A nanocomposite was synthesized using silica nanoparticles (SN) and Epoxy Vinyl Ester Resin (VE671). Nanoparticles were dispersed in the mixture by ultrasonic equipment to prevent the agglomera-tion. Transmission electron microscopy (TEM) was used to investigate the dispersion of the silica nanopar-ticles in the mixture. Non-isothermal differential scanning calorimetry (DSC) technique was used to study the cure kinetics of VE671 resin with and without adding silica nanoparticles. The activation energy (Ea) was determined by using Kissinger and Ozawa equations. The Ea values of curing for VE671 / 4% SN sys-tem showed a decrease with respect to the neat resin. It means that there is a catalytic effect of silica na-noparticles in the cure reaction. A dynamic kinetic model was obtained to predict the degree of cure and cure rate of resin. The results showed a good agreement between the model and the experimental data for different heating rates. The char yields increased with the addition of 4% of SN to the epoxy resin and im-proved the polymer flame retardancy and thermal resistance at high temperatures.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3545

Aghili, A.

Arabli, V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 3, 03NCNN26(4pp) (2013) 
 
 
2304-1862/2013/2(3)03NCNN26(4) 03NCNN26-1  2013 Sumy State University 
Effect of Silica Nanoparticles on the Curing Kinetics of Epoxy Vinyl Ester Resin 
 
V. Arabli*, A. Aghili† 
 
Department of Polymer Engineering, Shiraz Branch, Islamic Azad University, Shiraz, Iran 
 
(Received 07 June 2013; published online 01 September 2013) 
 
A nanocomposite was synthesized using silica nanoparticles (SN) and Epoxy Vinyl Ester Resin 
(VE671). Nanoparticles were dispersed in the mixture by ultrasonic equipment to prevent the agglomera-
tion. Transmission electron microscopy (TEM) was used to investigate the dispersion of the silica nanopar-
ticles in the mixture. Non-isothermal differential scanning calorimetry (DSC) technique was used to study 
the cure kinetics of VE671 resin with and without adding silica nanoparticles. The activation energy (Ea) 
was determined by using Kissinger and Ozawa equations. The Ea values of curing for VE671 / 4% SN sys-
tem showed a decrease with respect to the neat resin. It means that there is a catalytic effect of silica na-
noparticles in the cure reaction. A dynamic kinetic model was obtained to predict the degree of cure and 
cure rate of resin. The results showed a good agreement between the model and the experimental data for 
different heating rates. The char yields increased with the addition of 4% of SN to the epoxy resin and im-
proved the polymer flame retardancy and thermal resistance at high temperatures. 
 
Keywords: Silica Nanoparticles, Epoxy Vinyl Ester Resin, Nanocomposite, Curing Kinetics, Modeling. 
 
 PACS numbers: 82.35.Np, 82.20.Wt 
 
 
                                                        
* arably@yahoo.com 
† aghili@iaushiraz.ac.ir, alirezaaghili@yahoo.com 
1. INTRODUCTION  
 
The thermoset resins have many applications, be-
cause of their highly desirable properties such as stiff-
ness, suitable chemical resistance, wear resistance, 
excellent adhesion and low shrinkage after curing. It is 
important to study the cure kinetics and the correlation 
between the degree of cure and the thermal and me-
chanical properties to design the optimum curing con-
ditions [1-6]. To provide organic–inorganic hybrid ma-
terials, we can mix the silica nanoparticles with poly-
meric materials. The hybrid materials are also known 
as nanocomposites [5, 7]. Silica nanoparticles can be 
widely used in paints, plastics, color rubbers, reinforce 
fillers in epoxy molding compounds, adhesives and 
many other fields [5-8]. Liu et al have reported that 
nanoscale colloidal silica particles act as a curing agent 
in the curing state of epoxy–silica nanocomposite for-
mation [7]. It was showed an interesting reactivity of 
silica nanoparticles toward epoxy resins without the 
need of adding other catalyst in cure reaction.  
In this research, an epoxy―silica nanocomposite 
was synthesized and the cure kinetics of reaction for 
neat epoxy and nanocomposite was investigated by us-
ing non-isothermal DSC technique and a kinetic model 
was obtained. Thermal degradation of nanocomposite 
by using thermo gravimetric analysis (TGA) technique 
was studied and discussed. 
 
2. EXPERIMENTAL  
 
2.1 Materials 
 
Crystic® VE671 vinyl ester resin using bisphenol A 
epoxy, with viscosity of 430 ± 50 mPa @ 25°C, was pur-
chased from Scott Bader Co. (Dubai, U.A.E.). Silica 
nanoparticle with average diameter of 12 nm was pro-
vided by Nippon aero-sol Co. (Tokyo, Japan). Cobalt, 
Dimethyl Amine (DMA) and Methyl Ethyl Ketone Per-
oxide (MEKP) were purchased from Merck Chemicals 
Company.  
 
