(703g) Evaluating the Potential Benefits of Cohesion

Powders are often categorized as being free-flowing or cohesive, with a common perception that highly cohesive materials will experience more processing challenges whereas free-flowing powders will perform well. For example, Shear Cell testing generates a flow function coefficient which categorizes powders into discrete groups and primarily assesses their ability to discharge from hoppers and silos, where higher cohesion will typically inhibit flow. However, hopper discharge is often only the first stage of a manufacturing processes, with subsequent unit operations applying a range of stress conditions and flow regimes. Under these conditions less cohesive powders may not always deliver the best results.

Simple classifications do not describe the breadth of behavior that powders can exhibit, nor identify how this can change depending on the process environment. In order to optimize productivity and achieve high quality final products, powders must be selected based upon their compatibility with a process, rather than upon the generalization that cohesive powders exhibit poor performance.

This presentation demonstrates how a degree of inter-particular bonding and friction can be necessary for optimal performance in certain applications. Powder rheology is used to evaluate flow properties under a range of stress and strain conditions, and demonstrates how this data can be applied to better understand and predict performance in a number of unit operations. The potential benefits of greater absolute cohesion within coating, filling and dosing applications are discussed, for example to promote the formation of robust coatings and stable agglomerates.