Study on rough‐surface biaxially oriented polypropylene film. III. Influence of MFI of base resin

In Part II of this series we reported that by using a successively stretching tenter method, biaxially‐oriented polypropylene films (BOPs) with a crater‐like roughness have been obtained by selecting appropriate machine direction (MD) stretching conditions. In the present article, in order to select still better conditions, the influences of melt flow index (MFI) of base polypropylene (PP) and machine and transverse direction stretching conditions on the roughening of BOP have been studied. Rough‐surface BOPs were obtained by casting homoisotactic PPs, having MFIs of 1.3–9.8 dg/min and containing no β‐crystal nucleator, with a T‐die extruder into sheets which subsequently have been stretched successively in the machine direction (MD) and the transverse direction (TD) with a roll‐type stretching machine and a pantograph‐type stretching machine, respectively. The level of β‐crystal content in the cast sheet was maximum when a PP with MFI ≒ 8 dg/min was used as a raw material and the obtained roughness state of the BOP was the best. As for the MD‐stretching conditions, the roughness state of a BOP improved as the surface temperature of the heating roll was increased to a temperature range of 135–150°C, and there was the best roll rotation speed from the balance of the thermal conductivity of the sheet and the melt‐recrystallization rate of the β crystals. A temperature in a range of 150–155°C was best for the TD‐stretching.