By: Annjoest & Andrés Castro , collaborative work as part of the project more sustainable chemicals for the leather industry.
While leather traditionally has been perceived as a reliable and long-lasting material, it is increasingly associated with negative impacts on animal welfare, the environment, and the well-being of tannery workers and consumers. Because of that, there is a growing interest in alternative materials with a similar look or touch like leather, often advertised as “bio-based” leather alternatives and “purely synthetic” leathers. Bio-based materials can relate to materials that have a renewable resource base like pineapple, and synthetic materials can relate to materials made of fossil fuels mainly. Following, alternatives may be used for applications in which they can be less suitable, because the mechanical features differ, or because they are linked to better-perceived production processes.
Therefore, this blog highlights some of the dimensions of sustainable production processes and the importance of mechanical properties when it comes to material choices. Its aim is to shed more critical light on the perception of materials’ impacts and to better enable consumers and producers to make more conscious material choices.
1. Shortcomings and opportunities of current sustainability assessments for leather and alternatives
Due to leather’s linkage to animal farming, it is often associated as a material that contributes to large greenhouse gas emissions (GHGs). In contrast, consumers and producers may perceive that synthetic and “bio-based” alternatives, which are detached from these GHG emissions, have fewer emissions in their total production. However, to really understand product-related emissions, it is necessary to look into the entire product life-cycle, starting from the raw materials used, the production methods, and their end-of-life paths. To best assess how leather and alternatives compare, a holistic assessment should take into account all impacts of production phases from upstream to downstream activities. In this respect, a Life Cycle Assessment (LCA) is commonly used1.
Although LCAs can provide an overall impression on the impacts of materials and production processes, their results need to be interpreted cautiously. Currently, most LCAs consider different life-cycle phases, which means that the impact of products is analyzed in production boundaries. This can give a specific impression of a products’ impact, but it does not fairly compare materials for which production standards and assessment systems vary. To do so means that standardized system boundaries for product groups should exist. For instance, in the textile industry, the Higg Index is commonly used as a benchmark for textile materials. It measures the impact of production such as on climate change, the environment, the impact of chemicals used, and the extent to which resources for production are depleted2.
To compare the impact of different production processes, it sets similar production boundaries. It currently scores leather high (i.e., rather negative) on its environmental footprint (159 compared, e.g., with 44 for polyester and 98 for cotton)3. However, such a score can give a narrowed impression, because the Index allocates the total emissions from animal farming to leather, which does not happen with other products or their materials. To better compare materials and products means that an equal assessment for production processes and their boundaries has to take place. In analogy to cotton and polyester, it could be argued that the assessment boundary for leather should begin at a slaughterhouse as opposed to the cattle farm.
Assessment criteria also need to consider the total resources used in the production and the processing of materials. Synthetic materials are mainly based on fossil fuels with a greater linkage to carbon emissions, while “bio-based” alternatives have a renewable resource base that by nature stores carbon. However, these bio-based alternatives may use fossil fuel-based additives to achieve specific material features and therefore, increase their carbon emissions. This can also apply to leather if chemicals are used for processing and finishing (surfaces). It follows that assessments better have to consider all the materials in the processing used. Only then, the impact of production can be correctly measured and communicated.
Whether a product can be claimed as more sustainable or with certain advantages in environmental performance, should also depend on its materials’ toxicological impact. For leather, around 85% of hides are tanned using chrome III which can oxidize to chrome VI and be problematic for human health in particular4. If the leather has been processed and the final product handled properly, chrome VI may not occur. To avoid that tanning alternatives such as vegetable tannin can be used but if used in excess and not safely handled can promote other environmental impacts such as eutrophication and resource depletion. That is if the demand for vegetable tannins is higher than their “regrowth time” and or when unsafe handling of vegetable tannins results in eco-toxicological impacts of production.
When it comes to alternatives of leather, fossil fuel-based materials, and bio-based materials with fossil fuel additives, there is little knowledge about their toxicological impact. If we assume that these materials need additional substances (i.e PVC or TPU) to obtain a certain look or properties they may also have such an impact. In a certain regard, they can be linked to problematic additives that have been spotted to be regulated in chemical legislation. For example, phthalates are used as softeners in PVC production, or flame retardants as TBBPA (tetrabromobisphenol A) used in certain thermoplastics. Those intentionally added additives are often used in plastic production and they certainly represent a concern for human health and environment safeness.
Although this can also apply to leather when mixed with additives, high-quality leather have better natural occurring features and often does not need extensive use of additives that could be hazardous. This can also happen in certain bio-based materials alternatives. Therefore, it is important to get to know the natural properties of each material and avoid mixing them with other materials where possible.
Besides the already mentioned impacts, there are also concerns about the sourcing of materials. Leather often raises concern in relation to animal welfare and deforestation processes that might be linked to livestock. Thereby the perception exists that livestock is raised for the “skin” only. Although there is an indirect impact on how animals are raised in relation to skin quality, cattle are not raised for the hides only. Hides are considered as by-products, which represent around 8%-12% of the value of fed cattle (Gary & Swander, 2020), whereas around 3.5 % of byproducts are allocated to hides5.
As dairy and meat consumption is expected to rise, increasing availability of hides as by-products can even be expected. Although there are regions, where hide waste products are strictly regulated such as by having to enter other production and consumption systems (i.e. animal feed, fertilizer, and bulk pet food), other regions may lack such regulations. The latter might be one indication that industrial waste products such as hides more likely end up in landfills or are being incinerated. It could be assumed for as long as dairy consumption exists, the use of the hide is beneficial. This can also apply to other waste products, whereas their production and use impacts depend on how they are being processed to fulfill what type of product needs.
