Dynamic powder testing is a modern technique that directly measures the flow properties of consolidated, conditioned, aerated and even fluidised powders. Dynamic test methods complement shear and bulk property measurements to deliver a comprehensive level of powder characterisation. Used in combination, these three methodologies provide a powerful tool for evaluating the complex behaviour of powders.
What is a powder?
Referring to powders as a single phase can be somewhat simplistic. Powders are typically three-phase systems containing complex and variable solid particles as well as an unquantified amount of air and a degree of moisture. As a result, powder behaviour is an intricate interaction of all three phases and is influenced by a vast array of parameters. This makes both measurement and processing demanding.
Focusing on flow
The way in which a powder flows is one of the most important aspects of its behaviour. Ensuring that powders flow through a plant in a reliable, controlled way is a significant challenge. However, the properties that dictate flow also influence many other unit operations, such as blending and die filling. When a powder moves, the particles within it need to move with respect to each other. The ease with which this occurs determines how readily the powder will flow. This simple statement immediately helps identify factors that influence powder flow. These include the level of friction between particles, which is a function of shape and surface texture, and the likelihood of mechanical interlocking, where particles fit together like jigsaw pieces and exert significant resistance to movement. The degree of cohesion and adhesion is also highly influential. Van der Waals forces, electrostatics and magnetism, amongst others, will all influence the strength with which particles attach to one another. Adhesive forces will also arise from the interaction between particles and container walls as well as from liquid bridging between particles. With denser particles the influence of gravity tends to dominate, but for smaller, lighter particles the cohesive and adhesive forces are relatively large and can therefore define behaviour. These multiple mechanisms make it difficult to predict how a process changes may impact performance. For example, moisture may encourage agglomeration via liquid bridging, it may lubricate the relative movement of rough particles, or even earth an electrostatically charged system, resulting in a different outcome in each case.
The pragmatic response to this complexity has been to develop measurement techniques that classify flow behaviour with just a single figure, such as flow through an orifice, angle of repose or tapped density. These traditional techniques provide some measure of flow but are limited for the purposes of process and product optimisation as they typically suffer from poor repeatability and don’t necessarily generate data that correlate with process performance. Powder processors increasingly recognise the importance of employing a number of complementary techniques to rationalise flow behaviour and the need to adopt methods that generate accurate, reliable and relevant data.
Introducing dynamic testing
Dynamic powder testing involves measuring a powder in motion. A flow energy is determined by measuring the axial and rotational forces acting on a blade as it travels along a defined path through a powder sample. Rotating the blade downwards applies a compacting force by pushing the powder against the base of the test vessel while an upward traverse produces a low stress, lifting action. The resulting parameters are Basic Flowability Energy (BFE) and Specific Energy (SE) respectively which directly quantify how a powder flows under different conditions and have been shown to reliably correlate with process performance. Dynamic powder testing has the benefit of evaluating a flowing powder and, due to the fact that powders can be tested in a consolidated, conditioned, aerated or even fluidised state, the test conditions can be chosen to simulate the process environment. This approach provides a highly sensitive method which is capable of differentiating powders that are otherwise classified as identical.
Assembling a modern powder testing toolkit
Dynamic powder testing represents a significant advance in the field of powder characterisation and one with proven value for powder processors. However, it is most usefully applied in combination with complementary techniques. Modern powder testers incorporate dynamic, shear and bulk property methodologies in one device. Shear testing is a well-established technique for characterising powders previously consolidated under moderate to high stress conditions. Modern shear cell testers simplify the generation of data required to support the design protocols developed by Jenike. For this reason, they are an important part of the powder testing toolkit although the technique can be employed well beyond its original remit. Figures 1a and 1b show shear and dynamic test data for two samples of titanium dioxide. Shear data indicate that the two samples are identical but the flow energy identifies clear differences suggesting that under certain process conditions, these samples may behave identically but their performance under other conditions may vary significantly. For example, a hopper designed to handle each material may require the same outlet dimensions but the process parameters for a die filling rig or blender may need to be adjusted because of the distinct differences in dynamic flow properties. This highlights the need for a range of measurements when trying to understand and rationalise powder behaviour.
Bulk parameters such as density, compressibility and permeability are also highly relevant. They are required for certain design calculations but can also be extremely helpful in rationalising complex behaviour patterns. The permeability of a powder, for example, describes how easily a powder transmits or retains air and this has a significant impact on how it behaves in processes such as filling and pneumatic conveying. Information on compressibility is valuable in any environment where a powder is exposed to an external stress such as during storage or in a tablet press. Together, dynamic, shear and bulk properties describe the mechanisms that dictate flow behaviour and provide the information needed for successful process optimisation. This is exemplified by a study in which the dynamic, shear and bulk properties of a limestone sample were measured as a function of moisture content in order to assess the impact of humidity. The shear data remained relatively consistent as moisture content increases but BFE, Compressibility and Permeability exhibited significant changes, (see Figures 2a, b & c.) The increase in the flow energy of limestone suggests that it becomes more cohesive as moisture content increases. This could be due to water acting as a binder, forming liquid bonds that encourage the particles to attach to each other. Compressibility also increased reinforcing the theory that the limestone becomes more cohesive with increasing moisture content. Cohesive powders tend to form agglomerates and air becomes entrained within and between the agglomerates resulting in a more compressible material. Permeability however shows a more unusual response, first decreasing with the introduction of moisture and returning near to its original value as moisture content increases further. This means that in certain processes, performance may degrade as humidity increases but actually improve at higher humidity levels. The response illustrates how changes in process conditions may not affect performance in a predictable manner and demonstrates how multiple parameters are required to comprehensively describe a powder’s response.
There are many powder testing techniques available but in order to deliver value, methods must prove to be reliable, reproducible and relevant. Techniques that meet these essential criteria can guide formulation scientists and process engineers through the challenges of working with powders.
Figures & Images
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