2.2 Devices and Equipment  
 
DSC and TGA were measured by a Mettler Toledo 
(OH, USA) under nitrogen gas flow of 20 ml/min. 
Transmission electron microscopy (TEM) images were 
taken by CM 30 (Philips, Netherland). Ultrasonic 
Cleaner 7500S (Parsonic, Iran) was used for dispersion 
of silica nanoparticles in epoxy–silica nanocomposite. 
 
2.3 Preparation of Materials and Nanocompo-
site 
 
Epoxy Vinyl Ester Resin VE671 was cured by a cur-
ing agent, methyl ethyl ketone peroxide (MEKP 55%) 
and accelerated by three chemicals: cobalt (Co:6% solu-
tion), Dimethyl amine (DMA:10% solution) and cadox 
(L50A Catalyst). Samples of VE671 resin containing 4% 
of SN were mixed at 25°C and stirred for 20 min. The 
ultrasonic device was used for 20 min. Mixing and vibra-
tion by ultrasonic device were repeated three times to 
achieve a homogeneous and uniform mixture. Overall 
mixing and vibration time was about 2 hours. Then the 
mixture was well mixed with the stoichiometric amount 
of curing agent and accelerators at room temperature. In 
each case the mix ratio of the VE671 resin, hardener 
MEKP, accelerators Cobalt, Di-methyl amine (DMA) and 
cadox at 25°C were 100, 0.3, 1.0 and 1.25 wt.% for 25min 
gel time, respectively. The obtained samples were used 
for TEM, DSC and TGA tests. 
 
2.4 DSC and TGA and TEM Tests 
 
For starting the non-isothermal heating tests, 30 mg 
of the uniform viscous mixture was put in the DSC sam-
ple cell at room temperature. The sample was heated by 
 V. ARABLI, A. AGHILI PROC. NAP 2, 03NCNN26 (2013) 
 
 
03NCNN26-2 
constant heating rate (5, 10 and 15 °C /min) from 25 to 
160°C under nitrogen gas flow of 20 ml/min. Degradation 
and weight loss of the epoxy―silica nanocomposite was 
investigated by the TGA system under nitrogen gas flow 
of 20 ml/min and heating rate of 10 °C/min. Before be-
ginning TGA and TEM tests, samples were cured at 
100°C for 2 hours. 
 
3. RESULTS AND DISCUSSION  
 
3.1 Curing Kinetics and Modeling 
 
Non-isothermal (DSC) technique was used at differ-
ent heating rates to study the kinetics of cure reaction 
of VE671 resin with and without adding silica nanopar-
ticles. The results are shown in Fig. 1. It can be ob-
served that at each heating rate, the heat flow curve 
exhibited a peak at higher temperatures and a shoul-
der at lower temperatures. It can be assumed [10] that 
the curing is composed of two reactions, the first at 
lower temperatures and the second at higher tempera-
tures which showed the shoulder and the peak, respec-
tively. The exothermic peak temperature Tp for both 
systems containing VE671 and VE671+4% SN, shifted 
to lower temperatures with decreasing heating rates. 
The values of peak temperatures and heats of reaction 
are shown in Table 1. For the first reaction a turning 
point and for the second reaction the maximum point in 
DSC curves are reported as TP1 and Tp2, respectively 
[10]. A comparison of values for both systems shows a 
decrease in Tp for VE671+4% SN system. The exother-
mic heat of the samples containing SN is lower than 
that of the samples without SN and this result is in a 
good agreement with the previous reports [5, 11]. 
All kinetic models have a same basic equation: 
 )()( 

fTk
dt
d
  (1) 
 
where dα/dt is the cure reaction rate, k(T) is the rate 
constant and can be explained by the Arrhenius equa-
tion, α is the fractional conversion at a time t, f(α) is 
function of α and depends on the reaction mechanism. A 
kinetic model for a dynamic curing process with a con-
stant heating rate can be explained as [2, 5, and 10]: 
 
 
nmRTEa
Ae
dt
d
)1(
/




  (2) 
 
where k(T) and f(α) are replaced by Arrhenius equation 
and an equation based on an autocatalytic model, re-
spectively. A is the pre-exponential factor, Ea is the 
activation energy, R is the gas constant, and T is the 
absolute temperature. Autocatalytic model has inde-
pendent reaction orders m and n and an initial dα/dt of 
zero. The correlation between dα/dt and dα/dT can be 
explained by following equation: 
 
 
dT
d
dt
dT
dt
d 






  (3) 
 
where dT/dt is constant heating rate. Substituting Eq. 
(2) and (3) gives: 
 
   










 



RT
Eanm
dt
dT
A
dT
d
exp1
1


 (4) 
 
For determining the activation energy Ea, Kissinger 
and Ozawa kinetic methods were used. Following form 
is general linear eq. between the heating rate and peak 
temperature Tp as Ozawa [12] method: 
 
 
















 

TR
Ea
c
dt
dT 1
ln  (5) 
 
Equation (6) indicates Kissinger eq. [13].  
 
