Synthesis and properties of a bio-based epoxy resin from 2,5-furandicarboxylic acid (FDCA)

Article abstract of DOI:10.1039/c5ra00242g

A bio-based epoxy monomer, diglycidyl ester of 2,5-furandicarboxylic acid (DGF) was synthesized for the first time from the renewable 2,5-furandicarboxylic acid (FDCA). For comparison study, its petroleum-based counterpart, diglycidyl ester of terephthalic acid (DGT) was also prepared. Their chemical structures were confirmed in detail by ¹H NMR, ¹³C NMR and FT-IR before they were cured by methylhexahydrophthalic anhydride (MHHPA) and poly(propylene glycol)bis(2-aminopropyl ether) (D230), respectively. The curing behaviors were investigated using differential scanning calorimetry (DSC). The thermal mechanical properties and thermal stabilities of the cured resins were evaluated using dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). Results showed that DGF displayed higher curing activity, elevated glass transition temperature and similar mechanical properties compared with those of the cured DGT. This study indicated that FDCA had a huge potential to replace the petroleum-based terephthalic acid in the synthesis of epoxy resins with satisfactory performance.

Full text of DOI:10.1039/c5ra00242g

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Synthesis and properties of a bio-based epoxy resin
from 2,5-furandicarboxylic acid (FDCA)
Cite this: RSC Adv., 2015, 5, 15930
*
Jun Deng, Xiaoqing Liu, Chao Li, Yanhua Jiang and Jin Zhu
A bio-based epoxy monomer, diglycidyl ester of 2,5-furandicarboxylic acid (DGF) was synthesized for the
first time from the renewable 2,5-furandicarboxylic acid (FDCA). For comparison study, its petroleum-
based counterpart, diglycidyl ester of terephthalic acid (DGT) was also prepared. Their chemical
structures were confirmed in detail by 1H NMR, 13C NMR and FT-IR before they were cured by
methylhexahydrophthalic anhydride (MHHPA) and poly(propylene glycol)bis(2-aminopropyl ether)
(D230), respectively. The curing behaviors were investigated using differential scanning calorimetry
(DSC). The thermal mechanical properties and thermal stabilities of the cured resins were evaluated
using dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). Results showed that
DGF displayed higher curing activity, elevated glass transition temperature and similar mechanical
properties compared with those of the cured DGT. This study indicated that FDCA had a huge potential
to replace the petroleum-based terephthalic acid in the synthesis of epoxy resins with satisfactory
performance.
Received 6th January 2015
Accepted 27th January 2015
DOI: 10.1039/c5ra00242g
www.rsc.org/advances
2,5-Furandicarboxylic acid (FDCA), which can be derived
from polysaccharides or sugars through the oxidation of
hydroxymethylfurfural (HMF),10,11 has been identied as one of
1 Introduction
The conversion of biomass into useful polymeric materials has
been regarded as one of the effective methods to save fossil
resources and protect our living environment. According to
the origin and production of bio-based polymers, it could
be divided into three main categories: natural polymers,
synthesized bio-based polymers and those produced by micro-
organisms or bacteria.1 Undoubtedly, the synthesized bio-based
polymers are most popular and promising. A large quantity
of polyesters,2–7 such as polylactic acid (PLA), poly-
(hydroxyalkanoates) (PHAs), poly(butylene succinate) (PBS) and
polyethylene (PE) has been synthesized from biomass and
commercialized successfully already. However, compared with
the well-known engineering plastics, such as poly(ethylene
terephthalate) (PET), polycarbonate (PC) and diglycidyl ether of
biphenyl A (DGEBA), their mechanical and thermal properties
are still subjects to be improved upon for the applications of
structural or engineering materials. Based on the chemical
structure comparison in Scheme 1, the lack of aromatic or rigid
ring structure in current bio-based polymers' architectures
might be the reason for their relatively weak properties.
