Study of green epoxy resins derived from renewable cinnamic acid and dipentene: Synthesis, curing and properties

Article abstract of DOI:10.1039/c3ra47927g

An epoxy based on cinnamic acid (Cin-epoxy) and an anhydride curing agent based on dipentene were prepared. Both products are liquids of low viscosity at room temperature. For the synthesis of the epoxy, cinnamic acid was first converted to a diacid by reacting with maleic anhydride via Friedel-Crafts reaction, followed by allylation of the carboxylic groups and subsequent epoxidation of the allyl double bonds. The curing agent was the Diels-Alder adduct of dipentene and maleic anhydride (DPMA). The chemical structures of Cin-epoxy and DPMA were confirmed by ¹H NMR, ¹³C NMR, FT-IR and ESI-MS. Non-isothermal curing of Cin-epoxy was studied using differential scanning calorimetry (DSC). In addition to DPMA, two commercial anhydrides were also used to cure Cin-epoxy and the curing reactivity and properties of cured resins were compared. Thermal mechanical properties and thermal stability of the cured epoxy resins were studied using dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA), respectively. Results showed that Cin-epoxy was slightly more reactive than the bisphenol A type epoxy DER 332 and displayed good dynamic mechanical properties and thermal stability. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra47927g

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Page 1 of 22
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Study of green epoxy resins derived from renewable cinnamic acid and
dipentene: Synthesis, curing and properties
Junna Xin,1 Pei Zhang,1 Kun Huang,1,2 Jinwen Zhang1*
1Composite Materials and Engineering Center, Washington State University, Pullman, WA
99164, USA
2 Institute of Chemical Industry of Forestry Products, CAF; Nanjing 210042, China
* To whom correspondence should be addressed. Tel.: 509-335-8723; Fax: 509-335-5077;
Email: jwzhang@wsu.edu

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Abstract
An epoxy based on cinnamic acid (Cin-epoxy) and an anhydride curing agent based on dipentene
were prepared. Both products are liquid of low viscosity at room temperature. For the synthesis
of the epoxy, cinnamic acid was first converted to a diacid by reacting with maleic anhydride via
Friedel-Crafts reaction, followed by allylation of the carboxylic groups and subsequent
epoxidation of the allyl double bonds. The curing agent was the Diels-Alder adduct of dipentene
and maleic anhydride (DPMA). The chemical structures of Cin-epoxy and DPMA were
confirmed by 1H NMR, 13C NMR, FT-IR and ESI-MS. Non-isothermal curing of Cin-epoxy was
studied using differential scanning calorimetry (DSC). In addition to DPMA, two commercial
anhydrides were also used to cure Cin-epoxy and the curing reactivity and properties of cured
resins were compared. Thermal mechanical properties and thermal stability of the cured epoxy
resins were studied using dynamic mechanical analysis (DMA) and thermogravimetric analysis
(TGA), respectively. Results showed that Cin-epoxy was slightly more reactive than the
bisphenol A type epoxy DER 332 and displayed good dynamic mechanical properties and
thermal stability.
Keywords: Cinnamic acid, dipentene, biobased epoxy, Friedel-Crafts reaction

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Introduction
Epoxy resins are extensively used for coatings, composites, electronic materials and adhesives.
Mechanical, physical and chemical properties of epoxies can be greatly manipulated through
selection of the epoxy and curing agent pair. Bisphenol A (BPA) is by far the most widely used
intermediate chemical in epoxy synthesis. The rigid aromatic ring structure of BPA imparts high
mechanical properties and thermal resistance to epoxy resins. Because BPA is a chemical
estrogen,1, 2 the residual BPA in epoxy products, especially in food contact packaging, may pose
a potential risk to human immune and reproductive systems.3, 4 In recent years, there is a
growing interest in developing epoxy resins from the less toxic and renewable feedstocks.
Various natural chemicals have been attempted for the syntheses of epoxies and curing agents.
Vegetable oils are probably the most studied renewable feedstock for various polymers.5-7 Use
of epoxidized vegetable oils, such as soybean oil, linseed oil and castor oil, etc., for epoxy
applications have been attempted in many studies.8-13 However, the internal epoxy groups of the
epoxidized vegetable oils exhibited low curing reactivity and the presence of long aliphatic
chains often resulted in unsatisfactory thermal and mechanical properties for the crosslinked
materials. On the other hand, glycidyl carboxylate esters built on fatty acid-derived diacids or
triaicd exhibited sufficiently high curing reactivity and the stiffness of cured resins could be
greatly manipulated.14, 15 Glycerol, sorbitol and isosorbide were also attempted as feedstocks for
epoxies in the literature, but the resulting epoxies alone did not demonstrate the properties
suitable for structural and engineering polymer applications.16-18 The properties of epoxy resins
are highly dependent on their molecular architectures. If the chemical structure of an epoxy resin
contains aliphatic or other flexible chains, its thermal resistance would be inferior to that of those
containing aromatic or cycloaliphatic rings. Therefore, rigid molecular structures are desirable

