Compaction and Compression


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Compaction and Compression various forces involved, compression cycle, properties of materials, powders, various equations.


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A Seminar On “Compression And Compaction” :

A Seminar On “Compression And Compaction” PRESENTED BY Mr. Sudip M. Mulik M.PHARM .I (PHARMACEUTICS) Under The Guidence of Prof. Mr. K. K. Mali FACULTY OF PHARMACY YSPM’S-YTC WADHE, SATARA .




INTRODUCTION COMPRESSION Reduction in bulk volume of material due to displacement of gaseous phase. CONSOLIDATION Increase in mechanical strength of material due to particle-particle interaction. COMPACTION It is the compression & consolidation of two phases (solid & gas) system due to applied force.

Powder compression:

Powder compression It is defined as the reduction in volume of a powder owing to the application of a forces. Because of the increased proximity of particle surfaces accomplished during compression, bonds are formed between particles which provide coherence to the powder i.e. compact is formed.

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Single Punch Machine (Tablets)

Effect of Compression :

Effect of Compression When external mechanical forces applied to a powder mass there is reduction in bulk volume as follows Repacking Particles deformation Elastic deformation-e.g. acetyl salicylic acid, MCC Plastic deformation-at yield point of elastic. Brittle fracture – e.g. sucrose Micro quashing - irrespective of larger particles, smaller particles may deform plastically.


Difference Compaction Consolidation It is defined as the formation of solid specimen of defined geometry by powder compression . The compression takes place in a die by the action of two punches, the lower and the upper by which compression force is applied. It is increase in mechanical strength of material from particle-particle interactions.


PROPERTIES OF POWDER / GRANULES Volume True volume (Vi) Bulk volume (Vb) Granular volume (Vg) Density True density Bulk density Granular density 8


PROPERTIES OF POWDER / GRANULES cont.. Porosity (E) Void volume Void volume = Bulk Vol – True Vol E = Void volume / Bulk Vol = V / Vb The relation bet compression & porosity is important because it determines rate of dissolution, disintegration & absorption.


10 PROPERTIES OF POWDER / GRANULES cont.. Particle shape : Spherical smooth particles increases flow properties Density/Porosity : High density = Low porosity = Good flow Moisture High moisture = cohesion, adhesion = poor flow


DETERMINATION OF ANGLE OF REPOSE Angle of repose (in degrees) Type of flow Less than 25 0 Excellent 25-30 0 Good 30-40 0 Passable More than 40 0 Very poor Angle of repose = tan -1 (h/r) 11


CARRS INDEX Carr’s consolidation Index consolidation Index = Tap density – Bulk density/Bulk density Carr’s Index Flow 5-15 Excellent 12-16 Good 18-21 Pair to passable 23-35 Poor 33-38 Very poor More than 40 Very poor Very poor 12


COMPRESSION CYCLE Process of compression Appropriate volume of granules in die cavity is compressed between upper & lower punch to consolidate material into single solid matrix which is finally ejected from die cavity as tablet. 13


14 COMPRESSION CYCLE EVENTS Transitional repacking /Particle rearrangement Deformation at point of contact Fragmentation Bonding Decompression Ejection

Transitional repacking /Particle rearrangement:

Transitional repacking /Particle rearrangement During initial stage of compression particles are subjected to low pressure during this particles moves with respect to each other . Smaller particles enter voids bet larger particles as a result volume decreases & density increases. Spherical particles undergo lesser rearrangement than irregular particles. 15

Deformation at point of contact:

Deformation at point of contact Elastic : When force is applied deformation occurs & deformation disappears upon release of stress. Plastic : The deformation which does not completely recover after removal of stress. Yield strength : Force required to initiate plastic deformation


17 Fragmentation As compression force increases deformed particles start undergoing fragmentation due to high load particles breaks in to smaller fragments leading to formation of new bonding areas . The fragments undergo densification with infiltration of small fragments in to voids. Some particles undergo structural break down called as brittle fracture.


BONDING Cold welding : When particles approach each other close enough (at the distance of 50µm) the unsatisfied forces present on their surface lead to formation of strong attractive forces/bonding Formation of strong attractive forces. This is called as cold welding. Fusion bonding : In the powder mass particles are in irregular in shape ,size & applied force to mass must pass through this bed of particles this transmission may lead to generation of heat .If this heat doesn’t disappear the raise in temperature could be sufficient to cause melting of contact points. This melt is cooled & solidifies that gives rise to fusion bonding.


