Article
Characterization and Curing Kinetics of
Epoxy/Silica Nano-Hybrids
Cheng-Fu Yang 1 , Li-Fen Wang 2 , Song-Mao Wu 3 and Chean-Cheng Su 1, *
Received: 31 August 2015 ; Accepted: 10 October 2015 ; Published: 16 October 2015
Academic Editor: Teen-Hang Meen
1
2
3
*
Department of Chemical and Materials Engineering, National University of Kaohsiung, No. 700,
Kaohsiung University Rd., Nan-Tzu Dist., Kaohsiung 811, Taiwan; [email protected]
Department of Applied Chemistry and Materials Science, Fooyin University, 151 Jinxue Rd.,
Daliao Dist., Kaohsiung 831, Taiwan; [email protected]
Department of Electrical Engineering, National University of Kaohsiung, No. 700,
Kaohsiung University Rd., Nan-Tzu Dist., Kaohsiung 811, Taiwan; [email protected]
Correspondence: [email protected]; Tel.: +886-7-591-9338; Fax: +886-7-591-9277
Abstract: The sol-gel technique was used to prepare epoxy/silica nano-hybrids. The thermal
characteristics, curing kinetics and structure of epoxy/silica nano-hybrids were studied using
differential scanning calorimetry (DSC), 29 Si nuclear magnetic resonance (NMR) and transmission
electron microscopy (TEM). To improve the compatibility between the organic and inorganic
phases, a coupling agent was used to modify the diglycidyl ether of bisphenol A (DGEBA) epoxy.
The sol-gel technique enables the silica to be successfully incorporated into the network of the
hybrids, increasing the thermal stability and improving the mechanical properties of the prepared
epoxy/silica nano-hybrids. An autocatalytic mechanism of the epoxy/SiO2 nanocomposites was
observed. The low reaction rate of epoxy in the nanocomposites is caused by the steric hindrance in
the network of hybrids that arises from the consuming of epoxide group in the network of hybrids by
the silica. In the nanocomposites, the nano-scale silica particles had an average size of approximately
35 nm, and the particles were well dispersed in the epoxy matrix, according to the TEM images.
Keywords:
sol-gel technique;
autocatalytic mechanism
nanocomposite;
differential scanning calorimetry (DSC);
1. Introduction
Epoxies have many favorable properties such as high tensile strength and modulus, excellent
chemical resistance, and high thermal stability. These characteristics make them ideal matrices for
many applications, such as epoxy molding compounds (EMCs) [1], printed circuit boards (PCBs) [2],
adhesives [3], paints [4], and high-performance composites [5]. Epoxy composites, which are hybrid
organic-inorganic materials, have a wide range of engineering applications that require a high
strength to weight ratio, low cost and ease of fabrication [6,7].
Sol gel-derived products have various applications, such as thin films [8,9], protective
coatings [8], decorative coatings [10], electro-optic components [11] and composites [12].
Sol-gel methods are used to fabricate metal oxide from a chemical solution that acts as the precursor
of an integrated network of either discrete particles or network polymers. Typical precursors are
metal alkoxides and metal chlorides, which undergo various hydrolysis and polycondensation
reactions [13,14]. An extensively studied alkoxide is tetraethyl orthosilicate (TEOS). Alkoxides are
ideal chemical precursors for sol-gel synthesis because they react readily with water. The mechanism
of the sol-gel process is as follows.
Materials 2015, 8, 7032–7040; doi:10.3390/ma8105357
www.mdpi.com/journal/materials
Materials 2015, 8, 7032–7040
Hydrolysis:
Materials 2015, 8, page–page Si pORq4 ` H2 O Ñ HO–Si pORq3 ` R–OH
Condensation:
Hydrolysis: 2O→HO–Si(OR)
3 + R–OH Si(OR)4 + H
pORq3 –Si–OH + HO–Si–
pORq
3 Ñ rpORq3 Si–O–Si pORq3 s ` H–O–H
Condensation: Polymerization is associated with the formation of a one-, two- or three-dimensional network of
(OR)3–Si–OH+HO–Si‐(OR)3→[(OR)3Si–O–Si(OR)3] + H–O–H siloxane (Si–O–Si) bonds, accompanied by the formation of H–O–H and R–O–H species [15–19].
