A Comparison of Electric and Dielectric Properties of Standard and High-Crystallinity Polypropylene Films

The dielectric properties of two grades of Bioriented isotactic polypropylene (BOiPP) are studied, using a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents, dc conduction currents. Standard (STPP) and high-crystallinity (HCPP) polypropylene films are investigated. Measurements are carried out over a wide temperature range, up to (-150 degrees C/ +125 degrees C). Breakdown field in both materials show a very small difference. On the other hand, dielectric losses and dc conduction currents are significantly lower in HCPP. Both materials show a decrease of dielectric losses versus temperature in the range (20 90 degrees C), favorable for the application to ac power capacitors. The analysis of dc currents allows to evidence two main conduction mechanisms: (i) below 80 degrees C in both materials, a hopping mechanism due to motion of electrons occurring in the amorphous phase; (ii) above 80 degrees C, an ionic conduction in HCPP and a hopping conduction in STPP.