Biaxially oriented polypropylene blown films. I: Morphological analysis of orientation in the machine direction

In biaxial blown film technology of isotactic polypropylene, PP, the film tube drawn over the mandrel contains considerable morphological disorder. This disorder changes on further treatment to greater order i.e. the para‐crystallinity and the metastable crystal forms diminish as the monoclinic crystal form increases. The morphology change in PP was stimulated by further treatment of the drawn film tube over a mandrel corresponding to conditions of biaxial bubble forming. Analyses were by SEM, Wide‐angle X‐ray diffraction (reflection), DSC, and IR Dichroic measurements. A condenser quality PP, Mw/Mn = 7 and MFI = 3.7, shows a tensile yield at 110°C of 11.4 MPa. A tensile stress of 17.7 MPa was obtained at a blow up ratio of 7; the morphology of PP corresponds to deforming spherulites. The fraction of “perfectly” oriented molecules is 0.53.