Dielectric and microwave properties of carbon nanotubes/carbon black filled natural rubber composites O. A. Al-Hartomy1,2, A. A. Al-Ghamdi1, F. Al-Salamy3, N. Dishovsky*4, R. Shtarkova5, V. Iliev6 and F. El-Tantawy7 Natural rubber (NR) based nanocomposites containing a constant amount (50 phr) of standard furnace carbon black and carbon nanotube (CNT) at a concentration from 1 to 5 phr have been prepared. Their dielectric (dielectric permittivity and dielectric loss) and microwave properties (coefficients of absorption and reflection of the electromagnetic waves and electromagnetic interference shielding effectiveness) have been investigated in the 1–12 GHz frequency range. The results achieved allow recommending CNTs as second filler for NR based composites to afford specific absorbing properties. Keywords: Natural rubber composites, Carbon nanotubes, Dielectric and microwave properties
Introduction Since the documented discovery of carbon nanotubes (CNTs) in 1991 by Iijima1 and the realisation of their unique2 physical properties, including mechanical, thermal and electrical, many investigators have endeavoured to produce advanced CNT composite materials that exhibit one or more of these properties. For example, CNT are quite effective as a conductive filler of polymers, compared to traditional carbon black microparticles, primarily due to their high aspect ratios. Recently, the electrical percolation threshold has been reported to be at 0?0025 wt-% of CNT and conductivity at 2 S m21 at 1?0 wt-% of CNT in epoxy matrixes.3 Owing to their fibrous shape with extremely large aspect ratio, CNT may, at a very low concentration, yield composites of low resistivity, high permittivity and frequency dispersion.4 It is well known that the nanosized particles usually exhibit properties different from those of microsized particles of the same composition, which is the primary reason for the great attention currently paid to the radio and microwave frequency performance of CNT composites. A number of novel CNT features have been reported in the literature.5–11 These results demonstrate
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Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Department of Physics, Faculty of Science, University of Tabuk, Tabuk 71491, Saudi Arabia 3 Department of Mathematics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia 4 Department of Polymer Engineering, University of Chemical Technology and Metallurgy, 8 Kl. Ohridski Blvd., Sofia 1756, Bulgaria 5 Department of Chemistry, Technical University, 8 Kl. Ohridski Blvd., Sofia 1000, Bulgaria 6 College of Telecommunications and Posts, Sofia, Bulgaria 7 Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt 2
*Corresponding author, email
[email protected]
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ß Institute of Materials, Minerals and Mining 2012 Published by Maney on behalf of the Institute Received 24 June 2012; accepted 12 July 2012 DOI 10.1179/1743289812Y.0000000032
the possibility to design CNT composites with electric/ dielectric properties, which are more diverse than those obtainable with other carbon fillers. There are numerous investigations on nanocomposites based on elastomeric matrixes and CNTs as filler, although the researchers’ attention has been directed mainly to the reinforcement of polymer matrixes. The influence of this unique filler upon the dielectric and microwave properties of the elastomeric composites has been scarcely studied. Lately, there have been articles suggesting possible applications of such nanocomposites in microwave absorbers for solving problems of electromagnetic interference (EMI) and electromagnetic compatibility.4,12–20 The polymer matrixes used in these cases are usually epoxy resin, acrylonitrile–butadiene rubber, styrene–butadiene rubber, silicone rubber and polyurethane rubber. Only in the last years have appeared reports on the investigations on natural rubber (NR) based nanocomposites filled with CNTs.21–24 The price of CNTs is still significantly higher than that of standard furnace carbon black. In this context, the aim of this study is to determine whether the addition of small quantities (1–5 phr) of CNT in addition to a standard significantly greater than the amount of active furnace carbon black (50 phr) can be used as a way to modify and improve the dielectric (dielectric permittivity, dielectric loss) and microwave properties [coefficient of reflection, coefficient of attenuation and EMI shielding effectiveness (SE)] of NR based composites in the high frequency range (1–12 GHz). Data for such a study on the reported combination of fillers have not been found in the literature.
Experimental Characterisation of carbon nanofillers used Multiwalled CNTs as produced by Hayzen Engineering Co. (Ankara, Turkey) were used in our investigation.
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1 Images (TEM) of CNT in transmission regime and selected area electron diffraction regime (insert in the right corner)
The material purity was higher than 95% (density, 2150 kg m23). Carbon nanotubes were with average diameter of about 15 nm and length of 1–10 mm (Fig. 1). The pattern taken in an electron diffraction regime (insert in the right corner) shows the typical polycrystal structure of CNT. The specific structural features of the carbon black used (Corax N 220, produced by Evonik) are shown in Fig. 2. As seen, the primary particle size of the carbon black used is ,20 nm, but their capability of forming secondary aggregates and agglomerates is considerable. SAED indicates the absence of crystal structures. Natural rubber (SMR 10) was used as a rubber matrix in our investigations. Other ingredients such as zinc oxide (ZnO), stearic acid (SA), N-(1,3-dimethylbuthyl)-N9phenyl-p-phenylenediamine (Vulkanox 4020, produced by Lanxess), dibenzothiazyl disulphide, MBTS (Vulkacit DM, produced by Lanxess) and sulphur were of commercial grades and used without further purification.
Preparation and vulcanisation of rubber compounds Table 1 summarises the formulation characteristics of the rubber compounds (in phr) used for the investigations. Table 1 Composition of rubber compounds studied
Natural rubber Foaming agent Stearic acid ZnO Processing oil Carbon black N220 CNT MBTS* TMTD{ IPPD 4020{ Sulphur
NR 1
NR 2
NR 3
NR 4
100 8 1 4 10 50 0 2 1 1 2
100 8 1 4 10 50 1 2 1 1 2
100 8 1 4 10 50 3 2 1 1 2
100 8 1 4 10 50 5 2 1 1 2
*Mercapto benzothiazole sulphonamide. {Tetramethyl thiuram disulphide. {Dimethylbutyl-phenyl-p-phenylendiamine.
Carbon nanotubes/carbon black filled natural rubber composites
2 Images (TEM) of furnace carbon black Corax N220 particles together with SAED (insert in the right corner)
The rubber compounds were prepared on an open two-roll laboratory mill (L/D 3206360 and friction 1?27) by incorporating precharacterised CB and CNTs into an NR matrix at various loadings (Table 1). The speed of the slow roll was 25 min21. The experiments were repeated for verifying the statistical significance. The ready compounds in the form of sheets stayed 24 h before their vulcanisation. The optimal vulcanisation time was determined by the vulcanisation isotherms, taken on an oscillating disc vulcameter MDR 2000 (Alpha Technologies) at 150uC according to ISO 3417:2002. These composites were evaluated for their dielectric (dielectric permittivity, dielectric loss angle tangent) and microwave (reflection coefficient, attenuation coefficient and SE) properties in the 1–12 GHz frequency range.
Measurements Microwave properties
Reflection and attenuation Measurements of reflection and attenuation were carried out using the measurement of output (adopted) power Pa in the output of a measuring line without losses, where samples of materials may be included. Because of the wide frequency measurement, a coaxial line was used. Samples of the materials were shaped into discs with an external diameter D520?6 mm, equal to the outer diameter of the coaxial line and thickness of D
