Delamination mechanism of high-voltage coil insulators made from mice flakes and thermosetting epoxy resin

To clarify the delamination mechanism of high-voltage coil insulators made from mica flakes and epoxy resin due to static mechanical stress, the relat ionships between the shear strength of the insulator and the physical prope rties of the component materials were studied. The mechanism of their delam ination was thought to be either a lack of epoxy resin between the mica fla kes, interface failure between the mica flakes and the epoxy resin, or clea vage of the mica flakes. The first two mechanisms were discounted because t he shear strength of the insulator was found to be independent of both the contact angle of the corresponding liquid epoxy resin on the mica flakes an d the critical surface tension of the epoxy resin. Furthermore, the shear s trength of the model insulator was improved by using an epoxy resin with a higher bending elastic modulus, implying that the delamination mechanism in this system is the cleavage of mica flakes. Therefore, the epoxy resin sho uld have a high elastic modulus to ensure high delamination resistance, tha t is, the temperature to which the insulators are exposed should be lower t han the glass transition temperature of the corresponding epoxy resin. Opti cal microscope studies also supported these results. (C) 2001 John Wiley & Sons, Inc.