Effect of inhomogeneities in epoxy-glass-mica composites on conductive channel formation

Epoxy-glass-mica composites consisting of an epoxy matrix, glass fibers, and a mica paper are commonly used in insulation systems where high dielectric strength is expected. Such insulating systems are typically made from partially cured and flexible prepregs that are layered and shaped before final curing. Due to the extensive range of epoxy resins and various types of glass fibers and mica paper, the material parameters, including dielectric strength, can vary significantly between prepregs produced by various manufacturers. However, in addition to the composition, inhomogeneities formed due to the prepreg layering and curing, such as air microbubbles presented in the prepregs before curing or trapped between the prepregs during curing, must also be considered. These influences can significantly affect the resulting dielectric strength and the mechanism of formation and propagation of conductive channels in a structure prepared from several layers of prepregs. This study analyzes the mechanism leading to the breakdown of two types of epoxy-glass-mica prepregs with a very similar composition but supplied by different manufacturers. The different geometry and distribution of glass fibers and the structure of the mica paper were macroscopically visible at a glance for tested materials. The main monitored material parameter was the dielectric strength, and the specimens after breakdown were evaluated by imaging methods. The first composite with the lower dielectric strength was characterized by a dominant conductive channel and minimal branching. The second composite with the higher dielectric strength (average value higher by about 50% than the first one) showed significant branching in a larger specimen’s area.