Chapter 3 Morphology and Mechanical Properties of...
Transcript of Chapter 3 Morphology and Mechanical Properties of...
Chapter 3
Morphology and Mechanical Properties ofNylon Copolymer/EPDM Blends
Abstract
This chapter discusses the phase morphology and
mechanical properties of the nylon/EPDM blends. The phase
morphology of the blends was investigated using scanning
electron microscopy with special reference to blend ratio. The
results were compared with theoretical perditions. Much
attention has been paid to correlate the mechanical properties
with the phase morphology. Finally various theoretical models
were used to fit the experimental data of the mechanical
properties.
The results discussed in this chapter have been submitted for publication in Journal of Applied Polymer Science
118 Chapter 3
3.1 Introduction
Blending of an elastomer with a selected plastomer provides
thermoplastic elastomers (TPEs) of diverse nature. These materials have
significant commercial interest [1-5]. Today, polymer blending is a
versatile and widely used method for optimizing the cost-performance
balance and increasing the range of potential application. TPEs are
reprocessible materials [6]. The physical properties of the blends are
controlled by the size of the dispersed phase, its dispersibility and its
interfacial interactions [7,8]. The key factors affecting the mechanical
properties of TPEs are the morphology and the compatibility between the
blend components [9].
Nylon copolymer is high performance engineering thermoplastic
with good mechanical strength, abrasion resistance and good chemical
resistance. It is widely used for many industrial applications. But the high
cost, dimensional instability and moisture uptake make the polymer
unsuitable for industrial applications and this is considered as the main
limitation of nylon polymer.
EPDM rubbers are the fast growing elastomers because of its
excellent properties especially in an ozone environment. These rubbers
exhibit good chemical resistance, low density, resistance to oxidation and
good dielectric properties. We can make cost effective products by
incorporating EPDM with nylon copolymer. The high cost of the nylon
copolymer is however a limitation for widespread applications.
. The blending of two polymers would lead to the formation of a new
cost effective polymeric material with desirable properties. It is very
essential to replace as much as nylon by EPDM since the former is very
expensive. So a balance between cost and performance is inevitable for the
Morphology and Mechanical Properties of … 119
new material. The direct mixing of the polymers leads to the formation of
incompatible blends, having poor properties. At the same time, it is well
known that immiscible polymer blends are characterised by a coarse and
unstable morphology coupled with poor interfacial adhesion between the
individual phases.
Nylon and EPDM could form blends with very interesting
properties because nylon offers very good mechanical properties and
EPDM offers good barrier to moisture. Nylon/rubber blends have been
extensively studied by several researchers [10-19]. In contrast to nylon 6
and nylon 66 blends, less information is available on the properties of nylon
copolymer blends. Therefore it is challenging and interesting to develop
super tough thermoplastic elastomers with nylon copolymer. The effect of
blend ratio on the mechanical properties of nylon copolymer and nitrile
rubber (NBR) blends was studied by Kumar et al. [20]. They found that
morphology of blends have a strong influence on the mechanical properties.
Scott and Macosko [21] found that the time of mixing plays an important
role in blend morphology and related properties. It was observed that as the
time of mixing increases the dispersed domain size decreases in the case of
nylon/ EPDM blends. Thomas and Groeninckx. [22] studied the effect of
processing conditions on the morphology development of nylon6/EPM
blends in 1999. Paul and co-workers [23] studied the mechanical properties
of blends of nylon/EPM-g-MA. They observed strain hardening and cold
drawing for the nylon rich blend systems.
In the present work, a systematic study has been carried out to
investigate the effect of the incorporation of varying amounts of EPDM on
morphology and mechanical properties of nylon/EPDM binary blends. First
we have studied the effect of blend ratio on the mechanical properties of
120 Chapter 3
nylon/EPDM blends. From this we selected three blend ratios viz. 30/70
(EPDM rich blend), 50/50 (co-continuous blend) and70/30 (nylon rich
blend) for further studies. Characteristics of EPDM dispersion in the blends
were determined with scanning electron microscope (SEM). Mechanical
properties of the blends were correlated with the morphological parameters.
Finally various theoretical models were applied to evaluate and compare
the experimental values of tensile strength. Explanations and illustrations
based on the results of above studies are presented in this chapter.
3.2 Results and Discussion
3.2.1 Processing Characteristics
The processing characteristics of the blends have been studied from
the Rheomix time-torque curves, which are shown in the Fig. 3.1.
Figure 3.1: Rheographs showing torque-time relations
The time–torque curves of all blends have two peaks. The first peak
is due to the increase in the viscosity by the introduction of the cold nylon
granules into the mixer. The viscosity then decreases, showing the
complete melting of the nylon. Upon the addition of EPDM into the nylon,
the viscosity again increases which corresponds to the second peak.
0 2 4 6 80
5
10
15
20
25
30
35
Torq
ue (N
m)
Mixing time (min)
N0 N30 N50 N70 N100
Morphology and Mechanical Properties of … 121
Thereafter the viscosity comes down showing the complete melting of the
second phase and finally the curves level off to give uniform torque value
at the end of the mixing. The leveling off of the torque may be related to
the attainment of a good level of mixing.
The highest stabilization torque was obtained for EPDM and the
lowest for nylon. Higher torque values correspond to higher melt
viscosities. All the blends show higher mixing torque than nylon, and the
torque is found to increase with increase in EPDM content. The melt
viscosity of the EPDM decreases very little with time as compared to
nylon.
