A SEMINAR ON COMPACTION AND COMPRESSION 

Presented by Dharmendra chaudhary M.Pharm-1st Year Dept. of Pharmaceutics N.E.T college of pharmacy

COMPACTION AND COMPRESSION  Compaction of powder is the term used to

describe a situation in which these material are subjected to some level of applied mechanical force over the powdered solids.

Hence compaction can be defined as “the compression and consolidation of a two phases (particulate solid –gas) system due to an  applied force “.

Compression is a reduction in bulk volume of the material as a result of displacement of gaseous phase .

Consolidation is an increase in the mechanical strength of the material resulting from particle-particle interaction. 

Derived Properties of Powders or Granules: Some derived properties which help in quantification of imp. variables are

Volume Density Porosity Flow properties. Volume: Measurement of volume of powder is not easy  as the

measurement of mass of powders ,because in powders there will be inter and intra- particular voids . Hence three types of volume can be considered for a powdered mass, they are,

True volume Granular volume Bulk volume

 

True Volume of the powder (vp): It is a volume of the particles excluding the inter and intra particulate spaces in a powder or it is volume of powder itself 

Granular Volume of the powder(Vg): It is a volume of the particles including intra particulate voids or it is the volume of powder itself and volume of intra particulate spaces. 

Bulk Volume of the powder(Vb): comprises the true Volume and inter and intra particulate voids or it is volume of powder itself and volume of intra and inter particle spaces.

DENSITY Density (q); it is the ratio of weight to volume of substance. By

considering the three types of volume of powders, we can define the respective densities as,

True density: Mass of the powder/ True volume of the powder. 

Granular density:  Mass of the powder/ granule volume of the powder. 

Bulk density: It is the ratio of total mass of the powder to the bulk Volume of the powder. It is measured by pouring the weighed powder into a measuring cylinder and the volume is noted.

It is expressed in gm/ml and is given by

                          Db = M/Vo. 

Where , M is the total mass of the powder

             Vo is the Bulk Volume of the powder 

POROSITY Porosity: The space b/w the particles in a powder are known

to be voids. The volume occupied by such voids is known to be void volume. 

Void volume (v) = bulk volume –True volume  The porosity of the powders is defined as ratio of the void

volume to the bulk volume of the of the packing.

Porosity = void volume /bulk volume.

               =V/Vb

                =[vb-vp/vb] 

Porosity is frequently expressed in percent

                 =[1-vp/vb] x 100 

The relation b/w porosity and compression is important because porosity determines the rate of disintegration, dissolution and drug absorption.

FLOW PROPERTIES

To get uniformity of the weight of the tablet, the powder should possess a good flow property. Flow properties of the powders depend on the-

Particle size, Shape, Porosity and density, Moisture of the powder.

Particle size:The rate of flow of powder is directly proportional to the diameter to the particles.

Beyond particular point, flow properties decreases as the particle size in increases. Because in small particle (10µ) the vanderwaal’s , electrostatic and surface tension forces causes cohesion of the particles resulting poor flow .

As the particle increases, influence of gravitational force on the diameter increases the flow property. But appropriate blends of fines & coarses improves flow characteristic, as the fines get absorbed and coarse particle reduce friction. 

Particle shape:

Spherical, smooth particles improves flow properties, surface roughness leads to poor flow due to friction and cohesiveness , flat and elongated particles tend to pack loosely, obstructing the flow

Density & porosity:

Particles having high density and low internal porosity tend to posses good flow properties. 

Moisture:

The higher moisture content, the flow property will be poor owing to cohesion and adhesion. 

ANGLE OF REPOSE

The flow characteristic are measured by angle of repose . Angle of repose is defined as the maximum angle possible b/w the surface of a pile of powder and the horizontal plane.

                    tanØ= h/r

                         Ø = tan-1(h/r)

Where, h  = height of pile

             r =Radius of the base of pile.

