Quantitative Analysis of Micro-Structural Transformation in Extrusion Process

Publication Reference: 
46-02
Author Last Name: 
Windhab
Authors: 
Prof. Dr.-Ing, Erich J. Windhab,Paolo Arancio
Report Type: 
ARR
Research Area: 
Particle Formation
Publication Year: 
2003
Publication Month: 
03

The rheology of concentrated suspensions containing rigid fillers is important in many technological areas since it mirrors the dynamic behavior of the structuring units, such as disperse particle/particle aggregates as well as macromolecular compo- nents (binders) in the continuous fluid phase. For systems with high solid content, air is easily entrapped during different process steps, creating a compressible wet powder. The aim of the work is to investigate the phase transition between a 3-phase wet pow- der to a 2-phase system suspension (3P2S) occurring in the extrusion processing and to describe its mechanism. Extrusion processing of highly concentrated powder-binder systems always applies uni- or multiaxial shear and/or elongation flow fields, along the screw channel. The main Micro Structuring Mechanism (MSM) acting on the struc- turing units in such flow process are de-agglomeration, deformation, orientation and agglomeration. The increase of the pressure along the extruder channel and at the die zone, in combination with the high shear stresses present, compress and induce the plastification of the powder system. The 3P2S transition takes place in a well define layer of the final product creating concentric adjacent layer of wet powder and con- centrated suspension. Modifying the system components, process steps and operating condition can achieve a different microstructure and distribution of the wet powder/- suspension zones.

The rheology of the model system has been investigated and the solid filler extensively characterized with respect to the agglomerate strength. The 3P2S transition has been investigated in the High Pressure Powder Shear reactor and in extrusion processing. The influence of the mixing quality can show a strong influence on the transition; thus different mixing process has been proposed and NIR spectroscopy has been used to eval- uate the quality. Finally the product microstructure has been qualitatively analyzed with scanning electron microscopy (SEM).