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.