## 3 Results and Discussion
Table 2 shows that the Payne effect increases after addition of NDs. The change can partly be explained by the increased total surface area of the fillers but NDs may also suppress the interfacial interaction of CB particles by locating between them. This improves the dispersion of CB and enables the formation of a filler network. At 5 phr ND loading, the Payne effect decreases again indicating worse filler dispersion. The SEM images (Figure 2) show some differences in filler dispersion and distribution in different compounds, although ND particles cannot be distinguished separately. The reference compound (Figure 2a) containing only CB as filler has well dispersed areas but also unfilled and agglomerated areas. In addition, the compound containing 0.5 phr (Figure 2b) NDs has an irregular surface caused by the large agglomerates, which is evidence of insufficient dispersion. Figure 2c,d shows that the filler dispersion is improved. The surface is rather smooth, large filler agglomerates do not exist (except for the two  
\begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Apparent Crosslink** & \multirow{2}{*}{**Bound Rubber (\%)**} & **Payne Effect, G\({}_{0.56}\)–G\({}_{\infty}\)** \\  & **Density (-)** & & **(MPa)** \\ \hline ND 0 & 0.36 \(\pm\) 0.01 & 43.7 \(\pm\) 3.1 & 0.94 \\ ND 0.5 & 0.39 \(\pm\) 0.01 & 42.4 \(\pm\) 6.7 & 1.39 \\ ND 1 & 0.35 \(\pm\) 0.01 & 41.8 \(\pm\) 4.0 & 1.44 \\ ND 2.5 & 0.41 \(\pm\) 0.06 & 44.3 \(\pm\) 1.3 & 1.3 \\ ND 5-filler agglomerates in Figure 2d), and the fillers seem to be more evenly distributed than in the reference compound. The improved dispersion explains the earlier Payne effect results. At 5 phr ND concentration, the filler dispersion becomes poorer again and the large agglomerates are found (Figure 2e), which is the reason for the decreased filler-filler interaction observed from the Payne effect. Although the clear evidence of improved polymer-filler interaction was not obtained from the bound rubber results, the tensile strength of the rubber compound is improved remarkably after addition of NDs as seen in Figure 3. This is new evidence of the improved dispersion. The benefits of the hybrid filler system are partly lost with higher ND concentrations (>2.5 phr) due to the worse filler dispersion and decreased polymer-filler interaction, but the tensile strength is still at a higher level than without NDs. Furthermore, the standard deviation is the highest for the compounds containing 0 or 5 phr NDs, which indicates inhomogeneous compounds due to the poorer filler dispersion. Hence, the tensile strength results support the SEM analysis. The tensile modulus at 300% elongation (M300)  
Figure 2: SEM images from the fracture surface (tensile test) of the compounds (**a**) ND0, (**b**) ND0.5, (**c**) ND1, (**d**) ND2.5, (**e**) ND5. increases first but decreases when the amount of NDs is increased. Moreover, a minor decrease in elongation at break is observed after the addition of NDs for the compounds with the highest M300, but the changes are between the error bars. The decreased elongation at break is commonly attributed to the restricted mobility of the polymer chains. In tear resistance results the trend is opposite (Figure 3d). The tear resistance decreases when 0.5-2.5 phr CB is replaced by NDs but increases again at 5 phr NDs. This is related to the plasticizing effect of NDs and filler dispersion. The poor interface between NDs and rubber as well as the reduced mechanical trapping of polymer chains weakens the tear resistance at low ND loadings. The big standard deviation at higher ND loadings also indicates the poor dispersion of fillers. In addition, wear resistance is improved after addition of 2.5 and 5 phr NDs, although change in hardness is nonexistent (Figure 4). The harder surface of NDs helps rubber to slide on the smooth steel surface.