Therefore, the exploration of aromatic or rigid compounds
derived from renewable resources should be signicant for the
next generation of bio-based polymers.8,9
top 12 potential platform molecules likely to play an important
role in establishing the green chemistry industry by the US
department of Energy.12 Due to its structure similarity to that of
terephthalic acid (TPA) and rapid progress on large scale
production technology, FDCA might be one of the most prom-
ising bio-based compounds for TPA substitute. In fact, more
and more explorations on the synthesis of polyester,13–17 poly-
amide18–20 and polyurethane21–24 containing FDCA units have
been reported. FDCA has shown a huge potential in the
synthesis of bio-based plastics with enhanced properties.
For instance, including the comparable or even better thermo-
mechanical properties, the barrier properties with regard
to oxygen and carbon dioxide permeability of poly(ethylene
2,5-furandicarboxylate) (PEF) were much better than those of
PET.25
Epoxy resin has been widely used in the eld of aerospace,
coatings, adhesives and composites due to its excellent prop-
erties. Nowadays, the dominant commercial epoxy resin,
diglycidyl ether of bisphenol
A (DGEBA) was produced
completely from fossil resource and the toxicity associated with
bisphenol A has been recognized extensively.26 In addition,
compared with the activities and achievements on bio-based
thermoplastics, researches on the subject of bio-based ther-
mosetting resins need much more attention. Considering the
concept of bio-based polymers and their development tendency,
the thermosetting resins derived from biomass, especially the
high-performance ones, should also have a bright future.
Ningbo Key Laboratory of Polymer Materials, Ningbo Institute of Material Technology
and Engineering, Chinese Academy of Sciences, Ningbo 315201, China. E-mail: liuxq@
nimte.ac.cn; Fax: +86-574-86685925
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Scheme 1 Chemical structures of current popular bio-based polymers and petroleum-based engineering plastics.
Up to now, several bio-based recourse, such as plant oils,27–33
2 Experimental
2.1 Materials and reagents
lignin,34,35 rosin,36–39 starch,40 sucrose,41 gallic acid42,43 and iso-
sorbide44 have been employed to prepare the bio-based
epoxies.45–47 However, the resulting epoxy resins usually
2,5-Furandicarboxylic acid was purchased from Sichuan Dag-
aote Technology Co., Ltd, China. Allyl bromide, m-chloroper-
oxybenzoic acid (m-CPBA) (85%) and poly(propylene glycol)-
bis(2-aminopropyl ether) D230 were obtained from Aladdin
Reagent, China. 2-Ethyl-4-methylimidazole, methylhexahy-
drophthalic anhydride (MHHPA), epichlorohydrin (ECH) and
terephthalic acid (TPA) were obtained from Sinopharm Chem-
ical Reagent Co., Ltd, China. All the chemicals were used as
received.
exhibited lower glass transition temperature,48 poor mechanical
properties,31 slow curing reaction or unstable properties49 due to
the structures' defects of the bio-based compounds. For
example, the epoxy derived from plant oil or fatty acid usually
showed poor mechanical and thermal properties because of its
long so aliphatic chain.50 How to stabilize the properties of
lignin-based epoxy is still a challenge in front of us. The severe
brittleness of epoxy derived from gallic acid and rosin51,52
should gain much more attention. Apparently, the platform
building blocks selection and structure design are key factors to
determine the properties of bio-based synthetic epoxy resins.
Based on its unique structures and encouraging perfor-
mance in the synthesis of bio-based thermoplastics, the utili-
zation of FDCA might be a promising start to the bio-based
epoxy with satised properties. In this study, diglycidyl ester of
2,5-furandicarboxylic acid (DGF) and its petroleum-based
counterpart diglycidyl ester of terephthalic acid (DGT)
(Scheme 2) were synthesized. In order to compare their
performance and study the possibility of FDCA for high
performance epoxy synthesis, their curing reactions with
MHHPA and D230 as well as the properties of the cured resins
in terms of thermal stabilities, mechanical and thermal prop-
erties were investigated.
2.2 Preparation of bis(prop-2-enyl)furan-2,5-dicarboxylate
(FDCE)
FDCA (50 g, 0.32 mol), N,N-dimethylformamide (DMF, 140
mL), allyl bromide (143 g, 1.18 mol), acetone (150 mL), tri-
ethylamine (73 g, 0.72 mol) were placed in a round-bottomed
ask equipped with a magnetic stirrer, a thermometer and a
reux condenser. The mixture was heated and reuxed for 24
h aer the solid was dissolved completely in the solution.
