Compaction and compression of powder Physics of tablet compression, mechanism of tablet, bonding of tablets, the effect of compress ional force on tablet properties, effect of lubricants on tablet compression and binding, instrumented tablet machines and tooling, problems associated with large scale manufacturing of tablets.

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Compaction and Compression of Powder
Compaction of Powder:
Compaction of powder is a general term used to describe the situation in which these
materials are subjected to some level of mechanical force. In pharmaceutical industry the
effect of such process are particularly important in various ways. Such as:
a. In the manufacturing of tablet and granules,
b. In the filling of hard gelatin capsule shell,
c. In powder handling in general.
The physics of compaction may be simply stated as “the compaction and consolidation of
two phase (particulate solid and gas) system due to applied force”. Therefore
Compaction = Compression + Consolidation
Compression:
Compression means a reduction in the bulk volume of the material as a result of
displacement of air on gaseous phase. In another word, compression is the process of
applying external mechanical force to the pressing material to make it more firm and solid.
Consolidation:
Consolidation is an increase in the mechanical strength of the material result from particle
to particle interaction and which are accompanied by the following term—
a. Cold welding: forms a film due to the application of cold.
b. Fusion Welding: to maintain moisture content forms a film outside the particle.
Solid air interface:
Atoms or ions are located at the surface of any solid particle are exposed to a different
distribution of different of intermolecular and intramolecular bonding forces than those
within the particle. This is indicated as unsatisfied attractive molecular force, extending out
some molecular distance of the solid surface. This condition gives rise to free surface energy
of the solid which plays a major role between a particle and the environment. Many
important phenomena e.g. adsorption, addition, attrition, rate of dissolution, crystallization
are the fundamental properties. Because of these unsatisfied bonding forces identically
particle attract together. This phenomena is important in several operation –
a. Flow from the hopper,
b. Relative motion of the mixture,
c. Production of granules,
d. Compression to produce granule or tablets etc.

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The overall resistance to relative movement of the particle may be markedly affected by
two other factors:
Firstly, many powder of the pharmaceutical interest readily develop intrastatic force
specially when subjected to internal friction. Although these particle contact and separation
are pre-requisite. The charge develop depend on the particular material involved and the
type of motion produced in it. Usually electrostatic force is relatively small but may be
significant because they act over a greater distance of the molecular force.
The second factor namely the presence of an adsorbed near the moisture on the particle.
When particle approaches one another closely enough, this field of moisture can formed
liquid bridge which holds the particle together surface tension effect and a negative
capillary pressure.
Angle of repose:
It is the maximum angel that can be obtained
between the free standing surface of a
powder heap and horizontal plane. This is
expressed by and defined by the following
equation:
Tan = ⁄
Value of rarely less than 200
and value of up to 40 indicates reasonable flow potential.
Above 500
however the powder flows only with great difficulty.
Fig.: Angle of reposes for various particles

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Flow rate:
Resistance to movement of particle especially for granular powder with little cohesiveness
may be assist by determining their flow rate. Through a circular orifice for instance tablet
dies. Flow experiment with the mixture of different size fraction of the same material can be
particularly valuable because in many instances they exists optimum proportion that leads
to a maximum flow rate. For this system when the proportion of the fine particle exceeds
40% there is a dramatic fall in the flow rate.
A simple indication of the ease with which a material can be induced to flow is given by the
application of a compressibility index (I).
= ( ⁄ )
Where, V = Volume occupied by a sample of the powder after being injected to a
standardized tapping procedure.
V0 = Volume before tapping.
Values of I below 15% usually gives rise to good flow characteristics but reading above 25%
indicates poor flow ability.
0
500
1000
1500
2000
2500
20 40 60 80 100
flow
rate

Effect of fine on the rate of flow of mixture of coarse granules as a
function of increase amount of fine
0.158mm
0.09mm
0.059mm
0.048mm
Fine powder ( %w/w) 

