Stretching‐Regulated Homogenization of Lamellae Thickness in Biaxially Oriented Polypropylene Capacitor Films, and Its Effect on Breakdown Strength

Stretching fields give rise to the formation of reinforcing crystals in biaxially oriented polypropylene capacitor films, endowing them with excellent electrical properties. In this paper, the correlation between stretching temperature, ratio, and annealing temperature and film structure as well as electric breakdown strength (E_b) during biaxial stretching is investigated. Synchronous biaxial stretching ensures that the orientation of the prepared films is isotropic. However, the changes caused by the stretching field occur in the lamellae structure, with a gradation of thick and thin lamellae. First, an increasing stretching ratio leads to the homogenization of the crystal toward thick lamellae, increasing E_b. Second, the onset melting temperature (T_onset) is the key to the gradation of thick and thin lamellae. Stretching near or above T_onset results in a reduced order of the recrystallization precursors and forms thin lamellae, degrading E_b. Additionally, annealing below the melting point promotes chain migration, which facilitates the homogenization to thick lamellae and improves E_b. In conclusion, the improvement of E_b is related to the homogenization of lamellae thickness. These results provide new insights for optimizing film processing and crystal structure to enhance electrical insulation.