Why Are Powders Complex?

The Influence of External Variables

As illustrated in the previous example, powders can behave very differently when they are fully aerated, loosely packed or consolidated. Some powders are extremely sensitive to these variables whilst others less so. A given powder could flow well when aerated and loosely packed, but become a real problem if consolidated (toner, for example). A different powder could exhibit reasonable flow properties when loosely packed, not be too affected by consolidation, but show real improvements in flow properties following aeration (ceramic powder, see video). Considering this observation, it seems unlikely that a single number can be used to fully describe a powder’s response to the large range of aeration and consolidation levels that the powder will experience during processing and application.

The extent of consolidation stress and air content are two of the most influential variables as far as flow properties are concerned. However, powder behaviour is also affected by the speed at which it is processed, such as during mixing or when on a filling line for example, as well as by other variables such as ambient moisture level and storage time. A powder that was once performing well may exhibit poor behaviour if left in storage, or if processed in an environment with slightly higher humidity than normal.

External Variable When and Where Effect
Consolidation
  • Vibration / Tapping
  • Direct pressure (hopper, IBC, keg)
  • Increase in particle pressure, contact area and number of contact points
  • Reduction in air content between particles (reduced porosity)
Aeration
  • Gravity discharge
  • Blending
  • Pneumatic conveying
  • Aerosolisation
  • Reduction in particle pressure, contact area and number of contact points
  • Increase in air content between particles (increased porosity)
Flow (shear) Rate
  • Within powder
  • Powder against equipment wall
  • Mixing
  • Mostly non-Newtonian
  • Greater resistance to flow at lower flow rates
Moisture
  • Storage
  • Processing
  • Intentionally introduced (granulation)
  • Increase particle adhesion
  • Reduces particle stiffness - more compliant but increased contact surface area
  • Increase electrical conductivity
Electrostatic Charge
  • Discharge from hopper
  • Pneumatic conveying
  • High shear mixing
  • Increase bond strength between particles
  • Adhesion of powder to equipment
Storage Time
  • Raw materials / Intermediates
  • Consolidation
  • Caking
  • Permanently affecting downstream performance

Controlling certain variables may be relatively straight forward, but changes in air content and consolidation stress is usually unavoidable during processing. Even the most basic of transfer chutes has the potential to aerate and consolidate the powder moving through it. Aeration occurs as a powder dilates, with particles becoming further separated, and this occurs in many processes, including blending, filling and discharge operations, even though an external air supply is not being applied, as it would be in pneumatic conveying.

Recognising that any of these external variables has the potential to change the way the powder behaves is the first step in gaining a better understanding of process performance. Measuring a powder’s response to each external variable then provides the ability to understand why a powder behaves in a certain way, and the chance to optimise both formulation and manufacturing efficiency.

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