Leather dyeing is a transition process between tanning and finishing. Some kinds of leathers require penetration of dyestuff into collagen network while certain kinds of leather need not be dyed through, it is enough to dye them on the surface. This decreases consumption of dyestuff, which is an expensive material. It is important to know how penetration of dyestuff proceeds. The processes done affect the chemical properties of leather.The result of dyeing is not only dependent on tanning agent and method used, reactions of fat, surfactant and water should also be taken into consideration. 
Collagen contains functional groups, part of them bound in  the tanning process, and tanning agents which may possibly react with dyestuffs: in a vegetable leather these may be sugars and phenols and in mineral tanned leather these may be complexes of chromium or other metals as well as fats, surfactants or proteins. Isoelectric point of collagen lies in the range of 7-7.8 pH, that of chrome tanned collagen 6.5 pH, of vegetable tanned one 3.5pH, aldehyde, quinone and oil tanned about pH 4.5. It is essential to choose dyestuffs so as to ensure opposite charges on both substances. 
In conventional dyeing process the dye is dissolved in water, and the leather is treated with this solution. During dyeing the color fixes chemically to the leather, leaving the water colorless. Under ideal conditions all the dye offered is fixed to the leather, and subsequent washing of the dyed leather with water should not wash off any color. The dyeing is then said to be 100% wash fastness.
Manufacture of dyestuffs is an old branch of industry which is based or synthetic organic chemistry. Various intermediates need to be prepared using typical reactions such as nitration, sulphonation, caustic fusion, halogenation, reduction and oxidation.
Also some specialized techiques are necessary. Because aniline was prominent  as an intermediate in the early days of the industry, synthetic dyestuffs are still sometimes referred to as aniline dyes but this practice has been incorrect for many years. A very important publication in the field of dyestuffs is the Color Index.
Modern dyestuffs contain the benzene ring in their molecular structure. The significance of this is that the orbital arrangement of  the electrons is sensitive to light. Starting with relatively simple organic molecules, colored compounds are made by joining these together by such processes as diazotization and coupling, etc. Very large complex molecules are made where the electronic orbits are sensitive to light of specific wavelengths(certain colors). Functional groups on the aromatic ring are of chromophore character. These groups are shifting pi electrons in the rings thus forming some negatively or positively charged centers in an aromatic molecule.
A simple orange dyestuff made from aniline, diazotized and coupled with napthol would be insoluble in water. It is sulphonated to introduce strongly ionizing groups with high water stability. The means employed to make leather dyestuffs water soluble is of particular importance(carboxylic,sulfonic amino or hydroxylic groups are introduced to increase water solubility).
Anionic Dyes:
In the case of the orange dye shown above the dye is referred as an anionic dye, it has a negatice charge. It will tend to precipitate or “fix” on to cationic colloids which have a positive charge. The skin leathers come into this category under acid conditions (ie. if pH is below their isoelectric point). Consequently anionic dyes “fix” on the leather under acid conditions by ionic forces. These are powerful forces and the reaction or fixation can very rapid particularly at higher temperatures. Such rapid fixation can lead to unlevel dyeing of leather. This is even more important if one wishes the dye to penetrate through the thickness of the leather. If the fixation is rapid then the bulk of the dye will fix on the outer surfaces and the solution which eventually diffuses into the skin centre will be denuded of color. These factors may be controlled by controll of pH of the dyeing system. To obtain high degree of penetration it is usual to start the process under non-acid conditions, drumming or paddling until penetration is obtained, at the end, acid is added to cause fixation.

Dyes of anionic type are often termed “acid” dyes which refer to the desirability of using acid to fix them.There is a very wide range of colors available. However, the individual dyes may have vastly different structure and have different degrees of resistance (fastness), to light (fading in sunlight), water washing, soap washing, dry rubbing (marking off), or dry cleaning solvents. These properties are usually evaluated by the dyestuff supplier and graded in pattern books as specifications often by a system in which 5 means excellent properties and 0 means very bad. Generally acid dyes have good average properties, ie. are reasonably fast and water resistant. But there are outstanding exceptions.
