Coloring is a matter of chemical transformation and its use in the leather sector is no exception to the rule.
The process is highly resource-intensive, particularly in terms of water and energy, and it also draws criticism for the chemical agents involved which, without adequate treatment, can be released into the environment.
As for textiles, the leather industry is mobilizing to propose greener alternatives for leather that can be deployed throughout the production chain.
The different steps and possibilities of coloring
Giving leather its final color is possible at various stages of the transformation process. In order to understand the various coloring options and their resulting characteristics, here is a short recap of the 4 different stages of leather transformation.
- The first step, river work, includes cleaning the skins. This step prepares leather for tanning.
- Then comes the tanning which, thanks to the action of tannins, prevents the skin from putrefying. Tannins are a substance that can be of vegetable, mineral, synthetic or combined origin. At this stage, the skin does not yet have any specific behavior or surface appearance.
Specificities appear during the curing stage. This step involves many operations including retanning, nourishing and dyeing, with the leather immersed in a dye bath for dyeing in bulk. When the finished leather is dyed solely by this process, without additional surface repigmentation, it is called aniline leather (or dipped in the case of full-grain lambskin).
- Lastly, the finishing stage includes all the operations that give the leather its protective properties (against stains, rubbing, water or light), as well as aesthetic characteristics concerning relief, shine, color, or handle. This includes a whole range of finishing techniques that give the leather its ultimate surface appearance. It is at this stage that the hide can be colored or recolored on the surface to correct or intensify the color or to give it chromatic fantasy such as a patina, or dual tones. Leathers with additional surface coloring are called pigmented leathers.
Specificities of the two most common coloring techniques
- Dipped leathers or anilines: the color of the leather is only obtained by immersion in a dye bath, the color is worked in transparency and will acquire a patina over time. The skin’s surface and natural irregularities remain visible.
- Pigmented leathers: an opaque layer of pigment is applied to the surface of the hide, giving it a covered, regular, homogenized appearance. The pigmented finish allows for clear colors and optical white that are not altered by the leather’s original color.
Water and energy consumption as well as the chemical products used in these various stages have pushed companies to research more virtuous developments.
Today, new eco-responsible solutions are emerging, making it possible to obtain clear and intense colors while reducing environmental impact. These solutions derive from the diversification of tanning methods, notably metal-free tanning such as vegetable tanning, synthetic or chrome-free tanning.
While chrome tanning remains the most widely used method accounting for 80 to 85% of the world’s leather production its formulation loaded with heavy metals can impact the environment.
Vegetable tanning, a natural and traditional tanning method using plants with tanning properties, is now enjoying renewed interest. This ancestral method of transforming raw hide into finished leather allows for the development of a wide range of totally sustainable products. Beyond its eco-responsible aspect, the physical and aesthetic characteristics of vegetable tanning give it real singularity. Firm and sensual to the touch, it develops a patina over time and preserves the unique character of each skin.
Basanes – Original Color
Basane leathers are undyed, vegetable-tanned leathers. Without any finishing, they develop a patina and are natural and authentic. Originally pinkish-beige, basanes “tan” in natural light over time until they become honey-
toned. That photosensitivity gives them, and their natural tanned aspect, a noble appeal.
Diversification of tannins
- New basanes–Natural colorations and circular tannins
Today, new vegetable extracts with tanning properties are appearing, opening up fresh possibilities in terms of behavior and coloring. Their coloring virtues allow tanners allow to combine the retanning and coloring stages: hides are retanned and colored in the same bath thanks to pigments naturally present in those tannins.
These solutions save on the dyeing and pigmenting stages, thus reducing water and energy consumption, as well as the impact of chemicals on the environment, since pigmented finishes are sometimes composed of solvents and volatile organic compounds.
These tannins also expand the basanes’ range of nude tints and allow better control in obtaining the desired final color. Each of these tannins brings its own characteristics in terms of tint: thus, mimosa will render pinkish tints, acacia cream tones, and quebracho reddish nuances.
