COMPACTION, COMPRESSION & CONSOLIDATION

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COMPACTION, COMPRESSION

& CONSOLIDATION

CONTENT:

Compression

Consolidation

Decompression

Compaction of powders with a particular reference to distribution and measurement of forces within the powder mass undergoing compression

COMPRESSION:

Compression is the process of applying pressure to a material. In pharmaceutical tableting an appropriate volume of granules in a die cavity is compressed between an upper and a lower punch to consolidate the material into a single solid matrix, which is subsequently ejected from the die cavity as an intact tablet.

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Compactibility:

The ability of powder bed to cohere into or to form a compact or it is the ability of powder material to be compressed in to a tablet of specified tensile strength.

CONSOLIDATION:

Consolidation is an increase in mechanical strength of material from

particle- particle interaction.

Mechanism: i. Cold welding

ii. Fusion welding

iii. Recrystallization

i. Cold welding:

When the surface of 2 particles approach each other closely enough <50 nm, their energies result in a strong attractive forces. This mechanisms is cold welding.

ii. Fusion welding:

When load is applied, frictional heat can be generated. If this heat is not dissipated, the local rise in temperature could be sufficient to cause melting of the contact area of the particles, which would relieve the stress in that particular region. In that case, the melt solidifies giving rise to fusion bonding.

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iii. Recrystallization:

The actual solubility of solid also depends somewhat on the applied pressure, so that if a film of moisture is present on the solid surface then the high pressure at the point solid contact could force more material into solution. This dissolved solid would crystallize on relief of the applied stress to form a solid bridge whose strength would partly depend on the rate of recrystallization. In general, slow rate should produce a more prefect crystal structure with consequent higher strength.

Consolidation process is influenced by, Chemical nature of material Extent of available surface

Presence of surface contaminants

Intersurface distance

i. Chemical nature of material:

The type and degree of crystallinity in a particular material influences its consolidative behaviour under appreciable applied force. Substances possessing the cubic lattice arrangement were tableted more satisfactorily than those with a rhombohedral lattice.

ii. Extent of available surface:

The compressional process is affected by the extent of available surface.

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Effect of increasing compressional force on specific area of powder mass.

When a powder mass is subjected to increasing compressional force, there is initial particle fracture, which gives rise to increased surface area(O to A). At the point A, particles rebonding becomes dominant factor, and from then on surface area decreases unless tablet lamination begins.

iii. Presence of surface contaminants:

The compressional process is affected by the extent of presence of surface contaminant. For example, lubricants such as magnesium stearate form weak bond, so that over lubrication or even overmixing of lubricant into the tableting mass, results in a continuous coating of the latter, and hence in some cases weak tablets are formed.

iv. Intersurface distance:

At the low level of external forces, molecular and electrostatic forces are source of attractive tendencies between individual particles. Van Der Waal’s forces however may exert a significant effect at distance upto 100nm, so that once an agglomerate of particles has been formed, they may serve to prevents its breakdown.

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COMPRESSION PROCESSES

The various events that occur in the process of compression are, I. Transitional repacking

II. Deformation at the points of contact

III. Fragmentation and/or deformation

IV. Bonding

V. Deformation of the solid body

VI. Decompression and

VII. Ejection

I. Transitional repacking or Particle rearrangement:

The particle size distribution of the granules and the shape of the granules determine the initial packing(bulk density) as the granulation is delivered in to the die cavity. The granules flow with respect to each other, with the finer particles entering the void between the larger particles, and the bulk density of the granulation is increased. Spherical particles undergo less particle rearrangement than 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. Particle rearrangement and the energy expended in rearrangement are minor considerations in the total process of compression.

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II. Deformation at the points of contact:

When a stress is applied to a material, deformation (change of form) occurs. If the deformation disappears completely (returns to the original shape) upon release of the stress, it is elastic deformation. A deformation that does not completely recover after release of the stress is known as a plastic deformation. The force required to initiate a plastic deformation is known as the yield stress. When the particles of granulation are so closely packed that no further filling of the void can occur, a further increase of compressional force causes deformation at the points 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 the formation of potential bonding areas.

III. Fragmentation and/or deformation:

At higher pressure, fracture occurs when the stresses within the particles become great enough to propagate cracks. Fragmentation furthers densification, with the infiltration of the smaller fragments into the void space. Fragmentation increases the number of particles and forms new, clean surfaces that are potential bonding areas. In some materials fragmentation does not occur because the stresses are relieved by plastic deformation.

The influence of applied pressure on the specific surface area of sulfathiazole tablets is shown below, the specific surface of the starch and sulfathiazole granulation was 0.18 𝑚 𝑔⁄ ; the tablet compressed at a pressure of 1600 𝑘𝑔 𝑐𝑚 ⁄ had a specific surface of 0.9 𝑚 𝑔⁄ .

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IV. Bonding:

Several mechanisms of bonding in the compression process have

been conceived, but they have not been substantiated by experimentation

and have not been useful in the prediction of the compressional property

of material.

The three theories which describe the bonding process are,

a) The mechanical theory,

b) The intermolecular theory and

c) The liquid surface theory.

a) The Mechanical theory:

It proposes that under pressure the individual particles undergo elastic, plastic or brittle deformation and that the edges of the particles intermesh, forming a mechanical bond. If only the mechanical bond

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exists, the total energy of compression is equal to the sum of the energy of deformation, heat and energy adsorbed for each constituent. Mechanical interlocking is not a major mechanism of bonding in pharmaceutical tablets.

b) The Intermolecular theory:

The molecules (or ions) at the surface of a solid have unsatisfied intermolecular forces (surface free energy) which interact with other particles in true contact.

