Powder Characterization Techniques For Predicting Segregation

By Tim Freeman, Freeman Technology

The theme of this current series of editorials has been material characterization for specific processes or unit operations. This last 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! In the manufacture of pharmaceuticals, segregation in a feed hopper or feed frame, or during conveying/transport, can have a catastrophic effect on content uniformity. It may also impact processability.

Dynamic powder testing, using a powder rheometer, is one way to evaluate the tendency of a material to segregate. In addition the technique quantifies the impact of segregation on flow behaviour, a key determinant of processability.

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