Influence of hybrid nanostructures and its tailoring mechanism on permeability, rheology, conductivity, and adhesion properties of a novel rubber blend nanocomposite

Abstract

The present work provides an extensive insight into the effect of hybrid nanofillers and their structure–property relationship in nanocomposites based on bromobutyl rubber (BIIR)/polyepichlorohydrin rubber (CO) blends. TEM photomicrographs reveal high degrees of dispersion of the nanoclay with the formation of hybrid nanostructures. The rheological behavior of the nanocomposites displays a shear thinning nature and significant reduction of die swell (up to 13% reduction) is observed with increase in the dosage of nanoclay. The addition of the nanoclay drastically reduces the air permeability up to 17%, increases electrical conductivity and thermal conductivity of the rubber nanocomposites. Adhesion of rubber to the fabric ply is found to be good in the nanocomposite having a lower dosage of nanoclay. These unique attributes were found to stem from the fundamental viscoelastic characteristics i.e., increase in the entanglement density due to the hybrid nanostructures. The development of hybrid nanostructures and their significant contribution to the improvements of properties are schematically explained. Rubber formulations with such suitably tailored nanostructures will find their applications for next generation rubber based industrial products.

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1. Introduction

Recently, rubber nanocomposites have attracted researchers for their superior functional properties. Nanofillers such as layered silicates (LS), talc, silica, nanobiofibrils, carbon nanotubes, and graphene find various applications based on their structures and types.¹ However, layered silicates, due to their higher aspect ratio and platelet like structure, offer low gas permeability in the nanocomposites. Nielson and Gerlowski stated that nanoclay can effectively decrease diffusivity and overall transport of water across a film due to the layered platelets.² The layered silicates commonly used in the nanocomposites belong to the structural family known as the 2 : 1 phyllosilicates. Among the 2 : 1 layered clays, montmorillonite has higher swelling capabilities, cation exchange capacity (CEC) and basal spacing. Naturally occurring montmorillonite clay (MMT) is organically modified (OMMT) for improved compatibility with rubber. The selection of the surfactants for the modification of the clay depends on the type of the rubber and application of the nanocomposites. The dispersion of the nanoclay is significant in reinforcing the rubber and to improve the properties of the rubber nanocomposites. The effect of OMMT clay on the reinforcement of the black filled compound has already been reported in our earlier work.³ The partial replacement of the carbon black with the nanoclay may sufficiently reduce the component weight and increase the durability.

Bromobutyl rubber (BIIR) is commercially the most important derivative of butyl rubber. The advantage of using BIIR over butyl rubber (IIR) is to enhance its compatibility with unsaturated diene rubbers suitable for co-vulcanization. BIIR also offers better inter-ply adhesion, greater heat resistance and faster cure rate.⁴ BIIR is the most favored rubber for the construction of inner liners of tire, heat resistant tubes, bladders and pharmaceutical ware due to its lower permeability to gases.¹

Polyepichlorohydrin rubber (CO) offers excellent resistance to gas permeability,⁵ even lower than that of the butyl rubber. Frank et al. claimed that blending of epicholorohydrin rubber with brominated butyl rubber and having delaminated talc in the rubber composite gave rise to lower air permeability.⁶ It finds potential application in tire inner liners with low permeability. Hence, CO rubber is suitably blended with BIIR in order to further reduce the barrier properties of the rubber nanocomposites. However, the effects of hybrid fillers on comprehensive properties of novel BIIR–CO blend nanocomposites were hardly found in the contemporary literature.

In this paper, we intend to prepare nanocomposites of BIIR–CO blends filled with carbon black and nanoclay. The effect of the nanoclay and hybrid nanostructure on the morphology, rheology, permeability, conductivity (dielectric and thermal), viscoelastic, and adhesion properties were studied, keeping their wider scopes for industrial applications in mind.

