Flowability – the ease with which a powder flows – is arguably the most important characteristic of powder behavior. For pharmaceutical manufacturers, engineering the right flow properties has always been the key to many processes, one important example being the successful production of quality tablets at the required rate. Today though, the increasing emphasis on more efficient manufacture and continuous processing intensifies the need for reliable, relevant and precise flowability measurement. This changing climate makes it timely to review traditional flowability test methods, examining both their value and limitations.
The harmonized pharmacopoeial chapters on powder flowability testing (USP chapter 1174 and EP 2.9.36) describe four of the techniques most commonly applied within the pharmaceutical industry: shear testing; flow through an orifice; Compressibility Index/ Hausner Ratio; and angle of repose. These chapters were released to encourage more standardized practice, thereby clearly acknowledging a fundamental limitation with these techniques: imprecise definition. Three of the four methods listed above exemplify some of the simplest powder testing methodologies used to quantify behavior.
Angle of repose is ‘the constant three-dimensional angle (relative to the horizontal) assumed by a cone-like pile of material formed by any of several different methods’ (US Pharmacopoeia chapter <1174>). Larger angles are associated with stronger inter-particulate forces and therefore poorer flowability, ‘excellent’ being the classification routinely reserved for materials with an angle of 25 to 30o.
Compressibility Index and Hausner ratio are measured by comparing the unsettled volume of a sample with the tapped volume. As the sample is tapped the particles jostle to a closer packing, causing an increase in bulk density. Here, significant change on tapping is associated with poor flowability, with more free-flowing materials maintaining a more constant volume/density.
Flow through an orifice is perhaps the most intuitive and simply involves measuring mass or volumetric flow rate through a given geometry. There is no general scale of flowability because the geometries used can vary considerably depending on the device selected.
These tests reflect certain fundamental aspects of powder behavior. More cohesive powders have a greater tendency to form a steeply sided cone and do tend to flow relatively poorly. They also have a propensity to entrain significant amounts of air, which may be released by tapping, causing a dramatic change in volume. All three techniques therefore provide some insight into flowability and, for the most part, give a qualitative indication of comparative performance, when used appropriately. Where there is a need for more in-depth, process-relevant understanding, however, or a requirement to sensitively differentiate between two closely similar powders, these methods have less value.
For example, if a powder has a Hausner Ratio of 1.11 it would be classified as having excellent flowability, with an expectation of free-flowing behavior. The question is: what does this mean in terms of processing? If this powder were blended under low stress conditions it is likely each particle would disperse well and blend homogeneity would be readily achieved. However, when subjected to higher stresses and forced flow, such as in the feedframe of a tablet press, it could lock up and perform very poorly. The key to efficient powder handling is matching powder behavior to the processing environment, so labelling a powder as ‘good’ or ‘bad’ can be misleading. Furthermore, a powder with a Hausner ratio of 1.0 would be identically classified, although clearly not identical! If two such powders were capable of behaving differently in a process then it is important to apply a technique that differentiates them.
So, there are some limitations with the traditional toolkit and these are becoming more important as our need for information grows. In my next editorial I’ll be looking at the issue of reproducibility, a further limitation of thes...
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