Vrushali V. Sonawadekar
M-pharm 1st year
Dept. of pharmaceutics
KLE’U collage of pharmacy , Belagavi.
Mechanisms of consolidation
Factors affecting consolidation
Compression & Consolidation under high loads
Force volume relationship
An increase in the mechanical strength of the
material resulting from particle or particle
interaction. ( Increasing in mechanical strength
of the mass)
Cold welding : when the surface of two
particles approach each other closely enough ,
their free surface energies result in strong
attractive force , a process known as cold
E.g. At separation of less than 50nm
Fusion bonding : Multiple point contacts
of particles upon application of load produces
heat which causes fusion or melting. Upon
removal of load it gets solidified & increase
the mechanical strength of mass.
Mechanical theory :
It occurs btw irregular shaped particles.
Also increases the number of contact points btw the particles
As the particle undergo deformation , the particle boundaries
that the edges of the particles intermesh , forming a
Intermolecular force theory : Under pressure the
molecules at the points of true contact between new , clean
surface of the granules are close enough so that van der waals
force interact to consolidate the particles.
E.g , microcrystalline cellulose is believed to undergo
significant hydrogen bonding during tablet compression e
Liquid – Surface film theory :
Thin liquid films form which bond the particles
together at the particles surface . The energy of
compression produces melting of solutions at the
particles interface followed by subsequent
solidification or crystallization thus in the
formation of bonded surface.
Factors affecting consolidation
The chemical nature of the material.
The extent of the available surface.
The pressure of the surface contaminants.
The inter surface distance.
Role of moisture
Moisture is necessary for formation of tablets.
it can fill the small voids present between the
Moisture is also important in wet / moist
A small proportion of moisture content is
required in the formulation of tablet.
This moisture content is important for the
mechanical strength of tablet.
If moisture content is less there will be increase
in the die wall friction.
If moisture content is increased there will be
decrease in compact strength .
Moisture content is determined by loss on
drying given by formula
Percentage moisture content=
(loss in wt/ initial wt) x 100
Eg : As a results of increasing compressional
force , result in this water being squeezed out to
the surface of the tablet . This expelled moisture
may act as a lubricant at the die wall, but it
could be cause material to stick to the punch
Different types of states during
1. Pendular state (powder + binding agent)
2. Funicular state (powder + more binding agent)
3. Capillary state(powder + even more binding
4. Droplet state(powder + even more & more
Pendular state :
This state is occurs at low moisture level.
In this state particles are held together by
lens shaped rings of liquid .
These cause adhesion because of the
surface tension force of the liquid – air
interface & the hydrostatic suction
pressure in the liquid bridge.
Funicular state :
This state represents an intermediate stage
btw the pendular & capillary state.
When the air start to displace from btw
the particles , the particles arrange in
After the funicular state ,the particles
arrange themselves in capillary state &
there is no air btw them.
Capillary state :
When all the air has been displaced from
btw the particles the capillary state is
Moist granules tensile strength increases
about three times btw the pendular & the
capillary state .
This state is most desirable state in the
process of granulation.
This is another state in the process but this
step is undesirable .
This will be important in the process of
granulation by spray drying of a
Compression & Consolidation under
Effect of Friction
Development of radial force
1. Effect of Friction :
Frictional forces are inter particulate friction and
die wall friction
Inter particulate friction occurs due to particle-
particle contact and it is more significant at low
applied load .These forces are reduced using
Eg : colloidal silica , talc , corn starch,
Die wall friction forces occur from material pressed
against die wall and moved, it is dominant at high
applied load. These forces are reduced using
lubricants like magnesium stearate , talc , stearic
acid , waxes etc,
2 . Force distribution :
Most investigations are carried out on single
station presses or even on isolated punches &
die sets in conjugation with hydraulic press,
These must be an axial balance of forces.
A force is applied on cylinder on top of
cylinder of powder mass
FA…Applied force to the upper punch
FL…Force transmitted to lower punch
FD…Reaction at die –wall due to
friction at surface
3. Development of radial force :
As the compressional force is increased & the
repacking of tableting mass is completed ,the material
may be regard as a single solid body.
Then as with all the solids , compressive force applied
in one direction (eg: vertical) results in a decrease in
ΔH i.e. height
In case of unconfined solid body , this would be
accompanied by an expansion in the horizontal
direction of ΔD.
The ratio of these two directional changes is known as
poisson ratio (λ) of the material.
λ is characteristic constant for each solid
Consequently a radial die-wall force
F R develops.
Materials with larger values of λ gives rise to larger value of
The relation ship btw F D & F R is given by the expression :
µW =coefficient of radial die wall friction.
4. Die-wall lubrication
Most pharmaceutical tablets formulations requires
the addition of a lubricants to reduces friction at the
-Die-wall lubricants function by inter posing a film
of low shear strength at the interface btw tableting
mass & die-wall there is some chemical bonding
btw boundary lubricants & the surface of die-wall as
well as at edge of the tablets.
