' Chapter 10/Part 2: The Application of Ionic to Ionic Fibers: , and and Their Sorption of Anions

By J. R. ASPLAND. School of , 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, attractions might occur between oppo- bite. The mordant bites into the 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 of wool was concerned, the other business where outstanding light- and Acid and Mordant Dyes 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 superseded by the afterchrome method.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 . 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. These metal ions are like property charges on the metal ion, leaving it with ‘O+C” 42+‘0, owners who have rooms to rent to singles. only one positive charge. The third comes 7 long as the tenants can pay (with So from a donation of electrons (to share) H’ .f ’ ‘H electrons) the owners like to keep their from an azo nitrogen (+). In the process, rooms occupied. The maximum number of two new ring structures are formed, one H /o\H ligands (tenants) is known as the co- with six and the other with five members. Fig. 1. Blue hydrated cupric ion. ordination number (CN) and varies from Most other stable aromatic ring structures metal to metal. are made up of five and six-membered Two metallic ions widely used in dye rings. There is also a bald spot left on one a) 0.0’ dihydroxy technology are the trivalent chromium side of the chromium atom which will be and divalent copper ions (Cr3+, Cu2+) referred to in the next section. which have CN = 6 and CN = 4, respec- tively. Cobalt and nickel derivatives are Stability of Aromatic Rings also used and have CN 6 and 4 respec- Aromatic five and six-membered rings add tively. stability to molecules because of a phe- A simple example of a co-ordination nomenon known chemically as resonance. complex is to be found in the solid and Resonance is simply the movement of solutions of the widely known blue bonding electrons from one atom to an- compound, copper sulfate (hydrate), other within a molecule. Structures which CuSO4.5H20. When the water is driven provide electrons with a wide choice of off, the anhydrous copper sulfate, CuSO4, interconnected alternative places to con- b) 1:l chrome:dye complex is almost white. gregate are said to be resonance stabilized. When copper sulfate is dissolved in The classical examples are benzene and water, it gives a characteristic blue solu- naphthalene, Chapter 2, Fig. 1 (6). The tion, but it does not dissociate into cupric multiplicity of ways in which the bonds copper, Cu2+, and sulfate ions, SO4=, as may be drawn are indicative of resonance. might be expected. The sulfate ions are In Fig. 2b the metal still requires three there, but the cupric copper ion is sur- further ligands to satisfy the CN = 6 rounded (hydrated) by four molecules of requirement, and fill the bald spot. These water. These all act as ligands, with their could come from any available nondye, oxygen atoms each donating a share in two ligand forming molecule or anion. They electrons, to satisfy the co-ordination could come from water, as in Fig. 1, but c) 1:2 chrome:dye complex number, CN = 4, of the cupric ion. This they could also come from alkaline solu- contribution is known as dative-covalence tions as hydroxide ions, or from the amino, Fig. 2. Tridentate dye ligands forming 1:1 and and is, like hydrogen bonding, represented thiol, hydroxy or carboxylic acid groups in I :2, metakdyeco-ordination complexes.