             Ø = Angle of repose . The angle of repose is calculated by measuring the height and

radius of heap of powder formed. The frictional forces in a loose powder can be measured by

Angle of repose. The lower the angle of repose, better will be flow property. The values of angle of repose are given below: 

ANGLE OF REPOSE

Angle of repose (in degrees)

Type of Flow

<25

25-30

30-40

>40

Excellent

Good

Passable Very poor

CARR’S CONSOLIDATION INDEX It indicates powder flow properties. It is expressed in

percentage. It is defined as:  

Consolidation Index = I = Tapped density-Poured density / Tapped density

Therefore = Dt- Db/Dt x 100 

Where, Dt is the tapped density of the powder   Db is the Poured density of the powder  

        

Determination of Tapped density & Poured density. It is determined by passing a fixed quantity of powder into a

measuring cylinder and the volume is noted . CI can be calculated by founding out by tapped density and

Poured density of powder. 

Grading of the powder for their flow properties according to Carr’s index:

Carr’s index(%) Type of flow

5-15 Excellent

12-18 Good

18-21 Fair to passable

23-35 Poor

33-38 Very poor

>40 Very very poor

COMPRESSION PROPERTIES

This involves compressibility and compactability .

Compressibility can be defined as the ability of a powder to decease in volume under pressure.

Powders are normally compressed into tablets using a pressure of about 5.0kg/cm2. The process is called compaction & compression.

Compactability can be defined as ability of powder to be compressed in to a tablet of a certain strength or hardness.

These two relate directly to the tabletting performance.

For proper compression to occur the tablet should be plastic i.e., capable of permanent deformation and it should also exhibit certain degree of brittleness. 

Acc. If the drug is plastic , then the excipients chosen should be brittle (lactose, calcium phosphate) and if the drug is brittle, then the excipients should be plastic (Microcrystalline cellulose).

Plastic material: when materials are ductile they deform by changing the shape, since no fracture , no new surface are generated during compression, which leads to poorer bonding. Increase the dwelling time at compression will increase bonding strength. 

Elastic material: Some materials, paracetamol is an example and there is

very little permanent change caused by compression: the material rebounds when compression load is released.

If the bound is weak, the compact will self-destruct and the top will detach (capping) or the whole cylinder cracks into horizontal layers (lamination).

PROCESS OF COMPRESSION

    In pharmaceutical tabletting an appropriate volume of granules in a die cavity is compressed b/w an upper & lower punch to consolidate the material in to a single solid matrix, which is subsequently ejected from the die cavity as an intact tablet. The subsequent events that occur in the process are:

1.Transitional repacking.

2.Deformation at the point of contact.

3.Fragmentation .

4.Bonding.

5.Deformation of the solid body.

6.Decompression.

7.Ejection.

 

TRANSITIONAL REPACKING OR PARTICLE REARRANGEMENT

The particle size distribution and shape of granule determines initial packing. In the initial stages of compression, the punch and particle movement occur at low pressure.

During this particle move with respect to each other & smaller particles enter the voids b/w the larger particles. As a result the volume decreases and bulk density of granulation increases.

Spherical particles undergo less rearrangement than irregular particles as spherical particle 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 energy expended in rearrangement are minor consideration in the total process of compression.

DEFORMATION AT THE POINT OF CONTACT

When a stress is applied to a material, deformation(change of form) occurs. If the deformation disappears completely (returns to original state) upon the release of stress, it is an elastic deformation. If the deformation that does not completely recover after release of stress is known as plastic deformation.

The force required to initiate plastic deformation is known as yield stress. When the particles of the granulation are so closely packed so that no further filling of the void can occur, a further increase of compressional force causes deformation at the point of contact.

  Both plastic and elastic deformation may occur although

one type predominates for a given material.

Deformation increases the area of true contact and formation of potential bonding areas.

FRAGMENTATION As the compressional load increases the deformed particle

starts undergoing fragmentation. Because of the high load, the particle breaks into smaller fragments leading to the formation of new bonding areas. The fragments undergo densification with infiltration of small fragments into voids.

In some materials where the shear stress is greater than the tensile strength, the particles undergo structural break down. This is called brittle fracture.

Example: sucrose – shear strength is greater than the tensile strength. 

With some materials fragmentation does not occur because the stresses are relieved by plastic deformation. Plastic deformation may be thought of as a change in particle shape and as the sliding group of particles in an attempt to relieve stress(viscoelastic flow). Such deformation produces new, clean surface that are potential bonding areas.

