Characterization of flow properties of cohesive powders: A comparative study of traditional and new testing methods

• The state of compaction greatly affects the powder flow properties.
• The powder rheometer FT4 allows quick and reproducible measurements.
• The characterization methods have different working ranges.
• The combination of several characterization methods is required.

The characterization of powder flow properties is often required for reliable design and proper operation of industrial processes. The effect of the state of compaction and bed voidage on bulk solids flowability is probably the most critical area of understanding. The goal of the present study is to compare traditional characterization techniques with methodologies provided by the FT4 Powder Rheometer (Freeman Technology). The data from six different methods, covering low to high stress levels, were compared to examine the hypothetical relationships between them. These techniques were also evaluated with regard to their ability to discriminate between different powders. To make a comparison of the testing methods, a range of seven materials was selected to cover the entire range of fine powders, i.e., from nanoparticles to group B powders.The results showed that the characterization techniques clearly have different working ranges depending on the level of cohesiveness of the powder. The powder rheometer was found to allow quick and reproducible measurements of the powder response to various environments. The different blade testing methods provided data that were in good agreement with traditional characterization techniques. However, the powder rheometer measurements were difficult to interpret because they depend on many physical properties and environmental parameters. They were particularly useful to compare similar materials but did not allow good discrimination between very different materials. A more detailed understanding of the physical phenomena involved in blade testing techniques is still needed.Finally, this study showed that powders and bulk materials cannot be viewed as invariant entities. Their flow properties cannot be predicted by only one indicator. The connection of several characterization methods is required to ensure a complete understanding of the powder flow properties over a wide range of conditions. This approach allows better insight into the powder/process relationship.

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