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CHAPTER 5

NITRILE RUBBER (NBR) NANOCOMPOSITES BASED ON DIFFERENT FILLER GEOMETRIES (Nanocalcium carbonate, Carbon nanotube and Nanoclay)

5.1 Introduction

Nanocalcium carbonate (NCC) is a particulate nanofiller that has advantages in terms of

cost and commercial availability compared to nanoclay or nano-fibres. NCC has low

aspect ratio and large surface area [30]. There are reinforcing effect of nanometric

calcium carbonate in thermoplastic polymers has been demonstrated in various studies

[100 – 105, 112 – 114]. In recent years, similar studies have been done on thermosets

filled with NCC [133 – 135]. Though the volume of literature on NCC filled

thermoplastics and thermosets is relatively large, there are only few reported studies on

nanometric calcium carbonate filled elastomers. It has been reported that incorporation of

NCC increased the mechanical and thermal properties in natural rubber [292, 293],

butadiene rubber [295], styrene butadiene rubber [199] and EPDM rubber [215].

While several researchers reported tremendous property improvement in NBR – nanoclay

composites [26-32], there are very few studies on nanometric CaCO3 filled nitrile rubber

(NBR) composites. As discussed earlier, NCC gave remarkable improvements in rubbers

like natural rubber, SBR, EPDM and butadiene rubber. In this chapter, the effect of NCC

on mechanical, thermal and transport properties of nitrile rubber were studied and the

properties have been compared with those of NBR – precipitated CaCO3 vulcanizates.

The morphological studies on the NBR nanocomposites were carried out using

transmission electron microscopy.

The mechanical properties of polymer composites are affected by particle size, particle

content and particle/matrix interfacial adhesion [26]. The enhancement in properties of

polymer nanocomposites is attributed to the large interfacial area between the polymer

and the nanofiller. The surface area per unit volume of the nanofiller depends on its

geometry. For particles and fibres, the surface area per unit volume is inversely

proportional to the diameter of the particle [10]. To evaluate the effect of geometry on the

properties, NBR nanocomposites were prepared using three different nanofillers, viz.

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nanoclay (layered structure 1D), carbon nanotube (2D) and nanocalcium carbonate

(particulate 3D). The aspect ratio of the fillers followed the order nanoclay > carbon

nanotube > nanocalcium carbonate. The differences in the properties of nanocomposites

were attributed to the differences in structure and aspect ratio of the fillers.

5.2 Nitrile rubber (NBR) – nanocalcium carbonate (NCC) composites

5.2.1 Cure characteristics

The curing behaviour of the NBR – NCC nanocomposites was analyzed at 150˚C on a

TechPro Rheotech oscillating disc cure meter. The cure characteristics of NBR

nanocomposites are summarized in Table 5.1.

Table 5.1 Cure characteristics of NBR – NCC composites at 150°C

It was observed that addition of nanocalcium carbonate decreased the scorch time (ts2) of

the NBR – NCC composites. The cure time (t90) of the compounds showed a decrease on

incorporation of NCC up to 5 phr and thereafter increased at higher NCC content. It may

be inferred that the NCC activated the cure reaction up to 5 phr. At higher nanofiller

content, the activation may be lesser due to agglomeration of the particles as evident from

morphological studies. Cure rate index (CRI), a direct measure of the fast curing nature of

the rubber compounds, was calculated using equation (4.1).

For the NBR –NCC composites, CRI increased with addition of NCC and it facilitated the

activation of cure reaction up to 5 phr. On further increase in the filler content, cure time

increased while CRI decreased, but was higher than unfilled NBR. At higher loading the

t90 values increased indicating deactivation of the cure process which was due to

Name NCC

content (phr)

Scorch time (min)

Cure time (min)

τmin (Nm)

τmax (Nm)

Δτ (Nm)

Cure rate index

(min-1) NBRNCC0 0 6.8 11.1 0.60 2.50 1.90 22.80

NBRNCC2 2 3.5 7.9 0.40 3.02 2.62 22.99

NBRNCC5 5 3.5 7.1 0.31 2.79 2.48 28.17

NBRNCC7.5 7.5 3.3 9.1 0.36 2.86 2.50 17.30

NBRNCC10 10 3.3 8.4 0.38 2.78 2.40 19.61

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formation of aggregates of NCC that resulted in poor interfacial interaction between

rubber and NCC [291].

