The dyeing of wool in sheepskin production is conditioned by three main factors, namely: a) substratum to be dyed, b) dyestuffs used, and c) dyeing method. The substratum, which is the wool, consists of a wide variety of types in respect of fineness (merino, crossed, entrefino types).

Furthermore, the effect of the environment in which the animal has lived and its age both influence whether the wool is in better or worse condition.

All this means that the chemical composition and physical arrangement of the quality of the wool varies between skins, and even within a single skin, depending on the area concerned.

All these are the properties inherent to the animal, but there is another important factor and that derives from the treatment to which the skin has been subjected during the tanning process. This affects the physico-chemical transformation undergone by the wool, such as processes of oxidation reduction in bleaches or washes, and the quality of tanning product (chrome, aluminium etc) fixed or deposited on the wool, together with a certain quantity of grease whose nature or composition can have a varying influence on dyeing.

We may state, finally, that if the wool has undergone a process of ironing or glazing, the chemical and physical composition vary extraordinarily according to the composition of the ironing liquids and temperature of the ironing roller.

The types of dyestuff most widely used in wool dyeing are: a) acid dyestuffs, b) metal-complex dyestuffs, c) mordant dyestuffs, d) reactive dyestuffs and e) vat dyestuffs.

In this work, we will restrict ourselves to studying those of types a) and b), as they are the ones most frequently used.

Acid dyestuffs

The acid dyestuffs are generally sodium salts of sulfonic acids, typical ones being C.I. acid orange 10. C.I. acid blue 45, and C.I. acid blue 1, which represent the azo, antrochinoid and triarylmethane classes, respectively.

The molecular weight of most of the acid dyestuffs is of the order of 300 to 800. The molecules of the dyestuffs are composed of a large anion which contains from one to four acid sulfonic groups, associated with the corresponding number of small sodium cations. Despite their high molecular weight, the free sulfonic acids, whose dyestuffs are sodium salts, are acids almost as strong as sulfuric acid. Their pKa values range between 1 and 2, and the solutions of their solid salts are neutral.

Owing to their large size and their water-repellent character, the anions of acid dyestuffs show a considerable attraction to each other. This leads to the formation of aggregates.

In general, the higher the molecular weight and the lower the number of acid sulfonic groups, the greater the tendency to aggregation.

This aggregation is encouraged by the presence of acids or salts in the bath, while temperature increase reduces the tendency.

The molecule takes on an electronegative character, being considered as a weak acid insoluble in water.

The alkaline salts are partially soluble in water and can be used following dilution with a dispersant.

Solubility in water increases if groups which are not sulfonic groups as such but non-ionic groups, are added to the dyestuff molecular. The most common of these are the sulfamide groups: -SO2HN2-.

The 1:2 metal-complex dyestuffs can take four forms:

1 Acid form

2 Alkaline salt

3 With non-ionic water-solubilising agent

4 Monosulfonic mixed complex

Of these four types, the weaker their solubility in water, the better they will bond to the wool and less onto the leather (types 2 and 3); of these we must choose the most water-repellent and employ them under the best conditions for dyeing.

When we deal with the dyeing of leather, we will see the type of conditions best suited to application.

The studies we made in 1980 went more deeply into the response of the acid and 1:2 metal-complex dyestuffs in wool dyeing.

Spanish merino-type skins with wool characteristics as similar as possible to each other were taken as the starting point, so that the various methods used in the tests used an homogeneous substratum.

The chemical properties of these skins, both for wool and for leather are as given in Table 1.

The skins having been tanned and fatliquored, were dried and then conditioned following optimum setting out. The wool was then ironed using an alcohol solution, formic acid and formol, three times at 170ºC, and then sheared to 17mm.

Following the dyeing treatment of each variable, a part of the dyed skin was taken and the wool removed with a knife. This piece was then submitted to the extraction treatment described below.

We have calculated an ‘extractability factor’ for each type of dyestuff, based on the following method: de-woolled skin with identical tanning as the doubleface was treated with the same dye. We could use spectrophotometry to determine the quantity of dyestuff absorbed by the leather.

This leather was subjected to the following stripping process: prior treatment with 1% ammonia at 50ºC for four hours and three washings with distilled water at 50ºC for two hours for each washing.

Once the skin had dried it was submitted to treatment with perchloroethylene at 45ºC twice for three hours. In case of the acid dyestuffs we, thereby, extract 87.5% of the fixed dyestuff, of which 74.5% was obtained with the alkaline treatment and 13% in treatment with the solvent.

