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CHAPTER 4

NITRILE RUBBER (NBR) – NANOCLAY COMPOSITES

4.1 Introduction

Reinforcement of a polymer matrix using nanosized layered silicates results in dramatic

improvement in mechanical properties, abrasion resistance, barrier properties and flame

retardance. The outstanding properties of these nanocomposites result from the large

surface area and strong matrix – reinforcement interaction that the nanofiller provide [3 –

5, 13, 14]. Various nanoclays have been used for preparing polymer nanocomposites by

exploiting the ability of the clay silicate layers to disperse into polymer matrix.

Organoclays of montmorillonite family are widely used in both thermoplastic and

elastomeric systems [3-7, 10, 13 - 15]. In nitrile rubber (NBR), long chain surface

modified montmorillonite clay improved the mechanical properties of NBR

nanocomposites [165, 170, 175]. Gas barrier properties of NBR composites have been

found to show tremendous improvement on incorporation of organomodified nanoclay

[41, 42, 168, 171, 189].

Several methods of preparation of nanocomposites like in-situ polymerization, melt

intercalation and solvent intercalation have been extensively studied for elastomers [13,

14]. Most of the reported literature on elastomer based nanocomposites use solution

mixing technique, where a polymer is dissolved in a suitable solvent along with nanofiller

followed by evaporation of solvent to obtain the nanocomposite [41, 218, 173]. Solution

mixing can seldom be used for bulk production of nanocomposites as dissolution of

elastomer in the solvent and subsequent removal of the solvent can pose engineering

difficulties and environmental problems. For preparation of elastomer based

nanocomposites, mixing of latex and nanoclay followed by coagulation and drying is a

viable method in cases of rubbers that are available in latex form [145, 167, 196]. It has

been shown that open two roll mill mixing results in inadequate dispersion of the

nanofiller in the elastomer matrix compared to compounding in an internal mixer [17].

In this chapter the properties of NBR – nanoclay composites prepared by a two step

procedure are discussed. The NBR nanocomposites were prepared by first preparing a

rubber – nanofiller masterbatch followed by compounding neat NBR on a two roll mill

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along with the masterbatch and other compounding ingredients. This method will enable

bulk commercial production of nanocomposites (if masterbatch is available) in any

standard mixing device like two-roll mill eliminating the need for specialized equipments.

The cure characteristics and mechanical properties of NBR nanocomposites reinforced

with different levels of nanoclay were studied. The morphology of the nanocomposites

was analyzed using X-ray diffraction and transmission electron microscopy. The effect of

nanoclay content on mechanical, dynamic mechanical and thermal properties of the NBR

nanocomposites were studied. The viscoelastic behaviour of the nanocomposites was

studied by dynamic mechanical thermal analysis. Comparisons were made between

experimental data and the values predicted using various mechanics – based theoretical

models. The effect of nanoclay content on the gas permeation rate and transport

characteristics of NBR – nanoclay composites was also investigated.

4.2 Selection of nanoclay

In the preliminary studies, nitrile rubber nanocomposites with four different grades of

nanoclay were prepared. Nitrile rubber with medium acrylonitrile content (33%; supplied

by Apar Industries Ltd., Mumbai) was used through out this study. NBR was

compounded with 5 phr nanoclay and other compounding ingredients [sulphur (1.5 phr)

zinc oxide (5.0 phr), stearic acid (1.0 phr), MBTS (1.25 phr), TMTD (0.25 phr)] on a two

roll mill. The compounds were cured at 150°C and 200 MPa for the optimum cure time in

a hydraulic press to make ~ 2mm thick rubber sheets and tested for mechanical properties.

The mechanical properties of the different NBR – nanoclay composites are shown in

Table 4.1.

Table 4.1 Mechanical properties of NBR – nanoclay composites (5 phr nanoclay) with different grades of nanoclay

Property/ Grade of nanoclay Cloisite 10A Cloisite 20A Cloisite 30B Cloisite Na+

Tensile strength (MPa) 3.22 3.53 2.22 2.40

Elongation at break (%) 459 880 503 599

M100 (MPa) 0.61 0.32 0.34 0.30

Hardness (Shore A) 67 62 57 55

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It was found that among the different grades of nanoclay tested, the maximum

improvement in mechanical properties was for Cloisite 20 A. Hence further experiments

were conducted with Cloisite 20A as the nanofiller.

4.3 Masterbatch versus direct mixing

Since nanoclay consists of very small filler particles, the method of incorporating /

dispersion of nanoclay in the rubber would affect the mechanical properties of the

composites. Two sets of NBR nanocomposites with 5 phr nanoclay (Cloisite 20A) were

prepared by different methods. In the first set, nanoclay was added along with other

compounding ingredients on a two-roll mill. In the second set, a masterbatch of nanoclay

and nitrile rubber prepared by the method described in 3.3.1 was used along with other

compounding ingredients on a two-roll mill. The tensile properties of nanocomposites

[5phr nanoclay] prepared using masterbatch and by direct compounding methods are

compared in Table 4.2.

Table 4.2 Masterbatch versus direct compounding for NBR – Cloisite 20A (5 phr)

It was observed that the mean values of properties of composites prepared by masterbatch

technique were higher. On visual examination, agglomerates of nanoclay were observed

in the NBR – nanoclay compounds prepared by direct two – roll mill mixing. While this

is not a conclusive method for characterization, the evidence for greater homogeneity in

the masterbatch mixing was further strengthened by noting that there was smaller

variation (standard deviation) in properties for these composites. This may be attributed

to the uniform distribution of the nanoclay in the nanocomposite. Hence further studies on

nanoclay – NBR composites were carried out using the masterbatch mixing methodology.