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

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

viscosity of the compound, decreased as NCC loading increased to 5 phr and thereafter

showed an increase. However, no such trend was observed for the maximum torque

(τmax) of NCC filled NBR. The difference between maximum and minimum torques (Δτ),

an indication of the extent of crosslinking, was found to increase for the nanocomposites

up to 5 phr. [175, 275]. This may be due to the interaction between the NCC and the

rubber matrix, the interaction being enhanced by the larger interfacial area between the

nanofiller and the matrix. However at NCC content greater than 5 phr, Δτ decreased due

to the reduction in interaction between the nanofiller and NBR which was attributed to

agglomeration of NCC.

5.2.2 Morphology

Figure 5.1 TEM micrographs of NBR- NCC composites (a) 0 phr (b) 2 phr (c) 5 phr and (d) 10 phr

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Transmission electron microscopy was used to study the nanostructure of NBR – NCC

composites. The TEM micrographs of NBR nanocomposites containing 0, 2, 5 and 10

phr are shown in Figure 5.1(a), 5.1(b), 5.1(c) and 5.1(d) respectively. The NCC particles

are indicated by the arrow in the figure. It was observed that NCC was finely and

uniformly dispersed in composites containing 2 and 5 phr. However, as the filler content

was increased to 10 phr, the NCC formed aggregates. From the TEM micrographs, it may

be concluded that at higher NCC contents, strong particle-particle interaction caused the

formation of agglomerates. In EPDM rubbers, the formation of agglomerates was

attributed to the high surface energy of NCC [215].

5.2.3 Mechanical properties

The stress-strain characteristics of the NBR nanocomposites are shown in Figure 5.2.

Figure 5.2 Stress –strain characteristics of NBR – NCC composites

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

strain behaviour was typical of synthetic elastomers. Incorporation of NCC increased the

stress in the nanocomposites for the same strain level. The mechanical properties of NBR

nanocomposites with different NCC contents are given in Table 5.2.

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Table 5.2 Mechanical properties of NBR – NCC composites

Name

NCC content (phr)

Pptd CaCO3 content (phr)

Tensile strength (MPa)

Elongation at break

(%)

M100 (MPa)

M300 (MPa) M100f/M100u

NBRNCC0 0 0 2.19±0.49 558±58 0.54±0.13 1.18±0.06 1.00

NBRNCC2 2 0 2.20±0.26 458±60 0.87±0.15 1.08±0.11 1.61

NBRNCC5 5 0 2.62±0.34 519±44 1.01±0.10 1.30±0.14 1.87

NBRNCC7.5 7.5 0 2.72 ±0.32 436±46 1.16±0.11 1.64±0.12 2.15

NBRNCC10 10 0 3.11±0.36 503±52 0.93±0.13 1.27±0.11 1.72

NBRCC10 0 10 2.81±0.46 521±73 0.89±0.06 1.16±0.13 1.65

NBRCC20 0 20 3.09±0.35 582±63 1.03±0.07 1.29±0.06 1.91

NBRCC30 0 30 2.89±0.36 556±51 0.96±0.06 1.28±0.06 1.77

NBRCC40 0 40 2.78±0.28 562±45 0.95±0.07 1.19±0.04 1.76

It was observed that the tensile strength increased with the NCC content. The increase in

the tensile strength was 24% and 42% for 5 and 10 phr of NCC respectively. This was

equivalent to the increase in tensile strength from 10 phr and 20 phr respectively of

commercial precipitated CaCO3. To get an equivalent increase in tensile strength using

commercial precipitated CaCO3, the filler content should be two times that of NCC. The

modulus at 100% elongation increased up to 7.5 phr and then decreased. It may be noted

that by adding 5 phr of NCC, the modulus obtained was comparable to that obtained by

adding 20 phr of precipitated CaCO3. The reinforcing effect of the NCC was evident from

the increase in the ratio of modulus of filled compounds (M100f) to that of unfilled NBR