Polymerization is associated with the formation of a one‐, two‐ or three‐dimensional network of Epoxy/silica
hybrids that comprise nanoparticles that are dispersed throughout an epoxy
siloxane (Si–O–Si) bonds, accompanied by the formation of H–O–H and R–O–H species [15–19]. matrix have been widely studied because they exhibit a desired combination of the flexibility of
Epoxy/silica hybrids that comprise nanoparticles that are dispersed throughout an epoxy matrix epoxy and
the hardness of silica nanoparticles [20,21]. The sol-gel process is an efficient means of
have been widely studied because they exhibit a desired combination of the flexibility of epoxy and preparing metal oxide networks at low temperatures. The structure of the inorganic material can be
the hardness of silica nanoparticles [20,21]. The sol‐gel process is an efficient means of preparing controlled by optimizing the synthetic conditions, including concentration, solvent/alkoxide ratio,
metal oxide networks at low temperatures. The structure of the inorganic material can be controlled temperature,
pH, and the species of the catalyst and the solvent [22,23]. Whereas the curing kinetics
by optimizing the synthetic conditions, including concentration, solvent/alkoxide ratio, temperature, and pH, mechanisms
of reaction
ofcatalyst variousand epoxy
resins have
been
studied
and the species of the the solvent [22,23]. Whereas the using
curing various
kinetics methods,
and including
infrared spectroscopy and differential scanning calorimetry. In fact, differential scanning
mechanisms of reaction of various epoxy resins have been studied using various methods, including infrared spectroscopy and differential scanning calorimetry. In fact, differential scanning calorimetry calorimetry
is more commonly used. The effects of nano-scale silica in epoxy resins on their curing
is more commonly used. The effects of nano‐scale silica in epoxy resins on their curing kinetics have kinetics
have rarely been investigated. This study provides such critical information to elucidate the
rarely been investigated. This study provides such critical information to elucidate the reactivity and reactivity
and to predict the curing behavior of epoxy/silica hybrids.
to predict the curing behavior of epoxy/silica hybrids. 2. Experimental Section
2. Experimental Section 2.1. Materials and Methods
2.1. Materials and Methods The epoxy resin that is used in this work is the diglycidyl ether of bisphenol
The epoxy resin that is used in this work is the diglycidyl ether of bisphenol A (DGEBA) (shell 828, A (DGEBA) (shell 828, Shell Inc., Kawasaki, Japan). The epoxy resin was cured with
Shell Inc., Kawasaki, Japan). The epoxy resin was cured with 2.4‐methylhexanhydrophthalic 2.4-methylhexanhydrophthalic
anhydride
Inc.,4.3‐isocyanatopropyltriethoxysilane Kawasaki, Japan). The alkoxides,
anhydride (MHHPA) (Shell Inc., Kawasaki, (MHHPA)
Japan). The (Shell
alkoxides, 4.3-isocyanatopropyltriethoxysilane
(IPTES)
and
tetraethoxysilane
(TEOS),
were purchased
(IPTES) and tetraethoxysilane (TEOS), were purchased from Shin‐Etsu Inc. (Naoetsu, Japan). The from
Shin-Etsu
Inc.
(Naoetsu,
Japan).
The
chemical
structures
of
DGEBA,
MHHPA,
IPTES
and TEOS
chemical structures of DGEBA, MHHPA, IPTES and TEOS are as follows. are as follows.
H2 C
CHCH 2
CH 3
C
CH 3
(O
O
CH 3
O CH 2CHCH2 )n O
OH
C
CH 3
O CH 2 CH CH2
O
DGEBA epoxy O
OEt
O
O
EtO
Si
OEt
OCH3
O C N CH2CH2CH2 Si OCH3
OCH3
OEt
MHHPA TESO IPTES A mixture of epoxy and IPTES, with an epoxy/IPTES weight ratio of 100/5, was stirred
A mixture of epoxy and IPTES, with an epoxy/IPTES weight ratio of 100/5, was stirred vigorously at 80 ˝ C for 2 h to yield the silanized epoxy. Scheme 1 shows the preparation of silanized
vigorously at 80 °C for 2 h to yield the silanized epoxy. Scheme 1 shows the preparation of silanized epoxy
precursor.
