The Application of Ionic Dyes to Ionic Fibers: Nylon, Silk and Wool and Their Sorption of Anions
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' Chapter 10/Part 2: The Application of Ionic Dyes to Ionic Fibers: Nylon, Silk and Wool and Their Sorption of Anions By J. R. ASPLAND. School of Textiles, Clemson University, Clemson, S. C. 0 repeat the objectives of this chap- ionic and cationic charges associated with These compounds became known as mor- T ter, the sorption of anionic dyes will them, it is to be anticipated that ionic dants, from the Latin word meaning to be examined including acid, mordant attractions might occur between oppo- bite. The mordant bites into the dye and (chrome), premetallized and reactive sitely charged species in the dyebath and in the fiber and holds on. dyes. Disperse dyes cover chemical barre the fiber. But first, a look at the kinds of However, around 1850, chromium salts well on nylon, and for this reason are used dyes used for these fibers. were discovered, and as far as the mordant for a significant portion of the textile dyeing of wool was concerned, the other business where outstanding light- and Acid and Mordant Dyes mordants fell rapidly behind. The extent wetfastness are not required. The subject The name acid dyes is derived from the to which this happened is shown in Ref. (3) was treated in Chapters 7 and 8. Reactive fact that for centuries some water soluble in which Bird calls his chapter on mordant groups can either be added to a disperse dyes have been more readily applied to dyeing, Chrome Dyes. There are virtually dye structure or an acid dye structure and wool and silk from aqueous dyebaths no wool mordant dyes in use today that do their uptake properties, as opposed to their containing acids than from neutral dye- not use chromium compounds. fixation properties, will be determined baths (I). Those acid dyes of natural Chromium, in the form of sodium (or accordingly. origin-Le., which predate 1856 and Per- potassium) bichromate, NalCrl07, has It is already possible to write a useful kin-have been combined with other nat- been optionally applied to wool either general formula for all anionic dyes: ural coloring matters and are now listed as before, during or after dyeing with mor- naturaldyes in theColour Index (2).Most Dye"-. n. Naf dant dyes; but the first two choices, known synthesized acid dyes depend on pendant as the chrome mordant and the meta- which simply indicates the dye anions have sulfonate salt groups, -SOjNa, for their chrome methods respectively have been n charges associated with them (n is solubilityas do thedirect dyes. superseded by the afterchrome method. normally from one to four), and that these The name mordant dyes refers only to Here, the mordant dyes are applied as anions are normally accompanied by an those water soluble acid dyes which con- normal acid dyes to the wool, and enough equivqlent number of sodium cations. tain groups which are capable of reacting acid is added to exhaust the bath. The Since we have already seen that protein in the presence of the tiber with selected temperature is lowered to ca. 70C (1 60F), and polyamide fibers (PA) can have an- metallic cations, known as mordants, to 0.2-3.0% sodium bichromate, depending give metal-dye complexes with increased on the depth of shade, is added and the molecular size and improved fastness. temperature raised to the boil for 30-60 This is the conventional definition of minutes. mordant dyes, but strictly speaking mor- During the process the bichromate ion, ABSTRACT dants can complex with other classes of Cr207=, is absorbed by the wool, the dyes and do not have to be cationic or hexavalent chromium (written Cr6+ or Cr Acid, mordant (chrome) and metallic. For example, basic dyes (cat- VI) is reduced (gains three negative elec- premetallized dyes are introduced and ionic) can be dyed onto silk treated with a trons) to trivalent chromium (writen Cr3+ the chemical nature of metal-dye tannin (anionic) mordant. But, leaving the or Cr 111) by a complex series of interac- complexes is considered. The general question of what is a mordant, let tions with sulfur containing (cystine) mechanism of dyeing protein and polyamide fibers with anionic dyes is us answer another more specific question. groups in the wool, and chromium-dye discussed in terms of the Langmuir and What is a metallic mordant? complex compounds are formed in the Nernst sorption isotherms. A qualitative fiber. picture of the action of leveling or Metal Compounds as Mordants Mordant (chrome) dyes are still in use restraining agents is developed. Centuries before the development of sys- today-the AATCC Buyer's Guide lists tematic chemical knowledge, man had about 40 dyes (4). The afterchromed discovered that pretreating or post-treat- dyeings have