Fretter^a, , K.Freischlad^a, M. Hucke^b, W. Ludwig^c, J. Carstens^c, J. Pieper^c, R. F. Lammens^a

• ^a Solids Development Consult GmbH, Am Turm 38, D-53721 Siegburg
• ^b Hucke Software, Buchweizenberg 29, D-42699 Solingen
• ^c Romaco Kilian / Kilian Tableting GmbH, Scarletallee 11, D-50735 Köln

 

Purpose is to extend the possibilities for characterizing compaction properties of powders by performing ultrasonic (=US) measurements during tableting, whilst focusing on practical applications.

Methods.
Both the upper and the lower punch of a fast single stroke tablet press (StylOne Evolution, Romaco Kilian, Germany) have been instrumented with longitudinal and transversal US-transducers for determining the signals transmitted through the powder bed during tableting.

Every time a force and displacement signal has been collected, also an US-wave has been generated and its time of flight and its intensity have been recorded with a dedicated data acquisition system.

Results.
The speed of ultrasonic sound uniquely depends on the powder density. Upon an increase of density, the US-speed increases. However, during decompression the US speed is clearly lower than during compression, indicating that unexplainable changes in the density because of (elastic) recovery can be detected. Furthermore, US-speed during decompression turned out to be a clear indicator of the elastic powder properties.

There is evidence that during densification the constant of Poisson hardly changes for brittle and clearly increases for plastically deforming substances.

Moreover, the maximum density during tableting does not correspond to the maximum intensity of the US-signal, which is a hint for a still lasting deformation processes improving the coupling of US-waves.

Although not unexpected, various excipients showed a large difference with respect to their US-transmission properties. Whether and to what extent this relates to compaction relevant properties has not yet been investigated.

Conclusions.
Characterizing deformation properties seems to be possible without any assumptions regarding the densification model, although this has to be verified by investigating more substances.

The extent of the hysteresis loop in the curve of US-signal versus solid fraction might be a good indicator for the speed sensitivity of formulations.