These “fastness” properties are not entirely functions of the ionic charge on the dyestuff. The dyestuff molecules are large complexes and can exhibit high secondary valency forces due to dipole moment or H-bonding which play a significant role once the dye molecule is drawn into close proximity of the fibre by ionic forces. This accounts for the phenomenon that whilst the dye may be fixed by acid, if subsequently the acid is neutralised with alkali(eg. Ammonia) the dye is not entirely “unfixed” or stripped from the leather. Dyes which have strong secondary valency forces often aggregate in strong-concentrated solutions.
Temperature Effects:The normal method of dissolving acid dyestuff is to paste the powder with a little cold water until thoroughly wetted and then add boiling water to an amount 20 times the weight of dyestuff.
Secondary forces are weakened by the rise of temperature and large molecule aggregates are broken down to give good solution.
Thus the higher the temperature of dyeing, the smaller the dye molecules, the better penetration and distribution on the leather and the less fixation due to these forces.
However the ionic forces which are pH dependent increase with temperature, giving more rapid fixation and less penetration or levelness the higher the temperature. Temperature rise has an opposit effect on these two types of force.
To get good penetration the dyes would choose a “penetrating dyestuff”(ie. probably low secondary valency forces), adjust the pH of leather and dyebath to a point of minimum acidity by neutralizing or ammonia addition, eg. PH 6.0 thus reducing ionic fixation, dye warm(40-60 C) and later acidify to pH 3.7.
Effect of Concentration:Given a known quantity of dye and leather the less water used the stronger the surface shade obtained will be.
However, if one is drum dyeing, the less the amount of water used the much greater will be the mechanical action, giving rapid diffusion of he dye. Such penetration would reduce the accumulation of dye on the surface and give paler shades.
Effect of Tannages:Acid dyes being anions will combine ionically with the cationoc basic amino groups of the skin and also by secondary forces.
Vegetable Tannages and syntannages: These are also anionic and in such leathers the cationic groups will be largely blocked by their presence, thus reducing the number of sites on the leather molecule to which the acid dye can fix.
Consequently acid dyes on these tannages show a lower rate of fixation which gives good penetration and level dyeing, but paler shades result and it is imposible to get strong shades.
Auxiliary type of syntans will block these amino groups to a greater degree than most vegetable tans and therefore give paler shades. They are  often used as “level dyeing” assistants.
Acid addition will increase fixation, but lower pHs are needed than on other tannages to get equivalent fixation(usually pH 3.2-3.7).
Usually penetration is easily obtained at pH 4.0-4.5. One should be careful in adding ammonia to higher pHs to obtain penetration, for it will tend to strip the anionic vegetable tannage from the fibre.
Vegetable tanned leather may be quoted as having a shrinkage temperature of 62 C by lab test but it will not stand this temperature under drum dyeing conditions of an hour or more, when the normal safe maximum is 45 C. Vegetable tanned leather has a pale brown color, and with the transparent color of dyestuffs this is an important factor because the color of dyed leather will be a mixture of pale brown and that of dyestuff used. This pale brown tends to dull, or darken green and blue shades in particular.
Vegetable tannages do not have good “light fastness”, on exposure to light they become a darker brown. In many cases their light fastness is much worse than that of dyestuffs used. This can give rise to peculiar effects. Dyestuffs fade on exposure to light. Thus a brown dyed vegetable tan leather may appear to have good light fastness, because although the brown dye fades, he brown of the vegetable tan darkens.
With an equivalent blue on the same leather, the light effect would be most marked,ie. the blue color would disappear to be replaced by the darkening brown of the vegetable tan.
Vegetable tannages do not have very good resistance to water or soap washing and are not used for leathers requiring these properties.
Chrome Tannage: Chromium salts fix to the acid carboxyl groups of the skin protein and consequently tend to increase the cationic charge of the skin. Further, chromium salts hydrolyse giving acids which also increase the acidity of the leather.
