Ion conduction through paper and polypropylene foils impregnated with liquid dielectric

Abstract
In order to improve our knowledge of the impregnated materials, we have focused our work on polypropylene and capacitor grade papers. In addition to electrical measurements (permittivity, conductivity versus voltage, time, temperature), we made a study of the porous structure of the materials using electron microscopy, the bubble breakthrough method, permeation of gas and liquid under a hydrostatic pressure and mercury porosimetry. The adsorption of electrolytes, the diffusion of ions without voltage through polymer films, and the electrophoresis of microparticles of polymers were also studied. We conclude that paper and swollen polypropylene are porous materials; but in paper the pores are very tortuous and the liquid mainly spreads over the surface or into voids parallel to the surface; in polypropylene the pores seem to be numerous and slightly tortuous. Thus, impregnated paper mainly behaves like a series association of pure cellulose and free liquid, whereas polypropylene has a large parallel component. Another striking difference is that paper is a semipermeable material, able to block ions at its surface (according to the nature of the liquid and of the ions): this physico-chemical interpretation is preferable to the mechanical model proposed by Garton. Polypropylene is not (or but very feebly) selective, and ions of both signs can cross it, but their apparent DC mobility is 10−4to 10−5 smaller than in the free liquid: so with AC, ions of both signs are trapped producing the ‘field-enhanced adsorption’ effect described by Tobazeon and Gartner.

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