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PRAVEEN KUMAR sir M.Pharmacy Department of Pharmaceutics NALANDA College of PharmacyPowerPoint Presentation: CONTENTS Definitions Mechanisms of compression of particles Compression process Consolidation process Factors affecting consolidation Compression & Consolidation Under High Loads Effect of Friction. Force Distribution Development of radial Force. Die-wall Lubrication. Ejection Forces. Force volume relationships Heckle Equations & Plots Conclusion ReferenceDefinitions: Definitions COMPRESSION - Compression means reduction in the bulk volume of the material as a result of displacement of the gaseous phase . CONSOLIDATION - An increase in the mechanical strength of the material resulting from particle/particle interaction . 3PowerPoint Presentation: RADIAL FORCE : It is the force required to the attempt the material to expand horizontally 4PowerPoint Presentation: AXIAL FORCE : It is the force required to attempt of material to constrict vertically. 5 Mechanisms of compression of particles: Repacking Load Deformation If elastic If plastic Mechanisms of compression of particles Load Load Load 6Compression process: Compression process In particle deformation and rearrangement the following three principal modes of deformation are as follow 1. Elastic deformation : A spontaneously reversible deformation of the compact in which, upon removal of the load, the powder mass reverts back to its original form. Most materials undergo elastic deformation to some extent. Compression of rubber would be by elastic deformation. 7Compression process: Compression process 2.Plastic deformation : After exceeding the elastic limit of the material (yield point), the deformation may become plastic, that is, the particles undergo viscous flow. This is the predominant mechanism when the shear strength between the particles is less than the breaking strength. Plastic deformation is a time-dependent process. 3 . Brittle fracture: Upon exceeding the elastic limit of the material (yield point), the particles undergo brittle fracture if the shear strength between the particles is greater than the breaking strength. Under these conditions, the larger particles are sheared and broken into smaller particles. 8PowerPoint Presentation: 9Consolidation process: Consolidation process Cold welding : when the surfaces of two particles approach each other closely enough, their free surface energies results in strong attractive force, a process known as cold welding. Fusion bonding: Multiple point contacts of the particle upon application of load produces heat which causes fusion / melting. Upon removal of load it gets solidified and increase the mechanical strength of mass. 10The consolidation mechanisms : The consolidation mechanisms Mechanical theory : As the particles undergo deformation, the particle boundaries that the edges of the particle intermesh, forming a mechanical bond Intermolecular forces theory : Under pressure the molecules at the point of true contact between new, clean surface of the granules are close enough so that van der Waals forces interact to consolidate the particle. E.g microcrystalline cellulose is believed to undergo significant hydrogen bonding during tablet compression. Liquid-surface film theory: Thin liquid films form which bond the particles together at the particle surface. The energy of compression produces melting of solution at the particle interface followed by subsequent solidification or crystallization thus resulting in the formation of bonded surfaces 11Factors affecting consolidation: Factors affecting consolidation The chemical nature of the material The extent of the available surface The presence of surface contaminants The inter surface distance 12Compression & Consolidation under high loads: Compression & Consolidation under high loads Effect of Friction. Force Distribution. Development of radial Force. Die-wall Lubrication. Ejection Forces. 13Compression & Consolidation under high loads: Compression & Consolidation under high loads Effects of friction Inter particulate friction: This arises at particle-particle contacts & can be expressed as inter particulate friction coefficient μ i . It is more significant at low applied loads. Glidants reduce the inter particulate friction and promote the flow of powder. Die-wall friction: This results from material being pressed against the die-wall and moved down it; and it can be expressed as die-wall friction coefficient μ w. 14Compression & Consolidation under high loads: 2.Force distribution : Most investigations are carried out on Single station presses (eccentric presses) or even on isolated punches and die sets in conjugation with hydraulic press. These must be an axial balance of forces. F A = F L +F D F A = Applied force to the upper punch F L = Force transmitted to lower punch F D = Reaction at die-wall due to friction at surface. Compression & Consolidation under high loads 15Compression & Consolidation under high loads: Compression & Consolidation under high loads 16 Compression & Consolidation under high loads: Compression & Consolidation under high loads Development of Radial Force : As the compressional force is increased and the repacking of tabletting mass is completed, the material may be regarded as a single solid body. Then, as with all other solids, compressive force applied in one direction (e.g.: vertical) results in a decrease in ∆H i.e., height. In case of unconfined solid body, this would be accompanied by an expansion in the horizontal direction of ∆D. The ratio of these two dimensional changes is known as poisson ratio ( λ ) of the material λ = ∆D/ ∆H. λ is characteristic constant for each solid. 