The combination of both factors make chrome tanned leather very cationic giving rapid surface fixation with acid dyes. This strong surface dyeing is usually coupled with unlevel dyeing and poor penetration. Increase in temperature increase these effects.
Chrome tannages vary in this respect. Masked tannages are less cationic and give more level dyeing, penetration and paler shades. Chrome leather that has been dried out and then wetted back also should show loss of cationic charge and give less rapid dye fixation.
Levelling or penetrating agents usually consist of anionic syntans applied befoe or in the dyebath to reduce the affinity for anionic dyestuffs.
Many acid dyestuffs contain chemical groups which can coordinate with the chrome complex, in a similar way to “masking”salts. Loosely defined as “chrome mordant” dyes they give impoverished wash and water fastness.
Zirconium and aluminum tannages: The Zirconium and aluminum salts used for tannage hydrolyse to a greater extent than the corresponding chrome salts and thus give more highly cationic or more acid leathers. They also give co-ordinated complexes with many dyes which can be of high colour value. Normally the give strong surface shades of poor penetration and a tendency to unlevel dyeing. They are white tannages, of good light fastness and can give very clean brilliant shades. Wash and solvent fastness us usually improved.
Alum tannage can raise particular problems, if the alum is not well fixed, and tends to wash out in the dye bath. Traditionally alum tanages were dyed with dye woods or vegetable extracts.
Dye woods or vegetable extracts: These are obtained by extracting certain barks, leaves, fruits, etc. with water. Their reaction on the skin or hide is similar to that of tannins except that the dyewoods are especially selected because of their strong color value. They are applied in a similar manner to other acid dyes, but have two distinctive properties.
a)  their tanning power giving a fuller or more leathery feel,
b)  their ability to complex with many metal salts to give a range of colored complexes.
With aluminum salts or tanned leather they tend to give bright yellowish brown colours of fairly good wash-fastness, by comlexing with the aluminum, fixing to the fibre, and also giving some added leathering property.
Iron-salt strikers tend to give dark or black shades.Tin salts were used for red shades.
Aldehyde Tanneges: Formaldehyde and glutaraldehyde tannages combine with the basic amino groups of the skin and generally reduce its cationic charge and its anionic dyestuffs. They are usually tanned at high pHs eg. 6-8, which further neutralizes fixation of anionic dyes and they are generally sensitive to hot water, so that a maximum dyeing temperature of 40 is required.
Under these conditions they show poor fixation for acid dyes; only very pale shades of poor wash-fastness are possible. These tannages are white.
They are sometimes used in combination tannages with chrome or zirconium, when the leather will stand higher temperatures and the pH is more acid, eg. 4.5-5.0. Their effect here is to give a paler surface shade with improved level dyeing and penetration.
Vegetable/Chrome Semichrome leather is leather which is fully vegetable tanned and afterwards retanned with chrome salts. The chroming process improves the wet heat resistance so that higher bath temperatures can be used and increase the cationic charge on the leather.
The overall effect is to improve the strength of surface shade obtained and also the wash-fastness.
Chrome-Retan is a fully chrome tanned leather after treated with some vegetable tan or syntan. This retannage reduces the anionic charge on the leather surface giving rather paler shades but improved levelness and penetration.
Resin Retannages, where the resin is based on urea, melamine or dicyanamide formaldehyde condensate, are often cationic and lead to strong surface shades, reduced penetration or levelness.
Exceptions may be where the resin preparation contains anionic dispersing syntan, or free formaldehyde, when this cationic effect may be nullified.
Incorporation of polymeric acrylate, vinly or butadiene resins dispersions into the leather by drum application is also done.The effect on dyeing is not very great unless they are applied in excessive amounts(more than 5%). However the dispersing agent is usually anionic and may give rather paler shades.
Other types of Anionic dyes:
Direct or Cotton or Substantive Dyesstuffs so-called because they can give direct dyeings on cotton (without any mordant pre-treatment).
