Addition of graphene gives running shoes a boost

Composites made from graphene and polymers are being considered – and used – in a huge range of different applications. In many cases, the goal is to make those polymers electrically conductive, without unduly increasing their mass. Materials including polycarbonate (PC), Polystyrene (PS), and polyethylene terephthalate (PET) have all seen increases in conductivity through the addition of 2D carbon. But graphene is also known to improve mechanical properties, thanks to the strong interfacial bonds that form between the graphene and the polymer matrix.

Inspired by studies that indicated that graphene might increase both the tensile strength and Young’s modulus of EVA (Ethylene vinyl acetate co-polymer) foam, a team of researchers from Nanyang Technological University in Singapore decided to take a closer look. They’ve written about their findings in Polymer Testing [DOI: 10.1016/j.polymertesting.2022.107688].

EVA is a foam widely used in sports equipment; particularly, in the midsoles of high performance running shoes. It is lightweight, good at absorbing impact, and it can ‘bounce back’ in response to that impact. However, it’s not particularly robust in terms of its abrasion resistance. To test what impact the addition of graphene might have on these properties, the authors prepared samples of EVA foams with 0, 0.1, 0.2, 0.5, and 1.0 wt% graphene. Quantities of the blowing agent and nucleating agent were kept the same for all samples, while the amount of DCP – a common cross-linking agent – was varied.

These samples were put through a range of lab tests, with properties such as density, compression strength at 50% compression, and impact properties (impact peak force, absorbed impulse, and total impulse) all investigated.

Samples made with higher quantities of DCP tended to have much lower density than those made with less DCP (0.12 – 0.15 g/cm3 vs. 0.38 g/cm3). Given that the best performing running shoes tend to be very lightweight, the authors decided to set aside the low-DCP foams, and proceed only with those made with 3 part per hundred of rubber.

Graphene was found to greatly enhance the compressive strength of samples, with the highest value measured for 0.1 wt% graphene foam. An impact tester was used to analyse the absorption of mechanical energy by the samples. The sample with 0.1 wt% graphene displayed the highest value of impact peak force, while the lowest was seen for the 0.2 wt% sample. Interestingly, the zero-graphene sample sat somewhere in between. In terms of performance, the lower the force, the better the foam is at absorbing impact, so this result suggests that adding 0.2 wt% graphene had a positive impact on the foam’s energy-absorbing qualities.

The authors were ready to start prototyping real running shoes, which they did in collaboration with Luke Shoes Material Co., Ltd in Xiamen. Shoes were made with the reference material (0 wt% graphene), as well as with the best performing ones from lab tests (0.1 and 0.2 wt% graphene). Interestingly, the industrially-made samples had higher densities than the lab-made ones, possibly resulting from interactions between the nucleating agent and the crosslinking agent. However, this did not negatively impact performance.

The addition of both 0.1 and 0.2 wt% graphene significantly improves the energy absorption characteristics of the foam when used in real footwear. In compression tests, graphene soles were found to be softer than EVA-only soles, which suggests they could be more comfortable for runners. 0.2 wt% soles were also 30 % stiffer than the reference soles – a very useful property that aids in propulsion during running. And finally, samples with 0.1 wt% graphene performed well in abrasion tests – resistance to damage was 40% higher than the reference foam. This may mean that graphene soles will retain their appearance for longer than those made with EVA-only.


Andrey V. Lunchev, Aleksandr Kashcheev, Alfred ling YoongTok, Vitali Lipik. “Mechanical characteristics of poly (ethylene vinyl acetate) foams with graphene for the applications in sport footwear,” Polymer Testing Vol 113, September 2022, 107688. DOI: 10.1016/j.polymertesting.2022.107688