 














Ea
AR
TpR
Eaq
pT
lnln
2
 (6) 
 
where q is the heating rate. A plot of ln(q/Tp2) versus 
1/Tp was made as a Kissinger plot and also ln(q) ver-
sus 1/Tp was made as an Ozawa plot for both peaks of 
individual DSC curve.  
 
-0.5
0
0.5
1
1.5
2
20 40 60 80 100 120 140
Temperature (°C)
H
e
a
t 
F
lo
w
 (
W
/g
)
5ºC/min
10ºC/min
15ºC/min
 
 
 a 
 
-0.5
0
0.5
1
1.5
2
20 40 60 80 100 120 140
Temperature (°C)
H
e
a
t 
F
lo
w
 (
W
/g
)
5ºC/min
10ºC/min
15ºC/min
 
 
 b 
 
Fig. 1 – DSC curves of VE671 resin (a) and VE671+ 4% SN (b) 
at different heating rates 
 
Table 1 – Dynamic DSC data for the curing of resin at different heating rates 
 
Sample VE671 VE671 + 4% SN 
q (°C /min) 5 10 15 5 10 15 
Tp 1 (K) 321.75 334.15 342.55 319.75 331.75 340.15 
Tp 2 (K) 346.95 361.75 371.35 340.15 356.95 366.5 
Exo. Heat (J/g)  183 187 198 163.27 169.73 175.53 
 
 EFFECT OF SILICA NANOPARTICLES ON THE CURING KINETICS … PROC. NAP 2, 03NCNN26 (2013) 
 
 
03NCNN26-3 
As mentioned earlier, each peak indicates a reac-
tion. Values of activation energies for two reactions are 
shown in Table 2. 
 
Table 2 – Ea values from Kissinger and Ozawa methods 
 
Sample VE671 VE671 + 4% SN 
Ea(kJ/mol) Reac.1 Reac.2 Reac.1 Reac.2 
Kissinger 48.15 40.25 44.12 35.69 
Ozawa 53.65 46.86 49.87 42.7 
 
By comparing the Ea for both systems of epoxy resin 
(VE671 and VE671 + 4% SN), it can be suggested that 
silica naoparticles as a catalyst, improved the cure reac-
tion and decreased Ea values. Based on the logarithmic 
form of Eq. (4), the isoconversional plots were obtained 
from plotting the logarithm of heating rate versus recip-
rocal of the absolute temperature (T) and apparent Ea 
for each α was calculated from its slope [10] and values 
are listed in Table 3. For each system, the apparent acti-
vation energies for all range of conversions are between 
the two activation energies of reaction 1 and 2 calculated 
from Ozawa method. At lower conversions the apparent 
Ea is very close to the Ea for reaction 1 and at higher 
conversions, it is very close to that of reaction 2. The 
values also indicate a decrease in Ea for all range of con-
versions (α) when 4% nanosilica was used. 
Having the activation energies (Ea) for both reac-
tions, a multiple nonlinear least-squares regression 
method based on the Levenberg–Marquardt algorithm 
[10] was used to find the best values for pre-exponential 
factor (A) and reaction orders (m and n) for both reac-
tions 1 and 2. These dynamic kinetic parameters are 
listed in Table 4 for all heating rates. 
 
Table 3 – Ea Values Calculated from Isoconversional Plots at different degrees of cure 
 
Degree of Cure 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% 
Ea (kJ/mol) VE671 53.63 51.08 50.88 49.63 48.63 48.01 47.56 47.22 47.02 46.85 46.82 
Ea (kJ/mol) VE671 + 4% NS 49.87 47.56 47.35 45.81 44.92 44.18 43.65 43.23 42.94 42.73 42.70 
 
Table 4 – Dynamic Kinetic Parameters Determined using a Multiple Nonlinear Least-Squares Regression 
 
 Heating rate Reaction 1 Reaction 2 
sample (°c/min) A1 m1 n1 A2 m2 n2 
 5 3.6467E+05 0.395 2.502 8.5833E+04 1.63 1.045 
VE671 10 2.4387E+05 0.2785 2.998 1.2817E+05 1.711 1.247 
 15 2.2319E+05 0.3001 2.171 2.0209E+05 2.308 1.339 
 5 6.2387E+05 0.4035 2.973 1.6385E+05 2.469 1.045 
VE671+4% SN 10 3.8458E+05 0.3914 2.591 1.5647E+05 2.413 1.376 
 15 4.4823E+05 0.4469 2.504 1.4941E+05 2.567 1.393 
 
