## 3 Results and Discussion
Therefore, extra grafting experiments were performed with the PE and PP mixtures (Table 2) to check whether the presence of PP had an influence on the grafting degree. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & \multicolumn{2}{c}{**FD**} \\ \hline
**Inlet** & **LDPE** & **RPE** & \
\begin{tabular}{c} **10 wt.\% PP and** \\ **90 wt.\% LDPE** \\ \end{tabular} &
\begin{tabular}{c} **20 wt.\% PP and** \\ **80 wt.\% LDPE** \\ \end{tabular} \\ \hline
0.6 wt.\% DCP & 3.30 & 2.25 & 0.65 & 1.115 \\
10 wt.\% GMA & 2.96 & 2.39 & 0.79 & 1.08 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: FD of several substrates without the use of scCO\({}_{2}\). Figure 5: Grafting efficiencies for LDPE (**a**) and RPE (**b**). It can be concluded that the samples containing PP yielded significantly lower FD values. The different results might have been caused by the involvement of the polymer macro-radical via the chain scission (\(\beta\)-scission) of the unstable PP macro-radicals. This is highly undesirable if a high graft yield is desired for use in a secondary process. On the other hand, the difference in the composition led to the different viscosity of the melt. The rheology measurements pointed out that the viscosity of the RPE was six times higher than that of the LDPE at 130 \({}^{\circ}\)C. This high viscosity led to insufficient mixing in the extruder and favored the cage effect, which may explain the lower FD [14]. The exact reason for this difference is that viscosity was yet to be found. However, it probably stemmed from the post-consumer nature of RPE and the presence (vide supra) of different contaminants. In order to obtain a higher FD value, the use of scCO\({}_{2}\) during the reactive extrusion might help, due to its advantages. According to [PERSON] et al. (2002) [37], who investigated the influence of scCO\({}_{2}\) on the levels of MAH grafted onto polypropylene through reactive extrusion, scCO\({}_{2}\) had a positive impact on the grafting level when working with higher inlet concentrations of MAH (4 wt.%). Moreover, using certain levels of scCO\({}_{2}\) (2, 3 wt.%) led to less chain degradation compared to working with lower inlet concentrations of MAH (2 wt.%). On the other hand, [PERSON] et al. (2010) [29] discovered that both the grafting degree and efficiency drastically improved, with an efficiency up to 85%, when lower concentrations of MAH and DCP were used. The use of scCO\({}_{2}\) gave different results for the two types of PE used, as shown in Figures 2 and 3. ScCO\({}_{2}\) had a positive impact on the FD for the PE-g-GMA (0.6 wt.% DCP and 10 wt.% GMA) prepared with LDPE, reaching an FD of 6.3% compared to 3.2% without the use of scCO\({}_{2}\). Nevertheless, the influence of scCO\({}_{2}\) was more significant at higher concentrations of GMA compared to the results of [PERSON] et al. (2002) [37] and [PERSON] et al. (2010) [29]. On the other hand, scCO\({}_{2}\) had a negative impact on the FD for all the RPE-grafted samples, as can be seen in Figures 2 and 3. According to the gel content measurements with toluene, using higher concentrations of DCP favored a crosslinking instead of a grafting reaction. This observation was also reported by [PERSON] et al. (2017) [38], where an increase in the DCP concentration increased the gel content in HDPE. Furthermore, [PERSON] et al. (2012) [39] studied the use of scCO\({}_{2}\) during a reactive extrusion of PP/polystyrene blends and noticed that chain scission reaction by peroxides became more prominent at high DCP levels. This might also have occurred during our process, in which the use of scCO\({}_{2}\) during the reactive extrusion probably further enhanced the crosslinking effect and, therefore, resulted in lower FD values. Despite the negative initial results of using scCO\({}_{2}\) for the grafting of RPE, scCO\({}_{2}\) has the advantage of lowering the viscosity of the melt phase due to its swelling and plasticizing effects, which can result in a lower process temperature [23; 24; 25; 26; 27; 29].