With attributes ranging from good rebound resistance to high abrasion resistance and flexibility or stiffness that can be tailored to a wide range of properties, polyurethanes are widely used sports and safety footwear materials. As Craig Roberts, Huntsman’s commercial manager for global brands explains, PUs are commonly used in all manner of ways in the construction of footwear.
‘Both two-component polyurethanes and TPUs are used in footwear, and their use has been increasing in recent years,’ he said. ‘About 7–8% of soling materials are now PUs and TPUs.’ But these materials are starting to cover the tops of our feet as well as protect the soles. ‘We are now seeing them increasingly being used in upper components, too, which has driven some of the market share. PUs and TPUs are being used in new ways in footwear. You can now spray PUs, and make TPUs into yarns. This helps them get into places where they were not used so much before.
Govind Gupta, Dow’s marketing manager for polyurethane based in Maharashtra, India added that the opportunities for PU in footwear are significant. ‘PU can address the key drivers of the footwear industry; it provides great design freedom, comfort, and durability with superior productivity, to the OEMs and brand owners,’ he said.
But it has not always been so positive for polyurethanes, Roberts claimed. ‘The use of polyurethanes in footwear is a little cyclical,’ he said. ‘A lot of PU was used in sport midsoles 30 years ago, but much was replaced by EVA as it was lighter, did not discolour and was not subject to hydrolysis degradation.’
Swinging back to PU
The pendulum is moving the other way, and there has been a swing back to PU as people realise that it has better environmental aspects. He pointed to materials that can have fairly high levels of biobased content, with the added bonus that TPU is naturally recyclable.
Gupta agreed. ‘Sustainability is a common challenge for not only polyurethane, but for all other materials used in the footwear industry,’ he said, adding that Dow, in common with other materials suppliers, is working to reduce the carbon, energy, water and waste footprints of its polyurethane production. ‘We are optimising our supply chain deliveries, our R&D team is working continuously to design new formulations which are safer and more sustainable, and we are working actively to manage end-of-life issues for PU, and closing the loop for used products.’
Turning the focus back towards polyurethane technology, Huntsman’s Roberts there has also been a push recently to making expanded TPUs, such as in the Adidas Boost technology, and equivalent technologies from other brands. Expanded TPU beads, expanded TPU sheets, and injection moulded TPUs at fairly low densities are all finding their place.’
Expanding TPU helps to reduce its density, which is important for runners who greatly appreciate lighter shoes, and is also important in the athleisure market. It gives the shoes added bounce, or in more formal language, returns more energy to the user.
Two component systems
Although TPU is currently the subject of a lot of interest in the footwear sector, two-component polyurethane systems still retain a big place in the market. Roberts contrasted the two approaches: ‘There are several advantages of two-component PU in footwear. Generally, these have very good resistance to abrasion and compression set, they can be injection moulded and attached directly to the upper.’ The materials react in the mould which makes it easier to bond a PU midsole to a PU, rubber or TPU outsole, he said.
Gupta pointed out that this is a key benefit of using reactive polyurethanes. ‘Direct injection on uppers [can help processors] avoid using attachments to connect the upper with the sole. It can help producers. to save a lot of time.’
Roberts added that using a reactive two-component system can help to reduce the amount of in-process scrap that footwear manufacturers can develop. ‘You can also incorporate chopped-up recycled material into the PU,’ he said. ‘On the TPU side, it is thermoplastic, it can be relatively low density.’ He said that this can be below 0.2 kg/m¡ for blown TPUs. This helps to retain performance.
Additionally, TPUs can be processed by a number of methods. ‘They can be 3D printed, sprayed, injected or cast,’ he said. This makes TPU materials suitable for a number of footwear applications, ranging from a lightweight cushioning foam in the upper for extremely soft padded collars, right through to something very hard like a soccer plate or the tip of a stiletto heel. Low density, high rebound materials are what the sports casual trainer market wants, according to Dow’s Gupta. ‘In casual segments, we have a lot of development where light-weight and the high rebound performance are two of the key requirements,’ he said.
But there are downsides to using polyurethanes in footwear of all kinds. Firstly, they yellow with UV exposure and age, while EVA does so to a much smaller urethanes have suffered from hydrolysis, breaking down in moist environments, which can be a problem for long-term warehouse storage,’ Roberts said. ‘This has largely been addressed with better quality polyester formulations, and polyether technologies that are inherently stable to hydrolysis and therefore do not have that issue. The other drawback of PU is that the reaction is done in the factory; in contrast, TPU is already reacted and just needs melting and reprocessing. However, two-component materials do give huge versatility.’