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for high performance epoxy resins. Lignin is a naturally occurring aromatic polymer and is
abundantly available in many lignocellulosic biomass. Because lignin has some hydroxyl groups,
use of lignin as a reactive component in epoxy resins have received a lot of study.19-24 IBM
reported the incorporation of Kraft lignin into epoxies for the fabrication of printed wiring boards
in microelectronics industry.25-27 However, such lignin filled epoxy resins exhibited slow curing
and unsatisfactory properties due to the poor compatibility of lignin with other ingredients in the
resins.20, 21 Rosin acids are a family of isomeric natural chemicals and present in pines and
conifers. The characteristic rigid hydrogenated phenanthrene ring structure imparts rosin-based
epoxy resins excellent thermal and mechanical properties.14, 28-33 However, rosin-based epoxies
and curing agents are usually solids of high melting points.
Cinnamic acid (CA) or β-phenyl acrylic acid is one of the basic natural phenylpropanoids. It can
be obtained from oxidation of cinnamic aldehyde which is the major component in cinnamon
bark oil. Nowadays CA is primarily used in food additives, perfume and pharmaceutical
applications. On the other hand, the characteristic structure of CA also makes it an attractive
resource for renewable chemicals and monomers. CA derivatives were used as building blocks to
improve the properties of some polyesters in the literature.34-37 Semi-flexible and rigid main-
chain polyesters containing photoreactive mesogenic units derived from CA were synthesized by
high-temperature polycondensation.35 In addition, CA-derived amidoamines, polyamides or
amine adducts were used to cure the epoxy binders in the literature.38
In this work, CA was converted to a novel glycidyl ester type epoxy as shown in Scheme 1. The
resulting epoxy was a liquid of low viscosity and easy to mix with curing agents. Since the
resulting epoxy had a rigid aromatic ring and short aliphatic moieties, it was expected to
demonstrate moderate mechanical and thermal properties. Additionally, the adduct of maleic

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anhydride and renewable dipentene (DPMA) was prepared and used as a curing agent in this
study. Curing of Cin-epoxy with DPMA, hexahydrophthalic anhydride (HHPA) and methyl
nadic anhydride (MNA) was studied and the properties of the cured resins were compared. The
objective of this work is to develop a synthesis method of CA-derived epoxy and evaluate the
properties of the cured resins. To the best of our knowledge, a diglycidyl ester epoxy derived
from CA has not been reported elsewhere.
O
O
O
O
O
O
OH
O
HCOONH4
OH
HO
OH
AlCl3, rt,12 h
Friedel-Crafts
10% Pd/C, reflux, 2 d
O
diacid 2
Cinnamic Acid
1
O
O
Bu4N+I-
Br
m-CPBA
O
Zn, HAc/H2O
rt, 3 h
O
O
OH
rt, 48 h
K2CO3, DMF, 50 oC
24 h
O
HO
O
O
4
diacid 3
O
O
O
O
O
O
O
Cin-epoxy
O
O
O
O
isomerization
I2, 180 oC
200 oC, 3 h
O
O
DPMA
Dipentene
terpinene
Scheme 1. Synthetic routes of cinnamic acid-derived epoxy (Cin-epoxy) and adduct of dipentene
and maleic anhydride (DPMA).
Experimental
Materials

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Cinnamic acid (98.0%), allyl bromide (99.0%) and meta-chloroperoxybenzoic acid (mCPBA, 70-
75%) were purchased from ACROS. Maleic anhydride (95.0%), succinic anhydride (> 99.0%),
hexahydrophthalic anhydride (HHPA, 97.0%), palladium on carbon (10% Pd/C, loading 10 wt%),
ammonium formate (HCOONH4, 97.0%) and tetrabutylammonium iodide (98.0%) were
purchased from Sigma-Aldrich. Methyl nadic anhydride (MNA, 99%) was purchased from
Electron Microscopy Sciences. Dipentene was a product of technical grade obtained from China.
Sodium bicarbonate (NaHCO3, ≥ 95%), dimethylformamide (DMF, 99.8%), methylene chloride
(CH2Cl2, 99.8%), methanol (MeOH, 99.8%) and zinc powder were obtained from J. T. Baker.
Synthesis of Cin-epoxy
Hydrogenation of cinnamic acid: To the solution of cinnamic acid (2 g, 13.5 mmol) in MeOH
(50 mL) was added HCOONH4 (1.106 g, 17.55 mmol) and 10% Pd/C (170 mg). The reaction
1
mixture was refluxed for two days. The spectrum of H NMR indicated the reaction was
completed. After filtration, the filtrate was concentrated by removing methanol using a rotary
evaporator. The residual solid was extracted with ethyl acetate (EA, 50 mL) and the organic
phase was washed with 1N HCl solution (3 x 30 mL) and then with brine (3 x 30 mL). The
organic layer was dried by MgSO4 and concentrated to give a white solid hydrocinnamic acid 1
1
(1.8 g, yield 90%). H NMR (CDCl3, δ ppm) 7.34-7.20 (m, 5H), 3.00-2.95 (t, 2H), 2.72-2.67 (t,
2H). ESI-MS m/z 149, [M-H+].
Synthesis of diacid 2 (Friedel-Crafts reaction): To a solution of 1 (1.0 mmol) and maleic
anhydride (0.99 mmol) in dichloromethane (DCM) under stirring was slowly added AlCl3 (2.0
mmol). The reaction was continued at room temperature for 12 h. The reaction was followed
using thin layer chromatography (TLC). After completion of the reaction, DCM was removed

