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Kinetics and properties of diethyltoluenediamine type benzoxazine-cured diglycidyl ether of bisphenol-A
Accepted Manuscript Title: Kinetics and Properties of Diethyltoluenediamine Type Benzoxazine-cured Diglycidyl Ether of Bisphenol-A Author: Shitong Ren Xin Yang Xiaojuan Zhao Ying Zhang Wei Huang PII: DOI: Reference:
S0040-6031(15)00308-1 http://dx.doi.org/doi:10.1016/j.tca.2015.08.001 TCA 77299
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
19-5-2015 2-8-2015 3-8-2015
Please cite this article as: S. Ren, X. Yang, X. Zhao, Y. Zhang, W. Huang, Kinetics and Properties of Diethyltoluenediamine Type Benzoxazine-cured Diglycidyl Ether of Bisphenol-A, Thermochimica Acta (2015), http://dx.doi.org/10.1016/j.tca.2015.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights 1. Detailed curing kinetics of aromatic diamine benzoxazine/epoxy was first studied. 2. Non-crosslinkable benzoxazine can be effective hardener for epoxy.
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3. The reaction between benzoxazine and epoxy was not affected by the steric hindrance.
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4. The resulting benzoxazine/epoxy copolymer exhibited favorable properties.
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Graphical Abstract (for review)
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*Manuscript Click here to view linked References
1
Curing Kinetics and Properties of Diethyltoluenediamine Type
2
Benzoxazine-cured Diglycidyl Ether of Bisphenol-A
3
Shitong Ren a,b, Xin Yang a,*, Xiaojuan Zhao a, Ying Zhang a, Wei Huang a,*
4
a
5
Republic of China
6
b
7
China
8
*
9
E-mail address: [email protected] (X. Yang), [email protected] (W. Huang).
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Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s
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University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
an
Corresponding authors. Tel.: +86 010 62558109; Fax: +86 010 62558109.
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Abstract: A novel diethyltoluenediamine type benzoxazine (PDETDA) was used as a
12
hardener for diglycidyl ether of bisphenol-A (DGEBA), and the curing kinetics was
13
investigated by non-isothermal differential scanning calorimetry (DSC). The results
14
showed that PDETDA/DGEBA exhibited two curing processes, which were attributed
15
to the ring-opening polymerization of benzoxazine (reaction 1) and the etherification
16
between hydroxyl groups of polybenzoxazine and epoxide groups (reaction 2),
17
respectively. Both reactions were autocatalytic in nature and can be well described by
18
the proposed kinetic models. The average activation energies (Eas) of reaction 1 and
19
reaction 2 were determined to be 104.9 kJ mol-1 and 125.0 kJ mol-1, respectively. It
20
was concluded from Ea that the alkyl substituents in PDETDA significantly affected
21
the reaction 1 due to the steric hindrance, but had no influence on the reaction 2. The
22
PDETDA-cured DGEBA exhibited higher strength and modulus, higher char yield,
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Page 3 of 56
23
and lower water absorption than diethyltoluenediamine (DETDA)-cured DGEBA.
24 25
Keywords: benzoxazine; epoxy resin; curing kinetics; properties
27
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26
1. Introduction
Polybenzoxazine is a novel developed class of thermosetting resin, which is
29
based on the ring-opening polymerization of benzoxazine precursors [1-5].
30
Polybenzoxazines possess many attractive properties such as no small molecule
31
byproduct generated and near-zero shrinkage during curing, high modulus and high
32
char yield, low water uptake, excellent dielectric properties and extremely rich
33
flexibility of molecular design [6-12]. These characters make them promising
34
candidates in many applications like semiconductor encapsulations and composite
35
matrix resins, etc. [13-18].
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Recently, a novel aromatic diamine type benzoxazine (PDETDA) based on
37
diethyltoluenediamine (DETDA) and phenol was synthesized [19], as shown in
38
Scheme 1. Since the methyl and ethyl substituents break the structural regularity of
39
PDETDA molecule, PDETDA has the merit of low viscosity and can be conveniently
40
prepared through one-step solvent-less method. However, the steric hindrance effect
41
of alkyl substituent groups restrains the polymerization of PDETDA, therefore
42
non-crosslinked polymer with almost no strength is obtained after thermal
43
polymerization. To enhance the properties and explore the use of PDETDA, it is
44
considered to copolymerize PDETDA with epoxy resin, for the reason that the
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2
Page 4 of 56
phenolic hydroxyl groups from the ring-opening reaction of benzoxazine monomers
46
are able to react with epoxy groups at elevated temperature to form additional
47
crosslinking points [20-25]. From another point of view, benzoxazines can serve as
48
hardeners for epoxy [26, 27].
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45
In the past decades, the curing kinetics of benzoxazine has been extensively
50
investigated by non-isothermal DSC which is more precise and attractive to evaluate
51
the curing kinetic parameters in a relatively short period of time [19, 26, 28-31]. It
52
was found that the curing reaction of benzoxazine was autocatalytic in nature. The
53
reported activation energies (Eas) were in the range of 81~128 kJ mol-1 with an overall
54
reaction order about 2~4, depending on the specific kinetic method and benzoxazine
55
structure. However, the detailed kinetic study of benzoxazine/epoxy system was rarely
56
reported even though the corresponding materials have been vigorously prepared.
57
Recently, Chanchira et al. [26] studied the curing kinetics of a diphenol type
58
benzoxaine/novolac epoxy system using Flynn-Wall-Ozawa and Friedman methods,
59
and two autocatalytic reactions corresponding to the ring-opening polymerization and
60
the etherification between hydroxyl groups of polybenzoxazine and epoxide groups
61
were found during the curing process with Eas of 81 kJ mol-1 and 118 kJ mol-1,
62
respectively. No detailed curing kinetics of aromatic diamine type benzoxazine/epoxy
63
was reported to our knowledge.
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64
In the present paper, we utilized the aromatic diamine type benzoxazine
65
PDETDA to cure diglycidyl ether of bisphenol-A (DGEBA), in order to obtain a
66
crosslinked
and
mechanically
sound
material.
The
curing
kinetics
of
3
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PDETDA/DGEBA system was investigated in detail by non-isothermal DSC. The
68
thermal, mechanical and water absorption properties of the resulting material were
69
examined and compared with DETDA-cured DGEBA. The structures of DETDA,
70
PDETDA and DGEBA are shown in Scheme 1.
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Scheme 1. Chemical structures of DETDA, PDETDA and DGEBA
2. Experimental
75
2.1. Materials Diethyltoluenediamine
(DETDA,
mixture
of
76%
of
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76
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3,5-diethyl-2,4-diaminotoluene and 24% 3,5-diethyl-2,6-diaminotoluene, Huntsman
78
Advanced Materials), diglycidyl ether of bisphenol-A (DGEBA, epoxy equivalent
79
weight 185-196 g mol-1, Nantong Xingchen Synthetic Material Co. Ltd., China),
80
paraformaldehyde (95%, Sinopharm Chemical Reagent Co. Ltd., China), phenol (AR,
81
Beijing Chemical Works, China), chloroform (AR, Beijing Chemical Works, China),
82
sodium hydroxide (NaOH, AR, Beijing Chemical Works, China), anhydrous sodium
83
sulfate (Na2SO4, AR, Beijing Chemical Works, China). All of these reagents were
84
used without further purification.
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77
85
PDETDA was synthesized from DETDA, phenol and paraformaldehyde
86
according to the procedures described earlier [19], and the ring-closed benzoxazine
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Page 6 of 56
87
structure was about 80%.
88
2.2. Preparation of PDETDA/DGEBA copolymer 57 wt% PDETDA and 43 wt% DGEBA with equimolar quantity of oxazine
90
groups and epoxide groups were mixed at 120 oC. After degassed under vacuum,
91
small amount of the melt was taken out for DSC measurements, the left was poured
92
into a preheated stainless steel mold treated with a silicone-based mold-release agent,
93
and was cured according to the following procedure: 160 oC for 2 h, 180 oC for 4 h,
94
200 oC for 2 h, 220 oC for 2 h, 240 oC for 2 h.
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For comparison, neat PDETDA was cured at 160 oC for 2 h and 180 oC for 6 h
96
according to the reported procedure [19]. Stoichiometric DGEBA and DETDA were
97
also mixed and cured by the procedure of 140 oC for 2 h, 160 oC for 2 h and 180 oC
98
for 2 h. The properties were compared based on the fact that the samples were all
99
completely cured.
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2.3. Characterization
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100
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Fourier transform infrared spectroscopy (FT-IR) was performed on a BRUKER
102
TENSOR-27 FT-IR spectrometer at room temperature in the range of 4000-400 cm-1.
103
FT-IR spectra were obtained by casting a thin film on a KBr plate for the sample
104
before curing or using the KBr pellet technique for the cured samples at a resolution
105
of 4 cm-1.
106
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Differential
scanning
calorimetry
(DSC)
was
performed
on
a
SII
107
EXSTAR6000-DSC6220 instrument in N2 flow of 50 mL min-1, and at heating rates
108
of 3, 5, 7 and 10 oC min-1, respectively.
5
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109
Thermogravimetric
analysis
(TGA)
was
carried
out
on
a
SII
110
EXSTAR6000-TGA6300 instrument at a heating rate of 10 oC min-1 in N2 flow of 100
111
mL min-1. Parallel plate rheological measurement was performed using a TA AR-2000
113
rheometer under air atmosphere at a heating rate of 4 oC min-1 under an oscillatory
114
shear mode with at a frequency of 1.5 rad s-1. The plate diameter was 25 mm and the
115
measuring gap was 1.0 mm.
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Dynamic mechanical analysis (DMA) was performed on a TA Q800 instrument
117
in the double-cantilever mode under air atmosphere at a frequency of 1 Hz and a
118
heating rate of 5 oC min-1. The dimensions of samples were 60 mm×12 mm×2.5 mm.
