materials
Article
Epoxy Resins Toughened with Surface Modified
Epoxidized Natural Rubber Fibers by
One-Step Electrospinning
Joo Ran Kim 1 and Jung J. Kim 2, *
1
2
*
Fiber Science, Cornell University, Ithaca, NY 14853, USA; [email protected]
Department of Civil Engineering, Kyungnam University, Changwon-si 51767, Korea
Correspondence: [email protected]; Tel.: +82-55-249-6421
Academic Editor: Nicole Zander
Received: 4 April 2017; Accepted: 25 April 2017; Published: 27 April 2017
Abstract: Epoxidized natural rubber fibers (ERFs) are developed through one-step electrospinning
and directly deposited into epoxy resins without collecting and distributing of fibers. The shape
of ERFs shows rough surface due to different evaporation rate of solvent mixture consisting
of chloroform and dichloromethane and the average diameter of ERFs is 6.2 µm. The increase
of ERFs loading from 0 to 20 wt % into the epoxy resin increases the fracture strain
significantly from 1.2% to 13% and toughness from 0.3 MPa to 1.9 MPa by a factor of 7. However,
the tensile strength and Young’s modulus decrease about 34% from 58 MPa to 34 MPa and
from 1.4 GPa to 0.9 GPa, respectively. Due to the crosslinking reactions between oxirane groups of
ERFs and amine groups in the resin, surface roughness and the high aspect ratio of ERFs, ERFs result
in more effective toughening effect with the minimum loss of tensile properties in epoxy resins.
Keywords: natural rubber fibers; epoxy resin; toughening; surface modification
1. Introduction
Electrospinning as an efficient and versatile technology has been developed for generating
ultrafine fibers in a variety of materials, such as polymers, inorganic materials, and hybrid materials [1].
These fibers or non-woven fibrous membranes exhibit diameters ranging from a few nanometers
to a few micrometers and have advantages of large surface area-to-volume ratios and various
morphological modifications [2]. Electrospinning system typically consists of three basic components:
a high voltage supply, a Taylor cone and a metal collector [3]. The electrodes contain a high voltage
supply, with one connected to the polymer solution being metered and the other connected to the
collector. As electrostatic repulsion counteracts the surface tension on the Taylor cone, the polymer
solution is stretched to draw threads [4]. This stretched polymer solution evaporates the solvent to
form solid fibers before fibers reach to the collector [5]. During electrospinning, the morphology
or mechanical properties can be easily modified by tuning many parameters such as polymer
concentration, distance between a Taylor cone and a collector, applied voltage, syringe feeding rate, etc.
depending on applications such as biomaterials, reinforcement, barrier textiles and toughening [6–9].
Among these applications, brittle thermoset resins have been studied to increase the material toughness
using electrospinning technique [7]. Epoxy resins toughened with electrospun polysulfone nanofibers
showed an increase in fracture toughness (GIC ) of 158%, 261% and 281% at 1, 3 and 5 wt %
of fiber loadings, respectively [10]. The epoxy reinforced with carbon fibers composite showed
100% improvement in fracture toughness as 20% polycaprolactone nanofiber was incorporated into
the epoxy composites to increase the interlaminar fracture toughness [11]. Electrospun poly(vinyl
pyrrolidone)/tetraethyl orthosilicate/silica nanoparticle nanofibers (2% loading) improved the
Materials 2017, 10, 464; doi:10.3390/ma10050464
www.mdpi.com/journal/materials
Materials 2017, 10, 464
2 of 13
fracture toughness by 1.9 times that of neat polypropylene resin [12]. Nanofibers composed of
styrene-butadiene-styrene (SBS) block copolymer as a rubbery core and polyacrylonitrile (PAN) as a
hard sheath have been produced via coaxial electrospinning, and can be used to improve the toughness
of epoxy matrix increased impact energy up to 150% [13].
Many studies have investigated improving toughness of epoxy resin using synthetic
copolymers and metal derivatives as toughening agents such as carboxyl-terminated acrylonitrile
butadiene, synthetic ductile polymers such as the acrylic rubber, hydroxy-terminated butadiene or
acrylonitrile butadiene, epoxy-terminated acrylonitrile butadiensbse, epoxy-terminated butadiene and
amine-terminated acrylonitrile butadiene [14–17].
However, little research has been conducted on eco-friendly toughening agents to improve brittle
epoxy resins. For example, epoxy resins with 60% functionalized green halloysite nanotubes resulted in
increased impact strength to about 5.8 kJ m−2 , 1.5 times that of neat epoxy resin even metal conductivity
of 0.85 W m−1 K−1 [18]. After adding 10% hyperbranched polyurethane into epoxy resins, the impact
strength was increased to triple the amount of neat epoxy [19]. In another example, silicone skeleton
was used as a toughening agent added into the epoxy resin, showing increased impact, flexural
and tensile strength of epoxy resin by about 92.5%, 36.0% and 88.6%, respectively, compared to neat
epoxy resin [20].
The problem in ductile toughening agents is that most resins experience a decline in their tensile
properties with the addition of ductile materials. Hence, good interfacial bonding via chemical
reactions between the matrix and the toughening agent has been an active research area in order
to improve the toughness without loss of tensile properties in the matrix. For example, as 0.5 wt %
dodecane functionalized tannic acid incorporated into epoxy resin as the toughening agent increased
the impact strength by up to 196% compared to that of neat epoxy [21]. This tendency was due to good
interfacial bonding between terminal hydroxyl groups of dodecane functionalized tannic acid and the
epoxy matrix [21]. Glycerol (10 wt %) was used to improve toughness of poly(lactic acid) (PLA) matrix,
resulting in strain at break of up to 93% and fracture stress of 48 MPa compared to neat PLA matrix
with strain at break of 2.3% and fracture stress of 64 MPa. Again, this trend is due to esterification
reaction occurring at the interface between the hydroxyl groups of glycerol and the carboxylic groups
of PLA [22]. Eco-friendly natural toughening agents such as epoxidized natural rubber (ENR) and
liquid natural rubber have been found to be effective in improving the toughness in brittle resins
because of functional groups such as oxirane groups or hydroxyl groups [23–25]. One explanation is
that the oxirane groups of ENR are able to react with carboxyl, amine, glycidyl and hydroxyl groups
of matrices, crosslinking with ENR to offer good interfacial bonding in the matrix [26]. One example
is biodegradable PLA matrix and ENR blend via chemical reactions between oxirane groups and
carboxylic groups, resulting in improved toughness and tensile properties [27,28].
The primary objectives of this study were to develop micro-sized green elastic ENR fibers
(ERFs) with surface modification through one-step electrospinning. Figure 1 shows the one-step
electrospinning system developed in this study for the ERFs production and concurrently distribution
of ERFs with epoxy resin. ERF loading was varied at 5, 10, 15 and 20 wt % into epoxy resins (E-ERF 5,
E-ERF 10, E-ERF 15 and E-ERF 20, respectively) and its effect on toughness and other mechanical
properties was analyzed.
Materials 2017,
Materials
2017, 10,
10, 464
464
Materials 2017, 10, 464
of 13
13
33 of
3 of 13
Figure 1. One-step electrospinning to prepare ERFs into epoxy resins.
Figure
Figure 1.
1. One-step
One-step electrospinning
electrospinning to
to prepare
prepare ERFs into epoxy resins.
2. Results and Discussion
2.
