Anthony D Covington, British School of Leather Technology, The University of Northampton, discusses the possible future directions of tanning without chromium. This is an abridged version of the second part of the paper he gave at the Washington IULTCS congress 2007: ‘Quo vadit chromium?'
It has been suggested1 that the environmental impact of chromium could be alleviated by substituting all or part of the offer by other metal tanning salts, when the following options are the likeliest candidates:
Al(III), Ti(III)/(IV), Fe(II)/(III), Zr(IV), lanthanide(III)
The list of available options is limited to the few presented above by considerations of cost, availability, toxicity and reactivity towards carboxyl groups. The benefits of introducing another metal salt can be summarized as follows:
- The efficiency of chrome uptake is improved by reducing the offer
- The rate of chrome uptake can be improved by the presence of another mineral tanning agent, applied as a pretreatment, demonstrated in the case of aluminium(III)1.
The contra indications can be summarised as follows:
- In no case is the shrinkage temperature positively affected, the opposite applies. If it is required that the leather should be boilfast, the chrome offer must be 0.75% Cr2O3
- The properties of the leather are modified, changed by the presence of another mineral tanning agent, maximised if the substitution is total
- The change may be to the handle, in terms of collapsing (as with aluminium(III) salts) or filling (as with titanium(IV) or zirconium(IV) salts) the fibre structure
- The leather will become more cationic and, therefore, more surface reactive towards anionic reagents
- The colour may be changed (as with iron salts). At all proportions in a mixture with a colourless substitute, chrome tanned leather remains distinctly blue, even at very low chrome offers
- The hydrothermal stability is adversely affected, in terms of the pH window within which the mineral-collagen bonding remains intact.
The stability of mineral tanned leather can be enhanced if the approach demonstrated by Holmes is adopted, in which he grafted multifunctional complexing sites onto collagen2
. In this way, the linking part of the tanning mechanism is improved, fixing the matrix to the collagen more firmly. This is a chemical modification to the substrate that can be exploited further, but it clearly complicates the tanning operation. However, for some specialised applications, it could be appropriate.
Organic tanning options - Polyphenol chemistry
The best known example of mineral/plant polyphenol tanning for high hydrothermal stability is the semi metal reaction. Here, the requirement is a covalent complex between the linking polyphenol and the metal ion3: in practice, this means using the hydrolysable tannins, but some condensed tannins, the prodelphinidins (II) (eg myrica esculenta, pecan and green tea)4 and prorobinetinidins (IV) (eg mimosa) have the required structure in the B-ring. Many metal salts are capable of reacting in this way, even non-transition metals such as zinc5, so there may be useful applications for the future.
The reaction can take place even with the monomeric units of vegetable tannins, as is also the case with the polyphenolic components of the flavonoid ring system, as shown in Table I.
From Table I, the following conclusions can be drawn:
- Not all aldehydic locking agents react effectively
- Phloroglucinol is a model for the A-rings of the procyanidins and prodelphinidins, which are more reactive than the corresponding sites on the profisetinidins and the prorobinetinidins
- For oxazolidine locking reactions, the preferred B-ring structures are in the prodelphinidins and the prorobinetinidins
The link-lock mechanism can be exploited in other ways, even using reagents that at first do not appear to be tanning agents; an example of this is naphthalene diols, as shown in Figure 17.
The behaviour of naphthalene diols in the tanning process is highly dependent on the structure of the isomer, as indicated in Table 2.
It is clear that the presence of a hydroxyl in the 2-position activates the naphthalene nucleus: the 1-position does not work, shown by comparing the 1,5 with the 1,6 (2,5) diol. When there are two groups in the 2,6-positions, they act together. When the hydroxyls are in the 2,7-positions they act against each other. The basis is the inductive effect of the hydroxyl on the aromatic ring, activating the ortho positions to electrophilic attack or allowing those positions to engage in nucleophilic attack at the methylene group of the N-methylol group of the oxazolidine.
