Powder Flow Flashcards
Importance of Powder Flow
• Tablettling
• Granulation
• Capsulation
ALL ABOVE Filled by volume
• Transfer in the manufacturing process – Pneumatic transfer
– Hydraulic transfer
– Gravity or mechanically assisted feeding
Consequences of poor powder flow
• Uneven mix into feed
• Risks of segregation
• Non-uniformity of dose
• Problems with reproducibility of dosage forms that require powder transfer
– E.g. granulation, tabletting, capsulation
• Risks of capping and lamination (tabletting)
– Due to entrapped air within powders
• Increase in particle-die-wall friction and risks of dust contamination
– With fine particles
Resistance of powder flow
• Cohesion
– Cohesive forces between particles
– Due to Van der Waals forces (electrostatic)
• Dependant on nature of particles
• Forces increase as particle size decreases
– Due to moisture (relative humidity)
• Formed between particles when above critical humidity
• Adhesion
– Adhesive forces between two unlike surfaces
• E.g. a particle with the hopper wall of a tabletting press
Factors affecting powder flow
- Resistance to flow due to cohesion/ adhesion
- Particle size
• <10um particles extremely cohesive
• <50um irregular/ no flow due to VDW forces
• >50 um free flowing - Density of particles
• dense particles less cohesive
Density =
mass / vol
Particle shape
– Spherical particles less interparticulate contact than other uniform or irregular shapes.
Particle size distribution
– Smaller particles (> 50μm) settle below larger particles and lead to uneven powder flow
Particle surface
Electrostatic forces increase cohesion
Particle texture
– Rough surface more cohesive
– Packing geometry
Humidity
— Absorbed moisture on particle surface
Processing conditions
— Vibration of hopper (controlled)
— Require validation
Factors affecting powder flow
- Particle shape
- Particle size distribution
- Particle surface
- Particle texture
- Humidity
- Processing conditions
- Powder density
- Porosity of Powder
Powder density
• Rearrangement of particle packing geometry
– Change in bulk volume
– Change in density of the powder
• Different geometric packing’s can be achieved with particles of same shape
– E.g. Cubic or rhombohedra particle packing (more densely packed) compared with spherical particles
• Density of a powder sample is referred to as the bulk density
– Volume includes particulate volume and inter- particulate pores
Bulk density
• Minimum bulk density
• Poured bulk density
• Tapped bulk density
- Minimum bulk density–maximum volume occupied by powder
- Poured bulk density–volume of powder measured after pouring into a cylinder
- Tapped bulk density – maximum bulk density (in theory) that can be achieved without deformation of particles
Relative density
Relative density (k) also known as packing fraction or fractional solids content
k = bulk density/ true density
Porosity of powder bed
- Indication of the proportion of a powder bed that is occupied by pores
- A measure of the packing efficiency of the powder
Porosity = 1 - bulk density/ true density
Arching / Bridging
is bad, forms semi permanent structures leading to bad powder flow
- doesn’t allow consistent flow
Why do powders need to be formulated?
hard to administer powders and therefore they need to be formulated to control the dosage
Formulation of powders and examples
- Addition of glidants
- Anti-adherent properties
• Examples
– Magnesium stearate
– Colloidal silicon dioxide helps reduce bulk density of powder bed
– Sodium bicarbonate may improve flow of hygroscopic powder
Powder flow in the hopper
• Requirement
– Controlled and constant flow rate
– Uniformity of flow (no segregation of powder)
• Hopper design (geometry and wall material) – fixed?
• Powder properties - modifiable
• Two main types of flow:
- Mass flow
2. Core flow
Mass Flow
mass of bulk is flowing at a constant rate
• All material is in motion during discharge
• Whole bulk of powder moves downwards
• Generally Ɵ ͠ 20°
– But need to consider wall friction angle Ø
Core Flow
more adhesion to the sides
• Materials outside the falling region do not move
• Some powders remain undisturbed
• Dead space occurs during discharge
• Generally Ɵ ͠ 50°
– But need to consider wall friction angle Ø
Problems of core flow
• Flow from hopper can suddenly stop
– Arching
• Formation of arch between the hopper walls
• Arch strong enough to support material above it
– Piping, rat holing or funnelling
• Sudden rapid discharge of powder
– Collapse of the arch or pipe, rate hole or funnel results in ‘flooding’
• Segregation of powder
– Loss of mixing quality
– Non-uniformity of flow - dose irregularities
Powder travelling through an orifice
- Particles just above orifice flow freely
- A depression forms at upper surface and spreads outwards to side of the container
- A ‘tongue’ is formed above the orifice
Characterisation of powder flow
- Angle of repose
- < 30° good flow
- > 40° problematic
- Different methods exist using different angles can be used for same powder sample
- Characteristics can be variable not definite
- Take caution in data interpretation
Bulk density measurements (1)
• Ratio of the poured bulk density and tapped bulk density of powder
Hausner ratio = tapped bulk density (Df)/ poured bulk density (Do)
- free flowing powder ( 1.2)
- cohesive powder (>1.6)
Bulk density measurements (2)
Carr index (% compressibility) = Df - Do/Df x 100
Measure of tendency of arch formation
- 5-15 excellent
- 12-16 good
- 18-21 fair
- 23-28 poor
- 28-35 poor
- 35-38 very poor
- > 40 extremely poor
Tapped density tester
• BP
– Carry out 10,500 and 1250 taps and read the corresponding volumes (V10, V500, V1250) to the nearest mL
• Mechanically tapped at a constant speed • Allow measurements of D0 and Df
How can powder flow be measured
– Angle of repose
– Hausner ratio
– Carr Index
Factors that influence powder flow
– Particle
– Powder
– Manufacturing equipment
