Dielectric properties of PTFE-based composite insulating materials: interfaces effects

This paper relates to the study of the dielectric behavior of polymer-based insulating materials used in high voltage equipment. The materials tested are microheterogeneous, basically made out of a semicrystalline polymeric matrix (polytetrafluoroethylene) with mineral particles (calcium fluoride) dispersed in it. The dielectric properties are determined in a wide frequency (or time) range by a combination of two techniques: the short time response is assessed by measuring the complex permittivity in the frequency domain (20 Hz-1 MHz), and the long term response is studied by measuring the polarization current in the time domain (1s-600s) thus covering more than 8 decades in frequency or time. The modelling used to describe microheterogeneous dielectrics is shown to fit the experimental data. However, exposure to moisture is shown to induce strong relaxational effects. These effects vanish in a reversible way by drying treatment (i.e. removal of water molecules). These additional relaxations are discussed in term of conductivity effects occurring at the interface matrix-filler particles.