Dielectric features of two grades of bi-oriented isotactic polypropylene

The dielectric properties of two grades of bi-oriented isotactic polypropylene were studied with a variety of techniques: breakdown field measurements, dielectric spectroscopy, thermally stimulated depolarization currents (Is), and direct-current (dc) conduction I values. Standard polypropylene (STPP) and high-crystallinity polypropylene (HCPP) films were investigated. Measurements were carried out over a wide temperature range (2150C/1125C). The breakdown fields in both materials showed a very small difference. On the other hand, the dielectric losses and dc conduction I values were significantly lower in HCPP. Both materials showed a decrease in the dielectric loss versus temperature in the range 20–90C; this is favorable for application in alternating-current power capacitors. The analysis of the dc I value allowed us to find evidence of two main conduction mechanisms: (1) below 80C in both materials, a hopping mechanism due to the motion of electrons occurred in the amorphous phase, and (2) above 80C, ionic conduction occurred in HCPP, and hopping conduction occurred in STPP.