Hence detailed study of NBR –nanoclay composites prepared using NBR-nanoclay

masterbatch followed by two roll mill compounding (as described in 3.3.) were carried

out using Cloisite 20A as the nanoclay.

Mixing method Tensile Strength (MPa)

Elongation at break (%)

Modulus (MPa)

Two roll mill mixing 3.04±0.63 402±56 0.65±0.21

Masterbatch 2.87±0.41 527±38 1.22±0.07

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4.4 Cure Characteristics

The NBR – nanoclay compounds were tested for cure characteristics in an oscillating disc

rheometer. The rheograms for NBR nanocomposites obtained at 150°C are shown in

Figure 4.1. The rheograms showed that addition of nanoclay into the NBR matrix

increased the torque while reducing the scorch time.

Figure 4.1 Influence of nanoclay content on rheometric torque of NBR – nanoclay composites at 150°C

Various cure characteristics of NBR nanocomposites are summarized in Table 4.3. From

Table 4.3., it was evident that the addition of layered silicate to the NBR matrix altered its

cure characteristics.

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Table 4.3 Cure characteristics of NBR- nanoclay composites at 150°C

It was observed that the scorch time (ts2) decreased as nanoclay content in the

nanocomposite increased. The cure time (t90) of the compounds also sharply decreased on

addition of 2 phr nanoclay. Similar studies reported in literature suggested that amine

functionalities in the filler facilitated the curing reaction of rubber stocks and reduced

cure time [205, 274]. It has been suggested that the zinc – sulphur accelerator complex

reacts with the amine functionalities of the organomodifier during the curing reaction

thereby reducing the cure time of nanoclay composites [144, 151, 152, 171]. However, at

nanoclay content of 7.5 and 10 phr the cure time was higher than that at 5 phr. The –OH

groups on the surface of nanoclay had a retarding effect on the cure reaction. At higher

concentration of nanoclay, the retarding effect of the –OH groups was more than the

accelerating effect of the amine groups in the nanoclay. Cure rate index (CRI), a direct

measure of the fast curing nature of the rubber compounds, was calculated using the

following relation [53, 275]:

CRI = 100/ (t90 - ts2 ) (4.1)

For the NBR nanocomposites, CRI at 2 and 5 phr nanoclay contents were higher than that

of unfilled NBR and hence it can be inferred that nanoclay supported the activation of the

cure reaction up to 5 phr. At higher loading, a slightly higher cure time than that at 2 and

5 phr was observed. At these loadings, the nanofiller tended to agglomerate and increased

cure time compared to lower filler contents.

From the study of the effect of nanoclay content on rheometric torque of NBR

nanocomposites, it was observed that the minimum torque (τmin), an indirect measure of

Name Nanoclay content (phr)

Scorch time (ts2)

(min)

Cure time (t90)

(min)

τmin (Nm)

τmax (Nm)

Δτ (Nm)

Cure rate index

(min-1) NBRNCL0 0 6.8 11.1 0.6 2.5 1.9 22.8

NBRNCL2 2 4.5 7.7 0.6 2.7 2.1 31.6

NBRNCL5 5 4.0 7.9 0.7 2.7 2.0 25.3

NBRNCL7.5 7.5 2.0 9.5 0.5 3.1 2.6 13.4

NBRNCL10 10 2.7 8.8 0.8 3.4 2.6 16.3

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viscosity of the compound, increased gradually as nanoclay content increased. The

maximum torque (τmax) was more or less same up to nanoclay content of 5 phr. At 7.5 phr

there was an increase in the maximum torque. The difference between maximum and

minimum torques (Δτ), an indication of the extent of cross linking, was found to increase

with filler loading [175, 205]. This may be due to the incorporation of NBR chains into

the galleries of the nanoclay resulting in better interaction between the nanoclay and the

rubber matrix.

A general equation for rubber curing that follows nth order kinetics is given by [275]:

dαc/dt = k(T) (1 – αc)n (4.2)

where dαc/dt is the vulcanization rate, t is time, k(T) is specific rate constant at

temperature T and αc is the degree of cross linking. αc is defined in cure rheometer study

as

αc = (τt – τ0) / (τh – τ0) (4.3)

where τ0, τt and τh are the torque values at time zero, at time t and at the end of curing,

respectively. For first order kinetics (n = 1), equations (4.2) and (4.3) can be combined

and integrated with respect to t to give

ln (τh - τt) = k(T) t + ln (τh – τ0) (4.4)

To verify the compliance of experimental data to first order kinetics, ln (τh - τt) was

plotted against curing time t and fitted to the linear model given in equation (4.4). The

regression coefficients (R2) at all the filler loadings were found to be greater than 0.9 and

first order kinetic model was found to be appropriate to describe the cure reaction of

NBR-nanoclay systems.

4.5 Morphology

4.5.1 Wide angle X-ray diffraction studies

Wide angle X-ray diffraction has been used to characterize the state of dispersion and

exfoliation in nanocomposites [20, 22, 31]. In exfoliated composites, the silicate layers

are delaminated in the polymer matrix and this is indicated by disappearance of XRD

peaks. A shift in the basal reflection to 2θ corresponding to a larger d value indicated

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intercalation [15, 34]. XRD patterns for the NBR nanocomposites and the organoclay are

shown in Figure 4.2.

