Powder Flow: Powder characterization for QC

Quality control, of either an intermediate material or final blend, places a unique set of demands on an analytical tool. Sensitive differentiation between samples is essential, as is the need to measure a parameter that accurately reflects performance, during manufacture or within the customer’s application. Historically for powders, both these issues have presented a challenge, but the last decade has seen significant advances in testing technology. Universal powder testers now measure a comprehensive set of dynamic, shear and bulk properties with exemplary reproducibility, making them a suitable choice for sensitive pharmaceutical QC.

Powder behavior is complex and not aided by our inability to mathematically define the many variables that influence material characteristics.  The ease with which a powder flows, for example, depends on an array of primary variables, including particle size, shape and texture, but also on many system parameters such as degree of consolidation and moisture level. Poor control of any influential variable will compromise the reproducibility of an analytical technique.

Modern powder testers such as the FT4 Powder Rheometer from Freeman Technology, achieve excellent reproducibility by employing closely defined rigorous measurement protocols, with significant automation, and by conditioning the sample before analysis. Conditioning involves gently agitating the powder to release excess air and/or break up agglomerates, and leaves a homogeneous loosely packed bed. Ensuring that a powder is always measured in this same baseline state significantly enhances reproducibility and, consequently, sensitivity. Differences detected by these systems can be confidently attributed to real differences between samples, rather than variability in the technique.

Figure 1 – FT4 Powder Rheometer®

While precise and accurate measurement is critical for effective QC it is equally important to select a variable that will accurately reflect performance targets. Consider a pharmaceutical manufacturer sourcing alternative supplies of an excipient. The excipient has a specification defined in terms of composition and properties such as particle size and bulk density, but when a new material meeting this specification is introduced into the process, efficiency of production plummets. The new material causes blockages and the final product quality is inconsistent. Clearly the defined specification does not include key variables that directly impact processability and is therefore inadequate for identifying optimal quality/cost supply.

Powder testing instruments that offer multiple methodologies make it easier to quickly identify those parameters relevant to performance. Building a database of shear, bulk and dynamic properties for a material facilitates the correlation of specific variables with aspects of processability or product performance. For the excipient, for example, a short but detailed study may reveal that the specification is more precisely defined in terms of Basic Flowability Energy (BFE), a dynamic term, and permeability, a bulk property, as well as composition and particle size. With a well-defined specification, testing alternative sources of the excipient and identifying the best supplier becomes very much easier.

Simple powder characterization techniques such as angle of repose, Hausner Ratio and flow through an orifice, have a well-established place within the pharmaceutical industry but are limited. These techniques measure just a single property, often with poor reproducibility and are not very searching. In contrast, modern universal powder testers deliver multi-faceted powder characterization and exemplary precision and reproducibility. Automated, with well-defined measurement procedures these instruments enable sensitive specification setting and effective QC.

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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