A SEMINAR ON

COMPRESSION AND COMPACTION

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CONTENTS COMPRESSION PROPERTIES

AXIL FORCE AND RADIAL FORCE

PROCESS OF COMPRESSION

MEASURMENT OF FORCE

DISTRIBUTION OF FORCE

COMPRESSION PROPERTIES

COMPRESIBILITY:- It is the ability of powder to

decrease in volume under pressure.

COMPACTIBILITY:- It is the ability of a powder to be

compressed into a tablet of a certain strength or

hardness.

1. Plastic material

ex:kaolin, pvp.

1. Elastic material

ex:aspirine, microcrystalline cellulose

AXIAL FORCE :

it is the force required to attempt of material to constrict

vertically.

RADIAL FORCE : it is the force required to the attempt the material to

expand horizontally

PROCESS OF COMPESSION

1. Transitional repacking

2. Deformation at the point of contact

3. Fragmentation

4. Bonding

5. Deformation of solid body

6. Decompression

7. Ejection

Transitional repacking or particle

rearrangement.

Granules to be placed in the hopper of the tablet press.

Formulation and processing are designed to ensure that

at a fast production rate the weight variation of the final

tablet is minimal.

The particle size distribution of granulation and the

shape of the granules determine the initial packing as

the granules is delivered in to the die cavity.

In the initial event the punch and particle movement

occur at low pressure.

The granule flow with respect to each other, with the

finer particle entering the void between the larger

particle, and the bulk density of the granulation is

increased.

Spherical particle undergo less particle

rearrangement then irregular particle as the

spherical particle tend to assume a close

packing rearrangement initially.

To achieve a fast flow rate required for high-

speed presses the granulation is generally

processed to produce spherical or oval particles.

Thus, particle rearrangement and the energy

expended in rearrangement era minor

consideration in the total process of

compression

Deformation at point of contact

When the stress is applied to a material, deformation

(change of forms) occurs.

If the deformation disappears completely (return to the

original shape) upon release of stress , it is an Elastic

deformation.

A deformation that dose not completely recover after

release of the stress is known as a Plastic deformation.

The force required to initiate plastic deformation is

known as the yield stress.

In the initial event the punch and particle movement

occur at low pressure.

the granule flow with respect to each other ,with the

finer particles entering the void between the larger

particle, and the bulk density of granule is increased.

spherical particle under go less particle arrangement

then irregular particles as the spherical particles tend to

assume a close packing arrangement initially.

To achieve a fast flow rate required for high speed presses

the granulation is generally processed to produce spherical

or oval particles, thus particle rearrangement and the energy

expended in rearrangement are minor considerations in the

process of compression.

When the particles of a granulation are so closely packed

that no further filing of the void can occur, a further increases

of compressional force cause deformation at he point of

contact.

Both plastic and elastic deformation may occur although one

type predominates for a given material.

Deformation increase the area of true contact and the

formation of potential bonding areas.

Fragmentation and Deformation

At higher pressure, fracture occur when the stresses

within the particles become great enough to propagate

cracks.

fragmentation further densification, with the infiltration of

the smaller fragment in to the void space

Fragmentation increase the number of particle and form

new, clean surface that are potential bonding area.

Bonding

Several mechanism of bonding in the compression

process have been conceived, but they have not been

useful in in the prediction of the compressional

properties of material.

Three theory are

1 . Mechanical theory

2 . The intermolecular theory.

3. the liquid surface film theory

Mechanical theory:-

This theory proposes that under pressure the individual

particle undergo elastic, plastic or brittle deformation and

that the edges of the particle intermesh, forming a

mechanical bond.

If only the mechanical bond exists, the total energy of

compression is equal to the sum of the energy of

deformation, heat and energy adsorb for each

constituent.

Mechanical inter locking is not a major mechanism of

bonding in pharmaceutical tablets.

The inter molecular theory:-

The molecule (or ions) at the surface of the solid have

unsatisfied intermolecular force, which interacts with

other particles in true contact.

According to the intermolecular forces theory, under

pressure the molecules at the point of true contact

between new, clean surface of the granules are close

enough so that van der Waals forces interact to

consolidate the particle.

A microcrystalline cellulose tablet has been described as

a cellulose fibril in which the crystals are compressed

close enough together so that hydrogen bonding

between them occurs.

It appear that very little deformation or fusion occur in the

compression of microcrystalline cellulose.

Aspirin crystals under go slight deformation and

fragmentation at low pressure, it appear that hydrogen

bonding has strongly bonded the tablet, because the

granules retain their integrity with further increase in

pressure .

The liquid surface film theory:-

The liquid surface film theory attributes bonding to the

presence of a thin liquid film, at the surface of the

particle induced by the energy of compression.

During the compression an applied force is exerted on

the granules; however, locally the force applied to a

small area of true contact so that a very high pressure

exists at the true contact surface.