Figure 4. 2 XRD pattern of NBR – nanoclay composites

For the Cloisite 20A, the peak in XRD pattern occurred at 3.775° corresponding to d

spacing of 2.3 nm. The nanoclay – NBR composites, at lower concentration of nanoclay

(2 and 5 phr) showed negligible peaks. This indicated that the galleries were separated by

insertion of polymer chains. At higher nanoclay content (10 phr), a peak was observed at

2θ = 5.12° (corresponding to d = 1.7 nm) indicating formation of aggregates of clay that

were not dispersed well in the NBR matrix. It can also be inferred that there was a

decrease in interlayer distance between the layers of the nanoclay. In their studies on

rubber composites, Varghese et al suggested that the re-aggregation of the nanoclay and

the decrease in interlayer distance of the layered silicate may be attributed to participation

of the alkyl groups in organoclay in the curing reaction during vulcanization [144, 146].

Hwang et al proposed that during curing a zinc-sulphur accelerator complex that

“extracts” the amine intercalant of the organosilicates was formed which caused the

collapse of the layers [171]. It was also reflected in the decrease in cure time and increase

in maximum torque of the nanocomposites at higher nanoclay loading. The reduction in

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intensity of peak for 10 phr loading compared to that of Cloisite20A suggested that some

amount of dispersion of the clay has taken place. It can be seen from the XRD data that

the width of the XRD peak [measured as full width at half maxima (FWHM)] decreased

from 0.09 nm for nanoclay to 0.07 nm for NBRNC10. As the FWHM is inversely

proportional to the coherence length of scattering intensities, it can be inferred that the

coherency of the layers in the nanocomposite was higher than un-intercalated layers and

that incorporation of NBR modified the structure of the nanoclay [171]. Sadhu and

Bhowmick [173] had reported that the intercalation of nanoclay in NBR – nanoclay

systems was due to the interaction between the butadiene segments of NBR and organic

surface of the modified nanoclay. Based on the XRD results, it can be concluded that the

nanoclay - NBR nanocomposites formed a mixture of intercalated and exfoliated nano-

structures.

4.5.2 Transmission electron micrograph studies

The dispersion of layered silicate in the polymer matrix, whether they are partially/fully

exfoliated or in the form of disordered intercalates, cannot be exactly determined by XRD

measurements alone [276]. Transmission electron microscopy was used to study the

nanostructure of NBR – nanoclay composites. The TEM micrographs of NBR

nanocomposites containing 0, 2, 5 and 10 phr are shown in Figure 4.3(a), 4.3(b), 4.3(c)

and 4.3(d) respectively.

At 2 phr concentration, the nanoclay was dispersed as single platelets as well as small

aggregates (tactoids) consisting of few stacks of clay platelets. At 5 phr, the nanoclay was

uniformly dispersed in the NBR matrix as exfoliated single platelets along with few

stacks of clay platelets. At higher nanoclay content, i.e., 10 phr, the nanoclay formed

aggregates with a few platelets of clay layers. These stacks gave rise to the peak observed

in the XRD pattern. From the TEM and XRD studies it can be concluded that the

organomodified nanoclay was dispersed evenly in the NBR matrix at lower nanoclay

content while at higher loadings aggregation of the nanofiller occurred. The morphology

of NBR- nanoclay composites prepared by the masterbatch method was similar to that

prepared by solution technique [146, 277] and hence preparation of nanocomposite using

masterbatch can be a viable method for large scale production of elastomer

nanocomposites.

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Figure 4.3 TEM micrographs for NBR-nanoclay composites (a) 0 phr (b) 2 phr (c) 5 phr (d) 10 phr

4. 6 Static mechanical properties

4.6.1 Tensile properties

The stress-strain characteristics of the NBR nanocomposites are shown in Figure 4. 4. It is

seen that the stress continuously increased with deformation of NBR; the stress – strain

behaviour was typical of synthetic elastomers. Incorporation of nanoclay increased the

stress in the nanocomposites for the same strain level.

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Figure 4.4 Stress –strain characteristics of NBR – nanoclay composites

It was observed that the tensile strength increased rapidly with the clay content in the

range 2-5 phr; at 7.5 phr it showed a gradual decrease. The tensile moduli of

nanocomposites increased with increasing clay content up to 7.5 phr and with further

addition of nanoclay there was a drop in the values. The reinforcing effect of the nanoclay

was shown by the increase in the ratio of modulus of filled compounds (M100f) to that of

unfilled NBR (M100u). The mechanical properties of NBR nanocomposites with different

nanoclay loadings are given in Table 4. 4.

Table 4.4 Mechanical properties of NBR – nanoclay composites

Name Nanoclay Content

(phr)

Tensile strength (MPa)

Elongation at break

(%)

M100 (MPa)

M300 (MPa) u100

f100

MM

NBRNCL0 0 2.19±0.49 558±58 0.54±0.13 1.18±0.06 1.00

NBRNCL2 2 3.08±0.51 593±34 0.72±0.19 1.58±0.19 1.33

NBRNCL5 5 6.98±0.38 721±53 1.08±0.08 2.41±0.10 1.98

NBRNCL7.5 7.5 4.51±0.53 575±47 1.22±0.09 2.26±0.05 1.98

NBRNCL10 10 4.01±0.39 595±41 0.79±0.10 1.87±0.06 1.45

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The tensile properties of nanocomposites are influenced by the dispersion of nanoclay in

the matrix and the interfacial interaction between the matrix and the nanoclay [83, 181,

253]. The improvement in tensile strength can be attributed to the dispersion of nanoclay

in the rubber matrix, rigidity of the nanoclay and affinity between nitrile rubber and the

organomodified nanoclay. The high aspect ratio of the nanoclay resulted in larger

interfacial region and consequently efficient transfer of stress across the composite

components and reinforcement. As suggested by the XRD and TEM micrographs, at 10

phr nanoclay content, the formation of aggregates resulted in poorer dispersion of the

filler in the matrix than in the case of 2 and 5 phr clay loadings, and hence there was

reduction in the tensile strength.

