A reexamination of the relationship between β to α crystal transition and the surface crater formation in biaxially oriented polypropylene film

Abstract

The formation mechanism of the crater-like surface of biaxially oriented polypropylene (BOPP) film remains controversial. To clarify this issue, in this work, three isotactic polypropylene (iPP) samples with different tacticities are selected to prepare BOPP films. It is shown that the relative β crystal content (kβ) in the cast sheet increases with tacticity. The β crystal transforms to the α crystal when stretched at 145 °C along the machine direction. For all the samples, rough surfaces are obtained even if the kβ value of the cast sheet is very low (kβ < 0.1). The arithmetic mean roughness (Ra) of the BOPP films increases with the increase of the kβ in the cast sheet. While higher kβ in the iPP cast sheet contributes to a higher Ra value, the β crystallites hinder the enlargement of roughening rings, and cause surface defects (cavitation), which are detrimental to applications. Our results indicate that the β crystal in the cast sheet is not essential for the crater formation and a higher β crystal content is not always beneficial for BOPP capacitor films.