Having obtained the kinetic parameters for these 
two reactions, we could calculate the values for degree 
of cure (α) and cure rate (dα/dt) for each reaction by 
solving the a system of differential equations: 
 
 
    111 21211
1
1 1
nmRTEa
eA
dt
dT
dT
d








 
  (7) 
 
 
    222 21212
1
2 1
nmRTEa
eA
dt
dT
dT
d








 

 (8) 
 
where subscripts 1 and 2 are related to the first and 
second reactions, respectively. The system of ordinary 
differential equations has been solved using Matlab 
software. A program based on the Runge–Kutta (4, 5) 
algorithm was used to find the numerical solution for 
eqs. (7) and (8). As a typical example, the calculated 
values of degree of cure (α) versus temperature, along 
with the experimental results for VE671+4% SN at 
heating rate of 10°C/min, are shown in Figure 2. As 
shown in this figure, we have two calculations and 
two curves are plotted, because the overall curing 
consists of two reactions. The calculated degree of 
cure (α) agreed well with the experimental data for all 
heating rates. The cure rate dα/dt versus tempera-
ture was obtained by using eq. (3). The predicted re-
sults, along with the experimental data, are shown in 
Figure 3. 
 
0
0.2
0.4
0.6
0.8
1
20 40 60 80 100 120 140
Temperature (°C)
D
e
g
re
e
 o
f 
C
u
re
Reaction 1
Reaction 2
Model
Experiment
 
 
Fig. 2 – Comparison of model and experimental data for degree 
of cure as a function of temperature for VE671+4% nanosilica at 
heating rate of 10°C/min. 
 
0
0.01
0.02
0.03
0.04
0.05
20 40 60 80 100 120 140
Temperature (°C)
C
u
re
 R
a
te
 (
1
/s
)
Reaction 1
Reaction 2
Model
Experiment
 
 
Fig. 3 – Comparison of model and experimental data for Cure 
rate as a function of temperature for VE671+4% nanosilica at 
heating rate of 10°C/min. 
 V. ARABLI, A. AGHILI PROC. NAP 2, 03NCNN26 (2013) 
 
 
03NCNN26-4 
3.2 Dispersion of Nanoparticles in Nanocompo-
site 
 
Figure 4 shows TEM image of cured epoxy resin 
filled with 4% of silica nanoparticles. The nanoparticles 
were dispersed well in the matrix and there was no 
aggregation. 
 
 
 
Fig. 4 – TEM image of cured epoxy – silica nanocomposite with 
4 wt% of silica nanoparticles 
 
3.3 Thermal Stability 
 
Figure 5 shows the TGA curves for cured neat 
VE671 resin and the VE671+4%SN nanocomposite. It 
can be observed that the thermal degradation of 
nanocomposite takes place at higher temperatures 
than the neat resin. The thermal degradation data are 
also shown in Table 5. The char yields at 600°C in-
creased from 6.8% to 12.4% with the addition of 4% of 
silica nanoparticles to the epoxy resin. The increasing 
of char yields agrees with the mechanism of flame 
retardancy. Therefore silica nanoparticles improved 
the polymer flame retardancy and thermal resistance 
[14]. 
Table 5 – Thermal degradation data of the cured samples  
 
System IDT(°C)a T(°C)b Tmax(°C) c Chr.Y(%) d 
VE671 387 400 416 6.8 
VE671/SN 390 406 424 12.4 
a) Initial Degradation Temperature 
b) Temperature for 30% of weight loss 
c) Temperature of maximum weight loss 
d) Char Yield at 600ºC 
 
0
20
40
60
80
100
120
0 200 400 600
Temperature (°C)
w
e
ig
h
t 
(%
)
4% nanosilica
0% nanosilica
 
 
Fig. 5 – TGA curves of cured VE671 resin and its nanocompo-
site. 
 
4. CONCLUSION 
 
Effect of silica nanoparticles on the cure kinetics of 
epoxy resin in the presence of 4% nanosilica was stud-
ied. To determine activation energy Ea of cure reaction 
of VE671, non-isothermal DSC method, Ozawa and Kis-
singer equations were used. The Ea value of cure reac-
tion of VE671 in the presence of 4% silica nanoparticle 
decreased about 5 KJ/mol. It is concluded that silica 
nanoparticles acted as catalyst in the reaction of 
VE671/SN. DSC curves were modeled by Matlab pro-
gram. The models were agreed well with the experi-
mental data for all heating rates. The char yields in-
creased with the addition of 4% of silica nanoparticles to 
the epoxy resin and improved the polymer flame re-
tardancy and thermal resistance at high temperatures.  
 
