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Indian Journal of Fibre & Textile Research Vo l. 21, March 1996, pp. 30-34 Recent progress in indigo dyeing of cotton denim yam J N Etters Th e U ni vers it y o f Geo rgia, Athens, GA 30602 , USA During the past several years, significant progress has been made in understanding the important parameters th at in fl uence the sorption of indi go dye by cotton denim yarn. It has been observed that dye uptake by cotton fib er and the extent of penetration of indi go dye into the denim yarn cross- section are influenced strongly by the pH-controlled ionic species of both dye and cattaIl in the dye- ba th . High dyebath pH results in a hi gh level of ionization of both indigo dye and cotton cellulose, res ulting in decreased substantivit y and increased penetration of the yarn fiber bundle by the dye, with associated lower col or yie ld . On the other hand, mod.erate dyebath pH promotes lower ioniza- ti on of d ye and fiber, result in g in increased ring-dyed ya rn, greater color yield , and a denim garment th at washes dow n quickly and consiste ntl y to produce the current appearance demanded by fas hio n. Keyword s: Co tton yarn, Denim, Dyein g, Indigo 1 Introduction Much is yet to be lea rnt ab out the various para- meters th at influence indigo dye uptak e by cott on de nim yarn. If anything is certain, it is that a com- plete und ersta ndin g of a ll the fac tor s that influ- ence the mec hanism of dye so rption is not yet availabl e. Neve rtheless, a sigpjfi ca nt quantit y of new informa ti on has b ee n unc overed during re- cent investigations. For exa mple , the influence of dyebath pH on both pene tr a ti on of indigo into denim yarn a nd the resulting yield has b ee n shown to be of paramo unt imp ortance. Ind eed, the imp o rtanc e of dyebath pH was repo rted as ea rl y as 1 98 7 in a BASF technical bulletin I. Th e scientific basis of the empiri ca ll y ob se rved effect was not discussed and deta il ed quantifica ti on of thc phenomenon nOt given. It was noted in the publica ti on: " It is most imp ortant to ens ur e that a constant pH is maintained durin g the [indigo 1 dyeing process, so that the affinity and dyeing penetration also re- ma in co nstant a nd there is no 'e ndin g' within the dyed lo t. We reco mmend dyeing at pH 12; at pH I 1.5 , the dye in g is less pe ne tr ated, whereas ab ove pH 12 the dyeing penetra ti on is better but the dyeing li ghter". T he purp ose of the present inves ti ga ti on is to review th e results of studies at T he U ni versity of Geo rgia and elsewhere that have co ntribut ed to an incr eas ed und erstanding of the ph ys ico -chemi- cal rela ti onships involved in co mm ercial indigo d ye in g. It is hopcd that the discussion will reiter- ate the imp ortance of strict ' co ntrol of dyeb ath pH on the resulting color yi eld, extent of ring dyeing of denIm yarn, a nd the associated wash do wn char- acte ri s ti cs of denim fabric. Most of the data that have been o bt ained in studies mad e at Th e Uni- versity of Geo rgia was obtained un de r the experi- mental conditions given in the following section. 2 Experimental Procedure 2.1 S ubstrat e 100% knitted co tton tub es, prepared from 8s denim yarn a nd supplied by Auburn Univers it y, were l:I se d. Th e tub es had a flattened width of 4.5 cm and a weight of 7.2 g per 30 em leng th- the length used for laboratory dycings. Knitted tubes were used in the laboratory wo rk to facilit- ate subse qu ent re fl ectance measurement s. Uni- form re fl ectance meas ur emen ts on denim yarn are more difficult to obtain. 2.2 Oyehath s T hre e-liter infinite dyebaths were empl oye d to ass ur e that the conce ntr a ti on of dye in the bath did not ch ange signifi ca ntly durin g th e co ur se of the dyeings. Infinite dyebaths are maintained in co mmercial indigo dye ranges by co ntinuously f eeding con ce ntrat ed stock so lutions of dye into the dyeba th at a carefully maintained rale. 2,3 Dyeing Knitted co tton tub es were wet out 111 a roo m

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Indian Journal of Fibre & Textile Resea rch Vo l. 2 1, March 1996, pp. 30-34