Decompression The success & failure of intact tablet depends on stress induced by elastic rebound & the associated deformation produced during compression & ejection . As the upper punch is withdrawn from the die the tablet is confined in die cavity by radial pressure consequently any radial change during decompression must occur in axial direction. Thus capping is due to unaxial relaxation in die cavity at the point where punch pressure is released & some may occur at ejection. If decompression occurs in all directions simultaneously capping is reduced. 19


EJECTION As lower punch rises & push tablet upward there is continuous residual die wall pressure & energy may be expanded due to die wall friction . As the tablet is removed from die lateral pressure is relieved & tablet undergoes elastic recovery with increase (2-10%) in the volume of that portion of the tablet removed from the die. During ejection that portion of the tablet within die is under strain so if exceeds the shear strength of the tablet, the tablet caps adjustment to the region in which the strain has been removed .

Various forces involved in the Compression:

Various forces involved in the Compression Frictional force Distributional force Radial force Ejectional force

Frictional force:

Frictional force Frictional forces are interparticulate friction & die wall friction. Interparticulate friction forces occur due to particle-particle contact & it is more significant at low applied load . These forces are reduced by using glidants e.g. colloidal silica . Die wall friction forces occur from material pressed against die wall & moved it is dominant at high applied load These forces are reduced using lubricants e.g. magnesium stearate.

Distribution Forces:

Distribution Forces Most investigations of fundamentals of tableting have been carried out on single punch press or even isolated dies & punches with hydraulic press A force is applied on top of cylinder of powder mass consicder single isolated punch F A = Force applied to upper punch F L = Force transmitted to lower punch F D = Reaction at die wall due to friction at surface F A = F L + F D.

Compaction Force:

Compaction Force Because of difference between applied force at upper punch which affects material close to lower punch is called as Mean Compaction Force (F M ). F M = F A + F L /2 Radial Force As compressional force is increased any repacking of tableting is completed.


POISSON’S RATIO OF MATERIAL When force is applied on vertical direction which result in decrease in height (∆H) for unconfined solid body. The expansion is in horizontal direction (∆D). This ratio of two dimensional changes is known as Poisson’s ratio ( ג ) of material. ג D = ∆D / ∆H The material is not free to expand in horizontal plane because it is confined in die at the same time a radial die wall force (F R ) develops perpendicular to die wall surface. Higher Poisson's ratio ( ג ) higher F R value.

Ejection Force:

Ejection Force Radial die wall forces & die wall friction also affects ejection of the compressed tablet from the die. The force necessary to eject the finished tablet is known as ejection force. This force can eject tablet by breaking tablet/die wall adhesion. Variation also occurs in ejection force when lubrication is inadequate.

Heckel equation:

Heckel equation Heckel plot is density Vs applied pressure. Follows first order kinetics. As the porosity increases the compression force will increase. The Heckel equation is described as follows. It is based on the assumption that powder compression follows first-order kinetics, with the interparticulate pores as the reactant and the densification of the powder bed as the product. Where D= relative density of a powder P=compact at pressure P. Constant k = measure of the plasticity of a compressed material. Constant A =die filling and particle rearrangement before deformation and bonding of the discrete particles. Thus, a Heckel plot allows for the interpretation of the mechanism of bonding.

Kawakita equation:

Kawakita equation The Kawakita equation is described as follows. This equation describes the relationship between the degree of volume reduction of the powder column and the applied pressure. The basis for the Kawakita equation for powder compression is that particles subjected to a compressive load in a confined space are viewed as a system in equilibrium at all stages of compression, so that the product of the pressure term and the volume term is a constant. where C = degree of volume reduction of a powder compact at pressure P. constants (a and b) =evaluated from a plot of P/C versus P. a= total volume reduction for the powder bed [carr’s index] b= constant that is inversely related to the yield strength of the particles. The data from this study were modeled via the Kawakita equation in an attempt to evaluate the relationship between the volume reduction and applied pressure for each studied DC binder.


CONCLUSION Compression & Consolidation are important in tableting of materials. The importance of each will largely depend on the type of compact required whether soft or hard and on the brittle properties of the materials . Various mathematical equations have been used to describe the compaction process.


CONCLUSION CONT.. The particular value of heckel plots arises from their ability to identify the predominant form of deformation in a given sample. Kawakita equation is modified form of heckel’s equation.


REFERENCES The Theory and Practice of Industrial pharmacy Leon Lachman, Herbert E. Lieberman, Joseph L. Kanic Special Indian Edition 2009 Page no.71-83. Aulton’s Pharmaceutics The design and Manufacture of Medicines Michael E. Aulton Third Edition Page no.478,443,468-473,355-358.

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