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CHAPTER 8
Rheological behaviour and Viscoelastic and Thermal Properties of Immiscible NR/NBR
Blend Nanocomposites
Abstract
This chapter deals with the rheological behaviour and viscoelastic and
thermal properties of natural rubber/nitrile rubber blend nanocomposites with
nanoclay. The linear and non-linear viscoelastic characteristics of the blend
nanocomposites have been studied with reference to the filler loading, blend
composition and nanoclay modification. The effect of preparation method on
the viscoelastic behaviour has also been investigated. The blend composition
and filler loading etc. are found to be the key parameters influencing the
properties. The rheological properties have been analysed with special
reference to filler loading and frequency. Thermal analysis (TA) is a useful
tool to investigate a wide variety of properties of polymers and it can be also
applied to polymer blend nanocomposite in order to gain further insight into
the dispersion. This chapter also illustrates the thermal analysis of NR/NBR
blend nanocomposites reinforced with nanocaly. Thermal conductivity of the
different blend composites with varying filler content and clay modification
has been carefully studied.1
1 The results of this chapter has been communicated for publication in Rheologica Acta
246 Chapter 8
8.1 Introduction
Polymer composites and blends are effective solutions to the challenge of
developing new polymers with specific sets of properties. One of the main aims
of material researchers is to create novel materials with properties modified to a
particular application. Elastomer nanocomposites show remarkable change in its
properties once the filler particles are introduced into it. But the real effect of
these reinforcement can be understood once we gain insight into the detailed
nature of polymer/filler network. Rheological properties of a material determine
its behaviour when a shear force is exerted on it and has important implications
in many and diverse applications. The relationship between the structure and
rheology of a polymer is of practical interest for two reasons: First, rheological
properties are very sensitive to certain aspects of structure and they are simpler to
use than analytical methods, such as nuclear magnetic resonance. Second, it is
the rheological properties that govern the flow behaviour of polymers when they
are processed in the molten state. Applications of rheology are important in many
areas of industries involving metal, plastic, and many other materials. The results
from rheological investigations provide the mathematical description of the
viscoelastic behaviour of matter. An understanding of the rheology of a material
is important in the processing of composites. 1
The filler reinforcement can also be explained using strain sweep studies. For
polymer nanocomposites, strain sweep shows striking strain dependence of
storage modulus and loss modulus. This striking behaviour was first
determined by Fletcher and Gent and was later confirmed by Payne and is
called payne effect. Payne effect is believed to be a key viscoelastic signature
for filled rubber at low strain amplitude.
Another important feature that can contribute to the detailing of reinforcement
mechanism is stress softening effect or hysteresis loss seen during stress loading
Rheological behaviour and Viscoelastic and Thermal Properties of… 247
and unloading process. The energy dissipated during the cycles can be of
primary importance in many industrial applications.
The life span of rubber products depends on several key factors including the
polymer matrix, the ingredients added to the compound, the network that holds
the matrix together and the heat history of the matrix. Thus, the total effect of
these factors determines both the thermal history that the compound will
experience during its use. Knowledge of the thermal behaviour of a material
is an important requirement in the design of the material and the further
procedures.
An interesting area of thermal study is the thermal conductivity of cured
elastomer blends. The physical properties of immiscible two-phase blends
depend on the properties of each constituent phase. The other factors include
the morphology, dispersion and stiffness. Each factor plays a significant role in
the final properties of the blend. The transfer of heat through an elastomeric
matrix is very important for processing of the material and its lifetime. Many
elastomer products used in various industries have to experience a significant
heat build-up as a result of the heat produced by repeated deformation and the
friction between constantly moving parts. This heat build-up cause an increase
in operating temperature as the system is being worked. Such a heat build-up
in that part can cause deformity and can make that part less efficient. Therefore,
for many of the products heat transfer is a factor for the longevity of the product.
So addition of fillers which can enhance the thermal conductivity can serve as a
thermally conductive path for dissipation of heat from the product so as to
enhance the service life of the elastomer products2- 4. The three most important
physical properties of a material that are needed for heat-transfer calculations
are thermal conductivity, thermal diffusivity, and specific heat, that is, the
thermophysical properties and they can be used to evaluate the influence of
248 Chapter 8
different polymers and fillers on heat transfer. Additionally, the dispersion of
the filler which has an effect on heat transfer and thermal conductivity
measurements can be used to provide semi-quantitative estimations of filler
dispersion5-6. The magnitude of thermal conductivity is specific to each element
alloy or compound and is defined by relating the heat flow to the thermal
gradient produced by this flow7.
