A SEMINAR ONCOMPRESSION AND COMPACTION Nisargrx@hotmail.co.uk Wish to have more ,mail to above id Your e mail id :- Will send you skydrive link for free:-
CONTENTS COMPRESSION PROPERTIES AXIL FORCE AND RADIAL FORCE PROCESS OF COMPRESSION MEASURMENT OF FORCE DISTRIBUTION OF FORCE
COMPRESSION PROPERTIESCOMPRESIBILITY:- It is the ability of powder to decrease in volume under pressure.COMPACTIBILITY:- It is the ability of a powder to be compressed into a tablet of a certain strength or hardness.1. Plastic material ex:kaolin, pvp.1. Elastic material ex:aspirine, microcrystalline cellulose
AXIAL FORCE : it is the force required to attempt of material to constrict vertically.
RADIAL FORCE : it is the force required to the attempt the material to expand horizontally
PROCESS OF COMPESSION1. Transitional repacking2. Deformation at the point of contact3. Fragmentation4. Bonding5. Deformation of solid body6. Decompression7. Ejection
Transitional repacking or particle rearrangement. Granules to be placed in the hopper of the tablet press. Formulation and processing are designed to ensure that at a fast production rate the weight variation of the final tablet is minimal. The particle size distribution of granulation and the shape of the granules determine the initial packing as the granules is delivered in to the die cavity. In the initial event the punch and particle movement occur at low pressure. The granule flow with respect to each other, with the finer particle entering the void between the larger particle, and the bulk density of the granulation is increased.
Spherical particle undergo less particle rearrangement then irregular particle as the spherical particle tend to assume a close packing rearrangement initially. To achieve a fast flow rate required for high- speed presses the granulation is generally processed to produce spherical or oval particles. Thus, particle rearrangement and the energy expended in rearrangement era minor consideration in the total process of compression
Deformation at point of contact When the stress is applied to a material, deformation (change of forms) occurs. If the deformation disappears completely (return to the original shape) upon release of stress , it is an Elastic deformation. A deformation that dose not completely recover after release of the stress is known as a Plastic deformation. The force required to initiate plastic deformation is known as the yield stress. In the initial event the punch and particle movement occur at low pressure. the granule flow with respect to each other ,with the finer particles entering the void between the larger particle, and the bulk density of granule is increased.
spherical particle under go less particle arrangement then irregular particles as the spherical particles tend to assume a close packing arrangement initially. To achieve a fast flow rate required for high speed presses the granulation is generally processed to produce spherical or oval particles, thus particle rearrangement and the energy expended in rearrangement are minor considerations in the process of compression. When the particles of a granulation are so closely packed that no further filing of the void can occur, a further increases of compressional force cause deformation at he point of contact. Both plastic and elastic deformation may occur although one type predominates for a given material. Deformation increase the area of true contact and the formation of potential bonding areas.
Fragmentation and Deformation At higher pressure, fracture occur when the stresses within the particles become great enough to propagate cracks. fragmentation further densification, with the infiltration of the smaller fragment in to the void space Fragmentation increase the number of particle and form new, clean surface that are potential bonding area.
Bonding Several mechanism of bonding in the compression process have been conceived, but they have not been useful in in the prediction of the compressional properties of material. Three theory are 1 . Mechanical theory 2 . The intermolecular theory. 3. the liquid surface film theory
Mechanical theory:- This theory proposes that under pressure the individual particle undergo elastic, plastic or brittle deformation and that the edges of the particle intermesh, forming a mechanical bond. If only the mechanical bond exists, the total energy of compression is equal to the sum of the energy of deformation, heat and energy adsorb for each constituent. Mechanical inter locking is not a major mechanism of bonding in pharmaceutical tablets.
The inter molecular theory:- The molecule (or ions) at the surface of the solid have unsatisfied intermolecular force, which interacts with other particles in true contact. According to the 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.
A microcrystalline cellulose tablet has been described as a cellulose fibril in which the crystals are compressed close enough together so that hydrogen bonding between them occurs. It appear that very little deformation or fusion occur in the compression of microcrystalline cellulose. Aspirin crystals under go slight deformation and fragmentation at low pressure, it appear that hydrogen bonding has strongly bonded the tablet, because the granules retain their integrity with further increase in pressure .
The liquid surface film theory:- The liquid surface film theory attributes bonding to the presence of a thin liquid film, at the surface of the particle induced by the energy of compression. During the compression an applied force is exerted on the granules; however, locally the force applied to a small area of true contact so that a very high pressure exists at the true contact surface. The local effect of the high pressure on the melting point and solubility of a material is essential to bonding. The relation of pressure and melting point (clapeyron) dT T(V1-Vs) T-temperature dP H By analogous reasoning , the pressure distribution in compression is such that the solubility is increased with increasing pressure.
