Modelling the Gas-loss Creep Mechanism in EVA Foam from Running Shoes

Compressive creep experiments were performed on an EVA foam from a running shoe midsole of density 275 kg m ^-3 . Analysis revealed a 50% air loss from the cells on a time scale of 10 hours, and a significant polymer contribution to the creep response. The recovery process after creep is slow and incomplete, perhaps due permanent deformation or polymer recrystallisation. SEM shows that surface cells are ‘permanently’ compressed. Modelling was performed of gas diffusion perpendicular to the stress axis, to predict the creep curves, and along the stress axis to predict the development of a compressed, gas-depleted surface layer. The latter considers the interactions of cell strain, diffusivity, stress sharing between the polymer and cell gas, and strain rate. The number of gas-depleted cells is predicted to increase with the square root of the creep time.