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Bio-based Heat Resistant Epoxy Resin with Extremely High Biomass Content from 2,5-Furandicarboxylic Acid and Eugenol Jia-Tao Miao, Li Yuan, Qingbao Guan, Guozheng Liang, and Aijuan Gu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01222 • Publication Date (Web): 22 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017
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ACS Sustainable Chemistry & Engineering
1
Bio-based Heat Resistant Epoxy Resin with Extremely High Biomass
2
Content from 2,5-Furandicarboxylic Acid and Eugenol
3 4
Jia-Tao Miao, Li Yuan, Qingbao Guan, Guozheng Liang* and Aijuan Gu*
5
State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials,
6
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application
7
Department of Materials Science and Engineering,
8
College of Chemistry, Chemical Engineering and Materials Science,
9
Soochow University, 199 Ren'Ai Road, Suzhou, 215123, China
10 11
Abstract
12
Preparing a bio-based (biomass-based) high performance epoxy resin with
13
extremely large biomass content is of great important for sustainable development.
14
Herein,
15
bis(2-methoxy-4-(oxiran-2-ylmethyl)phenyl)furan-2,5-dicarboxylate (EUFU-EP), was
16
synthesized
17
(2,5-furandicarboxylic acid and eugenol) and the biomass content of EUFU-EP is as
18
large as 93.3%. In addition, a new bio-based epoxy resin, EUFU-EP/MHHPA, was
19
prepared by using methyl hexahydrophthalic anhydride (MHHPA) as the curing agent
20
and 2-ethyl-4-methylimidazole as the curing accelerator. The curing reactivity and
21
integrated performances including thermal and mechanical properties as well as flame
22
retardancy of the cured resin were systematically researched, and compared with
a
new
from
epoxy
two
resin
bio-based
green
with
and
a
low
precise
toxic
structure,
compounds
* Corresponding author. Tel: +86 512 65880967. Fax: +86 512 65880089. E-mail address: E-mail address:
[email protected] (Guozheng Liang),
[email protected] (Aijuan Gu). . 1
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those of petrochemical resource-based epoxy resin (DGEBA/MHHPA) consisting of
24
commercial
25
2-ethyl-4-methylimidazole.
26
DGEBA/MHHPA have similar curing reactivity, but cured EUFU-EP/MHHPA resin
27
shows better thermal properties, rigidity and flame retardancy than cured
28
DGEBA/MHHPA resin. Specifically, the glass transition temperature (Tg) of
29
EUFU-EP/MHHPA resin is as high as 153.4 oC, the storage modulus at 50 oC
30
increases by 19.8%, meanwhile both peak heat release rate and total heat release
31
reduce by 19.0%. The nature behind these outstanding integrated performances is
32
attributed to the unique structure of EUFU-EP, which is not only rich in aromatic
33
structure, but also has furan ring. The especially large biomass content and
34
outstanding thermal, mechanical and flame retarding performances clearly show that
35
EUFU-EP resin has a great potential in actual applications.
diglycidyl
ether
of
Results
bisphenol
A
show
that
(DGEBA),
MHHPA
EUFU-EP/MHHPA
and and
36 37
Keywords: Biomass; Epoxy resin; Thermal property; Flame retardancy; Structure.
38 39 40
Introduction It is arguable that the petrochemical resources are indispensable; however, 1, 2
41
energy depletion becomes one of greatest challenges of the world today.
42
petrochemical resources, biomass which can translate into petrochemical resources
43
through hundreds of millions of years is characterized to be abundant, renewable, and
44
high annual output.3-7 In order to meet the challenge of energy crisis and the 2
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requirement of sustainable development, it is urgent to pay an increasing attention to
46
biomass resource.8-11
47
Thermosetting resins, strongly dependent on petroleum resources, have been key
48
and basic materials in industry since the birth of the first synthetic resin in 1872.12
49
They play indispensable role from basic (chemical industry,13 coating
50
(aerospace,
51
occupies about 70% of the whole thermosetting resin market due to its outstanding
52
integrated performances.18 Note that more than 90% of epoxy resin is bisphenol A
53
epoxy resin (DGEBA),
54
epichlorohydrin in the presence of sodium hydroxide.17 Unfortunately, bisphenol A is
55
strongly dependent on petrochemical resources.