Irrespective of behavior of large particles, small particles may deform plastically, a process known as microsquashing, and the proportion of fine powder in a sample may therefore be significant.

BONDING AND CONSOLIDATION

The hypothesis favoring for the increasing mechanical strength of a bed of powder when subjected to rising compressive forces can be explained by the following theory.

I. Mechanical theory II. Intermolecular theoryIII. Liquid-Film surface theory  Mechanical theory: During compression the particles

undergoes elastic,plastic or brittle deformation and the edges of the particle intermesh, forming a mechanical bond.

If only mechanical bond exists, the total energy of compression is equal to the sum of energy for deformation, heat, and energy absorbed for each constituent.

Mechanical interlocking is not a major mechanism.

Intermolecular theory: The molecules at the surface of a solid have unsatisfied intermolecular force( surface free energy), which interact with other particles in true contact.

Absolutely clean surface will bond with the strength of the crystalline material, whereas adsorbed materials restrict bondings.

According to this theory, under pressure the molecules at the point of contact b/w new, clean surfaces of the granules are close enough to each other (separation by 50nm) so that the van der Waals force interact to consolidate the particles.

A MCC tablets are compressed close enough together so that H-bonding b/w them occurs. It appears that a very little compression or fusion occurs in compression of MCC. Although aspirin crystal undergo slight deformation at low pressure, it appears that H-bonding has strongly bonded the tablets, because the granules retain their integrity with further increase in pressure.

Liquid-surface film theory:Thin liquid films form which bond the particles together at the particle surface. The energy of compression produces melting or solution at the particle interface followed by subsequent solidification or crystallization thus resulting in the formation of bonded surfaces. Due to the applied pressure, the particles may melt (due to lowering of M.pt.) or dissolve (due to increased solubility). As the pressure is released, solidification and crystallization occur.

The intermolecular forces theory and the liquid-surface film theory are believed to be the major bonding mechanisms in tablet compression.

Many pharmaceutical formulations require a certain level of residual moisture to produce high quality tablets. The role of moisture in the tableting process is supported by the liquid-surface film theory.

DEFORMATION OF SOLID BODY

  As the applied pressure is further increased, the new bonded solid is consolidated towards a limited density by plastic &/or elastic deformation within the die.

DECOMPRESSION The success or failure to produce an intact tablet depends

on the stress induced by elastic rebound and the associated deformation process during compression and ejection.

As the upper punch is withdrawn from the die, the tablet is confined in the die by radial pressure. Consequently any dimensional change during decompression must occur in axial direction.

As the movement of tablet is restricted by the residual die wall pressure and the friction within the die wall, the stress from the axial elastic recovery and the radial contraction causes splitting (capping) of tablet unless the shear stress is relieved by plastic deformation.

Thus capping is due to uniaxial relaxation, in the die cavity at the point where the upper punch pressure is released & some may also occur at ejection. It has been demonstrated that if decompression occurs simultaneously in all directions, capping is reduced or eliminated.

Stress relaxation of plastic deformation is time dependent. Materials having slow rate of stress relaxation crack in the die upon decompression. The rate of stress relieve is slow for acetaminophen so cracking occurs within the die whereas with MCC the rate is rapid and hence intact tablet result.

A slower operational speed provides more time for stress relaxation and hence can prevent cracking.

A tablet press that provides precompression allows some stress relaxation before final compression.

Shape of tablet may be selected to reduce stress gradient within the tablet. In deep oval punches the material in dome expand radially, but main body of tablet cannot expand radially instead is constrained by the die wall, large shear stress develop. Flat faced punches can be used to reduce stress gradient.

EJECTION

As the lower punch raises & pushes the tablet upward there is a continued residual die wall pressure and considerable energy may be expanded due to die wall friction.

As the tablet is removed from the die, the lateral pressure is relived, and the tablet undergoes elastic recovery with an increase (2-10%) in the volume of that portion of the tablet removed from the die.

During ejection that portion the tablet with in the die is under strain, and if exceeds the shear strength of the tablet, the tablet caps adjacent to the region in which the strain had just removed. 

THANK YOU….

