Common performance challenges for leather alternatives
Matching up to leather.
Regardless of the material composition and process used for leather alternatives (plastic, plant, animal, microorganism, or other), meeting the look, feel, drape, performance and manufacturability of leather is not trivial to achieve.
In terms of performance and durability, leather is an amazing material (!).
Leather is tough and enduring due to the combination of complex fiber structure and tanning chemistry.
The reason so many ‘leather alternatives’ fail to meet the performance of leather is largely due to lack of fibrous STRUCTURE.
Leather is made up of intertwined collagen fibrils and fiber bundles in a multilayer structure. This structure has a density gradient that increases from the corium to the grain layer. During tanning this structure is cross-linked retaining the integrity of the structure while enabling elasticity so that the fibers may flex and recover.
Leather alternatives employ various methods to deliver similar properties. Materials that most closely mimic the fiber structure, along with cross-linking chemistry, are likely to be best performing across all factors: look, feel, touch, drape, elasticity, recoverability, durability and manufacturability.
PVC
Polyvinyl Chloride, often generically shortened to ‘pleather’ (plastic + leather) or ‘vinyl’.
Usually built in reverse; a plastic surface coating is laid down onto a textured release paper, followed by a foam layer and then a synthetic textile layer.
PVC materials have high durability, low breathability, do not biodegrade and are furthest from looking and feeling like natural leather. Cheap, lightweight and can have a strange smell.
Produces toxic dioxins which are harmful to humans and the environment. Multiple countries are enacting PVC bans.
Desserto
Marketed as ‘Vegan cactus leather’.
Same basic process as for other synthetic leathers. Reverse lay up; a PU based topcoat is laid down onto a textured release paper, followed by a foam layer consisting of powdered nopal cactus leaves in a polyurethane suspension then a final polyester textile layer.
Contentious for the lack of transparency on composition. Many brands bought into the idea this was ‘PVC-free’, not realizing it is mostly (up to 65%) made of polyurethane (PU) and has been found to contain several restricted substances.
Mycelium
Various methods exist for the production of mycelium materials using filamentous fungi.
Shown here is aerial mycelium grown statically on a solid ag waste substrate. The mycelium hyphae are more open the closer to the air interface and more compacted nearer to the substrate.
Typically the hyphae grow perpendicular to the surface resulting in poor mechanical stability. Using directed growth is one method to counteract this, a process used by MycoWorks.
Once growth is complete the mycelium is sliced from the substrate before heat-treatment, cross-linking chemistries, pressing and finishing. The exact process varies according to each company.
Textile scaffolds may be integrated within or added as a backing material. PU may also be present in high percentage.
NB: This image is from an academic paper not commercial production (there are no publicly available SEMs of mycelium leather materials at this time).
Bacterial Cellulose
Bacterial cellulose self-assembles into dense sheets at the air interface of static liquid culture.
Pure nano and microfibrils of cellulose are arranged in a 3D network as a gelatinous membrane. The fibers are naturally organized as stratified layers.
This membrane is harvested and undergoes washing or other treatment to prevent further growth. The material is hydrophilic (water-loving) and retains moisture. Depending on any chemistries applied, it may be more supple or drier in feel and still susceptible to humidity.
It is a strong, nano crystalline structure. While other fibers may be inserted the dominant characteristics lie with the nano structure itself. A final finish layer may be applied for durability, look and feel. The exact process varies according to each company.
Textile backers may be applied.
Clarino (scale unknown)
Clarino, a microfiber material made by Kuraray, intentionally mimics the structure of leather. Available as a top-grain leather mimic as well as a suede-like version.
Clarino is based on a non-woven fabric and uses the ‘Islands in the Sea’ process.
Multi-component fibers of nylon and polypropylene are intertwined in three dimensions and surrounded by a sponge-like water-based polyurethane binder. The fabric is microscopically perforated to give it breathability.
It is lighter than leather and breathable, with good durability, stretch and recovery. It is 100% petrochemical in origin.
Leather
Natural animal leather is the hide or skin of an animal tanned to preserve it and imbue specific characteristics around durability, look and feel.
The hide is a protein material composed of triple-helical collagen fibrils aligned into fiber bundles which interweave into a cohesive 3D matrix.
Leather has a gradient density whereby the thickest, loosest fibers are found in the corium layer, with the densest found in the grain layer and the two linked at the grain corium junction.
Various tanning chemistries can be used from chrome based through to veg tan. Many finishing chemistries will use PU in the top coat. Tanning and finishing inhibit biodegradation.
Leather is an intensely fibrous material. Its structure and chemistry forms a strong, durable, breathable, material with unique properties that are difficult to mimic.