While the thermal conductivity studies can be of importance during the
processing of the material compound, the determination of the glass transition
temperature is important while for assessing the miscibility of the polymer
blends. The classical case of immiscibility is marked by the appearance of two
unbroadened glass transition temperatures (Tg) which are unchanged from that
of the component (non-blended) polymers. Any shift of Tg could indicate partial
solubility8. In the case of partially miscibile blends, the Tg of the polymer with
the lowest glass transition temperature increases and, that of the polymer with
the highest glass transition temperature decreases, thus shortening the
temperature interval between the two glass transitions. The extent of this
shortening is a measure of miscibility and, in the ideal case of a miscible blend
there is a single Tg. The thermogravimetric analysis done to determine the
thermal degradation of the sample is also important when it comes to certain
applications where we need thermal stability.
Considerable work has been reported on the thermal studies of polymer
blends.9-12 Saxena et al.13 studied and measured the thermal conductivity of cured
SBR with particles of NR latex waste as filler. They reported a broad peak in the
thermal conductivity in the glass transition region of the compounds. A relatively
high thermal conductivity was observed by Fuji14 mixing styrene– butadiene–
styrene block copolymer elastomer with a specified amount (such as 15 phr) of
elastic graphite. Fan et al.15 in their study on nano-zinc oxide/solution
Rheological behaviour and Viscoelastic and Thermal Properties of… 249
polymerized butadiene styrene rubber (SSBR) composites found a gradual
increase in the thermal conductivity with nano-zinc oxide loading, and found that
the measured thermal conductivity was very close to the theoretical value
calculated by Nielsen model. Zhou et al.16 in their report on CNTs/SBR
composites prepared by spray drying method found that thermal conductivities of
the composites were gradually enhanced with the increase of CNT addition. The
influences of three types carbon black N330, N339, and N351 on the heat
conductivity of radial tire tread (SBR/BR, 80/20) were investigated by Ma et al.17
and found that the heat conductivity of tread increased with the total levels of
carbon black. Tang et al.18. reported that the thermal conductivity of the heat
conductive SBR composites with nano-alumina and micro-alumina. increased
with increasing alumina amount and nano-alumina had higher thermal
conductivity than those filled with micro-alumina at the same loading. The
variation of thermal conductivity with temperature for silicone rubber-SBR
blends was reported by Bhowmick et al.19
The thermal degradation and dynamic mechanical characteristics of many
polymer blends were investigated by various researchers20-22. Enhanced thermal
stability and dynamic modulus upon compatibilization were reported. This
enhancement in the properties has been explained as been due to with higher
morphological stability of the dispersed phase and interfacial interactions23.
Jana and Cho24 have reported in their work on thermal stability and molecular
interaction of polyurethane nanocomposites prepared by in situ polymerization
with functionalized multiwalled carbon nanotubes that the higher thermal
stability of in situ nanocomposites was ascribed to covalent bond formation
between MWNTs and PU chains, which could result in better dispersion of
MWNTs in the PU matrix for the in situ nanocomposites than for the
conventional nanocomposites..Nanosize filler was reported to enhance
250 Chapter 8
resistance against decomposition.25 Balachandran et al.26 have studied the effect
of expanded graphite (EG) filled with compatibilized and uncompatibilized
EPDM/FKM (50/50, w/w) blends in which maleic anhydride grafted EPDM
(MA-g-EPDM) was used as a compatibilizer ad have studied the thermal
stability of the EG loaded EPDM/FKM blends using thermo gravimetric
analysis (TGA). Paul et al.27 have reported the effect of fibre loading and
chemical treatments on thermo-physical properties of banana fibre reinforced
polypropylene commingled composites and observed that chemical
modifications on natural fibres improved the thermo-physical properties,
although a decrease in the thermal conductivity and thermal diffusivity was
shown on increasing fibre loading.
