Assessing powder flowability at low stresses using ball indentation method: Evaluation of constraint factor

Powders can exhibit different flow behaviour resulting from a combination of physical properties of the material and equipment design. Problems with powder flow are ubiquitous in process industry and become prominent when dealing with fine and cohesive powders. It is therefore of great importance to characterise the flowability of cohesive materials for better process control. Powder flowability is commonly assessed under relatively high preconsolidation loads using shear cell and uniaxial compression methods by which the unconfined yield strength (Y) is evaluated as a function of the applied load. However, these techniques are typically limited to applied stresses greater than 1 kPa and require a relatively large quantity of powder. To overcome these limitations, the recently developed Ball Indentation Method (BIM) is used in this work for assessing powder flow behaviour at low stress levels. The unconfined yield strength (Y) is inferred from the resistance to ball penetration into the surface of a powder bed, based on the method for measurement of hardness (H). This requires the flow resistance, represented by hardness, to be related to the unconfined yield strength by a proportionality factor termed the constraint factor, C, following the analogy with yield stress measurement in continuum solids, i.e. Y=H/C. The constraint factor for silanised glass ballotini, calcium carbonate, α-lactose monohydrate, Avicel and limestone is evaluated and reported here. It is shown that the unconfined yield strength inferred by this method correlates well with those from the uniaxial compression and shear cell measurement. The characterisation of the constraint factor makes it possible to use BIM for powder flowability testing at low stress levels and using a very small powder quantity. This is highly desirable for applications such as capsule filling, tableting and dry powder inhaler devices.