Mechanical recycling of TPU foams via microcellular injection molding - effects on hard segments crystallization, cell structure, and mechanical properties

Recycling thermoplastic polyurethane (TPU) foams offers a promising route toward sustainable footwear manufacturing, but the high thermal and shear sensitivity of TPU introduces new challenges. This study investigates industrial-scale microcellular injection molding of recycled TPU (rTPU) foam. The reorganization of the hard segments (HS) during processing led to a significant increase in crystallization onset temperature after recycling. The HS crystals acted as cell nucleation sites, increasing cell number density from 17.9 × 10 6 cells/cm 3 in the virgin foamed material, to 155.6 × 10 6 cells/cm 3 in rTPU parts. Simultaneously, the cell diameter decreased from 42.6 μm in the virgin foams to 21.2 μm in the recycled foams, on average. Despite this morphological refinement, rTPU foams exhibited inferior mechanical performance under cyclic compression, with a 14.8% higher energy loss coefficient compared to the virgin TPU parts. Overall, these results demonstrate that expanded TPU waste can be successfully processed via microcellular injection molding, although degradation remains a limiting factor that requires further process optimization.