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DYEING PROTEIN SUBSTRATES (LEATHER)


 1.  Summary

 2.  The dyer's objectives

 3.  Essential electro-chemistry: "Cation & Anion"

 4.  Iso-electric point of collagen

 5.  The dye molecule

 6.  The solution behavior of dyestuffs

 7.  The dyebath

 8.  The presence of salts

 9.  The attraction of dye by tanned collagen

10. The mechanism of dyeing

11. Scale of affinity for Dyestuffs

12. Wet migration

13. Complementary effects of dyestuff characteristics

14. Effects of temperature on dye migration

15. Factors in dye selection and level dyeing

16. Control Points for predictable results

17. INFLUENCE OF ANIONIC TREATMENTS (RETANS) ON RESULTING SHADE

18. Objectives for the color matcher

19. Computer assisted color match prediction.

20. Difficulties with different substrates

21. Research objectives as preconditions

22. Limitations of CCMP

23. Advantages of affinity numbers

24. Increasing demands on fastness properties of dyed leather

25. Conclusion

The following is based on experience of dyeing cross-linked collagen, leather in its many forms, but the information has relevance to dyeing keratin (wool), and polyamides and to a lesser extent with cellulosics.

Summary

This paper makes a brief clarification of the electro-chemistry and dyeing mechanisms necessary for control and consistent results in the dyehouse. Nineteen Control Points are listed, observation of which can lead to the data required for operation of a successful computer color match prediction system. The increasing fastness demands by leather consumers have caused tanners to strive for improvements in application techniques. As fastness can so easily deteriorate with any change or development of an existing process, the chemical industry has introduced new dyestuff ranges permitting specifications to be met with greater margins of certainty. These are briefly described.

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The dyer's objectives

To eliminate the random element which occurs in dyeing, by applying an understanding of the mechanism of dye fixation, in order to advance:

- dyehouse process control: actual process parameters and equipment status congruent with parameters specified in the 'Recipe'.

- predictable and consistent results: consistent process conditions produce consistent product quality test results.

- accurate color matching - within agreed tolerances.

- specific Fastness Properties achieved: Fastness to Light, Washing, Perspiration, Migration in PVC, Heat & Damp,

- economic process, with minimised waste, avoiding re-dyeing, correction dyeings and multiple shading additions.

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Essential electro-chemistry

The meaning and significance of two terms must be clarified:

"dissociation" and "undissociation".

It is often overlooked that products with "anionic charge" or "cationic charge" - especially organic chemicals - are frequently not in a charged state.

 

Cation

 (2 Ionic '+' Groups)

Anion

(2 Ionic '-' Groups)

Art-work by Günter Rutschmann

Biel-Benken - Basle, CH.

Cation's open hand able to grasp: dissociated charge: reactive!

Left hand holds a ball - an Anion. undissociated charge -

so ionic reaction not possible

Right hand holds a block - a Cation: undissociated charge -

so ionic reaction not possible

Anion's Open hand is able to grasp: dissociated charge:

 reactive!

In the charged state (open hand) the chemical group concerned can confer water solubility, and enter into ionic reactions to form salt-links. In this condition, it is referred to as dissociated. The ionic charge present can, like electrostatic charge, attract an opposite charge and repel a similar charge. The group, however, is in an equilibrium with its uncharged state; the electron imbalance being satisfied by a covalent bound atom or group. In the case of the usual anionic groups found in dyestuffs and leather (-SO3-, -C00- etc), if excess free H+ are added, the pH falls and the equilibrium swings to the undissociated form (-SO3H ,-C00H). The reverse action (e.g. adding OH- ions to cationic amino groups) causes a rise in pH with the same result: undissociation, and zero net charge.  

Being able to switch charges on and off, by a manipulation of pH is of great use in the wet processing of leather. It is as well to understand that the switch is not changed instantly, but can be controlled as a gradual, slowed-down change from 'ON' to 'OFF' and vice versa. This is of importance in ensuring uniform and "level" dyeings.

