Study on the synthesis of bio-based epoxy curing agent derived from myrcene and castor oil and the properties of the cured products

Article abstract of DOI:10.1039/c6ra24818g

Two novel bio-based epoxy curing agents derived from myrcene (MMY) and castor oil (CMMY) were prepared, respectively. Their chemical structures were confirmed by Fourier transform infrared spectrometry (FTIR) and ¹H nuclear magnetic resonance (¹HNMR). The two curing agents were used to cure a commercial epoxy resin (E-51). The MMY-cured epoxy resin had very poor toughness while the CMMY-cured epoxy resin had low strength, so the two curing agents were mixed at different weight ratios to form new curing agents for the E-51 epoxy resin. The tensile strength, impact strength, dynamic mechanical properties, thermal stability, micro-morphology of fracture surfaces and gel content of the cured epoxies were all investigated. The curing behaviors of the cured epoxies were studied by differential scanning calorimetry (DSC). Results show that the elongation at break is increased and the tensile strength and glass transition temperature are decreased with increasing weight ratio of CMMY, while the impact strength is increased gradually. The initial degradation temperatures of all the cured epoxy resins were above 367 °C. The gel contents of the epoxy resins cured with the mixed curing agents were above 87%. The activation energies for the systems with MMY and CMMY were 75.90 and 67.69 kJ mol^?1, respectively.

Full text of DOI:10.1039/c6ra24818g

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Study on the synthesis of bio-based epoxy curing
agent derived from myrcene and castor oil and the
properties of the cured products
Cite this: RSC Adv., 2017, 7, 238
Xuejuan Yang, Chunpeng Wang, Shouhai Li,*ab Kun Huang, Mei Li, Wei Mao,
a
ab
ab
ab
a
a
ab
Shan Cao and Jianling Xia*
Two novel bio-based epoxy curing agents derived from myrcene (MMY) and castor oil (CMMY) were
prepared, respectively. Their chemical structures were confirmed by Fourier transform infrared
1
1
spectrometry (FTIR) and H nuclear magnetic resonance ( HNMR). The two curing agents were used to
cure a commercial epoxy resin (E-51). The MMY-cured epoxy resin had very poor toughness while the
CMMY-cured epoxy resin had low strength, so the two curing agents were mixed at different weight
ratios to form new curing agents for the E-51 epoxy resin. The tensile strength, impact strength, dynamic
mechanical properties, thermal stability, micro-morphology of fracture surfaces and gel content of the
cured epoxies were all investigated. The curing behaviors of the cured epoxies were studied by
differential scanning calorimetry (DSC). Results show that the elongation at break is increased and the
tensile strength and glass transition temperature are decreased with increasing weight ratio of CMMY,
while the impact strength is increased gradually. The initial degradation temperatures of all the cured
Received 6th October 2016
Accepted 1st November 2016
ꢀ
epoxy resins were above 367 C. The gel contents of the epoxy resins cured with the mixed curing
DOI: 10.1039/c6ra24818g
agents were above 87%. The activation energies for the systems with MMY and CMMY were 75.90 and
ꢁ1
www.rsc.org/advances
67.69 kJ mol , respectively.
Chemicals and materials derived from renewable resources
have received considerable attention in recent years as the
1
. Introduction
Epoxy resin, which is an important type of thermosetting poly- petroleum depletion and environmental problem become
6,7
mer with a history of more than 60 years, is widely used in increasingly serious. For this reason, many biobased epoxy
protective coatings, adhesives, castings, composites, micro- curing agents have been prepared. For example, a novel
electronic encapsulated materials and printed circuit boards vegetable-oil-based polyamine was prepared from grape seed oil
8
due to their advantages of versatility, moisture, solvent and using cysteamine chloride by thiol–ene. Differential scanning
chemical resistance, mechanical properties, adhesion and calorimetry (DSC) showed that the light-color cardanol-based
1–3
electrical properties.