According to the intermolecular theory, under pressure the molecule at the points of true contact between new, clean surfaces of the granules are close enough so that Van der Waals forces interact to consolidate the particles.

c) The liquid surface theory:

The liquid-surface film theory attributes bonding to the presence of a thin liquid film, which may be the consequence of fusion or solution, at the surface of the particle induced by the energy of compression.

During compression an applied force is exerted on the granules; however, locally the force is applied to a small area of true contact so that a very high pressure exist at the true contact surface. The local effect of a high pressure on the melting point and the solubility of the material is essential for bonding.

The relation of pressure and melting point is expressed by the Clapeyron equation,

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Where, dT/dP = Change in melting point with change in pressure,

T = Absolute temperature,

∆H = Molar latent heat of fusion,

= Molar volume of the liquid melt and the solid respectively.

If the pressure at the point of true contact is exerted only on the solid and the liquid phase is subjected to a constant atmospheric pressure, the relationship simplifies to,

Where, dT/dP is positive, regardless of the expansion or contraction of the solid, the pressure acting locally at the point of true contact lowers the melting point.

V. Deformation of the solid body:

As the applied pressure is increased, the bonded solid is

consolidated towards a limiting density by plastic and/or elastic

deformation of the tablet within the die. The effect of applied pressure

on the apparent density of tablets of sulfathiazole is given below,

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VI. Decompression:

The success or the failure to produce an intact tablet depends on the stresses induced by elastic rebound and the association deformation process during decompression and ejection. Often, if capping or lamination of the eject tablet has occurred , the individual pieces are dense, hard and strongly bonds indicating that sufficient areas of true contact existed during compression. As the upper punch withdraw from the die cavity , tablet is confined in the die cavity by a radial pressure. Ideally, if only elastic deformation occurred, with the sudden removal of the axial pressure the granule would return to their original form breaking any bonds that may have formed under pressure. Also the die wall pressure will be zero as the elastic material recovered axially and contracted radially. As the movement of tablet is restricted by the residual die wall pressure and friction ,the stress from axial elastic recovery and radial contraction cause capping. Capping is due to uniaxial relaxation in the die cavity at the point where the upper punch pressure is release. If decompression occurs simultaneously in all directions capping is reduced. Stress relaxation of plastic deformation is time dependent. Material having slow rates of stress relaxation crack in the die upon decompression .

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In fig, the ratio of pressure at time to the maximum pressure is plotted against the logarithm of the time. The change of the initial slope suggest some prominent mechanism of bonding soon becomes negligible. The initial slope reflects the ability of the material to relieve stress during decompression. The rate of stress relieve is slow for acetaminophen so cracking occurs while the tablet is within the die. With microcrystalline cellulose the rate of stress relieve is rapid and intact tablets result.

If stress relaxation is slow and cracking is a problem, a slower operation speed provides more time for stress relaxation. A tablet press that provide precompression allows some stress relaxation before the final compression. To optimized the stress relaxation, before final compression the precompression pressure should be approach the maximum pressure.

VII. Ejection:

As lower punch rises and pushes the tablet upward there is continue residual die wall pressure and energy must be expanded due to die wall friction. As the tablet is removed from the die, the lateral

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pressure is relieved, and tablet under goes elastic recovery with increase in volume of that portion of the tablet removed from the die. During the ejection that portion of the tablet within a die is under strain, and if this strain is exceed the share strength of the tablet, the tablet caps adjacent to the region in which the strain had just been removed.

Description of process:

The process of compression has been described in the terms of

relative volume and applied pressure.

In transitional repacking the granules are packed to an arrangement in which the particles are immobile and the number of intergranular points of contact has increased. The decrease in the relative volume during transitional repacking is represented by the segment AE. With the further increase in the pressure, temporary supports between the particles are formed as represented by segment EF. Fragmentation and or plastic deformation is represented by segment FG. As some higher pressure bonding and consolidation of solid occur to some limiting value as indicated by segment GH.

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FORCE DISTRIBUTION

Tablet strength can be found:-

Particle deformation whether plastic or elastic will be proportional

to applied force. The porosity of tablet &hence its strength. The weakest

points in tablet structure will be those that receive the lowest forces

FA=F

L+F

D

FA = Force applied to the upper punch.

FL= Proportion of it transmitted to the lower punch

FD= Reaction at the die wall due to friction at this surface

FM

=FA+F

L/2

where FM = Mean compaction force .

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MEASUREMENT OF FORCE:

Instrumentation:

i. Strain gauge ii. Piezo electric transducer

iii. LVDT(linear variable differential transducer)

i. Strain gauge

It is the coil of highly resistant wire mounted on paper backing. During compression the force applied cause a very small elastic deformation of two punches. If suitable strain gauge is firmly bound to the punch shank as close to the compression site as practical, it is deformed as the punch is deformed. Due to deformation, there is a change in wire resistance as it decrease and the diameter is increased. The resultant decrease in electrical resistance can be measured by wheat stone bridge.

ii. Piezo electric transducer:

When piezo electric material is subjected to external force these

material develop an electric charge proportional to force. This transducer

is connected by a high impedence cable to a charge amplifier which

converts the charge into correctly proportional to dc voltage.

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iii. Linear variable differential transducer (LVDT)

Punch displacement is based on differential inductor principle.

Ferrous core of the transducer is rigidly connected to punch by link, so

that movement of punch unbalances the secondary circuit, the output is

attenuated to produce dc voltage which is directly proportional to

displacement.

REFERENCE:

Leon Lachman; Herbert A. Lieberman: Theory and practice of Industrial pharmacy; Special Edition 2009.

E.A. Rawlins; Bentley’s textbook of pharmaceutics; Eight edition.

Herbert. A. Lieberman, Leon Lachman; Pharmaceutical dosage forms: Tablets vol 2; Second edition 2005.