15
new energy,
16
18, 19
information
17
14
) to strategic
) fields. Among them, epoxy resin
which is synthesized from bisphenol A and
56
In recent years, attempts have been made to introduce some biomass-based
57
(bio-based) units into epoxy resins instead of petroleum-based units. Various
58
biomasses have been reported (such as vegetable oil,20 itaconic acid,21, 22 cardanol,1
59
rosin,23 eugenol,
60
raw materials in synthesizing bio-based epoxy resins, of which the biomass content of
61
epoxy ranges from 40.1% to 84% as summarized in Table S1 in Supporting
62
Information.
6, 24, 25
2,5-furandicarboxylic acid (FDCA),26 lignin,7, 27 etc.) to act as
63
FDCA and eugenol, both of which are common renewable resources in nature,
64
have aroused worldwide concern in recent years due to their rigid aromatic structure.
65
FDCA is classified as the top ten green chemical by US Department of Energy.28
66
Eugenol, occupying about 80% of clove oil, is a renewable biomass material with low 3
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toxicity and relatively low cost. 29 FDCA and eugenol have been used for synthesizing
68
bio-based epoxy resins, respectively. Zhang et al. reported a eugenol-based epoxy
69
resin with a biomass content of 62.7%, and the glass transition temperature (Tg) of
70
epoxy resin cured with hexahydrophthalic anhydride was only 114 °C.25 Wang et al.
71
reported two kinds of eugenol-based epoxy resins which link eugenol molecules
72
together with biomass-free α,α-dichloro-p-xylene
73
respectively. The former (melting point, 124oC) cured with 4,4’-diaminodiphenyl
74
methane has a biomass content of 70.2% and a Tg value of 114.4 °C, while the latter
75
cured with 3,3’-diaminodiphenylsulfone has a higher biomass content of 80% and
76
higher Tg (207 °C). Liu’s group prepared an epoxy resin based on FDCA, the Tg of the
77
epoxy resin cured with methyl hexahydrophthalic anhydride (MHHPA) was 152 oC,
78
the flexural strength was 96 MPa and the biomass content was 65.2%.26 These works
79
have significantly promoted the use of eugenol and FDCA in the field of bio-based
80
epoxy resins. However, all synthesis routes of them use dichloromethane as solvent;
81
moreover, the biomass contents and integrated performances of above bio-based
82
epoxy monomers still need increase.
24
and cyanuric chloride
6
,
83
Herein, a unique bio-based epoxy resin monomer, bis(2-methoxy-4-(oxiran-2-
84
ylmethyl) phenyl)furan-2,5-dicarboxylate (EUFU-EP), with extremely large biomass
85
content (93.3%), high thermal and mechanical properties as well as good flame
86
retardancy was designed and synthesized through building a precise structure from
87
FDCA and eugenol. The curing behavior and relationships between structure and
88
properties of EUFU-EP/MHHPA resins were intensively studied and compared with 4
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its petrochemical resource based DGEBA counterpart.
90
Experiment section
91
Raw materials
92
Eugenol, 3-chloroperoxybenzoic acid (mCPBA, 85%) and MHHPA were
93
purchased from Energy Chemical, China. FDCA was bought from Sichuan Dagaote
94
Technology
95
triethylamine, sodium sulfite, sodium carbonate, tetrahydrofuran (THF), ethyl acetate,
96
N,N-dimethylformamide (DMF) and other reagents were obtained from Sinopharm
97
Chemical Reagent Co., Ltd, China. Diglycidyl ether of bisphenol A (DGEBA) used
98
herein has an epoxy value of 0.44 mol/100 g, which was got from Nantong Xingchen
99
Synthetic Material Co., Ltd, China.
100
Co.,
Ltd,
China.
Oxalyl
chloride,
2-ethyl-4-methylimidazolet,
Synthesis of furan-2,5-dicarbonyl dichloride (FDCDCl)
101
A mixture of FDCA (15.6 g, 100 mmol), THF (100 mL) and DMF (catalyst, 0.05
102
mL) was added into a 100 mL round-bottomed flask, oxalyl chloride (19.04 g, 150
103
mmol) was added dropwise to above suspension at 0 °C. The reaction mixture was
104
slowly heated and reacted at 25 °C for 3 h until the solid was completely dissolved.
105
Then the resultant solution was rotary evaporated to obtain a white solid (19.14 g,
106
yield: 99.2%), which is FDCDCl. 1H NMR (400 MHz, DMSO-D6) δ 7.31 (s, 2H)
107
(Figure S1 in Supplementary Information).
