Information on any of the following application studies may be requested using the Contact Us form.
Application studies on flowability questions
The FT4 is capable of measuring flowability in relation to each of the factors that affect flow properties. These factors may be grouped into primary and secondary variables; the primary group containing those that are always relevant and the secondary containing the variables that may be significant, depending on the powder.
The primary factors that are defined in methodology are:
- Stability test to assess stability - expressed as a stability index (SI)
- The basic flowability energy (BFE)
- Flow rate sensitivity test expressed as a flow rate index (FRI)
- Sensitivity to consolidation condition, expressed as a compaction index (CI)
- Aeration ratio (AR) indicating sensitivity to aeration
An ideal powder might be defined as one that would flow freely with a minimal energy input requirement and which was unaffected by any of the usually significant factors such as flow rate, compaction or attrition. All flowability indices would be unity value for an ideal powder.
Secondary factors that may also affect flow performance include:
- Attrition - the affect of wear and tear
- Segregation
- De-aeration characteristics
- Moisture adsorption
- Electrostatic charge and the affect on flowability
- Bulk density dependence on attrition
- Wall effect studies - how different materials affect flow properties
Each of these factors may be investigated separately in relation to a particular powder.
Compaction studies - how packing condition affects flow properties
Flowability is usually more affected by the packing condition than by any other single factor. The relationship may be determined and quantified in terms of a compaction index at one extreme and by the aeration ratio at the other. The appropriate test may be applied to two samples, one of which is conditioned using the standard FT4 conditioning procedure, the other having been compacted by repeated tapping to consolidate the powder. The ratio of these two test results is the compaction index.
Those powders having a high compaction index are likely to be difficult to process consistently because the energy needed to establish flow when the powder is compacted, may not be available. Even if it is, this energy may be sufficient to cause even more compaction, so compounding the problem rather than moving the powder.
The way in which powders are compacted or consolidated is also important. Vibration tends to realign and interlock particles in the case of the freer flowing powders, and have little effect on cohesive powders. Dead weight compaction of cohesive powders on the other hand, such as may be imposed in a hopper, may produce a significant increase of bulk density and a radical change of flow properties. Both methods of compaction may need to be evaluated.
The graph above shows the results of tests on a lactose sample under a variety of different packing conditions, varying from aerated to de-aerated and finally to a consolidated state induced by mechanical tapping. This data shows a range of energies varying from 47.4mJ up to 4249mJ and a volume change from 200ml to 114ml.
Flow Rate Studies
Some powders such as washing powders, polymer powders and some pharmaceutical products, may have little sensitivity to flow rate, usually because this characteristic has been designed in at the formulation stage. Others are highly affected and are therefore likely to have inconsistent flow performance during processing. This sensitivity to flow rate may be easily quantified using the FT4 by a series of flowability tests over a range of blade tip speeds or flow rates.
The graph below shows the results of flow rate testing on a sample of talcum powder. The area beneath each of these energy gradient curves represents the total energy consumed during each test traverse and the results are plotted in right-hand graph below.
The sensitivity of a powder to flow rate may be expressed as a flow rate index calculated as follows:
The figure below shows the flow rate test data for a range of powders. For these powders, the flow rate index varies from 0.89 to 6.09.
High values are characteristic of cohesive powders and indicate potential processing difficulties, especially if combined with other adverse characteristics such as a high compaction flowability index. Some powders behave in a more Newtonian way and require less energy at lower flow rates. In these cases the flow rate index is less than one. This may be regarded as a more stable characteristic from the point of view of achieving consistent flow performance.
Aeration and de-aeration studies
Air is almost always present when powders flow and not surprisingly, the amount of air can have a dramatic affect on the flow properties. An abundance of air may be present when material is pneumatically conveyed and at the other extreme, a stored powder may become consolidated with time as the entrained air is gradually excluded, or at least minimised. In some cases, the material may be deliberately fluidised producing a dramatic change of flow properties, as for example with powder coating materials.
Evaluating the affect of air entrainment as well as fluidisation performance may be achieved by fitting a fluidisation accessory to the FT4.
The graph below shows the results of tests on a cohesive powder - a poor flowability limestone. The data points are individual repeats of the standard downward compaction test for different airflow rate values. The first series of tests was on a sample that was de-aerated at the start. Another sample was then tested in an aerated state, i.e. re-aerated before each of the seven test cycles.
The effect of aeration is dramatic; the basic flowability reducing from 530 to 50mJ. In this case a poor flowability powder has been transformed by aeration.
A further example of the effects of aeration is described in Bulk Density studies.
Fluidisation study
In this study a catalyst powder was fluidised using air. The air supply was controlled to obtain the data points shown in the graph below. The use of only the smallest amount of air flow is sufficient to reduce the energy required to establish flow from around 500mJ to less than 10mJ.
Application studies on processability questions
Segregation studies
The segregation of different size particles may affect powders in many ways, not least in relation to the possible separation of an important component. Very often in processing, samples of coarser material as well as samples of the fines will be readily available. In these cases, standard basic flowability and flow rate tests may be used on suitable samples of each, to determine the range of flow properties produced merely by segregation.
