Roller Compaction: Mechanistic understanding of ribbon splitting and sticking

Roller compaction is a dry granulation technique in which the feed powder is subjected to different types of stresses, which are applied between two counter-rotating rollers. The stress applied during this process mediates both desired (intact ribbon) and undesired (ribbon splitting/sticking) bond formation. The phenomenon of ribbon splitting/sticking could originate from either process or formulation related causes; due to the imbalanced combination of material mechanical properties and stress application which has the potential to result in different types of ribbon failures i.e. cohesion and/or adhesion-related ribbon failure. Despite current research work conducted in the field of roller compaction, there remains a lack of comprehensive understanding in relation to the development of ribbon failure during roller compaction which can hinder the use of roller compaction with many formulations. Therefore, an in-depth understanding of the causes of ribbon failure during roller compaction is required to design better formulations and incorporate steps which will mitigate/stop the occurrence of ribbon splitting/sticking.
The aim of this work is to build a mechanistic understanding of ribbon splitting/sticking phenomenon in terms of the ribbon-roller interactions in both the bulk and at the roller walls.
A range of materials was selected to cover a wide range of powder deformability, the minimum gap between the two smooth rollers was fixed and the maximum roll stress was varied. Ribbon splitting was observed to occur either transversally (through the ribbon thickness) or longitudinally (through the ribbon width). It was observed that transverse splitting is commonly associated with sticking of the split ribbons to the rollers. Longitudinal splitting is associated with an across-width distribution of the ribbon density which may cause an adverse effect on the mechanical strength and dissolution properties of the tablets formed from the milled granules. The observed trends of ribbon splitting were rationalised in terms of a splitting index which is a measure of the residual stresses driving crack growth relative to the tensile strength of the ribbons.
Furthermore, knurled rollers with higher roller/powder contact surface area have been investigated in terms of the occurrence of ribbon splitting/sticking. The predictability of the splitting index was improved by incorporating the ribbon solid fraction difference across the ribbon width. Using knurled rollers extends the range of transversal
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splitting/roller sticking when compared with the smooth rollers and reduces the extent of longitudinal splitting.
Attempts have been conducted to correlate powder and metal thermodynamic work of adhesion with its sticking tendency during roller compaction. The technology of inverse gas chromatography was utilised to characterise both powder and metal in terms of their surface energy. No clear correlation between the work of adhesion/cohesion and sticking probability was found which suggests that particle deformation is the dominant factor of sticking during compaction. However, the result could contribute to the elaboration of the lubrication mechanism of MgSt during powder compaction.
Finally, a data-driven model was developed to predict the ribbon porosity distribution using the artificial neural network approach (ANN). Various process-related parameters and material properties considered as the inputs of the ANN. While the outputs of the network were the porosity which was experimentally measured by X-ray tomography across the ribbon width, powder and ribbon porosity distribution have linked together using ANN approach as a novel approach to predict the heterogeneity of ribbon in terms of its porosity distribution. Results showed that the ANN was able to successfully map various material and process parameters to the ribbon porosity distribution across the ribbon width, which is considered to be one of the most important quality attributes in the roller compaction process. Due to its short processing time This is particularly useful as it allows potentially for future powders and formulations to be modelled to understand the propensity of undesired ribbon properties to occur.