OVERVIEW

Rubbers are usually electrically insulating and this property is widely exploited in cables and various components in electrical appliances. They can also be made anti-static and even conducting by the addition of suitable carbon blacks. In all cases, it is the combination of the electrical properties and the inherent flexibility of rubbers which makes them attractive for particular applications.

In principle, the measurement of the electrical properties of rubber is the same as for any other material yielding results of the same order of magnitude, but particular precautions have to be taken because of distortion of this relatively low stiffness material by applied electrodes, and the very high contact resistances which may exist between the electrodes and the rubber surface. In addition, the properties are often very sensitive to the past history of the rubber, including mechanical stress, which can lead to a large degree of variability. It is not intended to dwell here on the aspects of electrical measurement which are common to all materials, but to emphasise the aspects which are more particular to rubbers. Electrical testing is a speciahsed subject and, as much of the apparatus used is complex, the work is normally undertaken by those having suitable training and expertise, rather than in a typical rubber laboratory.

The tests most likely to be needed for rubbers can be classified as:

• Resistance or resistivity

• Surface charge

• Electric strength

• Tracking resistance

• Power factor and permittivity

Resistance or resistivity is by far the property most often measured. For...