Powder Analysis - A Review on the Confined and Unconfined Flow Properties of Powders by Freeman Technology

Topics Covered

Background
Understanding the Different Behaviors of Powders
Defining Confined and Unconfined Flow of Powders
Challenges with Confined and Unconfined Powders
Techniques for Quantifying the Flowability of Powders
Measuring Confined Flow Using Basic Flow Energy (BFE)
Measuring Unconfined Flow Using Specific Energy (SE)
Understanding Cohesivity in Classification of Powders
Understanding the Interparticle Forces Between Cohesive Powders
Differences Between Cohesive and Non-Cohesive Powders
The Mechanisms of Powder Flow
Summary

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Background

Freeman Technology is a specialist company pioneering the measurement and understanding of powders and their flow properties. Founded in 1989, the company developed the novel, patented technology that forms the core of its Powder Rheometer system at its design and manufacturing centre in Worcestershire, UK where all manufacturing takes place in an ISO 9001:2008 accredited environment. Research into understanding powder behaviour is central to the company's business strategy.

Freeman Technology's business is researching powder behaviour and designing instrumentation for powder characterisation. The FT4 Powder Rheometer is a universal powder tester that is really three instruments in one – combining a powder rheometer with a shear cell capability and a compression tester. This allows comprehensive characterisation of powders of all types, reflecting the complexity of powders in contrast to traditional single number assessments of flowability.

Understanding the Different Behaviors of Powders

Powders can exhibit very different behaviors depending on the conditions to which they are subjected during storage, handling or processing. For example, powders inside a hopper or bin are confined, whereas at the outlet of the hopper and during discharge, they are unconfined and free to flow under gravity. In unconfined conditions the powder may break into lumps formed of agglomerated powder held together by cohesive forces, or perhaps separate into individual particles if the material is non-cohesive. In the latter case, sufficient aeration may fluidize the powder with potentially serious consequences. The same powder confined in a hopper may release its air and become very resistant to flow after a certain period of confinement. In production such differences in the behavior of any given formulation can be very challenging – especially if the material characteristics are not well suited to the processing conditions. One of the keys to successful processing is to match a powder to the demands that a unit operation will place upon it.

This paper examines the mechanisms associated with powder flow, contrasting the way in which materials behave when confined and unconfined. Understanding this important aspect of behavior is a significant step towards knowledge-based processing, as advocated by QbD.

Defining Confined and Unconfined Flow of Powders

When a sample of powder is swirled in a jar, or tipped onto a surface, it exhibits unconfined flow behavior – flow is induced by gravitational forces only. The particles move freely or bond together to form larger agglomerates, depending on the characteristics of the material. Such conditions often apply in industrial processes – when powder is poured out of a sack into a hopper, is pneumatically conveyed, or flows freely into an empty die, for example.

In contrast, when a powder is confined and forced to flow in a certain way, the particles are less free to move, and the powder behaves quite differently. It may extrude or ooze into the available volume, or simply lock up in a formation that is essentially immobile. During processing, confined powder flow occurs in molding operations, when powder is forced into a partially full die or mold and during pelletization when a powder feed is extruded into a stable product form.

Challenges with Confined and Unconfined Powders

A powder that flows freely when unconfined is likely to be much more problematic when confined and, perhaps less intuitively, the converse is also true. Cohesive powders that flow poorly when unconfined can move relatively easily under more forcing conditions. It is therefore important when matching process and powder to understand what conditions will prevail in the unit operation, and also the characteristics of the material. So, how can these aspects of flow behavior be investigated reliably and what does testing tell us about the mechanisms governing powder flow?

Techniques for Quantifying the Flowability of Powders

The quantification of flowability is a topic that has exercised the minds of powder processors for decades. There are many different ways of assessing flow behavior but arguably the most sensitive is dynamic characterization with a powder rheometer. Here the principle is to measure the energy needed to cause three dimensional flow, established by rotating a blade through the sample to generate a prescribed flow pattern. The torque and force acting on the blade are measured to determine parameters such as Basic Flow Energy (BFE) and Specific Energy (SE). These flow energies describe the resistance to flow of the powder in the confined and unconfined state respectively. Conditioning the sample before measurement ensures that it is analyzed in a consistent baseline state, which is one of the reasons why the technique delivers such a high level of reproducibility.

Measuring Confined Flow Using Basic Flow Energy (BFE)

Basic Flow Energy (BFE) is measured by rotating the blade downwards through the sample. This applies a compacting motion, each particle interacting with its neighbor but unable to move far because of the container walls. This is why the value of BFE correlates well with confined flow behavior.

Measuring Unconfined Flow Using Specific Energy (SE)

Specific Energy (SE), in contrast, is measured by rotating the blade upwards through the sample. The powder particles are free to move up and over the twisting blade so measurements reflect the ease with which the material will flow when unconfined. The observed correlation between SE and shear strength [1], the parameter measured during shear testing, highlights the ability of this method to quantify cohesivity.

Understanding Cohesivity in Classification of Powders

Cohesivity is one of the most commonly used terms in the classification of powders. With a cohesive powder the particles have a greater tendency to stick to one and another, and often to other surfaces as well. These forces of attraction may be strong because of particle size or particle shape, or some combination of these and other factors. Generally speaking the strength of interparticle forces increases with decreasing particle diameter, and irregularly-shaped particles have a greater tendency to lock together than those that are more spherical. The least cohesive powders tend to have spherical, relatively large particles.