With an increase in solubility at the point of true contact, solution usually occur in the film of adsorb moisture on the surface of the granule. When the applied pressure is released and the solubility decrease, the solute dissolve in the adsorbed water crystallizes in small crystals between the particles. the strength of the bridge depend on the amount of material deposited and rate of crystallization. At higher rates of crystallization, a finer crystalline structure and a greater strength are obtained.
The poor compressibility of most water insoluble material and the relative ease of compression of water soluble materials suggest that pressure induced solubility is important in tableting. The moisture may be present as that retain from the granulating solution after drying or that adsorb from the atmosphere. Granulation that are absolutely dry have poor compressional characteristics.
Decompression:- After the compression and consolidation of the powder in the die, the formed compact must be capable of withstanding the stresses encountered during decompression and tablet ejection. The rate at which the force is removed (dependent on the compression roller diameter and the machine speed) can have a significant effect on tablet quality. The same deformation characteristics that come into play during compression, play a role during decompression. After application of the maximum compression force, the tablet undergoes elastic recovery.
While the tablet is constrained in the die, elastic recovery occurs only in the axial direction. If the rate and degree of elastic recovery are high, the tablet may cap or laminate in the die due to rapid expansion in the radial direction only. Tablets that do not cap or laminate are able to relieve the developed stresses by plastic deformation. Since plastic deformation is time-dependent, stress relaxation is also time-dependent. Formulations which contain significant concentrations of microcrystalline cellulose typically form good compacts due to its plastic deformation properties. However, if the machine speed and the rate of tablet compression are significantly increased, these formulations exhibit capping and lamination tendencies.
The rate of decompression can also have an effect on the ability of the compacts to consolidate (form bonds). Based on the liquid-surface film theory, the rate of crystallization or solidification should have an effect on the strength of the bonded surfaces. The rate of crystallization is affected by the pressure (and the rate at which the pressure is removed). High decompression rates should result in high rates of crystallization. Typically, slower crystallization rates result in stronger crystals. Therefore, if bonding occurs by these mechanisms, lower machine speeds should result in stronger tablets. The rate of stress relieve is slow for acetaminophen so cracking occurs while the tablet is within the die. with microcrystalline cellulose the rare of stress relieve is rapid, and intact tablets result.
Ejection As the lower punch rises and pushes the tablet upward there is a continued residual die wall pressure and considerable energy may be expanded due to the die wall friction. As the tablet removed from the die, the lateral pressure is relieved, and the tablet undergoes elastic recovery with an increase (2 to 10%) in the volume of that portion of the tablet removed from the die. During ejection that portion of the tablet within the die is under strain, and if this strain exceeds the sheer strength of tablet the tablet break as elastic recovery.
A large value of the heckel constant indicate the onset of plastic deformation at relatively low pressure. A heckel plot permits an interpretation of the mechanism of bonding. For dibasic calcium phosphate dihydrate, which undergoes fragmentation during compression, the heckel plot is nonlinear and has small value for its slope (a small heckel constant). As dibasic calcium phosphate dihydrate fragments, the tablet strength is essentially independent of particle size. For sodium chloride a heckel plot is linear indicating that sodium chloride undergoes plastic deformation during compression. no fragmentation occur.
Effect of friction At least two major component to the frictional force can be distinguished Interparticulate friction :- this arises at particle /particle contacts and can be expressed in term of a coefficient of interparticulate friction m1. it is more significant at low applied loads. Material that reduce this effect are referred to as glidants. Ex:- colloidal silica, talc, corn starch Die-wall friction :-this result from material being pressed against the die wall and moved down it ; it is expressed as mw, the coefficient of die wall friction.
This effect become dominant at high applied forces when particle rearrangement has ceased and is particularly important in tabletting operations. Most tablets contain a small amount of an additive design to reduce die wall friction; such additives are called lubricants. Ex:-magnesium stearate, talc, PEG, waxes, stearic acid Force distribution FA HO H FR FD D FL Diagram of a cross section of a typical simple punch and die assembly
This investigation carried on single station press. Force being applied to the top of a cylindric power mass and the following basic relationships apply. FA=FL+FD Where, FA =is the force applied to upper punch FL =is that proportion of it transmitted to the lower punch FD =is a reaction at the die wall due to friction at this surface Because of this difference between the force applied at the upper punch and that affecting material closed to the lower punch, a mean compaction force, FM where, FM=FA+FL/2 A recent report confirm that FM offer a practical friction- independent measure of compaction load, which is generally more relevant then FA.