2. Results
Results and
and Discussion
Discussion
Figure 2 shows the morphology of ERFs produced using 1% ENR polymer concentration.
Figure
shows
morphology of
produced
using
1% ENR
polymer
concentration.
Figureof22 diameter
shows the
the
of ERFs
ERFs
produced
using
polymer
concentration.
The range
of morphology
ERFs varied from
1.6 µm
to 13 µm
and1%
theENR
average
diameter
of ERFs is
The
range
of
diameter
of
ERFs
varied
from
1.6
µm
to
13
µm
and
the
average
diameter
of of
ERFs
is
The
of diameter
from
1.6 µm
to 13displays
µm and bumpy
the average
diameter
ERFs
aboutrange
6.2 µm
as shown of
in ERFs
Figurevaried
2a. The
surface
of ERFs
(spines-like)
and
rough
about
6.2
µm
as
shown
in
Figure
2a.
The
surface
of
ERFs
displays
bumpy
(spines-like)
and
rough
is about
µm as
2a. The surface
of ERFs
bumpy (spines-like)
and rough
edges
as 6.2
shown
in shown
Figure in
2b.Figure
This morphology
is related
to displays
a rapid evaporation
of the solvent
while
edges
as shown
shown in
inFigure
Figure2b.
2b.This
Thismorphology
morphologyisisrelated
related
to
a rapid
evaporation
of the
solvent
while
edges
as
to
a
rapid
evaporation
of
the
solvent
the fibrous jet is accelerated to the metal collector. Megelski et al. [29] have studied the while
effectthe
of
the
fibrous
jet
is accelerated
tometal
the metal
collector.
Megelski
et al.
[29]
have studied
the
effect of
fibrous
jet
is
accelerated
to
the
collector.
Megelski
et
al.
[29]
have
studied
the
effect
of
different
different solvents such as acetone, chloroform, tetrahydrofuran and their mixtures on the morphology
different
solvents
such as chloroform,
acetone, chloroform,
tetrahydrofuran
and
their mixtures
on the morphology
solvents
as polyethylene
acetone,
tetrahydrofuran
and their
mixtures
on the morphology
of fibers.
of
fibers.such
When
glycol (PEO)
was spun from
chloroform
displayed
rougher surface
of
of
fibers.
When
polyethylene
glycol
(PEO)
was
spun
from
chloroform
displayed
rougher
surface
of
When
polyethylene
glycol
(PEO)
spun [29].
fromSimilar
chloroform
displayed
fibers than
fibers than
other fibers
used
otherwas
solvents
studies
showedrougher
that thesurface
boilingof
temperature
fibers
than other
fibers
other
solvents
[29].
Similar
studies
showed
that the
boiling temperature
other
fibers
used
other used
solvents
[29].
Similar
studies
that
the[30].
boiling
temperature
ofshowed
solvent
of
solvent
was
associated
with surface
roughness
andshowed
diameter
size
Electrospun
fibers
of
solvent
was associated
with
surface roughness
andsize
diameter
size [30]. Electrospun
fibers
showed
was
associated
with
surface
roughness
and
diameter
[30].
Electrospun
fibers
showed
an
increase
an increase in surface roughness and pores with increasing boiling point of the solvent mixture since
an
increaseroughness
in surface and
roughness
and increasing
pores withboiling
increasing
boiling
of the
solvent
mixture
since
in surface
pores
with
point
of [30].
thepoint
solvent
mixture
since
thein
boiling
the
boiling temperature
is related
to volatility of solvents
As ENR
is dissolved
only
the
boiling
temperature
is
related
to
volatility
of
solvents
[30].
As
ENR
is
dissolved
in
only
temperature
is
related
to
volatility
of
solvents
[30].
As
ENR
is
dissolved
in
only
dichloromethane
dichloromethane while keeping other parameters constant, ERFs display smooth surface. As the
dichloromethane
while
keepingconstant,
other parameters
constant,
ERFs
display
smooth
surface.
As the
while
keeping
other
parameters
ERFs display
smooth
surface.
As for
the
mixture
of chloroform
mixture
of chloroform
and dichloromethane
was used
as the
solvent
ENR, the
mixture of
mixture
of
chloroform
and
dichloromethane
was
used
as
the
solvent
for
ENR,
the
mixture
of
and dichloromethane
usedroughness
as the solvent
forstudy.
ENR, the
mixture
of solvents
solvents
influences thewas
surface
in this
Since
ERFs are
directly influences
deposited the
intosurface
epoxy
◦
solvents
influences
the
surface
roughness
in
this
study.
Since
ERFs
are
directly
deposited
into
epoxy
roughness
in this
study.
Since ERFs
are directly deposited
into
resins heated
at 40 C boiling
during
resins
heated
at 40
°C during
electrospinning,
the mixture
ofepoxy
two solvents
has different
resins
heated
at
40
°C
during
electrospinning,
the
mixture
of
two
solvents
has
different
boiling
electrospinning,
of two
different (T
boiling
temperature
(Tboiling
rate
b ) and volatility
temperature
(Tb)the
andmixture
volatility
ratesolvents
becausehas
chloroform
b: 61 °C)
has higher
temperature
◦ C) has
◦ C) [29].
temperature
(Tb) and(Tvolatility
ratehigher
because
chloroform
(Tb: 61than
°C) dichloromethane
has higher boiling
temperature
because
chloroform
:
61
boiling
temperature
(T
:
39
b
b
than dichloromethane (Tb: 39 °C) [29].
than dichloromethane (Tb: 39 °C) [29].
Figure 2. The morphology of micro-sized ERFs produced using 1% ENR polymer concentration.
Figure 2.
2. The
The morphology
morphology of
of micro-sized
micro-sized ERFs
ERFs produced
produced using
using 1%
1% ENR
ENR polymer
polymer concentration.
concentration.
Figure
The internal structures of ERFs are shown in Figure 3. Left images are all brightfield images,
The internal structures of ERFs are shown in Figure 3. Left images are all brightfield images,
whereas right images are fluorescent images that indicate the presence of ENR stained with a
whereas right images are fluorescent images that indicate the presence of ENR stained with a
Materials 2017, 10, 464
4 of 13
The internal structures of ERFs are shown in Figure 3. Left images are all brightfield images,
Materials 2017, 10, 464
4 of 13
whereas right images are fluorescent images that indicate the presence of ENR stained with a
fluorescent
The ERFs
ERFs produced
produced using
fluorescent dye,
dye, appearing
appearing green
green colored.
colored. The
using 1%
1% polymer
polymer concentration
concentration
have
long
rod,
non-porous,
and
rough
surface,
as
shown
in
Figure
3a,c.
A
close
examination
have long rod, non-porous, and rough surface, as shown in Figures 3a,c. A close examination of
of the
the
white
square
area
in
Figure
3a,b
demonstrates
the
rough
surface
at
the
edges
of
the
ERFs.
Their
white square area in Figures 3a,b demonstrates the rough surface at the edges of the ERFs. green
Their
color
that thethat
ERFs
areERFs
composed
of ENR and
have rough
edges
as shown
3b,d.
greenindicates
color indicates
the
are composed
of ENR
and have
rough
edgesinasFigure
shown
in
The
weak
green
signal
indicates
ERFs
have
smaller
diameter
or
thinner
region
of
ENR
(less
ENR
Figures 3b,d. The weak green signal indicates ERFs have smaller diameter or thinner region of ENR
concentration);
however, the
strongthe
green
signal
indicates
larger diameter
or thickerorarea
of ERFs.
