AbstractRe‐cycled polymers can be used for food packaging with a layer of virgin polymer as a functional barrier protecting the food from contamination by material from the re‐cycled layer. A mathematical treatment of the diffusion of the contaminant (initially at a uniform concentration in the re‐cycled plastic) is made by considering the diffusion through the two layers with a constant diffusivity.In order to better define the role of the functional barrier, the case without transfer of contaminant through the external surfaces is considered. Dimensionless numbers are used so as to draw master curves either for the kinetics of contaminant transfer or for the profiles of concentration of contaminant in the material. The efficiency of the functional barrier can be defined by the lag time for the start of migr