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Air spaces:
The mass of the bulk powder sample can be determined with a great accuracy but the
measurement of volume is more complicated. The main problem arises in defining the
volume of bulk powder. Three types of air space can be distinguished:
a. Open intraparticular void:
Those are within the single particle but open to the external environment.
b. Closed intraparticular void:
Those are within the single particle but closed to the external environment.
c. Interparticulate void: Those are between the two or more particle.
Therefore at least 3 interpretation of powder volume may be proposed:
1. True volume:
True volume is the total volume of the solid particle which excludes all spaces
greater than the molecular dimension. It is denoted by Vt.
2. The granular volume or particle volume:
It is the cumulative volume occupied by the particle including all intraparticulate
voids. It is denoted by Vg.
3. The bulk volume:
It is the total volume occupied by the entire powder mass under the particular
packing achieved. When studying this phenomena resulting in a change of volume. It
may be convenient to consider volume V, of the sample under experimental
condition relative to the true volume Vt.
Relative volume, R = ⁄
R decreases and tends towards to unity as all the air is eliminated from the mass. This
phenomenon occurs in tableting.
Porosity:
The void present in the powder mass may be more significant than the solid component. For
this reason a second dimensionless quantity, the ratio of total of the void space to the bulk
volume of the material is often selective to monitor the progress of the compression. This
ratio is referred to as porosity of the material.
Porosity, Vv / Vb
As we know, Vv = Vb – Vt
Again, = ( Vb – Vt ) / Vb
Therefore, = ( 1 – Vt / Vb )

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Density:
The ratio of the mass to volume is known as the density of the material. It is denoted by .
True density,
t = M / Vt
Here, M = mass of the sample.
Deformation:
When any solid body is subjected to opposing force there is a slight change in its geometry.
Depending upon the nature on the applied load, the relative amount of change
(deformation) produced by such force is a dimensionless quantity called strain.
For example if a solid rod is compressed by forces acting by each ends to cause a reduction
in length of from an unloaded length of H0. Then the compressive strength Z is
expressed by following equation,
Compressive strength,
Z = / H0
The ratio of the force (F) necessary to produce this strength to the area (A) over which it is
act is called the stress.
Stress,
= F / A
Because most powder mass contain air space true analogue behavior to the solid body
should not be expected. Under low porosity condition compression do provide a useful way
of interpreting experimental observation.
Different stages of tablet compression:
Compression is the process of applying pressure to the material. There are six different
stages involved in the tablet formulation. These are:
1. Transitional repacking or particle rearrangement,
2. Deformation at the point of contact,
3. Fragmentation,
4. Bonding,
5. Deformation of the solid body,
6. Ejection.

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1. Transitional repacking or particle rearrangement:
When the granules are delivered into the die cavity the control of particle size
distribution occur, so initial occur. During the compression cycle most of the punch and
the particle movement occurs at low pressure. Particle flow with respect to each other
with the final particle entering the void between the larger particle and the density of
the granules is increased. Spherical particle undergoes less particle rearrangement than
do flaky particle.
2. Deformation at the point of contact:
As the pressure increases the arrangement become more difficult and further
compression involves some type of particle deformation at the point of contact resulting
in a sharp change in the shape and size of the particles. There are two types of
deformation:
a. Elastic deformation:
If the deformation disappears entirely on the removal of the load or pressure, it is
called elastic deformation. In another word, when deformation returns to the initial
stage, it is called elastic deformation.
b. Plastic deformation:
If the deformation does not disappear entirely on the removal of the load or
pressure, it is called plastic deformation. In another word, when deformation does
not return to the initial stage, it is called elastic deformation.
The force required to initiate a plastic deformation is known as yield value, when the
particles are so highly packed that is no further filling of the void may occur. The
compression force increases in the plastic deformation at the point of contact. By increasing
the area of contact and deformation of the potential bonding areas both plastic and elastic
deformation may occur, although one type generally predominate for a given materials.
3. Fragmentation:
When the pressure is further increased so that the share strength is greater than the
tensile strength particles undergo fragmentation and forms smaller fragments. These
are helps to fill up any adjacent air space in the tablet. This is most likely occurring with
hard brittle particle. Fragmentation is usually accompanied by an increase in the surface
area.
4. Bonding:
After fragmentation of the particles as the pressure increases there is a formation of
new bonds between the particles at the contact area. Bonding gives rise consolidation as
the new surfaces are press together.

