Problems encountered for food safety with polymer packages: chemical exchange, recycling.

When a polymer is in contact with a liquid, generally some matter transfers take place: the liquid enters the polymer, while some additives leave the polymer. This fact is responsible for a reduction in the properties of the polymer and pollution of the liquid food is obtained. However, it is sure that, on account of their outstanding properties, food packages are becoming more and more based on polymers. The process of mass transfers is controlled by transient diffusion, and these theoretical problems are so complex that only numerical models are able to resolve them. New difficult problems appear with the desire of recycling old food packages made of polymers by reusing them as new food packages. Of course, because of the potential contamination brought with this recycled polymer, the package is made of bi-layer or tri-layer films where the reused polymer is located between two virgin polymer layers. As it takes some time for the contaminant to diffuse through the virgin layer, this latter plays the role of a functional barrier to pollution. The various difficulties in the whole problem are considered in succession: the coextrusion of the films where a contaminant transfer already occurs; and the effect of the liquid food which may enhance the diffusion of the contaminant. Various master curves are drawn by using dimensionless numbers, so as to make the results quite general and of use from a practical point of view.