Nowadays, the most common and epoxy curing agent, synthesized from cardanol butyl ether,
important class is the epoxy resin synthesized from bisphenol A formaldehyde and diethylenetriamine, was less reactive than
and epichlorohydrin. Before curing, epoxy monomers or oligo- phenalkamine and had markedly higher impact strength and
mers have at least two epoxy groups, but aer curing, they lap shear strength attributed to the cavities formed in the curing
9
become cross-linked networks due to the reaction with a suit- process. Rosin-derived amine curing agent containing an
4
able curing agent or catalyst. Amine-type and anhydride-type imide structure displayed a similar modulus but higher glass
curing agents are commonly used for epoxy resin and they act transition temperature than the commercial aromatic amine
10
as an important role since the weight ratio of curing agents can curing agent. The fully biobased epoxy resins prepared from
go up as high as 50 wt% in the epoxy formulations and can terpene maleic anhydride and maleopimaric acid have compa-
determine the ultimate properties of the cured resin in large rable or even better mechanical properties and thermal stability
5
extent.
than the bisphenol A type epoxies cured by petroleum-based
curing agents.
5
Terpenes, which are natural monomers containing carbon
a
Institute of Chemical Industry of Forestry Products, CAF, Key Lab. of Biomass Energy skeletons of isoprene, can be extracted from many essential oils
and Material, Jiangsu Province, National Engineering Lab. for Biomass Chemical
Utilization, Key and Lab. on Forest Chemical Engineering, SFA, Nanjing 210042,
China. E-mail: yangxuejuan2008@126.com; lishouhai1979@163.com; xiajianling@
11
and used as a versatile chemical feedstock. The chemical
structures of several common terpenes and their derivatives are
shown in Fig. 1. b-Myrcene, a monoterpene that bears three
double bonds including conjugated diene, is extracted from the
1
b
26.com; Fax: +86-025-85482454; Tel: +86-025-85482453
Institute of Forest New Technology, CAF, Beijing 100091, China
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Fig. 1 The chemical structures of several common terpenes.
essential oils of many plants, such as laurel, verbena, cypress corporation of institute of chemical industry of forestry prod-
1
2,13
and hop.
cracking of b-pinene (production of 26 000 tons), which is ob-
Myrcene is commercially prepared from the ucts. All the reagents were used as received.
14
tained from turpentine oil. Myrcene can react with different
dienophiles to form a variety of fragrant compounds and
pharmaceutical intermediates.
that could be used in deodorants was recently synthesized from
myrcene, and the optimal catalyst for the addition of methanol
to myrcene was AlCl
resin was prepared from the reaction of myrcene with 1,10-
methylenedi-4,1-phenylene)bismaleimide, and its thermal,
mechanical and water-resistance properties were investigated.
In this study, a Diels–Alder adduct of myrcene and maleic
anhydride was synthesized and used to cure epoxy E-51. Despite
the excellent strength and modulus, the cured epoxy resin had
poor impact property and very low elongation at break. As we all
know, the toughness of epoxy resins can be improved by incor-
porating a exible epoxy resin, curing agent or reactive additive
2
.2 Synthesis
.2.1 Synthesis of adduct of myrcene and maleic anhydride
MMY). 98.00 g (1.00 mol) of maleic anhydride was charged to
2
15,16
For example, one fragrance
(
a four-neck 500 mL ask equipped with a magnetic stirrer,
a thermometer, a dropping funnel and a reux condenser. The
17
3
. Moreover, one biobased thermosetting
ꢀ
temperature was raised to 55 C and kept until maleic anhy-
dride melted completely. Then 136.23 g of distilled myrcene
(
(
MY, 1.00 mol) was added dropwise meanwhile maintaining the
18
ꢀ
ꢀ
temperature at 55–60 C. The mixture was heated to 70 C aer
all the myrcene was added and then kept reacting for 4 h. Next,
the crude product was distilled with a vacuum distillation
ꢀ
equipment (1.05 kPa) and the fraction of 190–195 C was
collected. Finally, 215.13 g (yield 91.85%) of adduct of myrcene
and maleic anhydride (MMY) was obtained. The synthesis route
was shown in Fig. 2.
19
into the network during curing. Castor oil is extensively used in
varnishes, paints, coatings and a variety of other products.