FORCE INVOLVED IN COMPRESSION AND FACTORS AFFECTING HARDNESS

OF TABLET

Presented by Dharmendra chaudhary M.Pharm-1st Year Dept. of Pharmaceutics N.E.T college of pharmacy

FORCES INVOLVED IN COMPRESSION     Forces, which influence the compaction of granules. 

Force distribution: The fundamentals of tabletting have been carried out on single-station

press or even on isolated punch & punches with hydraulic press. When force is being applied to top of a cylindric powder mass, the

following basic relationship applies, since there must be an axial (vertical) balance of forces.

the system is represented diagrammatically .

                            FA= FL + FD

Where,

                 FA = force applied to upper punch

                 FL = proportion of FA force transmitted to lower punch,

                 FD =reaction at the die wall due to friction at the surface 

Because of this inherent difference b/w the force applied at the upper punch & that affecting material close to the lower punch, a mean compression force applied is given by :

                          FM= FA + FL

                                   2

Fm gives practically friction independent measure of compaction load which is more relevant than FA.

In single station press applied force transmission decays exponentially (i.e. FL= FA.e –kh/d )to over come this, appropriate geometric force FG might be used,

                         FG= FA x FL 

Use of these parameters are more appropriate than FA when determining relationship b/w compression force and tablet properties like tablet strength.

DEVELOPMENT OF RADIAL FORCE ( FR)

As the compression 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. 

When compressive force is applied in one direction (vertical) results in the decrease in height (ΔH) but in case of unconfined solid body, this would be accompanied by an expansion in the horizontal direction of ΔD. The ratio of these two dimensional changes is known as poission ratio(λ) of the material, defined as: 

                                 λ  =  ΔD/ΔH The Poisson ratio is characteristic constant for each tablet.  Under the condition like compression the material is not free

to expand in horizontal plane bcoz its confined to die. Consequently, a radial die-wall force FR develops perpendicular to die wall surface.

Material with high Poisson ratio give higher FR value.

Classical friction theory can be applied to obtain a relationship b/w axial frictional force FD and radial force FR as;

FD =µw.FR

where, µw is coeff. of die wall friction.

Frictional effect represented by, µw arises from shearing of adhesions that occurs as the particles slide along the die wall. Its magnitude is related to shear strength S and effective area of cntct Ae b/w two surfaces.

Force transmission is maxm when FD is minimum which is achieved by adequate lubrication of die wall(lower S) and maintaining minm tablet ht.(reducing Ae ).

Degree of lubrication is compared to measure FA &FD and determine ratio of FL/FA. This is called the coeff. of lubrication efficeincy or R value.

It approaches1 for perfect lubrication, and in practice as high as 0.98 may be achieved.Values below0.8 indicate poor lubrication.

EJECTION FORCES Radial die forces & die wall friction also affect the ease with

which the compressed tablet can be removed from the die. The force necessary to eject a finished tablet follows a distintictive pattern of three stages.

The first stage involves distinctive peak force required to initiate ejection, by breaking of tablet/die wall adhesions.

A smaller force usually follows, that is required push the tablet up the die wall.

The final stage is marked by a decline in force of ejection as the tablet emerges from the die.

Variation on this pattern are sometimes found when lubrication is inadequate and/or “slip-stick” conditions occur b/w tablet and die wall.

Worn dies, which cause the bore to become barrel shaped gives rise abnormal ejection force and may lead to failure of tablet structure.

A direct connection exists b/w FD and force required to eject tablet from die, FE. For eg.well lubricated system(large R value) have been shown to have smaller FE values.

COMPACTION PROFILES       Many attempts have been made to minimize the

amount of applied force transmitted radially to the die walls. All such investigations lead to characteristic hysteresis curves called as compaction profiles. Radial pressure is developed due to the attempt of material to expand horizontally. The plot of radial pressure against axial pressure leads to hysteresis curve called as compaction profile.

    When the elastic limit of the material is high, elastic deformation may make the major contribution, and on removal of the applied load, the extent of the elastic relaxation depends on the value of the material’s modules of elasticity (young’s modulus). Lower the modulus higher will be the elastic relaxation. Then there will be the danger of structural failure. Higher the modulus value results in low decompression hence lesser risk of structural failure.