Then the salt precipitate was removed via ltration before the
acetone, unreacted allyl bromide and triethylamine were
removed from the ltrate by a rotary evaporator. In order to
get rid of DMF in the raw product, the solution was washed
by ionized water several times aer it was diluted with
dichloromethane. Aer that, dichloromethane was removed
before it was dried with anhydrous magnesium sulphate
(MgSO4). At last, a yellow solid product weighing (62 g,
82%) was obtained. The synthetic scheme is presented in
Scheme 3.
FT-IR (KBr, cmꢀ1): 3085, 979 and 931 cmꢀ1 (]CH2), 2946
cmꢀ1 (–CH2–), 1713 cmꢀ1 (C]O), 1650 cmꢀ1 (–C]C–), 1287
cmꢀ1 (–C–O–), 1578, 1223, 1018, 979, 825, and 765 cmꢀ1 (furan
ring C]O and –C–O–). 1H NMR (400 MHz, acetone-d6, d, ppm):
7.37 (s, 2H), 6.06 (m, J1 ¼ 22.7 Hz, J2 ¼ 10.9 Hz, J3 ¼ 5.72 Hz, 2H),
5.44 (m, 2H), 5.30 (d, J ¼ 10.5 Hz, 2H), 4.84 (d, J ¼ 5.6 Hz, 4H).
13C NMR (400 MHz, acetone-d6, d, ppm): 157.2 (2C), 146.7 (2C),
132.1 (2C), 118.6 (2C), 118.1 (2C), 65.5 (2C).
Scheme 2 Chemical structures of DGT, DGF, MHHPA and D230.
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before it was washed with deionized water ve times. The
organic layer was dried with anhydrous MgSO4 and concen-
trated. At last, the white powder weighting (90 g, 80%) was
obtained.
FT-IR (KBr, cmꢀ1): 3063 cmꢀ1 (benzene ring ]C–H), 3010
cmꢀ1 (oxirane ring C–H), 2959 and 2930 cmꢀ1 (oxirane ring
C–H), 1716 cmꢀ1 (C]O), 1580 and 1500 cmꢀ1 (benzene ring
C]C), 1280 cmꢀ1 (ester C–O), 1250 and 1073 cmꢀ1 (oxirane ring
C–O–C), 1115 and 980 cmꢀ1 (benzene ring C–H), 897 cmꢀ1
(the breathing absorption of oxirane ring), 864 cmꢀ1 (oxirane
ring C–O–C), 829 (benzene ring 1,4-disubstituted C–H). 1H NMR
(400 Hz, CDCl3-d6, d, ppm) 8.16 (d, J ¼ 20.8 Hz, 4H), 4.72 (d, J ¼
3.0 Hz, 2H), 4.70 (d, J ¼ 3.0 Hz, 2H), 4.20 (m, J1 ¼ 12.3 Hz, J2 ¼
6.4 Hz, 2H), 3.38 (m, J1 ¼ 6.5 Hz, J2 ¼ 2.9 Hz, 2H), 2.93 (t, J ¼ 4.5
Hz, 2H), 2.76 (m, J1 ¼ 4.8 Hz, J2 ¼ 2.6 Hz, 2H); 13C NMR
(400 MHz, CDCl3-d6, d, ppm): 165.4 (2C), 133.7 (2C), 129.7 (4C),
65.9 (2C), 49.3 (2C), 44.7 (2C).
Scheme 3 Synthesis procedure of DGF from FDCA.
2.5 Curing procedure
2.3 Preparation of DGF
DGF or DGT was cured with MHHPA in a 1 : 1 equivalent ratio,
together with 2-ethyl-4-methylimidazole (0.5 wt% on the basis
of the total weight of curing agent and epoxy). A small amount
of dichloromethane was added to dissolve the solid epoxy and
MHHPA powder before it was stirred for 30 min at room
temperature to get a homogeneous system. The mixture was
placed in a vacuum oven at 30 ꢁC for 2 h to get rid of the solvent
and then it was transferred to a mold with uniform cavity size
before curing at 120 ꢁC for 2 h, 150 ꢁC for 2 h and 180 ꢁC for 2 h.
