Increasing toughness and tensile strength of an epoxy-diamine system using an inorganic ultra-accelerator

Article abstract of DOI:10.1039/c5ra06908d

Epoxy resins are a class of thermoset materials with extensive structural and composite applications. A wide range of chemical species, of which room temperature curing agents are most favourable, due to their widespread applications, can easily cure these resins. The major drawback of low temperature curing agents is their relatively high curing time. To overcome this issue, accelerators are usually added to decrease curing time and temperature. In this study, we present a unique type of inorganic accelerator that has immensely reduced curing time and temperature as well as imparting major improvements in mechanical properties including tensile strength, izod impact strength and adhesion strength. The epoxy resins used in the following study were diglycidyl ether of bisphenol A (DGEBA) and Epikure F 205 (modified isophorone diamine). The curing kinetics of epoxy resin was determined in the presence of inorganic accelerator using FT-IR spectroscopy. Resins were cured with and without inorganic accelerator and the thermal properties were investigated using DSC (differential scanning calorimetry). FT-IR spectroscopy results showed that using 5% inorganic accelerator, forced 99.68% of the epoxy groups to open, while crosslinks were observed after 18 minutes at 25 °C. Results from DSC analysis showed that using 5% inorganic accelerator reduced the initial onset curing temperature from 68.3 °C to -2.4 °C, while significantly improved its mechanical properties in the polymer backbone.

Full text of DOI:10.1039/c5ra06908d

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Increasing toughness and tensile strength of epoxy-
diamine system using inorganic ultra-accelerator
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
b
Abdolreza Mirmohseni-Namin,* Saeid Nikafshar, Farid Mirmohseni
DOI: 10.1039/x0xx00000x
Epoxy resins are a class of thermoset materials with extensive structural and composite applications. A wide range of
chemical species, of which room temperature curing agents are most favourable, due to their widespread applications, can
easily cure these resins. The major drawback of low temperature curing agents is their relatively high curing time. To
overcome this issue, accelerators are usually added to decrease curing time and temperature. In this study, we present a
unique type of inorganic accelerator that has immensely reduced curing time and temperature as well as imparting major
improvements in mechanical properties including tensile strength, izod impact strength and adhesion strength. The epoxy
resins used in the following study were diglycidyl ether of bisphenol A (DGEBA) and epikure F 205 (modified isophorone
diamine). Curing kinetic of epoxy resin was determined in the presence of inorganic accelerator with FT-IR spectrum.
Resins were cured with and without inorganic accelerator and the thermal properties were investigated using DSC
(differential scanning calorimetry). FT-IR spectrum results showed that using 5% of inorganic accelerator, forced 99.68 % of
epoxy groups to open, while crosslinks were observed after 18 minutes at 25 °C. Results from DSC analysis showed that
using 5% of inorganic accelerator reduce the initial onset curing temperature from 68.3 °C to -2.4 °C, while significantly
improve its mechanical properties in the polymer backbone.
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chemical bonds, morphology and crosslink density within
composite structure8.
a
Introduction
Due to the combination of excellent chemical and mechanical
properties, epoxy resins are one of the most important classes of
thermosetting polymers. They are suitable for a wide range of
applications such as engineered composites, forming compounds,
protective coatings, casting and adhesives1-5. Epoxy resins are low
molecular weight pre- polymer that have one or more epoxy
groups.6. These resin form a cross linked network along with curing
agents7. There are many different types of curing agents including
aliphatic and aromatic amines, polyamides, anhydrides and
Curing agents that cure epoxy resins at room temperature or lower
are more demanding due to their widespread applications10
.
Examples of such curing agents include aliphatic and cycloaliphatic
amines, polyamides and polymercaptans which can ultimately cure
epoxy resins at ambient temperatures8. The curing time for
aliphatic amines are relatively high, while the density of the formed
crosslink at room temperature is low. When such curing agents are
used, due to the low degree of conversion, an extra step (post cure)
is carried out to complete cure reaction and enhanced chemical and
mechanical properties. Due to a stronger nucleophilicity of sulphur
in comparison with nitrogen, polymercaptans can cure epoxy resins
at much faster rate compared to aliphatic or cycloaliphatic
8
polymercaptans . A number of factors including the type of curing
agent and its curing temperature can affect the final properties of
an epoxy network9. The curing agent can also determine the type of
amines11
. Main disadvantages associated with the usage of
polymercaptans are their unpleasant smell and the relatively
complex and expensive synthesis procedures of mercaptans8.
