compression and comp action

8/6/2019 Compression and Comp Action

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Tiruveni. Movva (M.pharmacy)Chebrolu Hanumaiah institute of pharmaceutical sciences

chowdavaram


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Definition

Compression is the process of applying pressure to a material.

In pharmaceutical tabletting an appropriate volume of granules ina die cavity is compressed between an upper and lower punch toconsolidate the material into a single solid matrix, whichsubsequently ejected from the die cavity as an intact tablet.


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Steps Involved In Compression

Transitional Repacking

Deformation at points of contact

Fragmentation and /or deformation

Bonding

Deformation of the solid body

Decompression

Ejection


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Transitional repacking or particle rearrangement

` The particle size distribution of the granulation and the shape of thegranules determine the initial packing as the granulation is delivered intothe die cavity.

` The granules flow with respect to each other, with the finer particlesentering the void between the larger particles undergo less particles andthe bulk density of the granulation is increased

` Spherical particles undergo less particle rearrangement than irregular

particles


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Deformation at points of contact

` Elastic deformation` Plastic deformation` Deformation increases the area of true contact and the formation of

potential bonding areas.

Fragmentation and deformation

` Fragmentation increases the number of particles and forms new, cleansurfaces that are potential bonding areas.

`

In some materials fragmentation does not occur because the stress arerelieved by plastic deformation.


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Theories of Bonding:

Mechanical theory:

The mechanical theory that proposes that under pressure thatindividual particles undergo elastic, plastic, or brittle deformation and

that the edges of the particles intermesh, forming a mechanical bond.

Intermolecular theory:

The molecules at the surface of a solid have unsatisfied inter

molecular forces, which interact with other particles in true contact. van der walls forces interact to consolidate the particles.


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Liquid surface-film theory:

Bonding due to the presence of a thin liquid film, at the surface ofthe particle induced by the energy of compression.

The effect of pressure on the melting point and solubility of a

material is essential to bonding.

The relation of pressure and melting point is expressed by theclapeyron equation

dt/dp=T(V1-Vs)/H

If the pressure at the points of true contact is exerted only on thesolid, and the liquid phase is subjected to a constant atmosphericpressure, the relation ship simplifies to

dt/dps=VsT/H


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Deformation of the solid body

As the applied pressure is further increased, the bonded solid isconsolidated towards a limiting density by plastic or elasticdeformation of the tablet within the die

Decompression

The success or failure to produce an intact tablet depends on thestress induced by elastic rebound and the associated deformationprocesses during decompression and ejection.


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Ejection

As the lower punch rises and pushes the tablet upward there isa continued residual die wall pressure and considerable energymay be expended due to the die wall friction

As the tablet is removed from the die, the lateral pressure isrelieved, and the tablet under goes elastic recovery with anincrease in the volume of that portion of the tablet removedfrom the die


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Properties of tablet influenced by compression

Density and porosity

The apparent density of a tablet is the quotient of the weightand the geometric volume. The apparent density density of atablet is exponentially related to the applied pressure

Eg:

When equal weights of aspirin and lactose are compressed with10% starch, the porosity of the lactose tablet, is of magnitudebetween that of the individual lactose and aspirin tablets atcorresponding pressure.


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Hardness and tensile strength

The ability of a tablet to withstand mechanical handling andtransport has been evaluated by various types of testshowever, the data from these tests seldom can be correlated

in a precise manner.Eg: Lactose-aspirin tablets, compressed mixtures have a

hardness values between those of tablets composed of theindividual ingredients.


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Specific surface

Specific surface is the surface area of 1g of material

As the lactose granules, which were granulated by adding10% starch paste, are compressed, the specific surface isincrease to a maximal value, indicating the formation of newsurfaces due the fragmentation of the granules.

Further increase in applied pressure produce a progressivedecrease in specific surface as the particles bond.


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Disintegration

As the applied pressure used to prepare the tablet isincreased, the disintegration time is longer. Frequently,

there is relation ship between the disintegration time andthe applied pressure

Sulfadiazine tablets, the concentration of a disintegratingagent influences the relationship between applied pressureand disintegration time.


