The Effect of Dynamic Vulcanization on the Properties of Polypropylene Ethylene-Propylene Diene Terpolymer Natural Rubber (PP EPDM NR) Ternary Blend
Polymer-Plastics Technology and Engineering
ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20
The Effect of Dynamic Vulcanization on the
Properties of Polypropylene/Ethylene-Propylene
Diene Terpolymer/Natural Rubber (PP/EPDM/NR)
Ternary Blend
Halimatuddahliana & H. Ismail
To cite this article: Halimatuddahliana & H. Ismail (2008) The Effect of Dynamic Vulcanization
on the Properties of Polypropylene/Ethylene-Propylene Diene Terpolymer/Natural Rubber
(PP/EPDM/NR) Ternary Blend, Polymer-Plastics Technology and Engineering, 48:1, 34-41, DOI:
10.1080/03602550802539270
To link to this article: http://dx.doi.org/10.1080/03602550802539270
Published online: 17 Dec 2008.
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Date: 29 August 2016, At: 01:10
Polymer-Plastics Technology and Engineering, 48: 34–41, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0360-2559 print/1525-6111 online
DOI: 10.1080/03602550802539270
The Effect of Dynamic Vulcanization on the Properties
of Polypropylene/Ethylene-Propylene Diene Terpolymer/
Natural Rubber (PP/EPDM/NR) Ternary Blend
Halimatuddahliana1 and H. Ismail2
1
Department of Chemical Engineering, Universitas Sumatera Utara, Indonesia
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia,
Penang, Malaysia
2
Dynamic vulcanization has been extensively applied to
the vulcanization of the soft elastomer phase of a blend
with rigid thermoplastics. The process is carried out under
high shear and above the melting point of the thermoplastic at sufficiently high temperature to activate and complete
the vulcanization. During dynamic vulcanization of rubbers, the polymeric chains of the base elastomer become
interconnected, converting the viscous gum into an elastic
network. In dynamic vulcanization of the thermoplastics
elastomer, the structure is generated in small rubber particles dispersed in the thermoplastic polymer matrix. Various
types of curatives can generally be used in dynamic vulcanized blends, such as sulfur, phenolic resin, and peroxide.
This paper examines the effect of dynamic vulcanization
with sulfur on the process development and some properties, such as tensile properties, chemical and oil resistances,
and gel content of the PP=EPDM=NR blends, along with
the accompanying characteristics in the crystallinity and
morphology.
The effects of dynamic vulcanization on the process development
and some properties, such as tensile properties, swelling index,
gel content, crystallinity, and morphology, of the polypropylene
(PP)/ethylene-propylene diene terpolymer (EPDM)/natural rubber
(NR) blends were investigated. Dynamically vulcanized blends show
higher stabilization torque than unvulcanized blends. In terms of
tensile properties, the tensile strength and tensile modulus (stress
at 100% elongation, M100) of the vulcanized blends have been found
to increase as compared with the unvulcanized blends, whereas the
elongation at break is higher in the blend with richer EPDM content. These results can be attributed to the formation of crosslinking in the rubber phase. The formation of cross-links in the
rubber phase has also been proved by swelling index and gel content.
The percentage of crystallinity of the blends is decreased by
dynamic vulcanization. Scanning electron microscopy (SEM)
micrographs from the surface extraction of the blends support that
the cross-links occurred during dynamic vulcanization.
Keywords Crystallinity; Dynamic vulcanization; Gel content;
Swelling index
EXPERIMENTAL
INTRODUCTION
Several works have reported the dynamic vulcanization
of TPE, especially the PP=EPDM blends, and most of the
investigations focused on the processing and the improvements of mechanical and physical properties of the
blends[1–6]. However, replacement of EPDM with NR in
PP=EPDM blends has been considered due to the
reduction of the cost. It has also been observed that the
partial replacement of EPDM by NR decreased the properties of PP=EPDM blends[7]. Therefore, dynamic vulcanization has been introduced in the PP=EPDM=NR blend to
improve the properties of such blends.
Materials
Polypropylene (PP) homopolymer used in this study was
an injection-molding grade, supplied by Titan PP Polymers
(M) Sdn Bhd, Johor, Malaysia (TITANPRO 6331 grade),
with a melt flow index (MFI) value of 14 gr=10 min at
230C and 2.16 kg. Ethylene-propylene diene terpolymer
(EPDM-EPT 3072E), with Mooney viscosity, ML (1 þ 4)
74 at 100C was purchased from Luxchem Trading Sdn.
Bhd. Natural rubber (SMR L), with Mooney viscosity,
ML (1 þ 4), 73 at 100C was obtained from Hokson Rubber
Trading Sdn. Bhd, Seremban. Curatives, i.e., 5 phr of zinc
oxide (ZnO), 1.5 phr of stearic acid, 1 phr of N-cyclohexyl2-benzothiazol sulfenamide (CBS), 0.4 phr of antioxidant,
and curing agent, i.e., 2 phr of sulfur, were industrial grade
chemicals from Bayer (M) Sdn. Bhd.
Address correspondence to H. Ismail, School of Materials and
Mineral Resources Engineering, Universiti Sains Malaysia, 14300
Nibong Tebal, Penang, Malaysia. E-mail: hanafi@eng.usm.my
34
35
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
Preparation and Processing
Studies were conducted on PP=EPDM=NR blends
consisting of two systems viz. unvulcanized blend and
vulcanized blend, where each blend covered different blend
compositions viz. 50=50=0, 50=40=10, 50=30=20, 50=20=30,
50=10=40, and 50=0=50. Thermoplastic elastomer blends
were prepared by melt mixing in an internal mixer, Haake
Polydrive with Rheomix R600=610, at temperature and
rotor speed of 180C and 50 rpm, respectively.
During blending, thermoplastic PP were first loaded
into the internal mixer and pre-mixed for 2 min, followed
by the rubbers (EPDM and NR). For unvulcanized, the
blends were taken out after 6 min of mixing. The corresponding dynamic vulcanization blends were prepared in
the same manner except that the curatives were added after
5 min of mixing and followed by sulfur at 7 min, and the
mixing time was prolonged until 10 min. The samples were
then sheeted by passing through a 2 roll-mill and allowed
to cool at room temperature.
Specimens for testing were compression molded using
an electrically heated hydraulic press machine. The
machine was pre-heated at 180C for 6 min, followed by
another 4 min of compression under the same temperature.
The specimen was allowed to cool under pressure for a
further 4 min. A similar procedure was adopted for all
blend systems.
Tensile Properties
Tensile tests were carried out according to ASTM D412
on an Instron Machine, and 2-mm thick dumbbell specimens were cut from the molded sheets with a Wallace die
cutter. The specimen was tested using a constant rate
(50 mm=min) at room temperature of 25C. The results
were quoted based on the average value of five specimens
tested for each blend system.
