A renewable bio-based epoxy resin with improved mechanical performance that can
compete with DGEBA
Citation:
Nikafshar, Saeid, Zabihi, Omid, Hamidi, Susan, Moradi, Yousef, Barzegar, Saeed, Ahmadi, Mojtaba and
Naebe, Minoo 2017, A renewable bio-based epoxy resin with improved mechanical performance that
can compete with DGEBA, RSC advances, vol. 7, no. 14, pp. 8694-8701.
DOI: 10.1039/c6ra27283e
©2017, The Authors
Reproduced by Deakin University under the terms of the Creative Commons Attribution Non-
Commercial Licence
Available from Deakin Research Online:
http://hdl.handle.net/10536/DRO/DU:30093508
A renewable bio-based epoxy resin with improved
mechanical performance that can compete with
DGEBA
Saeid Nikafshar,
a
Omid Zabihi,
*
b
Susan Hamidi,
c
Yousef Moradi,
d
Saeed Barzegar,
a
Mojtaba Ahmadi
e
and Minoo Naebe
*
b
The aim of this study is to find a suitable substitution for diglycidyl ether bisphenol A (DGEBA) to avoid the
devastating side effects of bisphenol A. Vanillin, an aromatic compound, was used as a renewable material
to synthesize a bio-based epoxy resin. The structure of the vanillin-based epoxy resin was confirmed by
Fourier transform infrared spectroscopy (FT-IR) analysis. The major drawback of bio-based epoxy resins is
their poor mechanical properties preventing them from competing with petroleum based epoxy resins
such as DGEBA. Herein, a prepared calcium nitrate solution as an inorganic accelerator was used to
accelerate the curing reaction of bio-based epoxy resin which reduced curing times as well as improving
significantly the mechanical properties e.g., tensile strength, pull-off strength, and Izod impact strength.
Differential scanning calorimetry (DSC) analysis was used to investigate the curing process and thermal
properties of the vanillin-based epoxy resin with and without inorganic accelerators and also DGEBA
without accelerators. The results showed that in the presence of 2 wt% inorganic accelerator, the initial
onset curing temperature of vanillin-based epoxy resin was reduced from 60.1
Cto8.5
C, while the
initial onset curing temperature of DGEBA was 55.8
C. In addition, tensile strength and Izod impact
strength of the vanillin-based epoxy system in the presence of inorganic accelerators increased in
comparison to the DGEBA system. Moreover, in order to study the effect of inorganic accelerators on the
toughness of the synthesized vanillin-based epoxy resin, fracture surfaces from Izod impact strength tests
were observed using scanning electron microscopy (SEM) which confirmed improving mechanical properties.
Introduction
During recent years, bio-based polymers have attracted signi-
cant attention mainly due to the overuse of fossil fuels and
reservoirs as well as the increase of greenhouse gas emission,
causing serious environmental issues.
1,2
Bio-based polymers
can be synthesized from renewable, inedible, and sustainable
materials such as sucrose,
3
lignin,
4
and vegetable oils,
5,6
which
do not interfere with human and animal food.
DGEBA as an epoxy resin is one of the most used types of
thermoset polymers due to its excellent chemical and
mechanical properties.
7,8
Thermoset polymers are widely
utilized in a wide range of applications e.g., coatings, adhesives,
and composites.
9–11
However, a signicant AMOUNT of research
and development is dedicated to replacing DGEBA with more
environmentally friendly alternatives.
11
This is because of the
fact that more than 67% of the molar mass of DGEBA is strongly
dependent on fossil sources.
2
Moreover, bisphenol A (BPA)
which is the raw material used in production of DGEBA has
deleterious impacts on human health and environment, and it
has also been proven that it can be toxic regarding living
organisms and acts as an endocrine disruptor.
12
Use of BPA in
food packaging industry and food related materials has been
forbidden in some countries such as Canada and France.
13
Therefore, in this regard, there have been substantial efforts
concerning substituting bio-based and environmentally
friendly materials with BPA.
14
Epoxy resins can be synthesized from various types of bio-
based materials. Ferulic acid is a natural compound which is
found in numerous inedible bio-resources like bagasse, wheat
bran, beetroot pulp, and other bio-based diols under mild
conditions.
15
Currently, ferulic derivatives are used to synthe-
size copoly(ester-urethane)s,
16
isocyanate-based poly-
urethanes,
17
and poly(anhydride-ester)s.
18
Itaconic acid are also
produced from fermentation carbohydrates such as glucose,
a
Department of Appl ied Chemistry, Faculty of Chemistry, University of Tabriz, T abriz,
Iran
b
Institute for Frontier Materials, Deakin University, Geelong, Victoria, Australia.