The
silanized
water, ethanol, ethanol,and and
catalyst
were
mixed
stirred
epoxy precursor. The silanized epoxy,
epoxy, TEOS,
TEOS, water, catalyst were mixed and and
stirred vigorously
at room temperature for 1 h to obtain a homogeneous mixture. The amount of MHHPA
vigorously at room temperature for 1 h to obtain a homogeneous mixture. The amount of MHHPA that was
required to provide an overall –COOH/epoxide molar ratio of 1/1 was then added to the
that was required to provide an overall –COOH/epoxide molar ratio of 1/1 was then added to the mixture
at room temperature and mixing was continued for another 5 min. The curing process was
mixture at room temperature and mixing was continued for another 5 min. The curing process was carried out at various temperatures to obtain cured poxy/SiO
2 hybrids. Scheme 2 shows the process carried out at various temperatures to obtain cured poxy/SiO
2 hybrids. Scheme 2 shows the process
for preparation of epoxy/silica nano‐hybrids. for preparation of epoxy/silica nano-hybrids.
2 7033
Materials 2015, 8, 7032–7040
Materials 2015, 8, page–page Materials 2015, 8, page–page Scheme 1. Preparation of silanized epoxy precursor. Scheme 1. Preparation of silanized epoxy precursor.
H2C CHCH2
O
H2C CHCH2
O
CH3
CH3
Scheme 1. Preparation of silanized epoxy precursor. C
C
O CH2CH CH2
O CH2CHCH2)n O
(O
CH
O
CH
O
3
3
CH3
CH3
C O 2)n O
C
C
O CH2CH CH2
O CH2CHCH
(O
CH3
NH
O
CH3
O
(CH
C 2O)3
H3CO NH
Si OCH3
(CH
2)3
OCH
3
H3CO Si OCH3
Silane
OCH
3
H2C CHCH2 (O
O
H2C CHCH2 (O
O
CH3
C
CH3
CH
3
C
CH3
Silane
O CH2CHCH2)n O
O
C O ) O
O CH2CHCH
2 n
NH
O
CH3
C
CH3
CH
3
O CH2CH CH2
O
C
CH3
O CH2CH CH2
O
R
C O
(CH
2)3
R
NH
Si
O
O Si
R
(CH2)O
3 Si
O
Si O
O R
O
Si
Si
O O O Si
Si
SiO OSi
O
R
O
R
Si O Si
O
O
O
Si
O O
O
O
Si O Si
R
O OR
Si
O
O O
O
Scheme 2. Preparation of epoxy/silica nano‐hybrids. O
2.2. Characterization of Epoxy/Silica Nano‐Hybrids Scheme 2. Preparation of epoxy/silica nano‐hybrids. Scheme 2. Preparation of epoxy/silica nano-hybrids.
A differential scanning calorimeter (Perkin‐Elmer PYRIS I, equipped with an intracooler) was 2.2. Characterization of Epoxy/Silica Nano‐Hybrids used in the isothermal curing experiments to obtain data for analysis. The isothermal curing reaction 2.2. Characterization of Epoxy/Silica Nano-Hybrids
A differential scanning calorimeter (Perkin‐Elmer PYRIS I, equipped with an intracooler) was was conducted at five temperatures (100 °C, 110 °C, 120 °C, 130 °C, and 140 °C); it was considered to used in the isothermal curing experiments to obtain data for analysis. The isothermal curing reaction be complete when the isothermal DSC thermogram leveled off to the baseline. The total area under A differential scanning calorimeter (Perkin-Elmer PYRIS I, equipped with an intracooler) was
the exothermal curve, which was based on the extrapolated baseline at the end of the reaction, was used inwas conducted at five temperatures (100 °C, 110 °C, 120 °C, 130 °C, and 140 °C); it was considered to the isothermal curing experiments to obtain data for analysis. The isothermal curing reaction
be complete when the isothermal DSC thermogram leveled off to the baseline. The total area under used to calculate the isothermal heat of curing, ΔH
IO (Jg−1). To determine the residual heat of reaction, was conducted
at five temperatures (100 ˝ C, 110 ˝ C,
120 ˝ C, 130 ˝ C, and 140 ˝ C); it was considered to
the exothermal curve, which was based on the extrapolated baseline at the end of the reaction, was ΔHR (Jg−1), the cured samples were scanned at 10 °C/min from 40 °C to 200 °C. The sum of both the be complete
when the isothermal DSC thermogramIOleveled
off to the baseline. The total area under the
used to calculate the isothermal heat of curing, ΔH
(Jg−1). To determine the residual heat of reaction, isothermal heat (ΔHIO) and the residual heat (ΔHR) of reaction was taken to represent the total heat of exothermal
curve,
which
was
based
on
the
extrapolated
baseline
at the end of the reaction, was used
−1
ΔH
R (Jg ), the cured samples were scanned at 10 °C/min from 40 °C to 200 °C. The sum of both the reaction (ΔH
T). The isothermal conversion at time t was defined as αI(t) = ΔHI(t)/ΔHT. ´
1
to calculate
the isothermal
heat of curing, ∆HIO R(Jg
). To determine the residual heat of reaction,
isothermal heat (ΔH
IO) and the residual heat (ΔH
) of reaction was taken to represent the total heat of ´1 ), the cured
˝ C to 200
˝ C.T. The sum of both the
reaction (ΔH
T). The isothermal conversion at time t was defined as α
I(t)/ΔH
∆HR (Jg
samples were scanned at 10 ˝ C/min from 40I(t) = ΔH
3 isothermal heat (∆HIO ) and the residual heat (∆HR ) of reaction was taken to represent the total heat
of reaction (∆HT ). The isothermal conversion at time
3 t was defined as αI (t) = ∆HI (t)/∆HT .
