Vat Dyes Properties, Classification and Application
What is Vat Dye/ Vat Dye Definition
This chapter discusses vat, indigo and sulphur dyes together because their use in dyeing involves the same principles. Vat dyes are one of the oldest types of dye Vat dyes in particular give dyeings on cellulosic fibres with the best overall fastress properties. Because of the popularity of blue jeans, Indigo is still one of the most important of all dyes in present use. Natural Indigo was obtained by extraction from leaves, and by fermentation in wooden vats, the origin of the term ‘vatting Today, Indigo is synthesised from manufactured intermediates. Its application involves reduction to the water-soluble leuco compound, dyeing the cotton and re-oxidation of the leuco dye in the fibres to the insoluble pigment, The basic steps involved in vat dyeing
Vat dyes are water-insoluble pigments. They are called dyes because the chemical reduction in alkaline solution converts the pigment into a water-soluble leaf form with substantivity for cotton. The vat pigment and the leuco compound often have quite different colours — blue and pale yellow in the case of Indigo- and the progress of the reduction is often easy to observe. After dyeing with the lead compound, the pigment is regenerated in the dyed cotton by oxidation. The overall process therefore involves three key steps:
(1) reduction of the pigment to the soluble leuco compound, a process called vatting:
(2) absorption of the leuco compound by the cotton during dyeing.
(3) oxidation of the absorbed leuco compound in the cotton, reforming the insoluble pigment inside the fibres.
The use of strongly alkaline solutions (pH 12–14) for vatting and dyeing limits the use of most vat dyes on cellulosic fibres.
CHEMICAL CONSTITUTION OF QUINONE VAT DYES
In the case of Indigo, vatting involves reduction of the a pair of conjugated carbon groups (Figure 1.4). For most vat dyes, these carbonyl groups are present in an anthraquinone or polycyclic quinone. The parent quinone is an insoluble pigment. The leuco compound is the sodium salt of the diphenol or hydroquinone formed by reduction in NaOH solution (2). It is reasonably soluble in water, substantive to cotton and easy to oxidise back to the red pigment Reduction is carried out in a strongly alkaline solution to avoid the formation of the vat acid (3). This is the free diphenol formed by protonation of the anionic leuco derivative. The vat acid is malleable in water, has no substantivity for cotton and is more difficult to oxidise
Ci Vat Red 42
leuco compound formed by reduction
vat acid formed by protonation of the anionic leuco derivative
Quinone vat dyes are usually either anthraquinone derivatives (1) or polycyclic quinones. Indanthrone (4), the first quinone vat dye, was discovered by accident during attempts to make an Indigo-type derivative from 2-aminoanthraquinone The structures of most vat dyes are often quite complex and have no ionic groups. Many of the polycyclic derivatives have no attached substituents, but a few have halogen, hydroxyl, methoxyl or amide groups. The more complex dyes may have several reducible quinone groups
The molecular structure of a vat dye influences the water solubility, substantivity and rate of diffusion into the cotton of the leuco compound used in dyeing. A classification of quinone vat dyes according to dyeing characteristics is more useful for our purpose and will be given later in this chapter.
Molecular structures of complex anthraquinone and polycyclic vat dyes
THE REDUCTION OF QUINONE VAT DYES
Vat dye dispersions
Quinone vat dyes are available as fine powders or grains and as liquid dispersions or pastes. The insoluble pigment is extensively milled with dispersants such as sodium lignosulphonates to produce very small particles (Section 15.3). Typically, at least 50% of the particles will have diameters less than 1 um. All forms must disperse readily in water and any aggregates of particles must easily break up to give a fine, stable dispersion. The rate of reduction to the leuco form is usually faster, the finer the pigment dispersion.
