Characterization of water absorbed epoxy insulating coating material used in ZnO varistors by dielectric measurements

The water (moisture) absorption behavior of epoxy insulating coating material used in ZnO varistors is characterized by dielectric measurements. It is observed that the water absorption process in the epoxy insulating layer matches with Fick's second law in the initial stage, and then the water absorption approaches to a saturated value. The hardener plays an important role in determining saturated value of the water absorption process of the coating material. Further it is noticed that under the same experimental conditions the relative permittivity ɛr, and the dissipation factor (tan δ) increase in the frequency range 30 Hz ≤ f ≤ 30 kHz for dry and wet (water treated) samples. The increasing rate of dissipation factor of the samples cured with the anhydride hardener is higher in the low frequency range than it is in the high frequency range. It is suggested that an ionic conductance caused by water/moisture inside the samples that may have led to the obtained experimental results.