(M100u). The improvement in mechanical properties of the nanocomposites, which was

greater than that produced by equivalent quantity of precipitated calcium carbonate, was

due to the smaller size of the NCC, resulting in larger surface area of the filler and better

interfacial bonding with the rubber matrix. This, along with good dispersion and

homogeneity in bonding, improved the mechanical properties of the nanocomposites

[200, 275]. In natural rubber, the reinforcing ability of the NCC was attributed to the

smaller particle size, greater surface area and consequent increase in contact area between

the filler and rubber particles. This inhibited the movement of polymer chains while

increasing the ability to inhibit microcrack expansion [292]. But as the amount of the

reinforcing filler exceeded a critical value, the contact between the rubber and filler got

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saturated. Further increase in filler content caused the size of the agglomerates and the

distance between rubber particles and agglomerates to increase, thus making the filler and

rubber behave as different phases resulting in poorer reinforcing effect [292]. Thus, the

slight reduction in mechanical properties at NCC content greater than 7.5 phr was due to

the aggregation of the nanofiller. Similar improvement in tensile properties for NCC

based composites of natural rubber [293], butadiene rubber [294], polybutadiene rubber

[295] and NR and NR/NBR blends [296] was reported in literature.

5. 2.4 Dynamic mechanical behaviour

The dynamic elastic (storage) modulus E’ data for neat NBR and NBR-NCC composites

are plotted as a function of temperature in Figure 5.3.

Figure 5.3 Storage modulus (E’) as a function of temperature for NBR- NCC composites at 1 Hz

Above Tg, at all loadings of NCC, the nanocomposites showed clear increase in storage

modulus. The effectiveness of the filler on the moduli of the composites may be

represented by the coefficient C calculated using equation (4.5). 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 was noted from the values of

C, given in Table 5.3, that the effectiveness of the filler was optimum at 7.5 phr,

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comparable to the effectiveness of double the concentration of precipitated CaCO3. At

this concentration, there was maximum stress transfer between the matrix and the filler.

Table 5.3 Dynamic mechanical properties of NBR – NCC composites

Sample tan δmax

E”max (MPa)

Tg from tan δ (°C)

Tg from E” (°C) C at 1

Hz 1 Hz 10 Hz 1 Hz 10 Hz 1 Hz 10 Hz 1 Hz 10 Hz

NBRNCC0 1.30 1.40 221 301 -8.0 -4 -14.0 -12 1.00

NBRNCC2 1.35 1.47 166 211 -9.9 -4 -14.0 -12 0.72

NBRNCC5 1.27 1.35 222 282 -9.2 -6 -16.0 -14 0.43

NBRNCC7.5 1.44 1.53 248 312 -8.0 -5 -13.8 -12 0.40

NBRNCC10 1.26 1.38 177 247 -12.0 -6 -16.0 -14 0.67

NBRCC10 1.24 1.35 192 261 -12.8 -8 -18.0 -14 0.43

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

frequency of 1 Hz is shown in Figure 5. 4. It was observed that the effect of NCC content

on the peak value of tan δ and Tg values of the NBR-NCC composites was marginal. The

area under the peak in tan δ vs temperature curve is a measure of energy dissipated. As

observed from the figure, there was marginal narrowing of peaks and marginal reduction

of damping properties. The effect of NCC content on values of tan δmax , E”max and the Tg

values obtained for all the samples at 1Hz and 10 Hz are tabulated in Table 5.3. The

maximum value of tan δ was greater than that of unfilled NBR at 2 and 7.5 phr, indicating

better damping properties. But at other NCC contents it was lower than for unfilled NBR.

The maximum value of loss modulus was observed to be at 7.5 phr of NCC. The dynamic

mechanical properties of NBR-NCC composites at 7.5 phr may be different due to the

increased NCC – NBR interaction and interphase properties at this critical filler content.

