Physics of tablet compression

Physics of compaction & compression

1. PHYSICS OF TABLET COMPRESSION Presented By: Mahewash A Pathan

2. Contents: Introduction Compaction Compression Consolidation Stages of compression Forces involved in compression Compaction profiles Compaction equations Conclusion references 2

3. INTRODUCTION Tablets constitutes about 70-80% of all pharmaceutical dosage forms. Processes: wet granulation Dry granulation Direct compression Compaction represents one of the most important unit operations in the pharmaceutical industry. compaction is the situation in which the materials are subjected to some level of mechanical forces. The physics of compaction may be stated as the compression & consolidation of two phase system due to the applied force. Invention of tabletting machine : in 1843 by William Brockedon . 3

4. Properties of powders: Surface properties Porosity Flow properties: Angle of repose Carr’s index 4

5. Compression: Compression means a reduction in the bulk volume of a material as a result of the removal of the gaseous phase(air) by applied pressure. Consolidation : Consolidation is defined as an increase in the mechanical strength of a material resulting from particle-particle interactions. 5

6. Compaction: Compaction of powders is the term used to describe the situation in which the materials are subjected to some level of mechanical forces. 6

7. Compression When some external forces applied on the powder reduction in the bulk volume of the powder. 7

8. Stages involved are: Transitional repacking/ Particle rearrangement Deformation at points of contact Fragmentation Bonding Removal of pressure Deformation of solid bonding Decompression ejection 8

9. 1.Particle Rearrangement Particles are rearranged under compaction pressure to form a closer packing structure. The finer particles enter the voids between the larger ones & give a closer packing arrangement. The energy is evolved as a result of interparticulate friction & there is an increase in amount of particle surface area capable of forming interparticulate bonds. 9

10. 2 . Deformation : 3.Fragmentation: 10

11. 4. Bonding: After fragmentation of particles, as the pressure increases, formation of new bonds between the particles at the contact area occurs. There are 3 theories about bonding of particles in the tablet by compression Mechanical theory Intermolecular force theory Liquid surface film theory 11

12. 5. Deformation: Change in geometry of solid body. Deformation produced by force of Tensile strain, Compressive strain ,Shear strain. Two types of deformations : 1. Elastic deformation Ex:- Acetyl salicylic acid, MCC. 2. Plastic deformation Ex:- sucrose 12

13. 6. 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. 13

14. 7. Ejection 14

15. Forces involved in compression Frictional forces Interparticulate friction; reduced by adding glidants. Die wall friction; reduced by adding lubricants. Radial forces Ejection force; the force necessary to eject a finished tablet. Distribution forces 15

16. Consolidation: Increase in mechanical strength of the mass. Consolidation process- Cold welding : When surface of two particles approach each other (<50 nm), their free surface energies result in a strong attractive forces. Fusion bonding : contacts of particles at multiple points upon application of load, produces heat which causes fusion or melting. Upon removal of load it gets solidified giving rise to fusion bonding & increase in mechanical strength. 16

17. Compaction profiles Force-Time profile: Compression phase, dwell phase, decompression phase. Consolidation time Dwell time Contact time Peak offset time (toff) 17

18. 2. Force-displacement profile: NWC = GWC – WER GWC = Wf + Wp + We Work of elastic relaxation=WER net work of compaction=WOC Wf=work of fragmentation Wp=work of plastic deformation We= work of elastic deformation GWC= gross work of compaction WER= work of elastic relaxati on 18

19. Compaction equations 1. Kawatika equation: particles are subjected to compressive load is equilibrium at all stages of compression, so that the product of pressure term and volume term is constant. The Kawatika equation is- P / C = P/a + 1/ ab Where, P = Applied pressure C= degree of volume reduction of a powder compact. a = total volume reduction for the powder bed ( Carr’s index) b = constant that is inversely related to the yield strength of the particles. This equation holds best for soft fluffy pharmaceutical powders, and is best used for low pressures and high porosity situations 19

20. 2. Heckel equation: based on the assumption that densification of the bulk powder under force follows first-order kinetics. The Heckel equation is expressed as ln [1/1– D ] = KP + A where, D = relative density of the powder P = applied pressure k = constant, measure of the plasticity of a compressed material A = constant, die filling & particle rearrangement before deformation & bonding of the discrete particles. 20

21. In 1961, Heckel proposed a relationship between the constant K and the yield strength for a range of metal powders. K = 1/3 σ where, σ is the yield strength of the material. K is inversely related to the ability of the material to deform plastically. 21

22. 3. Walker Equation: The Walker equation is based on the assumption that the rate of change of pressure with respect to volume is proportional to the pressure, thus giving a differential equation Log P = – L x V ′ / V0 + C 1 where, V0 is the volume at zero porosity. The relative volume is V ′/ V0 = V =1/ D, C 1 is constant. The coefficient L is referred to as the pressing modulus.   22

23. 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 & on the brittle properties of the materials. Various mathematical equations have been used to describe the compaction process. The particular value of heckel plot arises from their ability to identify the predominant form of deformation in a given sample. Kawakita equation is modified form of heckel’s equation. 23

24. References: Sarsvat Patel and Arvind Bansal Compression Physics in the Formulation Development of Tablets, Critical Reviews in Therapeutic Drug Carrier Systems, February 2006. Lachman , L. liberman , H. A. and kanig , Compression and Consolidation, The Theory and Practice of industrial Pharmacy, J.L.;2009; Page No. 66-99. CVS Subramanyam , Textbook of Physical pharmaceutics, Page No. 224-227. Michael E. Aulton , Aulton’s Pharmaceutics The design and manufacture of medicines, Third Edition Page No. 478,443,468-473,355-358. www.wikipedia.com 24

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