LEATHER FRIENDS?

A myriad of new materials has flooded onto the market (backed by now diminishing funding) and early data suggests that instead of them disrupting the leather market, they have in fact started impacting the 3.1 trillion square metre textile/synthetic material market – possibly helping the leather industry. These biomaterial manufacturers have also adapted a friendly tone with existing natural product manufacturers.

The materials can be broadly categorised as follows:

• Reconstituted composites
• Grown sheets (or grown and re-composited)

The history of these materials goes back to the middle of the last century where materials were cast, calendared (formed between two rollers), or were coated onto a textile underlay. The use of fibre wastes was also inevitably going to see beneficiation into fabricated sheets that were bonded together to resemble other materials. Coated, laminated (and foils), and heavily finished materials meant manufacturers could move commodity materials into shoes and apparel.

Reconstituted fibres
The techniques used to sidestep a highly patented area shows the ingenuity and creativity that companies are prepared to go. The ability to convince the natural fibres to cooperate is one of the largest challenges. The materials often need coating to resemble genuine leather and mostly require a backing to ensure that they are not rejected at the first hurdle in failing to meet strength requirements.

The companies producing these materials often confuse the consumer as to what genuine recycling should look like. These companies claim they recycle the materials used in their products – they do upcycle natural fibres, but that is where the circular economy ends – after one short-lived cycle these products end up in landfill – friends of the leather industry are currently using leather wastes and are looking to increase their second and third generation circularity.

Woven/Knitted/Composites/Coated Textiles
These materials use one thread/yarn (for knitting) or two or more threads/yarn for woven materials either as the backing materials that they coat, or as the thread or yarn themselves. The material relies on a fibre that is reformed into a thread/yarn for processing from natural, semi-synthetic (highly processed natural materials), or from synthetic (usually petrochemical) sources.

Natural fibres have been competing with leather for thousands of years and both industries respect the definitions that each industry uses and have gone to great lengths not to confuse their consumers. Many natural fibre producers are now talking to the leather industry to co-operate against mistruths about their materials.

The fibres like wool (sheep, cashmere); hair (angora, mohair, camel, horse); and silk (spider, worm, sea). Cotton (non-Organic, Organic, Egyptian) are also heavily used natural fibres as well as hemp, jute, flax, abaca, pina, ramie, sisal, and bagasse. These fibres are then woven (warp and weft, interweave) or knitted (using a knot) to produce a fabric.

The woven or knitted fabric is then rolled into a bolt that can be unrolled for coating, such as taking it through a rollercoater or spray line that applies the pre-base/base coat. After being dried the coating is then top coated. Some coated textiles are applied in one go. Some coated textiles are coated using a foil or laminate that is stuck to the textile using a water- or solvent-based adhesive.

Man-made fibres, often called semi-synthetic, include materials like cellulose regenerated (rayon, Tencel, modal, cellulose acetate/nitrates). When the difficulty to acquire the natural fibres given above started to increase (rising population demand and shrinking farm resources) it was natural to see the innovation of new semi-synthetic fibres. The development of semi-synthetic fibres lagged the synthetic yarn materials (given below). The realisation that a large quantity of cellulose waste (plant and tree waste) could be converted using a chemical process into materials such as rayon, Tencel, or Lyocell. Using different solvents, the chemists solubilise the cellulose materials and then rapidly precipitate them into filaments, which can then be woven or knitted. These materials can be coated or uncoated.

New generation semi-synthetics
New generation semi-synthetics, like Mirum, use no petroleum-based polymers in their compositing process, relying on rubber, latex, or biopolymers to composite the modified or spun fibres into a matrix. These materials are susceptible to very long-term biodegradation (measured in tens if not hundreds of years) because they are based on modified natural materials, but again their breakdown could be influenced by the extent of their treatment or whether they have a non-biodegradable coating. The companies making these materials have been very friendly to the leather industry, choosing to compete in a positive marketing manner.

Compositing
Most new materials fall into the composite category. In effect the materials fall into the same formula – identify an ingredient (or ingredient waste stream) and include it in a polymer composite that holds the token ingredient in place. Too much of the ingredient and the polymer strength fails because its integrity is undermined.

Many materials use polyurethane as a matrix - as it provides strength. Other synthetic and semi-synthetic blends can be used, and many composite makers are including more biopolymeric and semi-synthetic polymers. Many of the materials use a fully synthetic (petroleum based) polymer to glue the material together – hence have attracted the wrath of environmentalists who brand these plastics as greenwashing. Professionals should be cautious of calling these materials as being PU-based, as many of them substituted the PU out for biodegradable, low microplastic shedding polymers. Activated Silk by Evolved by Nature (a company providing this as a leather finish too) started by incorporating PU in their formulation, but has moved into using biopolymers that can replace the PU. Activated Silk makes use of silkworm cocoons that are incorporated structurally and chemically into films and coatings. The use of novel peptides and “grown” proteins make for a very interesting future but the biogenic content of materials from companies making such claims should always be validated.

The long fibre materials like Piñatex, Barktex, and Fique Fabric are the most interesting here as they use calendaring, or layering, to form the sheet, then they intercalate, or felt the fibres, back them with a strong textile, and then coat them with a surface coating like leather would have. Piñatex have made it very clear they are not friends of the leather industry and have even gone so far to say they are the opposite. The material is coated in a fashion like the method outlined above for natural fibres.

Grown sheets (or grown and re-composited)
Microbes like bacteria or fungi can be grown on a nutrient medium to produce a sheet. The sheet can even be grown through a woven textile (or placed filament) to provide internal strength. In other methods the cells are grown and then sheets of material are 3-D printed using these grown cells. Materials include: collagen (BioVera, previously Zoa; and Bionic Fibre); Kombucha (Medusomyces gisevii, Komagataeibacter rhaeticus); or Mycelium (Reishi, Mylo, or Muskin). These companies have worked with the leather industry to make their products and seem to have a helpful alliance against petrochemical plastics.

Using biomimetic ideas, the producers of these materials conceptualise that a sheet of biomaterial that is grown in nature will replace leather – and there is merit to this concept. However, the material companies fail to see the hierarchical nature of leather and pre-suppose that a sheet containing fibrous materials, like mycelium, will be as strong as leather.

The grown materials need to use a range of coating and backing techniques to obtain the minimum strength required for a leather mimic. The materials also fail to impress the brands as the grown substrates lack shape memory, are either spongey, or are flat and empty. Brands report that the tensile and tear strength on these materials is low and need to be compensated for.

The last barrier for these new grown materials is they are grown; hence they are highly variable (density/thickness, structural composition) compared to synthetic or semi-synthetic materials. A material with variable density/composition needs to be pre-treated (like the beamhouse in leather) to ensure the consistency that the manufacturer will need. Herein lies the problem, the producers of these materials have stringent green agendas, which almost prohibits them using the chemistry needed for these grown sheets. To get around this problem, the sheet manufacturers discombobulate the sheets by maceration and then composite the materials using the techniques outlined above.

The companies and their products mentioned above have been included, not as a means of promotion, but as a way of identifying friends of the leather industry. It could be stated that the category of biomaterials could be a useful alliance for the leather and shoe industry that could work together to fight against a formidable plastics industry.