56 0.3 Vol. 25, No. 3 '(. Examples of meta1:dye complexes have (see later) they dye wool, silk and nylon groups, -NH3+. The electrical neutrality been given in previous chapters. It is from almost neutral solution, but with of the fiber phase must be maintained, so interesting to think that nomadic carpet poor leveling properties. They have excel- any anions present in the bath-such as dyers and weavers in ancient Persia (Iran) lent lightfastness but like all premetallized those associated with the acid, with any knew how to use this chemistry before the dyes (but not all dye-metal complexes) salts present and with the dye-begin birth of Christ. they have a shade gamut which is re- simultaneously to diffuse into the fiber to stricted by the absence of bright colors. neutralize the charge on the -NH3+ Premetallized Dyes Incidentally, metallization usually moves groups. The dye anions, being rather large, Dye manufacturers have long exploited the hue bathochromically and dulls the diffuse relatively slowly into the fiber, but the stability of meta1:dye complexes, and shade. because they alone are substantive, they the products are spread broadly through- Such dyes are illustrated by the struc- will eventually displace all the smaller and out dye and pigment technology. How- ture shown in Fig. 2c, and since this less or nonsubstantive anions which ever, the name premetallized dyes is usu- structure is already anionic, these prod- rushed in first. ally reserved for those dyes which can be ucts do not generally contain additional The chemical behavior of wool parallels applied to wool, in a similar manner to the sulfonate groups. However, they are large that of water soluble amino acids. Hydro- acid dyes, to achieve dyeings with fastness molecules, of relatively low solubility, and gen ions can be sorped at least until the properties which approach those of the groups which can hydrogen bond to water largest majority of the carboxylate groups mordant (chrome) dyes, and without sig- are used to improve this property; e.g., are in the free acid form, -COOH. At this nificant free metallic ions in the effluent. -S02-CH3 and -SOz-NH?, methyl stage almost all the charged ammonium The 1:l premetallized dyes are 1:l sulfonyl and sulfonamido groups, respec- groups, -NH3+, areavailable for eventual meta1:dye complexes. A number of them tively. These dyes have achieved consider- neutralization by dye anions (see Eq. 1). able importance in applications where were introduced in 1919. The structure of OH - the complex part of the molecule is shown very high lightfastness is desirable; e.g., in nylon automotive carpets. NH?-(X)-COOH - in Fig. 2b in which (in the absence of ionic H+ ligands filling the bald spot) the metal is Lest we forget, some of these 1:2 metal- shown to carry a cationic charge. In dye complexes are cobalt derivatives, from 0-H- addition, these dyes normally have either Co3+, and are very similar to the corre- NH~-(x)-coo@ -. one or two sulfonate groups, -s03O sponding chromium derivatives. They do H+ somewhere on the dye chromophore for tend to be duller in shade, slower to dye solubility. This would normally be ex- and have better lightfastness. Inciden- NHz--(X)-COO@ Eq. 1 pected to create two different kinds of dye. tally, it is not necessary for the dye When there is one metallic cationic charge chromophore on both sides of the metal to This understanding gave rise to the notion (+) and one sulfonate anion (-), the dye be the same. that the ammonium groups could be con- molecule should have no overall charge; Mechanism of Acid Dyeing sidered as dye sites for anionic dyes. i.e., it should behave as if it were nonionic. Further, the maximum possible concen- However, if there is one cationic and two Early work on the mechanism of acid dye tration of ammonium groups, which is anionic charges, the overall charge on the uptake by wool gave rise to the hypothesis equal to the concentration of amino groups dye molecule should be negative; Le., the that under acid conditions, the cationic present initially (in dry fiber), might also dye is anionic. But these mono- and hydrogen ions, being small and able to represent the upper limit of dye anion disulfonated dyes do not behave differ- diffuse readily, are first sorped from the concentration which might be sorped to ently in practice. dyebath by the water swollen fiber and saturate the fiber. Let this fiber saturation This raises unanswered questions about become associated with the amino groups concentration be written: (S)F whether other anionic ligands can attach in the wool to give cationic ammonium Carrying the idea a step further, the rate themselves to the nondye side of the metal of dyeing would be governed not only by (or the bald spot) in dyebath conditions. the concentration of dye in the bath but Certainly the dyebath properties of these also by the concentration of unoccupied dyes are somewhat abnormal, for they can dye sites in the fiber: be dyed on wool at the very low pH of ca. 2 (8% owg) and still level well. Rate of dyeing = kdye X (Dye), This leveling at low pH is most uncharac- x [(Q-(DY4fI Eq.2 teristic of other acid dyes with good Similarly, the rate of stripping can be wetfastness because such a low pH gener- given: ally increases acid dye substantivity very markedly. More recently, a range of 1: 1 Rate of stripping premetallized dyes has become available = kstrlp x (DYe)f Eq. 3 which dyes wool at pH 3.5-4, but it does At equilibrium the rates of dyeing and call for a special complex forming addi- stripping are equal and therefore: tive. Whatever is really going on with these (Dye)/. = k X (Dye), dyes in the bath and on the fiber, they do x [(Q-(DY4f)l Eq.4 dye from acid baths to give fastness where k is equal to the constant, kdye + approaching that of chrome dyes, with kstrlp.Here kdye and kdye and kstripare the relatively good leveling. For these reasons rate constants for dyeing and stripping they have a distinct market niche for wool respectively. The last equation can be dyeing and constitute about 7% of the rearranged to give an expression for the market. concentration of dye in the fiber, (Dye)f, in The 1:2 premetallized dyes are 1:2 terms of that in the solution, (Dye),: meta1:dye complexes which were not in- troduced until about 1951. Like most (Dye)f = k X (S)f X (Dye), other acid dyes of very good wetfastness + 1 + k X (Dye), Eq.5