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Tensile and flexural properties were measured on an Instron Universal Tester
120
Model 3365 (Instron, Canton, MA) at room temperature. The tensile property was
121
tested according to the China National Standard GB/T 16421-1996. The strain rate
122
was 2 mm min-1, and the samples were double-shovel shaped with a total length of 75
123
mm. The flexural property was tested according to the GB/T 16419-1996 in a
124
three-point bending mode by using samples with dimensions of 80 mm×10 mm×4
125
mm, the span was 64 mm, and the strain rate was 1 mm min-1. No-notch impact
126
strength was measured at room temperature with a JC-25 impact tester (Chengde,
127
China) according to China National Standard GB1043-79. The dimensions of samples
128
were 80 mm×10 mm×4 mm. At least 4 specimens for each sample were tested, and
129
the data were averaged.
130
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Water absorption measurements were conducted by immersing the cured samples
6
Page 8 of 56
in distilled water at room temperature. The samples were periodically taken out,
132
wiped dry, weighed, and then immediately returned to the water bath. The dimensions
133
of samples were 80 mm×10 mm×4 mm. The Water absorption was calculated from
134
the following equation:
135
Water absorption(%)
136
where Wo is the original sample weight and Wt is the sample weight at immersed time
137
t.
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131
Wt Wo 100 Wo
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(1)
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3. Results and Discussion
140
3.1. Curing behavior of PDETDA/DGEBA system
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139
According to our previous work, PDETDA cannot produce crosslinked polymer
142
through ring-opening polymerization because of the steric hindrance of the alkyl
143
substituents. However, it was reported that benzoxazine can be used to cure epoxy
144
resins due to the reactive phenolic hydroxyl groups which were generated during the
145
ring-opening polymerization of benzoxazine [26, 27]. The reaction between phenolic
146
hydroxyl groups and epoxide groups appeared to be not affected by the steric
147
hindrance of PDETDA, so crosslinked PDETDA/DGEBA copolymer and
148
mechanically sound materials were expected to be obtained.
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141
149
The curing behavior of PDETDA/DGEBA system was studied by DSC, FT-IR
150
and rheological measurement. Firstly, the curing behavior was examined by the
151
rheological measurement, and the viscosity change was recorded in Fig. 1. The
152
viscosity of PDETDA/DGEBA mixture was about 14 Pa s at 40 oC and decreased to 7
Page 9 of 56
0.13 Pa s at 90 oC, which was significantly lower than that of the reported
154
benzoxazine/epoxy systems [23]. The viscosity remained low values before 230 oC,
155
after which a sharp increase of the viscosity was observed because of gelation. This
156
result indicated that the PDETDA/DGEBA system had a wide processing window.
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157
Fig. 1. Rheological curve of PDETDA/DGEBA system
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158
The curing process was further monitored by DSC. Fig. 2 shows the DSC curves
160
of PDETDA/DGEBA at different curing stages. There were two exothermic peaks
161
appeared, and it was noted that with the elevation of curing temperature and the
162
extension of curing time, the amount of exothermic decreased gradually. The
163
exothermic peak at lower temperature disappeared after heating at 180 oC for 4 h,
164
while the exothermic peak at higher temperature did not die away until heated at 240
165
o
166
transition temperature (Tg) of the cured PDETDA/DGEBA arose after the 180 oC
167
curing stage and increased with the advancing of the cure procedure. The DSC data
168
are summarized in Table 1.
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159
C for 2 h, indicating the completely cure of PDETDA/DGEBA. Besides, the glass
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ip t Fig. 2. DSC curves of PDETDA/DGEBA system at different curing stages
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169
171
Table 1. Results of DSC at different curing stages for PDETDA/DGEBA system
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172
-
253.1
160 oC, 2 h
-
180 oC, 4 h
84.6
220 oC, 2 h
0
251.4
300.3
227
18.4
-
297.7
80.4
71.1
102.4
-
301.0
62.0
77.7
118.3
-
300.2
30.3
89.1
129.0
-
-
0
100
Ac
240 oC, 2 h
278
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200 oC, 2 h
300.5
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Before curing
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Curing procedures Tg (oC) Tp1 (oC) Tp2 (oC) ΔH (J g-1) Degree of curing (%)c
173
c
174
after a certain curing stage; ΔH0 is the reaction enthalpy of PDETDA/DGEBA before
175
curing.
Degree of curing = 1-(ΔHi/ΔH0). ΔHi is the reaction enthalpy of PDETDA/DGEBA
176 177
Fig. 3 shows the FT-IR spectra of PDETDA/DGEBA system at different curing
178
stages. It was seen that the characteristic absorptions corresponding to the
9
Page 11 of 56
out-of-the-plane C-H vibration of benzene to which an oxazine group was attached at
180
932 cm-1 and epoxide group at 914 cm-1 were consumed gradually with the
181
proceeding of curing. These peaks appeared to have almost completely disappeared
182
by the end of the 180 oC curing stage. Signals around 3400 cm-1 that were attributed to
183
the hydroxyl groups enhanced significantly due to the ring-opening polymerization of
184
oxazine rings and epoxide rings. These changes indicated that the copolymerization of
185
PDETDA and DGEBA happened.
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186
Fig. 3. FT-IR spectra of PDETDA/DGEBA system at different curing stages
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187 188
190
3.2. Curing kinetics of PDETDA/DGEBA system
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The
curing
kinetics
of
PDETDA/DGEBA system
was
studied
by
191
non-isothermal DSC. Fig. 4 shows the DSC curves of PDETDA/DGEBA at different
192
heating rates. Two overlapped exothermic peaks appeared on the curves, suggesting
193
that at least two reactions happened during the curing. This phenomenon was also
194
observed in other benzoxazine/epoxy systems [23, 26]. When the heating rate
195
increased, both peak temperatures of the exothermic reactions (Tp1 and Tp2) shifted to
10
Page 12 of 56
higher temperatures. In addition, slower heating rate separated the two peaks better as
197
reported in literature [26, 28].
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196
198
Fig. 4. DSC curves of PDETDA/DGEBA resin at different heating rates
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199
With the values of Tp1s and Tp2s obtained from DSC at different heating rates,
201
activation energies (Eas) of the two exothermic reactions, which were assigned as
202
reaction 1 and reaction 2 in order, can be calculated by Kissinger method and Ozawa
203
method according to Eq. (2) and Eq. (3), respectively [28].
204
Q AR Ea ln 2 ln p T E a RT p p
205
E AE ln ln a ln F ( ) 5.331 1.052 a RT R p
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(2)
(3)
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200
Kissinger and Ozawa plots of reaction 1 and reaction 2 are shown in Fig. 5. Good
207
linear relationships were observed by using both kinetic methods. From the slopes of
208
the straight lines of lnβ/Tp2 vs. 1/Tp and lnβ vs. 1/Tp, Eas of both reactions were
209
obtained and listed in Table 2, together with the Ea of PDETDA polymerization for
210
comparison.
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Page 13 of 56
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211
Fig. 5. Kissinger and Ozawa plots for Ea determination of (a) reaction 1 and (b)
213
reaction 2 of PDETDA/DGEBA system
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212
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214
Table 2. Eas of PDETDA/DGEBA and PDETDA obtained by Kissinger method and
216
Ozawa method
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Kissinger Ea Ozawa Ea Average Ea
Curing systems (kJ mol-1)
(kJ mol-1) (kJ mol-1)
Reaction 1 103.4
106.4
104.9
Reaction 2 123.6
126.4
125.0
104.7
107.5
106.1
PDETDA/DGEBA
PDETDA 217
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Page 14 of 56
The Kissinger Ea and Ozawa Ea were quite close to each other for both reactions.
219
The average values of Kissinger Ea and Ozawa Ea for reaction 1 and reaction 2 were
220
104.9 kJ mol-1 and 125.0 kJ mol-1, respectively. For pure PDETDA ring-opening
221
polymerization, the average Ea value was 106.1 kJ mol-1. Notably, the Ea of reaction 1
222
was almost the same to the Ea of PDETDA ring-opening polymerization. It was
223
therefore deduced that the mechanism of reaction 1 was the same as PDETDA
224
ring-opening polymerization which produced a non-crosslinked polymer with
225
–N=CH2 groups according to our previous work as shown in Scheme 2. Logically, the
226
reaction 2 can be attributed to the etherification reaction between the phenolic
227
hydroxyl groups which were generated from the ring-opening of PDETDA and the
228
epoxide groups (Scheme 2). This assumption was also supported by the similar Ea
229
value (129.6 kJ mol-1) of epoxide-phenol curing reaction calculated from density
230
functional theory [32].
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231
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Page 15 of 56
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232
Scheme 2. Possible curing reaction mechanism of reaction 1 and reaction 2 for
234
PDETDA/DGEBA system
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In order to get more kinetics parameters of PDETDA/DGEBA curing reaction,
236
the DSC curves at different heating rates in Fig. 4 were analyzed using PeakFit v4.12
237
program. The overlapped peak 1 and peak 2 were separated using PearsonVII
238
distribution
239
PDETDA/DGEBA at a heating rate of 10 oC min-1, as well as the calculated curves
240
simulated by Peakfit v4.12 with two resolved peaks corresponding to reaction 1 and
241
reaction 2, were illustrated in Fig. 6.
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235
As
an
exemplification,
the
DSC
thermogram
of
Ac
equation.
14
Page 16 of 56
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242
Fig. 6. DSC thermogram of PDETDA/DGEBA system recorded at 10 oC min-1 and
244
the corresponding calculated curves
us
243
With the data of the separated peak 1 and peak 2, a fuller assessment of the Ea of
246
PDETDA/DGEBA curing throughout the entire conversion range can be obtained
247
using the Flynn-Wall-Ozawa method [26, 28]. This method is based on Eq. (4) and Eq.
248
(5):
249
AE E ln ln a ln g ( ) 5.331 1.052 a R RT
250
g ( )
0
252
d f ( )
(4)
(5)
where g(α) is the integral conversion function.
Ac
251
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245
For a constant α, the plot of lnβ vs. 1/T obtained from DSC data with different
253
heating rates should render a straight line, from which Ea can be determined from the
254
slope. Fig. 7 shows the Flynn-Wall-Ozawa plot at various α of both reactions of
255
PDETDA/DGEBA system. Good linear relationships of lnβ vs. 1/T were observed
256
throughout the entire conversion range for both reaction 1 and reaction 2. From the
257
slopes of these lines, Ea values at various α were calculated.