Swelling Test
Determination of the index of swelling was carried out
according to ASTM D471. The specimens with dimensions
30 mm 5 mm 2 mm were cut and weighed using an
electrical balance. The test pieces were then immersed in
toluene for 12 h and in oil (IRM 903) for 48 h at room temperature. The samples were then removed from toluene and
oil, wiped with tissue paper to remove excess liquid from
the surface, and weighed. The swelling index of the blends
was then calculated as follows:
Swelling index ¼
W2
100%
W1
ð1Þ
Where W1 ¼ weight of specimen before immersion, and
W2 ¼ weight of specimen after immersion.
Gel Content
The degree of cross-linking in the rubber was measured
after extraction in boiling cyclohexane for 8 h. The samples
were dried at 80C for 30 min and subsequently weighed.
The percentage of gel content of the blends was then calculated as follows:
% gel content ¼
Wg
100%
Wo
ð2Þ
Where Wg and W0 are sample weights after and before
extraction, respectively.
Differential Scanning Calorimetry (DSC) Study
Thermal analysis measurements of selected blend systems were performed using a DSC-7 Perkin Elmer differential scanning calorimeter. The samples were programmed
and heated at 20C=min to about 200C and maintained
at this temperature for 10 min to ensure a complete melting
of the crystals. The melting temperature (Tm) and the heat
of fusion (DHf) were measured during the heating cycle.
Morphology Studies
Morphological evaluations of PP=EPDM=NR surfaces
were done using a scanning electron microscope (SEM),
model Leica Cambridge S-360. The unvulcanized blend
samples were solvent-extracted using n-hexane for 2 days
at room temperature to extract the rubber phase from
the blend. The samples were then dried to remove excessive
solvent. The vulcanized samples were etched with nitric
acid for 2 days, washed with water, and then dried. All
the samples were mounted on aluminum stubs and sputtercoated with a thin layer of gold to avoid electrostatic
charging during examination. The examinations were done
within 24 h of preparation.
RESULTS AND DISCUSSION
Process Development
The plastograms of sulfur vulcanized blends as compared to unvulcanized blends of PP=EPDM=NR blends
with 50=50=0, 50=30=20, and 50=0=50 blend compositions
are shown in Fig. 1. In sulfur vulcanization, curing ingredients minus sulfur were added at the 5th min, whereas sulfur
was added at the 7th min of mixing. The torque started to
rise after the 7th min, at which sulfur was added, and stabilized at higher values than corresponding unvulcanized
blends.
The raised torque values of sulfur vulcanized blends
over that of unvulcanized blends indirectly indicate the
increasing melt viscosity at the processing temperature.
This implies that curing occurred. As cross-links are
formed in the rubber phase, intermolecular slippage is
restricted and the deformability of the rubber particles is
decreased[8]. Consequently, melt viscosity of the blends
36
HALIMATUDDAHLIANA AND H. ISMAIL
FIG. 2. Comparison of stabilization torque between unvulcanized and
vulcanized PP=EPDM=NR blends.
FIG. 1. The effect of dynamic vulcanization on torque-time curve
obtained during blending of PP=EPDM=NR.
increases, requiring a higher energy requirement for the
melt processing of sulfur vulcanized blends than the
control counterparts.
Further, they show an increasing torque value as the NR
content increases. As can be seen from Fig. 1, the increase
in the torque with increasing NR content is evidence that
the high cross-link formation occurred more on the NR
phase than on the EPDM phase. This behavior may be
explained as follows: incorporation of NR involves simultaneous replacement of a portion of cross-linkable EPDM
in the blend. Consequently, the unsaturated bonds for formation of cross-link were increased, which are prepared by
NR. As well, EPDM, being a copolymer of two olefin
monomers (ethylene and propylene), is a partially saturated
elastomer, thus it is less cross-linked by the acceleratedsulfur cure systems that are commonly employed in the
vulcanization of NR. However, as mentioned previously,
interaction between NR and PP is weaker than PP and
EPDM. Therefore, the increasing viscosity with increasing
NR content and hence cross-link density may not be followed considerably with the tensile properties, which will
be discussed next.
It is also worth noting that on the vulcanized blend without NR content (50=50=0 PP=EPDM=NR) the torque did
not increase significantly. This could be due to the type
of the EPDM (EPT 3072E) used, which contained oil. This
allows the preparation of softer compositions with significant improvement in processability. In the melt, the oil partitions between the phases and greatly lowers the viscosity,
resulting in improved processing. In addition, the oil
expands the volume fraction of the solid cross-link rubber
phase, resulting in improvement of processability[3].
The comparison of stabilization torque values between
sulfur vulcanized blends with unvulcanized blends is presented as a function of blend compositions in Fig. 2. It
can be seen that the stabilization torque increased as NR
content increased for both vulcanized and unvulcanized
blends. When the NR is present as an additional dispersed
phase in the PP phase, an unstable melt of blends could be
expected because the PP becomes insufficient to provide a
melt flow. As more NR particles are present in the PP
continuous phase, the amount of rubber particles that
can exist as dispersed phase become more, and consequently, the raised stabilization torque becomes higher.
Further, it shows that the stabilization torque of the
dynamically vulcanized blend lies above the unvulcanized
blend for every blend composition. During mixing, the vulcanized molecule chains could not be easily deformed by
shearing action of the rotor, which is related to increasing
the power requirement and enhances the frictional resistance of the blend against deformation[1].
Tensile Properties
The effects of dynamic vulcanization on tensile strength,
tensile modulus (M100), and elongation at break of
PP=EPDM=NR blend as compared with the unvulcanized
blends are presented in Figs. 3, 4, and 5, respectively. It is
clear from the results that the dynamic vulcanization
process significantly enhances the tensile strength over
unvulcanized blends as can be seen in Fig. 3.
The improvement in tensile strength by dynamic vulcanization is well explained in the literature of elastomer=
plastic blends[9,10]. Formation of cross-links in the
elastomer phase facilitating stress transfer contributes to
such enhancement. This enhancement is a result of many
factors. First, cross-linking increases the molecular entanglements and forms a network structure resulting in an
enhancement in stress transfer between phases. Second,
under shear, the particle size of vulcanized rubber
decreases, so the smaller the rubber particle size, the higher
the tensile properties. Third, the particles distribute more
uniformly in the matrix and, therefore, stress can be transferred to the material more effectively. Finally, the dynamic
vulcanization of the rubber component restricts the rubber
particles coalescence and subsequently improves the
properties.
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
37
FIG. 3. Comparison of tensile strength between unvulcanized and
vulcanized PP=EPDM=NR blends.
FIG. 5. Comparison of elongation at break between unvulcanized and
vulcanized PP=EPDM=NR blends.
