Three techniques that score highly against these criteria and have proven especially relevant for process-related studies are shear, bulk and dynamic property measurement.
Shear analysis, which was developed in the 1960s , is particularly useful for hopper design and, more generally, for characterizing consolidated, cohesive powders. Modern instrumentation with well-defined methodologies and a high degree of automation has brought enhanced reproducibility and reliability, ensuring the place of shear analysis in the modern powder testing toolkit.
Bulk property measurement, i.e., the determination of bulk density, permeability and compressibility, though well-established, similarly has benefited from instrument refinement. Bulk property data may be used directly in process design calculations and provide a general insight into powder behavior that supports the prediction of performance in certain processes.
Dynamic powder testing methods, which were developed in the late 1990s, marked a step change in powder characterization, rather than refinement of an existing technique. Dynamic characterization involves measuring the axial and rotational forces acting on a blade as it traverses through a sample along a fixed helical path to generate a value for flow energy (Figure 2). This value directly quantifies powder flowability, i.e., the ease with which the powder flows. The technique is highly sensitive and has the distinct advantage of allowing powders to be characterized in a consolidated, conditioned, aerated or even fluidized state. It can directly measure the response of a powder to the introduction or release of air.
Let's now look at how shear, bulk and dynamic property measurement can provide insights on the impact of humidity on two different, industrially relevant powders.
THE IMPACT OF HUMIDITY
We assessed the impact of humidity on limestone (BCR116, a very fine material with a mean particle size of four microns, used as the standard reference powder for shear testing) and lactose (FlowLac100, an example of a widely used pharmaceutical excipient, which has a mean particle size of 140 microns).
First, we allowed samples to equilibrate in environments of varying relative humidity to assess how much water was taken up. For both materials, absorption and adsorption rates are quite low (Figure 3). However, the more interesting question for processors is whether the resulting moisture content of the powder can change behavior. To answer this question, we subjected each sample to shear, bulk and dynamic property testing using the FT4 Powder Rheometer. Reference 2 provides full details of the test methodologies.
Limestone. The dynamic measurements for limestone show that basic flowability energy (BFE) — the ease with which the powder flows under forcing, compacting conditions — increases with increasing moisture content (Figure 4a). This behavior may indicate the water acts as a binder, producing liquid bonds that raise the overall cohesivity/adhesivity of the system and promote the formation of loose agglomerates. The variations observed in the aerated energy (AE) data (Figure 4b) at first sight would seem contradictory to the BFE results. However, these are better understood when studied alongside the permeability results (Figure 4c), which are generated by measuring the pressure drop across the powder bed for a given air flow — higher pressure drop equates to lower permeability.
The limestone has very low permeability across all levels of moisture content, largely because of its fine particle size. In general, the strength of interparticle forces increases with decreasing particle size; this is why finer materials tend to have relatively high cohesivity. Strong interparticle forces result in a packing structure that resists the passage of air, causing low permeability. So, with cohesive powders the inclusion of water provides relatively little scope to reduce permeability further. This effect is illustrated clearly here, where increasing moisture content minimally changes permeability in absolute terms.
For analogous reasons, the limestone substantially resists aeration; any upward-flowing air tends to channel through to the surface rather than promoting steady fluidization. Therefore, the introduction of air has a limited and variable impact on flow energy, with the extent and influence of the channelling varying erratically with moisture content.
The compressibility data (Figure 4d) support the hypothesis that increasing cohesivity explains the trend in BFE. Cohesive powders have a tendency to trap air within them, making them relatively easy to compress. In contrast, less cohesive powders have particles that are efficiently packed together; compression is difficult because there's significantly less air to expel. Therefore, the increase in compressibility as moisture content goes up points to steadily rising cohesivity. The bulk density of the limestone also decreases with increasing moisture content (data not shown), which supports the idea that higher cohesivity leads to more air trapped within the bed.