The production of expanded polypropylene (EPP) beads is of great signi cance for its wide range of industrial applications. However, it requires special grades of polypropylene with high melt strength (HMS-PP) and the capability of forming double-melting expanded beads to enable sintering during post-processing of EPP. In this study, conventional Ziegler-Natta PP grades of heterophasic ethylene-propylene copolymer, random propylene- ethylene-butene-1 terpolymer, and their 50 wt% blends were studied in a wide range of foaming conditions from solid to semi-liquid state. While, all the polypropylenes were capable of producing EPP beads with double- melting behavior when foamed in a semi-liquid state, in solid-state foaming, the double-melting behavior ap- pears only in polymorphic polypropylenes with low temperature and high temperature melting peaks associated with γ and α phase crystals, respectively. The EPP samples foamed in the solid-state show a closed-cell morphology with high cellular density (>1010 number of cells/cm3) and low cell size (0.3–5 μ m) resulting from the high extensional strength of the polymeric matrix when foamed at a temperature prior to the initial melting. Using polymorphism to induce double-melting behavior and performing the expansion in solid-state is promising for producing nano cellular EPP beads.

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