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and the remaining mixture was added into ice-HCl, extracted by tetrahydrofuran (THF). The
organic phase was washed with brine, dried with MgSO4 and concentrated. The residue was
washed by EA/hexane (1/3 v/v) and filtered to give a yellow solid diacid 2. 1H NMR of diacid 2
(DMSO-d6, δ ppm) 7.96-7.93 (d, 2H), 7.89-7.84 (d, 1H), 7.43-7.41 (d, 2H), 6.67-6.62(d, 1H),
13
2.92-2.87 (t, 2H), 2.60-2.55 (t, 2H). C NMR of diacid 2 (DMSO-d6, δ ppm) 194.32, 178.96,
171.78, 153.21, 141.64, 139.63, 138.14, 134.40, 39.93, 35.74. ESI-MS m/z 247, [M-H+].
Synthesis of diacid 3: Zinc dust (6.028 g, 93.2 mmol) was added to a mixture of 2 (20 g, 81
mmol), AcOH (168 mL) and H2O (48 mL) at room temperature. The reaction was continued for
3 h. TLC shows the reaction was completed. The mixture was added THF (200 mL) and then
filtered to remove the white solid. The obtained filtrate was concentrated to give a white solid 3
1
(14.5 g, yield 72%). H NMR (DMSO-d6, δ ppm) 7.88-7.85 (d, 2H), 7.38-7.35 (d, 2H), 3.20-
13
3.16 (t, 2H), 2.89-2.84 (t, 2H), 2.49-2.48 (t, 4H). C NMR (DMSO-d6, δ ppm) 198.81, 175.22,
147.59, 135.17, 129.27, 128.63, 35.74, 33.88, 31.27, 28.79. ESI-MS m/z 249, [M-H+].
Synthesis of allyl 4-(4-(3-(allyloxy)-3-oxopropyl)phenyl)-4-oxobutanoate (compound 4):
K2CO3 (12.16 g, 88.1 mmol), tetrabutylammonium iodide (2.952 g, 8 mmol) and allyl bromide
(38.72 g, 320 mmol) were added to the solution of 3 (10 g, 40 mmol) in DMF (120 mL). The
reaction mixture was stirred at 50 °C for 24 h and then was poured into ice-H2O. The aqueous
phase was extracted with DCM and the combined organic phase was washed with H2O and brine,
dried over MgSO4, and the solvent was evaporated. The residue was purified via flash column
chromatography (0%-10% EtOAc/hexanes) to yield a light yellow oil 4 (10.3 g, 78%). 1H NMR
(CDCl3, δ ppm) 7.93-7.90 (d, 2H), 7.31-7.28 (d, 2H), 5.98-5.81 (m, 2H), 5.36-5.20 (m, 4H),
13
4.62-4.55 (m, 4H), 3.32-3.28 (t, 2H), 3.04-2.99 (t, 2H), 2.81-2.76 (t, 2H), 2.71-2.66 (t, 2H). C