Because of the long aliphatic chains in the triglyceride section,
the materials prepared from castor oil have excellent exibility,
while the strength was poor. Ricinoleic acid obtained from the
hydrolysis of castor oil was used to modify the myrcene-based
adduct. The modied myrcene-based adduct was also used as
the curing agent for epoxy E-51. The cured epoxy resins had
excellent exibility, but too poor strength, which limited their
applications. Therefore, the two curing agents at different weight
ratios were mixed and used to co-cure epoxy E-51, and the cured
epoxy material was supposed to have the balanced strength and
2
.2.2 Synthesis of castor oil modied adduct of myrcene
and maleic anhydride (CMMY). 40.00 g (1.00 mol) of NaOH was
dissolved in 330 mL of ethanol–H O (1 : 1, v/v), and then the
2
mixture was introduced into a 1000 mL four-necked round-
bottom ask equipped with a mechanical stirrer and a ther-
mometer. When the mixture was heated to 70 C, 311.15 g (0.33
mol) of castor oil was slowly added to the above reaction system
ꢀ
with a dropping funnel over half an hour. Reaction continued at
ꢀ
7
0 C for 2.0 h. Aer that, the reacting system was adjusted to
ꢁ
1
pH ¼ 2–3 by slowly adding a certain amount of 5 mol L HCl,
ꢀ
and the reaction continued at 70 C for 1.0 h. Aer standing for
3
0 min, the supernatant ricinoleic acid was taken out. The nal
product was washed with H O. Finally, the residual H O was
removed by vacuum distillation. Then 270.86 g (yield: 89.14%)
of light yellow liquid of pure ricinoleic acid was obtained, the
acid value is 173.84 (theoretical value 193.15 mg g , conversion
is 90.00%).
exibility. The tensile properties, dynamic mechanical proper-
2
2
ties, curing kinetics, thermostability and fracture morphology of
the cured resins were also studied.
ꢁ1
2. Experimental
80.00 g (0.27 mol) of ricinoleic acid and 62.79 g (0.27 mol) of
2.1 Materials and chemicals
adduct of myrcene and maleic anhydride were charged to a 250
ꢀ
Castor oil (>97%), 2,4,6-tris(dimethylaminomethyl)phenol were mL ask, the mixture reacted at 120 C for 2 h. Then the castor
purchased from Aladdin Industrial Corporation. Myrcene was oil modied adduct of myrcene and maleic anhydride was ob-
purchased from Jiangxi global natural spices Co., contains tained. The synthesis route was shown in Fig. 2.
about 79% of myrcene. Maleic anhydride (stabilized, 99.5%),
2.2.3 Preparation of the cured epoxy samples. MMY and
benzyltriethylamine chloride ($98.0%), sodium hydrate, CMMY were mixed well by weight ratios of 100/0, 75/25, 50/50,
hydrochloric acid and ethyl alcohol were purchased from the 25/75, 0/100, respectively, and they were used as the curing
Group chemical reagent Co. Distilled water was prepared in our agent. Then the mixed curing agent and the epoxy resin E-51
laboratory. Epoxy resin E-51 (epoxy value was 0.51/100 g) was were mixed uniformly in the stoichiometric balance (molar
provided by Nanjing science and technology development ratio of epoxy groups to curing agent was 1 : 1), and 2,4,6-
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Fig. 2 The synthesis route of MMY and CMMY.
Table 1 Formula of epoxy mixture for curing
The dynamic mechanical analysis (DMA) was performed on
a Q800 dynamic mechanical thermal analyzer (TA, USA).
Sample
E-51 (g)
MMY
CMMY
DMP-30 (g)
Samples were test in double cantilever mode with a frequency of
3
1
Hz and the dimension of the samples were 60 ꢂ 10 ꢂ 4 mm .
MMY100
33.48
32.46
31.46
30.46
29.46
40.00
30.00
20.00
10.00
0
0
0.73
0.72
0.71
0.70
0.69
ꢀ
MMY75/CMMY25
MMY50/CMMY50
MMY25/CMMY75
CMMY100
10.00
20.00
30.00
40.00
All the tests were swept from ꢁ70 to 150 C at a heating rate of
ꢀ
ꢁ1
3 C min
.