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5. Deformation of the solid body:
During bonding step any non-bonding solid particles are present that can be made
bonding by further increasing of pressure. As a result deformation (particle or elastic
deformation) takes slightly or largely which depends upon the characteristics of
material. With several pharmaceuticals such as, acetyl salicylic acid and microcrystalline
cellulose elastic deformation mostly occur and highly irregular particle plastic
deformation occur.
6. Ejection:
After completion of all stages the upper punch move upward and the lower punch also
move upward upto the top of the die wall and ejection of tablet occur.
Heckel Equation:
The changes in volume with applied pressure are defined by various equations of which
Heckels equation is most important.
Heckel consider that the reduction in volume obey first order kinetics relationship with
pressure. For the compression process Heckel has proposed the following equation and this
equation is known as Heckels equation.
V = bulk volume at applied pressure p,
V0 = original volume of the powder including void,
Vα = Volume of solid excluding void,
K = A constant related to the yield pressure of the powder.
Porosity is the ratio of the total volume of void space to the bulk volume of the material. So,
porosity,
= ( Vb – Vt ) / Vb
So, from the previous equation,
=
This is the rearranged or moderate form of Heckels equation.

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Category 4
Category 3
Category 2
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Category 1
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Heckel Plot:
When or is plotted against applied pressure (p) we get a plot which is known
as Heckel Plot. The nature of this plot depends on the characteristics of the material
compressed. The Heckel Plot explains the mechanism of bonding. For NaCl the plot is liner
indicating that, NaCl is not fragmented but undergoes plastic deformation during
compression. If fragmentation occurs initially during the compression then the plot of
Heckel equation is not linear.
In the experiment of Heckels plot the curves 1,2,3,4 presents decreasing particular size
fraction of the same material. In type A curves are atypically of plastically deforming
materials, so the plots are linear.
In the case of type B fragmentation occurs initially, so plots are not linear.
Application of Heckels equation:
a. Heckels equation has been used to distinguish between the mechanisms of tablet
formation.
b. By the Heckels equation a linear relationship is observed when the material
undergoes plastic deformation without fragmentation (e.g. NaCl). In this case the
strength of tablet depends on the original particle size of the material. On the other
hand a non-linear line is observed with particles which undergo fragmentation
initially during compression (e.g. lactose). In this case the strength of the tablet is
essentially independent of the original particle size. So by the Heckels equation we
can know about the particles fragmentation, either it occurs or not.
Limitations of Heckels equation:
a. The plot is linear only at pressure,
b. The plot can be influenced by the time of compression and the degree of lubrication.
Type A Type B

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Strength of tablet:
Tablet should possess sufficient strength to withstand mechanical handling and transport.
The mechanical strength of tablet is described by
1. Hardness,
2. Bonding strength,
3. Fracture resistance,
4. Friability,
5. Crushing strength.
Crushing Strength:
The most popular estimate of tablet strength is crushing strength which may be defined as
the compressional force which when applied diametrically to a tablet just to fracture it. It is
expressed by the following equation:
ST =
Where, ST = Tensile Strength,
Fc = Compressional force,
D = Diameter of the tablet,
H = Thickness of the tablet.
Crushing strength can be expressed as, Sc = Kd-a
Where, Sc = Crushing strength,
d = Mean particle diameter,
a = Constant of material dependent property.
Friability:
The crushing strength may not be the best measure of potential tablet test behavior during
handling and packaging. The resistance to surface abrasion may be more relevant
parameter that measure the weight loss or subjecting the tablet to a standardized agitation
procedure. The friability can be equated by,
f = 100
Values of 0.8-1.0% are regarded as the upper limit of acceptability. Here Wo and W is the
weight of tablet before and after agitation respectively.

