TABLETING
FIG. 3A. Impact of the specific press force on the re-compactability of two different substances
FIG. 3B. Example of the relationship of press force on the particle size distribution of granulates MCC in roller compaction
cm to 6 kN/cm increases the ribbon density by approximately 31% (0.70 g/ cm³ to 0.92 g/cm³) for the 2mm gap. Whereas maintaining the press force and increasing the gap by a factor of 2 from 2mm to 4mm, the density is reduced by approximately 13% (for 6 kN/cm: 0.92 g/ cm³ to 0.80 g/cm³). At higher specific roll forces, the impact
In addition to the re-compatibility of the material, the particle size distribution of the granule is affected by ribbon density. In general, a higher specific press force results in granules with coarser particles. In Fig. 4 an example is given for this relationship.
INFLUENCE OF THE GAP IN ROLLER COMPACTION Beside the specific press force, the gap has a strong impact on the ribbon density. For a constant press force, larger gaps result in ribbons with lower density. An example is given in Fig. 5, in which the ribbon density as a function of press force for two different gaps is displayed. For example, at an equal press force, the 4mm gap results in lower densities than the 2mm gap. Te influence of the gap on the ribbon density can be explained by the thin layer
model, which is described by Peter et al.[1]
A larger gap results in larger nip
area. Terefore, the applied press force is distributed over a larger area and thus the powder is densified less. Te extent of this reduction depends on the densification properties of the material which is being compacted and thus cannot be generalised. Consequently, roller compacting a material at the same specific roll force, but different gaps, will result in different ribbon densities. Tis also leads to different granule properties, especially regarding the recompactability and particle size distribution.
Comparing the extent of the influence
of press force and gap on ribbon properties, the specific press force has a larger influence than the gap. Based on the example in Fig. 5, doubling the force from 3 kN/
FIG. 4. Example of the relationship of press force on the particle size distribution of granulates MCC in roller compaction
References 1. Peter, S., Lammens, R. F., Steffens, K.-J., Powder Technology, 199, 2, (2010), pp. 165-175
FIG. 5. Example for the influence of press force and gap on ribbon density
of increasing the specific press force gets smaller, because the material is already highly densified and further densification requires disproportionally higher press forces to achieve higher densification. Te influence of the gap is also reduced at larger specific roll forces.
CONCLUSION Constant press force and gap are the main influencing factors for consistent ribbon density and therefore for the granulate properties of recompactability and particle size distribution. Whereas larger specific press forces result in higher ribbon densities, this is the opposite for the gap: larger gaps result in lower ribbon densities. Te influence of the specific press force is larger than the influence of the gap. Te extent of the influence depends on the properties of the material being compacted.
Hartmut vom Bey is with Gerteis & Dr Barbara Fretter is with Solids Development Consult.
www.gerteis.com
www.scientistlive.com 45
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