DGF or DGT was formulated with D230 in the same conditions,
mixed in a 1 : 1 equivalent ratio. The mixture was cured at
100 ꢁC for 2 h, 130 ꢁC for 2 h and 150 ꢁC for 2 h to achieve fully
cured polymers. All the samples were cured under the same
conditions and carefully removed from the mold for properties
study.
A
three-necked round-bottomed ask equipped with a
mechanical stirrer, a thermometer and a reux condenser was
charged with a solution of FDCE (60 g, 0.25 mol), dichloro-
methane (450 mL) and m-CPBA (110 g, 0.63 mol). The reactants
were mixed and kept at 40 ꢁC for 3 days. Aer that, the mixture
was ltered and washed with a solution of 10% sodium sulte
followed by 10% sodium carbonate and distilled water. The
organic part was dried with anhydrous MgSO4 and concentrated
by a rotary evaporator. The raw product was washed by ether
several times until it became white. The resultant product
weighting (50 g, 72%) was obtained aer recrystallization. The
synthetic route is shown in Scheme 3.
FT-IR (KBr, cmꢀ1): 3151 and 3109 cmꢀ1 (furan ring ]C–H),
3013 cmꢀ1 (oxirane ring C–H), 2949 cmꢀ1 (–O–CH2–), 1722 cmꢀ1
(C]O), 1575, 1447 and 1343 cmꢀ1 (furan ring C]C and its
breaking absorption), 1293 cmꢀ1 (ester C–O), 1267 and 1075
cmꢀ1 (furan ring C–O), 1232 and 866 cmꢀ1 (oxirane group
C–O–C), 909 cmꢀ1 (the breathing absorption of oxirane ring),
986, 815 and 766 cmꢀ1 (2,5-disubstituted in furan ring). 1H
NMR (400 MHz, CDCl3, d, ppm): 7.27 (s, 2H), 4.67 (d, J ¼ 3.2 Hz,
H), 4.64 (d, J ¼ 3.2 Hz, H), 4.20 (m, J1 ¼ 12.2 Hz, J2 ¼ 6.3 Hz, 2H),
3.34 (m, J1 ¼ 6.4 Hz, J2 ¼ 3.1 Hz, 2H), 2.90 (t, J ¼ 4.5 Hz, 2H), 2.73
(m, J1 ¼ 18.1 Hz, J2 ¼ 9.0 Hz, 2H); 13C NMR (400 MHz, CDCl3, d,
ppm): 157.5 (2C), 146.5 (2C), 118.9 (2C), 65.9 (2C), 49.0 (2C),
44.8 (2C).
2.6 Characterization
Fourier transform infrared (FT-IR) spectra were recorded on a
Thermo Nicolet 6700 Fourier transform infrared spectrometer
from Thermo-Fisher Scientic, scanning from 400 to 4000 cmꢀ1
and 32 scans were collected for each sample. NMR spectrum
was recorded on a Bruker 400 AVANCE III spectrometer oper-
ating at 400 MHz. It was conducted at 25 ꢁC with the solvent of
CDCl3 or acetone-d6.