Solid-state 29 Si NMR measurements were made at room temperature using a Bruker Avance
400 NMR spectrometer that was operated at 400 MHz. The chemically modified silica was examined
7034
Materials 2015, 8, 7032–7040
Materials 2015, 8, page–page 2929Si
Si cross-polarization
NMR measurements and
were magic
made at room spinning
temperature using a Bruker Avance Solid‐state usingMaterials 2015, 8, page–page solid-state
angle
nuclear
magnetic
resonance
400 NMR spectrometer that was operated at 400 MHz. The chemically modified silica was examined spectroscopy.
The morphology of epoxy/silica nanocomposites was analyzed by a transmission
29Si 29Si NMR measurements and were magic made angle at room temperature using a Bruker Avance Solid‐state using solid‐state cross‐polarization spinning nuclear magnetic resonance electron
microscopy
(TEM,
model JEOL JEM
1200 EX,
Tokyo,
Japan)
with an
accelerating
voltage
400 NMR spectrometer that was operated at 400 MHz. The chemically modified silica was examined spectroscopy. The morphology of epoxy/silica nanocomposites was analyzed by a transmission of 100 kV.
29
using solid‐state Si cross‐polarization and magic angle spinning nuclear magnetic resonance electron microscopy (TEM, model JEOL JEM 1200 EX, Tokyo, Japan) with an accelerating voltage of spectroscopy. The morphology of epoxy/silica nanocomposites was analyzed by a transmission 100 kV. and Discussion
3. Results
electron microscopy (TEM, model JEOL JEM 1200 EX, Tokyo, Japan) with an accelerating voltage of 100 kV. 3. Results and Discussion 3.1. Characterization
of Epoxy/Silica Nano-Hybrids
3. Results and Discussion 3.1. Characterization of Epoxy/Silica Nano‐Hybrids The
chemical structure of silica in the epoxy/silica nanocomposites was examined using
which
The chemical structure of silica in the epoxy/silica nanocomposites was examined using 29Si 3.1. Characterization of Epoxy/Silica Nano‐Hybrids 29
exhibited
major
chemical
shifts
at
´91
ppm,
´102
ppm,
and
´110
ppm,
which
were
attributed
NMR. Figure 1 shows the solid‐state Si NMR spectra of the epoxy/silica nanocomposite, which The chemical structure silica in ppm, the epoxy/silica nanocomposites was examined using 29to Si silica
to silsesquioxane
absorption
byof dihydroxy-substituted
(Q2 ),ppm, monohydroxy-substituted
exhibited major chemical shifts at −91 −102 ppm, silica
and −110 which were attributed 29Si 3
4
2
3) NMR. Figure 1 shows the solid‐state NMR spectra of the epoxy/silica nanocomposite, which silsesquioxane absorption by dihydroxy‐substituted silica (Q
), monohydroxy‐substituted silica (Q
(Q ) and
nonhydroxy-substituted silica (Q ), respectively. These
chemical shifts involved the Si–O–Si
4
exhibited chemical shifts at −91 ppm, −102 ppm, and −110 ppm, which were attributed to and of
nonhydroxy‐substituted silica (Q
), respectively. These shifts the Si‐O‐Si bonding
themajor silsesquioxane
[24,25].
