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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.

International audience; The purpose of this article is to study how processing parameters can promote adhesion in grafted polypropylene/aluminum laminates made by extrusion coating. The density of bonds created at the interface is quantified by Xray spectroscopy after dissolution of the polymer film. dynamic mechanical analysis (DMA) and tensile tests are performed to characterize the mechanical properties of polymer films. They are linked with the crystalline structure, as revealed by optical microscopy and X-ray diffraction. A numerical model of extrusion coating is used to quantify the impact of cooling conditions on adhesion. A good correlation is found between the temperature history, the formation of bonds, and the mechanical properties of the polymer films. High temperature conditions can improve adhesion by increasing the open time for the reaction and the rate of the chemical reaction between grafted polymer chains and aluminum. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 112: 2609-2624, 2009