Dyeing with Vat dyes: Part 1

Dyeing with Vat dyes: Part 1

Fundamentals of application: 

Vat dyes are insoluble in water and nonionic in nature; these are converted to leuco compounds on reduction followed by solubilisation with alkali in that state these show substantivity towards cellulose. Rate of exhaustion is excellent with higher strike rate which raises chances of unlevelled dyeing. After dyeing, parent dye structure is recovered by oxidising it within fibre, when the dye molecules get trapped in situ and establish linkage with fibre through H-bonds and Van der Waals forces. Carbonyl groups are the chromophores which are changed to >C‒OH groups on reduction and then to soluble sodium derivatives (>C‒ONa) in presence of alkali. The dyeing process is little complicated; generally involves three basic steps:  

(i) Preparation of the vat, i.e., reduction of dye to its leuco form followed by its conversion to sodium salt.  
(ii) Dyeing, and 
(iii) Oxidation to parent dye 

Figure 1:

Dyeing with Vat dyes, fabric dyeing with vat dyes

Although molecular size of dye is too small compared to pore size of cellulosics – after diffusion and oxidation – these crystallise to form big stable aggregates during soaping and cannot come out providing superior fastness properties.

Physical form of dye:
Vat dyes are available in various commercial forms. Powder form has large particle size, (ultra-concentrated or uc grade) and is designed for exhaust dyeing. Fine micro-form (small particle size, fm), fine ultra-dispersed form (fd, ud) and superfine (sf) qualities – all are suitable for continuous dyeing while paste form is suitable for printing.

All vat dyes are water insoluble, anthraquinoids are mostly soluble in hot dimethyl sulphoxide (DMSO) and can be extracted from dyed sample at boil without any change in λmax; C=O groups in anthraquinone dye act as chromophore while groups such as –NH2, –OH, alkylamino (NHR, NR2), benzamide (NH–CO–RH) and alkoxy (–OR) are auxochromes. Dyes possess excellent colour fastness properties. One characteristic feature of anthraquinone dyes is that on reduction these show typical change in colour and original colour is restored on oxidation. Dyes are costlier, full shade range is available; some of yellows, browns and oranges have marginal poor light fastness. 

Vat dyes are classified based on their chemical structure and method of application.
1. Chemical structure  

Figure 2:

Dyeing with Vat dyes, fabric dyeing with vat dyes

2. Method of application 
This classification is based on condition and concentration of chemicals required for reduction, solubilisation and dyeing. Based on these facts, selective vat dyes can be applied in any of these four methods, viz. 
(i) IK dyes (Indanthrene cold dyes): Dyes belonging to this class require less alkali, less Na2S2O4, low vatting (35–50°C) and dyeing temperatures (30°C) but lack affinity. To promote exhaustion, electrolyte applied in large quantity. 
(ii) IW dyes (Indanthrene warm dyes): These dyes are applied in the presence of moderate amount of alkali, moderate amount of Na2S2O4, lower vatting (45–50°C) and dyeing temperatures (40–45°C). Dyes possess moderate affinity and require moderate amount of salt to promote exhaustion. 
(iii) IN dyes (Indanthrene normal dyes): Dyes belonging to this class require relatively high alkali concentration, higher amount of Na2S2O4, high vatting (55–60°C) and dyeing temperatures (50– 55°C). Dyes show very good affinity for cellulosics and application of salt at very lower concentration completes exhaustion. 
(iv) IN special dyes: Dyes belonging to this group require exceptionally high alkali as well as Na2S2O4 concentration. Vatting and dyeing are done at or above 60°C, no electrolyte is used due to excellent affinity of reduced and solubilised dye for fibre.  

Table 1:

Dyeing with Vat dyes, fabric dyeing with vat dyes

Requirement of Na2S2O4 concentration (sodium hydrosulphite or sodium dithionite) is proportional to number of C=O groups to be reduced for a specific dye as well as to ensure reduced state till end of dyeing. The same is true for NaOH for solubilisation of reduced dye. It implies that an IK dye possesses less C=O groups, structure of dyes in this class are simple, favouring reduction at lower temperature with less reducing and solubilising agents. A vat dye possesses a few numbers of C=O groups in structure, but only a very few are to be reduced and solubilised for efficient dyeing, reduction of all C=O groups may cause over-reduction. While approaching from IK to IN special dye classes – in general – more C=O groups need to be reduced and structure becomes more compact; with obvious increase in concentration of Na2S2O4 and NaOH along with increase in vatting temperature. Indigo comes under IK class but follows completely different technique of application. Sulphurised vat dyes are applied either as vat dye or sulphur dye but reduction or dyeing is done as that used for a vat dye. 