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Iso-electric point of collagen

Tanned collagen fiber is a cross-linked fiber which has carboxylic and amino side groups. As a result it is amphoteric. Not all the groups are charged, because, as shown above, they may be undissociated and dissociated according to the prior treatment, and the actual pH of the environment. The Iso-electric point (IEP) is defined as the pH where the number of dissociated active charges balance, i.e. anionic charge density balances the cationic charge density. If the IEP is low, this implies that at a low pH many anionic groups have been "switched off", leaving the few which are still dissociated to balance the existing dissociated cationic charges. A low IEP therefore implies a strongly anionic leather - such as a vegetable/syntan retanned leather. This has an IEP frequently down to pH 3.0. Pure chrome leather is more cationic with an IEP at pH 7.0. The significance of this is that processes prior to tanning, and also during and after tanning may not penetrate the hide completely. According to the charge  (+ or -) these treatments import into the hide structure, and how deep they penetrate, they may leave the fiber with graded zones of IEP (e.g. relatively anionic surface - IEP = pH3.5; very cationic middle layer - IEP = pH7)

As an anion is not attracted by another anionic group, penetration is helped if the leather is uniformly anionic. If, however, it meets a cationic zone, there will be an effective barrier to further penetration. To achieve an anionic condition, it is necessary to adjust the pH of leather to a point above the IEP. To achieve fixation of an anion, acidification below the IEP is necessary. The contrary is the case for cations. It can be seen that this concept is important, not only for dyeing but also for fatliquoring.

In dyeing the importance of achieving a uniform IEP throughout the cross-section is related to consistent and level dyed shades when dyeing with dyestuff mixtures. This raises the issue of "Affinity" between the dye and the fibre. Because different colour dyes have different molecular shapes and sizes as well as charge density, they also therefore have different affinity for the substrate. Leather differs from other substrates in a way which makes its uniform treatment more difficult: its 3-dimenional structure. Thus higher affinity dyes in a shaded mixture will react and fix with lower affinity outer surfaces of a retanned leather; conversely, lower affinity dyes in that shaded mixture avoid reacting with the low affinity outer surfaces of the leather, but they do react and fix on meeting the middle layers which are more cationic (higher IEP). In practice dyestuff manufacturers try to screen dyestuff components marketed as a dye 'range' to behave in a similar way towards substrates of specified IEP, to try and minimise this separating of coloured components during the dyeing process. In practice, the best that has been achieved are ranges with dye affinity varying within a limited range - avoiding combinations of very high affinity dyes with those of low affinity.

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The dye molecule

Why do dyes have color?

Organic molecules have the property of absorbing radiation energy. When molecules contain chromophore groups, the wavelength of radiation which they absorb falls within the visible spectrum. The reflected light can no longer contain these wavelengths, so no longer appears white - but has color. Common chromophore groups are:

Azo  include the -N=N- group; 50% of all dyes are 'azo-dyes'.  90% of leather dyes fall in this group. Such dyes are increasingly under scrutiny by Governmental Health & Safety Authorities since many have tested positive as potential carcinogens. But the color range possible from azo dyes covers the entire visible spectrum, with the exception of the brightest greens, blues and violets, so they continue to be in great demand. As mentioned, the size and shape of the molecule can vary: small, compact molecules are mobile and form salt-linkages; these are represented by traditional penetrating acid dyes (anionic) and basic dyes (cationic). Large, long or planar molcules are slower, fixed by short range weak but numerous molecular forces (H bonds, van der Waals' forces, etc.); these are represented by direct dyes (anionic, neutral exhausting and surface dyeing).

Other categories of chromophores which are responsible for absorbing light energy, and conferring colour on dye molecules include 'Triarylmethane', 'Heteropolycyclic' and 'Anthraquinone'

The dyeing of protein substrates as textiles (wool) presents less of a challenge, since the initial fixation of molecules by salt-linkages can be re-arranged by promoting migration of the dyes within and throughout the substrate at high temperatures (at the boil). This is a strategy tanners cannot use, in view of the low shrinkage temperature of most leathers (even if some do stand boiling, the fibre loses strength which reflects in the final durability of the leather.) As a result, tannery dyers have to adopt strategies to ensure that the dyeing process is slow and gradual, and permits the mix of dye components enough time to properly distribute throughout the substrate.

Therefore the ideal anionic dyes which have been specially researched for the leather industry are intermediate in size and have properties between acid and direct dyes. On medium affinity leather (IEP = 4.5) they exhaust at pH 4.0-4.3 and so need less acid to achieve the gradual transfer from solution to leather. The process is therefore easier to control. Many such dyes are 'premetallized' with heavy metal atoms complexed at the heart of the molecule, to improve lightfastness, levelness, etc. Here again, the specific metal atom used is under scrutiny by the health and safety authorities - dye companies are always under pressure to ensure the safety of the consumer.