The degree of cure of each sample was tested by the Soxhlet
extraction. 1.5 g of cured sample was extracted for 24 h, using
2
00 ml acetone as solvent. For the accuracy, each sample tested
three times.
Thermogravimetric analysis (TGA) was performed using
a 409 PC thermogravimetric analyzer (Netzsch, Germany). Each
tris(dimethylaminomethyl)phenol (DMP-30) (1 wt% on the
basis of the total weight of curing agent and epoxy) used as the
catalyst, all the formulations were listed in Table 1. Then the
mixtures were poured into the polytetrauoroethylene mold.
The curing reaction was performed at 120 C for 2 h and 160 C
for 3 h. At last the cured samples were carefully removed from
the mold and polished for further test.
ꢀ
ꢀ
sample was tested from 25 to 700 C at a heating rate of 10 C
min under a nitrogen atmosphere.
ꢁ
1
ꢀ
ꢀ
The fracture surfaces of epoxy resin cured with different
MMY/CMMY ratios were observed with the scanning electron
microscopy (SEM). A thin gold layer which is a few nanometers
thick was coated on the fracture surfaces to aid in feature
resolution. Then a S-3400N scanning electron microscopy
2.2.4 Characterization. FTIR analysis was performed
using an IS10 spectrometer (Nicolet, USA) by an attenuated
(Hitachi, Japan) was used to collect SEM images for all samples.
total reectance method. Each sample was scanned from
ꢁ
1 1
4
000 to 400 cm . HNMR (300 MHz) spectra were recorded
on an ARX300 spectrometer (Bruker, Germany). The chemical
shis relative to that of deuterated chloroform (d ¼ 7.26) was
recorded.
3
. Results and discussion
3.1 Fourier transform infrared spectrometry (FTIR)
Tensile property test was performed following ASTM D638-03
using a CMT4303 universal test machine (SANS, China) with the
The spectra of myrcene and MMY are shown in Fig. 3. The
spectrum of myrcene shows several characteristic peaks corre-
sponding to the conjugated double bonds (1644 and 1594
ꢁ1
test speed of 3 mm min . Impact strength was test following
GB/T 1043.1 using ZBC1400-C impact tester. For the sake of
accuracy, ve replicates were measured and the average values
ꢁ
1
ꢁ1
cm ), terminal vinyl group (3086 and 1795 cm ) and isolated
ꢁ1
20
double bond (1644 cm ). All these peaks clearly verify the
molecular structure of myrcene. The spectrum of MMY shows
the peaks characteristic of carbonyl in the cyclic anhydride
ꢀ
were obtained. All the samples were tested at 25 C.
Non-isothermal curing kinetics was studied by differential
ꢁ
1
scanning analysis (DSC) using a 2920 MDSC (TA instruments).
(
1835 and 1770 cm ) and the bending vibration of C–H in the
ꢀ
ꢁ1
Samples were scanned from 25 to 230 C at heating rates of 5,
hexatomic ring (1311 cm ). However, the peaks at 3086, 1795
and 1594 cm appearing on the spectrum of myrcene have
ꢀ
ꢁ1
ꢁ1
1
0, 15, 20 C min , respectively.
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1
1
3
.2 H nuclear magnetic resonance ( HNMR)
1
The HNMR spectra of MMY and CMMY are shown in Fig. 5 and
6. MY: (CDCl
3
, d ppm) 6.43–6.53 (t, 1H), 5.24–5.35 (t, 1H), 5.03–
5.18 (d, 2H), 4.70–4.85 (q, 2H), 2.18–2.36 (m, 4H), 1.67–1.85
(
m, 6H).
MMY. (CDCl
.29–3.34 (m, 2H), 2.30–2.60 (m, 4H), 1.93–2.13 (m, 4H), 1.56–
.71 (m, 6H).
Ricinoleic acid. (CDCl , d ppm) 5.34–5.55 (m, 2H), 3.58–3.67
3
, d ppm) 5.30–5.45 (t, 1H), 5.00–5.10 (t, 1H),
3
1
3
(m, 1H), 2.17–2.37 (m, 6H), 2.00–2.09 (m, 2H), 1.57–1.67 (m,
2H), 1.42–1.50 (m, 2H), 1.22–1.42 (m, 12H), 0.84–0.94 (m, 3H).