E-mail: omid.zabihi@deakin.edu.au; Minoo.Naebe@deakin.edu.au; Tel: +61
469570372
c
Applied Chemistry Research Laboratory, Department of Chemistry, Faculty of Science,
University of Zanjan, Iran
d
Department of Organic Chemistry, Faculty of Chemistry, Isfahan University of
Technology, Isfahan, Iran
e
Department of Chemical Engineering, Isfahan University of Technology, Isfahan,
84156/83111, Iran
Cite this: RSC Adv.,2017,7, 8694
Received 24th November 2016
Accepted 19th January 2017
DOI: 10.1039/c6ra27283e
rsc.li/rsc-advances
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which can be converted to epoxy resin by an esterication
reaction.
2
Eugenol, containing 70–90% clove oil, is another
example for renewable phenol compounds, extracted form
plants. Moreover, Wan et al.
19
synthesized a high modulus and
hardness, and low ammability bio-based epoxy resin form
eugenol. Rosin is also a good candidate to synthesize bio-based
epoxy resins because it consists of aromatic rings, expected to
have high mechanical and thermal performances.
20
Aouf et al.
13
used gallic acid, which is a natural phenolic acid, to synthesize
bio-based epoxy resin via a two-step synthesis rout in the pres-
ence of oxygen transfer agents. In another study, bio-based
epoxy resins were synthesized from gallic and vanillic acids by
chemo-enzymatic epoxidation with high degree of conversion to
epoxides.
14
Epoxidized soybean oil was also synthesized from
vegetable oils and it was used as a thermosetting epoxy resin
system. Since vegetable oils are aliphatic structures, epoxy
resins based on epoxidized soybean oil have low thermal and
mechanical properties.
21
Among various sustainable and
renewable materials, lignin has received much attention. Aer
cellulose, lignin is the second most abundant natural material
on earth. Lignin can be obtained from bers of plants using
various extraction methods.
22
Aromatic groups of lignin poten-
tially make it an ideal candidate for polymer synthesis. It is
crystal clear that aromatic compounds have great thermal and
mechanical properties, compared to aliphatic compounds.
Lignin is a suitable natural material for obtaining low molar
mass compounds like 4-hydroxy-3-methoxybenzaldehyde
(vanillin) that could potentially be used in production of bio-
epoxy resins. It is extracted from lignin through various
methods
22
in which approaches based on oxidation is set to
result in a high yield.
23,24
Although various compounds can be
produced from lignin, vanillin is the most well-known valuable
product. Vanillin is usually used as avoring or chemical
materials in pharmaceutical elds.
25
But it can also be used as
raw material to synthesize epoxy resin and the resulted epoxy
resin from vanillin demonstrates good thermal and mechanical
properties due to the presence of the aromatic ring in its
structure.
26,27
The chemical structures of lignin and vanillin are
illustrated in Fig. 1.
The aim of this study is to synthesis a bio-based epoxy resin
from vanillin and compare its thermal and mechanical prop-
erties with the DGEBA resin. An inorganic accelerator for curing
process was also prepared to minimize the negative inuence of
low molecular weight of synthesized vanillin-based epoxy resin
on its nal performance. It is hypothesized that this inorganic
accelerator affects mechanical performance of the epoxy matrix
through inducing changes in physico-chemical events occurred
during curing process. The goal is to develop a vanillin-based
epoxy resin which gives the composites with competitive
advantages over DGEBA as its rival.
Experimental
Materials
Liquid diglycidyl ether of bisphenol A (Epon 828), with epoxy
equivalent weight of 185–192 eq. g
1
was supplied from E. V
Roberts. Epikure F 205 was purchased from Hexion. Epikure F
205 with amine equivalent weight of 105 g eq.
1
was utilized as
a curing agent. Epichlorohydrin was supplied from Parachem.
Vanillin (99%), benzyltriethylammonium chloride (99%),
sodium hydroxide (97%), ethyl acetate, sodium per carbonate,
tetrahydrofuran, acetic acid, methanol, nitric acid (65%), and
calcium carbonate were purchased from Sigma Aldrich.
Synthesis of vanillin-based epoxy resin
To convert all aldehyde and ketone functional groups of vanillin
to methoxyhydroquinone, Dakin oxidation was used according
to the following steps:
28
(1) vanillin (1 mol) and sodium percar-
bonate (1 mol) were dissolved in 100 ml of tetrahydrofuran and
40 ml distilled water in an ultrasonic bath for 15 min under
argon atmosphere. Next, the reaction was quenched by addition
of 10 ml acetic acid and the total solution was concentrated by
evaporating the solvent under vacuum until methoxyhy-
droquinone precipitation was obtained. (2) Methoxyhy-
droquinone (1 mol) and triethylbenzylammonium chloride (1
Fig. 1 Chemical structures of lignin and vanillin.
