Unveiling the Printability and Performance of 3D Printed Ethylene-Vinyl Acetate Stretchable Foams.

The rise of additive manufacturing has pivoted the trajectory of advanced materials with intricate geometries and precise design. Although various materials have been adapted to 3D printing, developing printable microcellular foams remains in its infancy. Herein, for the first time, we report 3D printed ethylene-vinyl acetate (EVA) foams enabled by expandable microspheres (EMs). Expandable filaments of EVA/EMs containing 0-10 wt % EM result in a tailored density in the range of 0.903-0.224 g/cm3. The viable temperature spectrum relies upon EM content, where higher EM loading leads to a wider printing window. The foam density further exhibits an optimal correlation with temperature, reaching its minimum at 230 °C, independent of EM content. Microstructural analyses reveal uniform morphologies wherein the foam's cell density increases and cell size slightly drops with EM loading. The decay rate of Young's modulus and tensile strength with density reduction is significantly lower compared to conventional foams, which is attributed to the reinforcing effect of the EM shell material. Strain rate and print pattern dependency of mechanical properties are also discussed. While unfoamed EVA exhibits strain hardening upon cyclic compressive loading, the addition of EMs introduces a counteracting strain-softening behavior proportional to EM content. The thermal analysis furthermore depicts that EM leaves a negligible impact on the crystallinity of EVA. The findings of this work highlight the potential of 3D printed EVA foams as lightweight and tunable materials for various applications.