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Institute for Measurement Technology
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Characterisation and enhancement of the conductivity of highly conductive elastomers

Dipl.-Ing. Helmut Wernick

In general, conductive plastics can be separated into intrinsically conductive plastics and nonconductive ones, which were doped with conductive additives. These additives may include metallic particles, nano particles like carbon nano tubes or carbon black. Intrinsically conductive plastics are using conjugated bands for conduction, but the conductivity is very low.

The emphasis of this project is to investigate the behaviour of conductive elastomers filled with conductive carbon black. The base material, natural rubber, is doped with highly conductive carbon black. In order to achieve exceedingly high conductivity within the material, the carbon black used must have special properties regarding the surface porosity and particle size. For example, Ketjenblack EC300J meets these requirements.

In addition, the production process consisting of mixing the materials, dispersing, calendaring and vulcanisation have great influence on the conductivity and material characteristics. Figure 1 shows the conductivity distribution of a thin specimen, which was manufactured in a hot moulding press. Clearly, areas with varying conductivity originating from the altered flow behaviour of the material around the vent holes of the press tool are visible. But for applications like flexible heating pads or sensors the homogeneity of the conductivity distribution of the conductive rubber elements is essential even on large areas.

Figure 1: Inhomogeneous conductivity distribution resulting from the vent holes of the press tool. Figure 2: Anisotropic conductivity introduced through dielectrophoresis.

In the opposite way, using the technique of dielectrophoresis, the conductivity characteristics of the material can be tuned specifically. This can be used to integrate invisible sensors or water marks within the material. Dielectrophoresis uses the fact, that a force is exerted on a dielectric particle when it is subjected to a non-uniform external electric field. The dielectric particles try to align themselves in the direction of streamlines of the electric field and create lines with altered conductivity. Figure 2 shows a specimen, which was altered partly with an electric field perpendicular to the surface area. The conductivity distribution of the specimen was estimated by applying an excitation current in planar direction and measuring the magnetic field and the thermal radiation. Using dielectrophoresis the material gets anisotropic and therefore the scalar conductivity must be substituted by a tensor quantity.

Within the framework of this project, methods and models for analysis and for intentional modifications of the conductivity distribution are developed to improve the commercial usability of the material.