The global footwear industry produces an estimated 24 billion pairs of shoes annually. Estimates vary, but a recent market study indicates that PU and TPU account for approximately 7-8% of the total footwear soling involved. In Europe this figure is higher (~30%). Some 35 years ago, the main use for PU was in casual and fashion shoes.

Today, with consumers demanding more lifestyle type shoes, its popularity has risen and looks set for future expansion. Estimates suggest a compound annual growth rate (CAGR) of 4.4% during 2022-2025 for PU and TPU, almost twice the rate expected for the total footwear industry. So, what lies behind this? Well, their popularity is largely due to their versatility.

PU is one of the few materials that can be used in almost every footwear application. From midsoles to outsoles and unit soles, for sockliners and uppers, and hard components such as soccer plates and stiletto tips, the application flexibility of PU is unparalleled. The fact that it can be manufactured in different ways has also boosted its popularity. Over the years, the ways in which PU and TPU can be processed have evolved. Beyond traditional production methods such as thermoplastic injection moulding for TPUs, and casting and direct moulding for PUs, innovative spray systems are now becoming more mainstream. Furthermore, specialist forms of PU can now be 3D printed while TPU is also being employed in yarns used to make woven uppers.

The environmental credentials of urethane-based systems are also fuelling a new phase of growth. Like the automotive sector, the footwear industry has been ahead of the sustainability curve for some time, shaping the narrative and pushing the boundaries of what is possible. Engaging closely with end consumers, many brands now demand that suppliers offer products that provide environmental performance ahead of what is actually required by legislators. While sustainability objectives differ between companies, there are some very clear themes that stand out.

Broadly speaking, brands want materials that:

• Are based on clean(er) chemistries
• Contain more bio-based materials and recycled content
• Help them actively reduce their carbon footprint and cut energy consumption
• Can be recycled when shoes reach the end of their life

PU and TPU tick all of these boxes, offering advantages at every step of the footwear production process.

Cleaner chemistries

When it comes to creating clean(er) chemistries, PUs and water-blown polyether-based systems in particular, have excellent credentials. Huntsman was the first company to introduce water-blown polyether-based CFC-free footwear systems and is continuing to innovate in this area. In 2019 it launched its Daltoped AquaPUR water-blown polyether range of PUs. Extremely robust, with good flow properties and a well-defined surface finish, this technology is used for midsoles and outsoles, typically in lifestyle shoes. Designed for manufacturers that want to switch to using a water-blown system but do not want to compromise on quality, it provides good cold flex properties plus hydrolysis and slip resistance. Suitable for both direct moulding and casting of unit soles and midsoles, it is compatible with standard PU processing machines and can enable the production of complex soling designs in multiple colour combinations. It also has superior blend stability in the machine tank and can be processed at room temperature, which helps reduce energy consumption and shortens the production cycle.

More bio-based content

As well as using chemistries that are cleaner, there is a big push from the footwear industry for systems that are bio-based. Huntsman is currently formulating TPU and PU systems that are up to 55% bio-based under the Avalon S and Daltoped  S product lines respectively. At this level, these systems can reduce carbon impact by around 25%. Looking ahead, the company is working to expand bio-content to 70% in the near term, while also investing in high impact projects where the goal is 85%. These projects utilise bio-based feedstocks that do not impact on the food chain. When it comes to sourcing bio-content, the priority is clear as only raw materials that come from agricultural waste streams are used, as for example, in the case of the company’s work with Keen, Inc.

Huntsman recently helped this company develop a range of sneakers with plant-based soles. The Field to Foot (F2F) sneakers were created by Keen’s Advanced Concepts Team, utilising a specially developed bio-based PU system that contains a by-product from agricultural processing waste. With a bio content ranging from 35% to 51%, the system has a lower carbon footprint than petroleum-based alternatives. Developed at Huntsman’s footwear centre in Belgium, this direct moulding system offers comparable performance to conventional ones when it comes to resilience, rebound, hydrolysis, comfort and durability but, crucially, is fully aligned with the brand’s commitment to incorporating clean, environmentally focused, high-performance technologies into its footwear. Keen is said to regard this as the most significant environmental technology launch in its history and a meaningful step towards setting a new industry standard for sustainable footwear.

Cutting virgin content

Alongside the development of bio-based systems, the use of materials containing recycled content is a major focus for many of the world’s leading footwear brands. Cutting the use of virgin materials is much easier when using PU, where its inherently adhesive properties make it ideal for incorporating ‘chopped up’ recycled content. Huntsman’s work with Blumaka, an innovative supplier of foam components, is a case in point. It is using Huntsman’s PU and TPU systems as part of its recycled technology platform, which produces the highest recycled foam content products in the footwear industry. Used in Sanuk’s SustainaSole footwear collection, its innovative recycled technology platform employs an Irogran TPU and a Suprasec PU system to deliver all the comfort and performance properties expected of conventional foam components for midsole and insole applications, with 85% recycled foam content by volume and 35% recycled content by weight.