The application of this NR/NBR blend system includes several industries like
automobile and sports industry where it is important to know the rheological and
thermal history. In this chapter on NR/NBR/O1Mt polymer blend
nanocomposites we sought to investigate the non-linear viscoelastic behaviour of
the blend nanocomposites and the thermal conductivity based on the effect of
filler loading, blend composition and clay modification. We have also tried to
investigate the effect of nanoclay modification into this immiscible NR /NBR
blend system. In a polymeric blend system the effective thermal conductivity
depends upon the thermal conductivity of the individual components. The study
is expected to help in gaining deeper insights into the nanoclay reinforcement and
in optimizing these blends composition for further studies.
Rheological behaviour and Viscoelastic and Thermal Properties of… 251
8.2 Results and Discussion
8.2.1 Rheological properties of NR/NBR blend nanocomposites
Rheological behaviour at high frequencies is normally used to estimate the
effect of the filler on processing properties. Low-frequency behaviour is
sensitive to the structure of the percolation state of nanofillers within the
composite28. The complex viscosities, |η*|, of the different NR/NBR blend
nanocomposites are shown in Fig. 8.1, 8.2 & 8.3. The complex viscosity
increases with the nanoclay loading for most of the compositions. The effect of
the nanoclay is most prominent at low frequencies and the relative effect
diminishes with increasing frequency due to shear thinning. This is in
accordance with earlier reports of theoretical expectations and experimental
observations for nanoclay filled elastomer nanocomposites29. For all the blend
nanocomposites series, for 10 phr nanoclay, the complex viscosity is increased
with increasing nanoclay concentrations, even at high frequency. This may be
due to strong filler/filler network at higher concentration.
0.1 1 10
1000
10000
100000
Com
plex
Vis
cosi
ty -
η∗ (P
as)
Frequency (Hz)
70/30(0) 70/30(1) 70/30(2) 70/30(5) 70/30(10)
Figure 8.1 Frequency dependence curves of complex viscosity η *for 70/30
NR/NBR blend.
252 Chapter 8
Figure 8.2 Frequency dependence curves of complex viscosity η *for 50/50
NR/NBR blend.
Figure 8.3 Frequency dependence curves of complex viscosity ƞ*for 30/70 NR/NBR blend.
0.1 1 10
104
105
106
Com
plex
Vis
cosi
ty
η∗ (P
a.s)
Frequency (Hz)
30/70(0) 30/70(1) 30/70(2) 30/70(5) 30/70(10)
Rheological behaviour and Viscoelastic and Thermal Properties of… 253
The storage modulus (G), of the composites measured at 1000 C are
logarithmically plotted as functions of frequency in Fig.8.4. For 50/50 and
30/70 NR/NBR blend composition the storage modulus increased with filler
loading. This increase in storage modulus at higher concentration can be
explained by the fact that at higher filler loading in the absence of polymer/
filler interaction, the interacting clay layers through the formation of physical
connectivity or a percolated network between clay layers can show a solid like
behaviour which is reported to be a pseudo solid like behaviour in the
literature30.
0.1 1 10
105
106
log
G' (
MP
a)
Frequency (Hz)
30/70(0) 30/70(1) 30/70(2) 30/70(5) 30/70(10)
Figure 8.4 Storage modulus vs. dynamic amplitude curves of for 30/70 NR/NBR blend with different filler loading.
254 Chapter 8
0.1 1 10
104
105
106
Frequency (Hz)
log
G'(M
Pa)
50/50(0) 50/50(1) 50/50(2) 50/50(5) 50/50(10)
Figure 8.5 Storage modulus vs. dynamic amplitude curves of for 50/50 NR/NBR
blend with different filler loading.
8.2.2 Payne effect-effect of nanoclay on the polymer-filler network formation
The linear viscoelastic region in elastomer nanocomposites can be determined
from the strain (amplitude) dependence of the dynamic viscoelastic properties
of polymer nanocomposites and is known as payne effect. At high strain
amplitudes, the rigid layer decomposes and the interactions of polymer chains
with the filler surface will break, leading to perturbation of the nanocomposite
structure and observation of the nonlinear strain dependence of storage
modulus.31 The different phenomenon like polymer-filler interaction energy,
chain entanglements and the surface of the filler which is in contact with
polymer chains,32 have substantial effects on the strain dependence and linear
viscoelastic properties of the samples33. Thus, an immobilization of polymer
Rheological behaviour and Viscoelastic and Thermal Properties of… 255
chains in comparison to the neat polymers yields a better interlocking in the
otherwise mobile polymer entanglements of the polymer melt. Payne effect
yields a good understanding of the polymer filler interaction and the concept of
filler networking34. The higher inter-particle forces render higher payne effect.
