| One way to obtain a tire that exhibits both low rolling resistance and high wet traction is to use precipitated silica together with solution polymers (rather than emulsion polymers) and a silane coupling agent in the tread. However, the use of silica compounds entails considerable disadvantages in terms of raw material costs and processability (before vulcanization) compared to carbon black-filled treads. Therefore, the improvement in the processability, especially of stored, uncured silica-filled rubber stocks, is an important issue. One of the objectives of this study is to suggest new additives for silica-filled tire tread compounds that retard the degradation of processability while maintaining the mechanical properties of the vulcanizates. To achieve this objective, we improved our understanding of the phenomenon of storage of uncured filled elastomers and its effects on both rheology and processability.; Effects of storage temperature and storage time on the rheological properties of model tire compounds were characterized. Results suggest that filler particles or bound rubber domains were agglomerated to form a network structure; this leads to an increase in a peak stress during a start-up flow and a drop of the extrudate quality, especially in the case of silica-filled compounds. A new method was developed for the evaluation of extrudate roughness by using optical microscopy combined with image analysis. A good correlation was found between the yield stress as measured with the sandwich viscometer, and the peak stress measured on the Mooney viscometer in a low-shear-rate start-up flow (i.e., Mooney peak). In addition, there is good correlation between the Mooney peak and extrudate roughness. Based on rheology and the measured fraction of bound rubber, a physical model for the morphology of filler-rubber structures was proposed, wherein the strength members of an open-cell sponge-like structure comprise filler particles and bound rubber. A mechanism for extrudate distortion was suggested. Moreover, we found that it is possible to improve the processability and increase the storage stability of silica-filled tire tread compositions by using small amounts of certain low-molecular-weight epoxides as additives. In addition, a rheological model for silica-filled uncured rubber compounds was proposed. |
