Effect of TPU hard segment content on TPU's bead foaming behavior

A lab-scale bead foaming reactor was used to investigate the foaming behavior of thermoplastic polyurethane (TPU) samples with different hard segment contents of around 39 %, 49 % and 57%. The saturation temperature, pressure, and time differently influenced the formation of hard segment crystals with different close-packed structures and sizes. Consequently, with the increase in hard segment crystallization during the saturation the heterogeneous cell nucleation was improved and the foam expansion was controlled by the stiffness of the TPU/CO₂ mixture, solubility of CO₂, and final shrinkage of the foamed samples. As the saturation temperature increased, the expansion ratio tended to increase with a tendency to shrink more due to the reduced crystallites content. This is while the cell density decreased due to the reduced hard segment crystallites generated during the saturation. Increase in pressure also improved the CO₂ solubility and hence the expansion ratio was increased more significantly. The saturation time also influenced the amount and perfection degree of hard segment crystals. The interrelation between the crystallization behavior of the TPU bead foams and process conditions was reported using differential scanning calorimetry (DSC) and scanning electron microscope (SEM) analysis.