108
Synthesis of bis(4-allyl-2-methoxyphenyl) furan-2,5-dicarboxylate (EUFU)
109
Eugenol (16.4 g, 100 mmol) and triethylamine (16.9 mL, 120 mmol) were 5
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dissolved in ethyl acetate (200 mL) with stirring to get a solution B, into which a
111
solution of FDCDCl (50 mmol, 9.65 g) in ethyl acetate (100 mL) was added dropwise
112
at 0 °C within 20 min, followed by maintaining at 25 °C for 30 min. Then, the
113
reaction solution was filtered. After that, removing solvent through rotary evaporation,
114
washing with water and filtering to get white solid (20.5 g, yield: 91.4%), which was
115
EUFU. 1H NMR (400 MHz, CDCl3) δ 7.45 (s, 2H), 7.06 (d, J = 8.0 Hz, 2 H), 6.85 -
116
6.77 (m, 4H), 5.98 (m, 2H), 5.16 - 5.08 (m, 4H), 3.82 (s, 6H), 3.40 (d, J = 6.6 Hz, 4H).
117
13
118
122.59, 120.93, 119.98, 116.46, 113.06, 56.05, 40.30. HRMS (ESI+): m/z calcd for
119
[C26H24O7Na]+: 471.1414, found 471.1402. Anal. calcd for C26H24O7: C 69.63, H 5.39,
120
found: C 69.67, H 5.607.
121
Synthesis of EUFU-EP
C NMR (151 MHz, CDCl3) δ 156.07, 151.04, 146.82, 139.79, 137.37, 137.13,
122
mCPBA (12.2 g, 60 mmol) was slowly added into a solution consisting of EUFU
123
(8.97 g,20 mmol) and ethyl acetate (100 mL) at 0 °C, the reaction mixture was slowly
124
heated and reacted at 40 °C for 48 h. After that the resultant solution was filtered to
125
get the filtrate, which was then washed with 10% Na2SO3 solution, 5% NaHCO3
126
solution and deionized water, successively. The organic layer was dried with
127
anhydrous sodium sulfate, and ethyl acetate was rotary evaporated to give a yellow
128
solid. The solid was then washed with ethanol to give a white solid, coded as
129
EUFU-EP (6.10 g, yield: 63.5%). Melting point: 99.7 °C (differential scanning
130
calorimetry, DSC). 1H NMR (400 MHz, CDCl3) δ 7.47 (s, 2H), 7.09 (d, J = 8.0 Hz,
131
2H), 6.91 (s, 2H), 6.87 (d, J = 8.0 Hz, 2H), 3.84 (s, 6H), 3.20 – 3.14 (m, 2H), 2.88 (d, 6
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J = 5.3 Hz, 4H), 2.83 (t, J = 4.2 Hz, 2H), 2.60 – 2.54 (m, 2H). 13C NMR (151 MHz,
133
CDCl3) δ 155.97, 151.12, 146.76, 137.84, 137.05, 122.74, 121.33, 120.04, 113.51,
134
56.10, 52.46, 47.00, 38.86. HRMS (ESI+): m/z calcd for [C26H24O9Na]+: 503.1313,
135
found 503.1306. Anal. calcd for C26H24O9: C 65.00, H 5.04, found: C 64.71, H 5.176.
136
Preparation of cured resins
137
EUFU-EP and MHHPA with an equivalent molar ratio as well as
138
2-ethyl-4-methylimidazole (0.5 wt% of the total weight of EUFU-EP and MHHPA)
139
were mixed thoroughly at 80 oC for 30 min to get a clear liquid (prepolymer). The
140
prepolymer was then transferred to a preheated mold and degassed under vacuum at
141
80 oC for 30 min, followed by putting into an oven for curing using the procedure of
142
130 oC/2 h+150 oC/2 h+170 oC/2 h. After that, the mold was naturally cooled to room
143
temperature to obtain cured resin, coded as EUFU-EP/MHHPA.
144
Similarly,
EUFU-EP
145
DGEBA/MHHPA resin.
146
Characterizations
147
1
was
replaced
by
DGEBA
to
prepare
cured
H NMR spectra of FDCDCl, EUFU and EUFU-EP, 2-D HHCOSY spectra of
148
EUFU and EUFU-EP were recorded with a Bruker AVANCE III 400 MHz
149
superconducting magnetic resonance spectrometer (USA). 13C NMR spectra of EUFU
150
and EUFU-EP were recorded with an Agilent DD2-600 MHz spectrometer (USA).
151
CDCl3 and tetramethylsilane were applied as the solvent and internal standard,
152
respectively. 7
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153 154 155 156 157 158
Fourier transform infrared (FTIR) spectra were obtained using a Bruker a Vertex 70 spectrometer (USA) over the wavenumber ranging from 600 to 4000 cm-1. High-resolution mass spectra (HRMS) were tested on a Bruker UltiMate 3000 high performance liquid chromatography (HPLC) system (Germany). The elemental analysis (C and H) was performed on a Vario EL III elemental analyzer (Germany).