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The solution behavior of dyestuffs

Due to the traditional drying of dyes, the grinding, powdering and standardizing with salts, most suppliers recommend boiling to ensure complete solution and separation of dye aggregates into individual molecules. Ideally, soft water should be used. The benefits of correct dye preparation are:

    avoid wastage of undissolved dye

    gain color yield (the single molecule has almost the same colour yield as an aggregate of many molecules)

    gain more brilliance (aggregates absorb additional wavelengths)

    improve fastnesses: bleeding, crock and migration into PVC

       (aggregates have loosely bound molecules, more able to rub off and migrate in perspiration and in the solvent-like plasticisers in PVC).

   predictable affinity for the leather (aggregates have variable affinity, depending on the number of molecules).

To overcome some of the practical problems, dyes are lately being marketed in liquid form - either as a dispersed SLURRY of fine particles or as a true solution. The slurry should be treated as a powder and dissolved with boiling water. The solutions are more practical for ease of application - needing only hot water, ca. 140°F.

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The dyebath

The dye mechanisms are influenced by

Temperature

at high temperature (~ 60°C):

greater solubility, better de-aggregation,

faster rates of reaction.

Note: the actual effect is very dependent on the dyestuff architecture (the chromophore, if premetallised, solubilising groups).

 

 

pH this affects the dissociation of the charged groups on the dye and on the leather.

As explained, an anionic dye in acid pH will tend to "switch off" its charge. If in the presence of a cationic group in the process of being "switched on" it will tend to form a salt-link. In the case of direct (anionic) dyes, the molecules are so large that salt-linkage is relatively unimportant, fixation taking place between pH 4-7.

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The presence of salts

The dissociation of the anionic and cationic groups can be affected by all anions and cations depending on molar concentration. The equilibrium is similarly displaced and the charge on the group is de-activated. This accounts for such effects as salting-out and electrolyte sensitivity (see Appendix 1).

In the process of reducing solubility, the molecules of dye are forced to the only medium where compatible material exists: other dye molecules. The resulting micelle formation increases the aggregation which causes associated problems, discussed under the dissolving of dyes.

 To avoid such problems it is necessary to:

      select the dyestuff carefully

      use consistent quality of soft water

      wash the leather, especially after neutralization

      drain the drums adequately; inefficient draining can leave up to 30% of the previous float in the drum.

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The attraction of dye by tanned collagen

Collagen's polymer chains of protein form a twisted rope-like structure with the core, triple helices internally and externally crosslinked. Tanning inserts additional crosslinks which help stabilize the structure against heat and bacteria. The monomeric units are amino acids with various side chains, both anionic (carboxylic) and cationic (amino).

Ionic species can form salt-links with the available dissociated charges and enter into strong fixation with the polarity of the peptide links (-CONH-) between the amino acids. Because these H bonds and van der Waals' forces are weak and short range, a molecule must be large to interact and fix with them.

The significance of the long range ionic forces in starting this process has been discussed. The IEP of the leather is critical for determining the ideal pH of penetration and fixation.

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The mechanism of dyeing

The anionic dye molecule is brought together with the substrate at a pH above the IEP and drummed for 20-40 minutes, so that penetration can occur. As the charge on the dye is anionic and the leather's cationic groups are undissociated, "switched off", the dye has no or little affinity or attraction to the substrate.

Cautious (slow, with rapid dispersal) addition of acid causes the pH to fall below the IEP during which time the increasing salt-linkage causes uptake onto the fibre and 'exhaustion'. The dye is not yet fixed on the fiber - a continuing wet migration takes place for an extended period whilst the leather is on the horse awaiting drying. This can be an advantage or a disadvantage. It will be considered later.

During this period, the originally salt-linked dye molecules can adjust into progressively tighter fit with the protein, allowing the short-range forces to bind the dyestuff more tightly.

Non-ideal leather dyes have molecules which are either too small to benefit from this secondary binding power, or too large so that these forces predominate and the dyeing proceeds too fast and out of control causing unlevellness and incomplete penetration.

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Scale of affinity for Dyestuffs

In the most comprehensive attempt yet to make dyestuff users aware of these differences and to allow appropriate use of the dye's properties, affinity number lists have been prepared placing the dyes in a hierarchy of attractiveness to two standard substrates: a cationic leather of high affinity and a retanned anionic substrate with low affinity. A scale of affinities 1 to 100 is used.