In tensile strength, the sulfur vulcanized blend has a
more pronounced effect in the EPDM richer blends over
the NR richer blends. This implies that the formation of
cross-links is not the only factor that governs this improvement. The dynamic vulcanization, which resulted in crosslinks in the rubber phase and is more pronounced in the
NR rich blend is not indicated in this possibility. The good
interactions between PP-EPDM over PP-NR contribute to
enhanced tensile properties. As the NR content increased,
the number of good interface regions available in the
blends was decreased. This indicates that the interfacial
adhesion between components plays an important role in
achieving enhanced properties. The deterioration of tensile
strength of the blend richer in NR content indicates the
lower capability of vulcanized NR particles to support
transfer of stress from the polymer matrix to the particles.
The effect of dynamic vulcanization on the tensile
modulus (M100) of PP=EPDM=NR blend as compared to
the unvulcanized blends is shown in Fig. 4. The tensile
modulus (M100) registers a marked improvement with the
dynamic vulcanization, which proves the progressive development in cross-linking. This behavior is easily attributed
to the increased strength in the rubber regions and the
entanglements in the rubber phase owing to cross-linking,
and the implication is increasing stiffness of the blend.
Here, vulcanized blends could not resist the deformability
of the macromolecules, hence increasing the M100. However, M100 of the 50=0=50 PP=EPDM=NR vulcanized
blend is not reported because it ruptured before 100%
elongation at break was achieved.
Figure 5 shows the comparison of elongation at break
between unvulcanized and sulfur vulcanized PP=
EPDM=NR blends. It can be seen that vulcanized blends
show higher elongation at break in the EPDM richer blend.
The elongation at break of the sulfur vulcanized blend containing richer NR shows an inverse relation with the
EPDM rich blend. It shows that the elongation at break
of the dynamically vulcanized blend is lower than the
unvulcanized one and decreases gradually as the NR takes
the dominant content over EPDM until minimum value is
attained (at 50=0=50 blend). This indicates that the vulcanized rubber phase does not significantly increase the initial
resistance to the deformation of the macromolecules in the
NR richer blend. The reason is as follows: although NR
content increases, the chemical cross-linking of rubber
restricts mobility of the polymer chain. In addition, when
the NR content is high, vulcanized NR particles may exist
mostly in the continuous phase, resulting in significant
resistance to the deformation of macromolecules. This will
narrow down the effect of rubber on the elongation at
break. In addition, the number of poor interface regions
available in the blends and lack of interfacial adhesion
may affect the elongation at break.
FIG. 4. Comparison of tensile modulus (M100) between unvulcanized
and vulcanized PP=EPDM=NR blends.
Swelling Index
Results presented in Table 1 show the comparison of
swelling index between sulfur PP=EPDM=NR vulcanized
blends with unvulcanized blends as a function of blend
composition. The swelling index values of blends in toluene
and oil are shown in the same table and indicate the chemical and oil resistance of the blends, respectively.
It can be seen that swelling index of PP=EPDM=NR
blends after immersion in toluene for 12 h increased with
increasing NR content. Here, EPDM has a low level of
38
HALIMATUDDAHLIANA AND H. ISMAIL
TABLE 1
Swelling index of dynamically vulcanized PP=EPDM=NR
blends as compared with unvulcanized blends after
immersion in toluene and oil IRM903
TABLE 2
Gel content of selected dynamically vulcanized
PP=EPDM=NR blends as compared with unvulcanized
blends
Swelling index
Gel content (%)
Unvulcanized blend Vulcanized blend
Blend ratio
(PP=EPDM=NR)
50=50=0
50=40=10
50=30=20
50=20=30
50=10=40
50=0=50
Toluene
(12 h)
Oil
(48 h)
Toluene
(12 h)
Oil
(48 h)
1.57
1.60
1.69
1.73
1.80
1.86
1.40
1.45
1.47
1.52
1.56
1.59
1.27
1.34
1.47
1.60
1.69
1.71
1.30
1.32
1.38
1.45
1.47
1.42
polarity, consisting of only carbon and hydrogen atoms
with little unsaturation (the main chain has no double
bonds). As compared with NR, which has double bonds
in the main chain, EPDM is considerably less polar than
NR[11]. Therefore, EPDM exhibits less swell in toluene
(polar liquid) compared with NR, and its blend consequently leads to high chemical resistance.
On the other hand, Table 1 also shows that the swelling
index of PP=EPDM=NR blends after immersion in oil
IRM 903 for 48 h increased with increasing NR content.
Even though EPDM has lower polarity than NR, which
swells easier in oil (nonpolar liquid), due to the good interaction between PP and EPDM, the 50=50=0 PP=
EPDM=NR blend possesses higher resistance to oil than
those of 50=30=20 and 50=0=50 PP=EPDM=NR blends.
Here, the good interaction between PP and EPDM inhibits
the penetration of oil into the rubber.
Table 1 depicts that the swelling index values of
dynamically vulcanized blends exhibit lower values than
their unvulcanized counterparts for toluene and oil as well.
The introduction of cross-links into the rubber phase interrupts this diffusion process and results in a reduction in the
swelling index. Table 1 shows that dynamically vulcanized
rubber is effective as evidenced by a similar decrease in the
swelling index. The reductions of swelling index signify the
different tensile properties of the vulcanizates as shown in
tensile strength.
However, the increase in NR content in the blends has
also increased the swelling index. Here, even though high
cross-link formations have occurred, the decreased
compatibility between PP and NR decreased the chemical
and oil resistance of the blends.
Gel Content
The effect of dynamic vulcanization on the percent gel
content of PP=EPDM=NR blend with respect to the
Blend ratio
(PP=EPDM=NR)
50=50=0
50=30=20
50=0=50
Unvulcanized
blend
Vulcanized
blend
61.2
57.2
52.3
89.2
86.6
83.0
unvulcanized blends is presented in Table 2. The results
show that the introduction of NR into the PP=EPDM
blend decreased gel content of the blends. This effect is
more pronounced for NR richer blends and indicates that
NR is easier to extract by cyclohexane compared with
EPDM. This can be attributed to the good interaction
between PP and EPDM, hence during extraction the rubber particles (EPDM) were not easily extracted from the
continuous phase (PP). This observation also confirms
the tensile properties as mentioned before, where the good
interaction between PP and EPDM is disturbed by NR
addition.
Table 2 also depicts that the dynamic vulcanization process gives rise to a sensible increase in gel content as well as
a considerable reduction of the percent of extractable elastomer by cyclohexane with respect to the corresponding
unvulcanized blends. A higher gel content means that fewer
main chains have been extracted, which can be attributed
to the cross-links formation. During the cross-linking reaction, the average molecular weight of the chain increases,
thus causing the sample to become gradually insoluble in
solvent.
