## 4 Hybrid Filler Networks in Compounds for Tire Application  
The effect of filler network structure is quantified with rheology and mechanical measurements and rationalized with the help of constitutive equations and models. For instance, in ref [36] the authors utilize a hybrid silica/sepiolite filler to tune the mechanical properties of natural rubber formulations. The Payne effect, or the magnitude of G\({}^{\prime}\) reduction upon strain, is related to the energy dissipation during the low-strain dynamic solicitation, representative of the compound contribution to tire rolling resistance. In Figure 4, the trend of G\({}^{\prime}\) for SepS9 and REF-SepS9 shows a lower reinforcing effect but also a lower Payne effect for SepB5 and REF-SepB5. The authors attribute this behavior to the filler network structure, which presents larger aggregated structures for SepB5 and REFSepB5 materials, responsible for a higher stiffness on the unstrained network that eventually breaks down giving an undesirably higher Payne effect. Compounds for tire applications shall contribute with their mechanical properties to the overall tire behavior. It is generally acknowledged that the tread compound contains the whole'magic' of the tire grip on the surface, with little interest in the other components  
Figure 4: Strain dependency of elastic (G\({}^{\prime}\)) and viscous (G\({}^{\prime\prime}\)) moduli for natural rubber silica/sepiolite nanocomposites from ref. [36]. (**a**) the trend of G\({}^{\prime}\) for SepS9 and REF-SepS9. (**b**) the trend of G\({}^{\prime}\) for SepB5 and REF-SepB5  
Figure 3: (**a**) Organomodified sepiolite (Pangel B5) as a single filler in rubber compound; (**b**) Smart-net Silica® featuring stronger interparticle attraction. From ref. [35]. that are supposed to have only secondary functions (e.g., inner liner compound for air retention, sidewall for ozone protection, etc.... ). The tread compound indeed features the most demanding overall set of properties, which has increased in complexity over the last few years: the magic triangle of properties, largely reported in the tread compound literature, shall be more realistically replaced by a'magic dodecagon'. Nevertheless, the tire technologist understands how deep is the interaction between tread pattern, tread compound, and tire structure, and in this holistic picture mechanical properties of each structural compound are expected to play a role. For that reason, the filler or mixture of fillers used in each component determines the main mechanical indicators, in particular in a function of (1) the stability of filler network upon strain, which determine the compound hysteresis, or its capability to dissipate heat under cyclic deformation (2) the stability of the filler network in the temperature range, which is relevant for the consistency of properties in the different conditions in which the tire is operating. The possibility of testing novel nanofillers in the particulate-filled networks of tire compounds has allowed Pirelli to discover new properties trade-off, which can become a precious development tool for the most diverse applications. This is the result of long-standing collaborations with selected universities, which allowed the development of specific knowledge and competencies to study key elastomeric materials topics that can be declined in everyday tire technologist's work of winning rubber compounds trade-offs. A basic example of this concept is patent application WO2012164433 [37] where a novel set of properties was claimed. The innovative formulation containing derivatives of Sepiolite together with the mixture of silica and carbon black showed a steep increase of dynamic modulus while keeping good tensile properties, as shown in Table 1. This allowed using stiffer undertread formulations for better handling performances without compromising high-speed safety. This was demonstrated on two different undertread compounds, to testify the generality of the approach.