Physics Of Tablet Compression

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this presentation comprises of mechanism of compression, euations of compression, forces of compression etc.

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PHYSICS OF TABLET COMPRESSION 1 PRESENTED BY: S.PRAVEEN KUMAR, 611285901027, II/II M.Pharm , Sri Vasavi Institute of Pharmaceutical Sciences.

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DEFINITIONS: COMPRESSION: Compression of powder means reduction in the bulk volume of a material as a result of displacement of the gaseous phase under pressure. COMPACTION: It is defined as the transformation of powder into coherent specimen caused by applied pressure. CONSOLIDATION: It is described as an increase in the mechanical strength of the particle resulting from particle to particle interaction. COMPRESSIBILTIY: It is the ability of a powder to decrease in volume under pressure. 2

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COMPRESSION: COMPACTION: 3

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COMPACTION CONSOLIDATION It is defined as the formation of solid specimen of defined geometry by powder compression. Compression takes place in the die by the action of two punches , the lower and the upper by which the compression takes place It is increase in mechanical strength of a material from particle -particle interactions COMPACTION AND CONSOLIDATION: 4

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STEPS INVOLVED IN TABLET COMPACTION: The complete cycle of tablet compaction occurs in four following stages: Stage1 : Top punch is withdrawn from the die by the upper cam. Bottom punch is low in the die so powder falls in through the hole and fills the die. Stage 2 : Bottom punch moves up to adjust the powder weight, it raises and expels the excess powder. Stage 3 : Top punch is driven into the die by upper cam and bottom punch is lowered by lower cam. Both punches heads pass between heavy rollers to compress the powder. 5

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Stage 4: Top punch is withdrawn by the upper cam . Lower punch is pushed up and expels the tablet . Tablet is removed from the die surface by surface plate. Stage 5 : Return to stage 1.,., 6

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STEPS INVOLVED IN TABLET COMPACTION: 7

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MECHANISM OF TABLET COMPACTION: The process of tablet compaction involves following identifiable phases like: Transitional repacking or particle rearrangement. Deformation. Fragmentation. Bonding. Deformation of the solid body. Ejection. 8

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1.PARTICLE REARRANGEMENT: Generally occurs at low pressures. It depends on particle size distribution and shape. Reduction in the relative volume of powder bed into closure packing structures. 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. As pressure increases, relative particle movement becomes impossible, inducing deformation. 9

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2.DEFORMATION: When the particles of the granulation are so closely packed that no further filling of the voids can occur, a further increase of compression force causes deformation at the point of contact. Change in the shape of the material occurs. Deformation can me observed by Confocal Laser Microscopy. At certain point, the packing characteristics of the particles, reduced space or porosity and inter-particulate friction will prevent any further rearrangement of the particles. 10

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At this point, further reduction in the compact volume results in elastic, plastic or viscoelastic deformation of particles. DIE-FILLING REPACKING 11

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MECHANISMS OF DEFORMATION: 12

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13 ELASTIC DEFORMATION: Second phase of tablet compaction process. It can be described as densification of particles due to movement of cluster of molecules or ions that forms the particles. Particles either whole or partly can modify their shape temporarily by elastic deformation. Deformation completely disappears on withdrawal of the force. Examples of compounds that undergo elastic deformation are acetyl salicylic acid and microcrystalline cellulose.

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14 PLASTIC DEFORMATION: Third phase of tablet compaction or irreversible deformation of the powder bed. A deformation that doesn’t completely recover after release of the stress is known as a plastic deformation. Plastic deformation occurs by sliding of the molecules along the slip planes within the particles. Deformation beyond the elastic limit leads to plastic deformation. Yield stress: the force required to initiate a plastic deformation is known as yield stress.

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15 3.FRAGMENTATION: Under high pressures the deformed particles may fragment resulting in new clean surfaces that have potential bonding areas. Fragmentation leads to further densification with the infiltration of the smaller fragments into the voids. The mechanism of fragmentation and plastic deformation are not independent because both the phenomena modify particle size distribution. With some materials fragmentation doesn’t occur because the stresses are released by plastic deformation.

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16 4.BONDING OF PARTICLES: Governed by several theories as follows: The mechanical theory. The intermolecular theory. The liquid surface film theory.

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17 THE MECHANICAL THEORY: It occurs between irregularly shaped particles. Also increases the number of contact points between the particles. The mechanical theory proposes that under pressure the individual particles undergo elastic/plastic or/& brittle deformation & that the edges of the particles intermesh deforming a mechanical bond. If only the mechanical bond exists, the total energy of compression is equal to the sum of the energy of deformation, heat & energy absorbed for each constituent. Mechanical interlocking is not a major mechanism of bonding in pharmaceutical tableting.

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18 The molecules [or ions] at the surface of solids have unsatisfied forces [surface free energy] which interact with the other particles in true contact. Under pressure the molecules in true contact between new clean surfaces of the granules are close enough so that vanderwals forces interact to consolidate the particles. Materials containing plenty OH groups may also create hydrogen bonds between molecules. INTERMOLECULAR THEORY:

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19 LIQUID SURFACE FILM THEORY: The liquid-surface film theory attributes bonding to the presence of a thin liquid film which may be the consequence of fusion or solution at the surface of the particle, induced by the energy of compression. SOLID BRIDGES: The formation of solid bridges, also referred to as the diffusion theory of bonding, occurs when two solids are mixed at their interface and accordingly to form a continuous solid phase. HOT WELDING: Under the influence of applied pressure, edges of the contact points between particles undergoes a possible melting due to generation of heat in case of low melting point solids.

