Physics of Tablet compression- Part I (Compression & Compaction)

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events of compression cycle, compression, consolidation mechanism, Force distribution through powde bed

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1 PHYSICS OF TABLET COMPRESSION Part I (Compression & Compaction) QIS College of Pharmacy Ongole, Andhra Pradesh Presented by Mr.S.Chellaram M.Pharm., Associate Professor , QIS College of Pharmacy

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2 The simplest process for tableting is direct compression, in which the drug(s) and excipient(s) are dry mixed and then compacted. For this process to be successful, the powder mixture requires certain properties, such as high flowability, low segregation tendency, and high compactibility. Pharmaceutical powders often lack these properties and must, therefore, be pretreated with a particle modification process before compaction. Generally, granulation is done as a pretreatment process before compaction. The primary drug(s) and the excipient particles are agglomerated into larger secondary particles (granules or agglomerates), usually of a higher porosity than the primary ones. Techniques to improve tabletability involve different granulation techniques, both wet and dry, and special wet granulation techniques, which yields almost spherical agglomerates, such as pelletization,or extrusion–spheronization

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3 COMPACTION: Essential step in the manufacturing of tablets which includes. COMPRESSION: (Volume reduction & particle rearrangement) Compressibility is the ability of the powder to deform under pressure. CONSOLIDATION: (interparticulate bond formation) Consolidation is the ability of the powder to form mechanically strong compacts. The success of the compaction process depends on Physico-technical properties of drugs & excipients, -moisture content, polymorphism, deformation behaviour Choice of instrument settings - tableting speed, pre/main compression force

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4 Mechanical aspects of tableting can be studied using, instrumented punches/dies, instrumented tableting machines, and compaction simulators . Compaction equations describe density–pressure relationships that predict the pressures required for achieving an optimum density. Compaction equations is useful in solving the analytical problems related to tableting such as capping, lamination, picking, sticking, etc. Mathematical models, force-time, force-distance, and die-wall force parameters of tableting are used to describe work of compaction, elasticity/plasticity, and time dependent deformation behavior of pharmaceuticals.

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5 Surface Properties The flow and intermolecular attraction of the powder depends upon Surface properties of the powder material Atoms or ions located at a surface experiences different intermolecular and intramolecular bonding forces than those present within a particle. The atoms present inside the solid experiences equal and opposite forces of attraction whereas atoms at the surface experiences unbalanced force of attraction due to the absence of layer above it. This unsatisfied attractive molecular forces that extend out to some small distance beyond the solid surface gives rise to free surface energy of solids, which plays a major role in interparticulate interaction. Attractive forces between like particles are called cohesive force , and those between un-like particles are called adhesive force The attractive forces resist the movement of constituent particles when subjected to an external force during compression. Other types of resistance are electrostatic forces , adsorbed moisture , and residual solvent on the surface of solid particles. PROPERTIES OF POWDERS

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6 Porosity The porosity of powder ( E ) is defined as the ratio of total void volume ( V ) to the bulk volume ( Vb ) of the material. The total void volume, V = Vb - Vt where, Vt is the true volume. E = Vb - Vt / Vb = 1 - Vt / Vb The compressibility of a powder bed is the degree of volume reduction owing to applied pressure , which is related to porosity and is assumed to be a first-order reaction. Porosity–pressure relationship is also explained by the Heckel equation and is commonly used as a measure of compressibility

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7 Flow Properties Good flow property of a pharmaceutical powder is essential to ensure proper die fill during compression, especially in direct compaction process. high percentage of fines , excess moisture , increased amount of lubricants , and electrostatic charge may contribute to poor flow of powders. Angle of repose is commonly used to measure flow of powders , and is the maximum angle ( F ) between the plane of powder and horizontal surface. Angle of repose less than 30° indicates free flowing material, up to 40° Indicates reasonable flow potential, above 50° the power flows with great difficulty. The increase in bulk density of a powder is related to its cohesivity . Bulk density and tap density relationship is another way to measure flowability. Indices such as the Hausner Ratio ( H ) and Carr’s Index ( CI ) are based on tapped and bulk densities. Hausner ratio is the ratio of tapped density to bulk density , and varies from about 1.2 for a free-flowing powder to 1.6 for cohesive powders.

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8 The percentage compressibility, also called as Carr’s Index is 100 times the ratio of the difference between tapped density and bulk density to the tapped density . Values of Carr’s index 5–12% indicate free-flowing powder, 23–35% indicate poor flow, and >40% an extremely poor flow. Additionally, flow rate is used to determine the resistance to movement of particles especially for granular powder with poor cohesiveness. A simple indication of the ease with which a material can be induced to flow is given by Compressibility index, I . where, I = [1 – Vt/Vo ] × 100 vt is the tap volume and V0 is the volume before tapping. Value of below 15% indicate good flow properties but values above 25% mean poor flow.