15
Page 17 of 56
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258
Fig. 7. Flynn-Wall-Ozawa plots at various α of (a) reaction 1 and (b) reaction 2
ed
259
The variations of Ea vs. α for reaction 1 and reaction 2 are shown in Fig. 8. It was
261
seen that both Eas of reaction 1 and reaction 2 increased with the proceeding of curing
262
reactions, which was interpreted by the decrease in molecular mobility or due to a
263
kinetic compensation effect [26, 28, 33]. The average Eas for reaction 1 and reaction 2
264
at various α were 109.1 kJ mol-1 and 125.2 kJ mol-1, respectively. This result was quite
265
close to the Eas obtained from Kissinger method and Ozawa method. Therefore, the
266
average value of Kissinger Ea and Ozawa Ea was selected for further determining the
267
reaction order. Besides, it was worth noting that the Ea of reaction 1 was obviously
268
higher than most of the reported values of ring-opening polymerization of
269
benzoxazines [26, 28] because of the steric hindrance of substituents in PDETDA,
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260
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Page 18 of 56
while the Ea of reaction 2 was close to the values of the reaction between phenols and
271
epoxide groups [26, 32], indicating that the etherification was not obviously affected
272
by the steric hindrance.
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270
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273
Fig. 8. Plots of Ea vs. α of reaction 1 and reaction 2 obtained by Flynn-Wall-Ozawa
275
method
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274
ed
Based on the above average Ea, the other parameters of the curing kinetics can be
276 277
determined by the Frideman method [26, 28, 31], which is based on Eq. (6):
278
ln
279
The curing reaction mechanisms of thermosetting resins are usually divided into an
280
n-order reaction and an autocatalytic reaction. In the case of the n-order reaction,
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f ( ) (1 )n
282
Combined with Eq. (6) results Eq. (8):
283
lnAf ( ) ln
(6)
(7)
Ac
281
d d E ln( ) lnAf ( ) a dt dT RT
d Ea ln A n ln(1 ) dt RT
(8)
284
Friedman suggested that the plots of ln[Af(α)] vs. ln(1-α) should yield a straight
285
line for the n-order reaction. The reaction rate is highest at the beginning of the curing
286
and decreases as the reaction proceeds. However, for autocatalytic process, the
287
Friedman plot shows a maximum of ln(1-α) approximately in the range of -0.51 to 17
Page 19 of 56
-0.22 which is equivalent to α of about 0.2-0.4. Fig. 9 shows the Friedman plots of
289
reaction 1 and reaction 2 of PDETDA/DGEBA system. Since ln[Af(α)] and ln(1-α)
290
were not linearly related and a maximum was evidently presented in the above
291
mentioned range of α, both reaction 1 and reaction 2 of PDETDA/DGEBA were
292
actually autocatalytic in nature. Based on the above results, the autocatalytic nature of
293
reaction 1 can be explained by the generation of free phenol groups while the oxazine
294
rings open, and these phenol groups can further accelerate the ring opening process.
295
Similarly, the secondary hydroxyl groups produced in the ring-opening reaction of
296
epoxy groups are able to participate in the curing reaction, which accounted for the
297
autocatalytic nature of reaction 2 [26].
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288
298
Fig. 9. Plots of ln[Af(α)] vs. ln(1-α) for reaction 1 and reaction 2 at heating rate of 10
300
o
Ac
299
C min-1
For the autocatalytic reaction,
301 302
f ( ) m (1 )n
(9)
303
Combined with Eq. (6) results Eq. (10):
304
ln(
305
Eq. (10) can be solved by multiple linear regression, in which the dependent variable
d d E ) ln( ) ln A ( a ) m ln n ln 1 dt dT RT
(10)
18
Page 20 of 56
is ln(dα/dt), and the independent variables are lnα, ln(1-α), and 1/T. Therefore, the
307
values of A, m, and n can be obtained using the calculated Ea. The α was chosen
308
between 0.05 and 0.50. The results of the multiple linear regression analysis for
309
reaction 1 and reaction 2 are listed in Table 3 and Table 4, respectively. Compared
310
with the diphenol type benzoxazine/novolac epoxy system [26], PDETDA/DGEBA
311
showed much higher Ea and lower total reaction order for reaction 1, but similar Ea
312
and total reaction order for reaction 2. This result further indicated that the reaction 1
313
was strongly affected by the steric hindrance while the reaction 2 was not.
314
Table 3. Kinetic parameters evaluated for reaction 1 of PDETDA/DGEBA system
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306
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Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m 3
19.95
0.46
1.32
20.03
0.48
1.28
ed
5
104.9
10 315
20.06
0.49
mean
1.28
20.10
0.51
1.28
20.14
0.52
1.23
Table 4. Kinetic parameters evaluated for reaction 2 of PDETDA/DGEBA system
Ac
316
ce pt
7
mean n
Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m 3 5
mean n
22.23
0.45
1.28
22.32
0.49
1.30
125.0
22.33
0.49
mean
1.26
7
22.33
0.50
1.24
10
22.42
0.53
1.22
317 19
Page 21 of 56
318
According to the results of the evaluated parameters, the curing kinetic equation
319
of reaction 1 can be expressed as:
320
d d 12617 0.49 exp( 20.06) exp( ) (1 )1.28 dt dT T
321
The curing kinetic equation of reaction 2 can be expressed as:
322
d d 15035 0.49 exp( 22.33) exp( ) (1 )1.26 dt dT T
ip t
(11)
(12)
The experimental curves and predicted curves based on the determined kinetic
324
parameters of reaction 1 and reaction 2 are shown in Fig. 10. It was clearly seen that
325
the calculated data from the models were in good agreement with the experimental
326
results.
Ac
ce pt
ed
M
an
us
cr
323
327 328
Fig. 10. Comparison of experimental values and calculated values for (a) reaction 1
329
and (b) reaction 2
20
Page 22 of 56
330
3.3. Properties of cured PDETDA/DGEBA The thermal stability, mechanical properties and water absorption of the cured
332
PDETDA/DGEBA were examined and compared with the samples of DETDA-cured
333
DGEBA (DETDA/DGEBA) and polymerized PDETDA (P-PDETDA). It was noted
334
that P-PDETDA did not have sufficient strength for mechanical measurements.
ip t
331
Fig. 11 shows the TGA curves of the three samples, and the TGA data are listed
336
in Table 5. The 5% mass loss temperature (T5%) and the decomposition temperature
337
(Td) of the cured PDETDA/DGEBA were comparable to the corresponding values of
338
the cured DETDA/DGEBA, and were much greater than those of P-PDETDA. Char
339
yield of the cured PDETDA/DGEBA at 800 oC was twice more than that of the cured
340
DETDA/DGEBA. Two reasons explained the higher char yield of the cured
341
PDETDA/DGEBA. First, PDETDA/DGEBA system contained more aromatic
342
structure than DETDA/DGEBA. Second, more stable hydrogen bonds can be formed
343
in the cured PDETDA/DGEBA. The above results indicated that the cured
344
PDETDA/DGEBA owned good thermal stability.
Ac
ce pt
ed
M
an
us
cr
335
345 346
Fig. 11. TGA curves of cured PDETDA/DGEBA, cured DETDA/DGEBA and
347
P-PDETDA 21
Page 23 of 56
348
Table 5. TGA results of different cured systems T5% (oC) Td (oC) Char yield at 800 oC (%)
DETDA/DGEBA
367.7
375.2
3.5
PDETDA/DGEBA 355.2
377.4
12.0
P-PDETDA
347.5
24.1
296.5
us
350
ip t
Cured systems
cr
349
Dynamic mechanical analysis (DMA) was applied to evaluate the dynamic
352
mechanical properties of the cured PDETDA/DGEBA and DETDA/DGEBA
353
(Fig. 12). The cured PDETDA/DGEBA had a lower Tg (146 oC) than that of the cured
354
DETDA/DGEBA (189 oC) as indicated by the peak temperature of tan delta. It is
355
known that Tg is closely related to the crosslink density, which can be estimated from
356
the following equation [34]:
357
G' RT
358
where ν is the crosslink density, G' is the storage modulus in the rubbery region
359
(Tg+50 oC), is the front factor which is taken as unity.
M
ed
ce pt
(20)
Ac
360
an
351
It should be noted that this equation is strictly valid only for lightly crosslinked
361
materials and therefore is used only to qualitatively compare the level of crosslinking
362
of the cured materials. The calculated crosslink densities for the cured
363
PDETDA/DGEBA and DETDA/DGEBA were 1.56×10-3 mol·cm-3 and 2.08×10-3
364
mol·cm-3, respectively. Lower crosslink density of the cured PDETDA/DGEBA
365
accounted for its lower Tg. 22
Page 24 of 56
The storage modulus in the glassy region was a reflection of the material’s
367
stiffness, and as shown in Fig. 12, the cured PDETDA/DGEBA had obviously higher
368
stiffness than that of the cured DETDA/DGEBA. It is well known that strong
369
hydrogen bonds are formed during benzoxazine polymerization. Therefore, even if
370
PDETDA/DGEBA had lower crosslink density, the resultant hydrogen bonds in the
371
cured PDETDA/DGEBA endowed it with higher stiffness.
cr
ip t
366
Tensile, flexural and impact properties of the two cured systems were measured
373
and the data were listed in Table 6. All the mechanical properties except the impact
374
strength were pronounced higher for the cured PDETDA/DGEBA than those of the
375
cured DETDA/DGEBA. Higher modulus and strength of the cured PDETDA/DGEBA
376
were probably resulted from the stronger hydrogen bonds existed in the polymer. It
377
was also the stronger hydrogen bonds in the cured PDETDA/DGEBA that had
378
adverse effect on the toughness, leading to its lower impact strength.
