Abstract
Biaxial-stretching-induced morphology development anddielectric properties of hydrophobic fumed silica-BOPPnanocomposite films were investigated. The precursor (cast)film morphology, pre-heating as well as biaxial stretchingtemperature and stretch ratio were found to profoundly affect thesilica-BOPP film bulk morphology, surface texture and theformation of particle/agglomerate-induced cavitation uponbiaxial stretching. Isothermal high-field conductivity andthermally stimulated depolarization current measurementsindicated decreasing high-field/high-temperature conductivityand modification of trap density of states upon incorporation ofnanosilica in comparison to neat BOPP. Large-area DCdielectric strength is, however, sensitive to thermal/mechanicalstresses upon biaxial stretching, hence necessitating carefuloptimization of the nanocomposite formulation and processing.
Highlights
Next-generation biaxially oriented polypropylene (BOPP)silica nanocomposite (NC) films are currently being developed in the European project GRIDABLE with the aim towards applications in metallized film capacitors for e.g. voltage source converters (VSC)
The importance of optimized compounding environment as well as the degree to which the surface texture, bulk morphology and dielectric performance can be controlled by the biaxial stretching process are demonstrated, highlighting the essential role of the above factors to reach the full potential of this technology for film capacitor applications
Mixed type α/β-form crystalline structure was confirmed by differential scanning calorimetry (DSC) analysis for each cast film, as is exemplified in Figure 1 for a 4.5 wt-% silica-PP and unfilled reference PP compounds
Summary
Next-generation biaxially oriented polypropylene (BOPP)silica nanocomposite (NC) films are currently being developed in the European project GRIDABLE with the aim towards applications in metallized film capacitors for e.g. voltage source converters (VSC). Especially for very thin films (5 μm or less), avoiding structural inhomogeneities caused by e.g. NP agglomeration and cavitation/void formation during biaxial stretching is imperative for ensuring high dielectric performance [4] This necessitates careful optimization of the NC formulation, compounding, film extrusion and biaxial stretching. This paper explores our recent structure–dielectric property studies of silica-BOPP NC films, with a particular focus on understanding and controlling the biaxial-stretching-induced morphology development and the associated effects on NC charge trapping properties. For this purpose, we analyzed pilotscale silica-BOPP NC films manufactured under varied biaxial stretching conditions (pre-heating time, stretching temperature, stretch ratio). The importance of optimized compounding environment as well as the degree to which the surface texture, bulk morphology and dielectric performance can be controlled by the biaxial stretching process are demonstrated, highlighting the essential role of the above factors to reach the full potential of this technology for film capacitor applications
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