(less ENR concentration);
however,
strong
green
signal indicates
larger diameter
thicker
area
The
results
from
a
Confocal
laser
scanning
microscopy
(CLSM)
images
confirm
the
morphology
of
of ERFs. The results from a Confocal laser scanning microscopy (CLSM) images confirm the
ERFs
as
shown
in
SEM
results
in
Figure
2.
morphology of ERFs as shown in SEM results in Figure 2.
Figure 3.
3. The
ENR concentration
concentration by
by
Figure
The internal
internal structures
structures of
of micro-sized
micro-sized ERFs
ERFs produced
produced using
using 1%
1% ENR
CLSM:
(a)
the
brightfield
image
of
ERFs;
(b)
the
fluorescent
image
of
ERFs
at
low
magnification;
CLSM: (a) the brightfield image of ERFs; (b) the fluorescent image of ERFs at low magnification;
(c) the
the brightfield
brightfield image
image of
of the
the white
white square
square area;
area; (d)
(d) the
the fluorescent
fluorescent image
image of
of the
the white
white square
square area
area
(c)
(green
color
corresponds
to
ENR
stained
with
fluorescent
dye).
(green color corresponds to ENR stained with fluorescent dye).
Figure 4 shows attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR)
Figure 4 shows attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR)
spectra of neat ERFs, neat epoxy and all E-ERF resins. Neat epoxy specimen shows transmittance
spectra of neat ERFs,
neat epoxy and all E-ERF resins. Neat epoxy specimen shows transmittance
peaks at 860 cm−1 (epoxy C–O–C ring deformation) [31], 1020–1250 cm−−11 range (C–N stretching),
peaks at−1860 cm−1 (epoxy
C–O–C ring deformation) [31], 1020–1250 cm range (C–N stretching),
−1 (CH and CH2 deformation), 1528 cm−1 (NH stretching) and 2862–2937 cm−1
1375 cm−1 and 1494 cm−
1
1375 cm and 1494 cm (CH and CH2 deformation), 1528 cm−1 (NH stretching) and−12862–2937 cm−1
range (CH and CH3 asymmetric stretching) [32]. The broad peak around 1600 cm corresponds to
range (CH and CH3 asymmetric stretching) [32]. The broad peak around 1600 cm−1 corresponds to
NH2 bending of excess amine group from the curing agent in the epoxy resin. Neat ERFs
NH2 bending of excess amine group
from the curing agent in−1the epoxy resin. Neat ERFs transmittance
transmittance peaks
at 751
cm−1, 852 cm−1
and 1246 cm corresponding to the oxirane groups
−
1
−
1
peaks at 751 cm , 852 cm and 1246 cm−1 corresponding to the oxirane groups appeared in the
appeared in the spectra [24].
spectra [24].
The E-ERF resins show additional peak at 3000–3600 cm−1 range to indicate the presence of
hydroxyl groups (–OH) produced by crosslinking reaction between oxirane groups of epoxy resin or
ENFs and amine of the curing agent, as shown in Figure 4b. The oxirane group is shown to open up
during curing process and the reaction product is secondary amines with hydroxyl groups [33]. Two
hydroxyl groups of secondary amines can react further to form ether linkages [33]. In Figure 4a, the
Materials 2017, 10, 464
5 of 13
The E-ERF resins show additional peak at 3000–3600 cm−1 range to indicate the presence of
hydroxyl groups (–OH) produced by crosslinking reaction between oxirane groups of epoxy resin
or ENFs and amine of the curing agent, as shown in Figure 4b. The oxirane group is shown to open
Materials
2017,
10, 464process and the reaction product is secondary amines with hydroxyl groups [33].
5 of 13
up
during
curing
Two hydroxyl groups of secondary amines can react further to form ether linkages [33]. In Figure 4a,
reaction
between
oxirane
groups
of ERFs
or epoxy
resinresin
and and
excess
amine
of theofcuring
agentagent
shows
the
reaction
between
oxirane
groups
of ERFs
or epoxy
excess
amine
the curing
−1 range
−
1
peak
at
1020–1250
cm
corresponding
to
C–N
linkages
[34].
The
increase
of
ERFs
loading
into
shows peak at 1020–1250 cm range corresponding to C–N linkages [34]. The increase of ERFs loading
−1
−1 range),
epoxy
resin
shows
C–N (1020–1250
(1020–1250cm
cm
range), C–O–C
C–O–C(ether
(etherstretching
stretching
into
epoxy
resin
showsincrease
increaseininpeaks
peaks of
of C–N
−1) and –OH (3000–3600 cm
−1), as shown in Figure 4a. The increased transmittance
−
1
−
1
vibration
at
945
cm
vibration at 945 cm ) and –OH (3000–3600 cm ), as shown in Figure 4a. The increased transmittance
peaks
demonstrate
that
E-ERF
resins
show
additional
crosslinking
reactions
between
oxirane
group
peaks
demonstrate
that
E-ERF
resins
show
additional
crosslinking
reactions
between
oxirane
group
of
of
ERFs
and
excess
amine
or
hydroxyl
group
in
epoxy
resins
[35].
ERFs and excess amine or hydroxyl group in epoxy resins [35].
Figure4. 4.(a)
(a)ATR-FTIR
ATR-FTIRspectra
spectraofofneat
neatepoxy
epoxyresin,
resin,neat
neatERFs
ERFsand
andallallE-ERF
E-ERFresins;
resins;(b)
(b)possible
possible
Figure
reactions
between
oxirane
group
amine
to form
the secondary
amines
with hydroxyl
reactions
between
oxirane
group
andand
amine
to form
the secondary
amines
with hydroxyl
groupsgroups
and
and
two
secondary
amines
form
ether
linkages
in
epoxy
resin.
two secondary amines form ether linkages in epoxy resin.
Figure 5 presents differential scanning calorimetry (DSC) thermograms of neat epoxy, neat
Figure 5 presents differential scanning calorimetry (DSC) thermograms of neat epoxy, neat ERFs,
ERFs, and all E-ERF resins at different loading of ERFs scanned from −50◦ to 200 °C. In the present
and all E-ERF resins at different loading of ERFs scanned from −50 to 200 C. In the present study,
study, neat ERFs show Tg1 (glass transition temperature of ENR) at −25 °C, whereas neat epoxy
neat ERFs show Tg1 (glass transition temperature of ENR) at −25 ◦ C, whereas neat epoxy matrix
matrix shows Tg2 (glass transition temperature of epoxy resin) at 55 °C. E-ERF resins, however, show
shows Tg2 (glass transition temperature of epoxy resin) at 55 ◦ C. E-ERF resins, however, show little
little change in Tg1 from −28 to −25 °C but significant change in Tg2 from 55 to 65 °C as ERFs loading
change in Tg1 from −28 to −25 ◦ C but significant change in Tg2 from 55 to 65 ◦ C as ERFs loading
increases from 0 to 20 wt %. At 5 wt % loading of ERFs in epoxy resin, there is little increase in Tg2.
increases from 0 to 20 wt %. At 5 wt % loading of ERFs in epoxy resin, there is little increase in Tg2 .
