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.
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FT4 powder rheometer®
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.
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®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
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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.
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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 MINIPACTOR with different roll gaps...
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This information has been sourced, reviewed and adapted from materials provided by Freeman Technology.
For more information on this source, please visit Freeman Technology.