## 3 Discussion
### Impact of Swelling  
Experimental compliance testing foresees use of simulants that are inappropriate in many cases and that the polymer swell in a way that is not representative of normal use [23]. This is especially the case for food simulants of similar polarities as the polymers that are tested and becomes of increasing relevance with increasing temperatures. An example of a practice that tends to suffer from polymer-swelling effects is testing styrene-based polymers with ethanol (alternative food simulant for vegetable oil) or ethanol-water mixtures (50% ethanol in water as food simulant for dairy products) [19]. Figure 12: Comparison of repeated-use scenarios (blue: continuous use, orange: intermittent use 8 h contact and 16 h without contact and green: occasionally use). Migration of Metilox (**a**) and Irganox 1076 (**b**) to 50% ethanol (\(K=10\)) at 60 \({}^{\circ}\)C.  
In practice, materials will come into contact with foods, which may cause the polymer to swell due to the polymer's uptake of food components. An example of this would be how non-polar polymers such as polyolefins take up vegetable oil, how polar polymers such as polyamides take up water, or how medium-polar polymers such as polyesters take up alcohol. Swelling involves plasticizing the polymer by lowering its glass transition temperature, such as by intentionally adding plasticisers to polymers, e.g., adding 15%, 30%, or 50% diethylhexylphthalate (DEHP) to PVC or PVC/PMMA blends [24]. A systematic study on the effect of the swelling of official food simulants on ABS was published by [PERSON] et al. [19]. From their data, it was concluded that the use of high concentrations of ethanol-water, especially at high temperatures, causes ABS to swell and results in significantly higher migration values compared to real foods. Similar swelling effects can be observed for PET when testing with ethanol 95% or ethanol 50% [25]. [PERSON] published a linear correlation of the polymer-specific constants of polymers with glass transition temperatures [13]. According to [PERSON] et al. [9], the polymer-specific constant is the main factor that determines the pre-exponential factor in the estimation procedure based on the Arrhenius relationship for the diffusion coefficients of the migrants in polymers. In summary, polymer swelling will lower its glass transition temperature and will consequently increase the diffusion rates of the migrants that are in the swollen polymer. According to [PERSON] [26], the glass transition temperature (Tg) of polyamide 6 strongly depends on humidity, i.e., the experimental values of the glass transition temperature for dry polyamide 6 are in the range from 50 to 75 \({}^{\circ}\)C; if they are exposed to 50% relative humidity, then Tg-values are in the range from 3 to 20 \({}^{\circ}\)C; and if they are exposed to 100% relative humidity, then Tg-values are in the range of \(-\)22 to \(-\)32 \({}^{\circ}\)C. The diffusion  
Figure 13: Cumulated migration of Metilox (**a**) and Irganox 1076 (**b**) to 50% ethanol at 60 \({}^{\circ}\)C by eliminating no-contact times (only migration cycles displayed, blue: continuous use, orange: intermittent use with 8 h contact and 16 h without contact and green: occasionally use). coefficients of the migrants in PA6 can be estimated independently of the degree of swelling that is related to relative humidity based on the linear correlation of the polymer-specific constants of the polymers with their glass transition temperatures. [13]. The results of an estimation that considered the impact of humidity on migration from PA6 are given in Figure 14. A further phenomenon that is already known qualitatively is the impact of material swelling by the regulated simulants on migration. According to today's knowledge, material swelling that is caused by simulants increases migration rates. The impact of partitioning on migration will become significant at lower temperatures and/or smaller molecules, as shown for PA6 when it is in contact with water.