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Powder Behaviour and the Nature of Powders PDF Print E-mail
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About powders

We think of powders as a mass of solid particles or granules. In fact these particles are usually surrounded by air (or other fluid) and it is the solids plus fluid combination that largely determines the bulk properties of the powder. It is perhaps the most complicating characteristic because the amount of fluid can be so variable.

Powders are probably the least predictable of all materials in relation to flowability because of the large number of factors that can change their rheological properties. Physical characteristics of the particles, like size, shape, angularity, surface texture, porosity and hardness will all affect flow properties. External factors such as humidity, conveying environment, vibration and perhaps most importantly, aeration, will compound the problem. The more common variables would include:

Powder or Particle variables External Factors influencing Powder Behaviour
  • Particle size
  • Size distribution
  • Shape
  • Surface texture
  • Cohesivity
  • Surface coating
  • Particle interaction
  • Wear or attrition characteristic
  • Propensity to electro-static charge
  • Hardness
  • Stiffness
  • Strength
  • Fracture toughness
  • Flow rate
  • Compaction condition
  • Vibration
  • Temperature
  • Humidity
  • Electro-static charge
  • Aeration
  • Transportation experience
  • Container surface effects
  • Storage time

Another characteristic of powders is that they are often inherently unstable in relation to their flow performance. This instability is most obvious when a free flowing material ceases to flow. This transition may be initiated by the formation of a bridge in a bin, by adhesion to surfaces or by any event that may promote compaction of the powder. The tendency to switch in this way varies greatly from one powder to another, but can even be pronounced between batches of the same material.

Powder behaviour will be very dependent upon particle size, the variation of size and the shape of the particles. In general powders with large particles (>100µm) will be non-cohesive, permeable and will probably fluidise and will have low compressibility and relatively low shear strength.

Conversely, fine powders <10µm say, are likely to be cohesive, compressible, contain much entrained air and yet have poor aeration characteristics. Generally they have high shear strength, high flow energy, low permeability and are very affected by being consolidated when entrained air is excluded.

There are many exceptions to the above – for example toner used in printers and copying machines are fine powders with an outstanding fluidisation characteristic. A small amount of aeration is sufficient to transform a consolidated powder into one with fluid like rheology. Another broad generalisation is that under forced flow conditions, where powders are made to move other than by gravity, fine powders can behave more like a fluid. They are able to extrude round corners or through holes, unlike coarse powders that are more likely to become solid like as particles realign and lock together and become very resistant to flow.

The nature of powders therefore is such that an adverse combination of environmental factors can cause an otherwise free flowing powder to block or flow with difficulty. Conversely, a very cohesive powder may be processed satisfactorily if the handling conditions are optimised.

Because processing is largely a matter of making things work, it is often not known how marginal a set up may be. This often results in a system operating well for hours, and suffering from stoppages from time to time. What is needed is knowledge of both the flow properties of the material and the processing characteristics of the machinery to be used.

Aluminium D50

Aluminium, D50 134µm

Tungsten D50

Tungsten, D50 4µm

Predicting powder flowability

Given the complex nature of powders, it is not surprising that processing difficulties are commonplace. Being able to predict flow performance would bring many operational advantages such as reducing stoppages and improving product quality.

To achieve this, we need to know how a given powder is affected by the variables mentioned above and also to have a reliable indicator of the potential instability of the powder. These are the primary functions of the FT4 Powder Rheometer.

The difficulties are well illustrated if we look at how transportation can affect the flow properties of a material. A powder could become highly aerated or even fluidised under some conditions of transportation. Alternatively, it might be gently agitated in a different situation, or as often happens, highly consolidated by being continuously vibrated. The energy needed to establish flow of a fluidised powder might be only 0.3% of that needed if that powder becomes consolidated by vibration or direct compaction. This energy requirement would reduce to 15% if the powder were in a lightly packed state, free of excess air and residual compaction.

