Herein, 10-fold expansion ratio foams were successfully fabricated for the first time by a core-back foam injection molding (FIM) technique using a newly developed long-chain branching polypropylene (LCBPP). Different rheological curves were investigated to distinguish the strain hardening behavior and melt strength of the LCBPP from isotactic polypropylene (iPP). Fast scanning chip calorimetry (FSC) results revealed that the long-chain branches could increase the crystallization temperature even at fast cooling rates. The FIM of LCBPP could notably not only reduce the cell sizes but also increase the cell densities through the capability of LCBPP to stabilize cells and prevent cells from coalescence. Furthermore, the foam processing window of manufacturing 10-fold expansion foams was significantly broadened for LCBPP foams. This study revealed that the introduction of longchain branches is a good approach to fabricate high expansion microcellular foams for different potential applications such as construction and transportation.

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A new foam injection-molding technology was developed to produce microcellular foams without using supercritical fluid (SCF) pump units. In this technology, physical blowing agents (PBA), such as nitrogen (N 2 ) and carbon dioxide (CO 2 ), do not need to be brought to their SCF state. PBAs are delivered directly from their gas cylinders into the molten polymer through an injector valve, which can be controlled by a specially designed screw configuration and operation sequence. The excess PBA is discharged from the molten polymer through a venting vessel. Alternatively, additional PBA is introduced through the venting vessel when the polymer is not saturated with PBA. The amount of gas delivered into the molten polymer is controlled by the gas dosing time of the injector valve, the secondary reducing pressure of the gas cylinder and the outlet (back) pressure of the venting vessel. Microcellular polypropylene foams were prepared using the developed foam injection-molding technology with 2-6 MPa CO 2 or 2-8 MPa N 2 . High expansion foams with an average cell size of less than 25 lm were prepared. The developed technology dispels arguments for the necessity to pressurize N 2 or CO 2 to the SCF to prepare microcellular foams. POLYM. ENG. SCI., 57:105-113, 2017. 109 FIG. 11. SEM micrographs of iPP/MD (0.5%) foams with N 2 at different expansion ratios. The foaming temperature is indicated in the upper left corner of each micrograph.