Cell Structure Regulation of Polypropylene/Ethylene-Propylene Rubber Bead Foams and Enhanced Mechanical Properties of Their Molded Products

To improve the foamability and steam-chest molding performance of polypropylene (PP) bead foams, ethylene-propylene rubber (EPR) was introduced into PP via melt blending. The role of EPR in the complete bead-foaming-to-molding process was systematically investigated by correlating phase morphology, crystallization behavior, melt viscoelasticity, CO2 dissolution and diffusion, cellular structure, inter-bead welding, and the mechanical properties of molded foam products. The incorporation of EPR refined the PP crystalline morphology, reduced the apparent crystallinity, and markedly enhanced the melt viscoelasticity, thereby broadening the foaming temperature window. The dispersed EPR phase functioned simultaneously as a CO2 reservoir and a high-diffusivity pathway of CO2, which promoted cell growth while suppressing excessive nucleation. The enhanced melt viscoelasticity and improved CO2 affinity promoted bead expansion and optimized the cellular structure. At 150 °C, the expansion ratio increased from 18.7 for neat PP to 21.1 with 10 wt% EPR. EPR also regulated the cellular structure. At 150 °C, the cell diameter increased from 83 to 176 μm as the EPR content increased from 0 to 20 wt%. EPR markedly changed the double-melting behavior of PP bead foams. The low-temperature melting enthalpy increased from 28.5 J/g for neat PP to 37.8 J/g with 10 wt% EPR, which served as an effective interfacial binder, significantly promoting inter-bead welding. Consequently, the optimized PP/EPR foam containing 10 wt% EPR exhibited a tensile strength of 1.13 MPa and an elongation at break of 22.1%. More importantly, excellent molding quality was achieved at a reduced steam pressure of 2.2 bar, demonstrating the great potential of PP/EPR bead foams for the energy-efficient manufacturing of high-performance lightweight products.