Many parts of the fashion industry, supported by the vegan lobby, continue to claim that non-leather alternatives are more desirable and sustainable than the real thing. The leather industry’s representative body in the European Union, COTANCE, has at long last decided to put some of the claims to the test with the help of testing and research centre FILK. The results were published earlier this year in a paper called the ‘Comparison of the Technical Performance of Leather and Trendy Alternatives’. Needless to say, the results come as no surprise.

In fact, none of the substitute materials exhibited all of the performance characteristics of leather and some contained chemicals of concern. On announcing the results, COTANCE said, “The fascination with one of the oldest natural materials in the world remains unchanged, and no wonder, because the material is a true all-rounder. It lends a noble touch to fashion, footwear, accessories and furniture, and it brings many functional properties such as durability, tear-resistance and breathability.”

FILK carried out standardised physical and chemical tests on the substitute materials, all of which have come to market in recent years. It concluded that technical progress among the manufacturers of synthetic materials has led to some of these having ‘individual properties that are similar here and there to those of leather. However, it also found that none of the tested substitutes could be called a true alternative to leather.

In the tests, leather proved superior in terms of water vapour absorption and water vapour permeability, with the other materials scoring significantly’lower. It also showed a superior performance in flex and tear-resistance tests, indicating greater durability. Another point that COTANCE made was that, although the producers of synthetic alternatives often present their products as being good alternatives, there is frequently a lack of transparency about exactly what they are made from. The FILK tests have uncovered more detail.

For example, one of the materials tested was Desserto. The tests showed that while the material does contain cactus fibre as claimed, in the sample tested the plant fibres were contained in a polyester carrier fabric that had two layers of polyurethane (PU) on top and was in effect actually 65% PU. The majority is actually fossil fuel-based which in view of all the furore over global warming can hardly be described as being ‘desirable’.

The alternatives

In all, nine alternatives were tested using a shoe upper leather as the control. In recent years, much has been published about these innovative products that, in their manufacturers’ terms, offer vegan, ‘cruelty-free’ and more sustainable alternatives. All can be considered fibrous, but their make-up differs physically and chemically. Some are naturally grown, making use of the dry mycelium of mushroom fruitbodies; others are made as alternatives to fossil-based raw materials, replacing them with more natural ones such a pineapple leaf fibre, the milled cactus leaves mentioned previously or apple pomace (solid residue from apple processing).

All are commercially available and are in use in products ranging from bags and small accessories to shoes and garments. The materials were analysed both physically and chemically. Based on their design, the more innovative materials can be sub-divided into three conceptual categories: animal-free grown materials, multi-layered coated fabrics with plant-based additives and non-woven fabrics with or without a surface finish.

Grown

Muskin – naturally grown single-layer fibre, made from mushroom fruitbodies. Due to the restricted availability of the fungus (Phellinus ellipsoideus), this material is only grown in small size pieces approximately 40cm x 20cm.

Kombucha – a dense and compact material, based on polysaccharides. Made from the fermenting process of the Kombucha beverage, it is a symbiotic community of bacteria and yeast that grows on the top of the fermenting liquid. Subsequently dried out to form a tough sheet.

Coated multi-layer

Desserto – made from nopal cactus (prickly pear). Dried and macerated leaves (pads) are incorporated into the material, which is essentially a coated polyester-based textile with a compact PU layer and partially foamed layer containing the organic content.

Appleskin – a PU-impregnated coated textile carrier with thin compact PU layers plus a foamed one incorporating apple waste. The material is made using a coagulation process and is 50% PU.

Vegea – coated textile combining compact and foamed PU layers on a cellulose-based carrier with organic constituents from grape waste in the foamed layer.

Teak Leaf – coated and laminated leaves from the teak tree with non-woven textile layers laminated onto the back and a topcoat based on polyolefin wax.

PUCF – made by a coagulation process, this is an alternative to leather that has long been used for ladies fashion shoes. It is a coated textile consisting of a thin, compact  PU topcoat on a modified cellulose mid-layer with (in test sample) a polyester textile carrier.

Noani – a composite material of three individual layers. The upper layer consisted of polyester microfibre, the core was leather fibreboard and the bottom layer a coated textile with a compact PU layer and foamed polyvinylchloride layer. Being a structured material, rather than a single material, no further tests were carried out.

Non wovens

Pinatex – made of cellulose-based natural fibres from the waste leaves of the pineapple plant. A felt-like material is made when these fibres are combined with polylactic acid. Final finishes vary but most incorporate resin topcoats.
SnapPap – marketed as vegan paper with a leather look, this cellulose-based material incorporates natural rubber and an acrylate-based polymer.

