Structure–Property Relationships in Polyester-Based Polyurethane Foams with Varying Isocyanate Index for Footwear Midsole Applications

Polyurethane (PU) foams are widely used in footwear midsoles because their cellular structure, density, impact-attenuation capability, and mechanical durability can be tailored through formulation design. In this study, polyester-based PU foams were prepared at different isocyanate indices while keeping the main formulation components constant, and their structure-property relationships were evaluated under midsole-relevant conditions. The samples were characterized by density, tensile and compression testing, standard abrasion wear testing, water absorption, temperature-dependent flexural resistance, Fourier transform infrared (FTIR), scanning electron microscope (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis/derivative thermogravimetry (TGA/DTG), and dry/wet tribometry. FTIR results confirmed the formation of urethane/urea-related linkages and the absence of detectable residual isocyanate groups, whereas DSC indicated broad heat-flow events typical of segmented PU systems, including high-temperature events that should be interpreted together with TGA. TGA/DTG analysis showed similar initial degradation behavior for all formulations; however, the 138-index sample exhibited the highest t90% value, indicating improved high-temperature mass retention. Tribometric tests revealed an environment-dependent coefficient of friction (COF) response: the 138-index sample exhibited the lowest steady-state COF under dry sliding (μss = 0.211), whereas the 113-index sample showed the lowest COF value under wet sliding conditions (μss = 0.176). Overall, among the three stable formulations investigated, the 113-index formulation exhibited the most balanced multi-property performance. These results suggest that, within the tested formulation range, midsole-relevant PU foam performance is associated with a balance of formulation characteristics rather than simply with increasing the isocyanate index.