70 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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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
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