Powder Flow Flashcards
Why is powder flow important ?
- powders are building blocks for many solid dosage forms
What can powder flow directly affect ?
- can affect manufacturing efficiency
- dosage form weight
- drug dose uniformity
What are the 2 driving forces for powder flow ?
- gravity
- mechanical agitation (external forces)
What are the 2 resistive forces for powder flow ?
- cohesion / adhesion
- friction
Gravitational force
- F = m x g
- F = gravitational force
- m = mass of the particle
- g = particle acceleration
Mechanical agitation
- external forces to induce powder motion and to affect its flow behaviour
What is cohesion ?
- attractive forces between 2 chemically similar surfaces (e.g particle + particle )
What are adhesion and cohesion a result of ?
- van der waals forces
- electrostatic interactions
- mechanical interlocking
What is adhesion ?
- attractive forces between 2 chemically different substances (e.g particle + equipment )
Van der Waals forces
- induced dipole - induced dipole interactions
- greater if particles are larger + closer together
What is tribocharging ?
- surface statics develop due to friction
Where does the tribocharging effect take place during the flow of powder in a device ?
- takes place at the contact between the grains + at the contact between the grains and the device
How does mechanical interlocking occur ?
- rough surface textures
- sharp edges
- angular shapes
How does mechanical interlocking affect particle movement ?
- particles with irregular complementary shape lock together
- so they strongly resist further movement
How can you reduce mechanical interlocking ?
- make particles more spherical by granulation
What is friction + how does it affect particle movement at contact surfaces ?
- present at contact surfaces where at least one surface is in motion
- acts in opposite direction versus direction of movement, thus inhibiting particle movement
5 particle properties that affect powder flow ?
- size
- shape
- density
- surface texture
- moisture content
How does size of particle affect powder flow ?
- as particle size decreases, flow is reduced
- due to greater adhesion forces
How does shape of particle affect particle flow ?
- optimal flow provided by spherical particles
- because there is no interlocking
How does density of particle affect powder flow ?
- influence gravity + surfaces forces
- denser particles have better flow
How does surface texture affect powder flow ?
- smooth surfaces increase powder flow
- more rough surface = more particle adhesion
How does moisture content affect particle flow ?
- higher amount of moisture = greater risk of adhesion and cohesion
- reduces particle flow
Optimal particle properties for powder flow ?
- large size
- spherical shape
- small surface area (large particles)
- high density
- moderate moisture content
Do smaller particles have a large or small surface area ?
- small particles have large surface area
Effects of large surface area
- greater surface free energy
- more bonding interactions
- more cohesive
- greater resistive forces
- poorer flowability
Why is spherical particle shape better for powder flow ?
- has the smallest surface area of any particles shape
- less likely to mechanically interlock with another particle
Effects of spherical particles shape
- smaller specific surface area
- lower surface free energy
- fewer bonding interactions
- less cohesive
- smaller resistive forces
- better flowability
What is packing geometry ?
- position of solid particles relative to one another
What is packing fraction ?
- volume fraction of solid particles (k)
What is porosity ?
- volume fraction of air (e = 1- k)
What does high porosity mean ?
- more air between particles
What is bulk density ?
- density of solid particles plus entrapped air when the powder is allowed to settle naturally under gravity
- bulk density = mass / bulk volume
What is tapped density ?
- density of solid particles with minimal amount of entrapped air, after consolidation using a pharmacopeial method
- tapped density = mass / tapped volume
What does particle density (true density) refer to?
- refers to the density of the solid material in a particle excluding air or pores
- assuming the particle is non-porous.
Characteristics of a denser particle ?
- greater mass at same volume
- greater gravitational force
- lower specific surface area
- smaller resistive forces
- better flowability
Arrange bulk density, tapped density, and true density in increasing order ?
- Bulk density < Tapped density < True density.
What role does moisture play as a binder in powders ?
- moisture acts as a binder, helping particles adhere to each other (cohesion ) + surfaces (adhesion)
What does moderate moisture form ?
- forms liquid bridges between particles
How does moisture affect lubrication in powders ?
- provides lubrication by reducing friction between particles
- improves flow
What problems can arise with dry particles ?
- more likely to experience friction + generated by statics
What does moderate moisture content provide a balance between ?
- between lubrication + cohesion
Mass flow
- first in, first out
- good flowability
Funnel flow
- last in, first out
- arching, ratholing
- poor flowability
What are some powder flow problems ?
- caking
- wall build up
- funnelling
- plugging
4 methods for testing powder flow ?
- flow rate
- density ratios
- angle of repose
- shear cell
What causes a powder to flow out of a funnel ?
- due to gravity
What is method 1 for measuring powder flowability ?
- measure amount of powder discharged over a fixed duration
What is method 2 for measuring powder flowability ?
- measure the time taken to discharge a fixed amount of powder
Equation for calculating flow rate ?
- flow rate = amount of powder discharged / discharge duration
How does cohesion affect powder flow ?
more cohesive powder :
- flows less readily
- takes longer to flow through an orifice
- lower flow rate
What 2 factors is flow rate influenced by ?
- hopper shape
- orifice shape
How does orifice diameter affect flow rate ?
- increased diameter = increased flow rate
How does height of powder in hopper effect flow rate ?
- has no effect when height is greater than orifice diameter
How do you measure powder flowability using density ratios ?
- measure degree of volume reduction upon powder consolidation
- standardised set of parameters (number of taps, drop of height )
What is the angle of repose ?
- angle between slope and horizontal
What does steepness of the slope represent ?
- represents equilibrium of driving and resistive forces
How does cohesion affect angle of repose ?
- causes particles to pile up (stop flowing)
- increases angle of repose
How does gravity influence the angle of repose ?
- cause particles to roll down the slope (flow)
- reduces angle of repose
What does higher angle of repose mean ?
- poor flowability