To win market share, TPU suppliers need to exploit the advantages of superior abrasion. ‘In the past, the big disadvantage of TPU was the density, but now this can be got right down,’ Roberts said. ‘Wrapping a PU midsole with an aliphatic TPU film can take away the issue of yellowing.’
Changing processing methods can help to turn a weakness into a strength, he added. ‘Air bubbles have been a problem for liquid PUs, because air becomes trapped as it flows in the mould, and the bubbles look like defects. But if you spray PU into the moulds instead, you do not see this, and you get extremely high definition. This also allows extremely thin profiles, picking out fi ne details in the mould.’
Make it safe
There is a great deal of focus on sports and recreational footwear, but safety footwear is another important sector for polyurethanes. Dow’s Gupta said that, traditionally, this type of footwear is very safe and heavy, but not very comfortable. ‘Now, as a new consumer requirement of comfort is coming up, Dow has introduced solutions for safety shoes which can be more comfortable and safe for consumers,’ he said.
This is an area where crossover from one type of footwear to another is noticeable. ‘We also have a number of products in our DaltoPed Energy range,’ said Roberts. ‘This includes higher-rebound systems that were originally designed for lifestyle casual products, but have taken off well in safety boots. A safety boot is worn all day; considering people have to work longer, so why shouldn’t they have comfort and energy return in the way that running shoes do?
Back in the world of polyurethane systems, Gupta pointed to some new variations in his company’s Enerlyte brand of water-blown polyurethane systems for footwear. These systems produce polyurethane materials with density between 320 and 380kg/m3, and the unit sole grade has abrasion resistance of <120mm3. The company also claims that there is no cut growth in the materials to DIN 53543. The material has a Tg below –20°C.
Another option is Huntsman’s AquaPur. ‘Our AquaPur water-blown polyether PU range is extremely robust, with good flow and a good surface fi nish,’ Roberts said. ‘It is used for outsoles, typically for casual shoes.’(
In the second half of 2021, Covestro announced a couple of footwear developments: a partly bio-based midsole for hiking boots, and a new adhesives system. The company has teamed up with Vaude, a European manufacturer of footwear and outdoor equipment, for the boots. Covestro’s Desmopan EC 33000 series materials are used to make durable foam materials that meet Vaude’s Green Shape criteria, without sacrificing performance, the company said.
‘With the new Lavi Eco, we want to introduce our first bio-based outdoor shoe in the 2022 summer collection and the first outdoor sports shoe in which all components are made from some bio-based and recycled materials,' said Clement Affholder, innovation manager at Vaude.
Covestro’s material is being used for the midsole. This has to be soft and cushion the foot, but give good grip. TPUs have been used in midsoles for active footwear such as mountain boots for a while now, said Sayma Hou, key account manager footwear at Covestro. This is the first time a hard–soft material combination has been produced with non-GMO raw materials, she claimed.
The raw material comes from start-up supplier Metabolic Explorer. The company uses palm oil as the starting point for the 1,3-propane diol that is used to produce the more complex polyols used in the formulation.
Using the EC materials, it is possible to produce polyurethanes for footwear that generate 20% less CO¥ in production than conventional oil-based polyols, said Covestro. It is not the first time that Vaude has chosen products from Covestro’s Desmopan EC range. The material has been used in the toe and heel components on Vaude’s TRK Skarvan hiking boots since 2018.
In a separate development, Covestro, has launched Desmomelt U thermoplastic polyurethane powder into the digital printing arena as a hot melt adhesive for footwear production. The material can be melted and applied precisely in complex patterns so that the sole of the shoe does not separate from the upper using digital printing processes. This is significantly faster than using polyurethane dispersions, and allows processing steps to be significantly reduced, Covestro said.
Turkish systems house Kimteks Poliuritan is another company that offers a range of footwear grades. Its Kimfoot sports shoe sole systems, for example, are three-component, low density polyester- based systems that offer high flex resistance, high flowability and short demoulding times, the company said. Typically, the high-gloss grades offer density between 380-420 kg/m3, with Shore A hardness of 45±5 and compression set of 35, according to TS EN ISO 1856.
