## 3 Results and Discussion
### Rubber Composites Preparation and Characterization
#### 3.2.5 High Pressure Capillary Viscosimeter (HKV) Tests  
As mentioned in the introduction, the rheological properties are a drawback of silica-based composites. Investigation into the rheological properties of the composites in Table 1 was completed by measuring the Mooney viscosity and by performing the HKV-tests, by applying the methods described in the Section 2. Mooney data are in Table 6.  
The replacement of silica with HSAG led to a slight lowering of the composite Mooney viscosity, whereas the reduction appears to be appreciable for the composite with HSAG-SP.  
The HKV tests were performed in order to study the processability of rubber compounds in the shear rate range typically experienced during the processing. The curves obtained from the HKV -tests are in Figure 7. They show the amount of compound flowing  
\begin{table}
\begin{tabular}{c c c c} \hline
**Composite** & **Silica** & **Silica/HSAG** & **Silica/HSAG-SP** \\ \hline M\({}_{\text{L}(100\text{ }\text{C},\,1+4)}\) & 73.8 & 71.1 & 68.7 \\ \hline \end{tabular}
\end{table}
Table 6: Mooney viscosity of vulcanized composites of Table 1. The Mooney Viscosity, registered as a torque in Mooney Units (MU), was measured at 100 \({}^{\circ}\)C after a preheat time of 1 min and a test running time of 4 min (M\({}_{\text{L}(100\text{ }\text{ }\text{C},\,1+4)}\)).  
Figure 6: Stress-strain curves of vulcanized rubber-based composites containing either Silica, Silica/HSAG, or Silica/HSAG-SP as fillers.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Composite** & **Silica** & **Silica/HSAG** & **Silica/HSAG-SP** \\ \hline \(\sigma_{\text{50}}\) (MPa) & 0.98 \(\pm\) 0.04 & 1.2 \(\pm\) 0.03 & 1.03 \(\pm\) 0.03 \\ \(\sigma_{\text{100}}\) (MPa) & 1.7 \(\pm\) 0.09 & 2.1 \(\pm\) 0.06 & 1.8 \(\pm\) 0.08 \\ \(\sigma_{\text{300}}\) (MPa) & 8.2 \(\pm\) 0.3 & 8.8 \(\pm\) 0.12 & 8 \(\pm\) 0.27 \\ \(\sigma_{\text{B}}\) (MPa) & 19.3 \(\pm\) 1 & 11.7 \(\pm\) 4.3 & 18.4 \(\pm\) 0.16 \\ \(\epsilon_{\text{B}}\) (\%) & 512.4 \(\pm\) 7.9 & 393.6 \(\pm\) 84 & 534.1 \(\pm\) 11.7 \\ \hline \end{tabular}
\end{table}
Table 5: Stress at 50% (\(\sigma_{\text{50}}\)), 100% (\(\sigma_{\text{100}}\)), and 300% (\(\sigma_{\text{300}}\)) strain, stress at break, and strain at break of vulcanized rubber composites of Table 1.  
through the capillary of the HKV versus time. Curves were taken immediately after the preparation of the composites and after 7 days of aging at room temperature.  
Tests performed on unaged samples reveal the appreciably higher flowability of the composite with HSAG-SP, whereas the amount of released silica in the presence of pristine HSAG appears to be only slightly higher. After 7 days storage, the improvement of composite flowability, due to the presence of HSAG-SP, appears to be more pronounced. It is worth observing that the flowability of the HSAG-SP filled composite, after 7 days storage, and of the not stored silica-based composite, are the same. This indicates a high polymer-filler interaction. Moreover, after 7 days storage, the effect of pristine HSAG appears to be lower, which means higher viscosity, due to reagglomeration of the filler.