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For the last few years a range of compounds called accelerators are the curing reaction rate. Although aqueous solution of Ca(NO3)2
utilised to overcome the issues with temperature and curing time of only reduces gel time and it does not affect curing time
epoxy systems 10. Examples of such compounds include substituted
Experimental
urea, N- Methyl secondary amines, transition metal chelates,
quaternary phosphonium, substituted thiodiphenol, tertiary amine,
Materials
8, 10,
imidazoles, Lewis acid and dicyandiamide
12-19. All of these
compounds, accelerate curing of epoxy resin at high temperature
and at the room temperature they are almost deactivate. On the
other hand, using the aforementioned accelerators could reduce
crosslink density of the network and ultimately decrease the
Liquid Diglycidyl Ether of Bisphenol A (Epon 828) (CAS No. 25068-
38-6), with an epoxy equivalent weight of 185-192 eq/g was
purchased from E.V Roberts. Epikure F205 was supplied by Hexion
which was used as the curing agent. This curing agent is based on
mechanical properties of the epoxy system 20
.
Isophoron
diamine
(3-
aminomethyl-3,
5,
5-
trimethylcyclohexylamine) (CAS No 2855- 13- 2) that was further
modified to cure epoxy resin at room temperature. 65 % Nitric acid
(CAS No 7697-37-2) and Calcium Carbonate (CAS No 471- 34- 1) was
supplied by Sigma Aldrich. The chemical structures of DGEBA and
IPDA are shown in Fig 1.
Amines are one of the main curing agents used in epoxy resin
systems8. Aliphatic and cycloaliphatic amines are widely used in
8,
protective epoxy coating and for adhesive applications 21. Our
results show that Epikure F205 base of isophorene diamine (IPDA)
can be used as a curing agent for diglycidyl ether of bisphenol A
(DGEBA) epoxy resin, with a novel accelerator precursor to make
hardener. This accelerator is based on an inorganic salt (calcium
nitrate solution). It has been shown that as the aliphatic and
cycloaliphatic diamine react; the accelerator increases the curing
rate of the epoxy system immediately. As a result, curing time and
curing temperature of epoxy dropped. Also it has been found that
curing reaction might take place in low (sub-zero) temperatures.
When a high concentration of an inorganic accelerator was used,
cure rate was high. It was observed that as the mixture of DGEBA,
Epikure F 205 and high concentration of inorganic accelerator
began to boil, the curing reaction took place immediately and foam
was formed (the boiling point of DGEBA is >260 °C). Previous works
conducted by Mark whiter et al in 2003, led to the invention of an
inorganic accelerator that could only reduce the gel time, but was
unable to alter the curing time of the epoxy system22. . In different
tests that were performed it was found that HNO3 does not affect
Methods
Synthesis of inorganic accelerator. 0.0263 mole (2.63 gr) of calcium
carbonate was added to 5.152 ml of 65% nitric acid in room
temperature with continuous stirring. After 2 minutes, a green
solution was observed. Reaction occurred according to equation (1).
ꢀꢁꢀꢂꢃ ꢄ 4ꢅꢆꢂꢃ → ꢀꢁꢇꢆꢂ3ꢈꢉ ꢄ ꢀꢂꢉ ꢄ ꢅꢉꢂ ꢄ 2ꢅꢆꢂꢃ
ꢇ1ꢈ
Average epoxy molar mass (EMM) of Epon 828, as reported by the
manufacturer, was 188.5 g/eq. The amine hydrogen molar mass
(AHMM) of Epicure F 205, which was used as the curing agent, was
reported to be 105 g/eq. Therefore, each amine hydrogen reacts
with one epoxy group and the stoichiometric ratio of the curing
agent for reaction with the epoxy resin can be calculated
accordingly as AHMM/EMM= 0.557. To prepare the samples, 100g
of epoxy resin was mixed with 55.7g of Epicure
F 205.