In single station presses, where the applied force transmission decay exponentially, a more appropriate geometric mean force FG, might be 0.5 FG=(FA . FL) Use of this force parameters are probably more appropriate then use of FA when determining relationships between compressional force and such tablet properties as tablet strength.
Development of radial force As the compressional force increased and any repacking of the tabletting mass is completed, the material may be regarded to some extent as a single solid body. Then as with all other solid, compressive force applied in one direction (e.g. vertical) result in decrease in H in the height, i.e. a compressive stress. In the case of an unconfined solid body, this would be accompanied 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 of the material, defined as: Poisson ratio = D/H The poisson ratio is a characteristic constant for each solid and may influence the tabletting process in following way.
Under the condition illustrated in figure , the material in not free to expand in horizontal plane because it is confined in the die. Consequently, a radial die wall force FR develops perpendicular to the die wall surface, material with larger poisson ratios giving rise to higher value of FR. Classic friction theory can then be applied to deduce that the axial frictional force FD is related to FR by the expression: FD = mw.FR Where mw is the coefficient of die wall friction. Note that FR is reduced when material of small poisson ratio are used, and that in such cases, axial force transmission is optimum.
Die wall lubrication Most pharmaceutical tablet formulation require the addition of a lubricant to reduce friction at the die wall . Die wall lubricant function by interposing a film of low shear strength at the interface between the tabletting mass and the die wall. Preferably, there is some chemical bonding between this boundary lubricant and the surface of the die wall as well as the edge of the tablet. The best lubricant are those with low shear strength but strong cohesive tendencies in direction at right angles to the plane of shear.
Ejection forces Radial die wall forces and die wall friction also effect the ease with which the compressed tablet can be removed from the die. The force necessary to eject a finished tablet follows a distinctive pattern of three stage. The first stage involves the distinctive peak force required to initiate ejection, by braking of tablet/die wall adhesions. A smaller force usually follows, namely that required to push the tablet up the die wall. The final stage is marked by declining force of ejection as the tablet emerges from the die.
Variation on this pattern are sometimes found, especially when lubrication is inadequate and/or “slip-stick” condition occur between the tablet and the die wall, owing to continuing formation and breakage of tablet die wall adhesion. A direct connection is to be expected between die wall frictional forces and the force required to eject the tablet from the die, FE. For e.g. well lubricated systems have been shown to lead to smaller FE values. Compection profiles Monitoring of that proportion of the applied pressure transmitted radially to the die wall has been reported by several groups of workers. For many pharmaceutical materials, such investigation lead to characteristic hysteresis curves , which have been termed compaction profiles.
The radial die wall forces arises as a result of tabletting mass attempting to expand in the horizontal plane in response to the vertical compression. The ratio of this two dimensional changes, the Poisson ratio, is an important material dependent property affecting the compressional process. When the elastic limit of the material is high, elastic deformation may make major contribution, and on removal of the applied load, the extent of the elastic relaxation depend upon the value of the materials modulus of elasticity (young’s modulus). If this value is low, there is considerable recovery, and unless a strong structure has been formed, there is the danger of structural failure. If the modulus of elasticity is high, there is small dimensional change on decompression and less risk of failure.
The area of the hysteresis loop (OABC’) indicate the extent of departure from ideal elastic behavior, science for perfectly elastic body, line BC’ would coincide with AB. In many tabletting operation the applied force exceed the elastic limit (point B), and brittle fracture and/or plastic deformation is then a major mechanism. For example, if the material readily undergoes plastic deformation with a constant yield stress as the material is sheared, then the region B to C should obey the equation. PR = PA – 2S Where S is the yield stress of the material The slope of this plot is unity, so that mark deviation from this value may indicate a more complex behavior.
Deviation could also be due to the fact that the material is still significantly porous. For e.g. since point C represent the situation at the maximum compressional force level, the region CD is therefore the initial relaxation response as the applied lode is removed. In practice, many compaction profiles exhibit a marked change in the slope of this line during decompression, and a second yield point D has been reported. Perhaps the residual redial pressure (intercept EO), when all the compressional force has been removed, is more significant, since this pressure is an indication of the force being transmitted by the die wall to the tablet.
As such, it provide a measure of possible ejection force level and likely lubricant requirements, it suggests a strong tablet capable of at least withstanding such a compressive pressure. A low value of residual redial pressure, or more significantly, a sharp change in slop (DE) is sometime indicative of at least incipient failure of the tablet structure. In practical term this may mean introducing a plastically deforming component (e.g.pvp as binder).
