Updated: Nov 22, 2021
The rise of plant-based leather alternatives seems unstoppable. There is a growing interest in cruelty-free, plant-based and low-carbon alternatives to traditional leather, especially as veganism and climate-conscious purchasing become top priorities.
The established media follow suit. Recently, The New York Times hosted an interactive article titled "Fabrics From Your Refrigerator". In it, he praised the properties of different materials, such as mushroom-based alternative leathers like Mylo and Mycoworks, "apple-based" leathers like Frumat, and "grape-based" leathers like Vegea.
In particular, the partnership between Stella McCartney and Bolt Threads with the Mylo leather alternative (itself a licensed technology US startup from Ecovative Design) has received a large amount of international media coverage. Despite the impression of the media campaign, you should read my other post to find out that this new material is neither fully bio-based, nor biodegradable nor plastic-free.
Mylo itself tells us it was mixed with a bio-based polymer, most likely a partially bio-based polyurethane. Confirmed that Mylo is 50-85% bio-based; this would fit the hypothesis of a natural material (mycelium) partially complemented by a bio-based polyurethane (i.e. a plastic composed of both petroleum and bio-based raw materials) but with properties similar to conventional petroleum-based polyurethanes.
I previously wrote an article called "Marketing Scam: Plant and Plastic Blends" in Textile Journey. Some brands and consumers are entirely concerned with biological processes. The idea of a material that can seamlessly decompose into nature and return its carbon to the natural carbon cycle. Sounds great.
On the other hand, while it is possible to have a recyclable material as well as Biodegradable and even compostable (degradable under realistic composting conditions), most “plant-based” leathers are neither natural nor fully plastic.
Products like this are far from the only material type and are applicable to various 'fruit' skins and mycelium-based leathers, which are often mixed with non-degradable binders and coatings to somehow approximate the performance of the leather. The main advantage of following this approach is the potential for a slightly lower carbon footprint due to the displacement of some of the weight of plastic found in traditional PU-based 'vegan' leathers.
In general, people don't like to be misled, and most are shocked to learn that plant-based leather isn't actually entirely plant-based.
A peer-reviewed scientific article has been published in the journal Coatings comparing various leather alternatives to natural leather. The article was written by the FILK Freiberg Institute, an independent institute specializing in testing leather and polymer composite materials.
They confirmed my previous claim that a large class of plant-based leathers are mostly attached to a polyurethane fabric, typically a polyester or cotton fabric. They also compared completely bio-based alternatives such as MuSkin and Kombucha, but the main finding was that their properties were far removed from real leather.
How did they manage to verify this?
In chemistry, an infrared spectrum is a type of "molecular fingerprint" of a chemical, polymer, or material that can show us the types of molecular bonds and thus the chemical structures that can be found in a material. It is not quantitative, but is used to give a rough qualitative determination of components or a mixture, especially when compared to reference spectra of other materials.
The only example they show is the material we discussed earlier:
Sweet Cactus Skin;
The infrared spectrum below confirms that the material is mostly polyurethane and not (mostly) a plant-based natural material biopolymer such as cellulose or other polysaccharides (shown below for comparison). However, specific proportions of the material are difficult to measure from infrared analysis alone, and this is the information the authors emphasize.
Figure 1 – IR spectrum of cactus skin (PU hybrid material).
Figure 2 – IR spectra of cellulose, the most abundant component of natural materials (for comparison).
In the article, we also see enlarged cross-sections of the studied materials. Here we can see the polymeric coating (a) and Sweet leather (c) textile backing and the layer of foamed polymer and natural material (b):
Figure 3 – A microscope view of Tatlı showing (a) the top polymeric coating, (b) the foamed polymer and natural material layer, and (c) the synthetic textile lining.
Even more surprising, the research found that some of the leather alternatives contain traces of explicitly prohibited chemicals, notably Desserto, Appleskin, Vegea, and Piñatex, a group of plastic-coated textile leathers. One of the weakest scoring materials was Desserto cactus skin, which actually contained five restricted substances including butanone oxime, toluene, free isocyanate, an organic pesticide called folpet, and traces of a phthalate plasticizer. Toluene and plasticizers and solvent DMF were detected in some of the other alternative leather samples. However, the article does not clarify the absolute levels at which these contaminants were detected.
The team also tested the physical properties of the materials and compared them to natural leather. Not surprisingly, none of the materials showed the properties of leather. Especially Muskin and Kombucha, which are uncoated or hybrid materials, fell short in terms of strength. Of course, depending on the application, some brands may tolerate different physical properties as they may want to incorporate the unique properties of certain materials into their product designs. In essence, it is quite difficult to replicate the natural fibrous collagen network found in animal skins, so it is quite obvious that the materials are substitutes for real leather.
There is a clear need for continuous innovation and improvement in this area. Conventional leathers are currently available chromium-free and can be tanned using plant-based processes, reducing impact in some key areas. And there are other innovators working on this, trying to create completely plant-based and plastic-free alternatives to animal skin.
What we need to be clearer about is the claims made about the materials and how these stories are told to the consumer.
If any new hybrid of plastics and natural materials is hailed as "breakthrough sustainable innovation," it will undermine consumer confidence in the long run. Innovators, manufacturers, and brands must honestly portray the limitations and challenges of certain approaches, without overlooking details or obscuring key pieces of information that will enable consumers to make informed decisions.