Microcellular Foam from Ethylene Vinyl Acetate/Shear-Stiffening Gel for Advancing Footwear Midsole Development

Ethylene-vinyl acetate (EVA) is widely used in shoe soles due to its foamability, affordability, and resilience. However, rising consumer demands for greater durability and energy rebound drive the need for performance enhancements. This study presents a novel approach by introducing a silicone-based masterbatch (MB), developed by incorporating a silicone-based shear-stiffening gel (SSG) (polyborosiloxane) with EVA through a reactive extrusion process. Two types of SSG/EVA are produced, i.e., crosslinked SSG/EVA (abbreviated as SSG/EVA-X) and non-crosslinked SSG/EVA (abbreviated as SSG/EVA-NX). The SSG/EVA-X reveals interconnected phases, including crosslinked and uncrosslinked SSG and EVA, and mixed regions. The developed SSG/EVA are further utilized as MB to modify and enhance EVA foam properties. Incorporating SSG/EVA MB into EVA foam significantly enhances its physico-mechanical properties. These improvements are pronounced in EVA foams incorporating with the SSG/EVA-X MB (abbreviated as EVA/MB-X), which outperforms the SSG/EVA-NX MB, (abbreviated as EVA/MB-NX) due to superior material integration. Dynamic impact energy return of EVA foam increases by over 10%, while abrasion resistance shows an improvement of more than 50% at the optimal SSG/EVA MB content. These findings suggest that crosslinking silicone MB with EVA presents a promising strategy for EVA foam modification, offering a pathway to enhance its performance.