Large-area dielectric breakdown performance of polymer films – Part II: Interdependence of filler content, processing and breakdown performance in polypropylene-silica nanocomposites

Abstract

In this study, large-area dielectric breakdown performances of various bi-axially oriented polypropylene (BOPP)-silica nanocomposite films are studied by utilizing the self-healing multi-breakdown method presented in the Part I of this publication. In particular, the effects of silica filler content, pre-mixing method, co-stabilizer content and film processing on the large-area breakdown performance are analyzed. Nanostructural and film cross-sectional analyses are correlated to the breakdown responses. The optimum silica filler content is found to reside at the low fill fraction level (~1 wt-%) and automatic pre-mixing of the raw materials and the optimization of the orientation temperature are found to be preferable. The co-stabilizer Irgafos 168 is found to have a significant effect on the breakdown distribution homogeneity of the reference BOPP films. The breakdown response of the silica nanocomposites is found to be not only dependent on the active measurement area but also on the voltage ramp rate, indicating that the silica nanocomposites exhibit altered internal charge behavior under DC electric field. The area-and ramp-rate-dependence results exemplify the importance of careful breakdown strength evaluation of dielectric polymer nanocomposites. Above all, the results emphasize the fact that a thorough understanding and the optimization of the film processing parameters are crucial for achieving improved breakdown response in dielectric polymer nanocomposite films.

The Role of Film Processing in the Large-Area Dielectric Breakdown Performance of Nano-Silica-BOPP Films

“…As also discussed in [3], [4], Figure 2a clearly shows that an increase in silica content reduces the breakdown performance in the >5 % breakdown probability region in comparison to the unfilled reference BOPP but also leads to a concurrent improvement of the breakdown distribution homogeneity. Interestingly, the Weibull lines of the 1.0 wt-% SiO 2 -BOPP PNC and the unfilled BOPP intersect in the 5-10 % breakdown probability range with the 1.0 wt-% SiO 2 -BOPP PNC showing higher strength in the low probability region.…”

“…The film processing was conducted at the VTT Technical Research Centre of Finland and at a Brückner pilot film line in Denmark. As the total amount of studied compounds was over 25, a detailed listing of all the materials, processing parameters and film specifications is out of the scope of this paper and can be found from the related publications ( [3], [4], [7]). Only the essential aspects are summarized in the following.…”

“…The SEM images were converted to binary images and the particle area distributions were determined by using ImageJ software. Moreover, a parameter describing the characteristic length of the unreinforced polymer domains, free-space length L f , was also calculated and used to quantify the dispersion quality [3], [10]. L f is defined as the width of the largest randomly placed square for which the most probable number of intersecting particles is zero.…”

“…A large-area multi-breakdown measurement approach based on the self-healing breakdown capability of metallized polymer films has been extensively applied for studying the breakdown properties, as it enables a large amount of breakdown data to be acquired from a relatively large total film area hence leading to a high statistical accuracy even at low breakdown probabilities. A detailed description and evaluation of the measurement method was recently given in [7] while studies exemplifying the large-area multi-breakdown characterization of various laboratoryscale, pilot-scale and commercial BOPP and BOPP-PNC films are presented in [3], [4], [8], [9].…”

Nanocomposite polypropylene for DC cables and capacitors: A new European project

“…Compared to the unfilled reference BOPP, it is shown that increasing silica content reduces the DC breakdown performance in the >5 % breakdown probability region, but it also leads to an increase of the shape parameter, that is to a decrease of the breakdown distribution dispersion [15]. Interestingly, the Weibull graphs of the 1.0wt-% silica-BOPP and the unfilled BOPP intersect in the 5-10 % breakdown probability range, with the nanostructured BOPP showing higher electric strength in the low probability region.…”