159
DSC curves were recorded on a TA instrument Q200 (USA) under a nitrogen
160
atmosphere with a flowing rate of 50 mL min-1. Each sample was weighed and sealed
161
in an aluminum crucible, and heated from room temperature to 250 oC at a heating
162
rate of 5, 10, 15 or 20 oC min-1.
163 164
Density was measured on a Shanghai Sunny Hengping FA1104J electronic balance with density device (China).
165
Dynamic mechanical analyses (DMA) were performed using TA DMA Q800
166
apparatus (USA) by multi-frequency-strain model in a single cantilever clamp at a
167
heating rate of 3 oC min-1. The tests were performed at a frequency of 1.0 Hz, a
168
deflection amplitude of oscillation of 20 µm, and a poisson’s ratio of 0.44. The
169
dimensions of specimen were (35 ± 0.02) mm× (13 ± 0.02) mm× (3 ± 0.02) mm. Tg is
170
regarded as the peak temperature of the tanδ-temperature curve.30
171
Flexural strengths were measured according to Chinese standard GB2570-95
172
using an electronic universal testing machine (SUST, Zhuhai, China) at a speed of 2
173
mm min-1.
174
Thermogravimetric (TG) analyses were performed on a TA Discovery TGA
175
Instrument (USA) under a nitrogen atmosphere (10 mL min-1) with a heating rate of 8
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10 oC min-1.
177
Micro-combustion calorimetery (MCC) was performed on a Govmark MCC-2
178
microscale combustibility calorimeter (USA). 5 mg sample was heated to 700 oC at a
179
heating rate of 60 oC min-1 in a mixed stream of oxygen and nitrogen flowing at 21
180
and 79 cm3 min-1, respectively.
181
Thermogravimetric analysis infrared (TG-IR) spectra were recorded using a
182
Netzsch TGA F1 thermogravimetric analyzer (Germany) that was interfaced to a
183
Bruker
184
spectrophotometer (Germany). Ten milligrams of a sample was put in an alumina
185
crucible and heated from 30 to 800 oC with a heating rate of 10 oC min-1 under a
186
nitrogen atmosphere.
187
Results and discussion
188
Design, synthesis and characterization of EUFU-EP
189
TENSOR
27
fourier
transform
infrared
spectroscopy
It is well known that there are two methods to synthesize epoxy resin.
(FTIR)
22, 31, 32
190
Taking DGEBA as the example, the first synthesis method (Method I) is derived from
191
bisphenol A and epichlorohydrin under sodium hydroxide (Figure S2a in
192
Supplementary Information).33 Note that this method produces complex oligomers but
193
not epoxy resin monomer with precise structure. Typically DGEBA monomers are
194
marketed with average repeat unit (n) in the range of 0.03−10. 18 In addition, the
195
obtained oligomers are hardly purified and separated. Obviously, this is not conducive
196
to control structure and performances of epoxy resin. The second method (Method II)
197
for synthesizing DGEBA contains two steps. An intermediate (allyl ether compound) 9
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198
from the reaction between bisphenol A and 3-bromopropene with the aid of sodium
199
hydroxide was synthesized, and then the double bond of allyl group is oxidized by an
200
oxidizing agent to obtain an epoxy resin monomer (Figure S2b in Supplementary
201
Information).
202
monomer with precise structure. However, when Method II is used to synthesize
203
bio-based
204
petrochemical resources will reduce the biomass content of epoxy monomer to some
205
extent.
34
Different from Method I, Method II can synthesize an epoxy
epoxy
monomer,
the
reactant
(3-bromopropene)
obtained
from
206
On the other hand, there are two major problems of bisphenol A, besides its
207
dependence on the petrochemical resources, bisphenol A has similar structure with
208
estrogen, which will reduce fertility and increase the risk of developing cancers and
209
other diseases.35,
210
bisphenol A-based materials for packing infant formula,37 and the demand for
211
bisphenol A-free products is booming. Therefore, derived from less toxic raw
212
materials, a new bio-based epoxy monomer with precise structure and extremely large
213
biomass content should be designed and synthesized.
