## 3 Results and Discussion  
### Influence of Ionic Coagents on Reinforcement  
A standard V-belt has to withstand high stresses during operation. Thus, EPDM compounds used for V-belt production typically contain a high amount of carbon black to meet the demands for high tensile strength. It should be noted that for the production of V-belt's, various textile fabrics are applied to further increase the strength of the final products [2; 28]. Herein, a reference EPDM compound was prepared using 50 phr CB (Belt-Ref). At this filler concentration, the mixing and processing performance of the EPDM compound is already negatively affected by the high viscosity of the filler system. In addition, due to the high viscosity of the compound, challenges with efficiently impregnating the reinforcing fabrics are expected to arise. To overcome the high viscosity without losing mechanical strength, it was evaluated if 20 phr of CB can be removed and replaced by the introduction of ionic coagents. Coagents are well known for their boost of cure rate and improved cross-link density. However, in nonpolar rubbers such as EPDM, they can in situ form small domains and generate reinforcing particles in the rubber matrix [20]. For the standard system Belt-Ref, the performance of a nonionic coagent TRIM was compared with ionic zinc di(meth) acrylates (ZDMA and ZDA). The used coagents are depicted in Figure 1 and are characterized by a different functionality. Whereas TRIM exhibits three methacrylate moieties, ZD(M)A only contributes to two functional groups. Therefore, the ZD(M)A content was set to higher levels than the TRIM compounds in the current study. Further, it should be noted that the peroxide content was adapted to lower contents for the ZD(M)A compounds to avoid reversion during rubber cure. A unique property of ZD(M)A coagents is the introduction of ionic bonds due to the present zinc ions. This yields a synergistic reinforcing system comprising covalent and ionic cross-links. The cure characteristics of nonvulcanized compounds containing coagents were determined via moving die rheometer (MDR) analysis and are depicted in Figure 2a and summarized in Table 3. Several critical parameters, including the optimum cure time (t\({}_{90}\)), scorch time (t\({}_{2}\)), minimum torque (M\({}_{\rm L}\)), and maximum torque (M\({}_{\rm H}\)), were studied. The rheogram of the studied EPDM compounds shows the influence of the coagent on cure rate and cross-link density. All samples comprise a decent cure rate, which is attributed to the presence of the respective coagents. However, the introduction of ZDA results in a significant improvement of the optimum cure time. Methacrylates are known to form tertiary radicals, which are much more stable compared with the secondary radicals of acrylates. Therefore, ZDA reduces the optimum cure to a greater extent than TRIM or ZDMA [20]. In addition, the scorch time is influenced, which is defined as the period before the vulcanization process starts. For EPDM-7.5 ZDA, the vulcanization is initiated at an earlier stage than the other samples due to the less stable acrylate radicals [20]. In general, the maximum torque is proportional to the cross-link density of compounds. Belt-Ref exhibits by far the highest maximum torque due to the reinforcing power of CB. As a result, the reduced maximum torque value of the EPDM compounds containing the selected coagents reflects the loss of 20 phr of CB. The cross-link density is strongly affected by the reactivity and functionality of the coagent. Thus, whilst compounds containing TRIM and ZDA exhibit a decent cross-link density, compounds with ZDMA suffer from a low cross-link density.