In the case of 1:2 metal-complex dyestuffs the results were: extraction of total of 59.8% of the fixed dyestuff, of which 9.5% pertained to the alkaline treatment and 50.3% to the treatment with solvent.

All the tests were carried out on pieces measuring 10 x 10cm, with the following analyses and the variables described above being found.

The following dyestuffs were used:

Acid orange 3

Acid red 88

Acid blue 25

The reference taken was the dry weight of the conditioned skin, with wool sheared to 17 mm, bath ratio of 1:20 and 0.5% dyestuff. This 0.5% does not refer to the weight of the commercial dyestuff since, of course, the concentrations of dyestuffs of equivalent absorbency are at the maximum of their spectrophotometic curve.

Wool dyeing process

Bath ratio 8l/skin

Soaking – retanning

Water at 40ºC

1.5g/l Celesal K

Run for 60 min

3.0g/l Chrome salt 33ºSch

3.0g/l Retanal CP Super

Run for 60 min

2.0g/l Sodium formate

Run for 60 min. pH = 4.2. Run off. Rinse for 15 min with cold water


Water at 30ºC

2.0g/l Sodium formate

Run for 10 min

2.0g/l Retanal NS

Run for 10 min

1.0g/l Sodium bicarbonate

2.0 Run for 60 min. pH = 6.0

Run off and rinse for 10 min

Wool dyeing

Water at 62ºC (maintained)

0.5g/l Dyeing auxiliary product

Run for 30 min

x g/l Acid or metal-complex dyestuff

Run for 15 min

0.5g/l Formic acid

Run for 30 min

0.5g/l Formic acid

Run for 30 min. pH = 4.2. Run off and rinse for 15 min.

Influence of pH

Six pH variables were carried out, these being 4.2, 6.2, 7, 7.5 8.1 and 8.6. The baths were set at these pH values, after having achieved equilibrium between bath and skin, that is, when at 90 minutes of treatment the pHs remained constant.

Samples were taken every 15 minutes and these were used to draw up the exhaustion curves of Figure 1, from which we can see that for each dyestuff exhaustion increases as pH decreases.

Comparatively, it can be seen that the yellow is exhaustion less in curve number 1 (95%), while the red and blue are almost totally exhausted (97%) at the same pH.

As the pH increases, the yellow is affected most, followed by the red and then the blue.

Fixing on the suede also increases with the pH, as can seen in the upper scales of Figure 1, both in absolute and relative values, at the set total quantity of fixed dyestuff.

Comparing them, for equal pH values the quantity of dyestuff fixed on the suede increases in the order, yellow, red and blue.

The pH of the bath is an aspect of vital importance. From the equation 4.1 below, it can be deduced that when the wool combines with the acid:

Equation (4.1)

R – +NH3 OO – C – R + ClH+ oNH+ H2Cl + HOOC – R

The ionisation of the carboxyl groups is inhibited and the fibre takes on a positive charge which is neutralised by the absorption of the inorganic anion. The positive charge increases with the quantity of acid present, reaching its maximum at pH1 approximately. In the case of the simpler acid dyestuff, that is those of a lower molecular weight and lower affinity, the dyeing is carried out at pH2.5-3.0, obtained with the addition of sulfuric acid.

In the case of dyestuffs of greater molecular weight and high affinity (equation 4.1). This will lead to a very fast adsorption of the dyestuff, which due to its high affinity will not travel to other basic groups. It may, therefore, be necessary to renounce the use of acid, in which case the reaction of the wool, on the basis of an essentially neutral bath (pH7), can be presented by the equation 4.2 shown next:

Equation (4.2)

R – +NH3 OCC – R + Na dyestuff o +RNH3 dyestuff + Na +OCC – R (4.2)

This means that the amino groups of the wool are now involved, although the non-polar forces are mainly responsible for the affinity of the dyestuff. It can, therefore, be established that the three dyestuffs studied are of medium affinity, increasing in the order yellow, red, and blue.

Influence of temperature

Six variables were implemented, with each of the three dyestuffs, at temperatures of 40, 45, 50, 55, 60 and 65ºC. The dyes started at pH 7.5, and after 15 minutes formic acid was added to pH6, while after 45 minutes acid was again added, this time to pH4.3. The treatment lasted another 45 minutes, until 90 minutes dyeing time was completed.

The curves obtained are shown in Figure 2, in which we can see that for three dyestuffs the six curves are exhausted at the end of the 90 minutes, the red and the blue showing similar response and becoming exhausted quickly, while the yellow became exhausted slightly more slowly.

The quantities of dyestuff fixed in the suede increase as the temperature diminishes, for each dyestuff, while comparatively, and at equal temperature, the amount of dyestuff fixed in the leather increases in the order yellow, red and blue.