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O
CH3
C
CH3
CH3
C
CH3
O
H2C
CH
CH2
O
O
CH2
CH CH2
OH
O
O
CH2
CH
CH2
n
Diglycidyl ether of bisphenol A (DGEBA) (CAS No 25068-38-6)
NH2
H3C
H3C
CH2NH2
CH3
Isophorone diamine (IPDA) (CAS No 2855-13-2)
Figure 1. Chemical structures of Didlycidyl ether of bisphenol A (DGEBA) and Isophoronediamine (IPDA)
All samples were prepared according to the ratio 1: 0.557 epoxy Differential scanning calorimetry (DSC). In this study DSC (Linseis,
resin and F 205 respectively and only the amount of inorganic PT10, and Germany) was used to report the rate and curing
accelerator in the samples was changed. The percentage of condition of the reaction. The DSC machine was calibrated with
inorganic accelerator in the epoxy resin for different samples has standard zinc and indium. Samples were prepared by mixing resin,
been shown in Table1.
curing agent and accelerator for 30 seconds, according to the recipe
shown in Table 1. Then 5 mg of each sample was sealed in an
aluminium pan and was placed in DSC machine for curing (because
of high curing rate of the system by accelerator, preparation of
sample and placing in DSC machine should be fast). A non-
isothermal test at a heating rate of 10 °C/min under a nitrogen flow
Analysis Techniques
Fourier transform infrared spectroscopy (FTIR). The FT-IR analysis
was used in FT-IR spectrometer, Tensor 27, Bruker with 40 scan
average at a resolution of 4 cm-1. In the present work, FT-IR
spectroscopy was applied to determine the degree of conversion of 40 ml/min was performed at the temperature range of -50 to 170
for epoxy resins in the isothermal mode at room temperature.
°C.
The glass transition (Tg). To measure Tg, DCS method (differential
scanning calorimetry) was used (DSC, Linseis, PT10, Germany).
Mixing, resin, curing agent and accelerator for 30 seconds, samples
were prepared according to the recipe shown in Table 1. Prepared
samples were left at room temperature for 7 days. Then 5 mg of
cured samples were placed in an aluminium pan. For each sample
Tg was measured at the temperature range of 0- 150 °C under
nitrogen flow of 40 ml/min and heating rate of 10 °C/min by DSC
machine.
Table 1. Amount of inorganic accelerator in different samples (percentage
was considered into to epoxy resin)
Inorganic accelerator
Sample Name
content (%)
A
B
C
D
E
0
1.25
2.5
3.75
5
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Tensile strength. Tensile strength test was performed according to
ASTM D256 with a Shimadzu 20KN-testing machine. Specimen
dimensions were chosen according to type 1 of ASTM (165 × 19 ×
Results and discussion
The inorganic accelerator reported in the study was synthesized by
using inexpensive materials following simple method of
a
3.2 mm). Crosshead speed was
2 mm/min. Specimens were
preparation. Unlike existing accelerators, the invented accelerator
increases the mechanical properties (adhesion, tensile strength and
impact strength) of epoxy resin. Following discussion investigates
the influence of inorganic accelerator (Calcium nitrate solution) on
the mechanical properties such as adhesion, tensile strength, and
izod impact strength on epoxy resins. The time required to
completely cure DGEBA and Epikure F 205 at 25 °C was 7 days (from
datasheet). In this study, it was found that by using 5% inorganic
accelerator in the system, curing time of resin at 25 °C can
significantly reduce to only 18 minutes, which is notably lower
compared to not using the inorganic accelerator.
prepared according to Table 1. DGEBA, Epicure F 205 and inorganic
accelerator were mixed for 30 seconds and the mixture was then
placed into a silicon mould. To allow a complete curing reaction,
following 8 hours at room temperature, specimens were removed
from the mould and stored at room temperature for 7 days, (the
complete curing time for sample A was 7 days). From each sample 5
replicates were prepared and tested.
Pull-off strength. The pull-off strength was measured in accordance
with ASTM D4541, by using a Posi Test AT-M from Deflesco.