The inter-particle forces between the filler particles are effective only when
distances between them are small.35 It has been reported by Poikelispe et al in
their work on carbon black in natural rubber-butadiene rubber blend, that, CB-
filled NR/NBR have the highest payne effect indicating the strongest filler–filler
network. When CB was replaced with CNT the filler–filler interactions become
weaker and the dispersion of the main filler is improved as a result of better
rubber–filler compatibility.
The present section focuses on the effect of nanoclay in forming a polymer filler
network, and on how the presence of nanoclay affects the organization of mobile
polymer chains in the blend. The viscoelastic behaviour vs. strain has been
investigated and the results for different blend composition with varying filler
loading is given in Fig.8.6 and 8.7. It is observed that the blend nanocomposites
show varying behavior generally associated to Payne effect. As expected, it is
observed that the unfilled is it shows almost a linear dynamic storage. For all
higher loadings, a typical payne effect was observed, with the sudden change of
the linear viscoelastic response of the storage modulus, at higher strain. In payne
effect, the modulus drop with strain amplitude is due to the combined effects of
polymer network, hydrodynamic effects in rubber structure and the filler-filler
interaction. For pure elastomers nanocomposites also it can be observed that (Fig.
8.6 & 8.7) the payne effect is prominent or the amplitude of payne effect increases
with filler loading. This can be due to the goog dipersiom of OIMt in the polymer
matrix. But, with the case of the blend nanocomposites, quite a different behavior
was shown by the nanocomposites at different filler loadings.
256 Chapter 8
1E-4 1E-3 0.01
4E6
5E6
6E6
7E6
8E6
9E6
log
G'(M
Pa)
Dynamic Strain (%)
100/0-0 100-0-2 100-0-5 100-0-10
Figure 8.6 Storage modulus vs. dynamic strain curves for 100/0 NR/NBR blend
with different filler loading
1E-4 1E-3 0.01
4E6
6E6
8E6
1E7
1.2E7
1.4E7
1.6E7
log
G'(M
Pa)
Dynamic Strain(%)
0/100-0 0-100-2 0-100-5 0-100-10
Figure 8.7 Storage modulus vs. dynamic strain curves of for 0/100 NR/NBR blend with different filler loading
Rheological behaviour and Viscoelastic and Thermal Properties of… 257
8.2.3 Hysteresis –damping efficiency of blend nanocomposites.
Highly Viscoelastic materials are gaining popularity in damping applications.
Although these materials have promising damping efficiency, they suffer from
several important limitations such as high weight penalty, compactness issues,
poor reliability, low thermal conductivity and poor performance at high
temperatures. Many of the composite materials incorporated with different
fillers show improved damping. They are still limited by the deficiencies of
the basic polymer and suffer weight gain and power disadvantages. Therefore,
there is a need to make a material with improved damping efficiency for
applications which need wear and tear resistance and that can overcome
limitations like weight gain and other existing disadvantages. Fillers are added
in rubber materials to improve variety number of properties that is required for
a particular application. For eg. in rubber products carbon black is added not
only for cost reduction but also as a reinforcing agent. For certain applications,
along with reinforcement, it is very crucial that properties like performance,
adhesion, durability, wearing, mileage, crack resistance, etc are also improved.
These properties can be improved if the properties like modulus and hysteresis
can be increased.