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NMR (CDCl3, δ ppm) 197.60, 172.65, 172.13, 146.44, 134.97, 132.39, 132.26, 128.76, 128.51,
118.42, 118.22, 65.39, 65.34, 35.31, 33.40, 30.93, 28.29. ESI-MS m/z 353, [M+Na].
Epoxidation of compound 4 with meta-chloroperoxybenzoic (mCPBA): To a solution of the
compound 4 (10.3 g, 31.2 mmol) in DCM (310 mL) was slowly added mCPBA (22.9 g, 99.86
mmol). After stirred at room temperature for 48 h, the reaction mixture was filtered to remove
the white solid produced during the reaction. The filtrate was washed by 0.1N Na2S2O3 solution,
saturated aqueous NaHCO3 and brine successively. The organic layer was dried over MgSO4,
1
filtered and concentrated to receive Cin-epoxy as a light a yellowish oil (10.117 g, 96.5%). H
NMR (CDCl3, δ ppm) 7.93-7.90 (d, 2H), 7.32-7.29 (d, 2H), 4.46-4.40 (m, 2H), 4.01-3.86 (m,
2H), 3.33-3.29 (t, 2H), 3.24-3.16 (m, 2H), 3.05-3.00 (t, 2H), 2.87-2.60 (m, 8H). 13C NMR
(CDCl3, δ ppm) 197.68, 172.87, 172.35, 146.38, 135.00, 129.52, 128.84, 128.61, 121.70, 65.38,
65.32, 49.56, 49.48, 44.92, 44.84, 35.26, 33.44, 30.95, 28.25. ESI-MS m/z 363,[M+H+]. The
viscosity of Cin-epoxy was 0.67 Pa·s.
Synthesis of adduct of dipentene and maleic anhydride (DPMA)
Maleic anhydride (9.8 g, 0.1 mol) was charged into a flask equipped with a stirrer, dropping
funnel, inert gas inlet, thermometer, and reflux condenser. The temperature was raised to 180 °C,
and dipentene (13.62 g, 0.1 mol) containing iodine (12 mg, 0.047 mmol) was added slowly. The
reaction continued for 3 h at 200 °C after all the dipentene and iodine were added. The crude
product (23.42 g) was purified by silica column using a hexane/ethyl acetate (6/1 v/v) as solvent,
1
resulting pure DPMA (yield: 18.03 g, 77%). H-NMR (CDCl3, δ ppm) 6.07-6.10 (d, 1H), 5.99-
6.02 (d, 1H), 3.20-3.23 (d, 1H), 2.83-2.86 (d, 1H), 2.48-2.62 (m, 1H), 1.49 (s, 3H), 1.29-1.48 (m,
4H), 1.06-1.08 (d, 3H), 0.99-1.01 (d, 3H).

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Curing of the epoxy resins: Cin-epoxy and curing agent (HHPA, DPMA or MNA) in a 1:0.9
molar ratio together with 2-ethyl-4-methylimidazole (1wt% on the basis of the total weight of
curing agent and epoxy) were mixed well (when HHPA was used, the mixture was first heated
up to 40 °C). The mixture was transferred into a mold of 5 cavities with dimensions of 50 mm x
18 mm x 5 mm. The curing was performed at 150 °C for 2 h and then at 180 °C for 2 h. The
cured samples were carefully removed from the mold and used for dynamic mechanical analysis
and thermal tests.
O
O
O
O
O
O
O
O
O
HHPA
MNA
DPMA
Characterizations
1H NMR spectra of the chemicals in CDCl3 or DMSO-d6 were recorded using a Bruker 300
MHz spectrometer at room temperature. Fourier transform infrared (FTIR) spectra were recorded
from 4000 cm-1 to 400 cm-1 using a NEXUS 670 FT-IR spectrometer (Nicolet). The FTIR
samples were prepared by dissolving a small amount of compound in DCM, followed by
smearing the solution onto a KBr crystal plate and evaporating the solvent completely. Mass
spectra were recorded with a LCQ advantage mass spectrometer system (Thermo Finnigan).
Viscosity of the epoxy was measured using a Discovery HR-2 rheometer (TA Instruments). Tests
were performed using a steel parallel-plate geometry (d = 25 mm) with a gap of 500 µm at 25 °C,
and the shear rate was swept from 10 to 1.0 s-1.

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Non-isothermal curing of the epoxy resins was performed by differential scanning calorimetry
(DSC) on a 2920 MDSC instrument (TA Instruments) under a nitrogen atmosphere. Heat scans
ranging from 30 to 280 °C were performed at heating rates of 2.5, 5, 10, 15, 20 °C min-1,
respectively. The formulations of the samples for DSC analysis were the same as those used for
the test specimens for dynamic mechanical analysis (DMA). Approximately 6 mg of each sample
was weighed and sealed in a 40 µL aluminum crucible and the DSC curing was conducted
immediately. DSC analysis for each sample was repeated twice.
DMA was carried out on a TA Q800 analyzer (TA instruments) in a single-cantilever mode with
an oscillating frequency of 1 Hz. All samples were tested from 30 to 180 °C at a heating rate of
3 °C min-1. Two repeats of DMA test were performed for each sample. Thermogravimetric
analysis (TGA) was performed on a TA Q600 analyzer (TA Instruments). Each sample was
scanned from 50 to 600 °C at a heating rate of 5 °C min-1 under a nitrogen atmosphere.
Results and Discussion
Synthesis of Cin-epoxy
Scheme 1 shows the synthetic route of Cin-epoxy. In order to synthesize the diacid 3
intermediate, two methods were investigated. First, the Friedel-Crafts reaction of succinic
anhydride and hydrocinnamic acid was attempted to produce the diacid 3. However, this method
gave a very low yield of conversion (~20%). 1H NMR analysis (spectrum not shown) revealed a
lot of unreacted succinic acid remained in the product. It was difficult to purify the product
because the diacid 3 and succinic acid had very similar properties. On the other hand, the
Friedel-Crafts reaction of maleic anhydride and hydrocinnamic acid gave a good yield of diacid
2 (81%). Furthermore, the resulting diacid 2 could be easily purified by washing with EA/hexane