OA

BC’

CD

E

Axial pressure →

Rad

ial p

ress

ure

→

<

< <

<

Dotted line O to A represents a highly variable response due to repacking, while at A, elastic deformation becomes dominant and continues until elastic limit B is reached.

From B to point of maxm compression C, deformation is predominantly plastic, or brittle fracture is taking place.

The decompression process C to D is accompanied by elastic recovery, and if a second yield point (D) is reached, by plastic deformation or brittle fracture D to E.

The decompression line B to C’ represents the behavior of largely elastic material.

The area of hysteresis loop (OABC’) indicates the extent of departure from ideal elastic behavior, since for a perfectly elastic body, line BC’ would coincide with AB.

PROPERTIES OF TABLET INFLUENCED BY COMPRESSION

1.Density and porosity: The apparent density of a tablet is exponentially related to

applied pressure (or compressional force) until the limiting density of the material is achieved.

As compressional force increases the density of tablet also increases as a result of decrease in bulk volume.

As the porosity and apparent density are inversely proportional, the plot of porosity against log of compression force gives linear plot with a negative slope.

2. Hardness and tensile strength

Specific surface area: Specific surface area initially increases to a maximal

value as the force increases, indicating the formation of new surface due to fragmentation of granules.

Further increase in force produce a progressive decrease in surface area due to bonding of particles.

Disintegration: Usually as the applied pressure used to prepare a

tablet is increased, the disintegration time increases (lactose/aspirin alone).

Frequently, there is exponential relationship b/w disintegration time and pressure (aspirin-lactose).

In some formulation there is minimum value when app.pressure is plotted against log of disintegration time (with 10% and 15% starch in sulfadiazine tablets)

For tablets compressed at low pressure, there is a large void, and the contact of starch grains in the interparticular space is discontinuous. Thus there is a lag time before the starch grains, which are swelling due to imbibitions of water, contact and exert a force on surrounding tablet structure.

For tablets compressed at certain applied pressure, the contact of starch grains is continuous with the tablet structure, and the swelling of starch immediately exerts pressure, causing the most rapid disintegration.

For tablets compressed at pressures greater than that producing minm disintegrtaion time, the porosity is such that more time is required for the penetration of water into the tablet, hence increase in disintegration time.

Dissolution: The effect of applied pressure on dissolution rate may be considered from viewpoint of disintegrating and non-disintegrating tablets.

Shah & Parrot showed that, the dissolution rate is independent of applied pressure from 53 to 2170 kg/cm2 for non-disintegrating spheres of aspirin, benzoic acid, salicycic acid, an equimolar mix.of aspirin & salicylic acid, aspirin & caffeine.

Mitchell and Savill found dissolution rate of aspirin disk to be independent of pressure over 2000 -13000 kg/cm2 and independent of particle size of granules used to prepare disks. Similar observation was found for benzoic acid disks.

The effect of applied pressure on dissolution of disintegrating tablet is difficult to predict.

If fragmentation of granules occur during compression, the dissolution is faster as the applied pressure is ↑ed , bcoz of ↑se in specific surface area.

If bonding of particle is predominate phenomena in compression, ↑se in applied pressure, ↓se dissolution.

The four most common dissolution-pressure relations are:

1. The dissolution is more rapid as pressure is increased.

2. The dissolution is slowed as pressure is increased.

3. The dissolution is faster, to a maxm, as force is increased, and further increase in force slows dissolution.

4. The dissolution is slowed to a minm as pressure is ↑ed, and then further an increase in pressure speeds dissolution.

Pressure(MN/cm2)

t50% (min)

Starch paste Methylcellulose solution

Gelatin solution

200400600800

10002000

54.042.035.010.07.03.3

0.50.81.11.21.41.8

10.04.53.04.64.96.5

Table: Effect of compressional force on dissolution of Sulfadimide tablets prepared with various garnulating agents

FACTORS AFFECTING STRENGTH OF TABLETS The ability of a tablet to withstand mechanical handling and

transport has been evaluated by various types of tests (abrasion, bending, idention, hardness, diametral crushing).

