Effect of film structure and morphology on the dielectric breakdown characteristics of cast and biaxially oriented polypropylene films

Effects of cast film extrusion and biaxial orientation on morphological development and DC dielectric breakdown performance of non-oriented and biaxially oriented polypropylene (BOPP) films were studied. Cast films, based on two capacitor-grade isotactic polypropylene (iPP) homopolymers, were manufactured using three different extruders under different crystallization conditions and subsequently biaxially oriented using a laboratory stretching machine. Analysis of polymorphic composition and microstructural features was carried out in conjunction with large-area multi-breakdown performance assessment under progressive DC voltage ramp conditions. Polymorphic α/β-form crystalline composition and spherulitic/trans-crystalline morphology formed during cast film extrusion were found to directly affect the biaxially oriented film morphology and breakdown characteristics. For the BOPP films, decrease in dielectric strength was traced back to β→α crystal transformation and subsequent microvoid/porosity formation upon biaxial stretching. Topographical features and increasing surface roughness were also found to correlate with lower BOPP film dielectric strength. Dependence of breakdown strength on the biaxial stretch ratio was demonstrated, with higher biaxial stretch ratio resulting in improved BOPP film breakdown performance. As an extreme case, the biaxially oriented films were compared against non-oriented cast films in similar thickness range, highlighting the essential role of biaxial orientation in the high-dielectric-performance of thin films in capacitor applications. Control of polymer design as well as optimization of film processing and morphological development was found to be necessary to avoid formation of structural defects and loss of electrical insulation integrity.