Recent progress in indigo dyeing of cotton denim yam

J N Etters

The Univers ity of Georgia, Athens, GA 30602, USA

During the past several years, significant progress has been made in understanding the important parameters that infl uence the sorption of indigo dye by cotton denim yarn. It has been observed that dye uptake by cotton fiber and the extent of penetration of indigo dye into the denim yarn cross­section are influenced strongly by the pH-controlled ionic species of both dye and cattaIl in the dye­bath . High dyebath pH results in a high level of ionization of both indigo dye and cotton cellulose, resulting in decreased substantivity and increased penetration of the yarn fiber bundle by the dye, with associated lower color yield. On the other hand, mod.erate dyebath pH promotes lower ioniza­ti on of dye and fiber, result ing in increased ring-dyed yarn, greater color yield, and a denim garment that washes down quickly and consistently to produce the current appearance demanded by fashion.

Keywords: Cotton yarn , Denim, Dyeing, Indigo

1 Introduction Much is yet to be learnt about the various pa ra­

meters that influence indigo dye uptake by cotton denim ya rn. If anything is certain, it is that a co m­plete understanding of all the factors that influ­ence the mechanism of dye sorption is not yet available. Nevertheless, a sigpjficant quantity of new info rmation has been uncove red during re­cent investigations. Fo r example, the influence of dyebath pH on bo th penetra tion of indigo into denim yarn and the resulting colO~ yield has been shown to be of paramount importance. Indeed , the importance of dyebath pH was repo rted as ea rly as 1 987 in a BASF technical bulletin I. The scientific basis of the empiricall y observed effect was no t discussed and de ta iled quantification of thc phenomenon wa~ nOt given. It was noted in the publi ca tio n :

"It is most important to ensure that a co nstant pH is maintained during the [indigo 1 dyeing process , so that the affinit y and dyeing penetration also re­main constant and there is no 'ending' within the dyed lo t. We recommend dyeing at pH 12; a t pH I 1.5 , the dye ing is less penetra ted , whe reas above p H 12 the dyeing penet ra tion is better but the dyeing lighte r".

T he purpose of the present inves tiga tion is to review the results of studies at T he U nive rsity of Geo rgia and elsewhere that have contributed to an increased understand ing of the physico-chemi­cal relationships involved in commercial indigo

dyeing. It is hopcd that the discussion will reiter­a te the impo rtance o f strict 'control of dyebath pH on the resulting colo r yield , extent of ring dyeing of denIm ya rn , a nd the associa ted wash down ch ar­acteri stics of denim fabric. Most of the data that have been obtained in studies made a t The Uni­versity of Georgia was obtai ned under the expe ri­mental conditions given in the fo llowing section.

2 Experimental Procedure

2.1 S ubstrate

100% knitted cotton tubes, prepa red from 8s denim ya rn and supplied by Auburn University, were l:Ised . The tubes had a flattened width of 4.5 cm and a weight of 7.2 g per 30 em length­the length used fo r labo rato ry dycings. Knitted tubes we re used in the labo ratory wo rk to facilit­ate subsequent refl ectance measure ments. Uni­form refl ecta nce measurements on denim ya rn are more difficult to obtain .

2.2 Oyehaths

T hree-liter infinite dyebaths were employed to assure that the concentration of dye in the bath did not change significantly during the course of the dyeings. Infinite dyebaths are maintained in comme rcial indigo dye ranges by continuously feeding concentrated stock solutions of dye into the d yeba th a t a ca refull y maintained rale .

2,3 Dyeing

Knitted cotton tubes we re wet out 111 a room

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ElTERS: INDIGO DYEING OF COTTON DENIM YARN 31

tempt. ature solution of 1.5 giL sodium dioctyl sulphosuccinate wetting agent and passed between the squeeze ro ll s of a Butterworth's pad manglc a t SO Ib/in ~ pressure. The tubes were rinsed in dei­onized water, passed through the mangle again , and placed into a fresh deionized water-hath where they were kept until they were dyed.

