TPU foam breakthroughs help shoe sustainability


The boundaries in shoe manufacturing are being stretched as foamed TPU soles receive attention from innovative companies in process technology and bio-based materials. David Eldridge reports


Decathlon reduced 20 production steps to just one with the Kiprun KipX midsole and outsole. Image: Decathlon


The rise of thermoplastic polyurethane (TPU) in foamed shoe midsoles has opened the door for the footwear industry to become more sustainable. As a thermoplastic, TPU can be mechanically recycled, unlike the thermoset EVA foam that is the main material for sports shoe soles. A manufacturing process that is growing in importance for TPU soles is super critical fluid (SCF) foaming, in which an inert gas such as nitrogen is dispersed in the thermoplastic melt during injection moulding, forming microcellular foam in the mould and creating soles with low density and high performance. This is a physical foaming process, while EVA soles are usually produced with a chemical foaming process. TPU SCF’s backers claim it is a new way


to manufacture shoes, drastically reducing the number of process steps, with all the savings implied by that, in terms of production time, carbon footprint, energy use and waste generation. SCF foaming improves recyclability as midsole and


outsole can be produced as an all-in-one piece, removing the need for glue which contaminates the recycling stream. Sustainability benefits, they say, also extend to eliminating the chemicals used as foaming agents. The Salomon Index.02 women’s running


shoe, featuring a midsole made with BASF’s Elastollan TPU and manufactured with SCF foaming technology, was launched two years ago when its recyclability was novel. Excitement about foamed soles that are recyclable and contain recycled TPU has been generated recently by Ortholite, pioneer of the open-cell polyurethane high-performance insole. Its Cirql subsidiary was set up to focus on insole and midsole end-of-life innovations (ie recyclability and biodegradability) and it has already made important product launches. Cirql rTPU30 midsole foam, made using SCF with 30% post-consumer recycled TPU material, has been followed by Cirql rTPU50 with 50% post-industrial recycled TPU, which the company unveiled this September.


Cirql highlights attractions for shoe


brands: with its 50% recycled content, rTPU50 exceeds 2025 compliance standards, it said. Also, SCF foaming is a chemical-free process. As well as midsoles, Cirql adds the option of a co-moulded TPU outsole which removes the current bonding process. Cirql’s midsole foams can also be conventionally bonded to all outsoles on the market today, it said.


Towards circularity “The big picture goal of Cirql is to help move the footwear industry toward true circularity, by reducing carbon footprints, delivering more sustainable materials solutions, and eliminating the use of hazardous chemicals in the manufacturing process,” said Matt Thwaites, Vice President and General Manager of Cirql, when Cirql rTPU30 was launched. The innovative potential of the SCF


foaming process for TPU soles was outlined in July by speakers at the Polymers in Footwear conference in Portland, Oregon,


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