COMPRESSION AND COMPACTION, introduction, principle

COMPRESSION AND COMPACTION, introduction, principle

1. COMPRESSION AND COMPACTION PRESENTED BY NIVEDITHA G M PHARM(1 st sem ) Dept of pharmaceutics

2. Introduction Tablets constitutes of almost 70-80% of the all dosage forms. Tablets are generally manufactured by 3 main processes. Compaction represent one of the most important unit operations in the pharmaceutical industry. Compaction is the situation in which materials are subjected to some level of mechanical force. The physics behind the compaction is stated as the compression and consolidation of the two phase system due to applied force. While considering the compaction and compression of tablets we had to taken the properties of powder into the consideration as they are involve in the progress of compression and compaction .

3. PHYSICS OF TABLET COMPRESSION,COMPACTION AND CONSOLIDATION Compression: Compression of powder means reduction of bulk volumes of material as a result of displacement of a gaseous phase under pressure. Consolidation : Consolidation is an increase in the mechanical strength of a material resulting from particle –particle interactions.(Increasing in mechanical strength of mass).

4. Compaction of powder Compaction of powder is a general term used to describe the situation in which these materials are subjected to some level of mechanical force. In pharmaceutical industry the effect such process are particularly important in various ways. such as: A. In the manufacturing of tablet and granules, B. In the filling of hard gelatin capsule shell, C . In powder handling in general. The physics of compaction may be simply started as “the compaction and consolidation of two phase (particulate solid and gas )system due to applied force” Therefore Compaction =compression=consolidation

5. Tablet compression machine makes the tablet by passing the granules in die with lower and upper punch. Tablets are being formed by compressing the granules using the compression machine. A tablet formation takes place by combined pressing action of two punches and die.

6. Solid air interface: Atoms or Ions are located at the surface of any solid particle are exposed to a different distribution of intramolecular and intermolecular bonding forces than those within the particle. This is indicated as unsatisfied attractive molecule force , extending out some molecular distance of the solid surface . This condition gives rise to free surface energy of the solid which plays a major role between a particle and the environment . Many important phenomena e.g. adsorption , attrition , rate of dissolution , crystallization are the fundamental properties. Because of these unsatisfied bonding forces identically particle attract together .

7. Flow from the hopper, Relative motion of the mixture, Production of granules, Compression to produce granule or tablets etc. The overall resistance to relative movement of the particle may be markedly affected by two other factors : Firstly , many powder of the pharmaceutical intersect readily develop intrastatic force specially when subjected to internal friction.

8. Although these particle contact and separation are pre-requisite . The charge develop depend on the particular material involved and the type of motion produced in it . Usually electrostatic force is relatively small but may be significant because they act over a greater distance of the molecular force .

9. The second factor namely the presence of an adsorbed near the moisture on the particle. When particle approaches one another closely enough , this field of moisture can formed liquid bridge which holds the particle together surface tension effect and a negative capillary pressure.

10. Angles of repose : It is the maximum angel that can be obtained between the free standing surface of a powder heap and horizontal plane. This is expressed by ϕ and defined by the following equation : Tan ϕ =2h/D Value of ϕ rarely less than 20 and value of up to 40 indicates reasonable flow potential .Above 50 however the powder flows only with great difficulty.

11. Angle of repose for various particles

12. Flow rate: Resistance to movement of particle especially for granular powder with little cohesiveness may be assist by determining their flow rate. Through a circular orifice for instance tablet dies. Flow experiment with the mixture of different size fraction of the same material can be particularly valuable because in many instances they exists optimum proportion that leads to a maximum flow rate . For this system when the proportion of the fine particle exceeds 40% there is a dramatic fall in the flow rate.

13. A simple indication of the ease with which a material can be induced to flow is given by the application of a compressibility index(I). I =(1-V/V O ) -100% Where ,V= Volume occupied by a sample of the powder after being injected to a standardized tapping procedure . V o =volume before tapping . Values of below 15% usually gives rise to good flow characteristics but reading above 25%indicates poor flow ability.

