Multifunctional Integration and High Dimensional Stability in EVA Foam via Dynamic Ionic Cross-Linking Network

The thermal shrinkage of ethylene-vinyl acetate (EVA) foam compromises its dimensional stability, limiting its suitability for high-performance fields. This work presents a method to construct a dynamic ionic cross-linking network (DAM) in EVA, forming a semi-interpenetrating structure that markedly boosts melt strength and dimensional stability. The DAM is formed by coordinating Zn2+ ions with carboxyl groups generated from the in situ hydrolysis of acrylate copolymer (ACR) ester groups, in the presence of maleic anhydride (MAH) and catalyzed by zinc acetate. The formation mechanism of the DAM was systematically investigated, and the effects of DAM on the thermal behavior, rheology, cell morphology, dimensional stability, and mechanical properties of EVA were comprehensively analyzed. Results show that the introduction of ACR and MAH successfully constructs the DAM, markedly improving the cross-linking density, melt strength, and thermal stability of EVA. Benefiting from the heterogeneous nucleation effect of ionic clusters and the increased melt strength, the EVA foam exhibits finer and more uniform cells, along with a higher foaming ratio that enables lightweight structures. The construction of DAM endows EVA with outstanding dimensional stability at 70 °C, reducing transverse and longitudinal thermal shrinkage by 66.77% and 68.62%, respectively, while increasing tensile and compressive strengths by 51.7% and 35.3%. Moreover, the EVA exhibits a low thermal conductivity of 0.051 W/m·K and a high contact angle of 147.79°, indicating excellent thermal insulation and self-cleaning potential. This work presents a simple method to construct dynamic ionic networks in EVA foam, effectively solving its thermal shrinkage problem. The resulting advanced foam is lightweight, stable, tough, and thermally insulating, holding great promise for use in sports footwear, precision packaging, and building insulation.