outstandingly good wash- KEY TERMS ing the available textile fibers with differ- and lightfastness on wool, although the Acid Dyes ent earths in the dyebath could lead to the colors are generally rather dull, with Anionic Dyes production of a range of different hues blacks, navies, browns and dull yellows Chrome Dyes from the same natural dye. Not only that, easily predominating. It is interesting to Leveling Agents but someof thecombinations of earths and note that C.I. Mordant Black 11 is one of Mordants dyes produced colors with more desirable the most widely used dyes in the world. Nernst Isotherms properties than others. And so, inevitably, However, several questions remain un- Nylon patterns began to emerge. Eventually it answered. For example, what is the nature Premetallized Dyes became known that metallic compounds of the chromium-dye complexes and what Silk of aluminum, cobalt, copper, iron and happens to the chromium? The next sec- Wool others were responsible for the improve- tion will address these questions. Do any of ments in the properties of the dyeings. the environmentally undesirable chro- March 1993 CCO 55 Nylon, Silk and Wool by an arrow pointing from the donor atom, wool fibers, although it is hard to establish or by a dotted line, Fig. 1. whether they do or not. Certain structures and configurations of The resulting structure in Fig. 2b is a mium salts end up in the effluent? The individual dye molecules enable them to 1: 1, meta1:dye co-ordination complex (see answer to this question is yes. And since it provide metal (mordant) ions with not the section on premetallized dyes). The is estimated that of all the dyes being used only one but two, three and even four overall charge on the complex will depend for wool worldwide, chrome dyes repre- ligands; e.g., the copper phthalocyanine on whether the nondye ligands were sent about 36%, the answer yes represents structure shown in Chapter 2, Part 11. nonionic or anionic. Unfortunately, the a substantial problem. Such dyes are known as polydentate resonance ensures that the resulting ionic Although there has been much effort ligands (with many teeth). They are also charge is delocalized and spread about expended to reduce the amount of chro- known as chelating agents, from the Greek within the structure, making it hard to mium in the aqueous ,effluent, it is still word for (crab) claw. Thechelating agents find. undesirably high. However, a more tanta- used to tie up the free metal ions in Fig. 2c shows that two suitable mole- lizing environmental question is what hap- water-e.g., EDTA-do indeed act in the cules of dye-e.g., of the type shown in pens to chrome-containing goods when same way, and are quite capable of wres- Fig. 2a-could each provide three ligands they end up as solid waste in landfills. tling some metal ions from premetallized and satisfy CN = 6, to give a 1:2, met- dyes. a1:dye co-ordination complex. These may Metal-Dye Complexes Fig. 2a shows the single most important alternatively be called 2:l dyemetal com- The present state of chemistry enables us dye structural feature for the formation of plexes. Now there are four anionic ligands, to understand what properties of certain metal-dye complexes, although there are a R-0-, whose negative charges, when metallic compounds (mordants) and se- number of others (4).It is called the o,o’ - added to the three positive charges on the lected acid dyes (mordant dyes) contrib- dihydroxyazo group, which simply means chromium cation, leave the complex with ute to the formation of metal-dye com- that two hydroxy groups are situated on one negative charge; Le., anionic. The plexes, known also as co-ordination com- carbon atoms adjacent to those holding the product is resonance stabilized by four pounds. azo group, one on either side. aromatic rings, and the anionic charge is The metallic compounds which are still Fig. 2b shows how this dye structure can delocalized. Some 1:2 metal-dye com- used to form metal-dye complexes are all provide three ligands to a trivalent chro- plexes have been extracted from mordant from a group known as the transitional mium ion (CN = 6). Two come from dyed, afterchromed wool. It seems likely metals. In simple terms, this means that ionized hydroxy groups in the dye mole- that both 1 : 1 and 1:2 meta1:dye complexes their metallic cations (+) not only attract cule, Fig. 2a; cf. the ionization of naph- may be present on the wool. anions (-) but also have electronic config- thols in alkaline solution, Chapter 6 (5): urations which enable them to form com- plexes with particular numbers of chemi- R - OH + OHO- R - oo+ H~O cal groups, whether the groups are anionic These two anionic ligands will also effec- H\o/H or nonionic. Such groups are known as tively neutralize two of three positive ligands.