Effect of hard segment content on the microcellular foaming behavior of TPU using supercritical CO₂

A lab-scale reactor was used to investigate the foaming behavior of three different thermoplastic polyurethane (TPU) grades with different hard segment (HS) contents of around 39%, 49% and 57 wt%. The effect of HS content on the foaming behavior of TPU was extensively analyzed while varying the foam processing parameters. The saturation temperature, pressure, and time differently influenced the formation of HS crystals in each TPU with different closed-pack structures and sizes. With the increase in HS crystallization during the saturation the heterogeneous cell nucleation was improved and the foam expansion was controlled by the melt strength of the TPU/CO₂ mixture, solubility of CO₂, and the final shrinkage of the foamed samples. In TPUs with higher HS content, the processing window was broadened and shifted to higher temperatures while the expansion ratio was limited and shrinkage was minimized.