In addition to the use of plants’ natural coloring properties, circularity is now at the heart of the process, with “circular tannins” derived from agri-food waste. Recently, initiatives promoting industrial symbiosis have begun to develop. Establishing collaborative networks between industries, encouraging cooperation between the agri-food and the textile or leather industry now allows for promising innovation, especially regarding the use of waste that was previously unused. These materials form part of the circular economy, in order to optimize all resources used and thus reduce ecological footprints.
This approach can be used with tea, henna, and coffee grounds, as well as tannins from purified grape marc and from the wine-making process for soft wine-colored hues. This approach valorizes local resources rich in polyphenols and offers an alternative to conventional dyes.
Bright new colors in vegetable tanning
If vegetable tanned leather’s color response made it more difficult to obtain vivid colors, innovations in the field now surpass that issue and open the field to wider color applications. Better dye ab-
sorption is now possible.
Circular tannins are also used here, such as rhubarb root or olive leaves from olive oil crops. Once those plants have been transformed into tanning agents, the result is a solution free of metals and chemical tanning agents. This new process significantly improves leather’s response to color compared to conventional vegetable tanning, and allows for intense colors through immersion in a vat of dye. For tanners exploring these new processes, finishes are also becoming low-impact thanks to water-based solvent-free primers or vegetable waxes that provide softness and suppleness. In addition to their coloring properties, these new renewable resources give leathers a true identity, subtle scents and unmatched tactile properties.
Metal-free & Chrome-free Tanning
Metal-free tanning is done without chrome or other metals, and is based on synthetic tanning agents developed from organic compounds (polymer, polyphenol, modified glutaraldehyde, triazine). This tanning process has no official formulation; each tanner develops its own recipes, sometimes patented and with environmental standards. There is increased interest in this type of tanning due to its metal-free transformation process.
This method of tanning results in leathers known as WET WHITE, for their characteristic pure, intense, bright white hue when they come out of the drum, which makes a perfectly adapted base for intense colors or bright pastels.
The name WET WHITE is opposed to WET BLUE
leathers resulting from chrome tanning and requiring re-pigmentation to obtain optical whites.
On the other hand, new generations of tanning without chromium, heavy metals, bisphenol-A or glutaraldehyde are currently being developed. Remember that glutaraldehyde, as well as bisphenol-A, both frequently used in synthetic tanning, have been registered on the SVHC (substances of very high concern) list.
These new solutions give skins properties resembling those of chrome tanning and also make it possible to achieve light or very intense, light-resistant colors.
A recent technology formulated with zeolite, a mineral that binds to the skin’s collagen, gives leathers properties almost similar to those of chrome tanning.
Other technologies that also aim to obtain a better method for dyeing and broad chromatic diversity are being developed in vegetable tanning, with tanning solutions mixing vegetable tannins and biopolymers. These are being developed in collaboration with other actors along the entire value chain: manufacturers of finishing solutions and fertilizers. Finished leathers using these new organic solutions considerably improve their potential recyclability. The absence of polluting chemical compounds allows finished leather scraps and production waste to be recycled, to become organic fertilizer, for example.
A 360° approach is necessary to achieve a product that can be recycled at the end of its life (biodegradable according to ISO 20136 criteria, or Cradle to Cradle Certified). This virtuous circle requires cleaner choices of dye combined with more respectful finishing.
Taking into account the end of the product’s life and the environmental impact of its manufacturing process is an essential factor in an eco-responsible approach, and the choice of coloring process will be an important factor.
The many innovations in this area are aimed at reconciling the desired leather characteristics with a reduced and non-polluting environmental impact, in order to obtain finished leather with an optimal lifecycle analysis. Research and development is now tending toward increasingly eco-friendly options in response to designers’ creative color proposals. These are varied and stabilized, and may be vivid, intense, or patinated …
Beyond a global approach, bringing together different kinds of expertise is indispensable, and transparency throughout the manufacturing chain is crucial in order to push research and extend solutions toward more quality, creativity and performance, with ever-greater virtue.
To go further, see our Technical know-how: The leather glossary
Discover the Première Vision’s Color Report AW 24-25
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