## 4 Hybrid Filler Networks in Compounds for Tire Application
Another example is WO2014068451 [38] where the real challenge for a new filler network takes place: high modulus compounds. The apex compound is a little insert in the tire structure but with a relevant impact on driving performance and some applications demand a very stiff compound for quick response under little deformation. Nevertheless, given the peculiarities of apex compound processing, the filler mixture used shall not imply an increase in compound viscosity. Using Sepiolite derivatives inside the filler mixture allowed a significant increase in the dynamic stiffness of the component without compromising the processability of the formulation. Nonetheless, the hysteresis value remained under control, to avoid excessive heat build-up in extreme conditions. This improved hysteresis behavior resulted from the improved filler network stability under strain, which is responsible for a lower Payne effect. The concept of temperature stability of the filler network assumes different meanings according to the specific application considered. Property stability at high temperature results in improved driving in extreme handling conditions, as for WO2014049516 [39] where Sepiolite derivatives allowed this achievement without implying any loss of struc \begin{table}
\begin{tabular}{l c c} \hline \hline  & **Reference** & **Hybrid** \\ \hline Natural Rubber & 100 & 100 \\ Carbon Black & 63 & 56 \\ Sepiolite Pangel B5 & & 9 \\ \hline Tensile modulus at 100\% strain (MPa) & 4.5 & 6.5 \\ Tensile Strength (MPa) & 19.5 & 20.1 \\ Elongation at break (\%) & 374 & 350 \\ \hline E\({}^{\prime}\) (70 \({}^{\circ}\)C, 100 Hz), MPa & 16.9 & 20.6 \\ tanD (70 \({}^{\circ}\)C, 100 Hz) & 0.19 & 0.188 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Properties of formulation for tire undertread taken from ref. [38]. tural strength or other properties correlated. Looking at the lower end of the possible tire operating temperature range, patent WO2019106562 [40] describes an application for a Sepiolite-derived material where the temperature stability of the modulus is verified over a wider temperature range, where wet, dry, and high-performance behavior can be evaluated. The use of Sepiolite derivatives provokes a decrease of compound stiffness temperature instability, keeping a high level of dynamic reinforcement, good mechanical properties, and low hysteresis in standard driving conditions, as reported in Table 2.  
Sepiolite has playful chemistry and when its surface is specifically modified, the trade-off game moves to one of the biggest challenges of the last 20 years in tire technology: decreasing the tire rolling resistance, which represents the tire contribution to the overall car fuel consumption as a result of its interaction with the road. The study of sepiolite surface modification led to the industrialization of a family of Pirelli customized materials represented by the Smart Net Silica(r) registered brand. When the Sepiolite is covalently bonded to the polymeric matrix, a huge advantage in hysteresis is achieved, as testified by patent applications WO2018207068 [41], EP3289013 [42], WO2018116125 [43] that cover different applications from tread to structural compounds (Table 3). Aside from the performance indicators, the approach is environmentally sustainable and represents a strategic asset for the tire developments of the next decades, in the frame of the company sustainability approach [44].