Blending & Blend Assessment Flashcards
What is blending/mixing?
Process whereby component constituents are well distributed & in close contact with one another
Ideal: Homogenous mixture but not realistic
Will mixed powders still exhibit some variation in the composition of samples?
Yes, but low standard deviation, lower than mixes that have not been mixed to the random state
What are the mechanisms of mixing?
1) Convective mixing
- Moving a group of adjacent particles from one location to another
2) Shear mixing
- Slip planes are formed & shear mixing occurs at the interfaces
3) Diffusive mixing
- Body of powder is lifted beyond angle of repose & the particles avalanche, tumbling over each other
* Involves indv particles*
Why does segregation (demixing) happen?
Differences in: particle size
particle shape
particle density
- Occurs during powder transfer
- Tendency to segregate increases when there is larger diff in form/size/density
What are the mechanisms of segregation?
1) Percolation Segregation
- Powder bed subjected to movement/vibration -> Finer/denser powder particles will percolate through large particles to the bottom
2) Elutriation Segregation
- An air steam rushing through the powder bed will sift out the finer/lighter powder particles (eg. empty tube filled with powder)
3) Projection Segregation
- When powder bed is projected into the air, the heavier particles will be projected further due to inertial forces
4) Feed/Heap Segregation
- Heavier/larger particles will roll further down a heap of powder
Ordered mixing:
- What is it?
- Why segregation may occur?
- Use
What: Fine particles are mixed with coarser particles, the fines may coat the carrier and form a stable blend that will not segregate
Why segregation may occur:
- Ordered unit segregation: Due to size diff, larger carrier particles have more of the adsorbed component
- Displacement segregation: Addition of another component (ie. MgSt) may compete for the adsorbed component
- Saturation segregation: Limited active sites on carrier particles for adsorption. Additional fines will be prone to segregation by percolation
Useful in: Dry powder inhalation
What is geometric mixing?
To thoroughly mix a small amount of ingredient with a larger entity
What blending equipments are used for small scale?
1) V-Cone
- Shaped bin blender
- Efficient (up to ~200kg)
- Cumbersome to turn around when goes to ~500-1000kg, very cumbersome for large scale
2) Y-Cone (Similar to V, with avalanche idea)
- Shaped bin blender
3) Double Cone
4) Drum & Post Hoist
What blending equipments are used for large scale?
1) IBC + Pedestal Type
2) IBC + Post Hoist Type
Examples of Industrial Mixers
- Higher shear mixers
- Paddle/screw/agitator mixers
Examples of Laboratory Mixers
1) Drum/Barrel Hoop Blender
- Efficient & flexible mixing chamber change
2) Turbula Mixer
- Best for free-flowing powders
- 3D movement leads to very efficient mixing
- H/w cumbersome for large scale (>50kg)
What is the mixing process like?
1) Expansion of bed of solids
2) Application of 3D shear forces to the powder bed
3) Allow sufficient time to achieve random mix
4) Maintain random state
Factors affecting Mixing
1) % of actives
- More difficult as % decreases (esp. < 1%)
2) Particle size
- More difficult as size decreases
- Though mixing potential improves, small sizes tend to agglomerate
3) Particle size distribution
- More difficult to mix densely packed particles
- Narrow size distribution flow & mix better but prone to segregation post-mix
4) Particle shape
- Optimal: Spherical
- Round easier to mix, but also tend to agglomerate
- Fibrous can interlock
What to test for blend homogenicity?
- Optimal mixing time
- Sample size ( = to unit dose that you are gg to make)
- No. of samples (ensure representation)
- Sampling devices
Suitable sampling devices
1) Scoop (to avoid)
2) Conventional thief keys (can be inaccurate)
3) Core samplers**
- Most efficient, go in powder once can take ~3-5 samples
- Improve accuracy