Ac
ce pt
ed
M
an
us
372
379 380
Fig. 12. Dynamic mechanical spectra of the cured PDETDA/DGEBA (●) and
381
DETDA/DGEBA (■) including storage modulus and dissipation factor tan delta
382 383 23
Page 25 of 56
384
Table 6. Tensile, flexural and impact properties of the cured PDETDA/DGEBA and
385
DETDA/DGEBA Flexural
Tensile
Tensile
Impact
strength
modulus
strength
modulus
strength
(MPa)
(MPa)
(MPa)
(MPa)
113.2±1.3
2.5±0.1
48.7±7.1
2.4±0.5
37.6±5.6
PDETDA/DGEBA 130.9±2.2
3.0±0.1
57.7±10.5
3.2±0.3
15.9±4.0
us
DETDA/DGEBA
(kJ m-2)
cr
Systems
ip t
Flexural
an
386
Fig. 13 shows the variation of water absorption of the cured PDETDA/DGEBA
388
and DETDA/DGEBA with the extension of immersion time in water at room
389
temperature. The results displayed that both the water absorption rate and the
390
saturated water absorption of the cured PDETDA/DGEBA were obviously lower than
391
that of the cured DETDA/DGEBA. The cured PDETDA/DGEBA absorbed water up
392
to about 0.9% of its conditioned weight, while the cured DETDA/DGEBA saturated at
393
approximately 1.5%.
Ac
ce pt
ed
M
387
394 395
Fig. 13. Plots of water absorption vs. time of the cured PDETDA/DGEBA and
396
DETDA/DGEBA 24
Page 26 of 56
397
The diffusion coefficients of the materials were determined according to the
398
Fick’s law through the following equation (21) [1],
399
M t 4 Dt M 0.5 l 2
400
where Mt and M∞ are the cumulative masses sorbed by a sample of thickness l at time
401
t and ∞, respectively. The diffusion coefficient D can be calculated from the initial
402
slope G of the plot of Mt/M∞ vs. t0.5/l (Fig. 14) through the following equation,
403
D G 2 16
0.5
us
cr
ip t
(21)
(22)
The calculated diffusion coefficients were 8.82×10-9 cm2 s-1 for DETDA/DGEBA
405
and 8.09×10-9 cm2 s-1 for PDETDA/DGEBA. This result indicated that the cured
406
PDETDA/DGEBA
407
DETDA/DGEBA. The lower saturated water absorption and diffusion coefficient of
408
PDETDA/DGEBA was speculated to be caused by the more amounts of inter- and
409
intramolecular hydrogen bonds in its structure, which shielded the hydroxyl groups
410
from interaction with water molecules [1].
412
M
slightly
lower
water
diffusivity
than
the
cured
ce pt
ed
had
Ac
411
an
404
25
Page 27 of 56
ip t cr
413
Fig. 14. Calculation of diffusion coefficients from the initial slope of the Mt/M∞, vs.
415
t0.5/l curves
us
414
4. Conclusions
M
417
an
416
The novel aromatic diamine-based benzoxazine PDETDA was proven to be an
419
effective hardener of DGEBA, and PDETDA/DGEBA mixture had the merit of low
420
viscosity and wide processing window. The curing kinetics of PDETDA/DGEBA was
421
studied by non-isothermal DSC, and two dominant curing reactions were observed.
422
The reaction 1 at low temperature was the ring-opening polymerization of
423
benzoxazine monomers, while the reaction 2 at high temperature corresponded to the
424
etherification between hydroxyl groups of polybenzoxazine and epoxide groups. Both
425
reactions were autocatalytic in nature, and the predicted curves from the proposed
426
kinetic models fit well with the experimental curves. Furthermore, the calculated Ea
427
values showed that the reaction 1 was greatly influenced by the steric hindrance of
428
substituents in PDTEDA while the reaction 2 was not, indicating that
429
non-crosslinkable benzoxazine can also be effective hardeners for epoxy resin. The
Ac
ce pt
ed
418
26
Page 28 of 56
430
properties of the cured PDETDA/DGEBA were examined and compared with the
431
cured DETDA/DGEBA. The results presented that PDETDA/DGEBA had higher
432
char yield, better mechanical properties and lower water absorption.
436
This study is financially supported by the National Natural Science Foundation
cr
435
Acknowledgments
of China (No. 51303084).
us
434
ip t
433
437
References
439
[1] Ishida, H., Allen, D. J. Physical and mechanical characterization of near-zero
440
shrinkage polybenzoxazines. J Polym Sci Part B: Polym Phys. 1996, 34: 1019-1030.
441
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444
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[4] Shen, S. B., Ishida, H. Dynamic mechanical and thermal characterization of
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5156-5162.
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[10] Yang, P., Gu, Y. A novel benzimidazole moiety-containing benzoxazine:
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allylamine. Polym Adv Technol. 2013, 24: 157-163.
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and polymerization of a novel benzoxazine based on diethyltoluenediamine. J Appl
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Polym Sci. 2015, 132, 41920.
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[20] Kimura, H., Murata, Y., Matsumoto, A., Hasegawa, K., Ohtsuka, K., Fukuda, A.
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New thermosetting resin from terpenediphenol-based benzoxazine and epoxy resin. J
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Appl Polym Sci. 1999, 74: 2266-2273.
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copolymers: effect of molecular weight and crosslinking on thermal and viscoelastic
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maleimide-functionalized benzoxazine/epoxy copolymers. J Appl Polym Sci. 2006,
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arylamine-based benzoxazine resins alloyed with epoxy resin. Polym Eng Sci. 2011,
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51: 1797-1807.
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[24] Kimura, H., Matsumoto, A., Sugito, H., Hasegawa, K., Ohtsuka, K., Fukuda, A.
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New thermosetting resin from poly(p-vinylphenol) based benzoxazine and epoxy
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resin. J Appl Polym Sci. 2001, 79: 555-565.
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epoxy, benzoxazine monomer system from cardanol: Thermal and viscoelastic
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properties. Eur Polym J. 2013, 49: 2365-2376.
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[26] Jubsilp, C., Punson, K., Takeichi, T., Rimdusit, S. Curing kinetics of
510
Benzoxazine-epoxy copolymer investigated by non-isothermal differential scanning
511
calorimetry. Polym Degrad Stabil. 2010, 95: 918-924.
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[27] Chang, S. L., Lin, C. H. One-pot synthesis of aromatic diamine-based
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benzoxazines and their advantages over diamines as epoxy hardeners. J Polym Sci
514
Part A: Polym Chem. 2010, 48: 2430-2437.
515
[28] Jubsilp, C., Damrongsakkul, S., Takeichi, T., Rimdusit, S. Curing kinetics of
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arylamine-based polyfunctional benzoxazine resins by dynamic differential scanning
517
calorimetry. Thermochim Acta. 2006, 447: 131-140.
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30
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519
resin by differential scanning calorimetry. Polymer. 1995, 36: 3151-3158.
520
[30] Hamerton, I., McNamara, L. T., Howlin, B. J., Smith, P. A., Cross, P., Ward, S.
521
Examining the kinetics of the thermal polymerization of commercial aromatic
522
bis-benzoxazines. J Polym Sci Part A: Polym Chem. 2014, 52: 2068-2081.
523
[31] He, X. Y., Wang, J., Ramdani, N., Liu, W. B., Liu, L. J., Yang, L. Investigation of
524
synthesis, thermal properties and curing kinetics of fluorine diamine-based
525
benzoxazine by using two curing kinetic methods. Thermochim Acta. 2013, 564:
526
51-58.
527
[32] Pham, M. P., Pham, B. Q., Huynh, L. K., Pham, H. Q., Marks, M. J., Truong, T.
528
N. Density functional theory study on mechanisms of epoxy-phenol curing reaction. J
529
Comput Chem. 2014, 35, 1630-1640.
530
[33] Zvetkov, V. L. Comparative DSC kinetics of the reaction of DGEBA with
531
aromatic diamines I: non-isothermal kinetic study of the reaction of DGEBA with
532
m-phenylene diamine. Polymer. 2001, 42: 6687-6697.
533
[34] Ishida, H., Allen, D. J. Mechanical characterization of copolymers based on
534
benzoxazine and epoxy. Polymer. 1996, 37: 4487-4495.
Ac
ce pt
ed
M
an
us
cr
ip t
518
31
Page 33 of 56
Table 1
Table 1. Results of DSC at different curing stages for PDETDA/DGEBA system
253.1
300.5
278
0
160 oC, 2 h
-
251.4
300.3
227
18.4
180 oC, 4 h
84.6
-
297.7
80.4
71.1
200 oC, 2 h
102.4
-
301.0
62.0
77.7
220 oC, 2 h
118.3
-
300.2
30.3
240 oC, 2 h
129.0
-
-
0
cr
-
us
89.1 100
Degree of curing = 1-(ΔHi/ΔH0). ΔHi is the reaction enthalpy of PDETDA/DGEBA
an
c
Before curing
ip t
Curing procedures Tg (oC) Tp1 (oC) Tp2 (oC) ΔH (J g-1) Degree of curing (%)c
M
after a certain curing stage; ΔH0 is the reaction enthalpy of PDETDA/DGEBA before
Ac
ce pt
ed
curing.
Page 34 of 56
Table 2
Table 2. Eas of PDETDA/DGEBA and PDETDA obtained by Kissinger method and Ozawa method Kissinger Ea Ozawa Ea Average Ea Curing systems
ip t
(kJ mol-1) (kJ mol-1)
Reaction 1 103.4
106.4
104.9
Reaction 2 123.6
126.4
104.7
107.5
cr
(kJ mol-1)
106.1
Ac
ce pt
ed
M
an
PDETDA
125.0
us
PDETDA/DGEBA
Page 35 of 56
Table 3
Table 3. Kinetic parameters evaluated for reaction 1 of PDETDA/DGEBA system Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m
mean n
3
19.95
0.46
1.32
5
20.03
0.48
1.28
20.06
0.49 0.51
10
20.14
0.52
1.28
1.28 1.23
Ac
ce pt
ed
M
an
us
cr
20.10
ip t
104.9 7
mean
Page 36 of 56
Table 4
Table 4. Kinetic parameters evaluated for reaction 2 of PDETDA/DGEBA system Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m
mean n
3
22.23
0.45
1.28
5
22.32
0.49
1.30
22.33
0.49 0.50
10
22.42
0.53
1.26
1.24 1.22
Ac
ce pt
ed
M
an
us
cr
22.33
ip t
125.0 7
mean
Page 37 of 56
Table 5
Table 5. TGA results of different cured systems T5% (oC) Td (oC) Char yield at 800 oC (%)
DETDA/DGEBA
367.7
375.2
3.5
PDETDA/DGEBA 355.2
377.4
12.0
P-PDETDA
347.5
24.1
Ac
ce pt
ed
M
an
us
cr
296.5
ip t
Cured systems
Page 38 of 56
Table 6
Table 6. Tensile, flexural and impact properties of the cured PDETDA/DGEBA and DETDA/DGEBA Flexural
Tensile
Tensile
Impact
strength
modulus
strength
modulus
strength
(MPa)
(MPa)
(MPa)
(MPa)
113.2±1.3
2.5±0.1
48.7±7.1
2.4±0.5
37.6±5.6
PDETDA/DGEBA 130.9±2.2
3.0±0.1
57.7±10.5
3.2±0.3
15.9±4.0
Ac
ce pt
ed
M
an
us
DETDA/DGEBA
(kJ m-2)
cr
Systems
ip t
Flexural
Page 39 of 56
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S0040-6031(15)00308-1 http://dx.doi.org/doi:10.1016/j.tca.2015.08.001 TCA 77299
To appear in:
Thermochimica Acta
Received date: Revised date: Accepted date:
19-5-2015 2-8-2015 3-8-2015
Please cite this article as: S. Ren, X. Yang, X. Zhao, Y. Zhang, W. Huang, Kinetics and Properties of Diethyltoluenediamine Type Benzoxazine-cured Diglycidyl Ether of Bisphenol-A, Thermochimica Acta (2015), http://dx.doi.org/10.1016/j.tca.2015.08.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Highlights (for review)
Highlights 1. Detailed curing kinetics of aromatic diamine benzoxazine/epoxy was first studied. 2. Non-crosslinkable benzoxazine can be effective hardener for epoxy.
