The time-delayed expansion profile of supercritical fluid extrudates

The typical time-delayed expansion behavior of supercritical fluid extrusion (SCFX) extrudates was investigated via video analysis. Cross-sectional, longitudinal and volumetric expansion characteristics were evaluated varying SCFX process conditions such as the ratio of SC-CO2 to feed and die size. The result showed that the initial cross-sectional expansion of SCFX extrudate was time, SC-CO2 concentration, and pressure drop rate dependent. The growth time of SCFX extrudates was approximately 30–200-fold longer compared to a typical growth time of steam-based extrudates. Die-swell in SCFX extrusion accounted for 32–45% of the maximum cross-sectional expansion but had insignificant effect on volumetric expansion. The pressure drop profile in the die was found to be critical in controlling not only the extent of volumetric expansion of SCFX extrudates but also the rate of SCFX expansion via nucleation and cell growth. Rapid initial cross-sectional and volumetric expansion of SCFX extrudates promoted faster structure collapse and strategies to enhance the volumetric expansion, which determines the textural properties of expanded products, were discussed. The retarded SCFX expansion is very unique and can be utilized for development of novel SC-CO2 expanded products with tailored texture.