CMMY. (CDCl , d ppm) 5.22–5.48 (m, 2H), 5.00–5.10 (t, 1H),
3
4
2
.83–4.94 (t, 1H), 4.08–4.15 (m, 1H), 2.90–3.11 (m, 2H), 2.42–
.64 (m, 6H), 2.18–2.40 (m, 12H), 1.90–2.11 (m, 4H), 1.53–1.68
(m, 6H), 1.16–1.40 (m, 12H), 0.79–0.92 (m, 3H).
Fig. 3 The FTIR spectra of myrcene and MMY.
3.3 Curing kinetics analysis
The nonisothermal DSC thermograms of the co-cured samples
basically disappeared. These evidences conrm the occurrence are shown in Fig. 7, and the DSC results are summarised in
of the Diels–Alder reaction between myrcene and maleic Table 2. Clearly, each sample exhibits only one exothermic
anhydride.
peak during the curing procedure. As the heating rate
The FTIR spectra of castor oil, ricinoleic acid and CMMY are increases, the initial temperature (Ti), peak exothermic
shown in Fig. 4. The spectrum of castor oil shows the peaks temperature (T ), and temperature at curing end (T ) all rise
p
e
attributed to the stretching vibration of –OH in fatty acid chain and the exothermic peak becomes sharper, which indicate
ꢁ
1
(
3450 cm ) and the stretching vibration of C–O–C in the a much more focused exothermic process and a quicker curing
ꢁ1
triglyceride section of castor oil (1161 cm ). Compared with reaction. The shi in the curing temperature is probably more
castor oil, the spectrum of ricinoleic acid shows no peak of methodological. The whole curing process is composed of
ꢁ
1
ꢁ1
C–O–C at 1161 cm , but a new peak at 3400–2500 cm
different elementary reactions depending on the heating
21,22
attributed to the stretching vibration of –OH in carboxyl group. rates,
while the DSC measures the overall reaction enthalpy.
The hydrogen bonding association makes the absorption peak Therefore, without comprehensive analysis of possible
wide and overlapped with the peaks of –CH and –CH at 3015, elementary reactions, DSC alone provides very limited infor-
3
2
ꢁ
1
2
927 and 2854 cm . In spectrum of CMMY, the associated mation on the mechanism. Nevertheless, the effects of curing
absorption peak is intensied obviously and a new peak rates on curing reaction can be eliminated by linear extrapo-
ꢁ1
23,24
assigned to the C]O of ester group appears at 1774 cm
.
lating the heating rate to innitely slow,
so that the curing
These changes of absorption peaks suggest the successful temperature at zero heating rate is determined. The T , T and
i
p
synthesis of the target object.
Te at zero heating rate are listed in Table 2, ranging from 110.38
1
Fig. 4 The FTIR spectra of MY and MMY.
Fig. 5 HNMR spectra of MMY.
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Table 2 DSC data of epoxy resin cured with pure MMY and CMMY
a
b
DH
( C) (J g
DH
ꢀ
ꢁ1
ꢀ
ꢀ
ꢀ
ꢁ1
ꢁ1
Sample ( C min
)
T
i
( C)
T
p
( C)
T
e
)
(kJ mol )
MMY
0
5
0
5
0
0
5
0
5
0
110.38 134.92 152.52
—
—
115.61 141.34 159.74 245.60 48.16
127.20 153.72 174.19 237.70 46.67
133.84 161.36 183.12 232.31 45.55
137.67 166.56 188.64 210.95 41.36
1
1
2
CMMY
103.47 127.48 157.28
—
—
109.46 134.72 165.16 169.48 33.23
117.07 147.80 179.52 174.04 34.13
123.76 155.74 188.35 173.51 34.02
129.06 162.28 195.28 179.54 35.20
1
1
2
a
On the basis of per mole of epoxide.
