## 3 Results and Discussion
### Preparation and Characterization of Rubber Composites
#### 3.2.6 Fracture Tests  
In the rubber field, natural rubber is the material of choice to prevent crack propagation. The fracture behaviour of the NR-HSAG and NR-HSAG-SP composites was studied by applying a fracture mechanics approach based on J-testing methodology [73]. Fracture tests allow to evaluate the resistance of rubber in the presence of a defect. Due to the innovative character of the materials and their limited availability, fracture resistance was characterized by the single specimen method based on the J integral parameter, described in Ref. [73] and in Materials and Methods, on quite small specimens. The J integral parameter describes the energy required to produce a unitary fracture surface. Due to the viscoelastic nature of rubber and the size of specimens--smaller than usual--the results are also  
The elongation before fracture decreases by increasing the amount of the graphitic material for all the composites--the material becomes stiffer. It is worth pointing out that the functionalization of HSAG with SP appears to improve the fracture behaviour: curves with and without SP at the same filler amount are similar, but fracture occurs at larger deformation and with higher values of load in the case of HSAG-SP based composites. This causes slightly higher values of fracture resistance, \(J_{\text{c}}\). Average fracture toughness (\(J_{\text{c}}\)) values obtained from Equation (2) are reported with the corresponding deviations in Table 6.  
Composites containing HSAG-SP are tougher; they show higher values of stored energy before fracture initiation with respect to composites containing pristine HSAG, and this difference is significant \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Composite n.** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline
**Filler Type** & HSAG & HSAG & HSAG & HSAG-SP & HSAG-SP & HSAG-SP \\
**Filler Amount** & 5 & 15 & 24 & 5 & 15 & 24 \\
**(phr)** & 5.2 \(\pm\) 0.7 & 3.5 \(\pm\) 0.3 & 2.6 \(\pm\) 0.3 & 5.4 \(\pm\) 0.6 & 4.0 \(\pm\) 0.4 & 3.6 \(\pm\) 0.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: \(J_{\text{c}}\) values for NR–HSAG and NR–HSAG-SP composites. Figure 8: Fracture curves for NR-based composites containing 5 (a), 15 (b), and 24 phr (c) graphitic nanofiller. Lighter continuous lines and darker dashed lines refer to composites with HSAG-SP and HSAG, respectively. Dots and triangles on the curves indicate the points of fracture initiation for HSAG and HSAG-SP, respectively. at 24 phr. It is acknowledged that in tensile tests, undispersed filler particles promote specimen failure. Hence, these experimental findings seem to suggest a better dispersion of HSAG-SP. The difference between HSAG and HSAG-SP based composites appears more relevant for 24 phr, as the filler content. As already reported, the percolation threshold of HSAG in a poly(isoprene) matrix occurs at about 21 phr [39]. Hence, the beneficial effect of a better filler dispersion seems to be more evident above the filler percolation. Similar conclusions were achieved in a recent study [54], which compared the fracture properties of NR/GO nanocomposites via melt and latex compounding. The same fracture method applied in the present work was used, but with different specimen dimensions and at very low filler content (\(<0.5\) phr). In spite of these differences, an improvement in fracture properties with latex blending due to better nanofiller dispersion was observed. It is reasonable that a polar filler such as GO benefit from the blending in rubber latex. This was also the hypothesis of the origin of this work, inspired also by the chance of using a polar graphite functionalized in a simpler and more sustainable way, with respect to GO. Values of pure NR are not reported, although the experiments were performed under the same experimental conditions. As a matter of fact, NR specimens were elongated up to about 700% at complete break during fracture tests. At such a high deformation, the specimen completely lost its shape and stress state completely changed.