Pharmaceutical Manufacturing Handbook: Production and

910 PHARMACEUTICAL PREFORMULATION
The true density is the material itself exclusive of the voids and interparticular
pores larger than molecular or atomic dimension in the crystal lattice. Particle
(granular) density, determined by the displacement of mercury, which does not
penetrate at ordinary pressure into pores smaller than 10 μ m, is the mass of the
particle divided by its volume. The different terms arise from the defi nition of
volume: True particle density is when the volume measured excludes both open and
closed pores and is a fundamental property of a material; apparent particle density
is when the volume measured includes intraparticulate pores; effective particle
density is the volume “ seen ” by a fl uid moving past the particles. Bulk density
(powder density) is the volume in a graduated cyclinder including both the particulate
volume and the pore volume. The bulk density will vary depending on the
packing of the powder. Based on the defi nition of volume, minimum bulk density is
when the volume of the powder is at a maximum, caused by aeration, just prior to
complete breakup of the bulk. Poured bulk density is when the volume is measured
after pouring powder into a cylinder, creating a relatively loose structure. Tapped
bulk density is, in theory, the maximum bulk density that can be achieved without
deformation of the particles.
Based on the defi nition of density, two new terms are defi ned. Porosity is defi ned
as the proportion of a powder bed or compact that is occupied by pores and is a
measure of the packing effi ciency of a powder and relative density is the ratio of the
measured bulk density and the true density:
Porosity
Relative density =
bulk density
= 1−
true density
bulk density
true density
Bulkiness The specifi c bulk volume, the reciprocal of bulk density, is often called
bulkiness or bulk . It is an important consideration in the packaging and fi lling of
powders for tablet production. The bulk density of calcium carbonate can vary from
0.1 to 1.3, and the lightest or bulkiest type would require a container about 13 times
larger than the heaviest type. Bulkiness increases with a decrease in particle size. In
a mixture of materials of different sizes, however, the smaller particles sift between
the larger ones and tend to reduce the bulkiness.
To defi ne bulkiness in detail, the porosity and density of powders should be
understood. Suppose a powder, such as zinc oxide, is placed in a graduated cylinder
and the total volume is noted. The volume occupied is known as the bulk volume
V b . If the powder is nonporous, that is, has no internal pores or capillary spaces, the
bulk volume of the powder consists of the true volume of the solid particles plus
the volume of the spaces between the particles. The volume of the spaces, known
as the void volume v , is given by equation
v= Vb−Vp where V p is the true volume of the particles.
The porosity or voids ( ε ) of a powder is also defi ned as the ratio of the void
volume to the bulk volume of the packing given below. Porosity is frequently
expressed in percent, ε × 100: