Samuel Devisme

Numerical simulation of extrusion coating

Abstract 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.

Influence of the Calendering Step on the Adhesion Properties of Coextruded Structures

Abstract The coupled coextrusion calendering process of multilayer thermoplastic sheets has been studied. The structure of the film has been observed, the density of chemical links at the interface and the adhesion between the coextruded layers have been measured. There are some controversial features about the influence of calendering parameters. Several models of increasing complexity are proposed to master the process and its influence on adhesion. A multilayer thermal model accounting for the crystallization kinetics of both polymers allows defining qualitatively the temperature field especially at the interface between the two layers. A multilayer thermo-mechanical model provides quantitative figures on the stress, shear, elongation and temperature fields in the coextruded film and especially at the interface. Relationships between these parameters, density of chemical links at the interface, crystalline structure of the sample and peeling forces are discussed.