Characterizing powders

It is no coincidence that the majority of powder processors share with tablet producers a heavy reliance on experience-based manufacture. Powders are challenging materials to characterize because their behavior depends on so many variables, including the properties of the particles themselves (e.g., hardness, porosity, size, shape, surface texture) and system variables such as air content and humidity, for example.

The complexity of powders makes a first-principles analysis of behavior beyond our current capabilities and severely challenges the relevance of single-number characterization. A more robust approach is to measure powders using reproducible methodologies that quantify certain aspects of behavior in process-relevant ways. Relating the resulting data to product performance allows the specification of powders that will perform in the required way in a given process environment.

Simply pouring these samples into a vessel or funnel confirms these observations. Nonetheless, if attempts are made to force the powders through an orifice (with a diameter significantly larger than the particle size of the materials), then behavior is markedly different. Under these conditions, the finely milled lactose will extrude through the orifice like a paste, and the spray-dried lactose will tend to resist this forced flow. Selecting a lactose with good flow properties may not be the straightforward task it initially appears.

Table I shows a selection of shear and dynamic parameters for both samples. Shear properties are determined by measuring the stresses required to rotate one plane of a (usually) consolidated sample against another. Dynamic characterization, on the other hand, involves measurement of the forces and torques required to maintain a specified flow pattern under various test conditions. With dynamic instruments (e.g., powder rheometers), samples can be measured in an aerated state, giving information for a broad range of applications. Comprehensive details of testing methodologies are provided in Reference 1.

Finely milled lactose is usually cohesive; that is, the particles stick to one another and to other surfaces such as container walls, thus inhibiting consistent flow. Shear testing measures parameters such as cohesion and unconfined yield strength, thereby quantifying the higher cohesive strength of the finely milled lactose. Here, though, the focus is flow, so which parameters define flow behavior rather than the more general parameter of cohesivity?

Specific energy (SE) is a flow parameter measured under noncompacting conditions by rotating a rheometer blade upward through an unconfined sample. Like most dynamic properties, it is measured using a conditioned sample—one that has been prepared by being made to flow in a low-stress manner to establish a homogenized, slightly aerated, bed of powder. Conditioning before the measurement ensures a consistent baseline state, essentially eliminating any effects associated with sample loading or processing history such as previous levels of consolidation. The conditioning step is essential to ensure the best possible reproducibility. The SE for the spray-dried lactose is much lower than that for the finely milled sample, which indicates that it will flow more readily when unconfined and when the flow is initiated by gravity, such as when tumbling in a jar or flowing into a die during tablet manufacture.

Table I: Bulk, shear, and dynamic parameters for a spray-dried lactose and finely milled lactose.

The compressibility of the finely milled lactose is much higher than that of the spray-dried sample. In this case, compressibility was defined as the change in bulk density induced through the application of a normal stress (<20 kPa). In general, finer cohesive materials tend to have a higher porosity or air content, so compression has a marked effect, forcing out air and increasing bulk density. Conversely, the spray-dried lactose has minimal cohesion and in its conditioned low-stress state, packs efficiently. The gravitational forces acting on each particle exceed the cohesive forces between them and thus minimizes the number of void spaces in the powder. The result is a stiff powder bed for which density changes little as a result of compression.

These differences in compressibility provide a rationalization of the measured BFE values. With the finely milled lactose, the compacting motion of the blade generates only localized compression and the flow zone is relatively small. The spray-dried sample, on the other hand, does not easily compress, so the compacting force is transmitted through the sample. A large flow zone is created with many particles in motion simultaneously, thereby resulting in a high BFE.

It is clear that flow behavior can vary and can be characterized in different ways, even when complicating factors such as aeration, consolidation, and humidity are not considered in any detail. Powders are not intrinsically good or bad, but it is the match between material and process that is crucial. The key to successful formulation and processing is identifying and controlling the parameters that define performance for any given application. This information is uncovered by correlating various powder properties with experience from the existing skill base or experimental work. The following examples highlight correlations between measurable parameters and aspects of tableting.