The rotor speed, time of mixing and temperature of mixing were
optimized in order to get maximum value for mechanical properties without
the material getting thermally or mechanically degraded. Even though a
leveling off in torque was observed after 5 minutes, we have selected 8
minutes for a good level of mixing as the number of ingredients is more in
compatibilised and dynamically vulcanized systems. Also it is clear from
the figure that there is no reduction in torque on continued mixing up to
8 minutes. This indicates that there is no degradation taking place. So it is
found that 180°C, 60 rpm and the mixing time of 8 minutes are the ideal
conditions for mixing.
3.2.2 Phase morphology of blends
The phase morphology of immiscible polymer blends depends on
several factors such as component ratio, viscosity ratio, elasticity ratio, rate
of shear during melt mixing, surface energy difference of the components,
interfacial tension and processing history. Under identical processing
conditions, the blend composition and their difference in melt viscosity
plays a significant role in determining the morphology [24]. If the
122 Chapter 3
individual polymers have similar melt viscosities, the resultant morphology
of the blend is expected to be very fine, and a uniform distribution of the
minor component in a region of major component takes place [25]. The
viscosity of the rubber component in blends is one of the important factors
that control the dispersion of the rubber phase in the polymer matrix.
Torque–time curves obtained from the internal mixer during processing
supports the viscosity difference of the individual polymers (Fig. 3.1). It is
seen that EPDM possesses the highest viscosity. Its high viscosity
apparently makes this material harder to disperse in the nylon matrix.
The main mechanism governing the morphology development in
the blends is believed to be the result of both droplet breakup and
coalescence. The scanning electron micrographs of nylon/EPDM blends
(N20, N30, N40 N50, N60, N70 and N80) are shown in the Fig. 3.2(a) to (g). All
the micrographs show a two-phase morphology due to the immiscible
nature of the blends as a result of strong unfavourable interfacial
interaction.
A careful evaluation of the micrographs suggests that, up to 30 wt
% nylon concentrations, the nylon phase is preferentially dispersed in the
high viscosity EPDM matrix with a notable difference in the size and its
size distribution (Fig. 3.2 (a & b)). Spherical, elliptical and elongated
elliptical domains of nylon can be observed in these blends. Ray and
Khastgir [26] explained the different shapes of the domains as follows. The
molten polymeric materials during melt mixing experience a high shearing
action. The induced shearing force deforms the dispersed molten polymer
into elongated, rod-like particles, which constricts progressively until
rupture.
Morphology and Mechanical Properties of … 123
(a) Nylon20/80EPDM (b) Nylon30/70EPDM
(c) Nylon40/60EPDM (d) Nylon50/50EPDM
(e) Nylon60/40EPDM (f) Nylon70/30EPDM
(g) Nylon80/20EPDM
Figure 3.2: Scanning electron micrograph of nylon copolymer/EPDM blends at a magnification of 250 times (a) N20, (b) N30,(c) N40, (d) N50 (e) N60 (f) N70 (g) N80
124 Chapter 3
This constriction is mainly due to Brownian motion. Now, when the
particles come out of the shearing zone, they may fully or partly relax to
regain their original spherical, elliptical or elongated elliptical shapes, and
may remain isolated from each other. However, there will be a tendency for
particle recombination leading to some intricate shape. The schematic
diagram for this sort of shearing action, and subsequent reaction on the
polymer domains, is given in Fig. 3.3.
Figure 3.3: Schematic diagram for polymer domains under shearing action during melting and subsequent relaxation [Ref.26]
Morphology and Mechanical Properties of … 125
As the concentration of the nylon increases there is an onset of co-
continuous morphology at N40. The blend systems N40, N50, and N60 have
got a co-continuous phase structure and all other blend systems have got a
typical matrix/droplet morphology. The nylon phase and EPDM phase are
completely continuous at the co-continuous region. One can see a
channel-like co-continuous phases of both components running through
one another in Fig.(3.2d).
In the nylon rich blends (N70 and N80) EPDM phase has been
extracted (Fig. 3.2 (f and g)), a phase inversion occurs where nylon forms
the continuous phase and EPDM exists as dispersed domains. The blend
N70 has got a clear and sharp interfacial boundary, which may be
attributed to high interfacial tension indicating poor adhesion at the phase
boundaries and is a manifestation of the incompatibility of the polymer
components in these blends. It is well known that blends based on
immiscible polymer components are characterized by high interfacial
tension, which makes the dispersion during the blending operation
difficult, and contributes to unstable morphology and poor adhesion.
In short from the morphology of nylon/EPDM uncompatibilised
blend, we can observe that all blends exhibit a non-uniform and unstable
morphology and as the wt% of the dispersed phase increases, the
morphology becomes less stable. This can be explained in terms of
interfacial tension and coalescence effect. Final morphology obtained is a
balance between deformation and disintegration phenomena on one hand
and coalescence on the other hand. For this reason, a verity of
morphological structures is obtained by varying the composition. Taylor
[27,28] studied the deformation and disintegration of the dispersed phase
126 Chapter 3
for Newtonian systems in simple shear-flow fields in the absence of
coalescence effects. He defined a dimensionless parameter E:
E = Ca {[(19 +16) / (16 + 16)]} ……….. (3.1)
Where Ca is the capillary number, is the viscosity ratio of the droplet
phase to the matrix.
Ca= /Rm ………… (3.2)
where m is the viscosity of the matrix, R is the radius of the droplet, is
the shear rate and is the interfacial tension. From the Taylor equations it
is seen that size of the dispersed particles is directly related to the
interfacial tension between the two phases. A direct experimental
confirmation of interfacial tension/particle size relationship predicted by
Taylor theory was applied to polymers, then made by Lepers et al. [29]
and Linang et al. [30]. According to the authors, there is a 1:1
relationship between droplet size and interfacial tension. So the high
interfacial tension situation due to the unfavorable interactions at the
interface between the components in nylon/EPDM blends is one of the
basic reasons for the existence of a non-uniform unstable morphology.