2. Experimental

2.1. Materials

Hydrin H 55 (homopolymer of epicholorohydrin (CO), specific gravity of 1.37) and Zisnet F-PT (2,4,6 trimercapto-sym-triazine) were provided by M/S Zeon Chemicals, Louisville KY. Bromobutyl rubber (BIIR-2255) was purchased from M/S Exxon Mobil. Nanoclay (Cloisite 20A) was purchased from M/S Southern Clay Products Inc. Gonzales TX. Carbon black (GPF N 660 grade) was procured from M/S Philips Carbon Black Limited (PCBL), India. Nytex 810 (Naphthenic oil) was supplied by M/S Nynas, Belgium lab. Span-60 was supplied by M/S Sigma Aldrich, USA.

2.2. Preparation of rubber blends nanocomposites

Rubber nanocomposites were mixed in a Brabender Plasticorder. Mixing of the nanocomposite was divided in to two stages for achieving better dispersion of nanoclay in the rubber matrix. First stage of mixing (master batch) includes melt mixing of BIIR rubber and pre-blended CO-Zisnet F-PT stock, followed by the addition of nanoclay (NC), process aids, carbon black (CB), oil and other functional additives. Nanoclay was added at the beginning of the mix in order to achieve better dispersion in the rubber matrix. The addition of curing agent and accelerators in the second stage of mixing constitutes the final batch. Compounding temperature was set at 55 °C and the rotor speed was maintained around 50 rpm. The total mixing time of the rubber nanocomposites was 10 minutes. Cure characteristics of the samples were obtained by using an oscillating disc rheometer (ODR) at 170 °C. The samples were cured for characterization based on the optimum cure time (OCT) obtained from ODR. Compounding formulations are tabulated in Table 1.

Table 1 Compound formulation (all ingredients are measured in phr, parts per 100 g of rubber)

Chemical ingredients	B90H10CB50NC0	B90H10CB50NC3	B90H10CB50NC5	B90H10CB50NC7	B90H10CB50NC10
a Cure package consists of ZnO-3.0, MBTS-1.0, sulphur-0.4, Zisnet F-PT-0.2, and PVI-0.2.
BIIR 2255	90	90	90	90	90
Hydrin H-55	10	10	10	10	10
Span 60	1.5	1.5	1.5	1.5	1.5
Mgo	0.2	0.2	0.2	0.2	0.2
Calcium stearate	2	2	2	2	2
Structol MS 40	7.3	7.3	7.3	7.3	7.3
Naphthenic oil	5.5	5.5	5.5	5.5	5.5
CI resin	2	2	2	2	2
CB N 660	50	50	50	50	50
Cloisite 20A	—	3	5	7	10
Cure package^a	4.8	4.8	4.8	4.8	4.8

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3. Characterization

The samples for HRTEM analysis were carefully made by ultra cryo-microtome using a Leica ultracut. Since the samples are elastomeric in nature, ultramicrotome was performed below the glass transition temperature of the blended rubbers (−75 ± 5 °C) using sharpened glass knives with cutting edge of 45°. Dispersion of nanoclay in the nanocomposites was extensively analyzed by using HRTEM. The cryotomed sections were supported on a copper grid for capturing photomicrographs. The microscopy was performed using JEM-2100, JEOL, Japan.

Capillary rheology of the samples was analyzed by capillary rheometer (Smart Rheo, Ceast, Italy). Die swell was also determined at different shear rate by using 20 : 1 L : D ratio capillary die.

Surface gloss of the capillary extrudate was analyzed with the help of the analytical scanning electron microscope (A-SEM) using ZEISS EVO 60, Carl Zeiss, Germany with Oxford EDS detector, UK. The microscope works at an accelerated voltage of 30 KV after sputter coating the samples with gold–palladium using POLARON-SC7620, UK.

The crosslink density was determined by employing kinetic theory of rubber elasticity and specific gravity of the vulcanizates was determined using Wallace digital densimeter. Green strength for all compounds was measured by Universal Testing Machine (UTM) using Hioks-Hounsfield UTM (Test equipment Ltd, Surrey, England) at a crosshead speed of 100 ± 10 mm min⁻¹ at 25 °C as per ASTM D6746-10.

The thermal conductivity of flat slab specimens was measured by using guarded hot plate apparatus in accordance with ASTM C-177. The test temperature ranges from RT to 100 °C working at 220/240 volts AC. Hot surface temperature of the specimen was regulated by electrical heating system. The thermal conductivity (α) is determined from Fourier's law as follows:

q = −α(dT/dx) = α[(T1 − T2)/x] (1)

where q is the heat flux (W m⁻²), T1 and T2 is the hot and cold surface temperature in °C, x is the separated distance or thickness (m), and α is the thermal conductivity (W (m K)⁻¹).

Dielectric properties were measured using Quadtech 7600 plus precision LCR meter, brass electrode (IET Labs, Inc. NY) with 0.05% basic measurement accuracy. The programmable frequencies were varied from 10 Hz to 1 MHz.

Dynamic mechanical property of the composites was determined by DMA using Metravib 50 N, France, using tension mode. The compounds were subjected to sinusoidal displacements with varying strain sweeps (0.05–25%) and temperature sweeps (−100 °C to +100 °C) at a frequency of 10 Hz at room temperature (∼25 °C).

Air permeability of the nanocomposites was measured following GB 1038 using Labthink permeability tester at 40 ± 1 °C. By measuring the pressure differences, the gas transmission rate (GTR) of the specimen was measured and average values were reported.

Adhesion strength between the composite and a reference rubber ply fabric was determined by a 180° peel test. Peel force of all composites were measured by Universal Testing Machine (UTM) using Hioks-Hounsfield UTM (Test equipment Ltd, Surrey, England) at a crosshead speed of 50 mm min⁻¹ at 25 °C as per ASTM D413-98 standard. The results reported are based on average values of three samples studied for reproducibility.

4. Results and discussion

4.1. Dispersion study by HRTEM

Dispersion of nanoclay in the rubber nanocomposites was analyzed by using HRTEM photomicrographs are shown in Fig. 1a–d. From the Fig. 1a–c, it is clear that the nanoclay is well dispersed along the entire matrix. The nanoclay has been randomly oriented and completely distributed throughout the entire matrix. However, TEM photomicrograph of B₉₀H₁₀CB₅₀NC₁₀ (Fig. 1d) confirms the formation of agglomerated clay-black clusters due to the overdose of nanoclay. The formation of hybrid nanostructures (NC–CB) and good interaction between the rubber and filler effects in the formation of ‘nano units’, ‘halo’ and ‘nano-channels’ corroborate to the overall improvement in filler connectivity.^7–9 The bending of the nanoclay around the carbon black particle (pink circle in Fig. 1b) reveals the signature of ‘nano unit’ whereas the blue box represents ‘halo’ in which the nanoclay are surrounded by carbon black particles. The formation of hybrid nanostructures in the nanocomposites is schematically narrated in Scheme 1. It can be seen that the nanoclay and carbon black interacts well with each other along with the rubber to form hybrid nanostructure which contributes to a significant improvement in the dispersion which in turn uplifts the overall properties.¹⁰ The formations of these nanostructures are frequent in the presence of well dispersed hybrid filler system.^11,12

Fig. 1 HRTEM photomicrograph of (a) B₉₀H₁₀CB₅₀NC₃; (b) B₉₀H₁₀CB₅₀NC₅; (c) B₉₀H₁₀CB₅₀NC₇ and (d) B₉₀H₁₀CB₅₀NC₁₀.

Scheme 1 Formation of hybrid nanostructures (nanounit and halo) in B₉₀H₁₀CB₅₀NC₅.