The user has the possibility of selecting dyes within a narrow range of affinity, which will be suitable for shade combinations, since the tendency of each dye to exhaust is similar to the other components, especially in terms of the amount of color deposited during a critical phase of the dyeing.

The information for two extreme substrates allows the dyer to judge how the dye will behave on his particular substrate. This is a significant advance on the old system of judging a dye's properties of penetration and levelness on an unknown substrate or on collective opinion.

The actual test method entails the interaction of the dye with the leather fiber, eliminating osmotic effects. The dyeing proceeds to recognized color-intensity, known as Standard Depth (the purpose being to eliminate the effects of different concentrations of commercial dyes). The uptake is followed towards the usual end-pH of dyeing - pH 4.0.

During extensive field work, I have found the list to be of immense practical help - designing compatible dyestuff groupings or explaining unsatisfactory dyeings.

Compatibility of dyes, however, is not only a question of equating affinity numbers. The already mentioned wet migration must also be balanced to ensure a positive result.

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Wet migration

As indicated, the dye, after fixation by salt-linkage, undergoes a cycle between being fixed on the fiber, re-entering the solution phase and refixing on the fiber. As a result there is a tendency for the dyestuff to wander - especially if the original site was over-subscribed! When it finds a site without other dye molecules it gradually uses the space available for the subsequent snug-fit of molecular accommodation representing final fixation.

Wet migration properties of established dye ranges have been categorized. Your supplier should be able to help you design dyestuff groupings which not only have similar affinity numbers but also similar tendency to migration. This is important to avoid a matched shade changing color or becoming patchy during horsing up. A yellow dye and a blue dye with similar affinity and migration properties will actually become a more level green as the period of horsing-up proceeds. If the yellow has a higher tendency to migrate, the green shade will develop blue patches as the yellow disappears inside the corium. This is especially noticeable at the pressure points of creases on horsed-up grain leather.

An anomaly apparent where dyes of different affinity have been satisfactorily combined to give fairly level results can be explained by the higher migrating power of the more rapidly and irregularly fixed component. This is however not to be relied upon to level out patchy dyeings resulting from unsuitable dye affinities in the color combination.

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Complementary effects of dyestuff characteristics

Affinity vs. power of migration

Dyestuff

Affinity

 Wet Migration - Property of the Dye

Powerful

Fairly strong

Weak Migration

High affinity

level dyeing

penetrating

level dyeing

less penetrating

unlevel dyeing

surface- penetrating

Medium affinity

level dyeing penetrating

level dyeing

less penetrating

not-so-level dyeing

less penetrating

Low affinity

unlevel dyeing penetrating

not-so-level dyeing

penetrating

fairly level dyeing

penetrating

 

Observed Characteristics of the Dyeing (above)

Effects of temperature on dye migration

In the dyeing of wool, the dye is brought onto the fiber, and then uniformly distributed over the substrate at the boil. Boiling promotes migration from the original site of fixation to undyed or less dyed areas. It is important to remember that temperature has such an influence, not only in older types of vacuum drying (90°C for several minutes), but also during lengthy horsing up.

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Factors in dye selection and level dyeing

Selecting compatible dyes for combination dyeings is a complex matter of balancing the temperature and pH and the variations of two important dyestuff properties: dyestuff affinity for the fiber and the subsequent wet migration behavior. Guidance is appropriately sought from the supplier.

In achieving level dyeings, a commitment from the dyer is essential to implement the suggested controls, before, during and after the actual dyeing operation.

Color matching: The relevant Control Points for predictable results in bulk dyeing

In the discussion so far 12 Control Points have been raised:

1. Washing and complete draining

2. Standard, unchanged pretreatments and prior processes

3. Uniform penetration of tans, retans and auxiliaries influencing IEP

4. Standard age of wet-blue

5. Correct preparation of dye solution

6. Quality of water

7. Temperature of dyeing

8. Time/pH profile

9. Compatible dye components (affinity and migration)

10. Final wash duration and temperature

11. Horsing up period

12. Drying conditions.

The difficulty with point 2. 'Standard, unchanged pretreatments and prior processes' is seen in the illustrations below.

The pre-treatments before dyeing - in this case retannage - has direct and pronounced effect on the shade which results.