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9 Compression of powdered solids Compression refers to a reduction in the bulk volume of materials as a result of displacement of the gaseous phase. Stages involved in the bulk reduction of powdered solids are shown in Fig. 1. Initial repacking of particles Elastic deformation of the particles until the elastic limit (yield point) is reached Plastic deformation and/or brittle fracture predominate until all the voids are virtually eliminated Compression of the solid crystal lattice then occurs

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10 During the I stage of compression process, the powder is filled into the die cavity, and the upper punch did not enter into the die cavity. At that time the forces that exist between the particles are those that are related to the packing characteristics of the particles , the density of the particles and the total mass of the material that is filled into the die. The packing characteristics of the powder mass will be determined by the packing characteristics of the individual particles During II stage of compression the upper punch enters into the die cavity. The external mechanical forces due to upper punch on a powder mass, produces a reduction in volume due to closer packing of the powder particles , and in most cases, this is the main mechanism of initial volume reduction . However, as the load increases in stage III of compression, rearrangement of particles becomes more difficult and further compression leads to some type of particle deformation

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11 If on removal of the load, the deformation is to a large extent reversible , i. e. it behaves like rubber, then the deformation is said to be elastic. All solids undergo elastic deformation when subjected to external forces. With several pharmaceutical materials such as acetylsalicylic acid, elastic deformation becomes the predominant mechanism of compression within the range of maximum force encountered in practice. In other groups of powdered solids, an elastic limit is reached, and loads above this level result in deformation not immediately reversible on the removal of the applied force. Bulk volume reduction in these cases results from plastic deformation and/or viscous flow of particles, which are squeezed into the remaining void spaces, resembling the behaviour of modelling clay.

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12 This mechanism predominates in materials in which the shear strength is less than the tensile or breaking strength . Plastic deformation is believed to create the greatest number of clean surfaces . Because plastic deformation is a time dependent process higher rate of force application lead to the formation of less new clean surfaces and thus resulting in weaker tablets . Since tablet formation is dependent on the formation of new clean surfaces, high concentration or over mixing of materials form weak bonds result in weak tablets. Conversely, in materials in which the shear strength is greater than the tensile strength , particles may be preferentially fractured, and the smaller fragments then help to fill up the adjacent air spaces. This is most likely to occur with hard, brittle particles and is known as brittle fracture ; sucrose behaves in this manner. The ability of a material to deform in a particular manner depends on the lattice structure ; in particular whether weakly bonded lattice planes are inherently present. Brittle fracture creates clean surfaces that are brought in intimate contact by applied load.

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13 Irrespective of the behaviour of large particles of materials, small particles may deform plastically through a process known as microsquashing , and the proportion of fine powders in a sample may therefore be significant. Asperities that are sheared off larger, highly irregular particles could also behave in this way; hence particle shape is an important factor. Consolidation Consolidation has been described as the increase in the mechanical strength of a material as a result of particle/particle interactions . Various mechanisms of powder consolidation are Cold welding Fusion bonding Asperitic melting The consolidation (i.e. bonding) mechanisms of the powder is influenced by its chemical nature , the surface area of the contact point, contamination (including film coating, such as magnesium stearate) and interparticulate distance

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14 Cold Welding When the surfaces of two particles approach each other closely enough (less than 50nm), their free surface energies produce a strong attractive force through a process known as cold welding . The nature of the bonds so formed are similar to those of the molecular structure of the interior of the particle surface , but the actual surface area involved may be small. This hypothesis is the major reason for the increasing mechanical strength of a bed of powder when subjected to rising compressive forces. Fusion Bonding: On the macro scale, most particles have an irregular shape, so that there are many points of contact in a bed of powder. Any applied load to the bed must be transmitted through this particle contacts. However, under appreciable forces, this transmission may result in the generation of considerable frictional heat . This heat produce, the local rise in temperature sufficient to cause melting of the contact area of the particles, which would relieve the stress in that particular region. When the melt solidifies, fusion bonding occurs, which in turn results in an increase in the mechanical strength of the mass.

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15 Asperitic melting During compression, the powder compact typically undergoes a temperature increase usually between 4 and 30 o C , which depends on the friction effects, the specific material characteristics , the lubrication efficiency , the magnitude and rate of application of compression forces , and the machine speed. As the tablet temperature rises, stress relaxation and plasticity increases while elasticity decreases and strong compacts are formed . Therefore, compression of material at elevated temperature with increase in ductility should result in stronger tablets . Asperitic melting is believed to be important only with relatively low melting point materials for which even very hard asperities are pushed into a more plastic material.

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16 Events of compression cycle (i) Consolidation time: time to reach maximum force. (ii) Dwell time : time at maximum force. (iii) Contact time : time for compression &decompression excluding ejection time. (iv) Ejection time : time during which ejection occurs. (v) Residence time : time during which the formed compact is within the die.