The elongation at break of nanocomposites approached the highest value near 5 phr and

decreased with further increments in nanoclay content. Jin-tae Kim et al attributed the

improvement in elongation partly to the plasticizing effect of alkylammonium ions that

are located at the clay–NBR interface [165]. Above 5 phr, this behaviour was attributed to

the formation of non-exfoliated aggregates which made these composites stiffer.

Therefore, with increasing organoclay content, the NBR nanocomposites showed a

substantial improvement in tensile strength, modulus and elongation at break compared to

unfilled NBR.

4.6.2 Theoretical prediction of tensile properties

The modulus of the polymer composite is determined by properties of the filler and the

matrix, the filler loading and the aspect ratio of the filler. Since the modulus of the

inorganic particles used as filler is much higher than that of the polymer matrix, the

addition of filler enhances the composite modulus [26]. There are several empirical or

semi-empirical micro-mechanics models proposed to predict the modulus of polymer

composites, though their applicability to nanocomposites has been subjected to debate.

However, several attempts in the recent past have reported meaningful results in the use

of these models [10, 29, 86, 141, 181, 240, 241, 278, 279]. In this work, the suitability of

Voigt upper bound rule (Rule of mixtures), Reuss lower bound rule (Inverse rule of

mixtures), Guth and Gold equation, modified Guth and Gold equation, Halpin-Tsai

equation and Hui-Shia model for predicting the modulus of NBR- layered silicate

composites were considered. These models have been discussed in Chapter 2. The

parameters of the nanoclay used for theoretical modelling are summarized in Table 4.5.

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Table 4.5 Modelling parameters for nanoclay [273]

Property Cloisite 20A

Modulus 170 GPa

Aspect ratio (length l/thickness t) 100 (1nm thick, 100 nm across)

Shape parameter ζ = 2 (l/t) 200

Inverse aspect ratio of dispersed fillers α = t/l 0.01

The experimental values of the static modulus of the nanocomposites were compared to

those predicted by mathematical models in composite theories. These values of the tensile

moduli for the clay - NBR nanocomposites are given in Table 4.6. A comparison of some

of the model results are shown graphically in Figure 4.5.

Table 4.6 Comparison of experimental and predicted values of modulus for NBR – nanoclay composites

Nanoclay Content

(phr)

Modulus (MPa)

Experimental

Predicted Voigt upper bound rule

Reuss lower bound rule

Guth Modified Guth

Hui-Shia

Halpin-Tsai

0 0.54 0.54 0.54 0.54 0.54 0.54 0.54

2 0.72 1750 0.55 0.56 1.85 0.59 1.68

5 1.08 4300 0.56 0.60 7.08 0.64 3.37

7.5 1.22 6350 0.56 0.62 14.2 0.66 4.77

10 0.79 8350 0.57 0.64 23.5 0.68 6.17

The tensile moduli predicted by Hui – Shia model were closest to the experimental values

with the deviation from the experimental values being 18 and 11% at 2 and 10 phr.

However, at 5 and 7.5 phr the deviation was higher (40% and 45% respectively). It may

be noted that at these nanoclay loadings, the nanocomposite exhibited enhanced

properties compared to other filler levels indicating better dispersion and interaction

between the matrix and the filler. The various theories considered here make several

assumptions like (i) uniformity in size, shape and alignment of fillers and that the filler

and matrix are linearly elastic, isotropic, and firmly bonded. These assumptions seldom

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hold true for nanocomposites. Another reason for large deviation in the predicted value at

these loadings of layered silicate could be the existence of an interphase with properties

different from those of the matrix and filler, which was not considered in the theoretical

models [3, 181]. The choice of composite theory determines how well the predicted and

experimental data agree.

Figure 4.5 Comparison of experimental and predicted values of modulus for NBR

– nanoclay composites

4.7 Dynamic mechanical analysis

Dynamic mechanical analysis (DMA) measures the response of a material to an

oscillatory deformation as a function of temperature. DMA analysis yields information

on storage modulus (E’), loss modulus (E”), tan δ (E”/E’) and occurrence of molecular

transitions like glass transition temperature and melting point of the material [31, 211]. It

is also an effective method to study material behaviour under various conditions of stress,

temperature and phase composition of composites and its role in determining mechanical

properties. The dynamic elastic (storage) modulus E’ for neat NBR and NBR nanoclay

composites are plotted in Figure 4.6.

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Figure 4.6 Storage modulus (E’) vs. temperature at 1 Hz of NBR – nanoclay composites

Above Tg, at all loadings of nanoclay the nanocomposites showed clear enhancement in

storage modulus indicating the strong effect of nanoclay on the dynamic properties of

NBR. The enhancement in E’ was due to the high aspect ratio of the nanoclay and the

formation of exfoliated and intercalated structures [20]. However, below Tg, the

difference in the values of E’ was less significant. This behaviour was due to the

intercalation of the copolymer chains into the galleries of the clay layers, which led to the

suppression of the mobility of the polymer segments near the interface in the rubbery

plateau region [31]. The effectiveness of the filler on the moduli of the composites can be

represented by the coefficient C calculated using equation (4.5) [280]

C = (E’G/E’R)composite / (E’G/E’R)resin (4.5)

where E’G and E’R are the storage modulus values in the glassy and rubbery region,

respectively. The higher the value of the coefficient C the lower the effectiveness of the

filler. The measured values of E’ at -50°C and +50°C were used as E’G and E’R,

respectively. It can be noted from the values of C, given in Table 4.7 that the

effectiveness of the filler increased up to 5 phr and thereafter decreased at 10 phr of

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nanoclay. The maximum stress transfer between the matrix and the filler was at 5 phr and

thereafter the effectiveness of stress transfer decreased.