The DSC measurement was conducted on a METTLER
TOLEDO-TGA/DSC I under a nitrogen atmosphere. Approxi-
mately 5–10 mg of each sample (epoxy monomer mixed with
curing agent) was weighed and sealed in 40 mL aluminum
2.4 Preparation of DGT
TPA (66.4 g, 0.4 mol), epichlorohydrin (ECH, 600 g, 6.48 mol) crucibles. The samples were heated from 25 ꢁC to 200 ꢁC at
and cetyl trimethyl ammonium chloride (CTMAC, 3.32 g, 0.01 heating rates of 5, 10, 15 and 20 ꢁC minꢀ1, respectively. The
mol) were added into a three-necked round-bottomed ask curves of heating ow as a function of time were recorded for
equipped with a magnetic stirrer, a thermometer and a reux activation energy analysis. DSC analysis for each sample was
condenser. With continuous stirring, the mixture was heated to repeated twice. TGA measurement was conducted on a Mettler-
90 ꢁC and maintained at this temperature for 3 hours. Then, the Toledo TGA/DSC1 thermogravimetric analyzer (METTLER
reaction was cooled to 30 ꢁC and 100 g aqueous solution of TOLEDO, Switzerland) with hꢀig1h purity nitrogen as purge gas at
ꢁ
ꢁ
sodium hydroxide (40 wt%, 1 mol) was added drop by drop. a scanning rate of 20 C min from 50 to 800 C. The sample
ꢁ
Aer that, the reaction was continued at 30 C for another 6 h weighted about 5 mg was applied for each test. Degradation
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temperatures at 5% (Td5) and 30% (Td30) weight loss and the respectively. No signicant absorption in the –COOH stretching
ꢁ
char yield at 800 C (R800) were recorded for the cured resins. region was detected, which conrmed a high reaction yield
DMA was performed on Mettler-Toledo DMA/SDTA861e under a between FDCA and allyl bromide. Aer FDCE was oxidized by m-
three points bending mode at a frequency of 1 Hz. The test CPBA, the characteristic peaks standing for the terminal C]C
samples wꢀer1e scanned from 25 ꢁC to 200 ꢁC at a heating rate of bonds and C]CH2 stretching absorption shown at 1650, 979
3
ꢁC min
and the glass transition temperature (Tg) was and 931 cmꢀ1 were all disappeared. At the same time, the
dened as the peak temperature of tan d curves. For each appearance of oxirane ring was indicated clearly by the arising
sample, multiple tests were performed in order to ensure the peaks at 3013, 1232, 866 and 909 cmꢀ1, reecting the stretching
accuracy.
of –CH2–, the stretching of C–O–C in oxirane group and the
The mechanical properties of the cured resins were carried breathing absorption peak in oxirane group. The characteristic
out on an Instron 5567 Electric Universal Testing Machine absorption bonds for the presence of oxirane ring and benzene
(Instron, America). The tensile properties of the specimens with ring of DGT were also detected in Fig. 1. Based on the FT-IR
the gauge length of 50 mm were measured at a speed of analysis, the structures of DGF and DGT could be conrmed
10 mm minꢀ1 according to GB/T 1040.2-2006. The exural preliminary. In order to make a further structure conrmation,
strength was evaluated by means of a three-point bending test at the NMR spectrum of FDCE, DGF and DGT were shown in Fig. 2
a speed of 2 mm minꢀ1 and ve replicates were tested for each and 3.
sample to obtain an average value.
In Fig. 2(a), the peaks between 4.5 and 6.5 ppm indicated
that the allyl groups were introduced into FDCA with high purity
and peak at 7.37 ppm was attributed to the furan ring's protons.
In Fig. 2(b), the peaks at 2.73, 2.90 and 3.34 ppm represented
the protons He, Hd and Hc in the ethylene oxide ring. The
characteristic peaks for protons Ha and Hb were appeared at
4.67, 4.64, 4.20, 4.17 ppm, respectively. This complicated
chemical shis pattern for the protons is due to the presence of
chiral carbon in the oxirane ring.37 In Fig. 2(c), the characteristic
peaks for the protons in DGT were also assigned and the
chemical shis data were very much in line with the theoretical
ones.
3 Results and discussion
3.1 Chemical characterization of DGT and DGF
It is well known that there are two alternative procedures for the
synthesis of epoxy resins. For the direct di-glycidylation, it is
difficult for us to control the reaction accurately so as to obtain
the products with specic chemical structures. Usually, the
complicated oligomers with different degree of polymerization
and epoxy values will be obtained and it is almost impossible to
make the purication or separation. In this study, in order to
compare the properties of epoxies derived from bio-based FDCA
(DGF) and petroleum based terephthalic acid (DGT), the two
step procedure was selected and it was shown in Scheme 3.
The chemical structures of bis(prop-2-enyl)furan-2,5-
dicarboxylate (FDCE), DGF and DGT were determined by
means of FT-IR, 1H NMR and 13C NMR spectroscopy. The typical
FT-IR spectrum of FDCE, DGF and DGT were presented in Fig. 1.
