Chapter 3: Vat Dyes and Their Application

BY J. R. ASPLAND, School of Textiles, Clemson University, Clemson, S. C.

ong before the advent of systematic 1 and 41 (2). Fig. 1 shows the structure of L physical sciences and the growth of both anthraquinone and indigo (2). The the chemical industry, as long as 5000 & present importance of the indifferent dye, years ago in India, dyeing methods were indigo, should teach a salutary lesson in being used for the application of the water 0 marketing. Satisfying the needs of the insoluble pigment, indigo, to natural fi- customer is generally more important bers. In the early nineteenth century, la) Anthraquinone than achieving the highest levels of techni- between the first cotton gin and the Civil cal performance in dyed and finished War, the value of natural indigo produced goods, although the two sometimes coin- in South Carolina exceeded that of rice or cide. Since the early 70s, customers have cotton. wanted indigo blue denim primarily for Indigo was first recovered from the the appearance which can result from the water soluble glucoside of indoxyl (indi- inferior fastness properties of the dyeings. can) present in the plants Indigofera and This is exploited by garment dyers and Zsatis tinctoria, and then the insoluble finishers in stonewashing and other wet blue product was dissolved in wooden vats lb) Indigo processes. by a natural fermentation process later Fig. I. Parent chemicalsof most vat dyes Let us now look at the general nature of known as vatting. This was the origin of vat dyes and vat dyeing so that the reasons the name vat dyes. Chemically speaking, for the vat dyeing processes in use today vatting is a reduction. after the section on functional groups in can be understood. In the right conditions, the known ani- vat dyes. mal and vegetable (cellulosic) fibers ac- Another natural vat dye, a brominated General Nature of Vat Dyes cepted the greenish-yellow material dis- derivative of indigo, was derived from the Vat dyes are sold as water insoluble solved in the vat, and on subsequent shellfish Murex brandaris, found in small pigments, so why are they called dyes? exposure of the dyed fibers to the oxygen in quantities at the eastern end of the Medi- Like all other dyes, under the right circum- the air, the color was converted back to terranean. It was known as Tyrian Purple stances, they can: that of the original indigo pigment. Resto- and was once important for its rarity and 0 Be dissolved to give solutions contain- ration of the indigo is an oxidation. Both its hue. The Roman emperors practically ing individual molecules or molecular oxidation and reduction will be discussed monopolized it, which gave rise to the term ions, royal purple. 0 Be transported to the surface of the Since the massive growth of indigo dyed fiber and denim, beginning in the 1970s, indigo is Be sorped on and diffuse into suitable ABSTRA'CT one of the most important single dyes in fiber. use today. However, little natural indigo is Note: Steps 1 and 2 are not necessarily Part 1 of this chapter covers the chemical produced now, for chemical synthesis placed in this order in dyeing processes. development of vat dyes and the general Vat pigments need alkaline reduction to principles of their application to cotton. gives a much more consistent and econom- The first vat dye discovered (and now the ical product. Synthetic indigo was first dissolve them, and they are treated to an most popular, indigo) does not exhibit the marketed in 1897. additional step (oxidation), which causes superior fastness properties or follow the In some ways, despite its long history the original insoluble pigments to be re- application methods typical of most other and current importance, it is almost unfor- generated, once the soluble ions have dyes in its class. Vat pigments, dissolved tunate that indigo must be classified as a diffused within the fiber. Nevertheless, the as leuco-vat dye anions, follow the gen- vat dye, because the present application third step, the sorption and diffusion of eral model for dye anion sorption and dif- methods faindigo and the fastness prop- suitable derivatives of vat dyes into cellu- fusion presented in Chapter 1. Practical erties of indigo dyeings are not typical of losics, parallels that of direct dyes quite dyeing principles of vat dyes will be dis- closely. cussed in Part 2 of this chapter to be pub- vat dyes as a whole. Since the discovery of lished next month. indanthrone (now C.I. Vat Blue 4) by R. One of the keys to successful dyeing Bohn in 1901, many vat dyes with no with vat dyes is to have control over how natural equivalents have been synthe- and when to convert the pigmentary colors KEY TERMS c sized. Dyeings of the products which have into water soluble coloring chemicals, survived in the marketplace normally under conditions in which they will diffuse Dyes show outstanding colorfastness proper- uniformly into the fibers. Dyeing ties, particularly to light, washing and Functional Groups chlorine bleaching, while indigo dyeings Functional Groups Indigo Leuco-Vat Anions have poor resistance to most of these and In the insoluble pigmentary form, all vat Oxidizing Agents poor abrasion resistance as well. dyes have a common chemical feature. Pigments The majority of vat dyes in use today They all contain one or more pairs of Reducing Agents are, like Vat Blue 4, derivatives of an- carbon atoms, all of which are doubly Vat Dyes thraquinone. Exceptions are C.I. Vat Reds bonded to an oxygen atom. These carbon