However, with the 10% addition of ERFs, Tg2 increases by 7 °C. This increasing trend continues with
However, with the 10% addition of ERFs, Tg2 increases by 7 ◦ C. This increasing trend continues with
ERFs loading of 15 and 20 wt % in epoxy resins to 63 and 65 °C, respectively. This trend is attributed
ERFs loading of 15 and 20 wt % in epoxy resins to 63 and 65 ◦ C, respectively. This trend is attributed
to cross-linking reactions between oxirane group of ERFs and excess amine or hydroxyl group in
to cross-linking reactions between oxirane group of ERFs and excess amine or hydroxyl group in
epoxy resins to create a 3D cross-linking structure. These cross-links increase the chain length and
epoxy resins to create a 3D cross-linking structure. These cross-links increase the chain length and
interactions between chains, resulting in limited freedom to rotate about the bonds and
interactions between chains, resulting in limited freedom to rotate about the bonds and consequently,
consequently, increases the Tg2 temperature of E-ERF resins [36,37]. The finding is in line with the
increases the Tg2 temperature of E-ERF resins [36,37]. The finding is in line with the observations
observations made by Thomas et al. [38] for epoxy resin toughened with liquid natural rubber. Other
made by Thomas et al. [38] for epoxy resin toughened with liquid natural rubber. Other researchers
researchers showed this shift of Tg with increasing loading concentrations of liquid rubber [39]. They
showed this shift of Tg with increasing loading concentrations of liquid rubber [39]. They explained
explained that the addition of liquid natural rubber into the epoxy resin increased the curing
that the addition of liquid natural rubber into the epoxy resin increased the curing temperature by
temperature by allowing crosslinking reactions to occur between the epoxy resin and hydroxyl or
allowing crosslinking reactions to occur between the epoxy resin and hydroxyl or carboxylic groups
carboxylic groups of rubber [40].
of rubber [40].
Materials
10, 464
Materials
2017,2017,
10, 464
Materials 2017, 10, 464
66ofof1313
6 of 13
Figure 5. DSC thermograms of neat ERFs, neat epoxy and all E-ERF resins.
Figure
5. DSC
thermograms
neatERFs,
ERFs,neat
neatepoxy
epoxyand
andall
all E-ERF
E-ERF resins.
Figure
5. DSC
thermograms
ofof
neat
Figure 6 presents dynamic mechanical analysis (DMA) plots of E-ERF resins scanned from −50 to
Figure 6 presents dynamic mechanical analysis (DMA) plots of E-ERF resins scanned from −50 to
100
°C.
storagedynamic
modulusmechanical
(E′) of E-ERFanalysis
resins is plotted
a function
of temperature
in Figure
6a.−50 to
Figure 6The
presents
(DMA)asplots
of E-ERF
resins scanned
from
100
°C.storage
The storage
modulus
(E′) of resin
E-ERF
resins
is plotted
aswhereas
a function
of resins
temperature
in
Figure 6a.
◦
0
The
modulus
of
neat
epoxy
shows
nearly
5
GPa,
E-ERF
display
around
100 C. The storage modulus (E ) of E-ERF resins is plotted as a function of temperature in Figure 6a.
The
of
neat resins,
epoxyas
resin
shows nearly
5 GPa, whereas
E-ERF
resins display
around
4.5storage
GPa.modulus
In modulus
the caseofofneat
E-ERF
the
temperature
there is aE-ERF
sharp decrease
in storage
The storage
epoxy resin
shows
nearlyincreases,
5 GPa, whereas
resins display
around
4.5modulus
GPa. In starting
the casearound
of E-ERF
as the temperature
there
is a sharp
decrease
storage
−28resins,
°C, corresponding
to Tg1 of increases,
ENR. The E′
for neat
ENR resin
showsinthe
4.5 GPa.
In the
case ofaround
E-ERF−28
resins,
as the temperature
increases,
there
is aneat
sharp
decrease
in storage
modulus
starting
corresponding
to Tg1E-ERF
of ENR.
The
E′ for
ENR
resin
shows
final storage
modulus close
to°C,
0 MPa
at 60 °C, whereas
resins
show
E′ values
above
4 GPa
at the
◦ C, corresponding to T of ENR. The E0 for neat ENR resin shows the
modulus
starting
around
−
28
g1
final
storage modulus close to 0 MPa at 60 °C, whereas E-ERF resins show E′ values above 4 GPa at
60 °C.
◦
0
◦
final60
storage
°C. modulus close to 0 MPa at 60 C, whereas E-ERF resins show E values above 4 GPa at 60 C.
Figure 6. DMA plots of neat epoxy resin, neat ENR sheet, and E-ERF resins at different loading of
ERFs in the temperature range of −50 to 100 °C: (a) the storage modulus; and (b) the tan δ curves.
Figure
6. DMA
of neat
epoxy
sheet,
and E-ERF
resins
at different
loading
of
Figure
6. DMA
plotsplots
of neat
epoxy
resin,resin,
neat neat
ENRENR
sheet,
and E-ERF
resins
at different
loading
of ERFs
◦
ERFs
in
the
temperature
range
of
−50
to
100
°C:
(a)
the
storage
modulus;
and
(b)
the
tan
δ
curves.
in the temperature range of −50 to 100 C: (a) the storage modulus; and (b) the tan δ curves.
Materials 2017, 10, 464
7 of 13
Materials 2017, 10, 464
7 of 13
The mechanical
factor
(tan (tan
δ) plots
E-ERF
areresins
shownare
in Figure
6b.in
TheFigure
intermolecular
The
mechanicalloss
loss
factor
δ) for
plots
forresins
E-ERF
shown
6b. The
interactions
between
ENR
and
epoxy
resin
brought
about
different
altering
trends
of
T
ENR
g because
intermolecular interactions between ENR and epoxy resin brought about different altering
trends
of
◦C
is
at
elastic
state
whereas
rigid
epoxy
resin
stays
at
glassy
state.
The
tan
δ
peak
at
around
−
25
Tg because ENR is at elastic state whereas rigid epoxy resin stays at glassy state. The tan δ peak at
represents
of therelaxation
neat ENR.ofAs
ERFs
increases
from 0 increases
to 20 wt %,
the 0Tg1
of
around
−25relaxation
°C represents
the
neatloading
ENR. As
ERFs loading
from
to values
20 wt %,
◦ C; however, T values of E-ERF resins increase from
E-ERF
resins
decrease
slightly
from
−
28
to
−
25
g2 °C; however, Tg2 values of E-ERF
the Tg1 values of E-ERF resins decrease slightly from −28 to −25
◦ C. The reason is that the chemical crosslinking reactions between oxirane groups of ENR
51
to
65
resins increase from 51 to 65 °C. The reason is that the chemical crosslinking reactions between
and excess
crosslinking
epoxycrosslinking
resin occur and
result
long molecular
chain
offerinhigher
oxirane
groups
of ENRagent
andinexcess
agent
in in
epoxy
resin occur
andand
result
long
than
intermolecular
hydrogen
bonding.
Therefore,
free
motion
of
epoxy
molecular
chain
segments
molecular chain and offer higher than intermolecular hydrogen bonding. Therefore, free motionare
of
restrained,
whichchain
results
in an increase
of Tg2 [41,42].
epoxy
molecular
segments
are restrained,
which results in an increase of Tg2 [41,42].
Figure 7. SEM images of fracture surface in tensile mode: (a) the fracture surface of neat epoxy resin;
Figure 7. SEM images of fracture surface in tensile mode: (a) the fracture surface of neat epoxy resin;
(b) the fracture surface of E-ERF 10 resins; (c,d) the top surface of E-ERF 10 resin; (e) bridging zone of
(b) the fracture surface of E-ERF 10 resins; (c,d) the top surface of E-ERF 10 resin; (e) bridging zone of
E-ERF 10 resin; (f) nanoweb formation in bridging zone.
E-ERF 10 resin; (f) nanoweb formation in bridging zone.
Figure 7 shows the tensile fracture surfaces of E-ERF 10 resins. For neat epoxy resin, many
Figure
7 shows or
thesharp
tensilecracks
fracturecan
surfaces
of E-ERF
10 resins.
For neat
epoxy have
resin, reported
many layered
layered
structures
be seen
in Figure
7a. Many
studies
this
structures
or
sharp
cracks
can
be
seen
in
Figure
7a.
Many
studies
have
reported
this
appearance
be
appearance to be the distinctive brittle fracture surface of thermoset resins [43,44]. However, to
after
the
distinctive
brittle
fracture
surface
of
thermoset
resins
[43,44].
However,
after
adding
ERFs
to
epoxy
adding ERFs to epoxy resins, the resins decrease in brittle features and sharp crests on the fracture
resins, theAs
resins
in brittle
featuresinto
andepoxy
sharp resins
crests on
the fracture
surfaces.
As the loading
surfaces.
the decrease
loading of
ERFs added
increase,
the fracture
surfaces
display
of
ERFs
added
into
epoxy
resins
increase,
the
fracture
surfaces
display
rougher
and
glossier
rougher and glossier features without the formation of cracks along the fiber directionfeatures
which
without
formation
cracks along of
theweak
fiber direction
which
indicates
the primary
characteristic
of
indicatesthe
the
primaryofcharacteristic
interfacial
bonding
between
fibers and
resins [45].
Figure 7b shows the tensile fracture surface of E-ERF 10 resin, clearly demonstrating that ERFs are
seen protruding long fibers from the epoxy resin. The ERFs show good dispersion with the wide
Materials 2017, 10, 464
8 of 13
weak interfacial bonding between fibers and resins [45]. Figure 7b shows the tensile fracture surface
of E-ERF 10 resin, clearly demonstrating that ERFs are seen protruding long fibers from the epoxy
resin. The ERFs show good dispersion with the wide range of diameter sizes in in E-ERF 10 resin.
The interface between ERFs and epoxy resin displays little fiber pullout, delamination or interface
failure induced from weak interfacial bonding between fibers and resins. The strong interfacial
bonding occurs by means of crosslinking reaction between oxirane groups of ERFs and amine groups
of excess curing agent, increasing the load transfer of ERFs to the epoxy resin. Chemical reactions such
as epoxy-amine reaction in the interface play a critical role for interfacial adhesion, specifically the
oxirane group opening in the interface and the formation of covalent bonds with ERFs [46]. Figure 7c
shows the top surface of the E-ERF 10 resin. ERFs display circular or entangled fibers on the surface;
however, they still maintain their rough surface to help ERFs to bond physically to epoxy resin,
as shown in Figure 7d. Figure 7e shows the bridging zone of the E-ERF 10 resin. It demonstrates an
effective toughening mechanism through the addition of ductile materials such as rubber materials.
ERFs effectively connect or bridge two fracture surfaces of the E-ERF 10 resins. Because of inherent
toughness and high aspect ratio of ERFs, cracks larger than 500 µm are stopped from propagating
into larger cracks. These results show that the adhesion between the ERFs and the epoxy resin are
reasonably strong with the absence of breakage or pullout features [47]. The enlarged image (Figure 7f)
shows the nanoweb (fishnet-like) formation in the bridging zone. These nanoweb formations have
been explained by Liu et al. [48]. The solvent remained in electrospun fibers caused by the incomplete
solvent evaporation led to form the connected fibers or nanoweb [48,49].
After curing, neat epoxy resin and all E-ERF resins are tested for their tensile properties. Figure 8
shows typical tensile stress–strain plots of neat epoxy resin and all E-ERF resins and their tensile
properties are summarized in Table 1. The increase in ERFs loading in the epoxy resin from 0 to 20 wt %
increase the fracture strain significantly from 1.2% to 12.8% and the toughness at break increase seven
times from 0.3 to 1.9 MPa. Remarkably, the tensile strength and Young’s modulus show only a minimal
decline of 34% from 58 to 34 MPa and from 1.4 GPa to 0.9 GPa, respectively. The neat epoxy specimens
are broken in a brittle manner with no apparent yielding, revealing the typical characteristic of a
thermoset resin. The tensile strength decreases with an increase in ERFs loading since the ERFs act as
stress concentrators, causing a decrease in facture strength and Young’s modulus [50]. E-ERF 5 and
E-ERF 10 show about 1.2 GPa and 1.1 GPa of modulus, and 52 MPa and 47 MPa of fracture strength,
respectively, compared to that of neat epoxy resin having modulus of 1.4 GPa and tensile strength of
58 MPa. Compared to other studies, epoxy resin containing ENR particles (not in fiber form) showed
much loss in tensile properties [24,51,52]. For example, epoxy resin with 12% liquid epoxidized natural
rubber (particle size 0.48 to 0.67 in diameter) increased 125% impact toughness, but decreased tensile
properties by 15 MPa from 65 MPa to 50 MPa, compared to neat epoxy resin [51]. As 20% ENR was
added into epoxy resin it have been reported to improve the impact strength of 2.55 J m−1 and KIC
value of 1.79 MPa m1/2 but the modulus decreased to half that of neat epoxy resin [52]. However,
the fiber form of ERFs results in significantly higher strain, fracture toughness, and minimal loss of
tensile properties when incorporated into the epoxy resin in this study. High aspect ratio due to its
fiber form contributes to better interfacial interaction, flexibility in surface functionalities, and superior
mechanical performance than the particle form of ENR [53]. The minimal loss in strength and modulus
can be explained by the crosslinking reactions between oxirane groups and amine which result in a
very effective 3D structure that helps to maintain the tensile properties.
Materials 2017, 10, 464
9 of 13
Materials 2017, 10, 464
9 of 13
Figure
Figure 8.
8. Typical
Typical stress–strain
stress–strain plot of neat epoxy resin and all E-ERF resins.
Table
of all
all E-ERF
E-ERF resins.
resins.
Table 1.
1. Tensile
Tensile properties
properties of
Type
Typeofof
Resins
Resins
E-ERF
E-ERF
Epoxy/ERF
Composition Tensile
Tensile
StrengthTensile
Tensile
StrainToughness
Toughness
at BreakYoung’s
Young’s
Modulus
Epoxy/ERF
Composition
Strength
Strain
at Break
Modulus
(w/w)
(MPa)
(%) (%)
(MPa)
(GPa)
(w/w)
(MPa)
(MPa)
(GPa)
100/0
100/0
95/5
95/5
90/10
90/10
85/15
85/15
80/20
80/20
58 (1.6)
1.2 (0.1)
0.3 (0.03)
58 (1.6)
* *
1.2 (0.1)
0.3 (0.03)
52 (1.2)
4.4 (0.3)
0.8 (0.1)
52 (1.2)
4.4 (0.3)
0.8 (0.1)
47 (1.0)
7.5 (0.5)
1.4 (0.1)
47 (1.0)
7.5 (0.5)
1.4 (0.1)
39 (0.8)
10.2 (0.4)
1.8 (0.2)
39 (0.8)
1.8 (0.2)
34 (1.1)
12.8 10.2
(0.8)(0.4)
1.9 (0.1)
34 (1.1)
12.8 (0.8)
1.9 (0.1)
* The values in parentheses are standard deviation.
