Chrome Tanning

Chrome Tanning:

When the Leather is tanned by chrome powder then the process is called chrome tanning & material called chrome tanning.

Basic chrome powder:

 The use of chromium (III) salts is currently the commonest method of tanning: perhaps 90% of the world’s output of leather in tanned in this way. Up to the end of the nineteenth century, virtually all leather was made by ‘vegetable’ tanning, i.e. using extracts of plant materials ,. The development of chrome tanning can be traced back to knapp’s treatise on tanning of 1858, in which he described the use of chrome alum: this is referred to as the single bath process, because the steps of infusion and fixing of the chromium (III) species are conducted as consecutive procedures in the same vessel. It is usually accepted that chrome tanning started commercially in 1884, with the new process patented by Sshultz: this was the ‘two bath’ process, in which chromic acid was the chemical infused through the hides or skins, conducted in one bath, the pelt was then removed to allow equilibration (but no fixation), then the chrome was simultaneously reduced and fixed in the second bath.

Brief Review of the development of chrome tanning:

The development of modern chrome tanning went through three distinct phases:

I)       Single bath process: The original process used chrome alum, Cr₂(SO₄)₃. K₂SO₄.24H₂O. applied as the acidic salt, typically giving pH ~ 2 in solution. Following penetration at that pH, when the collagen is unreactive, the system is basified to pH ~ 4, with alkalis such as sodium hydroxide of sodium carbonate to fix the chrome to the collagen.

II)    Two bath process: The first commercial application was an alternative approach to the single bath process: it was recognized that a more astringent, and consequently more efficient, tannage could be achieved if the technology of making chromium (III) tanning salts was conducted in situ. This means that the process was conducted in two steps. The pelt is saturated by chromic acid in the first bath, then it is removed, usually to stand overnight. At this time there is no reaction, because Cr(vi) salts do not complex with protein. Next, the pelt is immersed in a second bath containing a solution of reducing agent and enough alkali to ensure the final pH reaches at least 4. At the same time, processes were also devised that combined both valencies of chromium, exemplified by the Och’ process. However, the dangers of using chromium (v) drove change back to the single bath process. Not least of these consideration was the incidence of damage workers by chromium (vi) compounds:  the highly oxidizing nature of reagents typically caused unceration to the nasal septum. Notices warning of the dangers of chromium (vi) to health can still be found in UK tanneries, even though these compounds have long since ceased to be used.

III) Single bath process: With the development of masking (see below) to modify the reactivity of the chromium (III) salt and hence its reactivity in tanning, the global industry universally reverted to versions of the single bath process.

There have been attempts to reintroduce versions of the two bath process. Notably, the Gf process was developed towards the end of the twentieth century in Copenhagen. However, European tanners were reluctant to handle Chromium (vi), regardless of the advised measures to ensure complete reduction. For this reason, the method was never generally adopted.

Vegetable tanning:

When the leather is tanned by various types of natural organic material which obtained from the plant kingdom then the process is known as vegetable tanning and the materials are called vegetable tanning.

Some important vegetable tanning materials are Minosa, Oak, Hemlock, Scnec, Iaebracho, Mangrone, Myrobalar etc.

 Source:

The vegetable tanning materials one obtained from different ports of plant which is given below.

 Practical vegetable tanning

The history of vegetable tanning concerns the development of both the tanning agents employed and the equipment used for the process. In the former, changes involved the nature of the agent, gradually changing from the use of the plant material itself, to the availability of extracts. In the latter, pits have been the preferred vessel, but the way in which they were used changed. His-trically., tanning was conducted in pits in a process referred to as ‘layering’. A layer of the appropriate plant material. E.g. oak bark, was placed in the bottom of the pit, followed by a layer of hides or skins. Another layer of plant material was placed on the hides, followed by another layer of plant material: the alternate layering continued until the pit was full . Then the pit was filled with water.

The water leached the polyphenols out of the plant material and the dilute tannin diffused into the hides, converting them into leather. The dilute nature of the solution limits the reactivity of the tannins, allowing it to penetrate through the pelt cross section. The reaction is slow, in part due to the static nature of the process, so an early from of quality assurance applied more than half a

Millennium ago in England was the requirement for hides to stay in the pit for a year and a day.

Definition(Cobination tanning): 

The tanning process which is carried out by the use of two or more chemically different tanning agent in the some tanning process is know as combination tanning.

 Combination tannage:

When the leather is produced pure chrome tanning or pure vegetable tanning method. This leather can not show desirable properties to or user. So for providing maximum number of desirable properties of a leather for a particular uses the combination method should be followed.

When the leather is tanned combindly by both of chrome & vegetable tanning the process is known as combination tannage.

For example most of the chrome appear leathers are retanning by vegetable tannins. These leathers are known as chrome retanned leather.

FUTURE OF CHROME TANNING

The environmental impact of chromium (III) is low: as a reagent, basic chromium salts are safe to use industrially and can be managed efficiently, to the extent that discharges from tanneries can routinely be as low as a few parts per million. Therefore, clearly, the industry should be able to meet all the future requirements of environmental impact. Consequently, we can reasonably assume that the future of tanning will include a major role for chrome. Nevertheless, the technology can be improved: efficiency of use can be improved, as can the outcome of the reaction, in terms of the performance of the leather (shrinkage temperature) and the effectiveness of the reaciion (shrinkage temperature rise per unit bound chrome).

The role of masking in chrome tanning is an important feature, which can be technologically exploited. The use of specific masking agents, which gradually increase the astringeney of the chrome species, by increasing their tendency to transfer from solution to substrate, is an aspect of the reaction that has not received scientific attention. Here, the requirement is to match the rate of diminishing concentration of chrome in solution with the rate of masking complexation, to maintain or increase the rate of chrome uptake. This type of reaction is already technologically exploited by the use of disodium phthalate. although the degree of hydrophobicity conferred by even a very low masking ratio can cause an undesirably fast surface reaction. Other hydrophobic masking agents could be developed, to give a more controllable increase in astringency.

It is already recognised that the chrome tanning reaction is controlled by the effect of pH on the reacthity of the collagen substrate: a second-order effect is the increasing of the hydrophobicity of the chrome species by-polymerisation. In addition, the rate of reaction is controlled by temperature. However, the efficiency of the process is limited by the role of the solvent, in retaining the reactant in solution by solvation: this is typically only countered by applying extreme conditions of pH, which is likely to create problems of surface fixation, causing staining and resistance to dyeing 1.

The role of the counterion in chrome tanning offers potential for change. The chrome tanning reaction is controlled by the particular counterion present. It was fortunate for the leather industry thai chrome alum [potassium chromium (III) sulfale hjdrate] was the most readily available salt for the original trials of tanning ability: sulfate ion is highly effective in creating a stable supnimolecular matrix, because it- is a structure maker in water. The effect is very different if other salts are used. e.g. chloride or perchlorate, when the outcome is only moderate hydrothermal stability. However, even if these salts are used, the high hydrothermal stability can be acquired by treating the leather with another counterion. Since the effect is independent of a complexing reaction, the process of modifying the moderate tanning effect is fast. This opens up the chrome tanning reaction to modifications that exploit the separation of the link and lock reactions;

•  The environmentally damaging sulfate ion might be replaced by other less damaging counterions. such as nitrate.

•  Reactive counterions can then be applied: options include using the stoichiometric quantity of sulfate or organic anions.

•  The counterion might be replaced  with  polymeric agents,  including polyacrylates with the right steric properties.

OTHER MINERAL TANNING OPTIONS:

It has been suggested’ that the environmental impact of chromium can be alleviated by substituting all or part of the offer by other metal tanning salts; 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. All metals salts are mixable in all proportions in this context. However, the following general truism should he noted:

The dumiiging effet-i of any ( allegedj pollutant is nor eliminated and is barely significantly miligaied by reducing rhe degree to wich it is used..

The effects of substituting other metals into the chrome tanning process or even completely substituting it have been discussed, and are briefly reviewed below. There are circumstances when other mineral tanning systems might be appropriate, but it should be clear that there is no viable alternative for general applications and particular]) for those applications that require high hydrothermal stability-

 NON-CHROME TANNING FOR ‘CHROME FREE’ LEATHER:

In the current climate of emphasis on ecologically friendly processing and products, attention has been directed to the environmental impact of chromium (III) in tanning and leather. Therefore, this situation has fuelled the movement to produce so-called ‘natural’ leathers, perceived to be and marketed as ‘chemical free’ or using other appellations that are designed to indicate their ecological credentials. The perception of the abuse of resources in leather making has been exploited in the marketing of leather that is not mineral tanned. Hence, it is typically assumed that:

chrome free = organic.

Consequently, leathers and leather articles are offered to the market as non-chrome. The use of the term ‘chrome free1 leather implies that there is a problem underlying the inclusion of chromium(iii) salt in the production of leather. This concept has its origins in the (alleged) environmental impact of chromium (III) waste streams, liquid and solid, and the mobilisation and viability of chromium (III) ions from tanned leather waste. However, latterly the accusation that some chrome leathers are contaminated with chromium(vi) or that Cr(vi) can be generated during the use of the leather article has heightened the notion that chrome tanned leather may be undesirable.

The decision to produce leather by means other than chrome tanning depends on elements of technology and elements of economics: some of these aspects will clearly overlap. In each case, there are points for change and points against. If production methods are changed at the tanning stage, it must be accepted that every subsequent process step will be altered; some will benefit, others will not and additional technological problems will have to be overcome.

Technological aspects:

•  environmental impact of waste streams:

•  ‘just in time’ quick response, wet white technologies;

•  colour, may be counterproductive if vegetable tannins are used;

•  changed properties, e.g. HHB (hydrophobic-hydrophilic balance). IER (isoelectric point);

•  process timings may or may not benefit;

•  resource management will change, e.g. the continued programme of conventional retanning, dyeing then fatliquoring.

Economic aspects:              

•  waste  treatment  and  cost of treatments,  adding  value  and  use of byproducts;

•  recyclabilily’ of leather, which traditionally means compostability, i.e. returning the leather into the environment without adverse impact -although this must involve at least partial denaturation of the leather, either by pH or heat treatment or both.

•  thinking beyond ‘cradle 10 grave”, extending to ‘cradle to cradle’;

•  marketing of’natural’ leather:

•  public perception of leaiher making – the use of ‘chemicals’, renewable reagents:

•  eco-labelling, as a marketing tool for developed economies or just access for developing economies to developed markets.

It is important to retain a clear idea of what is required in the alternative leather. Typically what is required is ‘the same, but different’. Here the obvious difference is the absence of chromium (III) salt, but which elements or aspect? of chrome leather character performance, properties should or need to be retained is less simple. Here, the tanner needs to distinguish between the specific properties required in the leather and the more general property of ‘mineral character”: the former may be relatively easy to reproduce, at least with regard to individual properties performance, but the latter is more difficult to reproduce, since mineral character usually means chrome tanned character.

Clearly, no other tanning system will be able to reproduce all the features of chrome leather. Therefore, it is unlikely that a generic leather making process with the versatility of chromium (III) will be developed – any new leather making process must be tailored so that the resulting leather can meet the needs of the required application, to be ‘fit for purpose’.

SINGLE TANNING OPTIONS:

•  Other metals:

For example. Al(III). Ti(Iv), Zrd(Iv). possibly Fe(III), lanthanides(III). In each case, the leather is more cationic, creating problems with anionic reagents: the leather can be collapsed or over-filled depending on the metal. The HHB of leaiher depends on the bonding between the metal salt and the collagen. In all cases Ts is lower than Cr(ii).

•  Other inorganic reagents:

This category includes complex reagents, e.g. sodium aluminosilicate. silicates, colloidal silica, polyphosphates, sulfur, complex anions such as tungsiates. phosphotungstates. molybdates. phosphomolybdates and uranvl salts, i.e. those that have application in histology.

•  Aldehydic agents:

This group includes elutaraldeh;de and derivatives, polysaccharidc (starch, dextrin, alginate. ere.) derivatives and analogous derivatives of. for example, hyaluronic acid, oxazolidme and phosphonium salts. These reagents are not equivalent: they react in different polymerised states and their affinities are different, even though the chemistry of their reactivities is the same, similar or analogous. Jn each case, the leather is plumped up and made hydrophilic. The 1EP is raised. The colour of the leather depends on the tanning agent – glutaraldehyde and derivatives confer

Colour, others do not, Oxazolidine and phosphonium salts may produce formaldehyde in the leather or there is perception based on odour (Which may be the original reagent itself.)

Ploant polyphenols, vegetable tannis- hydrolysable, condensed:

Reactivity is a problem for a drum process, so the reaction may require syntan assistance. Filling effects and high hydrophilicity characterise these leathers. Colour saddening and light fastness may be a problem, particularly with condensed tannins. Migration of tannin can occur when the leather is wet. Ts is only moderate, not more than 80 C for hydrolysable, not more than 85 C for condensed.

Syntans retans or replacement syntans:

As for vegetable tannnis, although reactivity is more controllable by the choice of structure. Light fastness similarly controllable.

Polymers and resins:

This category includes: melamine-formaldehyde, acrylates, styrene maleric anhydride, urethanes, etc.

Miscellaneous processes, e.g. oil tanning. in situ polymerisation, etc.

Tanning Combinations:

Clearly, for the brief account of the individual tanning options set out above the number of available combinations is very large. Hence, it is important understand the principles of combination tanning. Analysis can commence with the effects of the components on the shrinkage temperature, as an indicator of the combined chemistries:

The components may react individually, i.e. they do not interact, primarily because they react with collagen via diffrent mechanisms and there is no chemical affinity between the reagents.

Here the contributions to the overall shrinkage temperature can be treated additively. Also, the properties of the leather will be a combination of the two tannages, although the first tannage applied will tend to dominate the leather character.

 The components may interact antagonistically, i.e. they interfere with each other. This may be due to competition for reaction sites or one component simply blocks the availlability of reaction sites for the other component. It can be assumed that the two components wither do not interact or that one component will react with the other in preference to reacting with collagen. Under these circumstances, the interaction does not constitute the creation of a useful new tanning species, i.e. the reaction of the second component does not involve crosslinking the first component. The outcome of the combined reaction is likely to be domination of the properties by the reagent that is in excess or has greater affinity for collagen.

Table 3: Indicative/possible interactions in combination tanning.

Here, the hydrothermal stability of the combination will be less than the anticipated sum of the effects of the individual tanning reactions. The effect of destabilising the leather is likely to be undesirable.

The components may react synergistically, i.e. they interact to create a new species, which adds more than expected to the overall hydrothermal stability. Since a new tanning chemical species is created, the properties of the leather are likely to reflect the sum of the individual tanning effects. but to differ significantly from a simple additive outcome.

The additional elevation of Ts may constitute a high stability process. as rationalised in the link-lock theory.

In making choices of combinations, it is important to consider how the reagents combine to create the leather properties other than hydrothermal stability: these could include handle ( softness, stiffness, fullness, etc.) hydrophilic-hydrophobic balance, strength ( tensile, tear, extensibility, etc.) and so on. If the components react individually, it might be expected that the properties of each would be conserved in the leather, dependent on the relative amounts fixed. If the components react antagonistically, the overall properties will be determined by the dominant reactant, although they may be modified by the presence of the second component, unless the antagonism means the second reagent is not fixed. Synergistic reaction means the tanning reaction is changed, so it can be assumed that the outcome will not be the same as the sum of the individual properties, although it can also be assumed that the leather will still reflect the individual properties.

The concept of the link-lock mechanism is useful in this context, because it offers a model of the tanning reaction that can be visualised and hence used in the analysis of reaction and outcome. The matrix model is easier to use than the previous model of  specific  interaction with the collagen sidechains.

All meials are mixable in all proportions for tanning and all metals react primarily at carboxvl groups, tjpically electrostatically. It is unlikely that there will be any positive impact on the tanning reaction from mixed speciation. The filling collapsing effects can be controlled by mixing the metal offers. There is no benefit with respect to Ts or cationic character. Any combination in which the components rely on hydrogen bonding for fixation is likely to be antagonistic. Hydrolysable  vegetable tannins and metals: This is established technology.

The product is full, hvdrophilic leather. Covalent fixation of metal by polyphenol complexation modifies the properties of the metal ions, but there is still cationic character.

Condensed tannin with aldehydic reagent: Prodelphinidin and profiseti-nidin tannins or non-tans are preferred. Not all aldehydic reagents work: oxazolidine is preferred. The result can be high TV Syntan with aldehydic reagent: The- industry standard is to use unspecified syntan with glutaraldehyde. There is apparently no attempt to define the syntan in terms of reactivity to create synergy; in general, reliance is probably placed on simple additive effects, so ihe rationale is not easy to discern.

The technology could be improved by analogy with the condensed tannins, by applying the structure-reactivity criteria known in that context. Polymer with aldehydic reagent: Melamine-formaldehyde polymer with phosphonium salt can be a synergistic combination, depending on the structure of the resin, especially the particle size.

There will be other polymeric reagents capable of creating synergistic tannages. Reliance on syntans and, or resins may carry ihe additionai problem of formaldehyde in the leather, which is subject to limits in specifications. It is possible to avoid this problem by scavenging the formaldehyde by the presence of condensed tanning, preferably prodelphinidin or profisetinidin.’

ORGANIC TANNING OPTIONS:    

Polyphenol Chemistry

The best known example of plant polyphenol exploitation for high hydro-thermal stability tanning is the semi-metal reaction. Here, the requirement is for

polyphenol and the locking metal ion: in practice, this means using the hydrolysable tannins, but alternatively some condensed tannins can be used  prodelphinidins (e.g. Myrica esculents, pecan and green tea) and prorobinetinidins (e.g. mimosa) each have the required structure in the B-ring. Many metal salts are capable of reacting in this way. so there may be useful applications for the future.

The condensed tannins can confer high hydrothermal stability by acting as the linking agent, with aldehydic crosslinker acting as the locking agent; other covaleni crosslinkers may also have application in this context. The reaction applies to all flavonoid polyphenols, when reaction always occurs at the A ring. In the case of the prodelphinidins and profisetinidins, additional reaction can take place at the B-ring. The effect is to increase the ease of attaining high hydro-thermal stability. It is useful to recall that the combination tannage does not rely-on conventional vegetable tannins, because it can work with the low molecular weight non-tans: here, there is advantage in terms of reducing or eliminating problems of achieving penetration by the primary tanning component.The link-lock mechanism can be exploited in other ways, even using reagents that at first do not appear to be tanning agents e.g naphthalene diols

Table-4:

Shrinkage temperatures (⁰C) hide powder treatede with dihy-droxynaphthols (DHNs) and oxazolidine.

The behaviour of naphthalene diols in the tanning process is highly dependent on the structure of the isomer. Clearly, the presence of a hvdroxyl in the 2-poshion 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 hydroxjls 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 electrophllic 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 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-dihydroxynaphlhalene produced the highest rise in shrinkage temperature for the range of this type of linking agents tested (by 26 °C, to 85 °C). Clearly, the locking reaction is more easily and effectively accomplished by applying a second reagent, rather than relying on direct reactions between linking molecules.

Polymer and Crosslinker:

Perhaps surprisingly, it is less easy to create high stability tannage with polymers than it is using oligomers or monomers, depending on the polymeric compound. High hydrothermal stability can be achieved using melamine resin crosslinked with tetrakis (hydroxymethyl) phosphonium salt. Several conclusions were drawn from these studies:

•  Not all melamine linking resins work. Therefore, the requirements for matrix formation are likely to be more important than possessing specific chemical reactivity.

•  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 1290C, thereby matching the maximum shrinkage temperature achieved by chromium in the presence of pyr-omellitate (1.2,4,5-tetracar boxy benzene). 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, whilst 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 is capable of forming a synergistic matrix with the polyphenol, but the reaction causes the shrinkage temperature to drop significantly (A.D. Covington, unpublished results). The clear inference is that the new reaction is antagonistic to the established matrix: by analogy with other antagonistic combination,” there is competition with the mechanism that binds the first matrix to the collagen, effectively loosening the binding belween the matrix and the collagen.

allowing shrinking to occur, with ihe consequence that the hydrothermal stability is lowered. It is difficult to rationalise the observation by any mechanism other than the formation of matrices.

COMPACT TANNING:

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. lime 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 relanning. 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 case of chromium (III) tanning is less clear. No technologies have been offered to the industry in which a retanning agent has been combined with conventional chrome tanning salt. The closest to that situation are products referred to ;is ‘chrome symans’. which are well known to the industry. However, these are primarily offered as filling versions of chrome relanning. rather than a new approach to prime tanning. Because chromium (III) has little affinily for phenolic hydroxide as a ligand. chrome synians are either mixtures of syntan and chrome salt or the syntan may be capable of complexation with chrome, by having some carboxvl functionality in its structure. In ihe latter case, the function of retanning might be accomplished with tanning. Similarly, an extended application of masking, to confer additional features, may be useful.

The application of compact processing to post tanning is discussed in Chapter 15. Clearly, there is potential for further development, using creative chemistry

ALTERNATIVE TECHNOLOGIES

It is conventionally assumed that tanning has to be conducted in aqueous solution. However, there are options available for the future. Wei has demonstrated the possibility of processing waler wet pelt in a tumbling medium of water-immiscible solvent. The technology is feasible, but brings problems of its own, notably the requirement for diffusion across the pelt, unlike the conventional requirement for diffusion through the cross section. However, such difficulties are solvable, if the willingness is there. The alternative is an essentially non-aqueous process, as indicated in the context of compact processing: this approach to processing, including the use of liquid carbon dioxide, has been discussed for some steps, but industrial development has not yet followed. The advantage of such an approach to one-step post tanning is that the substrate is dry (at least to the touch), so the moisture in the leather would have a limited influence on the process. It is less clear that such an approach would be feasible for primary tanning, in whatever way that might be done chemically. If chrome tanning were to be developed in that direction, it is not difficult to imagine the HHB properties of the chrome complex being tailor-made for the solvent by appropriate masking.

It is useful to speculate on the roles that enzymes might play in tanning. Clearly, biotechnology has an increasingly important part to play in the beamhouse, but it is less clear if it can contribute to collagen stabilisation. From the matrix theory of tanning and collagen stabilisation, the inability of transglutaminase to increase the hydrothermal stability is predictable and understandable, even though it is clearly capable of introducing crosslinking in the conventional sense. Similar reactions, which can introduce crosslinks into collagen, are unlikely to function as useful tanning agents, beyond altering the texture of the protein.

At the other end of the processing procedures, the role of drying remains to be exploited to advantage by industry . Recent studies have linked the properties of leather, including area yield, to the programme of drying conditions: “softness does not depend on the rate of drying, only the moisture content. Using conventional plant, it is possible to modify the drying programme into two stages, to obtain advantage from the relationship between the viscoelastic properties of leather and its water content, so that area gain of intact leather does not have to be at the expense of softness.

In considering alternative technologies, it is useful to examine whether there is an alternative mechanism to link-lock, i.e. whether there could be exceptions. If the mechanism of shrinking, as set out here, is right, then the impact of the tanning reaction on hydrothermal shrinking is right. Consequently, the use of conventional, penetrating reactants, in the way also outlined above, is also right. Therefore, the only exception to the link-lock mechanism would be a single step reaction that combines both features. The requirement would be the formation of the stabilising matrix from a single species polymerisation reaction, which would have to include reaction with the collagen, in this regard, polymerisation of cod oil is the closest reaction known in the art. However, in this case, despite the complexity of the chemistry of polymerisation by oxidation, there appears to be little direct interaction between the polymer and the collagen, despite the possibility of creating aldehyde groups. Hence, the leathering reaction does not elevate the shrinkage temperature significantly. Examples of chemistries apparently capable of multiple bonding with collagen do not react in that manner, because ihere is no evidence of an outcome that might be expected from such tanning reactions . Nevertheless, it is no! inconceivable that such a polymerising tannage might be developed, particularly if an extended range of practically useful solvents is made available to the industry.

OVERVIEW

The range of chemistries available to the tanner is widening. By considering ihe molecular basis of ihe 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 just by observing the following rules:

a.  Apply the first reagent, linking to the collagen wilh high stability bonding, preferably covalemly. The reagent must offer the potential for a second reaction, by possessing usefully reactive groups, but need not have high molecular weight.

b.  Apply the second reagent, to react with the first reagent by locking the molecules together: therefore, the second reagent must be multifunctional. 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 that incorporate a degree of vulnerability. In this way. high hydro thermally 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 ihe chemistry of the tannage. Alternately, tannages can be devised in which the stability is temporary, such as semi-metal chemistry that is vulnerable to redox effects . Other approaches are possible and should not be beyond the wit of the leather scientist.

CONCLUSIONS:

The expression of the link-lock mechanism has made it possible to take quantum steps forward in developments in tanning technology. This new

theory is a simpler and more powerful view of collagen stabilisation than the older model of direct crosslinking between adjacent sidechains; it is a more elegant view. It is no longer worth pursuing the single reagent alternative to chrome tanning, because it does not exist. Indeed, why should we seek an alternative to chrome tanning? It works well, it can be made to work even better and, anyway, by all reasonable judgements it causes little environmental impact. Nevertheless, organic options provide potential for new products from the leather industry. Now we can develop those options using sound logic, based on sound theory. The literature is littered with examples of multi-component tanning systems: the outcome of each is entirely predictable if we apply our new understanding of the theoretical basis of tanning.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 recognize that the scrutiny of current technology will often identify inconsistencies and misunderstanding of principles: (he 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.