ꢀ
ꢀ
to 134.92 C for MMY and 103.47 to 127.48 C for CMMY. These
data are references for temperature setting in study on
isothermal curing. The total reaction enthalpy (DH) of the two
system changed little with the heating rate (Table 2). The
enthalpy of E-51/MMY is close to the DGEAPA/NMA and
DGEDA/NMA system report by Huang et al., while the enthalpy
1
Fig. 6 The HNMR spectra of CMMY.
25
of E-51/CMMY system is lower than that.
The activation energy can be computed from various
methods. In our study, Kissinger's equation was used:
26,27
2
p
ln(b/T ) ¼ ln(AR/E ) ꢁ (E /RT )
a
a
p
p
where b is the heating rate, T is the peak temperatures on DSC
curves, A is a pre-exponential factor, R is the ideal gas constant
ꢁ
1
(
a
8.314 J (mol K) ) and E is the activation energy of curing
ꢁ
1
2
reaction (kJ mol ). The plot of ln(b/T ) against 1/T is a good
p
p
linear relationship and the slope is equal to ꢁE /R (Fig. 8). The
a
ꢁ1
E was calculated to be 75.90 kJ mol for the MMY system and
7.69 kJ mol for the CMMY system. On one hand, the lower E
a
a
ꢁ1
6
is probably due to the exible aliphatic chains that enable to
move more freely and thus increase the contact possibility
28
between the curing agent and the epoxy. On the other hand, in
Fig. 7 The nonisothermal DSC curves of samples cured with MMY (a)
and CMMY (b).
2
p
Fig. 8 Linear plots of ln(b/T
) vs. 1/T
p
based on Kissinger equation.
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Fig. 9 DSC curves of the cured samples with different MMY/CMMY
weight ratios.
E-51/MMY curing system, the anhydride group rstly reacted
with the hydroxyl of E-51, and a carboxyl group was produced.
Then the carboxyl group continued to react with the epoxy
group. However, in E51/CMMY curing system, the carboxyl
group directly reacted with epoxy group. Therefore, the E
1/MMY curing system is slight higher than that of E51/CMMY
curing system.
a
of E-
5
Besides, the cured samples were checked by DSC, the DSC
thermograms of them are shown in Fig. 9. There were no
exothermic peaks noted except a little peak at the DSC curve of
CMMY. This little peak was probably due to the crystalline part
melting peak of cured sample. The DSC results imply that the
sample have already cured completely. It is worthy noting that
there were no peaks on DSC curves of MMY100 and MMY75/
CMMY25-cured sample. This was probably due to the tested
Fig. 10 DMA curves of cured epoxy resin with different MMY/CMMY
weight ratios.
T
g
of the two samples were lower than the initial testing of rigid alicyclic structure is improved, which enhances the
temperature, thus, they were can not been detected.
hardness and rigidity of the cured resins. Under a given load,
the network containing more rigid alicyclic structure is more
resistant against deformation and thus has a higher storage
modulus. Besides the crosslink density is increased with the
increasing content of MMY when the MMY content is less than
3
.4 Dynamic mechanical analysis (DMA)
Fig. 10 shows the storage modulus curve (a) and loss factor
tan d, b) curve of the cured epoxy resins with different MMY/
(
7
5% wt. The higher crosslink density positively contributes to
CMMY weight ratios. The peak temperature of tan d is dened
31
higher storage modulus. On the other hand, with the increase
of CMMY weight ratio, the gel content rst rises and then
declines (Table 3). This is probably because the steric hindrance
effect of cycloaliphatic structure in MMY is gradually weakened
0
as the glass transition temperature (T
g
). Storage modulus (E ) in
the rubbery state can be used to calculate the crosslink density
29,30
according to the rubber elasticity theory:
0
E ¼ 3n RT
e
0
Table 3 The T and gel content of the cured sample
where E is the storage modulus of the cured sample in the
rubber state. R is the gas constant and T is the absolute
g
0
T
( C)
g
ꢀ
E at T
g
+
n
e
/
Gel content
(%)
0
ꢀ
temperature. In this study the E at T
g
+ 30 C was taken to
ꢀ
ꢁ3
Sample
30 C (MPa) (mol m
)
ensure the cured samples were in the rubber state. The cross-
linking density of the cured sample are shown in Table 3.
