Factors affecting the strength of the tablet-

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Factors affecting the strength of the tablet: 

Factors affecting the strength of the tablet Presented by PG- 1 st Year Department of Pharmaceutics

Content : 

5-Jul-12 Dept of Pharmaceutics 2 Content Introduction Factors Moisture Content Particle size & Particle shape Compaction force Lubrication Effect of entrapped air Binders

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5-Jul-12 Dept of Pharmaceutics 3 Conclusion Summary References

Introduction: 

5-Jul-12 Dept of Pharmaceutics 4 Introduction Definitions:- Tablet :- It is a solid unit dosage form consisting of one or more medicament with excipients prepared by either compression or molding.

Factors : 

5-Jul-12 Dept of Pharmaceutics 5 Factors Moisture content:- A small proportion of moisture content is required in the formulation of the tablet. This moisture content is important for the mechanical strength of the tablet. If the moisture content is less there will be a increase in the Die-wall friction, This increase in the friction is due to increase in the stress ratio.

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5-Jul-12 Dept of Pharmaceutics 6 At optimum moisture level, the Die-wall friction is reduced , This is due to the decrease in stress ratio. As the moisture content is increased, there will be decrease compact strength, This decrease is due to the reduction in interparticulate bonding.

Effect of moisture content on the strength of the tablet can be known by four states, namely :-: 

5-Jul-12 Dept of Pharmaceutics 7 Effect of moisture content on the strength of the tablet can be known by four states, namely :- Pendular state Funicular state Capillary state Droplet state

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5-Jul-12 Dept of Pharmaceutics 8 Pendular state :- Liquid films are formed on the surface of the powder particles, when they are wetted with the liquid, At the point of contact this liquid films are formed and later they combine to form discrete liquid bridges. In this state movement of liquid is not possible and migration of water takes place by evaporation followed by diffusion, flow of water vapour through pores of the powder bed.

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5-Jul-12 Dept of Pharmaceutics 9 Therefore these bridges provide cohesive force and result in a condition called the pendular state. This pendular state has low mechanical strength

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5-Jul-12 Dept of Pharmaceutics 10 Funicular state As the liquid content is increased several bridges coalesce giving rise to funicular state. This results in increase in the strength of the granules and thereby increasing the strength of the tablet. But in this state the movement of liquid is possible through continuous liquid bridges, but movement is restricted by friction.

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5-Jul-12 Dept of Pharmaceutics 11 As the amount of liquid is reduced the continuity of the film around the particle is lost and water is held only in lens like rings at the point of contact.

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5-Jul-12 Dept of Pharmaceutics 12 Capillary state:- The particles come closer to each other on addition of more amount of liquid, Due to this the void spaces within the granules are entirely eliminated. At this point there is a bonding due to interfacial forces and intragranular forces. This state is known as capillary state, which is considered as a optimum strength required for the granules to give a tablet of good strength

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5-Jul-12 Dept of Pharmaceutics 13

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5-Jul-12 Dept of Pharmaceutics 14 Droplet state:- Further addition of liquid results in droplet formation in which the particles are still held together by surface tension. But such structures are weaker. Ex:- Acetaminophen with 2% moisture content gives tablet of optimum strength.

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5-Jul-12 Dept of Pharmaceutics 15 Generally moisture content is usually determined by loss on drying. Formula for calculating moisture content in percentage:- % moisture content = ( loss in weight ) Χ 100 (Initial weight)

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5-Jul-12 Dept of Pharmaceutics 16 Particle size:- Decrease in particle size increases the strength of tablets. The powder should posses good flow property to have uniformity of weight of the tablet.

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5-Jul-12 Dept of Pharmaceutics 17 The flow property of powder depends upon:- - particle size - particle shape - porosity - Density - Moisture of the powder The rate of flow α Size of particles of powder

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5-Jul-12 Dept of Pharmaceutics 18 As the particle size increases there is an influence of gravitational force on particles leading to the increase in flow property of powder. Beyond particular point flow properties decrease as the particle size increases i.e.. Beyond 10 µm. This is due to Vanderwaals forces Electrostatic forces Surface tension forces Mechanical forces

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5-Jul-12 Dept of Pharmaceutics 19 - Particle Density:- Particles having high density and low internal porosity tend to posses good flow properties - Porosity:- The space between the particles is known as voids. And the volume occupied by this voids is called as void volume.

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5-Jul-12 Dept of Pharmaceutics 20 Void Volume = Bulk Volume – True Volume Porosity/ voids is the ratio of the void volume to the bulk volume of the packing. Therefore Porosity = Void volume Bulk volume = V/Vb, but, v = V b – V t therefore, porosity= V b – V t / V b

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5-Jul-12 Dept of Pharmaceutics 21 Porosity is generally expressed in percent ε = [1-vp/vb] X 100 Therefore there is a relation between compression & porosity, because porosity determines the rate of,disintegration dissolution,& drug absorption The flow property also depends upon angle of repose & flow property is measured by angle of repose.