Both anionic retans and anionic fatliquors are especially prone. This shows that the pretreatment agent itself interacts with the tanned-collagen, and the combination of pre-treatment molecule (e.g. retan) and tanned-collagen interacts with the dye to produce a variety of shades. The illustrations were made with homogeneous (single) dyes at 1% dyeing on shaved weight. This shows that it is not only the charge on the leather and the effect on the IEP which influences the final shade.

The need to standardize all prior processes should not impose rigidity on tanyard developments, provided clearly distinct batch numbering systems for trials are used. Once adopted it is imperative that the dyehouse is informed of process changes to allow for rematching.

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INFLUENCE OF ANIONIC TREATMENTS (RETANS) ON RESULTING SHADE

  1% 1% 1% 1% 1% 1% 1%  
  DOr 39 DRed239 DRed80 ARed119 ABlue90 DBlue15 ABlue312

Retanning System

No retan - Blank

(Full chrome)

6% Mimosa

6% Quebracho

6% Chestnut

6% Phenolic syntan

 

6% Acrylic syntan

 

4% Acrylic

4% Protein Fill

2% Methylol Urea

Other important points which should be considered as Control Points are:

Constant concentration of dyes and chemicals

Constant moisture content of hygroscopic dyes and chemicals

Constant moisture content of wet-blue and crust leather

Uniformity of pack weight

Accurate measurement of float ratio

Accurate weighing of dyes and chemicals - especially shading elements

Uniform dimensions and rotation speeds of all drums in the dye-yard.

Experience in controlling all the above 7 and the previous 12 potential sources of error will give information on the acceptable limits of process deviation, between which a process parameter can be permitted to change without practical noticeable effects.

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Objectives for the color matcher

1. Rapid results to meet the demands of the client / sales management

2. Small-scale, economic working (not requiring large areas of valuable leather)

3. Correlation factor to correct recipes for bulk working - this requires systematic practical records of the differences in shade when a recipe (e.g. 1% Acid Red 119 ) is run in the sample drum in the lab, and then repeated in the bulk tannery drum. The main difference is the mechanical energy transfer. In the lab probably less than 1,000g is dyed in a 60cm diameter drum rotating at 24 rpm. In the bulk the pack weight may be 800 Kg up to 3,000 Kg, in a drum 3.5m in diameter, rotating at 14rpm. In the bulk dyeing, the dyes get pounded into the thickness of the leather, so there is a colour dilution effect. This has to compensated when scaling up laboratory dyeings for bulk application

4. Feed-back results from bulk to correct correlation factor: bulk practical results need continuous monitoring until the technicians feel that the process is responding in a predictable manner. Only then can they rely on the reliability of the correlation factor.

5. Reduce/eliminate shade adjustments in bulk. Shade adjustments have two significant drawbacks for the tannery:

    a. getting the shade adjusted, so it conforms to the target pattern, prolongs the process an indeterminate amount of time.

        Longer run times affect the grain-looseness, softness and other properties of the leather - so inconsistency of quality results.

    b. the dyes and labour used in getting the shade right have not been budgeted, and probably won't be accounted for in the costing exercise.

The greater the success in implementing the 19 pre-conditions the closer the color matcher comes to meeting his objectives. Errors in the various Control Points lead to shade corrections in bulk, which is expensive, inconvenient and unnecessary. There are dyehouses which only dye by the process of guess, correction, overcorrection, dye into black and start again. Apparently the thrill is in the chase. Unfortunately the experience gained is not systematic and cannot be put to good use in the future developments in color matching, namely:

Computer assisted color match prediction.

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Color match prediction

Since leather became a fashion item and tanners met a requirement to produce color ranges as wide and seasonal as those found in textiles, a need has arisen to match colors very quickly and accurately. Any short cut to meet these requirements is obviously desirable. The provision of such a short cut to color matching in the leather industry would be a very marketable service - or a very acceptable supporting service for the marketing of raw materials, dyes or machines.

Difficulties with different substrates

The draw-back to application of computer color match prediction is the variability of the substrate. Whereas the variety and origins of textile substrate and blends are finite and specifiable, the types of pre-dyeing processes which affect the final leather color are infinitely variable and effectively result in an infinite variety of leather substrates. Even within the same tannery or tannery group, "standardized" pre-dyeing processes must be modified, up-dated and changed according to technological progress, changing economics of processing and changing fashion. Each change effectively negates the data held in shade and recipe libraries. The alternative of maintaining rigidly standardized processes for the benefit of the color matching program is a strong disincentive for technical change imposed upon the tannery production team: a situation clearly at odds with the needs of fashion.