Differential Scanning Calorimetry (DSC)
Figure 6 shows the effect of sulfur on DSC scans of
PP=EPDM=NR blends with different blend compositions
as compared with the unvulcanized blends. The area of
the melting endothermic was measured as the heat of
fusion (DHf). The parameter Xc determines the percentage
of crystallinity blends. The Xc in this study was calculated
by dividing the measured DHf by the DHf of 100% crystalline material. The DHf for polypropylene as a fully crystalline material is 209 J=g[12]. It can be seen that all blend
compositions have almost the same pattern of heating
thermogram, which showed no change in the curves on
inclusion of NR. The thermal properties of the blends are
governed by the crystalline PP=EPDM blend alone because
NR is an amorphous polymer. Therefore, a marginal
change in melting temperature (Tm) and onset temperature
39
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
TABLE 3
Thermal properties of dynamic vulcanization of PP=EPDM=NR blends as compared with unvulcanized blends derived
from DSC scan thermogram
Blend ratio
(PP=EPDM=NR)
Unvulcanized blend
50=50=0
50=30=20
50=0=50
Vulcanized blend
50=50=0
50=30=20
50=0=50
Onset temperature
(To) C
Melting temperature
(Tm) C
Fusion enthalphy
(DHf) (J=g)
% Crystallinity
(Xc)
150.7
151.0
152.6
161.0
162.4
163.7
42.3
40.5
39.8
20.24
19.38
19.04
151.2
152.3
153.7
160.8
161.0
163.0
41.4
39.5
38.5
19.81
18.90
18.42
(To) of PP=EPDM blend was observed with the addition of
NR, as can be seen in Table 3.
However, a slightly increased To in the 50=30=20 and
50=0=50 PP=EPDM=NR blends clearly indicates that the
introduction of NR in the blend results in delayed
nucleation and thus implies an increasing heterogeneity in
spherulite size. The reduction rate of crystallization in
the presence of NR due to the restricted mobility of
the PP=EPDM segments by NR addition thereby leads to
lacking alignment in the crystal lattice[13].
It is clear from the heating curves that the melting
temperature of PP does not change significantly by the
dynamic vulcanization with respect to the control blends.
It is also interesting to note that incorporation of NR does
not show any influence on melting temperature (Tm) of
dynamic vulcanized blends. However, careful investigation
of heating scans of the dynamic vulcanized blend, which
are shown in Table 3, show a shift of onset temperature
(To) toward a slightly higher value. It seems to be due to
the cross-link formation, which makes the chain unfolding
of PP plausible at a bit higher temperature.
FIG. 6. The effect of dynamic vulcanization on DSC scans of
PP=EPDM=NR blends as compared with unvulcanized blends with different blend compositions.
Vulcanized blends exhibit low fusion enthalpy over
control blends at each blend composition. This observation supports that the percentage of crystallinity is less
in the vulcanized blends. This result attests to the
increased chain entanglements due to vulcanization, which
may in turn interrupt the ability of PP macromolecules to
align. The cross-link acts as a local defect and this would
lead to the reduction in percentage of crystallinity. In
addition, the reduction due to the formation of cross-links
in the rubber phase caused an increase in its viscosity.
This restricted the process of crystallization and crystal
growth in PP. Hence the overall crystallinity of the blend
is decreased.
The result further shows that in control and vulcanized systems, fusion enthalpy (DHf) decreased slightly
as NR was introduced and hence the percentage of
crystallinity (Xc) of the blends also decreased. Here,
as NR content increases in the blends, such reduction
in fusion enthalpy of the blends can occur because
the content of nucleating agent component (EPDM)
is reduced. However, it shows that the degree of orientation does not much affect the strength property in
these blends. Though the PP orientation is one factor
that will impart tensile properties of these blends, the
continuous nature of the matrix (PP) allows the load
to be shared. And there is some evidence that percentage of crystallinity of the blends is less disturbed by
the dynamic vulcanization. Hence, blends still exhibit
higher tensile strength compared to the control blends
as explained before.
Morphology Studies
Figures 7a–f presents the SEM micrographs of extracted
selected PP=EPDM=NR blends for both dynamic vulcanized and unvulcanized blends. The honeycomb structure
of the micrographs on the unvulcanized blends (Figs. 7a,
c, and e) shows a large number of voids, which consist of
dark and light spots. The dark spots correspond to the
40
HALIMATUDDAHLIANA AND H. ISMAIL
Simultaneously, the holes in the vulcanized blends
are smaller and less obvious than those of unvulcanized
blends, and the dispersibility of rubbers in PP matrix are
markedly improved. Here, during the dynamic vulcanization process, stable domain morphology of vulcanized
particles is obtained and consequently produces better
blend properties.
The micrographs also show that the vulcanized rubber
particles are dispersed more uniformly in the continuous
phase of PP. This could be attributed to the immobilization
of the rubber particles by cross-linking and therefore
breaking down to small size under the applied shear
field[14]. According to Sabet et al.[3], during dynamic vulcanization of TPE, the rubbers and the thermoplastic
undergo a phase inversion to maintain the thermoplasticity
of the blend. In the initial stage of dynamic vulcanization,
two co-continuous phases are generated and as the degree
of cross-linking advances while mixing, the continuous
rubber phase becomes elongated further and further and
then breaks up into polymer droplets. As these rubber particles are forming as droplets, the PP becomes a continuous
phase.
FIG. 7. SEM micrographs of extracted PP=EPDM=NR blends:
(a) 50=50=0, (b) 50=50=0 vulcanized blend, (c) 50=30=20, (d) 50=30=20
vulcanized blend, (e) 50=0=50, and (f) 50=0=50 vulcanized blend.
empty holes left behind by the rubber after dissolving in
n-hexane. The light spots represent the insoluble PP phase.
There was an indication of the heterogeneous phase
structure of PP=EPDM=NR blends for every blend composition. Here, one component is expected to form a discrete
phase (rubber) dispersed in the other component forming
the continuous phase (plastic) and thus giving rise to a
two-phase system. In these systems, during processing the
low viscosity component tends to become the continuous
phase, whereas the high viscosity is a discrete phase. However, Fig. 7a shows that the rubber particles (EPDM) are
well dispersed in the continuous phase of PP indicating
good adhesion between PP and EPDM.
It can be seen that upon extraction, the rubber particles
of vulcanized blends (Figs. 7b, d, and f) were not easily
extracted from the continuous phase. These nature
micrographs reveal that the blends achieved vulcanized
rubber properties due to the formation of cross-links.
CONCLUSION
Dynamically vulcanized PP=EPDM=NR blends exhibit
higher stabilization torque, tensile strength, and tensile
modulus (M100) but lower elongation at break with EPDM
rich content than unvulcanized blends. The enhancement
of gel content and the reduction of swelling index have also
proved the formation of cross-links. Dynamically vulcanized blends show slightly lower percentage of crystallinity
than unvulcanized blends. The development of finer vulcanized rubber particles due to the cross-links formation
improves stability of the morphology, which improves the
properties of the blends.
ACKNOWLEDGEMENT
The authors would like to thank Yayasan Felda for the
research grant that made this research work possible.
