Impact of hollow glass microsphere addition to poly(ethylene vinyl) acetate foam on mechanical properties of sports footwear

Abstract

Samples of ethylene vinyl acetate (EVA) co‐polymer foams in form of plates and industrial outsoles of prototyped sport shoes containing 0% (reference), 2.5%, and 5.0 weight % (wt%) of hollow glass microspheres (HGM) were made. The goal was to study the influence of HGM on properties of EVA foam in laboratory conditions followed by the industrial trial, where the most promising laboratory result was applied. The physical properties of samples and behavior of material under dynamic load were examined. EVA foams with HGM filler demonstrated that they have better ability to reduce impact peak force and increase absorb impact impulse compared to the reference foam without HGM. Addition of 5.0 wt% of HGM in laboratory foam increased its absorbed impact pulse by two‐fold. Meanwhile for industrial prototype outsoles, the impact peak force was reduced by 5%–7% in walking simulated conditions and by 11% in case of simulated running. It is also observed that, addition of 5.0 wt% of HGM in EVA foam increased its abrasion resistance by three times, which generally helps to increase the lifetime of sport footwear. Under dynamic load, samples with the addition of HGM behave interestingly and result shows a small decrease in energy return (7%) which is logical, because this composite tends to absorb energy and not return it. Samples with 2.5 wt% HGM are stiffer than reference until 40% compression deformation, but beyond that point it becomes softer, and better absorbs impact energy than reference. Outsole with 2.5% of HGM in EVA outsole increased resistance of shoe at lateral bending of 50% and 20% rise of resistance was found at medial twist. Impact absorption and flexibility resistance are useful for protection against impact which are also helpful to prevent sport injuries. The obtained results show the addition of HGMs which are cheap by‐products in several technologies brings improvement to properties of EVA foams which is the most common materials for the sport footwear industry.