4.2. Air permeability

Air permeability of the samples were measured and presented in the Fig. 2. Addition of the nanoclay effectively reduces the air permeability up to 17% in the nanocomposites. Air permeability in the nanocomposites depends on the tortuous path offered by the nanoclay platelets. It is understood that due to their layered structure, the addition of nanoclay increases the tortuous path and substantially reduces the air permeability of the nanocomposites. The formation of hybrid nanostructures further facilitates the reduction of permeability. The air permeation process in the rubber nanocomposites is schematically shown in Scheme 2. It is noted that the formation of nanostructures and clay platelets effectively increases the tortuous path of air permeation (d < d′ < d′′). However, the dispersion of nanoclay is pivotal in reducing the air permeability of the nanocomposites. The permeability reduction can be further related to the restriction in the molecular mobility. The permeation in BIIR–CO rubber nanocomposites is due to the segmental chain mobility of the amorphous BIIR rubber. This possibly affects the bulk barrier properties of the nanocomposites. The tan delta (δ) of the nanocomposites obtained at temperature 25 °C and at frequency 10 Hz is reported in the Table 2. Increase in the nanoclay addition significantly lowers the tan delta. The mechanical loss or tan delta which measures the energy dissipation is related to the stress relaxation. A lower tan delta corresponds to a greater stress relaxation and therefore restricts the chain mobility. This reduction in the chain mobility considerably decreases the gas permeation in the nanocomposites. Increase in the crosslink density as well as the density of the composites (Table 3) lead to a considerable reduction in the air permeability.^13–15

Fig. 2 Air permeation characteristics of the samples with the addition of nanoclay.

Scheme 2 Extended tortuous path (d′′) due to the formation of hybrid nanostructures in BIIR–CO nanocomposites.

Table 2 tan delta (δ) values for the nanocomposites reported at 25 °C and 10 Hz

Compound	tan delta
B90H10CB50NC0	0.339
B90H10CB50NC3	0.311
B90H10CB50NC5	0.296
B90H10CB50NC10	0.266

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Table 3 Crosslink density and specific gravity of the nanocomposites

Compound	Crosslink density (mol m^−3)	Specific gravity
B90H10CB50NC0	0.00474	1.150
B90H10CB50NC3	0.00565	1.156
B90H10CB50NC5	0.00644	1.163
B90H10CB50NC7	0.00658	1.166
B90H10CB50NC10	0.00757	1.169

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4.3. Capillary rheology

Capillary rheology of the samples has been analyzed in order to understand the dynamic extrusion behavior of the sample under various shear rates. Viscosity, shear stress and die swell of the nanocomposites were measured and shown in Fig. 3a–c. Viscosity measurement (Fig. 3a) at various shear rates reveals shear thinning behavior of the nanocomposites. As the shear rate increases, viscosity of the nanocomposites increases marginally as compared to that of the base compound. It is to be noted that the addition of nanoclay does not significantly influence the processing viscosity of the composites. The pattern of shear stress (Fig. 3b) of the nanocomposites depicts interesting characteristics of the nanocomposites. In general, the shear stress is higher for the nanocomposites than that of the base compound at any particular shear rate. However, it is found that the composite with lower dosage of nanoclay possess higher shear stress at lower shear rate and crosses over a point at a shear rate of around 200 s⁻¹. After the cross over point, the shear stress decreases with further increase in the shear rate up to 1000 s⁻¹. This suggests that the formation of hybrid nanostructure is effective and thus responsible for the higher shear stress at higher shear rates. Addition of nanoclay decreases the die swell considerably (up to 13% reduction). However, the effect of nanoclay in reducing the die swell is much pronounced at higher shear rates (Fig. 3c). This is attributed to the reduction in elastic recoiling due to the formation of hybrid nanostructures in the rubber nanocomposites.¹⁶ It is claimed that the addition of nanoclay have decreased die swell and mill shrinkage, improved extrusion rates and hot air ageing resistance of the nanocomposites.¹⁷

Fig. 3 (a) Viscosity (b) shear stress and (c) die swell of the nanocomposites at different shear rate.

