Physics of Tablet compression- Part II (Compaction profiles)

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compaction simulators, instrumented tableting machines, force - displacement profile, force-time profile, die wall friction

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1 PHYSICS OF TABLET COMPRESSION Part II (Compaction Profiles) 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 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 INTRODUCTION

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3 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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4 INSTRUMENTATION FOR STUDYING COMPACTION PROFILES 1. Attachable Instrumentation (Instrumented punches) Instrumentation for rotary machines includes strain-gauge punches , displacement transducers and radio telemetry system for obtaining force and displacement signals for accurate measurement of the operational characteristics of high speed tabletting machines. The strain gauges are mounted as closely as possible to the tips of the punches to minimize errors resulting from longitudinal punch distortion during compression. The displacement transducer is mounted in a punch guide adjacent to a standard punch that is modified to couple it mechanically to a transducer. A battery powered transmitter rotating with the turret, and combined with an aerial bonded to the circumference of the turret, sends the signals to a receiver mounted on a tie bar of the machine. The data is then accumulated or transmitted via telemetry to a computer.

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5 The instrumented punches are limited to one station, limited to one size and shape of tooling. Roll pin instrument reports data for all stations and any tooling.

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6 2.Fixed Instrumentation Telemetric systems are inappropriate for monitoring routine production batches. The strain gauges can be mounted at various positions on tablet presses to measure peak compaction forces on both the top and bottom punches. The most accurate and convenient position for such strain gauges is on the roll pin or the carriage pins. Measurement of the ejection force , is equally important as that of compression force but difficult to measure. To measure the ejection force instrumented ejection cams are used. The system must be designed in such a way that the force output is independent of the contact position of the punch . This can be achieved by inserting strain gauges in a metal platform that is then mounted below a modified ejection cam. Instrumentation is also available to measure sweep-off force to predict the force of adhesion between a tablet and the lower punch.

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7 3. Die-wall Instrumentation gives information about transmitted radial stress that can be used to assess lubricating properties of materials. It is also useful for elucidating the friction phenomena during compaction and related tableting problems such as capping, lamination and tooling wear. In fact, capping and lamination often originate in the compression and decompression phases, but become evident at ejection phase. Compaction simulators The compaction simulators have certain advantages such as mimicing the cycle of many presses, and can be used for stress–strain studies Compaction simulators have potential application in pharmaceutical R&D, such as studying basic compaction mechanism , process variables , scale-up parameters , trouble shooting problem batches, creating compaction data bank, and fingerprinting of new active APIs or excipients.

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8 COMPACTION SIMULATORS

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9 COMPACTION ANALYSIS: Compaction is analysed by compaction simulators , which are attached to punching machines. These simulators collect or measure the data from forces on punches, displacement of punches, die wall friction, ejection force and temperature change. From these data we can study the impact of force on tablet. COMPACTION PROFILES: Compaction data obtained from tableting machine are 3 types: Force-time profile Force- displacement profile Die wall force profile .

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10 1.Force-Time profile: Compression force-time profiles are used to characterize the compression behavior of the active ingredients and excipients. On a rotary tablet press, the force-Time curves are segmented in to three phases; Phases of compression events a) Compression phase: horizontal and vertical punch movement b) Dwell phase only horizontal punch movement c) Decompression / Relaxation phase both punches moving away from upper & lower surfaces

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11 Compression phase: Compression is the process in which maximum force is applied on powder bed in order to reduce its volume. Dwell phase: When compression force reaches a maximum value, this maximum force is maintained for prolonged period before decompression. The time period between the compression phase and decompression phase is known as DWELL TIME. Decompression phase: Removal of applied force on powder bed i.e, both punches moving away from upper and lower surfaces. 1. Compression phase – horizontal and vertical movement of punch movement 2. Dwell phase - only horizontal punch movement (punch head is under compression roller

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12 Compression event divided in to series of time periods. Consolidation time : Time period to reach maximum force. Dwell time : Time at maximum force. Contact time : Time for compression and decompression. Ejection time : Time during which ejection occurs. Residence time : Time during which the formed tablet is with in die.

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13 The compression area A 1 and compression slope (sl c ) describe the compression phase. The area ratio A 6 /A 5 and peak offset time(t off ) characterize the dwell phase. The A 5 and A 6 are obtained by drawing a parallel line to X-axis from starting to end point of dwell phase. The decompression area A 4 and the decompression slope (Sl d ) describe the decompression phase. Compressioin force-time curve for MCC

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14 A 1 (compression phase) is small for powders having high density ( low volume; due to less void spaces) Example :Dicalcium phosphate dihydate. A 1 (compression phase) large for powders having low density (more void spaces) Example : Microcrystalline cellulose. Plastic materials show a decrease in force over dwell time , in contrast a plateau is observed for brittle materials. The dwell phase coefficient (A 6 / A 5 ) can be used to measure the plasticity of a substance mixture.

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15 PEAK OFFSET TIME (t off ): Peak offset time is the difference between the time of maximum pressure and middle of dwell time. At a given F max ,short t off values are characteristics of materials that consolidate mainly by brittle fracture, whereas large values indicates an increase in plastic flow.

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16 Assessment of the compaction behavior of materials is done by force-displacement profile. Force-displacement profile can be used to determine the behavior of plastic and elastic materials. Stress relaxation is observed to be minimal in case of plastic deformation; where as materials that undergoes elastic deformation tend to relax to a greater extent during and/or after decompression At a given f max the displacement area of plastic deformation is more when compared to the displacement area of elastic deformation. 2. Force - Displacement profile: Force-displacement profile showing the plastic deformation and frictional work, and the elastic deformation areas.

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17 Most of the materials undergo plastic and elastic deformation at different stages, hence the work required for compression is the sum of work necessary to rearrange the particles, deform and finally to fragment them. Net work of compaction (NWC) is calculated by subtracting the work of elastic relaxation (WER) from the gross work of compaction (GWC). NWC = GWC – WER GWC = W f + W p + W e +W fr W f = work against friction W e = work of elastic deformation W p = work of plastic deformation W fr = work of fragmentation. This information can be used to predict the compaction behavior of pharmaceutical materials. Ex: Lesser the amount of work needed to compress indicates higher the compressibility of material and vice versa.

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18 3.DIE WALL FORCE PROFILE: During tableting friction arises between the material and the die wall and also between particles (Interparticulate or internal friction). Internal friction is significant only during particle slippage and rearrangement at low applied pressures. The coefficients of friction related to tableting process are; a) Static friction b) Dynamic friction. Static friction : Force require to initiate sliding Dynamic friction: Force to maintain sliding between two surfaces

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19 Static friction (µ 1 ) = maximum axial frictional force / maximum radial force Dynamic friction (µ 2 ) = Ejection force / residual die wall force Friction phenomena can be determined from upper and lower punch force and displacement. Lubrication ratio (R value) is the ratio of the maximum lower punch force to the maximum upper punch force The die wall force reaches a maximum just after the maximum upper and lower force, and a constant residual value after upper and lower forces become zero. When the ejection process starts it increases again. The residual die wall force is the average of values in the constant region at zero upper punch force, The difference of displacement between upper and lower punch gives a measure of the tablet area contact with the die wall.

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21 S.No Material Residual die wall force 1 Plastic large 2 Brittle Medium 3 Elastic Low The high die wall force during ejection is a sign of adhesion of powders to the die.

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22 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

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23 THANK YOU