Étude de la cristallisation dans la mise en forme des polymères. Application à l’extrusion de films de polypropylène

Crystallization of polymers in industrial processes. Application to extrusion of polypropylene films

Ecole Nationale Superieure des Mines de Paris, Centre de Mise en Forme des Materiaux, U.A. CNRS N° 852 Sophia Antipolis, Valbonne

Abstract

Crystallization of a polymer in an industrial process is highly dependent on the mechanical (flow), thermal (cooling rates, thermal gradients) and geometrical (walls of the processing devices) conditions encountered by this polymer in the process. Cast film extrusion of polypropylene is chosen as an example of interrelations between processing conditions and microstructure. In this process, the polymer is extruded through a slit die, slightly stretched in air and then cooled on a chill roll. Our analysis is based on the combination of a thermomechanical model and morphological observations, which makes it possible to discuss the respective influences of stretching in air and cooling on the roll. Stretching in air essentially affects spherulite nucleation and growth during subsequent crystallization on the roll. The existence of a transcrystalline region is attributed to heterogeneous nucleation at the roll surface Finally, the observation of a zone near the roll with a finer microstructure is related to the cooling rates and the thermal gradients calculated by the thermo- mechanical model. A conclusion of this work is that a complete quantitative interpretation requires the development of crystallization models taking into account the actual processing conditions.