Top 10 Tips for Powder Characterisation


​Powders are widely recognised as presenting a tougher processing challenge than gases and liquids, and relevant characterisation can be equally demanding. Here Jamie Clayton, Operations Director at Freeman Technology shares some critical insights about powder behaviour and how to quantify it to enhance product performance and boost manufacturing efficiency.

Freeman Technology Provided byFreeman Technology
1

​Recognise the difference between powders and particles

Recognise the difference between powders and particles

The terms ‘particle’ and ‘powder’ are often used interchangeably but this is inaccurate and unhelpful when it comes to understanding powder behaviour.  Powders are bulk assemblies that contain particles but also, liquid (typically water) and gas (normally air). Bulk powder behaviour is influenced by all three phases, not just by the properties of the particles present. Recognising this point is fundamental when it comes to rationalising observed behaviour, whether processing powders or testing them.

2

​Appreciate the impact of environmental conditions

Appreciate the impact of environmental conditions

​The variability frequently attributed to powders is often a failure to recognise their sensitivity to environmental conditions. For example, a powder may take up water, release entrained air, and/or dissipate stored electrostatic charge, depending on external variables.  These processes are often relatively uncontrolled but, by changing one of the three phases that influence powder behaviour, they can dramatically alter processing performance and/or test data. Appreciating this is an important first step towards better process control and/or the establishment of more robust test protocols.

The variability frequently attributed to powders is often a failure to recognise their sensitivity to environmental conditions. For example, a powder may take up water, release entrained air, and/or dissipate stored electrostatic charge, depending on external variables.  These processes are often relatively uncontrolled but, by changing one of the three phases that influence powder behaviour, they can dramatically alter processing performance and/or test data. Appreciating this is an important first step towards better process control and/or the establishment of more robust test protocols.
3

​Focus on flowability

Focus on flowability

​The ease with which a powder flows is arguably its defining characteristic, with poor flowability the root cause of many processing efficiency issues.  This may be more immediately obvious when faced with a blocked hopper than with tablets of poor content uniformity but often proves to be true; flowability directly influences content uniformity by impacting how easily powders blend with one another.

The mechanisms that govern how easily one particle moves relative to another, and hence powder flowability, include friction, mechanical interlocking, liquid bridging, cohesion, and gravitational effects. Focusing on these mechanisms and their control will help achieve the flowability, and processing performance, required.

The mechanisms that govern how easily one particle moves relative to another, and hence powder flowability, include friction, mechanical interlocking, liquid bridging, cohesion, and gravitational effects. Focusing on these mechanisms and their control will help achieve the flowability, and processing performance, required.
4

Understand the limitations of a single number

​Powders can exhibit an impressive array of behaviours which is one of the reasons they are so industrially useful.  For example, they can flow, compress, aerate and/or fluidise, consolidate, agglomerate, cake and/or become electrostatically charged. Trying to capture this wealth of behaviour with just a single number is not realistic - a little like trying to describe a person with a single adjective. Powder testers that measure just one property, and deliver a single number, attempt to do just that.

​If you choose to adopt a single number approach then recognise it for what it is. You are looking at just one aspect of behaviour and quite possibly not the one of most relevance to your process. If you need more insight then adopt a more comprehensive multi-faceted approach. Measuring an array of properties provides a fuller understanding of your powder making it possible to identify the behaviours that truly govern performance.

Powders can exhibit an impressive array of behaviours which is one of the reasons they are so industrially useful.  For example, they can flow, compress, aerate and/or fluidise, consolidate, agglomerate, cake and/or become electrostatically charged. Trying to capture this wealth of behaviour with just a single number is not realistic - a little like trying to describe a person with a single adjective. Powder testers that measure just one property, and deliver a single number, attempt to do just that.

 

If you choose to adopt a single number approach then recognise it for what it is. You are looking at just one aspect of behaviour and quite possibly not the one of most relevance to your process. If you need more insight then adopt a more comprehensive multi-faceted approach. Measuring an array of properties provides a fuller understanding of your powder making it possible to identify the behaviours that truly govern performance.
5

​Reject the myth of the ‘good’ or ‘bad’ powder

Reject the myth of the ‘good’ or ‘bad’ powder

​A direct consequence of the preceding point is that recognising the demands of your process is an essential precursor to the identification of a powder that will be well-matched to it. Powders are processed in a myriad of unit operations including hoppers, feeders, blenders, mixers, granulators, dryers, conveyors, mills, extruders, and fillers. These may subject a powder to processes such as compression or aeration, they may exert forcing flow or permit free-flow under gravity, and they may be operated in ways that have a direct impact on powder behaviour. Understanding the demands of your process, and the conditions to which it subjects a powder allows you to test powders in a more relevant way and/or to knowledgably set processing parameters so as to handle potential problems.

A direct consequence of the preceding point is that recognising the demands of your process is an essential precursor to the identification of a powder that will be well-matched to it. Powders are processed in a myriad of unit operations including hoppers, feeders, blenders, mixers, granulators, dryers, conveyors, mills, extruders, and fillers. These may subject a powder to processes such as compression or aeration, they may exert forcing flow or permit free-flow under gravity, and they may be operated in ways that have a direct impact on powder behaviour. Understanding the demands of your process, and the conditions to which it subjects a powder allows you to test powders in a more relevant way and/or to knowledgably set processing parameters so as to handle potential problems.
6

​Know the demands of your process

Know the demands of your process

A direct consequence of the preceding point is that recognising the demands of your process is an essential precursor to the identification of a powder that will be well-matched to it. Powders are processed in a myriad of unit operations including hoppers, feeders, blenders, mixers, granulators, dryers, conveyors, mills, extruders, and fillers.