For dyeings made at each dye concentration and pH, a knitted tube was removed from the deionized water-bath , attached to a lead line o f polyester fabric, and passed between the squeeze rolls . The tube was then immersed into the dye­bath at room tempe rature for 15 seconds, passed through the mangle to about 70% pickup, and air oxidized for 45 seconds. The immersion, padding and oxidation sequence was repea ted five times in an attempt to simulate actual commercial dyeing. The dyed tube was then placed on a line and al­lowed to dry at room temperature. After all dy­eings at the various dye concentrations and pH had been completed , the tubes were rinsed to­gether with agitation in a large white, plastic bucket under a faucet of warm running water un­til the rinse water appeared to be free of loose, unfixed dye, as indicated by the absence of blue coloration . The tubes we re then dried on a line at room temperature.

2.4 Renectance Measurements

Each of the dyed tubes was fo lded to achieve opacity and reflectance measured over the wave­length range of 400-700 nm, using a LabScan 6000 spectrophotometer. Reflectance values a t 660 nm were converted to K IS, using Eq. (1 ). The selected wavelength of 660 nm has been shown to be the wavelength at which consistently minimum re tlec tance values a re obtained for many different shade depths. Other wavelengths near 660 nm also are appropriate to usc. A prac­(ieal form of the Kubelka-Munk equation can be uscd to measure depth of shade2

• The equation is given by

~_[l- ( Rc- RJf [l- (R,, -RJ f

S 2(Rc - RJ 2(R" - RJ . . . ( I )

where Rc is the fractional refl ectance of the sub­strate that contains a given concentration of indi­go; R(J, the reflectance of a mock dyed substrate th at contains no dye, and R" the reflectance of a substrate containing an infinite concentration of dye, i.c . a concentration so high that further in­crease in concentration does not result in a fur­ther lowering of retlectance at the wavelength of minimum reflectance. As is well known that, with-

in a commercially reasonable concentration range, K IS is linea rl y proportional to concentration of dyc in a tex tile substrate when the dye is uniform­ly distributed in the cross-section of the suhstrate. The relationship is given by :

K - = (1C S

. .. (2)

where 1/ IS the reflectance absorptivity coeffi­cient ; and C the concentration of indigo in the dyed kllitted tube. It has been es timated that the value of 1/ for dyeings in which indigo is uniform­ly distributed in the cross-section of the substrate is approximately 30-40 when the dye concentra­tion is expressed as grams of indigo per 100 g of fiber ' .

2.5 Dye Concentration Measurements

Small circular samples were punched out of each dyed knit tube and repeated ly ex tracted with warm (80°C) pyridine until all the dye was re­moved . The concentration of indigo in eat:;h cot­ton tube ( gra~s indigo/1 00 g fiher ) was then es ti­mated eolorimetrically from known concentra­tion/ absorbance curves and the dried sample weights.

3 Results and Discussion

3.1 Shade Depth at Constant Dyebath Concentration but Variable pH

Early in the investigation rather strange results were observed when dyeing cotton tubing in infi­nite dyebaths that contained a constant concentra­tion of dye but were adjusted to different pH . For example, typical results that were obtained from dyeings made in infinite dyebaths containing 1.0 gi L indigo but four different dyebath pH are shown in Fig. 1. It is obse rved from this figure that the ~hade depth of the dyed tubing, ex­pressed as KIS, increases as the dye.bath pH dec­reases (rom 13 to 11 and then decreases at the lowest pH of 10. The concentration of dye: in the various dyei ngs was determined hy pyridine ex­traction and is given on the bars in the figure. It is observed . that the measured concentration of dye (g/ 100 g fibre ) increases as the dyeha th pH dec­reases. H owever, when the concentration of dye found for each dyeing is used in Eg. (2 ) along with the estimated reflectance absorptivity coeffi­cient of 40, the calculated values of KIS are found to be much lower than the measured K IS values. It is now underst(')od that the difference between the measured values of K IS and those calculated by the lise of Eq. (2) is due to the fai-

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32 INDI AN 1. FIBRE TEXT. RES., MARC H 1996

lure of Eg . (2) for substrates in which dye is not uniformly distrihuted in the substrate cross-sec­tion . Microscopy revealed that the ex tent of ring dyeing of the ya rns increased as the dyehath pH decreased from 13 to 11 , with the ex tent of ring dyeing at pH 10 being about the same as that at pH 11 . In the case of ring dyed denim yarn, an expression has been found that more accurately describes the relationships between KIS, dye con­tent , and penetration~:

... (3)

where a is the "true" reflectance absorptivity coef­fici ent fo r uniformly distributed dye; C, the dye concentration in .the to tal cross-section of the de­nim yarn , expressed as grams of dye per 100 g of yarn; and p, the fractiona l penetration o f the yarn by dye, i.e. a parameter that takes into account the extent of ring dyeing of the denim yarn.