Table 4.7 Dynamic mechanical properties of NBR – nanoclay composites

Sample tan δmax E”max (MPa)

Tg from tan δ (°C)

Tg from E” (°C) C

1 Hz 10 Hz 1 Hz 10 Hz 1 Hz 10 Hz 1 Hz 10 Hz 1 Hz 10 Hz NBRNCL0 1.3 1.4 221 301 -8 -4 -14 -12 1 1

NBRNCL2 1.3 1.4 247 309 -8 -2 -14 -12 0.9 0.9

NBRNCL5 1.1 1.2 281 352 -10 -4 -16 -12 0.5 0.5

NBRNCL7.5 1.3 1.4 272 298 -9 -4.5 -14 -12 0.6 0.6

NBRNCL10 1.2 1.3 249 310 -8 -4 -16 -12 0.6 0.7

The influence of nanoclay content on dynamic properties can be explained by studying

the normalized storage modulus with temperature at different nanoclay content as

depicted in Figure 4.7. Normalized storage modulus can be defined as the ratio of the

storage modulus of the composite (E’c) to the storage modulus of the matrix (E’m) at the

same temperature.

Figure 4.7 Variation of normalised storage modulus with nanoclay content at different temperatures for NBR-nanoclay composites

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It was evident from the plot that as the nanoclay content increased the normalized storage

modulus increased, reached a maximum value and then decreased with increase in

nanoclay loading. Also, at a particular nanoclay loading, the normalized storage modulus

increased with increase in temperature. This indicated that the nanoclay restricted the

mobility of the elastomer molecules at elevated temperatures.

The effect of nanoclay content on loss factor (tan δ) as a function of temperature at a

frequency of 1 Hz is shown in Figure 4.8. Incorporation of nanoclay lowered the peak

value of tan δ and thereby reduced the damping properties of the system. The lowest

value of tan δmax was at 5 phr nanoclay loading. At 10 phr, the peak value was lower than

that of unfilled rubber but higher than that at 5 phr nanoclay. However, there was no

change in Tg values due to the addition of nanoclay. The area under the peak in tan δ vs.

temperature curve is a measure of energy dissipated [175]. As seen from the curve, there

was marginal narrowing of peaks and marginal reduction of damping properties.

Figure 4.8 Effect of nanoclay content on tan δ of NBR – nanoclay composites at 1 Hz

The storage modulus, loss modulus and damping peaks were analyzed at frequencies 1Hz

and 10 Hz. The variation of E’ and tan δ with frequency for NBRNCL5 as a function of

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temperature is shown in Figure 4.9. There was a slight increase in both modulus values

and tan δ with frequency. For a viscoelastic material subjected to constant stress, the

modulus decreased as time elapsed due to molecular rearrangements that resulted in

reduction of localized stresses. Hence modulus measurement at higher frequency (shorter

time interval) showed higher values compared to that taken at lower frequency (long time

period) [281]. The same trend was observed at all loadings of nanoclay. At higher

frequency, tan δ curve peak corresponding to the Tg was shifted for NBR

nanocomposites, while the maximum value of tan δ increased. The tan δ curves were

broadened, indicating restriction in segmental mobility at higher frequency. The effect of

nanoclay content on values of tan δmax, E”max and the Tg values obtained for all the

samples at frequencies 1 Hz and 10 Hz are given in Table 4.7.

Figure 4.9 Variation of storage modulus and tanδ with frequency for NBRNCL5

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t was observed that the peak of the loss modulus curve shifted to higher temperature at 10

Hz. Between -14°C and 30°C the loss modulus values at 10 Hz were much higher than

those at 1 Hz. This indicated that better viscous dissipation occurred when the

nanocomposite was strained for shorter time duration than for a longer time period.

The Cole –Cole plot of storage modulus E’ vs. loss modulus E” and modified Cole – Cole

plot (logarithmic plot of E’ against E”) have been successfully utilized to examine the

homogeneity of nanocomposites [282, 283]. Homogenous polymeric systems exhibit a

semicircle diagram in the Cole-Cole plot.

Figure 4.10 (a) Cole-Cole plot of storage modulus E’ vs. loss modulus E” and (b) modified Cole-Cole of log E’ vs. log E’’ plot for NBR – nanoclay composites at 1 Hz (temperature range -70°C to +70°C)

The Cole - Cole plot of storage modulus (E’) vs. loss modulus (E”) of NBR nanoclay

composites deviated from semicircular shape implying that the system was heterogeneous

(Figure 4.10 (a)). If there were no structural changes due to incorporation of nanofiller,

the modified cole-cole plot of log E’ vs. log E’’ for the nanocomposite would

superimpose on the plot of the neat matrix. As depicted in Figure 4.10 (b), the modified

Cole - Cole plot of the NBR – nanoclay composites indicated structural changes on

addition of nanoclay, the changes being more prominent at 5 and 10 phr filler loading.

These changes were consistent with the trends shown in static and dynamic moduli.

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4.8 Thermal behaviour

Thermogravimetric analysis was performed in nitrogen atmosphere to study the thermal

stability of NBR – nanoclay composites. The thermal stability factors, viz. initial

decomposing temperature (IDT), temperature at the maximum rate of heat loss (Tmax) and

the char content at 500°C were calculated from the TGA thermograms (see Figure 4.11)

and are listed in Table 4.8.

