## 3 Results and Discussion  
The replacement of small amounts of CB by NDs can be expected to shorten the vulcanization time due to the high thermal conductivity of NDs [37; 38] and thus due to the more efficient thermal energy transfer and faster temperature increase in rubber. This kind of behavior has been observed for CNTs [39] and for the CNT-CB filler combinations [14] as well as in the earlier ND studies [40]. However, the effect of NDs was opposite in the current case. Figure 1 and Table S1 (Supporting Information) shows that both the scorch time and the vulcanization time increases when the concentration of NDs increases. The carboxyl functional groups are known to adsorb the accelerator [41]. Further, NDs decrease the maximum torque values (Table S1). Although NDs have similar functional groups on the surface than CB, the surface of NDs is more polar. This has a negative influence on the polymer-filler interaction and affects the maximum torque together with the adsorption of curatives. This indicates a lower reinforcing ability of NDs. Although the rheometric torque indicated lower networking for the compounds containing NDs, the apparent crosslink density determined by the swelling study increases marginally when a part of CB was replaced by NDs, as seen in Table 2. Although swelling study is commonly accepted as a qualitative method for crosslink density, the filler volume and filler dispersion influence the swelling study results [42] as the swelling behavior of bound rubber and occluded rubber differ from the unfilled rubber. Replacing CB by the same weight amount of NDs decreases the total volume fraction of fillers as NDs have higher particle density than CB [43; 44], but probably increases the total interfacial area through the improved CB dispersion, which leads to the increased apparent crosslink density determined by the swelling study. The polymer-filler interaction was studied by the bound rubber content. The bound rubber describes the polymer fraction adhered to filler by strong covalent bonds or by weak physical bonds. The bound rubber decreases the mobility of the polymer chains and makes the adhered polymer insoluble to solvents and thus reinforces the polymer. The higher the bound rubber content, the better the polymer-filler interaction. Table 2 shows that the bound rubber content decreases slightly at 0.5 and 1 phr ND loadings but increases back to the same level with the compound containing only CB at higher ND loadings. However, on the basis of the standard deviation, the direct conclusions are impossible to make. In this study, the physical and chemical bonds were not separated. Thus, the bound rubber results contain also the physically bonded or mechanically trapped rubber, i.e., occluded rubber. Due to the polar functional groups on the surface of NDs, covalent bonds with non-polar NR and BR are not likely to occur [24; 45], and the rubber chains could slide along the surface of NDs showing some plasticizing effect. Thus, a decrease in bound rubber content is natural, especially if the ND particles are well dispersed and distributed. CB exists in aggregates and polymer chains can be trapped into the voids of these aggregates. In well dispersed NDs, these voids do not exist, and the amount of occluded rubber is reduced. The state of dispersion was analyzed by the Payne Effect and SEM. The Payne Effect is determined by the difference between the complex shear modulus at low strain and at high strain. At low strain, filler-filler interaction is strong, and fillers stay in agglomerated form. When the strain is increased, the filler particles are pulled apart and the filler agglomerates are broken into smaller aggregates, and the modulus of rubber decreases. Hence, the Payne effect describes the filler-filler interaction. The higher the Payne Effect, the higher the filler-filler interaction and, in the case of nanofillers, the more likely the formation of a three-dimensional filler network instead of an aggregated filler structure [46].