Energy involve in compaction Tablet machines, roller compactors, and similar types of equipment required a high input of mechanical work. The work involve in various phase of tablets operation includes, That necessary to overcome friction between particles, That necessary to overcome friction between the particles and machine parts, That required to induce elastic and/or plastic deformation of the materials, That required to cause brittle fracture within the materials, and That associated with the mechanical operation of various machine parts.
Nelson and associate, who compared the energy expenditure in lubricated and unlubricated sulfathiazole granules. Lubrication reduce energy expenditure by 70%, chiefly because of a lessening of the major component, namely energy utilized during ejection of the finished tablet. Lubricant has no apparent effect on the actual amount of energy required to compress the material. Compression Energy expended(joules) process Unlubricated Lubricated Compression 6.28 6.28 Overcoming die wall friction 3.35 -- Upper punch withdrawal 5.02 -- Tablet ejection 21.35 2.09 Total 36.00 8.37
By assuming that only energy expended in the process of forming the tablet cause a temperature rise, Higuchi estimated the temperature rise to be approximately 5 c. For a single punch machine operating at 100 tablets per min, and approximately 43 kcal/hr were required for unlubricated granules. Wurster and creekmore by use of an internal temperature probe found a 2 to 5 c rise in the temperature of tablet compressed from microcrystal cellulose, calcium carbonate, starch and sulfathiazole The temperature of compressed tablet is affected by the pressure and speed of tablet machine.
In non instrumented single punch tablet machine set at minimum pressure, the compression of 0.7 g of sodium chloride caused a temperature increase of 1.5 c ; when the machine was set near maximum pressure , the temp. increase was 11.1 c . When the machine was operating at 26 and 140 rpm the increase in temp. was 2.7 and 7.1 respectively. When the machine was operating at 26 and 140 rpm to compress 0.5 g of calcium carbonate, the increase in temp. was 16.3 and 22.2 c respectively.
Properties of tablet influence by compression Higuchi and train were the first pharmaceutical scientists to study the effect of compression on tablet characteristics. The relationship between applied pressure and weight, thickness, density, and the force of ejection are relatively independent of the material being compressed Density and porosity Hardness and tensile strength Specific surface Disintegration Dissolution
30 Lactose porosity 20 % lactose-aspirin 10 aspirin 500 1000 2000 4000 applied pressure, kg/cm 2 The effect of applied pressure on the porosity of various tablet with 10% of starch. Porosity and density inversely proportional to each other.
80 radial tensile 60 strength kg/cm 2 40 20 axial 200 4000 6000 8000 applied pressure, kg/cm 2The effect of applied pressure on tensile strengths of tablet of dibasic calcium phosphate granulated with 1.2% starch.
DISTRIBUTION OF FORCE : Fm = Fa + Fd 2 Fa = Fl +FdFa =force applied to upper punch. Fl =force transmitted to lower punch.Fd =is the reaction at the die wall due to friction at the surface. Fm = mean force
Relation between applied and transmitted force The relation between applied & transmitted forces Fa, Fl practically linear In case of single punch the force exerted by upper punch ↓ exponentially as depth ↑ The relation between Fa, Fl written as Fl = Fl٠ Fl e KH / D Rearranging the above equation Fa = Fl ٠ eKH / D
COMPACTION PROFILERadial pressure is due to theattempt of material to expandhorizontally.Axial pressure is due to theattempt of material to constrictvertically. OA =Shows Early repacking AB = elastic deformation BC = plastic deformation CD = elastic recovery DE = plastic recovery
MEASURMENT OF FORCES1) STRAIN GAUGE : A coil of high resistant with length width ratio 2:1 & resistant 100 ohm is suitable During compression the applied force causes a small elastic deformation of two punches Strain gauge are connected to punch as close to the compression site. it is deformed as the punch deformed With the deformation, the length of resistance wire ↓ & its diameter is ↑. The resulting decrease in resistance is measured by wheat stone bridge as a recording devise. Care must be taken to use low voltage so that heating effect do not interfere with the strain measurement.
2)PIEZO-ELECTRIC LOAD CELLS: Certain crystals like quartz may be used. When subjected to external force these develop an electrical charge proportional to the force. This transducer is connected to amplifier which converts the charge in to dc voltage. The small piezo-electrical transducer are connected to upper & lower punch holder of single station press. The disadvantage is the dissipation of charge with time, hence nit suitable for static measurement.
DIAGRAMATIC REPRASENTATION OF PIEZO-ELECTRIC CELLS
REFRENCES1. The theory & practice of industrial pharmacy By: Lachman2. The science of dosage form design edited by: Michael E. Aulton3. Text book of physical pharmacy by Alfred Martin, James Swarbric.4. By internet source.