Fundamentally they are made similarly  to acid dyestuffs with sulfonic groups to give water solubility. However, the molecule is so built that it has very large secondary valency potential and the minimum degree of sulphonation and hence of ionic affinity. The expression substantive refers to the dyes’ ability to fix on cotton which contains no ionic groups and hence refers to secondary valency bonds, eg. H-bonds or dipole fixation. The expression “acid-substantive” dye refers to one which has approximate equal bonding by these forces and ionic forces.
They are used exactly the same way as acid dyestuffs. On chrome leather, they can give very strong surface shades of little penetration withaout acid exhaustion at pHs 4-5. This is due to strong secondary valency forces on chrome tanned fibre. They have similar behaviour on Zr and Al tanned leathers. On vegetable tanned leathers they behave in a similar way to acid dyes.
Chrome-Mordant Dyes
These show outstanding ability to react with chrome salts to give dyeings of much improved wash-fastness. The leather is dyed and acid exhausted normally, when 0.5- 1 % potasiumbichromate is added to exhaust dyebath and running continued for 30 min.
Premetallised Dyes
The previous method is not very convenient particularly as the “after chrome” may alter the shade. Premaellised Dyes are supplied by the manufacturers as a metal salt (usually Cr or Cu) is already coordinated with anionic dyestuff molecules. They are classified as two distinct types:
a)  1:1 premetallized, inferring 1 anionic dyestuff molecule per 1 molecule of metal.1:1 dyestuffs are formed at pH 4. These have good wash-fastness and give pale, level penetrating dyeings.
b)  2:1 premetallised, 2 anionic dyestuff molecule per 1 molecule of metal. They are formed at pH>4. They are more anionic and therefore more sensitive to pH change and can be adjusted to give stronger surface shades, sometimes with the sacrifice of a little wash-fastness.

Amphoteric Dyes
These premetallised dyes has the potential to give the molecule a negative charge while the metal has the potential to give cationic charge. In this respect they resemble the skin protein, having a pH giving no charge(isoelectric-point). At pHs below this they will be cationic and above it anionic.
Sulphonated Basic Dyes
These are basic dyes sulphonated. They are useed in a similar way to acid dyes but have some of the brilliance of shade (and poor light-fastness) of basic dyes. They are amphoteric .At isoelectric point(~3.0), they have no charge thus give good penetration and levelness. At more acid pHs they are cationic and give good strong surface fixation on vegetable tanned leather.
Reactive-Triazinyl Chloride Dyes (Procion Dyes)
These give extremely good wash-fastness because the dyestuff is covalently linked to the protein of leather.
While they can be classed as anionic their fixation does not depend on anionic fixation.They are applied in warm water solution to the leather. Salt is added to the water to favour absorption on the leather fibre and then sodium carbonate is added to give an alkaline condition of pH 8.0-9.0. Under these conditions, the chlorine from the triazinyl chloride dye is split off to give sodium chloride with the alkali and the free bond created covalently links the dye to the leather.
They are of particular merit on glove or clothing leather which has to stand washing, and on woolskins. There are a limited number of shades and only pastel shades are obtainable. Light fastness is only average. They can be used on mordanted chrome but are unsuitable on vegetable tanned leathers which will not withstand the alkali dyeing conditions. They are particularly good for dyeing washable aldehyde leathers.
Sulphur Dyes
These are made by fusion of aromatic amines or phenols with sulphur or alkaline polysulphide. They are only soluble in alkaline solutions of sodium sulphide(pH 9-12).
This alkalinity seriously damages most tannages with the exception of chamois leather and aldehyde leathers, for which these dyestuffs can be used. After application of the dyestuff, acidification and oxidation, the sodium sulphide is destroyed and the remaining dyestuff is quite insoluble in water or soap solution-hence giving good wash-fastness. The range of shades available is rather limited.