17PowerPoint Presentation: Consequently a radial die-wall force F R develops perpendicular to the die-wall surface. Materials with larger values of λ give rise to larger values of F R The relation ship between F D and F R is given by the expression: F D = μ w .F R μ w = coefficient of die wall friction. 18Compression & Consolidation under high loads: Die-wall lubrication Die-wall lubricants function by inter posing a film of low shear strength at the interface between tabletting mass and die-wall there is some chemical bonding between boundary lubricant and the surface of die-wall as well as at edge of the tablet. The best lubricants are those of low shear strength but have strong cohesive tendency in directions at right angle to the plane of shear. Compression & Consolidation under high loads 19Compression & Consolidation under high loads: Ejection forces: Force necessary to eject a finished tablet. Ejection force for a finished tablet consists 3 stages. Stage1 :Peak force required to initiate ejection by breaking of tablet/die-wall adhesions. Stage2 : Small force, that required to push the tablet up the die-wall . Stage3: Declining force of ejection as the tablet emerges from the die. Compression & Consolidation under high loads 20Force volume relationships: In many tabletting processes, when appreciable force has been applied, the relationship between applied pressure (p) and volume parameter such as porosity E does become linear over the range of commonly used pressure. It can be expressed by Shapiro equation Log E = Log E 0 – K.P. Where E 0 = porosity when pressure is Zero K = Constant & P= Pressure Walker expressed similarly 1/1-E = K 1 - K 2 . Log P Force volume relationships 21Force volume relationships: Force volume relationships i- initial repacking of particles ii- elastic deformation until elastic limit is reached iii- plastic deformation and/or brittle fracture dominates iv – compression of solid crystal lattice formation 22HECKEL EQUATION: HECKEL EQUATION It is analogous to first order reaction, where the pores in the mass are the reactant, that is: Log 1/E = Ky.P + Kr Where Ky = material dependant constant inversely proportional to its yield strength ‘S’ Kr = initial repacking stage, hence Eo. 23HECKEL EQUATION: HECKEL EQUATION The applied compressional force F and the movements of the punches during a compression cycle and applied pressure P, porosity E. For a cylindrical tablet, P=4F/ π .D 2 where D is the tablet diameter. Similarly E can be calculated by: E = 100.(1-4w/ ρ t. π . D 2 .H) where, w is the weight of the tabletting mass, ρ t is its true density, H is the thickness of the tablet 24 HECKEL plots : HECKEL plots Materials that are comparatively soft and that readily undergo plastic deformation retain different degrees of porosity, depending upon the initial packing in the die. This in turn is influenced by the size distribution, shape, etc. of the original particles. Ex: Sodium chloride (shown by type a, in graph) Harder materials with higher yield pressure values usually undergo compression by fragmentation first, to provide a denser packing. Ex: Lactose (type b, in graph) 25HECKEL plots: HECKEL plots Type-a plots exhibits higher slope (Ky) then type b. Because type-a materials have lower yield stress. Type-b plots exhibits lower slope because brittle, hard materials are more difficult to compress. 26Conclusion:: Conclusion: The compression and consolidation are important in the tabletting of the materials. The importance of each will depend largely on the type of compact required i.e., whether soft or hard and on the brittle properties of the materials. Various mathematical equations have been used to describe the compaction process. The particular value of Heckel plots arises from their ability to identify the predominant form of deformation in a given sample. 27References:: References: Jones, T.M: J. Pharm. Sci.,57:2015,1968 Travers, D.N., Celik, M., and Buttery, T.C.: Drug Devel. Ind. Pharm., 9:139,1983. Rankell, A.S., and Higuchi, T.:J. Pharm. Sci. Ed.,48:26,1959. The Theory and Practice of Industrial Pharmacy Leon Lachman, Herbert A. Lieberman, Joseph L. Kanic Mehta, A.M., and Augsburger, L.L: Int.J.Pharm., 7:327,1981. Mehta, A.M., and Augsburger, L.L: Int. J. Pharm., 4:347,1980. Knudsen, F.P.: J. Amer. Ceram. Soc., 42:376,1959. Walker, E.E.: Trans. Farad. Soc., 19:60,1923. Hess, H.: Pharm. Tech., 2:36,1978. Wray, P.E.:Drug Cosmet. Ind., 105:58,1969. 28Thank you: Thank you 29 You do not have the permission to view this presentation. In order to view it, please contact the author of the presentation.