Strategies for Enhancing Expansion in Starch-Based Microcellular Foams Produced by Supercritical Fluid Extrusion

Figures & data

Table 1 Nozzle geometry, calculated pressure drop rate, experimentally determined bubble density, and expansion ratio for SCFX extrudates

Nozzle type	Radius (mm)	Length (mm)	Flow rate (m^3/s)	Pressure drop rate (MPa/s)	Bubble density (10^6/cm^3)	Expansion ratio
Nozzle 1	3.00	18.0	1.102 × 10^−5	41.3	1.14	7.50
Nozzle 2	2.25	18.0	1.102 × 10^−5	75.2	0.59	6.90
Nozzle 3	1.50	18.0	1.102 × 10^−5	22.5	0.27	2.80

Figure 1 Schematic of nozzle geometry. Three nozzle radii, R = 3.00, 2.25, and 1.50 mm, were used to vary pressure drop rates and final bubble density of the starch-based SCFX foam.

Figure 2 Theoretical pressure profiles in extruder nozzle for three different nozzle radii, R—(i) 3.00 mm, (ii) 2.25 mm, and (iii) 1.50 mm. In the flow restrictor valve and spacer element prior to the nozzle, P drops from 10 MPa to a value determined by R. ΔP/Δt refers to the calculated pressure drop rates in the straight section of the nozzles. The inset shows the nozzle profile with tapered (AB) and straight (BC) sections.

Figure 3 Average bubble diameter (D _bubble) and nucleation density (N _bubble) vs. nozzle radius for SCFX extrudates.

Figure 4 Expansion ratio (ER) and piece density (ρ) vs. nozzle radius for SCFX extrudates.

Figure 5 Scanning electron micrographs of SCFX extrudates showing the cellular structure obtained at two melt temperatures: (a) 60°C and (b) 40°C. Lowering the melt temperature led to an increase in average bubble diameter and greater overall expansion.

Figure 6 Effect of melt temperature (40 and 60°C) on (a) average bubble diameter, (b) expansion ratio, and (c) piece density of SCFX extrudates.