Testing
Focusing purely on material performance, the materials underwent the same tests, according to the appropriate internationally recognised specifications for shoes, gloves and apparel. They were also evaluated by 10 trained panellists for touch and feel. Additionally, they were also screened for hazardous substances.

Thickness – cross-section under micro­scopic analysis.

Tensile strength – tensile strength, elongation at a specified load and elongation at breaking point. Specification 15N/mm² minimum.

Tear strength – tear strength using trouser tear test.  40N/mm² satisfactory.

Water vapour permeability – ability to allow humidity from the body to be transported out and away from the material. Over 0.8mg/ (cm² x h) needed for comfort.

Water vapour absorption – amount of water vapour absorbed by the material.

Flex resistance – flex resistance of material when folded. Specification 80,000 flexes 

Results

Leather has the highest mechanical stability, representing the highest values for tensile and tear strength. Coated textiles’ tensile strengths ranged from 9 to 20N/mm² with the figures dependent on the properties of the textile carrier; leather’s score was 39.5. For the non wovens, physical properties relied on their fibre-bonding methods which, in the case of SnapPap, were compromised in tear strength by the short fibre length, with little compensation from the polymeric binder used.

In use, materials used for footwear undergo concave and convex flexing. In a flexometer test, once anything more than minor cracks in the topcoat are observed, the test must be stopped. It was noted that only the coated textile and pinatex had a similar performance to leather; all the others were deemed insufficient.

For a pleasant feel, humidity from the body must be transported away from the skin. Water vapour permeability is a measure of the efficiency of a material to allow this to happen. Leather, Muskin and SnapPap exceeded the limit set in the ISO standard for full chrome upper leather (ISO 20942) of sufficiently greater than 0.8mg/(cm² x h) and whilst the PUCF was satisfactory, none of the others were.

In the haptic test, the Muskin was said to feel pleasant while the Desserto, Pinatex and SnapPap felt rough. The materials that had synthetic surfaces, Desserto, Vegea, PU-coated textile and Appleskin, had a soft feel, but the touch felt artificial with a sticky tendency; this was even more apparent with the Kombucha sample.

Hazardous substances

Using thermal desorption analysis, which involves heating a material to release contaminants or chemicals of interest which are then identified and quantified by gas chromatography.

All the samples tested emitted volatile organic compounds. Restricted substances were found in the PUCF and also in the Appleskin, Pinatex, Desserto and Vegea materials, which could all also be classed as coated textiles. Appleskin was found to contain Butanone oxime and traces of dimethylformamide (DMFa) as well. Desserto contained five restricted substances, Butanone oxime, toluene, frees isocyanate, foltpet (an organic pesticide) and traces of the plasticiser Diisobutyl phthalate (DIBP), which was also found in the Pinatex. Toluene was also detected in the sample of Vegea.

Conclusions

Alternatives attempting to replicate the natural composition of leather, namely a graduating density of fibres through the structure increasing from the grain side towards the corium or flesh layer, was, in most instances of the coated textiles, attempted with multi-layered materials. The main strength of the structure was reliant on the composition of the textile backing or carrier. In most cases, water vapour permeability was all but lost, with a corresponding effect on flex resistance.

For the naturally grown materials, the best attributes appeared to be water vapour permeability or absorption, but to the detriment of the tensile or tear strength. For the Muskin sample, while the short fibre length aided the vapour permeability, this had a negative impact on the tensile and tear strengths. Fibre orientation was also a contributing factor and the probable cause of a lower flex resistance. Conversely, the Kombucha was almost impermeable but still highly hygroscopic, most likely due to the high sugar content and its gel-like consistency prior to drying.

In conclusion, hazardous substances aside, the challenge remains for alternatives to leather to replicate the full functionality of the bionic structure of the skin. Additions of biobased content to the already widespread PU coated textiles may give some sustainability credentials to the material, but their addition has yet to show any physical advantage to the product. As can be seen from the results table, leather outperforms all the other materials and while some come close to some of leather’s inherent properties, no single alternative product can match them all.

If, according to COTANCE’s findings  leather is so superior to all these alternatives, why is so much of it simply allowed to go to waste? According to the Leather and Hide Council of America (LHCA), there are around one billion head in the global cattle herd of which about 300 million will go to slaughter each year. From these, 55% of the hides will be turned into leather and 45% will go to waste meaning 135 million hides per year going to landfill. The LHCA assumes these average 25kg, amounting to almost 3.4 million tonnes of waste which, incidentally, could be creating as much as  three million tonnes of CO₂-equivalent emissions per year. If we want to save the planet, we should surely be using more leather, not less?

The prickly pear is used in the production of one alternative to leather. 
Credit: Shutterstock/BCARMONAR 

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