Flocculation, Reinforcement, and Glass Transition Effects in Silica-Filled Styrene-Butadiene Rubber

The introduction of silanes to improve processability and properties of silica-reinforced rubber compounds is critical to the successful commercial use of silica as a filler in tires and other applications.  The use of silanes to promote polymer-filler interactions is expected to limit the development of a percolated filler network and may also affect the mobility of polymer chains near the particles.  In this study, styrene-butadiene rubber (SBR) was reinforced with silica particles at a filler volume fraction of 0.19, and various levels of filler-filler shielding agent (n-octyltriethoxysilane) and polymer-filler coupling agent (3-mercaptopropyltrimethoxysilane) were incorporated.  Both types of silane inhibited the filler flocculation process during annealing the uncured rubber materials, thus reducing the magnitude of the Payne effect.  In contrast to the significant reinforcement effects noted in the strain-dependent shear modulus, the bulk modulus from hydrostatic compression was largely unaltered by the silanes.  Addition of polymer-filler linkages using the coupling agent yielded bound rubber values up to 71%; however, this bound rubber exhibited glass transition behavior which was similar to the bulk SBR response, as determined by calorimetry and viscoelastic testing.  Modifying the polymer-filler interface had a strong effect on the nature of the filler network, but it had very little influence on the segmental dynamics of polymer chains proximate to filler particles.