Table 3.1 presents the morphological parameters derived from
SEM analysis of cryogenically fractured etched surfaces of the blends.
Morphology and Mechanical Properties of … 127
Table 3.1: Morphological Parameters of nylon/EPDM Uncompatibilised Blends from SEM analysis
Samplecode
Composition of
nylon/EPDMnD (μm) wD (μm) wD / nD Ai
(μm)2/(μm)3 IPD(μm)
N20
N30
N70
N80
20/80
30/70
70/30
80/20
8.6
13.8
15.8
14.5
13.9
20.3
21.9
19.1
1.61
1.47
1.39
1.32
0.23
0.14
0.30
0.28
4.6
3.4
3.5
2.8
It can be seen from the table that the dispersed phase domain size increased
as the concentration of the dispersed phase increased. The extent of
increase in the particle size ( nD ) suggests that the phenomenon of
coalescence is more predominant at high concentrations of the dispersed
EPDM phase. However, on the other hand, when nylon is the dispersed
phase, the influence of increasing nylon concentration on the coalescence is
less predominant as compared to the situation where EPDM is dispersed
phase. This is associated with the high viscosity of the rubber phase
(matrix) which resists the agglomeration of the nylon domains. In fact
when the matrix phase is more viscous, the higher shear forces and, hence,
the decreasing collision times along with a more difficult matrix interlayer
film drainage between the colliding droplets reduce the coalescence
probability. The phenomenon of coalescence at a higher concentration of
one of the components was reported by several authors [31-33]. This is, in
general, related to the droplet agglomeration during melt mixing, which is
well known to be a random process. As the result of mixing, drops of
dispersed phase may tend to collide and coalesce eventually. The
distribution of dispersed particles in continuous matrix can be evaluated
128 Chapter 3
from the polydispersity, wD / nD . It is obvious from the table that N30 has
got lower value of interfacial area per unit volume than N20. Interfacial area
is a measure of interfacial thickness, which in turn is a measure of
compatibility of blends. N30 blend is highly incompatible. So it has got a
very narrow interface compared to other blends, which may fail to transfer
stress between the phases. The low values of the interparticle distance
(IPD) indicate that the blends are not very brittle.
3.2.3 Estimation of co-continuity level Co-continuous structures are complicated three-dimensional
interpenetrating and inter twining structures. It is much more difficult to
identify a co-continuous structure unequivocally in a melt mixed blend.
Solvent extraction results are summarised in the Table 3.2. It is observed
that the results are in good agreement with the morphology obtained from
SEM observation (Fig. 3.2).
Table 3.2: Results of disintegration tests of the different blend series
Solvent used Sample code
Formic acid Boiling xylene
N20
N30
N40
N50
N60
N70
N80
Precipitate (ND) Precipitate (ND) Colloidal (PD) Colloidal (PD) Colloidal (PD) Milky suspension (D) Milky suspension (D
Milky suspension (D) Milky suspension (D) Colloidal (PD) Colloidal (PD) Colloidal (PD) Precipitate (ND) Precipitate (ND
D-Disintegration; ND-No disintegration; PD-Partial disintegration
Morphology and Mechanical Properties of … 129
The test gives results in the shorter time for macroscopic characterization
so that it can be carried out in all blend samples. For the blends consisting
of matrix with dispersed particles, etching of the matrix causes a complete
disintegration of the blend material and a milky suspension is obtained. In
the case of co-continuous structures neither of the solvents could cause
complete disintegration of the blend. So a colloidal solution is obtained.
From the nylon rich blends i.e. N50, N60, N70, N80, EPDM phase was
extracted with boiling xylene and from EPDM rich blends i.e. N20, N30, N40,
N50 nylon phase was extracted using formic acid. In both cases a precipitate
is obtained indicating the dissolution of the minor phase. N40, N50, N60 do
not disintegrate completely showing onset of percolation and co-continuity.
Results of the dissolution test for the different blend series show the
effectiveness of dissolution. Table 3.3 shows the volume fraction of the
different blend series after disintegration tests.
Table 3.3: Disintegration test results of different blend series
Samplecode
Co-continuityindex of Nylon
Co-continuityindex of EPDM % Continuity
N20
N30
N40
N50
N60
N70
N80
0.08
0.17
0.45
0.72
0.82
0.95
0.97
0.97
0.92
0.81
0.80
0.70
0.27
0.10
35.9
57.0
95.2
97.3
97.8
98.2
98.3
At 20% of nylon, the level of continuity is nearly zero, which is confirmed
from the scanning electron microscope (Fig. 3.2 a). In N30 continuity
130 Chapter 3
increases considerably to a volume fraction of 0.17. Between N40 & N50
and N50 & N60, a semi-continuous morphology is observed (Fig. 3.2). At
N60 phase inversion takes place and beyond which EPDM is seen to be
dispersed in Nylon phase. From N60 the continuity of the phase increases
gradually and reaches a volume fraction of 0.97 at N80. Similar trend is
observed when xylene is used as a solvent for EPDM phase. The samples
containing nylon & EPDM content greater than 60%, when etched with
formic acid, a jelly like mass was obtained. The percentage continuity of
the blends at different blend ratios is given in Fig. 3.4. From the Fig. 3.4 it
is clear that above 40wt % of nylon, the nylon phase is continuous. So
during extraction with formic acid, dissolution of the nylon will take place
only up to 40wt % and for rest of the compositions (50-80 wt%) colloidal
and milky suspensions were obtained indicating the continuous nature of
the nylon matrix.
