The Influence of Roller Compaction Parameters on Granule Properties

A number of challenges may be faced when processing powder blends: the blends may be susceptible to segregation, the formation of agglomerates may affect homogeneity, or component powders may be poorly flowing in the process. In many industries and applications, granulation is used to combine multiple components of a blend into a more free-flowing, homogeneous intermediate product for downstream processing.

The technique is frequently performed as a wet process, but the resulting wet mass has to be dried and milled to produce a processable product. The process is expensive and time-consuming, and may not be possible in some cases because of the chemical and/or thermal degradation of the active ingredient.

FT4 powder rheometer® (Freeman Technology, UK)

FT4 powder rheometer® (Freeman Technology, UK)

There are significant benefits to dry granulation, in terms of cost-reduction and processing, and it can be used with sensitive materials. However, there is little indication as to which process parameters produce optimal granulate quality to achieve interruption-free processing and high-quality products. Therefore, many product manufacturers and equipment suppliers rely on traditional and ad-hoc trial information to identify suitable parameters.

®MINI-PACTOR® roller compactor (Gerteis Maschinen+Processengineering AG, Switzerland)

®MINI-PACTOR® roller compactor (Gerteis Maschinen+Processengineering AG, Switzerland)

This article discusses the joint study undertaken by Freeman Technology Ltd. and Gerteis Maschinen+Processengineering AG, to explore how the properties of the dry granulate of a placebo formulation are influenced by process parameters.

Methods

A Gerteis MINI-PACTOR® roller compactor, on which the roll gap, roller speed, and compaction force can be varied together with the sieve/screen size, was used to granulate a placebo formulation consisting of 0.5% magnesium stearate, 29.5% microcrystalline cellulose, and 70% lactose. A Freeman Technology FT4 Powder Rheometer® was used to evaluate the resulting granulates to quantify the bulk, dynamic, and shear properties.

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The Effect of Compaction Force

Six identical batches of the feedstock were processed in the MINI-PACTOR® at different compaction forces:

I II III IV V VI
Compaction Force (kN/cm) 3.0 4.5 6.0 7.5 9.0 12.0

A roll gap of 3 mm was maintained, the screen size at 1 mm, and the roller speed at 2.5 RPM. The FT4 was used for the subsequent evaluation of the resulting six batches of granules, to investigate the effect of compaction force on granule properties.

Conditioned Bulk Density and Compressibility

The linear correlations observed showed that the compressibility and conditioned bulk density (CBD) of the granulate varied with compaction force, with a higher force generating lower compressibility and higher CBD.

More uniform granules that pack more efficiently, are generated by a greater compaction force. As a result of this efficient packing, there are fewer air voids. This increases the material’s bulk density and results in less available space into which granules can move when subjected to an applied stress.

Permeability

Permeability and compaction force displayed a strong relationship, with a higher force generating higher permeability.

The granules produced at a higher compaction force, generate a bulk that is more resistant to compaction. As such, when an external stress is applied to the bulk, the channels between the granules can be maintained, allowing air to pass through more easily.

From the results, it is evident that there is a direct correlation between CBD and compaction force, permeability and compressibility. As compaction force increases, compressibility decreases and permeability and CBD of the resultant granulate increase. All of these properties are typically associated with more free-flowing materials and are indicative of more efficient packing.

Unlike the dynamic flow and bulk data, shear properties had little influence, with the shear cell test results providing no correlation between the compaction force and wall friction angle and no differentiation between the samples. The lack of correlation to a dynamic, low stress process is expected as shear cells were mainly designed to evaluate the onset of flow for continuous, cohesive powders under high stress.

The Effect of Roll Gap

Six identical batches of the feedstock were processed in the MINI-PACTOR® with different roll gaps:

I II III IV V VI
Roll Gap (mm) 1.5 2 2.5 3 4 5

The roller speed was maintained at 2.5 RPM, the compaction force at 4.5 kN/cm, and the screen size at 1 mm. An FT4 was used to evaluate the resulting six batches of granules, to find out the effect of roll gap on granule properties.

