10. Plots to determine activation energies by the Kissingermodel for DGEBA/AECO and its nanocomposites. DGEBA: digly-cidyl ether of bisphenol A; AECO: acrylated epoxidized castor oil.

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The accuracy of the kinetic model proposed by the

Malek method was verified by plotting d�/dt versus the

temperature with the calculated data in Table 4 and experi-

mental data. Figures. 18 and 19, show that the calculated

data (solid lines) coincide with the experimental values

(symbols) at different heating rates, which indicate that the

two-parameter Sestak–Berggren model deduced by the

Malek method could be used to study the entire curing reac-

tion process of 10 and 20 wt% of AECO-toughened

DGEBA epoxy nanocomposites.

Conclusions

The cure kinetics of AECO-toughened DGEBA epoxy sys-

tems were investigated by non-isothermal DSC analysis

with and without OMMT clay. The epoxy groups of ECO

Figure 15. Normalized function curve of y(�) for DGEBA/20%AECO/1% OMMT system. DGEBA: diglycidyl ether of bisphenolA; AECO: acrylated epoxidized castor oil; OMMT: organicallymodified montmorillonite.

Figure 16. Normalized function curve of z(�) for DGEBA/10%AECO/1% OMMT system. DGEBA: diglycidyl ether of bisphenolA; AECO: acrylated epoxidized castor oil; OMMT: organicallymodified montmorillonite.

Figure 14. Normalized function curve of y(�) for DGEBA/10%AECO/1% OMMT system. DGEBA: diglycidyl ether of bisphenolA; AECO: acrylated epoxidized castor oil; OMMT: organicallymodified montmorillonite.

Figure 13. Conversion rate versus temperature of 80% DGEBA/20% AECO/1% OMMT system. DGEBA: diglycidyl ether ofbisphenol A; AECO: acrylated epoxidized castor oil; OMMT:organically modified montmorillonite.

Figure 12. Conversion rate versus temperature of 90% DGEBA/10% AECO/1% OMMT system. DGEBA: diglycidyl ether ofbisphenol A; AECO: acrylated epoxidized castor oil; OMMT:organically modified montmorillonite.

Paluvai et al. 925

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were confirmed by 1H NMR spectra at 2.8 to 3.2 ppm

and by FTIR spectra at 1244 and 844 cm�1, respectively.

Further, the acrylation of ECO was confirmed through the

appearance of peaks in the FTIR spectra at 1639, 1462, and

1400 cm�1, respectively. The heat of reaction decreased for

EC1–EC4 due to the addition of the catalytic effect of

OMMT on DGEBA/AECO system. With the addition of 1

wt% OMMT clay to the DGEBA/10% AECO and

DGEBA/20% AECO systems, the Ea values obtained by the

Flynn–Wall–Ozawa method were reduced from 63 to 59 kJ

mol�1 and 69 to 61 kJ mol�1, respectively. The two-

parameter autocatalytic Sestak–Berggren model was used

to describe the curing process of the studied systems. The

model parameters were calculated by the Malek method.

The calculated kinetic parameter data show good agreement

with the experimental data of bio-based systems. This sys-

tem represents the DGEBA resin replaced by AECO/OMMT

and indicates a sustainable material for industrial use.

Table 4. Estimated kinetic parameters m, n, and ln A for Sestak–Berggren model.

Sample Heating rates (�C min�1) m n ln A (min�1)

EC1 5 0.96 1.56 15.7110 1.00 1.58 15.9415 1.01 1.55 16.0920 1.05 1.58 16.37

Mean 1.00 1.57 16.02EC2 5 0.88 1.36 15.37

10 0.89 1.34 15.6115 0.88 1.29 15.8020 0.92 1.29 15.97

Mean 0.89 1.32 15.68

Figure 18. Comparison of experimental (symbols) and calculated(lines) DSC curves for DGEBA/10% AECO/1% OMMT system.DSC: differential scanning calorimetry; DGEBA: diglycidyl ether ofbisphenol A; AECO: acrylated epoxidized castor oil; OMMT:organically modified montmorillonite.

Figure 19. Comparison of experimental (symbols) and calculated(lines) DSC curves for DGEBA/10% AECO/1% OMMT system.DSC: differential scanning calorimetry; DGEBA: diglycidyl ether ofbisphenol A; AECO: acrylated epoxidized castor oil; OMMT:organically modified montmorillonite.

Figure 17. Normalized function curve of z(�) for DGEBA/20%AECO/1% OMMT system. DGEBA: diglycidyl ether of bisphenolA; AECO: acrylated epoxidized castor oil; OMMT: organicallymodified montmorillonite.

Table 3. Characteristic peak conversions �M and �P1.

Sample Heating rates (oC min�1) �M �P1

EC1 5 0.38 0.6110 0.39 0.6115 0.39 0.6320 0.40 0.62

Mean 0.39 0.62EC2 5 0.39 0.63

10 0.39 0.5815 0.41 0.5920 0.41 0.62

Mean 0.40 0.60

926 High Performance Polymers 27(8)

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Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with

respect to the research, authorship, and/or publication of this

article.

Funding

The author(s) disclosed receipt of the following financial support

for the research, authorship, and/or publication of this article: The

authors are thankful to the Center of Excellence (CoE) under the

GREET project sponsored by the Department of Chemicals and

Petrochemicals, Govt of India.

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