Facile one-step method to manufacture polypropylene bead foams with outstanding thermal insulation and mechanical properties via supercritical CO2 extrusion foaming

Abstract

Expanded polypropylene (EPP) foam, is widely used in the field of packaging, automotive and toys, etc., and its preparation process is mainly obtained by semi-continuous autoclave bead foaming. Because foamed beads can be fabricated into complex 3D components and have excellent structural controllability. In contrast, a novel method for continuous production of expanded PP foamed beads is developed via supercritical CO2 (scCO2) extrusion foaming coupling with a wind cutting device, then the real-time cell growth and pelletizing processes of the extruded bead foams are revealed by using a high-definition camera for the first time. Interestingly, the processes of solubilization, compounding, pelletizing and foaming can happen in a one-step process through the above method, which can greatly shorten the production time, save energy consumption, and promote structural diversification. Besides, the transformation of foamed beads, like spherical, cylindrical and dumbbell-shaped, can be achieved by adjusting the pelletizing speeds based on the Barus effect. Furthermore, the steam-chest molding of the extruded foamed beads into foamed product shows that it is possible to sinter the beads at a low steam pressure (2.0 bar). In addition, the as-prepared foamed products can withstand constantly treading by an adult, and the surface of the foamed product rises only ~7.5 °C after prolonged heating, illustrating the outstanding mechanical and thermal insulation properties. Finally, it can be imagined that the related technical route has important scientific significance for the development of other polymeric bead foaming systems.