## 3 Results and Discussion
### Rheological Characterization and Curing Behavior of Hybrid Reinforced EPDM Compounds  
Resins are suitable for introducing reinforcing, rigid particles in a soft rubber matrix. PF has already been employed as a reinforcing agent for EPDM, but a distinct drawback is the scavenging effect of PF on peroxide-cured vulcanizates. In the previous section, it was demonstrated that ionic coagents are a suitable way to introduce reinforcement into the EPDM matrix. However, they also positively affect the curing efficiency of peroxide-cured rubbers, making them attractive for hybrid filler systems. In the following, the hybrid reinforcement (ionic and nonionic coagents in combination with PF) is compared to Belt-Ref and compounds only reinforced with the PF resin. The obtained curing characteristics for hybrid reinforced compounds are depicted in Figure 3a and are summarized in Table 3. The rheogram of Belt-Ref is characterized by a plateau, whereas solely PF reinforced compounds PF-5 PO and PF-7 PO exhibit a marching modulus. In addition, M\({}_{\text{H}}\) decreases in the presence of PF, which can be only partly compensated by increasing the PO content from 5 to 7 phr. The results confirm the radical scavenging effect of PF on peroxide cure and give rise to incomplete curing of the network. However, the cure rate is significantly improved in the presence of ionic coagents, which yields a plateau of the curve and reduces t\({}_{90}\). M\({}_{\text{H}}\) can be further increased by incorporating TRIM as a coagent, which comprises a higher functionality than ZD(M)A. Although the compound PF-TRIM benefits from a high M\({}_{\text{H}}\), a higher concentration of (meth)acrylate moieties is also known to increase the cure time, which is evident for the prolonged t\({}_{90}\) in PF-TRIM [20]. Again, the scorch time is reduced by the introduction of coagents to the base matrix. However, PF seems to act as a retarder which results in a higher scorch safety for the hybrid compounds. Therefore, a longer time frame for processability is given. A further distinctive parameter is \(\Delta\)M, which is related to the cross-link density of the compounds. The introduction of PF results in a lower cross-link density compared with Belt-Ref. However, the introduction of the coagents positively impacts the overall cross-link density of EPDM-PF compounds, following the trend of PF-TRIM \(>\) PF-ZDA \(>\) PF-ZDMA. M\({}_{\text{L}}\) is decreased significantly upon the addition of PF to the matrix. However, the type of coagent seems negligible for M\({}_{\text{L}}\), which appears to be mainly governed by the type of resin. Resins are already reported as suitable plasticizing agents and improve the general flow behavior of the compounds. Low values for M\({}_{\text{L}}\) and Mooney viscosity are therefore seen as beneficial for the processability of rubber compounds [32]. Especially PF-ZDA and PF-TRIM exhibit advantageous processing performance compared with Belt-Ref. For a better understanding of the overall network composition, swelling measurements were performed. Further, the swelling degree is also a suitable method to determine the influence of the post-curing process on the EPDM/PF compounds. The obtained values for hybrid-reinforced EPDM vulcanizates are summarized in Table 7.  
In general, highly cross-linked networks show lower swelling due to the tightly locked interface and high solvent resistance [33; 34]. As expected, EPDM-PF compounds without coagent exhibit the highest swelling degree due to their overall lower cross-link density. In addition, the swelling degree declines in the order of PF-ZDMA > PF-ZDA > PF-TRIM, which indicates the highest restriction of swelling for PF-TRIM. PF-ZDA and PF-TRIM are even in a similar range as Belt-Ref, indicating a suitable method for replacing 20 phr CB. Postcuring of the samples yields a further reduction of the swelling for all EPDM-PF compounds.