Since the maximum foaming temperature window is only about 4 °C for supercritical CO₂ (scCO₂) foaming of pristine polypropylene, it is important to raise the melt strength of polypropylene in order to more easily achieve scCO₂ foaming. In this work, radiation cross-linked isotactic polypropylene, assisted by the addition of a polyfunctional monomer (triallylisocyanurate, TAIC), was employed in the scCO₂ foaming process in order to understand the benefits of radiation cross-linking. Due to significantly enhanced melt strength and the decreased degree of crystallinity caused by cross-linking, the scCO₂ foaming behavior of polypropylene was dramatically changed. The cell size distribution, cell diameter, cell density, volume expansion ratio, and foaming rate of radiation-cross-linked polypropylene under different foaming conditions were analyzed and compared. It was found that radiation cross-linking favors the foamability and formation of well-defined cell structures. The optimal absorbed dose with the addition of 2 wt % TAIC was 30 kGy. Additionally, the foaming temperature window was expanded to about 8 °C, making the handling of scCO₂ foaming of isotactic polypropylene much easier.
Keywords: TAIC; foaming; polypropylene; radiation cross-linking; supercritical carbon dioxide.