Numerical simulation of extrusion coating
Résumé
In packaging industry, many structures are produced by extrusion coating. In this process, a polymer film is extruded through a slit die, then stretched in air, coated on a substrate (paper, aluminium or steel foil) between a chill roll and a flexible pressure roll, and finally cooled on successive chill rolls. Due to their non-polar character, polypropylenes are not suitable for extrusion coating on metallic surfaces. Adhesive properties can be improved by grafting on the polymer chain a polar group like maleic anhydride which may react with the aluminium surface. Our purpose was to develop a general model in order to predict the temperature field in the thickness of the multilayered structure along the stretching, laminating and cooling steps, and especially near the polymer/metal interface. This model includes crystallization kinetics and accounts carefully for the heat transfer coefficient with the successive rolls and surrounding air. Its predictions have been successfully compared to experimental temperature measurements along the coating line for various processing conditions (velocity, roll temperature, etc). Moreover, thermal history has a real impact on structure and morphology in the film. All these aspects are revealed by microscopic observations of thin microtomed sections of the film and X-ray diffraction experiments. Finally, adhesion properties of the laminate have been tested for the same process conditions. A good correlation has been established between adhesion properties and the thermal history experienced by the grafted polypropylene near the interface with aluminium before crystallization.