Peculiarities of Quasi-Isothermal Foaming of the SCF-Plasticized Polylactide: the Effect of Transition from Foam Expansion to Its Collapse

Abstract

The results of the experimental and theoretical studies of the behavior of the amorphous D,L-polylacide foam are reported. D,L-polylacide is preliminarily plasticized using sub- or supercritical carbon dioxide at a constant temperature of 313.3 K under various pressures in the range from 4 to 20 MPa; after plasticizing, it is foamed by quasi-isothermal depressurization with low depressurization rates (from 0.005 to 0.05 MPa/s). It is found that the transition from the expansion of the polylactide foam to its collapse occurs during depressurization when polylactide is plasticized under supercritical conditions (with the carbon dioxide pressure above 8 MPa). Using the equation of state first proposed by S. Ross for a thermodynamically equilibrated foam, the criterion for such a transition is established. It is shown that this phenomenon is caused by a substantial increase in the surface tension of the polymer when slowly reducing the carbon dioxide pressure.