## 3 Results and Discussion
In Figure 6, the results are shown of the temperature effect for different inlet concentrations. The residence time was set to 5 min, with a screw speed of 20 rpm. Again, the highest FDs obtained at 190 \({}^{\circ}\)C and 200 \({}^{\circ}\)C were achieved for the samples not treated with scCO\({}_{2}\). The reason why higher temperatures resulted in a decrease in FD might be that the processing temperature exceeded the ceiling temperature of the GMA (150-220 \({}^{\circ}\)C). At higher temperatures, the polymerization of GMA becomes reversible. Moreover, the residence time was set to 20 rpm, which is not the most suitable time for all the different temperatures due to the different half-life times of DCP at different temperatures. The influences of scCO\({}_{2}\) and a different process temperature on the FD of the RPE was not significant at all for the samples with higher inlet concentrations. However, a small increase was seen for the 3 wt.% GMA + 0.3 wt.% DCP at 175 \({}^{\circ}\)C compared to the results in Figure 3. This difference in FD values as functions of temperature with and without scCO\({}_{2}\) is relevant and yet difficult to explain. Generally speaking, the influence of temperature on FD values for melt processes is related to the occurrence of side reactions [5,6]. This can be also be the case in scCO\({}_{2}\) conditions, with one additional remark. The monomer used in this work (GMA) is soluble in scCO\({}_{2}\); this causes a dilution effect, according to which, side reactions as GMA homo-polymerization become less probable. At the same time, with GMA present in the supercritical phase, a mass transfer barrier has to be overcome in order to obtain grafting. The optimal residence time in the extruder is linked to the temperature profile and, therefore, the screw speed. An excessively long residence time is unfavorable and can result in a decline in GMA grafting due to the depolymerization of the GMA [7,9]. Therefore, the screw speed was adjusted to the ideal residence time and temperature, as shown in Table 3.  
The results are given in Figure 7.  
Adjusting the screw speed resulted in a significant improvement of the FD for the samples prepared without scCO\({}_{2}\) while working at 200 \({}^{\circ}\)C. The mixing was probably improved due to the increase in the rotational screw speed to 66 rpm, resulting in the exertion of a large amount of shear by the screws. The samples grafted under scCO\({}_{2}\) did not show any serious improvement. Presumably, the scCO\({}_{2}\) could have swollen the polymer more due to a lower viscosity at higher temperatures. Therefore, the initiator could diffuse better into the matrix, which would have led to the crosslinking of the PE instead of GMA grafting. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Temperature (\({}^{\circ}\)C)** &
\begin{tabular}{c} **Half-Life-Time** \\ **DCP (min)** \\ \end{tabular} & **Residence Time (min)** & **Screw Speed (rpm)** \\ \hline
175 & 0.79 & 8 & 4 \\
180 & 0.50 & 5 & 20 \\
190 & 0.21 & 2 & 42 \\
200 & 0.09 & 1 & 66 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Half-life-time DCP at different temperatures. Figure 6: Temperature influence of FD on RPE samples. Having considered the screw speed, as well as the temperature profile of the extruder, it was also reasonable to observe the different properties of the scCO\({}_{2}\) itself. The solubility, solvation power, and transport properties of the scCO\({}_{2}\) were all related to the temperature and pressure [24]. Some experiments were carried out with different pressures (90-170 bar) at a constant temperature (180 \({}^{\circ}\)C). The results are shown in Figure 8.  
Higher pressures did not favor the GMA grafting process. This was also reported by [PERSON] et al. (2004) [40] during the grafting of MAH onto isotactic PP. According to their findings, there was more polymer swelling due to the higher density (increasing solubility parameter) of the scCO\({}_{2}\).