22 co3 Vol. 24, No. 1 .. 1

atoms are always part of a condensed 0 o@ Na@ OH system of benzene or other aromatic ring alk. reductiy acid structures (which look like chicken wire - when drawn) with a closed system of acid oxidation alkali alternating double and single bonds be- a) b) C) tween adjacent carbon atoms. Since car- Insoluble sodium vat-acid bon is involved with four chemical bonds, pigment leuco-vat these carbon-oxygen structures can be drawn in the manner shown in Table Ia or Fig. 2. Vat dye functional groups and their dyeing reactions. Fig. 2a. The same features are shown in Fig. 1. Fig. 2 shows the structures of the > functional groups of vat dyes cut to the To understand what is happening when ments cannot be restored (over reduction). bone. For simplicity, Fig. 2 only shows one vat pigments are dissolved, or the nature Alkaline solutions of hydro generally of each functional group; but remember, and purpose of the aftertreatment of vat meet these requirements, although some these are present in pairs in vat colors. As is dyeings with chemicals, requires at mini- of the hydro is used up in the process and usual in organic chemistry, where the lines mum a rudimentary knowledge of what is more is destroyed by its reaction with representing chemical bonds intersect, meant by the terms reducing agents, re- oxygen in the air (oxidation) especially at there are carbon atoms. Also shown in Fig. duction, reduction potential, oxidizing higher temperatures. Since the hydro deg- 2 are the reactions of the functional groups agents, oxidation and oxidation potential. radation products can be acidic, and are necessary for dyeing (See the second Chemical systems which incorporate both neutralized by the alkali, the alkali is also portion of this chapter titled Practical reduction and oxidation reactions are used up during the process. Consequently, Dyeing.) known as redox systems. enough alkali must be present initially to Thestructureshown in Fig. 2b isderived insure that the bath remains strongly from the vat pigment functional group Reducing Agents alkaline and reasonably stable throughout shown in Fig. 2a and is indicative of the Reducing agents are compounds which any desired dyeing process. This strong water soluble sodium salt known as the either donate hydrogen to, subtract oxy- alkalinity is one of the factors which sodium leuco-vat dye. In strongly alkaline gen from or add electrons (negative effectively limit the application of conven- solutions (pH 12-15), the sodium salt of charges) to other chemicals. The affected tional vat dyes to the alkali-insensitive the leuco-vat dye is fully ionized to give a chemicals are said to be reduced. During cellulosic fibers, although in special cases negatively charged dye anion with associ- the reduction process, the reducing agent it is possible to dye other fibers with vat ated positively charged sodium cations. itself is changed (oxidized), often irrevers- dyes. Under acidic conditions, this may be ibly. For example, the vat pigment struc- Various reducing systems for vat dyes converted to the insoluble or sparingly ture in Fig. 2a is reduced to give the have been proposed and used but alkaline water soluble vat-acid form of the vat dye, structures shown in Fig. 2b the soluble hydrosulfite generally has proved to be the Fig. 2c. sodium leuco-vat, or 2c the sparingly most satisfactory. These include thiourea The reversibility of the reactions shown soluble leuco vat-acid, which have an dioxide, which is relatively expensive and in Fig. 2 are denoted by the double arrows added negative charge and a hydrogen can over-reduce some dyes, and several which point in opposite directions. atom respectively. hydroxyalkylsulfinates used selectively for The leuco-vat anion may be compared In vat dyeing, the reducing agent of printing and high temperature applica- with water soluble direct dye anions, for it choice is sodium hydrosulfite, commonly tions. is the anion which is responsible for vat known as hydro but more correctly known The stability of the alkaline solutions of dyeing. Incidentally, the word leuco as sodium dithionite, which has the chemi- reducing agent decreases with increased comes from the Greek word leucos, mean- cal formula Na2S204. This compound is temperature, increased surface exposed to D ing white, for this is the color of the not stable in neutral or acid solution but is the air, increased agitation of the bath and vat-acid of indigo. stable in strongly alkaline solutions in the decreased concentration. Table I lists some functional chemical absence of air. Unfortunately, oxygen in features of vat dyes and their derivatives, the air does react with hydro to give Oxidizing Agents along with the names of the compounds sodium sulfite and sodium sulfate, Oxidizing agents are compounds which and some of their characteristic proper- Na2SO3 and Na2S04. either add oxygen to, subtract hydrogen ties. The corresponding colors are shown To be practically useful, solutions of a from, or take electrons away from other for C.I. Vat Blue 1 and Green 1. Such vat dye reducing agent must have a level of chemical compounds. The overall effect marked color differences between vat- reducing power (reduction potential) suf- may beeither theloss of negativechargeor pigment, sodium leuco-vat and vat-acid ficient to reduce all commercial vat dyes to the gain of a positive charge. Such reac- are not unusual and can give a dyer some their water soluble forms, economically tions are called oxidations. Oxidation is visual help in assessing the chemical con- and quickly, without converting the dyes essentially theoppositeof reduction. In the ditions in thedyebath. to products from which the original pig- course of oxidizing any chemical, the oxidizing agent is reduced. Conversely, in the course of reducing any chemical, the reducing agent is oxidized. Oxidizing and Table 1. Chemical Features and Selected Properties of Vat Dye Derivatives reducing agents can be seen to go hand in I hand, much like acids and bases, positive C.I. Vat Name and negative, male and female. When the structure given in Fig. 2b is treated with an Chemical Compound Water Cellulose Blue 1 oxidizing agent, it loses an electron from Structures Name and Form Solubility Substantivity (Indigo) Green 1 \ the anion and reverts to the original a)C=O vat pigment none none indigo jade pigmentary form with the double bonded / blue c \ green oxygen, Fig. 2a. Similarly, 2c on oxidation b) C-0- NaC sodium leuco vat soluble substantive yellow blue 4 (anionic vat dye) is also restored to 2a, this time with the loss c$C-OH leuco compound sparing none white red of a hydrogen. (vat acid) To convert fibers dyed with sodium

