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FLODEX Technical Bulletin
Intrinsic flowability:
a new technology for
powder-flowability classification
ALBERT0 GIOIA
T
HE FLOWABILITY of the powdered materials used
in a tablet formulation is a major consideration in the
production of this popular dosage form. Flowability may be
defined as the powder's ability to flow evenly, by means of
gravity and other forces, from the top to the bottom of the
hopper and then on to the dosage, compaction, and
crushing chambers.
In tableting applications, powder with a high degree of
flowability offers several advantages. (1) A smooth down-
ward flow of the material will minimize air-pocket forma-
tion. (2) The dosage chamber may be filled very accurately,
which not only increases average weight and decreases
variation in average-weight coefficient but also creates
even pressure during compression, thereby lessening wear
on machine parts. (3) Flowable powder increases the repro-
ducibility of feed parameters, which results in consistent
tablet hardness, friability, dissolution rates, and blood
levels. (4) During compression, air is expelled well because
of the powder's high degree of permeability, a quality which
helps eliminate such tablet flaws as capping and splitting.
(5) Finally, high production speeds may be maintained.
If flowability of the powder has been obtained by
minimizing the percentage of fine powders, two additional
advantages result: the limited surface area of the powder
particles makes lubrication easy; and the reduced electro-
static and pneumatic dust sprays of fine powders help to
increase yields and to keep rooms and machines clean.
Weight Uniformity
Attainment of weight uniformity is the prime objective in
powder fractionation, but it is also something of a problem.
First, although many pharmacopeias indicate the weight-
variation limits acceptable in monodose forms, they do not
Teledyne Hanson Research
9810 Variel Avenue
Chatsworth, CA 91311
Phone: +1 818-882-7266
www.teledynehanson.
Reprinted from Pharmaceutical Technology, Feb. 1980
delineate procedures for fractionating the bulk material in
such a way that these limits are met. Moreover, the phar-
maceutical industry's practice of using volumetric dosage
in bulk fractionation ensures satisfactory uniformity only
when the bulk material is liquid and nonthixotropic. For
liquid materials, standard laboratory methodologies do
exist for quantitative viscosity control under both virtually
static and more or less dynamic conditions.
Powders to be fractionated should also have good flow-
ability, but until now no laboratory methodology has
been suitable for determining flowability indexes that were
applicable to actual production. Augsburger and Shangraw
perhaps came closest when, emphasizing that the main
objective should be to attain a uniform weight, they experi-
mented with some mixtures in tableting machines and con-
sidered the variation coefficient of the average weight as the
flowability index.' Unfortunately, this evaluation system
is flawed in that it is not absolute - it depends on working
conditions and machinery used. Furthermore, the method
requires that mixtures have reasonable flowability and
lubrication. The average weight (CV) obtained in the pro-
duction equipment, however, may be used as the point of
comparison in laboratory tests, just as blood levels are
used as points of comparison in dissolution tests.
Parameters Affecting Flowability
Many researchers have identified powder flowability with
interparticle friction, which can be measured by various
techniques - the flowability cone angle: the tilting-table
name a few. Gold et al. obtained a weight/time-variation
layout under standard vibration conditions by sacrificing
the clarity of data on a single variable.5
Teledyne Hanson Research Document 99-380-001 Rev. 06-19
com
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