Rubber has a viscoelastic characteristic which results in the energy loss during
a cycle or repeated stress/strain cycling. The energy loss is called hysteresis
(or hysteretic loss). Hysteresis can consume much energy, and therefore the
rate of crack propagation can be slowed down. This is because the
deformation of rubber, as a result of the applied mechanical energy, is
converted into heat and other forms of energy. Many properties like wear,
modulus, tear, heat generation, etc., are often correlated with hysteresis loss in
research literature36. However, it has been proved by many reports that the
energy loss in the compound plays a key role in enhancing better performance
258 Chapter 8
in applications, where an elastomer is subjected to repeated deformation by a
force of sufficient magnitude and frequency37. It is observed that, in the case
of elastomer nanocomposites, the hysteresis is also increased with increase in
filler loading added to a compound. This is due to the increase in total filler
surface area, which results in an increase in interfacial adhesion38. The
interfacial slippage is considered to be one of the main cause for energy
dissipation. It was proposed by Dannenberg and coworkers39 that the
elastomer located at the particle-rubber interface could undergo surface
slippage, resulting in both stress softening and hysteresis.
It has been reported that40-45 the quality of adhesion between the filler and
polymer chains results in the effective load transfer. There are a lot of ongoing
research which focuses on strengthening of the filler–polymer interface to
prevent interfacial slip. Although it can be detrimental to stiffness and
strength, a very high mechanical damping can be beneficial in certain
applications as, it can prevent protection to the materials covered by this or
beneath this. In the present chapter, NR/NBR blend nanocomposite, the effect
of blend composition and filler loading is studied with reference to hysteresis.
This effect is related to the nanoscale dimensions and high aspect ratio of
nanoclay, which results in a large interfacial contact area, leading to high
frictional energy dissipation during the sliding of nanoclay from the polymer
surface, by the continuous loading and unloading. Once these blend
nanocomposites improve damping without much sacrifice in the mechanical
properties and structural integrity, it can be further analysed in the future, to
ensure stability in a wide variety of structural components and systems.
A comparison of the hysteresis for different blend composition with varying
nanoclay loading, can be seen from the single cycle experiment given in Fig.8.8
to 8.11 for each compound. The area inside the hysteresis loop is a measure of
Rheological behaviour and Viscoelastic and Thermal Properties of… 259
damping and the values for it are given in Table 8.1. The values are dependent
on the viscous property of vulcanizates. It can be seen that, for all the
composites, the hysteresis value increases with increase in filler loading except
for certain blends, where it can be due to other intriguing factors which needs
further study. The damping is more for the blend nanocomposites than that for
the pure nanocomposites. This result suggests that the filler-polymer interaction
in blend sytem is weaker than that of the organoclay filled pure elastomers.
Also, in all cases, the loading curve of the first cycle was different from the
unloading curve, and so were the successive loading curves. On the first
loading-unloading cycle, the area of hysteresis loop, ie the dissipated energy,
significantly differed from each other and increased by increasing the amount
of nanoclays. The responsible mechanism that could be explained for the
observed increase in mechanical damping can be either energy dissipation
caused by interfacial sliding at the filler/polymer interface and the energy
dissipation caused by interfacial stick–slip sliding at the filler –filler interface.
It is also reported that filler-filler interaction also plays a major role in rubber
hysteresis behavior46. The higher hysteresis loss in the case of higher filler
compounds may also be due to the more number of filler- filler interactions,
well established by earlier reports47.
260 Chapter 8
0 20 40 60 80 100 1200
2
4
6
8
10 S
tres
s (M
Pa)
Strain(%)
100/0(0) 100/0(1) 100/0(2) 100/0(5) 100/0(10)
Figure 8.8 Effect of nanoclay content on single loading-unloading cycles 100/0 NR/NBR nanocomposites at a constant strain of 100%.
0 20 40 60 80 100 1200
2
4
6
8
10
12
Strain (%)
Str
ess
(MP
a)
70/30(0) 70/30(1) 70/30(2) 70/30(5) 70/30(10)
Figure 8.9 Effect of nanoclay content on the single loading-unloading cycles 70/30 NR/NBR nanocomposites at a constant strain of 100%
Rheological behaviour and Viscoelastic and Thermal Properties of… 261
Figure 8.10 Effect of nanoclay content on the loading-unloading cycles of 50/50
NR/NBR nanocomposites at a constant strain of 100%.
