Foaming behavior of melt compounded thermoplastic polyurethane in presence of butane

The hard segments in thermoplastic polyurethanes (TPUs) are statistically distributed in the soft matrix phase. This distribution can be changed by processing the TPU material at various conditions such as melt processing or isothermal treatments. Furthermore, the hard segments contribute to the microcrystalline phase in TPUs. In this study, the foaming behaviour of "as received" and "processed" TPUs has been investigated using n-butane as a blowing agent. It was found that changes in the hard segment distribution caused by melt compounding affected the foam morphology. Comparing the Differential Scanning Calorimetry (DSC) thermographs of unfoamed "as received" with "processed" TPUs, it was observed that the "processed" TPU showed a broader endothermic peak, suggesting a change in the hard segment length distribution. Furthermore, the low melting temperature peak as seen in the "processed" TPU can be improved to higher temperatures by an isothermal annealing procedure. The synergistic effect of annealing temperature and plasticizing effect of butane as a blowing agent may lead to the formation of microcrystalline hard segment domains that are dispersed in the soft segment domains. These microcrystalline domains could act as nucleating agents for the trapped gas molecules leading to the increase in the cell density as seen in the "processed" TPU foams.