Tanning with iron salts - an old system in a new light*

Summary Tannage using only vegetable extracts and synthetic tanning agents technically works fine for a limited set of articles such as belt or bag leather and, of course, soles. But for lightweight leather types such as upholstery or even garment, it is generally more expensive and aesthetically not always acceptable. Considering all these topics, a new cost effective two-step tanning process has been developed which can overcome a great part of these inherent problems. Without using any ecotoxicologically problematic mineral salt or toxic reactive reagents, a leather intermediate with a Tg of more than 80°C was obtained, which can be shaved like wet-blue. Effective retannage with optimised processes and some newly developed synthetic retanning agents delivers an automotive type crust leather which can easily compete with existing chrome-free systems for aesthetics, fastness and mechanical properties. Introduction 'Chrome-free tanning' are keywords which have triggered a large number of discussions in recent years. From a tanner's point of view, there are few motivations to change from the versatile, simple and cheap chrome tannage towards any of the other tanning systems, which are first choice only for a small number of special leather articles. Unfortunately, the decision has been taken out of the hands of the tanners by some downstream users such as the automotive industry or shoe producers, who have their own, different view on ecology. Ignoring the factors of the overall ecological balance of leather which starts with processing the raw hides, they demand the final article to fulfil their requirements. And the tanners have to follow suit, if they want to stay in business. Requirements and ways to meet them The requirements from the downstream users are mostly induced by two factors: legislation on recyclability and absence of hazardous substances in the article. In addition, the tannery might prefer to tan without chrome for easier disposal of leather waste, eg shavings. Finally, for some articles such as dashboard leathers or sole leathers, chrome-free is not requested for the absence of chrome, but for the leather properties derived from such processes. So what are the systems and products to transform raw hides into leather: The standard: * basic chrome sulfate Commercial alternatives: * aluminium sulfate * zirconium sulfate * glutardialdehyde * tetrahydroxymethyl phosphonium sulfate * syntan, vegetable, and/or acrylic resin Academic and/or outdated alternatives: * formaldehyde * isocyanates/polyurethane * sugar aldehydes * activated carboxylic acids Apart from 'chrome-free', descriptions such as 'heavy metal-free', 'mineral-free', 'organic tannage' or 'wet-white leathers' are often used, but all of them are somewhat misleading and do not fully identify the underlying problem. Aluminium is not a heavy metal, but has its ecological problems due to aquatoxicity, and magnesium or calcium are certainly mineral forming metals, but never considered a problem, not even in 'organic tannages'. Glutardialdehyde or tetrahydroxymethyl phosphonium salts (THPS), being reactive organic molecules, have their own problems with acute or latent toxicity. So what tanners and downstream users are dreaming of is: A tanning process which does not use any toxicologically problematic substance or produce ecological problematic residues in the processing wastes or final articles and is as versatile and convenient as chrome tannage Not surprisingly, so far nobody has come up with such a system. What seems to come closest is a tannage with glutardialdehyde, although there are several drawbacks which can be seen from the following comparison: The impossible dream: Chrome leather without chrome but with the thermodimensional stability of wet-white leathers * Shrinkage Temperature (Tg) > 100°C, lightfast and resistant to heat yellowing * foolproof, reproducible, cheap process * no problems with samming, shaving etc * versatile tannage for all types of articles The real benchmark: Glutardialdehyde pretanned leathers with synthetic/vegetable retannage * Tg 70-80°C, medium lightfastness * process much more sensitive * samming, shaving etc, a more delicate operation * more costly due to product intensive retannage Those drawbacks are quite substantial and there are only a few advantages to compensate for this. The demands of the downstream users, however, can be fulfiled with this system and tanners can live with it, even if it is not their first choice. So what are the specifications derived from this situation towards a new system: * No ecotoxicologically problematic substances * chrome: Cr limitations for landfills, wastewater * aluminum salts: aquatoxicity, wastewater * glutardialdehyde: toxicity * phosphonium salts: teratogenic, very toxic to aquatic organisms * Tg high enough to make samming and shaving easy, ie >75°C * Physical fastness properties and aesthetics competitive at least to glutardialdehyde