Choosing the best powder characterization techniques
Successful processing requires a powder that is compatible with all stages of the process. Because it is important to understand
how the powder will behave under many different conditions, applying a single powder testing technique may not give adequate
information. Historically, the pharmaceutical industry has relied on parameters such as Hausner Ratio and Carr's Index, which
are both derived from tapped density techniques. More recently, the industry has begun to use shear testing.
These approaches do have some relevance when trying to access information to support optimization of the press. Shear testing,
for example, was developed, and remains widely used, for the design and troubleshooting of hoppers. This test provides valuable
shear strength data that is relevant to design and operation of the feed hopper and to the compression stage, in which applied
stresses are even higher. However, shear testing may not be the best technique for generating data to support other parts
of the tablet manufacturing process, in which applied stresses are far lower.
For example, the shear data presented in Figure 2a for vanillin and ethylvanillin classifies them as closely similar. However,
as shown in Figure 2b, the dynamic flow energy measurements tell a different story, suggesting that the materials can behave
differently in certain circumstances. Flow energies are generated by measuring the axial and rotational forces acting on a
paddle as it rotates through a powder sample (3). Downward rotation of the paddle imposes a forcing, bulldozing action while
an upward traverse measures a flow energy more closely associated with gravity induced flow. The data suggests that while
these two materials may behave similarly in the hopper, and in terms of how they cohere in the finished tablet, they will
behave very differently in the feed frame. The ease with which the materials flow into the feed frame, and subsequently into
the die, as well as their response to a range of blade designs, are all likely to be different.
Figure 2: Shear data classifies these two excipients as closely similar while flow energy measurements show that in certain
circumstances they can behave quite differently.
Similarly, tapped density measurements can differ from flow energy measurements. Although tapped density measurements do detect
changes that indicate differences in powder properties, they are far less sensitive than dynamic techniques in specifically
measuring flow properties (see Figure 3). Variability in a material, which could go on to impact flow behaviour in the tablet
press, might be undetected by tapped density measurements.
Figure 3: For this system, tapped density data are less sensitive than flow energy measurements in detecting changes in powder
Looking across the tablet-making process in its entirety highlights the properties that could be measured to optimize performance
at each step. These include:
Shear properties—for the design, operation, and troubleshooting of hoppers
Bulk properties such as permeability and compressibility—to assess the response of the powder to air, the ease with which displaced air will be dispersed, and how the blend will
be impacted by compression
Dynamic properties that directly quantify flowability under different conditions—for optimizing flow through the feed frame, assessing the impact of different paddle designs, investigating the effect of
air on powder flowability, and quantifying de-aeration behaviour.
Dynamic testing can also be used to assess powder stability and is therefore a useful tool for investigating the likelihood
of attrition, segregation, and/or agglomeration.
Together these measurements form a database of properties that can be used to tightly define an optimal blend specification.
Furthermore, such data would support investigation into which parameters in upstream processes ultimately dictate performance
in the press and the quality of the final product. Such strategies are highly beneficial in pushing towards more efficient
manufacture on the basis of secure knowledge.