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 NBRNCC5 as a function of

temperature is shown in Figure 5.5.

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Figure 5.4 Effect of NCC content on tanδ of NBR- NCC composites at 1 Hz

Figure 5.5 Variation of storage modulus and tan δ with frequency for NBRNCC5

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There was a slight increase in both modulus values and tan δ with frequency. For a

viscoelastic material subjected to constant stress, the modulus decreases as time elapses

due to molecular rearrangements that result in reduction of localized stresses. Hence

modulus measurement at higher frequency (shorter time interval) will have higher values

compared to those taken at lower frequency (long time period) [281]. The same trend was

observed at all concentrations of NCC. The tan δ curve peak corresponding to the Tg was

shifted for NBR – NCC composites at higher frequency. The maximum value of tan δ

also increased. The tan δ curves were broadened, indicating restriction in segmental

mobility at higher frequency. The effect of NCC 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 5.3.

The Cole –Cole plot of storage modulus E’ vs. loss modulus E” was utilized to examine

the homogeneity of NBR – NCC composites (Figure 5.6). Homogenous polymeric

systems exhibit a semicircle diagram in the Cole-Cole plot [282, 283].

Figure 5.6 Cole-Cole plot of storage modulus E’ vs. loss modulus E” for NBR – NCC

composites at 1 Hz

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

composites deviated from semicircular shape implying that the system was

heterogeneous. It indicated structural changes on addition of nanoclay, the changes being

more prominent above 5 phr NCC.

5. 2.5 Thermal behaviour

The thermal stabilities of NBR/nano-CaCO3 composites were studied by means of TGA

in nitrogen atmosphere. The thermograms for the NBR nanocomposites at different NCC

content are given in Figure 5.7.

Figure 5.7 Thermograms of NBR- NCC composites

The thermal stability factors, viz. initial decomposing temperature (IDT), temperature at

the maximum rate of weight loss (Tmax) and the char content at 500°C were calculated

from the TGA thermograms and are listed in Table 5.4.

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Table 5.4 Thermal stability factors of NBR- NCC composites obtained from TGA

Name NCC Content (phr)

IDT ( °C)

Tmax (°C)

Char (%)

NBRNCC0 0 401 450 23.0

NBRNCC2 2 407 447 25.5

NBRNCC7.5 7.5 410 447 29.1

NBRNCC10 10 412 446 32.8

The thermal stability of the composites was enhanced on addition of NCC. In neat rubber,

IDT was around 400oC. On addition of NCC, the IDT increased by 6-11°C depending on

the content of the filler. This increase in the IDT was also visible in the shoulder region

of the derivative thermograms shown in Figure 5.8.

Figure 5.8 Derivative thermograms for NBR- NCC composites

Another notable feature from the derivative thermogram was that the maximum rate of

decomposition of the nanocomposites decreased significantly on addition of NCC. This

was clearly evident in the reduction in peak height of the derivative thermogram with

increasing NCC content. However, NCC content had no significant effect on the

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temperature at which maximum rate of decomposition occurs. Jin and Park [294]

postulated that the increased area of contact of the nanometric CaCO3 with the rubber

matrix increases the absorption of heat energy during decomposition, resulting in higher

IDT and lower decomposition rate for the nanocomposites. The char content of the

nanocomposites at 500°C increased with NCC content.

5. 2.6 Transport characteristics

The effects of NCC concentration on the diffusion, sorption and permeation of toluene

through NBR nanocomposites were studied. The transport behaviour through composites

depends on filler type, matrix, temperature, 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

behavior of NBR - NCC composites was assessed by calculating swelling coefficient (β)

using the equation 4.6. Table 5.5 shows that the swelling coefficient decreased with

increase in NCC content.

