Preparation of Polyvinyl Chloride Plastics for Electrical Measurements

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Abstract
The experimental precautions which must be considered in order to obtain reliable data on the electrical properties of plastic solids are discussed. Edge effects, such as fringing, creepage currents and stray capacitance, are eliminated by the use of a three‐terminal cell. Surface effects, the most important of which is a series air film due to imperfections in the surfaces of the sample and the electrodes, are eliminated by coating the surfaces of the sample with a conducting carbon film, with simultaneous attention to proper centering of the sample with respect to the guard and test electrodes. Dimensional changes in the sample, due to relaxation of elastic strains, must be determined and included in the calculation of volume properties. The fundamental criterion of the reliability of the results is the independence of calculated volume properties on the size and shape of the sample and on the details of mechanical construction of the cell in which the measurements are made. Some preliminary results of an investigation of the system polyvinyl chloride—tricresyl phosphate are given, using the methods outlined above. The properties are dependent on the complete thermal history of a given sample : the d.c. conductance increases with time and temperature of aging; and corresponding changes appear in the dielectric constant and its frequency coefficient, and in the a.c. loss factor. It is shown that the d.c. conductance, as an auxiliary independent variable, summarizes the effects of thermal history.

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