powder flow

Question 1

why should the powder be free flowing ?

Answer 1

1. Uniform feed from storage containers or machine hoppers into the tablet dies
   and capsule dosators, allowing uniform particle filling which maintains
   weight uniformity.
2. Uneven powder flow can result in excess entrapped air within powders,
   which may cause capping or lamination of tablets.
3. Flowability of powders also influences mixing

Question 2

what is cohesion and adhesion

Answer 2

Cohesion occurs between like surfaces, such as component particles of a
bulk solid.
Adhesion occurs between two unlike surfaces, for example, between a
particle and a hopper wall.
Adhesion and cohesion are important factors
that can limit powder flow.

Question 3

what are the factors that affect powder flow?

Answer 3

1. Particle size.
2. Particle shape.
3. Particle density.

Question 4

what is the effect of particle size on powder flow

Answer 4

 the smaller the particle size → the greater the surface area per unit mass
(specific surface area) → the greater the attraction forces between particles
→ higher adhesion/cohesion —– worse flowablility

 Particles larger than 250 micron are usually relatively free flowing,
 Particles below 100 micron powders become cohesive and flow problems
are likely to occur.
 Powders having a particle size less than 10 micron are usually extremely
cohesive and resist flow under gravity.

Question 5

what is the effect of particle shape on powder flow

Answer 5

Powders with similar particle sizes but dissimilar shapes can have different
flow properties.
This is due to differences in interparticle contact areas.
For example:
 a group of spheres has minimum interparticle contact and generally optimal
flow properties,.
 a group of particle flakes has a very high surface-to-volume ratio and
poorer flow properties.

irregularly shaped particles may experience mechanical interlocking

Particle shape affects powder inter-particle friction, and hence flow characteristics
of the powder. Generally, as the particle shape becomes irregular, the angle of repose
increases

Materials composed of particles with rounded edges as in a & b in the figure below
Will have better flow than those with sharper edges cubical shape (c) or two
dimensional flake-like particles (d). While, particles having an interlocking shape (e),
Or fibrous configuration as in (f) will have poor flow due to bridging which causes
stoppage of powder flow

Question 6

what is the effect of particle density on powder flow

Answer 6

 Given particles of the same size and shape, dense particles are generally
less cohesive than less dense particles.
 This is because powders normally flow under the influence of gravity.
Therefore particles of the same size and shape having the same surface area
but different density (and therefore mass) tend to have different flow
properties:
 the more dense particle will flow better than the less dense

Question 7

what are the factors in the HOPPER that influence flow rate

Answer 7

1 Orifice Diameter: Powder flow rate of through an orifice is proportional to a function of orifice diameter D0

2- Hopper width
If the bed strength at a given point in the hopper is great enough to resist the forces
which promote flow, then a stable arch will be formed.
In points where the bed strength is not high enough to support an arch against the
stresses within it, flow occurs.

3- Head Size
The pressure below the upper free surface increases to a constant controlled by frictional factors. The
pressure again drops off towards the hopper outlet falling below the atmospheric pressure at the orifice.
This will cause air to be drawn up
into the region close to the base to stabilize this negative pressure and hence allow flow to continue.

4- Hopper wall angle
A flat-bottomed bin retains a certain volume of powder centred around the orifice (a).
Hence, in order to ensure that all powder is discharged from the hopper, the walls
have to inwardly angled close to the outlet that is known as hopper angle (θ).

Question 8

what is the mass flow

Answer 8

The powder is in motion
whenever it is withdrawn. It moves en mass
from the centre and from the sides of the
hopper towards the outlet.
Mass flow provides:
1- a first in- first out flow sequence
2- It removes stagnant powder
3- provides a constant discharge with
consistent bulk density
4- It reduces the extent and the effect of segregation

Question 9

what is funnel flow

Answer 9

Funnel Flow (b): In this case, powders do not discharge freely
because of either high adhesion/cohesion or shallow hopper
angles and hence, an active flow channel is formed above the
outlet with stagnant powder at the periphery. This is a first inlast out flow sequence which can lead to erratic flow such as:
1- Arching: a stable obstruction which forms near the hopper
outlet and preventing the discharge of the remaining powder.
2- Rathole: Stable pipe or a vertical cavity which empties above
The bin outlet. Material is left stranded in stagnant zones ntil is dislodged using an
applied force. 3- Flooding: When a rathole collapses, powder can entrain large volumes
of air causing the particles to fluidize and flood out of the hopper.