January 1992 co3 23 Vat Dyes shade change on soaping may have to be Batch dyeing procedures are tailored to used. take this into account.

Leuco-Vat Dye Anions In Solution Freundlich Sorption Isotherms The physical principles treated at some Like other anionic dyes on the nonionic leuco-vats back to fibers containing im- length in Chapter 1, in the section titled cellulosic fibers, the vat dye-anion sorption bedded vat pigments, the oxidizing agent Dye-Anions: Solution, Sorption and Dif- isotherms are of the Freundlich type, is frequently an aqueous solution of a fusion, are as applicable to leuco-vat dye Chapter 1, Fig. 4b). This means that the per-compound such as sodium per borate anions as they are to any of the other concentration of dye on the fiber can or hydrogen peroxide in the presence of application classes of anionic materials for always be increased by addition of more acetic acid, which will also help to neutral- cellulosic fibers. But there are some points dye to the dyebath. But, practically speak- ize any alkali present in the residual still worth reviewing. ing, there is a dye concentration above reducing solution. It is important for any which the color change obtained by fur- acid treatment not to precede but to be Solution and Sorption ther dye addition cannot justify the added concurrent with the oxidation. As shown Relatively large amounts of alkali and cost. in Fig. 2, it is possible, but undesirable, to reducing agent, both sources of sodium form sparingly soluble vat-acids at this ions, are necessary to dissolve the vat Dyeing Equilibrium late stage of a vat dyeing process, for they pigment. Because of this, the sodium ion The simple arithmetical model of dyeing oxidize quite slowly. content and the chemical potential of presented in Chapter 1 is also applicable to For some vat dyes, in some circum- leuco-vat dyes in solution start from a high vat dye anions. But remember: the values stances, the oxidation potential of hydro- baseline. For many dyes there is no need of of constants such as the dyeing rate con- gen peroxide solutions is too great and additional common salt to insure that the stant, kdyP, the stripping or desorption over-oxidation-i.e., beyond the oxidation surface negative potential of the fibers is constant kstrrprand the substantivity or state of the original pigmentary form of overcome and that dye sorption can take partition coefficient, K, are all very depen- the dye-can take place. In these circum- place rapidly. In other words, the high dent upon the prevailing conditions in the stances, a compound known chemically as substantivity of leuco-vat anions is largely bath. These include the salt concentration, the sodium salt of metanitrobenzenesulfo- due to the high salt content of their the liquor-to-goods ratio, L, and the tem- nic acid is used as a much milder but more solutions. perature. The relationship between per- expensive alternative oxidizing agent, and But there is a range of substantivities centage exhaustion, liquor ratio and parti- this operates well in alkaline conditions. and solubilities which is largely dependent tioncoefficient, is worth reiterating: %E= The oxygen in the air is an adequate upon the individual molecular structure of 100 (K/(K+L)). Note: At the end of a oxidizing agent for returning the reduced the different dyes. Like direct dyes, some dyeing, %Eand K can both be increased form of vat dyes to the original pigmentary of the vat dye molecules are linear, some with the addition of salt and lowering the