In other cases it may be necessary to assess flowability before and after segregation. This requires a segregated sample to be produced by the use of aerating flow modes designed to promote the migration of fines downward, and large particles upward. This is followed by a standard test in order to ascertain the change of flowability properties. The advantage of this method is that the segregation procedure is exactly repeatable so that the method is suitable as a quality control technique.
The graph below shows the results of segregation testing on a 160g granulated sugar sample that had been repeatedly compacted earlier to promote attrition and the generation of fines, a process that consumed 183 joules. It was then sieved onto a tray in an attempt to uniformly distribute the particles by size before pouring into the test vessel.
Each segregation test was preceded by 3 aeration cycles that gently rearranged the particles and promoted segregation without imposing significant stresses. Each test phase was followed by the next without removing the sample from the vessel so that the segregation effect was cumulative. The results show a steady increase of the energy requirement, eventually levelling off when segregation stabilised. The final test followed sieving and reconditioning and shows that the energy requirement was then reduced to less that the starting figure.
Segregation may significantly affect flowability. This test method provides a quick and sensitive way of quantifying and classifying this. However, attrition affects may be contributing to the changes measured here and therefore further tests would normally be done to determine an attrition index, as described below.
Attrition studies
The flow properties of powders are nearly always altered by attrition - the wear process that occurs when particles rub against each other or against containing surfaces. Particles may change in size and shape, become more rounded or more angular and perhaps become electro statically charged. Surface coating may be lost and the bulk density may be change.
An FT4 attrition study dissipates a given amount of energy into a powder sample and then assesses the flowability change. This may be done repeatedly until a given total energy has been dissipated or until the flowability value levels off as shown in the graph below.
In this study a cooking salt sample was repeatedly worked by applying 10 compacting cycles before each test. In order to exclude segregation effects, the sample was then emptied and mixed before refilling and testing. The results show that after the sixth attrition cycle, and having consumed about 240J of energy, the powder stabilised and no further changes of flowability energy were seen.
Moisture adsorption studies
Exposure of a material to atmosphere, particularly when the relative humidity is high, may result in moisture adsorption and changes to the powder properties. The affect on flow properties is easily measured by standard testing on powders that have been prepared by exposure for different periods of time.
In many cases, hygroscopic materials will experience significant changes that will be identified as higher values of basic flowability index and different flow rate and compaction indices. In such cases, the affect of many secondary factors such as segregation, attrition and electrostatic charge may be radically altered.
Hence flow performance and processability characteristics may be changed by moisture adsorption to the extent that the powder is quite different to the original.
The graph below shows the affect of exposure to atmosphere of a hygroscopic surfactant powder.
Further applications studies - in summary
Wet granulation studies
The FT4 is capable of assessing the rheology of wet powder blends to determine the relationship between flowability characteristics and the proportions of liquid binder.
The detection of optimum levels of binder in a blend and the use of characterising data to assist scale up of a blending process, are both important needs in pharmaceutical processing. The photo shows a 120ml sample of granulated microcrystalline cellulose being tested.
Samples as small as 10ml may be characterised.
Cohesion testing
Cohesion or shear strength measurements may also be readily measured using the FT4 by using an appropriate test method. The method is essentially to firstly condition the powder sample to produce a standardised powder bed and then to shear through the powder bulk, with minimal compaction. This is achieved by testing upwards, rather than down as in the usual flowability tests.
Formulation studies - evaluating lubricants
The FT4 is usually able to differentiate between very small changes of additive in a powdered formulation. For example, the lubricating affect of magnesium stearate in lactose may be assessed using standard testing routines. A detailed study of this is available in Literature.
Bulk density studies
Bulk density is of course dependant upon packing condition, level of aeration and other factors. The widespread use of volumetric dosing means that the mass/volume relationship is required to be consistent so that the final product quantities are within the permissible weight tolerance.
The FT4 is able to provide automatic volume measurements of samples so that with knowledge of the sample mass, the bulk density may be calculated. This allows the sample densities to be determined throughout any testing programme, e.g. after pouring, after conditioning, after flow rate testing and when compacted.
It is straightforward therefore, to evaluate bulk density changes in relation to aeration, compaction, moisture adsorption, attrition affects and other factors.
The graph below shows the results of tests on a lactose sample under a variety of different packing conditions, varying from aerated to de-aerated and finally to a consolidated state induced by mechanical tapping. This data shows a range of energies varying from 47.4mJ up to 4249mJ and a volume change from 200ml to 114ml.
Penetrometer testing
The Penetrometer accessory allows a cylindrical plunger to be used to compress a test sample in order to measure force vs displacement characteristics.
Semi-solids
The FT4 is able to assess the rheological properties of a wide range of materials in addition to powders, including pastes, slurries, gels, emulsions, creams and other complex materials.
The graph below shows typical plots of energy consumed vs blade speed or flow rate, for a range of very different materials. These curves indicate flow rate sensitivity and display widely different functions as expected. The near straight line relationship of the Newtonian glycerine and syrup contrasts with the pseudo plastic behaviour of the aqueous cream and the pronounced sensitivity to flow rate shown by the dilitant cornflower paste.
This photo shows a dough mixture being tested following programmed blending.
