Polypropylene re-entrant foams with high energy absorption realized by vacuum-counter pressure-assisted supercritical CO2 foaming

Manipulating the cellular microstructure is an important means to alter the mechanical properties and expand the application of polymer foams. Herein, a vacuum-counter pressure-assisted supercritical CO2 foaming (V-CP foaming) process was developed to fabricate polypropylene (PP) foams with closed re-entrant cell structures. The re-entrant cellular structure was realized by the combined effect of the cell over expansion during the vacuum treatment stage right after the depressurization, and the rapid cell contraction under the counter pressure of the refilled air. The morphology of the re-entrant structure could be tuned by adjusting the key processing parameters. By using butanol and scCO2 as the co-blowing agent, the over expansion and rapid contraction of cells were greatly amplified due to the plasticizing, vaporization, and liquidation of the butanol in the V-CP process. The fabricated PP re-entrant foams that have a negative Poisson's ratio possess prominent compressive properties and impact energy absorption performance.