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.

Determination of Nucleus Density in Semicrystalline Polymers from Nonisothermal Crystallization Curves

“…Unfortunately, the number of nuclei cannot be evaluated directly from this simple equation. Only few attempts carried out to obtain nucleus density under non-isothermal conditions [31][32][33] . Boyer et al 32 introduced recently an excellent work for description of crystallization at high cooling rates.…”

Crystallization of polypropylene at high cooling rates: Microscopic and calorimetric studies

“…In this article, a different strategy is adopted to determine $ N_0 $ . It is based on the approach proposed by Duffo28 and already illustrated by Devisme et al27 If $ N_0 $ is constant, $ {\rm{\chi }}(T) $ is simply given by: …”

“…In conclusion, the future is obviously to develop more rigorous and physically based crystallization models, which requires important experimental efforts to measure the relevant parameters, but it must also be confessed that approximate expressions often give nice results, from an engineering and even scientific point of view 27, 30, 32. Therefore, in continuity with previous work,27 Ozawa's approach will be kept here. Two additional reasons are the following: (i) the polymer under investigation is isotactic polypropylene and Ozawa's model applies rather well to this polymer; (ii) the main part of the experimental work will be done by DSC.…”

“…Furthermore, it supposes that the crystallization phenomena, especially nucleation, remain the same at high cooling rates, which remains an open question. Such an extrapolation procedure has been applied to polypropylene to model structure development in the extrusion‐coating process 27. The model was able to predict structure development within the films and to give an interpretation of the adhesion properties in relationship with processing conditions.…”

“…The purpose of this article is to revisit and to improve the extrapolation procedure used by Devisme et al,27 which derives from an initial proposal of Duffo 28. A first route is to limit the extrapolation domain by acquiring data at higher cooling rates.…”