Compression and Compaction

Presented By:

Mr. Santosh A. Payghan (Asst. Professor)

Tatyasaheb Kore college of Pharmacy,

Warananagar.

Powder Properties
Solid particles are made up of molecules that are held in close proximity to each other by intermolecular forces. The strength of interaction between two molecules is due to the individual atoms within the molecular structure.

For example, hydrogen bonds occur as a result of an electrostatic attraction involving one hydrogen atom and one electronegative atom, such as oxygen. For molecules that cannot hydrogen bond, attraction is due to van der Waal's forces. dipole-dipole (Keesom),dipoleinduced dipole (Debye) and induced dipole-induced dipole (London) forces.

Derived properties of powdered solids

1. The solid-air interface 2. Angle of repose 3. Flow rates 4. Mass-volume relationships 5. Density

Mass-volumeRates AngleDensity The solid-air interface Flowrepose of relationships HeliumTan-1VOLUME 1. = Pycnometer (h/r) 2. = cos D/ Liquid displacement method Compressibility indexdensityPOROSITY -1 of (l1+l2) VCOHESION: Methods topossible Angle gravity bottle used.Flow Different-U ] : Consolidation/Repose The -U x[U types of = V /U maximum angle measure here, Pycnometer or specific here, (Carr's consolidationlike standing True density= w /(w -w = (w E = V / Vb Truevolume betweent)freeparticle. CarrsVIndex(% ) of) base-w )/(w -w ) volume and index) (V a. Attraction of sample Vt = Fixedheight of pile true = funnel h between the surface of pile , D = diameter cone[1-v/vo particles method. Experienced by]x100 spheres I r=volume of of the (V Vc=true= radius stainless base)of the pile w = wt. ofl1Pycnometer opposite sides of pile Granule volumesteel g in bulk. here 5-15 +l2 = the Excellent of the powder and the w = Wt. of Pycnometer + sample or angle of repose U1=Volume of empty cell Void volume here, =density: b) t=M/vt VV =Wt. of Pycnometer with powder glass beads b. Tilting Bulk volume (V True box method. w 12-16 & Wall is linedfilled with Good by sandpaper horizontal ADHESION: plane.FLOW U1-U2=Volume occupied by the std. sample VSTATICvolume KINETIC/DYNAMIC b = Bulk A.R. solvent ANGLE OF REPOSE V = Tapped Volume
t c 1 2 1 s

4=

Relative volume unlike particle. c. Attraction between (Vr) Revolving cylinder method.

U1-Us = volume occupied by Granule density:sample g=M/vg noww2-w1 Wt. of sample Fair To Passable w3 , () A.R. V0= Volumebytparticles at surface. w*18-21 before tapping Vr = V/ V Experienced 4-w2 Volume of liquid displaced by the solid Voidangle of (VV)It is anglet of repose detervolume = Vb V < 25 EXCELLENT b Bulk density: Resistance to movement bparticles It is of =M/v *23-35 Poor repose mined by the )/ method Therefore, Porosity (E) =(VbVt3rdVb is affected by two as all air is Vr tends to become unity factors:determined by It is preferred since they eliminated25-30 the mass GOODthe/ tPorosity when expressed as percentage relative density: duringr= from st
= = =

a. Electrostatic forces. compression process *30-40 PASSABLE

b. Adsorbed layer of moisture on >40 VERY POOR particles.

33-38 1 two methods (a. & b.)

Very mimic most closelyPoor the E =100.[(VbVt)/ Vb] manufacturing situation in >40 Very Very Poor Tapped which powder is in motion. density-tester Specific gravity bottle

Powder compression

COMPRESSION: The reduction in the bulk volume of a material as a result of the removal of the gaseous phase (air) by applied pressure. C ONSOLIDATION: Involves an increase in the mechanical strength of a material resulting from particle-particle interactions. C OMPACTION: The compression and consolidation of a 2 phase (solid + gas) system due to an applied force.

Compression
When external mechanical forces are applied to a powder Powder fluidity mass, there is reduction in bulk volume as follows, required to transport the material 1.Repacking 3.Brittle fracture: e.g., sucrose provide adequate filling of the dies to produce tablets of 2.Particle weight 4.microquashing consistent and strength. deformation Powder compression e.g., acetyl salicylic acid, MCC Elastic Depends on density and packing characteristics of deformation - when elastic limit or yield point powder is reached.
Plastic deformation

Microsquasing: Irrespective of the behavior of larger particles smaller

particles may deform plastically.

Stages involved in compression

1. Initial repacking of particles.

2. Elastic deformation of the particles until the elastic limit (yield point) is reached.
3. Plastic deformation and/or brittle fracture then predominate until all the voids are virtually eliminated. 4. Compression of the solid crystal lattice then occurs.
On Decompression

Stages involved in compression Elastic deformation: Plastic deformation

1.