Scheme
3 shows
the
Qchemical architecture
ofinvolved the
polysilsesquioxane
2), monohydroxy‐substituted silica (Q3) silsesquioxane absorption by dihydroxy‐substituted silica (Q
bonding of the silsesquioxane [24,25]. Scheme 3 shows the Q architecture of the polysilsesquioxane in the epoxy/silica nano-hybrids. Siloxane
bridges in the silica, which were highly crosslinked in the
and nonhydroxy‐substituted silica (Q4), respectively. These chemical shifts involved the Si‐O‐Si in the epoxy/silica nano‐hybrids. Siloxane bridges in the silica, which were highly crosslinked in the epoxy/silica
hybrid materials, were observed. Since the samples, having a Q3 and Q4 environment
bonding of the silsesquioxane [24,25]. Scheme 3 shows the Q architecture of the polysilsesquioxane epoxy/silica hybrid materials, were observed. Since the samples, having a Q3 and Q4 environment couldin the epoxy/silica nano‐hybrids. Siloxane bridges in the silica, which were highly crosslinked in the be considered to include a highly condensed silica phase, the formation of the SiO2 network
could be considered to include a highly condensed silica phase, the formation of the SiO2 network structure
waswas complete
in the prepared samples. The incomplete reaction
of 4siloxane
was not
epoxy/silica hybrid materials, were observed. Since the samples, having a Q
environment structure complete in the prepared samples. The incomplete reaction 3 and Q
of siloxane was not 4 was observed to increase
observed
in
the
epoxy/silica
nano-hybrids.
Additionally,
the
amount
of
Q
4 was observed to increase could be considered to include a highly condensed silica phase, the formation of the SiO
2 network observed in the epoxy/silica nano‐hybrids. Additionally, the amount of Q
with structure TEOS
content,
indicating
that
a a large
amount
of introduced introduced
TEOS
may
have
promoted
was complete in the prepared samples. incomplete TEOS reaction of siloxane was not with TEOS content, indicating that large amount The of may have promoted the the
4 was observed to increase observed in the epoxy/silica nano‐hybrids. Additionally, the amount of Q
formation
of the silica network.
formation of the silica network. 29 Si NMR. Figure 1 shows the solid-state 29 Si NMR spectra of the epoxy/silica nanocomposite,
with TEOS content, indicating that a large amount of introduced TEOS may have promoted the formation of the silica network. Figure 1. The solid‐state Si NMR spectra of epoxy/silica nanocomposite. 29
Figure 1. The solid-state 29 Si NMR spectra of epoxy/silica nanocomposite.
Figure 1. The solid‐state 29Si NMR spectra of epoxy/silica nanocomposite. Q2
Q3
Q4
Q2
Q3
Q4
Scheme 3. The Q structure of polysilsesquioxane. 4 Scheme 3. The Q structure of polysilsesquioxane. Scheme 3. The Q structure of polysilsesquioxane.
4 7035
Materials 2015, 8, 7032–7040
Materials 2015, 8, page–page 3.2. Cure Kinetics of Epoxy/Silica Nano‐Hybrids 3.2. Cure Kinetics of Epoxy/Silica Nano-Hybrids
A general equation for the autocatalytic curing reactions of many epoxy systems is as follows [7]. A general equation for the autocatalytic curing reactions of many epoxy systems is as follows [7].
r  dα / dt  ( k1  k 2α m )(1  α) n (1)
r “ dα{dt “ pk1 ` k2 αm qp1 ´ αqn
(1)
where α is the conversion; k1 and k2 are the apparent rate constants; r is the rate of the reaction, where
is the
conversion;
k1 and
k2 are the
rate constants;
r is the
of the reaction,
and m αand n are the kinetic exponents of apparent
the reactions. The constant k1 rate
in Equation (1) can and
be m and n are the kinetic exponents of the reactions. The constant k1 in Equation (1)1 and k
can be
calculated if
calculated if the initial reaction rate at α = 0 can be estimated. Kinetic constants k
2 are assumed thebe initial
reaction
rate at form: α = 0 can
constants
k1 and
to A be is of the
k1 beA1estimated.
EXP(Ea1 / Kinetic
RT ) and k2  A2 EXP
(Eka22are
/ RTassumed
) , where to of the Arrhenius the Arrhenius
form:
k
“
A
EXPp´
E
{RTq
and
k
“
A
EXPp´
E
{RTq,
where
A
is
the
pre-exponential
2
2
1
1
energy; R a2
is the gas constant, and T is the pre‐exponential constant; Ea is a1the activation constant; Ea is the activation energy; R is the gas constant, and T is the absolute temperature.
