Quantitative Analysis of Micro-Structural Transformation in Extrusion Process

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Author Last Name: 
Paolo Arancio and Erich J. Windhab
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Research Area: 
Particle Formation
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Summary of 3rd year

In this part of the project work a new mixing/extrusion concept was further developed to study the microstructure changes of a model system, which undergoes a transition from a 3- phase wet powder to a 2-phase concentrated suspension system (3P2S Transition) (Arancio and Windhab, 2003).

The further developed model system used consists of: powder/binder: liquid fat melt/oil (e.g. cocoa butter)(i), powder/solid filler: hydrophobic silica particles (Sip.D17, Degussa, (D)) (ii). The first advantage of using a fat melt, which is in the fully liquid state at moderately elevated temperatures (eg. 40°C for cocoa butter), was given by the ease of solidification close to room temperature. This made investigations of the wet powder/suspension microstructure possible without additional structural changes during sampling from the process and during sample preparation. The second advantage proved was the good homogeneous mixing ability of the hydrophobic silica powder particles within the fat binder, even at extremely low binder fraction due to the fact that the binder fat melt was cold sprayed, thus producing small solidified binder particles. These solidified binder particles were mixed with the filler particles under NIR-based homogeneity control. If this powder mixture was heated up to 40°C, a significantly better homogeneity of the mixture of powder and liquid binder was obtained compared to the conventional/alternative process of mixing by spraying the low fraction of liquid binder into the filler powder (formation of local lumps). The performed process consisted of the following steps: a) spray cooling of the binder-fluid phase in order to obtain a solid fine binder powder, b) mixing of this binder powder with the filler powder and mixing quality control by near infrared spectroscopy, c) pre-compaction of the model system “powder/binder powder mixture (PBPM)” in a mechanical testing machine (Zwick), d) ram extrusion of the pre-compacted system and e.) micro-structural investigation of the extruded product.

The powder/binder mixture in the solid powder state of the binder (A) was used to investigate the local stresses/stress distribution leading to deformation of the solid cocoa butter (binder) particles. This allows identifying the zones where the transformation from a three phase wet powder to a two phase concentrated suspension (3P2S) preferably happens. The solid liquid mixture above the melting temperature of the binder (cocoa butter) (B) reflects the powder binder system in the normally applied state, with air additionally entrapped. The acting pressure and shear stresses lead to a synergistic reduction/compression of the air included until the 3P2S transformation takes place. The impact of pressure, shear and pressure + shear on the resulting powder/binder microstructure were investigated in detail and 3P2S phase diagrams derived, which form a new basis for design calculations of extruders/extrusion processes and extrudate property optimization.