For the spectrum of FDCE, including the characteristic
Fig. 3 is the 13C NMR spectra of FDCE, DGF and DGT. In
Fig. 3(a), peaks at 132.1, 118.1 and 65.5 ppm represented the
appearance of C3, C5 and C6 in the allyl group. The spectrum of
DGF showed the disappearance of double bonds and the
appearance of carbon atoms on the glycidyl epoxides at 65.9,
49.0 and 44.8 ppm in Fig. 3(b). This also indicated the oxi-
dization of double bonds into oxirane rings. In Fig. 3(c), the
spectrum of DGT displayed similar information to that of DGF.
Therefore, it can be concluded that the result of the synthesis
was indeed as expected.
absorption bonds of furan rings (1578, 1223, 1018, 979, 825, 765
cmꢀ1
)
the peaks arising from C]O stretching vibration and
53
C–O stretching were appeared at 1713 cmꢀ1 and 1287 cmꢀ1
,
3.2 Curing behavior and curing activity
Fig. 4 is the DSC thermograms of DGT/MHHPA, DGF/MHHPA,
DGT/D230 and DGF/D230 systems at a heating rate of
5 ꢁC minꢀ1. All the systems displayed a single exothermic peak
which was attributed to the ring opening reaction of epoxy
groups in DGF and DGT with amino groups in D230 and
anhydride group in MHHPA. The peak temperatures of curing
curves might be taken as an indicator for the curing reactivity
comparison of epoxy resin and curing agent.54 The higher the
temperature of the peak is, the lower the reactivity is. Obviously,
the peak temperatures of DGF systems were lower than those of
DGT systems under the same curing conditions, no matter what
kind of the curing agent was applied, MHHPA or D230. This
result indicated that the reactivity of DGF was higher than that
of DGT.
The curing reactivity of epoxy was greatly dependent on the
heating rate and the shi of curing temperature with heating
Fig. 1 The FT-IR spectrum of FDCE, DGF and DGT.
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Fig. 2 1H NMR of FDCE (a), DGF (b) and DGT (c).
Fig. 3 13C NMR spectra of FDCE (a), DGF (b) and DGT (c).
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ln q ¼ ꢀ1.052 ꢂ Ea/RTp + ln(AEa/R) ꢀ ln F(x) ꢀ 5.331 (2)
here F(x) is a conversion dependent term. Therefore, the Ea
values are determined from the slopes of the plots of ꢀln q
versus 1/Tp.
Fig. 6 shows the plots of ꢀln(q/Tp2) versus 1/Tp based on
Kissinger's equation (a) as well as ln q versus 1/Tp based on
Ozawa's theory (b) for the DGT/MHHPA and DGF/MHHPA
systems. Table 1 summarized the results and showed that
Fig. 4 DSC thermograms of DGT/MHHPA, DGF/MHHPA, DGT/D230
and DGF/D230 systems at a heating rate of 5 ꢁC minꢀ1
.
rate was a typical methodological phenomenon for non-
isothermal curing. Therefore, the curing behaviors of DGF/
DGT cured with MHHPA at different heating rates were inves-
tigated and both Kissinger's55 and Ozawa's56 methods were
employed to calculate their activation energy (Ea) for accurate
results. Fig. 5 shows the DSC curves of DGF and DGT cured with
MHHPA at different heating rates. All the systems displayed
single exothermic peaks with the different heating rates and the
peak temperatures were increased with the accelerated heating
rates.
Curing activation energy was calculated based on the Kis-
singer's eqn (1):
ꢀln(q/Tp2) ¼ Ea/RTp ꢀ ln(AR/Ea)
(1)
here Tp is the peak exothermic temperature, q is the constant
heating rate, Ea is the activation energy of the curing reaction
and R is the gas constant with a value of 8.314 J molꢀ1 Kꢀ1. The
Ea values are determined from the slopes of the plots of
ꢀln(q/Tp2) versus 1/Tp. Ozawa's methods can also be introduced
to describe curing activation energy by eqn (2):
Fig. 6 Linear plot of ꢀln(q/Tp2) versus 1/Tp based on Kissinger's
equation (a) as well as ln q versus 1/Tp based on Ozawa's theory (b).