2. What role do mechanical properties play in material choice?
What type of materials are used and processed plays an important role in the longevity of a product and therefore its “sustainability”. That is how long different materials can be used to fulfill specific demands on a product (e.g. aesthetics, safety, water protection). The less likely the range of product demands can be met over its lifespan, the shorter the lifespan of a product will be. A consequence is that the same product is likely purchased more frequently and because of that the frequency of new production increases (i.e. fast fashion). Such a system pressures the environment because more materials and the energy to process them are needed. It also means that more money is spent on the buying of new products over a person’s life.
To avoid that products wear off more easily and that consumers repurchase them more often, the materials of which they are made are important. For products to last longer, they should be made with durable materials. That is where certain types of leather [e.g. genuine leather] are suitable. Because the leather has unique mechanical properties that allow it to age-long, other materials that are advertised as “vegan or synthetic alternatives” likely do not yet. Because these materials are different, they may not meet up to the specific features of leather over time. For instance, a couch made from a durable type of leather is less likely to wear off as opposed to a coach made from polyester.
Such a difference can be less important for products that are kept shortly or are used less intensely, but gain importance if products are intended to be kept for multiple years and where product longevity or other unique features play an important role, e.g., besides furniture, also a wallet, jacket, shoes.
“There is a balancing act between the properties you actually want from your material. And in many, many cases, different materials will have different physical performances of leather, particularly where there are a lot of stressing and bending examples in footwear.“Chemical Supplier
In the case of leather, it is made of a unique composition that is difficult to replace. It is made from hides, whereas hides consist largely of collagen – a structure-forming protein. During the processing of hides, leather tanning proteins (fibrils and fibers) are intertwined and because of that give the leather its strength and structure. The upper hide layers have very thin and tight collagen fibers and because of that show higher mechanical stability (tensile and tear strength). As a result of leather’s origin in the collagen network, its tensile strength is noticeably higher compared to other materials (Meyer, Dietrich, Schulz & Mondschein, 2021).
Although tensile strength is an important material criterion, not all products benefit from tensile strength. Therefore, bio-based alternatives can be suitable for products with unique demands. (see Figure 1: Comparison of physical properties of leather and alternatives). On the other hand, leather has a range of mechanical features such as elasticity, water vapor permeability, abrasion resistance, and durability. Because many alternatives only have a small fraction of these features, materials may be enhanced synthetically. A result is that these features may not last, which can decrease the value of a product over time. This can also be seen in lower-quality leather (i.e. split leather) which is often enhanced using synthetics. Therefore, the right choice of material is important to make it last and to use it in accordance with specific product needs.
3. How can the most suitable material for consumption and production be chosen?
In the light of sustainable development, the material choice depends on its use case and the system surrounding production and consumption processes. Regardless of the material used, production processes and the materials should be safe for humans and the environment and it should utilize as little as possible of total resources for production where possible. Holistic assessments such as LCAs should give a differentiated view on the impact of different materials.
From a mechanical point of view, different materials have different properties which makes them more or less beneficial for different use cases. This means the right material can depend on how long a product should be used and what expectations exist on it. Instead of competing for being “the best” material, materials might actually add value to each other in a product for which each material is most suitable; a bio-based shoelace that wears off quickly with a leather shoe topping to last.
1Kurian Joseph, N. Nithya,Material flows in the life cycle of leather,Journal of Cleaner Production,Volume 17, Issue 7,2009,Pages 676-682,ISSN 0959-6526,https://doi.org/10.1016/j.jclepro.2008.11.018
2Mertens, J. (2020, 31. July). Higgs Materials Sustainability Index (MSI) Methodology. Sustainable Apparel Coalition. https://howtohigg.org/wp-content/uploads/2020/07/Higg-MSI-Methodology-July-31-2020.pdf
3Davila, G. (2020, 2. November). SAC Responds to Leather Industry Concerns Over Higg MSI. Sustainable Apparel Coalition. https://apparelcoalition.org/sac-response-to-leather-industry-concerns/
4Hedberg, Y. S. (2020, 15. July). Chromium and leather: a review on the chemistry of relevance for allergic contact dermatitis to chromium – Journal of Leather Science and Engineering. SpringerOpen. https://JLSE.SpringerOpen.com/articles/10.1186/s42825-020-00027-y
5De Rosa-Giglio, Fontanella, Gonzalez-Quijano, Ioannidis, Nucci, Brugnoli. (2018). Product Environmental footprint Category Rules- Leather. On behalf of the Leather Pilot Technical Secretariat. Retrieved from: https://ec.europa.eu/environment/eussd/smgp/pdf/PEFCR_leather.pdf
CO (2021). Retrieved from: Leather Production Sustainability | Eco-Friendly Leather Alternatives (commonobjective.co)
Gary, W. and Swanser, K. (2020). Quantifying the relationship between U.S. Cattle hide prices/value and U.S. Cattle Production. PhD Research Report. Leather and Hide Council of America Response to: Cross-Price Elasticity of Demand RFP
Meyer, M., Dietrich, S., Schulz, H., & Mondschein, A. (2021). Comparison of the technical performance of leather, artificial leather, and trendy alternatives. Coatings, 11(2), 226.
Suski, P., Speck, M., & Liedtke, C. (2021). Promoting sustainable consumption with LCA–A social practice based perspective. Journal of Cleaner Production, 283, 125234.
United Nations Industrial Development Organization. UNIDO (2012). Brugnoli, F. Life Cycle Assessment, Carbon Footprint in Leather Processing. Retrieved from: https://leatherpanel.org/sites/default/files/publications-attachments/lca_carbonfootprint_lpm2012.pdf