Solid and semi-liquid state foaming of polypropylene copolymers towards production of expanded polypropylene beads with double melting peaks

The production of expanded polypropylene (EPP) beads is of great significance for its wide range of industrial applications, however, it requires special grades of polypropylene with high melt strength (HMS-PP) and capability of forming double melting peaks. In this study, conventional Zeigler-Natta PP grades of heterophasic ethylene-propylene copolymer, random propylene-ethylene-butene-1 terpolymer and their 50 wt% blends were foamed in a wide range of processing conditions from solid to semi-liquid state to produce EPP, using CO2 as the blowing agent. In solid state foaming, the double melting behavior is only observed in polymorphic polypropylenes with the lower and higher melting peaks associated with and phase crystals, respectively. All EPP samples showed the double melting behavior when foamed in semi-liquid state, which corresponds to monoclinic crystals with different levels of crystal perfection ( 1 and 2). The EPP samples foamed in solid state are characterized by a closed-cell morphology with high cellular density ( 107), low cell size (1-15 m) and medium expansion ratio (80-160) resulted by the high extensional strength of the polymeric matrix when foamed at below melting temperature. The semi-liquid foaming, which is the most conventional condition in EPP bead foam technology, can provide very high expansion ratio beads ( 250) and double melting behavior, but in a narrower temperature and pressure foaming window, compared to solid-state foaming, which can provide bead foams with fine closed-cell morphology in supercritical conditions (above 75 atm).