Cationic Dyes-Basic Dyes
These were the original “aniline” dyestuffs and are made from tar distillation products by similar methods to those used for acid dyestuffs, except that the sulphonation processes are omitted. Water solubility is conferred by the presence of amine groups which form strongly ionizing salts with acids. 
The colloidal dye ion carries a positive charge. Consequently there is a strong ionic attraction between these cations and colloidal anions.
Basic dyes give strong surface shades, no penetration and often unlevel dyeings on vegetable/syntan tanned leathers. In a similar way they will give a strong surface shade on leather previously dyed with anionic dyes. This fixation is rapid hot or cold and is relatively independent of pH between 3-9.
Conversely cationic dyes have very little affinity on cationic chrome leather, giving only the faintest tints. However, if the chrome leather have been heavily masked, dried out and vegetable retanned or syntan mordanted, and fatliquored with highly sulphated or sulphited oils, or is anionic dyed, these anions may increase the affinity for basic dyestuffs.
Basic dyestuffs give a range of strong briliant shades of yellow, orange, red, blue, violet and brown. Rich blacks are made from a mixture of these. Unfortunately they fade badly in sunlight, although some may be better. Some improvement can be made by an after-treatment with salts of phosphomolybdo-tungstic acids.
Basic dyes tend to be soluble in some oils, greases, waxes and solvents. The free bases or their oleates are very soluble and are used for coloring oils, waxes, etc( eg. Boot polishes, carbon paper, typewriter ribbons, etc.). This property is associated with their propert of “marking off”,ie. by simple contact, color may be transfered to another surface.
This must be avoided on many clothing leathers, etc.Their solubility in non-aqueous solvents may cause trouble in dry cleaning or where a leather finish contains such solvents.
The free base obtained under alkaline conditions is of poor water solubility. Hard water or alkaline conditions may cause precipitation. Basic dyestuffs are often dissolved by pasting with a little acetic acid, before adding boiling water to avoid this happening.
Nonionic dyestuffs(e.g. azoic ones) require longer dyeing times.Binding occurs at very broad pH. Nonionic dyestuffs are resistent to washing and abresion. However surfactants of glycols of polyether type remove them, probably due to formation of new hydrogen bonds. These dyestuffs may be completely washed out from pelt by acetone; in chrome-tanned leather, however, 1/3 of them will remain. One may conclude that in the binding of dyestuffs to chromium complexes strong bonds participate. Behavior of dyestuff in solution depends primarily on dissociation of its functional groups being responsible of its solubilty.
The effect of individual substituents on the pH has been known for long. The following are some rules worth mentioning:
1)  Sulfonic group is strongly dissociated when attached to an aromatic ring or in the presence of amino groups; its pK is 0.5.
2)  Carboxylic group is more dissociated when attached to benzene ring, than when linked to an aliphatic chain.Attacment of other groups to the ring changes the degree of dissociation of the group itself; direction of this change depends on the kind of group and on its position; nitro group in ortho position increases dissociation the strongest. 
3)  Hydroxylic group is affected by other groups; nitro groups and halogens in phenols increase significantly the degree of dissociation.
4)  Amino groups dissociate to a small extent, other groups influence it like,e.g.,hydroxyl.
Two groups of dyestuffs of very specific way of action have gained importance recently: metal complex and reactive dyestuffs.

Standardization of dyestuffs
Manufacturers standardize their dyestuffs so that they are of uniform strength and that repeat dyeing of a certain recepie give similar shades. The strength of dyestuff is adjusted by addition of common salt in case of anionic dyes and starch or dextrin it the case of basic dyes.
Standard shades may be designated by the letter S or 100 after their name. A stronger quality might be “200” or 200% stronger, in which case only half the quantity should be used in a standard recipe. In the same way 50% would indicate that this quality is only half standard strength. 
For leather,they usually standardise on chrome leather or vegetable tanned leather.

