Powder Characterisation for QC

Quality control, of either an intermediate material or final blend, places a unique set of demands on an analytical tool. Sensitive differentiation between samples is essential, as is the need to measure a parameter that accurately reflects performance, during manufacture or within the customer’s application. Historically for powders, both these issues have presented a challenge, but significant advances in characterisation technology have transformed this situation. Universal powder testers now measure a comprehensive set of dynamic, shear and bulk properties with exemplary reproducibility, making them a suitable choice for sensitive QC.

Reproducible measurement…

Powder behaviour is complex and cannot yet be mathematically described using the many discrete variables that influence it.  The ease with which a powder flows, for example, depends on an array of primary variables, including particle size, shape and surface texture, but also on many system parameters such as degree of consolidation stress, shear rate and moisture content. Poor control of any influential variable within the test will compromise the reproducibility of an analytical technique.

Modern powder testers reflect widespread recognition that achieving excellent reproducibility relies on employing closely defined measurement protocols and significant automation, and on ‘conditioning’ the sample before analysis. Conditioning involves gently agitating the powder to release excess air and/or break up agglomerates and leaves a homogeneous loosely packed bed. Ensuring that a powder is always measured in this same baseline state significantly enhances reproducibility and, consequently, sensitivity. Differences detected by instruments that employ these strategies to deliver exemplary reproducibility can be confidently attributed to real differences between samples, rather than variability in the technique.

…of a relevant variable

While accurate measurement is critical for effective QC it is equally important to select a variable that will best reflect performance targets. Consider a manufacturer sourcing alternative supplies of a raw material. The material has a specification defined in terms of composition and properties such as particle size and bulk density, but when an alternative supply meeting this specification is introduced into the process, production efficiency plummets. The new material causes blockages and the final product quality is inconsistent. Clearly the defined specification does not include key variables that directly impact processability and is therefore inadequate for identifying an optimal quality/cost supply.

Powder testing instruments that offer multiple methodologies make it easier to quickly identify those parameters relevant to performance. Building a database of shear, bulk and dynamic properties for a material facilitates the correlation of specific variables with aspects of processability or product performance. For the raw material, for example, a short study may reveal that the specification is more precisely defined in terms of Basic Flowability Energy (BFE), a dynamic term, and permeability, a bulk property, as well as composition and particle size. With a well-defined specification, testing alternative sources of the raw material and identifying the best supplier becomes very much easier.

Simple powder characterisation techniques such as angle of repose, Hausner Ratio and flow through an orifice, have a well-established place within the powder processing industries but typically measure just a single property, often with poor reproducibility. In contrast, modern universal powder testers deliver multi-faceted powder characterisation and exemplary precision and reproducibility. Automated, with well-defined measurement procedures these instruments enable sensitive specification setting and the highly effective QC needed to target premium product performance.