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The data obtained by the aforementioned laboratory
methods, however, do not represent a dependable estimate
of the powder's behavior in the machine.
tests are unsuccessful because they are not directed at the
true crux of the problem. In particular, it is incorrect to
identify flowability with interparticle friction, as though
powder particles were like glass or sand spheres.
The truth is that the parameters determining powder
flowability are numerous - particle size, fines, unit sur-
face, particle shape, actual density, bulk density, porosity,
air permeability through the powder, electrostatic charge,
humidity, settling effects, and cohesion forces (e.g., London
and hydrogen) - and they have contrasting and interde-
pendent influence. Flocculation alone - which is camused
by cohesion forces, all other parameters permitting - can
impair flowability, while very good flowability can be ob-
tained with high-bulk-density powders consisting of almost
spherical granules, without floccuii.
Flocculation and Its Origin
Flocculi are groups of low-cohesion fines; they cannot be
isolated in a laboratory because they disintegrate easily in
sieves. In production equipment. powder containing floc-
similar to that of a liquid gelatinizing in its container.
Flocculation occurs when the large surface area - and
therefore large contact area -- of fines favors cohesion
forces. These are the same cohesion forces that greatly
increase during compression to produce hard tablets; in
powder bulk, however, these forces are too weak to force
out the air created by the well-known embolic property of
capillaries. Distances between granules therefore remain at
100 A = 0.01 pm. Lactose, starches, and calcium carbonate
provide classic examples of flocculation.
Humidity favors the occurrence of flocculation by in-
creasing the contact surface among powder particles, by
reducing electrostatic repulsion force since the dielectric
constant of water is 80, and by favoring the leakage of
electrostatic charge. On the other hand, a light electrostatic
charge - obtained, for example, through sifting or by the
addition of Cabosil - might contribute to the elimination
of flocculi.
An Experimental Method
Principle behind the Method
The basis for this method is the powder's ability to fall
freely through a hole in a plate. The diameter of the smallest
hole through which the powder passes three times out of
three is taken as the flowability index. This method has
proved easily reproducible. Each trial is considered valid
when the powder that falls involves the entire height of the
powder (not to be less than 60mm).
Description of the Equipment
Very simply, the Dow-Lepetit device for testing intrinsic
powder flowability (Hanson Research Corp., Northridge,
California) consists of a cylinder with a series of replaceable
disks - of different diameters - in the bottom; the hole is
believe that these
closed by a mobile shutter (Figure 1). The actual compo-
nents
1.
2.
The Procedure
The ring is secured to the bearing to allow the bottom of
the funnel to be near but not touching the powder surface. A
powder load of 50 g is then poured through the funnel into
the middle of the cylinder. When loading is completed, 30
flocculi or mass flocculation of the whole load (Figure 2).
Now the lever device is operated to open the hole in the
disk quickly and without vibration. A very flowable pow-
der will slowly flow through the small-diameter holes, leav-
ing a cavity shaped like an upside-down, truncated cone
(Figure 3). A powder that flocculates in bulk, on the other
hand, will fall abruptly, forming a cylindrical cavity. If the
experiment is negative - i.e., if the powder falls as just
Figure 1: The equipment for testing powder jlowabilitj
consists of a cylinder with a series of replaceable disks of
different diameters - in the bottom; the hole is closed b?
a tnobile shutter.
of the system are as follows:
A stainless steel cylinder with an approximate ca-
pacity of 200 ml.
A series of stainless steel disks. Each disk has a pre-
cise hole in the center in graduated sizes differing l-2
ml in diameter and is easily attached to form a bot-
tom for the cylinder.
A shutter that covers the hole and that may be quick-
ly removed without vibration to allow the powder
to flow through the selected hole.
An adjustable funnel for loading the sample cylinder
with a free fall of the test powder.
A suitable container to collect the powder that flows
through the unit.
Teledyne Hanson Research Document 99-380-001 Rev. 06-19
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