Powder Flow: Reproducible Powder Testing

Reproducibility, measurement sensitivity and the usefulness of an analysis are inextricably linked. If a technique or instrument exhibits poor reproducibility then measurements are ‘noisy’, so only gross trends or differences can be detected. As the pharmaceutical industry strives for greater knowledge of its processes and products, this becomes increasingly limiting. In the area of powder testing, achieving the required level of reproducibility is especially challenging, but is now essential.

Many traditional powder testing techniques, such as angle of repose, Hausner ratio and flow through an orifice, remain largely manual. As a result, operator-to-operator variability can be an issue. Added to this is the imprecise definition of methods and equipment for such techniques, which has a tendency to inhibit data exchange across different areas of powder testing and powder processing. Beyond these generic issues we also have to consider those that are unique to powders and the effect they have on powder analysis.

A powder’s properties are not defined solely by the immediate environment, they are also influenced by processing history – an often under-appreciated but easily illustrated point. Imagine measuring the angle of repose of two samples, identical apart from their storage history. One has been held under consolidating conditions and the other extracted from a freely flowing process line. Even if the conditions applied during testing of the powders are identical, without appropriate sample preparation the results will be very different.

To accurately define and measure powder properties it is crucial to first establish and apply a reliable baseline state for testing. Eliminating processing history - as far as is possible and practicable - makes comparative testing much more informative, and able to identify the differences between the powders that have a bearing on their behaviour.

Using a powder rheometer, samples are analysed by measuring the forces acting on a helical blade as it rotates through the powder bed in a prescribed pattern. Values of flow energy, a dynamic parameter that quantifies powder flowability, are calculated from the resulting data. Methodologies are well-defined and testing is largely automated, both of which are important for reproducibility, but in addition, the sample is conditioned prior to analysis. This conditioning takes the form of gentle agitation and results in a uniform, loosely packed bed that defines a baseline state for measurement.

Figure 1 – Basic Flowability Energy (BFE)

Powder rheometry offers a level of sensitivity that means any differences detected can be reliably ascribed to a real difference in the sample. In fact, flow energy measurements can differentiate samples that other techniques classify as identical. Using conditioning during the application of other methodologies, such as bulk property measurement, also lends superior reproducibility to these measurements. From a practical perspective this simplifies investigating how a powder will respond to the processing environment: to the introduction of air, for example, or to consolidation; to moisture; to storage – will segregation occur; or to aggressive handling- will attrition be a problem? All important questions during development and into production. In answering them reliably, powder rheometry supports faster, more efficient commercialisation, and the evolution of better manufacturing practice.

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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