Measuring powder flowability can be a major problem,particularly for the pharmaceutical industry. Here Reg Freeman describes new powder rheometer technology that helps theearly stage assessment of powder property and therefore theability to design formulations with the required characteristics.
The challenge of accurately and reproducibly measuring the characteristics that affect the flow properties of wet and dry powders has occupied researchers, process scientists and QC analysts for many years.
Powders are notoriously unpredictable but are essential components in manufacturing processes across practically all industrial sectors worldwide. This article outlines a unique approach to powder flowability measurement using a laboratory instrument that is becoming the method of choice in many areas, and is proving especially valuable in pharmaceutical industry applications.
Powders mean problems
Powders often present difficulties. They are inherently changeable, making it extremely difficult to predict their behaviour and to optimise a manufacturing process.
Researchers, perhaps understandably, categorise powders as either agood' or abad' on the basis of their experience. But very many physical, chemical and environmental variables have an impact on the flowability of a given powder, so it's not possible to categorise a powder in this simple way, since flowability is highly dependent on both storage and handling conditions. Some powders, for example, may be free flowing under aerating conditions, but exhibit poor flowability through a hopper. When damp, the same powders might not flow well, even with aeration.
When working with powders the issues to be addressed include: whether or not the powder is fit for purpose; how it will change under various conditions of handling and processing; what factors will influence its behaviour; its performance within the plant.
Traditional flowability tests (cone angle test, funnel test) may be suitable for some powders, but mostly do not deliver consistently reliable and accurate information. These traditional methods attempt to deliver a single number to define a powder and fail to reflect its complexity. Quantifying the significant changes in powder rheology that can occur with aeration or consolidation, for example, is also difficult with these systems.
Use of a powder rheometer such as Freeman Technology's FT4 (Fig. 1.) overcomes these limitations, providing sensitive and reproducible results.
Each factor that affects a powder's flowability can be examined individually. Data on aeration, compaction and flow rate sensitivity can be obtained, as can information on secondary characteristics such as attrition, segregation, moisture adsorption, the effect of electrostatic charge, and bulk density dependence on attrition. The end result of this analysis is a comprehensive picture of the powder.
Powder rheometer
The FT4 can measure the flow properties of dry and wet powders in terms of the energy needed to make them flow. It then determines how this energy requirement changes in relation to the key variables, such as binder content, particle size, aeration, consolidation and attrition.
The rheometer measures all the relevant forces needed to give a complete and accurate measurement of powder flowability. Sensitive and reproducible data are obtained by measuring both axial forces (as the blade moves vertically through the sample) and rotational forces (or torque) to determine flow energies. Dry and wet powdersmay be evaluated in terms of each of the key variables.
The system works on a patented principle whereby a specially formed blade moves along a helical path and causes the powder to flow. Depending on the direction and speed of movement a range of flow patterns and rates can be achieved. The axial and rotational forces acting on the blade are measured and these data form the basis of the flowability assessment. Force measurements are converted into energy to determine the total energy consumed or work done during the traverse.
Standardisation
One of the most important parameters affecting flowability is the packing condition of the powder. Taking an aerated state at one extreme and a fully consolidated state at the other, the energy needed to establish flow varies greatly. Typically it can be by around 100 times, but in extreme cases up to a 5000-fold increase may be needed. Suchsensitivity to the presence or absence of air andthe effect that this has on particle interaction means that unless test samples start with an identical packing condition, repeat test results cannot be compared.
Pre-conditioning of powders before measurement is therefore an essential step. It ensures that each sample is tested from the same starting point and under the same conditions. Direct comparisons can be made between results from different samples. Information databases can be created, akin to those available for other materials such as solids and liquids.
Following pre-conditioning, the FT4 determines a powder's basic flowability energy (BFE). This is the amount of energy needed to move a conditioned powder in a programmed way using the instrument's patented helical blade principle. Having determined the BFE of a powder, the effect of different variables can be examined (Table 1).
Compaction index
Most powders reduce in volume under direct pressure or when subjected to vibration (or simply on storage). Particles move closer together and bulk density increases.
The FT4 subjects appropriately consolidated powder samples to the standard flowability test, omitting the pre-conditioning cycle and measuring the energy required to induce flow of the consolidated powder. The results are expressed in terms of the compaction index, the factor by which the BFE is increased as a result of consolidation.
Powder handling and processing also involves the removal or addition of air to the bulk. Introducing air into a powder bed reduces its packing density, the number of physical point contacts and the inter-particulate forces, giving a more free-flowing material needing less energy to promote flow.
Some materials fluidise with the addition of air. Most do not, but will aerate readily (Fig. 2). In general, agitation or disturbance of the powder bed during aeration produces a packing condition requiring considerably less energy. The FT4 can quantify this in terms of an aeration ratio - the factor by which the BFE is reduced by aeration.
Application of the standard FT4 measurement technique across a range of flow rates on a conditioned powder shows that the energy requirement also varies as a function of flow rate. This relationship is described using a flow rate index the factor by which the energy changes when the flow rate is reduced by a factor of 10.
An ideal powder would be insensitive to flow rate, having a flow rate index of one. In some powders it is less than one. Generally these are free flowing and comprise quite coarse particles. Most powders have flow rate indices greater than one typically three or four.
Wet granulation
The FT4 can be used to assess the rheological properties of a wide range of materials in addition to dry powders.
In one pharmaceutical company the powder rheometer has been used to develop a sensitive method of characterising wet granulated blends in relation to their rheological properties.
Samples for the study comprised 50/50 proportions of lactose and microcrystalline cellulose (Avicel) blended with varying amounts of water and all samples were of constant mass (11.4g).
Fig. 3 shows the results of seven tests on blends varying from 30 per cent to 70 per cent water. These indicate that sensitivity to water content is high, especially below 55 per cent. Repeatability of the technique has proved to be good, and the fact that samples can be tested within a few minutes has made it suitable for end point detection in wet granulation processes.
Conclusions
Resolving the problems of powder flowability and enhancing processing efficiency are priorities in many areas of development and production in pharmaceuticals and throughout manufacturing industry.
There is a universal desire to reduce stoppages and optimise throughput, and to improve product quality. The FT4 powder rheometer can be used to characterise both dry powders and wet granulation systems and experience to date indicates that this technology generates information that enables processes to be better understood and even optimised.
Characterising powders at the formulation or design stage of a new product, means that key decisions can be made with regard to the flow properties needed and the degree of robustness required. The ability to evaluate as little as 10ml of material, to determine the primary flowability indices and examine secondary flowability factors, makes possible the early stage assessment of powder properties and facilitates designing a formulation with the required characteristics.
Reg Freeman is the founder and CEO of Freeman Technology, Malvern, Worcestershire, England.Tel: +44 (0) 1684 310860, fax: +44 (0) 1684 310236, info@freemantech.co.uk
