Compression and Compaction

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),dipole-induced dipole (Debye) and induced dipole-induced dipole (London) forces.

Derived properties of powdered solids

1.The solid-air interface2.Angle of repose3.Flow rates

4.Mass-volume relationships

5.Density

COHESION:Attraction between like particle.Experienced by particles in bulk.

ADHESION: Attraction between unlike particle. Experienced by particles at surface.Resistance to movement of particles is affected by two factors:-

a. Electrostatic forces.b. Adsorbed layer of moisture on particles.

The solid-air interfaceAngle of repose

The maximum angle possible between the surface of pileof the powder and the horizontal plane.

Methods to measure Angle of Reposea. Fixed funnel and free standing

cone method.

b. Tilting box method.

c. Revolving cylinder method.Wall is lined by sandpaper

1. θ = Tan-1(h/r) here,

h = height of piler = radius of the base of the pileθ = angle of repose

2. θ = cos-1 D/ (l1+l2)

here, D = diameter of base l1+l2 = the opposite sides of pile

ANGLE OF REPOSE (Ø)

FLOW

< 25 EXCELLENT

25-30 GOOD

*30-40 PASSABLE

>40 VERY POOR

STATIC A.R.

KINETIC/DYNAMIC A.R.

It is angle of repose determined by 1st two methods (a. & b.)

It is angle of repose deter-mined by the 3rd methodIt is preferred since they most closely mimic the manufacturing situation in which powder is in motion.

Flow RatesCompressibility index(Carr's consolidation index)

I = [1-v/vo]x100here, V = Tapped Volume V0= Volume before

tapping

Consolidation/Carr’s

Index(% )

Flow

5-15 Excellent

12-16 Good

*18-21 Fair To Passable

*23-35 Poor

33-38 Very Poor

>40 Very Very Poor

VOLUMETrue volume (Vt)Granule volume (Vg)Bulk volume (Vb) Relative volume (Vr) Vr = V/ Vt

Vr tends to become unity as all air is

eliminated from the mass during the

compression process

POROSITY E = VV/ Vb

here,

VV = Void volume

Vb = Bulk volume

now,

Void volume (VV) = Vb –Vt

Therefore, Porosity (E) =(Vb–Vt)/ Vb

Porosity when expressed as percentage

E =100.[(Vb–Vt)/ Vb]

Mass-volume relationshipsHelium Pycnometer

Vt = Vc/U1-U2x[U1-Us]

Vt = true volume of sample

Vc=true volume of stainless steel spheres

U1=Volume of empty cell

U1-U2=Volume occupied by the std. sample

U1-Us = volume occupied by sample

Liquid displacement methodPycnometer or specific gravity bottle used. True density= w3/(w4-w2) = (w2-w1)/(w4-

w2)w1 = wt. of Pycnometer

w2 = Wt. of Pycnometer + sample or glass beads

w4 = Wt. of Pycnometer with powder & filled with solvent

w3 = w2-w1 = Wt. of sample

w4-w2 = Volume of liquid displaced by the solid

Specific gravity bottle

Different types of density :

True density: ρt=M/vt

Granule density: ρg=M/vg

Bulk density: ρb=M/vb

relative density: ρr= ρ/ ρt

Tapped density-tester

Density

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.

CONSOLIDATION:�Involves an increase in the mechanical strength of a material resulting from particle-particle interactions.

COMPACTION:�The compression and consolidation of a 2 phase (solid + gas) system due to an applied force.

Compression

Powder fluidity required to transport the material provide adequate filling of the dies to produce

tablets of consistent weight and strength. Powder compression

Depends on density and packing characteristics of powder

When external mechanical forces are applied to a powder mass, there is reduction in bulk volume as follows,

1.Repacking 3.Brittle fracture: e.g., sucrose

2.Particle 4.microquashing deformation e.g., acetyl salicylic acid,

MCC - when elastic limit or

yield point is reached.

Microsquasing: Irrespective of the behavior of larger

particles smaller particles may deform plastically.

Elastic deformation

Plastic deformation

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.

Stages involved in compression

On Decompression

1. The only forces that exist between the particles are those that are related to the packing characteristics of the particles, the density of the particles and the total mass of the material that is filled into the die

2. External force - reduction in volume due to closer packing of the powder particles- main mechanism of initial volume reduction

3. As the load increases, rearrangement of particles becomes more difficult and further compression leads to some type of particle deformation

Stages involved in compressionElastic deformation: On removal of the load, the deformation is reversible - it

behaves like rubber

All solids undergo elastic deformation when subjected to external forces.

Some materials, e.g. paracetamol, are elastic and There is very little permanent change (either plastic flow or fragmentation) caused by compression:

The material rebounds (recovers elastically) When the compression load is released. If bonding is weak the compact will self-destruct and the top will detach (capping) Else, whole cylinder cracks into horizontal layers (lamination).

Elastic materials require a particularly plastic tableting matrix or wet massing to induce plasticity.

Plastic deformation1. Deformation not immediately reversible on the

removal of the applied force.2. Predominant in materials in which the shear

strength is less than the tensile or breaking strength.

3. Believed to create the greatest number of clean surfaces

4. Plastic deformation is a time dependent process, higher rate of force application leads to the formation of less new clean surfaces - weaker tablets.

5. Since tablet formation is dependent on the formation of new clean surfaces, high concentration or over mixing of materials that form weak bonds result in weak tablets e.g. Mg stearate

Compression events

Consolidation time: Time to reach maximum force.

Dwell time: Time at maximum force.

Contact time: Time for compression decompress- ion excluding ejection time.