 
REFERENCES 
 
1. G. M. Karami, M. H. Beheshty, M. Esfandeh, 
A. M. Rezadoust, Iran polym. J. 21, 155 (2008). 
2. M. Hayaty, M. H. Beheshty, M. Esfandeh, J. Appl. Polym. 
Sci. 120, 62 (2010). 
3. M. Hayaty, M. H. Beheshty, M. Esfandeh, J. Polym. Adva. 
Techno. 22, 1001 (2011). 
4. M. Moshirnia, M. Kokabi, M. Homayun, Iran J. Polym. 
Sci. Tech. 17, 141 (2004). 
5. M. Ghaemy, M. Bazzar, Chinese J. Polym. Sci. 29, 141 
(2011). 
6. T. Mahrholz, J. Stangle, M. Sinapius, Appl. Sci. Manuf. 
40, 235 (2009). 
7. Y. L. Liu, S. H. Li, J. Appl. Polym. Sci. 95, 1237 (2005). 
8. J. Yuan, Sh. Zhou, G. Gu, L. Wu, J. Mat. Sci. 40, 3927 
(2005). 
9. Polymer Composites (Ed. K. Friedrich, S. Fakirov, 
Z. Zhang) (New York, Springer  2005). 
10. L. Sun, S. S. Pang, A. M. Sterling, I. I. Negulescu, 
M. A. Stubblefield, J. Appl. Polym. Sci. 86, 1911 (2002). 
11. J. Gonis, G. P. Simon, W. Cook, J. Appl. Polym. Sci. 72, 
1479 (1999). 
12. T. J. Ozawa, Therm. Anal. 2, 301 (1970). 
13. H. E. Kissinger, Anal. Chem. 29, 1702 (1957). 
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106 (2013). 
 