0 20 40 60 80 100 1200
2
4
6
8
10
12
14
Str
ess
(MP
a)
Strain (%)
0/100(0) 0/100(1) 0/100(2) 0/100(5) 0/100(10)
Figure 8.11 Effect of nanoclay content on single loading-unloading cycles of 0/100 NR/NBR nanocomposites at a constant strain of 100%
0 20 40 60 80 100 1200
2
4
6
8
10
12
Str
ess
MP
a
Strain %
50/50(0) 50/50(1) 50/50(2) 50/50(5) 50/50(10)
Strain (%)
Str
ess
(M
Pa
)
262 Chapter 8
Table 8.1 Area inside the hysteresis loop for different NR/NBR nanocomposites.
Filler loading (phr) Area inside the loop (sq.inch)
100/0 70/30 50/50 0/100
0 0.53 1.07 0.70 1.62
1 1.88 1.25 2.09 2.47
2 2.36 2.05 2.12 3.53
5 1.58 3.53 4.49 8.11
10 4.40 6.58 8.25 6.5
8.2.4 Effect of strain level on the hysteresis loss
Inorder to understand the hysteresis loss behaviour over a range of strains, the
stress-strain curves at different strain levels were recorded. The measurements
were continued upto 3 cycles. Representative curves of hysteresis loss of
different NR/NBR blend nanocomposites with varying filler loading are given
in Fig. 8.12 to 8.14. The nature of the plot shows expected behaviour of the
dependence of hysteresis loss on strains, i.e., the hysteresis loss increases with
an increase of and strain level. However, the rate of increase of hysteresis loss
is not same throughout the range of strain levels. To know the effect of blend
composition, a graph has been plotted for different blend composites with 2 phr
nanoclay at different strain (Fig.8.15). The hysteresis loss increases for most of
the blends in the second and the third cycles. This can be attributed to the fact
that, although we cannot overlook the large contribution of orientational aspects
in the stress-strain behavior of composites, there is a significant effect of the
strain in debundling of the agglomerates and it is worth to notice that the
magnitude of increase in hysteresis loss for second and third cycle is higher in
the case of samples with high content of filler. This interpretation is supported
by the fact that, the filler polymer slippage or the interfacial interaction between
Rheological behaviour and Viscoelastic and Thermal Properties of… 263
polymer and filler, will occur in the second stretching at a much higher value of
strain than that obtained in the first stretching. On considering different filler
loading, there is no appreciable change with the varying filler loading for pure
NR nanocomposite. For all the nanocomposites, the damping is the same except
for 0/100, 30/70 and 50/50 where there is a decrease in damping behaviour for
the third cycle after 2 phr loading.
Figure 8.12 Effect of nanoclay content on the 3 consecutive loading-unloading cycles 100/0 NR/NBR nanocomposites
0 100 200 300 400 500 600 700 8000
5
10
15
20
25
30
35
40
Str
ess
(MP
a)
Strain (% )
100/0(0) 100/0(1) 100/0(2) 100/0(5) 100/0 (10)
264 Chapter 8
0 40 80 120 160 200 2400
4
8
12
16
20
24S
tres
s (M
Pa)
Strain (%)
30/70(0) 30/70(1) 30/70(2) 30/70(5) 30/70(10)
Figure 8.13 Effect of nanoclay content on the 3 consecutive loading-unloading cycles 30/70 NR/NBR nanocomposites
Figure 8.14 Effect of nanoclay content on the 3 consecutive loading-unloading cycles 50/50 NR/NBR nanocomposites
0 80 160 240 320 400 4800
4
8
12
16
20
24
Str
ess
(MP
a)
Strain (%)
50/50(0) 50/50(1) 50/50(2) 50/50(5) 50/50(10)
Rheological behaviour and Viscoelastic and Thermal Properties of… 265
8.2.5 Effect of blend composition on hysteresis loss
The hysteresis curves of the NR/NBR/OMt nanocomposite with two different
types of nanoclays are given in Fig 8.15. In all cases, the loading-unloading
cycle, the area of hysteresis loop and dissipated energy, were significantly
different from each other for all samples and increased with increasing amount
of NR. These successive cycles seemed to nearly overlap in the compressive
stress-strain curves, exhibiting better elasticity.
