The dielectric properties of cellulose acetate

Abstract

The possibilities of cellulose acetate as a dielectric have received attention recently from several investigators, at home and abroad. This material has been under consideration by the Electrical Research Association for the past two years, not only from the point of view of its industrial applications, but as a medium for the investigation of fundamental phenomena. The policy of the Electrical Research Association in regard to protracted researches of this nature is to release as much information as possible in the form of interim reports, rather than waiting until the final conclusions are available. The report which follows is one of a series on dielectric phenomena which, for convenience, are related to materials or groups of materials widely used in industry. Other reports in this series relate to ebonite, paper, and varnished cloth. The dielectric properties of cellulose acetate have been studied over the temperature range 25° C. to 90° C, using alternating voltages at frequencies covering the range 50 to 4000 cycles per second, and also direct voltage. Preliminary tests showed that the power factor of commercial cellulose acetate may vary from 2 percent to about 15 percent. The detailed investigation was confined to samples of low power factor. The material was found to absorb water from the atmosphere, and its power factor was reduced by drying. Samples which were dried at a temperature of 90° C. were regarded as completely dry, and such samples were used for most of the measurements. Other samples dried at 25° C. possessed a slightly higher power factor and therefore presumably contained traces of water. Measurements were made also on these samples in order to find out the effect of the water. Measurements of permittivity and power factor were made by means of the Schering bridge fitted with a Wagner earth connection. Mercury electrodes were used. It was found that as long as the voltage gradient does not