Theoretical Treatment with a Bi-layer Package

The problem is different from the case shown with the single layer package. 

As no analytical solution is obtained from the mathematical treatment, only a numerical method should be used. Thus there are no limitations concerned with the assumptions. 

Nevertheless, calculations have been made by using a constant diffusivity, and a uniform concentration of the diffusing substance, considered as the contaminant, is assumed to stand initially in the recycled polymer layer. In fact, these assumptions are not necessary, provided that the data are known beforehand. In the same way, one-dimensional diffusion is considered, leading to an easier problem. 

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