Figure 3.4: Effect of blend ratio on the continuity of one of the phases in nylon/EPDM blends
3.2.4 Mechanical propertiesTensile stress–strain behaviour of the simple blends at a crosshead speed
of 50mm/min is shown in Fig. 3.5. The difference in the deformation
0 20 40 60 80 1000
20
40
60
80
100
120
% C
ontin
uity
Weight % of Nylon
Morphology and Mechanical Properties of … 131
characteristics of the blends under an applied load is evident from the stress-
strain curves. Addition of non-crystalline elastomer in small amounts to
semicrystalline nylon changes the nature of the curve considerably. At the
crosshead speed of 50mm/min, neat nylon has got a well defined stress-strain
curve typical that of a flexible plastic. Blends of varying component ratio show
different failure characteristics. Stress- strain curves of nylon and nylon rich
blends (>50%) show a linear elastic region followed by yielding in the inelastic
region. The curve up to the yield point shows clear elastic deformation,
thereafter the plastic deformation predominates. In the case of neat nylon, the
sharp increase in stress with strain beyond the yield point is associated with the
orientation of the crystalline hard segments of the nylon. As the rubber content
increases, the initial modulus as well as the yielding tendency decrease. The
phase change morphology can be understood from the stress-strain curves. In the
case of N30 the stress initially increases slightly and then decreases till the failure
occurs.,
Figure 3.5: Tensile stress-strain curves of nylon/EPDM blends
The blend N50 which is having a co-continuous morphology exhibits a
stress-strain behaviour, which is intermediate to those of the other blend
0 50 100 150 200 250 3000
10
20
30
40
50
St
ress
(MPa
)
Strain (%)
N0 N20 N30 N50 N70 N80 N100
132 Chapter 3
compositions. It is also observed that upon the addition of EPDM the strain
increases and the stress decreases. Various tensile properties such as tensile
strength ( m), elongation at break (Eb) and Young’s modulus (E) were
determined from the stress-strain curves. The variation of tensile strength
with wt% of nylon is shown in the Fig. 3.6.
Figure 3.6: Variation of tensile strength with weight % of nylon
The tensile strength of the nylon/EPDM blends depends on the strength of
the nylon matrix which in turn depends on the crystallinity of the nylon
phase. As evident from the Fig. 3.6, nylon is a semi crystalline material
having very good tensile strength, while EPDM is an amorphous material
having very poor tensile strength. The curve shows a negative deviation.
The blends show much lower tensile strength than projected from the
additivity line. The negative deviation is due to the poor interfacial
adhesion between the crystalline polar nylon and amorphous non-polar
EPDM rubber, which prevents the stress transfer between the matrix and
the dispersed phase. The failure stress depends on the interfacial interaction
between the two polymer phases. The lowering of the tensile strength in the
nylon/EPDM blends may be attributed to the presence of rubbery EPDM
0 20 40 60 80 1000
10
20
30
40
50
Max
imum
tens
ile st
reng
th (M
Pa)
Weight % of nylon
Morphology and Mechanical Properties of … 133
particles acting as stress concentrators. It is clear from the Fig. 3.6 that
tensile strength increases as the nylon content increases. A sudden increase
in the tensile strength is seen in blends where the nylon concentration is
greater than 40%. This sharp increase in the tensile strength is associated
with the phase inversion of nylon from dispersed to continuous phase. A
clear change in the slope of the tensile strength-composition curve is seen
between the composition ranges N30-N50. Danesi and Porter [24] for
PP/EPDM system have reported such deviation in the slope of mechanical
property-composition curves.
The Young’s modulus of nylon/EPDM blends as a function of blend
ratio is given in the Fig. 3.7.Young’s modulus values followed a trend
opposite to the strain at break. Modulus is a measure of the strength of the
material at low strains. So nylon rich blends give comparatively good
Young’s modulus values. Pure nylon has got a Young’s modulus of 205
MPa. Addition of EPDM decreases the Young’s modulus. The curve has
got a negative deviation. This is due to the high interfacial tension between
the two phases and the low modulus value of EPDM phase. From 60wt% of
nylon onwards the modulus increases remarkably due to the presence of
high modulus of nylon as continuous phase. The yield stress also got the
same trend as that of young’s modulus.
The decrease in the tensile modulus in the blends may is due to the
softening effect of the EPDM copolymer, since the tensile modulus of
EPDM is considerably lower than that of pure nylon. The introduction of
EPDM, a low modulus material, in to the nylon matrix causes an overall
lowering in the tensile modulus of the blends, and this in fact is contributed
by the low interfacial adhesion between the two mixtures.
134 Chapter 3
Figure 3.7: Effect of blend composition on the Young’s modulus and yield stress of nylon/EPDM blends
As seen from the Fig. 3.8 the elongation at break also shows a
negative deviation. EPDM has got high elongation at break value than
nylon. The value decreases as the nylon content increases and is found to
have more or less same values for N40, N50 and N60 composition. Thereafter
the elongation at break is found to be increased. The blends have
intermediate values which are much lower than projected from additive
level. The low value of the elongation at break for the blends are due o the
incompatibility and the poor adhesion between the phases.
The tension set after failure also increases as the nylon content
increases (Fig. 3.8). The considerable increase in the tension set values for
the blends of high nylon content greater than 40% is attributed to the poor
elastic recovery of the nylon phase after deformation.
