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Electrical Properties

Filled polymers are used in applications requiring both electrical insulation (e.g., cable coating) and electrical conductivity. Their effects on the electrical properties of composites can thus be very important. The properties of most interest are conductivity and dielectric properties.

Conductivity

Most particulate fillers have low electrical conductivity and this is good where insulation properties of a composite are being exploited. In this case, the most concern is over the presence of trace amounts of water-soluble ionic impurities, which can markedly increase composite conductivity under humid conditions, even if only present in trace amounts. Sodium chloride is a particular example. Tests on the filler, such as conductivity of aqueous extract, are often used for quality control purposes when this is an issue.

At the other end of the scale, there are a range of applications where fillers are used to make a polymer electrically conducting (antistatic and EMF shielding are examples). Carbon blacks are the predominant particulate (as opposed to fibrous) fillers used here. Others include metals, graphite, and doped zinc and tin oxides.

Unlike most other properties, electrical conductivity does not vary smoothly with increased filler content. Conductivity is dominated by percolation meaning that adding conductive fillers has little effect until enough is present to form a continuous particle pathway through the material. This concentration is known as the percolation threshold and is accompanied by a dramatic increase in the electrical conductivity of the composite. This is illustrated in Fig. 9. For mechanical properties, good dispersion is usually the goal, but for electrical conductivity, it can be counterproductive (indeed, electrical conductivity is used as a measure of dispersion quality in carbon black-filled elastomers, good dispersion being indicated by low conductivity). Perfectly separated particles cannot percolate so conductivity remains low. Instead, the goal is to have a controlled amount of agglomeration and to be able to keep it exactly the same from lot to lot. Particle size and shape have a big effect on the volume fraction at which the percolation starts, with smaller fillers and more anisotropic particles leading to lower percolation thresholds.

Percolation of electrically conductive particles

Fig. 9 Percolation of electrically conductive particles

 
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