The local effect of the high pressure on the melting point

and solubility of a material is essential to bonding.

The relation of pressure and melting point (clapeyron)

dT T(V1-Vs) T-temperature

dP H

By analogous reasoning , the pressure distribution in

compression is such that the solubility is increased with

increasing pressure.

With an increase in solubility at the point of true contact,

solution usually occur in the film of adsorb moisture on the

surface of the granule.

When the applied pressure is released and the solubility

decrease, the solute dissolve in the adsorbed water

crystallizes in small crystals between the particles.

the strength of the bridge depend on the amount of material

deposited and rate of crystallization.

At higher rates of crystallization, a finer crystalline structure

and a greater strength are obtained.

The poor compressibility of most water insoluble material

and the relative ease of compression of water soluble

materials suggest that pressure induced solubility is

important in tableting.

The moisture may be present as that retain from the

granulating solution after drying or that adsorb from the

atmosphere.

Granulation that are absolutely dry have poor

compressional characteristics.

Decompression:-

After the compression and consolidation of the powder

in the die, the formed compact must be capable of

withstanding the stresses encountered during

decompression and tablet ejection.

The rate at which the force is removed (dependent on

the compression roller diameter and the machine speed)

can have a significant effect on tablet quality.

The same deformation characteristics that come into

play during compression, play a role during

decompression.

After application of the maximum compression force, the

tablet undergoes elastic recovery.

While the tablet is constrained in the die, elastic recovery

occurs only in the axial direction. If the rate and degree

of elastic recovery are high, the tablet may cap or

laminate in the die due to rapid expansion in the radial

direction only.

Tablets that do not cap or laminate are able to relieve

the developed stresses by plastic deformation.

Since plastic deformation is time-dependent, stress

relaxation is also time-dependent.

Formulations which contain significant concentrations of

microcrystalline cellulose typically form good compacts

due to its plastic deformation properties.

However, if the machine speed and the rate of tablet

compression are significantly increased, these

formulations exhibit capping and lamination tendencies.

The rate of decompression can also have an effect on

the ability of the compacts to consolidate (form bonds).

Based on the liquid-surface film theory, the rate of

crystallization or solidification should have an effect on

the strength of the bonded surfaces. The rate of

crystallization is affected by the pressure (and the rate at

which the pressure is removed).

High decompression rates should result in high rates of

crystallization. Typically, slower crystallization rates

result in stronger crystals.

Therefore, if bonding occurs by these mechanisms,

lower machine speeds should result in stronger tablets.

The rate of stress relieve is slow for acetaminophen so

cracking occurs while the tablet is within the die. with

microcrystalline cellulose the rare of stress relieve is

rapid, and intact tablets result.

Ejection

As the lower punch rises and pushes the tablet upward

there is a continued residual die wall pressure and

considerable energy may be expanded due to the die

wall friction.

As the tablet removed from the die, the lateral pressure

is relieved, and the tablet undergoes elastic recovery

with an increase (2 to 10%) in the volume of that portion

of the tablet removed from the die.

During ejection that portion of the tablet within the die is

under strain, and if this strain exceeds the sheer strength

of tablet the tablet break as elastic recovery.

A large value of the heckel constant indicate the onset of

plastic deformation at relatively low pressure.

A heckel plot permits an interpretation of the mechanism

of bonding.

For dibasic calcium phosphate dihydrate, which

undergoes fragmentation during compression, the heckel

plot is nonlinear and has small value for its slope (a

small heckel constant).

As dibasic calcium phosphate dihydrate fragments, the

tablet strength is essentially independent of particle size.

For sodium chloride a heckel plot is linear indicating that

sodium chloride undergoes plastic deformation during

compression. no fragmentation occur.

Effect of friction

At least two major component to the frictional force can

be distinguished

Interparticulate friction :- this arises at particle /particle

contacts and can be expressed in term of a coefficient of

interparticulate friction m1. it is more significant at low

applied loads.

Material that reduce this effect are referred to as

glidants.

Ex:- colloidal silica, talc, corn starch

Die-wall friction :-this result from material being pressed

against the die wall and moved down it ; it is expressed

as mw, the coefficient of die wall friction.

This effect become dominant at high applied forces

when particle rearrangement has ceased and is

particularly important in tabletting operations.

Most tablets contain a small amount of an additive

design to reduce die wall friction; such additives are

called lubricants.

Ex:-magnesium stearate, talc, PEG, waxes, stearic acid

FA

FL

FR FD

HOH

D

Force distribution

Diagram of a cross section of a typical simple punch and die assembly

This investigation carried on single station press.

Force being applied to the top of a cylindric power mass

and the following basic relationships apply.