REFERENCES
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EPDM-polypropylene thermoplastic vulcanizates. Rubb. Chem.
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ISSN: 0360-2559 (Print) 1525-6111 (Online) Journal homepage: http://www.tandfonline.com/loi/lpte20
The Effect of Dynamic Vulcanization on the
Properties of Polypropylene/Ethylene-Propylene
Diene Terpolymer/Natural Rubber (PP/EPDM/NR)
Ternary Blend
Halimatuddahliana & H. Ismail
To cite this article: Halimatuddahliana & H. Ismail (2008) The Effect of Dynamic Vulcanization
on the Properties of Polypropylene/Ethylene-Propylene Diene Terpolymer/Natural Rubber
(PP/EPDM/NR) Ternary Blend, Polymer-Plastics Technology and Engineering, 48:1, 34-41, DOI:
10.1080/03602550802539270
To link to this article: http://dx.doi.org/10.1080/03602550802539270
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Date: 29 August 2016, At: 01:10
Polymer-Plastics Technology and Engineering, 48: 34–41, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0360-2559 print/1525-6111 online
DOI: 10.1080/03602550802539270
The Effect of Dynamic Vulcanization on the Properties
of Polypropylene/Ethylene-Propylene Diene Terpolymer/
Natural Rubber (PP/EPDM/NR) Ternary Blend
Halimatuddahliana1 and H. Ismail2
1
Department of Chemical Engineering, Universitas Sumatera Utara, Indonesia
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia,
Penang, Malaysia
2
Dynamic vulcanization has been extensively applied to
the vulcanization of the soft elastomer phase of a blend
with rigid thermoplastics. The process is carried out under
high shear and above the melting point of the thermoplastic at sufficiently high temperature to activate and complete
the vulcanization. During dynamic vulcanization of rubbers, the polymeric chains of the base elastomer become
interconnected, converting the viscous gum into an elastic
network. In dynamic vulcanization of the thermoplastics
elastomer, the structure is generated in small rubber particles dispersed in the thermoplastic polymer matrix. Various
types of curatives can generally be used in dynamic vulcanized blends, such as sulfur, phenolic resin, and peroxide.
This paper examines the effect of dynamic vulcanization
with sulfur on the process development and some properties, such as tensile properties, chemical and oil resistances,
and gel content of the PP=EPDM=NR blends, along with
the accompanying characteristics in the crystallinity and
morphology.
The effects of dynamic vulcanization on the process development
and some properties, such as tensile properties, swelling index,
gel content, crystallinity, and morphology, of the polypropylene
(PP)/ethylene-propylene diene terpolymer (EPDM)/natural rubber
(NR) blends were investigated. Dynamically vulcanized blends show
higher stabilization torque than unvulcanized blends. In terms of
tensile properties, the tensile strength and tensile modulus (stress
at 100% elongation, M100) of the vulcanized blends have been found
to increase as compared with the unvulcanized blends, whereas the
elongation at break is higher in the blend with richer EPDM content. These results can be attributed to the formation of crosslinking in the rubber phase. The formation of cross-links in the
rubber phase has also been proved by swelling index and gel content.
The percentage of crystallinity of the blends is decreased by
dynamic vulcanization. Scanning electron microscopy (SEM)
micrographs from the surface extraction of the blends support that
the cross-links occurred during dynamic vulcanization.
Keywords Crystallinity; Dynamic vulcanization; Gel content;
Swelling index
EXPERIMENTAL
INTRODUCTION
Several works have reported the dynamic vulcanization
of TPE, especially the PP=EPDM blends, and most of the
investigations focused on the processing and the improvements of mechanical and physical properties of the
blends[1–6]. However, replacement of EPDM with NR in
PP=EPDM blends has been considered due to the
reduction of the cost. It has also been observed that the
partial replacement of EPDM by NR decreased the properties of PP=EPDM blends[7]. Therefore, dynamic vulcanization has been introduced in the PP=EPDM=NR blend to
improve the properties of such blends.
Materials
Polypropylene (PP) homopolymer used in this study was
an injection-molding grade, supplied by Titan PP Polymers
(M) Sdn Bhd, Johor, Malaysia (TITANPRO 6331 grade),
with a melt flow index (MFI) value of 14 gr=10 min at
230C and 2.16 kg. Ethylene-propylene diene terpolymer
(EPDM-EPT 3072E), with Mooney viscosity, ML (1 þ 4)
74 at 100C was purchased from Luxchem Trading Sdn.
Bhd. Natural rubber (SMR L), with Mooney viscosity,
ML (1 þ 4), 73 at 100C was obtained from Hokson Rubber
Trading Sdn. Bhd, Seremban. Curatives, i.e., 5 phr of zinc
oxide (ZnO), 1.5 phr of stearic acid, 1 phr of N-cyclohexyl2-benzothiazol sulfenamide (CBS), 0.4 phr of antioxidant,
and curing agent, i.e., 2 phr of sulfur, were industrial grade
chemicals from Bayer (M) Sdn. Bhd.
Address correspondence to H. Ismail, School of Materials and
Mineral Resources Engineering, Universiti Sains Malaysia, 14300
Nibong Tebal, Penang, Malaysia. E-mail: hanafi@eng.usm.my
34
35
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
Preparation and Processing
Studies were conducted on PP=EPDM=NR blends
consisting of two systems viz. unvulcanized blend and
vulcanized blend, where each blend covered different blend
compositions viz. 50=50=0, 50=40=10, 50=30=20, 50=20=30,
50=10=40, and 50=0=50. Thermoplastic elastomer blends
were prepared by melt mixing in an internal mixer, Haake
Polydrive with Rheomix R600=610, at temperature and
rotor speed of 180C and 50 rpm, respectively.
During blending, thermoplastic PP were first loaded
into the internal mixer and pre-mixed for 2 min, followed
by the rubbers (EPDM and NR). For unvulcanized, the
blends were taken out after 6 min of mixing. The corresponding dynamic vulcanization blends were prepared in
the same manner except that the curatives were added after
5 min of mixing and followed by sulfur at 7 min, and the
mixing time was prolonged until 10 min. The samples were
then sheeted by passing through a 2 roll-mill and allowed
to cool at room temperature.
Specimens for testing were compression molded using
an electrically heated hydraulic press machine. The
machine was pre-heated at 180C for 6 min, followed by
another 4 min of compression under the same temperature.
The specimen was allowed to cool under pressure for a
further 4 min. A similar procedure was adopted for all
blend systems.
Tensile Properties
Tensile tests were carried out according to ASTM D412
on an Instron Machine, and 2-mm thick dumbbell specimens were cut from the molded sheets with a Wallace die
cutter. The specimen was tested using a constant rate
(50 mm=min) at room temperature of 25C. The results
were quoted based on the average value of five specimens
tested for each blend system.
