Comparison of granule properties produced using Twin Screw Extruder and High Shear Mixer: A step towards understanding the mechanism of twin screw wet granulation

The wet granulation process plays an important role in the pharmaceutical industry. With the introduction of a Twin Screw Extruder (TSE), it allows the possibility of wet granulation to be run continuously in contrast to a conventional batch process using a High Shear Mixer (HSM). However, the mechanism of Twin Screw Extruder wet granulation is not well understood and the aim of this study is to investigate this process and compare it with the High Shear Mixer granulation process with regard to the granule properties.Granules of pure microcrystalline cellulose (MCC) were produced using a Twin Screw Extruder and a High Shear Mixer with water as liquid binder under different process conditions and formulations. The properties (particle size distribution, shape, surface morphology, internal structure, internal porosity and strength) of the granules were then compared. It was found that the granules produced by the two methods have different physical properties. Granules produced by the HSM are spherical in shape and are made up of smaller granules through coalescence process. It is believed that the granules produced by HSM in the present study are over-granulated which leads to the production of strong dense granules. On the other hand, granules produced by TSE have irregular shapes with tiny pores spread uniformly throughout the granules. The granules produced by HSM are affected by process conditions and are stronger at higher impeller speed. However, granules made by TSE are consistent in strength and relatively independent of process conditions. Due to over-granulation in HSM process, the granules produced result in the production of weak tablets whilst granules made by TSE manage to produce stronger tablets.Finally, by observing the structure of the granules, it is believed that the mechanism of HSM and TSE granulation is different. Although the granules made by HSM were over-granulated, it shows the existence of consolidation process that is apparently absent in TSE granulation. Nevertheless, more work will be needed to further understand the mechanism in continuous TSE wet granulation process.