-The best lubricants are those of low shear
strength but have strong cohesive tendency
in direction at right angle to the plane of shear
The shear strength of some
Stearic acid 1.32
Calcium stearate 1.47
Hard paraffin 1.86
Magnesium stearate 1.96
Potassium stearate 3.07
5. Ejection force:
Force necessary to eject a finished tablets.
Ejection force for a finished tablets consists of 3
stage-1: Peak force required to initiate ejection
by breaking tablets / die wall adhesion.
stage-2: Small force , that required to push the
tablets up the die wall .
stage-3: Declining force of ejection as the
tablets emerges from the die.
Radial die wall forces and die wall friction also
affects ejection of compressed tablets from die.
The force necessary to eject finished tablet is
known as ejection force
This force can eject tablet by breaking tablet/ die
Variation occurs in ejection force when lubrication
Measurements of forces
1) Strain Gauge
A coil of high resistant with length width ratio 2:1
& resistant 100 ohm is suitable.
During compression the applied force causes a
small elastic deformation of two punches.
Strain gauge are connected to punch as close to the
compression site . It is deformed as the punch
With the deformation , the length of resistant wire
decreases & its diameter is increases.
The resulting decrease in resistance I measured by
wheat stone bridge as a recording devise.
Care must be taken to use low voltage so that
heating effect do not interfere with the strain
2) Piezo electric load cells :
Certain crystals like charge quartz may be used.
When subjected to external force these develop
an electrical charge proportional to the force.
This transducer is connected to amplifier which
convert the charge in to dc voltage.
The small piezo – electrical transducer are
connected to upper & lower punch holder of
single station press.
The disadvantage is the dissipation of charge
with time , hence nit suitable for static
Diagrametic representation of
Force volume relationship
In many tablets processes , when appreciable force
has been applied, the relationship btw applied
pressure (p) & volume parameters such as porosity E
dose become linear over the range of commonly used
- It can be expressed by shapiro equation
Log E =Log E0-K.P.
Where….E0=porosity when pressure is zero
- Walker expressed similarly
i. initial repacking of
ii. Elastic deformation until
elastic limit is reached.
iii. Plastic deformation or
iv. Compression of solid
crystal lattice formation.
The heckel analysis is a most popular
method of deforming reduction under
compression pressure .
Powder packing with increasing
compression load is normally attributed to
particles rearrangement , elastic & plastic
deformation & particle fragmentation.
It is analogous to first order reaction ,where the
pores in the mass are the reactant , that is:
Log 1/E= Ky . P + Kr
Where….. Ky =material dependent constant
inversely proportional to its yield
Kr=initial repacking stage hence E0
The applied compressional force F & the
movement of the punches during
compression cycle & applied pressure P ,
For a cylindrical tablets
Where… D is the tablet diameter
similarly E can be calculated by
Where…w is the weight of the tableting
ρt is its true density ,
H is the thickness of the tablets.
Heckel plot is density v/s applied pressure
Follows first order kinetics
As porosity increases compression force also
Thus the Heckel’s plot allows for the
interpretation of the mechanism of bonding.
Materials that are comparatively soft & that
readily undergo plastic deformation retain
different degree of porosity , depending upon
the initial packing in the die.
This in turn is influenced by the size distribution
, shape etc of the original particles.
Ex: sodium chloride (shown by type a in graph)
Harder material with higher yield pressure
values usually undergo compression by
fragmentation first , to provide a denser packing.
Ex: Lactose, sucrose ( shown in type b in
Type-a plots exhibits higher slop (Ky) then type-b.
because type-a materials have lower yield stress.
Type-b plots exhibits lower slop because brittle ,
hard materials are more difficult to compress.
Pa /C = 1/ab + Pa /a
Where C= degree of volume reduction
a,b= constants characteristic to powder
it is a plot of p/c v/s p
This equation describes the relationship between
the degree of volume reduction of the powder
column and the applied pressure
The basis for the kawakita equation for powder
compression is that the particles are subjected to
a compressive load in a confined space are
viewed as a system in equilibrium at all stages
of compression, so that the product of the
pressure term and volume term is a constant.
The compression & consolidation are important in
the tableting of the materials.
The important of each will depends upon largely on
the type of compact required i.e whether soft or
hard & on the brittle properties of the materials.
Various mathematical equations have been used to
describe the compaction process.
The particular value of Heckel plots arises from
their ability to identify the predominant form of
deformation in a given sample.
Aulton’s pharmaceutics: The design and
manufacture of medicines by Micheal Aulton
The theory and practice of industrial pharmacy
by Leon lachman, Herbert A Liberman
Pharmaceutical dosage forms :tablets , vol :1 by
Larry Augsburger & Stephen Hoag.
Remington : the science & practice of pharmacy
,21st edition by lippincott willams & wilkins.
Dekker series volume 71
By internet sources.