Fig. 5 Typical DSC thermograms of DGF/MHHPA (a) and DGT/MHHPA (b) at different heating rate (5 ꢁC minꢀ1, 10 ꢁC minꢀ1, 15 ꢁC minꢀ1
,
20 ꢁC minꢀ1).
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Table 1 Activation energy (Ea) for the different systems determined by
Kissinger and Ozawa methods
the presence of furan ring structure in DGF systems. In Fig. 8(b),
a sharp loss factor (tan d) peak was noted in all cases and Tg is
determined by the peak temperature of the tan d curve. Appar-
ently, the same trend was also notable where the DGF systems
demonstrated signicantly higher Tg than those of the DGT
systems when cured with the same agents. The Tg data for the
different samples were collected in Table 2. The glass transition
temperature of epoxy networks is mainly dependent on its
crosslink density and the chemical structure of the chain
segment. The following equation for rubbery elasticity was
employed here to calculate the crosslink density (ve) of the
different cured systems:
Ea (kJ molꢀ1
Kissinger
58.82
)
Heatingꢀr1ates
Peak tem.
(ꢁC)
Samples
(ꢁC min
)
Ozawa
62.43
DGF/MHHPA
5
10
15
20
5
10
15
20
126.6
141.7
150.3
156.7
135.6
149.1
157.7
163.5
DGT/MHHPA
66.39
69.78
E0 ¼ 3veRT
(3)
where E0 is the storage modulus of the thermosetting resin in
the rubbery plateau region at Tg + 30 C, R is the gas constant,
DGF exhibited a little lower activation energy values compared
with DGT, which indicated DGF has a higher curing reactivity.
The synthesized DGF and DGT were cured by the anhydride-
based curing agent (MHHPA) and amine-based curing agent
(D230), respectively. And the commonly used curing procedures
(120 ꢁC for 2 h, 150 ꢁC for 2 h plus 180 ꢁC for 2 h for DGF/
MHHPA and DGT/MHHPA systems; 100 C for 2 h, 130 C for
2 h plus 150 ꢁC for 2 h for DGF/D230 and DGT/D230 systems) for
the similar systems were employed. In order to make sure that
the samples were cured fully under this condition, the cured
resins were applied to the DSC measurement (Fig. 7). Obviously,
there were no exothermic peaks detected in the scanning curves
at higher temperature, which veried the full curing process.
ꢁ
and T is the absolute temperature. The calculated ve were shown
in Table 2. Obviously, when DGF and DGT were cured with the
same curing agents, the resulting resins have indistinguishable
crosslink density due to their similar chemical structure
volume. However, the remarkable difference between DGF and
DGT is the presence of oxygen atoms in furan rings, which could
form the intermolecular hydrogen bond with hydroxyl groups
generated during the curing reaction. This result have been
conrmed by the near-infrared spectra (N-IR) in the previous
literatures.9 In addition, the rotation of ve-membered furan
ring structure in DGF was deemed to be more difficult
compared with the six-membered benzene rings in DGT.
Therefore, the higher Tg and higher storage modulus in DGF
systems were reasonable.
ꢁ
ꢁ
3.3 Dynamic mechanical properties of the epoxy networks
The dynamic mechanical analysis (DMA) was used to determine
the glass transition temperature of the cured systems. Fig. 8 is
the DMA plots of DGF and DGT cured with MHHPA and D230.
