General Introduction to the Various Problems

But plastics contain low molecular compounds such as oligomers and sometimes monomers, and additives such as plasticisers, lubricants, stabilisers and antioxidants, which are absolutely necessary either for the processing or the stability of the final polymeric materials. The drawback of using these is the potential migration of these additives from the package into the packed material. This is a problem of major interest where food or drugs are concerned. 

Moreover, as the use of plastic packages is now widespread in all countries, a large amount of old packages are discarded in the garbage heaps, if not anywhere. Thus a few solutions are given in order to answer the question what to do with various kinds of old plastics A few ways have already been explored, if not widely used  

The process of contaminant transfer from the package into the food can be generally described by transient Fickian diffusion through the package, and either convection in the food in a liquid state or diffusion in the food when it is highly viscous or solid. 

The theoretical treatment of the process of mass transfer controlled by diffusion-convection with liquid food or even by diffusion-diffnsion when the food is solid is described precisely. Emphasis is placed on the fact that an infinite rate of convection cannot exist at the package-liquid interface in our finite world, and conseqnently we have to deal with more complex equations. The question of the volume of the package as a fraction of the volume of the liquid is also considered. In fact, the problems of diffnsion are not simple to resolve, either by considering the experiments or by making calcnlations. 

The problem of bi-layer systems is also examined, when the package consists of the layer made of the recycled polymer bound to the virgin polymer layer. As this virgin layer is placed in contact with the food, it plays the role of a functional barrier. The effect of the various parameters is determined, without forgetting the effect of the mass transfer taking place between these two layers during the stage of co-extrusion. 

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