March 1993 Oca 57 This is the equation which governs the rate of dyeing by controlling the available To ensure the solubility of such com- shape of the so-called Langmuir sorption dye sites, [ (S)f- (Dyeh) I. Any other plexes, the cationic leveling agents are isotherm. When k X (Dye), is much anion, which can compete with the dye normally polyethoxylated quaternary larger than unity-Le., at high dye con- anions for the available sites, effectively compounds which derive much of their centrations in the bath-Eq. 5 reduces to lowers the value of this term, and by so solubility not from their cationic character (Dye)f = (S>f. doing lowers the rateof dyeing (Eq. 2). but from their ethoxy groups, In acid wool dyeing it is quite usual to -(-CZH~O-)~-. Langmuir Sorption Isotherms dye in the presence of 2-3% owg sulfuric If only things were this simple! Many If the equilibrium uptake of acid dyes by acid, but it is normal to include 10% owg proprietary leveling agents containing wool, silk or nylon were governed solely by sodium sulfate as a leveling agent. The both weakly acidic and weakly basic the electrostatic or coulombic interaction large concentration of sulfate anions com- groups (like amino acids) and whether between the dye anions and the fiber petes economically and effectively with they are anionic or cationic in the dyebath cations, then the sorption isotherms would the dye anions for the cationic groups depends on the pH. Such compounds are have shapes of the Langmuir type, Fig. 3b. within the fiber, although ultimately the referred to as being amphoteric. The curves should tend to level out to a dye will prevail by virtue of its added constant (Dye)f value which indicates the property-substantivity . Dye-Fiber interactions maximum amount of dye which it is On the other hand, if a leveling agent The danger inherent in generalizing about

I possible to get on these fibers and which is were chosen which was colorless, water dye and fiber molecules lies in assuming equivalent to the concentration of avail- soluble and which dissociated into an that they are homogeneous. Certainly able amino groups in the fiber. aipmatic anion with some substantivity, wool, silk and nylon fibers have an ionic Some acid dyes do show such behavior the dye anion would have more serious character at some locations, as do acid on wool, but since the advent of nylon, competition. Ideally such an agent should dyes, but this does not limit the interaction many more do not. In fact, the hypothesis diffuse more rapidly and have less substan- between dyes and fibers to electrostatic of attraction of anionic dyes to cationicdye tivity than the dye anion; otherwise the interactions. sites is an oversimplification. But it should normally high equilibrium exhaustion of Water soluble dyes have hydrophilic not be rejected out of hand, since it enables the dyebath could be in jeopardy. locations, such as the sulfonate groups, other well known technical phenomena to 0 Cationic leveling agents control the -SO3Na, but they also have regions which be explained in a simple, qualitative way. rate of dyeing by controlling the effective are made up primarily of benzenoid and concentration of dye anion in the dyebath. aliphatic hydrocarbons, which gives them Leveling Agents Eq. 2 shows that the rate of dyeing is a tendency to aggregate into dye-dye The leveling agents used in wool and nylon proportional to the dye concentration in micelles. The magnitude of this tendency dyeing are usually of the type whose the bath, [Dye],. depends on the number and location of the effectiveness relies on controlling the rate When an organic leveler cation reacts ionic groups and on the size and hydropho- of dye uptake, so that adequate circulation with an organic dye anion in water, the bic character of the rest of the molecule. of the dye liquor can ensure that dyeings result is an ionic complex. Such complexes Wool fibers have a higher moisture are uniformly level from the start. are in equilibrium with the free ions: regain than cellulosic fibers, and they are There are two principle types of leveling filled with ionic groups. But there are (Lev.') (Dye-) 2 (Lev.+Dye-) agents-anionic and cationic-and the + locations where particular amino acid action of both types can be understood by Here the concentration of added leveler substituents in the chain include phenyl examining the two concentration terms in and the dissociation constant of the com- (benzenoid) and other hydrocarbon sub- Eq. 2. plex will control the effective concentra- stituents. Nylon fibers are not very hydro- 0 Anionic leveling agents control the tion of the free dye anion and hence the philic overall (contrast wool) and they are rate of dyeing. filled with hydrocarbon segments which

_t __f t r 1

L -1 ca. 75 6 carbons' 6 carbons n Fig. 3. A graphical representation of how the Langmuir and Nernst isotherms can be combined to reflect the equilibrium acid dyeing Fig. 4. Comparison of repeat units in protein and polyamide fibers. There characteristicsof wool, silk and nylon. are more than 16 different possibilities for R in wool; more than 1 1 for silk.