Dye is pasted with T R oil, water is added into it and the mixture is heated up to required vatting temperature depending on the class of dye in jigger, one-third of total NaOH is added followed by addition of one-third of Na2S2O4; the solution is stirred till colour of the solution is changed with coloured foam at the top layer of solution; 10 min is allowed for complete reduction at vatting temperature. Vatting and dyeing conditions as well as relative concentration of chemicals.

Table 2:

Dyeing with Vat dyes, fabric dyeing with vat dyes

In this reduced and solubilised dye, another one-third of total NaOH and Na2S2O4 each are added and a specific dyeing temperature is maintained as shown above. Pretreated wet cotton is dyed in this bath for 30 min after which salt is added and dyeing is continued for further 1–2 hrs depending on the depth of shade. Throughout dyeing, remaining one-third of Na2S2O4 and NaOH are added in small amounts at regular intervals to retain dye in reduced and solubilised form and to prevent precipitation of oxidised dye on goods. A too low vatting temperature causes incomplete reduction but if it is high, over-reduction of dye may occur. IN special dyes are not generally over-reduced if concentration of Na2S2O4 and NaOH are in excess. After dyeing, a cold wash is imparted followed by oxidation of dyed material at 50–60°C with 1–2 ml/l H2O2 (35%) for 15–20 min, soaped or steamed at boil for 15–30 min, finally a thorough wash completes the process.

All vat dyes are to be essentially reduced and solubilised before dyeing. 

Figure 3:

Dyeing with Vat dyes, fabric dyeing with vat dyes

Indigo requires around −700 to −750 mV and anthraquinoid dyes require −800 to −1000 mV reduction potential (r p) for complete reduction depending on the class of vat dye, i.e., IK, IN etc; IK requires the lowest range of reduction potential while IN special the highest. Sodium hydrosulphite (Na2S2O4) is the universally accepted reducing agent. It reduces insoluble vat dye to partially soluble leuco dye, counter-acts effect of dissolved oxygen in water, which otherwise may precipitate a part of reduced dye through oxidation. Little excess of it retains stability of reduced liquor required for levelled dyeing, and it produces a sediment free clear reduction bath. Na2S2O4 – if in too excess – retards the rate of dyeing and large excess results over reduction as well as wastage of dye. It releases nascent hydrogen required for reduction when added in water in presence or absence of alkali.
Na2S2O4 + 4H2O = 2NaHSO4 + 6H
Na2S2O4 + 2NaOH = 2Na2SO3 + 2H
Na2S2O4 reduces indigo at 30–50°C, resulting in formation of mostly biphenols (leuco-indigo). The reduced form is quite stable in presence of NaOH to carry out dyeing at room temperature. Other vat dyes are reduced at a temperature depending on the class of dye. However Na2S2O4 is very unstable, at the time of its reduction, it decomposes through hydrolysis, thermal decomposition, oxidative decomposition and in various other ways to produce a list of toxic sulphur products requiring 2–3-times higher amount over the stoichiometric requirement.  
(i) Hydrolytic decomposition
2Na2S2O4 + H2O → Na2S2O3 + 2NaHSO3
(ii) Thermal decomposition 
2Na2S2O4 + 2NaOH → 2Na2S2O3 + 2Na2SO3 + H2O 
3Na2S2O4 + 6NaOH → 5Na2SO3 + Na2S + 3H2O 
(iii) Oxidative decomposition 
Na2S2O4 + O2 + 2NaOH → Na2SO3 + Na2SO4 + H2O 
2Na2S2O4 + O2 + 4NaOH → 2Na2SO3 + 2H2O 
(iv) Other possible reactions 
Na2S + H2O → NaHS + NaOH 
4NaHS + O2 → 2Na2S2 + 2H2O 
2NaHS + 2O2 → Na2S2O3 + H2O 
2Na2SO3 + O2 → 2Na2SO4 
Na2S2O3 + H2 + 2NaOH → Na2S2O3 + Na2S + 2H2O
A few by-products such as Na2S, NaHS etc pollute air by forming H2S and salts of sulphur (e.g. sulphates and sulphites; Na2SO3, NaHSO4, Na2SO4, Na2S2O3), contaminate sewage, lowers its pH and show corrosive action on concrete pipes.. Other problems related to use of Na2S2O4 are its cost and storage stability. A very attractive feature of Na2S2O4 as reducing agent is that it can generate −700 to −1000 mV of reduction potential and required reduction potential can be varied by changing its concentration, alkali or temperature. Consumption of Na2S2O4 can be reduced by 15% with 0.25–0.5 g/l dextrin or sodium borohydride, by 30–35% with Na2S (65–70 parts Na2S2O4 and 35–30 parts Na2S) or by 50% with a mixture of dextrin and NaHSO3 (1.5–2.0 g/l each). These additives cannot reduce vat dyes rather show a synergistic effect on stability of reduced bath and so are to be added in the bath to start dyeing but vatting must be done with Na2S2O4. 

