Compaction and Compression


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Compaction and Compression :

Compaction and Compression BY :- Mr. Nishant V.Chandak M.Pharm P’ceutics 2 nd Semester Department of Pharmaceutics , ACP, Chikhli, Maharashtra .



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Compaction means some level of mechanical force applied over the powdered solids. Hence, Compaction can be defined as the compression, consolidation of two phases (particulate solid – gas) system due to the applied forces. It is also known as Physics of Compaction. Compression is the reduction in the bulk volume of material due to the displacement of the gaseous phase. Consolidation is the increase in the mechanical strength of the material due to particle – particle interaction. While studying the physics of compaction, we have to consider certain factors such as angle of repose, density, flow rate, Carr’s index, etc; of the material. INTRODUCTION

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Process of Compression In pharmaceutical tableting, an appropriate volume of granules in die cavity is compressed between an upper and lower punch to consolidate the material into a single solid matrix, which is finally ejected from the die cavity as a tablet. Steps involved in process of Compression Transitional Repacking/Particle Rearrangement. Deformation at Point of Contact. Fragmentation. Bonding. Decompression. Ejection.

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Initially Repacking Deformation

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Decompression Fig :- Effect of Compression Elastic Plastic

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Transitional Repacking At initial stage of compression the particles are subjected to the low pressure, during this particle moves with respect to each other. Small particles enters void space between the larger particles. Thus volume decreases and density increases. Spherical particle under goes lesser rearrangement than irregular particles. Deformation at Point of Contact When the particles of granules are so closely packed no further filling of voids can occur increase in the compressional force causes Deformation at the point of contact . If the deformation disappears completely upon release of the stress it is said to be Elastic deformation . A deformation that does not recover completely after release of stress it is known as Plastic Deformation . The force required to initiate plastic deformation is known as Yield stress .

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Fragmentation As the compressional forces increases, the deformed particles starts undergoing Fragmentation because of high load, the particles breaks into smaller fragments leading to formation of new bonding areas. The fragments undergo densification with infiltration of small fragments into voids. Some materials undergo structural breakdown called as Brittle Fracture . Bonding Cold Welding • When particles approach to each other close enough at a separation of 50 µm the unsatisfied forces present on their surfaces lead to formation of strong attractive forces / bond. This process is known as cold welding . Fusion Bonding • In the powder mass, particles are irregular in shape and size, any applied force to mass must pass through this bed of particles.

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• The transmission may result in the generation of heat. If this heat does not disappear, the raise in temp may cause melting of contact points. • Thus, melt is cooled and solidifies give rise to Fusion bonding and increase in mechanical strength. Factors affecting the Bonding • Chemical nature of material. • Available surface. • Presence of surface contaminants. • The interface distance.

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Decompression The success or failure of intact tablet depends upon the stress induced by elastic rebound and the associated deformation produced during decompression and ejection. Any dimensional change during the decompression must occur in the axial direction. The capping may be result of unaxial relaxation in the die cavity. If decompression occur simultaneously in all direction, capping may be reduced or diminished. Ejection As the lower punch rises and pushed the tablet upward, there is continuous residual die wall pressure and energy may be expanded due to die wall friction. During ejection that portion of tablet within die is under strain, so if exceeds the shear strength of tablet, the tablet caps to the adjacent region in which the strain has been removed.

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Collar locker Single Punch Machine (Tablets) Upper and Lower Collar

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Various Forces Involved in Compaction Frictional Forces ● Interparticulate ● Die-wall Distribution Forces Radial Forces Ejection Forces

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Frictional Forces The forces which are produced due to friction are called as frictional forces. ● Interparticulate frictional forces • The forces which arises at particle/particle contacts are of this type. • Denoted by coefficient of Interparticulate friction µ i . • It is more significant at low applied loads. • Materials used to reduced this effect are referred to as glidants. e.g. colloidal silica. ● Die-wall frictional forces • This results from material being pressed against the diewall & moved down it. • Denoted as coefficient of die wall friction; µ w . • It is dominant at high applied forces. • Reduced by adding additives called as lubricants. e.g. magnesium stearate.

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Distribution Forces Most investigation of the fundamentals of tableting are carried out on single station presses. The force exerted by the upper punch diminishes exponentially at increasing depths below it. The following relationship apply F A = F L + F D Where, F A – Force applied to upper punch . F L – Force transformed to lower punch. F D – Reaction at the die wall to friction Mean Compaction Force ; Fm Fm = (F A + F L ) ∕ 2

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Radial Force As the compression force is increased & any repacking of mass is completed compression force applied in vertical direction results in decrease in height ∆H. In case of unconfined body expansion occurs in horizontal direction ∆D occurs. The ratio of two dimensional changes is known as Poisson’s ratio, λ = ∆D∕ ∆H But the material is not free to expand in the horizontal plane because it is confined in the die cavity. Consequently, a radial die wall force F R develops perpendicular to the die wall surface. Materials with larger Poison ratio gives higher value of F R .

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Ejection Forces Radial die wall force & die wall friction also affect the ease with which the compressed tablet can be removed from the die. The force necessary to eject the tablet is called as Ejection Force F E . It occurs in 3 stages ● First stage involve distinctive peak force required to initiate ejection by breaking tablet ∕die wall adhesion. ● Second smaller force required to push the tablet up the die wall. ● The final stage marked by declining force of ejection as tablet emerges from the die. Variation may occur due to inadequate lubrication or “slip-stick” condition between the tablet & die wall giving abnormal ejection force & failure of tablet structure.

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F E F A F D F R D H H 0 F L Various Forces in Compaction In this ; F A Force applied to upper punch. F L Force transmitted to lower punch. F D Reaction at Diewall due to friction at surface. F R Radial diewall force. F E Force of ejection. D Diameter. H o Initial height. H Height after compression. Fig :- Various forces in compaction

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Conclusion The process of tableting by the compression involves the application of massive compression force which causes deformation in the solid particles. These forces are large enough to exceed the elastic limit of the solids. Plastic deformation and brittle fracture results in formation of new, clean surface ; which when pressed on each another undergoes cold welding. When the compaction force reaches its maximum ; there is formation of a bulk solid structure with overall strength which we called as a Tablet.

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REFERENCE :- Theory and practice of Industrial Pharmacy by Leon Lachman Edition ,page no.66-97.

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