THE FLOW PROPERTIES OF POWDERS AND GRANULES
The flow properties of powders and granules are of critical importance in the production of pharmaceutical dosage forms.
It is recommended that pharmaceutical powders and granules be free flowing (i.e. of good, rapid and regular flowability) for the following reasons:
1. Uniform feed from storage containers (as machine hoppers) into the tablet dies and capsule dosators, allowing uniform particle filling that maintains weight uniformity.
2. Uneven Powder flow can result in excess entrapped air within powder which may cause capping or lamination of tablets.
3. Uneven powder flow can result from the presence of excess fines that are suggested to increase particle - die wall friction and increase dust contamination risks during powder transfer and processing.
Factors affecting powder Flowability
1. Particle size. 2. Moisture content. 3. Particle shape. 4. Surface roughness (texture). 5.Particle density.
6. Surface forces, e.g. Hydrophobicity or electrostatic Charges.
** To Predict the Flowability of a powder quantitatively the following indirect Parameters are to be measured :
1. Bulk density 2. Flow Rate. 3. Angle of repose.
I- Determination of the Bulk density (gm/cm
3):
It is defined as the mass of a powder divided by the bulk volume.It depends on: particle size distribution, particle shape and the tendency of particles to adhere together.
Procedure:
1. Gently introduce an accurately weighed amount (5g) of each powder into a 10 ml graduated cylinder.
2. Record the volume (V0) of the powder and calculate "ρ0” which is the initial bulk density
before tapping.
Wt. (g) 5
ρ0 = --- = --- g/cm3
cm.
4. Record the volume (V10) of the powder and calculate “ρ10” which is the loose bulk
density.
5. Continue dropping and tapping the cylinder until no further reduction in volume (Vf) is
noticed (usually 40 – 50 times) and calculate “ρf” which is the final bulk density.
6. Calculate the Hausner’s ratio:
Hausner’s ratio = loose bulk volume (V10)
final bulk volume (Vf)
Hausner’s ratio is related to inter-particle friction and so could be used to predict the powder flow properties.
7. Calculate the percentage compressibility (Carr's Index):
% compressibility (Carr's index) = ρf – ρ0 x 100
ρf
Interpretation of values:
Carr's index Hausner’s ratio Flow behaviour
< 10 1 – 1.11 Excellent 11 – 15 1.12 – 1.18 Good 16 – 25 1.19 – 1.34 Pass 26 – 31 1.35 – 1.45 Poor > 32 > 1.46 Very poor
8. Repeat the experiment three times for each sample and record your results in the following tables. Sample 1: Exp. No. V0 V10 Vf Ρ0 Ρ10 ρf Hausner’s ratio Carr’s index 1 2 3
Mean Sample 2: Exp. No. V0 V10 Vf Ρ0 Ρ10 ρf Hausner’s ratio Carr’s index 1 2 3 Mean Comment:
II- Measurement of angle of repose:
Angle of repose is the maximum angle possible between the surface of the pile (heap) of powder and the horizontal plane. It gives a measure of the frictional forces which oppose the flow of a loose powder.
Tan Ө = H = 2H 0.5D D
Where, Ө is the angle of repose,
H is the height of a heap of powder and D is diameter the powder.
The rougher and more irregular surface granules, the higher will be the angle of repose.
Factors affecting the angle of repose:
Particle Size. Particle shape. Particle density. Presence of moisture. Techniques of measurement.
* Relationship between angle of repose (Ө) & powder flowability
Angle of repose (Ө) Flow behaviour < 25 Excellent
41 - 45 Poor > 46 Very poor
Measurement of angle of repose using funnel and plate method
Procedure:
1. Fix a funnel to a stand vertically above the center of a piece of paper so that the distance between the funnel tip and the paper is 5 cm.
2. Allow 5 g of each sample to slide on the funnel wall slowly with a fixed rate by gentle tapping on the powder container so that the powder forms conical heap ( pile ) on the paper. 3. Measure the diameter "D" of the bed in two planes at right angles taking the mean value. Also determine the bed height “H" using a ruler.
4. Repeat the same steps for at least three times. 5. Record your data in the following table
Results:
Sample 1 Exp.
N0.
H (cm) D (cm) Tan Ө Angle of repose (Ө) 1 2 3 Mean Sample 2
Exp. N0. H (cm) D (cm) Tan Ө Angle of repose (Ө)
1 2 3 Mean
Which sample shows the lowest angle of repose and why?
2.IMPROVEMENT OF POWDER FLOWABILITY
Flowability of powders could be improved by one or more of the following approaches: I. Alteration of particle size and size distribution by removal of fines or by granulation. II. Alteration of particle shape and texture by spray drying which produce spherical free flowing granules.
III. Alteration of surface electrostatic forces by:
A) Reduction of frictional forces between particles.
B) Discharging the electrostatic charges in powder containers by connection to the earth.
IV. Drying of powders and storage under low humidity to minimize moisture. V. Alteration of process condition by:
1. Using of force feeders i.e. by fitting vibrating baffles at the base of the feeding hopper. 2. Using of mechanically vibrated hoppers.
VI. Addition of flow activators (glidants) e.g., talc, magnesium stearate or silicon dioxide.
The use of glidants to improve powder flowability:
Glidants may act through different mechanisms to improve powder flowability, including: 1. Making the surface of the particles more smooth.
2. Reducing electrostatic charge adhesion and cohesion. 3. Reducing the moisture adsorbed on the particle surface.
Determination of the optimum glidant concentration Procedure:
of glidants, as mentioned previously.
2. Mix well the powder with the glidant (1 %, w/w) and re-measure the angle of repose. 3. Repeat the same experiment with the addition of (3 and 5%, w/w) of the glidant, respectively.
4. Record your results in the following table: Sample Diameter
(cm)
Height (cm)
Tan Ө Angle of repose (Ө) no glidant Mean + 1% glidant Mean + 3% glidant Mean + 5% glidant Mean Comment:
