In the manufacture of the most common pharmaceutical dosage forms, tablets, and capsules, powders and granules are compacted into solid masses of a given dosage unit.
Compactibility -
Analyses of powders
In
the manufacture of the most common pharmaceutical dosage forms, tablets, and
capsules, powders and granules are compacted into solid masses of a given
dosage unit. This process of compaction involves application of pressure on a
fixed quantity of the powder within a die using stainless steel punches. The
ability of a powder to form a compact on application of pressure is important
for the ability to manufacture tablets.
As
illustrated in Figure 19.2, the ability of a
powder to form a compact on application of pressure can be defined in terms of
three parameters:
·
Compressibility is defined as the ability of material to
reduce in vol-ume under applied pressure and can be measured by plotting
tab-let porosity as a function of compression pressure. Tablet porosity is
measured by the solid fraction of the compact.
·
Compactibility represents the ability of material to produce
tablets with sufficient strength under the effect of densification.
Compactability can be measured by plotting the mechanical strength of the
compact (tensile strength) as a function of its porosity.
·
Tabletability is the ability of powder material to be
transformed into tablets of a specified strength under compaction pressure.
Tabletability is measured by plotting the mechanical strength of the compact
(tensile strength) as a function of the compaction pressure used.
Figure 19.2 An illustration of the interrelationship between the concepts of compactibility,
compressibility, and tabletability.
The
ability of a powder blend to form a strong and physically stable com-pact
depends on its interparticle adhesion characteristics, and the balance of
plastic deformation and elastic recovery under mechanical stress. Plastic
deformation refers to the ability of a powder blend to permanently deform under
pressure. Elastic recovery, on the other hand, represents the percent expansion
of the compact from its most consolidated state under pressure.
Under
compressive stress, powder particles may maintain their size but only deform in
shape (plastic deformation, e.g., microcrystalline cellulose) or may break into
several smaller particles (brittle fracture, e.g., dibasic calcium phosphate).
Such material behavior can affect bonding between dif-ferent components of the
powder and affect adhesion of the powder blend. For example, lubricated powder
particles—where the surface has been coated with hydrophobic magnesium
stearate—show better interparticle bonding for materials that exhibit brittle
fracture due to particle breakage and exposure of new uncoated surfaces upon
compression compared to the materials that undergo plastic deformation.
Powder
blends that show high elastic recovery or lack of adhesive bonding with other
components of the powder tend to form physically unstable com-pacts. Such
compacts tend to show problems such as capping and lamination of the tablets.
Capping refers to breakage and separation of one layer of tablet close to the
edge, whereas lamination refers to breakage in the middle.
During
pharmaceutical development, compactibility of a powder blend is estimated using
simulated tableting equipment, such as Presster® tab-let press simulator or a
compaction simulator. These equipments apply well-defined and controlled
compression pressure within a defined period of time on the powder blends. They
allow the study of a powder blend’s compaction characteristics under a range of
compression pressures, dwell times (duration of time for which the blend is
subjected to the compression pressure), and compaction speeds (speed of
compaction of powder as deter-mined by the speed of punch movement during
compression).
Compactibility
of a powder is a function of its intrinsic mechanical behavior, such as
plastic deformation or brittle fracture, and surface interac-tions with other
powder particles. Compactibility of a powder can also be affected by its particle
size and moisture content, which act by impacting particle packing and
interparticle interactions, respectively.
In
pharmaceutical operations, usually powder blends are used for tablet-ing. The
pharmaceutical unit operations, such as granulation, and the use of excipients
in the dosage form can adjust the compaction characteristics of a powder blend.
The compactibility of a blend is a result of the compac-tion behavior of its
individual components, which can be influenced by changing the composition of
the blend.
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