compression and compaction

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COMPRESSION AND COMPACTION:

COMPRESSION AND COMPACTION BY AKSHAY MUNDHE 1

PHYSICS OF TABLET:

PHYSICS OF TABLET It is also known as Physics of Compaction . It may be stated as Compression Consolidation of two phases (Solid-Gas) system due to applied force. Compression is the reduction in bulk volume of material due to the displacement of gaseous phase. Consolidation is the increase in the mechanical strength of material due to particle- particle interaction. While studying the Physics of Compaction, we have to consider certain factors such as angle of repose, bulk density, flow rate, Hausner’s ratio & Carr’s index of the material :- .

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Process of Compression :- In pharmaceutical tableting, an appropriate volume of granules in die cavity is compressed between an upper & lower punch to consolidate the material into a single solid matrix, which is finally ejected from the die cavity as a Tablet. The various events that occurs in the process of compression are, Transitional Repacking or Particle Rearrangement Deformation at point of contact Fragmentation Bonding Decompression Ejection

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Transitional Repacking or Particle Rearrangement :- The particle size distribution determination initial repacking. During initial stage of compression the particle are subjected to the low pressure, during this particle moves with respect to each other & smaller particles enter voids between larger particles, As a result the volume decreases & density increases. Spherical particles undergo lesser rearrangement than irregular particles. .

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Deformation at the point of Contact :- When a force (stress) is applied on a material, deformation occurs. If the deformation disappears occurs completely (return to original shape) upon release of the stress, it is said to be Elastic Deformation. A deformation that does not completely recover completely after release of stress it is known as Plastic Deformation. The force required to initiate plastic deformation is known as Yield Stress. When the particles of granulation are so closely packed that no further filling of voids can occur. A further increase of compressional force causes deformation at the point of contact

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Fragmentation :- As the compressional force increases, the deformed particles starts undergoing Fragmentation because of high load, the particles breaks into smaller fragments leading to formation new bonding areas. The fragments undergo densification with infiltration of small fragments into voids. In some materials where Shear Stress is greater than Tensile Stress, then the particle undergoes structural breakdown called as Brittle Fractur e.

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Bonding :- Cold Welding :- When particles approach each other close enough (At a separation of 50µm) 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 & size, any applied force to mass must pass through this bed of particles. The transmission may result in generation of heat. If this heat doesn’t disappear, the raise in the temperature could be sufficient to cause melting of contact points. This melt is cooled & solidifies that gives rise to fusion bonding.

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v) Decompression :- The success or failure of intact tablet depends on the stress induced by Elastic Rebound & the associated deformation produced during decompression & ejection. As the upper punch is withdrawn from the die. The tablet is confined in the die cavity by radial pressure. Consequently any dimensional change during decompression must occur in the axial direction. Thus capping is due to unaxial relaxation in the die cavity at the point where the punch pressure is released & some may occur at ejection. If decompression occurs simultaneously in all directions, capping is reduced or diminished. vi) Ejection :- As the lower punch rises & pushed the tablet upward, there is continues residual die wall pressure & energy may be expanded due to die wall friction. As the tablet is removed from the die, the lateral pressure is relieved & the tablet undergoes elastic recovery with increase (2-10%) in the volume of that portion of tablet removed from the die. During ejection that portion of tablet within die is under strain, so if exceeds the shear strength of tablet, the tablet caps adjustment to the region in which the strain has been removed.

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Various Forces involved in the Compression Frictional Forces Distribution Forces Radial Forces Ejection Forces

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) Frictional Forces :- Frictional forces are interparticulate friction & die-wall friction. Interparticulate friction forces occur due to particle-particle contacts & 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 the material pressed against the die-wall & moved down. It is dominant at high applied load. These forces are reduced by using Lubricants . E.g.- Magnesium Stearate.

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B) Distribution Forces :- Most investigations of Fundamentals of Tableting have been carried out on Single Punch Presses or even on Isolated Dies & Punches with Hydraulic Press. A force is applied on the top of cylinder powder mass, consider 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 Because of difference between applied force at upper punch which affects the material close to lower punch is called as Mean Compaction Force (F M ). F M = F A + F L /2

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D) Radial Force :- As compressional force is increased any repacking of tableting is completed. When force is applied on vertical direction which results in the 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 (λ) of material. λ = ∆D/∆H The material is not free to expand in horizontal plane because it is confined in the die, at the same time a radial die-wall force (F R ) develops perpendicular to die-wall surface. Higher Poisson’s Ratio (λ) higher F R values .

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

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