Performance metrics for a modern BOPP capacitor film

In this paper, a set of performance metrics for modern biaxially oriented polypropylene (BOPP) capacitor films is established. The fundamental and applied properties of BOPP films required for application in state-of-the-art DC metallized film capacitors are reviewed, highlighting aspects related to high temperature operation, base PP properties and film processing. Commercial BOPP films - both base films and metallized films based on classic isotactic PP - are studied comprehensively, encompassing structural-morphological characterization and short- to medium-term dielectric characterization. Dielectric spectroscopy results demonstrate the negligible dielectric losses of BOPP, being in the range of 10- ⁴ or less in the expected operation temperature regime. Thermally stimulated depolarization current (TSDC) measurements indicated a modest density of shallow traps (~0.75 eV) and a high density of deep traps (~1.08 eV) in the 5 μm and 10 μm film variants showing differences presumably arising from film processing. Such an electronic structure was found to be connected with ultra-low conductivity (in the range of 10- ¹⁷ -10- ¹⁶ S/m), high breakdown strength (~700 V/μm) and negligible space charge accumulation up to temperatures of ~70 °C. It is shown that at current design stresses (~200 V/μm at ~60 °C) BOPP is operated close to its fundamental thermal and electrical limitations. Voltage endurance tests at higher fields revealed the onset of high-field degradation and drastically reduced insulation life, and thermal activation of deep traps in the high temperature region (~100 °C) was found to result in reduced dielectric performance.