Dyeing of silk and wool with mordant dyes and metal-complex dyes

Dyeing of silk and wool with mordant dyes and metal-complex dyes

Metal-complex dye:

In dyeing wool with acid dyes, the ionic bond between –NH3+ and DSO3- is weaker and as a result these bonds are easily broken and reformed under favourable circumstances allowing dye molecules to migrate. Stripping out of colour or staining of adjacent white wool during domestic washing remains a problem with acid dyeings. If the bond strength between dye and fibre can be remarkably enhanced – the dye structure can be made sufficiently larger – this nature of migration of acid dye can be reduced or arrested. Metal-complex dyes are mostly acid dyes possessing chelating sites to enable these to be combined with metal atoms; invariably used for dyeing of wool, silk and nylon to produce colourfast shades. The dye–metal complex – when produced during dyeing is called a mordant dye and when produced at the dye manufacturing plant – is called a premetallized dye. 

Mordant dyes:

A mordant is a simple chemical which possesses affinity for both fibre as well as dye. If a dye has less or no affinity for fbre then mordant – already applied on fbre – improves affinity of dye and makes a dye–mordant–fbre complex. Mordants are basically metal salts and electrically cationic. In contrast, all acid dyes are anionic in nature and their affinity is increased many folds for fibre when mordant is applied on fibre before application of dye. Various natural dyes, e.g., logwood black, madder etc., are also applied on wool with the help of mordant, e.g., alum, chrome, iron and tin salts – brighter colours and better fixation are obtained only by pre-mordanting. A few substantive or nonmordant dyes, e.g., turmeric, berberis, dolu, annatto and henna contain tannin in the colouring matter itself as natural mordant and produce fast colours at boil. In this process of complex formation, out of six coordination sites of chromium, three are used by dye and rest three by water molecules in acidic/neutral pH or hydroxyl ions in alkaline pH. .Poor wash fastness of dyeings with acid dyes (specifically strong acid dyes) is due to smaller size of dye molecule as the ionic attachment is weaker in nature. Acid dyes possessing chelating sites in its structure can be reacted with metal salts in various ways to develop bigger dye–metal complex through coordinate and covalent bond formation, they are known as mordant dyes All mordant dyes are acid dyes but the reverse is not true. In simpler techniques, chromium salts are invariably used for complex formation and the reactions are carried out in shop-floor level to make dye–metal complex and are also called chrome dyes. Chromium salts act as efficient mordant, e.g., Na2Cr2O7, K2Cr2O7 etc., and not only promote dye–metal complex formation to improve fastness but also enhance acid dye uptake due to its cationic nature when applied on wool before dyeing. These mordant dyeing methods are useful tools for cottage industries to dye wool with superior wash fastness. A single mordant with different mordant dyes or vice versa produces a long range of hues. Mordant dyes are classified chemically as azo, anthraquinone, oxazine, xanthene, triphenyl methane, nitroso and thiazine types, in which only azo dyes cover whole range of spectrum and are of paramount importance Mordanting is taken into action before, after or concurrently with the dyeing process with efficient mordants. Depending on the stage at which mordants are used, chrome dyeing methods are classified as: 
(i) on-chrome
(ii) metachrome and
(iii) after-chrome. 

On-chrome method
Fibre is chromed frst followed by dyeing with selected mordant dye. Chromium in higher valency state is applied on woollen material, by treating in K2Cr2O7 solution either in neutral condition – known as sweet chrome – or in acidic condition – known as sour chrome method. Reduced chrome method is used for dyes which are discharged by oxidising agents.
In ‘sweet chrome’ method, wool is treated with K2Cr2O7 solution (2%) at neutral pH at 60°C for 30 min, when chromium in higher valency state is absorbed by wool forming a wool–chromium complex consuming only half the amount of total chromium. In ‘sour chrome’ process, K2Cr2O7 (1–2%) is applied with H2SO4 (1%) at 60°C for 30 min, when all the chromium is absorbed by wool. In both these cases, chroming is succeeded by dyeing with selective mordant dyes. ‘Reduced chrome’ method is especially useful for those dyes which are susceptible to oxidation. The process is carried out using K2Cr2O7 (2%) and
lactic acid (3–4%) at 60°C for 15–20 min. All ‘on-chrome’ methods are a two bath process, expensive, usually only one lot of material can be chromed, washed-off and dyed in a day, more suitable for light and medium shades. In contrast – matching of colour is relatively
easy – regular building of shade occurs and not complicated by sharp changes
in shade of material during dyeing. 

