Abstract
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.
Highlights
Polypropylene (PP) foam has been considered as a substitute for other thermoplastic foams, as its mechanical properties are enhanced; these include increased toughness, impact strength, stiffness-to-weight ratio, fatigue life, and thermal stability, as well as its relatively high service temperature [1,2,3,4,5,6]
A higher gel content corresponds to a higher portion of the network formation in the gel content corresponds to a higher portion of the network formation in the amorphous region amorphous region polypropylene, which was insoluble in solvents
Was shown to result in an obvious change in the gel content and crystallinity. These variations had a positive effect on the cell structure—including cell size distribution, cell size, and cell density—and the foaming rate for supercritical CO2 (scCO2) foaming of PP
Summary
Polypropylene (PP) foam has been considered as a substitute for other thermoplastic foams, as its mechanical properties are enhanced; these include increased toughness, impact strength, stiffness-to-weight ratio, fatigue life, and thermal stability, as well as its relatively high service temperature [1,2,3,4,5,6]. The application of PP foam is determined by its mechanical properties, which are dependent on the cell structure—such as cell type, cell size, cell size distribution, and cell density—as well as the foaming rate of PP [1,7,8,9]. The low melt strength and narrow foaming temperature window are key constraints for the scCO2 foaming of polypropylene [14]. Radiation cross-linking can be applied to improve the melt
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