0 20 40 60 80 1000
50
100
150
200
250 Young's modulus (MPa) Yield stress (MPa)
Weight % of nylon
You
ng's
mod
ulus
(MPa
)
0
5
10
15
20
25
30Y
ield Stress(MPa)
Morphology and Mechanical Properties of … 135
,,,Figure 3.8: Effect of blend ratio on elongation at break and tension set of nylon and EPDM blends
The variation of tear strength with weight % of nylon is shown in the
Fig. 3.9. The tear strength values of the blends also exhibit same trend of
tensile strength. Tear strength decreases as the rubber content increases. This is
due to the decreases in the crystallinity caused by the incorporation of the
rubber phase. Nylon copolymer is a semi crystalline plastic with much better
strength and EPDM is an amorphous elastomeric material with poor strength.
From the Fig.3.9, it is clear that blends with higher wt % of nylon has got
higher tear strength. In these blends nylon behaves as a continuous phase.
Figure 3.9: Effect of blend composition on the tear strength ofnylon /EPDM blends
0 20 40 60 80 1000
50
100
150
200
250
300
350
400 Elongation at break Tension set
Weight % of nylon
Elon
gatio
n at
bre
ak (%
)
80
120
160
200
240
280
320
Tension set after failure %
0 20 40 60 80 1000
50
100
150
200
250
Tear
stre
ngth
(N/m
m)
Weight % of nylon
136 Chapter 3
One of the important advantages of TPEs is that, they exhibit wide
range of hardness. In Fig. 3.10 the Shore A hardness as a function of blend
composition is given. The hardness values ranges from 31 to 99 Shores A.
Figure 3.10: Variation of Shore A hardness with wt% of nylon
The neat nylon shows the highest value of Shore A hardness, while EPDM
shows the lowest. The curve shows a slope change beyond 50wt% of
EPDM. The reduction in the hardness and the slope change in the curve at
higher concentration of EPDM can be explained by the phase inversion of
EPDM from dispersed to continuous phase. The useful working range [34]
of the Shore hardness measurements is in between 10 and 90 for Shore A.
Therefore reliable results were obtained for blends. It is interesting to note
that the hardness values show a positive deviation. The values lie above the
additive line because it is a surface property and is much less related to the
interfacial bonding.
It is interesting to note that as the wt % of the minor phase increased
the properties decreased. This is in good agreement with the morphological
parameters, which showed that as the weight % of the minor phase
increased, the morphological stability decreased. In short, all the properties
0 20 40 60 80 10020
30
40
50
60
70
80
90
100
Har
dnes
s,sho
re A
Weight % of nylon
Morphology and Mechanical Properties of … 137
except hardness show a negative deviation from additivity line. The inferior
mechanical properties of the uncompatibilised nylon/EPDM blends are due
to the lack of interfacial interactions between the phases.
3.2.5 Effect of testing speed on mechanical properties
Tensile stress–strain behaviour of the uncompatibilised blends at a
crosshead speed of 500mm/min are shown in Fig. 3.11. Various tensile
properties such as stress at break, tensile modulus, and elongation at break
determined from these curves are presented in Table 3.4. It is observed
from Fig. 3.11 that pure nylon shows a brittle behaviour when the testing
speed is high. But EPDM shows a ductile failure in tension. Nylon exhibits
a prominent yield point, whereas, in the case of EPDM, the stress–strain
curve is typical of that of a rubbery polymer, having no yield point with
400% elongation at break. On incorporation of EPDM the yield point
disappeared. The blends containing higher proportion of nylon (50%) show
higher initial modulus. As the rubber content increases, the initial modulus
decreases. The mechanical properties of the blends are maximum when the
testing speed is 500mm/min.
The ultimate elongation is found to be less at higher strain rate as
the testing speed increases. Nylon shows high initial modulus followed by a
necking tendency at the fracture point. To avoid the fracture of the material
under increased strain ratio, the accumulated stress should be relieved
through some dissipation of energy, such as yielding or necking. When the
apparent tensile strength increases under increased deformation rate, the
possibility for the relief of the involved stress becomes less and the fracture
should occur at the less strain position.
138 Chapter 3
Figure 3.11: Tensile stress-strain curves of nylon/EPDM blends
This accounts for the decrease of elongation at high strain rates. Moreover
the accumulated stress in the polymer material required to produce some
micro crazes in it is considered to be constant. Some of the crazes will grow
to be crack and causes the fracture may be the reason for the lower
elongation at higher strain rates.
Table 3.4: Mechanical properties of Nylon/EPDM blends at Cross head speed of 500 mm/min
Property N100 N70 N50 N30 N0
Tensile strength (MPa) 46.36 1.0 13.51 .12 8.33 .01 2.44 0.9 0.672 0.005
Youngs Modulus (MPa)
203.3 3.3 63.33 5.0 33.05 1.3 4.83 1.0 0.933 0.2
Elongation at break (%)
190 4 80 3 37 0.8 120 3 353 10
Stress at break (MPa) 35.18 0.8 13.43 0.8 8.3 0.9 0.82 0.7 0.46 1.2
Tear strength (N/mm) 120.26 2 60.61 2 35.81 1.3 14.1 .1 5.55 0.8
Hardness Shore A 99 1.8 98 2.4 91 3 75 2 31 1.8
0 100 200 300 4000
10
20
30
40
50
Stre
ss (M
Pa)
Strain %
N0
N20
N30
N50
N70
N80
N100
Morphology and Mechanical Properties of … 139
Fig. 3.12 shows the load–displacement curves of nylon-EPDM
blends during tearing. Nylon tears at higher load and at a small
displacement. This shows the high resistance offered by nylon to the
tearing force. EPDM undergoes largest displacement with the minimum
tearing force. Nylon/EPDM blends show the intermediate behaviour. As the
EPDM content in the blend increases the load required to tear the sample
decreases and the displacement increases. The increase in the displacement
with rubber content may be due to the increased stretching of the rubber
particles. From the figure it is also seen that modulus of the blends
decreases with the increases in the rubber content and this reduction is
more pronounced in the case of N30. Similar behaviour has been reported in
nylon/NBR blends by Kumar et al. [20].