Increasing the dyeing temperature has a triple purpose: 1) to prevent aggregation of the dyestuff; 2) to increase swelling of the fibre and thus make the dyestuff more permeable; 3) to accelerate diffusion of the dyestuff within the fibre.

That is, the higher the temperature, the more the dyeing will take place with a higher concentration of dyestuff at the wool-bath surface.

Of the three dyestuffs studied, the yellow showed a greater tendency to form aggregates than the red and the blue.

Influence of auxiliary products

Four variables were implemented, with each of the three dyestuffs, corresponding to curves 1, 2, 3 and 4 (Figure 3).

The treatment was carried out, respectively, with the following products: curve nº1, oleilamine, nº2 nonylphenol (8 moles of ethylene oxide), curve nº3 without any product, and curve nº4 with sodium sulfate.

As can be seen in Figure 3, with the three dyestuffs, the speed of increase followed the same order as their curve number, with the final exhaustion being the same order as their curve number, with the final exhaustion being the same for the red and blue, while for the yellow curve nº4, that is, with the addition of sodium sulfate, the rise of the dyestuff is retarded a little, and the final exhaustion is slightly lower than in curves 1, 2 and 3.

The fixing of the dyestuff in the leather increases according to the order of the curve, and if we make a comparison between the three colours we see that for a given auxiliary product the quantity of dyestuff fixed in the leather increases in the order yellow, red and blue.

In general the wool absorbs the acid dyestuffs due to three different forces of attraction: 1) Ionic attraction between positive acid groups of the dyestuff and amino groups of the wool also charged positively; 2) Van Der Waal’s forces, non-polar and exercised between the water-repellent dyestuff anion and the parts of the wool of the same type adjacent to the positively charged amino groups, and the covalent bonds, which are responsible for the strongest unions and, therefore, play a direct role in fastness levels.

In the case of the acid dyestuff we are looking at, the three types of bond are important, and the acid must, therefore, be present to ensure the existence of a positive charge in the fibre, while the presence of additional sulfate ions promotes the competition of the basic points, leading to de-adsorption and levelled dyeing.

Furthermore, the function of the oleilamine is to react with the sulfonic groups of the dyestuff, transporting the molecule of said dyestuff to the positively charged amino groups of the wool in acid medium, thus facilitating their fixing.

Metal-complex dyestuffs

Chemically, the metal-complex (or premetallised) dyestuffs are very closely related with the metal complexes produced in the fibre by mordant dyestuffs, so that from the classification point of view, they are acid dyestuffs, treated as such in the Colour Index.

From the earliest times, mordanting has been associated with good dyeing fastness.

As we know, oxidation dyeing consists in depositing the metal atoms on the fibre and then producing the complex in situ by the oxidation of various organic compounds.

The mordanting operation naturally prolongs the dyeing process, so it was only normal that a method should be sought to permit a combination of metal (normally chromium, cobalt or iron) and the colour prior to dyeing.

Given that the 1:2 types are the ones mainly used for dyeing, our study will look at these.

We shall give the term 1:2 metal-complex dyestuff to those which have a metallic atom, forming a complex with two molecules of dyestuff (4). These generally correspond to acid/dihydroxyazoic or dihydrox/carboxyazoic dyestuffs; the metallic atom can be any of those mentioned above.

If the two dyestuff molecules are of the same constitution, the 1:2 metallic dyestuff is termed ‘symmetrical’, if not, it is ‘asymmetrical’ or mixed.

Its general structure corresponds to the schema in Figure 7.

The following dyestuffs were used:

Acid yellow 118

Acid orange 89

Acid black 63

The same procedure was followed as for the acid dyestuffs.

Influence of pH

Six variables were implemented (curves 1, 2, 3, 4, 5 and 6), with the corresponding pHs of 4.2, 6.2, 7.0, 7.5, 8.1, 8.6.

In Figure 4, it can be seen that with the three dyestuffs, the exhaustion increases as pH diminishes; the three, therefore, show a very similar response.

The distribution of the dyestuffs between the suede and the wool takes place in such a way the quantity fixed in the leather increases as pH increases. The increase is only in proportion to the total fixed, however, since in absolute terms it diminishes.

Compared with the acid dyestuffs, they can be said to undergo less exhaustion for similar pH levels, and their curves have less slope.

Likewise, as the pH rises there is a reduction in the absolute value of the quantity of dyestuff fixed in the suede, while the acid increases, although the performance on wool of the same quantity of dyestuff at the same pH is greater with the acid dyestuffs.

