Lecture 1- Powder Flow Flashcards
What is flowability?
The ability of a powdered material to flow from a stationary to a moving condition
*free flowing powder= moves consistently and steadily with particles moving independently of one another
*non-free flowing powder= contains cohesive particles moving as agglomerates
What is floodability?
Unstable, liquid-like flow of powder
*flooding caused by fluidising a mass of particles by air or other stimuli
*type of flow is uncontrollable and undesirable
Producing free-flowing powders
-improves mixing
-uniformity of feed- maintains tablet weight
-reproducible filling of tablet- improves weight uniformity
-uniformity of dose
-reproducibility of the finished product
Particle properties
*adhesion
- unlike surfaces; e.g. powder and feed hopper and die wall
-forces of electrostatic attraction
*cohesion
- like surfaces such as component particles of a bulk solid
-short range forces.g. Van der waal’s forces
Assessment of powder flow
*direct methods- hopper flow rate or flow meter
*indirect methods- angle of repose, angle of difference, angle of spatula, carr’s consolidation index etc
Flow through hopper
-time taken for a known amount of powder to flow through the hopper
- Shutter over outlet and hooper filled
- Remove the shutter
- Time to discharge completely
Discharge; mass/time = flow rate
Recording flow meter
Similar to flow through hopper method however the powder is allowed to discharge from a hopper onto a balance
- Powder onto balance
- Increase in mass with time
- Determine mass flow rates
- Quantifies the uniformity of flow
*sodium chloride= uniform flow pattern, easy flowing powder
*lactose regular= more cohesive, localised disruption in flow rate
Indirect methods of measurement
Angle of repose;
When a powder is poured through a funnel, it forms a cone-shaped pile making the angle horizontal
Large angle of repose- poor flow properties
Small angle- free flowing powder
Angle of fall
-results after disturbing the initial angle of repose heap
-INCREASES flowability and DECREASES angle of fall
Angle of difference
Angle of repose- angle of fall= angle of difference
INCREASES angle of difference INCREASES floodability
Equations
Carr’s consolidation index=
CCI= tapped identity- initial density/ tapped density x 100
Fluff density=
Powder weight/initial volume
Tap density=
Powder weight/ final volume
Consolidation index dec, flowability inc
Values- slide 22
Physical and environmental factors influencing powder flowability
Physical=
•Particle size distribution
•The presence of fine particles
•Shape of particles
•Friction Coefficient of the particle surface
•Cohesiveness of powder
•Adhesiveness of powder
•Electrostatic charge on particles
•Compressibility of powder
•Moisture content of powder
•Temperature of powder
Environmental=
•Humidity of the atmospheric air
•External pressure
•Vibrations
Particles properties- free-flowing powders/ good flowability
oNarrow particle size distribution
oSmall surface area/unit weight
oSpherical shape
oLow cohesiveness
oHigh hardness, not easily deformed
oMinimal induced electrostatic activity
oNon-hygroscopic
oHigh particle density
oLow particle compressibility
Particle properties- floodable flow
-Larger available surface area
-Spherical shape
-Uniform size
-Exist as individual particles
-Does not agglomerate
-Low density
-Non hygroscopic
Shear strength
Cohesion- stress (force per unit area) necessary to shear a powder bed under condition of zero normal load
-can be determined from the resistance to flow caused by cohesion/friction + can be measured using a shear cell
Tensile strength
-Tensile strength of a powder bed is characteristic of the internal friction or cohesion of the particles
-powder bed is caused to fail in tension by splitting rather than failing in shear by sliding
Measurement= using a tilting table method - slide 32
Formulation of flow activators
-glidants
-some lubricant and anti-adherent
-reduce adhesion and cohesion
-talc, maize starch and magnesium stearate
-colloidal silicone dioxide - very high specific surface area - low density
Bridging / arching + rat-holing
Bridging/arching;
Powder material that is being discharged forms a bridge/arch above the discharge point in a hopper
Rat-holing;
Powder material forms a hole or narrow channel above the feed outlet in a hopper while the remaining material is stationary against the wall
Ways to combat product flow issues; hopper design
Angle of repose; drained method
The drained method allows powder flow by lifting a powder filled cyclinder (with a canister inside-) allowing material to drain away, leaving a cone (on the cannister)
Method must be tested with multiple powders
Angle of repose dec, flowability inc
- angle of repose is indicative of bulk flow properties but does not indicate the flow pattern/flow rate
Values- slide 16
Indirect methods; hausner ratio
Tapped density/ fluff density
<1.25= good flow
>1.25= poor flow
1.25-1.5= suggests addition of glidant
Angle of spatula
- Immerse spatula in powder pile
- Pull spatula up vertically and measure angle
- Apple external force and measure second angle
- Average of 2 angles= angle of spatula
