Abstract
Expanded thermoplastic polyurethane (ETPU) beads were prepared by a supercritical CO2 foaming process and compression molded to manufacture foam sheets. The effect of the cell structure of the foamed beads on the properties of the foam sheets was studied. Higher foaming pressure resulted in a greater number of cells and thus, smaller cell size, while increasing the foaming temperature at a fixed pressure lowered the viscosity to result in fewer cells and a larger cell size, increasing the expansion ratio of the ETPU. Although the processing window in which the cell structure of the ETPU beads can be maintained was very limited compared to that of steam chest molding, compression molding of ETPU beads to produce foam sheets was possible by controlling the compression pressure and temperature to obtain sintering of the bead surfaces. Properties of the foam sheets are influenced by the expansion ratio of the beads and the increase in the expansion ratio increased the foam resilience, decreased the hardness, and increased the tensile strength and elongation at break.
Highlights
In order to obtain supercritical CO2, the temperature was set at 90, 100, 105, or 110 ◦ C and the pressure was set at 75, 80, or 90 bar; the thermoplastic polyurethane (TPU) was kept in the autoclave for 30 min, the pressure was quickly released to atmospheric pressure by opening a ball valve to prepare expanded TPU (ETPU) beads
The cell diameter and density measured from Figure conditions used in this study, the expanded thermoplastic polyurethane (ETPU) foam has a closed cell structure
The TPU used in this study exhibits closed cell structures when foamed at 75–90 bar
Summary
Polymer foams [1,2] are widely used for light weight polymer molded products. Typical processes for making light weight polymer molded products are the Mucell process [3,4]. Incorporation of the foaming agent into the polymer pellet can be carried out by addition in the polymerization process [15,16], by using a high temperature and pressure autoclave to introduce the foaming agent in the supercritical fluid state to polymer pellets [17,18], or by adding the foaming agent to the polymer melt in the extruder and preparing expandable or expanded beads by controlling the cooling condition [19]
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