The surface smoothness of the extrudate sample was determined by performing SEM analysis. SEM photomicrographs of the samples extruded at 1000 s⁻¹ are shown in Fig. 4a–c. In order to distinctively understand the effect of nanoclay on the surface roughness, the analysis was done for the samples B₉₀H₁₀CB₅₀NC₀, B₉₀H₁₀CB₅₀NC₅, and B₉₀H₁₀CB₅₀NC₁₀. In order to further understand the nature of the extruded surface and the flow marks, the higher magnification SEM photomicrographs are shown in Fig. 4d–f. Extrudate of B₉₀H₁₀CB₅₀NC₀ has more clusters (yellow circle in Fig. 4a) on the surface indicating that the rubber debris has been removed out during the extrusion process. This may be due to the rupture at the particle–matrix interface and decohesion at the weakest points.^18,19 However, the clusters are significantly less in B₉₀H₁₀CB₅₀NC₅ and there is no visible rubber debris in the extrudate of B₉₀H₁₀CB₅₀NC₁₀. As the dosage of the nanoclay increases, the particulate clusters tend to decrease substantially. It corroborates that the addition of nanoclay effectively reinforces the rubber. The effect of nanoclay in reducing the die swell can be observed from the figures. B₉₀H₁₀CB₅₀NC₁₀ has the lowest die swell. It is noted that the addition of nanoclay marginally reduces the surface smoothness. However, the higher dosage of nanoclay in B₉₀H₁₀CB₅₀NC₁₀ exhibits visible flow marks (Fig. 4g). It attributes to the formation of clay agglomerates in the rubber matrix. Hence, it can be inferred that the addition of nanoclay significantly reinforces the rubber, decreases the die swell without affecting the surface smoothness of the extrudate.

Fig. 4 SEM analysis of capillary extrudate at shear rate: 1000 s⁻¹ with magnification 100× (a) B₉₀H₁₀CB₅₀NC₀; (b) B₉₀H₁₀CB₅₀NC₅; (c) B₉₀H₁₀CB₅₀NC₁₀; and with magnification 150× (d) B₉₀H₁₀CB₅₀NC₀; (e) B₉₀H₁₀CB₅₀NC₅; (f) B₉₀H₁₀CB₅₀NC₁₀, (g). A closer view (200×) of B₉₀H₁₀CB₅₀NC₁₀ depicts flow marks during extrusion.

4.4. Green strength

In order to understand the processibility, creep and tear resistance of the unvulcanized compounds, it is important to determine the green strength and elongation of the compound. The green strength of the compound is defined as the resistance to deformation and fracture before vulcanization.²⁰ Green strength and elongation of the unvulcanized compound was measured and shown in Fig. 5. It is evident that as the dosage of the nanoclay increases, the green strength increases and the elongation of the compound decreases significantly. It relates to the increase in the storage modulus and the filler content.²¹ However, it is desirable to have a balance between the green strength and elongation of the unvulcanized compound for improved processibility. B₉₀H₁₀CB₅₀NC₅ displays optimum properties among all other nanocomposites.

Fig. 5 Green strength and elongation of the unvulcanized compounds.

