## 1 Introduction  
A rubber compound consists of several components. One of the most important additives are fillers that are added for several purposes, such as to enhance the mechanical properties of rubber, reduce costs or improve a specific property, e.g., electrical conductivity. Carbon black (CB) has been the main reinforcing filler in rubbers over a century. A high reinforcing ability of CB is related to small particles and aggregate size and high structure (shape and level of branching of aggregates) of the filler resulting in a large surface area available to interact with polymer chains and create chemical and physical bonding with rubber [1]. However, CB has a negative influence on dynamic properties compared to silica, the second traditional reinforcing filler, which is currently used, for example, in tire treads. This, together with a high carbon footprint of the CB manufacturing process, increases the requirement for new filler solutions [2]. Nanosized fillers, such as carbon nanotubes (CNTs) and layered silicates, have been widely studied during the last decades [3; 4; 5; 6; 7; 8; 9; 10; 11]. They are promising fillers due to their small particle size and high surface area, and thus they have, in theory, a high reinforcing potential already at low filler concentrations. The practice has shown that the mixing of these nanofillers into rubber is challenging due to the high surface energy and their tendency to form large agglomerates. Several publications have claimed that solution mixing is required for a good dispersion of CNTs [3; 12; 13], but it is not yet an industrially viable method. Instead, the CB-nanofiller hybrid filler systems have been found to lead to improved filler dispersion and improved mechanical properties [14; 15; 16]. For example, by combining up to 5 parts per hundred rubber (phr) of CNTs with CB, a good dispersion of CNTs is achieved in a conventional rubber mixing process [14; 16]. In addition, the hybrid filler system improves the CB dispersion and results in improved mechanical properties [17; 18] leaving, however, the dynamic properties still compromised [14]. Furthermore, natural rubber latex containing a filler combination comprising of CNTs, graphite oxide and CB was found to have better fatigue, crack growth resistance and mechanical properties than the single CB filler system [19]. This was also related to the improved dispersion and synergistic effect. Similar conclusions were made in a study where a graphene nanoplatelet and CB hybrid filler system was studied in ethylene propylene diene rubber [20]. The partial replacement of CB by graphene nanoplatelets was mentioned to reduce the CB aggregation. Moreover, a surface modified expanded graphite was reported to improve the mechanical properties of the CB-filled NR composite [21] as well as an intercalated graphene--CB hybrid filler system in styrene-butadiene rubber (SBR) [22]. A less studied nanofiller type, nanodiamonds (NDs), are considered as a non-reinforcing filler but together with CB it could result in good dynamic properties, as they have been found to decrease the losses of rubbers [23], including low mechanical hysteresis [24]. Such an effect may be related to the reduced internal friction due to the dry lubricating behavior of NDs facilitating the orientation of macromolecules upon the applied stress [23; 25]. Single detonation-produced NDs are 4-6 nm spherical particles consisting of sp\({}^{3}\)-carbon core, thin sp\({}^{2}\)-carbon transition layer and an active surface mostly containing carboxyls, hydroxyls, lactones, ketones and ethers [26; 27]. NDs are known to exhibit synergistic effect with other carbon-based fillers in SBR [28] and epoxy, and the addition of NDs to some rubbers has been shown to improve e.g., elongation at break [29; 30]. The CNT-ND hybrid filler system improved the dispersion of individual filler particles but had a negative influence on mechanical properties compared to CNT filled SBR [28].