Application Note using:
Small doses analysis
Granular materials and fine powders are widely used in industrial applications. To control and to optimize processing methods, these materials have to be precisely characterized. The characterization methods are related either to the properties of the grains (granulometry, morphology, chemical composition, …) and to the behaviour of the bulk powder (flowability, density, blend stability, electrostatic properties, …).
However, concerning the physical behaviour of bulk powder, most of the techniques used in R&D or quality control laboratories are based on old measurement techniques. During the last decade, we have updated these techniques to meet the present requirements of R&D laboratories and production departments.
In particular, the measurement processes have been automatized and rigorous initialization methods have been developed to obtain reproducible and interpretable results.
Moreover, the use of image analysis techniques improves the measurements precision.
A range of measurement methods has been developed to cover all the needs of industries processing powders and granular materials.
However, in this application note, we will be focused on the GranuHeap instrument.
When a powder is poured onto a surface, a heap is formed. It is well known that both the repose angle and the heap shape strongly depend on grain properties.
In particular, a cohesive powder forms an irregular heap while a non-cohesive powder forms a regular conical heap.
Therefore, a precise measurement of the heap shape provides useful information about the physical properties of the powder sample.
The GranuHeap instrument is an automated heap shape measurement method based on image processing and analysis. A powder heap is created on a cylindrical support.
In order to obtain reproducible results, an initialization tube with an internal diameter equal to the circular support is installed on the support.
After filling the initialization tube by hand with a fixed volume of powder (typically 100 ml), the tube goes up at the constant speed.
Thereby, the powder is flowing from the tube to form a heap on the cylindrical support.
A controlled rotation of the support allows obtaining different heap projections corresponding to different heap orientations.
A custom image recognition algorithm determines the position of the powder/air interface.
The repose angle refers to the angle of the isosceles triangle with the same surface than the powder heap projected image.
This isosceles triangle corresponds to the ideal cohesiveness heap shape.
The repose angle is computed for each image, i.e. for each heap orientation. Afterwards, an averaged value is computed.
In general, the lower the repose angle is, the better the powder flowability is.
The deviation between the real heap interface and the isosceles triangular heap provides the static cohesive index.
The static cohesive index is computed for each image, i.e. for each heap orientation.
Afterwards, an averaged value is computed. This static cohesive index is close to zero for a non-cohesive powder and increases when the cohesive forces inside the powder strengthen.
The next Table summarizes the empirical relation between the results obtained with the GranuHeap instrument and powder flowability.
For this application note, three different powders were selected: FlowLac 100, GranuLac 70 and InhaLac 400. They are lactose samples provided by Meggle Pharma.
GranuLac types consist of fine, sharp-edged lactose particles, having cohesive powder properties that can be beneficial during granulation processes.
InhaLac is a high quality crystalline lactose powder, designed for DPIs.
FlowLac powder is produced by spray-drying a suspension of fine milled alpha-lactose monohydrate crystals in a solution of lactose. When lactose in solution is spray-dried, a rapid removal of water is taking place, whereby amorphous, non-crystalline lactose is formed in addition to crystalline lactose.
For each experiment with the GranuHeap instrument, the 40 / 30 / 20 and 10mm diameter supports were selected. 16 pictures were taken during the heap rotation (1 picture each 11.25°) to increase measurements accuracy. At the end of analysis, one final picture is taken to check the pile integrity when measurement is done.
All experiments were carried under the same humidity/temperature values (30% RH and 21°C). All tests were repeated five times.
The excipients were stocked in our laboratory during several months. Therefore, the history of the samples is unknown and their properties could not correspond to the specifications of the producers for fresh powders.
The following figure represents the Angle of Repose obtained with every powder and support diameter. Straight lines do not correspond to a model, they are just displayed to guide the eye.
Error bars are displayed (they correspond to the standard deviation on the dynamic angle of repose obtained for the five repeatability tests):
Figure 1 allows to conclude about the support influence on the static Angle of Repose measurement. Indeed, we can see that for powders with a “poor” flowability (Angle of Repose bellow 55°), the lower the support diameter, the lower the Angle of Repose. Nevertheless, for powders with a “very very poor” flowability (Angle of Repose above 66°), there is no support influence on the results (if we take into account the error bars).
However, for one diameter, the trends in terms of powders flowability classification is the same. Thus, powders classification can be easily achieved with great accuracy (average error close to 3.5%). Finally, since the heap shape analysis is relative measurements, it allows to conclude that changing the heap support will not directly affect the analysis.
The GranuHeap instrument allows to measure powder flow ability with a wide range of support and with a great accuracy (average error close to 3.5% for lactose powders).
There is a slight support influence on the measurements.
However, for one support, the flow ability trends are the same.
Therefore powders classification is unchanged.
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