The introduction of even relatively small amounts of moisture may transform a free-flowing powder into something far more difficult to handle. This well-recognized behavior reinforces the tendency to label moisture as always being detrimental to efficient powder handling. However, there are notable occasions when the introduction of water can have a positive effect. For example, in the process of wet granulation, the ability of moisture to promote adhesion between particles is highly beneficial, and leads to the production of free-flowing granules from cohesive fine powders. In certain systems water acts as an interparticle lubricant, thereby improving flow characteristics; in others it enhances conductivity, discharging the electrostatic forces of attraction between particles that otherwise would increase cohesivity.
So, understanding the effect of humidity on the material being handled and stored is essential for developing cost-effective operating strategies. Where moisture is a problem, steps usually can be taken to control it — e.g., maintaining a storage facility at lower humidity or drying a stream exiting a wet unit operation, such as crystallization or wet ball milling, before further processing. However, such strategies incur cost. Optimizing operation depends upon ensuring humidity levels are controlled adequately but not excessively; this, in turn, relies on knowing how easily the powder takes up water and, most importantly, the impact of that moisture on behavior.
CRUCIAL STARTING POINT
An understanding of powder flow characteristics is essential. They define how easily and reliably material will move through a plant but, beyond this, they directly influence the efficiency of important unit operations such as blending and vial/die filling. In many instances controlling powder flow behavior is the key to achieving manufacturing excellence.
The mechanisms of powder flow are complex. They are influenced by an array of different parameters; some relate to the particles' physical attributes, such as size and shape, and others, such as humidity, to the system itself. Although there is a general understanding of these individual mechanisms, the multitude of interactions that govern the specific behavior of a given powder prevent the prediction of flow properties from first principles. The pragmatic alternative is to measure powder properties that correlate with in-process performance and use knowledge of the mechanisms of powder flow to develop a consistent rationale for these observed behaviors.
When a powder flows the particles within it are moving relative to one another. The ease with which this happens is governed by the strength of interparticular forces that arise from friction, mechanical interlocking, adhesion/liquid bridges, cohesion and gravity. The interaction and relative magnitude of these forces dictate the behavior exhibited by a powder in any specific environment.
Frictional forces inhibit movement, either between particles or between particles and the walls of the confining vessel. Their strength is strongly influenced by surface roughness, with smoother particles and surfaces exerting less resistance to flow, all other factors being equal.
In contrast, mechanical interlocking is more closely correlated with overall particle shape. Irregular particles, if oriented in a certain way, may slot together like pieces of a jigsaw puzzle, significantly resisting further movement (Figure 1).
Liquid bridging often accounts for the negative impact that moisture can have on flow behavior. By bridging the gap between particles, or particles and the vessel wall, a liquid can increase adhesive forces and inhibit particle motion.
Cohesive forces, such as Van der Waals forces and electrostatics, tend to be especially important in defining the behavior of fine powders. Gravitational forces, on the other hand, have a much greater impact on systems containing large high-mass particles because the force imposed by gravity is function of mass.
The complexities of powder behavior have led to the development of many alternative testing methods that seek to summarize this behavior in the form of just a single number. The diversity of these techniques underscores that many different approaches can provide some insight into powder behavior. However, processors are increasingly recognizing that reaching the levels of manufacturing performance now demanded requires a focus on methods that:
• are reliable and reproducible;
• generate process-relevant data that correlate with performance; and
• allow sensitive assessment of the impact of environmental variables such as moisture and degree of aeration.
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.
In this instance, bulk and dynamic property testing identified some significant effects but shear analysis provided little differentiation between the samples. This observation underlines the value of multifacted powder characterization and the greater sensitivity of certain powder testing techniques for specific applications.
Lactose. The very different behavior of the lactose is immediately obvious from the BFE data (Figure 5). The lactose shows a fall in BFE with increasing moisture levels, suggesting that here the presence of water actually may lubricate interparticular interactions. However, the specific energy (SE) data for lactose show the opposite effect: SE rising with increasing moisture content. This interesting behavior highlights an important, industrially relevant issue — namely, the processing environment strongly influences powder flow behavior. The BFE testing regime subjects the powder to a compacting action that is more representative of the forced-flow conditions that would apply, e.g., during extrusion or the pushing of powder into a partially filled die. In contrast, the upward motion of the blade during SE testing subjects the powder to a gentle, lifting action that produces values that reflect the unconfined flow behavior that would be observed when a powder pours freely from or into a vessel.
As with the limestone, the presence of moisture very likely will produce liquid bridges that would tend to increase the cohesivity of the system. This fits with the observed trend in SE. However, the BFE data suggest that under forcing conditions this effect is more than offset by a competing lubricating mechanism that makes interparticular movement easier. Therefore, under compacting conditions the net impact of the moisture is beneficial. AE values also decrease with increasing moisture content, suggesting that here too, water reduces the strength of cohesive bonds.
Evaluating bulk properties (Figure 5), the permeability data are perhaps most revealing. The steady rise in pressure drop observed indicates the powder becomes less permeable to the flow of air as moisture content increases. This supports the view that liquid bridges form within the system, inhibiting the passage of air. In contrast, both compressibility and bulk density (data not shown) change very little as a function of moisture content. The variation in bulk density (only 2 to 3 % across the experimental conditions) is particularly noteworthy because it suggests that in this instance bulk density/packing changes aren't responsible for the observed trends in flowability (as quantified by the dynamic test data). This indicates that powder testing methods based on bulk density could easily fail to detect the changes in behavior induced by moisture. Shear analysis is similarly insensitive for the lactose as for the limestone.
The effective management of humidity to ensure optimized processing relies on understanding and quantifying the effect of moisture in a way that's relevant to the process. Experimental data presented here for limestone and lactose illustrate the very different responses that moisture can induce and highlight the insight provided by multifaceted powder characterization, most especially incorporating dynamic measurement.
The results demonstrate that even for materials that exhibit low moisture uptake, exposure to humidity levels typical of an industrial environment can significantly affect performance. Furthermore, they provide evidence dispelling the idea that moisture always degrades powder behavior. For example, under certain conditions moisture improves the flow properties of lactose, a result attributed to the lubrication of interparticular movement.
As many processors already recognize, moisture's effects are neither linear nor predictable. So, it's essential to apply appropriate powder testing strategies to build a secure basis for intelligent decision-making around design and operation. Achieving the highest levels of manufacturing excellence and profitability requires keeping powders just dry enough to ensure optimal processing. Relevant powder testing provides the information needed to achieve this goal.
JAMIE CLAYTON is operations manager for Freeman Technology, Tewkesbury, U.K. E-mail him at Jamie.Clayton@freemantech.co.uk.
1. Jenike, A. W., "Storage and Flow of Solids," Bulletin of the Utah Engineering Experiment Station, 123 (November 1964, revised 1980).
2. 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, pp. 25–33 (2007).