Metachrome method
Milling and super-milling acid dyes with chelating sites are most suitable for
this process, which exhaust well at pH around 6–8.5. In this single bath process, dye and mordant are simultaneously applied from the same bath. In spite of being electrically opposite in nature, dye and mordant do not react initially rather both are exhausted on the substrate after which dye–mordant reaction occurs in situ under favourable conditions to develop the hue. If conditions are not maintained properly during exhaustion, dye and mordant react in bath and get precipitated. Chromium is deposited on or combines with fibre followed by reduction of CrO3 to Cr2O3 at boil. At the same time, dye combines with wool in the
same way as that with a milling acid dye. Dye and reduced chromium then combine with fibre to give dye–chromium–wool complex. All three reactions probably occur simultaneously in bath.  
(NH4)2SO4 → H2SO4 + 2NH3
2Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O
Na2Cr2O7 + H2SO4 → Na2SO4 + 2Cr2O3 + H2O
Cr2O3 + dye → Cr–dye lake

Chances of formation of other metal–dye–fibre complex due to the presence of other metals in water used for dyeing rather than with chromium is prevented by chromates which form protective oxide film on the surface of the metal and does not allow it to take part in the reaction. Dyebath contains dye, (NH4)2SO4, Na2CrO4 (sodium chromate) or Na2Cr2O7 usually. (NH4)2SO4 promotes deposition of CrO3 on wool without damaging it and generates a pH of around 6–8.5.

Na2CrO4 + H2O → 2NaOH + CrO3
2NaOH + (NH4)2SO4 → Na2SO4 + 2H2O + 2NH3
Na2CrO4 + (NH4)2SO4 → Na2SO4 + 2NH3 + CrO3 + H2O

Due to generation of more and more NH4OH, bath is likely to be more alkaline with time in a closed dyeing machine. Pretreated wool is treated with chrome mordant (2–5%), Glauber’s salt (5–10%) and a surfactant (0.5 g/l) at 40–50°C for 20 min. Dissolved dye is added, temperature is raised to boil over a period of 45 min and treatment is continued at this temperature for further 60–90 min. Complete exhaustion may be affected by adding 0.5–1% of CH3COOH (40%) or half the amount of HCOOH (85%), 30 min before dyeing is completed. (NH4)2SO4 may be substituted with CH3COONH4 when dyes in use are known to cause uneven dyeing. CrO3 is reduced to Cr2O3 by wool during this process.
Metachrome process is a single bath process, simple to apply with higher rate of production. Shade matching is easier because the final shade is produced during dyeing. Chromium does not form complex with dye in chromate form and so chromate and dye can be applied from the same bath; pH of dye bath is around 6–8.5 causing minimum damage to wool too. Dulling of shade due to iron or copper is absent. One prerequisite is that dyes which do not
react with Cr6 can be used in this method. However, range of shade is limited
as only a very few dyes are suitable for this method. Uneven dyeing cannot be
rectified. In deep shades, the dye is apt to rub due to precipitation of it. Due to
relatively higher pH of bath – exhaustion of dye remains incomplete – particularly with deeper shades – blacks, navy blue and other deep shades are seldom dyed in this method 

After-chrome method
Chrome dyes are first applied on wool in the presence of acid followed by
K2Cr2O7 (1–2%) in the same bath at 60–80°C. Dyeings show superior fastness to milling and potting. Shade matching is not easy and any change in shade at the last moment causes trouble.