Question 10

what are the methods used in the charaterization of powder flow

Answer 10

These includes indirect methods:
1. Angle of repose.
2. Measurements based of bulk and tapped density.
3. Critical orifice diameter.
and direct methods:
4. Hopper flow rate.
5. Recording flowmete

Question 11

what happens when we pour powder on a surface

Answer 11

1. Initially the particles stack
    Cohesive forces greater than gravitational forces
   due to angle of inclination.
    Any added particle does not slide down.
2. At some point, the approach angle for subsequent particles joining the stack is
   large enough to overcome cohesive forces, and the particles slip and roll over
   each other.
    Cohesive forces smaller than gravitational forces
   due to angle of inclination.
    Added particle slides down

Question 12

what is the angle of response

Answer 12

 The sides of the heap formed in this way make an angle with the horizontal
surface which is called the angle of repose and is a characteristic of the
cohesion of the particles.

 The value of the angle of repose will be high if the powder is cohesive and
low if the powder is non-cohesive.

Question 13

how to measure the angle of repose

Answer 13

One of the most commonly used consists in
pouring the powder from a funnel at a fixed height
and then measuring the angle of repose

A funnel is fixed with its tip at a given height (H)
above a graph paper which is attached at a
horizontal flat surface. Powder is poured through
The funnel until the apex of the conical pile
touches the tip of the funnel. Hence,

tanθ= 2h/r

 High θ : poor powder flow and particles
are in the range of > 100μm in size
 Low θ : good powder flow and particles
are normally > 250μm in size

Question 14

Factors affecting angle of repose And the
flow characteristics of the powde

Answer 14

1) Particle size
Generally, the repose angle increases
as the particle size is reduced

2) Particle shape: Discussed earlier

3) The addition of glidants
Glidants such as, talc, magnesium stearate,
magnesium oxide fumed silicon dioxide and
corn starch are materials which increase the
flow properties of powders.
The opposite figure shows the effect of talc on the
angle of repose of 60-80 mesh sulfathiazole
Granules. Generally, the repose angle goes through
a minimum (in this figure at about 1 of % talc) and
then increases as glidant concentration increases.

4) Moisture effects
When materials take up moisture from the atmosphere as they are exposed
to high humidifies, they generally become more cohesive and hence exhibit
very poor flow properties

Question 15

how do glidants act

Answer 15

a) Reduction of the interparticulate friction: Many glidants are lubricants and often
possess a coefficient of friction less than that of the bulk
solid to which they are added

b) Change in surface rugosity: lubricants can reduce face rugosity which will minimize
mechanical interlocking of the particles and hence reduce rolling friction.

c) Separation of coarse particles: glidants by adhering to the surface of coarse particles can provide a physical
separation of coarse particles in the bulk solid

d) Reduction of liquid or solid bridging: This Physical separation which results from
agglomerated glidant particles being adhered to the surface of coarse particles is
thought to reduce the action of capillary adhesion forces and also prevents
formation of solid bridges between particles.

e) reduction in static charge

Question 16

what are bulk and tapped density

Answer 16

Bulk density
 The bulk density of a powder is the ratio of the mass of an untapped powder
sample and its volume including the contribution of the interparticulate void
volume.
Tapped density
 The tapped density is an increased bulk density obtained after mechanically
tapping a container containing the powder sample.

Question 17

whats the correlation between powder flow and tapped density

Answer 17

 Interparticulate interactions influence in the same way:
 the bulking properties of a powder.
 the powder flow.
 A comparison of the bulk and tapped densities can give a measure of the
relative importance of these interactions in a given powder.
 Therefore comparison of bulk and tapped density are often used as an index of
the ability of the powder to flow.