(0.09)
1.4 1.4
(0.09)
1.2 1.2
(0.05)
(0.05)
1.1 (0.05)
1.1 (0.05)
1.0 (0.04)
(0.04)
0.9 1.0
(0.05)
0.9 (0.05)
* The values in parentheses are standard deviation.
3. Experimental Section
3. Experimental Section
3.1. Materials
3.1. Materials
Araldite® GY 6010 (MW: 11,000–14,000, unmodified DGEBA liquid) was purchased from
®
® 956-2 (modified
Araldite
11,000–14,000,
unmodified
DGEBA
liquid)amines)
was purchased
from
Huntsman
Co. GY
(Salt6010
Lake (MW:
City, UT,
USA). Aradur
aliphatic
was purchased
® 956-2 (modified aliphatic amines) was purchased
Huntsman
Co.
(Salt
Lake
City,
UT,
USA).
Aradur
from Krayden Inc. (Denver, CO, USA). Epoxidized natural rubber (ENR, 50 mol % conversion)
from
Krayden by
Inc.Malaysian
(Denver, CO,
USA).Board.
Epoxidized
natural rubber
(ENR,
50 mol % dye,
conversion)
was
was donated
Rubber
Adirondack
green 520
fluorescent
chloroform,
donated
by Malaysian
Rubber from
Board.
Adirondack
green Co.
520 (St.
fluorescent
chloroform,
dichloromethane
were purchased
Sigma
Aldrich Chemical
Louis, MO,dye,
USA).
dichloromethane were purchased from Sigma Aldrich Chemical Co. (St. Louis, MO, USA).
3.2. Electrospinning of ENR Fibers
3.2. Electrospinning of ENR Fibers
ENR (1 wt %) was dissolved in the mixture of 1:2 (v/v) of dichloromethane and chloroform
ENR
wt %)for
was
the mixture of
1:2 (v/v)
of dichloromethane
anda chloroform
and
and then (1
stirred
12dissolved
h at roomintemperature
(RT).
The solution
was put into
10 mL syringe,
then
stirred for 12
h fibers
at room
temperature
(RT).
The solution
into feeding
a 10 mLrate
syringe,
and
and electrospun
into
(ERFs)
with a high
voltage
of 12 kV.was
Theput
syringe
used was
−
1
electrospun
into
fibers
(ERFs)
with
a
high
voltage
of
12
kV.
The
syringe
feeding
rate
used
was
30
µL
30 µL min and the distance between the syringe needle tip (diameter: 1 mm) and the collector was
−1
◦C
min
the distance
syringe
needle
tip (diameter:
mm) collector
and the collector
wasto15
15 cm.and
Precured
epoxy between
resin wasthe
placed
above
the bath
containing1metal
and heated
40cm.
Precured
epoxy
placed
thedeposited
bath containing
with stirred
at 50resin
rpm.was
ERFs
wereabove
directly
into themetal
epoxycollector
resin. and heated to 40 °C with
stirred at 50 rpm. ERFs were directly deposited into the epoxy resin.
3.3. Fabrication Process of Epoxy Composites at Different Loadings of ERFs
3.3. Fabrication Process of Epoxy Composites®at Different Loadings of ERFs
The curing agent, 35 wt % Aradur 956-2, was added into neat epoxy matrix (DGEBA) and
stirred
50 rpmagent,
for 3035min.
ERFswas
using
1 wtinto
% ENR
concentration
Theatcuring
wt %Electrospun
Aradur® 956-2,
added
neat polymer
epoxy matrix
(DGEBA)were
and
directlyatdeposited
into30
themin.
epoxy
resin. Then
the epoxy
ERFs atconcentration
different loadings
stirred
50 rpm for
Electrospun
ERFs
using 1resin
wt containing
% ENR polymer
were
was poured
into a into
Teflon
mold (127
127 mm)
and precured
for 3 h atERFs
40 ◦ C.
The epoxy
matrix
directly
deposited
the® epoxy
resin.×Then
the epoxy
resin containing
at different
loadings
Materials 2017, 10, 464
10 of 13
containing ERFs was cast in the form of sheets and the epoxy sheets were then cured in the oven at
1 h for 100 ◦ C. Afterwards, all specimens were conditioned at 21 ◦ C and 65% RH for 72 h prior to
their characterization.
3.4. Thermal Analysis
All epoxy resins containing ERFs at 5, 10, 15 and 20 wt % (E-ERF 5, E-ERF 10, E-ERF 15,
and E-ERF 20, respectively) were tested their thermal properties. E-ERF resins were scanned on
differential scanning calorimeter (TA-DSC 2000, TA instruments Inc., New Castle, DE, USA) at a
heating rate of 5 ◦ C min−1 from −50 to 200 ◦ C to evaluate the changes in their thermal properties
under a nitrogen flow rate of 100 mL min−1 .
3.5. Tensile Properties of E-ERF Resins
All E-ERF resins were cut by a laser cutter to obtain the dimensions of 50 mm × 10 mm and
were characterized for their tensile properties according to ASTM D882-02 standard at a strain rate of
0.6 mm min−1 and gauge length of 30 mm. All tests were performed on Instron universal tensile tester
(Instron, Model 5566, Instron Co., Canton, MA, USA) using at least ten specimens [54].
3.6. Dynamic Mechanical Analysis of E-ERF Resins
Dynamic mechanical properties of E-ERF resins were analyzed using DMA (DMA Q800,
TA Instruments, New Castle, DE, USA) in 3-point bending mode. Resins were cut into the final
specimen dimensions of 30 mm × 10 mm and scanned from −50 to 100 ◦ C at a ramp rate of
5 ◦ C min−1 . All tests were performed at 5 µm amplitude and sinusoidal tensile stress (frequency,
1 Hz), and conducted at least three times [55].
3.7. Morphological and Chemical Analyses
Microstructures of the tensile fracture surfaces of all E-ERF resins were analyzed using Tescan
Mira3 FESEM. ATR-FTIR (Magna 560, Nicolet Instrument Technologies, Fitchburg, WI, USA) was
used to confirm crosslinking reactions of E-ERF resins. Neat ENR, neat epoxy matrix and all E-ERF
resins were scanned from 700 to 4000 cm−1 wavenumbers with a total of 300 scans and a resolution of
2 cm−1 . ENR solution was stained with 0.5% Adirondack green 520 fluorescent dye of ENR before
electrospinning. Then, it was electrospun directly into epoxy matrix. The resins were analyzed using
a confocal laser scanning microscope (CLSM; Zeiss LSM710, Thornwood, NY, USA) and 520 nm
excitation filter with 25× objective lens.
4. Conclusions
A convenient one-step electrospinning was developed for green elastic natural rubber fibers.
ERFs were directly deposited to epoxy resins without collecting or redepositing of fibers. The surface
of ERFs displayed bumpy (spines-like) and rough edges due to different volatility rate of solvents.
The range of diameter of ERFs varied from 1.6 µm to 13 µm and the average diameter was
approximately 6.2 µm. The increase of ERFs loading from 0 to 20 wt % into epoxy resins increased
the fracture strain significantly from 1.2% to 12.8% and toughness at break from 0.3 MPa to 1.9 MPa,
an increase by a factor of 7. However, Young’s modulus decreased minimally from 1.4 GPa to 0.9 GPa.
