PHYSICS OF TABLET COMPRESSION
Compression: compression of a powder means reduction in the bulk volume of a material as a result of displacement of the gaseous phase under pressure.
Compaction: compaction of a powder is a general term used to describe a situation in which powdered material is subjected to some level of mechanical force. The physics of compaction may be simply stated as the compression & consolidation of two phase (particulate solid & gas) systems due to the applied force.
Consolidation: consolidation is an increase of a material resulting particle-particle interaction.
Decompression, in tablet manufacturing:
During tablet manufacturing the compression process is followed by a decompression stage, as the applied force is removed. This leads a new set of stresses within the tablets as a result of elastic recovery which is augmented [increased] by the forces necessary to eject the tablet from the die.
Deformation: change of geometry of a solid when it is subjected to opposing forces. The amount of deformation is called strain.
The process of compression: in pharmaceutical tablet manufacturing an appropriate volume of granules in a die cavity is compressed between an upper & lower punch to consolidate the material into a single solid matrix which is subsequently ejected from the die cavity as an intact tablet.
The events that occur in the process of compression:
[a] Transitional repacking.
[b] Deformation at point of contact.
[c] Fragmentation &/or deformation.
[e] Deformation of the solid body.
[a] Transitional repacking :
The particle size distribution of the granulation & the shape of the granules determine the initial packing [bulk density] as the granulation movement occurs at lower pressure. The granules flow with respect to each other with the finer particles entering the void between the larger particles & the bulk density of the granulation is increased. Spherical particles undergo less particle rearrangement then irregular particles, as the spherical particles tend to assume a close packing arrangement initially.
[b] Deformation at the point of contact:
when the particles of the granulation are so closely packed that no further filling of the void can occur, a further increase of compression force causes deformation at the point of contact.
Elastic deformation: if the deformation is disappear completely [returns to the original shape upon release of stress, it is an elastic deformation. plastic deformation: a deformation that doesn’t completely recover after release of the stress is known as a plastic deformation.
Yield stress: the force required to initiate a plastic deformation is known as yield stress.
[c] Fragmentation & deformation:
At higher pressure, fracture occurs when the stress within the particle become great enough to propagate cracks. Fragmentation cause furthers densification with the infiltration of the smaller fragments into the void space. With some materials fragmentation doesn’t occur because the stresses are released by plastic deformation. Plastic deformation may be thought of a change in particle shape & as the sliding of groups of particles in an attempt to release the stress (visco-elastic flow). Such deformation produces new clean surfaces that are potential areas.
Bonding: several mechanism of bonding in the compression process has been conceived but they have not been substantiated by experimentation & have not been useful in the prediction of the compressional properties of the materials.
Three theories are
* The mechanical theory.
* The intermolecular theory.
* The liquid surface film theory.
[e] Deformation of the solid body:
As the applied pressure is further increased the bonded solid is consolidated toward a limiting density by plastic &/or elastic deformation of the tablet within the die
The success or failure to produce an intact tablet depends on the stresses induced by elastic rebound & the associated deformation processes during decompression & ejection.
As the upper punch is withdraw from the die cavity the tablet is confined in the die by a radial pressure. Consequently any dimensional change during decompression must occur in the axial direction.
As lower punch rises & pushes the tablet upward there is a continued residual die wall friction. As the tablet is removed from the die the lateral pressure is relived & the tablet undergoes elastic recovery with an increased (2-10 %) in the volume of that portion of the tablet removed from the die.