Solid forms contain much dispersing agent and more may be added to the dyebath to keep any unreduced dye in suspension. The addition of The additionydrosulphite (dithionite) (Na, S, O,) to the alkaline aqueous dispersion of the vat dye brings about rapid reduction. In pigmentation dyeing methods, the fine pigment particles are uniformly deposited in the goods by padding or liquor circulation before reduction occurs. Very fine dispersions are needed for these
pigmentation processes and the proportion of fine particles is higher than for what pigments in the traditional vetting process. Dye powders for pigmentation dyeing have higher dispersant content. Partes and liquid forms are useful for printing and continuous dyeing. They will usually have less dispersant but contain appreciable amounts of water. Some ranges of dyes are deliberate mixtures of water and dispersed dyes for dyeing cotton/polyester blends
Reducing VAT dyes with hydros
Reduction of quinones is not easy and vatting requires a strong reducing agent The dihydrate of sodium dithionite, or hydrosulphite. Na2S2O4.2H, O. is used almost exclusively and goes by the common name of these. Vatting U came out under strongly alkaline conditions for several reasons
(1) The reduction of the quinone (QO2) with hyds to give the leuco dye (QO2-), and the oxidation of the hydros by oxygen, consumed alkali.
(2) It is important that the insoluble vat acid does not precipitate
(3) Hydros is unstable in solution, particularly under acidic or neutral conditions and at higher temperatures. Its decomposition forms thiosulphate and bisulphate. This decomposition reaction consumes hydroxide ions and it is faster as the solution becomes more acidic
QO2 + Na2S204 + NaOH = QO2- + 2Na+ + 2Na2SO3 + 2H2O
Na S₂O + 2NaOH+ 0₂ = Na2SO3 + Na₂SO4+ H₂O
2Na2S204 +2NaOH = Na2S2O3 + 2Na2SO3+ H₂O
The decomposition of hydros is exothermic. Decomposition of solid Na2S2O4.2H, O can lead to spontaneous ignition. Cool, dry conditions are therefore necessary for storage. The decomposition of hydros occurs rapidly in an acidic solution. Alkaline hydros solutions are more stable provided oxygen is absent;
Because hydros reacts readily with oxygen (Scheme 17.1), the material being dyed with the leuco dye should not be over-exposed to air. Oxidation of the excess hydros can lead to subsequent oxidation of the leuco dye and precipitation of the quinone, giving uneven surface deposits of pigment on the goods. A significant excess of hydros in the bath is usual. The amount used depends on the dyes and the type of machine. The suppliers give suitable recommendations for their shades. Other more stable reducing agents, such as sodium sulphoxylate-formaldehyde (HOCH, SO, Na), are used in printing with VAT dyes to ensure the stability of the pastes. Steaming of the printed material activates this reducing agent and the vat dye in the print paste is reduced. Sodium sulphoxylate-formaldehyde is a poor reducing agent at 25 °C and is more stable than hydros Other reducing agents such as thiourea dioxide and glucose are now being suggested as more environmentally friendly alternatives to hydros.
Vatting
Traditional vatting involves adding hydros to a fine, relatively concentrated aqueous dispersion of the vat dye pigment containing NaOH. The vatting temperature is often higher than the subsequent dyeing temperature. This gives rapid reduction. The concentrated solution of the leuco form is then diluted by adding it to water in the dye bath. The bath water also contains NaOH and some hydros to avoid any oxidation from dissolved atmospheric oxygen. Vatting may also be carried out directly in the dye bath if the dye is readily reduced.
Dyeing takes place at temperatures lower than the vatting temperature to avoid decomposition of the hydros. This also decreases the risk of over-reduction of polyquinone dyes such as Indanthrone. Over-reduction is the reduction of more than one pair of conjugated carbonyl groups in a polyquinone and often results in poor colour yields and off-shade dyeings. Reduction inhibitors, such as sodium nitrite or glucose, prevent over-reduction during vatting. Anthraquinone and polycyclic vat dyes give leuco solutions with intense colours, often very different from those of the original pigment dispersion. The vat dye flavanthrone is a yellow pigment that produces a deep blue vat. It is often used in paper strips for the detection of excess hydros.
Vatting conditions for quinone vat dyes vary widely. Three main types of vatting process are used depending upon the dyes being used. These involve
(1) a concentrated NaOH solution and a high vatting temperature (60 °C);
(2) a moderate concentration of NaOH and a lower Vatting temperature of 50C
(3) a low concentration of NaOH and a low vatting temperature (30–40) C
THE SUBSTANTIVITY AND DYEING CHARACTERISTICS OF VAT DYES FOR CELLULOSIC FIBRES
Basic steps in the dyeing process
The dyeing of cellulosic materials with quinone vat dyes follows a four-step sequence:
(1) preparation of the vat containing the leuco forms of the dyes;
(2) dyeing of the material, in which the fibres absorb the water-soluble leuco compound;
(3) oxidation of the absorbed leuco compound back to the parent pigment inside the fibres;
(4) Soaping of the dyed material to remove pigment loosely adhering to the fibre surfaces and to develop the true shade and fastness properties.