Premetallised/metal-complex dyes

To avoid obvious complications associated with reaction of metals with dye possessing chelating sites in shop-floor level – in many cases – the reaction is carried out in the dye manufacturing plant itself and the dye–metal complex is supplied to the dyeing sector, known as premetallised dye. Selective acid dyes are complexed with copper, chromium, cobalt or nickel at suitable proportions
to develop these dyes. Reaction of one atom of metal with one molecule of
dye produces 1:1 metal-complex dye, whereas reaction of one atom of metal
with two molecules of dye produces 1:2 metal-complex dyes. All the valencies of a metal are not coordinated with only one dye molecule and so unsaturation exists in the 1:1 complex, leading to formation of 1:2 unsymmetrical complex with two different dye molecules – if such scopes exist.
Premetallised dyes are produced mainly from o-o′-dihydroxyazo, o-carboxy o′-hydroxyazo, o-amino o′-hydroxyazo and o-hydroxy o′-methoxyazo types of acid dyes. These dyes are applied from a very strong acidic bath (8% H2SO4). Due to the absence of dulling effect caused by wool–chromium complex, shades are brighter and matching is simple compared to mordant dyes. All light to heavy shades can be easily produced with these dyes. Light fastness is parallel to mordant dyes with slight deterioration in wash fastness. Dyes are water soluble because of –SO3H groups in structure. Dyeing at boil only gives the true shade and tone. Higher dosing of acid may be reduced to just half with application of cationic levelling agents, e.g., Palatine fast salt O and Neolan Salt II, which are fatty alcohol – ethylene oxide condensates possesses a general formula CH3–[CH2]x–O–[CH2–CH2O]–H. Levelling agents form loose complex with dye as these are of opposite electrical nature. The complex is absorbed by the fibre and at boil it breaks to release levelling agents. Thorough wash and neutralisation at the end are essential to remove acid from fibre. Dyeings can be discharged to white and so are extensively used in fancy prints.1:1 dyes exhaust too less from a neutral bath with poor rubbing, milling and light fastness. Being lighter in structure, these possess affinity for vegetable and acetate fibres too. However, strong acidic pH used during application of these dyes causes damage in wool as well as poor exhaustion of bath due to higher solubility of –SO3H groups (oxygen in –SO3H is electron donor to H of water). This led to the birth of 1:2 metal-complex dyes with the following pre-conditions: (i) the complex should carry a negative charge, (ii) any residual valency of the metal atoms must be fully or nearly satisfied, (iii) solubilising groups should be less hydrophilic, may be by introducing sulphonamide groups in place of –SO3H, (iv) complex should retain no or minimum hydrophilic groups and (v) all dyes in range should have identical exhaustion property. These dyes have too little affinity for fibre; being salts of strong acids, these ionise in water with lesser solubility and applied from mild acidic or neutral baths. The degree of exhaustion and wash fastness are function of molecular weight of dye and around a weight of 600, both these functions remain fairly good. Inclusion of water solubilising groups, e.g., –SO2–CH3, –NHCOCH3, –SO3–CH2 etc., improve their solubility in bath.1:2 metal-complex dyes are simple to apply, have shorter dyeing time with perfect levelled shade, possess excellent fastness and exact reproduction of 1:2 metal-complex dyes is easy. These dyes have practically no migrating power necessitating proper control over pH and slow rate of heating as rate of dyeing is rapid just below boil; the degree of migration depends on temperature, nature of dye and fbre. Remarkable migration occurs below 82°C, beyond which it decreases. Rate of exhaustion depends on pH, temperature and agitation of bath. An optimum pH around 6.5 at boil may be sufficient for level dyeing with maximum dye uptake, to increase rate of dyeing, pH may be lowered down to 4 or below it. Due to poor migrating power, agitation of bath is essential from start of dyeing for even distribution of dye. Matching of shade and required addition may be done at boil after adding little –NH3 to raise pH just to slow down rate of exhaustion. Salt alone is not capable of showing any retarding or levelling action but in the presence of levelling agents like Lyogen SMK (Sandoz), it promotes levelling effect.

Dyeing of silk and wool with metal-complex dyes:

Silk loses a part of its inherent lustre when treated in boiling water and to preserve aesthetic appeal, 1:2 metal-complex dye is the best choice for dyeing silk at 90°C in the presence of either acetic acid alone or in combination with ammonium sulphate (5%) – presence of ammonium sulphate as buffer
increases dye uptake significantly. After-treatment with cationic dye-fixing agent (2–4 g/l) at 40–50°C for 15–20 in indeed improves wash fastness. Metal-complex dyes adsorbed on silk inhibit marginal migration due to high stability of the dye–fibre linkages and produce invariably dull shades with good light and wash fastness.

Dyeing of silk and wool with mordant dyes:

Mordant dyes are acid dyes having chelating sites to form stable coordination
complex with metal ions from metal salts (mordants). Dyes can form chelates with different mordants to develop various shades with superior wash fastness. Na2Cr2O7 and K2Cr2O7 are the commonly used mordants for wool because of the presence of reducing thiol groups; absence of such reducing
groups in silk can be overcome by using basic salts of chromium to form dye–chromium complex. The dye–mordant reaction or chelation requires presence of salt forming groups, such as hydroxy or nitroso groups as well as presence of oxygen or nitrogen containing groups, such as carbonyl, carboxyl, azo etc., so that a lone pair electron can be donated to the chromium atom. Dye bath is set up with Glauber’s salt (10%) and acetic acid (4%). Temperature is raised to 50°C and silk is entered in the bath, temperature is raised to 90°C over 30 min after which formic acid (2%) is added for complete exhaustion of dye. Dyeing is continued for further 15 min and then dried ithout washing. Mordanting is done with mordant solution (3%) for 16 hours at 30°C followed by washing and drying – amount of mordant exactly required depends on the number of dye molecules on fbre as the chelation occurs at stoichiometric ratio. However, mordant dyes have restricted application on silk due to formation of dull shades and deposition of chromium on silk imparts rigidity to it with loss of its characteristic scroop.
Direct dyes on wool and silk, nylon produce only dull shades, reactive dyes unlevelled shades and basic dyes poor light fast shades and are not applied on these fibres if not in blends.
Dyeing of silk and wool with mordant dyes and metal-complex dyes Dyeing of silk and wool with mordant dyes and metal-complex dyes Reviewed by Suraj Gupta on April 25, 2020 Rating: 5

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