Over-reduction of vat dyes: 
When Na2S2O4 is applied in too-excess or when temperature of vatting is not maintained properly, few vat dyes get over-reduced and pose problem in developing true shade. Over-reduction is of several types and depends on structure of dye. Most of these over-reduction processes are irreversible and cause permanent change in dye structure. 
Simple over reduction 
Simple over reduction of some vat dyes occur either at high concentrations of Na2S2O4 or at high temperature. Dyes containing heterocyclic rings with nitrogen atoms and where not all the keto (>C=O) groups present in the molecule are to be reduced and are liable to be over-reduced. Indanthrene blue and yellow dyes are susceptible to this type of change, e.g., only two >C=O groups of indanthrene blue dyes should be reduced under ideal conditions of reaction, but when over-reduction occurs, all four >C=O groups are reduced.

Figure 4:

Dyeing with Vat dyes, fabric dyeing with vat dyes

Vat dyes containing benzyl amino or acyl amino group (yellow GK and gold orange 3G) are hydrolysed at high temperatures and prolonged vatting time resulting duller and less fast shades. During hydrolysis, –NHCOC6H5 group is changed into –NH2 group. 

Figure 5:

Dyeing with Vat dyes, fabric dyeing with vat dyes

Halogenated dyes such as violet 2R, violet 3B, violet 2RN, blue RC, blue BC and green 2G, loose halogen in its structure at high temperature vatting/dyeing. Dehalogenation results poor bleach fastness of dyeings.  
Brill. Violet 2R  Violet B
Green 2G  Green XBN

Molecular rearrangement 
Certain vat dyes like khaki 2G, olive T, grey M, when dyed at or above 66°C show permanent change in their hue caused by the irreversible change of some >C=O groups to >CH or >CH2 groups. 

Figure 6:

Dyeing with Vat dyes, fabric dyeing with vat dyes
Chances of over-reduction may be reduced partially by adding HCHO or glucose in bath. Manufacturer’s literature is to be consulted to prescribe right concentration of reducing agent and dyeing parameters. Different vat dyes possess several number of C=O groups and not all but only a few of these are to be reduced. Also – as a simple rule – if x kgs of Na2S2O4 is used in dyeing, 0.5x kg is to be used for reduction, 0.25x kg to be added in bath at the start of dyeing and rest at different times of dyeing to retain dye in reduced form; same technique may be applicable for NaOH also. Presence of little excess of Na2S2O4 may be tested by dipping vat yellow paper in dyebath; change in yellow colour to deep blue shows presence of excess Na2S2O4. The yellow paper is basically a filter paper dyed earlier with vat yellow 3RT dye (C I Vat Orange 11, C I 70805) and the blue hue is due to reduction of this dye. Based on this fact, other vat dyed papers may also be used to check presence of Na2S2O4 accordingly. 
Dyeing with Vat dyes: Part 1 Dyeing with Vat dyes: Part 1 Reviewed by Suraj Gupta on April 26, 2020 Rating: 5

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