ip t
3. The reaction between benzoxazine and epoxy was not affected by the steric hindrance.
Ac
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4. The resulting benzoxazine/epoxy copolymer exhibited favorable properties.
Page 1 of 56
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Graphical Abstract (for review)
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*Manuscript Click here to view linked References
1
Curing Kinetics and Properties of Diethyltoluenediamine Type
2
Benzoxazine-cured Diglycidyl Ether of Bisphenol-A
3
Shitong Ren a,b, Xin Yang a,*, Xiaojuan Zhao a, Ying Zhang a, Wei Huang a,*
4
a
5
Republic of China
6
b
7
China
8
*
9
E-mail address: [email protected] (X. Yang), [email protected] (W. Huang).
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Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s
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University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of
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Corresponding authors. Tel.: +86 010 62558109; Fax: +86 010 62558109.
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Abstract: A novel diethyltoluenediamine type benzoxazine (PDETDA) was used as a
12
hardener for diglycidyl ether of bisphenol-A (DGEBA), and the curing kinetics was
13
investigated by non-isothermal differential scanning calorimetry (DSC). The results
14
showed that PDETDA/DGEBA exhibited two curing processes, which were attributed
15
to the ring-opening polymerization of benzoxazine (reaction 1) and the etherification
16
between hydroxyl groups of polybenzoxazine and epoxide groups (reaction 2),
17
respectively. Both reactions were autocatalytic in nature and can be well described by
18
the proposed kinetic models. The average activation energies (Eas) of reaction 1 and
19
reaction 2 were determined to be 104.9 kJ mol-1 and 125.0 kJ mol-1, respectively. It
20
was concluded from Ea that the alkyl substituents in PDETDA significantly affected
21
the reaction 1 due to the steric hindrance, but had no influence on the reaction 2. The
22
PDETDA-cured DGEBA exhibited higher strength and modulus, higher char yield,
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23
and lower water absorption than diethyltoluenediamine (DETDA)-cured DGEBA.
24 25
Keywords: benzoxazine; epoxy resin; curing kinetics; properties
27
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26
1. Introduction
Polybenzoxazine is a novel developed class of thermosetting resin, which is
29
based on the ring-opening polymerization of benzoxazine precursors [1-5].
30
Polybenzoxazines possess many attractive properties such as no small molecule
31
byproduct generated and near-zero shrinkage during curing, high modulus and high
32
char yield, low water uptake, excellent dielectric properties and extremely rich
33
flexibility of molecular design [6-12]. These characters make them promising
34
candidates in many applications like semiconductor encapsulations and composite
35
matrix resins, etc. [13-18].
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Recently, a novel aromatic diamine type benzoxazine (PDETDA) based on
37
diethyltoluenediamine (DETDA) and phenol was synthesized [19], as shown in
38
Scheme 1. Since the methyl and ethyl substituents break the structural regularity of
39
PDETDA molecule, PDETDA has the merit of low viscosity and can be conveniently
40
prepared through one-step solvent-less method. However, the steric hindrance effect
41
of alkyl substituent groups restrains the polymerization of PDETDA, therefore
42
non-crosslinked polymer with almost no strength is obtained after thermal
43
polymerization. To enhance the properties and explore the use of PDETDA, it is
44
considered to copolymerize PDETDA with epoxy resin, for the reason that the
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Page 4 of 56
phenolic hydroxyl groups from the ring-opening reaction of benzoxazine monomers
46
are able to react with epoxy groups at elevated temperature to form additional
47
crosslinking points [20-25]. From another point of view, benzoxazines can serve as
48
hardeners for epoxy [26, 27].
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45
In the past decades, the curing kinetics of benzoxazine has been extensively
50
investigated by non-isothermal DSC which is more precise and attractive to evaluate
51
the curing kinetic parameters in a relatively short period of time [19, 26, 28-31]. It
52
was found that the curing reaction of benzoxazine was autocatalytic in nature. The
53
reported activation energies (Eas) were in the range of 81~128 kJ mol-1 with an overall
54
reaction order about 2~4, depending on the specific kinetic method and benzoxazine
55
structure. However, the detailed kinetic study of benzoxazine/epoxy system was rarely
56
reported even though the corresponding materials have been vigorously prepared.
57
Recently, Chanchira et al. [26] studied the curing kinetics of a diphenol type
58
benzoxaine/novolac epoxy system using Flynn-Wall-Ozawa and Friedman methods,
59
and two autocatalytic reactions corresponding to the ring-opening polymerization and
60
the etherification between hydroxyl groups of polybenzoxazine and epoxide groups
61
were found during the curing process with Eas of 81 kJ mol-1 and 118 kJ mol-1,
62
respectively. No detailed curing kinetics of aromatic diamine type benzoxazine/epoxy
63
was reported to our knowledge.
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64
In the present paper, we utilized the aromatic diamine type benzoxazine
65
PDETDA to cure diglycidyl ether of bisphenol-A (DGEBA), in order to obtain a
66
crosslinked
and
mechanically
sound
material.
The
curing
kinetics
of
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PDETDA/DGEBA system was investigated in detail by non-isothermal DSC. The
68
thermal, mechanical and water absorption properties of the resulting material were
69
examined and compared with DETDA-cured DGEBA. The structures of DETDA,
70
PDETDA and DGEBA are shown in Scheme 1.
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Scheme 1. Chemical structures of DETDA, PDETDA and DGEBA
2. Experimental
75
2.1. Materials Diethyltoluenediamine
(DETDA,
mixture
of
76%
of
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76
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3,5-diethyl-2,4-diaminotoluene and 24% 3,5-diethyl-2,6-diaminotoluene, Huntsman
78
Advanced Materials), diglycidyl ether of bisphenol-A (DGEBA, epoxy equivalent
79
weight 185-196 g mol-1, Nantong Xingchen Synthetic Material Co. Ltd., China),
80
paraformaldehyde (95%, Sinopharm Chemical Reagent Co. Ltd., China), phenol (AR,
81
Beijing Chemical Works, China), chloroform (AR, Beijing Chemical Works, China),
82
sodium hydroxide (NaOH, AR, Beijing Chemical Works, China), anhydrous sodium
83
sulfate (Na2SO4, AR, Beijing Chemical Works, China). All of these reagents were
84
used without further purification.
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77
85
PDETDA was synthesized from DETDA, phenol and paraformaldehyde
86
according to the procedures described earlier [19], and the ring-closed benzoxazine
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87
structure was about 80%.
88
2.2. Preparation of PDETDA/DGEBA copolymer 57 wt% PDETDA and 43 wt% DGEBA with equimolar quantity of oxazine
90
groups and epoxide groups were mixed at 120 oC. After degassed under vacuum,
91
small amount of the melt was taken out for DSC measurements, the left was poured
92
into a preheated stainless steel mold treated with a silicone-based mold-release agent,
93
and was cured according to the following procedure: 160 oC for 2 h, 180 oC for 4 h,
94
200 oC for 2 h, 220 oC for 2 h, 240 oC for 2 h.
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For comparison, neat PDETDA was cured at 160 oC for 2 h and 180 oC for 6 h
96
according to the reported procedure [19]. Stoichiometric DGEBA and DETDA were
97
also mixed and cured by the procedure of 140 oC for 2 h, 160 oC for 2 h and 180 oC
98
for 2 h. The properties were compared based on the fact that the samples were all
99
completely cured.
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2.3. Characterization
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100
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Fourier transform infrared spectroscopy (FT-IR) was performed on a BRUKER
102
TENSOR-27 FT-IR spectrometer at room temperature in the range of 4000-400 cm-1.
103
FT-IR spectra were obtained by casting a thin film on a KBr plate for the sample
104
before curing or using the KBr pellet technique for the cured samples at a resolution
105
of 4 cm-1.
106
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101
Differential
scanning
calorimetry
(DSC)
was
performed
on
a
SII
107
EXSTAR6000-DSC6220 instrument in N2 flow of 50 mL min-1, and at heating rates
108
of 3, 5, 7 and 10 oC min-1, respectively.
5
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109
Thermogravimetric
analysis
(TGA)
was
carried
out
on
a
SII
110
EXSTAR6000-TGA6300 instrument at a heating rate of 10 oC min-1 in N2 flow of 100
111
mL min-1. Parallel plate rheological measurement was performed using a TA AR-2000
113
rheometer under air atmosphere at a heating rate of 4 oC min-1 under an oscillatory
114
shear mode with at a frequency of 1.5 rad s-1. The plate diameter was 25 mm and the
115
measuring gap was 1.0 mm.
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Dynamic mechanical analysis (DMA) was performed on a TA Q800 instrument
117
in the double-cantilever mode under air atmosphere at a frequency of 1 Hz and a
118
heating rate of 5 oC min-1. The dimensions of samples were 60 mm×12 mm×2.5 mm.
