Quantifying Humidity's Impact on Powder Processing

For MCC, the minimum in the SE values mirrors that observed in the BFE data suggesting that the same mechanisms dictate behavior under both conditions. For lactose, in contrast, it seems that in low stress conditions, the increased cohesion induced by higher levels of moisture may dominate over any lubricating effect.

Aerated Energy (AE) is measured using the same principles as the BFE measurement, but with air flowing through the sample at a controlled velocity. The data is especially relevant for fluidization and pneumatic conveying applications, but also tells us a lot about the strength of the cohesive bonds between particles. Free-flowing materials will often fully aerate to a point where the measured AE stabilizes at a level of a few millijoules. The flow energy of cohesive powders, on the other hand, is not significantly affected by the introduction of air because the particles remain strongly linked together, resulting in minimal change in the packing structure of the powder bed.

Figure 5 shows that, for lactose, AE decreases steadily as moisture level rises, mirroring the BFE curve, and supporting the idea that moisture may lubricate particle-particle movement, and furthermore break cohesive bonds between the particles, under these flow conditions. The results for MCC are also consistent with the BFE data, though the effect at the highest humidity levels is even more apparent. This supports the suggestion that increases in flow energy are caused by the formation of relatively large, meta-stable agglomerates. Large particles or agglomerates can resist aeration as their increased mass means that higher air velocities are required to ‘lift’ the particles/agglomerates and change the packing structure of the powder sample. Large particles can also pack more efficiently, which results in greater resistance to the motion of the rotating blade (see Figure 6). This behavior can be observed as an increase in flow energy.

Bulk and shear properties
Bulk properties such as permeability, density and compressibility provide further insight into how powder behavior changes with the presence of moisture. Permeability quantifies the ease with which air can pass through the powder bed, with lower permeability typically associated with more cohesive materials.

MCC shows a steady increase in permeability with increasing moisture content (Figure 7). This is consistent with the dynamic data, which suggest a decrease in cohesivity caused by a reduction in electrostatic forces, followed by agglomeration, as moisture levels rise. Both effects are associated with reduced resistance to air flow; in the case of agglomeration this is because larger particles pack uniformly and with significant voids between particles which support the transmission of air. Although these large particles may be difficult to aerate due to their mass, they offer little resistance to the transmission of air.

Conversely, the permeability of lactose decreases with increasing moisture content. This is attributable to the formation of liquid bridges which, although they may lubricate flow, restrict the easy passage of air between the particles.

Compressibility data indicate the extent to which an applied force reduces the volume of a powder sample. High compressibility is usually associated with more cohesive powders which form loose agglomerates that then entrain air within the powder bed. Under a compressive force, this air can be squeezed out, resulting in a significant change in the sample volume. Less cohesive powders pack more efficiently and closely, making further compression difficult. Bulk density is similarly influenced by particle packing and tends to be a function of particle size and shape.

Lactose and MCC both show little change in compressibility with moisture content (Figure 8) and only small variations (2–3%) in Conditioned Bulk Density (CBD) (Figure 9) as a function of moisture content. Since the dynamic measurements have already revealed large changes in flowability, this suggests that for these materials, changes in packing behavior and, more importantly, bulk density may not be as influential as is often assumed in defining  the flow properties of a powder.

Shear cell testing measures the stress required to shear one critically pre-consolidated powder plane relative to another, and generates parameters such as the Angle of Internal Friction and shear stress that can be used to quantify certain flow characteristics. Despite the fact that the prevailing stresses are very different, the trends observed in the shear data, for both MCC and lactose, reflect those seen in the SE testing, a result that highlights the dependence of both sets of data on the strength of frictional forces and mechanical interlocking within the powder. Like the density measurements, the shear data show relatively modest changes with humidity (Figure 10) underlining the greater sensitivity of the dynamic data to changes in the process conditions.

Multi-faceted powder characterization maximizes understanding
To accurately manage the impact of humidity on process performance, it is important to understand the extent to which powders adsorb/absorb moisture and, more importantly, how this moisture affects powder properties. In this study, the MCC took up 10 times as much water as the lactose, but both materials showed significant changes in properties relevant to processing — changes that could neither be predicted from first principles nor reliably determined from any single measurement.

The data suggest that it is inappropriate to assume that moisture is always detrimental to powder behavior. For example, moisture improved the flowability of MCC under certain conditions, possibly because it dissipated electrostatic charge, and also had a positive effect on the flow properties of lactose, through its ability to lubricate the particles.

However, it is also clear that small quantities of moisture can have significant, non-linear and potentially detrimental effects on powder behavior, even with hydrophobic powders such as lactose.

These findings show that to optimize powder processing, it is essential to characterize powders using a variety of techniques and under representative conditions.

Although powders respond to moisture in complex ways that are hard to predict, appropriate measurements can quickly and reliably provide the data needed to understand and optimize process behavior.

References
[1] Freeman R. “Measuring the flow properties of consolidated, conditioned and aerated powders – A comparative study using a powder rheometer and a rotational shear cell”, Powder Technology 174 (2007) 25–33.
[2] Storage and flow of solids, Bulletin 123 of the Utah Engineering Experiment Station, November 1964 (Revised 1980), A.W.Jenike, University of Utah.