Powder Flow: Evaluating the Powder Testing Toolkit: Tapped Density

In other editorials I’ve been examining modern industrial requirements for powder characterisation and assessing how well different test methods answer to the resulting brief.  In this editorial I want to focus on tapped density methods - their advantages and limitations when it comes to investigating powder flow properties.

Tapped density is based on measurement of the increase in bulk density induced by tapping a powder sample. Bulk density of the sample is first measured in a baseline state, and then again after a defined tapping process. Carr’s Index and Hausner ratio are other ways of representing the ratio of tapped to untapped density, and they enable classification of the powder according to a predefined scale: a Carr’s Index of less than 15, for example, would indicate good flowability.

Such techniques have a number of practical advantages. They are quick, relatively easy to carry out and often inexpensive. They assess an important aspect of powder behaviour - density change as a result of vibration or unidirectional tapping, which always happens during transport and processing However, they only coarsely differentiate cohesive from free-flowing samples.  Moreover, attempting to apply such data to predict the flowability of different samples within the processing environment, quickly highlights some important limitations of the technique.

The use of tapped density measurements to assess powder flowability is based on the idea that the interactions that influence the packing or bulking properties of a powder are the same as those that control flow behaviour. I would argue that although this is broadly true it is not the whole story. The factors influencing bulk density and flowability are not exclusively matched nor do they impact both to the same extent, as the figure below shows.

In this simple experiment the change in bulk density induced by tapping is contrasted with the change in flowability measured directly using a powder rheometer. Flowability changes by an order of magnitude, while the change in bulk density is much more modest.

This comparison highlights two important limitations of tapped density methods. Firstly, they are far less sensitive to changes in flowability than alternative techniques such as powder rheometry. And secondly, they may be extremely misleading regarding the magnitude of the change in flowability that consolidation will cause. So, although such techniques have a place in the modern testing toolkit, these limitations would suggest that they are less than ideal for detailed process design, optimisation, troubleshooting and QC to the standards now required for successful manufacture.

Author Biography

Tim Freeman, Managing Director, Freeman Technology

Tim Freeman is Managing Director of powder characterisation company Freeman Technology for whom he has worked since the late 1990s. He was instrumental in the design and continuing development of the FT4 Powder Rheometer® and the Uniaxial Powder Tester. Through his work with various professional bodies, and involvement in industry initiatives, Tim is an established contributor to wider developments in powder processing.

Tim has a degree in Mechatronics from the University of Sussex in the UK. He is a mentor on a number of project groups for the Engineering Research Center for Structured Organic Particulate Systems in the US and a frequent contributor to industry conferences in the area of powder characterisation and processing. A past Chair of the American Association of Pharmaceutical Scientists (AAPS) Process Analytical Technology Focus Group Tim is a member of the Editorial Advisory Board of Pharmaceutical Technology and features on the Industry Expert Panel in European Pharmaceutical Review magazine.  Tim is also a committee member of the Particle Technology Special Interest Group at the Institute of Chemical Engineers, Vice-Chair of the D18.24 sub-committee on the Characterisation and Handling of Powders and Bulk Solids at ASTM and a member of the United States Pharmacopeial (USP) General Chapters Physical Analysis Expert Committee (GC-PA EC).

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