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Tensile and flexural properties were measured on an Instron Universal Tester
120
Model 3365 (Instron, Canton, MA) at room temperature. The tensile property was
121
tested according to the China National Standard GB/T 16421-1996. The strain rate
122
was 2 mm min-1, and the samples were double-shovel shaped with a total length of 75
123
mm. The flexural property was tested according to the GB/T 16419-1996 in a
124
three-point bending mode by using samples with dimensions of 80 mm×10 mm×4
125
mm, the span was 64 mm, and the strain rate was 1 mm min-1. No-notch impact
126
strength was measured at room temperature with a JC-25 impact tester (Chengde,
127
China) according to China National Standard GB1043-79. The dimensions of samples
128
were 80 mm×10 mm×4 mm. At least 4 specimens for each sample were tested, and
129
the data were averaged.
130
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Water absorption measurements were conducted by immersing the cured samples
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in distilled water at room temperature. The samples were periodically taken out,
132
wiped dry, weighed, and then immediately returned to the water bath. The dimensions
133
of samples were 80 mm×10 mm×4 mm. The Water absorption was calculated from
134
the following equation:
135
Water absorption(%)
136
where Wo is the original sample weight and Wt is the sample weight at immersed time
137
t.
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131
Wt Wo 100 Wo
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(1)
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3. Results and Discussion
140
3.1. Curing behavior of PDETDA/DGEBA system
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139
According to our previous work, PDETDA cannot produce crosslinked polymer
142
through ring-opening polymerization because of the steric hindrance of the alkyl
143
substituents. However, it was reported that benzoxazine can be used to cure epoxy
144
resins due to the reactive phenolic hydroxyl groups which were generated during the
145
ring-opening polymerization of benzoxazine [26, 27]. The reaction between phenolic
146
hydroxyl groups and epoxide groups appeared to be not affected by the steric
147
hindrance of PDETDA, so crosslinked PDETDA/DGEBA copolymer and
148
mechanically sound materials were expected to be obtained.
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141
149
The curing behavior of PDETDA/DGEBA system was studied by DSC, FT-IR
150
and rheological measurement. Firstly, the curing behavior was examined by the
151
rheological measurement, and the viscosity change was recorded in Fig. 1. The
152
viscosity of PDETDA/DGEBA mixture was about 14 Pa s at 40 oC and decreased to 7
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0.13 Pa s at 90 oC, which was significantly lower than that of the reported
154
benzoxazine/epoxy systems [23]. The viscosity remained low values before 230 oC,
155
after which a sharp increase of the viscosity was observed because of gelation. This
156
result indicated that the PDETDA/DGEBA system had a wide processing window.
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153
157
Fig. 1. Rheological curve of PDETDA/DGEBA system
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158
The curing process was further monitored by DSC. Fig. 2 shows the DSC curves
160
of PDETDA/DGEBA at different curing stages. There were two exothermic peaks
161
appeared, and it was noted that with the elevation of curing temperature and the
162
extension of curing time, the amount of exothermic decreased gradually. The
163
exothermic peak at lower temperature disappeared after heating at 180 oC for 4 h,
164
while the exothermic peak at higher temperature did not die away until heated at 240
165
o
166
transition temperature (Tg) of the cured PDETDA/DGEBA arose after the 180 oC
167
curing stage and increased with the advancing of the cure procedure. The DSC data
168
are summarized in Table 1.
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159
C for 2 h, indicating the completely cure of PDETDA/DGEBA. Besides, the glass
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ip t Fig. 2. DSC curves of PDETDA/DGEBA system at different curing stages
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cr
169
171
Table 1. Results of DSC at different curing stages for PDETDA/DGEBA system
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172
-
253.1
160 oC, 2 h
-
180 oC, 4 h
84.6
220 oC, 2 h
0
251.4
300.3
227
18.4
-
297.7
80.4
71.1
102.4
-
301.0
62.0
77.7
118.3
-
300.2
30.3
89.1
129.0
-
-
0
100
Ac
240 oC, 2 h
278
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200 oC, 2 h
300.5
ed
Before curing
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Curing procedures Tg (oC) Tp1 (oC) Tp2 (oC) ΔH (J g-1) Degree of curing (%)c
173
c
174
after a certain curing stage; ΔH0 is the reaction enthalpy of PDETDA/DGEBA before
175
curing.
Degree of curing = 1-(ΔHi/ΔH0). ΔHi is the reaction enthalpy of PDETDA/DGEBA
176 177
Fig. 3 shows the FT-IR spectra of PDETDA/DGEBA system at different curing
178
stages. It was seen that the characteristic absorptions corresponding to the
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Page 11 of 56
out-of-the-plane C-H vibration of benzene to which an oxazine group was attached at
180
932 cm-1 and epoxide group at 914 cm-1 were consumed gradually with the
181
proceeding of curing. These peaks appeared to have almost completely disappeared
182
by the end of the 180 oC curing stage. Signals around 3400 cm-1 that were attributed to
183
the hydroxyl groups enhanced significantly due to the ring-opening polymerization of
184
oxazine rings and epoxide rings. These changes indicated that the copolymerization of
185
PDETDA and DGEBA happened.
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186
Fig. 3. FT-IR spectra of PDETDA/DGEBA system at different curing stages
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187 188
190
3.2. Curing kinetics of PDETDA/DGEBA system
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189
The
curing
kinetics
of
PDETDA/DGEBA system
was
studied
by
191
non-isothermal DSC. Fig. 4 shows the DSC curves of PDETDA/DGEBA at different
192
heating rates. Two overlapped exothermic peaks appeared on the curves, suggesting
193
that at least two reactions happened during the curing. This phenomenon was also
194
observed in other benzoxazine/epoxy systems [23, 26]. When the heating rate
195
increased, both peak temperatures of the exothermic reactions (Tp1 and Tp2) shifted to
10
Page 12 of 56
higher temperatures. In addition, slower heating rate separated the two peaks better as
197
reported in literature [26, 28].
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196
198
Fig. 4. DSC curves of PDETDA/DGEBA resin at different heating rates
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199
With the values of Tp1s and Tp2s obtained from DSC at different heating rates,
201
activation energies (Eas) of the two exothermic reactions, which were assigned as
202
reaction 1 and reaction 2 in order, can be calculated by Kissinger method and Ozawa
203
method according to Eq. (2) and Eq. (3), respectively [28].
204
Q AR Ea ln 2 ln p T E a RT p p
205
E AE ln ln a ln F ( ) 5.331 1.052 a RT R p
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(2)
(3)
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206
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200
Kissinger and Ozawa plots of reaction 1 and reaction 2 are shown in Fig. 5. Good
207
linear relationships were observed by using both kinetic methods. From the slopes of
208
the straight lines of lnβ/Tp2 vs. 1/Tp and lnβ vs. 1/Tp, Eas of both reactions were
209
obtained and listed in Table 2, together with the Ea of PDETDA polymerization for
210
comparison.
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Page 13 of 56
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211
Fig. 5. Kissinger and Ozawa plots for Ea determination of (a) reaction 1 and (b)
213
reaction 2 of PDETDA/DGEBA system
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212
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214
Table 2. Eas of PDETDA/DGEBA and PDETDA obtained by Kissinger method and
216
Ozawa method
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Kissinger Ea Ozawa Ea Average Ea
Curing systems (kJ mol-1)
(kJ mol-1) (kJ mol-1)
Reaction 1 103.4
106.4
104.9
Reaction 2 123.6
126.4
125.0
104.7
107.5
106.1
PDETDA/DGEBA
PDETDA 217
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Page 14 of 56
The Kissinger Ea and Ozawa Ea were quite close to each other for both reactions.
219
The average values of Kissinger Ea and Ozawa Ea for reaction 1 and reaction 2 were
220
104.9 kJ mol-1 and 125.0 kJ mol-1, respectively. For pure PDETDA ring-opening
221
polymerization, the average Ea value was 106.1 kJ mol-1. Notably, the Ea of reaction 1
222
was almost the same to the Ea of PDETDA ring-opening polymerization. It was
223
therefore deduced that the mechanism of reaction 1 was the same as PDETDA
224
ring-opening polymerization which produced a non-crosslinked polymer with
225
–N=CH2 groups according to our previous work as shown in Scheme 2. Logically, the
226
reaction 2 can be attributed to the etherification reaction between the phenolic
227
hydroxyl groups which were generated from the ring-opening of PDETDA and the
228
epoxide groups (Scheme 2). This assumption was also supported by the similar Ea
229
value (129.6 kJ mol-1) of epoxide-phenol curing reaction calculated from density
230
functional theory [32].
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231
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232
Scheme 2. Possible curing reaction mechanism of reaction 1 and reaction 2 for
234
PDETDA/DGEBA system
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233
In order to get more kinetics parameters of PDETDA/DGEBA curing reaction,
236
the DSC curves at different heating rates in Fig. 4 were analyzed using PeakFit v4.12
237
program. The overlapped peak 1 and peak 2 were separated using PearsonVII
238
distribution
239
PDETDA/DGEBA at a heating rate of 10 oC min-1, as well as the calculated curves
240
simulated by Peakfit v4.12 with two resolved peaks corresponding to reaction 1 and
241
reaction 2, were illustrated in Fig. 6.
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235
As
an
exemplification,
the
DSC
thermogram
of
Ac
equation.
14
Page 16 of 56
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242
Fig. 6. DSC thermogram of PDETDA/DGEBA system recorded at 10 oC min-1 and
244
the corresponding calculated curves
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243
With the data of the separated peak 1 and peak 2, a fuller assessment of the Ea of
246
PDETDA/DGEBA curing throughout the entire conversion range can be obtained
247
using the Flynn-Wall-Ozawa method [26, 28]. This method is based on Eq. (4) and Eq.
248
(5):
249
AE E ln ln a ln g ( ) 5.331 1.052 a R RT
250
g ( )
0
252
d f ( )
(4)
(5)
where g(α) is the integral conversion function.
