By Tim Freeman, Managing Director, Freeman Technology

There are many variables that influence powder behaviour. In this article I’m going to take a look at the importance of a another critical particle parameter - shape.  Many powder processors recognise that particle shape influences powder behaviour, perhaps most significantly flowability, but until relatively recently it has been difficult to access any form of quantitative correlation. However, modern advances in shape measurement, as well as those in powder characterisation, have changed that situation and so understanding in this area is now advancing rapidly.

The mechanisms of powder flow

For a powder to flow, the particles within it have to move relative to one another. Many factors influence the ease of this movement, with shape being one of the easiest to understand in a qualitative way. The particles in the figure below have a highly irregular form that makes them prone to mechanical interlocking. This means that they can slot together like pieces of a jigsaw to exert significant resistance to particle-particle movement, an effect that will inhibit powder flow.

Figure 1 – Mechanical Interlocking

The particles on the left are orientated so as to make interlocking likely. However, if reoriented, as shown on the right, the particles are now more likely to glance past each other in a lower energy interaction.  Clearly this is simply a matter of chance. The likelihood of mechanical interlocking can only be reliably reduced by making the particles more regular in shape. A powder constituted of completely spherical particles, for example, has no potential for mechanical interlocking, and, all other factors being equal, would be expected to flow more easily than a powder with irregularly-shaped particles.

Investigating correlations between particle shape and flow energy

A recent study of two different grades of lactose – Spherolac 100 and Flowlac100 – provides evidence of this effect and quantifies the impact of changing particle form. These two powders are closely similar in every way, including particle size, but differ substantially in terms of shape. The Flowlac 100 is the more spherical, a feature that can be quantified using the shape parameter High Sensitivity Circularity (HS Circularity) which is described by the formula:

HS Circularity = (4 x π x Area / perimeter) ²

Perfect spheres have a HS circularity of 1 while less regular forms have values closer to zero. In this study HS Circularity was measured for both samples using an automated image analysis system (Morphologi G3, Malvern Instruments). The Flowlac 100 was found have a median value of 0.91, while the less regular Spherolac 100 had a median value of 0.83.

The flow properties of these two materials were quantified with the dynamic parameter Basic Flowability Energy (BFE) which was measured using an FT4 Powder Rheometer? (Freeman Technology). In dynamic powder characterization the axial and rotational force acting on a blade, as it rotates through a sample, is precisely measured to generate flow energy values that directly quantify the ease with the material flows. The BFE value measured for the Flowlac 100 was around 1200mJ compared to around 2500mJ for the Spherolac 100 illustrating here that the flowability of one lactose grade is approximately half as good as the other, solely as a consequence of differences in particle shape.

Figure 2 - FT4 Powder Rheometer? from Freeman Technology

The practical relevance of shape

This correlation between shape and flow is, of course, of very practical relevance since engineering powders with desirable flow characteristics is essential, both for efficient processing and in many instances for product performance, too. For example, recent research has shown that the flow properties of particles correlate closely with blending performance. The figure below shows how the flow energies of microcrystalline cellulose and sodium benzoate change with impeller speed i.e. whether the powders flow more or less easily at higher flow rates.

The MCC, which has roughly spherical particles, flows more easily at faster rates. The sodium benzoate on the other hand, which has platelet shaped particles exerts more resistance to flow at faster flow rates. Faster impeller speeds simply induce the platelets to lock together more enthusiastically.  In blending trials, the MCC blended more rapidly at higher impeller speeds but with sodium benzoate the opposite effect was observed, where lower blending speeds were more successful. This shape-related result can be readily predicted from the flow energy data.

These examples illustrate how our understanding of the impact of shape on powder flowability is starting to crystallise, and how that knowledge may enhance our ability to control powder behaviour. Such research marks the start of particle shape taking its place alongside particle size as one of the variables that formulators routinely manipulate. This advance, which is underpinned by developments in both powder characterization and shape measurement technology will strengthen our ability to successfully achieve powder processing goals.

Please take a look at the other articles in this series where I examine the effect of other variables, such as particle size.

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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Introduction

Following Tim Freeman's articles on Powder Flow,

understanding powder behaviour to optimise process performance, increase productivity and improve quality.

More Articles by Tim Freeman:

Tim Freeman's Powder Flow Feature: Exploring the Impact of Particle Size on Bulk Powder Properties

Tim Freeman's Powder Flow Feature: What Makes a Powder Flow?

Tim Freeman's Powder Flow Feature: Understanding Powders