## 3 Results and Discussion
### Influence of Ionic Coagents on Reinforcement
\begin{table}
\begin{tabular}{l c c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \(\mathbf{t_{90}}\) & \(\mathbf{t_{2}}\) & \(\mathbf{M_{H}}\) & \(\mathbf{M_{L}}\) & \(\boldsymbol{\Delta}\)M & **ML1+4** \\  & **(min)** & **(min)** & **(dNm)** & **(dNm)** & **(dNm)** & **(MU)** \\ \hline EPDM–7.5 ZDMA & 5.3 & 0.66 & 13.5 & 1.55 & 12.0 & 81 \\ EPDM–15 ZDMA & 5.9 & 0.92 & 11.4 & 1.30 & 10.1 & 83 \\ EPDM–7.5 ZDA & 4.2 & 0.50 & 15.8 & 1.59 & 14.2 & 76 \\ EPDM–15 ZDA & 4.1 & 0.67 & 16.7 & 1.51 & 15.2 & 76 \\ EPDM–TRIM & 5.4 & 0.68 & 16.0 & 1.62 & 14.4 & 75 \\ PF–ZDA & 6.5 & 0.72 & 12.6 & 1.11 & 11.6 & 67 \\ PF–ZDMA & 6.2 & 0.77 & 11.4 & 1.11 & 10.3 & 84 \\ PF–TRIM & 8.4 & 0.72 & 14.3 & 1.14 & 13.2 & 59 \\ PF–7 PO & 11.0 & 1.04 & 10.7 & 1.19 & 9.5 & 73 \\ PF–5 PO & 11.6 & 1.33 & 8.4 & 1.19 & 7.2 & 74 \\ Belt–Ref & 5.4 & 0.61 & 19.6 & 1.95 & 17.6 & 89 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: MDR rheometer characteristics and Mooney viscosity of reinforced EPDM compounds under investigation. Figure 2: (**a**) Cross-linking rheograms, (**b**) stress–strain curves, (**c**) storage moduli versus temperature, and (**d**) tan \(\delta\) versus temperature of EPDM compounds with selected coagents. In a further step, the content of the ionic coagents was increased from 7.5 to 15 phr to evaluate how a higher amount of ionic reinforcing domains influences the cure behavior and the viscosity of the compounds. The results revealed that both Mooney viscosity and cross-link density increase with increasing ZDA content, which is related to a higher number of available functional moieties. In contrast, ZDMA does not provide a suitable reinforcing capacity, and 15 phr even contributes to inferior properties. The cross-link density can also be qualitatively assessed by the swelling degree of compounds. A low swelling generally relates to a higher cross-link density due to the higher resistance of the network against swelling. Table 4 summarizes the obtained values for EPDM vulcanizates reinforced with coagents. The results are in good agreement with the MDR data. The reduction of 20 phr CB results for all samples in an increase of swelling degree, indicating a lower cross-link density. Interestingly, 15 phr ZDMA show increased swelling, which leads to the assumption of an inferior curing behavior. Similarly, the gel content of the rubber is reduced for EPDM-15 ZDMA, revealing an inferior cure compared with the other vulcanizates. These assumptions are backed by the calculated cross-link density \(\mathrm{M_{c}}\). The given results refer to the average molecular weight between cross-links, meaning a lower \(\mathrm{M_{c}}\) refers to higher cross-link density [29]. Unsurprisingly, Belt-Ref exhibits the lowest cross-link density due to the reinforcing effect of 50 phr CB. The other samples exhibit reduced M\({}_{\mathrm{c}}\), which reflects the loss of 20 phr CB. However, it is most prominent for EPDM-15 ZDMA, indicating a suppressed network formation. The impact of the coagents on the mechanical and dynamical properties was evaluated and is depicted in Figure 2b-d and summarized in Tables 5 and 6. A stiff material behavior characterizes the stress-strain of Belt-Ref due to the presence of 50 phr CB. The reduction to 30 phr is reflected by lower maximum stress but enhanced elongation at break for EPDM-TRIM. Due to the present ionic and covalent bonds, the molecular chains of ZD(M)A-reinforced compounds can orient along the direction of the external force. Thus, higher elongation at break and improved stress at break than EPDM-TRIM is observed. A higher ZD(M)A content leads to a deterioration of the tensile properties, which is only minor for EPDM-15 ZDA but significant in EPDM-15 ZDMA. This behavior is also explicable with the low cross-link density of EPDM-15 ZDMA.