36
The US Federal Drug Administration has banned the use of
214
Figure 1 illustrates the route to synthesize such bio-based epoxy monomer,
215
EUFU-EP, with precise structure and extremely large biomass content. Specifically,
216
eugenol with allyl group itself is taken as the reactant, so it is not necessary to use
217
bromopropene anymore, endowing EUFU-EP with very large biomass content
218
(93.3%). The specific route is composed of three steps. First, FDCDCl was obtained
219
from acylation of FDCA with oxalyl chloride in the presence of catalytic DMF; FDCA 10
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will be fully converted to FDCDCl and directly used for the next step without
221
purification. Second, triethylamine was used as base, FDCDCl and eugenol were
222
esterified to produce intermediate (EUFU) with a high yield (91.4%). Third, the target
223
product EUFU-EP was obtained by oxidizing allyl group in EUFU by mCPBA; the
224
yield of this step is 63.5%, and it was not necessary to purify EUFU-EP by column
225
chromatography, showing great potential for large-scale production. As no bisphenol
226
structure existed in the target product (EUFU-EP); moreover no bisphenol structure
227
was used and formed during the whole synthesis process, so EUFU-EP is a healthier
228
substitute compared with DGEBA.
229
230 231
Figure 1. Synthesis of EUFU-EP
232 233
Figure 2 shows 1H NMR spectra of EUFU and EUFU-EP. In the spectrum of
234
EUFU, the characteristic peaks at 5.98 ppm (Hg) and 5.08-5.16 ppm (Hh, Hi)
235
correspond to protons on double bond of allyl in EUFU, the characteristic peaks at
236
3.40 ppm (Hf), 3.82 ppm (He), and 7.45 ppm (Ha) represent protons on methylene
237
group of allyl adjacent to the double bond, methoxy group and furan ring, respectively.
238
In the spectrum of EUPU-EP, the characteristic peaks of proton on epoxy group are 11
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239
observed, such as 2.54-2.60 ppm (Hi’), 2.83 ppm (Hh’) and 3.14-3.20 ppm (Hh’), while
240
all peaks of protons (Hg, Hh and Hi) on double bond are not found. Other peaks in the
241
spectrum of EUFU-EP are similar as those of protons of EUFU, indicating that double
242
bonds of allyl groups in EUFU have been successfully oxidized to epoxy groups.
243
These assignments of protons have been also confirmed by the 2-D HHCOSY NMR
244
spectra (Figure S3 in Supplementary Information).
245
246
247
Figure 2. 1H NMR spectra of EUFU (top) and EUFU-EP (bottom)
248 249
Above statement is further confirmed by
13
C NMR spectra of EUFU and
250
EUFU-EP shown in Figure 3. The characteristic peaks at 139.79 and 116.46 ppm in
251
the spectrum of EUFU indicate C12 and C13 on allyl groups, respectively. These peaks
252
do not appear in the spectrum of EUFU-EP, instead, characteristic peaks representing
253
carbon atoms on epoxy groups appear at 52.46 ppm (C12’) and 47.00 ppm (C13’), also
254
proving that double bonds of allyl groups in EUFU have been successfully oxidized. 12
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Figure 3. 13C NMR spectra of EUFU (top) and EUFU-EP (bottom)
257 258
FTIR spectra of EUFU and EUFU-EP are shown in Figure S4 (in Supplementary
259
Information). The characteristic peak standing for C=C bonds of EUFU at 1639 cm-1
260
disappears. Instead, the characteristic peak standing for the oxirane rings of EUFU-EP
261
at 930 cm-1 appears, further proving that double bonds of allyl groups in EUFU have
262
been successfully oxidized.
263
In addition, the HRMS spectrum of EUFU-EP (Figure S5 in Supplementary
264
Information) shows that the experimental [M+Na+] is 503.1306, which is consistent
265
with the theoretical value (503.1313). The DSC curve of EUFU-EP (Figure S6 in
266
Supplementary Information) shows a clear endothermic melting peak at 99.7 oC.
267
Above characterizations fully prove that a unique bio-based epoxy resin with
268
precise structure has been facilely synthesized, its theoretical molecular weight is
269
480.47, and the epoxy value is 0.416 mol/100 g. 13
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Curing behavior and aggregation structure of EUFU-EP/MHHPA
271
Figure 4a shows DSC curves of EUFU-EP/MHHPA and DGEBA/MHHPA
272
prepolymers at different heating rates. Each curve displays a single exothermic peak
273
corresponding to the ring-opening reaction between epoxy and acid anhydride. From
274
fitting curves of heating rate vs. exothermic peak temperature of prepolymers (Figure
275
4b), the actual peak temperature (142.1 oC) of EUFU-EP/MHHPA is found to be
276
slightly higher than that of DGEBA/MHHPA (139.6 oC). At the same time, according
277
to Kissinger equation (Equation 1), dynamic curing parameters of prepolymers were
278
calculated. It is found that EUFU-EP/MHHPA has higher activation energy (71.15 kJ
279
mol-1) than DGEBA/MHHPA (61.21 kJ mol-1), so the former has lower reaction
280
activity than the latter. This is because epoxy groups of DGEBA connect with oxygen
281
atoms, increasing the polarization degree of epoxy groups due to the inductive effect,
282
and thus is more conducive to the ring opening reaction of epoxy groups.24
283
β AR E ln 2 = ln − E RTP TP
284
where Tp is the peak temperature, β is the heating rate, E is the activation energy of
285
the curing reaction, A is the pre-exponential factor, and R is the universal constant
286
(8.314 J mol-1 K-1).