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251
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245
For a constant α, the plot of lnβ vs. 1/T obtained from DSC data with different
253
heating rates should render a straight line, from which Ea can be determined from the
254
slope. Fig. 7 shows the Flynn-Wall-Ozawa plot at various α of both reactions of
255
PDETDA/DGEBA system. Good linear relationships of lnβ vs. 1/T were observed
256
throughout the entire conversion range for both reaction 1 and reaction 2. From the
257
slopes of these lines, Ea values at various α were calculated.
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Page 17 of 56
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258
Fig. 7. Flynn-Wall-Ozawa plots at various α of (a) reaction 1 and (b) reaction 2
ed
259
The variations of Ea vs. α for reaction 1 and reaction 2 are shown in Fig. 8. It was
261
seen that both Eas of reaction 1 and reaction 2 increased with the proceeding of curing
262
reactions, which was interpreted by the decrease in molecular mobility or due to a
263
kinetic compensation effect [26, 28, 33]. The average Eas for reaction 1 and reaction 2
264
at various α were 109.1 kJ mol-1 and 125.2 kJ mol-1, respectively. This result was quite
265
close to the Eas obtained from Kissinger method and Ozawa method. Therefore, the
266
average value of Kissinger Ea and Ozawa Ea was selected for further determining the
267
reaction order. Besides, it was worth noting that the Ea of reaction 1 was obviously
268
higher than most of the reported values of ring-opening polymerization of
269
benzoxazines [26, 28] because of the steric hindrance of substituents in PDETDA,
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260
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Page 18 of 56
while the Ea of reaction 2 was close to the values of the reaction between phenols and
271
epoxide groups [26, 32], indicating that the etherification was not obviously affected
272
by the steric hindrance.
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270
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Fig. 8. Plots of Ea vs. α of reaction 1 and reaction 2 obtained by Flynn-Wall-Ozawa
275
method
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274
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Based on the above average Ea, the other parameters of the curing kinetics can be
276 277
determined by the Frideman method [26, 28, 31], which is based on Eq. (6):
278
ln
279
The curing reaction mechanisms of thermosetting resins are usually divided into an
280
n-order reaction and an autocatalytic reaction. In the case of the n-order reaction,
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f ( ) (1 )n
282
Combined with Eq. (6) results Eq. (8):
283
lnAf ( ) ln
(6)
(7)
Ac
281
d d E ln( ) lnAf ( ) a dt dT RT
d Ea ln A n ln(1 ) dt RT
(8)
284
Friedman suggested that the plots of ln[Af(α)] vs. ln(1-α) should yield a straight
285
line for the n-order reaction. The reaction rate is highest at the beginning of the curing
286
and decreases as the reaction proceeds. However, for autocatalytic process, the
287
Friedman plot shows a maximum of ln(1-α) approximately in the range of -0.51 to 17
Page 19 of 56
-0.22 which is equivalent to α of about 0.2-0.4. Fig. 9 shows the Friedman plots of
289
reaction 1 and reaction 2 of PDETDA/DGEBA system. Since ln[Af(α)] and ln(1-α)
290
were not linearly related and a maximum was evidently presented in the above
291
mentioned range of α, both reaction 1 and reaction 2 of PDETDA/DGEBA were
292
actually autocatalytic in nature. Based on the above results, the autocatalytic nature of
293
reaction 1 can be explained by the generation of free phenol groups while the oxazine
294
rings open, and these phenol groups can further accelerate the ring opening process.
295
Similarly, the secondary hydroxyl groups produced in the ring-opening reaction of
296
epoxy groups are able to participate in the curing reaction, which accounted for the
297
autocatalytic nature of reaction 2 [26].
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288
298
Fig. 9. Plots of ln[Af(α)] vs. ln(1-α) for reaction 1 and reaction 2 at heating rate of 10
300
o
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299
C min-1
For the autocatalytic reaction,
301 302
f ( ) m (1 )n
(9)
303
Combined with Eq. (6) results Eq. (10):
304
ln(
305
Eq. (10) can be solved by multiple linear regression, in which the dependent variable
d d E ) ln( ) ln A ( a ) m ln n ln 1 dt dT RT
(10)
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Page 20 of 56
is ln(dα/dt), and the independent variables are lnα, ln(1-α), and 1/T. Therefore, the
307
values of A, m, and n can be obtained using the calculated Ea. The α was chosen
308
between 0.05 and 0.50. The results of the multiple linear regression analysis for
309
reaction 1 and reaction 2 are listed in Table 3 and Table 4, respectively. Compared
310
with the diphenol type benzoxazine/novolac epoxy system [26], PDETDA/DGEBA
311
showed much higher Ea and lower total reaction order for reaction 1, but similar Ea
312
and total reaction order for reaction 2. This result further indicated that the reaction 1
313
was strongly affected by the steric hindrance while the reaction 2 was not.
314
Table 3. Kinetic parameters evaluated for reaction 1 of PDETDA/DGEBA system
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306
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Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m 3
19.95
0.46
1.32
20.03
0.48
1.28
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5
104.9
10 315
20.06
0.49
mean
1.28
20.10
0.51
1.28
20.14
0.52
1.23
Table 4. Kinetic parameters evaluated for reaction 2 of PDETDA/DGEBA system
Ac
316
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7
mean n
Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m 3 5
mean n
22.23
0.45
1.28
22.32
0.49
1.30
125.0
22.33
0.49
mean
1.26
7
22.33
0.50
1.24
10
22.42
0.53
1.22
317 19
Page 21 of 56
318
According to the results of the evaluated parameters, the curing kinetic equation
319
of reaction 1 can be expressed as:
320
d d 12617 0.49 exp( 20.06) exp( ) (1 )1.28 dt dT T
321
The curing kinetic equation of reaction 2 can be expressed as:
322
d d 15035 0.49 exp( 22.33) exp( ) (1 )1.26 dt dT T
ip t
(11)
(12)
The experimental curves and predicted curves based on the determined kinetic
324
parameters of reaction 1 and reaction 2 are shown in Fig. 10. It was clearly seen that
325
the calculated data from the models were in good agreement with the experimental
326
results.
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cr
323
327 328
Fig. 10. Comparison of experimental values and calculated values for (a) reaction 1
329
and (b) reaction 2
20
Page 22 of 56
330
3.3. Properties of cured PDETDA/DGEBA The thermal stability, mechanical properties and water absorption of the cured
332
PDETDA/DGEBA were examined and compared with the samples of DETDA-cured
333
DGEBA (DETDA/DGEBA) and polymerized PDETDA (P-PDETDA). It was noted
334
that P-PDETDA did not have sufficient strength for mechanical measurements.
ip t
331
Fig. 11 shows the TGA curves of the three samples, and the TGA data are listed
336
in Table 5. The 5% mass loss temperature (T5%) and the decomposition temperature
337
(Td) of the cured PDETDA/DGEBA were comparable to the corresponding values of
338
the cured DETDA/DGEBA, and were much greater than those of P-PDETDA. Char
339
yield of the cured PDETDA/DGEBA at 800 oC was twice more than that of the cured
340
DETDA/DGEBA. Two reasons explained the higher char yield of the cured
341
PDETDA/DGEBA. First, PDETDA/DGEBA system contained more aromatic
342
structure than DETDA/DGEBA. Second, more stable hydrogen bonds can be formed
343
in the cured PDETDA/DGEBA. The above results indicated that the cured
344
PDETDA/DGEBA owned good thermal stability.
Ac
ce pt
ed
M
an
us
cr
335
345 346
Fig. 11. TGA curves of cured PDETDA/DGEBA, cured DETDA/DGEBA and
347
P-PDETDA 21
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348
Table 5. TGA results of different cured systems T5% (oC) Td (oC) Char yield at 800 oC (%)
DETDA/DGEBA
367.7
375.2
3.5
PDETDA/DGEBA 355.2
377.4
12.0
P-PDETDA
347.5
24.1
296.5
us
350
ip t
Cured systems
cr
349
Dynamic mechanical analysis (DMA) was applied to evaluate the dynamic
352
mechanical properties of the cured PDETDA/DGEBA and DETDA/DGEBA
353
(Fig. 12). The cured PDETDA/DGEBA had a lower Tg (146 oC) than that of the cured
354
DETDA/DGEBA (189 oC) as indicated by the peak temperature of tan delta. It is
355
known that Tg is closely related to the crosslink density, which can be estimated from
356
the following equation [34]:
357
G' RT
358
where ν is the crosslink density, G' is the storage modulus in the rubbery region
359
(Tg+50 oC), is the front factor which is taken as unity.
M
ed
ce pt
(20)
Ac
360
an
351
It should be noted that this equation is strictly valid only for lightly crosslinked
361
materials and therefore is used only to qualitatively compare the level of crosslinking
362
of the cured materials. The calculated crosslink densities for the cured
363
PDETDA/DGEBA and DETDA/DGEBA were 1.56×10-3 mol·cm-3 and 2.08×10-3
364
mol·cm-3, respectively. Lower crosslink density of the cured PDETDA/DGEBA
365
accounted for its lower Tg. 22
Page 24 of 56
The storage modulus in the glassy region was a reflection of the material’s
367
stiffness, and as shown in Fig. 12, the cured PDETDA/DGEBA had obviously higher
368
stiffness than that of the cured DETDA/DGEBA. It is well known that strong
369
hydrogen bonds are formed during benzoxazine polymerization. Therefore, even if
370
PDETDA/DGEBA had lower crosslink density, the resultant hydrogen bonds in the
371
cured PDETDA/DGEBA endowed it with higher stiffness.
cr
ip t
366
Tensile, flexural and impact properties of the two cured systems were measured
373
and the data were listed in Table 6. All the mechanical properties except the impact
374
strength were pronounced higher for the cured PDETDA/DGEBA than those of the
375
cured DETDA/DGEBA. Higher modulus and strength of the cured PDETDA/DGEBA
376
were probably resulted from the stronger hydrogen bonds existed in the polymer. It
377
was also the stronger hydrogen bonds in the cured PDETDA/DGEBA that had
378
adverse effect on the toughness, leading to its lower impact strength.
