Elsevier

Polymer

Volume 49, Issue 24, 10 November 2008, Pages 5276-5283
Polymer

Modified and unmodified multiwalled carbon nanotubes in high performance solution-styrene–butadiene and butadiene rubber blends

https://doi.org/10.1016/j.polymer.2008.09.031Get rights and content

Abstract

The outstanding properties of carbon nanotubes have generated scientific and technical interests in the development of nanotube-reinforced polymer composites. Therefore, we investigated a novel mixing approach for achieving a good dispersion of multiwalled carbon nanotubes (CNTs) in a rubber blend. In this approach the CNTs were incorporated into a 50:50 blend of solution-styrene–butadiene rubber and butadiene rubber. First, the CNTs were predispersed in ethanol and then this CNT–alcohol suspension was mixed with the rubber blend at elevated temperature. The rubber nanocomposites prepared by such method exhibit significantly enhanced physical properties already at very low nanotube concentrations. Additionally, we have analysed the dielectric and thermal properties of the compound. The high aspect ratio of the carbon nanotubes enabled the formation of a conductive percolating network in these composites at concentrations below 2 wt.%. In contrast to the electrical conduction behaviour, the thermal conductivity of the composites has not been influenced significantly by the presence of carbon nanotubes. Dynamic mechanical analysis indicates that the incorporation of CNTs affects the glass transition behaviour by reducing the height of the tan δ peak considerably. Above the glass transition temperature the storage modulus has been increased after incorporation of a small amount of CNTs. Finally, the ‘Payne effect’, an indication of filler–filler interactions, was observed at very low concentrations of CNT in the rubber matrix.

Introduction

The length of a carbon nanotube is from a few microns up to millimetres, with a diameter of the order of nanometers. A single nanotube is hundred times stronger and six times lighter than steel and exhibits good electrical and thermal conductivities. Hence, the future of carbon nanotubes application can be easily envisaged from electrical sensors to reinforcing fillers, especially in the vast world of high performance materials. Tremendous attention is now paid from all over the world to the use of carbon nanotubes as electrical conducting and reinforcing fillers for polymers [1], [2]. However, it is very difficult to disperse CNTs in polymers because they produce highly entangled agglomerates like felted threads. Owing to the strong van der Waals interaction and the inert graphite-like surface carbon nanotubes are very prone to form strong agglomerates.

Comparing with the enormous number of studies on the application of CNTs in epoxides, thermoplastics and fibres, there are rather few reports dealing with applications of CNTs in elastomers [3], [4], [5], [6], [7], [8]. Till now, it is a challenging task to disperse carbon nanotubes properly, to realise the full potential of this filler in improvement of the mechanical properties of different elastomers. Considerable improvement of physical properties was reported when multiwalled carbon nanotubes (MWCNTs) were incorporated in styrene–butadiene rubber (SBR) [3], [4]. In another work, the reinforcing effect of single walled carbon nanotubes (SWCNTs) in natural rubber was revealed by dynamic mechanical analysis and Raman spectroscopy [5]. Here, a noticeable decrease of the loss tangent (tan δ) peak height, as well as a marked shift of glass transition temperature (Tg) towards higher temperature was observed. Fakhrúl-Razi et al. [6] showed that the initial modulus of a natural rubber (NR) composite was increased up to 12 times in relation to pure NR. Kim et al. [7] evaluated the mechanical, thermal and electromagnetic shielding properties of ethylene propylene diene monomer rubber (EPDM)–MWCNT composites. They concluded that the alignment of tubes in a EPDM rubber matrix, arising from the mill processing, resulted in significant improvements in mechanical, electrical and thermal properties. Wagner et al. [8] reported the improvement of the mechanical properties of MWCNT filled silicone rubber. In their work it was reported that with the increase of the amount of carbon nanotubes a remarkable enhancement of the initial modulus (Young's modulus) was observed, accompanied by a reduction of the ultimate tensile strength. However, at higher strains (≈20%) the modulus was found not to change upon further increase of CNT concentration. Hydrogenated nitrile rubber was also used to prepare nanocomposites with CNT [9]. The authors reported a serious breakage of the CNTs during the ultrasonic dispersion, which resulted in a poor electrical conductivity of such nanocomposites. In order to get thermoplastic elastomer–CNT composites for tribological applications EPDM of a high ethylene content was spray-coated with an aqueous dispersion of the CNTs, which was then dried and melt blended with the thermoplastic elastomer [10]. Synthetic polyisoprene–CNT composites were also prepared by a solution method and the effect of stretching on the electrical properties has been discussed [11].