Table 4.8 Thermal stability factors of NBR-nanoclay composites obtained from TGA

Name

Nanoclay Content

(phr)

IDT

(°C)

Tmax

(°C)

Char

(%)

NBRNCL0 0 390 441 9.42

NBRNCL2 2 392 435 10.11

NBRNCL5 5 394 457 10.72

NBRNCL10 10 397 456 18.41

Figure 4.11 Thermograms for NBR- nanoclay composites

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The thermal stability of the composites was enhanced on addition of nanoclay. In neat

rubber, the initial decomposition temperature (IDT), the temperature at which the

degradation starts, is around 400° C. On addition of nanoclay, change in IDT was not

significant. However, the temperature at which maximum rate of decomposition occured

increased with increased nanoclay content. The enhanced thermal stability of NBR

nanocomposites was due to the restricted thermal motion of the polymer chains in the

silicate layers of the nanoclay [181]. The char content of the nanocomposites at 500°C

increased with nanoclay content.

The effect of nanoclay content on glass transition temperature (Tg) of NBR – nanoclay

composites was studied by differential scanning calorimeter (DSC). The Tg values

obtained from DSC are shown in Table 4.9. This study also confirmed that the effect of

nanoclay content on Tg was marginal. The values of Tg from DSC were lower than those

obtained from DMA techniques [118].

Table 4.9 Effect of nanoclay content on Tg of NBR – nanoclay composites by DSC

Sample Tg from DSC (°C)

NBRNCL0 -22.5

NBRNCL2 -22.4

NBRNCL5 -24.2

NBRNCL7.5 -24.6

NBRNCL10 -24.8

4.9 Gas permeability

The oxygen permeation rate values through the NBR nanocomposites are given in Table

4.10. It was observed that at lower nanoclay contents (2 and 5 phr) the permeation rate

decreased appreciably. The dispersion and exfoliation of the nanoclay platelets increased

the path length required to transport the permeating molecule through the rubber matrix

thus providing tortuous path for permeation and thereby decreasing the rate of transport

[40 – 44]. At higher nanoclay contents, the lengths of the tortuous path decreased due to

formation of aggregates. The lesser extent of exfoliation and decrease in permeation path

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length resulted in increased gas permeation rate through the composites at higher

nanoclay content.

Table 4.10 Oxygen permeation rate of NBR – nanoclay composites

Sample Oxygen permeation rate

(mL/m2/day)

NBRNCL0 932 NBRNCL2 600 NBRNCL5 611 NBRNCL7.5 779 NBRNCL10 862

The suitability of Nielson model to predict the barrier property of NBR nanocomposites

was studied. The model is given by equation (2.14). The aspect ratio of the filler was

taken as 100 [273].

Figure 4.12 Plot of oxygen permeability ratio of NBR – nanoclay composite to matrix, Pc/Pm as a function of nanoclay content

91

Figure 4.12 shows the relative permeability of (Pc /Pm - ratio of permeability of composite

to that of matrix) of NBR nanocomposites, both experimental and those predicted by

Nielson’s theory as a function of nanoclay content. It was found that Nielson’s theory was

satisfactory at lower nanoclay concentrations, whereas at higher nanoclay contents, the

experimental values were well above the theoretical prediction. Nielson’s model assumes

uniform arrangement of clay platelets in the polymer matrix. This assumption was not

valid in the case of nanocomposites with higher nanoclay content as the nanoclay formed

agglomerates and the dispersion of nanofiller was not uniform.

4.10 Transport characteristics

The effects of nanoclay content on the diffusion, sorption and permeation of toluene

through NBR-nanoclay composites were studied. The transport behaviour through

composites depends on the type of filler, matrix, temperature, size of the penetrant,

polymer segment mobility, reaction between solvent and the matrix, etc. Hence the study

of the transport process through composites can be used as an effective tool to understand

the interfacial interaction and morphology of the system. The swelling behaviour of NBR

- nanoclay composites was assessed by calculating swelling coefficient, β using the

equation [284]

( ) 1s

o

o xM

MM −∞ρ

−=β (4.6)

where Mo and M∞ are the mass of the sample before swelling and after equilibrium

swelling respectively and ρs is the density of the solvent. Table 4.11 shows that the

swelling coefficient decreased with increasing nanoclay content.

The diffusion coefficient of a polymeric sample immersed in an infinite amount of

solvent can be calculated using the equation [154]

( )∑∞=

=∞

π+−

+

π−=

n

0n2

2222

t

ht)1n2(Dexp

1n2181

QQ (4.7)

where Qt is the mole percent uptake for solvent at time t, Q∞ is the mole percent uptake

for solvent at equilibrium swelling, t is the time, h the initial thickness of the sample, D

the diffusion coefficient and n is an integer. From equation (4.7), it can be seen that a plot

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of Qt versus √t is linear at short time interval and D can be calculated from the initial

slope. For short time limit, the equation 4.7 can be modified as

2121

t tDh4

QQ

π=

∞

(4.8)

The sorption curves (Qt (moles of solvent sorbed per 100 g of rubber) vs. √t) at 30°C are

shown in Figure 4.13 for varying nanoclay content.

Figure 4.13 Sorption isotherms for NBR- nanoclay composites at 30°C

Figure 4.13 shows the effect of nanoclay content on the toluene uptake with time. The

sorption of toluene was reduced for the nanofilled composites compared to unfilled

rubber. The dispersion of nanoclay in the rubber matrix created tortuous path for the

transport of the solvent [40-44]. The Qt vs √t curve showed two distinct regions - an

initial steep region with high sorption rate due to large concentration gradient and a

second region exhibiting reduced sorption rate that ultimately reaches equilibrium

sorption. The sorption rate and equilibrium solvent uptake of NBR nanocomposites

reduced with increased nanoclay content. Beyond 5 phr the solvent uptake increased

slightly. This was due to the formation of agglomerates of nanoclay as evident from the

TEM micrographs.