Measurements were carried out following 12 hours of curing
process. Each test was repeated 5 times on an aluminium surface.
The curing mechanism of epoxy resins are different in the presence
of an inorganic accelerator, where homopolymerization reaction
occurs8. In the acidic environment, cationic polymerization takes
Izod impact strength. To measure izod impact strength, five
replicates from each sample were prepared. Specimens were un-
notched with the dimensions of 63.5 × 12.7 × 7.2 mm, according to
ASTM D256 using Zwick/Roell 6103 impact tester at room
temperature. DGEBA, Epicure F 205 and inorganic accelerator were
mixed for 30 seconds with the ratios indicated in Table 1, and then
the homogeneous mixture was poured into a silicon mould. After 8
hours, samples were removed from the silicon mould and kept for 7
days at 25 °C to complete curing.
24
place which causes an increase in the chain length23, . In the
presence of a curing agent such as Epikure F 205, without the
addition of a catalyst and accelerator, the curing reaction is in the
form of a polyaddition reaction forming a branched structure 8. The
molecular architecture is directly related to the physical and
mechanical properties of the epoxy structure 25, such that as the
chain length increases, the tensile strength also increases. This is
mainly due to the fact that the elongation and yield points are
directly related to the length of polymer chains.
Scanning electron microscopy (SEM). To investigate the effect of
inorganic accelerator on the toughness of epoxy samples, fracture
surfaces from izod impact strength were observed using Scanning
electron microscopy (Tescan, MIRA3 FEG-SEM, Czech Republic). To
enhance the resolution, samples were coated with gold vapour.
SEM was operated at an accelerating voltage of 5.00 KV.
The inorganic accelerator synthesized in this study was acidic in
nature, while the curing agent contained amine groups and was
basic. Upon reacting the inorganic accelerator with the curing
agent, the acid-base reaction caused an increase in the temperature
of the system, with the formation of inorganic salts. This increase in
the system temperature fastens the curing reaction. The
mechanism of curing epoxy in the presence of this inorganic
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accelerator can be categorised into the following 3 mechanisms: (inorganic accelerator has an acidic role while Epicure F 205 has an
poly addition, cationic polymerization and anionic polymerization. alkaline role) (Figure 3). Due to the exothermic nature of the acid-
The mentioned mechanisms can take place simultaneously, or occur base reaction, the system temperature and curing rate were
one after another depending on the system condition. Figure 2 increased.
illustrates the 3 mechanisms of curing epoxy as discussed earlier
Upon further investigation, it was discovered that the reaction of
(
Polyadditional reaction). Initially, ring opening took place by the
calcium nitrate with the amines resulted in the formation of
protons and nitrate ions. This was important as protons had the
ability to lead to cationic polymerization, while the negative nitrate
ions could initialise anionic polymerization. The proposed
mechanisms of cationic and anionic polymerization are
attack of primary amine groups to the epoxy ring. The second
amine group reacted with another epoxy group and also with the
hydroxyl group that was formed during the curing reaction. This
resulted in the formation of an etherification reaction, as shown in
8,20
Figure 2
.
schematically shown in Figure
4 and Figure 5 respectively.
Cationic and anionic polymerization. Cationic polymerization using Experimentally, it was found that during the anionic polymerization,
Lewis acids and protonic acids has been discussed in many different nitrate ions were attached to the epoxy group, causing the rings to
studies23, 24. In this investigation, nitric acid was used as the open in the presence of [HA]. The mentioned process could be
protonic acid compound. Upon mixing the inorganic accelerator referred to as a proton supplying curing reaction.
with DGEBA and Epikure F 205, an acid-base reaction took place
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H
R
NH2 + R' CH
CH2
R'
CH
OH
CH2
N
R
(1)
O
OH
HC
H
N
R'
CH2
R'
CH
OH
CH2
R
+
R'
CH
CH2
(2)
(3)
R'
R'
CH
OH
CH2
N
R
O
OH
HC
OH
HC
R'
CH
O
CH2
N
R'
CH2
N
CH2
R
R'
R'
CH
OH
CH2
R
R'
CH
CH2
+
O
CH2 CH
OH
H
H
N
CH2
CH3
CH3
R=
H3C
CH3
C
CH2
O
O
CH2 CH
CH2
R'=
O
CH3
Figure 2. Polyadditional reaction with epoxy resin and polyamine.