4.5. Dielectric properties

The application of the electric field induces molecular polarization in the material. Fig. 6 explains different types of molecular polarizations which occur at different frequencies. Dielectric properties of the nanocomposites are measured in a wide frequency range and were depicted in the Fig. 7a–d. Dielectric constant, dielectric loss and dissipation factor of the nanocomposites were determined by the frequency sweep. BIIR–CO rubber nanocomposites exhibit orientation polarization due to the permanent dipole moment exerted by the polar CO rubber.²³ From the Fig. 7a–c, it is evident that the dielectric responses of the nanocomposites decrease with increase in frequency at all loadings. This decrement is attributed to the delay in the dipole orientation to get align themselves in the direction of the applied electric field. The increase in the dosage of nanoclay gives rise to superior dielectric properties in the nanocomposites. It significantly increases the dielectric constant or dielectric permittivity ε′, dielectric loss ε′′, and dissipation factor of the nanocomposites. The addition of nanoclay platelets further increases the net polarization of the nanocomposites. The OMMT clay due to its layered TOT structure contains mobile ions on the platelet surface and exchangeable counter-ions between them. The application of electric field ionically polarizes the silicate platelets due to the conductivity mismatch with the rubber matrix. In addition to these polarizations, which are very effective at low frequency band, the interfacial polarization at BIIR–CO–CB–NC interfaces greatly contributes to the increase in dielectric permittivity. The difference in the dielectric permittivity of the rubbers and fillers must increase Maxwell–Wagner–Sillar interfacial polarization. However, this has less effect due to the surface functionalization of the nanoclay. Organically modified nanoclay interacts with the rubber matrix and restricts the molecular mobility of the rubber chain in the nanocomposites to a significant extent. This restriction will reduce the effective permittivity of the nanocomposites. It can be inferred that the large increase of both ε′ and ε′′ (Fig. 7a and b) is greatly related to the formation of hybrid nanostructures between the rubber chains at percolation threshold. The effect of hybrid nanostructure on the mechanism of polarization can be explained schematically (Scheme 3a and b). Depending on the interaction between the rubber and the nanoclay, an interfacial rubber nanolayer tends to form on the nanoclay surface which is highly immobile due to the strong bonding of the rubber chains and the filler surface.²⁴ When these immobile nanolayer formations are integrated to all the nano fillers in a polymer matrix, it can be anticipated that the mobility of the polymer segments or chains interacting with these nanoparticles in the nanocomposite are restricted. However, the formation of hybrid nanostructure minimizes the nanolayer due to the haloing/shielding effect of carbon black around the nanoclay. The shielding of the nanolayer by hybrid nanostructure makes them available to get polarized in response to the applied electric field. The increase in the dissipation factor (Fig. 7c) is attributed to the dissipation of electrical energy for the interfacial polarization and dipole orientation.²⁵ Fig. 7d illustrates a comparison between dielectric constant and dielectric loss for the samples at higher frequency (1 MHz). It is found that the percolation threshold is obtained for B₉₀H₁₀CB₅₀NC₅ after which the property shoots off. Interestingly, B₉₀H₁₀CB₅₀NC₃ and B₉₀H₁₀CB₅₀NC₅ display similar trend in the properties. The orientations of dipoles are effective to respond at longer time or at lower frequency of the applied field. The low frequency responses relate well with the electrical conductivity of the composites.²⁶ Fig. 8a depicts the electrical conductivity of the nanocomposites at low frequency (50 Hz–1000 Hz). It is observed that increase in the dosage of nanoclay increases the electrical conductivity of the matrix (brown dotted circle). It corresponds to the increase in the conductive paths due to the high conductivity of the polar layered silicates.²⁷ Electrical conductivity at higher frequencies (618.9 kHz and 1 MHz) was also studied and it is shown in Fig. 8b. The electrical conductivity of B₉₀H₁₀CB₅₀NC₃ is comparable with the base compound B₉₀H₁₀CB₅₀NC₀, increases for B₉₀H₁₀CB₅₀NC₅ and significantly drops beyond that. The increase in the electrical conductivity signifies the formation of the well dispersed hybrid nanostructure in the nanocomposites. This correlates well with increase in the dielectric responses of the composites.

Fig. 6 Different types of molecular polarizations at various frequencies.²²

Fig. 7 (a) Dielectric constant and (b) dielectric loss plotted versus frequency (c) dissipation factor (d) a comparison between dielectric constant and dielectric loss at a frequency of 1 MHz for the nanocomposites.

Scheme 3 (a) The formation of interface rubber nanolayer in the nanocomposite and (b) the haloing/shielding effect of carbon black around nanoclay increase the polarization responses of hybrid filled nanocomposite to the applied electric field.

Fig. 8 (a) Electrical conductivity of the nanocomposites at low frequency range, (b) electrical conductivity of the nanocomposites at higher frequencies.