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(v/v=1/3) to remove the small amount of unreacted CA (cinnamic acid). No maleic acid was
noted in the 1H NMR and 13C NMR (SI) spectra of the crude product. Because the conjugation of
C=C and C=O in maleic acid helped to form much more stable carbocation than succinic acid,39
this Friedel-Crafts reaction yielded a high conversion. The subsequent reduction of the double
bond of maleate in diacid 2 using Zn powder in HAc/H2O also gave a high yield (80%)40 and the
obtained diacid 3 could be used directly for the next step of reaction without further purification.
The synthesis of the glycidyl ester was first attempted by reacting diacid 3 with epichlorohydrin
(ECH) and this process was a two-step reaction involving ring opening of the oxirane of ECH
and dehydrochloration in the presence of NaOH. However, the first step produced more than
three compounds including two mono- and one di-substituted compounds. Additionally, these
intermediates continued to react with ECH. Therefore, the reaction of diacid 3 with ECH led to a
non-controlled glycidylation. In order to receive a purer product, the epoxy was prepared by first
forming allyl ester followed by epoxidation of the allyl double bonds. The allylation of diacid 3
by allyl bromide gave a decent yield (78%) of compound 4 after purification using column
1
13
chromatography. The molecular structure of 4 was identified by H NMR and C NMR (SI).
The epoxidation was carried out using mCPBA. When the equivalent of mCPBA was higher than
1.6 times that of 4, byproduct would be formed. A high yield of 96.5% was achieved at 1.6 eq
mCPBA. The epoxy equivalent weight was found to be 200 g/mol by titration using the acetone
hydrocloride method.
1
1H NMR confirmed the structure of Cin-epoxy. In the H NMR spectrum of Cin-epoxy (Figure
1), the protons attributed to the allylic double bonds at 5.0-6.0 ppm disappeared completely,
instead, the new peaks attributed to the protons of oxirane at 3.15-3.24 (c, d) and 2.60-2.87 (e, f)
ppm presented. In addition, the chemical shift of allyl proton (-CH2-) in compound 4 was shifted

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from 4.50-4.66 ppm (SI-Figure S3) to 3.86-4.01 and 4.40-4.46 ppm (a, b) in the glycidyl groups.
The chemical structure of Cin-epoxy was further verified using 13C NMR (Figure 2). After
epoxidation, the peaks of the four carbons (c, d, e, f) in double carbon bond were at 49.43 - 49.56
and 44.84-44.92 ppm.
O
i
l
k
O
O
g
j
O
f
g
b
l
O
a
d
k
h
a, b
O
c
c,d
e
O
Cin-epoxy
4.6
4.4
4.2
4.0
3.8
3.6
3.4
3.2
h, j, e, f
i
l
C D C l3
k
D C M
ppm
8
6
4
2
0
Figure 1. 1HNMR spectrum of Cin-epoxy.
O
i
l
k
O
o
O
n
p
g
j
m
O
O
f
b
q
l
O
a
d
h
k
k,l
c
e
C D C l3
O
Cin-epoxy
c,d
e,f
a,b
i,j,g,h
p
q
o,n
m
200
150
100
50
0
Figure 2. 13C NMR spectrum of Cin-epoxy.

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Synthesis of adduct of dipentene and maleic anhydride (DPMA)
The synthetic route of DPMA is illustrated in Scheme 1. The reaction first involved the
isomerization of dipentene to terpinene in the presence of iodine, and then terpinene reacted with
maleic anhydride via Diels-alder reaction.41 Nonetheless, these two reaction processes were
carried out in a one-step reaction, and the reaction gave a yield of 77%. The obtained DPMA was
1
a yellowish liquid and was able to mix with Cin-epoxy easily. Figure 3 shows the H NMR
spectrum of DPMA. The chemical shift of the protons i was higher than that of the protons
because the shielding effect of the methyl group was weaker than that of the isopropyl group.
h
g
h
O
a
i
f
b
O
j
e
c
d
O
DPMA
c,d
a, b
i
j
g,f
C D C l3
e
ppm
7
6
5
4
3
2
1
0
Figure 3. 1H NMR spectrum of DPMA.