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17 Decompression In tabletting, the compression process is followed by a decompression stage, as the applied load is removed. Complete tabletting cycle involves compression, decompression and ejection stages . It is now realized that the decompression stage is as important as (but not independent of) the compression stage in determining whether or not a tablet formulation will form satisfactory tablets. For example, some deformation processes are time-dependent and occur at various rates during the compaction sequence, so that the tablet mass is never in a state of stress/strain equilibrium during the actual tabletting process. This means that the rate at which load is applied and removed may be a critical factor in materials for which dependence on time is significant. More specifically, if a plastically deforming solid is loaded (or unloaded) too rapidly for the process to take place, the solid may exhibit brittle fracture. Capping and lamination tendencies of tablet formulations are related to their plastic and elastic behaviour during the compression/ decompression/ ejection cycle .

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18 Force transmission through a powder bed The resistance to differential movement of particles caused by cohesiveness results in the applied force not being transmitted uniformly throughout the entire mass. In the case of single-station press, the force exerted by the upper punch diminishes exponentially at increasing depths below it. Thus, the relationship between upper punch force, F A , and lower punch force, F L , may be expressed in the form: F L =   F A .   e - kH/D where k is an experimentally determined material-dependent constant that includes a term for the average die-wall frictional component. H and D are the height and diameter of the tablet respectively

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19 The difference between the two punch forces should be minimized in pharmaceutical tabletting operations, so that there is no significant difference in the amount of compression and consolidation between one region of the tablet and another. The effect of die wall friction can be reduced by having smaller tablet-to-diameter ratios and by adding lubricants The distribution of compression force being applied to the top of a cylindrical powder mass is given as F A =   F L +  F D where F A is the force applied to the upper punch, F L is that proportion of it transmitted to the lower punch, and F D is the reaction of the die wall due to the friction at  this surface.

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20 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, F M , has been proposed, where: F M =   (F A +  F L )  /  2 F M offers a practical friction-independent measure of compaction load, which is generally more relevant than F A . In single-station presses, where the applied force transmission decays exponentially then, a more appropriate geometric mean force, F G , might be: F G =    ( F A .   F L ) 0.5 The use of these parameters is probably more appropriate than the use of F A when determining the relationships between compression force and such properties as tablet strength

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21 As the compressional force is increased and the repacking of the tabletting mass is completed , the material may be regarded as a single solid body. Then, the compressive force applied in one direction (e.g. vertical) results in a decrease, DH, in the height, i.e. a compressive stress. In the case of an unconfined solid body, this would be accompanied by an expansion in the horizontal direction of DD. The ratio of these two dimensional changes are known as the Poisson ratio of the material, defined as: l     =   D D   /   D H The Poisson ratio is a characteristic constant for each solid material and may influence the tabletting processes.

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The material is not free to expand in the horizontal plane because it is confined in the die. Consequently, a radial die-wall force F R develops perpendicularly to the die-wall surface, materials with larger Poisson ratios giving rise to higher values of F R . Classical friction theory can be applied to deduce that the axial frictional force FD is related to F R by the expression: F D =    m W   .  F R where  m W is the coefficient of die-wall friction. F R is reduced when materials of small Poisson ratios are used, and in such cases, axial force transmission is optimum.

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A common method of comparing degrees of lubrication is to measure the applied and transmitted axial forces and determine the ratio F L / F A . This is called the coefficient of lubrication , or R value. The ratio approaches unity for perfect lubrication (no wall friction), and in practice, values as high as 0.98 may be realized. Values of R should be considered as relating only to the specific system from which they are obtained, because they are affected by other variables, such as compressional force and tablet H/D (height / diameter) ratio. Decompression leads to a new set of stresses within the tablet as a result of elastic recovery, which is augmented by the forces necessary to eject the tablet from the die. Irrespective of the consolidation mechanism, the tablet must be mechanically strong enough to withstand these new stresses, otherwise structural failure will occur.

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In particular, the degree and rate of stress relaxation within tablets, immediately after the point of maximum compression have been shown to be characteristic of a particular system. This phase of the cycle can provide valuable insight into the reasons behind inferior tablet quality and may suggest a remedy. If the stress relaxation process involves plastic flow , it may continue after all compression force has been removed, and the residual die wall pressure will decay with time. It is possible to interpret plastic flow in terms of viscous and elastic parameters in series. This interpretation leads to the relationship of the form: Log F t =   Log F m -  kt F t is the force left in the visco-elastic region at time,t, F m is the total magnitude of the force at time t=0 (i. e. when decompression begins) and k is the visco-elastic slope and a measure of the degree of plastic flow.

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Materials with higher k values undergo more plastic flow and such materials often form strong tablets at relatively low compaction forces. On the other hand, the changing thickness of the tabletting mass due to the compactional force and subsequently due to elastic recovery (ER) during unloading can be used to obtain a measure of plastoelasticity ER/PC; Where PC is the plastic compression of the material under constant load.

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REFERENCES: Compression Physics in the Formulation Development of Tablets Sarsvatkumar Patel, Aditya Mohan Kaushal, & Arvind Kumar Bansal Department of Pharmaceutical Technology (Formulations), National Institute of Pharmaceutical Education and Research(NIPER), S.A.S.Nagar, India