Classification of vat dyes
For quinone vat dyes, there is no single classification according to dyeing properties as is the case for the direct dyes. The German Interessen Gemeinschaft für Farbenindustrie (IG) developed one popular classification for their
Indanthrene range of vat dyes based on leuco compound substantivity and the required dyeing conditions.
There were three main types:
(1) The IN (indanthrene normal) group of dyes require the use of concentrated NaOH and high vatting (60 °C) and dyeing temperatures (60 °C). No salt is added to the dyebath because of the high substantivity of the leuco dyes for cotton;
(2) The IW (indanthrene warm) group of dyes require only moderate amounts of NaOH and lower vatting (50 °C) and dyeing temperatures (50 °C). The leuco forms of these dyes have moderate substantivity for cotton and some addition of salt is needed during dyeing to aid exhaustion;
(3) The IK group of dyes only need a low concentration of NaOH with low vatting (40 °C) and dyeing temperatures (20 °C). These dyes have low substantivity for cotton and need considerable salt for good dyebath exhaustion. Some have amide groups that would be hydrolysed under the vatting and dyeing conditions used for IN and IW dyes.
There are special processes for some black vat dyes that require an oxidative after treatment to develop the full black colour. Table 17.1 compares the characteristics of these three types of vat dye. The required concentrations of hydros, caustic soda and salt increase with increasing amounts of dye in the bath and with increasing liquor ratio.
Table 17.1 Characteristics of the three main types of vat dye
Vat dye type
Conditions IN IW IK
NaOH concentration (g 1-¹) 4.5–7.5 2.0–4.0 1.5–2.5
NaCl concentration (g 1-¹) 0 5–25 10–40
Dyeing temperature (“C) 50–60 45–50 20–25
Vatting temperature © 50–60 45–50 40
There are various other classifications of vat dyeing methods. The SDC recommend tests to determine the best dyeing method [1]. In this, the colour strengths of dyeings produced under different dyeing conditions are compared with those of standard dyeings using a grey scale. This test applies only to anthraquinone dyes. There are also SDC tests for determining the strike. migration and levelling characteristics of vat dyes. Different companies have different classification systems for their vat dyes. Because vatting and dyeing conditions vary from one dye to another, the suppliers’ recommendations should be consulted.
The high substantivity of leuco vat dyes
The origin of the substantivity of leuco vat dyes for cotton is far from clear Hydrogen bonds between cellulose hydroxyl groups and phenolate ion groups, or amino or amide substituents, in the leuco dye may be involved in dye-fibre binding. The substantivity of the leuco dye is very dependent on the position of the phenolate ion groups and on the actual molecular structure. Substantivity tends to be greater the larger the surface area of the leuco dye molecule and van der Waals interactions between the dye and fibre are important. Molecules of leuco vat dyes tend to be large and, although coplanarity of the structure is essential, the structures are often not as long and linear as for direct dyes. Although the planar molecular structures of leuco vat dyes should promote aggregation, there is not much evidence of this in dyeing solutions.