Swelling Test
Determination of the index of swelling was carried out
according to ASTM D471. The specimens with dimensions
30 mm 5 mm 2 mm were cut and weighed using an
electrical balance. The test pieces were then immersed in
toluene for 12 h and in oil (IRM 903) for 48 h at room temperature. The samples were then removed from toluene and
oil, wiped with tissue paper to remove excess liquid from
the surface, and weighed. The swelling index of the blends
was then calculated as follows:
Swelling index ¼
W2
100%
W1
ð1Þ
Where W1 ¼ weight of specimen before immersion, and
W2 ¼ weight of specimen after immersion.
Gel Content
The degree of cross-linking in the rubber was measured
after extraction in boiling cyclohexane for 8 h. The samples
were dried at 80C for 30 min and subsequently weighed.
The percentage of gel content of the blends was then calculated as follows:
% gel content ¼
Wg
100%
Wo
ð2Þ
Where Wg and W0 are sample weights after and before
extraction, respectively.
Differential Scanning Calorimetry (DSC) Study
Thermal analysis measurements of selected blend systems were performed using a DSC-7 Perkin Elmer differential scanning calorimeter. The samples were programmed
and heated at 20C=min to about 200C and maintained
at this temperature for 10 min to ensure a complete melting
of the crystals. The melting temperature (Tm) and the heat
of fusion (DHf) were measured during the heating cycle.
Morphology Studies
Morphological evaluations of PP=EPDM=NR surfaces
were done using a scanning electron microscope (SEM),
model Leica Cambridge S-360. The unvulcanized blend
samples were solvent-extracted using n-hexane for 2 days
at room temperature to extract the rubber phase from
the blend. The samples were then dried to remove excessive
solvent. The vulcanized samples were etched with nitric
acid for 2 days, washed with water, and then dried. All
the samples were mounted on aluminum stubs and sputtercoated with a thin layer of gold to avoid electrostatic
charging during examination. The examinations were done
within 24 h of preparation.
RESULTS AND DISCUSSION
Process Development
The plastograms of sulfur vulcanized blends as compared to unvulcanized blends of PP=EPDM=NR blends
with 50=50=0, 50=30=20, and 50=0=50 blend compositions
are shown in Fig. 1. In sulfur vulcanization, curing ingredients minus sulfur were added at the 5th min, whereas sulfur
was added at the 7th min of mixing. The torque started to
rise after the 7th min, at which sulfur was added, and stabilized at higher values than corresponding unvulcanized
blends.
The raised torque values of sulfur vulcanized blends
over that of unvulcanized blends indirectly indicate the
increasing melt viscosity at the processing temperature.
This implies that curing occurred. As cross-links are
formed in the rubber phase, intermolecular slippage is
restricted and the deformability of the rubber particles is
decreased[8]. Consequently, melt viscosity of the blends
36
HALIMATUDDAHLIANA AND H. ISMAIL
FIG. 2. Comparison of stabilization torque between unvulcanized and
vulcanized PP=EPDM=NR blends.
FIG. 1. The effect of dynamic vulcanization on torque-time curve
obtained during blending of PP=EPDM=NR.
increases, requiring a higher energy requirement for the
melt processing of sulfur vulcanized blends than the
control counterparts.
Further, they show an increasing torque value as the NR
content increases. As can be seen from Fig. 1, the increase
in the torque with increasing NR content is evidence that
the high cross-link formation occurred more on the NR
phase than on the EPDM phase. This behavior may be
explained as follows: incorporation of NR involves simultaneous replacement of a portion of cross-linkable EPDM
in the blend. Consequently, the unsaturated bonds for formation of cross-link were increased, which are prepared by
NR. As well, EPDM, being a copolymer of two olefin
monomers (ethylene and propylene), is a partially saturated
elastomer, thus it is less cross-linked by the acceleratedsulfur cure systems that are commonly employed in the
vulcanization of NR. However, as mentioned previously,
interaction between NR and PP is weaker than PP and
EPDM. Therefore, the increasing viscosity with increasing
NR content and hence cross-link density may not be followed considerably with the tensile properties, which will
be discussed next.
It is also worth noting that on the vulcanized blend without NR content (50=50=0 PP=EPDM=NR) the torque did
not increase significantly. This could be due to the type
of the EPDM (EPT 3072E) used, which contained oil. This
allows the preparation of softer compositions with significant improvement in processability. In the melt, the oil partitions between the phases and greatly lowers the viscosity,
resulting in improved processing. In addition, the oil
expands the volume fraction of the solid cross-link rubber
phase, resulting in improvement of processability[3].
The comparison of stabilization torque values between
sulfur vulcanized blends with unvulcanized blends is presented as a function of blend compositions in Fig. 2. It
can be seen that the stabilization torque increased as NR
content increased for both vulcanized and unvulcanized
blends. When the NR is present as an additional dispersed
phase in the PP phase, an unstable melt of blends could be
expected because the PP becomes insufficient to provide a
melt flow. As more NR particles are present in the PP
continuous phase, the amount of rubber particles that
can exist as dispersed phase become more, and consequently, the raised stabilization torque becomes higher.
Further, it shows that the stabilization torque of the
dynamically vulcanized blend lies above the unvulcanized
blend for every blend composition. During mixing, the vulcanized molecule chains could not be easily deformed by
shearing action of the rotor, which is related to increasing
the power requirement and enhances the frictional resistance of the blend against deformation[1].
Tensile Properties
The effects of dynamic vulcanization on tensile strength,
tensile modulus (M100), and elongation at break of
PP=EPDM=NR blend as compared with the unvulcanized
blends are presented in Figs. 3, 4, and 5, respectively. It is
clear from the results that the dynamic vulcanization
process significantly enhances the tensile strength over
unvulcanized blends as can be seen in Fig. 3.
The improvement in tensile strength by dynamic vulcanization is well explained in the literature of elastomer=
plastic blends[9,10]. Formation of cross-links in the
elastomer phase facilitating stress transfer contributes to
such enhancement. This enhancement is a result of many
factors. First, cross-linking increases the molecular entanglements and forms a network structure resulting in an
enhancement in stress transfer between phases. Second,
under shear, the particle size of vulcanized rubber
decreases, so the smaller the rubber particle size, the higher
the tensile properties. Third, the particles distribute more
uniformly in the matrix and, therefore, stress can be transferred to the material more effectively. Finally, the dynamic
vulcanization of the rubber component restricts the rubber
particles coalescence and subsequently improves the
properties.
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
37
FIG. 3. Comparison of tensile strength between unvulcanized and
vulcanized PP=EPDM=NR blends.
FIG. 5. Comparison of elongation at break between unvulcanized and
vulcanized PP=EPDM=NR blends.
