Influence of storage time and amplitude sweeps on flocculation behavior in silica-filled SSBR compounds

The Payne effect reflects the breakdown of filler-filler networks in rubber compounds, particularly in silica-filled systems. However, during storage, time-dependent flocculation of silica clusters leads to an increase in storage modulus, thereby compromising compound processability and making direct comparisons with freshly prepared materials unreliable. While the Payne effect is observed in both unvulcanized and vulcanized compounds, this work concentrates on the storage hardening of unvulcanized silica-filled SSBR (solution styrene-butadiene rubber) compounds, where both the storage duration and temperature affect the degree of filler flocculation. Also, this effect is significantly more pronounced in unvulcanized compounds, where the absence of a permanent crosslinked network allows greater mobility and reorganization of filler structures over time. The overall goal of this investigation is to develop a simplified, time-efficient test method that eliminates the impact of storage conditions on uncured Payne effect measurements in silica-filled compounds. To address this challenge, two sequential strain amplitude sweeps (low-to-high and high-to-low) were applied using the Rubber Process Analyzer (RPA). These two-sweep measurements help to distinguish between the storage-time-dependent and strain-dependent contributions of the Payne effect, establishing the differences between filler network rebuilding and filler flocculation. Furthermore, it was found that a single sweep from high to low strain effectively removes the storage-time influence and provides a reliable measure of the Payne effect. This offers a practical and time-efficient alternative for quality control in industrial rubber compounding.