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 Waals forces. dipole-dipole (Keesom),dipole- induced dipole (Debye) and induced dipole-induced dipole (London) forces.
Derived properties of powdered solids 1. The solid-air interface 4. Mass-volume relationships 2. Angle of repose 5. Density 3. Flow rates AngleDensity Mass-volumeRates of relationships The solid-air interface FlowreposeHeliumTan-1VOLUME 1. θ = Pycnometer 2. θ = cos D/ Liquid displacement method Compressibility indexdensityPOROSITY -1 of (l1+l2) (h/r)VCOHESION: Methods topossible Angle gravity bottle used.Flow = V /U maximum angle measure here, The -U x[U types of Different-U ] : Consolidation/Repose t here, c 1 2 1 s Pycnometer or specific Truevolume betweent)freeparticle. Carr’sVIndex(% ) of) base-w )/(w -w ) volume and index) (V (Carrs consolidationlike standing True density= w /(w -w = (w a. Attraction of sample Vt = Fixedheight of pile true = funnel E = V / Vb between the surface of pile , D = diameter h 3 4 2 2 1 4 2Vc=true= radius stainless base)of the pile w = wt. ofl1Pycnometer opposite sides of pile Experienced by]x100 spheres I r=volume of of the (V cone[1-v/vo particles method. Granule volumesteel g in bulk. here 1 5-15 +l2 = the Excellent ofθ the powder and the w = Wt. of Pycnometer + sample or here, =density: b) ρt=M/vt VV =Wt. of Pycnometer with powder glass beads angle of repose 2 Bulk volume (VTrue box method. U1=Volume of empty cell b. Tilting Void volume w 12-16 Wall is linedfilled with & ADHESION: plane.FLOW horizontal U1-U2=Volume occupied by the std. sample 4= VSTATICvolume KINETIC/DYNAMICGood by sandpaper b = Bulk A.R. Relative volume unlike particle. V = Tapped Volume ANGLE OF REPOSE c. Attraction between (Vr) solvent Revolving cylinder method. U1-Us = volume occupied by (Ø) ρGranule density:sample g=M/vg noww2-w1 Wt. of sample Fair To Passable w3 , = = A.R. V0= Volumebytparticles at surface. w*18-21 Vr = V/ V Experienced before tapping 4-w2 Volume of liquid displaced by the solid = Vb –V = Voidangle of (VV)It is anglet of repose deter- volume Resistance to movement ρbparticles It isBulk density:< 25 of =M/v EXCELLENT b *23-35 Poor Therefore, Porosity (E) =(Vb–Vt3rdVb repose mined by the )/ methodVr tends to become unity factors:- is affected by two as all air iseliminated25-30 the mass GOODtheρ/ ρtPorosity when expressed as percentage relative density: duringr= from ρ determined by It is preferred since they a. Electrostatic forces. compression process st 33-38 1 two methods most closelyPoor the Very mimic *30-40 PASSABLE E =100.[(Vb–Vt)/ Vb] b. Adsorbed layer of moisture on (a. & b.) manufacturing situation in >40 VERY POOR >40 Very Very Poor particles. Tapped which powder is in motion. density-tester Specific gravity bottle
Powder compressionCOMPRESSION:The reduction in the bulk volume of a material as a resultof the removal of the gaseous phase (air) by appliedpressure.„ CONSOLIDATION:Involves an increase in the mechanical strength of amaterial resulting from particle-particle interactions.„ COMPACTION:The compression and consolidation of a 2 phase (solid +gas) system due to an applied force.
CompressionWhen 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 Elastic e.g., acetyl salicylic acid, MCC Depends on density and packing characteristics of deformation powder - when elastic limit or yield point is reached. Plastic deformationMicrosquasing: Irrespective of the behavior of larger particles smaller particles may deform plastically.