Effect of Silica Nanoparticles on the Curing Kinetics of Epoxy Vinyl Ester Resin

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 3, 03NCNN26(4pp) (2013)   2304-1862/2013/2(3)03NCNN26(4) 03NCNN26-1  2013 Sumy State University Effect of Silica Nanoparticles on the Curing Kinetics of Epoxy Vinyl Ester Resin  V. Arabli*, A. Aghili†  Department of Polymer Engineering, Shiraz Branch, Islamic Azad University, Shiraz, Iran  (Received 07 June 2013; published online 01 September 2013)  A nanocomposite was synthesized using silica nanoparticles (SN) and Epoxy Vinyl Ester Resin (VE671). Nanoparticles were dispersed in the mixture by ultrasonic equipment to prevent the agglomera-tion. Transmission electron microscopy (TEM) was used to investigate the dispersion of the silica nanopar-ticles in the mixture. Non-isothermal differential scanning calorimetry (DSC) technique was used to study the cure kinetics of VE671 resin with and without adding silica nanoparticles. The activation energy (Ea) was determined by using Kissinger and Ozawa equations. The Ea values of curing for VE671 / 4% SN sys-tem showed a decrease with respect to the neat resin. It means that there is a catalytic effect of silica na-noparticles in the cure reaction. A dynamic kinetic model was obtained to predict the degree of cure and cure rate of resin. The results showed a good agreement between the model and the experimental data for different heating rates. The char yields increased with the addition of 4% of SN to the epoxy resin and im-proved the polymer flame retardancy and thermal resistance at high temperatures.  Keywords: Silica Nanoparticles, Epoxy Vinyl Ester Resin, Nanocomposite, Curing Kinetics, Modeling.   PACS numbers: 82.35.Np, 82.20.Wt                                                           * arably@yahoo.com † aghili@iaushiraz.ac.ir, alirezaaghili@yahoo.com 1. INTRODUCTION   The thermoset resins have many applications, be-cause of their highly desirable properties such as stiff-ness, suitable chemical resistance, wear resistance, excellent adhesion and low shrinkage after curing. It is important to study the cure kinetics and the correlation between the degree of cure and the thermal and me-chanical properties to design the optimum curing con-ditions [1-6]. To provide organic–inorganic hybrid ma-terials, we can mix the silica nanoparticles with poly-meric materials. The hybrid materials are also known as nanocomposites [5, 7]. Silica nanoparticles can be widely used in paints, plastics, color rubbers, reinforce fillers in epoxy molding compounds, adhesives and many other fields [5-8]. Liu et al have reported that nanoscale colloidal silica particles act as a curing agent in the curing state of epoxy–silica nanocomposite for-mation [7]. It was showed an interesting reactivity of silica nanoparticles toward epoxy resins without the need of adding other catalyst in cure reaction.  In this research, an epoxy―silica nanocomposite was synthesized and the cure kinetics of reaction for neat epoxy and nanocomposite was investigated by us-ing non-isothermal DSC technique and a kinetic model was obtained. Thermal degradation of nanocomposite by using thermo gravimetric analysis (TGA) technique was studied and discussed.  2. EXPERIMENTAL   2.1 Materials  Crystic® VE671 vinyl ester resin using bisphenol A epoxy, with viscosity of 430 ± 50 mPa @ 25°C, was pur-chased from Scott Bader Co. (Dubai, U.A.E.). Silica nanoparticle with average diameter of 12 nm was pro-vided by Nippon aero-sol Co. (Tokyo, Japan). Cobalt, Dimethyl Amine (DMA) and Methyl Ethyl Ketone Per-oxide (MEKP) were purchased from Merck Chemicals Company.   2.2 Devices and Equipment   DSC and TGA were measured by a Mettler Toledo (OH, USA) under nitrogen gas flow of 20 ml/min. Transmission electron microscopy (TEM) images were taken by CM 30 (Philips, Netherland). Ultrasonic Cleaner 7500S (Parsonic, Iran) was used for dispersion of silica nanoparticles in epoxy–silica nanocomposite.  2.3 Preparation of Materials and Nanocompo-site  Epoxy Vinyl Ester Resin VE671 was cured by a cur-ing agent, methyl ethyl ketone peroxide (MEKP 55%) and accelerated by three chemicals: cobalt (Co:6% solu-tion), Dimethyl amine (DMA:10% solution) and cadox (L50A Catalyst). Samples of VE671 resin containing 4% of SN were mixed at 25°C and stirred for 20 min. The ultrasonic device was used for 20 min. Mixing and vibra-tion by ultrasonic device were repeated three times to achieve a homogeneous and uniform mixture. Overall mixing and vibration time was about 2 hours. Then the mixture was well mixed with the stoichiometric amount of curing agent and accelerators at room temperature. In each case the mix ratio of the VE671 resin, hardener MEKP, accelerators Cobalt, Di-methyl amine (DMA) and cadox at 25°C were 100, 0.3, 1.0 and 1.25 wt.% for 25min gel time, respectively. The obtained samples were used for TEM, DSC and TGA tests.  2.4 DSC and TGA and TEM Tests  For starting the non-isothermal heating tests, 30 mg of the uniform viscous mixture was put in the DSC sam-ple cell at room temperature. The sample was heated by  V. ARABLI, A. AGHILI PROC. NAP 2, 03NCNN26 (2013)   03NCNN26-2 constant heating rate (5, 10 and 15 °C /min) from 25 to 160°C under nitrogen gas flow of 20 ml/min. Degradation and weight loss of the epoxy―silica nanocomposite was investigated by the TGA system under nitrogen gas flow of 20 ml/min and heating rate of 10 °C/min. Before be-ginning TGA and TEM tests, samples were cured at 100°C for 2 hours.  