Natural Pumice-Filled Flexible Polyurethane Composites: Effect of Filler Content on Mechanical Performance and Cyclic Durability

This study investigates the effect of natural pumice filler content on the physical and mechanical performance of flexible polyester-based polyurethane (PU) composites. Composites containing 0, 2, 5, 10, and 20 wt.% pumice were produced using a constant PU matrix formulation and identical processing conditions. Microstructural and structural characteristics were examined using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and Fourier transform infrared spectroscopy (FTIR), while density, water absorption, tensile behavior, abrasion resistance, and cyclic flexural durability were evaluated. Increasing pumice content resulted in greater local microstructural heterogeneity and increased density and water absorption. Tensile strength exhibited a non-linear numerical response to filler content, with the highest mean value obtained at 5 wt.% pumice, although the differences among formulations were not statistically significant. In contrast, elongation decreased significantly overall with increasing filler content, while mean abrasion mass loss progressively increased with filler loading. Cyclic flexural testing revealed that the unfilled PU and 2 wt.% composite completed 30,000 cycles without measurable crack propagation at 20 and 0 °C, whereas higher filler contents exhibited severe crack growth or complete fracture. At −20 °C, all formulations fractured completely. Overall, pumice content produced a property-dependent response, revealing a trade-off among tensile response, deformability, abrasion resistance, and cyclic durability. These findings provide a basis for tailoring flexible PU composites for applications involving repeated deformation and mechanical contact, including footwear and cushioning components.