Stages involved in compression1. 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. that areof the load, the deformation reversible on the the like On removal related to immediatelycharacteristicsbehaves Deformation not the packing is reversible - it of removal rubber of the applied force. particles, the density of the particles and the total mass of 2. the material elastic deformation whendie All solids undergo that materials in which the shear strength is Predominant in is filled into the subjected to external forces. 2. External forcetensile or breaking strength. closer packing less than the - reduction in volume due to Some materials, e.g. paracetamol, are elastic and There is very little 3. of the powder(eitherthe greatest mechanismclean surfaces permanent change particles- main number of of caused by Believed to create plastic flow or fragmentation) initial 4. volume deformation is a time dependent process, higher Plastic reduction compression: 3. As the load increases, rearrangementformation ofbecomes leads to the of particles less rate of force application elastically) When the compression load The material rebounds (recovers is released. If bonding is weak the compact will self-destruct some type more clean surfaces - weaker tablets. leads to and the top new difficult and further compression will detach (capping) Else, whole cylinder cracks into horizontal layers 5. of particle deformation is dependent on the formation of Since tablet formation (lamination). new clean surfaces, high concentration or over mixing of Elastic materials require a particularly plastic tableting matrix or wet materials that form weak bonds result in weak tablets massing to induce plasticity. 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 interactionsVarious Hypothesis: During compression, the powder compact typically undergoes a If this heatincrease usually isthe local rise in temperature temperature is dissipated,betweenWhen the sufficientthetwo particles4 approach each area of Any applied load to of cause melting andthe C surfaces to bed transmitted 30 contact other could be on of through particle Depends contacts.closely enough (e.g. at a separation of less than 50nm), theFriction effects particlestheirMaterial characteristics, free surface energies result in a strong attractiveforceLubricationa process known as cold welding .result in Under appreciable forces, this transmission may When the melt solidifies, fusion bonding occurs, which in through efficiency the generationin an increase in the mechanical strength of results of considerable frictional heat. turn This hypothesisapplication of compression forces Magnitude and rate of is favoured as a major reason for theMachine speed mass.the increasing mechanical strength of a bed of powder As the tablet temperature rises, stress relaxation and plasticitywhen 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 Mechanismsa) As Occurs at low a result the bonded solid is consolidated MajorThe compact pressures.melting, crystallization, axial 1.1. deformation increases, of allowing radial and sintering, Solid bridges (as pressure is ejected, Material chemical reaction, and binder hardening) mechanism(s) 2. Reduction in the relativeby plastic and/or elastic toward a limiting density volume of powder bed. recovery. Ascorbic acid, Dicalcium Fragmentation as a result of movable liquids (capillary and surfaceb)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 Fragmentation and relative particle movement becomes adsorption layers) elastic impossible, inducing deformation deformationd) Attraction betweenmonohydrate, Fragmentation and Lactose solid particles (molecular and electrostatic plastic deformation Microcrystalline cellulose forces) Plastic deformation NaHCO3, NaCL, Pree) Mechanical interlocking (irregular particle size and size gelatinized starch distribution) Starch Elastic deformation
Compaction data analysis The ideal requirements for a compression / compaction equation The compaction monitored relates compaction of densification these A The parameters equation model should cover during some measure of the state of the whole range vary widely in with studies. accuracy.a powder, such as porosity, volume (or relative consolidation of 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 , Kawakita and of the upperandbeen validated at least proportional to minor displacement Adams have lower punches, for pharmaceutical systems. experimental variables and insensitive or axial to radial load transmission, changes in normalisation factors like density or initial volume. However,friction, die wall it is highly unlikely that a single compaction equation will fit 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- which mechanisms are operating, or not, over different region of dissimilar compression characteristics. strain, pressure-volume or pressure –density since the natural pressure. strain, for example, model should be evaluatedof thecombinationinitial The quality of the is equal to the natural log by a ratio of the of the bed height or volume tocovered and height orto indicate to the observed range of compaction curve current be able volume respectively in the A good densification the should the goodness-of-fit changes data. compression mechanism
Heckel equationwhere ρR 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 materialThe 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 - ρ R force where V p and V are the volume at any applied load and the volume attheoretical zero porosity, respectively. powder compression follows Based on the assumption that Thus, equationkinetics kE cantheexpressed as: first order dρR/dP= with be interparticulate pores as the reactants andR /dP= k( 1-ρ R ) dρ the densification of the powder as the product. and then transformed to: According to [1/(1-ρR)]= kP+A i.e (y = mx +c) In the analysis, the degree of compact Plotting the value of In [1/(1-ρR)] 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: dρ 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 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
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. In [1/(1-ρR)]=kP + A 4. This is in turn influenced by the size distribution, shape, e. t. c., of the original particles.
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 In [1/(1-ρR)]=kP + A typical example of such materials.
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-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 equationThe Kawakita equation was developed to study powdercompression using the degree of volume reduction, C, aparameter equivalent to the engineering strain of theparticle bed C =(V0-Vp)/V0=abP/(1+bP)can be rearranged to give: P/C=P/a+1/abWhere,C is the degree of volume reduction,V 0 is the initial volume of the powder bed andV p is the powder volume after compression;a and b are constants which are obtained from the slope and intercept of theP/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
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