FA=FL+FD

Where, FA =is the force applied to 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 difference between the force applied at

the upper punch and that affecting material closed to the

lower punch, a mean compaction force, FM where,

FM=FA+FL/2

A recent report confirm that FM offer a practical friction-

independent measure of compaction load, which is

generally more relevant then FA.

In single station presses, where the applied force

transmission decay exponentially, a more appropriate

geometric mean force FG, might be

0.5

FG=(FA . FL)

Use of this force parameters are probably more

appropriate then use of FA when determining

relationships between compressional force and such

tablet properties as tablet strength.

Development of radial force

As the compressional force 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 solid, compressive force applied in

one direction (e.g. vertical) result in decrease in H in

the height, i.e. a compressive stress.

In the case of an unconfined solid body, this would be

accompanied 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:

Poisson ratio = D/H

The poisson ratio is a characteristic constant for each

solid and may influence the tabletting process in

following way.

Under the condition illustrated in figure , the material in

not free to expand in horizontal plane because it is

confined in the die.

Consequently, a radial die wall force FR develops

perpendicular to the die wall surface, material with larger

poisson ratios giving rise to higher value of FR.

Classic friction theory can then be applied to deduce that

the axial frictional force FD is related to FR by the

expression:

FD = mw.FR

Where mw is the coefficient of die wall friction.

Note that FR is reduced when material of small poisson

ratio are used, and that in such cases, axial force

transmission is optimum.

Die wall lubrication

Most pharmaceutical tablet formulation require the

addition of a lubricant to reduce friction at the die wall .

Die wall lubricant function by interposing a film of low

shear strength at the interface between the tabletting

mass and the die wall.

Preferably, there is some chemical bonding between this

boundary lubricant and the surface of the die wall as well

as the edge of the tablet.

The best lubricant are those with low shear strength but

strong cohesive tendencies in direction at right angles to

the plane of shear.

Ejection forces

Radial die wall forces and die wall friction also effect the

ease with which the compressed tablet can be removed

from the die.

The force necessary to eject a finished tablet follows a

distinctive pattern of three stage.

The first stage involves the distinctive peak force

required to initiate ejection, by braking of tablet/die wall

adhesions.

A smaller force usually follows, namely that required to

push the tablet up the die wall.

The final stage is marked by declining force of ejection

as the tablet emerges from the die.

Variation on this pattern are sometimes found, especially

when lubrication is inadequate and/or “slip-stick”

condition occur between the tablet and the die wall,

owing to continuing formation and breakage of tablet die

wall adhesion.

A direct connection is to be expected between die wall

frictional forces and the force required to eject the tablet

from the die, FE.

For e.g. well lubricated systems have been shown to

lead to smaller FE values.

Compection profiles

Monitoring of that proportion of the applied pressure

transmitted radially to the die wall has been reported by

several groups of workers.

For many pharmaceutical materials, such investigation

lead to characteristic hysteresis curves , which have

been termed compaction profiles.

The radial die wall forces arises as a result of tabletting

mass attempting to expand in the horizontal plane in

response to the vertical compression.

The ratio of this two dimensional changes, the Poisson

ratio, is an important material dependent property

affecting the compressional process.

When the elastic limit of the material is high, elastic

deformation may make major contribution, and on

removal of the applied load, the extent of the elastic

relaxation depend upon the value of the materials

modulus of elasticity (young’s modulus).

If this value is low, there is considerable recovery, and

unless a strong structure has been formed, there is the

danger of structural failure.

If the modulus of elasticity is high, there is small

dimensional change on decompression and less risk of

failure.

C

D

radial

pressure

E B

c’ A

O axial pressure

The area of the hysteresis loop (OABC’) indicate the

extent of departure from ideal elastic behavior, science

for perfectly elastic body, line BC’ would coincide with

AB.

In many tabletting operation the applied force exceed the

elastic limit (point B), and brittle fracture and/or plastic

deformation is then a major mechanism.

For example, if the material readily undergoes plastic

deformation with a constant yield stress as the material

is sheared, then the region B to C should obey the

equation.

PR = PA – 2S

Where S is the yield stress of the material

The slope of this plot is unity, so that mark deviation from

this value may indicate a more complex behavior.

Deviation could also be due to the fact that the material

is still significantly porous.

For e.g. since point C represent the situation at the

maximum compressional force level, the region CD is

therefore the initial relaxation response as the applied

lode is removed.

In practice, many compaction profiles exhibit a marked

change in the slope of this line during decompression,

and a second yield point D has been reported.

Perhaps the residual redial pressure (intercept EO),

when all the compressional force has been removed, is

more significant, since this pressure is an indication of

the force being transmitted by the die wall to the tablet.

As such, it provide a measure of possible ejection force

level and likely lubricant requirements, it suggests a

strong tablet capable of at least withstanding such a

compressive pressure.

A low value of residual redial pressure, or more

significantly, a sharp change in slop (DE) is sometime

indicative of at least incipient failure of the tablet

structure.

