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A series of crosslinked polypropylene samples with increased melt strengths were prepared via a copolymerization reaction, followed by melt processing. These crosslinked PP samples (PP-Cs) were foamed by a temperature rising process using supercritical CO2 as the physical blowing agent. The introduction of crosslinking structure resulted in PP-Cs foams with well-defined closed cell structure, decreased cell size, and increased cell density in comparison with a linear PP, which were attributed to the suppressed cell coalescence due to the significant increase in melt strength of PP-Cs. Further increasing the crosslinking degree tended to enhance the suppression effect on the cell coalescence, and hence increase the cell density of PP foams under the same foaming conditions,
especially at the longer foaming times. The welldefined closed cell structure was observed at the foaming temperature of 170–2508C and saturation pressure of 12–20 MPa.