powder flow

• what is a powder?

  a collection of solid particles of the same or different chemical compositions having equivalent diameters less than 1000 μm. (a material)
• what does flowability (powder flow) mean?

  the ability of powder to flow in the desired manner in a specific piece of equipment
• What is the importance of free-flowing powder?

  • uneven powder flow can result in excess entrapped air within powders, which in some high-speed tableting conditions may promote capping or lamination.
  • uneven powder flow can result from excess fine particles in a powder, which increases particle– die-wall friction, causing lubrication problems, and increased dust contamination risks during powder transfer
  • the powder gets stuck on the punch
• good powder flow properties are important for what?

  for manufacturability and quality of solid dosage forms
• examples of manufacturing operations that include powder transfer...

  • gravity feeding
  • mechanically assisted feeding
  • pneumatic transfer
  • fluidisation - suspending a powder in a gas
  • hydraulic transfer
• what forces allow adhesion and cohesion to occur?

  • van der Waals forces
  • increase as particle size decreases
  • vary with changes in relative humidity
  • surface tension forces (bridging/humidity) between adsorbed liquid layers at the particle surfaces
  • can increase the flowability by adding a bit of water, which increases the interaction between the fine particles. too much can cause dissolution
  • electrostatic forces arising from contact or frictional charging
• inter particulate forces are influenced by what?

  by the particle size and shape
• powder flow properties are dependent on the inter-particulate forces, what do these forces allow?

  adhesion between two different objects:

  • between two different particles
  • between a particle and a container wall - they stick together so decreases flowability

  cohesion between like surfaces:

  • same component particles in a bulk solid
• what is the force balance that is needed to improve the flowability of powder?

  fill in the banks

  A) driving forces

  B) drag forces

  C) gravitational

  D) particle

  E) adhesive

  F) cohesive

  
• what are the factors that affect powder flowability?

  • particle size
  • particle shape
  • density
  • relative humidity
  • process conditions
• what are the typical measurements for particle size?

  • course powders - the majority of particles have an average particle size 355um
  • moderately fine powders - it is in the range between 180-355um
  • fine powders - it’s tin the range between 125-180um
  • very fine powders - it’s in the range of 125um
• what is ideal for the particle size of powders?

  • need a narrow distribution not a wide for particle size
• what can be added as excipients to improve flowability? (in relation to particle size)

  fine particles
• what are the 3 methods that can be used to analyse particle size?

  Sieve analysis, laser diffraction, microscopy.
• describe what microscopy gives (particle size analysis)

  gives the clear shape of a particle so diameter can be measured
• sieve analysis...

  • the upper one has the bigger mesh and the lowest one has the smallest mesh
  • shows how your sample is distributed in size
  • assume all particles are spherical
• laser diffraction...

  gives the distribution of particle size in a log scale by exposing the particles in a solid or liquid state
• particle shape and its influence on flowability...

  • flowability depends on shape and chemical structure
  • a spherical powder has good flowability
  • minimum contact = better flow
  • and high contact = poor flow
  • rods and flakes - SA is high so more interactions and poor flowability
  • irregular shape (interlocking) = poor flow
• how can particle shape be measured?

  • optical microscopy
  • scanning electron microcopy - 3D image
• how do true density, bulk density and porosity affect flowability?

  • the bulk density of a powder is always less than the true density of its component (inter particulate voids filled with air and intraparticulate pores)
  • powder particle can only possess a single true density
  • higher-density particles are generally less adhesive/cohesive than less dense particles of the same size and shape
  • more air = decreases density
  • Higher density particles are generally less adhesive/cohesive so have better foldability than less dense particles
• The area of discharge of powder or granules from a hopper depends on what?

  • hopper width
  • height of powder in the hopper
  • hopper wall angle
  • funnel flow - better for the industry cause mass flow machine taller
  • poor flowability can still flow with mass flow but will get stuck if its funnel flow

  
• what are the techniques that can be used to measure powder flow?

  angle of repose,

  compressibility index and Hausner ratio and

  flow through an orifice
• angle of repose method...

  • an indirect method
  • quantifying powder flowability (inter-particulate cohesion)
  • tan (∝) = height ÷ radius
  • The smaller the angle - the better the flowability of the powder
• the angle of repose can be an indicator of powder flow properties by:

  
• Compressibility index and Hausner ratio method...

  • True density: Only the solid fraction of the particles is included
  • Bulk density: includes the volume of the solid fraction of particles and intra- and inter-particulate volumes
  • indirect method
  • measure of the tendency of arch or bridge formation
  • if the particle is too small in size - after tapping there will be a layer of air which is called an arc
  • arch formation can mean that it will occur in the hopper
• what is the Hausner ratio?

  Hausner ratio is a measure of the flowability of a powder, calculated as the ratio of tapped density to bulk density.

  • 𝐻𝑎𝑢𝑠𝑛𝑒𝑟 𝑟𝑎𝑡𝑖𝑜 = 𝑇𝑎𝑝𝑝𝑒𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 ÷ 𝐵𝑢𝑙𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦
• what is the compressibility index equation?

  • 𝐶𝑜m𝑝𝑟𝑒𝑠𝑖𝑏𝑖𝑙𝑖𝑡𝑦 𝑖𝑛𝑑𝑒𝑥 = (𝑇𝑎𝑝𝑝𝑒𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦−𝐵𝑢��𝑘 𝑑𝑒𝑛𝑠𝑖𝑡𝑦) ÷ 𝑇𝑎𝑝𝑝𝑒𝑑 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 × 100
  • poor powder flow = less compressible
  • tapped density - the density after tapping
• values for compressibility index and hausner ratio

  
• Flow through an orifice...

  • a direct method
  • measure the flow rate of the powder using a laser beam or a digital balance
  • example → 100g of powder to pass through the orifice to the nearest tenth of a second or the amount of powder passing through the orifice in 10 s to the nearest tenth of a gram
  • in g/s or g/ml
• how can the particle size and particle size distribution be altered?

  • big particle size → grind it
  • too little → granulation
• how do you improve powder flow properties?

  • alteration of particle size and particle size distribution
  • distribution needs to be narrow can be done by sieving
  • alteration of particle shape or texture
  • alteration of surface forces - can add additives to change surface forces during the grinding method
  • formulation additives (flow activators)
  • alteration of process conditions
• what alterations can be done to process conditions to improve powder flow properties?

  • use of vibration - assisted hoppers - prevents the powder from sticking to walls and each other
  • use of force feeders
  • powder needs to have an acceptable flowability for these alterations to work
• Interparticle forces: particle size

  The smaller the particles, the stronger the interparticle forces
• Interparticle forces: particle shape

  Irregularly shaped particles tend to have stronger forces than spherical particles
• Interparticle forces: particle composition

  Particles with a high polarity have stronger forces than particles with a low polarity