0 100 200 300 400 500 6000
5
10
15
20
25
30
35
Str
ess
(MP
a)
Strain (%)
100/0(2) 70/30(2) 50/50(2) 30/70(2) 0/100(2)
Figure 8.15 Effect of nanoclay content on the 3 consecutive loading-unloading cycles of different NR/NBR nanocomposites with 2phr nanoclay
8.2.6 Effect of clay modification on the hysteresis loss of the blend nanocomposites
The hysteresis plot of 70/30 NR/NBR nanocomposite with two different
modifications of nanoclay is given in Fig. 8.16. It can be observed from the
figure that for the blend nanocomposite with mercapto silane modification, the
area of the hysteresis loop is much lower. This, as reported in chapter 4 is due
to the stronger interaction between O2K and NR and the enhanced number of
266 Chapter 8
cross-linking caused by the mercapto silane group. The area inside the loop is
given in Table 8.2 for further clarification.
0 100 200 300 400 500 6000
10
20
30
40
Str
ess
(MP
a)
Strain %
70/30(5) O2K 70/30(5) O1Mt
Figure 8.16 Effect of nanoclay content on the 3 consecutive loading-unloading cycles 70/30 NR/NBR nanocomposites with two types of nanoclay.
TABLE 8.2 The area inside the hysteresis loop for the 3 consecutive loading-unloading cycles 70/30 NR/NBR nanocomposites with two types of nanoclay
Sample Area inside the loop(sq.inc)
Cycle 1 Cycle 2 Cycle 3
70/30/5 (O1Mt) 0.65 0.82 1.94
70/30/5 (O2K) 0.14 2.36 4.75
8.2.7 Thermo physical properties
8.2.7.1 Effect of blend composition on thermal conductivity
In many applications the accumulation of static electric charges on a polymer
surface can be reduced by improving its thermal conductivity. Usually elastomers
have inherently poor thermal and electrical conductivities and the accumulated
Rheological behaviour and Viscoelastic and Thermal Properties of… 267
internal heat is the major ageing mechanism for the elastomers products used in
dynamic loading conditions. Example includes like tyres, conveyor belts and
rubber rollers. Enhancing the thermal conductivity can solve this problem to a
certain extent. The reason for thermal conductivity in them is the lattice
vibrations. The hysteresis loss of elastomer and the internal friction between
polymer-polymer networks etc can be considered to be the reason for thermal
conductivity48. So, in order to improve the thermal conductivity for a particular
application it needs to be modified. One method is to add another elastomer with
higher thermal conductivity to suite the application.
Here, in this blend the higher thermal conductivity of NBR improves the
thermal conductivity of the elastomer blend to an appreciable level. The
morphological change also can be considered to impart some contribution.It
can be observed that the thermal conductivity of NR/NBR blend increased
with increasing of (NBR) content.(Fig 8.17). The increase in thermal
conductivity of NR with the addition of NBR may be attributed to differerece
in thermal conductivities of the two elastomers. This higher thermal
conductivity can be explained. It has been reported that the thermal
conductivity of NBR is 0.24 λ/m.K and that for NR is 0.148 λ/m.K49. Therefore
the higher thermal conductivity increases with increasing NBR content as
expected. Since the NBR is highly polar, it shows higher thermal conductivity
than NR50.The improvement in thermal conductivity at 50/50 blend
composition can be due to the continuous phase of NBR which shows that the
morphological development of the blend composites also influence the
improvement in conductivity. The blending of the rubbers having different
thermal conductivity values (NR is less thermally conductive than NBR) helps
to design a material which has either higher or lower thermal conductivity.
268 Chapter 8
100/0 70/30 50/50 0/1000.16
0.17
0.18
0.19
0.20
0.21
0.22λ
W/m
.k
NR/NBR blend composition
Figure 8.17 Thermal conductivity versus blend composition of NR/NBR blends.
8.2.7.2 Effect of nanoclay loading on thermal conductivity
The effective thermal conductivity of the composites depend on the thermal
conductivity of the constituent elements51. According to Agari model52-54 the
thermal conductivity of the composites is linearly related to the volume
percentage of the filler. The experimental values is shown in Fig. 8.18 shows
a linear growth. It is clear that the thermal conductivity of NR improves to an
appreciable level, slightly more than the thermal conductivity of NR in clay
composites. Thermal conductivity increase directly with the loading level of
nanoclay depending on the rule of mixtures. The thermal conductivity of
composite is thus governed by its component amounts and their properties.
