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Abstract from Reg Freeman's presentation: 'Powder Rheology in Relation to Air and Water Content - an Empirical Study' |
The rheological properties of powders are dependent upon a great number of variables and consequently the prediction of flow performance is usually difficult and unreliable. This paper describes a new way of characterising powders in relation to these variables by means of the FT3 Powder Rheometer.
The instrument uses a twisted blade to establish patterns of flow through a powder sample whilst measuring the total amount of work done. These energy totals that are exceptionally reproducible, are regarded as a measurement of the flowability of the powder and are used to derive a variety of flowability indices.
Extensive studies on a wide range of materials has shown that different powders have different sensitivities to variables such as particle size, shape, consolidation, tribo-charging and the many other physical and environmental factors. One of the most important is the amount of air present in a powder bulk since this may change the energy required to produce flow by a factor of 10, 100 or in extreme cases by 1000. This paper focuses in particular on the effect of aeration and de-aeration on a range of powders showing how some are readily fluidised. At the other extreme, cohesive materials such as limestone and titanium dioxide, whilst not fluidisable, may still be aerated to improve their flow properties.
Also studied are blends of micro-crystalline cellulose, lactose and water, where we are seeking to characterise the rheology of a wet granulated mass. The technique described is sensitive to better than 1% of water addition and offers new, more sensitive ways of determining the end point in wet granulation processes, as well as an effective technique for verifying the rheology when scaling-up.
All the data referred to is represented graphically and the tests are illustrated using video film.