Figure 5.9 Sorption isotherms for NBR- NCC composites at 30°C

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The sorption curves were plotted as Qt (moles of solvent sorbed per 100 g of rubber)

against √(time). Figure 5.9 shows the effect of NCC content on the toluene uptake with

time. The sorption of the solvent was reduced for the nanofilled composites compared to

unfilled rubber. The dispersion of NCC in the rubber matrix introduced a tortuous path

for the transport of the solvent. The Qt vs. √t curve showed two distinct regions - an initial

steeper region with high sorption rate due to large concentration gradient and a second

region exhibiting reduced sorption rate that ultimately reached equilibrium sorption. The

sorption rate and equilibrium solvent uptake of NBR nanocomposites reduced with

increasing NCC content. However, at 10 phr the sorption rate was reduced. Beyond 7.5

phr there was no appreciable change in equilibrium solvent uptake. This was due to the

formation of agglomerates of NCC which was evident from the TEM micrographs.

The crosslink density and the molecular weight of the polymer between the crosslinks

(Mc) were calculated from the sorption data using equation 4.9 – 4.12. The estimated

values of Mc for NBR - NCC composites are tabulated in Table 5. NCC filled systems

exhibited lower Mc values , i.e. lower molar mass between crosslinks than unfilled NBR

and Mc decreased with increasing NCC 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 10 phr was due to the

aggregation of the nanofiller as seen in the TEM micrographs. The calculated values of

crosslink density supported this observation. The crosslink density of NBR – NCC

composites is tabulated in Table 5.5. As the nanofiller content in the composites

increased, the crosslink density also increased.

Table 5.5 Swelling coefficient and crosslink densities of NBR - NCC composites

Sample Swelling

coefficient (β) (cm3/g)

Molar mass between crosslinks (Mc)

(g/mol)

Crosslink density x 104 (gmol/cm3)

NBRNCC0 2.77 3445 1.45

NBRNCC2 2.49 2985 1.68

NBRNCC5 2.43 2982 1.68

NBRNCC7.5 2.32 2852 1.75

NBRNCC10 2.28 2873 1.74

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The diffusivity (D), the sorption coefficient (S) and permeability (P) of NBR – NCC

composites were calculated using equations 4.13 – 4.15. To study the effect of

temperature on transport coefficients, the experiment was conducted at 30°C, 50°C and

75°C. The diffusion, sorption and permeability coefficients of NBR – NCC composites at 30°C,

50°C and 75°C are given in Table 5.6.

Table 5.6 Transport coefficients of NBR- NCC 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

NBRNCC0 6.58 8.11 8.46 2.39 2.30 2.16 15.7 18.6 18.3

NBRNCC2 6.19 8.00 8.43 2.14 2.22 1.89 13.2 17.7 15.9

NBRNCC5 5.92 7.96 8.32 2.11 2.01 1.79 12.5 16.0 14.9

NBRNCC7.5 5.53 7.04 8.03 2.01 1.95 1.84 11.1 13.7 14.8

NBRNCC10 5.93 7.78 7.78 1.97 1.87 1.83 11.7 14.5 14.2

The diffusion of solvent through a composite depended 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]. The diffusion and permeation

coefficients of the nanofilled composites were considerably lower than the unfilled NBR. The

diffusion of the penetrant solvent depended on the concentration of available space in the

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

NCC reduced the availability of these spaces and also restricted segmental mobility of the

rubber matrix. However at 10 phr, there was an increase in diffusion and permeation

coefficients. This increase was due to the aggregation of the nanofiller. Similar trends were

observed for sorption studies conducted at 50°C and 75°C. As the temperature increased, the

coefficient of diffusion also increased for all the samples. As temperature increased, 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 with temperature. The transport

coefficients at 50°C and 75°C are also tabulated in Table 5.6. Maiti et al had reported similar

reduction in transport coefficients for fluoroelastomer-clay nanocomposites [287].

