Powder Mixing Flashcards
What is the importance of powder mixing?
All solid dosage forms
Content uniformity
Therapeutic efficacy
Patient safety
What are the challenges of powder mixing?
1) Homogeneity difficult to achieve
Especially when mixing small amounts into large bulk
2) Demixing
Segregation of components over time.
3) Overmixing
Prolonged mixing degrades material properties (e.g.particle size, charge, surface texture).
What are the 3 types of mixtures?
Positive mixture- mixes spontaneously
Negative mixture- Segregates spontaneously
Neutral mixture- Neither mixes nor segregates spontaneously
What can the three types of mixtures be made into?
Perfect mixture- Completely homogenous
Random mixture- 80% homo, low content variability
Ordered mixtures- small particles adsorbed on to larger ones, inter-dependently move
How is powder homogeneity assessed and what type of mixture is it?
It is assessed using a statistical approach
Robust sampling is important
Random
What is ‘Sample size’ and ‘Scale of scrutiny’?
Sample size: Amount of powder to sample
Scale of scrutiny: The most suitable sample size, equivalent to the amount of mixture present in one dosage unit (e.g. mass of a tablet)
What are the 5 essential characteristics of mixers?
1) Homogeneity without degradation
2) Short mixing time
3) Reproducible
4) High yield
5) Cleanable
What are desirable features of mixers?
1) Versatile
2) Scalable
3) Small footprint (size)
4) Integration within manufacturing process (downstream process)-flow in factory
5) Cost and energy efficient
6) In-line motoring- info about mixer (70% mixed)
7) Environmental controls-more control and stability
What are the 5 types of mixture?
Tumbling Agitator Shear Fluidised Bed Static
What is a tumbling mixer?
Rotating vessel contains
powder
What is an agitator/nauta mixer?
Low-shear rotating paddles move powder
What is a shear mixer?
High-speed rotating blade split agglomerates
What is a fluidised bed mixer?
High speed air disperses particles
What is a static mixer?
Particles flow through static mixing elements
What are the mechanisms of the a tumbling mixer?
Particles set in motion by rotation of a vessel Mainly shear and diffusive mixing Performance affected by: Fill volume- max 50-60% Residence time- how long it mixes Rotation speed Inclination angle of mixer
What are the three different types of mixing?
Convective mixing: Mixing large groups of particles(move in groups)
Shear mixing: (rubbing )
Mixing at the interface between large groups of particles
Diffusive mixing:
Mixing within large groups of particles
What are the three types of tumbling mixer?
Rotating cube
Double cone
V-cone mixer
What are the characteristics of shear mixers?
High-velocity rotating blades High shear forces Breaks up agglomerates Shear and diffusive mixing Unsuitable for friable materials
What is a nauta/agitator mixer?
Contains a rotating screw in a conical vessel
Screws rotate axially and in an orbital motion
What is a fluidised bed mixer?
High-velocity air is passed through the bottom of the powder bed, causing particles to float/mix
Suitable for medium-density friable powders
Free-flowing powder, not sticky ones
What is a static mixer?
No moving parts
Powder flows through motionless mixing elements
What are the practical considerations?
Ensure that the mixer matches the material properties
Ensure fill volume and mixing mechanism
Determine mixing time
Statics cause clumping, (electrostatic attractions between particles), you can coat the particle to dissipate statics by earthing mixture
Use geometric-pre blending (1:99 drug to excipient, begin with 50:50- 25:25)
