Whitepaper: Zeolite & metals

What is metal tanning?

When it comes to tanning, and possible undesirable side effects, media attention is mostly focussed on chrome tanning. There is a type of chromium, which is NOT typically used in regular leather manufacturing, called chromium VI, which is potentially toxic, sensitizes skin and is potentially carcinogenic (when inhaled). This causes chrome tanning to be often perceived as an environmentally unfriendly production process. Moreover, chrome tanning yields a blue-greyish colored leather intermediate, which is usually re-colored to arrive at the desired leather color. Any extra processing needed, is disadvantageous for the environment.

Chromium is extensively used for the manufacturing of stainless steel (90% of all chromium ore). It is defined by chemists as a heavy metal, thanks to its density and atomic mass. The type of chrome used in chrome salts is called chromium (III).

Another type of tanning, is aluminium tanning. Also using a metal to produce leather, in this case aluminium. Aluminium is found in the periodic table between the elements of magnesium and silicon. It is described as a light metal, because of its low density, and low atomic mass. Aluminium is the third most common element on planet Earth. Humans encounter aluminium daily, through skin contact and through their diet. Plants readily absorb aluminium from soils, especially clays. Most aluminium ingested will pass through the digestive system unabsorbed. Meaning aluminium tanning is perceived to be much less harmful.

What is a metal?

Metals can occur as an individual element, such as the metal encountered in pure metal products, such as copper wires. They can appear as compounds, where the metal is linked to other atoms, which sometimes separate when they are dissolved. They can also appear in compounds where the metal is very tightly held, e.g., in coloured pigments, organic-bound metals, or inside huge compounds that are barely soluble, e.g., soils and clays. Aluminium is often bound to oxygen (known as oxides), silicon, magnesium, or calcium to form soil. Unless placed into acids, the aluminium is generally difficult to separate from the other minerals in the soil. Toxicologically, the negative effects of metals diminish as the metal complex moves from simple compounds into composites of metal oxides and organic structures.

Zeolites are porous structures that are part of the ceramic materials. These structures are metal oxides, linked in a cage that make the metal elements behave as a non-metal. It is unlikely that anyone would ever describe clay (or zeolites) as a metal, even though they contain metal elements like magnesium, metalloids like silicon, and the light metal aluminium.

Zeology, creating sustainable leather without harming the environment

Sustainability is high on the agenda today. Material producers therefore emphasize the environmental aspects of their input, processes and methodologies. They also closely monitor the safety of their output. Eco-benign products, or products that lack impact on the environment, become more and more important. What happens to the product at the end-of-life and the chemistry going into products, are subject to close monitoring. In order to create products sustainably, you can either focus on the end-of-life credentials or the circular bioeconomy, as a strategy.
Non-metal tanning with Zeology, by its zeolite-based nature, makes use of a more natural chemistry. A natural mineral, in the same manner as a natural vegetable tanning agent would be used, is placed into the leather. If, at the end of the product life cycle, the leather would returned to a natural ecosystem, it would break down into an aluminosilicate containing soil – identical to clay. Therefore, zeolite-based tanning is basically a natural cycling of minerals.

Zeology, a non-metal tanning concept

The zeolites used in tanning are crystalline, non-fibrous compounds that are negatively charged and will go into the hide or skin after deliming and bating (with a small amount of acid). The zeolite forms a network that will have alternating silica zones next to alumina zones. This alternating repetition of alumina and silica of the tanning agent links to the hide proteins and helps to stabilise the leather.

The main reason Zeology is described as a non-metal tanning agent, is that the metals (aluminium and silica) do not bind directly with the protein. The silica and alumina groups bind to the protein indirectly, using other atoms. The type of bond is also quite different. The Zeology tanning agent binds in a way that is associated with a ‘natural glue’ between the groups, or the substrate they are sticking to. It is also similar to the way that water molecules in ice are held together. This is vastly different to the way in which chromium, aluminium tanning, titanium, or zirconium, will tan leather.

In a way similar to the zeolite surface reacting with the protein, the post-tanning chemicals added after tannage will interact with the zeolite, because they have a similar affinity. To be clear: the dyes, fats and retanning agents can bind to the zeolite surface as well as they do with any other type tanning agent. Uptake of the zeolite is good and little zeolite will leave the tanning bath in the effluent, because of the attraction between the zeolite and the leather protein.

Zeology tanning may increase the shrinkage temperature of leather to 70-76°C. Moreover, it produces a pure white leather, by non-metal tanning. The tanning agent is negatively charged. This means that, after the tanning agent has bound to the leather protein, the leather has a character that allows dyeing, fattening, and retanning at lower pH levels than a tanner would normally expect (it lowers the isoelectric point). This has great benefits for process times and chemicals used.

Analyses of zeolite and aluminum in leather

There is no method to determine the zeolite contents per se. It is only through the determination of its individual constituents, that the zeolite content of the leather can become apparent.
In leather, the aluminium is tightly bound within the zeolite structure and the leather. However, during the analyses, leather and its constituents are completely destroyed before the individual constituents are determined. Under the conditions applied in ISO 17072-2, leather samples are broken down into the natural elements by a process called digestion. During the digestion of the leather, all (metal) elements are released. The same holds for minerals, clays and zeolites. As such, the determined aluminium content will be significant for zeolite based leathers.

The same applies for the determination of the unbound tanning agent, the so-called extractable part, which is extracted from the leather. For this purpose, method ISO 17072-1 is applied. This method also includes the total digestion of the extracted material before the individual constituents can be determined.