The Flowability of Powders and the Effect of Flow Additives
The flow properties of powders are often intentionally modified by the addition of flow additives, lubricants or glidants in order to improve their processability. The flow additive has the effect of reducing the frictional resistance of particle to particle movement and therefore changes the resistance to shearing and the flowability of the bulk powder.
This study investigates how the bulk and flow properties of a potato starch and a corn starch are affected by small amounts of a fumed silica flow additive (AEROSIL® 200). A powder rheometer was used to measure dynamic or three dimensional flow properties as well as shear properties, including their yield loci when pre-consolidated and their shear strength (cohesion) at near zero consolidation stress.
Earlier studies have indicated that optimum levels of additive may exist for a given flow regime but that there is not likely to be a single level that optimises flowability in consolidated, conditioned and aerated or fluidised flow regimes. This study determines optimum levels for each powder type in each of these regimes and reviews the consequences of using flow additives in flow regimes for which they were not intended. The effects upon a range of other bulk properties such as bulk density, compressibility and permeability characteristics are also determined and reviewed.
Keywords: powder, flowability, powder rheometer, flow additive, shear, permeability, compressibility, wall friction.
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