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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.

We present an investigation of the progressive building of copolymer layers formed by chemical reaction at interfaces in multilayer coextruded polymer films. Analyzing the surface density of copolymers in the final films, we show that the interface always remains under‐saturated in copolymers, even for coextrusion parameters such that the time open to the interfacial chemical reaction is well above the time necessary to reach saturation without flow. Based on a numerical analysis of the flow all along the coextrusion line, we show that this under‐saturation of the interface, which may strongly affect the final adhesion between the different polymers, results from a competition between the interfacial chemical reaction and the dilution of yet formed copolymers in all the zones of the coextrusion line where surface of interface is increased, due to convergent, divergent or elongational flow. We also show that the coupling between flow and reaction kinetics needs be taken into account to control the final surface density of copolymers and precisely optimize the interfacial properties, through an optimization of the coextrusion parameters POLYM. ENG. SCI., 59:E44–E50, 2019. © 2018 Society of Plastics Engineers

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