## 4 Conclusions  
In this work, the grafting of GMA onto LDPE and RPE was investigated. Several process conditions, such as concentrations of DCP and GMA, the use of scCO\({}_{2}\), and process temperature and reaction time, were adjusted in order to provide a better insight into which of the settings influenced the grafting process and adhesion properties the most. Increasing the DCP and, especially, GMA concentrations initially resulted in a higher FD in both substrates but, at higher concentrations, a decrease was shown due to the occurrence of more unfavorable side reactions. The influence of scCO\({}_{2}\) clearly showed different outcomes for the two substrates used. For all the grafted samples, the LDPE samples showed the highest FD and using scCO\({}_{2}\) had a positive impact. However, the scCO\({}_{2}\) had a negative impact on the FD of all the grafted samples made out of RPE, enhancing more unfavorable side reactions. Adjusting the temperature and rotational screw speed in accordance with the half-life time of the DCP resulted in higher FD, with the highest FD recorded as 6.7%. This value was obtained under the following conditions: 10 wt.% GMA, 1 wt.% DCP, a temperature of 200 \({}^{\circ}\)C, and a screw speed of 66 rpm. The tensile test showed that all the grafted samples appeared to have better adhesive properties. However, the tensile property increase for the recycled LDPE was less pronounced. An increase in FD for the grafted LDPE resulted in higher maximum stress. Even better were the results for the grafted RPE. These samples did not even need extremely high FDs to make the aluminum plates break.  
On a more general level, the purpose of this work was twofold: on the one hand, to investigate the possibility of using a post-consumer RPE for grafting purposes, and, on the other, to use scCO\({}_{2}\) as a processing aid. Both questions were shown to have a positive answer as far as the mere possibility is concerned. However, the scCO\({}_{2}\) seemed to have a slightly negative effect on the FD values, although the validity of this statement needs to be checked by analyzing other polymers before it can be stated generally. The use of a post-consumer RPE was shown to have little influence, if any, on the quality of the end-product, envisioned here as an adhesive. However, form a purely scientific point of view, the presence of contaminants does require extra effort in terms of characterization and dedicated extra experiments in order to fully elucidate the influence of different processing parameters on the final product structure. The strategy used in this work, including, for example, the use of pure LDPE/PP blends to clarify the obtained FD values, might be seen, in this context, as a general approach to the presence of polymeric contaminants.  
Conceptualization F.A.V. and F.P.; methodology F.A.V. and F.P.; investigation F.A.V., B.B.B., J.A.J. and F.P.; resources F.P.; data curation B.B.B. and F.A.V.; writing- original draft F.A.V.; writing-editing and review F.A.V. and F.P.; supervision F.P.; funding acquisition F.P. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by SNN (Northern Netherlands Alliance) and the Province of Groningen via an EFRO (European Fonds for Regional Development) subsidies program.  
Not applicable.  
The authors declare no conflict of interest.