PhD Work of Pauline Grau
Various routes are used to obtain a new material : the synthesis of a new species, the copolymerization of at least two monomers, or a mixture of at least two species. The latter route provides new properties by combination of those of each component. For a lower cost, this technology is widely used in the industry to get the balance of properties targeted for customers. This is particularly the case in the tire materials for which we have to optimize properties often antagonists: rolling resistance, traction and wear.
Solvay, the world leader of precipitated silica for tire applications, offers a complete range of silica (ZEOSIL) to greatly reduce the rolling resistance of the tread while maintaining excellent adhesion and wear properties. In a binary mixture of elastomers, however, the impact of the preferential localization of the filler in one or the other phase or at the interface, on the properties is not fully understood while it is a way to control the tread performances.
The first part of our study was therefore to identify the parameters affecting the microstructure of the material and to quantify their respective contributions. We have shown that the matrix-filler interactions are a powerful force guiding the charge organization in the blend but that these effect could be countered by changing the process. Due to their high viscosity, filled elastomers can exhibit morphologies which are very far from thermodynamic equilibrium ; by playing, for example, on the order of introduction of the individual components, various “kinetic” morphologies can be generated.
To examine the morphologies obtained, an innovative and original microscopy technique has been developed. Traditional methods of electron microscopy does not generally differentiate polymers having very similar chemical structures, and visualization techniques in transmission often lead to misinterpretation of the location of loads in such mixtures due to high volume fractions studied (about 20%).
Thus, with this methodology, we were able to clearly demonstrate the impact of filler-polymer interactions on the location of the particle (silica is systematically present in the more polar elastomer when all components are introduced at the same time) but also that we could "force" the location of the particles in the phase for which it has the lower affinity thanks to a "masterbatch" process (with the condition that the viscosity of the polymer is sufficient). We also show that depending on the viscosity level of the masterbatch in the nonlinear regime, and that of the second polymer, the uniformity of the silica distribution is strongly impacted.
Measuring dissipative properties of these materials indicates that it is possible to widely tune the range of dissipation (in frequency and / or temperature) of the material by adjusting the location of the silica in the polymeric phases. Reinforced material modulus in the linear regime is completely correlated with the generated microstructure. The drivers for optimizing material performances were identified and controlled, and thus allow the development of reinforced materials with properties approaching the expectations of the tire manufacturers.