To create these products, waste foam is ground into granulated particles and mixed with Suprasec PU to create the midsole unit. A special Irogran TPU film is then used to help to ensure the finished product is fashionable and stylish, but also functional and high performing. In addition to using recycled foam content, Blumaka’s manufacturing process has a greenhouse effect that is roughly four times less than conventional footwear foam manufacturing. It uses no silicone, far less water, does not require the use of heating ovens to cure the foam and, as part of running a ‘leave no trace’ foam manufacturing facility, the company also takes any scrap or trimmed foam from its footwear operation and uses it to create anti-fatigue mats, which help reduce pressure on the knees and feet of people who have to stand up all day at work.

Alongside its work with environmentally conscious innovators, Huntsman has other recycled content initiatives in the pipeline that will transform what footwear producers can achieve in this area. The company has recently started to incorporate its Terol specialty polyols into its footwear systems. These are made using a proprietary process that upcycles scrap polyethylene terephthalate (PET) bottles. Historically, these polyols have been supplied to the insulation market. Now, the footwear sector is a firm focus, fitting neatly with the environmental ethos of some of the world’s biggest brands and conversations across the sector are ongoing.

Improving energy efficiency

The environmental benefits of specifying PU do not stop at the point of system formulation. As well as delivering materials that are cleaner and can incorporate more bio-based and recycled content, PU and TPU can also help companies cut their carbon footprint during the production process. PUs have a lower carbon footprint than some other materials which have longer cycle times and take a lot of energy to process. Not satisfied with this, Huntsman is innovating further and has recently developed a range of speciality Daltoped systems that have a carbon footprint 30% lower than traditional polyester-based PU ones. Due to a combination of chemistry know-how and technology fit, these materials can be processed at significantly lower temperatures to save energy. In addition, PUs can also be processed in multiple ways, many of which are also less energy intensive. From direct moulding processes that eliminate the use of adhesives and reduce manufacturing steps, to spray systems that use materials more economically and generating less waste, PUs are delivering a new wave of manufacturing flexibility that is both freeing up time and unlocking new creative possibilities.

When it comes to manufacturing efficiency, PUs are helping to disrupt conventional forms of footwear production. Spanish company Simplicity Works working together with Huntsman and DESMA has developed a revolutionary new shoe production method that offers game-changing possibilities to manufacturers looking to produce products closer to customers in Europe and North America (World Footwear September/October 2018). In collaboration, the three companies have created a highly automated, cost-effective way of bonding together two-dimensional components in a single shot to form a seamless three-dimensional upper. Requiring no stitching or lasting, the process connects all pieces of a shoe simultaneously and in just a few seconds. Faster and cheaper than conventional footwear manufacturing techniques, the new technology can be customised and is already proving popular with a number of big brand shoe companies, helping them bring local production overheads into line with lower labour cost countries.

Closing the loop on waste

The story doesn’t stop there. PUs also come into their own at the end of the footwear life cycle, helping the footwear industry address one of its biggest environmental issues: what to do with shoes once consumers no longer want them. The average shoe contains multiple materials from different sources. In the most complex shoe designs, this can include as many as 40 different materials. When it comes to recycling, the use of so many materials can prove problematic. As a result, some brands are now actively designing shoes for disassembly. Others are exploring the development of mono material shoes, which are far easier to recycle.

The footwear industry is now at the point where it is possible to make a whole shoe out of TPU. This could be 3D printed or, alternatively, the shoe might have different component parts made from TPU using different techniques such as a TPU outsole with an expanded TPU midsole, a TPU sock-liner or footbed and an upper made out of TPU either 3D-printed or using TPU yarns. In 2020, Huntsman announced details of a footwear project with OESH, the US footwear brand that makes women’s shoes that are ‘healthy by design’. Using Huntsman TPU materials, OESH created a 100% recyclable shoe sole via 3D printing with positive results. Motivated to offer environmentally conscious customers a fully recyclable shoe, it worked with Huntsman to create a whole shoe that could be shredded and recycled for reuse.  A series of soles were produced using its unique 3D printing technique which creates parts directly from pellets. The soles were then recycled into pellets and put back into the 3D manufacturing process used for three regular OESH styles. Tests showed that the recycled soles had the same functionality as the original sole with no drop in performance properties.

Helping define the future

As in all industries, sustainability in the footwear industry is multi-faceted and there are numerous ways in which companies can shift the dial to reduce the impact of their activities. As the world works to reverse climate change and make the production of consumer products more carbon neutral, PU and TPU will become increasingly important assets in the footwear industry’s armoury. Delivering a truly flexible material solution, PU and TPU can add value at every stage of the footwear manufacturing process, offering the footwear industry multiple solutions to help deliver a brighter future with a better environmental footprint, while raising the bar on what’s possible in terms of material performance, comfort and processability.

SustainaSole™ is a registered trademark of Deckers Outdoor Corporation.
SimplicityWorks™ is a trademark of Simplicity Works Europe S.L.
Blumaka™ is a trademark of Invonu LLC.
Avalon® S, Daltoped®, Irogran®, Suprasec® And Terol® are registered trademarks of Huntsman Corporation or an affiliate thereof in one or more, but not all, countries.

KEEN’s F2F sneaker features a sole that uses a bio-based PU system containing by-product from agricultural processing waste.
Credit: Keen Inc

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