Thermal conductivity is defined as the quantity of heat that passes through a plate of a particular area and thickness per unit time when its opposite faces differ in temperature by 1°. In metals, the thermal transport follows approximately the pathway of conduction as the electrical conductivity. In this case freely moving valence electrons transfer not only electric current but also heat energy. However, the mechanism of the heat conduction is not governed only by the valence electrons, but also by a phonon mechanism. A phonon is a quantized mode of vibration, occurring in a rigid crystal lattice, such as the atomic lattice of a solid. The thermal conductivity of a multiwalled carbon nanotube is about ten times higher than that of a common metal [12]. Therefore, one can expect that the incorporation of CNT can significantly enhance the thermal conductivity of a polymer composite. Many efforts have been done to see the effect of CNT on the thermal conductivity of a CNT filled polymer. It is reported that the thermal conductivity of CNT–epoxy resin composites increases up to 60–125% depending on the type of loading of CNT in the matrix [13], [14]. Liu et al. [14] reported an enhancement of 65% in thermal conductivity with 4 wt.% CNT loading in a silicone elastomer. In order to understand the thermal behaviour of CNT–epoxy composites, Gojny et al. [15] measured the thermal conductivity of composites of various types of CNTs in an epoxy matrix. They claim that in contrast to the improvement of the mechanical properties of the epoxy resin by CNT, where the big surface area of the CNT and good interfacial adhesion is necessary, a contradictive requirement is essential for an enhancement of thermal properties. The occurrence of an electrical conductivity can be attributed to the formation of conductive pathways when the filler content exceeds a critical volume fraction. But the thermal conductivity on the base of phonon transport depends on different pathways from one carbon tube to another one.

CNTs have also a graphite-like inert surface. Therefore, several approaches [16], [17], [18] have been developed to generate some chemical functional groups on the surface of the tubes in order to get more filler–polymer interactions in the polymer composites. Very recently it has also been demonstrated that carboxyl modification of CNTs offered a highly reinforced and conducting natural rubber compound made by a latex coagulation method [19].

The present study reports about the preparation of MWCNT containing rubber nanocomposites with a very low loading of carbon nanotubes by a novel technique. In this technique predispersed ethanol mixtures of carbon nanotubes were mixed with rubber blends at elevated temperature using a two-roll mixing mill. In the mixing mill the ethanol vaporises and, simultaneously, the CNTs can be impregnated into the rubber matrix. The rubber matrix, considered here, is a 50/50 blend of solution-styrene–butadiene rubber (S-SBR) and polybutadiene rubber (BR). Similar compositions of rubber blends are widely used in a tire industry for high performance silica filled tire treads for passenger car applications (so called ‘green’ tire concept). In addition, in our present study hydroxyl modified carbon nanotubes were used as a functional filler with hydroxyl groups on the tube surface. A standard type silane-coupling agent was used to ensure the chemical linkage between the rubber and the hydroxyl modified carbon nanotubes.

Section snippets

Experimental

Solution-styrene–butadiene rubber (S-SBR, CE 3418-01) and polybutadiene rubber (BR, high cis Nd BR, CB25), were obtained from, Lanxess, Germany. Zinc oxide, stearic acid, diphenyl-guanidine (DPG), n-cyclohexyl-2-benzothiazole-sulfenamide (CBS) and soluble sulphur employed in this study were of industrial grades. Hydroxyl modified multiwalled carbon nanotubes and unmodified tubes were purchased from Nanocyl S.A., Sambreville, Belgium. Details are given in Table 1. The solvent used for the

Morphology and physical properties

The structure of CNTs and the state of their dispersion in the S-SBR–BR blend (mixed in the wet method by ethanolic dispersion) were visualized using transmission electron microscopy. It can be observed from Fig. 2a and b that the diameter of the tube is varying from 6 to 27 nm; whereas the length of the tubes is varying from a few to several hundred nanometers. It is also revealed from Fig. 2a that the carbon nanotubes are forming percolating networks at 5 phr loading. However, with smaller

Conclusions

A huge progress in carbon nanotube applications based on polymer composites can only be realized when a proper dispersion of the entangled agglomerates of as-prepared CNT products will be achieved, without damaging their unique properties. The aim of this study was to obtain a good dispersion of CNTs by debundling or isolation of individual tubes from highly entangled primary agglomerates by a novel process of predispersing the nanotubes in a liquid medium. This method proved to be a promising

Acknowledgements

This work has been supported by the German Federal Ministry of Education and Research (BMBF Grant 03X0002E). We are thankful to Dr. Petra Pötschke, IPF Dresden, for her valuable support.

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