The only forces that exist between the particles are those 1. removal Deformation not immediately reversible on the removal On that areof the load, the deformation is reversible - it behaves like related to the of the applied force. packing characteristics of the rubber 2. particles, the density of thein which the sheartotal mass of Predominant in materials particles and the strength is less undergo that is filled into the subjected All the material elastic deformation whendie solidsthan the tensile or breaking strength. to external forces. 3. Believed to create the greatest number of clean surfaces 2. External force - reduction in volume due to closer packing 4. Plastic deformation is a time dependent process, little Some materials, e.g. particles- main mechanism of very higher paracetamol, are of the powder(either plastic flowelastic and There is caused by permanent change or fragmentation) initial rate of reduction compression: volume force application leads to the formation of less new clean surfaces - weaker tablets. 3. As the tablet formation is dependent on particles becomes 5. Since load increases, rearrangement of the compression load The material rebounds (recovers elastically) When the formation of more clean surfaces, high compact will self-destruct some type is released. If bonding is weak theconcentrationleads to mixing top new difficult and further compression or over and the of will materials that formwhole cylinder cracks intoweak tablets detach (capping) Else, weak of particle deformation bonds result in horizontal layers (lamination).stearate e.g. Mg
Elastic materials require a particularly plastic tableting matrix or wet

massing to induce plasticity.

Compression events
Consolidation time: Time to reach maximum force. Dwell time: Time at maximum force. Contact time:

Time for compression decompression excluding ejection time.

Ejection time: Time during which ejection occurs. Residence time: Time during which the formed compact is within the die.

Consolidation
Definition: increase in the mechanical strength of a material as a result of particle/particle interactions Various Hypothesis:
If this heatincrease usually isthe local rise in temperature temperature is dissipated,between Any applied load to of cause melting andthe C When the sufficientthetwo particles4 approach each area of surfaces to bed transmitted 30 contact other could be on of through particle Depends
During compression, the powder compact typically undergoes a

contacts. closely enough (e.g. at a separation of less than 50nm), theFriction effects particles theirMaterial characteristics, free surface energies result in a strong attractive Under appreciable forces, this transmission may through efficiency forceLubricationa process known as cold welding .result in When the melt solidifies, fusion bonding occurs, which in Magnitude and rate of is favoured as a major reason for the generationin an increase in the mechanical strength of turn This hypothesisapplication of compression forces results of considerable frictional heat. theMachine speed mass. the increasing mechanical strength of a bed of powder As the tablet temperature rises, stress relaxation and plasticity when subjected elasticity decreases and strong compacts are increases while to rising compressive forces.
formed

Compaction:

Steps involved in compaction of powders under an applied force.

Stages of Compaction
Particle rearrangement/inter particle slippage Deformation of particulates Bonding/Cold welding Deformation of the solid body Elastic recovery/expansion of the mass as a whole Bonding/Cold Welding Deformation of the Solid Body Particle Rearrangement Deformation Recovery: of Materials Mechanisms a) As Occurs at low a result the bonded solid is consolidated Solid bridges (as MajorThe compact pressures.melting, crystallization, axial 1.1. deformation increases, of allowing radial and sintering, Material pressure is ejected, mechanism(s) chemical reaction, and binder hardening) toward a limiting density volume of powder bed. 2. Reduction in the relativeby plastic and/or elastic recovery. Ascorbic acid, Dicalcium Fragmentation as a result of movable liquids (capillary and surface b)deformation.phosphate, Maltose, Bonding 3.2. Small particles flow intoto revert the compact particles Elastic character tends voids between larger to its tension forces) Phenacetin, Sodium leading to Citrate, Sucrose packing arrangement As pressure a closer originalmovable binder bridges (viscous binder and shape. c) Non freely increases, Ibuprofen, Paracetamol relative particle movement becomes Fragmentation and adsorption layers) elastic impossible, inducing deformation deformation d) Attraction betweenmonohydrate, Fragmentation and Lactose solid particles (molecular and electrostatic plastic deformation Microcrystalline cellulose forces) Plastic deformation NaHCO3, NaCL, Pre e) Mechanical interlocking (irregular particle size and size gelatinized starch distribution) Starch Elastic deformation

Compaction data analysis

The ideal requirements for a compression / compaction equation A equation some measure of the state of The compaction monitored relates compaction of densification these The parameters model should cover during the whole range vary widely in with consolidation of studies. accuracy.a powder, such as porosity, volume (or relative sufficient volume), density or void ratio, with a function of the compaction Various parameters havebe related to physical the compaction behavior The parameters should been used to assess relevant properties of the pressure. of a variety of pharmaceutical powders and formulations powder. Forces on the punches Many compaction equations have been proposedformulation and The parameters should be sensitive to changes in like; Heckel , displacement Adams have lower punches, for pharmaceutical systems. and Kawakita and of the upperandbeen validated at least proportional to minor experimental variables insensitive or axial to radial load transmission, changes in normalisation factors like density or initial volume. die wall it is highly unlikely that a single compaction equation will fit However,friction, The model and itsmechanisms.should be easily estimated by general the compaction parameters all ejection force, available computer programs. temperature changes In interpreting compaction curves, it is therefore essential to know The model should significantly differentiate between powders and Resulting data may be expressed equivalently in term of stress-strain, which mechanisms are operating, or not, over different region of dissimilar compression characteristics. pressure-volume or pressure density since the natural strain, for pressure. example, is equal tomodel should be evaluated of the initial bed height The quality of the the natural log of the ratio by a combination of the or volumedensification covered or volume respectively to the observed range of compaction curve should be able to indicate changes in the A good to the current height and the goodness-of-fit data. compression mechanism