As shown in Fig. 2, when Eg. (3 ) is used to es­timate the shade depth, a much closer agreement with measured values is obtained, at least for dye­bath pH of 13, 12 and II . At pH 10, the agree­ment is not good. The va lues of p, roughly esti­mated by mic roscopy, for the pH values o f 13- 10 are respec tively O.nS, 0 .33, 0 .20 and 0.20. Since the measured concentration of dye in the tubing dyed at pH 10 is higher than that made at pH II , the lower than expected K IS value found fo r the dyeing at pH 10 must he the result of a lack of contribution to color depth o f dye present on the dyed yarn. In fact , microscopy revealed crystalline particles of indigo present between fibers in the ya rn i·nterio r. Such dye would ofcourse be ex tract­ed during the determination of fixed dye content, but would not contribute to shade depth.

3.2 Effect of Dyebath pH on Substantivity

Fig. 1 shows that the substantivity of indigo for cotton fiber - is highly pH dependent. Technical sorption isotherms or distribllltion coefficients found by the use of the disclosed dyeing tech­nique at room temperature are given in Fig. 3. Al­though the technical values are fo und to be ap­proximately linear for the dyeing technique used , it is known that true equilibrium sorption iso­therms for indigo on cotton fiber fo llow the Freundlich form '. It is clear that the substantivity of indigo fo r cotton fiber increases as the dyebath pH decreases up to a point. For example, Leupin and Hartmark observed about fifty years ago that a pH of about 9.9 is necessary to maintain indigo in solution formh. In addition, the work 7 of Rus-

120 O MeaslJred

100 ~ Calculated

0 522

80 0 63 1

(J) 60 ~

0390 40

10

Oyebath pH

Fig. I - Measured and calculated va lues of K IS for dyeings conducted with I giL indigo from infi nite dyebaths. Calculat­ed values of KIS made using Eq. (2 ) and a reflectance

absorptivity coefficient of 40

Oyebath pH

Fig. 2-Measured and calcu lated values of KIS for dyeings conducted with . 1 giL indigo from infinite dyebaths. Calculat­ed values of KIS made using Eq. (3), measured values of "p",

and a reflectance absorptivity coefficient of 40

2~--~-----------------'----~

c; 0 0 ~

1.5 :§ C 2 co 0 ., u Q)

>-0 co

0.5 ro >-

Dyebath Concentration (gi L)

Fig. :1 - Tec hnical distribution coefficients for indigo/ cotton denim yarn as a functio n of dyebath pH

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ETTERS: INDIGO DYEING OF COTTON DENIM YARN 33

sian scientists in the area of dyeing protein fibers at low pH and the work ~ on the effect o f pH on the presence of hydrolyzed indigo suggest that the minimum pH found by Leupin and Hartmark represents only a roughly approximate lower limit. The fractional amount of each ionic species of in­digo as a function of pH has been estimated from a knowl edge of the apparent pK" values o f the two-step ionization of indigo'l.

So far, the experimental work has revealed that a dyebath pH o f abo ut 11 is optimum from the standpoint of maximizing ring dyeing and color yield. This finding is consistent with the pH re­ported by Greer and Turner that is present in an innovative Japanese indigo dyeing range I II. Ac­cording to the authors, the unique quality of the machine is that "five dips and oxidation steps take place in each o f the fi ve boxes". The pH o f the bath in the unique Japanese machine is a no n­conventional 11 .2-11 .3 .

The UGA investigations have established that there is a solid scientific basis fo r the empirically obse rved relationship between pH and resulting colo r yield II. At a pH of about II , indigo exists as a mono-ionic fo rm having a grea te r substantivity fo r co tton than does the di-ionic form of indigo that exists at much highe r pH . In additio n, at p H 11 , cotton c~lIulose is hardly io nized at all , and this fact minimizes ionic repulsion between negat­ively charged dye and fibe r that occurs at highe r pH. The lower level of io nic repulsion between fiber and dye contributes to an even greater sub­stantivity of indigo fo r cotton. Associated with the increased substantivity that occurs ' a t pH 11 is an increased strike rate of the dye for the fibers in the yarn surface. The increased strike rate results in a mo re ring-dyed ya rn than that which would be obtained with lower strike rates found at high­er dyebath pH . With the increasing ring dyeing, there is greate r shade depth for a given amount o f dye fixed pe r 4.nit weight of ya rn .

