Hydrophobic/hydrophilic powders: Practical implications of screw element type on the reduction of fines in twin screw granulation

Abstract

This study focuses on the role that various screw elements play in the reduction of fines in twin screw wet granulation (TSG) when using increasing amounts of hydrophobic component in the formulation. The elements used are conveying-type elements (CE-type), kneading elements (KE-type) and tooth mixing-type elements (TME-type)). Fines are defined as <150 μm granules. Further, this work attempts to rationalize the positioning of the various screw elements along the barrel by studying the granule size, granule porosity, liquid binder distribution and residence time distribution results. The length of the screw elements was kept constant to ensure constant and comparable mixing quality.It was found that CE-type elements promoted poor mixing through concentration of activity in the intermeshing region of the twin screw, which resulted in large agglomerates being formed, together with large amounts of <150 μm granules. At increased formulation hydrophobicity, the distribution of the liquid binder droplets using the CE-type elements became unachievable, forming liquid marbles - ‘solid particles spreading around the liquid droplet’. The order of superior cross-sectional mixing behavior across the barrel channel width was observed to be CE < KE < TME, particularly with increase in formulation hydrophobicity. This was due to progressively improved liquid distribution with increase in material residence time, which resulted in less production of <150 μm granules and greater extent of granule formation with that order of elements. The axial cuttings through the flights of the CE-type elements are necessary to reduce the internal free volume of the intermeshing gap and cross-sectional area which promote material compression and higher degree of fill before the mixing zone in TSG. Whilst, the increase in internal free volume of the mixing elements provide more cross-sectional free volume for granules to be compacted.