leathers * Process cost and handling comparative at least to a glutardialdehyde based chrome-free system => Not a replacement for chrome, but an improvement to modern chrome-free systems A new approach Most new approaches over the past two decades have been concentrated on organic tanning materials such as phosphonium salts or isocyanates but these have not resulted in a commercially successful system. New developments using metal salts have been somewhat neglected, since their properties are all well known and documented and nothing promising seemed to be in line. Only very recently, some institutional researchers have been reviving tanning methods with iron salts, addressing the properties and problems of iron tanned leathers. Common knowledge and misconceptions on tanning with iron Pelts can be tanned with various iron salts and different success. First patented in 1770 in Great Britain, Fe (III) is believed to be the 'active' form. Tg of 80-97°C can be achieved as described in literature, eg by Heidemann or Stather. Some questions have been raised over the years regarding: * The stability to ageing of iron tanned leathers * The stability of the iron tanning complex towards hydrolysis * The best way to create iron tanned leathers without stains or discolouration Recent results with iron tannages, however, are quite encouraging and indicate that those questions can be answered satisfactorily. Outlines of the process Liming, deliming, bating: conventional system Pickle: pretreatment with special polymer based carboxylic acid Tannage: basic iron (III) sulfate, solution with 13% Fe Basification: conventional chemicals for slow basification Retannage, dyeing: process with special synthetic retanning agents and auxiliaries At first look this seems to be quite a standard process and, indeed, the time line and process steps are very similar to a chrome tannage. So why has nobody so far found this? The answer lies in the detail, and especially in the products which are used during the pickle and retanning steps. It has been published that iron (III) has to be masked to be used as an effective tanning agent. Most publications so far have been doing this with different sorts of carboxylic acids, which have been reacted with iron salts prior to the addition to the tanning float. Given the nature and solubility of those compounds, this can create a lot of problems, and high costs. On the other hand, it has been known that pelts can be thermodynamically stabilised using polymeric carboxylic acids, as demonstrated in Figure 1. This knowledge has been transformed into a pickling process, using specially selected carboxylic compounds for masking the collagen in situ, so that a simple solution of basic iron (III) sulfate can be added to the pickled pelt at a normal pH without precipitation. After a suitable time for penetration, basification is conducted using an optimised formulation of classical chemicals for high exhaustion to obtain a leather intermediate with shrinkage temperature above 80°C, that can be sammed and shaved like wet-blue. A typical compact iron tannage able to produce an intermediate suitable for automotive upholstery leather is included in the process in Table 1. The uptake of the iron tanning salt has been controlled and shows quite impressive results with a remaining iron content of less than 500ppm in the spent float at pH of 4.0 (Figure 2). This intermediate with superior lightfastness, named 'wet-tan' due to its tan colour, behaves through samming and shaving as trouble free as wet-blue. Since such intermediates are commonly shipped around or stored for some time, they have to be stable against bacterial and fungal attack. Trials for conservation with various phenol-based products** showed a good resistance against the common threats of mould. It can then be retanned using a variety of retanning, dyeing, and fatliquoring products. Unfortunately, the softness and sometimes the tear strength has not been very satisfying, especially for soft leather articles for upholstery and automotive purposes. It took the development of a new group of synthetic tanning agents for the neutralisation and retannage to overcome this problem. With them, a compact retannage to obtain an automotive upholstery leather can be run as in Table 2. In addition to giving good fullness and superior softness to the iron tanned crust leather, this new group of synthetic tanning agents shows excellent lightfastness and resistance to heat yellowing. The overall amount of products used for tannage and retannage is in the range typical for a chrome system. Accordingly, the load of the effluent with organic materials is in a similar range and, therefore, reasonably lower than for classical chrome-free systems relying on glutardialdehyde or pure syntan/vegetable tannins as are shown in figure 3. Although it is impossible to obtain a really white crust, light shades of finished leathers can be achieved, since the migration properties of all products necessary are