absolute temperature. The epoxy resin contained 10 phr (based on 100 parts of the epoxy) of silica and was cured at
The epoxy resin contained 10 phr (based on 100 parts of the epoxy) of silica and was cured at ˝ C, 110 ˝ C, 120 ˝ C, 130 ˝ C, and 140 ˝ C. Kinetic analysis was
five
isothermal
five isothermal temperatures
temperatures of
of 100
100 °C, 110 °C, 120 °C, 130 °C, and 140 °C. Kinetic analysis was performed using the above kinetic models. Figure 2 plots the rate curves that were obtained at the
performed using the above kinetic models. Figure 2 plots the rate curves that were obtained at the five isothermal temperatures. These rate curves were distinctly autocatalytic, and the maximum rates
five isothermal temperatures. These rate curves were distinctly autocatalytic, and the maximum rates were reached 5 min, 7 min, 11 min, 18 min, and 31 min after the start of the reaction at isothermal
were reached 5 min, 7 min, 11 min, 18 min, and 31 min after the start of the reaction at isothermal reaction temperatures of 100 ˝ C, 110 ˝ C, 120 ˝ C, 130 ˝ C, and 140 ˝ C, respectively. The results in these
reaction temperatures of 100 °C, 110 °C, 120 °C, 130 °C, and 140 °C, respectively. The results in these figures also indicate that the presence of silica in the epoxy does not change its autocatalytic nature.
figures also indicate that the presence of silica in the epoxy does not change its autocatalytic nature. Figure 2. Plot of the reaction rate vs. time for epoxy/silica hybrids at five various temperatures. Figure 2. Plot of the reaction rate vs. time for epoxy/silica hybrids at five various temperatures.
Table 1 presents the rate constants, which were obtained by iterative and graphic procedures. Table 1 presents the rate constants, which were obtained by iterative and graphic procedures.
The orders of the reaction, m and n, were approximately 0.5 and 1.4, respectively, and their values The not orders
of much the reaction,
and
n, were approximately
andΔE
1.4,
respectively, and their values
did vary among m
the epoxy‐silica hybrids. The 0.5
value 1 and ΔE2 for anhydride‐cured did
not
vary
much
among
the
epoxy-silica
hybrids.
The
value
∆E
and
∆E2 for
anhydride-cured
−1
−1, respectively. 1
DGEBA epoxy that were obtained in this study were 80 kJmol and 60 kJmol
The ´1 and 60 kJmol´1 , respectively.
DGEBA
epoxy
that
were
obtained
in
this
study
were
80
kJmol
autocatalytic kinetic model and the obtained rate constants were used to calculate the empirical The autocatalytic
kinetic
and the
obtained
rateepoxy/silica constants were
used at to five calculate
the empirical
curves of conversions as model
a function of time for the hybrids isothermal curing curves
of
conversions
as
a
function
of
time
for
the
epoxy/silica
hybrids
at
five
isothermal
curing
temperatures. Figure 3 shows that the empirical conversion curves fit the experimental data quite temperatures.
Figure
3
shows
that
the
empirical
conversion
curves
fit
the
experimental
data
quite
closely until the curing reactions had progressed to the point of vitrification. The model apparently closely until the curing reactions had progressed to the point of vitrification. The model apparently
describes the kinetics accurately, but diffusion control in the vitrified state limited the extent of the describes the kinetics accurately, but diffusion control in the vitrified state limited the extent of the
epoxy reactions. epoxy
reactions.
Figure 4 plots the curves of rate vs. conversion for the epoxy‐silica hybrids with 10 phr silica, Figure
4 plots the curves of rate vs. conversion for the epoxy-silica hybrids with 10 phr silica,
which were cured at five isothermal temperatures. The conversion rate above which the curing rate which
were
cured at five isothermal temperatures. The conversion rate above which the curing rate
varied coincided with the formation of the gel point, at which the curing reaction of the gelled epoxy network began to slow down. 7036
5 Materials 2015, 8, 7032–7040
varied coincided with the formation of the gel point, at which the curing reaction of the gelled epoxy
network began to slow down.
Table
Materials 2015, 8, page–page 1. Autocatalytic model constants for epoxy–silica hybrids.
