Rheological properties and the foaming behavior of a commercial linear polypropylene and two long-chain branched polypropylenes were investigated. The latter were prepared by electron-beam irradiation of the linear polypropylene (PP-0) with radiation doses of 5 kGy (PP-5) and 60 kGy (PP-60). In elongational flow strain hardening was found for the irradiated PP, while the linear PP did not show any strain hardening. The strain hardening is more pronounced the higher the radiation dose and thus the higher the amount of long-chain branching. In foaming tests PP-5 already showed a significantly higher expansion ratio than the linear PP, but still a large amount of cell coalescence. PP-60 exhibited the best foaming behavior concerning the density reduction, the homogeneity of the cell structure and the fraction of open cells. These results demonstrate the advantages long-chain branching offers with respect to foaming of polypropylenes.
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