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Research objectives as preconditions

This is the current situation. It does not mean that computer color match prediction has not found a wide application in the leather industry. It does, however, suggest that tanneries must invest in research and preparatory work to evaluate and develop processes:

      The leather must be able to be rapidly classified by analysis, in order to ascribe an affinity classification to each leather

            (resulting from the variety of tannages, retannages and other pre-dyeing processes);

      Broad classifications of leather types (depending on the pre-dyeing processes) may simplify the variety to manageable proportions;

       Leather affinity classification and type of retannage can be linked to predict the interaction with dyestuffs of specified affinity number.

            The interaction has to be studied with respect to color tone, buildup and strength of fixation;

       Develop a pretreatment to the dyeing process to effectively obscure the effects of all pre-dyeing processes.

            The goal is to achieve a substrates with a known affinity class, and hence predictable reactivity towards dyes of known affinity.

        The tannery must work in close partnership with a Dyestuff Researcher, in a suitable Institute - or, more commonly, as part of a Dyestuff Manufacture Corporation or a Consultant with specialised background. A range of colors (dyes, pigments or systems not yet developed) must be identified or developed which have a uniform interaction with any substrate leather, independent of tannage, retannage or pre-dyeing process. As a first objective, it would be desirable that the new dye ranges produce uniform color tones on as wide a range of retannage as possible even if color yield should vary (see illustration above). Secondly, the affinity values of the dyes and their relative positions in the affinity list should not change - whatever retannage or pretreatment was applied. Thus mixtures of compatible dyes having the same or similar affinity could remain unchanged for different types of retanned leather.

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Limitations of CCMP

In the textile field computer color match prediction has been found to be accurate - giving exact dye recipes - when matching with ternary (red, yellow, blue) combinations. However, when dyeing leather, often due to deficiencies in software and matchings involving dyes with different hues of the same basic color (e.g. reddish-blue dye mixed with greenish-blue dye, or different shades of brown dyes) have produced disappointing colour-matching accuracy. A further limitation has been the accuracy of matches to dark full color patterns. In light to medium depths accuracy is often acceptable.

In view of the importance of brown tones to the tanner, the range of brown dyes available and the preponderance of full and dark shades demanded of the leather industry by the consumers, software is undergoing further development by the makers of spectrophotometers used in CCMP.

The introduction of dye combinations, as suitable for computer assisted color matching, will certainly gain favor if they speed up reliable color matchings, and give level dyeings. So far, the problem of variation in dye affinity towards different substrates has not been overcome. At present, a particular dye which is suitable as a main element on a full chrome leather may only be used as a shading element (in small proportions) on a veg/syntan retanned leather. Whereas it had the same affinity as the other components of the combination towards, say, full chrome and is therefore combinable in any proportion, its affinity towards veg/syntan retanned leather can be very much lower than the affinities of the other dyes in the same combination. [see the note on Research Objectives - relative positions in the affinity list.]. In this case, if the proportion is increased to a level where it becomes the majority component, the higher affinity dyes strike first, leaving the lower affinity dye in solution. This causes patchy dyeing, especially if there is a strong color-tone difference between the dye components. The dyer must, therefore, restrict the range of shades which he attempts to dye with such a combination. This situation is frequently encountered: the experienced dyer knows that what he considers a "compatible" dye combination for one retannage, will not give uniform level dyeings on another leather. The color must be rematched using different dyes. Thus the universality of application of the dyer's favorite dye combination and its suitability for wide application in computer assisted color matching are called into question.

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Advantages of affinity numbers

Advances in characterizing the affinity values of dyes for different substrates do, however, provide the dyer with a practical guide to dyestuff selection and color matching. Color triangles constructed using, say, tobacco, rust and chocolate brown tones where the three elements have practically identical affinity to the leather, are of value in helping the dyer arrive at his starting recipe.

The practical advantage is that a leather with uneven uptake of anionic retans or irregularly charged chrome complexes, when dyed, will show a reduced amount of non-uniform color. Areas of tone-in-tone variation (paler or deeper tone) are more acceptable than patches of distinctly different color. Overall dye levelness is, therefore, perceived to be improved and the rate of rejection at crust-sort is reduced.

Although this is a long way from the facilities of computer assisted color matching enjoyed by the textile industry, it illustrates that the complex problems of the leather dyer are receiving attention.