14. Air space: The mass of the powder sample can be determined with a great accuracy but the measurement of volume is more complicated. The main problem arises in defining the volume of bulk powder. Three types of air space can be differentiated: Open intraparticulate voids: Those within a single particle but open to the external environment. b.Closed intraparticulate voids: Those are within a single particle but open to the external environment. Interparticulate voids: The air spaces between individual particles.

15. Therefore atleast 3 interpretation of “powder volume” may be proposed 1.True volume: True volume is the total volume of the solid particle, which excludes all spaces greater than the molecular dimension. It is denoted by V t. 2.The granular volume or particle volume: It is the cumulative volume occupied by the particle including all intraparticulate voids. It is denoted by V g. 3.The bulk volume : It is the cumulative volume occupied by the entire powder mass under the particular packing achieved during the measurement .

16. When studying this phenomena resulting in a change in volume. It may be convenient to consider volume V,of the sample under experiment condition relative to the true volume V t. Relative volume , v R =V/Vt V R decreases and tends towards to unity as all the air is eliminated from the mass .This phenomenon occurs in tabletting.

17. Porosity: The voids present in the powder mass may be more significant than the solid components .For this reason, a second dimensionless quantity , the ratio of the total volume of void space(V V ) to the bulk volume of the material is often selective to monitor the progress of compression . This ratio is referred to as porosity of the material. Porosity, E = V v / V b V V = V b -V t E=( V b –V t )/ V b E=(1-V t / V b ).100%

18. Density: The ratio of mass (weight)to volume is known as the density of the material .Three different densities for powdered solids ,based on the following ratios, M/V t =p t the true density M/V g = p g the granular density M/ V b =p b the bulk density M is the mass of the sample . Comparing the density p of a sample under specific test conditions with the true density(sometimes called theoretical density) of the material leads to the dimensionless quantity p r, the relative density ,

19. P r = p/p t During compressional processes ,relative density to a maximum of unity when all air spaces have been eliminated. Effect of Applied Forces: Deformation: When any solid body is subjected to opposing forces, there is a slight change in its geometry. Depending upon the nature on the applied load, the relative amount of change (deformation) produced by such forces is a dimensionless quantity called strain.

20. For example, if a solid rod is compressed by forces acting by each ends to cause a reduction in length of H from an uploaded length of H o. Then the compressive strength Z is expressed by following equation, Z= H/H O The ratio of the force (F)necessary to produce this strength to the area (A)over which it is called the stress.

21. Stress, ᵟ =F/A Because most powder mass contain air space true analogue behavior to the solid body should not be expected. Under low porosity condition compression do provide a useful way of interpreting experimental observation.

22. Process of Tablet Compression Compression is the process of applying pressure to the material. There are six different stages involved in the tablet formulation. These are: 1. Transitional repacking or particle rearrangement 2.Deformation 3.Fragmentation 4.Bonding 5.Deformation of the solid body 6.Ejection.

23. Process of tablet compression : 1.Transitional repacking /particle rearrangement. Generally it occurs at low pressure. It depends on particle size of 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 final particles entering the void between the larger particles and the bulk density of the granulation is increased.

24. Spherical particle undergo less rearrangement than the irregular particles. As spherical particles tends to assume close packaging. As pressure increases, relative particle movement become impossible, inducing deformation.

25. 2.Deformation When the particles of the granulation are so closely packed that no further fillings of voids can occur ,a further increase in the compression force causes deformation at that point of contact. Change in shape of material occurs. At a certain point, the packing characteristics of the particles, reduced space or porosity of inter-particulate friction will prevent any further rearrangement of particles. At this point further reduction in the compact volume results in elastic or plastic deformation of particles .

26. Both plastic and elastic deformation may occur although one type predominates for a given material. Elastic deformation : If the deformation disappears entirely on the removal of the load or pressure, it is called elastic deformation. In another word ,When deformation returns to the initial stage, it is called elastic deformation. Plastic deformation :If the deformation does not disappear entirely on the removal of the load or pressure ,it is called plastic deformation. In another word ,When deformation does not return to the initial stage ,it is called plastic deformation.

27. yield stress: The force required to initiate a plastic deformation is known as yield stress. Plastic deformation is believed to create the greatest number of clean surfaces . Because it is a time dependent process. Higher rate of force application should lead to the formation of less new clean surfaces and thus resulting in weaker tablets. Furthermore, since tablet formation is dependent on the formation of new clean surfaces , high concentrations or over mixing of materials that form weak bonds result in weak tablets.