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Bonding compression:
During compression, particles are bonded to each other. Several mechanisms of bonding are
employed on it, depending on the properties of the material. 3 theories are available for
particle bonding by compression for tablet. Any or all of these theories are valid during
bonding. These are –
1. Mechanical bonding theory:
According to this theory, under pressure the individual particle undergo elastic, plastic or
brittle deformation and edge of the particle intermesh forming a mechanical bonding.
Particle with more surface area having more point of contact, hence there is no chance
of interlocking. Under tremendous pressure or compression granules break down into
powder and cause less bonding.
2. Intermolecular bond theory:
According to this theory, the molecule, atoms or ions are at the point of contact
between newly shared surfaces of the granules that are close enough. So that Van-der
Waal force can be interact under compressional pressure. Crystals or molecules of
smaller size and individual particle get closed. As a result Van-der Waal forces become
active.
Hydrogen bonding can also be occur between atoms and thus by both Van-der Waal
force and hydrogen bonding are kept close.
3. Liquid surface bond theory:
This theory attributes bonding to the particular interface of thin liquid film which may be
consequence of fusion of absorption of moisture, according to this theory the
phenomena of bonding this particle is called welding. Which is two types, hot welding
and cold welding.
a. Hot welding:
Under high pressure of compression there is a friction between machine and the
particle cause the generation of heat. If heat is not dissipated melting of the contact
area is occurred. Moreover low melting point crystals may get liquefied or melted by
the frictional heat. This melting or liquefactions are done on the surface of the
particle. Other than heat of the compressional pressure itself also help in melting.
But as the pressure is withdrawn the melted ingredient solidified and causing fusion
of particles. Generally a normal proportion of moisture should present in particles
and in some cases this is require a coherent tablet. When the compression force is
applied then those moisture is squashed out on the surface of the particles and form
thin films of moisture. Therefore particles can interact with each other.

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Series1
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b. Cold welding:
The granules are given to compression having a little moisture in it. In a solution dosage
form drugs and excipients remain solvated in the formulation. In some cases, when
pressure is applied on it, the solubility is decreased. So those, some precipitation of
drugs or crystals are observed in the formulation. Sometimes these crystals are floated
on the surface of the formulation and make a thin film. These are known as cold
welding.
Factors affecting the strength of tablet:
a. Effect of entrapped air:
b. Effect of moisture content of the granule:
c. Effect of binder:
d. Effect of lubricant:
e. Effect of particle of the granule:
f. Effect of applied pressure:
A. Effect of applied pressure:
The strength of tablet is directly related to the log of the mean applied pressure, which
is expressed by,
log P = nFc + C
Fc = Crushing force,
P = Mean compressional pressure.
n and C = Constant depended on the substrate used,
Thus from above equation it is observed that if
the pressure is increased then the strength is
increased. A plot of Fc Vs log P is linear which is
shown beside this para.
The tablet of strength of tablet depends on the
area of bonding within the mass. It is related to
the relative volume (Vr) and the porosity (ε).
The Vr and ε of the compact tablet related as,
ε
The relationship between the relative volume and pressure are as follows,
Vr = C – K log Pa
Relationship of related volume Vr and strength are as follows,

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Series1
Series2
Series3
Series4
Fc = Fc0Vr–m
So, log Fc = logFc0 – m log Vr
Fc0 = Strength of tablet when Vr = 1, that is completed packed.
m = Constant for particular system.
So, if the relative volume is decreases compressional force is increased. Since porosity is
increased as the relative volume is increased. So equation were developed to relate the
strength of the tablet in terms of porosity,
Fc = Fc0
From this equation, we can conclude that applied force is directly related to the strength of
the tablet.
B. Effect of particle of the granule:
The strength of the tablet is inversely proportional to the square root of the mean
diameter. This relationship is shown by the following equation,
Fc0
√
⁄
or, Fc0 = Kd-1/2
Where, K = constant,
d = mean diameter.
A more general relationship resultant indicates that the power term of the above equation
is material dependent
Fc0 = Kd-a
So, log Fc0 = logK – a log d
Where, a = constant dependent on the material
computed.
From this above equation we observed that if
particle size is increased the strength of tablet is
decreased. Graphically it may be represented by
this way.
The effect of particle size on the strength of the tablet is altered from material to material.
For example, with NaCl, a decreased in particle size resultant in the increase of strength of
tablet, at any pressure whereas with aspirin particle size has little effect on tablet strength.