form, but clearly, in practical circum- are planar and some are both. These temperature. Batch dyeing procedures are stances, where it is necessary to oxidize the factors encourage dye aggregation at the also tailored to take this into account. goods rapidly and uniformly, one cannot fiber surface. Indigo dyeing depends on always afford to wait for the relatively slow this surface dye aggregation. Review action of the air upon the goods, particu- In 1901, C.I. Vat Blue 4, indanthrone, was larly when the goods are in the form of Kinetics of Sorption synthesized from anthraquinone and compact structures such as raw stock The sorption of the dye anions at or near to spurred the discovery of the whole range of cakes, cloth on beams and yarn packages. thefibersurfaces, shownin Chapter 1, Fig. vat dyes, of mostly superior fastness, In general one can say that vat dyes are 3a, is so relatively rapid for leuco-vat dye which are available todav The diwovery relatively difficult to reduce but relatively anions that this stage of dyeing may be of hydro (sodium dithionite) in 1904 was easy to oxidize. This contrasts nicely with considered as separate and distinct from equally significant. the sulfur dyes which are just the re- the much slower process of gradual diffu- Vat dyes are in fact pigments. In alka- verse-easy to reduce but more difficult to sion of the anions inside the fibers (3).So line reduction they dissolve in water as the oxidize. much so, that 80-9096 of the color can be leuco-vat dye anions. These anions have exhausted onto the fiber surfaces within substantivity for cellulosic fibers from the Soaping the first ten minutes of dyeing. Note: The reducing baths and their dyeing properties After sodium leuco-vat anions have dif- dyeing rate constant, kdye, and the strip- closely resemble those of the simpler direct fused into cellulosic fibers, after the excess ping or leveling rate constant, kstrip, both dye anions. alkali and reducing agents have been increase with increasing temperature. Once the leuco-vat anions are within the washed-off, after the oxidation has been cellulose, the original pigment is regener- accomplished and the pigmentary form of ated by removing the reducing system and the vat dye is ieft imbedded within the replacing it by an oxidizing system. This cellulosic fibers, then comes soaping. procedure gives vat pigment molecules Soaping is peculiar to vat dyes and it is within the fiber. The fastness properties of /I misnomer in that it does not require the use the dyeings may be enhanced by soaping, of soap. It can describe any treatment, in during which individual pigment mole- near boiling aqueous solutions containing cules are rearranged, aggregate and crys- a surfactant, during which the isolated ~ tallize. ccx, molecules ofvat pigments are reorientated and associate into a different, more crys- References talline form. This new form can have a (1) AATCC Buyer’s Guide for fhe Texlile Wet significantly different shade and gives the Processing Industry, AATCC, published annually as dyed goods materially improved fastness the July issue of Texfile Chemist and Colorist. II to light and washing. Unfortunately, on (2) Colour Index, Society of Dyers and Colourists continuous dye ranges, there is rarely and AATCC, Vols. 3 and 4, Third Edition, 197 I (3) The Dyeing of Cellulosic Fibres, edited by enough time to complete a soaping pro- Clifford Preston, Society of Dyers and Colourists. cess. Vat dyes which undergo minimal 1986, Chapter 6.

24 Cco Vol. 24, No. 1