Questions and Answers
What does the shear strength of a powder primarily depend on?
What is the purpose of the yield locus in shear strength analysis?
How is the shear stress calculated in a shear cell?
What characteristic does the Mohr diagram represent in the analysis of shear strength?
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What does a Mohr semicircle represent in the context of shear strength?
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What is the primary reason for ensuring uniform particle packing in pharmaceutical powders?
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Which factor is NOT a consequence of uneven powder flow during tablet production?
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Which method is NOT listed for moving powders from one location to another?
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What primarily contributes to interparticle cohesion within a bulk powder?
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What characteristic of powders can increase frictional forces and impede flow?
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Study Notes
Powder Flow in Pharmaceutical Production
- Flowability of powders is crucial in pharmaceutical dosage production for uniformity in weight and properties.
- Free-flowing powders ensure consistent feed from storage to tableting and capsule-filling machines.
- Proper flow minimizes entrapped air in powders, reducing issues like capping or lamination during high-speed tableting.
Powder Transfer Methods
- Powders can be transferred using:
- Gravity feeding
- Mechanically assisted feeding
- Pneumatic transfer
- Fluidization in gases and liquids
- Hydraulic transfer
- Efficiency of flow depends on particle properties and process design.
Cohesion and Adhesion
- Cohesion: Attractive forces between like particles; increases as particle size decreases.
- Adhesion: Attractive forces between unlike surfaces (e.g., particle and hopper wall).
- Key forces contributing to cohesion include:
- Van der Waals forces
- Surface tensional forces from adsorbed liquids
- Electrostatic forces from contact or friction.
Measuring Adhesive and Cohesive Properties
- Common measurements include:
- Shear strength
- Tensile strength
- Angle of repose
Shear Strength
- Shear strength defines the stress needed to shear a powder bed under zero normal load.
- Measured using a shear cell, which applies and measures shear and normal stresses.
- Mohr diagrams visualize the relationship between shear and normal stress, defining the yield locus specific to the powder's characteristics.
Tensile Strength
- Characterizes internal friction and cohesion, defined by the powder bed's resistance to splitting.
- Measured using a tilting table method; the angle at which the mobile plate breaks away determines tensile strength.
Angle of Repose
- The angle at which particles slide over each other, indicating interparticle cohesion.
- Cohesive powders have a higher angle of repose; non-cohesive powders have a lower angle.
- Very cohesive powders can demonstrate multiple angles of repose depending on packing conditions.
Particle Properties and Flow
- Forces promoting vs. preventing flow must be balanced at equilibrium.
- Driving forces include gravitational force, particle mass, and external mechanical forces.
- Drag forces involve adhesive, cohesive, and other surface forces affecting flow dynamics.
Particle Size Effects
- Smaller particles have higher cohesion due to greater surface area-to-volume ratios, affecting flowability.
- Particles larger than 250 µm tend to flow freely, while those under 100 µm are increasingly cohesive.
Particle Shape and Density
- Different shapes of similar-sized particles can significantly impact flow properties.
- Spherical particles exhibit optimal flow, while flakes create more interparticle contact and increased cohesion.
- Denser particles are typically less cohesive than less dense ones of the same size and shape.
Packing Geometry and Flow Rates
- Static equilibrium exists in a packed powder bed influenced by gravitational and adhesive/cohesive forces.
- Vibration can mobilize particles, rearranging the packing geometry and thus changing bulk volume.
Factors Affecting Flow Rates Through Orifices
- Flow rates are influenced by:
- Particle size
- Orifice diameter
- Hopper width
- Head size
- Hopper wall angle
- The flow rate through an orifice is generally independent of the powder head height when it is significantly higher than the orifice diameter.
Hopper Dynamics
- Different locations within a powder bed experience varying consolidating stresses, affecting flow.
- Stable arches can form when bed strength exceeds driving forces, influencing flow dynamics through the hopper design.
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Description
This quiz covers the critical aspects of powder flowability in pharmaceutical production, highlighting its importance for ensuring uniformity in dosage forms. You will explore various powder transfer methods and the forces of cohesion and adhesion that affect flow efficiency. Test your knowledge on how these factors contribute to successful pharmaceutical manufacturing processes.
