## 1 Introduction  
Ethylene-propylene-diene rubber (EPDM) is a terpolymer of ethylene, propylene, and a nonconjugated diene, and characterized by a fully saturated, hydrocarbon main chain with low levels of unsaturation in the side groups. The presence of the diene enables sulfur vulcanization of EPDM and enhances peroxide cross-linking efficiency. This low unsaturated, nonpolar rubber polymer is known for its good resistance against ozone, oxygen, irradiation, and heat and against polar media, such as aqueous solutions and polar solvents [1]. In industry, EPDM finds application in the manufacturing of high-performance automotive products. A prominent example is the V-belt, which is mainly used for the power transmission between pulleys. Due to its excellent physical attributes, EPDM is a suitable material for such a demanding application [2]. However, the EPDM matrix needs to be modified to unfold its full potential. To obtain superior vulcanizate properties, most rubbers, including EPDM, need to be reinforced with fillers. Today, carbon black (CB) and silica are the most common reinforcing fillers in the rubber industry [3, 4]. The reinforcing effect is not only governed by the chemical nature of the filler but also by several other factors, for example, the size, surface area, shape, and structure of the filler particles, and their dispersion and distribution [5, 6, 7]. In particular, the dispersion of the filler within the rubber matrix plays a distinctive role in the overall reinforcement. Silica fillers are especially known for their strong filler-filler networking, leading to agglomeration and, thus, poor filler dispersion. Furthermore, the mixing and processing of rubber compounds are aggravated by the viscosity increase upon the introduction of solid fillers [8, 9]. EPDMV-belts, on the one hand, require sufficient reinforcement to satisfy the application requirements, but on the other hand, they should also show a good flow for wetting the reinforcing textile during belt production. Achieving a sound balance between these contradictory application and production needs requires some subtle optimization of the rubber compound composition or a specialized approach. Rigid, reinforcing particles can also be introduced in a soft rubbery matrix by the in situ formation of an interpenetrating network (IPN). An IPN is defined as a combination of two cross-linked polymers, where one polymer network is cross-linked in the presence of a second polymer network [10]. For industrial applications, the generation of simultaneous IPN (SIN) is the most feasible approach. SIN formation occurs within a \"one-pot\" reaction, where all components are mixed and subsequently cross-linked simultaneously. The advantage of the IPN/SIN approach for EPDM V-belt applications is that on the one hand, the viscosity of the rubber compound is reduced, and on the other hand, the final vulcanized part is reinforced. The key to successfully forming SINs is the combination of two independent and noninterfering reaction mechanisms. This would be applicable for the radical cross-linking of EPDM polymer and the step-growth mechanism of phenolic resin [10; 11; 12]. Phenolic resins are already of high industrial relevance and are usually applied as tackifiers, cross-linking aids, or reinforcing agents of rubbers [13]. Especially phenol-formaldehyde (PF) novolac resins have been identified as useful reinforcing agents due to their high storage stability. Even though PF is a relatively polar resin, it can be modified with fatty acid tails to blend with nonpolar EPDM [14]. In addition, it has been reported that the further addition of CB and silica has an advantageous effect on the overall reinforcement. The synergistic effect of the filler and PF leads to a more uniform resin distribution with superior mechanical properties [15; 16; 17]. However, a distinct drawback of PF is their interference with peroxide cross-linking.