The presence of O1Mt improves the thermal conductivity of rubber blend
composite due to higher thermal conductivity of nanoclay than the thermal
conductivity of both NR and NBR rubbers.
Rheological behaviour and Viscoelastic and Thermal Properties of… 269
0 2 4 6 8 10
0.165
0.180
0.195
0.210
0.225
0.240
λW/m
.k
Filler loading (phr)
100/0 70/30 50/50 0/100
Figure 8.18 Thermal conductivity versus filler volume fraction for O1Mt filled NR/NBR nancomposites.
It is reported that55 to produce highly conductive composites(thermal
conductivity)containing a low content of filler, the filled phase should be
continuous or in, the filled phase should form the matrix of a dispersed blend or
exhibit a continuous phase in a cocontinuous blend. It has been also established
that thernal conductivity of polar sytems are higher. Thus in the case of NR/NBR
composite with different O1Mt loading the one which have continuous NBR
forms the conductive chains. Thus the reason for the improved thermal
conductivity for 50/50 blend nanocomposites can be justified.
Now, as the loading is increased, many O1Mt clay layers touch each other to
begin to form conductive chains, which greatly contribute to the thermal
conductivities of composites. Further it can be explained that the high surface
270 Chapter 8
área of the nanoclay facilitates more contact area between the polymer and filler
thus improving the rate of transfer of heat by increasing the thermal transport
across the interface56. Also, at higher filler loading the presence of large amount
of agglomerates, also can take place. It has been reported that particles with an
aspect ratio>1 exhibit better heat conduction in one direction, compared with
spheres (aspect ratio=1), with the same volume fraction57. Also, as reported in
earlier chapters the presence of nanoclay at the interface (Fig 8.19) confirmed by
the TEM images also leads to the conclusion that the transfer of heat through the
filler–polymer interface increases.
Figure 8.19 TEM images of 50/50 (10)NR/NBR bled nanocomposites a) co-continuous morphology with naoclay layer dispersion in both phases and b) the presence of nanoclay at the interface of two elastomers.
A three-phase model is developed in a polymer nanocomposite which includes
the matrix, the filler and the interfacial layer between the matrix and the filler
with distinct thermal conductivity and volume. So the overall thermal
conductivity of the polymer blend also will be influence by these.
Rheological behaviour and Viscoelastic and Thermal Properties of… 271
8.2.7.3 Effect of clay modification on thermal conductivity
The thermal conductivity values of the 50/50 NR/NBR blend nanocomposite
with two different nanoclays were observed and is given in Fig 8.20. It was
found that the thermal conductivity is not affected by the clay modification. It
has been reported that a change in thermal properties in clay can be brought
out only if the modification is thermal more active.
0 2 4 6 8 10
0.185
0.190
0.195
0.200
0.205
0.210
λ (W
/m.k
)
Filler loading (phr)
50/50(M) 50/50 (C )
Figure 8.20 Thermal conductivity versus filler volume fraction for O1Mt and O2Mt filled NR/NBR nanocomposites.
8.3 Conclusion
The chapter includes a detailed analysis on the viscoelastic properties and
thermal properties of nanoclay filled NR/NBR nanocomposites. Mainly two
essential properties that are very important in the characterization of filled
rubber are investigated. The high frequency modulus of NR/NBR/O1Mt
272 Chapter 8
nanocomposites were enhanced upon adding higher percentage of nanofiller.
The slight enhancement of the modulus by incorporating nanoclay into the
rubber blend was observed at high filler loaded samples. The payne effect,
was shown by NR and NBR nanocomposites only. The hysteresis loss is the
ratio of energy loss to the energy expended on deformation. The comparison
of the behavior of different blend composites could correlate with the
assumptions of earlier chapter 4 explaining the better property for 50/50 blend
nanocomposites.
Thermal properties was found to vary with the blend composition and filler
loading. The thermal conductivity increased with the NBR content. The
addition of nanoclay could also improve the conductivity. It can be explained
that the high surface área of the nanoclay facilitates more contact area between
the polymer and filler thus improving the rate of transfer of heat by increasing
the thermal transport across the interface.
Rheological behaviour and Viscoelastic and Thermal Properties of… 273
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