Effect of compression on tablet strength
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This document discusses factors that affect the strength of tablets. It defines tablet strength as the force necessary to break a tablet. Two main methods to measure tablet strength are described: diametral compression test and axial tensile strength test. Several factors influence tablet strength, including particle size and shape, compaction pressure, binders, lubricants, moisture content, entrapped air, and porosity. In general, smaller particle size, higher compaction pressure, and less lubricant and moisture lead to stronger tablets by promoting bonding between particles.
Effect of compression on tablet strength
- 1. Salwa (Re-Edired by Suraj C. ) PPM Page | 1 Effects of various factors on Strength of tablets Contents : Definition Introduction to tablet strength Measurement of tablet strength Theoretical models Factors affecting tablet strength References
- 2. Salwa (Re-Edired by Suraj C. ) PPM Page | 2 Definition :- Tablets are the solid unit dosage forms consisting of one or more medicaments with or without the excipients prepared by compaction. Introduction :- Tablets are manufactured by applying pressure to a powder bed, which compresses the powder into a coherent compact. The powder may consist of either primary particles or aggregated primary particles (i.e. granules). When these are compressed, bonds are established between the particles or granules, thus conferring a certain mechanical strength to the compact. Strength of the tablets The strength of pharmaceutical tablets is frequently assessed as an in-process control during manufacturing and as a means to understand the compaction behavior of a material. The strength of a tablet can be characterized by the force necessary to break the tablet. When a tablet is subjected to such a force, the response can be interpreted on the basis of the bond summation or fracture mechanics concepts. Measurements of strength :- Methods for measuring the mechanical strength of tablets : Diametral compression test Axial tensile strength 1. Diametral compression test - o The most common strength test in pharmaceutical applications. o Used to calculate the radial tensile strength of a tablet. o During radial tensile strength measurements, the fracture occurs through a diametral cross section of the tablet. o Reflects the average strength of tablet rather than the strength of the weakest plane in the tablet. 2. Axial tensile strength - o The force necessary to break the tablet is obtained by pulling the tablet parallel to the applied force during the formation of the tablet and this force is then used to calculate the axial tensile strength.
- 3. Salwa (Re-Edired by Suraj C. ) PPM Page | 3 o During axial tensile strength measurements, the fracture will occur through the weakest plane in the tablet. Theoretical models of mechanical strength :- Bond summation concept Fracture mechanics concept 1. Bond summation concept - the bonds will be holding the particles together and the breakage of these bonds during strength testing are emphasized. 2. Fracture mechanics concept - focus is on the propagation of cracks in the tablet during strength testing. Factors affecting strength of the tablets :- 1. Particle size 2. Particle shape & surface roughness 3. Compaction pressure 4. Binders 5. Lubricants 6. Entrapped air 7. Moisture content 8. Porosity 1. Particle size Extensive fragmentation during compaction of a brittle material may result in a large number of interparticulate contact points, which in turn provide a large number of possible bonding zones. The tablets made of these materials can have a high mechanical strength. Extensive elastic deformation, on the other hand, may cause a pronounced decrease in the mechanical strength of the tablet, due to breakage of interparticulate bonds when the compaction pressure is released. Plastic deformation is considered beneficial to the mechanical strength since it enables the particles to move very close to each other, thereby creating a large surface area over which bonds may be established. As the distance between the particles is decreased during compaction of plastic materials, particle interactions are also favoured. ↓ particle size → → ↑ mechanical strength The increase in mechanical strength is attributed to an increase in the surface area available for interparticulate attractions, as the particles become smaller.