R
NH2
HNO3
R
NH3 NO3
+
+ Ca(NO3)2 (aq)
NH2
R
R
NH3 NO3
Ca(OH)2
+
Figure 3. Reaction between amine curing agent and inorganic accelerator (acid- base reaction)
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R'
CH
CH2
R'
CH
CH2
+ H
O
H
O
R'
CH2
CH
R'
R'
CH
CH2
R'
CH
CH2
O
CH
O
R'
CH
CH2
O
R'
CH
CH2
OH
CH2
O
H
OH
O
n
CH2 CH
R'
R'
R'
CH
O
R'
CH
CH2
O
CH
CH2
C
H2
OH
n
Figure 4. Proposed cationic polymerization of epoxy groups in the presence of inorganic accelerator
(1)
(2)
+
O
O
CH2 CH
ONO2
R'
R'
CH CH2
O
O2NO
CH2 CH
R'
+
NO3
O2NO
O
CH2 CH
O
O2NO
CH2 CH
O
CH2 CH
R'
O
+
R'
CH CH2
R'
R'
CH2 CH
R'
O
CH2 CH
R'
O
O
CH2 CH
R'
ONO2
ONO2
O2NO
O
CH2 CH
R'
O
CH2 CH
R'
O
R'
CH CH2
O
n
(3)
n
CH2 CH
R'
O
CH2 CH
R'
ONO2
n
+
A
+
R'
O2NO
CH2 CH
R'
OH
(1)
CH CH2 + HA
NO3
O
R'
CH CH2
O
n
H
C
O2NO
CH2 CH
R'
OH
(2)
O2NO
CH2 CH
R'
O
CH2 CH
CH2
OH
n
R'
R'
Figure 5. Proposed anionic polymerization of epoxy groups in the presence of inorganic accelerator
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The newly formed hydroxyl groups continue the reaction until FTIR analysis
completion. In this reaction [HA] can be water or nitric acid. Both
FT-IR spectroscopy is a sensitive method to determine crosslink
anionic and cationic polymerizations increase the chain length, and
epoxy chains grow more in the length direction23, 24, 26. This increase
of chain length gives the network of epoxy new properties. The final
epoxy structures were assumed to be similar to Figure 6 for both
cases of with and without inorganic accelerator. The epoxy network
1, 27-30
density and curing rate of epoxy resin
. To study the effect of
inorganic accelerator on the curing reaction and determining the
degree of conversion of epoxy resin, sample No 5 (sample E) was
selected. Required amounts of epoxy resin (DGEBA), Epicure F 205
and inorganic accelerator were prepared based on Table 1. The
materials were transferred into a container and were mixed for 30
second in 25 °C. Immediately after mixing, a drop of mixture was
poured between two KBr tablets for FT- IR spectroscopy and a
sample was taken out every two minutes from the spectrum for 18
minutes, preparing a total of 9 spectrums.
crosslink of DGEBA/ Epicure
F 205 after curing, covered all
directions and lacked any special orientation, however the in the
presence of inorganic accelerator, it was found to have a lengthy
orientation, increased chain length and with most of the crosslinks
aligned across one line.
Figure 6. Cure epoxy architect with and without of inorganic accelerator
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Spectra with an area of 3800-400 cm-1 were obtained, whilst a In equation (2), A indicates the absorbance of the functional group
range of 1300-400 cm-1 was used to determine the degree of and the subscripts 0 and t refer to the initial and current reaction
conversion. The absorption bond of 915 cm-1 indicated the presence time respectively. According to Figure 8
, after 2 minutes from the
of epoxy groups. After XXXXX minutes, the intensity of absorbent reaction initiation, 49.942% of the epoxy groups were opened and
with a peak of 915 cm-1 was reduced due to the opening of the formed crosslink, while after 18 minutes, the percentage of
epoxy rings. The peak at 1182 cm-1 showed C-O stretching of crosslink degree reached 99.6779%, which indicated almost a
aromatic ring of the epoxy resin (DGEBA)30, 31. As it can be seen complete curing reaction of DGEBA and high activity rate of the
from Figure 7, the absorbance intensity of the epoxy group at 915 inorganic accelerator. However in the case of curing the sample
cm-1 was halved after 2 minutes, which indicated a very high curing without an accelerator (DGEBA, Epicure F205), the curing time
rate for the reaction. It should be noted that the reaction required was 7 days at room temperature (25°C). Since the rate
temperature was 25 °C.