Ac
ce pt
ed
M
an
us
372
379 380
Fig. 12. Dynamic mechanical spectra of the cured PDETDA/DGEBA (●) and
381
DETDA/DGEBA (■) including storage modulus and dissipation factor tan delta
382 383 23
Page 25 of 56
384
Table 6. Tensile, flexural and impact properties of the cured PDETDA/DGEBA and
385
DETDA/DGEBA Flexural
Tensile
Tensile
Impact
strength
modulus
strength
modulus
strength
(MPa)
(MPa)
(MPa)
(MPa)
113.2±1.3
2.5±0.1
48.7±7.1
2.4±0.5
37.6±5.6
PDETDA/DGEBA 130.9±2.2
3.0±0.1
57.7±10.5
3.2±0.3
15.9±4.0
us
DETDA/DGEBA
(kJ m-2)
cr
Systems
ip t
Flexural
an
386
Fig. 13 shows the variation of water absorption of the cured PDETDA/DGEBA
388
and DETDA/DGEBA with the extension of immersion time in water at room
389
temperature. The results displayed that both the water absorption rate and the
390
saturated water absorption of the cured PDETDA/DGEBA were obviously lower than
391
that of the cured DETDA/DGEBA. The cured PDETDA/DGEBA absorbed water up
392
to about 0.9% of its conditioned weight, while the cured DETDA/DGEBA saturated at
393
approximately 1.5%.
Ac
ce pt
ed
M
387
394 395
Fig. 13. Plots of water absorption vs. time of the cured PDETDA/DGEBA and
396
DETDA/DGEBA 24
Page 26 of 56
397
The diffusion coefficients of the materials were determined according to the
398
Fick’s law through the following equation (21) [1],
399
M t 4 Dt M 0.5 l 2
400
where Mt and M∞ are the cumulative masses sorbed by a sample of thickness l at time
401
t and ∞, respectively. The diffusion coefficient D can be calculated from the initial
402
slope G of the plot of Mt/M∞ vs. t0.5/l (Fig. 14) through the following equation,
403
D G 2 16
0.5
us
cr
ip t
(21)
(22)
The calculated diffusion coefficients were 8.82×10-9 cm2 s-1 for DETDA/DGEBA
405
and 8.09×10-9 cm2 s-1 for PDETDA/DGEBA. This result indicated that the cured
406
PDETDA/DGEBA
407
DETDA/DGEBA. The lower saturated water absorption and diffusion coefficient of
408
PDETDA/DGEBA was speculated to be caused by the more amounts of inter- and
409
intramolecular hydrogen bonds in its structure, which shielded the hydroxyl groups
410
from interaction with water molecules [1].
412
M
slightly
lower
water
diffusivity
than
the
cured
ce pt
ed
had
Ac
411
an
404
25
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ip t cr
413
Fig. 14. Calculation of diffusion coefficients from the initial slope of the Mt/M∞, vs.
415
t0.5/l curves
us
414
4. Conclusions
M
417
an
416
The novel aromatic diamine-based benzoxazine PDETDA was proven to be an
419
effective hardener of DGEBA, and PDETDA/DGEBA mixture had the merit of low
420
viscosity and wide processing window. The curing kinetics of PDETDA/DGEBA was
421
studied by non-isothermal DSC, and two dominant curing reactions were observed.
422
The reaction 1 at low temperature was the ring-opening polymerization of
423
benzoxazine monomers, while the reaction 2 at high temperature corresponded to the
424
etherification between hydroxyl groups of polybenzoxazine and epoxide groups. Both
425
reactions were autocatalytic in nature, and the predicted curves from the proposed
426
kinetic models fit well with the experimental curves. Furthermore, the calculated Ea
427
values showed that the reaction 1 was greatly influenced by the steric hindrance of
428
substituents in PDTEDA while the reaction 2 was not, indicating that
429
non-crosslinkable benzoxazine can also be effective hardeners for epoxy resin. The
Ac
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26
Page 28 of 56
430
properties of the cured PDETDA/DGEBA were examined and compared with the
431
cured DETDA/DGEBA. The results presented that PDETDA/DGEBA had higher
432
char yield, better mechanical properties and lower water absorption.
436
This study is financially supported by the National Natural Science Foundation
cr
435
Acknowledgments
of China (No. 51303084).
us
434
ip t
433
437
References
439
[1] Ishida, H., Allen, D. J. Physical and mechanical characterization of near-zero
440
shrinkage polybenzoxazines. J Polym Sci Part B: Polym Phys. 1996, 34: 1019-1030.
441
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5156-5162.
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[10] Yang, P., Gu, Y. A novel benzimidazole moiety-containing benzoxazine:
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Synthesis, polymerization, and thermal properties. J Polym Sci Part A: Polym Chem.
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New thermosetting resin from terpenediphenol-based benzoxazine and epoxy resin. J
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arylamine-based benzoxazine resins alloyed with epoxy resin. Polym Eng Sci. 2011,
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New thermosetting resin from poly(p-vinylphenol) based benzoxazine and epoxy
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resin. J Appl Polym Sci. 2001, 79: 555-565.
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epoxy, benzoxazine monomer system from cardanol: Thermal and viscoelastic
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Benzoxazine-epoxy copolymer investigated by non-isothermal differential scanning
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calorimetry. Polym Degrad Stabil. 2010, 95: 918-924.
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benzoxazines and their advantages over diamines as epoxy hardeners. J Polym Sci
514
Part A: Polym Chem. 2010, 48: 2430-2437.
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[28] Jubsilp, C., Damrongsakkul, S., Takeichi, T., Rimdusit, S. Curing kinetics of
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arylamine-based polyfunctional benzoxazine resins by dynamic differential scanning
517
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[29] Ishida, H., Rodriguez, Y. Curing kinetics of a new benzoxazine-based phenolic
519
resin by differential scanning calorimetry. Polymer. 1995, 36: 3151-3158.
520
[30] Hamerton, I., McNamara, L. T., Howlin, B. J., Smith, P. A., Cross, P., Ward, S.
521
Examining the kinetics of the thermal polymerization of commercial aromatic
522
bis-benzoxazines. J Polym Sci Part A: Polym Chem. 2014, 52: 2068-2081.
523
[31] He, X. Y., Wang, J., Ramdani, N., Liu, W. B., Liu, L. J., Yang, L. Investigation of
524
synthesis, thermal properties and curing kinetics of fluorine diamine-based
525
benzoxazine by using two curing kinetic methods. Thermochim Acta. 2013, 564:
526
51-58.
527
[32] Pham, M. P., Pham, B. Q., Huynh, L. K., Pham, H. Q., Marks, M. J., Truong, T.
528
N. Density functional theory study on mechanisms of epoxy-phenol curing reaction. J
529
Comput Chem. 2014, 35, 1630-1640.
530
[33] Zvetkov, V. L. Comparative DSC kinetics of the reaction of DGEBA with
531
aromatic diamines I: non-isothermal kinetic study of the reaction of DGEBA with
532
m-phenylene diamine. Polymer. 2001, 42: 6687-6697.
533
[34] Ishida, H., Allen, D. J. Mechanical characterization of copolymers based on
534
benzoxazine and epoxy. Polymer. 1996, 37: 4487-4495.
Ac
ce pt
ed
M
an
us
cr
ip t
518
31
Page 33 of 56
Table 1
Table 1. Results of DSC at different curing stages for PDETDA/DGEBA system
253.1
300.5
278
0
160 oC, 2 h
-
251.4
300.3
227
18.4
180 oC, 4 h
84.6
-
297.7
80.4
71.1
200 oC, 2 h
102.4
-
301.0
62.0
77.7
220 oC, 2 h
118.3
-
300.2
30.3
240 oC, 2 h
129.0
-
-
0
cr
-
us
89.1 100
Degree of curing = 1-(ΔHi/ΔH0). ΔHi is the reaction enthalpy of PDETDA/DGEBA
an
c
Before curing
ip t
Curing procedures Tg (oC) Tp1 (oC) Tp2 (oC) ΔH (J g-1) Degree of curing (%)c
M
after a certain curing stage; ΔH0 is the reaction enthalpy of PDETDA/DGEBA before
Ac
ce pt
ed
curing.
Page 34 of 56
Table 2
Table 2. Eas of PDETDA/DGEBA and PDETDA obtained by Kissinger method and Ozawa method Kissinger Ea Ozawa Ea Average Ea Curing systems
ip t
(kJ mol-1) (kJ mol-1)
Reaction 1 103.4
106.4
104.9
Reaction 2 123.6
126.4
104.7
107.5
cr
(kJ mol-1)
106.1
Ac
ce pt
ed
M
an
PDETDA
125.0
us
PDETDA/DGEBA
Page 35 of 56
Table 3
Table 3. Kinetic parameters evaluated for reaction 1 of PDETDA/DGEBA system Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m
mean n
3
19.95
0.46
1.32
5
20.03
0.48
1.28
20.06
0.49 0.51
10
20.14
0.52
1.28
1.28 1.23
Ac
ce pt
ed
M
an
us
cr
20.10
ip t
104.9 7
mean
Page 36 of 56
Table 4
Table 4. Kinetic parameters evaluated for reaction 2 of PDETDA/DGEBA system Heating rate (oC min-1) Ea (kJ mol-1) lnA (s-1) mean m
mean n
3
22.23
0.45
1.28
5
22.32
0.49
1.30
22.33
0.49 0.50
10
22.42
0.53
1.26
1.24 1.22
Ac
ce pt
ed
M
an
us
cr
22.33
ip t
125.0 7
mean
Page 37 of 56
Table 5
Table 5. TGA results of different cured systems T5% (oC) Td (oC) Char yield at 800 oC (%)
DETDA/DGEBA
367.7
375.2
3.5
PDETDA/DGEBA 355.2
377.4
12.0
P-PDETDA
347.5
24.1
Ac
ce pt
ed
M
an
us
cr
296.5
ip t
Cured systems
Page 38 of 56
Table 6
Table 6. Tensile, flexural and impact properties of the cured PDETDA/DGEBA and DETDA/DGEBA Flexural
Tensile
Tensile
Impact
strength
modulus
strength
modulus
strength
(MPa)
(MPa)
(MPa)
(MPa)
113.2±1.3
2.5±0.1
48.7±7.1
2.4±0.5
37.6±5.6
PDETDA/DGEBA 130.9±2.2
3.0±0.1
57.7±10.5
3.2±0.3
15.9±4.0
Ac
ce pt
ed
M
an
us
DETDA/DGEBA
(kJ m-2)
cr
Systems
ip t
Flexural
Page 39 of 56
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