How is dynamic powder characterisation different from other techniques and what insight does it provide?
Although relatively new, dynamic powder characterisation is appealing when dealing with the practicalities of powder handling.
Dynamic measurements are made using a powder rheometer, which precisely records the force and torque acting on a blade as
it rotates through a powder sample, quantifying flow energy. Automation, in combination with sample conditioning, ensures
high reproducibility. Flow energy measurements are extremely differentiating, especially for process–related studies.
A unique feature of dynamic testing is its ability to characterise powders in consolidated, conditioned, aerated or even fluidised
states, allowing direct study of a powder's response to air, which is frequently a defining characteristic. In tabletting,
for example, any entrained air not rapidly released as the powder settles in the die will be compressed with the dose, with
post-compression expansion of the air resulting in capping or lamination. With DPIs, however, response to air affects the
intensity of the aerosolisation process that drives drug delivery. Here, research has shown a direct correlation between the
dynamic term, aeration energy (the flow energy measured in a sample whilst air is passed though it), and fine particle dose,
a measure of delivery.1
In the examples above, aeration or deaeration is intentional, but often it occurs inadvertently. For example, during an unplanned
plant stoppage, powder settles and deaeration begins. Some powders tolerate this, but others rapidly assume a solid-like state,
complicating subsequent start-up. In another scenario, highshear blending will entrain air, potentially affecting the powder's
behaviour in downstream processes.
While these points emphasise the need to quantify a powder's response to air, dynamic testing offers more than this alone.
The robust flow energy measurement provides a firm baseline for exploring other variables. The propensity of a powder to segregate,
cake or attrite can be assessed directly in simple backtoback dynamic studies. A similar approach quantifies the impact on
flow properties of process variables such as flow rate, moisture and vibrational consolidation.
Dynamic testing is therefore highly complementary to traditional powder measurement techniques, such as shear and bulk testing.
Used in combination, these methods generate a rich data set from which to identify the variables that best describe powder
behaviour in any situation. Shear testing, for example, is highly appropriate for understanding powder behaviour in hoppers
and acceptable for classifying the flow properties of cohesive powders. However, with more free-flowing materials, sensitivity
becomes an issue as the measured shear forces are low and similar. Dynamic measurement maintains sensitivity across the cohesivity
spectrum and, in many cases, provides more informative flow data.
How can powder characterisation techniques be used to improve formulation?
Consider the process of vacuum drum vial filling used to produce pre-metered powdered doses for in situ reconstitution for intravenous delivery (Figure 1). Powder is transferred from the hopper under vacuum into the port at position 1. The vacuum draws powder into the die and
keeps it there as the drum rotates to bring the filled port directly above the vial. Then, the powder plug is ejected using
positive pressure and the filled vial passes down the line for sealing.
Fundamental compatibility between material and processing equipment is the key to powder processing success, easing manufacture
and supporting consistent product quality. During formulation, the powder properties are not fixed and can still be tailored
for a specific unit operation. This demands knowing which powder properties are relevant to the process; if the proposition
is to manufacture using a vacuum drum filling system, the question is "what powder attributes are advantageous?".