Long-Term Electrical Conduction in Films of Alkyd Resin and Graphite Mixtures

Partially conducting coatings, used to suppress discharge activity in the slot region of high-power turbogenerators, are subjected to long-term electrical, l, thermal and mechanical stresses which may eventually reduce a coating's effectiveness. To study their long-term properties, permittivity and dc conductivity measurements were made on graphite loaded samples with a modified alkyd-resin base over a range of filler content and at elevated temperatures. It was shown that the dielectric response was modified ied by the lamellar shape of graphite particles. A rapid transition from dielectric to high-conductivity behaviour occurred with increasing volume fraction, in accordance with the Scarisbrick-Kusy model, and was accompanied by a shift from polymer-type to metal lic-type conduction. Erratic and irreproducible switching from one state to another was also noted. This was attributed to poor interparticle contact. A limiting value of resistivity occurred at a volume fraction of about 0.35 when many graphite chains completely bridged the electrodes. It was also shown that long-term thermal aging was unlikely to deteriorate rate electrical properties through loss of binder, until a coating became mechanically unstable.