Performance of ethylene vinyl acetate waste (EVA-w) when incorporated into expanded EVA foam for footwear

Abstract

Ethylene vinyl acetate copolymer blends (EVA-B), with vinyl acetate set at 19 % and 28 % by weight (wt%), were mixed with EVA waste (EVA-w), (a reticulated and micronized ethylene vinyl acetate copolymer) from injection branches and defective midsoles. The EVA-B/EVA-w composites were prepared with 15, 25, and 35 per hundred rubber (phr) EVA-w using a Banbury mixer to mix all components. Afterwards, the mixture was submitted to compression molding to obtain expanded EVA-B/EVA-w composite plates. A standard compound, provided by a Brazilian footwear industry (EVA-ref), was used as reference for thermal and mechanical property comparisons. During the thermal analysis, when increasing the EVA-w content, a delay in the deacetylation of the EVA-B composites in relation to the EVA-ref was observed. There was also an increase in the maximum degradation rate temperature and a reduction in loss of mass in the first decomposition event. In the second event, the maximum temperature did not vary in the composites with EVA-w added as compared to the EVA-ref. The residue content increased at 600 °C, indicating that EVA-w acts as a filler for EVA-B. The mechanical properties evaluated were hardness, tension at break, elongation at break, tear strength, compression set, and shrinkage. Addition of EVA-w to the composites (comparing 15 phr–35 phr) increased tear strength properties by 4 %, (EVA-w acts as a nucleation cell agent, increasing the EVA foam cell density). However, shrinkage increased by roughly 35 % above the specifications for footwear applications. Hardness increased by 5 %, tensile strength increased by 10 %, and elongation at break decreased by roughly 4 %. The compression set property increased by nearly 5 % with the EVA-w content (due to decreasing matrix elongation). Thus, EVA-w acts as a reinforcement filler for EVA-B. A statistical study based on composite performance indices for the mechanical properties, and a cost analysis was performed. Except for the average linear shrinkage (35 phr EVA-w composite), all other studied composites met the mechanical property specifications for sports shoe midsoles. The best combination for process performance and cost analysis was for the EVA-w composite at 25 phr.