Preformulation - Powder Flow Flashcards
What are the different time of powders?
Coarse Moderately coarse Moderately fine Fine Very fine Microfine Superfine
What are powders?
Complex fluid compromising a mixture of solid particles + air
What do driving forces do?
Promote flow
What do resistive forces do?
Inhibit flow
What are the driving forces?
Gravity
Mechanical agitation
What are the resistive forces?
Cohesion/adhesion
Friction
What is adhesion?
Attractive forces between 2 chemically different surfaces
What is cohesion?
Attractive forces between 2 chemically similar surfaces
What are adhesion + cohesion result of?
London forces
Electrostatic attraction
Mechanical interlocking
How do you calculate gravitational force?
Mass X particle acceleration
What are London forces?
Induced dipole-dipole interactions
When are London forces greater?
Larger + closer together
What does electrostatic force depend on?
Magnitude of charge + separation distance between surfaces
When do mechanical interlocking happen?
Physical obstruction Rough surface texture Sharp edges Angular edges One particle stops movement of another
When is friction present?
At contact surfaces where at least one surface is in motion
How does friction inhibit movement?
Acts in opposite direction to direction of movement
How do you calculate friction force?
Coefficient of friction X reaction force
What size should the particle be to favour powder flow?
Large
What density should the particle be to favour powder flow?
High
What shape should the particle be to favour powder flow?
Spherical
What specific surface area should the particle be to favour powder flow?
Small
What packing geometry should the particle be to favour powder flow?
Loose
What moisture content should the particle be to favour powder flow?
Moderate
What do smaller particles have compared to larger particles?
Smaller SA per particle
Larger total SA
Larger specific SA
How do you calculate specific SA?
Mass
What does a larger specific SA have?
Greater surface free energy More bonding interactions More cohesive Greater resistive forces Powder flow ability
Why should the particles be spherical?
Smallest specific SA of any particle shape
Less likely to mechanical interlock
What does a more spherical particle have?
Smaller specific SA Lower surface free energy Fewer bonding interactions Less cohesive Smaller resistive forces Better flow ability
What is packing geometry?
Position of solid particles relative to one another
What is packing fraction?
Vol fraction of solid particles
What is porosity?
Vol of air
What is bulk density?
Density of solid particles plus entrapped air when powder is allowed to settle naturally under gravity
How do you calculate bulk density?
Mass/ bulk vol
What is trapped density?
Density of solid particles with minimal amount of entrapped air, after consolidation using a pharmacopeial method
How do you calculate trapped density?
Mass/ trapped vol
What is true density?
Density of solid particles only (discounting air)
How do you calculate true density?
Bulk density/ packing fraction
What does it mean if there is a denser particle?
Greater mass at same vol
Greater gravitational force
Better flowability
Why do denser particles have better flowability?
Lower specific SA
Less cohesive
Smaller resistive forces
What makes particles stick together?
Binding properties
What are dry particles most likely to experience?
Friction + generate statics
Why do we need a moderate moisture content?
Balance between lubrication + cohesion
What do liquid bridges between particles provide?
Lubrication
What are the different powder flow patterns?
Mass flow
Funnel flow
Describe mass flow
First in, first out
Good flowability
Describe funnel flow
Last in, first out
Arching, ratholing
Poor flowability
Describe flow through orifice
Powder discharged from hopper through orifice under gravity
How do you calculate flow rate?
Discharge duration
What is method 1 of measuring flow rate?
Measure amount of powder discharged over fixed duration
What is method 2 of measuring flow rate?
Measure time taken to discharge fixed amount of powder
Describe more cohesive powder flow rate
Flows less readily
Takes longer to flow through orifice
Lower flow rate
What is flow rate influenced by?
Hopper shape
Orifice shape
Orifice diameter
What has no effect on flow rate?
Height of powder in hopper
What does the compressibility index + Hausner ratio measure?
Measures degree of vol reduction upon powder consolidation
How is compressibility index + Hausner ratio measured?
Measure bulk vol of graduated cylinder
Consolidate powder on tapped vol tests to a standardised set of parameters
Measure tapped col in graduated cylinder
What are the units of Hausner ratio?
NONE
What does a smaller value of Hausner ratio mean?
Better flowability
What is the equation for Hausner ratio?
Bulk vol Tapped density
———— = ———————- > 1
Tapped vol Bulk density
What are the units of compressibility index?
%
What does a smaller value of compressibility index mean?
Better flowability
What is the equation for the compressibility index?
100 (Vf-Vt) 100(Pt-Pf)
—————– = —————– <100
Vf Pt
Vf = bulk vol Vt = tapped vol Pf = bulk density Pt = tapped density
What is the angle of repose?
Angle between slope + horizontal
What does the steepness of the slope represent?
Eqm of driving + resistive forces
What does cohesion cause particles to do?
Pile up (stop flowing)
What does gravity cause particles to do?
Slope (flow), reducing angle
What will happen if you have a more cohesive powder?
Greater angle of repose = poorer flowability
How do you calculate the angle of repose?
tan-1 (h/r)