Angle of repose: 

5-Jul-12 Dept of Pharmaceutics 22 Angle of repose Definition:- angle of repose is is defined as the maximum angle between pile of powder & the horizontal of plane. tan θ = h/r θ = tan -1 h/r where, h = height of pile r = radius of the pile ө = angle of repose Frictional forces in powders is measured by Angle of repose.

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5-Jul-12 Dept of Pharmaceutics 23 Lower the angle of repose, better will be the flow property. Decrease in particle size increases the angle of repose Finer particles ( which passed through 100 mesh) increases the angle of repose. Angle of repose in degrees Type of flow < 25 excellent 25-30 Good 30-40 Passable > 40 poor

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5-Jul-12 Dept of Pharmaceutics 24 Carr’s consolidation index :- it is also called as compressibility index. Def :- it is an increase in the mechanical strength of material due to particle particle interaction Consolidation index = tapped density –fluff density/fluff density. Consolidation index measures the potential strength that is built up by powder in its arch in hopper & also the ease with such an arch could be broken, so it is indirectly related with flow rate, cohesiveness & particle size

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5-Jul-12 Dept of Pharmaceutics 25 Consolidation index can be calculated by finding out fluff density & tapped density. Fluff volume :- fluff volume is a volume occupied by a powder when gently poured into a measuring cylinder & corresponding density is called as fluff density. Fluff density is the ratio of mass of powder to the fluff volume . Tapped volume is a volume occupied by a same mass of powder after a standard tapping, so tapped density is the ratio of mass of powder to the tapped volume.

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5-Jul-12 Dept of Pharmaceutics 26 Then, Consolidation index = D t – D f /D t Grading of powder according to Cars index Cars index Type of flow 5 - 15 Excellent 12 -16 Good 18 - 21 Fair to passable 23 - 25 Poor 33 - 38 Very poor > 40 Very very poor

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5-Jul-12 Dept of Pharmaceutics 27 Ex :- Decrease in particle size of sodium chloride results in increase in strength of tablet. But with aspirin decrease in particle size results in decrease in strength of tablet

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5-Jul-12 Dept of Pharmaceutics 28 Particle Shape :- Particle shape also affects the strength of tablet. Generally spherical shape is considered ideal for the tablet, because spherical shape increases the bulk density Particle shape affects the inter particle friction & consequently the flow properties of the powder. It also affects the the angle of repose of powder with low magnitude surface forces such as particles greater than 100µm

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29 Particle shape Effect on powder flow 1) Spherical Flow easily 2) Oblong with smooth edges Normally flows easily 3) Equidimentional with sharp edges like cube Does not flow as radially as 1 or 2 4) Irregular shape interlocking particle Poor flow & bridges easily 5) Irregular shape with two dimensional particles such as flakes Poor flow & causes bridging 6) Fibrous particles Very poor flow & bridges easily

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5-Jul-12 Dept of Pharmaceutics 30 Bridging :- Bridging refers to the stoppage of powder flow due to formation of rigid or semirigid structures within powder bulk Particle shape affects angle of repose of powder particularly powder with low magnitude surface forces such as paricles greater than 100µm. ex:- fine powders such as Talc & corn starch.

Compaction force :-: 

5-Jul-12 Dept of Pharmaceutics 31 Compaction force :- This involves……… Compaction Compactibility powders are normally compressed into tablets by applying pressure of about 5.0kg this process is called as compaction & compression.

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5-Jul-12 Dept of Pharmaceutics 32 Compressibility :- Compressibility is defined as the ability of powder mass to reduce in its volume under pressure. Compactibility :- It can be defined as ability of powder to be compressed into tablet of certain strength or hardness. Compression of powder material is depends upon…… 1) Plastic material 2) Elastic material

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5-Jul-12 Dept of Pharmaceutics 33 Deformation :- means change in form i.e. compression depends upon deformation of powder material. 1) Plastic material :- Plastic material undergoes irreversible deformation & shows permanent change during compression. this permanent change is due to formation of new bonding surfaces leads to proper binding of tablet material. Ex :- Paracetamol requires a plastic tableting matrix or wet massing to induce plasticity.

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5-Jul-12 Dept of Pharmaceutics 34 2) Elastic material :- Elastic material undergoes reversible deformation it is because of the elastic nature of the material & no fracture, no new surfaces are generated because of this very little permanent change is caused during compression. ex:- Aspirin & microcrystalline cellulose shows elastic deformation. elastic material tends to form the tablet of poor mechanical strength.

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5-Jul-12 Dept of Pharmaceutics 35 so for proper compression the tablet material should be plastic & also exhibit certain degree of brittleness . so accordingly if drug is plastic then the excepients should be brittle Ex :- lactose , calcium phosphate. so accordingly if drug is elastic then the excepients should be plastic Ex:- microcrystalline cellulose.