(1)
287
EUFU-EP was then cured by MHHPA using the curing procedure of 130 oC/2
288
h+150 oC/2 h+170 oC/2 h. In order to make sure that the samples were fully cured
289
under this condition,
290
EUFU-EP/MHHPA resin. As shown in Figure S7 and S8 in Supplementary
291
Information, there is no exothermic peak in DSC curve, and no characteristic peak of
292
epoxy groups in FTIR spectra, verifying that EUFU-EP/MHHPA resin has been
DSC and FTIR techniques were
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293
completely crosslinked.
294
295 296
Figure 4. DSC thermograms (a) and fitting curves of exothermic peak temperature vs.
297
heating rate (b) of EUFU-EP/MHHPA and DGEBA/MHHPA prepolymers 15
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298
Crosslinking density is an important index for characterizing the aggregation
299
structure of thermosetting resins,38 which is often calculated according to the classical
300
rubbery elasticity theory as shown in Equation 2.26 E'=3υeRT
301
(2)
302
where E’ is the storage modulus of the thermosetting resin in the rubbery plateau
303
region, R is the universal constant, and T is the temperature at which the storage
304
modulus is minimum.30
305
Crosslinking densities of EUFU-EP/MHHPA and DGEBA/MHHPA resins are
306
found to be 1373 and 2081 mol m-3, respectively. These results are attributed to the
307
fact that EUFU-EP has higher molecular weight, smaller epoxy value and lower
308
reactivity than DGEBA.
309
310
Thermomechanical properties
311
DMA thermograms of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins
312
are shown in Figure 5. It can be seen that EUFU-EP/MHHPA has higher storage
313
modulus (E’) at 50 oC (2229 MPa) than DGEBA/MHHPA, it is expected because E’
314
in the glassy state is affected synthetically by chemical structure and chain packing. It
315
is well known that the use of furan building block usually increases the stiffness of the
316
resins, 39,40 while higher density
317
packing, both of which are good for increasing E’. These two factors exist in
318
EUFU-EP/MHHPA resin. Specifically, on one hand, EUFU-EP/MHHPA resin has
319
rigid rod-like aromatic ester and furan structures. On the other hand,
27
and hydrogen bonding
27
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EUFU-EP/MHHPA has higher density (1.25 g/cm3) than DGEBA/MHHPA resin (1.18
321
g/cm3) reflecting a greater chain packing; moreover, the hydrogen and oxygen atoms
322
of methoxy substituents on the aromatic rings have the ability to form hydrogen bond
323
with carbonyl of anhydride and methylene resulting from the epoxy-anhydride
324
reaction, respectively.41
325
326 327
Figure 5. DMA thermograms for storage modulus and Tan delta against temperature
328
of EUFU-EP/MHHPA and DGEBA/MHHPA resins
329 330
Generally, Tg is regarded as the peak temperature of the tanδ-temperature
331
curve.30, 42 EUFU-EP/MHHPA has higher Tg (153.4 oC) than DGEBA/MHHPA (144.1
332
o
333
Specifically, an increased crosslinking density and a structure with chemical bonds
334
that are hard to rotate can lead to a higher Tg. Compared with DGEBA/MHHPA,
335
EUFU-EP/MHHPA has lower crosslinking density, tending to get reduced Tg; while
C). Tg depends on combined effects of chemical structure and aggregation structure.
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336
EUFU-EP/MHHPA has rigid rod-like aromatic ester and furan structures that are very
337
difficult to rotate. The combined effect of above opposite and positive factors
338
determine the final Tg. Figure 5 suggests that the positive factor plays the domain
339
influence.