The results in Table 2 also illustrate the principle that the linking agent may only exhibit a very weak effect in tanning terms, but successful locking of the linking species, combined with the ability to link the matrix to collagen in the locking reaction, can result in high hydrothermal stability.
Non-chemical polymerisation is less effective. Applying laccase (phenol oxidase enzyme) to hide powder treated with 2,6-dihydroxy naphthalene produced the highest rise in shrinkage temperature for the range of this type of linking agents tested, elevating the shrinkage temperature by 26°C, to 85°C7
. It is clear that the locking reaction is more easily and effectively accomplished by applying a second reagent, rather than relying on direct reactions between linking molecules.
4.2. Polymer and crosslinker
Perhaps surprisingly, it is less easy to create high stability tannage with polymers than it is using oligomers or monomers. It has been shown that high hydrothermal stability can be achieved using melamine resin crosslinked with tetrakis hydroxymethyl phosphonium salt. Several conclusions were drawn from these studies8
- Not all melamine linking resins work. Therefore, the requirements for matrix formation are likely to be more important than possessing specific chemical reactivity. In this case, there appears to be an optimum particle size for the resin of about 80nm
- Not all aldehydic locking agents work. The locking function does depend on creating stable bonding between the linking molecules and forming a rigid species capable of resisting the collapsing triple helices
- The linking reaction is dependent on physical parameters; particle size may be critical. It is not sufficient to provide space filling
- The ability to form the basis of a supramolecular matrix must depend on the stereochemistry of the linking agent. This requirement is more easily satisfied with lower molecular weight species.
This reaction provided an interesting aspect of the matrix theory of tanning, when an attempt was made to accumulate hydrothermal stability by adding a matrix to a matrix. Here, the sequence of reagent additions was as follows: melamine resin, phosphonium salt, condensed tannin (mimosa), oxazolidine. The observation was the achievement of high hydrothermal stability from the melamine resin and phosphonium salt, added to by the condensed tannin, reaching a shrinkage temperature of 129°C, thereby matching the maximum shrinkage temperature achieved by chromium(III) in the presence of pyromellitate.
The melamine and phosphonium salt create a matrix in which the melamine polymer reacts with the collagen via hydrogen bonds, the phosphonium salt crosslinks the polymer, while probably linking the matrix to the collagen. The introduction of condensed polyphenol raises the shrinkage temperature by a small additive effect, since it is applied after the matrix formation, it has no affinity for the resin, but has limited affinity for the phosphonium salt. The subsequent addition of oxazolidine causes the shrinkage temperature to drop significantly. It is difficult to rationalise these observations by any mechanism other than the formation of matrices.
Natural tanning agents
Recently, new tanning chemistries have come to light: they have the characteristic of being biomimetic, using natural reactions in a new context. Such organic tanning reactions are of interest from three points of view. First, they offer new methods of making leather, to yield new products, which may contribute to lessening the environmental impact of tanning. Second, they offer new opportunities for high hydrothermal stability tanning, by acting as new linking agents, then allowing manipulation of the chemistry of the locking step. Third, they may involve the novel use of enzymes in tanning, operating as catalysing activating agents, so the rate of reaction is highly controllable.
Starch derivatives are well known in the industry, but dextrin is an alternative starting material; it is a chain of 11-12 glucose units, obtained by partial hydrolysis of starch, and the tanning effects are comparable with the best effects of starch dialdehyde.9
However, there can be direct reactions between carbohydrate and proteins, glycosylation-type reactions. Komanowsky effectively generalised these reactions in his studies of the reactions within collagen at low moisture contents10
. The basis of the formation of permanent bonds is the Maillard reaction. The reaction depends on reducing the moisture content, in order for the groups to approach close enough to react: it works better at higher temperature and lower pH.
The stabilising reaction is illustrated in Figure 2. It is assumed that the ketoamine product can react further with collagen, to form a crosslink (of unknown structure), a reaction that becomes more feasible when the interacting chains are close, unlike the situation in wet collagen and leather.
The natural products obtained include sugars and the glycosaminoglycans, hyaluronic acid, dermatan sulfate and chondroitin. These compounds or their derivatives might be applied as reagents as part of a two-step organic tannage. Hyaluronic acid and dermatan sulfate are byproducts of the leather making process and offer the potential for useful new derivatives. In this case, the residual reactivity is enhanced by the greater variety of chemical moieties in the molecules.
A related stabilising process for protein that has not previously been reviewed in a leather context is the preservation of so called ‘bog bodies', eg ‘Tollund Man'11
. Perhaps surprisingly, the preservation mechanism is not an example of vegetable tanning. The environment in which the tissues of the body are preserved is typically a sphagnum peat bog: the effect is thought to be due to a Maillard reaction, and could be developed to the advantage of the tanner.
- Nor dihydroguaiaretic acid (NDGA)
NDGA is a naturally occurring polyphenol, isolated from the creosote bush; it has been proposed as a stabilising reagent for collagen, when it exhibits the unusual phenomenon of actually increasing the strength of the fibre12
, rather than the more common effect of lessening the weakening effect of processing. The mechanism of polymerisation of NDGA, Figure 3, is thought to align with the protein chains, constituting a supporting and strengthening structure. Clearly, the polymeric species offers opportunities for additional reactions.
Genepin is an iridoid derivative isolated from the fruits of Gardenia jasminoides. Its reaction with protein is characterised by the generation of deep blue colouration under the alkaline conditions required for fixing.
The reaction mechanism by which protein is stabilised is not clearly understood, but it has been shown that the shrinkage temperature of hide powder can be raised to 85°C13
, characteristic of a single tanning reagent. That in itself is not important, but the incorporation of new reactive groups into collagen by a linking reaction using genepin may provide new sites for reaction in a locking step.
Alternatively, genepin may provide another approach to the locking step in a combination tannage. There are many other iridoids; nearly 600 are known14
, characterised by the presence or absence of glucose moieties and the presence or absence of a cyclopentane ring.
Oleuropein is a natural product, a secoiridoid glycoside, found in privet and olive vegetation, where its function is part of the self defence mechanism against infections and herbivores: it contains glucose, but not a cyclopentane ring in its structure. The mechanism of reaction with protein is thought to involve the activation of oleuropein to the aglycone form, which reacts with lysine residues on proteins. From the work of Antunes et al, the stabilising effect can be compared with that of glutaraldehyde, as shown in Table III, in which the collagen was recast as a film from solubilised collagen.
As with genepin, the chemistry of oleuropein offers the potential for acting as either a linking agent or a locking agent, using an appropriate complementary combination reagent.
These biomimetic approaches to tanning constitute a new, potentially powerful aspect to applications of biotechnology in leather making. However, it is equally clear that they do not offer the opportunity for a single step, high hydrothermal stability tannage. The advantages lie in the new, covalent binding reactions, which create the linking part of the supramolecular matrix, which is firmly bound to the triple helix, then is capable of undergoing a variety of locking reactions.
There is a remaining class of reagents that might be classified as either linkers or lockers; the requirement for new components for tanning is that they should have the ability to react covalently under the typical conditions of tanning. They are options already known in the leather industry in the form of reactive dye chemistries; it is merely a matter of widening the exploitation. Some of the reactive species are presented in Figures 4 and 5.
The chemistries represented in Figures 4 and 5 could be exploited in the form of multiple reactive groups in the same molecule. This notion has already been initiated by BASF in the form of their product (dye) Fixing Agent P, triacryoyl triazine, but there is clearly potential for developing the approach further.
The concept of ‘compact processing' is the condensing or shortening of processing by combining two or more reactions into a single process step. In this way, time is saved and consistency is created in the interaction between the process steps involved, because only one reaction takes place, instead of two or more. This is a powerful contribution to innovation in the tannery. The idea of compact processing can be applied at any stage in processing, although it is more commonly applied in the later stages of wet processing. Therefore, thinking about this concept should not be limited in scope.
If it is accepted that high stability can only be achieved by a two stage tanning action, then there are opportunities to exploit such an approach to tanning in compact processing.
The use of a non swelling acid incorporates the notion of pretanning. It may be possible to substitute pretanning for retanning. However, it should be recognised that the first tanning agent controls the character of the leather. In the case of prime tanning being conducted with vegetable tannins, the requirement for further tanning is uncommon. An exception would be organic tanning, which might be based on polyphenols, when there would be a requirement for applying the second component of a combination process for high hydrothermal stability. Such an approach is feasible for the combination of gallocatechin-type condensed tannins with oxazolidine, when the reaction is activated by elevated temperature.
The conventional components of post tanning are: neutralise, retan, dye and fatliquor. In each case, an industrial process will use one or more agents for the purpose. The permutations and combinations are straightforward and some have found application already. Notable technologies include: neutralising syntans, vegetable tanning with bound dye (Forestal Quebracho) and retanning with fatliquoring and water resistance (Lubritan by Rohm and Haas).
An alternative approach to colouring leather is to use the chemistry of melanin formation: using model reactant polyphenols and the aid of polyphenol oxidase, it is possible to achieve a tanning reaction and develop colour at the same time16
. Here, the potential role of enzymes in tanning becomes apparent.
Dyeing itself might be considered to be a retanning reaction. The type of dye determines the nature of the binding interaction with leather: chromium(III) 1:1 premetallised dyes fulfil this dual function. The use of reactive dyes would clearly constitute a form of retanning, because they bind via one or two covalent links to basic sidechains. The outcome will depend on the amount of reactive dye and the ability of the structure to interact with the aqueous supramolecular matrix.
It has been shown that it is feasible to create a mixture of reagents for retanning, dyeing and fatliquoring in a single post tanning processing step17
: the development of a single reagent is clearly more difficult, but should not be chemically impossible.
The concept of compacting does raise the question of the role of the solvent18
If the tanner could use a more exotic solvent, simultaneously to solubilise a wider range of reagents than is possible in water, it might be feasible to apply all of the post tanning reagents at once, in a single step, with no residual effluent. According to the choice of solvent, there may be no need for neutralisation. Alternatively, neutralisation may have to be retained as an aqueous process, to achieve the charge required by the new mixed process.
It is conventionally assumed that tanning has to be conducted in aqueous solution. The alternative is an essentially non-aqueous process, including the use of liquid carbon dioxide. This has been discussed for some steps18, but industrial development has not yet followed. Wei demonstrated the possibility of processing water- wet pelt in a tumbling medium of water-immiscible solvent19
. The technology is feasible, and the difficulties it presents are solvable, if the willingness is there.
The range of chemistries available to the tanner is widening. By considering the molecular basis of the tanning mechanism, especially those requirements to confer high hydrothermal stability, the options open to the tanner are widened. If high stability organic tannages are desired, they can be created by observing the following rules.
- Apply the first reagent, linking to the collagen with high stability bonding, preferably covalently. The reagent must offer the potential for a second reaction, by possessing usefully reactive groups, but need not have high molecular weight.
- Apply the second reagent, to react with the first reagent by locking the molecules together; therefore the second reagent must be multi-functional. Contributing to linking the matrix to the collagen is useful.
The properties of the resulting leather can be controlled by the choices of reagents: this applies to both mineral and organic tanning options. In the absence of a polymerising tannage capable of meeting the stabilising matrix criteria, we are limited to the two-step process. But the two-step process does not have to extend processing times: this approach can contribute to compact processing, moreover it can offer specifically required properties and therefore offers the basis of the production of bio-vulnerable or so-called recyclable leathers.
The latter category has not been adequately explored. The definition of tanning refers to the resistance to biodegradation of a previously putrescible protein material: here the resistance refers to proteolytic attack. Therefore, it is feasible to consider tanning processes which incorporate a degree of vulnerability. In this way, high hydrothermally stable leather could be chemically or biochemically destabilised, to allow denaturation of the collagen at moderate temperatures, so that proteolytic degradation can be achieved. Targeting specific groups in the matrix may be sufficient to degrade its effectiveness: options are hydrolases, oxidases, reductases etc, depending on the chemistry of the tannage.
The continuation of developing tanning and leather technology depends on constant reappraisal of all aspects of the subject. This is the role of leather science. Conventional, received wisdom should not be relied upon without critically reviewing exactly what it means, what it contributes to processing and products and what the wider implications are for the practical tanner. It is important to recognise that the scrutiny of current technology will often identify inconsistencies and misunderstanding of principles: the technology may work, but the science may not. However, this is not always a bad thing, because it can lead to new thinking, new developments and more profitability in an environmentally sound, sustainable industry.
The author acknowledges and thanks The University of Northampton, the British School of Leather Technology, his academic and industrial colleagues and his research students.
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3. R A Hancock, S T Orszulik, R L Sykes (1980). Tannage with aluminium salts. Part 2. Chemical basis of the reactions with polyphenols. J. Soc. Leather Technol. Chem, 64(2), 32.
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5. J M Morera, E Bartoli, M D Borras, A Marsal, A (1996). Vegetable-zinc combination tannage on lambskin. J. Soc. Leather Technol. Chem, 80(4), 120.
6. L Song 2003). PhD Thesis, The University of Northampton.
7. A D Covington, C S Evans, O Suparno (2007). Novel combination tanning using diphenols and oxazolidine for high stability leather. J. Soc. Leather Technol. Chem, in press.
8. A D Covington, M Song (1997). New high stability synthetic organic tannages. Proceedings, IULTCS Congress, London, September 1997.
9. R Sugiyama, Y Chonan, H Okamura (1997). Effect of pre-treatment of hide powder by bifunctional dextrin compound on chrome fixation. Hikaku Kagaku, 43(1), 55.
10. M Komanovsky (1989). The Maillard reaction - its possible influence on the physical properties of leather. J. Amer. Leather Chem. Assoc, 84(12), 369.
11. T J Painter (1991). Review Paper: Lindow man, Tollund man and other peat bog bodies: the preservative and anti-microbial action of sphagnan, a reactive glycuronoglycan with tanning and sequestering properties. Carbohydrate Polymers, 15, 123.
12. T J Koob, J Hernandez (2003). Mechanical and thermal properties of novel polymerized NDGA - gelatine hydrogels. Biomaterials, 24, 1285.
13. K Ding, M M Taylor, E M Brown (2006). Effect of genepin on the thermal stability of hide powder. J. Amer. Leather Chem. Assoc, 101(10), 30.
14. K Konno, C Hirayama, H Yasui, M Nakamura (1999). Enzymatic activation of oleuropein: A protein crosslinker used as a chemical defense in the privet tree. Proc. Natl. Acad. Sci. USA, 96, 9159.
15. A P M Antunes, G E Attenburrow, A D Covington, J Ding (2007). Utilisation of oleuropein as a crosslinking agent in collagenic films. J. Soc. Leather Technol, Chem, submitted for publication.
16. A D Covington, C S Evans, T H Lilley, O Suparno (2005). Collagen and polyphenols: new relationships and new outcomes. Part 2. Phenolic reactions. J. Amer. Leather Chem. Assoc, 100(9), 336.
17. C Gaidau, M Ionescu, M Crudu, L Miu, M Giurginca, A Meghea (2006). New dyeing, fatliquoring and retanning compact material for ecological leather processing. Proceedings, IULTCS Conference, Istanbul, Türkiye.
18. G Gavend (1995). Leather and supercritical fluids, Industrie du Cuir, Aug/Sept.
19. Q-Y Wei (1987). Dry tannage in solvent medium. J. Soc. Leather Technol. Chem, 71(6), 195.
For more information including diagrams and tables, please see the July 2008 edition of Leather International