93

The crosslink density (υ) was calculated from the sorption data using equation (4.9) [179,

284]

υ = 1/Mc (4.9)

where Mc is the molecular weight of the polymer between the crosslinks. Mc is calculated

using equation (4.10) [179, 284]

2

3/1sP

c )1ln(V

Mχϕ+ϕ+ϕ−

ϕρ−= (4.10)

where Vs is the molar volume of the solvent, ρP is the density of the polymer, χ is the

interaction parameter and φ is the volume fraction of rubber in the solvent-swollen filled

sample. φ is given by Ellis and Welding equation as [179, 284]

( )( ) SPPoiD

PoiD

AMfMMfM

ρ+ρ−ρ−

=ϕ (4.11)

where MD is deswollen weight, fi is fraction of insoluble components, Mo is weight of

sample taken, ρP and ρs are densities of the polymer and solvent respectively and AS is

the weight of the absorbed solvent. The solvent interaction parameter χ is obtained from

the equation [179]

( )2PS

S

RTV

δ−δ+γ=χ (4.12)

where δs is the solubility parameter of the solvent (18.2 MPa1/2 for toluene) [285], δP is the

solubility parameter of the polymer (19.4 MPa1/2 for NBR) [285], γ is the lattice constant

(generally taken as 0.34 for elastomer – solvent systems), Vs is the molar volume of the

solvent (106.3 mL/gmol), R is the universal gas constant and T is the temperature in

Kelvin. The estimated values of Mc for NBR - nanoclay composites are tabulated in

Table 4.11. Nanoclay filled systems had lower Mc values , i.e. lower molar mass between

crosslinks than unfilled NBR and Mc decreased with increasing nanoclay content. As the

value of Mc decreased, the available volume between adjacent crosslinks decreased. The

decrease in volume restricted the diffusion process. The slight increase in Mc at nanoclay

content greater than 5 phr was due to the aggregation of the nanofiller as seen in the TEM

micrograph (Figure 4.3). The calculated values of crosslink density supported this

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observation. The crosslink density values of NBR – nanoclay composites are tabulated in

Table 4.11. As the nanofiller content in the composites increased, the crosslink density

also increased, the peak value occurring at 5 phr.

Table 4.11 Swelling coefficient and crosslink densities of NBR – nanoclay composites

Sample Swelling

coefficient (β) (cm3/g)

Molar mass between crosslinks (Mc)

(g/mol)

Crosslink density x 104 (gmol/cm3)

NBRNCL0 2.77 3445 1.45

NBRNCL2 2.31 2622 1.91

NBRNCL5 2.26 2643 1.89

NBRNCL7.5 2.29 2787 1.79

NBRNCL10 2.28 2872 1.74

By rearranging equation (4.8), the diffusivity (D) of the nanocomposites was calculated

using equation (4.13) given below [93, 41 - 43]

2

Q4hD

∞

θπ= (4.13)

where h is the thickness of the sample, θ is the slope of the sorption curves before

attaining 50% equilibrium (the initial linear portion of the curve) and Q∞ is the

equilibrium solvent uptake.

The sorption coefficient was calculated using equation (4.12) [154, 179, 286, 287]

o

s

MMS ∞= (4.14)

where Ms∞ is the mass of solvent taken up at equilibrium swelling and Mo is the mass of

the sample.

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The net diffusion through polymer depends on the difference in the amount of penetrant

molecules between the two surfaces. Hence, the permeability can be expressed as [154,

179, 286, 287]

P = D x S (4.15)

where D is the diffusivity and S is the solubility. Solubility is taken as mass of solvent

sorbed per unit mass of the sample.

The diffusion, sorption and permeability coefficients of NBR – nanoclay composites at 30°C,

50°C and 75°C are given in Table 4.12.

Table 4.12 Transport coefficients of NBR- nanoclay composites

Sample

Diffusion coefficient (Dx107) m2/s

Sorption coefficient (S)

(g/g)

Permeability coefficient (P x107)

(m2/s) 30°C 50°C 75°C 30°C 50°C 75°C 30°C 50°C 75°C

NBRNCL0 6.58 8.11 8.46 2.39 2.32 2.16 15.7 18.8 18.3

NBRNCL2 6.02 7.77 8.18 2.00 1.94 1.88 12.0 15.1 15.4

NBRNCL5 5.20 6.64 7.51 1.96 1.81 1.73 10.2 12.0 13.0

NBRNCL7.5 5.36 6.89 7.77 1.98 1.87 1.76 10.6 12.9 13.6

NBRNCL10 5.37 7.10 7.81 1.98 1.87 1.76 10.6 13.3 13.8

The diffusion of solvent through a composite depends on the geometry of the filler (size, shape,

size distribution, concentration, and orientation), properties of the filler, properties of the matrix,

and interaction between the matrix and the filler [287]. As shown in Table 4.12 the transport

coefficients for the nanocomposites were considerably lower than those of the unfilled NBR.

The diffusion of the penetrant solvent depends on the concentration of available space in the

matrix that is large enough to accommodate the penetrant molecule [154]. The addition of

nanoclay reduced the availability of these spaces, restricted segmental mobility of the rubber

matrix and created tortuous path for transport of solvent molecules through the nanocomposites

[40-44]. However, at nanoclay content greater than 5 phr, there was an increase in diffusion and

permeation coefficients. In this case also the increase in transport coefficients can be attributed

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to the aggregation of the nanofiller. Similar trends were observed for sorption studies conducted

at 50°C and 75°C also. As the temperature increased, the coefficient of diffusion also increased

for all the samples. With increase in temperature, the thermal energy increased and

consequently molecular vibration of solvent molecules, the free volume in the polymer matrix

and flexibility of the polymer chains increased [288]. As a result, the diffusion coefficients of

the NBR composites increased at higher temperatures. The transport coefficients at 50°C and

75°C are also shown in Table 4.12.