Figure 3.12: Tear load-displacement curves of nylon/EPDM blends
3.2.6 Free volume studies
Free volume is an important parameter for understanding many of
the characteristic properties of polymers [35-38]. The free volume indicates
the unoccupied regions available in the system. The positron annihilation
0 20 40 60 80 100 1200
20
40
60
80
100
120
140
160
180
200
Load
(N)
Displacement (cm)
N0
N30
N50
N70
N100
140 Chapter 3
lifetime spectroscopy (PALS) is a non distractive technique for the
determination of free volume in polymers.
The measured lifetime spectra were resolved into three lifetime
components by free volume analysis. The three lifetimes are generally due
to different state of positron annihilation in the following ways. The short
lived component 1, with the intensity I1 is attributed to p-Ps and free
annihilations. 2, the intermediate component with intensity I2 is mainly due
to the positron trapped in the defects present in the crystalline regions or
trapped in the crystalline-amorphous regions. The longest lifetime
component 3, with intensity I3, is due to o-Ps pick off annihilation in the
free volume sites present mainly in the amorphous regions of the polymer
matrix.
According to Tao [39] and Nakanishi et al. [40] models, positronium
is assumed to be localized in a spherical potential well having an infinite
potential barrier of radius R0 with an electron layer in the region R< r< R0,
and predicts the connection between 3 and the free volume hole size R.
Using this model the average radius (R) of the free volume holes can be
determined from the relation,
(1/ 3 ) = 2[1-(R/R0) + (1/2 ) Sin (2 R/R0)] ………… (3.3)
where R0=R+ R, R being the thickness of the electron layer. The value of
the R=0.1656nm was determined by fitting the above equation with
experimental 3 values of molecular materials with known hole size like
zeolites [41].
The free volume measurements were successfully used to study the
compatibility of nylon/EPDM blends. PALS can give direct information
about the free volume sites present in the system. The variation of free
Morphology and Mechanical Properties of … 141
volume with weight percentage of EPDM in nylon/EPDM blends is shown
in the Fig. 3.13.
In this case free volume values are found to increase with the addition of
EPDM phase. Since EPDM and nylon are very dissimilar polymers the
physical and chemical interactions across phase bounders are very weak.
As a result, a high extent of void formation can occur. This accounts for the
increase in free volume in nylon/EPDM blends.
Figure 3.13: Effect of addition of EPDM on the free volume of nylon/EPDM blends
3.2.7 Effect of Processing An important advantage of TPEs over the conventional
thermosetting rubbers is that TPEs can be reprocessed by all common
equipment for plastic processing, such as extruders, injection moulders, and
low moulders without significantly changing the physical properties of
TPEs. To illustrate this reprocessing ability of the nylon/EPDM TPE, the
TPE is processed five times by compression moulding with the product
being cut into small pieces after each moulding cycle. It was observed that
the tensile strength and elongation at break of the TPEs are almost the same
0 20 40 60 80 10060
80
100
120
140
160
Free
Vol
ume(
A^°
)
Weight % of EPDM
142 Chapter 3
after reprocessing as shown in Fig. 3.14. This indicates that the
nylon/EPDM TPE has good processing ability.
Figure 3.14: Retention of tensile properties of reprocessed 70/30 nylon/EPDM thermoplastic elastomer
,
3.2.8 Theoretical modeling In a two-phase system, theoretical model could correctly predict the
way by which the phases interact or respond towards a particular
deformation. In an incompatible blend, some anomaly may be seen in the
property-composition curve. We could account for this by carrying out
model calculations. The best fit curve to that of the experimental curve tells
us about the way in which the blend components respond towards an
applied stress. Several theoretical models were analysed to explain the
experimental variation of tensile strength with composition. These include
Parallel, Series, Halpin–Tsai, Coran, Takayanagi, Kerner and Kunori
models. The application of the various models gives insight into the
properties of individual components. It also helps to check the assumptions
regarding the structure and properties of the interface. The highest upper
bound parallel model is given by the rule of mixtures. According to parallel
combinations, the property of the blend M is given by
1 2 3 4 50
20
40
60
80
100
Elon
gatio
n at
bre
ak %
& T
ensi
le st
reng
th(M
Pa)
Reprocessing cycle
Elongation at break Tensile strength
Morphology and Mechanical Properties of … 143
M = + M2 ……………… (3.4)
where M is the tensile strength of the blend and M1 and M2 are the tensile
strength of the components 1 and 2 respectively. and represent the
volume fraction of the component 1 and 2 respectively. This equation is
applicable to the models in which the components are arranged parallel to
the applied stress. The applied stress elongates each component by the
same amount. In the lowest lower bound series model, the blend
components are arranged in series (Reuss prediction) perpendicular to the
direction of the applied force. The equation for the series combination of
the components is given by
1/M = /M1+ /M2 ……………. (3.5)
According to Halpin-Tsai model [42,43] the equation that relates the
morphology of the polymer blend to the properties is,
M1/M = ( 1+AiBi i ) ………….. (3.6)
Where Bi = (M1/M2-1)/(M1/M2+Ai ) …………… (3.7)
In this equation the subscripts 1 and 2 refer to the continuous and dispersed
phases respectively. The constant Ai is determined by the morphology of
the system. For elastomer domains, dispersed in a continuous hard matrix,
Ai = 0.66 and when the hard material forms the dispersed phase, Ai is 1.5.
