## 4 Discussion  
The residual content of styrene monomer in the PS sample materials (pellets, sheets, and pots) ranged from 248 to 505 \(\upmu\)g/g of polymer. These results are consistent with the previous reported values for various food-grade non-expanded PS resins [22; 23; 24]. Three different grades of GPPS (GPPS-1, GPPS-2, GPPS-3) and two grades of HIPS (HIPS-1, HIPS-2) in the form of granules were analyzed in this study. The concentrations of styrene in the GPPS pellets were found to be slightly higher than those in the HIPS pellets. However, when also considering the data available in the literature, GPPS does not generally show a higher residual content of styrene compared to HIPS. The differences may be related to the different polymer grades used. The pellets were converted to sheets by extrusion, blending a 50:50 mixture of GPPS/HIPS pellets, which is a typical composition of PS pots for food contact. Three types of extruded sheets were produced: PS-S-11 (blending GPPS-1 and HIPS-1), PS-S-22 (blending GPPS-2 and HIPS-2), and PS-S-32 (blending GPPS-3 and HIPS-2). The residual content of styrene monomer found in the extruded sheets reflects their composition. In fact, blending the GPPS and the HIPS grades with higher styrene residuals also resulted in higher styrene residual content in the sheets manufactured therefrom and vice versa. The sheets were further processed to create three types of pots (PS-P-11, PS-P-22, and PS-P-32) by thermoforming. The residual styrene contents in both the granulate and sheet were similar, at the most slightly decreasing during thermoforming. A decrease of approx. 10% in the styrene content was found in pot PS-P-11, obtained from sheet PS-S-11; however, in the other pots PS-P-22 and PS-P-32, it corresponded only to 1% and 4%, respectively, which can be regarded as insignificant. Previous studies involving the processing of PS granules into either cups or containers for dairy products support the evidence that there is a general tendency for residual styrene levels to decrease during thermoforming, which reflects and depends upon the temperature and vacuum conditions applied during processing [25]. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Food** & **Testing** & & & & **Styrene Migration [\(\upmu\)g/kg] \({}^{1}\)\(\pm\) SD** & & \\
**(Simulant)** & **Conditions** & **PS-S-11** & **PS-S-22** & **PS-S-32** & **PS-P-11** & **PS-P-22** & **PS-P-32** \\ \hline
20\% Ethanol & 10 days/20 \({}^{\circ}\)C & 38 \(\pm\) 3 & 53 \(\pm\) 2 & 68 \(\pm\) 1 & 16 \(\pm\) 1 & 22 \(\pm\) 3 & 21 \(\pm\) 2 \\
20\% Ethanol & 10 days/40 \({}^{\circ}\)C & 83 \(\pm\) 2 & 110 \(\pm\) 2 & 116 \(\pm\) 12 & 65 \(\pm\) 1 & 93 \(\pm\) 3 & 98 \(\pm\) 1 \\
50\% Ethanol & 10 days/40 \({}^{\circ}\)C & 188 \(\pm\) 13 & 231 \(\pm\) 4 & 241 \(\pm\) 12 & 155 \(\pm\) 4 & 204 \(\pm\) 7 & 212 \(\pm\) 5 \\ Yoghurt & 10 days/8 \({}^{\circ}\)C & 17 \(\pm\) 1 & 22 \(\pm\) 3 & 23 \(\pm\) 5 & 5 \(\pm\) 1 & 4 \(\pm\) 1 & 7 \(\pm\) 1 \\ Yoghurt & 20 days/8 \({}^{\circ}\)C & 23 \(\pm\) 2 & 39 \(\pm\) 8 & 38 \(\pm\) 3 & 7 \(\pm\) 1 & 5 \(\pm\) 1 & 10 \(\pm\) 1 \\ Yoghurt & 30 days/8 \({}^{\circ}\)C & 21 \(\pm\) 3 & 53 \(\pm\) 4 & 64 \(\pm\) 1 & 9 \(\pm\) 1 & 12 \(\pm\) 1 & 7 \(\pm\) 1 \\ Yoghurt & 40 days/8 \({}^{\circ}\)C & 24 \(\pm\) 3 & 53 \(\pm\) 3 & 59 \(\pm\) 10 & 9 \(\pm\) 1 & 7 \(\pm\) 1 & 11 \(\pm\) 1 \\ Yoghurt & 50 days/8 \({}^{\circ}\)C & 24 \(\pm\) 4 & 73 \(\pm\) 8 & 94 \(\pm\) 4 & 13 \(\pm\) 2 & 14 \(\pm\) 1 & 15 \(\pm\) 1 \\ \hline \hline \end{tabular} \({}^{1}\) The experiment was performed by applying the EU cube model of 6 dm\({}^{2}\)/kg.  
\end{table} 
Table 2: Migration of styrene in food simulants and in yoghurt under different testing conditions.