Dielectric Constant, Power Factor and Structure

The materials in Table 6.1 may be divided roughly into two groups  

Polymer Volume resistivity (fl m) Dielectric strength (kV/cm) (1 in sample) Dielectric constant Power factor  

It is not difficult to relate the differences between these two groups to molecular structure. In order to do this the structure and electrical properties of atoms, symmetrical molecules, simple polar molecules and polymeric polar molecules will be considered in turn. 

The influence of a particular dielectric on the capacitance of a condenser is conveniently assessed by the dielectric constant, also known as the relative permittivity or rarely specific inductive capacity. This is defined as the ratio of the relative condenser capacity, using the given material as a dielectric, to the capacity of the same condenser, without dielectric, in a vacuum (or for all practical intents and purposes, air). 

In the case of symmetrical molecules such as carbon tetrachloride, benzene, polyethylene and polyisobutylene the only polarisation effect is electronic and such materials have low dielectric constants. Since electronic polarisation may be assumed to be instantaneous, the influence of frequency and temperature will be very small. Furthermore, since the charge displacement is able to remain in phase with the alternating field there are negligible power losses. 

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Dielectric Constant, Power Factor and Structure 111 Table 6.1 Typical electrical properties of some selected plastics materials at 20°C... 

The electrical properties of materials are important for many of the higher technology applications. Measurements can be made using AC and/or DC. The electrical properties are dependent on voltage and frequency. Important electrical properties include dielectric loss, loss factor, dielectric constant, conductivity, relaxation time, induced dipole moment, electrical resistance, power loss, dissipation factor, and electrical breakdown. Electrical properties are related to polymer structure. Most organic polymers are nonconductors, but some are conductors. 

The electrical properties of polymers have been widely studied and the results subjected to a number of excellent reviews (e.g. Seanor (1972), Link (1972) and Parker (1972)). The writer has discussed, in qualitative terms, the effect of molecular structure on some electrical properties such as dielectric constant and power factor (Brydson, 1975). The rubbers show no particular distinctive features in respect of electrical properties over other polymers so this topic will be dealt with very briefly here. Perhaps the main point to be made is that the electrical properties of rubber compounds are often more dependent on the additives than on the rubbery polymer itself. 

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