Pharmaceutical Manufacturing Handbook: Production and

1138 TABLET COMPRESSION
P P
= +
C a ab
1 (4)
V0−V C =
V
In the Kawakita equation the particle density is not introduced in the calculations
since the model operates on the relative change in volume, which gives the same
result whether the relative or the absolute volume is used. The problem in the calculation
of this equation is to fi nd the correct initial volume V 0 .
6.6.3
COMPACTIBILITY
Compactibility of a powdered mixture is defi ned as the ability of the material to be
compressed into a tablet of a specifi ed strength without changing its composition.
Investigations have demonstrated that binary mixes of identical composition could
have different organizations, depending on the surface energy and particle size of
the fraction used. Actually, it has been demonstrated that it is possible to control
the organization of binary mixes by modifying the particle sizes of the fractions
blended if they have the appropriate surface energies [6] .
Generally, only powders that form hard compacts under an applied pressure
without exhibiting any tendency to cap or chip can be considered as readily
compactible. The compactibility of pharmaceutical powders can be characterized
by its tensile strength and indentation hardness, which can be used to determine
three dimensionless parameters: strain index, bonding index, and brittle fracture
index.
To calculate the work of compaction during tableting, it is necessary to have
accurate values of force and punch displacement. Differences in the dynamics of
powder densifi cation between eccentric and rotary machines were pointed out by
Palmieri et al. [14] after compression of microcrystalline cellulose, lactose monohydrate,
and dicalcium phosphate dehydrate at different compression pressures. The
effect of the longer dwell time of the rotary machine press on the porosity reduction
after the maximum pressure is reached is more noticeable in a ductile material such
as microcrystalline cellulose. It has been shown that Heckel parameters obtained in
the rotary press are in some cases different from those recovered in the eccentric
machine because of the longer dwell time, machine defl ection, and punch tilting
occurring in the rotary press, although theoretically they could better describe the
material densifi cation in a high - speed production rotary machine.
Williams and McGinity [15] studied and compared the compaction properties of
microcrystalline cellulose from six different sources using tableting indices. It was
demonstrated that storage of compacts at elevated humidity conditions prior to
determining the tableting indices decreased the magnitude of the tensile strength,
dynamic indentation hardness, and bonding index. Based on the differences in
physicomechanical properties observed for the tableting indices, the authors stated
that microcrystalline cellulose products from different sources are not directly interchangeable
and showed that the tensile strength, indentation hardness, bonding
index, and brittle fracture index for compacts composed of microcrystalline cellu-
0
(5)