Electrical Properties of Elastomers and Related Polymers

Interest in the electrical behavior of elastomers stems from several widely different sources. From the theoretical standpoint electrical measurements provide a valuable tool for the study of the molecular structure of elastomers and other polymers and the relation of structure to properties. From a practical point of view an understanding of the electrical behavior enables the manufacturer of wire and cable to produce insulation that will better withstand the severe conditions of space flight, or that will meet price competition and show a profit. The present day applications of elastomers are so many and varied that nearly any type of compound is likely to be employed for some practical purpose. A cable for x-ray equipment, for example, may be made wholly from elastomers with conductor, insulation, and jacket each from a different compound. At one time when almost the only electrical use of rubber was to provide the highest practical degree of electrical insulation it was correct to speak of “good” and “poor” electrical properties. Now, however, an elastomer that is a poor insulator may be excellent in an antistatic application. Communication cables require an insulation of the lowest practical dielectric constant, but for power cables a layer of insulation of high dielectric constant next to the conductor may be essential to prevent excessive electrical stresses. Modern technology not only calls for a wide diversity of electrical properties but it often requires further that elastomers having these properties be available in a wide range of mechanical properties. For example, the insulation on a cable for use in an airplane must be as thin and light as possible to save weight while an unarmored cable for laying in shallow water must have insulation that is thick and tough for mechanical protection and of high specific gravity to prevent the cable's being moved by waves or tide. Thus, the diversity of present and possible future applications is such that no one in the industry is likely to escape for long some contact with an application involving an electrical property. Accordingly, this review has been prepared to acquaint the rubber chemist and technologist with current information in the field. In the 25 years that have elapsed since an earlier review was prepared by the same author a great deal of work has been done on the relation between the properties of polymers and their molecular composition and structure. It is now possible to predict the properties of some polymers from their structural formulas, and a beginning has been made in relating the properties of simple elastomeric compounds to the properties of the different ingredients. However, knowledge in the field is still far from the state at which it would be possible to compile a table of functions such that the electrical properties of a multi-ingredient insulating compound could be computed from the properties of the individual ingredients.