4.6. Thermal conductivity

Thermal conductivity of a material is defined as the ability to conduct heat. Thermal conductivity is required for such materials which are subjected to dynamic applications in order to dissipate the heat. For non-metallic materials the heat gets propagated by phonon carriers. The thermal conductivity is measured by calculating the heat flux developed through the rubber matrix due to the thermal concentration gradient. In general, rubber is a poor conductor of heat or has very less thermal conductivity due to the phonon scattering effect.^28–30 Addition of organically modified nanoclay reduces the phonon scattering by minimizing the interfacial thermal resistance and thereby makes the rubber composite thermally conductive.³¹ Nanoclay has higher thermal conductivity than the rubber and transmits the heat faster. Fig. 9 illustrates the thermal conductivity of the nanocomposites at different dosage of nanoclay. It is noted that thermal conductivity is higher for B₉₀H₁₀CB₅₀NC₅. It reflects good interaction between filler to rubber matrix, reducing the thermal expansion mismatch. The formation of well dispersed hybrid nanostructure is pivotal in increasing the thermal conductivity of the nanocomposites. The functioning of the hybrid nanostructure in effectively increasing the thermal conductivity is explained in the Schemes 4 and 5. Scheme 4 explains the possible phenomenon of the thermal conductivity of the base compound B₉₀H₁₀CB₅₀NC₀. When the sample is heated, heat is transmitted in the form of thermal vibrations or phonons. The phonon acting as an elastic wave moves with a mean free path (λ) before it gets scattered due to the bulky side chains of the amorphous BIIR rubber. The scattering effect is attributed to the irregularity in the lattice of atoms located at the rubber interface through which they move. Due to the low free path of the phonon and frequent phonon scattering effect at the rubber interface, the net thermal conductivity of B₉₀H₁₀CB₅₀NC₀ is less. Scheme 5 explains the effect of hybrid nanostructure in enhancing the thermal conductivity of the nanocomposites. The dispersed hybrid nanostructures bridges between the BIIR–CO interface and acts as an extended path for the phonons to travel. The extended mean free path of phonons have now become to (λ + x) and (λ + y) from λ before it gets scattered. The increase in the mean free path corresponds to the increase in the thermal conductivity of the nanocomposites. However, relative decrease in the thermal conductivity of B₉₀H₁₀CB₅₀NC₁₀ with increase in the temperature corresponds to the increase in the number of structural defects in the nanocomposites at higher temperatures.³²

Fig. 9 Thermal conductivity as a function of temperature for the nanocomposites.

Scheme 4 Phonon scattering effect in BIIR–CO composite (B₉₀H₁₀CB₅₀NC₀).

Scheme 5 Increase in the mean free path due to the formation of hybrid nanostructure in BIIR–CO nanocomposites.

4.7. Adhesion strength

Adhesion between the components is a basic requisite of any composite product when it is subjected to dynamic applications. The effect of nanoclay on the adhesion of the nanocomposites with neighboring fabric compound were analyzed and shown in Fig. 10. The addition of nanoclay has minimal effect on the adhesion properties of the nanocomposites. However, agglomeration of clay deteriorates the adhesion strength of B₉₀H₁₀CB₅₀NC₁₀ considerably. The reduction in adhesion strength is attributed due to the reduction in the chain mobility. At lower dosage of the nanoclay, the effect of reinforcement predominates over the effect of decrease in the chain mobility. However, at higher dosage of the nanoclay, the effect of decreased chain mobility predominates over this level.³³ An increase in the elastic stiffness or Young's modulus also reduces the peel strength.³⁴

Fig. 10 Peel strength between the reference rubber ply fabric and nanocomposites. Inset: increase in the rubber plateau modulus of the nanocomposites decreases adhesion.

The reduction in the adhesion strength can be better explained by understanding the viscoelasticity and chain diffusion mechanism.

4.7.1. Entanglement molecular weight (M_e) and interfacial diffusion. The effect of the nanoclay in the plateau region is analyzed by determining the entanglement molecular weight (M_e).³⁵ The entanglement molecular weight can be related to the rubber plateau modulus using the following equation:

M_e = ρRT/E⁰ (2)

where ρ is the density of the rubber or blend, R is 8.31 × 10⁷ dyn cm/mol K, T is the absolute temperature where E⁰ is located, and E⁰ is calculated from the storage modulus (E′) at the onset of the rubber plateau region [see inset Fig. 10]. From Table 4, M_e value decreases with increase in the dosage of the nanoclay. B₉₀H₁₀CB₅₀NC₁₀ has the lowest value. This confirms that the higher rubber plateau modulus exerted on the rubber chain restricts its segmental mobility. The addition of nanoclay reduces the mobility of the rubber chains to get effectively diffuse in the interface between nanocomposite and fabric compound. The reduction in the adhesion strength is also due to polarity differences at the interface. The ionic nature of the nanoclay increases the net polarity of BIIR–CO rubber nanocomposites. The interfacial bonding is weak due to the polarity differences between BIIR–CO nanocomposites and NR ply compound. The increase in the green strength or tensile stress (Fig. 5) reduces the contact area and decreases the bond strength. Adhesion or interfacial bonding is therefore dependent on the chain mobility and molecular diffusion across the interface. The significant reduction in the green elongation break, tan delta and increase in rubber plateau modulus at higher dosage of nanoclay reiterates the same.

