Mikael Ritamaki

DC ramp rate effect on the breakdown response of SiO 2 -BOPP nanocomposites

The effect of voltage ramp rate on the short-term dielectric breakdown strength of polymer nanocomposites is not well-documented. In this paper, the effect of DC field ramp rate on the large-area breakdown performance of melt-extruded bi-axially oriented polypropylene (BOPP) films incorporating 4.5 wt-% of nano-silica is studied. By utilizing a self-healing multi-breakdown measurement method with a variable DC voltage ramp rate, a statistically large amount of breakdown data was obtained from a large total sample film area as a function of DC field ramp rate (0.1-50 Vs-1μm-1)). With a decreasing ramp rate, Weibull statistical analysis of the breakdown data suggests a systematically decreasing trend in the breakdown strength (Weibull α) and an increase in the Weibull shape parameter of time (>1) for the nanocomposite. The observed behavior is speculated to be attributable to highly altered internal charge dynamics of the silica-BOPP nanocomposite. The results exemplify the importance of careful breakdown strength assessment when dielectric films of more complex internal structure are studied.

Effects of thermal aging on the characteristic breakdown behavior of Nano-SiO 2 -BOPP and BOPP films

The effects of thermal stress on the breakdown behavior of biaxially oriented polypropylene (BOPP)-silica nanocomposite thin film and commercial BOPP film was evaluated with a step stress experiment in the temperature range of 50...110 °C. Weak spots were measured in thermally aged unfilled BOPP films but not in the PP-silica, which in turn displayed increasing scatter of the breakdown voltages and the scattering seemingly settled to a certain level. No weak spots were measured in un-filled BOPP films removed after highest temperatures. The behavior of un-filled films is speculated to result from either actual polymer aging in addition to the statistical nature of breakdown phenomena or from thermally-activated processes possibly involving antioxidants. The different behavior in the nanocomposite was speculated to result from polymer-nanoparticle interactions possibly countering the effects of antioxidants and hindering weak spot formation. The results demonstrate the different behavior of polymer-nanocomposites and traditional BOPP films under thermal stress and highlight the importance of evaluating the aging behavior of new materials.

Large-area approach to evaluate DC electro-thermal ageing behavior of BOPP thin films for capacitor insulation systems

A large-area electro-thermal ageing test setup was developed and utilized to age several laboratory-scale biaxially oriented polypropylene (BOPP)-hydrophobic silica nanocomposite films. The films were aged in test capacitors with self-healing metallized film electrodes, which enabled the ageing test to continue beyond the first breakdowns. Eight different films were aged for 1000 hours under 100 V/μm DC stress at 75 °C, and large-area DC breakdown measurements, dielectric spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and dielectric spectroscopy were used to detect localized and bulk degradation after the ageing period. The effects of antioxidant contents, different PP grades and compounder screw speed were evaluated. Material characterization indicates no bulk degradation had occurred during ageing, which was associated with moderate temperature stress and inert nitrogen atmosphere. On the other hand, low-field breakdowns (weak points) were observed in all but two of the aged materials, indicating that ageing was dominated by localized degradation which may have introduced new breakdown mechanisms. Weak points were also measured in a similarly aged commercial capacitor-grade BOPP film aged at a lower field, supporting this conclusion. The importance of long-term characterization in material development is demonstrated, and it is shown that long-term properties of the evaluated nanocomposites were at least on the same level compared to neat BOPP films.

Differences in AC and DC large-area breakdown behavior of polymer thin films

A large-area multiple breakdown measurement using self-healing gold sputtered electrodes was evaluated in determining the AC breakdown characteristics of insulating biaxially oriented polypropylene (BOPP) and BOPP-silica nanocomposite thin films. Results were compared with conventional small area measurements and it is shown that the large-area approach could be extended into AC measurements as long as good quality film-electrode interface in ensured. Special electrode arrangements are needed due to the absence of electrostatic force. AC small-area breakdown strength of pilot-scale films was ramp rate dependent but no such effect was clearly noticed for a commercial product. Conventional DC small-area breakdown measurements were also conducted and the results were compared to previously published large-area multiple breakdown data. The consistent results support the dependency between ramp rate and DC breakdown strength in the studied BOPP-silica nanocomposite.

Dielectric breakdown properties of mechanically recycled SiO 2 -BOPP nanocomposites

This paper reports on the effects of multiple compounding cycles on the structural, chemical, rheological and DC dielectric breakdown properties of bi-axially oriented silica-polypropylene nanocomposite films. While the effect of multiple compounding cycles on the silica particle agglomeration remains seemingly modest according to the quantitative structural analysis, the recompounding is found to markedly affect the steady shear viscosity and film processability. Fourier transform infrared spectroscopy is utilized to ascertain that no thermo-oxidative degradation is occurring during the reprocessing. The large-area multi-breakdown measurements indicate a ~13 % shift towards higher DC dielectric strength after the second recompounding cycle, an effect which is believed to be attributable mainly to the changes in film processability. Surprisingly, a 75:25 mixture of the virgin and three-times-reprocessed nanocompounds exhibits a similar increase in dielectric strength. Particularly from the processing perspective, the results emphasize the necessity of careful consideration of property modifications upon incorporation of nanofillers.