TMaterials 2015, 8, page–page (˝ C)
m
n Table 1. Autocatalytic model constants for epoxy–silica hybrids. k1 (min´1 )
k2 (min´1 )
Ea1 (kJmol´1 )
Ea2 (kJmol´1 )
100 T (oC) 0.5
110 100 0.6
120 T (
0.6
oC) 110 130
0.6
120 100 140
0.6
130 110 140 120 130 140 m 0.9
0.5 1.0
m 1.0
0.6 1.0
0.6 0.5 0.9
0.6 0.6 0.6 0.6 6.4−1)
k10.3
(min−1) k2 (min
Ea1 (kJmol–−1)
Ea2 (kJmol−1–) n Table 1. Autocatalytic model constants for epoxy–silica hybrids. 0.60.3 14.0
–
0.9 6.4 – –
– 18.8−1)
−1) k11.2
(min
Ea1 (kJmol
Ea2 (kJmol
n 1.0 0.6 −1) k2 (min
14.0 – 5.6−1)
– 5.4
1.6
30.0
–
–
1.0 1.2 18.8 5.6 5.4 0.9 0.3 6.4 – – 1.9
45.2
–
–
1.0 1.6 30.0 – – 0.6 14.0 0.9 1.9 45.2 – – 1.0 1.2 18.8 5.6 5.4 1.0 1.6 30.0 – – 0.9 1.9 45.2 – – A1 – A– 1 287 – – – 287 – – A1
A2
–2
–
A
–– –
287
A– 2 4699
–
–
4699 – –
–
– – 4699 – – Figure 3. Plot of the reaction rate vs. time for epoxy/silica hybrids at five various temperatures. Figure 3. Plot of the reaction rate vs. time for epoxy/silica hybrids at five various
temperatures.
Figure 3. Plot of the reaction rate vs. time for epoxy/silica hybrids at five various temperatures. Figure 4. Plot of the reaction rate vs. conversion for epoxy/silica hybrids at five various temperatures. 3.3. Morphological Analysis Figure 4. Plot of the reaction rate vs. conversion for epoxy/silica hybrids at five various temperatures. Figure 4. Plot of the reaction rate vs. conversion for epoxy/silica hybrids at five various temperatures.
Figure 5 shows a TEM image of the silica/epoxy hybrid with 10 phr silica and the histogram of 3.3. Morphological Analysis the particle sizes of silica that was cured at 140 °C for 60 min. TEM data reveal that the sizes of the Figure 5 shows a TEM image of the silica/epoxy hybrid with 10 phr silica and the histogram of 7037
6 the particle sizes of silica that was cured at 140 °C for 60 min. TEM data reveal that the sizes of the 6 Materials 2015, 8, 7032–7040
3.3. Morphological Analysis
Figure 5 shows a TEM image of the silica/epoxy hybrid with 10 phr silica and the histogram
of the particle sizes of silica that was cured at 140 ˝ C for 60 min. TEM data reveal that the sizes
of the nano-scale silica particles that were synthesized by the sol-gel method varied in the range
15–60 nm and the average size was 35 nm. In the epoxy/silica hybrid, nano-scale silica particles
Materials 2015, 8, page–page were dispersed uniformly throughout the epoxy matrix. This morphology reveals that 10% TEOS
sufficednano‐scale silica particles that were synthesized by the sol‐gel method varied in the range 15–60 nm to form silica particles during the gel reaction. However, the amount of silica particles
Materials 2015, 8, page–page formed and the average size was 35 nm. In the epoxy/silica hybrid, nano‐scale silica particles were dispersed increased with the amount of charged TEOS. The sol-gel reaction changed the sizes of
uniformly throughout the epoxy matrix. This morphology reveals that 10% TEOS sufficed to form nano‐scale silica particles that were synthesized by the sol‐gel method varied in the range 15–60 nm the nano-silica
particles. The reactions involved various degrees of hydrolysis and condensation,
silica particles during the gel reaction. However, the amount of silica particles formed increased with and the average size was 35 nm. In the epoxy/silica hybrid, nano‐scale silica particles were dispersed yielding nano polysilsesquioxane particles of various sizes. Alkoxide addition to epoxy resins is
the amount of charged TEOS. The sol‐gel reaction changed the sizes of the nano‐silica particles. The uniformly throughout the epoxy matrix. This morphology reveals that 10% TEOS sufficed to form thus expected
to change not only their morphology but also the curing kinetics. Phase separation
reactions involved various degrees of hydrolysis and condensation, yielding nano polysilsesquioxane silica particles during the gel reaction. However, the amount of silica particles formed increased with occurs and
a heterogeneous morphology develops if the silica and the crosslinked epoxy are not
particles of various sizes. Alkoxide addition to epoxy resins is thus expected to change not only their the amount of charged TEOS. The sol‐gel reaction changed the sizes of the nano‐silica particles. The misciblemorphology but also the curing kinetics. Phase separation occurs and a heterogeneous morphology after
curing. Additionally, phase morphology development during curing is expected to
reactions involved various degrees of hydrolysis and condensation, yielding nano polysilsesquioxane develops if the silica the crosslinked epoxy epoxy
are not resins.