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

ConsolidationDefinition: increase in the mechanical strength of

a material as a result of particle/particle interactions

Various Hypothesis:When the surfaces of two particles approach each other closely enough (e.g. at a separation of less than 50nm), their free surface energies result in a strong attractive force through a process known as cold welding .

This hypothesis is favoured as a major reason for the increasing mechanical strength of a bed of powder when subjected to rising compressive forces.

Any applied load to the bed is transmitted through particle contacts.

Under appreciable forces, this transmission may result in the generation of considerable frictional heat.

If this heat is dissipated, the local rise in temperature could be sufficient to cause melting of the contact area of the particles

When the melt solidifies, fusion bonding occurs, which in turn results in an increase in the mechanical strength of the mass.

During compression, the powder compact typically undergoes a temperature increase usually between 4 and 30 C

Depends on Friction effectsMaterial characteristics, Lubrication efficiencyMagnitude and rate of application of compression forces Machine speed

As the tablet temperature rises, stress relaxation and plasticity increases while elasticity decreases and strong compacts are formed

Steps involved in compaction of powders under an applied force.

Compaction:

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

wholeParticle Rearrangement

1. Occurs at low pressures.2. Reduction in the relative volume of powder bed.3. Small particles flow into voids between larger

particles leading to a closer packing arrangement As pressure increases, relative particle movement becomes impossible, inducing deformation

Deformation Mechanisms of MaterialsMajor deformation

mechanism(s)Material

Fragmentation Ascorbic acid, Dicalcium phosphate, Maltose, Phenacetin, Sodium Citrate, Sucrose

Fragmentation and elastic deformation

Ibuprofen, Paracetamol

Fragmentation and plastic deformation

Lactose monohydrate, Microcrystalline cellulose

Plastic deformation NaHCO3, NaCL, Pre gelatinized starch

Elastic deformation Starch

Bonding/Cold Weldinga) Solid bridges (as a result of melting, crystallization,

sintering, chemical reaction, and binder hardening) b) Bonding as a result of movable liquids (capillary and

surface tension forces) c) Non freely movable binder bridges (viscous binder

and adsorption layers) d) Attraction between solid particles (molecular and

electrostatic forces) e) Mechanical interlocking (irregular particle size and

size distribution)

Deformation of the Solid Body

As pressure increases, the bonded solid is consolidated toward a limiting density by plastic and/or elastic deformation.

Recovery:1.The compact is ejected, allowing radial and

axial recovery. 2. Elastic character tends to revert the

compact to its original shape.

Compaction data analysis

The parameters monitored during compaction vary widely in these studies.

Various parameters have been used to assess the compaction behavior of a variety of pharmaceutical powders and formulations Forces on the punches displacement of the upper and lower punches, axial to radial load transmission, die wall friction, ejection force, temperature changes

Resulting data may be expressed equivalently in term of stress-strain, pressure-volume or pressure –density since the natural strain, for example, is equal to the natural log of the ratio of the initial bed height or volume to the current height or volume respectively

A compaction equation relates some measure of the state of consolidation of a powder, such as porosity, volume (or relative volume), density or void ratio, with a function of the compaction pressure.

Many compaction equations have been proposed like; Heckel , Kawakita and Adams have been validated for pharmaceutical systems.

However, it is highly unlikely that a single compaction equation will fit all the compaction mechanisms.

In interpreting compaction curves, it is therefore essential to know which mechanisms are operating, or not, over different region of pressure.

A good compaction curve should be able to indicate changes in the compression mechanism

The ideal requirements for a compression / compaction equation

The model should cover the whole range of densification with sufficient accuracy.

The parameters should be related to physical relevant properties of the powder.

The parameters should be sensitive to changes in formulation and experimental variables and insensitive or at least proportional to minor changes in normalisation factors like density or initial volume.

The model and its parameters should be easily estimated by general available computer programs.

The model should significantly differentiate between powders and dissimilar compression characteristics.

The quality of the model should be evaluated by a combination of the range of densification covered and the goodness-of-fit to the observed data.

Heckel equation Powder packing with increasing compression

load is normally attributed to particle rearrangement, elastic and plastic deformation and particle fragmentation

The Heckel analysis is a popular method of determining the volume reduction mechanism under the compression force

Based on the assumption that powder compression follows first order kinetics with the interparticulate pores as the reactants and the densification of the powder as the product.

According to the analysis, the degree of compact densification with increasing compression pressure is directly proportional to the porosity as follows:

                            dρ R / dP    =    k E    

where ρR is the relative density at pressure P, and E is the porosity.The relative density is defined as the ratio of the density of the compact at pressure, P, to the density of the compact at zero void or true density of the material The porosity can also be defined as:                             E =(Vp –V)/V p= 1 - ρ R                

where V p and V are the volume at any applied load and the volume at theoretical zero porosity, respectively. Thus, equation dρR/dP= kE  can be expressed as:

                           dρR /dP= k( 1-ρ R )                            

and then transformed to:  In [1/(1-ρR)]= kP+A   i.e  (y = mx +c)

Plotting the value of In [1/(1-ρR)] against applied pressure, P, yields a linear graph having slope, k and intercept, A.                         

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 multi-component systems.

The intercept of the extrapolated linear region, A, is a function of the original compact volume.

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  

Heckel equation

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-ρR)]=kP + A 

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.

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

In [1/(1-ρR)]=kP + A 

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.

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

In [1/(1-ρR)]=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

Compaction profiles (Force-time, �Displacement-time)

Tablet expansion� Pressure-Volume relationships� Pressure transmission� Energy of Compaction� Radial vs Axial Force� Acoustics� Temperature