The right chemistry - alternatives to traditional tanning methods13 February 2017
With sustainability becoming an increasingly important criterion for leathermaking, the onus is on tanners to find ever cleaner and more efficient ways to process hides. Dietrich Tegtmeyer, head of the IULTCS Research Commission, explores some of the most promising alternatives to traditional manufacturing methods.
Chrome tanning remains the best-known available technology to thermally stabilise collagen, the main structural protein of any hide or skin. Multiple research studies of all mineral tanning processes over several years have shown that chromium(III) sulphate performs best due to two key reasons. The first is that, as an ion, it forms an extremely stable complex with the inner structure of the protein triple helix. Second, the tanning production process of penetration and fixation is easily controlled.
Tanners, logically enough, have traditionally focused on the quality of wet-blue and crust leather, and have designed processes accordingly. Unfortunately, this has tended to result in sub-optimal efficiency and sustainability. Only about 40% of the initially offered amount of chrome tanning material (CTM) ends up in the final leather. The remainder is either washed during processing, or is in the solid waste (mainly shavings), most of which is recyclable.
Chrome-containing sludge can be –safely, legally and usually economically – disposed of, so it is acceptable from an environmental perspective. However, in terms of sustainability (and probably also longer-term economics), this endless dumping of finite resources must be questioned, and the sooner the better.
Two of the most notable alternatives to the standard chrome method are high-exhaustion and/or hybrid tanning.
These would not only lead to a major improvement in the mass balance for chrome tanning material, but would also immediately benefit the public perception of the leather sector, and probably also prove critical to the long-term future of the tanning industry.
The main operating step in leather manufacturing is the tannage, which stabilises the collagen and chemically converts this protein into leather.
Due to the ideal chemical characteristics of chromium in the valence state III, this element can enter inside the triple-helical structure of collagen, where it uses its special complexing and cross-linking behaviour to stabilise the protein matrix.
The geometric dimensions of the resulting complex fit in perfectly with the surrounding 3D protein structure of the collagen. The macroscopic result is a significant increase of the denaturation point (shrinking temperature) of the leather above 100°C that can only be achieved with a CTM and not with any other metal tanning agent on its own.
Decades of research were necessary to understand the ideal, complex chemistry of chrome tanning scientifically, and this phenomenon has been explained in various ways. One famous theory is the principle of covalent tanning with CTMs within the link-lock mechanism. Another concerns the principle of olation/oxolation of metal ions.
Reference is often made to the cross-linking of CTM, and rather less frequently to olation, a type of in-situ inorganic polymerisation, but this is still fundamental and critical for understanding the tanning process. At a low pH (that is to say, pickle), many metal ions exist in solution as complexes with a low molecular weight that are unreactive, or rather less reactive, to collagen. As the pH increases (‘basification’), these complexes start to react with each other to form larger molecules (metal ion oligomers) with higher reactivity towards the collagen subunits, up to a level at which they become insoluble.
The tendency is to form bigger molecules – rather than cross-linking collagen – until no reaction with the protein occurs, and the ability to steer this with suitable ligands depends on the nature of the metal. Here, only chromium(III) has the right pH-dependent balance between increasing molecular weight to the right size and reactivity toward collagen, also steering this easily with suitable ligands, like sulphate or acetate.
Metals such as aluminium, zirconium or iron can provide stabilisation effects. These follow a different mechanism to chromium(III) and even the pH-dependent olation/oxolation phenomenon has different (and in this context, unfavourable) characteristics. Subsequently, all these alternative metal tanning systems result also in a lower denaturation point of the resulting substrate. So far, no alternative tanning agent for leather exists that fulfils the unique property profile of CTM. Therefore, non-chrome systems usually require some degree of ‘compensation’ especially in the retanning step.
This requires additional conventional chemicals, affecting sustainability as well as the haptic and technical performance of the material, although a chrome-like character is possible in terms of application.
Advantages and challenges
Chrome is also the dominant tanning system because it is relatively easy, stable and robust, and results in reliably safe, high-performance technical substrates. The speed of penetration of CTM can easily be steered and controlled, even visibly.
The step involving the cross-linking reaction of the chromium in the leather can again mainly be managed by pH adjustment with harmless basification agents. Therefore, about 85% of the world’s leather production is still based on chrome-tanned leather.
This has been the case for more than a century and is expected to continue for the foreseeable future.
To continue the future-proof development of chrome tanning in general, and especially from a sustainability point of view, a closer look at two important aspects is required: the process rules to avoid the formation of hexavalent chromium (Cr(VI)) in leather, and concepts to increase the use of CTS in order to minimise the release of chromium into effluent.
For the first topic, clear process rules have already been established and are well known. In order to have a closer look at the second issue, a simple mass balance needs to be performed to compare different chrome tanning process alternatives.
In a standard chrome tanning operation, up to 8% of CTM is offered (in relation to pelt weight), corresponding to an absolute amount of 17–20kg Cr2O3 based on a 1,000kg pelt. About 60% of this chromium will be chemically fixed (via covalent bonding) to the protein while approx. 40% remains still unfixed (either in the float, or in the liquid within the leather matrix after the tanning operation).
