## 3 Results and Discussion
### Mechanical Properties of EPDM-PF Compounds  
For the characterization of the reinforcing efficacy, several mechanical tests were conducted, which are summarized in Tables 5 and 6. As visualized in Figure 3a, PF and the type of coagent significantly influence the stress-strain behavior of the compounds. Belt-Ref is characterized by fast growth in modulus, resulting in high maximum stress. By replacing 20 phr CB with PF, the stress-strain curves significantly change towards lower modulus and maximum stress but increased elongation at break (\(e_{\text{b}}\)). Upon adding the (meth)acrylate-based coagents, a stiffening of the compounds can be observed. This is explicable with the increase in cross-link points due to the introduction of the coagents. The mobility of the chain is restricted, yielding a decrease in elongation at break (\(e_{\text{b}}\)). Furthermore, the increased cross-link density is also reflected by the enhanced modulus at 300% strain (\(\sigma_{300}\)) for all hybrid filler systems. While the stiffening of PF-TRIM is pronounced, the other coagents yield stress-strain curves, which are characterized by enhanced elongation at break. The increased stiffening of PF-TRIM could be attributed to the trifunctionality of the coagent, which results in a tight network structure. Unlike TRIM, ZD(M)A can additionally introduce ionic bonds, contributing to a synergistic system of covalent and physical cross-links within the rubber matrix. Especially in saturated rubbers, the physical absorption dominates filler-rubber interactions, resulting in divergent stress-strain curves compared with PF-TRIM [37, 38]. Therefore, ZD(M)A seems suitable for further toughening of EPDM-PF compounds. In addition, postcuring leads to further stiffening of all compounds, which is most pronounced for sole PF-reinforced compounds PF-7 PO and PF-5 PO. Such change indicates the proceeding curing reaction of PF. The tear strength is significantly enhanced upon the introduction of phenol novolac resin compared with Belt-Ref. Due to the presence of in situ formed rigid particles in EPDM-PF compounds, the crack needs to propagate around them and requires higher energy. In the absence of PF, the cracks can propagate more easily, yielding inferior tear properties. Regarding Shore A hardness, the loss of CB can be compensated by adding PF. With the further introduction of ionic coagents, the Shore A hardness is even increased for the hybrid filler compounds. In general, the Shore A hardness is linked to the cross-link density of the rubber compounds, which would suggest the highest hardness for Belt-Ref. However, the rigid resin particles significantly enhance Shore A hardness. Further, [PERSON] et al. reported that ZD(M)A is prone to result in high hardness values [21]. However, the compression set slightly deteriorates upon the introduction of a hybrid filler system. The addition of ionic coagents yields a reduced elastic recovery, which is more pronounced at 100 \({}^{\circ}\)C. As previously mentioned, the ionic clusters dissociate at elevated temperatures resulting in inferior _CS_ values [31]. Contrary to EPDM-TRIM, the sample PF-TRIM also exhibits a worsening of the _CS_ at elevated temperatures. This finding is attributed to the further cross-linking of PF, resulting in the development of a tight and interlocked network. The formation of such a network in the compressed state reduces the overall elastic recovery of the compounds. The dynamic mechanical properties were determined from 24 h postcured samples and are shown in Figure 3c,d. The temperature corresponding to the maximum of tan \(\delta\) (Figure 3d) was taken as the T\({}_{\text{g}}\) of the samples, which was in a similar range for all compounds under investigation. Above T\({}_{\text{g}}\), the storage modulus (E') showed a significant decrease due to the enhanced mobility of the polymer chain. E' loss is superior for compounds containing the phenol novolac resin, which highlights the reinforcing effect of PF.