Study on rough‐surface biaxially oriented polypropylene film. VI. Roughening by laminated cast sheet

When the roughening of biaxially oriented polypropylene film (BOP) is performed by forming β crystals in a monolayer cast sheet, occasionally voids are formed in the BOP. In order to prevent this and to roughen both sides of the BOP, rough‐surface BOPs were obtained by casting two‐resin, three‐layer sheets consisting of two surface layers with a high‐melt flow index (MFI) polypropylene (PP) and a core layer with a low‐MFI PP, by stretching the cast sheets five times in the machine direction (MD) with a roll‐type stretching machine, and by stretching the MD‐stretched sheets 10 times in the transverse direction (TD) with a pantograph‐type stretching machine. Comparatively large spherulites were observed in the surface layers of the cast sheets and structural differences between the surface layers and core layer were recognized. Both sides of the cast sheets could be roughened. The influence of processing conditions on the roughness state of BOP was generally similar to that in the case of monolayer cast sheet.