The particle size of silica can vary depending on the purpose for which it is to be used. For most applications it is the agglomerate size that is particularly important. Agglomerates are structures composed of aggregates and have sizes ranging from a few micrometers to a few millimeters, depending on the silica product. If the mixing intensity is sufficiently high the agglomerates may again be reduced to aggregates in the target application. The size of silica aggregates generally lies in the region of 100 nanometers. Further reduction in size to a value below aggregate size is possible only under special conditions. Silica primary particles in the size range of about 7 – 50 nanometers play a role in silica production only as short-lived intermediates; they fuse irreversibly with one another within a fraction of a second to give the aggregates described above.
Primary particles of silica are formed as intermediates in production. Within a split second of their formation they sinter irreversibly through covalent bonding, forming aggregates. Although the primary particles can be inferred visually by electron microscopy they cannot be isolated individually and there are no physical boundaries within an aggregate. Primary particles therefore are just a theoretical concept to describe the primary structures within an aggregate. The aggregates thus formed have a diameter of the order of one hundred nanometers. The aggregates subsequently combine to form agglomerates. The agglomerates are the particles that can be found in the product as it is brought to the market. Binding within agglomerates is much weaker than within aggregates and is ascribed mainly to hydrogen bonding and van der Waals forces. For precipitated silicas the production process allows production of agglomerates of definite size; the particle sizes are listed in the product information sheets. In fumed silicas the agglomerates are more easily dispersed and more sensitive than in precipitated silicas; for this reason, no definite agglomerate size can be specified here. For both fumed and precipitated silicas, the aggregates remain unchanged during dispersion under normal conditions: Only with very high energy input is it possible to further reduce the size of the aggregates. Complete fragmentation down to primary particles is not possible.
Technical Overview AEROSIL® - Pyrogene Kieselsäure, Evonik Industries, 2015.
Product Overview Specialty Silica, Evonik Industries, 2018.
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