miscible Figure
after curing. Additionally, influence
significantly
the and properties
of the cured
6 show
the siliconphase mapping of
particles of various sizes. Alkoxide addition to epoxy resins is thus expected to change not only their morphology development during curing is expected to influence significantly the properties of the morphology but also the curing kinetics. Phase separation occurs and a heterogeneous morphology silica/epoxy hybrids with 10 phr silica. The silicon distribution mapping of the epoxy/silica hybrids
cured epoxy resins. Figure 6 show the silicon mapping of silica/epoxy hybrids with 10 phr silica. The if the silica and suggesting
the crosslinked are not miscible curing. Additionally, phase revealeddevelops well mixed
silicon,
thatepoxy the inorganic
silicaafter nano-particles
in the epoxy/silica
silicon distribution mapping of the epoxy/silica hybrids revealed well mixed silicon, suggesting that morphology development during curing is expected to influence significantly the properties of the nanocomposites
were distributed uniformly on the nanometer scale and their nano-architectures were
the inorganic silica nano‐particles in the epoxy/silica nanocomposites were distributed uniformly on cured epoxy resins. Figure 6 show the silicon mapping of silica/epoxy hybrids with 10 phr silica. The well defined.
the nanometer scale and their nano‐architectures were well defined. silicon distribution mapping of the epoxy/silica hybrids revealed well mixed silicon, suggesting that the inorganic silica nano‐particles in the epoxy/silica nanocomposites were distributed uniformly on the nanometer scale and their nano‐architectures were well defined. (a) (b)
Figure 5. Transmission electron micrograph (TEM) of epoxy/silica hybrids with 10 phr silica: Figure 5. Transmission electron
(a) micrograph (TEM) of epoxy/silica hybrids
(b) with 10 phr silica: (a) TEM
(a) TEM image and (b) the particle sizes distribution of silica. image and (b) the particle sizes distribution of silica.
Figure 5. Transmission electron micrograph (TEM) of epoxy/silica hybrids with 10 phr silica: (a) TEM image and (b) the particle sizes distribution of silica. Figure 6. The silicon mapping of epoxy/silica hybrids with 10 phr silica. Figure 6. The silicon mapping of epoxy/silica hybrids with 10 phr silica. Figure 6. The silicon mapping of epoxy/silica hybrids with 10 phr silica.
7 7038
7 Materials 2015, 8, 7032–7040
4. Conclusions
A new class of epoxy/silica nano-hybrids was prepared using a sol-gel technique. To improve
the compatibility between the organic and inorganic phases, a coupling agent was used to modify
the diglycidyl ether of bisphenol A (DGEBA) epoxy. Siloxane bridges in the silica, which were highly
crosslinked in the epoxy/silica hybrid materials, were observed. The formation of the SiO2 network
structure in the prepared samples was complete. The difference in polarity between the inorganic and
organic materials may have formed the nanophase-separated structure, which is shown in the TEM
microphotographs. The sizes of the nano-scale silica particles in the nanocomposites varied in the
range of 15–60 nm, averaging 35 nm, and the particles were well dispersed in the epoxy matrix,
according to the TEM images. An autocatalytic mechanism of the curing of 10 phr epoxy/silica
hybrids at various isothermal temperatures was observed. Kinetic parameters for the epoxy/silica
hybrids were obtained and the proposed kinetic model was found to describe accurately the curing
behavior of the epoxy/silica hybrids up to the vitrification point. A kinetic modeling approach is used
to demonstrate how thermosetting epoxy resins that have been modified with silica behave during
curing. The curing kinetics were corrected for the morphological development of the heterogeneous
epoxy/silica hybrids during the curing progress.
Acknowledgments: This study is sponsored by the National Science Council of Taiwan under contract Nos.
MOST 104-2221-E-390-025 and MOST 104-2622-E-390-003-CC3.
Author Contributions: The study was designed by Chean-Cheng Su. The data collection was performed by
Cheng-Fu Yang, Li-Fen Wang and Song-Mao Wu. The data analysis was performed by Chean-Cheng Su.
The overall planning was directed by Chean-Cheng Su.
Conflicts of Interest: The authors declare no conflict of interest.
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