E-ERF resins showed the presence of strong interfacial bonding between the ERFs and the epoxy
resin without breakage or pullout of fibers. Inherent rubber toughness and high aspect ratio of fiber
form of ERFs could contribute to efficient energy dissipation, since effective crosslinking reactions
between the oxirane groups and amine or hydroxyl groups in epoxy resin allow the 3D structure to
serve as an effective toughening agent. Green elastic ERFs could be useful in many applications such
as construction materials, biomaterials, smart textiles, automotive and aerospace, therefore replacing
petroleum-derived elastic fibers.
Materials 2017, 10, 464
11 of 13
Acknowledgments: The authors would like to acknowledge the National Research Foundation (NRF) Grant
funded by the Korean government (No. 2017R1A2B4010594).
Author Contributions: Joo Ran Kim performed all experiments, measurements and biological data analysis;
and Jung J. Kim oversaw the work, directed the research and performed the data analysis. Both authors have
contributed to writing and discussions.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
Moghe, A.; Gupta, B. Co-axial electrospinning for nanofiber structures: Preparation and applications.
Polym. Rev. 2008, 48, 353–377. [CrossRef]
Teo, W.E.; Ramakrishna, S. A review on electrospinning design and nanofibre assemblies. Nanotechnology
2006, 17, R89. [CrossRef] [PubMed]
Deitzel, J.; Kosik, W.; McKnight, S.; Tan, N.B.; DeSimone, J.M.; Crette, S. Electrospinning of polymer
nanofibers with specific surface chemistry. Polymer 2002, 43, 1025–1029. [CrossRef]
Huang, Z.-M.; Zhang, Y.-Z.; Kotaki, M.; Ramakrishna, S. A review on polymer nanofibers by electrospinning
and their applications in nanocomposites. Compos. Sci. Technol. 2003, 63, 2223–2253. [CrossRef]
Theron, A.; Zussman, E.; Yarin, A. Electrostatic field-assisted alignment of electrospun nanofibres.
Nanotechnology 2001, 12, 384. [CrossRef]
Zussman, E.; Yarin, A.; Weihs, D. A micro-aerodynamic decelerator based on permeable surfaces of nanofiber
mats. Exp. Fluids 2002, 33, 315–320. [CrossRef]
Xiang, C.; Frey, M.W. Increasing mechanical properties of 2-D-structured electrospun nylon 6 non-woven
fiber mats. Materials 2016, 9, 270. [CrossRef]
Magniez, K.; Chaffraix, T.; Fox, B. Toughening of a carbon-fibre composite using electrospun poly
(hydroxyether of bisphenol a) nanofibrous membranes through inverse phase separation and inter-domain
etherification. Materials 2011, 4, 1967–1984. [CrossRef]
Kim, J.R.; Michielsen, S. Synthesis of antifungal agents from xanthene and thiazine dyes and analysis of their
effects. Nanomaterials 2016, 6, 243. [CrossRef] [PubMed]
Li, G.; Li, P.; Zhang, C.; Yu, Y.; Liu, H.; Zhang, S.; Jia, X.; Yang, X.; Xue, Z.; Ryu, S. Inhomogeneous toughening
of carbon fiber/epoxy composite using electrospun polysulfone nanofibrous membranes by in situ phase
separation. Compos. Sci. Technol. 2008, 68, 987–994. [CrossRef]
Van der Heijden, S.; Daelemans, L.; De Schoenmaker, B.; De Baere, I.; Rahier, H.; Van Paepegem, W.;
De Clerck, K. Interlaminar toughening of resin transfer moulded glass fibre epoxy laminates by
polycaprolactone electrospun nanofibres. Compos. Sci. Technol. 2014, 104, 66–73. [CrossRef]
Liang, Y.; Wen, S.; Ren, Y.; Liu, L. Fabrication of nanoprotrusion surface structured silica nanofibers for the
improvement of the toughening of polypropylene. RSC Adv. 2015, 5, 31547–31553. [CrossRef]
Guo, T.; Zhou, Z.; Guo, H.; Xiao, G.; Tang, X.; Peng, M. Toughening of epoxy resin with functionalized
core-sheath structured pan/sbs electrospun fibers. J. Appl. Polym. Sci. 2014, 131. [CrossRef]
He, J.; Raghavan, D.; Hoffman, D.; Hunston, D. The influence of elastomer concentration on toughness
in dispersions containing preformed acrylic elastomeric particles in an epoxy matrix. Polymer 1999, 40,
1923–1933. [CrossRef]
Chikhi, N.; Fellahi, S.; Bakar, M. Modification of epoxy resin using reactive liquid (ATBN) rubber.
Eur. Polym. J. 2002, 38, 251–264. [CrossRef]
Kunz, S.; Sayre, J.; Assink, R. Morphology and toughness characterization of epoxy resins modified with
amine and carboxyl terminated rubbers. Polymer 1982, 23, 1897–1906. [CrossRef]
Yamanaka, K.; Takagi, Y.; Inoue, T. Reaction-induced phase separation in rubber-modified epoxy resins.
Polymer 1989, 30, 1839–1844. [CrossRef]
Mo, H.; Yang, K.; Li, S.; Jiang, P. High thermal conductivity and high impact strength of epoxy nanodielectrics
with functionalized halloysite nanotubes. RSC Adv. 2016, 6, 69569–69579. [CrossRef]
Zou, Z.-P.; Liu, X.-B.; Wu, Y.-P.; Tang, B.; Chen, M.; Zhao, X.-L. Hyperbranched polyurethane as a highly
efficient toughener in epoxy thermosets with reaction-induced microphase separation. RSC Adv. 2016, 6,
18060–18070. [CrossRef]
Materials 2017, 10, 464
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
39.
40.
12 of 13
Zhang, D.; Liang, E.; Li, T.; Chen, S.; Zhang, J.; Cheng, X.; Zhou, J.; Zhang, A. Environment-friendly synthesis
and performance of a novel hyperbranched epoxy resin with a silicone skeleton. RSC Adv. 2013, 3, 3095–3102.
[CrossRef]
Fei, X.; Wei, W.; Zhao, F.; Zhu, Y.; Luo, J.; Chen, M.; Liu, X. Efficient toughening of epoxy-anhydride
thermosets with a bio-based tannic acid derivative. ACS Sustain. Chem. Eng. 2017, 5, 596–603. [CrossRef]
Coativy, G.; Misra, M.; Mohanty, A.K. Microwave synthesis and melt blending of glycerol based toughening
agent with poly (lactic acid). ACS Sustain. Chem. Eng. 2016, 4, 2142–2149. [CrossRef]
Tan, S.; Ahmad, S.; Chia, C.; Mamun, A.; Heim, H. A comparison study of liquid natural rubber (LNR) and
liquid epoxidized natural rubber (LENR) as the toughening agent for epoxy. Am. J. Mater. Sci. 2013, 3, 55–61.
Kim, J.R.; Netravali, A.N. Comparison of thermoset soy protein resin toughening by natural rubber and
epoxidized natural rubber. J. Appl. Polym. Sci. 2017. [CrossRef]
Salehabadi, A.; Bakar, M.A.; Bakar, N.H.H.A. Effect of organo-modified nanoclay on the thermal and
bulk structural properties of poly (3-hydroxybutyrate)-epoxidized natural rubber blends: Formation of
multi-components biobased nanohybrids. Materials 2014, 7, 4508–4523. [CrossRef]
Cosme, J.G.; Silva, V.M.; Nunes, R.R.; Picciani, P.H. Development of biobased poly (lactic acid)/epoxidized
natural rubber blends processed by electrospinning: Morphological, structural and thermal properties.
