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PSFC2 > Powder Flow > Flashcards

Powder Flow Flashcards

1
Q

Importance of Powder Flow

A

• Tablettling
• Granulation
• Capsulation
ALL ABOVE Filled by volume
• Transfer in the manufacturing process – Pneumatic transfer
– Hydraulic transfer
– Gravity or mechanically assisted feeding

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2
Q

Consequences of poor powder flow

A

• 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

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3
Q

Resistance of powder flow

A

• 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

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4
Q

Factors affecting powder flow

A
  • 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
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5
Q

Density =

A

mass / vol

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6
Q

Particle shape

A

– Spherical particles less interparticulate contact than other uniform or irregular shapes.

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7
Q

Particle size distribution

A

– Smaller particles (> 50μm) settle below larger particles and lead to uneven powder flow

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8
Q

Particle surface

A

Electrostatic forces increase cohesion

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9
Q

Particle texture

A

– Rough surface more cohesive

– Packing geometry

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10
Q

Humidity

A

— Absorbed moisture on particle surface

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11
Q

Processing conditions

A

— Vibration of hopper (controlled)

— Require validation

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12
Q

Factors affecting powder flow

A
  • Particle shape
  • Particle size distribution
  • Particle surface
  • Particle texture
  • Humidity
  • Processing conditions
  • Powder density
  • Porosity of Powder
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13
Q

Powder density

A

• 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

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14
Q

Bulk density
• Minimum bulk density
• Poured bulk density
• Tapped bulk density

A
  • 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
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15
Q

Relative density

A

Relative density (k) also known as packing fraction or fractional solids content

k = bulk density/ true density

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16
Q

Porosity of powder bed

A
  • 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

17
Q

Arching / Bridging

A

is bad, forms semi permanent structures leading to bad powder flow
- doesn’t allow consistent flow

18
Q

Why do powders need to be formulated?

A

hard to administer powders and therefore they need to be formulated to control the dosage

19
Q

Formulation of powders and examples

A
  • 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

20
Q

Powder flow in the hopper

A

• Requirement
– Controlled and constant flow rate
– Uniformity of flow (no segregation of powder)
• Hopper design (geometry and wall material) – fixed?
• Powder properties - modifiable

21
Q

• Two main types of flow:

A
  1. Mass flow

2. Core flow

22
Q

Mass Flow

A

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

23
Q

Core Flow

A

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

24
Q

Problems of core flow

A

• 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

25
Q

Powder travelling through an orifice

A
  • 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
26
Q

Characterisation of powder flow

A
  • 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
27
Q

Bulk density measurements (1)

A

• 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)
28
Q

Bulk density measurements (2)

A

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
29
Q

Tapped density tester

A

• 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

30
Q

How can powder flow be measured

A

– Angle of repose
– Hausner ratio
– Carr Index

31
Q

Factors that influence powder flow

A

– Particle
– Powder
– Manufacturing equipment

PSFC2 (31 decks)

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