curing reaction declined dramatically after 6 minutes, presenting all
9 spectra will cause an overlapping of spectra and ultimately make
it impossible to determine. Other results of spectra was used for
calculation of degree of crosslink.
Equation (2) was used to determine the degree of conversion.
ꢇ
ꢈꢇ
ꢈꢇ
ꢈ
ꢈ
A 915, t A 1182,0
(2)
ꢊ ꢋ ꢌ1 ꢍ ꢎ
ꢏꢐ ꢑ 100
ꢇ
A 915,0 A 1182, t
0.35
0.3
915 cm-1
1182 cm-
0.25
0.2
0.15
0.1
0.05
0
300
500
700
900
1100
1300
Wavenumber (cm-1)
0 min
2 min
4 min
6 min
Figure 7. The partial FT-IR spectra of epoxy resin and Epikure F 205 with 5% Inorganic accelerator
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two steps, the first step at the presence of inorganic accelerator,
which initiated early. Hence the concentration of accelerator
was low, curing reaction was partially gone and consequently
majority of the accelerator was consumed, causing the curing
reaction to stop at the low temperature. The curing reaction was
paused until the temperature of the system increased.
Consequently at the second onset temperature 54.7 °C (T2), the
remainder of the curing reaction continued. Total heat of curing
(∆H) was raised from 56.9692 J/g (sample A, without
accelerator) to 329.2118 J/g (sample 2, with 1.25% accelerator),
which shows the release of heat from curing reaction in the
100
90
80
70
60
50
40
30
20
10
0
0
2
4
6
8 10 12 14 16 18 20
Time (minutes)
Figure 8. Process of epoxide reaction in DGEBA/ Epikure F 205/ Inorganic
presence of accelerator. In sample C (2.5% inorganic
accelerator system by FT-IR spectroscopy at 25 °C
accelerator), similar to sample B (1.25% inorganic accelerator),
the curing reaction occurred in two steps. By increasing the
accelerator concentration, the initial onset temperature (T1), the
first temperature peak (Tp1), the second onset temperature (T2)
and the second temperature peak (Tp2) were decreased (T1: 0.1
°C, Tp1: 8.3 °C, T2: 37.9 °C, Tp2: 52.4 °C). As a result total heat of
curing (∆H) of sample C (2.5% inorganic accelerator) decreased.
By increasing inorganic accelerator concentration to 3.75% and
5%, the curing reaction of epoxy system had occurred at
negative temperature and in one step. Results of various
samples have been summarised in table 2. In general, by using
inorganic accelerator in the DGEBA/ Epicure F 205 system, the
curing reaction would be faster and would occur at the lower
temperature.
DSC analysis
Information obtained from DSC (differential scanning
calorimety) can be related to the curing reaction of epoxy
system 14. Many studies have been conducted on the epoxy
resin to obtain kinetic data with the DSC1, 5-7, 12, 13, 32-36
.
