Press force and gap improving dry granulation

Hartmut vom Bey & Dr Barbara Fretter discuss the influence of press force and gap on ribbon and granule properties in roller compaction

Press force and the gap between the press rollers are the main drivers in dry granulation also known as roller compaction. 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. The impact of the gap is the opposite. A larger gap results in less densified ribbons, which leads to smaller granules and stronger tablets. The extent of this influence depends on the compacted material itself.

A roller compactor consists of three different functional units: the dosing unit, the densification unit and the milling unit. The 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. The 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 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. The increase of density is non-linear and flattens towards higher densities, because more and more force is required to further densify the material.

The 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.

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 compactors

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.

The 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. Therefore, the applied press force is distributed over a larger area and thus the powder is densified less. The 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. This 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/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 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. The influence of the gap is also reduced at larger specific roll forces.

Roller compactors influencers 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. The influence of the specific press force is larger than the influence of the gap. The extent of the influence depends on the properties of the material being compacted.

References1. Peter, S., Lammens, R. F., Steffens, K.-J., Powder Technology, 199, 2, (2010), pp. 165-175

Hartmut vom Bey is with Gerteis and Dr Barbara Fretter is with Solids Development Consult

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