The throwaway culture of fast fashion means much footwear is destined for landfill, as it is difficult to recycle. But end-of-life concerns are not the only sustainability issues facing the sector, as Stuart Nathan reports
It isn’t known how long humans have been wearing shoes, but the earliest known leather pair dates back 5500 years – older than both Stonehenge and the Pyramids. Ever since then, shoe manufacture has been bad for the environment. Until the late 19th century, this was probably more a matter of it being very unpleasant to be around than damaging on a global scale – the substances used to tan leather included human and dog faeces.
Subsequently, noxious chemicals, notably chromium compounds, were developed that produced better leather, but they polluted watercourses and were hazardous to humans. Now that synthetic materials for footwear are readily available, the environmental hazard is different, but much more pervasive.
The polymers used in shoemaking – mainly but not exclusively in the sports and leisure footwear sectors – are normally derived from petrochemicals. Therefore, they share all the environmental drawbacks of every synthetic polymer: links to the extractive and polluting oil industry; high impact on emission of carbon dioxide and other greenhouse gases from the energy needed to make the materials; diffculty of disposal. In the case of polyurethanes, which are used in all parts of shoes but particularly as foam in the soles, another environmental hazard comes in the form of emissions of volatile organic compounds. The footwear and chemical industries have recognised these issues and, spurred on by growing customer concerns, are attempting to reduce the environmental impact of their products. The fact that they can charge a premium for ‘eco-sneakers’ is another bonus.
This is not a minor issue. According to the European Environment Agency’s March 2021 report, Textiles in Europe’s Circular Economy, clothing, footwear and household textiles is the fourth-highest pressure category for use of primary raw materials and water (behind food, housing and transport), the second-highest for land use, and the fifth-highest for greenhouse gas emissions. Moreover, this is a situation where European countries essentially export their environmental damage; 85% of the primary raw materials use, 92% of the water use, 93% of the land use and 76% of the greenhouse gas emissions take place outside Europe, where the manufacture and processing of these products takes place, primarily in Asia. The report argues that a systematic change towards circularity will be necessary to relieve the environmental and climate pressures and impacts from textile production and consumption while maintaining economic and social benefits.
But the barriers to circularity are high. Europeans discard about 11 kg of textiles per person per year, most of which is exported to Eastern Europe, Asia and Africa. Those not exported tend to be incinerated or landfilled. Textile recycling is low. According to the report, every year, each person in the EU consumes an average of 1.3 tonne of primary raw materials in clothing, footwear and household textiles.
Only about 15% of these products are made within the 28 EU nations; the majority of the rest is in Asia. In terms of greenhouse gas emissions in the upstream supply chains to meet this demand, the report estimates that 650kg equivalent of CO2 is emitted every year, with 75% of these emissions outside the EU.
The industry itself contributes to detrimental social impacts, the report adds. The business models of industry players tend to be linear and competitive, and are often categorised by low rates of pay, poor working conditions, and poor environmental conditions inside factories. These issues apply inside as well as outside the EU. Outside the EU, some producers use child labour and safety conditions are often poor, leading to injuries and fatalities.
To encourage circularity, the report recommends regulation and policy options such as taxes, levies and VAT manipulation to favour reduced use of virgin raw materials. Better consumer education through labelling to enable people to make informed choices about sustainability of products. It also recommends systems for reuse and repair of clothing place. Policy instruments to encourage circularity should not only be small scale but also spur systemic change within the whole textile system towards a circular economy. This must be fuelled by sustainable and safe materials and products, it concludes.
Studies focusing on footwear and its impact on the environment show a considerable problem. A lifecycle assessment by Massachusetts Institute of Technology in 2013 concluded that a typical pair of running shoes generates about 15kg of carbon dioxide emissions, equivalent to keeping an incandescent 100W light bulb on for a week. Two-thirds of this carbon footprint comes from manufacturing processes, with a much smaller proportion resulting from extracting raw materials. Part of the problem arises from the origin of a large proportion of the materials: they come from China, where a lot of energy generation depends on coal. The study also found that consolidating parts and reducing production steps is a good way to reduce energy use.