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Curing behavior
Cin-epoxy appeared a colorless liquid with a viscosity of 0.67 Pa·s which was much lower than
that of DER332 (3.31 Pa·s). Therefore, it was mixed easily with all curing agents selected.
Figure 4 shows the DSC thermograms of non-isothermal curing of Cin-epoxy with HHPA,
DPMA and MNA systems. Each sample displayed a main exothermic peak in its DSC curing
thermogram which corresponded to the ring-opening reaction between epoxy and anhydride
groups. With increasing heating rate, the peak exothermic temperature shifted to higher
temperatures. This shift of peak curing temperature with heating rate was a typical
methodological phenomenon for non-isothermal curing. The peak temperature of exothermic
peak usually reflects the reactivity of the compound in curing reaction. HHPA seemed to be
slightly more active than DPMA and MNA under the same heating rate (Figure 4a-4c). By
comparing the curing results of Cin-epoxy and bisphenol A type epoxy resin (DER 332) with the
same MNA that reported in our early study,15 it was noted that the peak curing temperature of
Cin-epoxy (146.5 °C) was much lower than that of DER332 (152.9 °C) at heating rate 5 °C/min.
This result indicates that the reactivity of Cin-epoxy was higher than that of DER 332.
The activation energy of the curing reaction was determined by the Kissinger’s method. The
activation energy could be obtained from the Kissinger’s equation:
d[ln(q /Tp2 )]
Ea
= −
(1)
d[1 / Tp ]
R
Here Tp is the peak exothermic temperature, q the constant heating rate, Ea the activation energy
of the curing reaction, and R is the gas constant with the value of 8.314 J mol-1·K-1 (Figure 5). At
the same time, the activation energy was also determined by the Ozawa method. Table 1 lists the

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activation energies of curing of Cin-epoxy with three different curing agents. As it can be seen,
the two methods gave very similar Ea values. In curing with MNA, Cin-epoxy exhibited a lower
activation energy than DER322 (63.3 KJ mol-1 vs. 79.9 KJ mol-1),15 which indicated Cin-epoxy
was more reactive. Because Cin-epoxy has a more flexible molecular structure than DER 332,
therefore, it is likely to exhibit high reactivity that DER 332 under the same reaction conditions.
4
(b)
(a)
170.5 oC
157.5 oC
5
1
2
3
4
5
2.5 oC /min
oC /min
10 oC /min
15 oC /min
20 oC /min
E xo
E xo
1
2
3
4
5
2.5 oC /min
oC /min
10 oC /min
15 oC /min
20 oC /min
177.0 oC
5
151.9 oC
5
4
5
143.6 oC
3
132.6 oC
MNA/C in-epoxy
2
C in-epoxy/HHP A
157.5 oC
3
146.5 oC
2
120.33 oC
1
132.1 oC
1
60
80
100
120
140
160
180
200
220
80
100
120
140
160
180
200
220
T emperature (oC )
T emperature (oC )
(c)
5
165.5 oC
E xo
1
2
3
4
5
2.5 oC /min
oC /min
10 oC /min
15 oC /min
20 oC /min
5
159.3 oC
4
C in-epoxy/DP MA
150.7 oC
2
139.5 oC
3
127.9 oC
1
80
100
120
140
160
180
200
220
T emperature (oC )
Figure 4. DSC thermograms of non-isothermal curing of Cin-epoxy with HHPA (a), MNA (b)
and DPMA (c).

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3.0
2.5
2.0
1.5
1.0
0.5
H H P A
MNA
D P MA
0.00224
0.00232
0.00240
1/(T p)
0.00248
0.00256
2
Figure 5. Linear plot of –ln(q/Tp ) versus 1/Tp based on Kissinger’s equation.
Table 1. Curing activation energy of Cin-epoxy and thermal properties of the cured resins
Ea /KJ mol-1
Ozawa Kissinger Tg (oC)
DMA
Eʹ at Tg+50
oC(MPa)
9.69
TGAa
ve (x103
mol mm-3)
0.95
T5%
T10%
Sample
(oC)
(oC)
Cin-epoxy/DPMA
Cin-epoxy/HHPA
Cin-epoxy/MNA
77.6
75.7
66.8
74.7
72.9
63.3
85.4
90.4
116.6
301.1
331.5
328.0
325.3
352.3
355.0
13.99
21.9
1.36
2.00
a T5% and T10%: temperature of 5% degradation and 10% degradation.
Dynamic mechanical analysis
Figure 6 shows the storage modulus (Eʹ) and damping (tan δ) as functions of temperature for
Cin-epoxy resins cured with different curing agents. The glass transition temperatures (Tgs) of
the cured samples were determined from the peak temperatures of the tan δ. The ve of the cured