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5-Jul-12 Dept of Pharmaceutics 36 So if compaction force is high it results in rupture of tablet or hard tablet will be formed that will have longer periods of disintegration & dissolution. Ex:- when coloured sucrose coating is applied to preformed spherical granules ,the coating remain intact at low compaction force, but if high force is applied granules distorted & the coating ruptures this phenomenon is called as Mottling.

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5-Jul-12 Dept of Pharmaceutics 37 At higher force due to fragmentation new surfaces are formed causing an increase surface area, hence more is available for bond formation hence more will be hardness of the tablet, so according to Balshin equation F c = F co Vr -m where, F co = strength of tablet when V r =1 m = constant here V r is relative volume.

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5-Jul-12 Dept of Pharmaceutics 38 Relative volume = I/(1-e) e = porosity of compact. Shotton & Ganderton gave a general equation for the effect of applied pressure on the strength of tablet Log p = nFe + c, Where, p = applied pressure Fe = strength of compact c = constant.

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5-Jul-12 Dept of Pharmaceutics 39 Lubricants :- Lubricants are intended to reduce the friction between walls of tablet & die wall during tablet ejection from die cavity. Lubricants also prevents the adhesion of tablet material to the dies & punches Lubricants are spread over the surface of particle which reduces the bond formation between the particles, which leads to reduction in interparticle friction, leads to improve the flow property of the granules.

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5-Jul-12 Dept of Pharmaceutics 40 Poor selection or excessive amount of lubricants results in water proofing the tablet, resulting in the poor tablet disintegration & dissolution. The addition of proper lubricant is highly desirable if tablet material tends to stick to the dies & punches. The level at which the lubricants should be employed & the degree to which they need to be added in the granules should be determined experimentally

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5-Jul-12 Dept of Pharmaceutics 41 Lubricants at low concentration decrease the angle of repose leads to increase the flow property. In selecting lubricants proper attention must be given to its compatibility with the drug agent. Ex :- different talc significantly varied the stability of aspirin. talc with high calcium content & high loss on ignition was associated with increased aspirin decomposition.

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5-Jul-12 Dept of Pharmaceutics 42 Ex :- Magnesium stearate is one of the most widely used lubricants but due to its hydrophobic properties it retards the disintegration & dissolution. To overcome the water proofing sodium lauryl sulfate is used. Types of Lubrication :- 1) Intrinsic Lubrication. 2) Extrinsic lubrication.

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5-Jul-12 Dept of Pharmaceutics 43 1) Intrinsic Lubrication :- In this method lubricants like magnesium, calcium, zinc salts of stearic acid are directly added to granulation, mix & then compress. This is the most efficient & most commonly used method. 2) Extrinsic Lubrication :- In this method lubricants are directly apply to the tablet tool surface. In this the melted

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5-Jul-12 Dept of Pharmaceutics 44 wax is sprayed to the tool surface, after one tablet is ejected & before the granulation for the next tablet enters the die cavity. Commonly used lubricants :- Talc Magnesium stearate Calcium stearate Stearic acid Hydrogenated vegetable oil. Polyethylene glycol

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5-Jul-12 Dept of Pharmaceutics 45 Binders :- Binders also affect the strength of tablet. Binders are the agents which provides or imparts cohesive qualities to the powdered material & also to the tablet, which is remained intact after compression. Binders are used in dry as well as in solution form depending upon the other ingredients & method of preparation. The quantity of binder used has considerable influence on the characteristics of the tablet

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5-Jul-12 Dept of Pharmaceutics 46 The use of too much binder leads to formation of hard tablet that will not disintegrate easily & will cause excessive wear of dies & punches, while less amount leads to too soft tablet that tends to crumble. Ex:- If the Gelatin (1-5%) content of tablet is increased, leads to increase in hardness disintegration & dissolution time of tablet. Binders added to avoid tablet problems like picking, sticking.

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5-Jul-12 Dept of Pharmaceutics 47 Ex:- Ethyl cellulose should be used as alcoholic solution which otherwise may lead to retard disintegration & dissolution time of tablet. Commonly used Binders :- Starch Gelatin Acacia Sodium alginate Methyl cellulose PVP Tragacanth

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5-Jul-12 Dept of Pharmaceutics 48 Effect of Entrapped Air :- On rapid compression, air may be entrapped in the pores & if that air did not escape or removed force may seal the tablet. This leads to Capping & Lamination

References: 

5-Jul-12 Dept of Pharmaceutics 49 References Henry G.; “Physical characterisation of pharmaceutical solids”, Vol 70, page no 178-183, Dekker Series, New-York. Goran Alderborn, Christer Nystrom; “Pharmaceutical powder compaction”, Vol 71, Dekker Series, New-York. Leon Lachman; “The theory and practice of industrial pharmacy”, 3 rd edition, Varghese Publication, page no 66-121

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5-Jul-12 Dept of Pharmaceutics 50 ANY QUERIES?

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5-Jul-12 Dept of Pharmaceutics 51 THANK YOU