During the subsequent washing and processing steps (samming, retanning), about 40% of the initially offered CTM will be washed out in the effluent and should be separated by further treatment. In a modern tannery, the majority of this discharge could be separated and recycled.
However, good equipment, and proper and intensive quality controls of the recycled chrome material are necessary to ensure that the quality of the wet-blue is comparable to the use of only CTM from a reliable supplier. If the chrome is not recycled, it must be safely and legally deposited at appropriate, usually municipal, approved facilities.
Furthermore, up to about 20% of the initially offered CTM will end up in the solid waste as shavings. The majority of the chrome in these will either end without a specific functional requirement in an upstream basic raw material (for construction or energy generation, for example) or end up as waste in landfill.
Considering these two ‘losses’ in a normal retanning process, without additional rechroming only around 40% of the initially offered amount of CTM ultimately ends up in the final leather. From this remaining chromium in the final leather matrix, only a very small part (a few parts per million) is not strongly bonded, and can be extracted with artificial sweat from the dried-crust leather. This is what is usually referred to as the ‘leachable’ chrome part.
As an alternative to standard chrome tanning processes, the mass balance of a typical high-exhaustion chrome tannage is illustrated in table 2. The sustainability is considerably improved because the majority of chrome originally offered ends up in the leather.
Furthermore, this process can be run with the same equipment as the standard chrome process; no special investment needs to be made and, in principle, every tannery worldwide should be able to switch to a high-exhaustion process.
As less chromium will be washed out into the effluent with this process, the initial offer can be reduced by as much as 5% (based on pelt weight). This alone represents an almost quarter reduction in CTM use compared with the standard process. Through the addition of special fixing agents – high-exhausting chrome tanning agents – the exhaustion can be improved significantly, not only in the tanning, but also in the retanning step. Subsequently, the resulting amount of chrome in the raw effluent is so low that most legal requirements for the chromium content of the discharge can be met without any problem. This process also results in a high score in the Leather Working Group (LWG) audit, which requires an uptake of at least 97%. The total amount of chromium in the final leather is comparable to the standard chrome tanning process.
The key difference is that the unfixed proportion – which normally gets washed out during processing and ends up in the effluent in the standard process – is effectively eliminated.
‘Hybrid tanning technology’ refers to the combination of wet-white pretanning with a synthetic tanning agent. The most sustainable wet-white techniques don’t even require pickling. Obviously, no chromium can end up in the effluent, or any by-products such as shavings; the process up to the wet-white intermediate stage is completely chrome-free. In order to get a similar intermediate (like a wet-blue) the retanning process must begin with rechroming. A high-exhausting rechroming agent is added, usually 5–7% (based on shaved wet-white weight), which correlates to only around 2.0–2.5kg absolute Cr2O3 based on 1,000kg split pelt (shaved wet weight has about 30–50% of the weight of a pelt, depending on water content and thickness.)
With a special fixing rechroming agent, the exhaustion in this process step is extremely high. Close to 100% of the initially offered Cr2O3 ends up in the crust leather, with only a tiny amount of Cr2O3 washed out in the retanning floats, which are virtually transparent and virtually colourless. After rechroming, the typical organic retanning process completes the process, usually with only very slight adjustments to the recipe necessary in order to reach a similar crust to that which can be achieved with a traditional wet-blue intermediate.
The comparison of the mass balances of the three tanning processes show that, in terms of sustainability, the two suggested alternative methods are more favourable compared with the standard chrome technique. Since there are so many parameters that can influence the outcomes – such as local conditions, recipes, raw material quality and beamhouse preparations – only rough numbers for chromium content in the effluent have been used.
However, many production-scale examples have shown that running a high-exhaustion process already reduces the chromium content of the effluent to such an extent that, in most cases, the effluent can be discharged directly to a wastewater treatment plant without any pretreatment or chrome separation. Also, in the case of sludge, its chromium content is so low that it is not (legally) considered hazardous.
The issue remains of chromium-containing shavings, of which there are massive volumes. The ultimate sustainable process and best available compromise that retains the advantages of a chrome-tanned article without this issue is considered to be the ‘hybrid’ approach. Zero chrome in the effluent and the shaving by-product is obviously guaranteed during the wet-white pre-tanning process.
A special high-exhausting rechroming agent means the leather will virtually fix all offered active chromium ions afterwards. Therefore, nearly all offered amounts of chromium will end up in the final leather sold, and not in any waste.
Just as stainless steel can only be produced with chromium, as no other element in the periodic system gives a similar stabilisation of the iron. Similarly, in leathermaking, chrome-tanning is the only known means of stabilising protein.
With that in mind, the high-exhaustion and hybrid methods described currently represent the best means by which to make the production process as sustainable as possible.
Should you have any questions about the lecture or this article, please contact Leather International editor Carl Friedmann at firstname.lastname@example.org.