How to measure powder flow with powder rheology

Bulk solids like powders and granular media can be found in many processes and in almost any industry. Working with powders can be difficult as their complex behavior depends on their properties as well as on a multitude of external influences. Powder behavior can change during a manufacturing process, especially when the conditions or environment change – for instance when a powder is fluidized during discharge from a silo and consolidated during storage.

Influences on powder flow

Powder flow properties depend on a multitude of influences. On the one hand, internal parameters like particle size, size distribution, and particle morphology can strongly influence how well a powder can be handled. On the other hand, a multitude of external influences can also change its behavior – from environmental changes like humidity and temperature, to the way the bulk material is being handled and processed. To ensure smooth processing, efficient quality control is important. A powder’s characteristics can be determined with the help of rheology, as is done in the Anton Paar Powder Cell.

What is cohesion strength?

Cohesion strength describes the internal resistance of the powder to flow, and is thereby a measure of powder flowability. It is defined as a measure for the strength of the bonding forces between powder particles. Based on cohesion strength, you can predict whether the powder will flow through your process smoothly and whether the quality of your powder has changed. These measurements are an ideal tool for quality control as they are fast and efficient while also providing a high reproducibility helping to predict powder behavior. The cohesion strength measurements can also be used as a basis for a more elaborate quality control scheme in which further measurements can pinpoint problems that might occur during processing or handling. In addition, it also gives information on further parameters like the air retention capacity and the aeration behavior through mechanical agitation. This can be used when powders are pneumatically conveyed, compacted to tablets or filled into containers, for example.

What are flowability indices?

Flowability indices like the Carr Index and Hausner Ratio are generally measured by looking at the compressibility of powders. Compressibility and thereby also powder flow can be influenced by many factors, such as size and form, elasticity, water content, and temperature. Predicting the exact impact of these factors on a powder’s flow behavior is difficult and complex. It is therefore usually easier to directly characterize the flow behavior. To this end, the compressibility is usually analyzed by putting into relation the bulk densities of the freely settled and of the tapped powder, giving a value for the Carr Index. The Hausner Ratio is determined the same way, by putting the bulk densities into relation, but the calculation is carried out slightly differently. The Hausner Ratio yields a different numerical value, but has the same meaning as the Carr Index. The Hausner Ratio and the Carr Index are then used to classify the powder’s flow character – from “excellent” to “very, very poor flow”. Despite being a simple test, it can be another useful method for powder flow analysis. Usually the flow indices are determined by tapping the powders repeatedly, but a more precise way to determine these indices is to look at the compressibility (or relative volume change) of a sample as a function of an applied normal stress. These flowability indices are for example often used in the pharmaceutical industry to estimate powder flow.

How is powder flow measured with the Anton Paar Powder Cell?

There are many different ways to measure powder flow. Often empirical methods, as in a Hall flow meter, are used, where the time of the outflow through a funnel is measured to estimate powder flow. As also the importance of more advanced methods is becoming increasingly recognized, such as in shear testers, where powders are sheared under normal loads for the design of silos and hoppers, or also in fluidization testers designed to understand the fluidization properties of bulk solids for transport, filling, and spraying processes.

Anton Paar MCR rheometers enable powder rheology with unmatched sensitivity

The Anton Paar Powder Cell offers a variety of measurement methods in order to determine a wide range of powder properties in conditions near to those of the application. This combination of improved empirical methods and advanced methods allows you to study powder behavior (not only flow behavior) in any state – from fluidized to deaerated (collapsed) to consolidated. The cohesion strength of powders can be determined using two distinct methods, one for tests in a deaerated state for more free-flowing samples and the other for tests in a consolidated state for cohesive powders. Both procedures are very simple and can be conducted in a few minutes only. As for the flowability indices, the Anton Paar Powder Cell can also control the normal stress applied on the bulk solid, enabling a precise analysis of a powder’s bulk density and of its flowability indices as a function of the controlled normal stress.

Conclusion

Understanding the behavior of powders or granular media can be a complex undertaking. It is especially relevant in industrial processes, where a powder’s behavior can strongly influence the quality and the output of a process. The Anton Paar Powder Cell offers a toolbox of measurement methods for a thorough powder analysis, for both quality control and scientific purposes.

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