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Leather produced by tannage must now be prepated for sale. Obviously it must be dried, but the color may need be made uniform or changed by dyeing, the thickness may need modification, oils or fats may be needed to improve suppleness or water resistance, and the “handle” of the leather may be modified by drying methods or by squeezing, rolling, plating and flexing. The grain may be required smooth or pebbled, shiny or dull or the leather may be finished on the flesh side as suede.
Removal of surplus tan liquor: after tannage it is common to allow excess tan liquor to drain off the hides or skins, and to let them stand in a damp condition for a day or more. With most tannages further fixation of tan and setting of the fibres occur. When a flat leather is required, the skins are drained flat to avoid any tendency for the fibres to set in a creased condition. Common methods are horsing up or cessing or piling. The pack may be covered to prevent surface drying or soiling.
Washing: The aim is to remove the loose, surplus tan. To chrome leather and vegetable tanned light skins rigorous washing  may be applied. In the case of heavy leathers rigorous washing  with large quantities of water is avoided.
Neutralizing: Chrome leather is acid and develops acidity on standing. Consequently neqatively charged colloids such as dyestuffs, vegetable tans, sulphated oils, etc. will readily precipitate on the skin surface. The washed chrome leather is therefore neutralized by mild alkalis (drumming in 1-2% sodiumbicarbonate or borax, 200-300% water, ~30 mins.). The leather is then washed again and the next process ( dyeing or fatliquoring, etc.) should be carried out immediately.
It follows that degree of neutralizing needed will depend on the tannage. Thus masked chrome tannages will give improved dye penetration, as will, combination or retannages with vegetable tans or syntans.
Adjustment of thickness: 
splitting: if a hide is thick enough (3mm), it may be split into two layers(grain layer2mm, and flesh layer 1mm).
Shaving: levelling off of thicker areas.
Removal of excess water: before drying, the leather may in many cases be bleached, or treated with oil by stuffing or fatliquoring. Many chrome leathers are neutralized, dyed and fatliquored before drying; whilst most vegetable-tanned leathers are dried out after tannage and are then wetted back for dyeing, because fresh vegetable tannage tends to wash out in the dye-bath, giving a thinner, emptier leather.
Chrome-tanned leather for suedes and gloving is often dried out before dyeing, in the former case to allow buffing of the dry leather to be carried out before dyeing, and in the latter case to allow the skins to be staked and very carefully sorted before dyeing. 
a)  Light leather: as much water as possible is squeezed out before drying (sammying machine)
Heavy Leather: also sammed but less easy. Setting out is performed to give a flat and wrinkle-free finish.
DRYING
Hide protein has associated with it a large amount of water and is in fact a hydophilic colloid.. Under slow drying conditions, evaporation from the surface proceeds at a slow enough rate for the water being removed from the surface to be replaced by that migrating from the inside.With high speed evaporation, however, the water from the inside cannot migrate rapidly enough, and the surfaces become dehydrated. The outer surface becomes a different material from the inside and it becomes a hard mass.
The attraction of the leather fibers for one another will result in some stiffness upon drying and a physical shrinkage of the leather.Drying methods that involve mechanically holding the leather in an extended position will result in a larger area. Tacking, pasting, toggling and vacuum drying all employ this principle.
Leather is dried to a very low moisture content so as to bring about permanent fixation of the materials within it.Drying, therefore, is a chemical as well as a physical activity.
It is very important to adjust the temperature in accordance with the moisture content of the leather at stages of drying when using a dryer.
Leather has a  characteristic moisture content in accordance with its equilibrium with the air around it. The curve has a significant S- shape for all types of leather.The moisture content of the leather will be very low at very low relative humidities and will increase as the relative humidity increases.With a gradual increase in humidity, the moisture content of the leather will level off at a fairly constant level until the humidity of the air reaches approximately 80% relative humidity. At 80% and above, additional moisture will be taken up by the leather. At low moisture contnt the leather is stiff and will shrink in size.As the relative humidity is incresed, the moisture content, flexibility and area of the leather is increased. The normal characteristics of the leather are achieved near 50% relative humidity.