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Cin-epoxy resin was estimated using the following equation based on the theory of rubber
elasticity:
ꢀ
3ꢀꢁ
ꢀ! =
(2)
where E is the elastic modulus of the thermoset in the rubbery state, R is the gas constant and T
is the absolute temperature. Since measuring the elastic modulus of thermoset in rubbery state is
a very tedious experiment, it is a convenient custom to use Eʹ in the rubbery state, e.g., at Tg +
50 °C, to roughly substitute E in the calculation.
The DPMA and MNA cured Cin-epoxy resins had similar Eʹs at room temperature, which were
both higher than that of the HHPA cured resin. This was probably because DPMA and MNA had
more bulky structures than HHPA and therefore resulted in more rigid glassy resins at room
temperature. In Table 1, it is noted that the Tgs of the HHPA and DPMA cured resins were
similar but both were lower than that of the MNA cured resin. It is understood that the Tg of an
epoxy network is dependent on both crosslink density and the chemical structure of the chain
segment. Higher crosslink density and more rigid chain segment both lead to a higher Tg. The
former is determined by the curing reactivity, molecular structure and mobility of curing agent
and epoxy. As suggested by the activation energy of curing, MNA had the highest reactivity
among the three curing agents. In addition, MNA was already a liquid at room temperature and
was probably still more accessible in the late stage of curing when the curing was controlled by
diffusion. Therefore, the MNA cured Cin-epoxy exhibited the highest ve and Tg in the three cases.

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1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
(a)
(b)
HHP A
MNA
1000
100
10
MNA
DP MA
DP MA
HHP A
40
60
80
100
120
140
160
40
60
80
100
120
140
160
T emperature (oC )
T emperature (oC )
Figure 6. Storage modulus (a) and tan δ (b) versus temperature for Cin-epoxy cured with DPMA,
HHPA and MNA.
Thermogravimetric analysis (TGA)
TGA is a convenient method to evaluate the thermal stability and degradation behaviors of
polymers. Figure 7 shows the TGA curves of the cured Cin-epoxy resins under nitrogen
atmosphere. Table 1 gives the temperatures at which 5% weight loss (T5%) and 10% weight loss
(T10%) were incurred. These three cured resins displayed very similar weight loss curves at the
initial stage. Cin-epoxy/MNA exhibited a T5% of ~328.0 °C which was about equal to that of
DER332/MNA (327.1 °C).15 This result suggested that Cin-epoxy had a comparable thermal
stability to that of BPA-based epoxy resins. The Cin-epoxy resins cured with HHPA and MNA
exhibited almost the similar T5% (331.5 °C, 328.0 °C) and T10% (352.3 °C, 355.0 °C), which were
higher than that (301.1 °C, 325.3 °C) of the Cin-epoxy cured with DPMA. In other words, the
Cin-epoxy resins cured with HHPA and MNA were more thermally stable than the Cin-epoxy
resin cured with DPMA. This was probably because the former two resins had higher crosslink
densities than the latter one (Table 1).

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100
80
60
40
20
0
D P MA
MNA
H H P A
100
200
300
400
500
600
700
T emperature (oC )
Figure 7. TGA plots of the cured epoxy resin under nitrogen atmosphere.
Conclusions
A cinnamic acid-derived epoxy and a dipentene-derived anhydride curing agent were
successfully synthesized. Unlike the traditional method for the synthesis of glycidyl ester which
involves the reaction of carboxylic acid and epichlorhydrin, in the synthesis of Cin-epoxy a two-
step method involving allylation of the carboxylic groups and subsequent epoxidation was
employed. This two-step method resulted in a high yield and few byproducts. Cin-epoxy and
DPMA were both liquid products and displayed comparable properties in curing compared to
some petroleum-based commercial epoxies and curing agents. The Tg of the renewable DPMA
cured Cin-epoxy was similar to that of the resin cured with HHPA but was lower than that of
MNA cured one. In curing with MNA, Cin-epoxy exhibited slightly higher reactivity than DER