As shown in Fig. 8(a), at a room temperature, DGF cured with
MHHPA or D230 exhibited higher storage modulus when
compared with that of DGT cured with the same curing agent. It
is reported several times that the use of furan building block
usually increases the stiffness of the resins.9,57,58 The relatively
higher storage modulus of cured DGF systems might be due to
3.4 Mechanical performance
The chemical structures of epoxy monomers and curing agents
have signicant inuences on morphology and crosslink
density of the cured networks, affecting their mechanical
properties. The mechanical properties of cured DGF and DGT
systems were tested by the Instron universal testing machine
and the data were listed in Table 2. During the tests, all of the
samples demonstrated the typical brittle fractures similar to
those of the common thermosets. For the DGF/MHHPA system,
the tensile strength, tensile modulus, exural strength and
exural modulus (84 MPa, 3000 MPa, 96 MPa and 3100 MPa)
were all similar or comparable to those of the DGT/MHHPA
system (78 MPa, 3080 MPa, 90 MPa and 2950 MPa). When the
more exible curing agent D230 was applied, the predictable
lower mechanical properties were shown and no signicant
difference was detected between DGF/D230 and DGT/D230
system. Besides compared with those of DGT, the mechanical
properties of DGF/MHHPA and DGF/D230 have also been con-
trasted with the common diglycidyl ether of bisphenol A
(DGEBA with the epoxide equivalent weight of 182–192 g eq.ꢀ1
)
cured with the same curing agents. As shown in Table 2, the
FDCA based systems possessed the similar or comparable
thermal and mechanical properties to those of the DGEBA
based systems. According to above mechanical performance
investigation and comparison, FDCA demonstrated a big
Fig. 7 Heating DSC curves for the different cured resins.
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Fig. 8 DMA curves of storage modulus versus temperature (a) and tan d versus temperature (b) for DGF/MHHPA, DGF/D230, DGT/MHHPA and
DGT/D230.
chance to serve as the TPA substitution in the synthesis of epoxy
resins with satised properties.
3.5 Thermal properties
The thermal stability of cured epoxies in nitrogen was investi-
gated by TGA and the typical curves for their thermal degrada-
tion behaviors were shown in Fig. 9. The values of the
degradation temperature for 5% weight loss (Td5) and 30%
weight loss (Td30) as well as the residual weight percent at 800 ꢁC
(R800) are presented in Table 3. In order to make a quantitative
comparison of thermal stability, the statistic heat-resistant
index (Ts) determined by the following equation was
applied (4):60,61
Fig. 9 TGA curves of cured systems under N2 with a heating rate of
20 ꢁC minꢀ1
.
Ts ¼ 0.49[Td5 + 0.6(Td30 ꢀ Td5)]
(4)
here, Ts is a popular index to evaluate the thermal stability of the
cured resins. From Table 3, the Ts values of DGF/D230 and DGF/
MHHPA systems were all similar to those of DGT/D230 and
DGT/MHHPA systems, respectively. That indicated a similar or
comparable thermal stability between DGF and DGT. Their
remarkable difference we should pay attention to was the char
residues at 800 ꢁC under the nitrogen atmosphere. The R800
values of cured DGF were lower than those of DGT when the
same curing agent was used. The reason might be that the
oxygen atom in furan ring was nally turned into CO2 which
Table 3 Thermal properties of the cured epoxy resins
Sample
Td5 (ꢁC)
Td30 (ꢁC)
R800 (%)
Ts
DGF/D230
DGT/D230
DGF/MHHPA
DGT/MHHPA
267
266
293
284
320
322
381
378
6.8
8.2
1.9
5.2
146
147
169
167
Table 2 Thermal and mechanical properties of the cured resinsa
DMA
Tensile (MPa)
Flexural (MPa)
Strength Modulus
Sample
Tg (ꢁC)
E0(MPa) Tg + 30 ꢁ
C
ve (mol mꢀ3
)
Strength
Modulus
DGF/MHHPA
DGT/MHHPA
DGF/D230
DGT/D230
DGEBA/MHHPA
DGEBA/D230
152
22
18
9
6
NA
NA
1930
1690
893
751
NA
84 ꢃ 4
78 ꢃ 2
68 ꢃ 3
64 ꢃ 2
68
3000 ꢃ 50
3080 ꢃ 80
2700 ꢃ 110
2800 ꢃ 60
2900
96 ꢃ 3
90 ꢃ 5
75 ꢃ 2
73 ꢃ 3
135
3100 ꢃ 110
2950 ꢃ 40
2500 ꢃ 90
2400 ꢃ 100
3400
128.8
101.2
91.8
125
97
NA
NA
NA
121
2950
a
Data for the DGEBA/MHHPA and DGEBA/D230 was obtained from the ref. 52 and 59.
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