Influence of Stretching and Cooling Conditions in Cast Film Extrusion of PP Films

Abstract

Abstract In cast film extrusion, the polymer melt is extruded through a slit die, slightly stretched in air and then cooled on a chill-roll. An important part of the present work was to study the effect of the roll temperature on the polymer crystallization in the thickness of extruded films. A thermomechanical model of the whole process taking into account the polymer crystallization was used to determine the mean stretching stress in air, local temperature, local cooling rate and local crystallization development. The morphologies in the thickness of the films were observed by optical microscopy on microtomed slices and the crystalline structures were investigated by wide-angle X-ray diffraction and DSC measurements. From DSC melting curves of thin sections it is possible to know the local crystallization temperature and to check the thermomechanical model. The model is validated and explains the influence of the roll temperature on the formation of structures and morphologies. Nevertheless neither the existence of three different zones in the thickness nor the high p phase concentration are predicted by the model, which needs new developments.