340
Note that for previously reported bio-based epoxy resins cured with MHHPA,
341
their Tg values are within 101.9-152 oC (Table S1 in Supplementary Information),
342
meaning that EUFU-EP/MHHPA reported herein shows high Tg that falls in the high
343
end range of resins reported, this is very attractive because Tg is the upper temperature
344
of service for thermosetting resins,43 that is, EUFU-EP has the biggest ability of
345
applying in strict environment requiring high temperature among all bio-based epoxy
346
resins so far.
347
348
Mechanical performance
349
Figure 6 gives flexural properties of EUFU-EP/MHHPA and DGEBA/MHHPA
350
resins. Analogical with storage modulus in the glassy state, flexural modulus reflects
351
the rigidity of resins. The flexural modulus of EUFU-EP/MHHPA is 3.33 GPa, higher
352
than that of DGEBA/MHHPA (3.05 GPa), this is consistent with the storage moduli in
353
the glassy state.
354
The flexural strength is usually used to characterize overall mechanical
355
properties of a material because the flexural loading contains multiple types of
356
loadings
357
EUFU-EP/MHHPA has slightly lower flexural strength than DGEBA/MHHPA, the
such
as
bending,
stretching,
compression,
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etc.
44
Although
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flexural strength of EUFU-EP/MHHPA is as high as 129.2 MPa, comparable with that
359
of reported epoxy resin with high mechanical properties,45 and is at a high level in
360
bio-based epoxy resins reported (Table S1 in Supplementary Information), so
361
EUFU-EP/MHHPA still has excellent mechanical properties.
362
363 364
Figure 6. Flexural moduli and strengths of EUFU-EP/MHHPA and DGEBA/MHHPA
365
resins
366
Flammability and mechanism of cured resins
367
Figure 7 shows curves of heat release rate (HRR) versus temperature, the
368
characteristic parameters including have longer time-to-ignition (TTI), peak heat
369
release rate (PHRR) and total heat release (THR) temperature are listed in this figure.
370
Different from about 70 s shortened TTI in eugenol-based epoxy resins
371
EUFU-EP/MHHPA resin has similar TTI as DGEBA/MHHPA; in addition,
372
EUFU-EP/MHHPA has significantly reduced PHRR and THR compared with
373
DGEBA/MHHPA,
and
the
reduction
is
about
19.0%,
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indicating
6, 24
,
that
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EUFU-EP/MHHPA has better flame retardancy than DGEBA/MHHPA. This
375
attractive result is attributed to the fact that the char yield (Yc) at 800 oC of
376
EUFU-EP/MHHPA resin is 10.9 wt%, about 1.7 times of that of DGEBA/MHHPA
377
(Figure 8). This is benefited from the highly compact aromatic rings of EUFU-EP,
378
which can promote carbonation,6 resulting in more residual carbon, and thus
379
displaying condensed phase mechanism of flame retardation.
380
381 382
Figure 7. Heat release rate-time curves from MCC tests for cured EUFU-EP/MHHPA
383
and DGEBA/MHHPA resins
384
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385 386
Figure 8. TGA curves of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins
387 388
To further investigate the flame retarding mechanism, TG-IR technology was
389
used to directly detect components in the gas phase during degradation process.
390
Figure 9 shows three-dimensional IR spectra of pyrolysis products for
391
EUFU-EP/MHHPA and DGEBA/MHHPA resins throughout the whole thermal
392
degradation process. Since the same size, morphology and weight were taken in tests,
393
the intensity of the absorption peak in the three-dimensional spectra can reflect the
394
amounts of degradation products. Figure 9 shows that the absorption peaks of
395
EUFU-EP/MHHPA resin are much lower than those of DGEBA/MHHPA resin,
396
proving that EUFU-EP /MHHPA resin releases much less pyrolysis products than
397
DGEBA/MHHPA resin.
398
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399 400
Figure 9. Three-dimensional FTIR spectra of pyrolysis products of cured
401
EUFU-EP/MHHPA and DGEBA/MHHPA resins
402 403
Figure 10 shows FTIR spectra of pyrolysis products of EUFU-EP/MHHPA and
404
DGEBA/MHHPA at the maximum decomposition rates. It can be seen that most
405
pyrolysis products of DGEBA/MHHPA are organic combustible gases, including
406
various hydrocarbons (2968 cm-1), carbonyl compounds (1807 cm-1), aromatic
407
compounds (1604 and 1510 cm-1) and C–O–C (1221 and 1176 cm-1);46, 47 while a
408
small amount of water (3656 cm-1), CO2 (2351, 2308 and 669 cm-1) and CO (2180
409
cm-1) were also found in pyrolysis products. Differently, pyrolysis products of
410
EUFU-EP/MHHPA show less organic combustible gases (various hydrocarbons,
411
carbonyl compounds and aromatic compounds) and more none-flammable CO2, this
412
is beneficial to retard further combustion, and thus EUFU-EP/MHHPA resin has a
413
better flame retardancy.
