Influence of foaming-induced crystallization behavior on the structure of poly(butylene adipate-co-terephthalate) supercritical foamed beads

The crystallization behavior during foaming directly affects the foaming properties. For crystalline polymers, there is no consensus on the influence of the crystallization behavior during foaming process on the stabilization of the cell structure. In this work, PBAT foamed bead and unfoamed pellets were prepared by controlling the saturated temperatures in supercritical CO2, soaking step (one or two) and the depressurization rate, respectively. Double melting peaks were observed in the DSC curve of supercritical CO2 foamed PBAT beads. By comparing the outgassing rates we find that the stretching-induced crystallization caused by the rapid expansion of the gas during foaming plays an important role in the stabilization of the cells. Although the crystalline perfection or crystal size at this time is much smaller than that of the crystalline grains formed during static cooling, the rapid crystallization is effective in stabilizing the cell structure of the foamed pores. Compared to normal supercritical foaming processes, the two-step foaming process of soaking CO2 at high temperatures followed by foaming at low temperatures results in an increase in cell size and expansion ratio. At high temperatures, more CO2 diffuses into the PBAT pellets, increasing the instantaneous gas concentration in the pellets for foaming, and the rapid stretching produces stretching-induced crystallization that raises the average size of the cells, further increasing the expansion multiplicity of individual cells. The average cell size of foam beads rises from 37.8 to 48.8 µm and the expansion ratio also increases to 8.6 with saturated temperature increasing form 95 to 105°C. The two-step soaking foaming method is a more efficient way of manufacturing industrial foamed beads, allowing for the preparation of better foam beads at low temperatures.