3. RESULTS AND DISCUSSION   3.1 Curing Kinetics and Modeling  Non-isothermal (DSC) technique was used at differ-ent heating rates to study the kinetics of cure reaction of VE671 resin with and without adding silica nanopar-ticles. The results are shown in Fig. 1. It can be ob-served that at each heating rate, the heat flow curve exhibited a peak at higher temperatures and a shoul-der at lower temperatures. It can be assumed [10] that the curing is composed of two reactions, the first at lower temperatures and the second at higher tempera-tures which showed the shoulder and the peak, respec-tively. The exothermic peak temperature Tp for both systems containing VE671 and VE671+4% SN, shifted to lower temperatures with decreasing heating rates. The values of peak temperatures and heats of reaction are shown in Table 1. For the first reaction a turning point and for the second reaction the maximum point in DSC curves are reported as TP1 and Tp2, respectively [10]. A comparison of values for both systems shows a decrease in Tp for VE671+4% SN system. The exother-mic heat of the samples containing SN is lower than that of the samples without SN and this result is in a good agreement with the previous reports [5, 11]. All kinetic models have a same basic equation:  )()( fTkdtd  (1)  where dα/dt is the cure reaction rate, k(T) is the rate constant and can be explained by the Arrhenius equa-tion, α is the fractional conversion at a time t, f(α) is function of α and depends on the reaction mechanism. A kinetic model for a dynamic curing process with a con-stant heating rate can be explained as [2, 5, and 10]:   nmRTEaAedtd)1(/  (2)  where k(T) and f(α) are replaced by Arrhenius equation and an equation based on an autocatalytic model, re-spectively. A is the pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is the absolute temperature. Autocatalytic model has inde-pendent reaction orders m and n and an initial dα/dt of zero. The correlation between dα/dt and dα/dT can be explained by following equation:   dTddtdTdtd   (3)  where dT/dt is constant heating rate. Substituting Eq. (2) and (3) gives:      RTEanmdtdTAdTdexp11 (4)  For determining the activation energy Ea, Kissinger and Ozawa kinetic methods were used. Following form is general linear eq. between the heating rate and peak temperature Tp as Ozawa [12] method:    TREacdtdT 1ln  (5)  Equation (6) indicates Kissinger eq. [13].    EaARTpREaqpTlnln2 (6)  where q is the heating rate. A plot of ln(q/Tp2) versus 1/Tp was made as a Kissinger plot and also ln(q) ver-sus 1/Tp was made as an Ozawa plot for both peaks of individual DSC curve.   -0.500.511.5220 40 60 80 100 120 140Temperature (°C)Heat Flow (W/g)5ºC/min10ºC/min15ºC/min   a  -0.500.511.5220 40 60 80 100 120 140Temperature (°C)Heat Flow (W/g)5ºC/min10ºC/min15ºC/min   b  Fig. 1 – DSC curves of VE671 resin (a) and VE671+ 4% SN (b) at different heating rates  Table 1 – Dynamic DSC data for the curing of resin at different heating rates  Sample VE671 VE671 + 4% SN q (°C /min) 5 10 15 5 10 15 Tp 1 (K) 321.75 334.15 342.55 319.75 331.75 340.15 Tp 2 (K) 346.95 361.75 371.35 340.15 356.95 366.5 Exo. Heat (J/g)  183 187 198 163.27 169.73 175.53   EFFECT OF SILICA NANOPARTICLES ON THE CURING KINETICS … PROC. NAP 2, 03NCNN26 (2013)   03NCNN26-3 As mentioned earlier, each peak indicates a reac-tion. Values of activation energies for two reactions are shown in Table 2.  Table 2 – Ea values from Kissinger and Ozawa methods  Sample VE671 VE671 + 4% SN Ea(kJ/mol) Reac.1 Reac.2 Reac.1 Reac.2 Kissinger 48.15 40.25 44.12 35.69 Ozawa 53.65 46.86 49.87 42.7  By comparing the Ea for both systems of epoxy resin (VE671 and VE671 + 4% SN), it can be suggested that silica naoparticles as a catalyst, improved the cure reac-tion and decreased Ea values. Based on the logarithmic form of Eq. (4), the isoconversional plots were obtained from plotting the logarithm of heating rate versus recip-rocal of the absolute temperature (T) and apparent Ea for each α was calculated from its slope [10] and values are listed in Table 3. For each system, the apparent acti-vation energies for all range of conversions are between the two activation energies of reaction 1 and 2 calculated from Ozawa method. At lower conversions the apparent Ea is very close to the Ea for reaction 1 and at higher conversions, it is very close to that of reaction 2. The values also indicate a decrease in Ea for all range of con-versions (α) when 4% nanosilica was used. Having the activation energies (Ea) for both reac-tions, a multiple nonlinear least-squares regression method based on the Levenberg–Marquardt algorithm [10] was used to find the best values for pre-exponential factor (A) and reaction orders (m and n) for both reac-tions 1 and 2. These dynamic kinetic parameters are listed in Table 4 for all heating rates.  