Synthetic leather manufacturing process
I. Pouring of first, thin, polymer* layer
I. Pouring of first, thin, polymer* layer
FLIPPED
Leather alternatives employ various methods to deliver similar properties. The most basic is a coated textile such as vinyl or PVC roller-coated onto a synthetic non-woven textile. Frequently a textile backing material is used to meet tensile and tear strength which is glued to a fibrous, foam or gel-like mid filler layer that contributes substance and hand feel, while a finishing layer or top coat provides appearance (often with an embossed grain) and durability.
Durability and biodegradability are often at odds
With new material development, especially in regard to leather alternatives, innovators often receive mixed feedback from brands regarding end of life. Some brands are looking for a 100% biocontent, monolithic material that can either be composted or recycled at end of use. Others are looking for a material that prioritizes durability and performance so that the product has an extended lifespan.
With leather, one function of tanning is to prevent the hide from decomposing. Cross-linking chemistries suppress the breakdown of fibers. So the process that preserves leather also inhibits degradability.
The oldest example of leather footwear is 5,500 years old*.
To require a material to be both ultimately durable and biodegradable in most cases is oppositional.
Just like nature’s materials (or leather), biomaterials want to decompose. If they are comprised of nature's building blocks they will be susceptible to mold, water absorption, changes in humidity, brittleness, fading, and so on. To preserve biomaterials innovators may turn to chemistry or some variation on traditional tanning to prevent decomposition and to impart extra durability.
Much of the performance specifications established for leather are met using specific surface chemistries, most of which contain polyurethane (PU). These finishing chemistries are supplied by a few leading chemical suppliers including Stahl*, TFl* and Lanxess*. Product developers turn to these same chemical companies whether the material is animal, vegetable, mycelium or microbially derived. Even if the material is 100% biocontent, the addition of a PU finish will mean the 100% claim is no longer valid. This however is also true of leather - if it has a PU finish, it too cannot be classed as 100% biobased - a double-standard?
Defining and working on material performance
The simple truth is these new material technologies are not the same as leather biologically, chemically or structurally. They are unlikely to match all the properties of leather because they are made of different building blocks (cellulose, chitin, proteins etc) arranged in different structures. Instead they offer something new.
However we have yet to fully understand the properties, beneficial or challenging, of these new biomaterials. Importantly, for such a young field, we have yet to establish standards and certifications relevant to these specific biological compositions and technologies.
With leather as the target, any replacement is a compromise in some dimension. So in a world seeking to replace leather with a biobased alternative, how to think about requirements?
Standardized tests formulated specifically for leather or PU materials (plastic) are hard to meet with biobased materials. To get closer, innovators look to the same solutions as some leathers or synthetics employ, for example, using PU in the top coat and a textile backer.
Bearing in mind innovators with radically different compositions and structures are being asked to meet standards not developed with those materials in mind, how can brands meet innovators where they are today?
Guidance for brands encountering new biomaterials
Understand the material in front of you for what it is, not what you want it to be…
Design and product development teams are set up for success if new materials are approached with an open mind - these creative teams can contribute to determining the best use of each new discovery by asking questions anew:
• What is this material?
• What is its composition?
• What is its structure?
• How does it look and feel?
• How does it drape or move?
• How does it sew?
• How does it recover?
• How does it wear?
• What does it lend itself to?
Once you understand what it does well, what you like, and what you don’t, you can start to match it to application. If you are looking for a leather replacement, what type of leather are you wanting to compare to?
• Light, supple, lambskin
• Sturdy, dense, veg tan
• Slouchy, full, pebble grain
• Soft, aniline, calf nappa
• Water and scratch-resistant, textured, epsom
• Strong, granular, goat
• Firm, glossy, box calf
And so many more…one material cannot hope to replace all of the above. Each biomaterial has its own unique attributes and each will be more suited to certain kinds of product. Design and construction need to account for the properties each material exhibits. Some might make a better shoe than a bag, or small leather goods rather than a jacket, or a car seat rather than a sofa etc.
The Structure of Ultrasuede®. 2023. TORAY
SEM images of the side view of polyurethane artificial leather. 2020. Journal of Engineered Fibers and Fabrics. DOI: 10.1177/1558925020916458
Cryo-SEM microscopy showing an overview and detailed morphology of two mycelium layers 2018. Scientific Reports. DOI: 10.1038/s41598-018-23171-2
Patents Assigned to Mycoworks, Inc. 2023. Justia Patents
Comparison of the Technical Performance of Leather, Artificial Leather, and Trendy Alternatives. 2021. DOI: 10.3390/coatings11020226
Dioxins and their effects on human health. 2016. World Health Organisation.
Global ban on PFOA enters into force for most countries. 2020. Chemical Watch
Clarino™ Microfiber - How it's made. 2022. Kuraray Co., Ltd.
World's Oldest Leather Shoe Found Stunningly Preserved: Areni-1 shoe. 2010. National Geographic
Stahl - Chemical Supplier
TFL Chemicals
Lanxess - Chemical Supplier
Sources
