The evolution of granule fracture strength as a function of impeller tip speed and granule size for a novel reverse-phase wet granulation process

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The feasibility of a novel reverse-phase wet granulation process has been established previously and several potential advantages over the conventional process have been highlighted (Wade et al., 2014a,b,b). Due to fundamental differences in the growth mechanism and granule consolidation behaviour between the two processes the reverse-phase approach generally formed granules with a greater mass mean diameter and a lower intragranular porosity than those formed by the conventional granulation process under the same liquid saturation and impeller tip speed conditions. The lower intragranular porosity was hypothesised to result in an increase in the granule strength and subsequent decrease in tablet tensile strength. Consequently, the aim of this study was to compare the effect of impeller tip speed and granule size on the strength and compaction properties of granules prepared using both the reverse-phase and conventional granulation processes. For the conventional granulation process an increase in the impeller tip speed from 1.57 to 4.71 m s−1 (200–600 RPM) resulted in an increase in the mean granule strength (p < 0.05) for all granule size fractions and as the granule size fraction increased from 425–600 to 2000–3350 μm the mean fracture strength decreased (p < 0.05). For the reverse-phase process an increase in impeller tip speed had no effect (p > 0.05) on mean granule strength whereas, like the conventional process, an increase in granule size fraction from 425–600 to 2000–3350 μm resulted in a decrease (p < 0.05) in the mean fracture strength. No correlation was found between mean granule fracture strength and the tablet tensile strength (p > 0.05) for either granulation approach. These data support the rejection of the original hypothesis which stated that an increase in granule strength may result in a decrease in the tablet tensile strength. The similar tablet tensile strength observed between the conventional and reverse-phase granulation processes indicated that while mechanistic differences exist in the formation of the granules, which resulted in significant granule-scale fracture strength differences, the granule compaction properties at pharmaceutically relevant tableting pressures were unaffected.

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