Powder Characterisation Techniques for Predicting Segregation

In recent posts I’ve been discussing material characterisation for specific processes or unit operations. Today’s contribution, on the topic of segregation, fits into this category, even though, unlike the other processes discussed, segregation tends to be unplanned and/or undesirable - or indeed both! Segregation is particularly problematic in situations where a small amount of an active ingredient is distributed within a powder blend, with pharmaceutical formulations being the obvious example. In the manufacture of pharmaceuticals, segregation in a hopper or feed frame, or during conveying and transport can have a catastrophic effect on content uniformity. It may also impact processability, although this would be considered a relatively low priority in light of content uniformity issues.

Segregation generally means the separation of one group from another. In powder processing it usually refers to the physical reorganisation of a previously homogeneous blend, most commonly on the basis of size, although other properties such as particle density and shape can also be influential. Segregation proceeds via different mechanisms depending on whether it is promoted by vibration, shear and flow, or aeration of the sample. Understanding these effects is important for the successful identification and resolution of a problem.

Gentle vibration of a sample may encourage smaller particles to move downward, and larger particles to rise to the surface. As the powder is agitated, finer material is able to travel downward filling inter-particulate gaps and forcing larger particles upwards. Segregation of this type is often driven by differences in particle size.

On the other hand, with an aerated sample, especially one close to the point of fluidisation, larger and/or heavier particles tend to sink to the bottom of the sample leaving fines disproportionately distributed in the upper layers. Here, the bulk of the powder behaves like a fluid through which heavier particles fall in the same way as they would through a low viscosity, lower density liquid. With samples containing similarly sized particles of different density it will be those that are denser that concentrate in lower regions of the sample via this sedimentation process.

Dynamic powder testing, using a Powder Rheometer, is one way to evaluate the tendency of a material to segregate and quantifies the impact of segregation on flow behaviour, a key determinant of processability. Two features of the technique are particularly beneficial for segregation studies.

Firstly, it is possible to submit the sample to controlled segregation cycles, well-defined low stress agitation for a set period of time. Determining the extent and rate of change of flow energy as a function of number of segregation cycles assesses the tendency of the sample to segregate. Generally speaking, with samples that segregate easily, flow energy will change significantly and quickly as the number of segregation cycles increases. Secondly, dynamic measurements can be carried out after the sample has been aerated or fluidised. This makes it possible to apply a test methodology that measures if and how the powder tends to segregate as a function of aeration, meaning that alternative segregation mechanisms can be investigated.

Segregation is a major issue, particularly for pharmaceutical manufacturers, with non-uniform distribution of an active ingredient the most pressing driver for its avoidance. Dynamic powder characterisation enables the investigation of both the tendency towards, and the mechanisms of, segregation providing useful information to enable formulators and process engineers to mitigate its effects.