BIAXIAL DEFORMATION OF SPHERULITE AND MULTI-LAYER STRUCTURE IN THE BIAXIALLY STRETCHED POLYPROPYLENE FILMS

Abstract

In this report effects of spherulite size and of various stretching processes on the characteristics of the deformation and fracture of polypropylene spherulites were investigated at both the room and higher temperature in connection with the stretchability of films. The exfoliatical property of biaxially stretched film is discussed in terms of spherulite deformation.Film containing spherulite of various sizes were prepared by hot-press technique under the various cooling conditions. These films were stretched with the film-stretcher, in the open bath or the hot-air bath. The following stretching methods were adopted: uniaxial stretchings under the free or constant width, and two-way successive or simultaneous biaxial stretchings.The biaxial stretchability is higher in the films with small size spherulites than in the films with large size.The deformation and the fracture mechanisms of spherulite could be phenomelogically classified as follows: (1) brittle break-down, (2) fracture with the cleavage of interfacial boundary, (3) uniaxial plastic deformation, (4) deformation with necking, (5) deformation with pleating, (6) deformation into fibril structure, and (7) biaxial plastic deformation.The characteristis of the deformation and the fracture of spherulites are summarized in Table 1.We have deduced that the appearance of exfoliatical property as mica was owing to the multilayer structure of the spherulite and the planar orientation of molecular chains and crystalls in the film.