Investigation into Layer Number Effect on Breakdown Strength of Multi-Layer Polymer Films

The layer number effect on electric breakdown strength (E-BD) of multi-layer polymer films is investigated using 10-mu m polypropylene (PP) films under a dc condition. The layer number, n, of the films during the test is as large as 120. It is observed that the relation between E-BD and n conforms to a minus power law, i.e., E-BD(n) = E(1 ')n(-a), where the power exponent, a, is 0.27, E-1 ' is a constant. By reviewing the experimental data in references, it is found that the power law holds true for different types of polymers in different test conditions, but the value of a varies from 0.072 to 0.5. The variation of a is explained in perspective of the discontinuous structures within films and those between films. A small value of a means a good purity level of the film, which is due to the decrease of the size of the inter-layer defects. A large value of a means a poor purity level of the films, which is due to the increase of the amount of intra-layer defects. Both factors influence the value of a, leading to the variation of a.