Impact of the Biaxial Stretching on the Breakdown Strength of Polypropylene Films

Biaxially oriented polypropylene (BOPP) film, as one of the most widely used energy storage materials in capacitor applications, exhibits excellent dielectric properties that are critically influenced by its stretching processes. This study investigates the effects of the biaxial stretching process, including stretching modes and stretching ratios, on the breakdown strength of BOPP films. By analyzing the crystalline characteristics of polypropylene films with different processing parameters, we explore the underlying mechanism between microstructures and breakdown strength of BOPP films. The experimental results demonstrate that under the condition of sequential biaxial stretching, the breakdown strength of BOPP films with a higher stretching ratio is significantly enhanced because of the increased crystallinity and reduced grain size. Specifically, the BOPP film sample prepared with sequential stretching at a stretching ratio of $3.0 \times 3.0$ exhibits the highest breakdown strength of 696.3 $\text{kV} / \text{mm}$.