MMY100
MMY75/CMMY25 58.03 19.00
MMY50/CMMY50 54.03
MMY25/CMMY75 45.09
CMMY100
61.59 19.06
2096
2110
469
313
140
84.46 ꢃ 1.20
87.34 ꢃ 0.96
91.89 ꢃ 1.03
88.22 ꢃ 0.86
57.71 ꢃ 0.98
Clearly, all the cured samples show one T , and the storage
g
4.18
2.72
1.11
modulus of the cured resins are elevated with the increase of
ꢀ
MMY weight ratio, particularly above ꢁ25 C (Fig. 10a). This is
15.14
because with the increase of MMY weight ratio, the proportion
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with the decrease of MMY weight ratio. As the CMMY weight Table 4 Detailed data of tensile and impact property of cured epoxy
resin
ratio exceeds 50 wt%, the inuence of steric effect is weakened,
while the chemical activities of the anhydride and carboxyl
groups play a dominant role in the curing process.
Tensile strength Elongation at Impact strength
ꢁ2
Samples
(MPa)
break (%)
(kJ m )
Nevertheless, the pure-CMMY-cured sample exhibits higher
storage modulus than other samples at low temperature. This MMY100
48.74 ꢃ 1.18
7.54 ꢃ 0.53 8.55 ꢃ 0.80
6.47 ꢃ 0.42 13.87 ꢃ 0.66
6.60 ꢃ 0.40 17.29 ꢃ 1.10
259.43 ꢃ 2.30 62.51 ꢃ 1.32
565.83 ꢃ 8.23 Unbroken
MMY75/CMMY25 42.11 ꢃ 0.92
MMY50/CMMY50 35.55 ꢃ 0.56
MMY25/CMMY75 11.11 ꢃ 0.34
is probably because the long-aliphatic-chain moieties in
CMMY are packed more regularly. The optimally compacted
conguration has very high interaction and cohesive energy,
CMMY100
0.43 ꢃ 0.05
3
2,33
which improves the modulus of the cured epoxy resin.
However, the non-covalent interaction is weakened, as the
exible chains get enough kinetic energy to overcome the

strength of 48.74 MPa, an elongation at break of 7.54% and
energetic barrier for their motion with the temperature rises.
Under this circumstance, the storage modulus decreases
dramatically.
ꢁ
2
impact strength of 8.55 kJ m . Although the strength is
lower than a reported commercial glycidyl ether type epoxy
DER332, the elongation at break is 20% improved and the
impact strength is 11% improved. With the increase of
MMY weight ratio, the tensile strength is improved while the
elongation at break rst decreases and then increases. When
the CMMY weight ratio is less than 50 wt%, the elongation at
break is low (6.47–7.54%) and does not vary largely. In
addition, with the CMMY weight ratio increasing from 50 to
With the increase of CMMY weight ratio, T
g
decreases
3
4
ꢀ
from 61.59 to 15.14 C, this is because the long exible
aliphatic chains and esters in CMMY increase the free volume
in the cured epoxy networks. With the temperature rises, the
ester group and the exible aliphatic segment move more
freely at lower temperature. Thus, the sample containing
g
more CMMY has a relatively lower T . Besides, the tan d peak
of pure CMMY-cured sample is more wider than that of other
samples, this is because under the alternative stress, the
7
5 wt%, the tensile strength is decreased by more than two-
thirds from 35.55 to 11.11 MPa, while the elongation at
break is dramatically increased by 40 times from 6.60 to

exible segments in CMMY could begin to move at lower
2
59.43%. The reason is that CMMY contains a long aliphatic
temperature than the alicyclic moieties do. In other words,
the E-51/CMMY100 system is more heterogeneous than other
system.
chain and an ester group which could enhance the exibility
3.5 Tensile properties
The tensile curves of the co-cured epoxy resins are shown in
Fig. 11 and the detailed data of tensile and impact property
are listed in Table 4. Clearly, the samples containing more
than 50 wt% of MMY exhibit a rigid behavior and an elon-
gation at break below 8%, while the samples containing less
than 50 wt% of MMY show a exible behavior without
a yielding point. The pure-MMY-cured sample has a tensile
Fig. 12 TGA curves of cured epoxy resin with different MMY/CMMY
ratios.