Heckel equation
where R is the relative density at pressure P, and E is the porosity. Powder packing with increasing compression load is
The relative density is definedparticle rearrangement, the compact normally attributed to as the ratio of the density of elastic and at pressure, P, to the density of particle fragmentation true density of plastic deformation and the compact at zero void or the material The porosity cananalysis is a popular method of determining The Heckel also be defined as: the volume reduction mechanism under the compression E =(Vp V)/V p= 1 - R force where V p and V are the volume at any applied load and the volume at theoretical zero porosity, respectively. powder compression follows Based on the assumption that Thus, equationkinetics kE cantheexpressed as: first order dR/dP= with be interparticulate pores as the d the densification of the powder as the reactants andR /dP= k( 1- R ) and then transformed to: product. In the analysis, the degree of compact According to [1/(1-R)]= kP+A i.e (y = mx +c) Plotting the value of In [1/(1-R)] against applied pressure, P, yields a is densification with increasing compression pressure linear graph having slope, k and intercept, A.

directly proportional to the porosity as follows: d R / dP = kE

Heckel equation
The reciprocal of k yields a material-dependent constant

known as yield pressure, Py which is inversely related to the ability of the material to deform plastically under pressure.
Low values of Py indicate a faster onset of plastic

deformation.
This

analysis has been extensively applied to pharmaceutical powders for both single and multicomponent systems.
function of the original compact volume.

The intercept of the extrapolated linear region, A, is a

Heckel equation
From the value of A, the relative density, D A , which represents the total degree of densification at zero and low pressures can be calculated using the equation A =In 1/(1-DA ) DA=1-e - A The relative density of the powder bed at the point when the applied pressure equals zero = D0 Describes the initial rearrangement phase of densification as a result of die filling. D0 is determined experimentally and is equal to the ratio of bulk density at zero pressure to the true density of the powder The loose packing of granules at zero pressure tends to yield low D0 values

Heckel equation
The relative density, DB describes the phase of rearran-

gement of particles in the early stages of compression Indicates the extent of particle or granule fragmentation, The extent of the rearrangement phase depends on the theoretical point of densification at which deformation of particles begins. D B can be obtained from the equation: DB=DA- D0

Based on Heckel equation 3 types of powder-A, B & C

1. With type A materials, a linear relationship is observed, with the plots remaining parallel as the applied pressure is increased indicating deformation apparently only by plastic deformation

2. An example of materials that exhibit type A behavior is sodium chloride. 3. Type A materials are usually comparatively soft and readily undergo plastic deformation retaining different degrees of porosity depending on the initial packing of the powder in the die. 4. This is in turn influenced by the size distribution, shape, e. t. c., of the original particles.

In [1/(1-R)]=kP + A

Based on Heckel equation 3 types of powder-A, B & C

1. For type B materials, there is an initial curved region followed by a straight line 2. This indicates that the particles are fragmenting at the early stages of the compression process 3. Type B Heckel plots usually occur with harder materials with higher yield pressures which usually undergo compression by fragmentation first, to provide a denser packing. Lactose is a typical example of such materials.

In [1/(1-R)]=kP + A

Based on Heckel equation 3 types of powder-A, B & C

1. For type C materials, there is an initial steep linear region which become superimposed and flatten out as the applied pressure is increased 2. This behavior to the absence of a rearrangement stage and densification is due to plastic deformation and asperity melting.

In [1/(1-R)]=kP + A

Application of Heckel equation

The crushing strength of tablets can be correlated with the

values of k of the Heckel plot .

Larger k values usually indicate harder tablets. Such information can be used as a means of binder

selection when designing tablet formulations.

Heckel plots can be influenced by the overall time of

compression, the degree of lubrication and even the size of the die, so that the effects of these variables are also important and should be taken into consideration.

Kawakita equation
The Kawakita equation was developed to study powder compression using the degree of volume reduction, C, a parameter equivalent to the engineering strain of the particle bed C =(V0-Vp)/V0=abP/(1+bP) can be rearranged to give: P/C=P/a+1/ab Where,
C is the degree of volume reduction, V 0 is the initial volume of the powder bed and V p is the powder volume after compression; a and b are constants which are obtained from the slope and intercept of the P/C versus P plots

Methods of Evaluating the Compaction Process

ompaction profiles (Force-time, Displacement-time) C Tablet expansion Pressure-Volume relationships Pressure transmission Energy of Compaction Radial vs Axial Force

Acoustics
Temperature