The energy required for the diffusion or permeation of solvent molecule was computed using

Arrhenius equation [154],

RTE0

DeDD −= (4.16)

RTE0

PePP −= (4.17)

where Do and Po are diffusion and permeation coefficients extrapolated to zero permeant

concentration respectively, R is the gas constant, T is the temperature in Kelvin and ED and

EP are the activation energies for diffusion and permeation respectively. The activation energy

for diffusion (ED) was obtained from the slope of ln D versus 1/T plot. The activation energy of

diffusion of toluene through NBR nanocomposites was found to be higher than that of unfilled

polymer. The nanofillers have higher specific surface area which led to enhanced rubber-filler

interaction resulting in enhanced reinforcement. As the nanofiller content increased, the

activation energy needed for diffusion also increased. The activation energy for permeation

(EP), evaluated using the Arrhenius equation showed similar trends as ED. The enthalpy of

sorption ΔHs was determined by van Hoff equation [289].

EP = ΔHs + ED (4.18)

It was observed that the sorption was an exothermic process. The value of ΔHs increased with

increasing nanofiller content. The thermodynamic parameters for transport of toluene through

NBR – nanoclay composites are given in Table 4.13.

97

Table 4.13 Thermodynamic parameters for transport of toluene through NBR – nanoclay composites

Sample ED (kJ/mol) EP (kJ/mol) ΔHs (kJ/mol)

NBRNCL0 4.86 2.88 -1.98

NBRNCL2 5.93 4.73 -1.20

NBRNCL5 7.15 4.77 -2.38

NBRNCL7.5 7.20 4.83 -2.36

NBRNCL10 7.24 4.99 -2.25

NBRNCL0 4.86 2.88 -1.98

To evaluate the mechanism of sorption, the solvent uptake data of the nanocomposites were

fitted to the equation [154, 286]

log(Qt/Q∞) = log k + nlog t (4.19)

where Qt is the mol% increase in uptake at time t, Q∞ is the mol% increase in uptake at

equilibrium and k is a constant characteristic of the sample which indicates the interaction

between the sample and solvent. The values of n and k were determined by linear

regression analysis are shown in Table 4.14.

Table 4.14 n and k values for diffusion of toluene through NBR – nanoclay composites

SAMPLE 30°C 50°C 75°C

n k (g/gmin2) n k

(g/gmin2) n k (g/gmin2)

NBRNCL0 0.48 0.076 0.495 0.068 0.507 0.071

NBRNCL2 0.49 0.058 0.507 0.063 0.514 0.068

NBRNCL5 0.49 0.061 0.528 0.054 0.489 0.076

NBRNCL7.5 0.47 0.070 0.519 0.057 0.486 0.079

NBRNCL10 0.50 0.055 0.505 0.062 0.496 0.075

Generally, the diffusion behaviour of polymeric composites can be classified according to

the relative mobility of the penetrant and of the polymer segments into (i) Case I or

Fickian diffusion (ii) Case II diffusion and (iii) non Fickian or anomalous diffusion. For a

Fickian mode of diffusion, the value of n is equal to 0.5 and the predominant driving

force for diffusion is the concentration gradient. In Fickian diffusion, the rate of diffusion

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is much less than the rate of relaxation of polymer chains. In Case II diffusion, n is equal

to 1 and the rate of diffusion is much higher than the relaxation process. When the value

falls between 0.5 and 1, the diffusion is anomalous and the rate of diffusion becomes

comparable with the rate of relaxation of polymer chains [290]. In the case of NBR-

nanoclay the value of n at room temperature (30°C) was almost equal to 0.5 and the

diffusion is Fickian type, controlled by concentration dependent diffusion coefficient. In

rubbery polymers, well above their glass transition temperatures, the polymer chains

adjust quickly to the presence of penetrant molecules and hence they do not exhibit

anomalous behaviour. It was observed that as the temperature increased, the value of n

increased for NBR- nanoclay composites, implying that the diffusion tended to be of

anomalous type.

4.11 Conclusion

NBR – layered silicate composites were obtained by a two-step process involving

preparation of a masterbatch of NBR and nanoclay in an internal mixer followed by

mixing on a two-roll mill. Rheograms indicated reduction in cure time and scorch time on

addition of organo-modified layered silicate. The cure kinetics for NBR – layered silicate

composites were found to be of first order. The tensile strength and modulus of NBR –

nanoclay composites increased with nanoclay content, up to 5 phr. XRD and TEM

investigations showed exfoliated and few intercalated structures at low nanoclay content.

At higher concentrations, the nanoclay had tendency to form agglomerates. Addition of

nanoclay enhanced the storage modulus, loss modulus and thermal stability of

nanocomposites. Incorporation of nanoclay lowered the tan δ peak value without

affecting the Tg of the nanocomposites. The experimental values of both static and

dynamic moduli were compared with those predicted by various composite theories.

Experimental static moduli of the NBR – nanoclay composites were close to those

predicted by Hui – Shia model at low (2 phr) and high (10 phr) nanoclay content. The

transport behaviour of solvent through the nanocomposites was investigated using

sorption isotherms. The diffusion, sorption and permeation coefficients for diffusion of

toluene through NBR- nanoclay composites were evaluated and found to decrease with

nanofiller content. The activation energies for diffusion and permeation for the

nanocomposites were higher than that for neat NBR. The diffusion of toluene through

NBR – nanoclay composites was Fickian in nature.