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332. The Cin-epoxy resin cured with MNA exhibited a moderate Tg of 116 oC. In general, these
results suggest that Cin-epoxy is a potential alternative to some of the petroleum-based epoxies.
References
1.
2.
3.
4.
L. W. Dodds E.C., Nature, 1936, 137, 996.
S. Sasaki, T. Ooya and T. Takeuchi, Polymer Chemistry, 2010, 1, 1684-1688.
J. C. O'Connor and R. E. Chapin, Pure Appl Chem, 2003, 75, 2099-2123.
H. Okada, T. Tokunaga, X. Liu, S. Takayanagi, A. Matsushima and Y. Shimohigashi, Environ Health
Persp, 2008, 116, 32-38.
5.
6.
P. Zhang and J. Zhang, Green Chemistry, 2013, 15, 641-645.
S. N. Khot, J. J. Lascala, E. Can, S. S. Morye, G. I. Williams, G. R. Palmese, S. H. Kusefoglu and R. P.
Wool, J Appl Polym Sci, 2001, 82, 703-723.
7.
8.
9.
10.
11.
12.
L. Hojabri, X. Kong and S. S. Narine, Biomacromolecules, 2009, 10, 884-891.
H. Miyagawa, A. K. Mohanty, M. Misra and L. T. Drzal, Macromol Mater Eng, 2004, 289, 629-635.
H. Miyagawa, M. Misra, L. T. Drzal and A. K. Mohanty, Polym Eng Sci, 2005, 45, 487-495.
S. Park, F. Jin and C. Lee, Materials Science and Engineering: A, 2005, 402, 335-340.
S. Park and F. Jin, Polym. Int., 2005, 54, 705-709.
M. Stemmelen, F. Pessel, V. Lapinte, S. Caillol, J. P. Habas and J. J. Robin, J. Polym. Sci., Part A:
Polym. Chem., 2011, 49, 2434-2444.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
J. R. Barrett, Environ Health Perspect, 2006, 114, A352-A358.
K. Huang, J. Zhang, M. Li, J. Xia and Y. Zhou, Industrial Crops & Products, 2013, 49, 497-506.
K. Huang, P. Zhang, J. Zhang, S. Li, J. Xia and Y. Zhou, Green Chemistry, 2013, 15, 2466-2475.
M. Chrysanthos, J. Galy and J. P. Pascault, Polymer, 2011, 52, 3611-3620.
M. Shibata and K. Nakai, J. Polym. Sci., Part B: Polym. Phys., 2010, 48, 425-433.
Y. Takada, K. Shinbo, Y. Someya and M. Shibata, J. Appl. Polym. Sci., 2009, 113, 479-484.
K. Hofmann and W. G. Glasser, J Wood Chem Technol, 1993, 13, 73-95.
K. Hofmann and W. G. Glasser, J Adhesion, 1993, 40, 229-241.
K. Hofmann and W. Glasser, Macromol Chem Physic, 1994, 195, 65-80.
M. N. El, Q. Yuan and F. Huang, BioResources, 2011, 6, 2492-2503.
M. N. El, Q. Yuan and F. Huang, BioResources, 2011, 6, 2647-2662.
T. Koike, Polym Eng Sci, 2012, 52, 701-717.
J. M. Shaws, S. L. Buchwalter, J. C. Hedrick, S. K. Kang, L. L. Kosbar and J. D. Gelorme, Printed
Circuit Fabrication, 1996, 19, 38-44.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
L. L. Kosbar, J. D. Gelorme, R. M. Japp and W. T. Fotorny, J. Ind. Ecol., 2000, 4, 93-105.
J. H. Lora and W. G. Glasser, J Polym Environ, 2002, 10, 39-48.
L. Deng, M. Shen, J. Yu, K. Wu and C. Ha, Ind. Eng. Chem. Res., 2012, 51, 8178-8184.
X. Liu, W. Xin and J. Zhang, Green Chemistry, 2009, 11, 1018-1025.
J. Qin, H. Liu, P. Zhang, M. Wolcott and J. Zhang, Polym Int, 2013, DOI: 10.1002/pi.4588.
X. Liu and J. Zhang, Polym. Int., 2010, 59, 607-609.
H. Wang, B. Liu, X. Liu, J. Zhang and M. Xian, Green Chemistry, 2008, 10, 1190-1196.
H. Wang, X. Liu, B. Liu, J. Zhang and M. Xian, Polymer International, 2009, 58, 1435-1441.
W. G. Kim, J. Appl. Polym. Sci., 2008, 107, 3615-3624.
R. M. Tejedor, L. Oriol, M. Pinol, J. L. Serrano, V. Strehmel, B. Stiller and J. Stumpe, J. Polym. Sci.,
Part A: Polym. Chem., 2005, 43, 4907-4921.

Page 21 of 22
RSC Advances
View Article Online
DOI: 10.1039/C3RA47927G
36.
37.
38.
V. Mannari, P. Ziemer, A. Saraf and F. Jones, Int. J. Polym. Mater., 2007, 56, 13-26.
R. Hany, C. Boehlen, T. Geiger, M. Schmid and M. Zinn, Biomacromolecules, 2004, 5, 1452-1456.
A. Klippstein, M. Cook and S. Monaghan, In Elsevier: Amsterdam, Oxford, Waltham, 2012, 519-
540.
39.
40.
R. E. Lutz, P. S. Bailey, C.-K. Dien and J. W. Rinker, J. Am. Chem. Soc., 1953, 75, 5039-5044.
K. Kameo, K. Ogawa, K. Takeshita, S. Nakaike, K. Tomisawa and K. Sota, Chem. Pharm. Bull., 1988,
36, 2050-2060.
41.
M. C. Kloetzel, in Organic Reactions, John Wiley & Sons, Inc., 2004.

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An epoxy based on cinnamic acid and an anhydride curing agent based on dipentene
were prepared.