White Paper: Assessing Powder Stability

Powders sometimes behave in unpredictable ways – a once free-flowing powder sets solid, or a homogeneous feed delivers a product of inconsistent composition. The material appears to have changed, with severe implications for operation, but the underlying causes are far from obvious. The changes may be reversible, but in some cases are not, in which case original performance is unlikely to be regained. Such variability is unique to powders - other materials exhibit much greater consistency and predictability.

Reversible and irreversible instability can be detected using a powder rheometer, an instrument that allows dynamic characterization of a sample i.e. measurement of a powder in motion. The force and torque acting on a blade as it rotates through the material is measured, to quantify the energy needed to induce and maintain flow under different conditions.

A key dynamic measure is Basic Flow Energy (BFE). To determine BFE the blade is rotated down through the sample, compacting the powder against the base of the sample vessel. Conditioning prior to measurement eliminates the effects of inherent powder variability so analysis is both reproducible and repeatable. As a result BFE is a highly differentiating parameter.

Repeating the BFE measurement cycle (condition and test) a number of times, typically around seven, detects instability. If BFE remains constant then the powder is stable. If, on the other hand, it either decreases or increases then testing or processing is changing the material. A powder exhibiting this characteristic is inherently unstable.