Abstract
This paper outlines our recent study on effects of cast film extrusion and biaxial orientation on the morphological development and dielectric performance of biaxially oriented polypropylene (PP) films based on two capacitor-grade isotactic PP (iPP) raw materials. Results on polymorphic composition, melting behavior, microstructure and dielectric properties are reported. Morphological development during film manufacturing is found to have a profound effect on film structure and dielectric characteristics. Formation of structural defects was traced back to beta --> alfa crystal transformation upon biaxial stretching.
Highlights
Oriented isotactic polypropylene (BOPP) film produced by tenter frame process is the current state-ofthe-art dielectric medium in both oil-impregnated filmfoil capacitors and metallized film capacitors
The structural and morphological development during Biaxially oriented isotactic polypropylene (BOPP) film manufacturing is in turn largely dictated by the processing parameters used in cast film manufacturing and biaxial stretching [4]
Surprisingly few systematic studies have been published on the dependence of dielectric properties on the base PP properties, film processing parameters and the resulting morphology—such knowledge would provide further means to optimize dielectric performance of BOPP capacitor films
Summary
Oriented isotactic polypropylene (BOPP) film produced by tenter frame process is the current state-ofthe-art dielectric medium in both oil-impregnated filmfoil capacitors and metallized film (dry) capacitors. BOPP film exhibits superior dielectric strength (smallarea breakdown strength of ~700 V/μm), very low dielectric loss (tan ~10−4) and excellent self-healing dielectric breakdown capability in metallized film capacitors [1]. While these properties arise essentially from the weakly polar nature and high purity of commercially available capacitor-grade PP resins, the film structure and morphology play a very important role in determining the mechanical, thermal and dielectric properties of BOPP. This paper extends the above structure–property analysis by introducing supportive differential scanning calorimetry, dielectric spectroscopy and small-area dielectric breakdown data
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