## 3 Discussion  
The investigated GPPS and HIPS sheets showed normal levels of residual monomer and oligomers when compared with previous reported values for PS food contact materials and articles [24; 32; 33; 34]. Additionally, the sheets were artificially spiked during extrusion with model compounds that are widely used for migration and functional barrier studies [35; 36; 37] as well as for so-called challenge tests for determining the cleaning efficiency of recycling processes [38; 39; 40]. These artificially added substances represent the following general categories: volatile and non-polar (toluene and chlorobenzene), non-volatile and non-polar (phenyl cyclohexane), non-volatile and polar (benzophenone and methyl stearate). The artificially spiked substances were included in the study in order to increase the amount and variety of substances for the migration kinetics as well in order to add polar substances. Isooctane is known to induce the swelling of non-polar and medium polar polymers like polystyrene according to the principle \"like dissolves like\" and consequently, to modify the transport properties of the material [41]. The results of this study show that contact with isooctane caused a clear weight increase (i.e., uptake of isooctane) of the GPPS and HIPS polymers, which is most probably due to this swelling effect. At 60 \({}^{\circ}\)C the polymers are completely swollen with a weight increase after 10 days of 45% for GPPS and 30% for HIPS, respectively. The swelling affects the migration kinetics. At 60 \({}^{\circ}\)C testing temperature, the initial slope (only visually appreciated) of the migration curves (cf. Figure 1) in isooctane was very high and substantially similar for all the substances, regardless of their molecular size and polarity. From the diffusion theory [42], the level of migration from polymer material depends on the initial concentration (C\({}_{p,0}\)) and nature of the substance with its molecular size and solubility, the type of material and the food or food simulant in contact. In this study, the migration kinetics obtained using isooctane reflect more an extraction from the material rather than a diffusion-controlled migration process. When testing GPPS and HIPS using isooctane at 40 \({}^{\circ}\)C or 20 \({}^{\circ}\)C (cf. Figures 2 and 3), differences for these two polymers were observed. GPPS was not significantly affected at 40 \({}^{\circ}\)C and 20 \({}^{\circ}\)C by isooctane with a weight increase below 1%. Migration levels from GPPS were significantly lower compared to 60 \({}^{\circ}\)C. At 40 \({}^{\circ}\)C migration of the larger components especially of the oligomers from GPPS does not follow diffusion theory. The percentage of migration of the dimers (MW 208 g/mol) is not higher than that of the trimers (MW 316 g/mol), but in a similar range. This suggests a swelling in the first micrometers of the test sheets. In contrast to GPPS at these lower temperatures, the HIPS polymer was still strongly swollen with a weight increase of 12% and 3% respectively after 10 days at 40 \({}^{\circ}\)C and 10 days at 20 \({}^{\circ}\)C. Correspondingly, migration values in isooctane at 40 \({}^{\circ}\)C are similar or only slightly lower compared to the one obtained at 60 \({}^{\circ}\)C. Only when testing HIPS at 20 \({}^{\circ}\)C some differences in the migration behavior according to the nature of the substances (as expected from diffusion theory) were noted. These results are in agreement with previous studies [43], which indicate that the rubber content (styrene-butadiene-copolymer) in HIPS can be an influencing factor for migration of chemicals such as styrene. At 60 \({}^{\circ}\)C in isooctane HIPS was less swollen than GPPS, which can be an effect of its increased heat resistance. However, at lower temperatures HIPS was more swollen than GPPS, possibly because of the interactions of isooctane with the rubber.