 In a free flowing powder, such interactions between particles are less
significant, and the bulk and tapped densities will be closer in value.
 For poorer flowing materials, there are frequently greater interparticulate
interactions, and a greater difference between the bulk and tapped densities will
be observed.

Question 18

how do we measure tapped density

Answer 18

Once bulk density and tapped density for a powder have been measured
using a tapped density tester, as part of the procedure, comparison
between bulk density and tapped density is often carried out using either:
 Hausner ratio.
 Compressibility index.

Hausner ratio
It is the ratio between tapped density (Df
)/ bulk
density (D0).
Powders with low interparticle friction, such as coarse
spheres, have ratios of approximately 1.2.
More cohesive, less free-flowing powders such as
flakes have Hausner ratios greater than 1.6.
As Hausner index increases powder flow decreases

Compressibility index (Carr’s index)

compressibility% = Df-D0\Df x 100

Question 19

what is critical orficie diameter?

Answer 19

 Powder is filled into a shallow tray to a uniform depth.
 The base of the tray is perforated with a graduated series of holes, which are
blocked either by resting the tray on a plane surface or by the presence of a
simple shutter.
 The critical orifice diameter is the size of the smallest hole through which
powder discharges when the tray is lifted or the shutter removed.

Question 20

what is a hooper flow rate

Answer 20

The simplest method of determining powder flowability directly is to measure the
rate at which powder discharges from a hopper.
Procedure:
1. A simple shutter is placed over the hopper outlet and the hopper filled with
powder.
2. The shutter is then removed and the time taken for the powder to discharge
completely is recorded.
3. By dividing the discharged powder mass by this time, a flow rate (e.g. g/s) is
obtained which can be used for quantitative comparison of different powders.

Question 21

what is recording flowmeter

Answer 21

A recording flowmeter is essentially similar to hopper flow rate method, except
that powder is allowed to discharge from a hopper or container directly onto a
balance.
The continuous increase in powder mass with time is recorded.
Recording flowmeters allow to be determined
1. powder flow rates.
2. uniformity of flow (this cannot be done using the hopper flow rate method).

Question 22

How to improve powder flow

Answer 22

1. Alteration of particle size and particle size distribution.
2. Alteration of particle shape or texture.
3. Alteration of surface forces.
4. Addition of formulation additives.

Question 23

how do we Alter the particle size and particle size distribution

Answer 23

Because coarse particles are generally less cohesive than fine particles, there
is a distinct disadvantage in using a finer grade of powder than is necessary.
 The process of granulation (that you will study in the next Chapter)
consist actually in forming large granules made of finer particles of
powder. (i.e. ↑ size = ↑flow)
 The size distribution can also be altered to improve flowability by removing
a proportion of the fine particle fracti

Question 24

how do we Alter the particle shape

Answer 24

 In general, for a given particle size more spherical particles have better flow
properties than more irregular particles.
 The texture of particles may also influence powder flowability, as particles
with very rough surfaces will be more cohesive and have a greater tendency
to interlock than smooth-surfaced particles

Question 25

how do we alter surface forces

Answer 25

 Reduction of electrostatic charges can improve powder flowability and this
can be achieved by altering process conditions to reduce frictional contacts.
For example, when a powder is poured down along pipes, the speed and
length of transportation should be minimized.
 The moisture content of particles is also of importance to powder flowability.
In cases where moisture content is excessive powders should be dried and, if
hygroscopic, stored and processed under low-humidity conditions.

Question 26

what additives should we add

Answer 26

 Flow activators are commonly referred to pharmaceutically as ‘glidants’.
 Flow activators improve the flowability of powders by reducing adhesion
and cohesion.
 Some commonly used glidants include talc, maize starch, magnesium
stearate and colloidal silicon dioxide.
 Most flow activators have their effect by reducing electrostatic interactions

Question 27

Remember from the mixing chapter:
Alterations made in order to improve powder flow might lead to demixing or
segregation.
Improving powder flow to improve weight uniformity may reduce content
uniformity through increased segregation!