Quantitative Analysis of Structural Transformation in Extrusion Processing

Publication Reference: 
Author Last Name: 
Tim Althaus, Erich J. Windhab
Report Type: 
Research Area: 
Wet Systems
Publication Year: 
Publication Month: 
United Kingdom

Wet powders consist of solid particles which are bound by a binder liquid. In contrast to pastes they have a non-negligible volume fraction of air which signifcantly influences their flow behavior. Wet powder systems are highly cohesive. Capillary and surface tension forces are re-sponsible for bonding forces exceeding Van-der-Waals adhesion forces by about one order of magnitude. At direct particle contact points normal force dependent frictional forces are present whereas particle surfaces which are lubricated by binder liquid facilitate a relative movement between particles. Due to high packing densities, sterical effects become essential when wet powders are subjected to high normal stresses. During extrusion the wet powder is forced into a reduced cross-sectional area (die). This accounts for a relative movement between the particles in the die inlet region. While the core of the powder in the (cylindrical) barrel flows in the form of an accelerated plug into the die, the particles in the outer regions are subjected to shear resulting in a structural change. The packing density is increased, air inculsions are compressed or pressed into the core resulting an increased saturation. The particles form a kind of shear layers. In ideal case the wet powder is transformed from the powdery state (3P) into a suspension state (2S). This is reered to as 3P2S transition. In real systems air is often still present but the system shows suspension like flow behavior with drastically reduced shear stresses (3S).

The transformed material in the die inlet region constitutes a shear zone along which material slides into the die. Before a steady state is reached this shear zone grows with time because of the lower flow velocity near the wall compared to the core. Within the scope of this IFPRI project hydrophilic wet powder systems were systematically investigated with respect to their flow behavior during extrusion and their microstructure as a function of process and material parameters. As solids the silica powders SEPASILr B 5/63, SEPASILr B 20/100 and SIKRONr B 800 and the glass ballotini SPHERIGLASSr 3000 were used. They vary in particle size distributions, particle shapes and in solid-liquid interfacial tensions. Watery Nutrioser solutions with varying concentrations were applied as binder liquids. They show Newtonian flow behavior. In the experimental part of this project critical stresses were quantifed which have to be surmounted during ram extrusion before a steady state is reached. These stresses depend on process and material parameters and are a measure of the flowability of wet powders. It was shown that shear in the barrel and the die can be neglected. The die length has no impact on flow profiles and on critical stresses. The ability of a wet powder to flow can be significantly improved by subjecting them to a mechanical energy input. Shear helps to cover the particles by uniform liquid layers resulting in reduced interparticle friction. Uniaxial compaction can improve the flowability if air cells are compressed to such an extent that the saturation exceeds about 80%. However, in case of kneaded wet powder systems no reduction of the resistance against flow by uniaxial compaction is observed. At low strain rates there is no impact of the binder liquid viscosity on shear stresses.