Non- isothermal DSC curves of sample A (without inorganic
accelerator), showed one exothermic peak (Figure 9, a). The
initial onset temperature (T1) and the first peak temperature
(Tp1) were seen at 68.3 °C and 93.2 °C respectively. By adding
inorganic accelerator the initial onset temperature (T1) and the
first peak temperature (Tp1) were dramatically reduced and the
curing of epoxy resin was divided into two parts: the first part
occurred in the lower temperatures and the second part
occurred in the higher temperatures. As low concentration of
inorganic accelerator was used, curing reaction was divided into
two parts. In sample B (with 1.25% inorganic accelerator), the
initial onset temperature (T1) was 11.9 °C compare to the
sample without accelerator (sample A) (the second onset
temperature (T2), was 54.7 °C). This curing reaction occurred in
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14
4
A
-6
-16
-50
-30
-10
10
30
50
70
90
110
130
150
170
190
Temperature [°C]
B
4
-16
-50
-30
-10
10
30
50
70
90
110
130
150
170
190
Temperature [°C]
C
14
4
-6
-16
-50
-30
-10
10
30
50
70
90
110
130
150
170
190
Temperature [°C]
D
14
4
-6
-16
-50
-30
-10
10
30
50
70
90
110
130
150
170
190
Temperature [°C]
E
4
-16
-50
-30
-10
10
30
50
70
90
110
130
150
170
190
Temperature [°C]
Figure 9. Heat flow versus temperature was obtained by DSC from samples A, B, C, D and E at 10 °C heating rate
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Table 2. DSC information from the epoxy system with and without inorganic accelerator at 10 °C/min heating rate
Inorganic
Sample Name
accelerator
(%)
T1 (°C)
Tp1 (°C)
T2 (°C)
Tp2 (°C)
∆H (J/g)
A
B
C
D
E
-
68.3
11.9
0.1
93.2
30.5
8.3
-
54.7
37.9
-
-
78.9
52.4
-
56.9692
329.2118
165.3273
123.4849
66.3832
1.25
2.5
3.75
5
-1.7
-2.4
1.7
-0.9
-
-
Glass Transition
The mentioned process is similar to the experiments where
inorganic particles like TiO2, ZnO and SiO2 were used37-40. Inorganic
materials have high melting point, so it is obvious that they increase
thermal properties such as Tg. As mentioned, increasing
temperature cause formation of the inorganic salts within the gaps
of the chains, which prevents polymer chains to move freely.
The glass transition temperature (Tg) relates to chain mobility of a
polymer. Findings of the present work show that in the present of
an inorganic accelerator, the polymer chain length had increased.
When the chains are able to move more freely, Tg is found to be
decrease. However it should also be noted that the formation of
inorganic salt during the curing reaction decreases mobility of the By increasing the amount of inorganic accelerator, Tg will grow. In
chains by filling spaces between the chains, causing Tg to increase.
the present of inorganic accelerator, inorganic salt will be formed
that will make it harder for the polymer chains to move freely.
Changes of Tg are shown in figure 10.
70
60
50
40
30
20
10
0
65.7
Tensile strength
60.9
57.2
55.6
According to table 3 by adding inorganic accelerator, tensile
strength, tensile modulus and elongation were increased. This
behavior shows that the chains grow in length direction and can be
more tensile. When the chains grow in lengthwise direction (in the
present of inorganic accelerator), the structure of the cured epoxy
can tolerate more loads compared to a chain growth in a disordered
direction. Indeed, when the structure is placed under tension, long
chains have higher tear resistance, as they are able to stretch
further.
52.1
0
1.25
2.5
3.75
5
All samples in this work were cured at room temperature and
without post cure. (Sometimes to increase the mechanical
properties, after curing the sample at room temperature, the
sample is placed at high temperatures of approximately 90 °C for a
duration of 1 hour). For this reason, the tensile strength and
Inorganic Accelerator (phr)
Figure 10. Tg of the samples as a function of inorganic accelerator content
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modulus of the cured epoxy resin in this work are much lower than Izod impact strength
the works done by other researches (High temperatures were never
Figure 11, shows the impact of izod strength versus inorganic
accelerator content. As the concentration of the inorganic
accelerator increases, the izod impact strength rises as well. For
instance with 5% content of inorganic accelerator, izod impact
strength was found to have increases by a multiple of 1.76 times.
The inorganic salt present in the gaps of the polymer chains can
absorb force. Izod impact strength test was performed to examine
fractured surface of samples. It was observed that the fracture
surface of the samples that contained inorganic accelerator was
rough, indicating that they had higher izod impact strength.
used in this work during curing reaction or even after that). This is
in strong agreement with one the aims of the proposed work, that
is to synthesis an epoxy system independently, without utilizing an
external sources of heat.