This behaviour of the metal-complex 1:2 dyestuffs is explained by the negative charge of the complex, which makes them sensitive to the pH variation, although less so than the acid dyestuffs, probably because the charge is not localised. As the molecule is large, the covalent bonds and Van der Waal’s forces act powerfully, providing good fastness properties.

Influence of temperature

Six variables were implemented (curves 1, 2, 3, 4, 5 and 6), corresponding to temperatures of 65, 60, 55, 50, 45 and 40ºC. Figure 5 shows how exhaustion increases as temperature increases, for all three dyestuffs.

The curves are also similar in all three, although for the yellow at 90 minutes the curves become more horizontal as temperature increases, while for the orange and black they retain a certain slope, which indicates that the dyestuff could become even more exhausted.

The distribution of the dyestuffs between wool and suede takes place in such a way that for all three, the quantity of dyestuff fixed in the suede decreases slightly as the temperature decreases in absolute values, as it does in relation to the total quantity of dyestuff fixed. These differences increase in the order yellow, orange and black.

If we compare against the influence of pH with temperature, we see that the pH variation has a greater effect than temperature variation on total fixation of the dyestuff; and the quantity of dyestuff fixed in the suede is higher than in the case of pH variation.

As they have large molecular size, the 1:2 metal-complex dyestuffs show a great tendency to form aggregates and the temperature in such cases, therefore, has to be high to provide the dyestuff molecules with sufficient energy to overcome the energy barrier at the wool-bath surface divide.

Influence of auxiliary products

Four variables were implemented (curves 1, 2, 3 and 4), corresponding to the products: tributyl phosphate; phosphoric ester in emulsion with nonyl phenol with 6 moles of ethylene oxide; without any product; and, finally, with sodium sulfate. The exhaustion curves for each of these dyestuffs increase to 90%.

The curves of the different products (Figure 6) cut across each other owing to the kinetics of the reaction being different according to the medium in which the dyestuff is placed; that is, a dyestuff in a certain medium begins with fast exhaustion and then rises only slowly, while the same dyestuff under the same conditions, but in a different medium, may show the opposite pattern of exhaustion, that is, first rising slowly and then doing so more rapidly.

In any case, the four curves differ little from each other, though the distribution between wool and suede is different.

We can generalise by stating that for the three dyestuffs, with 90% of dyestuff fixed, the phosphoric ester with nonyl phenol mixture leads to most fixing of dyestuff on the wool (at 33%), followed by tributyl phosphate (with 30%), then the blank test without product (20% fixed), and lastly the sodium sulfate, which fixes the least quantity of dyestuff on the wool (10%).

The influence of the auxiliary products on wool dyeing with metal-complex dyestuffs is very large. Given the considerable complexity of this particular subject, however, a special study would have to be devoted to the use of solvents and/or emulsions.

The exact role of the solvent is difficult to determine, although it is considered to be essentially a physical action, without direct participation in the reaction between the dyestuff and the wool fibre. It has a disaggregation action on the dyestuff, but above all it enhances contact between the dyestuff and the wool, dyestuff being absorbed mainly into the fibre of the latter, which leads to stronger concentration of the dyestuff in that place.

Where an emulsifying agent is used it plays an important role, as it orients itself on the surface of the wool and maintains the solvent in suspension, and this is the factor in determining a concentration of solvent on the interface surface sufficient to really influence the dyeing speed. Furthermore, the emulsifying agent must not combine with the metal complex or with the wool. This is so that the dye distribution mechanism between the two solvents is not disturbed.


1. In sheepskin dyeing with acid dyestuffs, as the pH diminishes fixation increases, reaching 98%; there is also an increased quantity of dyestuff fixed in the wool.

2. As temperature increases in dyeing with acid dyestuffs between 40 and 65ºC, the exhaustion rates do not change but dyestuff distribution does, fixing more on the wool as temperature increases.

3. The addition of various auxiliary products does not alter the exhaustion rate of the acid dyestuffs but it does alter their distribution between wool and suede.

4. In dyeing with 1:2 metal-complex dyestuffs, as the pH diminishes the total fixation of dyestuff increases, as does its distribution, this being greater in the wool as pH decreases.

5. As temperature is varied between 40 and 65ºC in dyeing with 1:2 metal-complex dyestuffs, the total quantity of dyestuff fixed alters considerably; however, the distribution between wool and suede retains the same proportions for each dyestuff.

6. The addition of solvents, emulsions of solvents or a sodium sulfate type salt does not alter the final exhaustion rate, but it does alter the distribution between wool and suede.

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