Mater. Sci. Appl. 2016, 7, 210. [CrossRef]
Akbari, A.; Jawaid, M.; Hassan, A.; Balakrishnan, H. Epoxidized natural rubber toughened polylactic
acid/talc composites: Mechanical, thermal, and morphological properties. J. Compos. Mater. 2014, 48,
769–781. [CrossRef]
Saito, T.; Klinklai, W.; Yamamoto, Y.; Kawahara, S.; Isono, Y.; Ohtake, Y. Quantitative analysis for reaction
between epoxidized natural rubber and poly (l-lactide) through 1h-NMR spectroscopy. J. Appl. Polym. Sci.
2010, 115, 3598–3604. [CrossRef]
Megelski, S.; Stephens, J.S.; Chase, D.B.; Rabolt, J.F. Micro-and nanostructured surface morphology on
electrospun polymer fibers. Macromolecules 2002, 35, 8456–8466. [CrossRef]
Moroni, L.; Licht, R.; de Boer, J.; de Wijn, J.R.; van Blitterswijk, C.A. Fiber diameter and texture of electrospun
peot/pbt scaffolds influence human mesenchymal stem cell proliferation and morphology, and the release
of incorporated compounds. Biomaterials 2006, 27, 4911–4922. [CrossRef] [PubMed]
He, R.; Zhan, X.; Zhang, Q.; Chen, F. Toughening of an epoxy thermoset with poly [styrene-alt-(maleic
acid)]-block-polystyrene-block-poly (n-butyl acrylate) reactive core–shell particles. RSC Adv. 2016, 6,
35621–35627. [CrossRef]
Wang, N.; Zhang, L. Preparation and characterization of soy protein plastics plasticized with waterborne
polyurethane. Polym. Int. 2005, 54, 233–239. [CrossRef]
Acocella, M.; Corcione, C.E.; Giuri, A.; Maggio, M.; Maffezzoli, A.; Guerra, G. Graphene oxide as a catalyst
for ring opening reactions in amine crosslinking of epoxy resins. RSC Adv. 2016, 6, 23858–23865. [CrossRef]
Sam, S.; Ismail, H.; Ahmad, Z. Effect of epoxidized natural rubber on the processing behavior, tensile
properties, morphology, and thermal properties of linear-low-density polyethylene/soya powder blends.
J. Vinyl Add. Technol. 2010, 16, 238–245. [CrossRef]
Nikolic, G.; Zlatkovic, S.; Cakic, M.; Cakic, S.; Lacnjevac, C.; Rajic, Z. Fast fourier transform ir characterization
of epoxy GY systems crosslinked with aliphatic and cycloaliphatic eh polyamine adducts. Sensors 2010, 10,
684–696. [CrossRef] [PubMed]
Di Liello, V.; Martuscelli, E.; Musto, P.; Ragosta, G.; Scarinzi, G. Toughening of highly crosslinked
thermosetting resins by blending with thermoplastic polyether imide. Die Angew. Makromol. Chem. 1993,
213, 93–111. [CrossRef]
O'Driscoll, K.; Sanayei, R.A. Chain-length dependence of the glass transition temperature. Macromolecules
1991, 24, 4479–4480. [CrossRef]
Thomas, R.; Yumei, D.; Yuelong, H.; Le, Y.; Moldenaers, P.; Weimin, Y.; Czigany, T.; Thomas, S. Miscibility,
morphology, thermal, and mechanical properties of a dgeba based epoxy resin toughened with a liquid
rubber. Polymer 2008, 49, 278–294. [CrossRef]
Park, S.-J.; Jin, F.-L.; Lee, J.-R. Thermal and mechanical properties of tetrafunctional epoxy resin toughened
with epoxidized soybean oil. Mater. Sci. Eng. A 2004, 374, 109–114. [CrossRef]
Jong, L. Characterization of soy protein/styrene–butadiene rubber composites. Compos. A 2005, 36, 675–682.
[CrossRef]
Materials 2017, 10, 464
41.
42.
43.
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
13 of 13
Zhou, Q.; Zhang, L.; Zhang, M.; Wang, B.; Wang, S. Miscibility, free volume behavior and properties of
blends from cellulose acetate and castor oil-based polyurethane. Polymer 2003, 44, 1733–1739. [CrossRef]
Liu, D.; Tian, H.; Zhang, L.; Chang, P.R. Structure and properties of blend films prepared from castor
oil-based polyurethane/soy protein derivative. Ind. Eng. Chem. Res. 2008, 47, 9330–9336. [CrossRef]
Lodha, P.; Netravali, A.N. Thermal and mechanical properties of environment-friendly ‘green’plastics from
stearic acid modified-soy protein isolate. Ind. Crops Prod. 2005, 21, 49–64. [CrossRef]
Rahman, M.; Netravali, A.; Tiimob, B.; Rangari, V. Bioderived “green” composite from soy protein and
eggshell nanopowder. ACS Sustain. Chem. Eng. 2014, 2, 2329–2337. [CrossRef]
Kinloch, A.; Shaw, S.; Tod, D.; Hunston, D. Deformation and fracture behaviour of a rubber-toughened
epoxy: 1. Microstructure and fracture studies. Polymer 1983, 24, 1341–1354. [CrossRef]
Mäder, E.; Pisanova, E. Interfacial design in fiber reinforced polymers. Macromol. Symp. 2001, 168, 189–212.
[CrossRef]
Brodowsky, H.; Mäder, E. Jute fibre/epoxy composites: Surface properties and interfacial adhesion.
Compos. Sci. Technol. 2012, 72, 1160–1166.
Liu, M.; Guo, B.; Du, M.; Chen, F.; Jia, D. Halloysite nanotubes as a novel β-nucleating agent for isotactic
polypropylene. Polymer 2009, 50, 3022–3030. [CrossRef]
Ding, B.; Li, C.; Miyauchi, Y.; Kuwaki, O.; Shiratori, S. Formation of novel 2D polymer nanowebs via
electrospinning. Nanotechnology 2006, 17, 3685. [CrossRef]
Rooj, S.; Das, A.; Thakur, V.; Mahaling, R.; Bhowmick, A.K.; Heinrich, G. Preparation and properties of
natural nanocomposites based on natural rubber and naturally occurring halloysite nanotubes. Mater. Des.
2010, 31, 2151–2156. [CrossRef]
Cizravi, J.C.; Subramaniam, K. Thermal and mechanical properties of epoxidized natural rubber modified
epoxy matrices. Polym. Int. 1999, 48, 889–895. [CrossRef]
Mathew, V.S.; George, S.C.; Parameswaranpillai, J.; Thomas, S. Epoxidized natural rubber/epoxy blends:
Phase morphology and thermomechanical properties. J. Appl. Polym. Sci. 2013, 131. [CrossRef]
Ondarcuhu, T.; Joachim, C. Drawing a single nanofibre over hundreds of microns. Europhys. Lett. 1998, 42,
215. [CrossRef]
Wu, Q.; Selke, S.; Mohanty, A.K. Processing and properties of biobased blends from soy meal and natural
rubber. Macromol. Mater. Eng. 2007, 292, 1149–1157. [CrossRef]
Kim, J.R.; Sharma, S. The development and comparison of bio-thermoset plastics from epoxidized plant oils.
Ind. Crops Prod. 2012, 36, 485–499. [CrossRef]
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).