Fig. 2 Synthesis approach for bio-based epoxy resin from vanillin.
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mol) were poured into a ask and epichlorohydrin (10 mol) was
charged into the ask. The resulting solution was mixed for 1 h
at room temperature and 30 min at 80
C. Aer cooling down to
ambient temperature, 800 ml of sodium hydroxide (5 molar) and
triethylbenzylammonium chloride (0.1 mol) were added into the
mixture and it was stirred for 30 min at 25
C. Aerwards, a two-
phased mixture of ethyl acetate/distilled water was added to the
above mixture, followed by 5 min further stirring. The extraction
was carried out two times in an aqueous phase with ethyl
acetate. Organic phase containing vanillin-based epoxy resin was
rinsed again with an aqueous solution of sodium chloride and
dried over manganese sulfate. Excess of ethyl acetate and
epichlorohydrin was eliminated using rotary evaporator. Silica
gel ash chromatography was applied to purication using
mixtures of cyclohexane/ethyl acetate. The chemical approach of
vanillin-based epoxy resin synthesis is shown in Fig. 2.
Synthesis of calcium nitrate as inorganic accelerator
Calcium carbonate (26.3 mmol) and 5.15 ml of nitric acid 65%
were stirred at room temperature for 2 min. A green solution
containing calcium nitrate was obtained. The reaction
happened according to the following equation:
CaCO
3
+ 4HNO
3
/ Ca(NO
3
)
2
+CO
2
+H
2
O + 2HNO
3
Epoxy resin system preparation
Firstly, the epoxy equivalent weight (EEW) of synthesized bio-
based resin was determined. For this propose, ASTM D1652
was used and its EEW was calculated to be 247.6 eq. g
1
. The
amine hydrogen molar mass (AHMM) of Epikure F 205 is 105 g
eq.
1
and it is supposed that each amine hydrogen reacts with
one epoxy group. Therefore, the stoichiometry ratio of epoxy
resin and curing agent is calculated according to AHMM/EEW
ratio. To prepare the samples, 100 g of epoxy resin was mixed
with a calculated amount of Epikure F 205 as presented in Table
1. To make sure that all remaining liquids were removed from
vanillin-based epoxy resin before adding curing agent, it was
heated in silicon mold for 10 min at 120–130
C before con-
ducting curing process. A er mixing all components for 30 s
using a mechanical stirrer at a high speed stirring which was
resulted in a uniform mixture, the curing process was accom-
plished in room temperature for 7 days.
Measurements
The FT-IR analysis was performed using a FT-IR spectrometer,
Tensor 27, Burker with 40 scan average at a resolution of 4
cm
1
. The DSC analysis was conducted using a Linseis, PT10,
Germany to investigate the curing conditions and the reaction
time. DSC instrument was rstly calibrated using standard zinc
and indium, and 5 mg of each prepared sample was used. As the
curing reactions are started in room temperature in particular
for the sample containing inorganic accelerator, aer mixing
the epoxy resin and curing agent with and without inorganic
accelerator, the DSC tests should be done immediately on the
mixtures. Non-isothermal DSC tests with a heating rate of 10
C
min
1
under nitrogen ow of 40 ml min
1
at temperature range
between 50 to 170
C were performed. To determine the glass
transition temperature (T
g
) of the fully-cured epoxy systems,
a DSC scan was conducted on 5 mg of each cured epoxy systems
with a heating rate of 10
C min
1
at the temperature range of
0–150
C under nitrogen ow of 40 ml min
1
. Tensile strength
test was conducted in accordance with ASTM D638 by using
Shimadzu 20KN-testing machine. Specimen dimensions of
samples were selected according to type 1 of ASTM (165 19
3.2 mm). Crosshead speed was adjusted to be 2 mm min
1
.At
least ve specimens were prepared and tested for each sample.
The pull-off strength test was performed according to ASTM
D4541 with a Posi Test AT-M from Deesco. Each test was
replicated ve times on an aluminum surface. Izod impact
strength was measured according to ASTM D256 by a Zwick/Roll
6103 impact tester at room temperature using specimens with
dimensions of 63.5 12.7 7.2 mm. For each sample, ve
replicates were performed. Fracture surfaces of the epoxy
systems aer Izod impact strength test were observed using
SEM analyses by Tescan, MIRA3 FEG-SEM, Czech Republic at an
accelerating voltage of 5.00 kV. Samples were coated with gold
vapor to increase the resolution.
