Characterisation of dispersion of carbon black in high density polyethylene using dielectric measurements

The use of dielectric measurements as a means of characterising the dispersion of carbon black in high density polyethylene has been investigated. The dispersion of carbon black is an important parameter to quantify because of its significant effect on the electrical properties of the material. Samples were prepared at 26 and 31% carbon black level by compounding on a twin screw extruder using different designs of screw to produce three degrees of mixing: low, medium, and high. Dielectric properties were measured as a function of temperature in the solid phase and through into the melt phase at various frequencies. For a given level of carbon black, and at a fixed temperature, the frequency sensitivity of the alternating current (AC) resistivity was related to the dispersion of the carbon black. The AC resistivity and its frequency sensitivity decreased as the distance between the carbon black aggregates decreased, i.e. the frequency sensitivity decreased as the level of carbon black increased, as the degree of mixing decreased, and at lower measurement temperatures (especially below the melting point as opposed to above it). The frequency sensitivity of the AC resistivity is explained qualitatively in terms of a resistor-capacitor network model. In the manufacturing environment, the frequency sensitivity of the AC resistivity may be used as a practical measure of the dispersion of carbon black in high density polyethylene. PRC/1633 (C) 2000 IoM Communications Ltd.