Rheological behaviour and Viscoelastic and Thermal Properties of ...

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


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.


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)


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


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


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


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%


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


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)



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)


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


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.


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.


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.


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