CONSOLIDATION

Presented By

Vrushali V. Sonawadekar

M-pharm 1st year

Dept. of pharmaceutics

KLE’U collage of pharmacy , Belagavi.

Content..Definition

Consolidation process

Mechanisms of consolidation

Factors affecting consolidation

Compression & Consolidation under high loads

Force volume relationship

Heckel Equation

Heckel plot

Kawakita equation

Conclusion

Reference

Consolidation

An increase in the mechanical strength of the

material resulting from particle or particle

interaction. ( Increasing in mechanical strength

of the mass)

Consolidation Process

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

welding.

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.

Consolidation Mechanism

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 mechanical bond.

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 waalsforce interact to consolidate the particles.

E.g , microcrystalline cellulose is believed to undergo significant hydrogen bonding during tablet compression e pa

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

particles.

Moisture is also important in wet / moist

granulation.

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

faces .

Different types of states during

moist granulation1. Pendular state (powder + binding agent)

2. Funicular state (powder + more binding agent)

3. Capillary state(powder + even more binding

agent)

4. Droplet state(powder + even more & more

binding agent)

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

funicular state.

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

reached .

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.

Droplet state

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

suspension.

Compression & Consolidation under

high loads

Effect of Friction

Force Distribution

Development of radial force

Die-wall lubrication

Ejection 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 glidants.

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=FL +FDWhere…

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=ΔD/ΔH

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

F R

The relation ship btw F D & F R is given by the expression :

FD=µw. FR 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,

-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

lubricants

Material Shear

strengthStearic 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

stages

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

wall adhesion

Variation occurs in ejection force when lubrication

is inadequate

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 deformed.

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 measurement.

Diagrametic representation of strain

gauge

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 measurement.

Diagrametic representation of

piezo-electric cell

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

pressure.

- It can be expressed by shapiro equation

Log E =Log E0-K.P.

Where….E0=porosity when pressure is zero

K=constant

P= pressure

- Walker expressed similarly

1/1-E=K1-K2.Log P

i. initial repacking of particles

ii. Elastic deformation until elastic limit is reached.

iii. Plastic deformation or brittle fracture dominates.

iv. Compression of solid crystal lattice formation.

Heckel Equation

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

strength ‘s’

Kr=initial repacking stage hence E0

The applied compressional force F & the

movement of the punches during

compression cycle & applied pressure P ,

porosity E.

For a cylindrical tablets

p=4F/л. D2

Where… D is the tablet diameter

similarly E can be calculated by

E=100.(1-4w/ρt .л.D2.H)

Where…w is the weight of the tableting

mass ,

ρt is its true density ,

H is the thickness of the tablets.

Heckel Plots

Heckel plot is density v/s applied pressure

Follows first order kinetics

As porosity increases compression force also

increases

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

graph).

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.

Kawakita Equation

Pa /C = 1/ab + Pa /a

Where C= degree of volume reduction

a,b= constants characteristic to powder

being compressed

P= pressure

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.

conclusionThe 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.

Referance 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.

Thank you