excellent and an undyed crust shows fastness against migration into pvc of 5 and higher. Brilliant shades can be obtained as well as various browns without any problem. For producing black articles, the colouration reaction of iron with vegetable tanning extracts can be used to give really deep black with as little as 2% of the dyestuff combined with 2% tara extract. The wetrubfastness of 3-4 is due to the complex nature and fixation of the dye. The crust leathers finished with classical automotive finishing systems based on polyacrylate and polyurethane show unimpaired quality of the finish with high physical properties. To answer one of the most important questions about the stability of the iron tannage, tests about the hydrolytic stability have been performed. After seven days at 70°C and 95% humidity, crusts as well as finished leathers were firmer but still flexible. While chrome tanned leather survives this treatment practically without change, a classical chrome-free leather based on glutardialdehyde and vegetable tannage will be completely destroyed under those conditions. Testing according to FAKRA-specification was passed without damaging the leather. The thermodimensional stability under dry conditions is also comparable to chrome-tanned leather, as can be seen from the chart (Figure 4) which shows degree of shrinking [%] after one hour at 150°C in relation to the amount of retanning materials offered [% calculated on wet-blue]. While a full grain automotive upholstery leather is certainly the most desired article for a chrome-free tanning system, others might be as interesting. In the future, it may be possible to manufacture and fit split leathers into the car after finishing, eg with a reverse coating system ***. Trials have been carried out which proved that iron tanned split leathers are very suitable for reverse coating due to their compactness and very short fibres after buffing. The physical and fastness properties of such leathers, especially finish adhesion and flexibility, are good. Finally, there is a last problem of chrome-free tannages which can be solved using iron salts for tannage: waterproofing. So far there are no products on the market that are able to overcome the hydrophilicity imparted to leather by a classical chrome free tanning process. Since the wet-tan material is basically a mineral tanned intermediate, it can be transformed into a waterproof crust if the waterproofing system does not need chrome for fixation. Using a recently developed waterproofing system****, several different chrome-free tanned waterproof leather articles have been prepared from 1.3mm sport shoe split suede up to 3mm leathergoods full grain leather which resist water penetration in the Bally-penetrometer for several hours with water uptakes of less than 15-20%. While most of the commonly accepted misconceptions about iron tanned leathers were refuted, some new aspects came up during the development that should not be omitted completely. The presence of ferric ions enhances the reactivity of double bonds. Due to this fact, fatliquors with a high iodine content and, especially, fish oil-based fatliquors, develop a stronger smell than usual and should be avoided. For the production of very soft leathers, a somewhat higher offer of fatliquor during the tanning process has proven useful. However, this can lead to fogging values which might create problems with some of the automotive specifications even together with an optimised retannage. Nevertheless, both of these problems can be overcome by optimised process conditions to achieve acceptable crust leathers. As a resumé, the properties of the iron tanned leathers can be summarised as follows: Wet-tan intermediate (3.6% Fe): * Tg 80 +/-2°C; pH3.7-3.9; differential number 0.2 * samming and shaving like wet-blue * lightfastness 4-5, pvc migration fastness 5 * resistance to biological attack comparable to wet-blue Crust: * tear strength ok * hydrolytic stability (7d, 70°C, 95%) better than aldehyde, not as good as chrome Finished leather: * flexes and friction unimpaired * finish adhesion more difficult * embossing ok Given the amount of products offered during the process, the overall costs are not as low as for a chrome-tanning system, but very competitive with any of the chrome free processes currently on the market. This is why we consider an optimised tannage with iron to have the potential to become a valid alternative to the existing chrome-free systems. However, like any other chrome-free system, it will probably never be able to match all the properties of a classical chrome tannage. * Ferrotan ** Preventol *** Levacast **** Xeroderm P-AF + Xeroderm S-AF Acknowledgments To Dr A El Amma, D N Price, Dr J Whiteman, Rohm and Haas Company, and Dr D Tegtmeyer for valuable ideas and discussions. To F Heinzelmann, C Henzel and A Kaplan for practical help. Preservation testing was done by H Rehbein and Dr O Kretschik of Bayer Chemicals business unit 'material protection products MPP'.