Table 4 Entanglement molecular weight of the nanocomposites

Compound	Entanglement molecular weight, Me, (g mol^−1)
B90H10CB50NC0	5714
B90H10CB50NC3	4892
B90H10CB50NC5	4143
B90H10CB50NC10	3491

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4.7.2. tan delta and Payne effect. Viscoelastic behavior of the nanocomposites was analyzed using DMA. tan delta and Payne effect of the rubber blend nanocomposites were measured. The addition of nanoclay lowers the tan delta and broadens the tan delta curve without shifting the T_g values (Fig. 11a). Interestingly, it is noted that the plateau modulus of B₉₀H₁₀CB₅₀NC₃ is lesser than that of the B₉₀H₁₀CB₅₀NC₀. This attributes to the formation of hybrid nanostructures which bridges the rubber chains without affecting the chain mobility. Thus the formation of hybrid nanostructure effectively counterbalances by maintaining the adhesion strength. However, higher dosage of nanoclay in B₉₀H₁₀CB₅₀NC₁₀ causes significant increase in the plateau modulus. This signifies that, at higher dosage, the breakdown of hybrid nanostructures is more effective than the formation of hybrid nanostructure. In other words, the formation of agglomerates predominates over the nanostructure development. Further, the nature of filler–filler interaction was analyzed by determining the Payne effect using strain sweep measurements (Fig. 11b). The rate of decrease of the storage modulus (E′) of the nanocomposites was determined in the strain sweep test. It is noted that the B₉₀H₁₀CB₅₀NC₃ exhibits lower Payne effect as compared to that of B₉₀H₁₀CB₅₀NC₀. The higher dosage of nanoclay leads to the structural breakdown of hybrid nanofillers. This is due to the agglomeration of the nanoclay at higher dosage. This again confirms that the formation of hybrid nanostructure is very effective in B₉₀H₁₀CB₅₀NC₃ and reaches percolation threshold in B₉₀H₁₀CB₅₀NC₅. Importantly, this balances the adhesion properties up to the percolation threshold. Higher dosage of nanoclay deteriorates adhesion due to the formation of more agglomerated structures.

Fig. 11 Viscoelastic properties of nanocomposites (a) tan delta (b) Payne effect.

5. Conclusions

The influence of organically modified platelet-type nanoclay and its nanostructure on the functional properties of BIIR–CO nanocomposites were thoroughly investigated. TEM photomicrographs of the nanocomposites reveal good dispersion of nanoclay and formation of hybrid nanostructures. Capillary rheological behavior of the nanocomposites exhibits lower die swell without affecting the surface gloss of the extrudate. SEM analysis of the die extrudate provides an insight into the good interaction between rubber and the filler in the nanocomposites. The development of hybrid nanostructure substantially contributes to the air barrier, thermal, and electrical conductivity of the rubber nanocomposites by increasing the diffusion path of the permeant, mean free path of the conduction phonon, and interfacial polarization respectively. The adhesion strength balances on the percolation threshold of the nanostructure. In a nutshell, the drastic improvement in the functional properties invokes the possibility of application of these materials in new generation tire inner liners, highly impermeable membranes and durable bladders. This is due to the formation of hybrid nanostructures those leads to unique viscoelastic attributes leading to increased overall entanglement density of the system.

Acknowledgements

The authors acknowledge gratefully the management of CEAT LIMITED, Vadodara for funding this work. Special thanks to Mr C. Cable (Zeon Chemicals, USA) for supplying free samples of CO rubber and Zisnet F-PT chemicals in this work. We also thank Mr M. Praveen Kumar, Research Scholar (Chemical Engineering Department, IIT Kharagpur, India) for his contributions in the thermal conductivity measurements.

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