28. 3.Fragmentation As compression force increase deformed particles start fragmentation due to high load, particles breaks into smaller fragments leading to formation of new bonding areas. Fragmentation leads to further densification with the infiltration of the smaller particles in 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 occurs because the stresses are released by plastic deformation.

29. Some particles undergo structural breakdown called as brittle fracture.

30. Fragmentation do not occur when applied stress is balanced by a plastic deformation. Change in shape. Sliding of groups of particle (viscoelastic flow).

31. 4.Bonding of particles A fter the fragmentation of the particles, as the pressure increases, formation of new bonds between the particles at that contact area occurs. The hypothesis favoring for increase in mechanical strength of bed of powder when subjected rising compressive forces can be explained by theories. The Mechanical Theories The Intermolecular Theories. Liquid –Surface Film Theory

32. THE MECHANICAL THEORIES: 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 Deformation and that the edges of the particle intermesh deforming a mechanical Bond. Mechanical interlocking is not a major mechanism of bonding in pharmaceutical tableting. Total energy of compression=Energy of deformation+ Heat+ Energy absorbed for each constituent .

33. THE INTERMOLECULAR THEORIES: The Molecules at surface of solids have unsatisfied forces which interact with the other particle in true contact. Absolutely clean surface will bond with the strength of the crystalline material ,whereas adsorbed materials restrict bonding. Under Pressure Molecule in true contact between new clean surface of the granule are close enough so that vanderwal forces interact to consolidate the particles. Material containing plenty OH group may also create hydrogen bond between molecule. e .g., MCC is believed to undergo significant hydrogen bonding during tablet compression.

34. The intermolecular forces theory and the liquid-surface film theory are believed to be the major bonding mechanisms in tablet compression. LIQUID –SURFACE FILM THEORY: Due to the applied pressure ,the particles may melt (due to lowering of melting point)or dissolve (due to increased solubility). Many pharmaceutical formulations require a certain level of residual moisture to produce high quality tablets.

35. The role of moisture in the tableting process is supported by the liquid –surface film theory. Thin liquid films forms ,Which bond the particles together at the particle surface. The energy of compression produces melting or liquefaction of the particles at the contact areas. As the pressure is withdrawn the melted ingredients solidifies causing fusing of the particles. In addition the solubility of the solution at the particle interface under pressure is increased and as the pressure is released it gets super saturated and followed by subsequent solidification or crystallization thus resulting in the formation of bonded surfaces.

36. 5.DEFORMATION OF SOLID BODY: On further increases of the pressure ,the non- bonded solid is consolidated towards a limiting density by plastic/elastic deformation. When upper punch is withdrawn, tablet is confined to the die by a radial pressure therefore any dimensional change during decompression should occur in axial direction. Capping can occur if decompression does not occur simultaneously in all direction.

37. 3 Stages of force necessary to eject a finished table, Peak force required to initiate ejection. Small force required to push tablet up to die-wall. Decline force as tablet emerge from die.

38. When lubrication is inadequate and /or “slip-stick” conditions occur between the tablet and the die wall, owing to continuing formation and breakage of tablet /die wall adhesions. Worn dies, which cause the bore to become barrel-shaped ,give rise to a similar abnormal ejection force trace and may lead to failure of the tablet structure.

39. Various forces involved in compression: Frictional forces Distribution forces Radial Forces Ejection Forces

40. Various Forces involved in compression: Frictional forces Inter-particulate friction: Reduced Glidants( E.g:Colloidal silica ) Die wall friction Reduced Lubricants ( E.g.Mag.stearate ) 2.Distribution forces:

41. Consolidation: An increase in the mechanical strength of the material resulting from particle or particle interactions.(Increasing in mechanical strength of the mass) Consolidation process: Cold welding Fusion bonding

42. CONSOLIDATION Consolidation process: Cold welding : when the surfaces of two particles approach each other closely enough,( e.g .,at separation of less than 50 nm),their free surface energies result in a strong attractive force , this process is known as cold welding . Fusion bonding: contact of particles at multiple points upon application of load, produces heat which fusion or melting .If this heat is not dissipated ,the local rise in temperature could be sufficient to cause melting of the contact area of the particles. Upon removal of load it gets solidified giving rise to fusion bonding and Increase the mechanical strength of mass.