Pull-off strength
According to the results of pull-of strength (Table 4), adhesion has
increased by adding inorganic accelerator. By increasing the
amount of inorganic accelerator, the pull-off strength is also
increase. It is assumed that formed inorganic salt improves the pull
off strength. During the pull off force, the inorganic salt prevents
formation of cracks at the surface in initial step. Many studies
confirmed this by showing that the fine inorganic particles can
indeed increase pull of strength41-43
.
Table 3. Effect of inorganic accelerator on the tensile strength, tensile modulus and elongation at break
Inorganic accelerator
Sample Name
Tensile strength (Mpa)
Tensile modulus (Mpa)
Elongation at break (%)
(%)
-
A
B
C
29.63 ± 1.3
1770.86 ± 46.4
1544.63 ± 41.1
1.79 ± 0.41
2.11 ± 0.62
1.25
2.5
32.12 ± 2.53
32.76 ± 4.2
33.97 ± 1.6
36.00 ± 1.4
1365.46 ± 52.8
1294.27 ± 49.5
1257.92 ± 44.7
2.25 ± 0.33
2.36 ± 0.19
2.88 ± 0.21
3.75
5
D
E
Table 4. The pull-off result from epoxy system with different amount of accelerator
Sample name
Inorganic accelerator (%)
Pull-off strength (Mpa)
4.41 ± 0.3
A
B
C
D
E
-
1.25
2.5
3.75
5
4.69 ± 0.2
5.03 ± 0.5
5.17 ± 0.4
5.45 ±0.2
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50
45
40
35
30
25
20
15
10
5
0
0
1
2
3
4
5
6
Inorganic accelerator content (%)
Figure 11. Izod impact strength of epoxy/ Epikure F 205/inorganic accelerator (%)
SEM morphology
Holes can be seen clearly in figure 12- D and 12- E. When the two
parts (epoxy resin Epicure F 205 and inorganic accelerator) are
mixed air bubbles tend to be introduced into the system. In samples
that contain greater amounts of inorganic accelerator such as
samples D and E, rate of curing is much higher and air bubbles are
trapped into the structure of the epoxy. Hence it can be concluded
that with the addition of inorganic accelerator, the surface fracture
of the sample becomes more rough and rigid.
Figure 12 shows the surface of sample A (without inorganic
accelerator) was completely slick and flat. Upon addition of an
inorganic accelerator, the surface specimen changes from its initial
flat condition and become rougher. Jagged surface absorbs the
energy ultimately and forces the samples to break. As the amount
of inorganic accelerator is increased, the surface of sample
becomes more jagged. Increase in toughness is shown in figure 12.
Figure 12. SEM images of the fractured surface (A: without inorganic accelerator, B: 1.25% inorganic accelerator)
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Continue of Figure 12. SEM images of the fractured surface for samples (C: 2.5% inorganic accelerator, D: 3.75% inorganic accelerator, E: 5% inorganic
accelerator)
curing agents such as Epicure F 205. The degree of conversion after
The presence of such air bubbles can alter the mechanical
properties, but it should be noted that the invented inorganic
accelerator changed the architect of the chain polymers, and the
formed inorganic salts acted as fender within the chain gaps. Hence,
even though air bubbles may be present within the mixture, the
mechanical strength is unaltered due to the inorganic accelerator.
18 minutes of curing reaction at 25 °C with 5% inorganic accelerator
reached 99.68%. While in the absence of inorganic accelerator, the
complete degree of conversion of epoxy system (DGEBA and
Epicure F 205) required a period of 7 days at room temperature to
cure. It was also found that mechanical properties of the resin
(tensile strength, izod impact strength and pull-off) have increases
significantly in the presence of accelerator. It is assumed that an
inorganic accelerator changes the architecture and structure of the
epoxy network in a way that the length of the chains becomes
longer. Subsequently the amounts of hydroxyl groups are increased,
Conclusions
In conclusion, results of this study indicate that inorganic
accelerator is an effective compound to reduce curing time and
curing temperature of epoxy resins. This accelerator can be used at
negative temperatures with cycloaliphatic and aliphatic diamine
providing the epoxy resin with the required adhesion property
.
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Acknowledgements
The authors would like to thank the University of Tabriz for
financially supporting this research project.
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