Pharmaceutical Manufacturing Handbook: Production and

during compression, which will also relate to the fracture toughness and the tensile
strength of the tablets, are the compression mechanics of the particles and their
dimensions. Generally, all materials have the ability to store some elastic strain;
however, its extent will greatly vary for different materials and will depend upon
the intrinsic nature of the material. There are many instances where a brittle material,
or its surface, reduces signifi cantly its cohesion or adhesion compared to that
of a ductile material [4] .
6.6.2
THEORY OF PARTICLE COMPACTION
THEORY OF PARTICLE COMPACTION 1135
Basically, the process of tablet compression starts with the rearrangement of particles
within the die cavity and initial elimination of voids. As tablet formulation is a
multicomponent system, its ability to form a good compact is dictated by the compressibility
and compactibility characteristics of each component. Compressibility
of a powder is defi ned as its ability to decrease in volume under pressure, and compactibility
is the ability of the powdered material to be compressed into a tablet of
specifi c tensile strength [1, 2] . One emerging approach to understand the mechanism
of powder consolidation and compression is known as percolation theory. In a
simple way, the process of compaction can be considered a combination of site and
bond percolation phenomena [5] . Percolation theory is based on the formation of
clusters and the existence of a site or bond percolation phenomenon. It is possible
to apply percolation theory if a system can be suffi ciently well described by a lattice
in which the spaces are occupied at random or all sites are already occupied and
bonds between neighboring sites are formed at random.
The transitional repacking stage is driven by the particle size distribution and
shape. This will determine the bulk density as the powder or granulation product is
delivered into the die cavity. In this phase, the punch and particle movements occur
at low pressure. The particles fl ow with respect to each other, with the fi ner particles
entering the void between the larger particles, and thus the bulk density of the
granulation is increased. Various techniques have been utilized to determine the
degree of the two consolidation mechanisms in pharmaceutical solids (initial packing
of the particles and elimination of void spaces), namely the rate dependency technique.
By applying this technique, stress relaxation data based on the Maxwell
model of viscoelastic behavior indicate virtually no rate dependency for elastic or
brittle materials. There is also an increase in the calculated yield pressure with an
increase in punch velocity for viscoplastic materials such as maize starch and polymeric
materials. This is attributed to the reduction of time necessary for the plastic
deformation process to occur [6] . For brittle materials such as magnesium and
calcium carbonates there is no observed change in the yield pressure with increasing
punch velocity [6] .
When a force is applied to a material, deformation occurs. When this deformation
completely disappears after cessation of the external force, further deformation
occurs. Deformations that do not completely recover after release of the stress are
known as plastic deformations. The force required to initiate a plastic deformation
is known as the yield stress. When the particles are so closely packed that no further
fi lling of the voids can occur, a further increase of the compressional force causes
deformation at the points of contact. Both plastic and elastic deformation may occur,