Effects of process variables and their interactions on melt pelletization in a high shear mixer

Abstract

The applicability of a newly designed high shear mixer with a seamless polytetrafluorethylene liner for melt pelletization was examined. The effects of impeller speed, massing time, binder concentration and mixer load were investigated using lactose monohydrate and polyethylene glycol 3000. Interactions between controlling variables were illustrated using surface response methodology. Orthogonal design was applied to optimize the melt pelletization process. The principal agglomerate growth mechanisms were identified as coalescence, crushing and layering. With a higher binder content, coalescence was promoted due to the greater ease of achieving sufficient surface plasticity and deformability. The high shearing forces at high impeller speeds gave rise to more rapid consolidation and agglomerate growth rates. The high shear mixer was able to produce melt pellets of the desired size and a narrow size distribution, provided that the process and formulation variables were well controlled. The mixer load was interactive with impeller speed and massing time, and should be carefully adjusted to obtain an appropriate energy input into the system. A high mixer load and a low binder content were conducive to a more controllable pellet forming process.