Process dynamics of starch-based microcellular foams produced by supercritical fluid extrusion. I: model development

Supercritical fluid extrusion (SCFX) has been used successfully to produce biopolymeric foams with bubble size in the range of 50-200 microns, and bubble density to the order of 10(6) per cm(3). Final bubble size and expansion ratio of extrudates depend on process and material parameters like CO2 injection rate, nozzle temperature, oven temperature, melt viscosity, melt yield stress, etc. The objective of this study was to describe SCFX process dynamics and post-extrusion drying mechanism by using a mathematical model for bubble growth at the microscopic level in conjunction with a macroscopic model for flow of starch melt through the extruder nozzle, bulk diffusion of CO2 and water to the atmosphere and heat transfer in the extrudate. The model was written in Visual Basic. The model provided the basis for a good understanding of the mechanisms of bubble growth and collapse, post-extrusion drying and open cell formation during SCFX processing, and is the only such one developed so far for extrusion puffed products. Simulation results and comparison with experimental data are presented in Part 11 of this paper. (C) 2003 Elsevier Science Ltd. All rights reserved.