Powders are employed in many pharmaceutical processes.
Powders
INTRODUCTION
Powders
are employed in many pharmaceutical processes. They are more dif-ficult to
handle and process than liquids and gases primarily because their flow
properties are fundamentally different. Unlike fluids, a particulate mass will
resist stresses less than a limiting value without continuous deformation, and
many common powders will not flow because the stresses imposed, for exam-ple,
by gravity are insufficiently high. Often additional processes that improve
flow, such as granulation and fluidization, are adopted to facilitate powder
transport and powder feeding.
Another
important property of powders is the manner in which the par-ticles of a powder
pack together to form a bed and its influence on bulk density. The latter is
the ratio of the mass of the powder to its total volume, including voids.
Unlike fluids, it varies greatly with the size, size distribution, and shape of
the particles because these affect the closeness of packing and the fraction of
the bed that is void. Vibration and tapping, which cause rearrangement of the
particles and a decrease in the void fraction, increase the bulk density. In
several processes, these factors are important because the powder is subdivided
and measured by volume. Variation of bulk density then causes variation in
weight and dose. The variation in the weight of compressed tablets is an
excellent example of this effect. The manner of packing also influences the
behavior of a bed when it is compressed.
Finally,
in a static condition, there is no leveling at the free surface of a bed of
powder. Nor is pressure transmitted downward through the bed. Instead, the
walls of the containing vessel carry the weight of the bed.
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