Formulations for safety shoes have also recently been updated, including a grade AA 00900 which is hydrolysis resistant. The range of three-component systems are designed for use in double or single sole formulations. When the products are used in a dual-sole system, the PU midsole is soft and breathable, while the outer PU sole provides high resistance to slip and corrosion, the company said.
Kimteks added that the outers can be fixed to PU or TPU. The soling formulations can be moulded via injection or cast techniques. Single density formulations yield materials with density between 500 and 550kg/m3, with a hardness of 58±5 Shore A. These demould in 150-210 sec. The antistatic double density outsole formulations are between 900-1000kg/m3, with hardness of 55±% Shore A and typically demould in between 90-120s.
Another Turkish systems house, Flokser Chemical, has introduced a new soft segment production group which it said has developed a product that allows soft, low density products to be made in slipper formulations which retain their dimensional stability. The company said soles with density between 230 and 320kg/m3 can be produced with its products by changing catalysts. Traditional formulations are typically 270kg/m3, the company said. Additionally, the system is designed to work with a lower ratio of diisocyanate, which can help to reduce costs. The company added that this could compete effectively with EVA materials.
Meanwhile, David Pascual, global 3DP commercial lead at Lubrizol Advanced Materials, outlined the kind of product offering that materials companies need to compete successfully in this sector. ‘We supply adhesives, and TPU for injection, extrusion and blown film,’ he said. ‘We already supply TPU for different applications and are working with customers to understand where we can be different from the current solutions in the marketplace, and we see designers, the key stakeholders in the footwear value chain, making a lot of developments.’
New life for old shoes?
Recently, Lubrizol launched a 100% TPU prototype shoe, which it claims meets the high-performance demands in the footwear industry, while achieving greater sustainability through enhanced recyclability. The shoe is made using the company’s TPUs yarns to keep the construction light weight. BounCell TPU foam is used for cushioning, and Estane TRX thermoplastic polyurethane is the abrasion resistant outer material.
Like other polyurethane applications, using TPU helps to glue the shoe together during production. This can simplify production by removing additional gluing steps and also make it easier to automate the production process. The thermoplastic nature of the polyurethane makes it much simpler for the footwear producers to recycle in-process waste back into the production stream. This saves emissions from incineration, the traditional way of dealing with in-process scrap.
As well as being recyclable in production and, in principle, at the end of the product’s life, the polyurethanes used to make the shoes contain a significant proportion of bio-based or renewable raw materials. Estane ECO TPU outers are derived from natural resources and deliver the same mechanical physical and chemical performance properties as conventional TPUs, Lubrizol said. The grades are also between 5% and 10% less dense than equivalent petrochemical grades and o. er the same hardness. The TPU resists yellowing as well, said the company.
Elsewhere, BASF has moved its Infinergy expanded TPU material out of pure sports footwear and into Hotter comfort shoes. The German company said Hotter chose the material because it complements the shoemaker’s own Cushion+ technology. The footwear is being marketed as offering what it described as ‘athlete-endorsed energy return’ to help make wearing the footwear less fatiguing. Hotter’s key demographics are the over 55s and professionals who have to stand for a long time in their jobs or commuting. Another BASF material is being used to make Haptex faux leather materials with footwear applications, particularly in the uppers of sports shoes. Haptex materials are claimed to have significantly lower energy and water demands than either water- or solvent-based competitors. By using these Haptex materials, synthetic leather producers can make faux leather with 52% lower greenhouse gas emissions and 20% less energy consumed per kg of chemicals used, BASF said. These figures were provided by Intertek, which made a lifecycle analysis of the materials its chemical production to the finished goods.
Traditionally, a lot of PU-based faux leather products are made by dissolving polyurethane (30% by weight) in DMF, and spreading the paste over the substrate with a knife. Although 99% of the DMF solvent can be recovered, the remainder is lost to the atmosphere via the factory’s extraction systems, or diffuses out of the faux leather over time. DMF is flammable, irritates the eyes and skin, and could damage fertility, according to GHS warnings for the chemical.
BASF said the Haptex polyurethanes resist yellowing, have good abrasion and peel properties. They are polyurethanes that are reacted in situ, and currently used by two companies in China, Warren Synthetic Leather (Suzhou) and Suzhou Greentech. BASF said it designed the material to help these types of company move away from dimethylformamide-based solvent coating systems. It is working with other Chinese companies who are