Non-Cross-Linked Foaming of Styrene–Ethylene–Butylene–Styrene Enabled by Long-Chain Branched Polypropylene with Triple Effects

The demand for thermoplastic elastomer foams continues to grow in applications such as sporting goods and automotive interiors, with styrene–ethylene–butylene–styrene (SEBS) block copolymers attracting considerable attention due to their stable elastic recovery properties. However, SEBS is primarily linear and lacks long-chain branching, leading to low melt strength and limited cell nucleation, which hinders the formation of high-expansion, stable foams under non-cross-linked conditions. Although current modification approaches improve foaming behavior, they often rely on complex processes, offer limited expansion ratios, and hinder recyclability, which constrains their use in sustainable applications. In this study, we introduce long-chain branched polypropylene (L-PP) to enable a continuous synergistic mechanism of “melt strengthening–heterogeneous nucleation–crystalline skeleton formation”: L-PP enhances melt strength during saturation and early growth, provides heterogeneous interfaces during nucleation, and improves dimensional stability through recrystallization during cooling. Under optimized conditions (145 °C, 14 MPa, 1 h), foams with a 10.7-fold expansion were obtained; compared with cross-linked reference samples, compressive strength increased by over 440%, and volume change after 70 °C × 1 h thermal treatment was negligible. The material can be remelted and refoamed, with key cell and mechanical properties retained within experimental error after a single recycling cycle. This study provides a strategy for enhancing the foamability of SEBS, effectively overcoming the technical limitations associated with supercritical nitrogen foaming under non-cross-linked conditions. It offers an important theoretical foundation and practical pathway for the development of green, high-performance, and recyclable thermoplastic elastomer foams, thereby contributing significantly to the advancement of sustainable polymer materials.