In practical term this may mean introducing a plastically

deforming component (e.g.pvp as binder).

Energy involve in compaction

Tablet machines, roller compactors, and similar types of

equipment required a high input of mechanical work.

The work involve in various phase of tablets operation

includes,

That necessary to overcome friction between particles,

That necessary to overcome friction between the

particles and machine parts,

That required to induce elastic and/or plastic deformation

of the materials,

That required to cause brittle fracture within the

materials, and

That associated with the mechanical operation of various

machine parts.

Nelson and associate, who compared the energy

expenditure in lubricated and unlubricated sulfathiazole

granules.

Lubrication reduce energy expenditure by 70%, chiefly

because of a lessening of the major component, namely

energy utilized during ejection of the finished tablet.

Lubricant has no apparent effect on the actual amount of

energy required to compress the material.

Compression Energy expended(joules)

process Unlubricated Lubricated

Compression 6.28 6.28

Overcoming die wall friction 3.35 --

Upper punch withdrawal 5.02 --

Tablet ejection 21.35 2.09

Total 36.00 8.37

By assuming that only energy expended in the process

of forming the tablet cause a temperature rise, Higuchi

estimated the temperature rise to be approximately 5 c.

For a single punch machine operating at 100 tablets per

min, and approximately 43 kcal/hr were required for

unlubricated granules.

Wurster and creekmore by use of an internal

temperature probe found a 2 to 5 c rise in the

temperature of tablet compressed from microcrystal

cellulose, calcium carbonate, starch and sulfathiazole

The temperature of compressed tablet is affected by the

pressure and speed of tablet machine.

In non instrumented single punch tablet machine set at

minimum pressure, the compression of 0.7 g of sodium

chloride caused a temperature increase of 1.5 c ;

when the machine was set near maximum pressure , the

temp. increase was 11.1 c .

When the machine was operating at 26 and 140 rpm the

increase in temp. was 2.7 and 7.1 respectively.

When the machine was operating at 26 and 140 rpm to

compress 0.5 g of calcium carbonate, the increase in

temp. was 16.3 and 22.2 c respectively.

Properties of tablet influence by compression

Higuchi and train were the first pharmaceutical scientists

to study the effect of compression on tablet

characteristics.

The relationship between applied pressure and weight,

thickness, density, and the force of ejection are relatively

independent of the material being compressed

Density and porosity

Hardness and tensile strength

Specific surface

Disintegration

Dissolution

1.5

1.4

Density

g/cm 3 1.3 sulphathiazole tablet

1.2

1.1 500 1000 2000 4000

logarithm applied pressure, kg/cm 2

30

Lactose

porosity 20

% lactose-aspirin

10 aspirin

500 1000 2000 4000

applied pressure, kg/cm 2

The effect of applied pressure on the porosity of various tablet with 10% of starch. Porosity and density inversely proportional to each other.

30

Lactose

hardness 20 lactose-aspirin

s.c unit

10 aspirin

500 1000 2000 4000

applied pressure, kg/cm 2

80 radial

tensile 60

strength

kg/cm 2 40

20 axial

200 4000 6000 8000

applied pressure, kg/cm 2

The effect of applied pressure on tensile strengths of tablet of dibasic calcium phosphate granulated with 1.2% starch.

DISTRIBUTION OF FORCE :

Fm = Fa + Fd

2

Fa = Fl +Fd

Fa =force applied to upper punch.

Fl =force transmitted to lower punch.

Fd =is the reaction at the die wall due

to friction at the surface.

Fm = mean force

Relation between applied and transmitted

force

The relation between applied & transmitted

forces Fa, Fl practically linear

In case of single punch the force exerted by

upper punch ↓ exponentially as depth ↑

The relation between Fa, Fl written as

Fl = Fl٠ Fl eKH / D

Rearranging the above equation

Fa = Fl ٠ eKH / D

COMPACTION PROFILE Radial pressure is due to the

attempt of material to expand

horizontally.

Axial pressure is due to the

attempt of material to constrict

vertically.

OA =Shows Early repacking

AB = elastic deformation

BC = plastic deformation

CD = elastic recovery

DE = plastic recovery

MEASURMENT 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 deformed

With the deformation, the length of resistance wire ↓ & its

diameter is ↑.

The resulting decrease in resistance is 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 measurement.

DIAGRAMETIC REPRASENTATION OF

STRAIN GAUGES

2)PIEZO-ELECTRIC LOAD CELLS:

Certain crystals like 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 converts 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 measurement.

DIAGRAMATIC REPRASENTATION OF

PIEZO-ELECTRIC CELLS

REFRENCES

1. The theory & practice of industrial

pharmacy By: Lachman

2. The science of dosage form design

edited by: Michael E. Aulton

3. Text book of physical pharmacy by Alfred

Martin, James Swarbric.

4. By internet source.

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