Compression & Consolidation of Powder


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Compression and consolidation of powder solids. : 

1 Compression and consolidation of powder solids. Presented by PAYAL H. PATIL (M.pharm 1st yr.) Dept. of pharmaceutics and Q.A R.C.Patel Institute of Pharmaceutical Education and Research, Shirpur. 1

Contents : 


Definitions : 

3 Definitions Compression- Compression means reduction of bulk volume of material as a result of displacement of gaseous phase. Consolidation – Consolidation is an increase in mechanical strength of material from particle - particle interactions. 3

Derived properties of powdered solids : 

4 Derived properties of powdered solids The solid-air interface Angle of repose Flow rates Mass-volume relationships Density 4


5 THE SOLID-AIR INTERFACE COHESION: Attraction between like particle.Experienced by particles in bulk. ADHESION: Attraction between unlike particle.Experienced by particles at surface. Resistance to movement of particles is affected by two factors:- a) Electrostatic forces. b)Adsorbed layer of moisture on particles. 5


6 ANGLE OF REPOSE DEFINITION: The maximum angle possible between the surface of pile of the powder and the horizontal plane. 6


7 METHODS TO MEASURE ANGLE OF REPOSE Fixed funnel and free standing cone method. Tilting box method. Revolving cylinder method. Wall is lined by sandpaper 7

Formula for measuring angle of repose. : 

8 Formula for measuring angle of repose. θ = Tan-1(h/r) here, h = height of pile r = radius of the base of the pile θ = angle of repose 2. θ = cos-1 D/ (l1+l2) here, D = diameter of base l1+l2 = the opposite sides of pile Adding glidant,0.2% aerosil may improve flow. 8


9 FLOW RATES Compressibility index (Carr's consolidation index) I = [1-v/vo]x100 here, V = Tapped Volume V0 = Volume before tapping Adding glidant,0.2% aerosil may improve flow. 9


10 MASS-VOLUME RELATIONSHIPS TYPE OF VOIDS OR AIR SPACES: Open intraparticulate voids Closed intraparticulate voids Interparticulate voids 10


11 TYPES OF VOLUME True volume (Vt) Granule volume (Vg) Bulk volume (Vb) Relative volume (Vr) Vr = V/ Vt Vr tends to become unity as all air is eliminated from the mass during the compression process 11

Porosity (E) : 

12 Porosity (E) porosity, E = VV/ Vb here, VV = Void volume Vb = Bulk volume now, Void volume (VV) = Vb –Vt Therefore, Porosity (E) =(Vb–Vt)/ Vb Porosity when expressed as percentage E =100.[(Vb–Vt)/ Vb] 12

Methods to measure volume of powder. : 

13 Methods to measure volume of powder. Helium pycnometer Liquid displacement method (specific gravity bottle method) 13

Helium pycnometer : 

14 Helium pycnometer Vt = Vc/U1-U2x[U1-Us] Here, Vt = true volume of sample Vc=true volume of stainless steel spheres U1=Volume of empty cell U1-U2=Volume occupied by the std. sample U1-Us = volume occupied by sample 14

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Liquid displacement method : 

16 Liquid displacement method Solvent used are e.g., ethyl alcohol ,water, mercury , etc. Pycnometer or specific gravity bottle used. True density= w3/(w4-w2) = (w2-w1)/(w4-w2) Here , w1 = wt. of Pycnometer w2 = Wt. of Pycnometer + sample or glass beads w4 = Wt. of Pycnometer with powder & filled with solvent w3 = w2-w1 = Wt. of sample w4-w2 = Volume of liquid displaced by the solid Specific gravity bottle 16


17 DENSITY DIFFERENT TYPE OF DENSITY : True density ρt=M/vt Granule density ρg=M/vg Bulk density ρb=M/vb relative density ρr= ρ/ ρt Tapped density-tester 17

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18 Effect of applied forces DEFORMATION: Strain: The relative amount of deformation produced on a solid body due to applied force. It is dimensionless quantity. Compressive strain, Z = ∆H/Ho Stress(σ): σ = F/A here, F is force required to produce strain in area A 18

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19 a)Tensile strain b)Compressive strain c)Shear strain FIG. Diagram shows changes in geometry (strain) of solid body resulting from various types of applied forces. *( In fig. dash line is original shape & solid line is deformed shape) H H0 D D0 19

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20 COMPRESSION: When external mechanical forces are applied to a powder mass, there is reduction in bulk volume as follows, 1. Repacking 3.Brittle fracture: e.g., sucrose 2.Particle 4.microquashing deformation e.g., acetyl salicylic acid, MCC - when elastic limit or yield point is reached. Microsquasing: Irrespective of the behavior of larger particles smaller particles may deform plastically. Elastic deformation Plastic deformation 20

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22 CONSOLIDATION: Mechanism, 1.Cold welding (particle distance <50nm) 2.Fusion welding (caused due to frictional heat) 3.Recrystallization Consolidation process is influenced by, - Chemical Nature of materials - Extent of available surface - presence of surface contaminants - Intersurface distance 22

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23 Effect of increasing compressional force on specific surface area of powder mass, Increased surface area (from O to A), initial particle fracture due to increased compression point A ,Particle rebonding predominates & then surface area decreases (from A to B). Compressional force 23

Granulation : 