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

Arrhenius equations 4.16 – 4.18. The activation energies for diffusion, sorption and permeation

are listed in Table 5.7

Table 5.7 Thermodynamic parameters for transport of toluene through NBR- NCC

composites

Sample ED (kJ/mol)

ED (kJ/mol)

ΔHs (kJ/mol)

NBRNCC0 4.86 2.88 -1.98

NBRNCC2 5.95 3.51 -2.44

NBRNCC5 6.56 3.41 -3.15

NBRNCC7.5 7.23 5.55 -1.68

NBRNCC10 6.21 3.66 -2.55

The activation energy of diffusion of toluene through NBR – NCC composites was found to be

higher than that of unfilled NBR. The nanofillers have higher specific surface area resulting in

greater rubber-filler interaction and enhanced reinforcement. As the nanofiller content

increased, the activation energy needed for diffusion also increased. The activation energy for

permeation (EP), also evaluated using the Arrhenius equation, showed similar trend as ED. It is

observed that the sorption is an exothermic process. The value of ΔHs increased with increasing

nanofiller content.

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

fitted to the equation 4.19 The values of n and k were determined by linear regression

analysis are shown in Table 5.8. In the case of NBR-NCC the value of ‘n’ at room

temperature (30°C) was close to 0.5 and the diffusion was Fickian type, controlled by

concentration dependent diffusion coefficient [290]. In rubbery polymers, well above

their glass transition temperatures, the polymer chains adjusted quickly to the presence of

penetrant molecules and hence they did not exhibit anomalous behaviour. It was observed

that as the temperature increased, the values of n increased for NBR-NCC composites,

implying that the diffusion tended to be of anomalous type. The constant k decreased with

increasing temperature.

114

Table 5.8 Parameters for diffusion mechanism of toluene through NBR – NCC composites

SAMPLE 30°C 50°C 75°C

n k (g/gmin2)

n k (g/gmin2)

n k (g/gmin2)

NBRNCC0 0.48 0.076 0.50 0.068 0.51 0.071

NBRNCC2 0.47 0.082 0.49 0.071 0.51 0.073

NBRNCC5 0.49 0.073 0.49 0.067 0.50 0.067

NBRNCC7.5 0.48 0.076 0.50 0.065 0.51 0.069

NBRNCC10 0.48 0.082 0.48 0.073 0.53 0.069

5.3 NBR nanocomposites with nanofillers of different geometries

The nanofillers used in this study are (i) one dimensional layered silicate (nanoclay)

having aspect ratio 1:100 (ii) two dimensional carbon nanotube (CNT) and (iii) three

dimensional nanoparticle nanocalcium carbonate (NCC). For comparison of properties of

NBR nanocomposites formed using the above nanofillers, were prepared as per the

procedure described in 3.2. The nanofiller content in each case was 2 phr. The

nanocomposites prepared using nanoclay, CNT and NCC were designated as NBRNCL2,

NBRCNT2 and NBRNCC2 respectively. The unfilled NBR was designated as NBR0.

5.3.1 Mechanical properties of NBR nanocomposites

The mechanical properties of NBR nanocomposites with different fillers are tabulated in

Table 5.9.

Table 5.9 Mechanical properties of NBR nanocomposites

Name

Nanofiller content (phr)

Tensile strength (MPa)

Elongation at break (%)

M300 (MPa) Mf/Mu

NBR0 0 2.19±0.49 558±58 1.18±0.06

NBRNCL2 2 3.08±0.51 593±34 1.58±0.19 1.34

NBRCNT2 2 3.06±0.41 499±49 1.54±0.18 1.30

NBRNCC2 2 2.20±0.26 458±60 1.08±0.11 0.92

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It was observed that among the three nanofillers, the highest tensile strength and modulus

were obtained for NBR – nanoclay composites while the lowest was for NBR – NCC

composites. This can be explained using the aspect ratio and the surface area per unit

volume of the filler given in Table 5.10. The higher tensile strength in nanoclay

composite was an indication of good intercalation/exfoliation of nitrile rubber segments

into the silicate layers as evident from the TEM micrographs and XRD diffraction pattern.

