Thermoplastic Polyurethane Bead Foams for Light Weight Flexible Applications

This chapter investigates the bead foaming behavior of various thermoplastic polyurethane (TPU) types in a lab-scale reactor using supercritical CO2 as the blowing agent. This includes the bead foaming behavior of an ether- and an ester-based TPU, TPUs with a given soft segment (SS) molecular length but varying hard segment (HS) contents of around 39%, 49% and 57wt%, and TPUs with similar HS contents but different SS molecular weights of 1000, 2000, and 3500g/mol. Overall, the saturation temperature, pressure, and time could differently influence the formation of HS crystals in each TPU with different structure, which could eventually differently affect the TPUs' bead foaming behavior. While the ester-based TPU could be foamed within a wider saturation temperature range and revealed an easier cell growth and foam expansion, the ether-based TPU showed a more limited cell growth behavior and hence processing window. On the other hand, 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. The variation of SS length did not only enhance the flexibility of the TPU samples but could also ease the HS crystallization with various size/closed packed structure with much wider HS crystal melting peak. Such broadened melting peak expanded the foam processing window in TPUs with higher SS length. The increase in SS length could not only enhance the foam expansion behavior but could also enhance the heterogeneous cell nucleation and more importantly minimize the shrinkage of the TPU foams as a result of broadened HS crystallization.