414
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415 416
Figure 10. FTIR spectra of pyrolysis products of cured EUFU-EP/MHHPA and
417
DGEBA/MHHPA resins at their maximum decomposition rates
418
419
Conclusion
420
Starting from green bio-based FDCA and eugenol, a new epoxy resin
421
(EUFU-EP), of which the biomass content is as high as 93.3%, was successively
422
synthesized using an eco-friendly route. Compared with DGEBA/MHHPA,
423
EUFU-EP/MHHPA expresses 9 °C and 19.9% enhancement in Tg and storage
424
modulus at 50 oC, respectively, as well as considerably high mechanical properties
425
and better flame retardancy. Abundant amounts of aromatic and furan structures in the
426
main chain endow cured EUFU-EP resin with much better integrated performances
427
than petrochemical resource-based DGEBA resin. Due to these attractive properties as
428
well as the advantages of non-toxic and renewably source, EUFU-EP holds a great
429
potential as a sustainable alternative for DGEBA. 23
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430
Acknowledgements
431
We thank National Natural Science Foundation of China (21274104), the Priority
432
Academic Program Development of Jiangsu Higher Education Institutions (PAPD),
433
China, and Scientific Innovation Research of College Graduate in Jiangsu Province of
434
China (KYLX16_0120), for financially supporting this project.
435 436
Supplementary data
437
Figure S1 gives 1H NMR spectra of FDCA and FDCDCl, Figure S2 shows synthesis
438
routes of commercial DGEBA, Figure S3 gives 2-D HHCOSY NMR spectra of
439
EUFU and EUFU-EP, Figure S4 gives FTIR spectra of EUFU and EUFU-EP. Figure
440
S5 gives the high-resolution mass spectrum (HRMS) of EUFU-EP, Figure S6 gives
441
DSC curve of EUFU-EP, Figure S7 gives DSC curve of cured EUFU-EP/MHHPA
442
resin, Figure S8 gives FTIR spectra of EUFU and EUFU-EP/MHHPA resin. Table S1
443
summarizes biomass contents and integrated performances of bio-based epoxy resins
444
in literature.
445 446
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447
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Ester Resin with Significantly Reduced Postcuring Temperature While Improved Toughness,
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Rigidity, Thermal and Dielectric Properties Based on Manganese-Schiff Base Hybridized
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Containing Monomer. RSC Adv. 2016, 6 (55), 49633-49642. DOI: 10.1039/C6RA06544A.
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For Table of Contents Use Only
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Brief synopsis
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Novel heat-resistant epoxy resin with precise structure and extremely
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high biomass content was prepared from renewable 2,5-furandicarboxylic
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acid and eugenol.
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Figure 1. Synthesis of EUFU-EP 50x18mm (300 x 300 DPI)
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Figure 2. 1H NMR spectra of EUFU (top) and EUFU-EP (bottom) 66x55mm (300 x 300 DPI)
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Figure 3.
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C NMR spectra of EUFU (top) and EUFU-EP (bottom) 75x71mm (300 x 300 DPI)
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Figure 4. DSC thermograms (a) and fitting curves of exothermic peak temperature vs. heating rate (b) of EUFU-EP/MHHPA and DGEBA/MHHPA prepolymers 119x176mm (300 x 300 DPI)
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Figure 5. DMA thermograms for storage modulus and Tan delta against temperature of EUFU-EP/MHHPA and DGEBA/MHHPA resins 55x37mm (300 x 300 DPI)
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Figure 6. Flexural moduli and strengths of EUFU-EP/MHHPA and DGEBA/MHHPA resins 50x31mm (300 x 300 DPI)
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Figure 7. Heat release rate-time curves from MCC tests for cured EUFU-EP/MHHPA and DGEBA/MHHPA resins 64x51mm (300 x 300 DPI)
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Figure 8. TGA curves of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins 59x44mm (300 x 300 DPI)
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Figure 9. Three-dimensional FTIR spectra of pyrolysis products of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins 57x22mm (300 x 300 DPI)
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Figure 10. FTIR spectra of pyrolysis products of cured EUFU-EP/MHHPA and DGEBA/MHHPA resins at their maximum decomposition rates 63x49mm (300 x 300 DPI)
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Graphic Abstract 35x14mm (300 x 300 DPI)
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