## 3 Results and Discussion
### Preparation and Characterization of Rubber Composites
#### 3.2.4 Quasi-Static Tensile Tests  
Quasi-static tensile measurements were performed on vulcanized composites, based on HSAG and HSAG-SP. A gum stock, with vulcanized NR and without any filler, was tested for comparison. Data from the measurements, with standard deviation, are given in Table 5 and representative stress-strain curves in Figure 6. Upon adding nanographite (with or without SP) to NR, higher stresses at every strain were obtained. HSAG and HSAG-SP based composites show comparable values of stress for a given value of deformation. As far as the ultimate properties are concerned, the strain at break decreased for all the composites loaded with the graphitic filler. The stress at break of the composites with 15 and 24 phr of graphite has substantially the same value as the NR gum stock, whereas it is lower for the composite with 5 phr, particularly with HSAG as the filler. As a matter of fact, at all the filler loadings, HSAG-SP led to higher values of elongation at break and stress at break, with respect to pristine HSAG. As far as the strain at break is concerned, the results in Figure 5 and Table 5 are in line with those reported in the literature, although the level of filler content here explored is much larger. In most literature works based on composites from NR latexes, the strain at break was found to decrease, to a different extent, with the content of GRM, by using thermally [45] or chemically [46] reduced graphene oxide, mechanically exfoliated graphite [53] or GO [48; 51]. In these works, the maximum amount of the graphitic material was 4-5 phr (or %). When the GRM content was lower than 1%, the strain at break was substantially unaltered [51; 54; 55]. An increase of \(\varepsilon_{\rm B}\) was also reported, however with graphene nanosheets prepared through the oxidation of graphite worms and dispersed with the help of ultrafine carbon black and silane as coupling agent and adopting 4 phr as the maximum content of the filler [57]. In the case of composites based on synthetic isoprenne rubber and the same HSAG used \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Composite n.** & **0** & **1** & **2** & **3** & **4** & **5** & **6** \\ \hline
**Filler Type** & NR & HSAG & HSAG & HSAG & HSAG-SP & HSAG-SP & HSAG-SP \\
**Filler Amount** & \(0\) & \(5\) & \(15\) & \(24\) & \(5\) & \(15\) & \(24\) \\
**(phr)** & & & & & & & \\ \hline \(\sigma_{50}\) (Mpa) & \(0.57\pm 0.02\) & \(0.6\pm 0.01\) & \(0.7\pm 0.10\) & \(1.1\pm 0.06\) & \(0.5\pm 0.02\) & \(0.8\pm 0.03\) & \(1.0\pm 0.02\) \\ \(\sigma_{100}\) (Mpa) & \(0.8\pm 0.02\) & \(0.8\pm 0.01\) & \(1.2\pm 0.06\) & \(1.9\pm 0.10\) & \(0.8\pm 0.01\) & \(1.3\pm 0.03\) & \(1.9\pm 0.04\) \\ \(\sigma_{300}\) (Mpa) & \(1.93\pm 0.08\) & \(2.53\pm 0.04\) & \(6.04\pm 0.31\) & \(10.5\pm 0.6\) & \(2.31\pm 0.02\) & \(6.19\pm 0.22\) & \(9.91\pm 0.24\) \\ \(\sigma_{\rm B}\) (Mpa) & \(24.5\pm 3.4\) & \(13.6\pm 1.9\) & \(21.4\pm 1.1\) & \(24.3\pm 1.8\) & \(19.6\pm 0.8\) & \(23.1\pm 1.6\) & \(25.4\pm 0.6\) \\ \(\epsilon_{\rm B}\) (\%) & \(760\pm 37\) & \(561.2\pm 3\) & \(516.5\pm 10\) & \(468.7\pm 35\) & \(633.7\pm 18\) & \(536.1\pm 4\) & \(493\pm 1\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Stress at 50% (\(\sigma_{50}\)) and 100% (\(\sigma_{100}\)) strain, stress at break, and strain at break of NR/HSAG and NR/HSAG-SP vulcanized samples. in the present work and prepared via melt blending, a consistent reduction of \(\varepsilon_{\text{B}}\) was observed with a filler content up to 60 phr [44]. The examination of literature stress at break data does not allow a straightforward summary and this hampers the comparison with the results reported in this manuscript. With reduced graphene oxide, the value of stress at break increases and then decreases, moving from 1 to 4 phr in ref [45]; an optimum level of 0.5 phr is reported in [46], where the strain induced crystallization of NR is supposed to play a main role.