In tensile strength, the sulfur vulcanized blend has a
more pronounced effect in the EPDM richer blends over
the NR richer blends. This implies that the formation of
cross-links is not the only factor that governs this improvement. The dynamic vulcanization, which resulted in crosslinks in the rubber phase and is more pronounced in the
NR rich blend is not indicated in this possibility. The good
interactions between PP-EPDM over PP-NR contribute to
enhanced tensile properties. As the NR content increased,
the number of good interface regions available in the
blends was decreased. This indicates that the interfacial
adhesion between components plays an important role in
achieving enhanced properties. The deterioration of tensile
strength of the blend richer in NR content indicates the
lower capability of vulcanized NR particles to support
transfer of stress from the polymer matrix to the particles.
The effect of dynamic vulcanization on the tensile
modulus (M100) of PP=EPDM=NR blend as compared to
the unvulcanized blends is shown in Fig. 4. The tensile
modulus (M100) registers a marked improvement with the
dynamic vulcanization, which proves the progressive development in cross-linking. This behavior is easily attributed
to the increased strength in the rubber regions and the
entanglements in the rubber phase owing to cross-linking,
and the implication is increasing stiffness of the blend.
Here, vulcanized blends could not resist the deformability
of the macromolecules, hence increasing the M100. However, M100 of the 50=0=50 PP=EPDM=NR vulcanized
blend is not reported because it ruptured before 100%
elongation at break was achieved.
Figure 5 shows the comparison of elongation at break
between unvulcanized and sulfur vulcanized PP=
EPDM=NR blends. It can be seen that vulcanized blends
show higher elongation at break in the EPDM richer blend.
The elongation at break of the sulfur vulcanized blend containing richer NR shows an inverse relation with the
EPDM rich blend. It shows that the elongation at break
of the dynamically vulcanized blend is lower than the
unvulcanized one and decreases gradually as the NR takes
the dominant content over EPDM until minimum value is
attained (at 50=0=50 blend). This indicates that the vulcanized rubber phase does not significantly increase the initial
resistance to the deformation of the macromolecules in the
NR richer blend. The reason is as follows: although NR
content increases, the chemical cross-linking of rubber
restricts mobility of the polymer chain. In addition, when
the NR content is high, vulcanized NR particles may exist
mostly in the continuous phase, resulting in significant
resistance to the deformation of macromolecules. This will
narrow down the effect of rubber on the elongation at
break. In addition, the number of poor interface regions
available in the blends and lack of interfacial adhesion
may affect the elongation at break.
FIG. 4. Comparison of tensile modulus (M100) between unvulcanized
and vulcanized PP=EPDM=NR blends.
Swelling Index
Results presented in Table 1 show the comparison of
swelling index between sulfur PP=EPDM=NR vulcanized
blends with unvulcanized blends as a function of blend
composition. The swelling index values of blends in toluene
and oil are shown in the same table and indicate the chemical and oil resistance of the blends, respectively.
It can be seen that swelling index of PP=EPDM=NR
blends after immersion in toluene for 12 h increased with
increasing NR content. Here, EPDM has a low level of
38
HALIMATUDDAHLIANA AND H. ISMAIL
TABLE 1
Swelling index of dynamically vulcanized PP=EPDM=NR
blends as compared with unvulcanized blends after
immersion in toluene and oil IRM903
TABLE 2
Gel content of selected dynamically vulcanized
PP=EPDM=NR blends as compared with unvulcanized
blends
Swelling index
Gel content (%)
Unvulcanized blend Vulcanized blend
Blend ratio
(PP=EPDM=NR)
50=50=0
50=40=10
50=30=20
50=20=30
50=10=40
50=0=50
Toluene
(12 h)
Oil
(48 h)
Toluene
(12 h)
Oil
(48 h)
1.57
1.60
1.69
1.73
1.80
1.86
1.40
1.45
1.47
1.52
1.56
1.59
1.27
1.34
1.47
1.60
1.69
1.71
1.30
1.32
1.38
1.45
1.47
1.42
polarity, consisting of only carbon and hydrogen atoms
with little unsaturation (the main chain has no double
bonds). As compared with NR, which has double bonds
in the main chain, EPDM is considerably less polar than
NR[11]. Therefore, EPDM exhibits less swell in toluene
(polar liquid) compared with NR, and its blend consequently leads to high chemical resistance.
On the other hand, Table 1 also shows that the swelling
index of PP=EPDM=NR blends after immersion in oil
IRM 903 for 48 h increased with increasing NR content.
Even though EPDM has lower polarity than NR, which
swells easier in oil (nonpolar liquid), due to the good interaction between PP and EPDM, the 50=50=0 PP=
EPDM=NR blend possesses higher resistance to oil than
those of 50=30=20 and 50=0=50 PP=EPDM=NR blends.
Here, the good interaction between PP and EPDM inhibits
the penetration of oil into the rubber.
Table 1 depicts that the swelling index values of
dynamically vulcanized blends exhibit lower values than
their unvulcanized counterparts for toluene and oil as well.
The introduction of cross-links into the rubber phase interrupts this diffusion process and results in a reduction in the
swelling index. Table 1 shows that dynamically vulcanized
rubber is effective as evidenced by a similar decrease in the
swelling index. The reductions of swelling index signify the
different tensile properties of the vulcanizates as shown in
tensile strength.
However, the increase in NR content in the blends has
also increased the swelling index. Here, even though high
cross-link formations have occurred, the decreased
compatibility between PP and NR decreased the chemical
and oil resistance of the blends.
Gel Content
The effect of dynamic vulcanization on the percent gel
content of PP=EPDM=NR blend with respect to the
Blend ratio
(PP=EPDM=NR)
50=50=0
50=30=20
50=0=50
Unvulcanized
blend
Vulcanized
blend
61.2
57.2
52.3
89.2
86.6
83.0
unvulcanized blends is presented in Table 2. The results
show that the introduction of NR into the PP=EPDM
blend decreased gel content of the blends. This effect is
more pronounced for NR richer blends and indicates that
NR is easier to extract by cyclohexane compared with
EPDM. This can be attributed to the good interaction
between PP and EPDM, hence during extraction the rubber particles (EPDM) were not easily extracted from the
continuous phase (PP). This observation also confirms
the tensile properties as mentioned before, where the good
interaction between PP and EPDM is disturbed by NR
addition.
Table 2 also depicts that the dynamic vulcanization process gives rise to a sensible increase in gel content as well as
a considerable reduction of the percent of extractable elastomer by cyclohexane with respect to the corresponding
unvulcanized blends. A higher gel content means that fewer
main chains have been extracted, which can be attributed
to the cross-links formation. During the cross-linking reaction, the average molecular weight of the chain increases,
thus causing the sample to become gradually insoluble in
solvent.
