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| Applications | |||||||||||||||||||||
| Direct compression (DC) | |||||||||||||||||||||
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The big advantage of DC is the use of only one production line (in contrast to wet granulation) combining the dry blending and tabletting process. However, DC also has some disadvantages : a. More requirements for the excipients with respect to tabletting and flowability b. Segregation of the blend before tabletting c. Sensitivity of tabletting for lubricants d. Lamination and capping behaviour a. Requirements for excipients for DC Excipients for DC must show excellent tabletting properties and flowability. Spray-dried lactose flows very well because of its spherical particle shape and has good tabletting properties due to the ductile behaviour of amorphous lactose. The tabletting properties of anhydrous lactose are good, because its particles behave in a ductile manner. Flow may be less for this material, which is corrected by the use of filler, for example non-milled lactose monohydrate. Most excipients for DC other than lactose show excellent tabletting behaviour and poor flow. b. Prevention of segregation In a granulate, segregation can only occur within a granule. Consequently, segregation is never a problem for tablets produced by wet granulation. In contrast, in a powder blend for DC, segregation can occur throughout the blend. The result will be an inhomogeneous powder and poor content uniformity of the tablet charge. To obtain a homogeneous powder blend, the mixing time needs to be optimised, which is often a matter of trial and error. Segregation is prevented by choosing excipients with comparable particle sizes wherever possible. Given the fact that the filler takes the major share in the volume of a tablet, it is especially this excipient for which the particle size is important. c. Influence of lubricants on tabletting properties To prevent the finished tablet from sticking to the punches and to facilitate ejection of the tablet from the die, a lubricant is added to the tabletting blend or granulate. By far the most popular lubricant is magnesium stearate. The lubricant forms a continuous film around particles, decreasing tabletting properties. This is especially a problem for ductile materials, since there is no fragmentation in finer particles. A lubricant film around brittle materials is generally destroyed during tabletting. Around particles with irregular surface, it is more difficult to form an intact lubricant film, which is why anhydrous lactose is less sensitive for lubrication. Generally, lubricants are added in as low concentrations as possible (0.5-2% of the tabletting blend) and lubrication time is as short as possible (generally 2-5 minutes). Amorphous lactose has self-lubricating properties; for spray-dried lactose, lubrication levels of 0.2% are sufficient. d. Lamination and capping behaviour When ductile materials deform, a small part of this deformation is elastic. This means that the material tends to return to its original shape when the tabletting pressure is released when the punches return to their original position. In practice, this means that at the end of the compaction, when the upper punch moves up and pressure is released, the tablet contains elastic energy. Generally, the quantity of elastic energy is larger with increasing tabletting speed. Most materials for DC are, therefore, sensitive to tabletting speed. Elastic energy is relieved when the upper punch moves up and is a driving force for porous expansion. As a result, tablets with a higher porosity are obtained by an increase in tabletting speed (the speed at which the punches move through the die). In some cases porous expansion is disabled because of too much friction of the powder with the die wall or because the original powder particles are too large. Since the elastic energy has to be relieved, cracks are formed in the tablet, weakening the tablet. This is called lamination. The cracks may become so large that the tablet will be visibly damaged or a complete part of the tablet breaks off when it comes out of the machine. The latter process is called capping. Granules that are too hard will easily fragment into the individual particles and therefore have a tendency to show capping behaviour.
Brittle and ductile behaviour |
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