This model was also useful in determining the properties of polymer blends
that contained both continuous and discontinuous phases.
144 Chapter 3
The properties of an incompatible blend usually are in between the
upper bound parallel model (MU) and the lower bound series model (ML).
According to Coran’s equation [44].
M = f ( MU-ML )+M …………(3.8)
Where ‘f ‘can very between zero and unity. The value of ‘f’ is given by
f = V nH (nVS+1) …………..(3.9)
where n contains the aspects of phase morphology. VH and VS are the
volume fractions of hard phase and soft phase respectively. It can be seen
from the Fig. 17 that the experimental data is very close to the Coran’s
modal in which the value of n=5.9. This result is almost consistent with our
experimental results from morphology and mechanical properties studies.
Thakayanagi proposed a series-parallel model [45] in which the
concept of percolation is introduced. According to this model ,
M = (1- ) M1+ [(1- )/M1+/ /M 2]-1 …………..(3.10)
Where M1 is property of the matrix phase, M2 is the property of the
dispersed phase and the values of and are related to the degree of
series-parallel coupling.
The degree of parallel coupling of the model can be expressed by
% Parallel = [ (1- x100 ………….. (3.11)
Kunori and Geil [46] developed a model to account for the tensile
failure of a blend due to lack of adhesion between the blend components.
When there is no adhesion between the blend components, the tensile
strength of the blends ( b) can be written as
Morphology and Mechanical Properties of … 145
b = m (1-Ad) …………….(3.12)
where b and m are the tensile strength of the blend and the matrix,
respectively and Ad represents the area occupied by the dispersed phase in
transverse cross-section. Kunori and Geil assumed that when a strong
adhesive force exists between the blend components, the dispersed phase
will contribute to the strength of the blend and therefore parallel model may
be modified as
b = m (1-Ad) + d d ……………(3.13)
If the force propagates mainly through the interface the above equation may
be written as
b = m (1- d2/3) + d d
2/3 ……………….(3.14)
and if the force propagates through the matrix then the equation becomes
b = m (1- d ) + d d ……………………..(3.15)
which is same as parallel model equation.
Another important model for perfect adhesion is the Kerner [47] model. In
this model a new factor, Poisson’s ratio, is coming into picture. The
equation for this model is
)1(15/)108()57(/)1(15/)108()57(/
mmdmmmmd
mmdmmmddmb EEE
EEEEE ……(3.16)
146 Chapter 3
Where Eb is the blend property, m is the Poisson’s ratio (0.44) of the
matrix, and is the volume fraction. The subscripts m, d and b stands for
the matrix, dispersed phase, and blend respectively.
The curves resulting from different models and that of the
experimental data for the variation of the tensile strength with volume
fraction of EPDM is shown in the Fig. 3.15.
Figure 3.15: Comparison of the experimental tensile strength and theoretical predictions for binary blends of nylon copolymer and EPDM
From the figure it is clear that Coran’s model gives the best fit
curve and the Takayanagi model to some extent. The best fit in Coran’s
model may be due to the fact that this model takes into account of the
morphology of the blend. The parallel and series models show extreme
deviations because in an incompatible blend like nylon and EPDM uniform
stress distribution is virtually impossible. Also, in such blends premature
failure may occur due to the stress concentration at the interface.
0.0 0.2 0.4 0.6 0.8 1.00
10
20
30
40
50
Tens
ile st
reng
th(M
Pa)
Volume fraction of nylon
Experimental Parallel Series Halpin-Tsai Takayanagi
(25% parallel coupling) Coran(n=5.9) Kerner Kunori1
Morphology and Mechanical Properties of … 147
3.3 Conclusions Morphology of nylon/EPDM blend system indicated a two-phase
structure in which low viscosity nylon phase was dispersed as domain in the
continuous high viscosity EPDM matrix up to 40 w t% of nylon concentration.
A co-continuous morphology was obtained for 40/60, 50/50 and 60/40
nylon/EPDM compositions. At high nylon concentrations (70wt%), the
EPDM phase was dispersed as domains in the continuous nylon matrix.
The size of the dispersed phase was found to increase with increasing the
concentration of that phase; this is associated with coalescence. All the
results confirmed that the blends of nylon with EPDM show poor
mechanical properties because of their immiscibility and owing to their
poor interfacial adhesion due to the coarse morphology and lack of favourable
interactions at the interface between nylon and EPDM. We observed a definite
correlation between the phase morphology and mechanical behaviour. The
mechanical properties of the blends were found to be strongly influenced
by the blend ratio. Mechanical properties such as tensile strength, Young’s
modulus, tear strength, and hardness increased with the increase in nylon
content. The increase was sharper when the nylon content was more than
60% where it formed a continuous phase. It is verified that when the
elastomer content increased, Young’s modulus decreased and elongation at
break increased. All the mechanical properties except hardness were found
to have a negative deviation due to the high level of incompatibility. The
influence of the testing speed on the mechanical properties was also noted.
When the testing speed was 500mm/min, nylon behaved similar to that of
brittle plastic.
Free volume measurements using PALS showed a positive
deviation indicating the incompatibility of the blends. Effect of
148 Chapter 3
reprocessing indicated that the nylon/EPDM thermoplastic elastomer could
be reprocessed without adversely affecting the properties. Comparison of
experimental results with various theoretical models has been made in
order to predict the tensile strength of the blends. It is found that Coran’s
model and Thakayanagi model with 25% parallel coupling fit the
experimental results. Finally, it is important to mention that the present
study revealed that compatibilisation is essential in these blends to stabilize
the blend morphology and to improve the interfacial adhesion.