Table 5.10 Comparison of surface area to volume ratio of nanofillers

Nanofiller Surface area / Volume Surface area / Volume*

Nanoclay

l4

t2

+ t = 1 nm

l = 100 nm 2.04

Carbon nanotube

l2

r2

+ r = 10 nm

l = 10 μm 0.2002

Nanocalcium carbonate

r3 r = 20 nm 0.15

* calculated using material data given in 3.1.2 – 3.1.4

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In NBR – NCC composites, the NCC particles were individual species having lesser

interaction with the nitrile rubber segments due to the lower surface area per unit volume

of the filler resulting in less reinforcement [179]. The overall tensile performance of

different fillers in nitrile rubber was in the order of NBR – nanoclay > NBR – CNT >

NBR – NCC > unfilled NBR. The elongation at break for nanocomposites was lower than

unfilled NBR except for NBR – nanoclay composites. In NBR – nanoclay composites, the

alkylammonium ions that are located at the clay–NBR interface imparted plasticizing

effect thereby increasing the elongation at break [165].

For particles and fibres, the surface area per unit volume is inversely proportional to the

diameter of the particle [10]. The improvement in mechanical properties of NBR-

nanoclay composites over NBR – CNT and NBR – NCC composites can be explained on

the basis of the higher surface area to volume ratio of the nanoclay. The larger surface

area to volume ratio of nanoclay resulted in larger interfacial area between the nanofiller

and the rubber matrix.

As mentioned earlier, Cloisite 20A is an organo-modified nanoclay. The organic modifier

in the nanoclay reduces the surface energy of the nanoclay and improves the wetting

characteristics of the polymer. As observed by Kim et al in their studies on NBR –

organoclay systems [181], it improves the strength of the interface between the inorganics

and the polymer. Surface treatment of nanocalcium carbonate with lipophilic substances

like stearic acid or calcium stearate is used to improve dispersability and surface

interaction with the polymer [67]. In this study, the nanocalcium carbonate used was of

uncoated grade and hence there was no surface interactions between the NBR matrix and

the nanocalcium carbonate filler. Functionalization of the CNT surface leads to increased

dispersability of the CNTs in polymers and also increases the strength of the interface

between the CNT and the polymer matrix [52, 108, 139, 162]. As the CNT used in the

study was non-functionalized, filler surface – NBR matrix interactions were not

prominent in the case of NBR-CNT composites. As the structural arrangements on the

molecular scale and the efficient transfer of stress across the composite components

depend on the interfacial region, nanoclay composites exhibited higher reinforcement

than the CNT and NCC based composites [30].

117

5.3.2 Dynamic mechanical properties of NBR nanocomposites

The effect of temperature on the storage modulus of NBR nanocomposites are compared

in Figure 5.10. Above Tg nanoclay the nanocomposites showed 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, for CNT and NCC composites, the

enhancement in storage modulus was less than that of NBR – nanoclay composites. This

behaviour can also be attributed to the increased interfacial surface area of the NBR –

nanoclay composites. Below Tg, the difference in the value of E’ was less significant.

Figure 5.10 Storage modulus (E’) vs. temperature of NBR nanocomposites at 1 Hz

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Table 5.11 Dynamic mechanical properties of NBR nanocomposites evaluated at 1 Hz

Sample tan δmax E”max (MPa) Tg (°C) from

tanδ E” NBR0 1.31 221 -8.0 -14.0

NBRNCL2 1.31 247 -8.0 -14.0

NBRNCNT2 1.32 198 -8.0 -14.0

NBRNCC2 1.35 166 -9.9 -14.0

Similarly, the maximum value of loss modulus (E”max) was the highest for NBR -

nanoclay composites. It was observed that the nanofillers had no significant effect on the

glass transition temperature. The values of tan δmax, E”max and Tg for NBR

nanocomposites evaluated at 1 Hz is tabulated in table 5.11.

5.3.3 Transport characteristics

The effects of type of nanofiller on the diffusion, sorption and permeation of toluene

through NBR nanocomposites were studied. The swelling behaviour of NBR - nanoclay

composites was assessed by calculating swelling coefficient, β using equation (4.6).

