The dielectric properties of some thermoplastics

The paper describes an investigation of the electrical properties of a series of pure thermoplastics, five of them being phenolics prepared by the condensation of phenol, o-, m- and p-cresol, and m-5 xylenol respectively with formaldehyde, and one being derived from benzyl alcohol. It is shown that the dielectric behaviour of all these materials is essentially similar, and that it is characterized by a sharp peak in the curves representing power factor as a function of either temperature or frequency, accompanied by a relatively rapid change of dielectric constant. The changes both of power factor and of dielectric constant are approximately proportional to the concentration of hydroxyl groups in the resin, showing that these properties depend on molecular structure. They are interpreted as the effects of orientation of the molecules under the influence of an applied electric field according to Debye's theory, the maximum power factor occurring when the period of the applied voltage bears a definite relation to the relaxation time of the molecules, which is determined by their effective radius and the viscosity of the material. The effective radius of the molecules, calculated from observed values of viscosity and relaxation time, is in every case of the same order as the radius of the OH group obtained from x-ray measurements. It is therefore believed that the dielectric behaviour of resins of this type is mainly determined by the rotation of the OH groups in their molecules. The only important differences between the electrical properties of the phenol, cresol and xylenol thermoplastics are those associated with differences of viscosity.