Conditioned Bulk Density

As roll gap increased, the bulk density of the granules decreased, indicating that the larger gap generates less consistent granules with a wider size distribution.

Typically, materials with a wide PSD pack less efficiently, entraining more air and reducing the density of the bulk.

Consolidation Index and Permeability

Robust relationships in which permeability and consolidation index (CI) of the granulate varied with roll gap were also observed; larger roll gap resulted in lower permeability and a greater sensitivity to vibrational consolidation.

This suggests that a larger roll gap leads to the generation of granules with a wider particle size distribution, packing less uniformly and entraining more air within the bulk.

The particles readily re-align and repack into a more efficiently packed structure when vibration is applied to the granules, expelling the air and causing a large increase in flow energy. Additionally, the less uniform packing structure does not allow stable air channels to be established, which leads to a reduction in permeability.

With an increase in the roll gap, the resulting granules are less uniform as a result of the less consistent consolidation regime established between the rollers. This can lead to greater variation in size distribution, surface texture, and shape of the granules, manifested by a reduction in particle packing efficiency as shown by the higher CI value, and lower CBD and permeability values.

Variation of Compaction Force and Roll Gap

At varying levels of roll gap and compaction force, nine identical batches of the feedstock were evaluated to see if these followed similar trends compared to those observed when varying only roll gap or compaction force.

I II III IV V VI VII VIII IX
Roll Gap (mm) 1.5 2 2.5 3 4 5 1.5 3 5
Compaction Force (kN/cm) 4.5 4.5 4.5 4.5 4.5 4.5 9 9 9

Conditioned Bulk Density and Compressibility

The granules that were produced with a 9 kN/cm compaction force across a range of roll gap values consistently have a lower compressibility and a higher CBD than those produced at 4.5 kN/cm; this supports the observations made in the initial investigations.

There were comparable linear relationships between CBD and roll gap at 4.5 kN/cm and 9 kN/cm compaction force. This suggests that the relationship between CBD and roll gap is independent of compaction force.

However, the relaionship between roll gap and compressibility does not show the same independence. At 9 kN/cm, compressibility shows a sharper increase as the roll gap increases. This suggests that the roll gap has a greater influence on granule properties at higher compaction forces.

Permeability

Compared to the granules produced at 4.5 kN/cm, those produced at 9 kN/cm compaction force have a higher permeability. This indicates that the granules are more uniformly packed and more permeable when the compaction force is higher. The previously observed relationship between roll gap and permeability was confirmed by the results.

However, the curve is more significantly pronounced at the higher compaction force, which reinforces the suggestion that roll gap has more influence on granule properties at higher compaction forces.

Conclusion

To control and optimize process performance and ensure final product quality, Quality by Design dictates that the relationship between processes and materials should be well understood. From the results generated here, it is clear that the required critical process parameters can be identified to optimize a roller compaction process to generate granules with properties that directly influence performance in downstream operations and critical quality attributes of the end product.

In the flow properties measured by the FT4, distinct and repeatable trends have been observed, showing how the process parameters predictably influence the rheological properties of the granules.

The compressibility, permeability, and CBD showed robust correlations with modes of operation of the roll compactor, suggesting that a combination of higher compaction force and smaller roll gap is more likely to result in more consistent/uniform granules that form a more efficiently packed powder bed generally associated with free-flowing powders.

This study shows the importance of powder rheology in a comprehensive, multivariate approach to powder characterization. Although it is not a fundamental material property, flowability reflects how multiple properties contribute to the overall ability of a powder to perform in a particular piece of equipment.

A significant difference in process performance can occur due to subtle variations in an individual property. This means that several characterization methodologies are essential and the results from which can be correlated with process ranking to create a design space of parameters that correspond to acceptable process behavior.

This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.

For more information on this source, please visit Freeman Technology.

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