Effect of drying on the structure and dispersion of precipitated silica

We use a combination of light scattering and ultra-small-angle X-ray scattering (USAXS) to establish the effect of drying on the structure of reinforcing silica. The data show that drying substantially changes the morphology on two different length-scales corresponding to aggregates and agglomerates. The rather gentle process of oven drying leads to deeply interpenetrated aggregates that are clustered into agglomerates that exceed 100 mum in radius of gyration. Spray drying, on the other hand, produces agglomerates in the 20 mum range, made up of aggregates that are weakly interpenetrated. On incorporation in organic rubber, the fillers look structurally identical as far as aggregate morphology is concerned. These results, along with light scattering data on sonicated samples, show that oven-dried powders are made up of robust agglomerates. Spray-dried agglomerates are less interpenetrated and substantially more fragile, making them easier to disperse in rubber by mechanical means. This observation was confirmed by the measuring the modulus and elongation at break measurements of silica filled rubber.