Fabrication of Sustainable Composite Foam from Ethylene Vinyl Acetate-Based Sole Waste via Solid-State Shear Milling and Supercritical Carbon Dioxide Foaming Technologies

Abstract

Ethylene vinyl acetate (EVA)-based foam has been widely used in the footwear industry, but high-value recycling of EVA-based sole waste (ESW) is still a big challenge, due to its cross-linked structure or complex component. Herein, a facile route to reuse ESW was developed, i.e., adopting solid-state shear milling (S3M) technology to realize the surface-activation and partially de-crosslinking of ESW and then blending ESW active powders (ESW-aps) with thermoplastic polyurethane (TPU) to prepare a TPU/ESW composite foam with good resilience via supercritical carbon dioxide (ScCO2) foaming technology. After 20 milling cycles, ESW-aps with 11.6% reduction in cross-linked chains and an average particle size of ∼400 μm were obtained. This greatly improved the hydrogen bonding between the carbonyl groups of ESW-aps and the amino groups of TPU, so endowed ESW-aps with good dispersion in the TPU matrix. The addition of ESW-aps enhanced the carbon dioxide (CO2) solubility and appropriately reduced the complex modulus of the system, leading to better foaming performance and cell structure of TPU/ESW composite foam. Compared with TPU foam with 20 wt % ESW, the residual strain of the foam with the same content of ESW-aps decreased from 6.8 to 3.6%, close to that of pure TPU foam (3.5%), indicating its potential application in the fields of foamed soles.