Process of contaminant transfers during coextrusion of food packages made from recycled layer and virgin polymer layer

The process of coexlrusion of bilaycr films made from a recycled and a virgin polymer is rather complex. Not only the heat transfer by conduction through the film thickness and convection at the surface is considered, but also the transfer of the contaminant by diffusion through the film thickness and evaporation on the surface of the recycled layer. The problem has been resolved using a numerical method with finite differences taking all these facts into account. With bilayer films of same thickness, the midplane where the layers are injunction is a plane of symmetry for heal transfer. The axis in this midplane at half the initial uniform concentration is the axis of symmetry for the contaminant transfer by diffusion. Profiles of temperature developed through the thickness of the sheet and the temperature-time histories at the midplane arc shown. Concentration profiles of contaminant developed through the thickness of the film are shown for various times by considering diffusion through the sheet midplane and evaporation from the contaminated surface. In the same way the kinetics of the contaminant transferred into the virgin polymer layer and of the contaminant evaporated are considered. The effect of the sheet thickness on these transfers is studied within the 200-600 (mu)m range.