3.4 References
1. M.C. Reed, J. Harding, Ind Eng., 41,675, 1949.
2. I. Bohn, Rubber Chem. Technol., 41, 495, 1968.
3. M. Hara, J.A. Sauer, J. Macrom. Sci. Rev. Macromol. Chem. Phys., 38,
327, 1998.
4. P.F. Hartman, C.L. Eddy, G.P. Koo, Rubber world, 59, 163, 1970.
5. R. Ranalli, A. Whelan, K.S Lee, Barking, Applied science 3, 21,1982.
6. Encyclopedia of polymer science and engineering. New York, Wiley, 12,
399, 1998.
7. D.R. Paul and S. Newmann, Polymer blends, Academic press, New York,
1978.
8. C.D Han, Multiphase flow in polymer processing, Academic press, New
York 1981.
9. G.E Molan, I.N. Aggarwal SH. editor. Block polymers. New York:
Plenum press 79,1970.
10. Z.Oommen, S.R. Zachariah, S. Thomas, I. Aravind, G. Groeninckx, J.
Macromol. Sci., B: Phys., 43, 1, 2004.
Morphology and Mechanical Properties of … 149
11. Z. Oommen, G. Groeninckx, S. Thomas, J. Appl. Polym. Sci., 92, 252,
2004.
12. J. Zhang, T.P. Lodge, C.W. Macosko, Macromolecules, 38, 6586, 2005.
13. H. Liu, T. Xie, L.Y. Hou, G. Ou, Yang, J. Appl. Polym. Sci., 99, 3300,
2006.
14. R. Scaffaro, F.P. La Mantia, Macromol. Chem. Phys, 207, 265, 2006.
15. J.S. Wang, X.D. Chen, M.Q. Zhang, M.Z .Rong, Polym. Polym. Comp., 14,
1, 2006.
16. H.T. Chiu, Y.K. Hsiao, J. Polym. Res, 13, 153, 2006.
17. Y. Matsuda, M. Hara, T.Mano, K. Okamoto, M. Ishikawa, Polym. Eng. Sci.,
46, 29, 2006.
18. S. Filipe, M.T. Cidade, M. Wilhelm, J.M. Maia, J. Appl. Polym. Sci., 99,
347, 2006.
19. K. Wang, C. Wang, J. Li, J. Su, Q. Zhang, R. Du, Q. Fu, Polymer,
48,2144,2007.
20. C. Radhesh Kumar, K.E George, S. Thomas, J. Appl. Polym. Sci., 64,
2383, 1996.
21. C.E. Scott, C.W. Macosko, Polymer, 35, 5425, 1994.
22. S. Thomas, G. Groeninckx, J. Appl. Polym. Sci., 71, 1405,1999.
23. D.R. Paul, O. Okada, H. Keskkula, Polymer, 42, 8715, 2001.
24. S. Danesi, R.S. Porter, Polymer, 19, 448, 1978.
25. S. Wu, Polym. Eng. Sci., 27, 335,1987.
26. L. Ray, R.D. Khastgir, Polymer, 34, 2030. 1993.
27. G.I. Taylor, Proc.R Soc. London Ser A, 138, 41, 1932.
150 Chapter 3
28. G.I. Taylor, Proc.R Soc. London Ser A, 146, 501,1934.
29. J.C. Lepers, B.D. Favis, R.J. Tabar, J. Polym. Sci. B: Polym. Phys., 35,
2271, 1997.
30. H. Liang, B.D. Favis, Y.S. Yu, A. Eisenberg, Macromolecules, 32, 1637,
1999
31. J.J. Elmendrop, A.K. Van Der Vegt, Polym. Eng. Sci., 26, 1332, 1986.
32. I.K. Fortelny, Ovar, Eur. Polym. J., 25, 317,1989.
33. U. Sundararaj, C.W. Macosko, Macromolecules, 28, 2647, 1995.
34. Techinical Literature, Operation instructions, Zwic 3102, UTM 1975.
35. G. Dulbek, H.M. Fretwell, M.A. Alam, Macromolecules, 33, 187, 2000.
36. C. Nagel-Gunther-ade, K. Fritsch, T. Strunskus, F. Faupel,
Macromolecules, 35, 2071, 2002.
37. S. Ranimol, V. Siby, J. Kuruvilla, O. Zachariah, S. Thomas, Polymer, 47,
858, 2006.
38. N. Djourelov, Z. Ate , O. Güven, M. Misheva, T. Suzuki, Polymer, 48,
2692, 2007
39. S.J. Tao, J. Che. Phys., 56, 5499, 1972.
40. H. Nakanishi, Y.C. Jean, D.M. Schrader, Y.C. Jean, Positron and Positronium
Chemistry. Amsterdam: Elsevier, 1998.
41. P. Ramachandra, R. Ramani, G. Ramgopal.C. Ranganathaiah, Eur. Poly.
J., 33, 1707,1997.
42. N.E. Nielson, Rheol. Acta , 13, 860, 1974.
43. J.C. Halpin, J. Compos. Mater., 3, 732, 1970.
44. A.Y. Coran, Hand book of elastomers: New Development in Technology;
A.K. Bhowmick, H.L. Stephens, Marcel Dekker: New York, P-249. 1988.
Morphology and Mechanical Properties of … 151
45. R.A. Dickie, J. Appl. Polym. Sci., 17, 45, 1975.
46. T. Kunori, P.H. Geil, J. Macromol., 36, 218, 1960.
47. E.H. Kerner, Proc. Phys. Soc. London, 69B, 808, 1956.