The majority of today's electro-photographic toners incorporate an external additive(s) to fine tune their desired performance attributes. One of the key additives found to be effective at influencing powder flow, charging behavior, and charge stability of conventionally milled or chemically produced toners is fumed silica and more so if the silica has been rendered hydrophobic.Fumed silica is produced via a pyrogenic process and results in primary particles coalescing, fusing and sintering to form particles with an aggregated structure. This secondary particle has a size ranging from approximately 100 – 200 nm depending on the process conditions for the specific silica grade. These aggregates form hydrogen bonds via the silanol groups and also become mechanically intertwined forming a tertiary structure called agglomerates. The size of these agglomerates can span from several microns to several hundred microns. Due to the much larger size and irregularities associated with an agglomerated particle, more aggressive shear conditions would be required to distribute the silica more uniformly on the toner surface. Such conditions could lead to silica becoming embedded and affect the toner itself. In the end, the structure and size of the agglomerate results in particle dispersibility unto the toner being compromised. Poor dispersibility of the external additive can lead to non-uniform behavior with respect to the toner's flow, charge and stability leading to sub-standard print performance and longevity.In an earlier paper we presented a study comparing structure modified de-agglomerated fumed silica to that of traditional fumed silica. The conclusion then was that de-agglomerated silica dispersed more efficiently and thereby led to improvements in toner flow, charging characteristics and charge stability. In this study, the work is a continuation and extension of the effects structure modification has on performance.
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