Use of thermal flow analysis and X-ray microtomography to model microstructure evolution in extruded biopolymeric foams.

摘要

Foaming of polymeric and biopolymeric materials is important in both food and industrial applications. For development of new foams, understanding of the underlying mechanisms of microstructure formation is essential. This work was carried out to aide in understanding the relationships between extrusion processing parameters and material properties of starch-based biopolymer foams. A new technique, X-ray microtomography (XMT), was utilized to investigate foam microstructure. Thermal flow analysis was performed on raw materials using a Phase Transition Analyzer(TM) (PTA). In the first experiment, non-invasive imaging using XMT allowed measurement of cell size distribution and average cell size (0.582 to 2.27 mm), open wall area ratio (0.100 to 0.842) and cell wall thickness (0.047 to 0.087 mm). In the second experiment, combinations of corn starch and whey protein concentrate (WPC, 34% protein) were extruded at two moisture contents (26 and 34% moisture, wb). Product temperature behind the die (Td) was recorded, and differences between Td and softening (Ts) or flow point (Tf) temperatures (T ds and Tdf, respectively), obtained via thermal flow analysis, were used to explain expansion and collapse phenomena, as well as results for physical properties and microstructure of the foams. Relationships existed between Tdf and parameters such as average cell diameter ( R2 = 0.94) and physical void fraction (R 2 = 0.80). In the final experiment, corn starch was extruded at various screw speeds and moisture contents, and XMT and thermal flow analysis were used in combination to develop a conceptual model for extrudate expansion, collapse and microstructure formation as a function of material properties and processing parameters. However, Tds and Tdf did not appear to adequately explain microstructure evolution, so Td was used individually with either Ts or Tf in response surface modeling. Significant relationships were observed between Td and Tf and many physical and microstructure parameters (e.g., R2 = 0.93 and 0.86 for average cell diameter and physical void fraction, respectively). The combination of XMT with thermal flow analysis proved to be a novel and useful way of studying and understanding relationships between extrusion processing parameters, raw material properties and final foam microstructure, expansion and water absorption.