Processing Powders and predicting flowability performance in a particular plant therefore requires knowledge of the handling and processing conditions as well as the flowability characteristics of the material under these conditions. It means that the process conditions relevant to flowability need to be determined. These might include the level of static and dynamic head produced in a storage bin or hopper, the amount of aeration that occurs, the opportunity to adsorb moisture, become electrostatically charged or be consolidated due to vibration.

Other factors could be segregation and attrition that may cause fines to collect, rounding of particles and so on - all potentially affecting the flow characteristics of the material. All, or at least the most important of these factors then needs to be quantified regarding how they affect flowability. Again, this is the function of the FT4 Powder Rheometer and the methodology that has been developed.

How many numbers do we need to describe a cup of coffee?

Arguably the answer is at least 4 or 5, probably more! So how many do we need to describe powders which are very much more complex? How do they flow under gravity when consolidated, unconsolidated, aerated or even fluidised? How readily will a powder entrain air and release it again? How is it affected by moisture, vibration, storage, the accumulation of fines or flow additives? How compressible is it, can it be forced to flow, will it extrude? Is it prone to segregation or attrition? How does the variation of particle size affect flowability? What are its shear properties, compression strength, internal angle of friction and cohesion values?

Yes it is complex and we cannot ignore this reality. Fortunately it is possible to identify those parameters that are almost always important from those that maybe sometimes. For example key parameters are usually:

  • Flow energy when consolidated, non-consolidated, aerated and fluidised
  • Cohesivity – or Specific flow energy
  • Permeability
  • Sensitivity to flow rate
  • Stability
  • How flow energy is affected by aerating the powder

Although there might be 100 or more factors that influence powder behaviour, there are 10 or 20 that can give a reasonable indication of how a powder will flow if we assume it is stable and not affected by segregation, moisture or fines variability. What is certain is that the traditional single number description is certainly inadequate and probably misleading.

Powder testing and the need for innovation

We have seen that a powder is a blend of particles and air and that the amount of entrained air can transform the flow properties of the powder bulk. Typically, the difference in energy needed to establish flow in a compacted powder may be 100 times that needed when the powder is aerated and possibly fluidised. For some powders, this ratio can be more than 1000 and in the most extreme case, 5000.

Slight compaction, a small vibration, or the smallest amount of aeration can significantly affect flowability. This is the main reason why traditional methods of flowability measurement have not been suitable as a basis for repetitive testing. In all traditional techniques, the packing condition and the air content, are largely unknown quantities and so the results will vary accordingly. When making an assessment it is essential to know what was tested and the condition of the powder when tested.

The first innovation needed, therefore, is a way of producing a standardised packing state as a preliminary to testing (Conditioning). It is then possible for measurements to be compared - whether between different batches, dates or sites.

In addition to the packing problem, traditional flowability measurements are prone to operator error, have poor repeatability and, for the most part, are very time-consuming. An automated test and analysis system is needed that takes only minutes, is very repeatable and is independent of the operator.

A further need is to measure flowability performance at flow conditions that are representative of real processing situations. Hence a dynamic flow condition is required that may be established and maintained for a period of time, during which measurements and observations are made.

Powder Characterisation

Classification of powders

The most important innovation required in relation to traditional techniques, is a way of classifying powders so that flowability performance of each powder can be measured and recorded along with processing experience. Eventually, such a database of information could remove much of the uncertainty from the Processing of Powders and provide a reference-base for the development of new powders. It would allow each piece of production equipment to be classified in terms of the powders that could be efficiently processed.

Ideally, the classification of powders would provide more than just flowability data, such as flow rate and compaction indices. It would also include data describing the robustness and stability of the powder - for example, vulnerability to segregation, attrition and vibration. Given this, then the two key issues of powder processing could be addressed. Firstly, will the powder flow satisfactorily - does it have flowability properties that suit the process? And secondly, is the powder robust - will it be adversely affected by being processed?

Classification is now possible and by providing answers to these questions is set to revolutionise the design, processing and quality control of powders.

 
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