As we have seen, the use of polyurethanes comes with considerable environmental challenges, and the industry is tackling these on an ongoing basis, often motivated by regulation. For example, the US Environmental Protection Agency mandated the elimination of methylene chloride in polyurethane foam manufacture in 2019 (it had been introduced as a blowing agent following the banning of CFCs under the Montreal protocol), leading to the development by Air Products and Chemicals of silicone surfactants that enable of a full range of PU foams using liquid carbon dioxide as a blowing agent. This also requires less energy and lower CO2 emissions than the process it replaced. In addition, PU manufacturers have almost eliminated the use of polybrominated diphenyl ethers, which were used as flame retardants. The main remaining environmental concern issue with polyurethane foam is the use of toluene diisocyanate (TDI), because of concerns over its toxicity.
What consumers want
More specifically in the footwear sector, increasing demands from consumers is also driving change and Eco-sneakers are now a growth sector for many footwear brands. With more than 300m pairs of sports shoes thrown out every year, and each pair taking 30 to 40 years to decompose in landfill, consumers are increasingly choosing to purchase shoes with better environmental credentials.
Major companies, including Adidas and Nike, are producing lines made from recycled plastics, sometimes reclaimed from ocean waste; Reebok produces a shoe with a cotton upper, a sole derived from corn, and insoles based on castor bean oil. Some newer companies are increasingly using vegan and bio-based materials in their products: AllBirds, for example, uses Merino wool and eucalyptus-based fibres in its uppers, and sugarcane-derived foam insoles.
In the chemicals industry, producers are keen to talk about their efforts to reduce the environmental impact of PU. ‘I feel that one of the common challenges we have across all of our industry, not just footwear, is how to handle the sustainability issues, and this is a common challenge for all the materials used that we produce,’ said Govind Gupta, marketing manager for polyurethane at Dow Chemical. The company is embarking on a range of initiatives, starting with ensuring that raw materials are sustainable, and working to reduce water and energy consumption and waste generation in production plants. ‘When it comes to developing new product, our R&D team is working continuously to design a new formulation which is more safe and sustainable for the industry, for the environment and for the consumers,’ he said.
One change that Gupta highlights is the increasing use of polyurethane dispersions use of organic solvents. Driven by regulation, he said, processes that led to the emissions of VOCs are being displaced by those based on solid dispersions in water. ‘Overall, PU is helping to make more sustainable and safe processes for synthetic leather,’ he said. End-of-life treatment for polyurethane products is also on the agenda, although it is not currently focused on footwear, but on foam used in mattresses. ‘In our new mattress recycling programme, we are able to recover the base polyols and the raw material of polyurethane foam from scrap mattresses,’ Gupta said.
‘We can then reuse these polyols to make new foam, completely closing the loop for the mattress. We are working in the same direction to establish a similar kind of circular economy paradigms for footwear as well, but we are in the early stage on this.’
Moreover, he said, the quality of foam made using recycled polyols meets the overall performance target for the application.
At Huntsman, commercial manager for global brands Craig Roberts is also keen to highlight the sustainability potential of PU over natural rubber and EVAs, which, he said, have long cycle times and require much more energy to produce. ‘A lot of the change is being driven by the brands, as they are closest to the consumer,’ he said. ‘Anything that is more sustainable can improve their image.’ On the producer side, however, reducing energy usage, cutting waste and developing more recycled materials are also priorities, he said.
Huntsman is also looking at producing more durable footwear materials. ‘If a pair of running shoes becomes compressed after a certain number of miles, they will be replaced,’ he said.
‘But this is also important for safety boots – if they last longer they’ll be replaced less often. Some brands are already promoting shoe types for repair and second life; PU/TPU is the material of choice for excellent wear resistance.’ Another innovation in Huntsman’s plans is to develop materials that can be used in all parts of shoes, as this will make them much easier to recycle at their end-of-life, Roberts said. A PU upper can be combined directly with the TPU sole without the use of adhesives, while filaments or powders of TPU can be used in 3D printing and could be derived from recyclates.