24 Granulation Addition of granulating liquid to mass of powder mass leads to following stages, Or droplet 24

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25 FIG. Plot of change in torque of mixer shaft during addition of granulating fluid. point F indicates exact end point to wet massing Granulation equipment can be instrument with the torque measuring devices, which senses the change in agitator power. METHOD TO DETERMINE STRENGTH OF GRANULE: Compressive strength or crushing strength. Abrasion tests 25

Compression and consolidation under high load : 

26 Compression and consolidation under high load Relationship between upper punch FA & lower punch forces FL: FL = FA × e-KH/D here, K = constant, H & D = height & diameter of tablet Effects of friction: 1.Interparticulate friction(μi ): Glidants used e.g., colloidal silica 2.Die-wall friction(μw ): lubricants used e.g., magnesium stearate 26

Slide 27: 

27 Force distribution -Axial balance of forces in punches is given by, FA = FL+FD -Mean compaction force (FM), FM = FA+FL/ 2 here, FA = upper punch force, FL =lower punch force, FD =axial friction force. 27 FIG. Cross section of a typical simple punch & die assembly used for compaction studies

Slide 28: 

28 Development of radial force: - Radial force (FR) develops perpendicular to die-wall surface. Poisson ratio , λ = ∆D / ∆H here, ∆D =change in horizontal direction, ∆H=change in height. According to classic friction theory, FD = μw × FR here, FD =axial friction force. μw = Die-wall friction - Coefficient of Lubricant efficiency (R), R = max. FL min. FA 28

Slide 29: 

29 Ejection forces - 3 stages of force necessary to eject a finished table, 1. Peak force required to initiate ejection 2. Small force required to push tablet up to die- wall 3.Decline force as tablet emerge from die. 29

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30 Die-wall lubrication Best lubricant has low shear strength & strong cohesive tendencies. Lubricant forms a film of low shear strength at the interface between tabletting mass & die-wall. 30

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31 End of compressional process is when bulk volume = tapped volume & porosity = 0 Decrease in porosity is due to two process: filling of large spaces by Interparticulate slippage Filling of small voids by deformation or fragmentation at high loads. A more complex sequence of events during compression process involves four stage as shown in fig., FIG. Decreasing porosity with increasing compressional force for single ended pressing i) initial repacking ii)Elastic deformation iii)Plastic deformation iv)compression Force volume relationship 31

Slide 32: 

32 Heckel plot It follows 1st order The pore in the mass are the reactant. log 1/E = KyP + Kr here, E = porosity P = Applied pressure Ky = material dependent constant Ky inversely proportional to it’s yield strength (S) (Ky = 1/3S) Kr = related to repacking stage & hence E0 For cylindrical tablet, P = 4F / ∏×D2 here, P = applied pressure D = tablet diameter F = applied compressional force 32

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33 E = 100×[1 – 4w/ρt ×∏×D2×H] here, w = weight of tabletting mass ρt = true density H = thickness of tablet. Type a:Soft material(e.g., NaCl) Type b:Hard material(e.g., lactose) Crushing strength of tablet is directly proportional to Ky APPLICATION OF HECKEL PLOT: Used to check lubricant efficacy. For interpretation of consolidation mechanisms Duberg & nystom distinguish between plastic and elastic deformation characteristics of a material. 33 FIG. Example of heckel plot.

Slide 34: 

34 Kawakita Equation C = Vi – Vp/ Vt = abPa / 1+ bPa here, C = degree of volume reduction, Vi = initial apparent volume, Vp =powder volume under applied pressure Pa, Vt = true volume, a & b = constants. LIMITATION: Compaction process can be described upto certain pressure, above which the equation is no longer linear. Vi – Vp/ Vi – Vt = C2 exp (-K2/Pa ) + C3 exp (-K3/Pa ) here, C2,C3, K2,K3 = constants LIMITATION : Applies only to single component system. Cooper and Eaton Equation 34

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35 Decompression Occurs on removal of applied force after compression. Tablet must mechanically strong to withstand stresses produce during decompression. Plastoelasticity(γ), γ = [Ho / Hm - (Hr-Hm) / Ho - Hm] here, Ho ,Hm, Hr = thickness of tablet mass at onset of loading, at max. applied pressure & on ejection from die γ > 9 produce tablets that are laminated or capped. 35

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36 Compaction profile Monitoring of applied pressure transmitted radially to die-wall gives compaction profile as follow, 36

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37 Energy involved in compaction Tablet machine, roll compactor, extruder requires high input of mechanical work Work involve in various phases of compaction are, 1.To overcome interparticulate friction 2.To overcome friction between machine parts and particles 3.To induce deformation 4.For brittle fracture 5.Mechanical operation of various machine parts Lubrication reduce energy expenditure by 75%. 37

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38 REFERENCES Leon Lachman, Herbert A.Liberman, & Joseph kanig ,THE THEORY AND PRACTICE OF INDUSTRIAL PHARMACY, third edition. Herbert A.Liberman, Leon Lachman & Joseph B. Schwartz ,PHARMACEUTICAL DOSAGE FORMS, TABLETS, volume II. ENCYCLOPEDIA OF PHARMACEUTICAL TECHNOLOGY, second edition,volume-3. C.V.Subrahmanyam ,TEXTBOOK OF PHYSICAL PHARMACEUTICS, second edition. Gilbert S. Banker , Christopher T. Rhodes, Modern Pharmaceutics , Fourth Edition. 38

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