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Dissolution

If the fragmentation of the granules occurs during compression, thedissolution is faster as the applied pressure increases and thefragmentation increases the surface area.

If the bonding of the particles is more, the increase in the appliedpressure causes a decrease in dissolution.


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Dissolution-Pressure Relationship:

` The dissolution is more rapid as the applied pressure is increased

` The dissolution is slowed as the applied pressure is increased.

` The dissolution is faster, to a maximum, as the applied force is

increased and then further increase in applied pressure slowsdissolution.

` The dissolution is slowed to a minimum as the applied pressure isincreased, further increase in applied pressure speeds dissolution.


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Compression and consolidation under high loads

` Forces are large enough to exceed the elastic limit of the solid

` Plastic deformation and/or brittle fracture results in the generation ofnew, clean surfaces , which being pressed against one another, undercold welding

` The structure must be strong enough to withstand the new stress

induced during release of the applied load those generated by ejectionform the die


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` Consolidation is accentuated in those regions adjacent to theintense shear to which material is subjected, as it is compressedaxially and pushed long the wall surface

` Smaller tablet height-to-diameter ratios are preferred

` The resistance to differential movement of particles in the appliedforces not being transmitted uniformly throughout the entire mass

FL=F A . e-KH/D


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Effect of friction

Interparticulate friction: This arises at particle /particle contacts More significant at low applied loads collide silica is a common example

Die-wall friction: This results from material being pressed against the

die wall and moved it down This effect become dominant at high applied forces magnesium stearate is a common choice as a lubricant


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Force distribution

FA =FL + FDFA = force applied to upper punchFL = proportion of it transmitted to the lower punchFD= reaction at the die wall due to friction at this surface

Force volume relation ships

The end of the compressional process may be recognizedas being the point at which all air spaces have been

eliminated i.e.,Vb=Vt and therefore E=0.


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Methods of Decreasing Porosity:

The filling of spaces by interparticulate slippage.

The filling of the small voids by deformation orfragmentation at higher loads.

This process can be expressed mathematically.E0-E/E0(1-E)=K1e-K2/p+K3e-K1/p


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The degree of compression achieved for a given load depends uponthe initial porosity

once appreciable force has been applied, the relationship betweenapplied pressure and some volume parameter such as porosity is

given byLogE =LogEo-K.P.

When the pressure is zero, and K is constant1/1-E= K1- K2 . Log p


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Heckel plots:

The Heckel equation is basedupon analogous behavior toa first order reaction,

Log1/E=KyP+Kr

Measuring the appliedcompressional force F and the

movements of the punches duringa compression cycle

for cylindric tablet, P is given byP=4F/.D2

D is the tablet diameter

E=100.[1-4W/t..D2.H]


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Compaction profile

Represents the proportion of the

applied pressuretransmitted radially to the die wall

Poisson ratio, is a material-dependent property affecting thecompressional processes

When elastic limits of the material ishigh, elasticdeformation occurs

Elastic relaxation depends upon thevalue of youngsmodulus.

If the material readily undergoesplastic deformation with aconstant yield stress


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Energy involved in compaction

Lubrication has no apparent effect on the actual amount ofenergy required to compress the materialThe total work involved, WT,

D max

WT = Df=0 F.dD

Strength of tablet

The mechanical strength of tablet has been described in

variety of ways, including of hardness, bending strength,fracture resistance friability, and crushing strength.


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Crushing strength

That compressional force which when applied diametrically to atablet, just fracture it.

ST =2FC/D.H The work Wfrequired to cause tablet failure

Wf=2/ DH.Fdz

Linear proportionality with disintegration time and log FA and inverseproportionality with porosity, over normal ranges of compressionalforce

Failure may be propagated through the granules themselves,Sc = K.d-a

The inherent cohesiveness of very fine particles lead to evenstronger tablets for a particular compressive force.


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Friability

It measures the weight loss on subjecting the tablets to astandardized agitation procedure

The friability, f, is given by:

f=100(1-wo /w)

Values of f from 0.8 to 1.0% are regarded as the upper limit ofacceptability.

