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for 5 min to eliminate the thermal history, and then the temp-
erature was cooled to 25 °C at a rate of 50 °C min−1, and kept
at 25 °C for another 5 min, and finally heated to 250 °C at a
3. Results and discussion
3.1. Synthesis and characterization of FAE and GEFA
rate of 10 °C min−1 for the second heating scan. The glass In this work, ferulic acid-based epoxy (FAE) was synthesized
transition temperature (Tg) was obtained from the peak temp- via a heavily used two-step reaction (ring-open and ring-close)
erature of the differential curve of the second heating curve of with epichlorohydrin (Scheme 1). It is worth noting that epoxi-
the cured samples. The non-isothermal curing kinetics of dation of phenolic hydroxyl and carboxyl groups was carried
FAE-GEFA systems were studied by DSC, and Kissinger’s out simultaneously in one pot, which manifests that there is
method (eqn (1))38 and Ozawa’s method (eqn (2))39 were used no need to extract intermediate products during the reaction.
to calculate the apparent activation energy during the curing A similar epoxidation process of gallic acid, a monomer with a
process.
phenolic hydroxyl group and carboxylic acid, was reported by
Aouf et al.43 They first allylated gallic acid, and then the result-
ing product reacted with meta-chloroperbenzoic acid to afford
the epoxy monomer. The whole process is complicated. In con-
trast, our synthesis route is simple and efficient.
2
ꢀln ðq=Tp Þ ¼ Ea=RTp þ ln ðAR=EaÞ
ð1Þ
ln q ¼ ꢀ1:052 ꢁ Ea=RTp þ ln ðAEa=RÞ ꢀ ln FðxÞ ꢀ 5:331 ð2Þ
FTIR, 1H NMR, 13C NMR and TOF MS spectra were recorded
to identify the chemical structure of FAE. Fig. 2a presents the
1H NMR spectrum of FAE. The peaks at around 2.68 ppm,
2.84 ppm, 3.26 ppm and 3.35 ppm belong to the six protons of
the two epoxy groups, and the peaks at around 3.85 ppm,
3.94 ppm, 4.37 ppm and 4.52 ppm correspond to the four
protons of the –CH2– next to the epoxy groups. The peaks at
around 7.00 ppm, 7.26 ppm, and 7.41 ppm represent the three
protons on the benzene ring, the peak at 3.83 ppm belongs to
the three protons of the –OCH3 on the benzene ring, and the
peaks at 6.66 ppm and 7.65 ppm are ascribed to the two
protons of –CHvCH– conjugated with the benzene ring.
Besides, the integral areas of all the peaks are matched well
with the chemical structure of FAE. With the interest of con-
firming the chemical structure of FAE, 13C NMR, FTIR and
TOF MS spectra were also recorded. Both the chemical shifts
and the number of peaks in the 13C NMR spectrum (Fig. 2b)
are in agreement with the chemical structure of FAE; the
characteristic peak of the epoxy group appears at 910 cm−1 in
the FTIR spectrum of FAE (Fig. 2c); and the TOF MS spectrum
in Fig. 2d presents a molecular ion peak of FAE at m/z 307.12,
corresponding to its molecular weight of 306 g mol−1 for FAE.
These results indicate that the target compound FAE has been
successfully synthesized. Similarly, we used a similar method
to synthesize glycidyl ether of furfuryl alcohol (GEFA)
(Scheme 2), and its chemical characterization was also deter-
mined by 1H NMR, 13C NMR and GC-MS spectra (Fig. S1†).
where q is the heating rate, Tp is the peak exothermic tempera-
ture, Ea is the average activation energy of the curing reaction,
A is the pre-exponential factor, R is the gas constant, and F(x)
is a conversion-dependent term. Thermogravimetric analysis
(TGA) was performed with a Mettler-Toledo TGA/DSC thermo-
gravimetric analyzer (Switzerland) at a heating rate of 20 °C
min−1, from 50 °C to 800 °C, under a nitrogen atmosphere.
Dynamic mechanical analysis (DMA) was carried out on a TA
Instruments Q800 DMA system in tension mode to measure
the dynamic mechanical properties of the thermosetting plas-
tics. The thermosetting plastics with dimensions of 25 mm
(length) × 6 mm (width) × 1 mm (thickness) were prepared and
tested from −50 °C to 300 °C at a heating rate of 3 °C min−1
and a frequency of 1 Hz. For the gel content measurement,
four groups of the cured FAE-GEFAs (around 200 mg per
group) were placed in a Soxhlet extractor and extracted with
acetone for 60 h, and then taken out and placed in a vacuum
oven at 60 °C for 12 h. The original mass is recorded as m, the
final mass after extraction is recorded as m′, and the gel
content is calculated using m′/m. The theoretical cross-link
density of the cured epoxy was estimated by calculating the
average molecular weight between the cross-link points (Mc)
using eqn (3) and (4).26,40
nFAE ꢁ MFAE þ nDDS ꢁ MDDS
MC1
¼
ð3Þ
ð4Þ
nDDS
1
The chemical shift of proton peaks and integral area in the H
nDGEBA ꢁ MDGEBA þ nDDS ꢁ MDDS
NMR spectrum (Fig. S1a†) and the number of peaks and
chemical shifts in the 13C NMR spectrum (Fig. S1b†) are all in
agreement with the target chemical structure of GEFA. The
GC-MS spectrum in Fig. S1c† presents a molecular ion peak of
GEFA at m/z 154.1, corresponding to its molecular weight of
154 g mol−1 for GEFA.
The appearance, melting point of FAE and viscosity of the
melted FAE at temperatures below the melting point are illus-
trated in Fig. 3. As illustrated, the melting point of FAE by DSC
is 104 °C (Fig. 3a). For the most solid epoxy, their melted
MC2
¼
nDDS
where n and M represent the molarity and molar mass of the
corresponding component in the epoxy formulations.
Meanwhile, the actual cross-link density (νe) of the cured
epoxies was calculated using eqn (5) based on the data
obtained by DMA.41,42
E′ ¼ 3νeRT
ð5Þ
where E′ is the storage modulus of the samples in the rubbery samples would return to the solid state immediately after
plateau region at Tg +50 °C, R is the gas constant, and T is the cooling to the temperatures below their melting points.
absolute temperature.
Interestingly, when FAE was melted from solid to liquid, its
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Green Chem., 2021, 23, 1772–1781 | 1775