Back in 2013, Adidas revolved a new line of running shoes with their all new material which they called Boost. This has been implemented into all of Adidas’ running line (e.g. Adios Boost 2) as a result with the view that it returns energy bak to you when running. However, there is much speculation over whether it is true or just a marketing myth.
This material is called Expanded Thermoplastic Polyurethane or E-TPU for short. This is a relatively new material which has been produced by the chemical company BASF in cooperation with sports manufacturer Adidas where the BASF trademark material Infinergy has been branded Boost by Adidas.
Adidas use this material specifically in Running shoes and are also looking to expand it into other sporting areas like basketball. From this video, you will see why it is such an attractive material for Adidas. This promotional video by Adidas shows the springy, elastic and yet tough properties of E-TPU in comparison to EVA (Ethylene-vinyl acetate) which is the main running shoe sole material; and also concrete which represents barefoot running.
How E-TPU Works:
- Expanded version of the traditional Thermoplastic Polyurethane.
- Particle, closed cell foam.
- Pockets of air.
- Rebound effect.
- Expended energy returned.
The material is a special one of a kind material which is the first to add foam-like properties to a thermoplastic polyurethane. BASF have done this by expanding the original TPU in a new disclosed procedure (1). The chemistry of the original Thermoplastic Polyurethane gives E-TPU some of its main properties. Different chemicals are added together, but the two main ones of long chain diols and chain extender which give elasticity and elastomeric properties (resilience, flexibility and ability to be malleable) along with toughness respectively. The material is also very malleable due to the thermoplastic properties. These come from the physical cross links, which melt when heated but re-form when cooled allowing the material to be shaped without the loss of its strength (2, 3).
The reason why it is used within running shoes is the foam-like properties gained from the expansion process. By expansion, particle, closed cell foam is created; here, pockets of air are enclosed by solid material. This means the pockets are closed off, subsequently creating a resilient, hard and water resistant material. This is one of the most important parts of E-TPU as due to large amounts air in the form of particle foam, it is very light, even at the high densities created for the shoes (7).
Three Levels of Structure:
There are three different structure levels which make up the E-TPU. The first, and smallest layer, is the microstructure which makes up the solid surroundings of the air pockets at the macrostructure level. The rebound effect is created from the microstructure due to the way in which the long chains can rebound when pressure is applied.
This structure continues in the macrostructure by building the walls around the air. These are random pockets which compress upon pressure and instantly bounce back again. Even the effect of large forces when running doesn’t effect the air pockets as their random arrangement means they work similarly to alloys meaning the pockets cannot slide past each other making it a rigid, strong structure (5).
Lots of the air pockets finally make up the final visible beads of foam which are then welded with multiple other beads to create the sole. These beads are tightly, randomly packed so again cannot slide past each other, but still retain the elastic properties coming all the way from the microstructure.
- Malleable and ductile.
- Light, yet strong.
- Tough, 250 kPa, 40,000x = deformation from 40mm-37mm.
- Tensile Strength 600 kPa.
- Recovery Behaviour, up to 55% rebound height.
- Strain of 100-150%.
The properties of E-TPU come from how it is made up, such as the light particle foam and elasticity from the long chain diols (as shown on slide). The fact that E-TPU is a thermoplastic material is very useful commercially as it can be moulded multiple times to gain the best shape and size whilst maintaining its strength. It is also very tough as there is very small deformation, demonstrated in a sample which went from 40mm – 37mm after 40,000 repetitions of 250 kPa.
This represents the longevity of the material and usefulness within a running shoe. In comparison however, the same test was carried out of EPE (Expanded Polyethylene) which is similar to EVA and also used in running shoes and it performed 75% worse with permanent deformation of 9 mm. As shown in the earlier video, the E-TPU material can have up to 55% rebound height in the metal ball rebound tests (1).
The main selling point on recovery behaviour however is the level at which the E-TPU performs in various temperatures. The material doesn’t become too stiff at temperatures as low as -20.C and not too soft at +40.C; this is in comparison to the EVA material which performs three times worse in temperature resistance and deformation behaviour.
Context within running:
- Used commercially by Adidas in Boost Running shoes, light and durable.
- Made replacement for traditional use of EVA foam, delivering better results with runners demonstrating 1% less oxygen consumption when using E-TPU.
- Perfect material for use due to excellent toughness and maintained properties
The use of E-TPU has been specifically used by Adidas who worked closely with BASF in the production of the best possible material for running shoes. The material is the perfect use for the sole of a running shoe which is under constant, repetitive impact as it retains its shape even under great loads of up to 3 or 4 times the body weight. Due to the particle cell foam, it allows the foam sole to be extremely light while giving back significant energy given by the runner giving the best results.
The energy return does have significance and is a breakthrough in comparison to the other material of EVA on the market. It has been shown that for the same weight of E-TPU vs EVA, the runner’s oxygen consumption is 1% less than for the usual EVA shoes. By having a lower oxygen consumption, it then means that the runner has greater oxygen reserves so subsequently reach their VO2Max (% value of maximal volume of oxygen uptake) slower so there is a longer aerobic energy production period (9).
Suitability to running shoes:
This graph here shows why the properties of E-TPU have been created and why it is the best choice of material. The plastic has foam-like properties meaning that it isn’t very stiff which is perfect for the need of elastic properties as it can be compressed and then expand freely continuously. The material also has enough strength for its use as great forces can be applied to the material without it breaking. During running up to 350 kPa can be acting upon the shoes and with the values shown on the graph, this is well within the amount of force needed to be applied for it to break (8, 10, 11)
E-TPU vs EVA Foam:
- EVA causes injuries with continuous loads due to deformation and change in properties from compression.
- E-TPU returns instantly to original state.
- You gain a much greater proportion of expended energy from E-TPU than EVA due to particle, closed cells.
Overall, the material of expanded thermoplastic polyurethane is an innovative material which will revolutionise running. Its benefits are directly seen within the comparison to it predecessor of EVA; which running shoe manufacturers have been using for many years. E-TPU may be a way of solving injuries too as according to ‘The Science of Footwear’, EVA increases peak plantar pressure (greatest force through leg) by 100% after 500km, meaning they no longer have a supportive effect. It has also been found that EVA takes up to 24 hours to return to its original state meaning it is greatly compressed, unlike the E-TPU which springs back to its original state after compression almost instantaneously (10).
There have also been reports which found great structural damages to EVA after continuous loads. At 500km of running, the cell edges wrinkle, losing their strength to the point at which they tear into holes after 750km. The strength of the closed particle cells within E-TPU mean that the solid material enclosing the air is tougher to break, whereas the open celled EVA foam is susceptible to tears and loss of strength after continuous load due to the way they are connected to one another. The final key benefit of the E-TPU is the fact that because there are closed cells, the EVA open cells can be contaminated with water, replacing the air holes and affecting performance (10).
Overall, for me the effects of the material are very little felt within running. Yes, it is true that you will be able get a much greater percentage of energy returned from the material as shown y the drop test. However, over a period of time running and in the specific context that it is in, you find that actually the returns are very minimal and actually won’t be necessarily felt during running. This is specifically backed up by the fact that there is a 1% reduced oxygen consumption rate which in reality is very small.
This concludes for me that as a material used in running shoes, it is definitely the best option, however, the advertised benefits over the traditional EVA have yet to be seen, showing the majority of the myths about the material to be marketing gimmicks.
- The Science of Footwear ( edited by Ravindra S. Goonetilleke) – Material Properties