Understanding the Interparticle Forces Between Cohesive Powders

The strong interparticle forces of more cohesive powders give rise to a range of distinctive characteristics. Such powders have low permeability, high compressibility and, of course, high shear strength. They are more prone to forming agglomerates and the resulting 'structure’ traps air within the bed, which is not easily released. It can be quite difficult to aerate a cohesive material since these agglomerates are not easily broken down and when air does penetrate it has a tendency to channel rather than flow more uniformly around individual particles, leaving flow properties largely unchanged (see figure 1).

Figure 1. Aeration of cohesive and non-cohesive powders.

So, in summary, with a cohesive bed the forces of attraction between particles are relatively high, giving poor flowability. The bed resists aeration so this poor flowability is not readily alleviated by blowing air through the material, although continuous aeration may improve performance. Unless actively consolidated, cohesive material will tend to entrain a great deal of air, reducing its bulk density and making it potentially highly compressible.

Quite differently, with less cohesive materials the forces of attraction between particles are weak to begin with and gravitational forces dominate. If the powder is aerated, flow energy becomes extremely low, very quickly. Particles are readily separated by the air, which substantially reduces the frictional or shear forces between them, so that the material behaves increasingly like a fluid rather than a powder. Less cohesive materials have lower compressibility and higher permeability. Such materials are generally closely packed and contain little entrained air, apart from the substantial void space between the particles. This means that compressibility is limited by the elastic and plastic properties of the particles. Permeability tends to be high because air can flow easily through the void spaces. However, vibration can promote closer packing of particles in a non-cohesive powder bed increasing both resistance to flow and bulk density.

Differences Between Cohesive and Non-Cohesive Powders

These fundamental differences between cohesive and non-cohesive powders have another important consequence with respect to processing. Cohesive powders release their air reluctantly – usually by being consolidated over time. On the other hand non-cohesive powders, especially those greater than 50 µm in size, release entrained air easily – sometimes so quickly that it compromises flowability.

The Mechanisms of Powder Flow

Comprehensive characterization of two powders at either end of the cohesivity spectrum – spray dried lactose and finely milled lactose (Table 1) - allows us to investigate the mechanisms governing flow behavior.

Table 1. Data for finely milled and spray dried lactose.

Finely milled lactose is clearly the more cohesive material, as would be expected from its size and shape. When unconfined it has the higher flow energy (SE) so under these conditions it will flow less easily than the spray dried lactose; however, under compaction or confined conditions the situation reverses. The BFE of the finely milled lactose is less than half that of the spray dried lactose. Why is this?

When the blade pushes down through the finely milled lactose in a compacting motion it compresses the material. The finely milled lactose, because it holds air, is relatively easy to compress, the whole bed having an almost 'spongy’ quality to it. The movement of the blade impacts only a small portion of the sample since the induced displacement can be absorbed quite easily. Here then the 'flow zone’ is relatively small so the energy needed to move the powder (the BFE) is quite low.

With spray dried lactose the flow zone is very much larger. The spherical, 130 µm (D50), spray dried lactose bed becomes tightly packed, remaining permeable due to the large void space, but experiencing high inter-particle friction throughout the flow zone. In this zone, the bed locks up and becomes difficult to shear as the blade forces its way through the powder. Therefore a large fraction of the bed moves as the blade rotates giving rise to a high BFE value (figure 2). Under compaction, beds of this type can easily lock up due to mechanical interlocking and wedging mechanisms which make shearing (flow), very difficult.

Figure 2. Forced flow behavior of non-cohesive and cohesive powders.

From a processing point of view these two powders will obviously behave very differently. The spray dried lactose will flow freely when unconfined and given the chance will fluidize, but under forced flow conditions it will perform poorly. Such powders tend to perform well in a tablet press, flowing freely into the die and forming a strong tablet; compressive forces are transmitted effectively through the powder plug. In extrusion applications, on the other hand, performance will be poor. Studies confirm that high pressures are required to push even a small amount of spray dried lactose through a die, and extrusion tends to be erratic.

Conversely the finely milled lactose, and similar cohesive powders, flow quite poorly when unconfined, so flow additives could be useful to achieve acceptable flowability for many processing steps. However, the finely milled lactose can flow well when extruded, providing the entrained air is not removed by excessive squeezing.

Summary

An understanding of the mechanisms affecting flow properties is essential in order to achieve optimal powder processing. Matching powder properties with the demands of a given unit operation minimizes the potential for processing problems, designing in manufacturing performance. As industry increases its focus on processing the need for relevant characterization at an early stage increases.

More-cohesive powders flow relatively poorly when unconfined but under the right conditions, may be extruded. In contrast, less-cohesive materials tend to flow more freely when unconfined but can easily lock up under forcing conditions, in which case they are able to withstand significant applied pressure. Dynamic powder characterization coupled with an analysis of the demands of the process allows the developer to avoid a mismatch between powder and plant. Building quality into the manufacturing process in this way is highly beneficial in terms of long-term production economics and an important goal in the current climate.

Source:Investigating the confined and unconfined flow behavior of powders

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