Hardness and tensile strength data are used to define threedimensionless indices (strain index, bonding index, and brittlefracture index) that were used to quantify the relative tablettingperformance


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Lamination of tablets

The tablet structure fails on ejection from the die or duringsubsequent operations such as coating.

Occur with fine particle size of materials, which tend to pack poorly.

Slow compressional rates and use of multi stage compression

presses often reduce capping tendencies.

Compressed at high speed partial vacuum, classical capping wasminimized

Inclusion of components in the formula to enhance bonding andprovide a matrix of plastically deforming material for stress relief.


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Instrumentation of tablet machine

Instrumenting tablet provide information on the compactioncharacteristics of the major components in a formulation, and onthe effect of additives upon them.

Mechanism of tabletting process involves the utilization of unsatisfied bonds at the solid surface

This process is enhanced by the generation of large areas of cleansurface, which are then pressed together, as a might occur if

appreciable brittle fracture and plastic deformation were introducedinto the system.


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Single-station presses

Strain gauge network as a transducer for measuring the magnitudeof the force operating during the compression cycle.

They should be bonded as near to the active site.

The bonding must be over the entire area of the gauge where theelastic change in linear dimension of the stress-bearing member canbe measured.


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The die wall instrumentation requires matching of the die-wall toaccommodate the gauges and reduce the thickness to a point atwhich adequate sensitivity is achieved

The original geometry is restored with silicon rubber or similar

material.

When subjected to external forces, these materials develop anelectrical charge proportional to the effect of the force


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Piezo-electric Transducer:

Disadvantage:The charge inevitably dissipates with time.

Advantages:High sensitivityRobust constructionNo bonding to machine fabric, which therefore allows easy

changing of tooling.


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Multi-station presses

Inherent difficulty in retrieving electrical signals from a revolving

turret.

Overcame this difficulty by employing radiotelemetry to transmit theforce signal from strain-gauged upper and lower punches to externalrecorders.

For a give machine, the best location probably depends upon itsactual design.

` Sufficient to induce an adequate change in gauge resistance while

not exceeding its elastic limit.

Cast iron components are unsuitable because of variability in theirmodulus of elasticity and poisson ratio. Parts constructed with steel


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Signal processing

The signals from the instrumentation are DC voltages and can beretrieved, stored, and processed by a common means.

Display the signals on the cathode ray oscillograph since thisenables instant visualization of the instrumentation out put.

Ultraviolet recording oscillographs provide better definition of tracesand can facilitate a larger number of simultaneous recordingchannels.

The analog signals can be fed by an A-to-D converter into memorylocations.


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Role of instrumentation in production

The design of instrumentation exercises some degree of controlover the tableting process.

The weight and hardness deviations can be minimized by:1. The formulation is homogeneous(i.e has uniform density)

2. The compressional force/tablet weight function iscontact.

3. The volume of the die cavities at the point of maximumcompression is constant.

The system produces a series of voltage pulses of short duration,each proportional to the weight of an individual tablet.


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The individual compression pulses can also be used to drivecounting mechanisms.

One can set upper and lower limits for acceptable tablet weight andthen distinguish pulses from tablet weight lying outside these out-of-specifications tablets exceeds some preset value, a relay can betripped to activate an alarm and/or the machine can be

automatically stopped.

The second approach is to take the amplified output signals andfeed them intoAC signal.

This average DC voltage is compared with a reference voltage, and

any difference is converted into an AC signal, which is amplified andused to drive a two-phase servomotor.


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The response of the instrumentation is a direct function of theproperty needing to be monitored.

Stresses generated in certain parts of the machine frame aredirectly proportional to the punch forces, which in are related tocompressional weights.

Instrumented tablet machine technology better understanding of

the tabletting process will assist in formulation development andbatch quality control.

The motor can be connected to the weight or pressureadjustment control of the press , so that any change in the

average compressional force is reflected in an adjustment ofeither the weight or force control.


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References:

The TheoryAnd Practice Of Industrial Pharmacy ByLachman, Liberman, kanig.pg.no.66-95

pharmaceutical dosage forms tablet volume-2 edited byherbertA.liberman, leon lachman, andjousph B. schwartz.pg.no.201-221


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compression and comp action

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