Compression

COMPRESSION
 When external mechanical forces are applied to a powder mass, there is

normally a reduction in its bulk volume as a result of one or more of the
following effects.
 The onset of loading is usually accompanied by closer repacking of the
powder particles and it most cases this is the main mechanism of initial
volume reduction as shown in the diagram
 As the load increases however rearrangement becomes more difficult and

further compression involves some type of particle deformation. If on
removal of the load the deformation is to a large extent spontaneously
reversible i.e if it behaves like rubber, then the deformation is said to be
elastic.
 All solid undergo some elastic deformation when subjected to external
forces. With several pharmaceutical materials such as ASA and MCC elastic
deformation becomes the dominant mechanism of compression within the
range of maximum forces normally encountered in practice.
 In other groups of powdered solids and elastic limit or yield point is reached
and loads above this level result in deformation not immediately reversible
on removal of the applied force.
 Bulk volume reduction in these cases results from plastic deformation and or
viscous flow of the particles which are squeezed into the remaining void
spaces resembling the behaviour of modelling clay.
 This mechanism predominates in materials in which the shear strength is less
than hte tensile or breaking strength.
 Conversely when the shear strength is greater particles may be preferentially
fractured and the smaller fragments then help to fill up any adjacent air
space.
 This is most likely to occur with hard, brittle particles and in fact is known
as brittle fracture. Sucrose behaves in this manner. Irrespective of the
behaviour of large particles of the material small particle may deform
plastically a process known as micro squashing and the proportion of fine
powder in a sample may therefore significant.
 All the deformation effects may be accompanied by the breaking and
formation to new bonds between the particles which give rise to
consolidation as the new surfaces are pressed together.
 Some deformation processes (plastic deformation) are time dependent and
occur at various rates during the compaction sequence so that the tablet mass
is never in state of stress or strain equilibrium during the actual tabletting
event.
 This means that the rate at which load is applied and removed may be
critical factor in materials for which dependence on time is significant. More
specifically deforming solid is loaded too rapidly for this process to take
place, the solid may exhibit brittle fracture.
 Conversely if the dwell time under the compressive load is prolonged then
plastic deformation may continue leading to more consolidation.
 This phenomenon has recently studied using a compaction simulator
whereby it was shown that the expansion of acetaminophen tablets ( a
material with laminating tendency) during decompression was particularly
sensitive to dwell time under a maximum load.
 For this reason relatively slower machine speeds and compression rolls of
large diameter sometimes help with troublesome tablet formulations
EFFECT OF FRICTION
Two major components to the frictional forces can be distinguished
1. Inter-particulate friction
2. Die wall friction
Inter-particulate friction: This arises at particle /particle contacts and can be
expressed in terms of a coefficient of inter-particulate friction i, it is more
significant at low applied loads. Materials that reduce this effect are referred to as
glidant. Colloidal silica is a common example.
Die-wall friction: These results from material being pressed against the die wall
and moved down it, it is expressed as w, the coefficient of die wall friction. This
effect becomes dominant at high applied forces when particle rearrangement has
ceased and is particularly important in tableting operations. Most tablets contain a
smaller amount of an additive designed to reduce die wall friction such additives
are called lubricants. Magnesium stearate is common choice.
FORCE DISTRIBUTION
Most investigations of the fundamentals of tabletting have been carried out on
single station presses (eccentric presses) in conjunction with a hydraulic press.
The above figure is typical of such arrangements with force being applied to the
top of a cylindric powder mass. Since there must be an axial (vertical) balance of
forces
FA = FL + FD
Where, FA is the force applied to the upper punch
FL is that proportion of it transmitted to the lower punch
FD is a reaction at the die wall due to friction at this surface. Because of this
inherent difference between the force applied at the upper punch and that affecting
material close to the lower punch , a mean compaction force FM had been
proposed where,
A recent report confirms that FM offers a practical friction independent measure of

compaction load which is generally more relevant than FA . In single station presses
where the applied force where the applied force transmisstion decays exponentially
as in equation (the above equation K = material dependent constant which includes

a term for the average die wall frictional component. The values H and D are the
height and diameter of the tablets respectively) a more appropriate geometric mean
force FG might be
Use of these force parameters are probably more appropriate than use of FA when
determining relationships between compressional force and such tablet properties
as tablet strengths.
Development of radial force
As the compressional force is increased and any repacking of the tabletting mass is
completed, the material may be regarded to some extent as a single solid body.
Then, as with all other solids compressive force applied in one direction (e.g.
Vertical) results in a decrease H in the height i.e a compressive stress as in
Following figure
In the case of an unconfined solid body this would be accompanied by an
expansion in the horizontal direction of D. The ratio of these two dimensional
changes is known as poisson ratio  of the material defined as
 = D/ H
The poisson ratio is a characteristic constant for each solid and may influence the
tabletting process in the following way.
Under the conditions illustrated in Figure 4.14 the material is not free to expand in
the horizontal plane because it is confined in the die. Consequently a radial die
wall force FR develops perpendicular to the die wall surface materials with larger
poisson ratios giving rise to higher values of FR.
Classic friction theory can then be applied to deduce that the axial frictional force
FD is related to FR by the expression
Where µW is the coefficient of die wall friction. Note that FR is reduced when
materials of small Poisson ratios are used, and that in such cases, axial force
transmission is optimum.
The frictional effect represented by µW arises from the shearing of adhesions that
occurs as the particles slide along the die wall. It follows that its magnitude is
related to the shear strength S of the particles (or the die wall particle adhesions if
these are weaker) and the total effective area of contact Ae between the two
surfaces. Therefore force transmission is also realized when FD values are reduced
to a minimum, which is achieved by ensuring adequate lubrication at the die wall
(lower S) and maintaining a minimum tablet height (reducing Ae).
A common method of comparing degrees of lubrication has been to measure the

applied and transmitted axial forces and determine the ratio FL/FA. This is called
the coefficient of lubricant efficiency or R value. The ratio approaches unity for
perfect lubrication (no wall friction) and values as high as 0.98 may be realized.
Values below 0.8 probably indicate a poorly lubricated system.
Die wall lubrication

Most pharmaceutical tablet formulations require the addition of a lubricant to
reduce friction at the die wall. Die wall lubricants function by interposing a film of
low shear strength at the interface between the tabletting mass and the die wall as
illustrated in Figure 4-15
Preferably there is some chemical binding between this boundary lubricant and the
surface of the die wall as well as at the edge of the tablet. The best lubricants are
those with low shear strength but strong cohesive tendencies in directions at right
angle to the plane of shear. The following table gives the shear strength of some
commonly used lubricants as measured by a punch penetration test. By utilizing
materials with low shear strength as lubricants shear failure occurs in the lubricant
layers and not at the compressed powder or resultant wall interfaces.
Force volume relationships
The end of the compressional process may be recognized as being the point at which all air spaces have
been eliminated

Compression

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COMPRESSION

 When external mechanical forces are applied to a powder mass, there is

 As the load increases however rearrangement becomes more difficult and

Where, FA is the force applied to the upper punch

FL is that proportion of it transmitted to the lower punch

A recent report confirms that FM offers a practical friction independent measure of

as in equation (the above equation K = material dependent constant which includes

Development of radial force

A common method of comparing degrees of lubrication has been to measure the

Die wall lubrication

Force volume relationships

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