Progress in answering the suggested research objectives will improve the predictability of leather color matching to the point where the leather industry can fully profit from the advantages of computer instrumentation.

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Increasing demands on fastness properties of dyed leather

In recent years color fastness standards have been considerably upgraded, to the extent that in conventional dye ranges only a few dyes can be found to meet these requirements.

In Europe, EEC requirements were established as far back as 1976 . In adapting to the specifications, the leather industry has reacted according to the needs of the end-user.

The shoe industry, for example, has been known to accept leathers of up to one full point below the EEC standards. For upholstery leather, the standards acceptable actually exceed the original specifications.

To a certain extent these standards have been met by changes in application techniques: changes in retanning, sandwich dyeings, after-fixing, etc. - however, the improvements have been limited to a restricted range of shades and intensities.

This has left the leather industry on a knife-edge, since the developments in other aspects of quality (softness, for example) cause the fastness properties to deteriorate noticeably. A reduction of even half a unit of fastness is grounds for rejection in borderline cases.

As a result, great efforts have been made in recent years to develop leather dyes which are "ideal" from the standpoint of application and which help the tanner to meet the enhanced quality specifications.

1        New anionic metal complex dyes

            The conventional 1:2 metal complex dyes had acceptable lightfastness,

            but their inherent solvent solubility caused an unacceptable migration into PVC and poor dry-cleaning resistance.

            The new dyes are distinguished by:

       very good level dyeing properties; grain damage is covered, with no accentuation of scar-tissue;

            the improved levelness is noticeable within the skin and from skin to skin, giving a distinct upgrading in selections

        lightfastness is excellent

       dry cleaning fastness is superior to conventional metal complex dyes

        migration into PVC fastness is excellent.

       Due to these special properties, such dyes offer the most reliable means of coloring high quality:

2        Reactive 1:2 metal complex dyes

The most outstanding feature is their outstanding levelling power - especially important when dyeing crust leathers. Again, grain faults are not accentuated as is the case with conventional dyes. The result in an improvement in grading and selection at the sorting table.

Although these are classed as reactive dyes, their reactivity cannot be fully utilized at the normal temperatures for dyeing leather. Fastness properties are somewhat superior to the conventional 1:2 metal complex dyes.

These dyes have been very well received for the level dyeing of crust garment leather. Even residues of subcutaneous tissue are dyed completely on-tone!

3        Liquid dye systems for drum dyeing (see Appendix 5)

Various systems have been introduced to overcome several of the difficulties encountered with powder dyes. These difficulties include the problem of powder contamination and ingestion by operators. Liquids are far safer from this point of view.

Another aspect in favor of liquid dyes is their suitability for automatic metering, measuring and pumping. This aspect of material handling has undergone intensive development, with the objective of automating many of the Control Points which we have discussed.

The main benefits of Liquid Dyes, immediately appreciated by the dyer, are the improved color yield, brilliance and electrolyte stability.

As mentioned in our discussion on dyestuff solubility, the leather dyer should verify which system of liquid dyes he is using: there are the particle dispersions or slurries, which still need dissolving in boiling water, and there are the true solutions. True solutions have the advantage of being directly dilutable in hot water, before addition to the dyeing drum.

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Conclusion

The present needs of the dyehouse, to obtain swift, accurate color-matchings and to reproduce these consistently in bulk, may be met by diligent attention to Control Points. If carried out by hand, it needs motivation and/or strict discipline. But to err is human, and many modern tanneries have shown that modern systems of process-control help to achieve consistency of process conditions. They have also shown, and frequently are known to speak up about the savings in wastage: chemical and dyestuff costs are know to reduce by 15% or even more - a big contribution towards a rapid amortisation of the automation.

Once process parameters have yielded to automated actuation and monitoring, and become consistent, correlations, equations and data-bases can be established, which help to establish the greatest dye-house aid yet: computer assisted color match prediction CCMP.

The needs of the leather consumers for high fastness properties are being met by team-work between the leather industry and the chemical companies. New improved application techniques are being reinforced by new dyestuff ranges and automated process controls.

These developments allow the inherent aesthetic value of leather to be marketed with the assurance of consumer satisfaction, and improved operating margins for the tanner.                                             To DYEING Index (Top of Page)

Author's Copyright: John C. Crowther

Swystem Logic GmbH 1995 -2003

email: John C. Crowther - Swystem Logic GmbH

Your feedback, comments and questions to the Author are welcome!