The strength of a tablet may be expressed as a tensile strength (breaking stress of a solid unit in kg/cm2).

Factors affecting tablet strength areI. Particle sizeII. Moisture contentIII. LubricantsIV. Applied pressure

1.PARTICLE SIZE:- A decrease in particle size resulted in the increase in the tablet

strength. Very large particle often exists as agglomerates of small crystal on

compression such agglomerates , being more friable than the crystal, breakdown in smaller units. The strength of the tablets prepared from such aggregates is higher.

With very fine particle , such as those produced by a fluid energy mill , the powder are very cohesive even in the uncompressed state. On compaction strong compact of tablet can be formed .

At a given pressure the use of a very small particle increases the chances of grapping & the volume of air entrapped also increases.

General equation formed for the effect of particle size is

Fc = Ka/√d Where, K= constant

a= material constant lies between (0.2 to 0.47)

Fc= hardness of the impact

d= diameter of the granule

2. MOISTURE CONTENT:- In the preparation of the pharmaceutical tablet , it is

generally accept that a small proportion of the moisture is present and in some cases this is required to form a coherent tablets.

Wet granulation of the powder material with hydrophilic additive was shown to yield tablet whose mechanical strength is dependant on the optimum content above or below with the tablets strength was reduced

With the optimum moisture content there is :Die wall lubricationInter-particulate lubricationHydro-dynamic resistance to consolidationExpression of intestinal liquid to the die wall

At low moisture content: ↑ed die wall friction due to ↑ed stress ratio, poor tablet hardness.

At high moisture content: moisture acts as lubricant , hence ↓ed die wall friction

At further ↑se in moisture content: Further ↑se in moisture, ↓se in compact strength due to ↓se in interparticulate bond.

Hence a granulation should contain an optimum

moisture content. It has been reported that the optimum moisture

content for starch granulation of lactose is approximately 12% and that of phenacetin is 3%.

3. LUBRICANTS The chief purpose of a lubricant is to minimize friction at die

wall, although they often enhances flow of granules by decreasing inter-particular friction.

Lubrication mechanism: The polar portion of lubricant adhere to oxide-metal surface and interpose a film of low shear strength at interface b/w die wall and tablet.

A lubricant reduces ejection force. Although a lubricant is added to facilitate its process of

tableting, its presence affects several properties of tablet. The effect of lubricant on mechanical strength of tablet

depends on mechanism of bonding. The strongest bonds are formed b/w clean, new surface; and

for material that undergo plastic and/or elastic deformation. In such cases lubricants acts as a physical barrier b/w new surface. Hence strength decreases.

Eg. A tablet of MCC, whose bonding occurs primarily through plastic deformation and flow, is mechanically weakened by lubricant. The addition of Mag.stearate markedly decrease axial and tensile strength in MCC as well as Lactose tablet.

For materials that are brittle and fragment, new, clean surfaces are formed and readily bond during compression, and the lubricant has little detrimental effect on strength of tablets.

Dibasic calcium phosphate dihydrate is consolidated by brittle fraction, and its axial and radial tensile strength are not significantly changed by addition of as much as 3% of mag.stearate.

Stearic acid, hydrogenated veg.oil, talc and PEG 4000 may be used in concn as great as 8% for brittle material with only a slight to moderate change in tensile strength.

4. EFFECT OF APPLIED PRESSURE

At higher forces due to fragmentation new surfaces are formed causing an increase in surface area, hence more area is available for bond formation, hence more will be the hardness of the compact.

There is a linear relationship b/w tablet hardness and the logarthim of applied pressure except at high pressures.

According to Balshin eqn. Fc = Fc0 Vr

-m

Where, Fc0 = strength of the tablet when Vr =1 (i.e. completely consolidated)m = is a constant for particular system

Vr is the relative volume defined as Vr = 1/1-ε Where ε is the porosity of the compact And, shotton and Ganderton gave a general

equation for the effect of applied pressure on the strength of the compact.

Log P = nFc + C

Where, P= applied pressure Fc= strength of the compact

C= constant When we extraplot the plot of logP vs Fc ,the

intercept gives the value of C, which probably represents the minimum pressure required for the formation of tablet.

Thank you