- 4. Salwa (Re-Edired by Suraj C. ) PPM Page | 4 ↑ particle size → → ↑ mechanical strength Eg: sodium chloride. This is attributed to an increased ability to bond with solid bridges as the particle size increases. During the compaction of coarse particulate sodium chloride, the pressure applied will be concentrated at a relatively small number of contact points. Thus, large stresses will be created there, which will facilitate the formation of solid bridges. For materials with a high fragmentation tendency, such as dibasic calcium phosphate dihydrate and saccharose, the strength of the tablet seems to be almost independent of particle size. Smaller particles have larger surface area & when these are exposed to atmosphere may be prone to oxidation and moisture absorption takes place which effects the strength of tablet. 2) Particle shape & surface roughness The mechanical strength of tablets of materials with a high fragmentation tendency are less affected by particle shape and surface texture. Particle shape affects the inter particulate friction & flow properties of the powder. Spherical particles are considered to be ideal. General particle shapes and their effect on powder flow are as follows: Spherical particles - Good Oblong shaped particles - Poor Cubical shaped particles - Poor Irregular shaped particles - Medium 3) Compaction pressure The compaction pressure and speed affects the strength of the resulting tablet. A material which undergoes time-dependent plastic or elastic deformation is referred to as a viscous or viscoelastic material. For such a material, a change in the compression speed significantly affects the deformation of the material. The effect of compression speed has been evaluated by calculating the strain rate sensitivity (relative difference between yield pressure values at a high and low compression speed). For a material known to deform by plastic flow, the volume reduction generally decreases as the compression speed increases, thus allowing a shorter time for plastic deformation and a subsequent decrease in mechanical strength of tablets. A fragmenting material, has been shown to be less affected by variations in compression speed.
- 5. Salwa (Re-Edired by Suraj C. ) PPM Page | 5 The behaviour of granules during compaction, the extent to which they bond together & the strength of the inter granule bonds relative to the strength of the granules determine tablet hardness. Eg : Colored coating was applied to preformed spherical granules of sucrose. Low compaction pressure - The coating remained intact & the granules largely retained their shape. Highest compaction pressure - The granules distorted & the coating had ruptured to such an extent that it appeared as colored fragments against the white underlying sucrose. 4) Binder A binder is a material that is added to a formulation in order to improve the mechanical strength of a tablet. In direct compression, it is generally considered that a binder should have a high compactibility to ensure the mechanical strength of the tablet mixture. Alternatively, amorphous binders which undergo pronounced plastic deformation have been suggested to provide an effective means of creating a large surface area available for bonding. The addition of a binder to a compound changes the surface properties of the coarse compound particles as they are covered by the small binder particles. This leads to increase in surface coverage & the surface area available for inter particulate bonding, thus increasing the number of bonds and also possibly creating stronger bonds, with a subsequently increased mechanical strength. Addition of a binder which increases elasticity can decrease tablet strength because of the breakage of bonds as the compaction pressure is released. Eg : MCC , starch , etc 5) Lubricants Used to improve granule flow, minimize die wall friction & prevent adhesion of the granules to the punch faces. Lubricant decreases the strength of the tablets. When lubricants are added as dry powder to granules, they adhere & form a coat or a film around the host particles during the mixing process. The Lubricant film interfere with the bonding properties of host particle by acting as a physical barrier. When the tablet is blended lightly, the lubricant is present as a free fraction.
- 6. Salwa (Re-Edired by Suraj C. ) PPM Page | 6 Prolonged mixing time will produce a surface film of lubricants over the drug particles due to which inter particulate bonding is reduced. This can be seen when the Tablet crushing strength is plotted as a function of the Log of the mixing time with the lubricant. 6) Entrapped air When the air does not freely escape from the granules in the die cavity, the force created by the expansion of the entrapped air may be sufficient to disrupt the bonds. The presence of entrapped air will produce a tablet which can be broken easily & it lowers the tablet strength. 7) Moisture content A small proportion of moisture content is desirable for the formation of a coherent tablet. Some amount of moisture present on the powder surface is just sufficient to fill the remaining voids in the bed. Further increase in compression force results in this water being squeeze out to the surface of tablet. This expelled moisture may act as a lubricant at die wall, but it causes material to stick to punch faces. At low moisture content there will be increase in die wall friction due to increased stress, hence the tablet hardness will be poor. At high moisture level the die wall friction is reduced owing to lubricating effect of moisture. At further increase in moisture content there will be decrease in compact strength due to reduction in inter particulate bond. Optimum moisture content is in the range of 0.5 – 4%. 8) Porosity When particles of large size is subjected to light compression the tablet will be highly porous – low tablet strength. Reduction in porosity is due to granule fragmentation giving smaller particles which may be more closely packed & plastic deformation which allows the granules to flow into the void spaces.