Table 3 – Ea Values Calculated from Isoconversional Plots at different degrees of cure  Degree of Cure 5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 95% Ea (kJ/mol) VE671 53.63 51.08 50.88 49.63 48.63 48.01 47.56 47.22 47.02 46.85 46.82 Ea (kJ/mol) VE671 + 4% NS 49.87 47.56 47.35 45.81 44.92 44.18 43.65 43.23 42.94 42.73 42.70  Table 4 – Dynamic Kinetic Parameters Determined using a Multiple Nonlinear Least-Squares Regression   Heating rate Reaction 1 Reaction 2 sample (°c/min) A1 m1 n1 A2 m2 n2  5 3.6467E+05 0.395 2.502 8.5833E+04 1.63 1.045 VE671 10 2.4387E+05 0.2785 2.998 1.2817E+05 1.711 1.247  15 2.2319E+05 0.3001 2.171 2.0209E+05 2.308 1.339  5 6.2387E+05 0.4035 2.973 1.6385E+05 2.469 1.045 VE671+4% SN 10 3.8458E+05 0.3914 2.591 1.5647E+05 2.413 1.376  15 4.4823E+05 0.4469 2.504 1.4941E+05 2.567 1.393  Having obtained the kinetic parameters for these two reactions, we could calculate the values for degree of cure (α) and cure rate (dα/dt) for each reaction by solving the a system of differential equations:       111 2121111 1nmRTEaeAdtdTdTd   (7)       222 2121212 1nmRTEaeAdtdTdTd  (8)  where subscripts 1 and 2 are related to the first and second reactions, respectively. The system of ordinary differential equations has been solved using Matlab software. A program based on the Runge–Kutta (4, 5) algorithm was used to find the numerical solution for eqs. (7) and (8). As a typical example, the calculated values of degree of cure (α) versus temperature, along with the experimental results for VE671+4% SN at heating rate of 10°C/min, are shown in Figure 2. As shown in this figure, we have two calculations and two curves are plotted, because the overall curing consists of two reactions. The calculated degree of cure (α) agreed well with the experimental data for all heating rates. The cure rate dα/dt versus tempera-ture was obtained by using eq. (3). The predicted re-sults, along with the experimental data, are shown in Figure 3.  00.20.40.60.8120 40 60 80 100 120 140Temperature (°C)Degree of CureReaction 1Reaction 2ModelExperiment  Fig. 2 – Comparison of model and experimental data for degree of cure as a function of temperature for VE671+4% nanosilica at heating rate of 10°C/min.  00.010.020.030.040.0520 40 60 80 100 120 140Temperature (°C)Cure Rate (1/s)Reaction 1Reaction 2ModelExperiment  Fig. 3 – Comparison of model and experimental data for Cure rate as a function of temperature for VE671+4% nanosilica at heating rate of 10°C/min.  V. ARABLI, A. AGHILI PROC. NAP 2, 03NCNN26 (2013)   03NCNN26-4 3.2 Dispersion of Nanoparticles in Nanocompo-site  Figure 4 shows TEM image of cured epoxy resin filled with 4% of silica nanoparticles. The nanoparticles were dispersed well in the matrix and there was no aggregation.    Fig. 4 – TEM image of cured epoxy – silica nanocomposite with 4 wt% of silica nanoparticles  3.3 Thermal Stability  Figure 5 shows the TGA curves for cured neat VE671 resin and the VE671+4%SN nanocomposite. It can be observed that the thermal degradation of nanocomposite takes place at higher temperatures than the neat resin. The thermal degradation data are also shown in Table 5. The char yields at 600°C in-creased from 6.8% to 12.4% with the addition of 4% of silica nanoparticles to the epoxy resin. The increasing of char yields agrees with the mechanism of flame retardancy. Therefore silica nanoparticles improved the polymer flame retardancy and thermal resistance [14]. Table 5 – Thermal degradation data of the cured samples   System IDT(°C)a T(°C)b Tmax(°C) c Chr.Y(%) d VE671 387 400 416 6.8 VE671/SN 390 406 424 12.4 a) Initial Degradation Temperature b) Temperature for 30% of weight loss c) Temperature of maximum weight loss d) Char Yield at 600ºC  0204060801001200 200 400 600Temperature (°C)weight (%)4% nanosilica0% nanosilica  Fig. 5 – TGA curves of cured VE671 resin and its nanocompo-site.  4. CONCLUSION  Effect of silica nanoparticles on the cure kinetics of epoxy resin in the presence of 4% nanosilica was stud-ied. To determine activation energy Ea of cure reaction of VE671, non-isothermal DSC method, Ozawa and Kis-singer equations were used. The Ea value of cure reac-tion of VE671 in the presence of 4% silica nanoparticle decreased about 5 KJ/mol. It is concluded that silica nanoparticles acted as catalyst in the reaction of VE671/SN. DSC curves were modeled by Matlab pro-gram. The models were agreed well with the experi-mental data for all heating rates. The char yields in-creased with the addition of 4% of silica nanoparticles to the epoxy resin and improved the polymer flame re-tardancy and thermal resistance at high temperatures.    REFERENCES  1. G. M. Karami, M. H. Beheshty, M. Esfandeh, A. M. Rezadoust, Iran polym. J. 21, 155 (2008). 2. M. Hayaty, M. H. Beheshty, M. Esfandeh, J. Appl. Polym. Sci. 120, 62 (2010). 3. M. Hayaty, M. H. Beheshty, M. Esfandeh, J. Polym. Adva. Techno. 22, 1001 (2011). 4. M. Moshirnia, M. Kokabi, M. Homayun, Iran J. Polym. Sci. Tech. 17, 141 (2004). 5. M. Ghaemy, M. Bazzar, Chinese J. Polym. Sci. 29, 141 (2011). 6. T. Mahrholz, J. Stangle, M. Sinapius, Appl. Sci. Manuf. 40, 235 (2009). 7. Y. L. Liu, S. H. Li, J. Appl. Polym. Sci. 95, 1237 (2005). 8. J. Yuan, Sh. Zhou, G. Gu, L. Wu, J. Mat. Sci. 40, 3927 (2005). 9. Polymer Composites (Ed. K. Friedrich, S. Fakirov, Z. Zhang) (New York, Springer  2005). 10. L. Sun, S. S. 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Effect of Silica Nanoparticles on the Curing Kinetics of Epoxy Vinyl Ester Resin

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