Quality Control of Cohesive Bulk Solids with the Powder Cell
Stress and strain history is known to strongly affect rheological behavior of materials, and this includes powders. Consolidation stresses occur, for instance, during transport or handling of the powder. Such stresses are usually non-uniformly distributed, leading to inhomogeneous distributions of the packing-density and agglomeration-state. Furthermore, during handling, air can become trapped in the powder bed, which also strongly influences the flow behavior. Therefore, it is important to include a homogenizing preparation step prior to making rheological measurements on cohesive powders.
One homogenization method is to fully fluidize the powder. This is the approach used in the Anton Paar Powder Cell for bulk solids that are sufficiently fluidizable. However, it is difficult to fluidize very fine particles because their cohesive interparticle forces can dominate over body forces and hydrodynamic forces. Such samples are termed "Geldart group C" particles. Thus, an alternative homogenization method is required for Geldart group C powders. Modified starch is an example of a Geldart group C powder. It serves as a thickening agent or stabilizer in foods, as an excipient (inactive component) in pharmaceutical formulations, as an additive in paper coatings, and as feedstock for fermentation processes. The size and size distribution of starch particles depend strongly on the type of crop the starch was extracted from. The mean particle size can vary from 5 μm (rice starch) through 20 µm (corn starch) to 50 μm (potato starch).
Modified starch is starch that has been chemically modified to improve specific properties (e.g. processing behavior). However, chemical modification can also affect rheological properties. Therefore, diverse powder flow behavior can be encountered when dealing with different modified starches. We report here a repeatable and robust method for homogenizing cohesive powders prior to rheological measurements.
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