Results and discussion
FTIR analysis
To conrm the molecular structure of the synthesized vanillin-
based epoxy resin, FT-IR spectroscopy was carried out in wave-
lengths of 3400–400 cm
1
. The FT-IR spectrum of vanillin-based
epoxy resin is shown in Fig. 3. The most important functional
group of this compound, is the epoxy group which has an
absorption peak in 915 cm
1
, conrming the formation of
epoxide rings.
29
As can be seen in Fig. 3, the intensity of this peak
is medium, due to the low amount of epoxy groups in the bio-
based resin. The absorption bond of 1185 cm
1
represents the
presence of C–O stretching of aromatic rings,
30
and the peak
around 3500 cm
1
indicates the hydroxyl groups of epoxy resin.
Two absorption peaks at 2969 cm
1
and 2878 cm
1
are assigned
to the stretching CH
2
and CH of aromatic and aliphatic,
Table 1 The fabricated epoxy systems with various compositions and their sample codes
Sample code Epoxy resin type
Epoxy resin
amount
Epikure F 205
amount
Wt% inorganic
accelerator
A DGEBA 100 g 55.7 g —
B Vanillin-based epoxy resin 100 g 42.4 g —
C Vanillin-based epoxy resin 100 g 42.4 g 2%
8696
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respectively.
31
Absorption peaks appeared at 1508–1607 cm
1
and 1200–1250 cm
1
are characteristic peaks of aromatic C]C
bonds and phenols C–O groups, respectively.
32
The peaks at 1035
and 831 cm
1
are also related to stretching C–O–C of ether and
stretching C–O–C of oxirane groups, respectively.
31
Moreover,
vibration bond at 1458 cm
1
corresponds to methylene groups.
33
Epoxy matrix formation
It has been frequently reported that bio-based epoxy resins
cannot compete with DGEBA in terms of mechanical perfor-
mance, in particular, regarding their brittleness due to the
rigidity of benzene rings.
34–36
To overcome this issue, an inor-
ganic accelerator which affects curing process can be used that
not only cures epoxy resins faster, but also improves mechanical
properties of epoxy structure through changing the network
architecture. In our previous study,
37
we have used a calcium
nitrate solution as an accelerator which showed positive inu-
ence on the mechanical performance as well as curing time of
DGEBA. It has been shown that DGEBA/Epikure F 205 system
was cured only in 18 min at room temperature, whereas curing
reaction without accelerator took more than 8 h. Additionally,
tensile strength, pull-off strength, and Izod impact strength for
samples with the accelerator were enhanced by 17.7%, 19.1%
and 44.7%, respectively.
37
Here, in order to reinforce the
vanillin-based epoxy resin, the same inorganic accelerator was
used to improve mechanical properties. In Fig. 4a, arrange-
ments of epoxy structure with and without inorganic accelerator
are compared. In the presence of inorganic accelerator, it is
supposed that chains themselves align in a linear direction and
such orientation helps mechanical properties to increase,
whereas without inorganic accelerator polymeric chains do not
have any specic direction.
DSC analyses provide information regarding formation of
the epoxy matrix during the curing reaction, which can be
useful to interpret the effect of accelerator on the matrix
formation.
8,38
Using DSC analysis, many studies have been
conducted on thermal curing characteristics of various epoxy
resin/curing agent systems.
39,40
For sample A (DGEBA/Epikure F
205), the initial onset temperature (T
1
) is 55.8
C and rst peak
temperature (T
p1
) is 93.1
C, while non-isothermal DSC analysis
for sample B (vanillin-based epoxy resin and Epikure F 205)
shows that initial onset temperature (T
1
) and rst peak
temperature (T
p1
) are 60.1
C and 97
C, respectively. T
1
and T
p1
of sample B are higher than those for sample A because the
amount of epoxy groups in sample B are less than that in
sample B. Comparison on the total heat of curing reaction (DH)
of samples A and B conrms this claim since DH of sample A is
higher than that of sample B. It is worth mentioning that
opening of epoxy rings by amine groups is an exothermic
reaction; therefore, the more epoxy groups are presented in
structure, the higher the heat release is set to be. For sample C
(vanillin-based epoxy resin, Epikure F 205 and 2 wt% inorganic
accelerator), the curing process of epoxy resin is divided into
two steps. In the rst step, as presented in Table 2, T
1
and T
p1
Fig. 4 Comparison of arrangement of epoxy structure with and without inorganic accelerator (top), and heat flow versus temperature obtained
for the various samples at heating rate of 10
C min
1
(bottom).
Fig. 3 FT-IR spectrum of synthesized vanillin-based epoxy resin.
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