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


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.


(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]


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.


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


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


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


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)


,,,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


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


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


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


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


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


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.

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