Sustainable Susterra
Eco-efficiency has been proposed as a key tool in promoting a transformation from unsustainable development to one of sustainable development. It is based on the concept of creating more goods and services while using fewer resources and creating less waste and pollution. It can be said to measure the ratio between the added value of what has been produced expressed as GDP and the added environmental impacts of the product or service involved as, for example, carbon dioxide (CO2) emissions from fossil fuels.
We now realise the extent to which CO2 adversely affects the environment in so many ways, not least of which is global warming and all the implications this holds for the world and its people. Unfortunately, CO2 emissions from petrochemicals production continue to grow substantially, despite all the efforts to recycle petrochemical materials. We therefore need to decarbonise the planet, reduce dependence on petroleum and seek renewable, sustainable sources for the materials we use. One way to do this is by using renewably sourced feedstocks that take CO2 out of the air and put it into bio-based materials and recycle those rather than recycle petrochemical based materials.
A report by the Ellen MacArthur Foundation says that besides decoupling virgin feedstock from finite resources, renewably sourced plastics can, under certain conditions, decrease carbon dioxide emissions and potentially act as a carbon sink throughout their life cycle. For plastics sourced directly from captured greenhouse gases, such as methane and carbon dioxide, this link is clear. For bio-based plastics, this happens indirectly as plants capture carbon dioxide from the atmosphere as they grow and this carbon is then harnessed in the polymer.
The message is beginning to strike home and the global demand for eco-efficient materials across industry as a whole is rising and the footwear industry is undergoing a pivotal shift to reduce its environmental footprint. There is an increasing realisation that we need to transform the way in which footwear is made and to increasingly use more sustainable materials from natural resources. Dupont Tate & Lyle Bio Products (DTL) is at the forefront of this movement.
Renewably sourced feedstock
The US Midwest produces enormous quantities of corn (maize) of which 99.7% is industrial field corn used for animal feed and processing into a variety of other products. First generation carbohydrate crops, and field corn in particular, are currently the most resource efficient and renewable feedstock available. Field corn’s high carbohydrate efficiency leads to relatively low land use, it has high performance potential and is scalable using existing technology and infrastructure. In addition, it is capable of usage in a wide variety of applications and yields valuable co-products.
Depending on location, natural fibre crops such as cotton, flax, beech and bamboo can be resource intensive due to high water and/or energy use. Fossil fuels such as crude oil are, while widely used for a myriad of products, non-renewable and have environmental and public health impacts from their extraction and usage. When taken holistically, field corn is positioned very favoroubly (field corn used to make Susterra is covered under the Tate & Lyle sustainable agriculture partnership with Truterra Insights, an interactive on-farm digital platform to help farmers advance their stewardship goals and return-on-investment in real time, acre-by-acre and help food companies measure sustainability progress).
DTL uses corn to produce Susterra, a 1,3-propanediol (Bio-PDO) which can be used as a building-block for bio-based polyurethane chemistry. The variety of corn involved is known as yellow dent and has a high starch content. After harvesting and drying, a wet milling process is used to separate it into its four basic components: starch, germ, fibre and protein. The nutrient rich components are used for animal feed while glucose is derived from the remaining starch content and is the raw material used for making 1,3-propanediol.
The company says that in addition to using a renewably sourced raw material, this sustainable manufacturing process on a ‘cradle-to-gate’ basis, produces 47% less greenhouse gas emissions and consumes 49% less non-renewable energy than equivalent petroleum-based diols. Compared to the typical petrochemical building block found in polyurethane materials in footwear butanediol (BDO), the figures are 48% and 46% respectively on the same ‘cradle-to-gate’ basis. The result is a building block for bio-based polyurethane chemistry that allows any footwear material or component made from petroleum-based polyurethane to be replaced by one that is partially bio-based. Applications for footwear include outsoles, insoles, synthetic upper materials, waterproof breathable membranes and adhesive films.
Going to the limit
A company that has sought to go as far as possible in the use of bio-based materials is UK brand Vivobarefoot. In its Primus Light Bio shoe introduced in March 2019, Susterra based materials feature prominently in the equation. Analysis demonstrates how far the use of plant-based material can extend in an athleisure type shoe. The inclusion of Bloom EVA is also significant as this is derived from toxic algae harvested from lakes and used to produce a sustainable natural additive to EVA (ethylene vinyl acetate) to improve its performance while at the same time helping to offset an environmental problem. Although such an extensive use of bio-based materials is not possible with all types of footwear, it does show the potential as far as unstructured casual and athleisure shoes are concerned.
All about performance
Susterra is very much all about performance and problem solving for different parts of the shoe. Paired with the right chemistry, DTL claims it can perform as well as–or better than–traditional TPUs based on those performance requirements. In laboratory tests, it has been evaluated against traditional polyols as a polyester and polyether thermoplastic polyurethane for outsole applications with a bio-content of up to 75%. Compared to traditional petrochemical based TPU, Susterra-based TPUs can have better low-temperature flexibility and performance, as well as better hydrolysis resistance. They also offer excellent slip resistance and come in a wide range of colours including fully transparent. Susterra-based TPU membranes also offer enhanced stretch and flexibility characteristics while maintaining high performance.
Combined with adipic acid (AA), it exhibited excellent low temperature flexibility when compared to butanediol (BDO) combined with AA. When combined with sebacic acid (Sb), a bio-based alternative to AA, it further improved low temperature flexibility. Dupont says that wear trial participants were impressed by its performance, even in blizzard conditions. In snowy conditions, hydrolysis performance is important as well and the company says that Susterra propanediol can be paired with the right chemistry to again equal or even outperform traditional TPUs.
Hot melt adhesive films
Another area where Susterra is increasingly found is in the production of TPU-based hot melt adhesives and laminating films also known as no-sew films. Depending on the nature of the substrate involved, these can then be used as backers for uppers and vamps or toe and heel reinforcements in the form of toe puffs and counters which cover a huge range of footwear types.
TPU films using Susterra PDO offer the same sort of tensile strength, hydrolysis resistance, bonding strength and easy processability shown in its other applications. Given the performance enhancing nature of TPU hot-melt films, DTL states that compared to traditional petrochemical-based films, those based on Susterra demonstrate higher bonding strengths at lower temperatures, better hydrolysis resistance, easy processing excellent bonding strength as well as a high bio-based content of up to 70%.
Going forward
Influential pressure group EU Bioplastics (EUBP) in a report on the industrial use of agricultural feedstock states the discussion about the use of biomass for industrial purposes is often linked to the question whether the conversion of potential food and feed to materials is ethically justifiable. It also says that this emotional debate lacks empirical research to support these claims with actual facts. As it happens, enough food to feed the world is being produced and, unfortunately, wasted each year.97% of the global agricultural area is used to grow food and feed or used for pasture. The area needed to grow biomass for material use accounts for approximately 2% and, within this share, bio-materials only account for about 0.02%.
Using sugar, starch or oil for bio-based chemicals, plastics or fuel still leaves all the plant-based proteins which are an important feedstock for the food and animal feed industries.
What will happen to footwear manufacture post covid-19 is still a matter of conjecture. Vivobarefoot has shown that there is enormous scope for the use of bio-materials in footwear and, as Susterra amply demonstrates, they are more than capable of matching or exceeding the performance of petro-chemical derived materials. One thing is certain however and that is there will be some changes and, with an ever increasing realisation that many of the world’s natural resources are fast running out, we must turn to those that have a truly sustainable future and this is where bio-materials can play an important part.
CO2 emissions are still growing.
Credit: SHUTTERSTOCK / NYC ROSS
