3.5. Mathematical migration modeling
Migration of a substance is strongly depending on the mobility of the molecule within its surrounding host matrix, which can be assessed by the diffusion coefficient of the substance in the polymer. Although the nano scale substance used in this study is not a molecule but a particle, it can be reasonably assumed that an increasing particle size results in a higher free cross section of the particle which leads to a lower mobility of the nano scale migrant. The size dependent mobility has recently been demonstrated for chemical substances in polyethylene terephthalate (Welle, 2013). Therefore, we consider mono chemical nano particles (existing of carbon) as quasi-molecules for which, depending on the particle size quasi molecular weights can be calculated. This allows to use Eq. (2) to calculate NP size dependent diffusion coefficients and from this the corresponding migration of these NPs.