Morphology of Highly Dispersing Precipitated Silica: Impact of Drying and Sonication

Abstract

Light and X-ray scattering are used to examine the structure of two commercial precipitated silicas (Zeosil 1165 and Ultrasil 7005) and one developmental precipitated silica, Dimosil 288. All three products have a four-level hierarchical structure consisting of primary particles, aggregates, hard agglomerates and soft agglomerates, with Dimosil 288 showing the clearest evidence of the four structural levels. The impact of sonication and drying protocol was explored by light scattering for Dimosil 288. With the exception of the large-scale soft agglomerates, all the structural levels are robust under sonication of aqueous suspensions. Sonication breaks down soft agglomerates leaving hard structures approximately 11 μm in radius-of-gyration. Drying plays a critical role in hardening the soft agglomerates. If the product is never dried, sonication reduces the agglomerate size to 3.5 μm, which is identified as the size of the hard agglomerate. Although agglomerates larger than 3.5 μm are present in the never-dried product, they are friable. Large-scale agglomerates are marginally more robust when dried at 150 °C compared to room temperature drying. Given the similarity of mechanical properties of rubbers filled with these silicas, it appears that the four-level structure with friable large-scale soft agglomerates is a characteristic of highly dispersing silica products.