Table 5 The detailed thermogravimetric datas of the cured epoxy
resin
ꢀ
ꢀ
Sample
T
i
( C)
T
5% ( C)
Char yield (%)
MMY100
372.60
374.70
370.30
373.90
367.30
347.93
351.53
341.88
336.21
332.72
10.06
6.93
6.46
5.47
4.57
MMY75/CMMY25
MMY50/CMMY50
MMY25/CMMY75
Fig. 11 The tensile stress–strain of the cured epoxy resin with different CMMY100
curing agents weight ratios.
244 | RSC Adv., 2017, 7, 238–247
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of the cured epoxy resins. When the cured samples are impact test results are in accordance with the tensile and DMA
stretched, the long chain could move more freely to offset analysis discussed above.
and balance with the external forces, so the samples con-
taining more CMMY are more extendable. Although MMY
and CMMY have the same numbers of reactive groups,
3.6 Thermal gravimetric analysis (TGA)
CMMY has a molecule weight about 2.3 times larger than The curves of weight ratio against temperature are displayed in
MMY. Besides, the gel content of sample cured with CMMY is Fig. 12 and the details are listed in Table 5. Clearly, all the cured
much lower than that cured with MMY. Therefore, the samples show one-stage degradation. The initial degradation
crosslink density of an MMY-cured sample is higher than temperatures (T ) of the samples are all very close and around
i
ꢀ
that of a CMMY-cured sample. In turn, a higher crosslink 370 C. The T (temperature corresponding to 5% of weight loss)
5%
density would improve the strength and decrease the elon- of the samples cured with the different MMY/CMMY weight ratios
gation at break. Moreover, the structure of MMY contains rises with the increase of MMY weight ratio. This is because the
a hexatomic ring, which could bestow the cured epoxy resin alicyclic structure in the MMY is more stable than the aliphatic
with high strength and rigidity. Thus the tensile strength is structure in CMMY. However, the T5% of the pure-MMY-cured
enhanced gradually with the increase of MMY weight ratio.
sample is slightly lower than that of the MMY75/CMMY25 cured
The impact strength is improved gradually with the sample. This is probably because the latter has a better cross-
increasing of CMMY content, this is also mainly due to the long linked network than the former. Usually, the char yield is
35
aliphatic chain in the ricinoleic acid part. The exible aliphatic inversely proportional to the hydrogen content of cured resins. As
chain can move more freely when the sample is impacted, CMMY contains more methyls and methylenes than MMY does,
which could consume large amount of impact energy. The the char yield decreases with the increase of CMMY weight ratio.
Fig. 13 SEM images of fracture surfaces of cured epoxy resin. (a) MMY100 (250ꢂ); (b) MMY75/CMMY25 (250ꢂ); (c) MMY50/CMMY50 (250ꢂ); (d)
MMY25/CMMY75 (250ꢂ); (e) CMMY100 (250ꢂ); (f) MMY25/CMMY75 (1000ꢂ).
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.7 Morphology
Paper
3
Acknowledgements
Fig. 13 shows the tensile fracture surface morphology of epoxy
resins cured with different MMY/CMMY weight ratios. The
fracture surface of MMY-cured sample (Fig. 13a) presents
ladder-like structure. This ladder-like structure was due to the
concentration of stress during the stretch which in turn would
This research project was supported by Five-year science and
technology support project (Grant number: 2015BAD15B04), the
Basic research funding earmarked for the Key Lab of Biomass
Energy and Material of Jiangsu Province, China (Grant number:
JSBEM-S-201508) and National Nonprot Institute Research
Grant of CAFINT (Grant number: CAFINT2014C08).
3
6
causing crack propagated along different directions. On the
contary, the fracture surface of pure CMMY-cured sample
(
Fig. 11e) is at and featureless. On one hand, this is because
ꢀ
the T of the pure CMMY-cured sample is 15.14 C, which is References
g
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