Differential Scanning Calorimetry (DSC)
Figure 6 shows the effect of sulfur on DSC scans of
PP=EPDM=NR blends with different blend compositions
as compared with the unvulcanized blends. The area of
the melting endothermic was measured as the heat of
fusion (DHf). The parameter Xc determines the percentage
of crystallinity blends. The Xc in this study was calculated
by dividing the measured DHf by the DHf of 100% crystalline material. The DHf for polypropylene as a fully crystalline material is 209 J=g[12]. It can be seen that all blend
compositions have almost the same pattern of heating
thermogram, which showed no change in the curves on
inclusion of NR. The thermal properties of the blends are
governed by the crystalline PP=EPDM blend alone because
NR is an amorphous polymer. Therefore, a marginal
change in melting temperature (Tm) and onset temperature
39
DYNAMIC VULCANIZATION EFFECT ON PP/EPDM/NR
TABLE 3
Thermal properties of dynamic vulcanization of PP=EPDM=NR blends as compared with unvulcanized blends derived
from DSC scan thermogram
Blend ratio
(PP=EPDM=NR)
Unvulcanized blend
50=50=0
50=30=20
50=0=50
Vulcanized blend
50=50=0
50=30=20
50=0=50
Onset temperature
(To) C
Melting temperature
(Tm) C
Fusion enthalphy
(DHf) (J=g)
% Crystallinity
(Xc)
150.7
151.0
152.6
161.0
162.4
163.7
42.3
40.5
39.8
20.24
19.38
19.04
151.2
152.3
153.7
160.8
161.0
163.0
41.4
39.5
38.5
19.81
18.90
18.42
(To) of PP=EPDM blend was observed with the addition of
NR, as can be seen in Table 3.
However, a slightly increased To in the 50=30=20 and
50=0=50 PP=EPDM=NR blends clearly indicates that the
introduction of NR in the blend results in delayed
nucleation and thus implies an increasing heterogeneity in
spherulite size. The reduction rate of crystallization in
the presence of NR due to the restricted mobility of
the PP=EPDM segments by NR addition thereby leads to
lacking alignment in the crystal lattice[13].
It is clear from the heating curves that the melting
temperature of PP does not change significantly by the
dynamic vulcanization with respect to the control blends.
It is also interesting to note that incorporation of NR does
not show any influence on melting temperature (Tm) of
dynamic vulcanized blends. However, careful investigation
of heating scans of the dynamic vulcanized blend, which
are shown in Table 3, show a shift of onset temperature
(To) toward a slightly higher value. It seems to be due to
the cross-link formation, which makes the chain unfolding
of PP plausible at a bit higher temperature.
FIG. 6. The effect of dynamic vulcanization on DSC scans of
PP=EPDM=NR blends as compared with unvulcanized blends with different blend compositions.
Vulcanized blends exhibit low fusion enthalpy over
control blends at each blend composition. This observation supports that the percentage of crystallinity is less
in the vulcanized blends. This result attests to the
increased chain entanglements due to vulcanization, which
may in turn interrupt the ability of PP macromolecules to
align. The cross-link acts as a local defect and this would
lead to the reduction in percentage of crystallinity. In
addition, the reduction due to the formation of cross-links
in the rubber phase caused an increase in its viscosity.
This restricted the process of crystallization and crystal
growth in PP. Hence the overall crystallinity of the blend
is decreased.
The result further shows that in control and vulcanized systems, fusion enthalpy (DHf) decreased slightly
as NR was introduced and hence the percentage of
crystallinity (Xc) of the blends also decreased. Here,
as NR content increases in the blends, such reduction
in fusion enthalpy of the blends can occur because
the content of nucleating agent component (EPDM)
is reduced. However, it shows that the degree of orientation does not much affect the strength property in
these blends. Though the PP orientation is one factor
that will impart tensile properties of these blends, the
continuous nature of the matrix (PP) allows the load
to be shared. And there is some evidence that percentage of crystallinity of the blends is less disturbed by
the dynamic vulcanization. Hence, blends still exhibit
higher tensile strength compared to the control blends
as explained before.
Morphology Studies
Figures 7a–f presents the SEM micrographs of extracted
selected PP=EPDM=NR blends for both dynamic vulcanized and unvulcanized blends. The honeycomb structure
of the micrographs on the unvulcanized blends (Figs. 7a,
c, and e) shows a large number of voids, which consist of
dark and light spots. The dark spots correspond to the
40
HALIMATUDDAHLIANA AND H. ISMAIL
Simultaneously, the holes in the vulcanized blends
are smaller and less obvious than those of unvulcanized
blends, and the dispersibility of rubbers in PP matrix are
markedly improved. Here, during the dynamic vulcanization process, stable domain morphology of vulcanized
particles is obtained and consequently produces better
blend properties.
The micrographs also show that the vulcanized rubber
particles are dispersed more uniformly in the continuous
phase of PP. This could be attributed to the immobilization
of the rubber particles by cross-linking and therefore
breaking down to small size under the applied shear
field[14]. According to Sabet et al.[3], during dynamic vulcanization of TPE, the rubbers and the thermoplastic
undergo a phase inversion to maintain the thermoplasticity
of the blend. In the initial stage of dynamic vulcanization,
two co-continuous phases are generated and as the degree
of cross-linking advances while mixing, the continuous
rubber phase becomes elongated further and further and
then breaks up into polymer droplets. As these rubber particles are forming as droplets, the PP becomes a continuous
phase.
FIG. 7. SEM micrographs of extracted PP=EPDM=NR blends:
(a) 50=50=0, (b) 50=50=0 vulcanized blend, (c) 50=30=20, (d) 50=30=20
vulcanized blend, (e) 50=0=50, and (f) 50=0=50 vulcanized blend.
empty holes left behind by the rubber after dissolving in
n-hexane. The light spots represent the insoluble PP phase.
There was an indication of the heterogeneous phase
structure of PP=EPDM=NR blends for every blend composition. Here, one component is expected to form a discrete
phase (rubber) dispersed in the other component forming
the continuous phase (plastic) and thus giving rise to a
two-phase system. In these systems, during processing the
low viscosity component tends to become the continuous
phase, whereas the high viscosity is a discrete phase. However, Fig. 7a shows that the rubber particles (EPDM) are
well dispersed in the continuous phase of PP indicating
good adhesion between PP and EPDM.
It can be seen that upon extraction, the rubber particles
of vulcanized blends (Figs. 7b, d, and f) were not easily
extracted from the continuous phase. These nature
micrographs reveal that the blends achieved vulcanized
rubber properties due to the formation of cross-links.
CONCLUSION
Dynamically vulcanized PP=EPDM=NR blends exhibit
higher stabilization torque, tensile strength, and tensile
modulus (M100) but lower elongation at break with EPDM
rich content than unvulcanized blends. The enhancement
of gel content and the reduction of swelling index have also
proved the formation of cross-links. Dynamically vulcanized blends show slightly lower percentage of crystallinity
than unvulcanized blends. The development of finer vulcanized rubber particles due to the cross-links formation
improves stability of the morphology, which improves the
properties of the blends.
ACKNOWLEDGEMENT
The authors would like to thank Yayasan Felda for the
research grant that made this research work possible.
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