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20 CONT,.,. Under unloading of stress these melted point of contacts undergo re-solidification, forming a solid bridge between the particles. COLD WELDING: Cold or contact welding is a solid state welding process in which joining takes place without fusion or heating at the interface of the two surfaces to be welded. When the surfaces of two particles come close to each other there will be a generation of surface attractive forces which cause bonding between two particles.

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21 5.DEFORMATION OF THE SOLID BODY: On further increase of the pressure, the non-bonded solid is consolidated towards a limiting density by plastic and/or elastic deformation. 6.EJECTION: Finally as the lower punch rises and pushes the tablet upward, there is a continued residual wall pressure and considerable energy may be expanded due to the die wall friction. After the pressure has been removed, there is a lateral pressure on the die wall. As the tablet is ejected it undergoes elastic recovery, with an increase in volume as it is removed from the die.

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22 Force distribution during compaction: The force distribution when compaction system takes place in a single station press is given by F A = force applied to upper punch F L = force transmitted to lower punch F D = reaction at die wall due to friction Mean compaction force is given by, F A = F L +F D F M = (F A + F L )/2

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23 Axial to radial stress transmission: Axial pressure: The force per unit area being applied in the direction in which the punch moves during compression. Radial pressure: It is the pressure being transversely being transmitted to the die-wall at right angles to the longitudinal punch axis. During decompression, some pressure still remains on the die-wall, it is given as the residual die-wall pressure(RDWP). Ejection force(EF): the force required to overcome the friction between the die-wall and tablet is directly related to the RDWP resulting from compression. EF= RDWF x μ w Where μ w coefficient of die-wall friction.

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24 Heckel equation Kawakita equation Cooper-Eaton model Walker and Balshin equations Leunberger equation Shapiro equation An overview of compaction equations:

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25 Heckel equations: The heckel analysis is a most popular method of determining the volume reduction under the compression pressure in pharmacy. Powder packing with increasing compression load is normally attributed to particle rearrangement, elastic and plastic deformation and particle fragmentation. Heckel analysis is based on the assumption that powder bed densification follows first order kinetics.

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26 In heckel equation the decrease in porosity of powder bed as the compression force increases is assumed to follow the following equation: Where, D= ρ ave / ρ true ρ ave = the average density of the powder bed. ρ true = true density of the powder. (1-D) = porosity. K = constant. Upon integration: In heckel plot we draw ln[1/1-D] as a function of compression pressure P. ( dD / dP )=K(1-D) ln [1/1-D] = K.P+A

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27 Heckel plots: Type A : For plastic deforming bodies. It is possible to distinguish the three different types of powder behavior by compressing different size fraction of the same material. For plastically deforming materials the Heckel plots, drawn from different size fractions remain parallel over the entire pressure range. Examples: MCC, Starch

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28 Type B: For fragmenting materials. For fragmenting materials the plots become coincidental as the compression pressure increases. Examples: Lactose, Sucrose.

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29 Type C: After initial linear part the plots become coincidental because the material packing fraction approaches unity at quite low compressive stress level. Examples : Fattyacids or Lactose mixed with high percentage of fattyacids.

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30 Significance of Heckel Plots: 1.The Heckel constant k, has been related to the reciprocal of the mean yield pressure, which is the minimum pressure required to cause deformation of the material under compression. 2. The intercept of the curve portion of the curve at low pressure represents a value due to densification by particle rearrangement. 3. The intercept obtained from the slope of the upper portion of the curve is a reflection of the densification after consolidation. 4. A large value of the Heckel constant indicates the onset of plastic deformation at relatively low pressure. 5. A Heckel plot permits an interpretation of the mechanism of bonding

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31 Weakness of Heckel Plots: Shape of the plot is very sensitive for small errors in the determination of powder true density. Linear part of the plot is sometimes difficult to determine. Heckel plot determination need very accurate data. Even the deformation of the tablet compression machine has to be recorded.

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32 Kawakita equation : The Kawakita equation was developed to study powder compression using the degree of volume reduction, C, a parameter equivalent to the engineering strain of the particle bed and is expressed as: In practice, the Kawakita equation can be rearranged to give: C  =  (V 0 -  V p ) / V 0 =  a b P / (1+ bP ) P / C   =   P / a   +   1 / ab

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33 where C is the degree of volume reduction, V 0 is the initial volume of the powder bed, V p is the powder volume after compression, P is the compression pressure, a and b are constants which are obtained from the slope and intercept of the P/C versus P linear plots respectively. The constant a is equal to the minimum porosity of the bed prior to compression, b , which is termed the coefficient of compression, is related to the plasticity of the material.

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34 Conclusion: The understanding of the principles involved in compressibility and compactibility are required to characterize the compaction profiles of pharmaceutical materials. Various mathematical equations have been used to describe the compaction process. The use of one single equation is unlikely to be adequate since different materials consolidate by different mechanism depending on their properties.

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35 The Theory and Practice of Industrial Pharmacy by Leon Lachman and Herbert A. Lieberman special Indian edition 2009. Pg No:66-99. http://alihasan-polas.blogspot.com/2009/07/physics-of-tablet-compression.html as dated on 28-04-2013 at 7pm http://www.pharmacy.utah.edu/pharmaceutics/pdf/Tabletting.pdf http://www.authorstream.com/Presentation/sandeepgunjal-1136535-tablet-compression/ as dated on 28-04-2013 at 4pm http://www.med.ut.ee/orb.aw/class=file/action=preview/id=1010624/Tablet_compression_B_13_5.pdf as dated on 28-04-2013 at 5pm http://www.pharmainfo.net/reviews/compaction-pharmaceutical-powders as dated on 28-04-2013 at 6pm REFERENCES:

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36 QUERIES,.,…?

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