Structural evolution of biaxially oriented polypropylene films upon asynchronous stretching

Abstract

AbstractTo understand the structural behavior of biaxially oriented polypropylene (BOPP) films upon asynchronous stretching, synchrotron X‐ray diffraction/scattering method is employed to in situ monitor the structure evolution of the cast films in the machine direction (MD) and the longitudinally stretched films in the transverse direction (TD) at high temperatures. By comparatively analyzing the simultaneous stress–strain curves and multiscale crystalline parameters, the structural evolution regularities and mechanisms during the two‐step asynchronous stretching courses are discussed. It shows that the structural evolution upon the MD drawing at 130°C, 140°C, and 150°C of the PP cast films has a poor temperature dependence, while the structural developing upon the TD stretching of pre‐oriented, longitudinally stretched films exhibits the different behavior under 155°C as compared to that at 135°C and 145°C. This study not only discloses the structural evolution mechanisms of the two‐step asynchronous stretched BOPP films, but also demonstrates the more important impact of processing temperature on the TD drawing than the MD stretching, providing the theoretical guidance for the structural control and property optimization under the industrial production of BOPP films.