These may subject a powder to processes such as compression or aeration, they may exert forcing flow or permit free-flow under gravity, and they may be operated in ways that have a direct impact on powder behaviour. Understanding the demands of your process, and the conditions to which it subjects a powder allows you to test powders in a more relevant way and/or to knowledgeably set processing parameters so as to handle potential problems.

7

​Define a specification that detects batch variability

Define a specification that detects batch variability

​Batch-to-batch variability in a powder is too often detected by sub-optimal process performance or via a customer complaint. This clearly represents a failure of QC, the root cause of which is that the associated specification does not robustly detect relevant differences.  A powder specification typically includes particle size and bulk density data, perhaps a measure of particle shape and/or a simple flow test result. If a powder can pass this specification but still fail then you need to add in parameters that reliably and relevantly detect differences. A more sensitive and relevant measure of flow is often the answer here, because flowability is critical to so many processes.

Batch-to-batch variability in a powder is too often detected by sub-optimal process performance or via a customer complaint. This clearly represents a failure of QC, the root cause of which is that the associated specification does not robustly detect relevant differences.  A powder specification typically includes particle size and bulk density data, perhaps a measure of particle shape and/or a simple flow test result. If a powder can pass this specification but still fail then you need to add in parameters that reliably and relevantly detect differences. A more sensitive and relevant measure of flow is often the answer here, because flowability is critical to so many processes.
8

​Invest in suitable storage

Invest in suitable storage

​Storing a raw material or final product in sub-optimal conditions can have a major impact on its value. Powders cake, a reversible or sometimes irreversible process of conversion from a relatively free-flowing powder to a solid. Caking can be influenced by the geometry of a storage container, the duration of storage, ambient storage conditions (temperature and humidity) and any vibrational consolidation induced by, for example, extended periods in shipment. Identifying test methods that allow you to determine whether your powder is prone to caking or not, under what conditions, and with what result, is the key to adopting cost-effective storage practices.

Storing a raw material or final product in sub-optimal conditions can have a major impact on its value. Powders cake, a reversible or sometimes irreversible process of conversion from a relatively free-flowing powder to a solid. Caking can be influenced by the geometry of a storage container, the duration of storage, ambient storage conditions (temperature and humidity) and any vibrational consolidation induced by, for example, extended periods in shipment. Identifying test methods that allow you to determine whether your powder is prone to caking or not, under what conditions, and with what result, is the key to adopting cost-effective storage practices.
9

​Share learning

Share learning

​An instruction to operate a hopper at level X for powder A and level Y for powder B may appear illogical. Compliance may be more easily assured if there is an understanding that powder A is more susceptible to compressive forces and that by keeping the hopper at a lower level, when using it, you’ll avoid a blockage. Providing operators with an understanding of the characteristics of the powder they are processing supports more effective operation and a greater contribution to process optimisation.

Listening to day-to-day observations of powder behaviour is equally important. Correlating these observations with measured powder properties can make it easier to, for example, establish effective QC criteria, understand and address the causes of batch-to-batch variability and/or define the characteristics of powders that will process well on a given line. The net result is an enhanced ability to match powder and processing equipment/conditions to boost manufacturing efficiency and to reduce the risk associated with making new products.

An instruction to operate a hopper at level X for powder A and level Y for powder B may appear illogical. Compliance may be more easily assured if there is an understanding that powder A is more susceptible to compressive forces and that by keeping the hopper at a lower level, when using it, you’ll avoid a blockage. Providing operators with an understanding of the characteristics of the powder they are processing supports more effective operation and a greater contribution to process optimisation.

 

Listening to day-to-day observations of powder behaviour is equally important. Correlating these observations with measured powder properties can make it easier to, for example, establish effective QC criteria, understand and address the causes of batch-to-batch variability and/or define the characteristics of powders that will process well on a given line. The net result is an enhanced ability to match powder and processing equipment/conditions to boost manufacturing efficiency and to reduce the risk associated with making new products.
10

​Evaluate sensitivity

Evaluate sensitivity

​If you are carrying out powder testing in an industrial setting then success has a commercial value. Understanding the reasons for poor process performance, for example, is a precursor to solving them and may be associated with debottlenecking the process, releasing manpower for more valuable activity, achieving more reliable product quality and/or commercialising a new product. All of these outcomes can be monetarised.  And they are all dependent in the first instance on being able to develop the necessary understanding of powder behaviour.

​Powder testing techniques differ in terms of their inherent sensitivity and their ability to detect differences. Two testers offering the same technique will also deliver different reproducibility and sensitivity. A tester that can’t differentiate powders that perform differently cannot help solve process or product related issues. Be realistic about how rigorously you need to test your powders and the value of the resulting data to identify a tester that is optimally matched to your needs.

If you are carrying out powder testing in an industrial setting then success has a commercial value. Understanding the reasons for poor process performance, for example, is a precursor to solving them and may be associated with debottlenecking the process, releasing manpower for more valuable activity, achieving more reliable product quality and/or commercialising a new product. All of these outcomes can be monetarised.  And they are all dependent in the first instance on being able to develop the necessary understanding of powder behaviour.

 

Powder testing techniques differ in terms of their inherent sensitivity and their ability to detect differences. Two testers offering the same technique will also deliver different reproducibility and sensitivity. A tester that can’t differentiate powders that perform differently cannot help solve process or product related issues. Be realistic about how rigorously you need to test your powders and the value of the resulting data to identify a tester that is optimally matched to your needs.

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