Abstract :
[en] Tires are essential to our societies as transportation of people and goods are keys to our
economies and social interactions. They need to provide satisfying performances in terms of safety,
service life and fuel consumption simultaneously. The famous “magic triangle of tire tread” is
constituted of the three main parameters that define a tire tread performance level: wet traction, abrasion
resistance and rolling resistance. To overcome the boundaries of this triangle requires constant research
and engineering work, as these properties work against one another. For instance, lowering the rolling
resistance allows to reduce fuel consumption, but also reduces wet grip performances and therefore
compromises safety.
The introduction of silica fillers in combination with silane coupling agents is the major
innovation that helped to mitigate this issue. It provides better wet traction as well as rolling resistance
compared to regular carbon black compounds. The drawback of silica is its tendency to form aggregates
in rubber via its polar hydrophilic groups: silanols. They prevent silica particles from interacting with
non-polar hydrophobic polymers commonly used in tires. The effectiveness of a reinforcing filler arises
from its ability to create a percolating network and to interact with the polymer chains of the matrix.
Therefore, silanes are essential to silica for rubber reinforcement. They chemically react with both the
silica particles and the polymer chains and grant silica a great dispersibility by converting polar silanols
to non-polar organic moieties. They also enable silica particles to become crosslink points in the rubber
composites through the reaction of the silane with the polymer chains. Currently, state of the art silanesilica
filler systems consist in 10-50 nm silica particles. Upon mixing in rubber matrix, they form a
fractal percolating network. The silane carries a sulfide moiety that allow the silica-rubber crosslinking
reaction. The limitation of such system resides in two aspects. First, the use of a single silane providing
simultaneously coupling and dispersion of the filler prevents to optimize both of these parameters.
Secondly, the constitutive aggregates of the percolating network may be subjected to dislocation under
stress. This causes unnecessary energy dissipation and irreversible weakening of the material over time.
Addressing these two problematics is of great interest for the improvement of tire rubber performances.
For this reason, the present doctoral thesis aims to study and understand the influence of silica
surface modification and morphology over mechanical properties of a tire tread rubber compound and
more specifically its wet grip, abrasion and rolling resistance indicator. Firstly, we investigated the
activation potential of two strong bases, namely sodium hydride and DBU, on silica surface reactivity
toward silanization. Then, we studied the effect of coupling and dispersion role dissociation as well as
silane length on rubber mechanical properties by synthesizing new dual-silane pre-treated silica fillers.
Finally, we present the synthesis of a unique regio-selectively modified dendritic silica and studied the behavior of the subsequent composite materials by assessing the effect of particle porosity and chemical
modification.
Our findings show that the dissociation of coupling and dispersive silane greatly impact rubber
properties. The combination of short mercapto and alkylsilane has proven to improve wet grip of
composite with a stable rolling resistance indicator. On the other side, the use of longer alkylsilane
completely changed the polymer-filler dynamic and resulted in phase segregation around fillers and
poor reinforcement. Finally, the spatial segregation of coupling and dispersive function with the newly
synthesized regio-selective modified dendritic silica particles induced remarkable changes in the
reinforcing behavior. When compared to equivalent non-porous and non-modified silica particle, the
permeation of polymer chains in the porous structure and their crosslinking with the coupling silane
enable reinforcement while near-perfect dispersion is ensured by the dispersive silane on the outer
surface. Unfortunately, this reduces the filler-filler interaction and consequently limit the reinforcement
of the rubber. The introduction of a small amount of the fractal filler allowed to fill the gaps between
the bigger porous particles. It proved to drastically improved the inter-particles interactions, and thus
the reinforcement and dynamic properties of the material beyond previous levels. This work puts light
on the potential of porous silica and dual-fillers systems as potent filler for tire tread rubber.
The outcomes of this doctoral thesis participate to better understand the impact of
chemical and morphological modifications of silica filler surface, and how it can serve the improvement
of the filler technology for the tire industry. It develops the use of porous silica particles as potent fillers
for tire tread rubber and brings new insights on reinforcement mechanisms as well as new possibilities
in filler architecture and chemistry.
