Powder Characterization: Beyond Shear Testing

When it comes to powder characterization few would argue with the credentials of shear testing, which has been in use since its development by Jenike in the 1960s. However, with the intense focus on processing efficiency that prevails across all sectors its limitations as a tool for process-related investigations of powders are becoming increasingly noticeable. The difficulties of shear testing for more free-flowing powders is one issue. Another is the inability of the technique to directly measure a powder’s response to air.

In this article, dynamic testing using a powder rheometer is examined as an approach with much to offer in areas where shear testing begins to lose relevance. The best powder rheometers incorporate both dynamic and shear testing, enabling the complementary application of both techniques. Data from one such instrument highlights the ability of dynamic data to differentiate samples that shear testing classifies as closely similar, to provide greater insight for process development and troubleshooting.

Reflecting on Shear Testing

Jenike’s pioneering work in the 1960s tackled one of the key design challenges for powders: reliable discharge from storage vessels. Here, powders that lie towards the free-flowing end of the flowability spectrum are relatively easy to handle but those that are more cohesive present significant difficulties. Erratic flow from hoppers and silos, as a result of bridging and blockages, was, and in fact still is a major problem for many powder processors.

The design methodologies that shear testing supports stem from a mathematical analysis of flow/no flow condition at the exit of a hopper (see figure 1). Under certain conditions, a stable arch can form across the exit of a hopper preventing further flow. This situation can be avoided by selecting the right hopper angle and outlet size for a given powder; more cohesive and lower density materials calling for steeper hopper angles and larger outlets.

Shear testing measures the force required to shear one consolidated powder plane relative to another so its relevance in hopper design is clear. At the base of the hopper, powder consolidated by the weight of material above it, is subject to this type of stress. When correctly applied, the Jenike methodologies move hopper design well beyond the almost entirely empirical approach that previously prevailed.

The Limitations of Shear Testing

Over the intervening decades the need for process-relevant powder characterization has encouraged the application of shear testing well beyond this original intent. These attempts have brought into focus some fundamental limitations of the technique in answering to more general requirements for powder characterization.

First, shearing more cohesive powders results in relative high forces that can be measured with a high degree of accuracy. This is not the case with more free-flowing materials. Powders with very low cohesivity are more challenging to measure accurately or differentiate via shear testing, the ‘free-flowing’ classification of flow function in fact covering a broad range of flow behaviors.

The second, arguably more important issue, is that while the conditions applied during shear testing may reflect those at the base of hopper, they are far from representative of the environment in, for example, a pneumatic conveyer, a fluidized bed, a dry powder inhaler, or a high-shear granulator. Stretching the application of shear analysis to try and infer how a powder will behave under such conditions is a big task and is achieved with a variable degree of success, especially where the response of the powder to air is critical. The underlying truth is that powders that are classified by shear testing as closely similar, whether cohesive or free-flowing, may not in fact behave similarly in certain unit operations.

Introducing Dynamic Powder Characterization

The powder rheometer was developed as a direct response to the need to measure powders in a way that enables the prediction of in-process behavior. Dynamic characterization, with a powder rheometer, involves the precise measurement of both the axial and rotational forces acting on a blade as it rotates through a powder sample in a defined way. These measurements are used to calculate values of flow energy, a parameter that directly quantifies the resistance of the powder to flow.

One of the core attractions of dynamic characterization is that the sample can be measured when aerated or fluidized, as well as in the conditioned (loosely packed) or consolidated state. Furthermore dynamic methodologies are well-defined and largely automated with the result that the technique delivers high reproducibility. Measurements of flow energy are extremely sensitive and precisely differentiating, for all powders, across the cohesivity spectrum.

Comparing Dynamic and Shear Test Data

Figure 2 shows two sets of comparative shear and dynamic test data for samples of titanium dioxide and two common pharmaceutical excipients. Shear stress values have been generated using standardized shear testing methodologies [1]. For dynamic characterization flow energy has been repeatedly measured under baseline conditions and then as a function of flow (or shear) rate, by testing at different blade speeds [1]. Dynamic testing highlights marked differences between the excipients (vanillin/ethylvanillin) and the two grades of titanium dioxide, that shear testing is unable to detect.

The immediate question that such analysis raises is: How important are these differences that dynamic characterization detects? Any analysis only needs to be sensitive enough to detect differences of relevance to the application that it is supporting. The answer depends on how the powders are being used. While samples that exhibit the same shear strength may behave similarly in a hopper, their behavior in other unit operations is likely to be very different, if flow energy measurements differentiate them.

Moving Forward

Powders are difficult to handle and process, therefore intensifying the need for sensitive, reliable, and relevant powder testing techniques. Shear testing continues to have an important role to play but it is important to recognize and understand its limitations too. A more intuitive and direct approach for process-related study, dynamic powder testing continues to differentiate when shear testing reaches these limits. The result is greater insight that promotes the more efficient design and operation of the widest range of powder processes.

Tim Freeman is director of operations, Freeman Technology Ltd (Worcestershire, UK). For more information, call +44 (0) 1684 310860 or visit www.freemantech.co.uk.

References

[1] “Measuring the flow properties of consolidated, conditioned and aerated powders — A comparative study using a powder rheometer and a rotational shear cell”, Freeman R., Powder Technology 174 (2007) 25–33.