TABLETING
Hartmut vom Bey & Dr Barbara Fretter discusse the influence of press force and gap on ribbon and granule properties in roller compaction
G BETTERRANULES PRODUCING
Both have an influence on the ribbon and granule properties. In general, a larger specific press roller force results in ribbons with higher density. Depending on the material’s properties, the resulting granules are often coarser, but generate mostly softer tablets. Te impact of the gap is the opposite. A larger gap results in less densified ribbons, which leads to smaller granules and stronger tablets. Te extent of this influence depends on the compacted material itself. A roller compactor consists of
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three different functional units: the dosing unit, the densification unit and the milling unit. Te dosing unit is responsible for providing material to the densification unit, which densifies the material between the two counterrotating press rollers into a powder band called a ribbon. Te applied press force is called the specific press roller force, specified in kN/cm and the minimum distance between the press rollers is the gap [mm]. Force and gap are the main drivers affecting ribbon density in roller compaction. Further, ribbon density is the main determining factor for granule properties. To understand the influence of press force and gap, both ribbon density and granule properties must be considered. Additionally, the properties of the
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ress force and the gap between the press rollers are the main drivers in dry granulation also known as roller compaction.
FIG. 1. Diagram of a roller compaction process
material which is densified and milled also has a huge impact on the final granule properties.
An example of the influence of the
specific press force on the ribbon density is given in Fig. 2. Starting from the initial bulk density at 0kN/cm, a larger press force results in ribbons with higher density. Te increase of density is non-linear and flattens
FIG. 2. Influence of specific press force[kN/cm] on ribbon density [g/cm³]
towards higher densities, because more and more force is required to further densify the material.
Te ribbon density itself influences the granule properties, especially their recompactability, which is the ability of the granules to generate strong tablets. As the material is densified once into the ribbon, the re-densification into a tablet is therefore called the recompaction or second densification. In Fig. 3 the recompactability is shown for two different substances depending on different roller forces. It is obvious, that the impact of the press force is strongly substance specific. Whereas microcrystalline cellulose (MCC) shows a strong decrease of achievable tablet strength, lactose shows hardly any impact. Most substances, which are roller compacted, show a certain loss in binding capacity, because the bonding sites of the particles are used for making the ribbon. Nevertheless, both substances can achieve 2 MPa tablet strength, which is generally the target value for tablets.
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