In the dyeing process, the influence of salt addition and increasing dyeing temperature are similar to the effects found in dyeing cotton with direct dyes. Unlike direct dyes, however, dyeing with many leuco vat dyes is very rapid at relatively low temperatures and usually gives high exhaustion at equilibrium. The ability to control the dyebath exhaustion by salt addition is very limited since the bath initially contains a high concentration of sodium ions from the added hydros and caustic soda. Because of this, many dyes have a rapid strike. The debate maybe 80–90% exhausted within 10 minutes. Because there is little diffusion into the fibres within such a short period, there is a strong tendency for cotton yarns and fibres to be ring-dyed in the early stages of dyeing. Diffusion only becomes important well after the time of half dyeing. The high strike must be controlled to avoid unlevel dyeing since the low dyeing temperatures do not favour migration The behaviour of the leuco dyes is assessed by the usual migration and strike tests in comparison with the behaviour of standard dyes [1]. The dyeing methods recommended correspond to the IN, IW and IK methods described. The rate of diffusion of a typical leuco vat dye into cotton is about half that of direct dye at the same temperature. The high rate of dyeing of leuco vat dyes is therefore a consequence of their high substantivity for cellulose. Since the standard affinities of leuco vat dyes are no higher than those of direct dyes, the
high substantivity and rate of dyeing are caused more by the high electrolyte content of the dyebath than by high inherent affinity. The initial strike can be retarded using polyethylene oxide complexing res The initially formed dye-auxiliary complex presumably slowly breaks down and gradually liberate free leuco dye, thus controlling the initial rate of adsorption Both non-ionic and cationic retardants are used but these decrease the exhaustion. They can also be used for levelling of uneven dyeings. Soch dying auxiliaries should be added to the dyebath after vatting. The dye manifester’s recommendations should be consulted since these agents are often dye-specific. Similar chemicals can be used for stripping faulty dyeings. This is aavy because of the high substantivity of the leuco vat dyes and the stripping auxiliaries send form much more stable complexes with the leuco dyes than those obtained from levelling agents.
DYEING COTTON WITH LEUCO VAT DYES
Preparation
As always, good preparation of the goods is essential to avoid a non-uniform distribution of residual impurities that might hinder level dyeing or that might interact with the dyes. Water quality is important since the calcium and magnesium salts of some leuco dyes may precipitate if hard water is used. The we of sequestrants or protective colloids is often recommended.
Dyeing conditions
Quinone vat dyes are used on all forms of cellulosic materials. The vat is all pre-prepared and added to the dyebath containing the goods, possibly in successive portions. Vat dyes of high substantivity are often used for dyeing deeper shades where the effect of poor levelling is less visible. They usually follow a Freundlich adsorption isotherm (Section 11.1.1) so that using more dye gives
promotes deeper shades, but with diminishing returns. The dyeing temperature may be increased to promote migration if the dye is stable. This gives better penetration into tightly constructed materials. Subsequently cooling the dyebath p better exhaustion. Any required salt may be added in portions towards the end d the process for dyes of lower substantivity.
The liquor ratio is usually in the range of 10:1 to 20:1. The actual an of caustic soda and hydros required for vatting, and for addition to the dacha depend on many factors, such as the dyeing temperature and time, and particularly on the degree of exposure to air during dyeing. The latter is mainly a character of the particular dyeing machine used. As in dyeing cellulosic fibres with some direct dyes, the rate of dyeing with leuco vat dyes is higher at higher temperatures lower liquor ratio and lower dye concentration. It is not much affected by the relatively high NaOH and hydros concentrations. Many leuco compounds have high substantivity and level dyeing requires careful control of the dying temperature, the total salt concentration and the gradual addition of the concentrated leuco dye vat to the dyebath. Figure 17.4 shows a typical procedure for dying cotton with a quinone vat dye. amount
A typical procedure for dyeing cotton with a quinone vat dye
Problems with anthraquinone dyes.
Several chemical problems arise with some quinone var dyes. These include
(1) Multiple reduction steps for poly quinones such as indanthrones
(2) isomerism of leuco compounds to xanthones,
(3) hydrolysis of amide groups;
(4) over-oxidation after dyeing
(5) dehalogenation of some dyes.
To minimise these types of problems, the supplier’s recommendations for voting and dyeing must be followed.
Indanthrone (CI Vat Blue 4) and some of its derivatives show a number of disease problems. Indanthrone has two anthraquinone residues in its molecule. The m blue leuco compound used in dyeing that corresponds to the reduction of one of the anthraquinone groups (5, in Figure 17.5). If both anthraquinone groups are reduced, the final product (6) gives a brownish yellow solution, pe substantivity for cotton and is more difficult to residue. Such over-refin produces duller blue dyeings of lower colour yield. Indantimne sales one of the quinone vat dyes whose leuco compound will tautomerase via the mat sed ox anthrone (7) if the concentration of NaOH in the solution is not sufficiently g Oxanthrones are not easily oxidised to the parent quinine and therefore results in dyeings of poor colour yield. Mild oxidation of learn var dyes of the indanthrone type with hypochlorite produces a much greenery derivative
Anthraquinone Vat Dyes