Structure-Property Relationships in Polyester-Based Polyurethane Foams with Varying Isocyanate Index for Footwear...

Area of Science:

• Materials Science
• Polymer Chemistry

Background:

• Polyurethane (PU) foams are integral to footwear midsoles due to tunable properties like cellular structure and impact resistance.
• Formulation design is key to optimizing PU foam performance for specific applications.

Purpose of the Study:

• To investigate the structure-property relationships of polyester-based PU foams at varying isocyanate indices.
• To evaluate foam performance under conditions relevant to footwear midsoles.

Main Methods:

• Synthesized PU foams with controlled isocyanate indices.
• Characterized foams using density, mechanical testing (tensile, compression, abrasion), water absorption, thermal analysis (DSC, TGA/DTG), FTIR, SEM, and tribometry.
• Assessed dry and wet sliding friction (coefficient of friction - COF).

Main Results:

• FTIR confirmed urethane/urea linkages and no residual isocyanate groups.
• TGA/DTG showed the 138-index sample had superior high-temperature mass retention.
• Tribometry revealed environment-dependent COF: 138-index was best for dry sliding (μss = 0.211), 113-index for wet sliding (μss = 0.176).

Conclusions:

• The 113-index PU foam formulation demonstrated the most balanced multi-property performance among the stable formulations tested.
• Optimal midsole performance in PU foams is achieved through a balance of formulation characteristics, rather than solely increasing the isocyanate index.