If mono-material construction is not viable for certain shoe types, the fundamental design of shoes will have to be considered, Roberts believes. ‘We also have to look at aspects such as design for disassembly. How can I make products that are easier to pull apart? Is it possible to put some form of label or tracking device on a shoe that can tell end-of-life processors what’s in it?’
Recycle that shoe
Lubrizol Advanced Materials’ global 3D printing commercial lead David Pascual is also concerned with the recyclability of polyurethane. ‘It is very much linked to the sustainability approach that we see more and more from shoemakers,’ he said. ‘They want to reduce their CO2 emissions and have a cleaner process, with parts that can be recycled and made more environmentally friendly. This could have real impact.’
Generally, some polyurethane raw materials such as diisocyanates are coming under greater regulatory pressure but it is hard to remove them commercially from polyurethane production. MDI is used to make self-skinned foams which are used in specific parts of shoes.
And although MDI is very important in footwear production, attempts to replace TDI are also instructive. For example, a 2018 paper from a team of Portuguese researchers explained that TDI replacement has received less attention than the replacement of polyols, and that most of the studies have focused on non-fossil fuel derived, more sustainable routes to manufacture of TDI than its replacement on toxicity grounds.
The researchers mention synthesis of TDI from amino acids, and the development of non-isocyanate polyurethane (NIPU). These, they say, have increased chemical resistance and thermal stability, along with improved porosity and water absorption. They are produced by reacting a reagent such as ethylene diamine, hexamethylenetetramine or trys (2-amino ethyl) amine with cyclocarbonates in the form of an oligomer or mixture of oligomers with several terminal cyclocarbonate groups.
Waste to shoes
In 2018, researchers from the UK's University of York’s Green Chemistry Centre of Excellence produced renewable self-blowing NIPUs from lysine and sorbitol, while a team from Russia and Iran synthesised non-isocyanate poly(ester amide/urethane) networks derived entirely from vegetable oil without using solvent.' The latter showed good thermal stability, low water absorption and degradation. Moreover, these materials are biodegradable. Whether they are cost effective and have sufficient performance to be used in footwear is yet to be shown commercially. The Portuguese group warned in 2018 that the PU industry must prepare for the possible banning of TDI, which could be as disruptive as the Montreal protocol or the banning of phthalate plasticisers was to the PVC industry.
The EU-funded project BIONIPU aims to develop bio-based and non-toxic polyurethanes for end products in the textile and elastomer industries, using residues from sugar refineries and the processing of biofuels, natural oils and fats. Among its industrial partners are coatings specialist Stahl and polyurethane producer and processor LUC Group. Currently, the project is focused mostly on producing PU textile coatings, although sectors including footwear are targets for future research. In its October 2021 newsletter, BIONIPU reports that it has produced the first water- based dispersions of NIPU, designed to be applied as a various coating layers. Another growing trend is the use of carbon dioxide to make polyols, pioneered in products such as Covestro’s Cardyon, which is made with up to 20% CO2 using a catalyst developed as part of a collaboration with Aachen University. Econic, a company spun out of Imperial College London, is also investigating this area using catalysts designed to react CO2 with vegetable oil-based epoxides. Currently, the main target market for CO2-derived polyols is furniture but, considering the size and growth potential of the footwear market, it is more than likely to be at least considered for applications here as well.
By contrast, footwear is the initial target market for another start-up, Novoloop, founded in California earlier this year by a biochemist and molecular biologist. Based on a process that converts waste polyethylene into diacids that can themselves be used as raw materials for polyols, which the company claims can be used to make TPU of equivalent quality to that made from virgin raw materials.
Targeting the production of one tonne batches by 2022, Novoloop is currently using conventional isocyanates, but is investigating renewable and sustainable alternatives.