Table 5.12 shows that the swelling coefficient was lowest for NBR – nanoclay

composites and highest for NBR – CNT composites. The sorption curves i.e., Qt (moles

of solvent sorbed per 100 g of rubber) vs. √t curves at 30°C for NBR nanocomposites are

shown in Figure 5.11.

It was observed that the sorption of toluene was reduced for the nanofilled composites

compared to unfilled rubber. It was the lowest for nanoclay composites. The sorption rate

and equilibrium solvent uptake also were the lowest for NBR – nanoclay composites. As

evident from the TEM micrographs of NBR – nanoclay composites (Figure 4.3), the

dispersion of nanoclay forming exfoliated and intercalated structure in the rubber matrix

created tortuous path for the transport of the solvent, resulting in lower sorption.

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Figure 5.11 Sorption isotherms for NBR nanocomposites at 30°C

The crosslink density (υ) and molecular weight of the polymer between the crosslinks

(Mc) calculated from the sorption data using equation (4.9) and (4.10) respectively are

tabulated in Table 5.12.

Table 5.12 Swelling coefficient and crosslink densities of NBR nanocomposites

Sample Swelling coefficient

(β) (cm3/g)

Molar mass between crosslinks (Mc)

(g/mol)

Crosslink density x 104 (gmol/cm3)

NBR0 2.77 3445 1.45

NBRNCL2 2.31 2622 1.91

NBRCNT2 2.60 3190 1.57

NBRNCC2 2.49 2985 1.68

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The transport coefficients for sorption of toluene trough NBR – nanocomposites were

calculated using equations 4.11 – 4.13 and are tabulated in Table 5.13.

Table 5.13 Transport coefficients of NBR nanocomposites at 30°C

Sample Diffusion coefficient

(Dx107) (m2/s)

Sorption coefficient (S)

(g/g)

Permeability coefficient (P x107) (m2/s)

NBR0 6.58 2.39 15.7

NBRNCL2 6.02 2.00 12.0

NBRCNT2 6.23 2.25 11.5

NBRNCC2 6.19 2.14 13.2

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]. 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 free space

available in the matrix to accommodate the penetrant molecule [154]. The addition of

nanoclay reduced the availability of these free 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 to the aggregation of the nanofiller.

5.4 Conclusion

NBR – nanocalcium carbonate (NCC) composites were prepared from a masterbatch of

NBR and NCC in an internal mixer followed by mixing on a two-roll mill. The cure time

of the compounds showed a decrease on incorporation of NCC upto 5 phr and thereafter

increased at higher NCC content. The dispersion of NCC in NBR matrix was analyzed

from transmission electron micrographs. The tensile strength and modulus increased with

NCC content. The mechanical properties of NBR - NCC nanocomposites were found to

be superior to those of NBR-CaCO3 microcomposites. Investigation of dynamic

mechanical properties showed that addition of NCC increased and lowered the tan δ peak

121

value, without altering the Tg of the nanocomposite. Addition of NCC to NBR resulted in

enhanced thermal stability. The sorption of toluene through the nanocomposite decreased

with increasing NCC content. The values of the transport coefficients were slightly higher

at 10 phr NCC content than at 7.5 phr because of aggregation of the nanofiller. Activation

energy for diffusion and permeation for the nanocomposites were higher than that

forunfilled NBR. Diffusion of toluene through NBR-NCC composites was found to

exhibit Fickian behavior.

The properties of NBR nanocomposites prepared using three different nanofillers viz.

nanoclay (layered structure 1D), carbon nanotube (2D) and nanocalcium carbonate

(particulate 3D) were compared in this chapter. The aspect ratio and the surface area per

unit volume of the fillers were in the order nanoclay > carbon nanotube > nanocalcium

carbonate. The larger surface area to volume ratio of nanoclay resulted in larger

interfacial area between the nanofiller and the rubber matrix and consequently nanoclay

composites exhibited higher reinforcement than the CNT and NCC based composites.

The equilibrium solvent uptake and transport coefficients for the diffusion of toluene

through the NBR nanocomposites were lowest for NBR – nanoclay composites.