A longway from chrome12 October 2018
Speaking in June at the 7th Freiberg Leather Days conference, Florian Döppert, the head of wet-end screening at TFL Ledertechnik, used his presentation to examine the industry’s choices when it comes to retanning systems for chrome-tanned and chrome-free leather. As he tells Ross Davies, we need to think outside the box.
Your presentation at the 7th Freiberg Leather Days conference was on the different impact of retanning systems for chrometanned and chrome-free leather. What made you choose to speak on this subject?
Florian Döppert: The production of chrome-free leathers based on glutaraldehyde pre-tanned material has become a state-of-the-art technology, and we are constantly working on further optimisation of the processes and products. We believe this technology to be the most effective and practical for the production of wet-white. Although chrome-free is working fine in general and has benefits – such as dyeing of brilliant shades and dry shrinkage behavior – there are some points where it can be improved, like the higher consumption of synthetic and natural retanning agents and the consequently increased chemical oxygen demand (COD) in the wet-end effluent.
We observe, in certain areas, increased production of chrome-free leather, which often comes with wastewater problems in wet-end due to tougher wastewater regulations. This means a higher output of the likes of COD and lower biological oxygen demand (BOD), which often results in poorer biodegradability of the wet-end wastewater stream in general.
This was the initial reason to develop a system where we can improve on this, in contrast to existing wet-white standard pre-tanning and retanning processes. At the 7th Freiberg Leather Days conference we had the opportunity to present the results, based on a non-toxic, specialmasked zirconium-based tanning agent.
Why are high amounts of synthetic and natural tanning agents used in producing chrome-free leathers?
Well, in today’s standard process for pre-tanning of chrome-free leather, you normally use glutaraldehyde-based tanning agents, with about 2.5–4% needed to stabilise the pelt. This allows them to reach shrinkage temperatures of anywhere between 72–75°C, which is still lower than those of chrome-tanned material. Practically, this causes no problem for shaving operations on wet-white.
However, the main tanning of chrome-free leathers is actually done in the retanning; and to really stabilise the material, you traditionally do this with a synthetic vegetable tanning process, which, unfortunately, requires a lot of materials compared with the already stabilised and well-tanned chrome leather.
So, when it comes to processing chrome-free leather, you have to shift your tanning process further from the tan yard to the wet-end, which results in noticeably higher effluent loads and COD levels compared with wet-blue.
What are some of the potential improvements for chrome-free leather production – from an ecological and performance perspective?
Chrome-free leather is produced regularly on an industrial scale, so, in general, it has good performance levels, with some advantages and some disadvantages compared with chrome leather.
From a performance point of view, there are many positive things, but tanners are also looking for improved substance control of their wet-white material. This means you can shave the material to, let’s say, 1.1–1.2mm, but during the traditional retanning process your substance might increase up to around 1.3–1.4mm. Also, you don’t get 100% the same leather substance through the whole hide; sometimes your neck ends up thicker than the main area.
So we would like to have better substance control, similar to processing wet-blue, in which you shave it to around 1.2mm and it more or less stays there – or perhaps goes up by 0.1mm.
Due to the high amount of synthetic vegetable tanning, you can also have a rather dry fibre, which can at some stage be negative when it comes to things such as tear, stitch and tensile strength.
From an ecotox point of view, the absence of chrome is the key benefit, avoiding the possible formation of unwanted chromium (VI) and chrome allergies and making the wet-white shavings more compostable. But the high amount of retanning agents needed means that we have one weaker point, which is the relatively high level of COD in the wastewater making biodegradability more difficult.
What about the dyeing aspect in all of this?
Chrome-free leather, in general, is fantastic to dye, and you can get these really brilliant yellow-orange shades – unlike the slightly duller and dirtier dye behaviour you get with chrome. So on the plus side, you get these great clear shades, but it does take a lot of dye.
This is needed because most of the vegetable and synthetic extracts tend to bleach, and the fixation of this high amount of dye, together with the absence of cationic charge, can influence things such as the perspiration fastness. It would be nice to see some improvement on this.
You talked about zirconium as a tanning agent. What is its potential in regard to retanning?
First of all, we really need to free our minds and look at the possibilities here. For the past 25–30 years, it has been acknowledged that pre-tanning with glutaraldehyde works really well. There are many tanneries out there using glutaraldehyde on thousands of hides every day, and then retanning in the standard way.
But things can be improved, and in order for this to happen we need to think outside the box. We have tried many things, such as using amphoteric polymers and special fixation agents, as well as different synthetic tanning materials. We have worked on eliminating fully vegetable extracts from the retanning of chrome-free leathers, and while we’ve witnessed certain improvements from this, we haven’t seen major changes in COD reduction of wetend wastewater loads from the chromefree processes.
It got us thinking about what the true difference is between chrome-tanning material and chrome-free-tanning material. It’s essentially the absence of metal cationic charges, which come from the chrome. You don’t have this in the chrome-free leathers and most people don’t want to rechrome their chrome-free leather or their wet-white. This means you can technically retan the glutaraldehyde pre-tan material with chrome, but then you end up with chrome, which, if your goal is to achieve chrome-free leather, is obviously not an option.
We believe the use of a non-toxic special-masked zirconium offers a great alternative. If you look at zirconium’s properties, it is not only non-toxic and eco-friendly, but, in general, is an interesting tanning agent and, for us, a new approach.
Most effective is the application in small amounts in the retanning of wetwhite, where you have already done your pre-tanning with glutaraldehyde. The addition of zirconium helps create a completely different cross-linking in the fibre and provides a nice cationic charge, which helps to improve fixation later on in the process.
Our trials showed that we can significantly reduce the amount of traditional synthetic and natural retanning agents and have a strong, positive effect on the COD, total dissolved solids and total organic carbon load of the wet-end wastewater for chrome-free leather by maintaining an excellent ratio between COD and BOD. This measurably improved the biodegradability of the effluent.
We believe that, by using non-toxic, special-masked zirconium tanning agents in the retanning of chrome-free leathers, we can offer a lot of interesting technical and ecological benefits to the industry.