43. Factors affecting consolidation Both “cold” and “fusion” welding ,the process is influenced by several factors, including The chemical nature of the materials The extent of the available sources The presence of surface contaminants The intersurface distances Friction: The action of one surface or object rubbing against another.

44. EFFECT OF FRICTION In this two major components of frictional forces are; 1.Interparticulate friction 2.Die wall friction 1.Interparticulate friction : This arises at particle/particle contacts and can be expressed in terms of a coefficient of inter particulate friction. It is expressed in term of It is more significant at low applied loads. Materials that reduce this effect are referred as glidants . eg ; colloidal silica .  

45. 2 .Die wall friction : This results from material being pressed against the die wall and moved down it; It is expressed in terms This effect becomes dominant at high applied forces when particle rearrangement has ceased and is particularly important in tabletting operation. Most tablets contain a small amount of an additive designed to reduce die wall friction; such additives are called lubricants. Eg :Magnesium stearate.  

46. Distribution Forces: Most investigations of the fundamentals of tabletting have been carried out on single-station presses(sometimes called eccentric presses),or even on isolated punch and die sets in conjunction with a hydraulic press. This simple compaction system provides a convenient way to examine the process in greater detail. More specifically ,the following basic relationships apply. Since there must be an axial (vertical)balance of forces:

47. Force applied to the upper punch reaction at the die wall due to friction at this surface.  

48. Because of this inherent difference between the force applied at the upper punch and that affecting material close to the lower punch ,a mean compaction force been proposed, where F M = F A +F L \2 F M is the friction –independent measure of compaction load.  

49. In single –station presses, where the applied force transmission decays exponentially . Then the more appropriate geometric mean force will be. F G =(F A . F L ) 0.5 Use of these force parameters are probably more appropriate than use of F A. While determining the relationship between compressional force and such tablet properties such as tablet strength.

50. Development of radial force: The compressional force is increased and any repacking of the tabletting mass is completed, the material may be regarded to some extent as a single solid body. Compression force applied in one direction ( e.g.,Vertical )results in a decrease H in the height. In the case of an 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 the poisson ratio of the material, defined as:

51. The material is not free to expand in the horizontal plane because it is confined in the die. consequently ,a die radial die wall force F R develops perpendicular to the die wall surface, materials with larger poisson ratios giving rise to higher values of F R . Classic friction theory can then be applied to deduce that the axial frictional force F D is related to F R by the expression.

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

53. FORCE VOLUME RELATIONSHIP: End of compressional process is when bulk volume=tapped volume. Porosity(E)=0 Decrease in porosity is due to two process 1.Filling of large spaces by interparticulate slippage. 2.Filling of small voids by deformation or fragmentation at higher loads. A more complex sequence of events during compression process involves four stages as shown in fig.

54. Decreasing porosity with increasing compressional force Single ended pressing Initial repacking Elastic deformation Plastic deformation compression

55. Heckel plots: The Heckel equation is based upon analogues behavior to a first order reaction ,Where the pores in the mass are the reactant, that is: Log1/E= K y P =K r K y is a material dependent constant inversely proportional to its yield strength S. K r is a related to the initial repacking stage.

56. For cylindrical tablet , P=4F/Π.D 2 P=applied pressure D=tablet diameter F=applied compressional force E=100.(1-4W/p t . ᴫ .D 2 .H) Here, W=Weight of tabletting mass . P t =true density H=thickness of tablet. Type a:Soft material ( e.g , Nacl ) Type b:Hard material (e.g., lactose)

57. Crushing strength of tablet is directly proportional to K Y. Application of Heckel plot : Used to check lubricant efficacy. For interpretation of consolidation mechanisms . Duberg and nystom distinguish between plastic and elastic deformation characteristics of a material.

58. LIMITATINS: The plot is linear only at pressure, The plot can be influenced by the time of compression and the degree of lubrication .

59. Reference : Share Lachman &Liebermann Industrial pharmacy. THANK YOU