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At low compressional pressure the behavior of methanamine is similar to NaCl. At higher
compressional pressure causing decreases strength due to the formation of capping.
C. Effect of lubricant:
Lubricating agent are assist particle movement and consolidation of tablet by reducing die-
wall friction. During compression lubricant is spread over the surface of the particle and
therefore reduces strength of the bond between the particles. Actually lubricating agent can
smooth the surface of the granules and therefore reduces the area of actual bonding. So,
excess lubricant causes marked decrease in the tablet strength.
D. Effect of binder:
Binder increases the strength of tablet as the concentration of the binder is increases the
strength of the tablet also increases but the properties of binders also increases the
strength of the tablet, because the capabilities of tablet for different binders are not same.
For example, molecule having more binding capability must differ from starch with respect
of binding property.
E. Effect of moisture content of the granule:
In the preparation of pharmaceutical tablets it is generally accepted that small proportion of
moisture must present and in some cases it is required to form a tablet. The optimum
moisture content varies from compound to compound. If the above level is below from the
optimum range the strength of tablet is reduced.
F. Effect of entrapped air:
On rapid compression the air may be trapped in the pore of the tablet. Since share at wall or
punch space may seal the tablet before air is trying to flow out. The pressure build up in this
pore may be so considerable that has to cause failure of the compact on release of the
applied load. The effect of entrapped air is more marked for fine particle which packed
comparatively poor. For this reason entrapped causing tablet capping.
Properties of tablet influenced by compressional forces:
Higuchi and Train were probably the first pharmaceutical scientist to study the effect of
compression on the tablet characteristics. The important properties of tablet that are
influenced by compression are –
a. Density and porosity,
b. Hardness and strength,
c. Specific surface area,
d. Disintegration time,
e. Dissolution time.

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Porosity

Compressional force 
Series1
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Apperent
density

Compressional force 
Series1
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Hardness

log Fc 
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Strength

Compressional force 
Series1
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A. Density and porosity:
The apparent density of tablet is the ratio of the weight and geometric volume of the
tablet. The apparent density of tablet is related to the compressional pressure or force
as shown in the following figure until a limiting density of the material is approach that is
increase in the pressure, increases the apparent density of the tablet.
Fig.: influence the Fc on density. Fig.: effect of Fc on porosity.
The relation between density and porosity are:
ε
B. Hardness and strength:
Although hardness is not the fundamental property it is used in the in process quality
control during tablet compression. There is a linear relationship between log of
compressional force and hardness expect at high pressure.
Strength of tablet is expressed as a tensile strength. As shown in the following figure the
tensile strength of crystalline lactose is directly proportional to the compressional force that
is if compressional force is increased, the strength of tablet is also increased.
Fig.: Effect of Fc in hardness Fig.: Effect of Fc in hardness

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Series 1 Series 2 Series 3 Series 4 Series 5
Specific
Surface
area

compressional force 
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Series 1 Series 2 Series 3 Series 4 Series 5
Disintigration
time

compressional force 
C. Specific surface area:
Specific surface area depends on the material. When the granules are compressed to tablet,
the specific surface areas are increased to the maximum value, indicating the fragmentation
of the granules, which leads to the formation of new surface area. Further increase the
compressional force leads to the decrease in specific surface area due to the bonding of the
particles. The influence of the compressional force on the specific surface area of a tablet is
shown below in the following figure:
fig.: Influence of Fc on specific surface area fig.: Effect of Fc on disintegration time.
D. Disintegration time:
Usually as the compressional force used to prepare a tablet increased the disintegration
time is longer. Frequently there is an exponential relationship between the compressional
force and the disintegration time.
For tablet compression, by small forces there is a large void and the content of starch in the
interparticular space is discontinuous, that increases the disintegration time.
For tablets, compressed at a certain force, the contact of the starch is continuous with the
tablet structure and swelling of the starch immediately exerts pressure, causing the most
rapid disintegration as well as decrease disintegration time. When compression pressure
increases, porosity decreases and more time require for the penetration of water in to the
tablet that result increase of disintegration time.
E. Dissolution time:
The effect of compressional force on dissolution rate may be considering form the view
point of non-disintegrating tablets. For a conventional tablet, dissolution depends on
a. Pressure range,
b. Dissolution medium,

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c. The excipients,
d. The properties of medicinal compounds.
If fragmentation of the granules occurs during the compression increase the specific surface
area, results further the dissolution. If bonding of particle is occur during compression,
dissolution rate is decreased.
In the case of non-disintegrating tablets, if larger particles are disrupted and the coating
material are fragmented dissolution increases.