58 CCO Vol. 25, No. 3 are definitely hydrophobic. These are the Sumner (11). Both use an approach based tures suitable for application to wool, silk paraffinic hydrocarbon, -(-CH*-),, -, onDonnan membrane equilibria (12). The and nylon, with their localized hydropho- groups where n = four, five or six (Fig. 4). more rigorous approaches allow the effect bic and hydrophilic regions, coupled with So in addition to ionic interactions some of changing the concentration of amino the mixture of hydrophobic and hydro- level of hydrophobic, dye-fiber bonding is and carboxylic acid end groups to be philic locations within the fiber molecules, to be expected, particularly between spar- studied, as well as the blocking effects, calls for a dyeing mechanism which can ingly water soluble dyes with large mole- where the greater substantivity of one dye take into account both ionic and hydro- cules and nylon. But what is hydrophobic in a mixture can effectively prevent an- phobic dye-fiber bonding. bonding? It is a term which does not imply other dye from being sorped by nylon. These more comprehensive approaches a particular kind of bond, and has been Perhaps the biggest surprise to the to modeling the dyeing system tend to be described as “the tendency of hydrophobic traditionalist is that whole families of dismissed as too complex. However, it is groups . . . to associate together and es- different sorption isotherms, resembling now more than 50 years since the develop- cape from an aqueous enyironment”(7). curves Fig. 3a and b, can be predicted ment of nylon and it is not too soon to The hydrocarbon regions dye-dye or dye- without the need to assume either that attempt to raise the general level of under- fiber are held in close proximity by van der anionic dyes become associated with cat- standing what is involved when dyeing Waals forces and are comparable with the ionic dye sites or that addition of a ionic fibers with anionic dyes. bonding between the hydrophobic dis- separate mechanism for making allow- perse dyes and polyester fibers. ance for hydrophobic dye-fiber bonding is The reality of acid dye to fiber bonding necessary. is that it is heterogeneous overall, varying In other words, “Ionic dyeing systems References from situation to situation, dye to dye. It are extremely complex” (10). Those who lies somewhere on a line between the wish to pursue this matter further should (1) Colour Index, AATCC and the Society of idealized electrostatic (coulombic) attrac- consult the appropriate references Dyers and Colourists, Vol. 1, Acid Dyes, Third Edi- (IO, tion, 1971. tion of some acid dye molecules for wool 11). (2) Colour Index, AATCC and the Society of and nylon (Langmuir) and the idealized Dyers and Colourists, Vol. 3, Natural Dyes, Third hydrophobic bonding of disperse dyes for Review Edition, 197 I. nylon (Nernst). Bothsituations couldexist For some, the preceding sections may have (3) Bird, C. L., Theory and Practice of Wool Dyeing, Fourth Edition, Society of Dyers and Colour- to different extents within the same fibers. contained more than enough general back- ists, Bradford, U.K., 1972. Where on this line might one expect the ground chemistry. However, it should now (4) Buyer’s Guide for the Textile Wet Processing large, sparingly soluble 1:2 premetallized be apparent that much chemical technol- Industry, published as the July issue of Textile dye molecules (with delocalized ionic ogy has gone into the development of those Chemist and Colorist,July 1992. (5) Aspland, J. R., Textile Chemist and Colorist, charge) to fall? acid, mordant (chrome) and premetal- Vol. 24, No. 9, September 1992, p74. These two extremes have been repre- lized dyes in use today. (6) Aspland J. R., Textile Chemist and Colorist, sented in Fig. 3b and c as Langmuir and A generalized mechanism for acid dye- Vol.23,No. 12,December 1991,p30. Nernst sorption isotherms respectively ing based primarily on the coulombic (7) Giles C. H., The Theory and Coloration of Textiles, Second Edition, Edited by A. Johnson, (8). There is an infinite number of inter- (electrostatic) attraction of anionic dyes Society of Dyers and Colourists, 1989, Chapter 2. mediate or combination sorption iso- by oppositely charged sites in the fibers has (8) Aspland J. R., Textile Chemist and Colorist, therms, one of which is illustrated in Fig. the benefit of simplicity and can be used to Vol.24,No. 10,October 1992, p39. 3a. The relative importance of the ionic qualititatively explain the different ionic (9) Iijima, T. et al., “Kinetic Behavior in Mixture Dyeing: Acid Dye/Polyamide,” proceedings from and hydrophobic interactions may be as- types of leveling agents suitable for use AATCC International Dyeing Symposium, Char- sumed to vary with the particular dye and when dyeing wool, silk and nylon. But such lotte, N. C., June 1992, p23. fiber involved (9). a mechanism is not entirely valid, and it (IO) McGregor, R., “Ionic Dyeing Systems,” pro- The whole subject of the thermodynam- cannot fully explain the widely known ceedings from AATCC International Dyeing Sympo- sium, Washington, D. C., May 1977, p126. ics of sorption of acid dyes by polyamides phenomena of overdyeing or the blocking (1 1) Sumner, H. H., Ref. 7, Chapter 4. has been approached with much more of one dye from the fiber by another. (1 2) Aspland, J. R., Textile Chemist and Colorist, scientific rigor by McGregor (IO) and The variety of colored anionic struc- Vol. 24, No. 5, May 1992, p34.

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