Understanding powder flow in materials testing

Tapping a powder sample encourages particles to jostle closer together, assuming a higher bulk density. The extent to which bulk density changes is influenced by the inter-particulate forces that, in turn, influence a powder's flowability - how it moves through processing machinary during manufacture. 

How do tapped density measurements work, and what are the alternatives?

To carry out a tapped density measurement a fixed volume of powder is poured into a measuring cylinder which is then tapped either for a certain number of taps or until there is no further change in volume. Flow performance is inferred from HR – the ratio of tapped to bulk (or poured) density – in accordance with the classifications shown below:

The premise of this technique has some merit. More cohesive powders tend to pack inefficiently when unconsolidated and exhibit more pronounced settling than more free-flowing analogues.

However, there are important limitations. The difficulty of achieving a consistently packed, level powder sample makes it difficult to read volume precisely, initial or final. This is particularly true for more cohesive powders; for free-flowing powders repeatability may be better but differentiation limited. Also, there are some powders that just don’t adhere to the ‘large change in volume – poor flowability correlation’, colloidal silica being a prime example.

Our eBook ‘Choosing a Powder Tester’ surveys alternative powder testing techniques including angle of repose, shear cell analysis and uniaxial testing but here I’m focusing on comparisons with dynamic testing because these are helpful in underlining the potential pitfalls of tapped density measurements.  Dynamic powder testing involves measurement of the axial and rotational forces acting on the blade of a powder rheometer as it rotates along a precisely defined path through a homogeneously packed powder sample. It is a highly sensitive technique and applicable to all types of powders.

Tapped density measurements seem an intuitively good way to assess the impact of vibrational consolidation? Is that correct?

It is certainly true that testing powders under conditions that reflect how they are being used is the best way to gain relevant information. So, let’s take a look at how tapped density and dynamic property measurements compare when it comes to assessing the impact of vibrational consolidation, as could easily occur during transportation.

The figure above shows HR and Consolidation Index (CI) data for a wide range of powders. Here, HR values are the ratio of density after 50 taps to conditioned bulk density since they were measured using the FT4 Powder Rheometer®, in combination with a Copley JV Autotapper. Conditioning is carried out as a precursor to dynamic property measurement to ensure samples are measured in a homogeneously packed state. These HR values are therefore likely to exhibit high repeatability relative to those measured using standard tapped density apparatus/techniques which do not incorporate conditioning.  CI is the ratio of Consolidated Energy, flow energy measured after 50 taps, to Basic Flowability Energy (BFE), a baseline dynamic property measured with a downward traverse of the powder rheometer blade.

Looking at the interquartile range (IQR), the middle 50% spread of the data highlights the extent the which the two techniques differentiate the samples. For CI, IQR = 1.0, while for HR, IQR = 0.1 indicating that dynamic testing provides far higher levels of differentiation. None of these materials exhibited an increase in density of more than 25% as a result of tapping but flow energy increased in some instances by more than 300%. In essence, the tapped density technique involves measuring smaller relative changes, with less precise technology, an inherently limited combination. Furthermore, changes in bulk density do not fully inform on all relevant changes associated with particle packing. For example, the reorientation of particles can affect interlocking, and by extension flowability, without having a substantial impact on bulk density.

The answer to the question is therefore that even for the specific application of assessing likely flow behaviour in a low stress environment, for powders subject only to vibrational stress, tapped density measurements are inherently limited.

Will tapped density measurements reliably predict trends in flowability, even if they lack sensitivity?

This is an important question with respect to the reliance placed on tapped density measurements, and by extension their value.

The two techniques broadly demonstrate equivalent trends in this study; materials with a high HR also have a high CI which means that for most materials, when vibration results in a significant change in density, It also causes a marked change in flowability.

Crucially though there are exceptions.

Let’s take a closer look at Lactose and Talc. These both have an HR in the range 1.19 – 1.25 giving them a ‘Fair’ flowability characterisation. In fact, tapped density measurements provide no differentiation. However, CI values tell a different story with a value of <1.5 for Talc and close to 3.5 for Lactose.

The figure above shows compressibility data alongside the CI figures. Compressibility is determined by measuring change in bulk density as a function of applied normal pressure, 15 kPa in this case. These results highlight Talc as the most compressible powder tested, much higher than Lactose.

So, we some complexity here that tapped density data can neither detect or elucidate. The density and flowability of the Lactose sample changes substantially with vibrational consolidation. Lactose particles, possibly as a result of their morphology, assume much denser packing efficiency and flow energy increases. This has practical implications, indicating that the flowability of the Lactose may be poor following transportation.

Talc particles also assume denser packing following vibration but there is no corresponding increase in flow energy. The high compressibility of Talc suggests it may entrain appreciable levels of air, some of which may remain, post tapping, potentially reducing resistance to flow but further investigation is needed to elucidate these behaviours. What is clear is that the tapped density measurements do not capture the differences between materials or the trends in flowability associated with vibrational consolidation.   These are important pitfalls to be aware of, for anyone using the technique. Dynamic testing offers greater insight.

For experimental details and other insights from this study please read the full application note: ‘Evaluating Consolidation Using the FT4 Powder Rheometer.’

Author: Dr Katrina Brockbank is Head of Laboratory and Powder Technologist at Freeman Technology: freemantech.co.uk