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 (2150 C/1125 C). 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 20-90 C; 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 80 C in both materials, a hopping mechanism due to the motion of electrons occurred in the amorphous phase, and (2) above 80 C, ionic conduction occurred in HCPP, and hopping conduction occurred in STPP.

This work reports on the relationship between structure and dielectric properties of biaxially oriented polypropylene. The morphology of semicrystalline bioriented isotactic polypropylene films is investigated using wide angle X-ray diffraction and Polarized Optical Microscopy. A b-orthorhombic structure, with a crystallinity ratio of about 46%, and "Crater" morphology of the b-form is identified. Dielectric properties are measured by Broadband Dielectric Spectroscopy over a wide temperature range (2150 to 1258C). Since the dissipation factor of the PP is very low, special care was taken to obtain valid data. Two main relaxation processes are observed: a a-relaxation peak associated to the glass transition temperature (Tg) at temperature about 278C, and a broad b*-relaxation at about 2608C, partly attributed to CH orientation. The variation of the dissipation factor versus sample thickness (from 3.8 to 11.8 mm) is correlated and partly explained by the increase of crystallinity ratio and lamella size at larger thicknesses. It comes out that the thinnest film seems perfectly meet the application requesting, namely lowest dissipation factor and highest permittivity.

Charging and discharging currents have been measured in a diglycidyl ether of bisphenol-A epoxy resin with and without silica fillers, below and above its glass transition temperature Tg = 65 °C. Both transient and steady-state current densities have been analysed. The average applied fields ranged from 3 to 35 kV mm−1 with a sample thickness of 0.5 mm. Above Tg, transient currents suggested a phenomenon of charge injection forming trapped space charges even at low fields. Steady-state currents confirmed that the behaviour was not Ohmic and suggested Schottky-type injection. Below Tg, the current is not controlled by the metal–dielectric interface but by the conduction in the volume: the current is Ohmic at low fields and both transient and steady-state currents suggest a phenomenon of space-charge limited currents at high fields. The field threshold is similar in the filler-free and the filled resin. Values in the range 12–17 kV mm−1 have been measured.

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours. Physical backgroundThe presence of particles in the material can alter space charge measurements in two ways: firstly by perturbing the diffusion of heat or the propagation of pressure waves, and secondly by adding spurious signals. These different alterations are studied in the following subsections. Heat diffusion and elastic wave propagationThe thermal methods rely on thermal diffusion to perform space charge measurements, and the pressurewave-propagation and the electro-acoustic methods rely on wave propagation. Any perturbation to the diffusion of heat or to the propagation of pressure waves may therefore alter these measurement methods.

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