Even though modern research has indicated the value of using a dyebath pH of about 11 , many indigo dye rs continue to employ a dyebath pH range of 12-13 simply because sueh'a pH range is easily obtained by the usc of safe concentratio ns of the conventio nal alkali sodium hydroxide. Commercial dyers who have attempted to obtain the more desirable dyebath pH of 11 by the use of a NaOH deficiency have experienced difficulty. Many have discovered that the technique is ex­tremely dangerous, producing intense shade varia­tion and streaking. As shown in Fig. 4, the con­centration of sodium hydroxide nonnally used by indigo dyers (2-4 giL) produces a rather limited

13.5

12 .5

11 .5

I 10.5 a.

9.5

8.5

7 .5 0 0.5 1.5 2 2.5 3 3.5 4 4 .5

Sodium Hydrox ide (g i L )

Fig. 4 - pH of unbuffered soEiium hydroxide Soluli o ns as a function of co ncentra tio n

pH response to small changes in NaOH concen­tration. However, as shown in the figure, when ve ry low levels of NaOH arc used in an attempt to achieve a dyebath pH of II , small changes in NaOH concentration lead to ve ry large changes in dyebath pH . It is critical that a trul y buffered al­kaline system be used to achieve th L' optimum pH Of 11 . Dyebath alkalinity must be maintained at a safe level, while dyebath pH is controlled at the lowe r, optimum level. Simpl y using a deficiency of sodium hydroxide in the dyebath to obtain a pH of 11 is fraught with dange r and should not be at­tempted.

3.3 Washdown Characteristics or Modern Denim

Denim garments that are produced fro m denim yarn dyed at pH 11 tend to wash down more quickly during "stone-washing" and other launder­ing processes designed to produce the popular worn o r distressed look . It is known that the rea­son fo r this behavior is the increased level o f ring dye ing found in ya rns dyed at pH II . Such ya rns cont ain much less dye to produce a given initial shade depth than do ya rns dyed at conventio nal j)H values. Less dye, therefo re, needs to be rL'­moved to obtain the desired fashion look.

4 Conclusion The ancient art of indigo dyeing slowly is being

transfo rmed into a much more predictable pro­c(:'ss . Much progress has been made in under­standing the important variables that influence dye uptake, co lor yield, and washdown character­istics of denim fabric, but much more is to be learnt. Although the modern forms of denim are being made possible through a greater level of technical control of the dyeing process, it must be recognized that dyers have successfully produced

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34 IN DI AN 1. FIBRE T EXT. RES., MARC H 1996

indigo dyed denim yarn for tho usands of years. It is hoped that as the art of indigo dyeing is re­placcd by scientific methods, appreciation of the artistic c1cments in indigo dyeing and the signifi­cant contributions made by traditional dyer .. will fl o t be forgotten.

References BASF. Tcchllical Ilifurl/Ullioli. T I/ T 0 17. Novem ber It)X7 (AJM ) 13.

2 Kuehni R G, Computer colorallf fo rmulation (Lexingto n Books. Lexingto n. Massachusetts ). 1<f75. II .

3 Annis PAct al., Can Text J, I O~ (4 ) (May I t)t) I ) 20. 4 E tters J N, AmDyestRep,8 J (3 )( 1l} 'J2 ) 17.

5 E tters J N & Ho u M, Text Res J; 6 1 ( 12 ) (1 991 ) 773.

6 LCllpin 0 & Hartma rk B. Melliand Textilher. 24 ( 1943 ) 3Y4.

7 Golomb L M & Shalimova G V, Tcchllol Texl 111£1.48 (5 ) ( 1965 ) 105.

H Mishche nko A V & A rt ym M I. Text Ind USSR. 54 (4 ) ( 1Y7 1) t)5.

Y E tte rs J N, J Suc Dyers Coluur, I (It) (7 / 8 )( 1t)':I3 ) 25 1.

10 Greer J A & Turner G R. Text Chen; Color. 15 (6 ) ( 19R3 ) I (I I .

II E tters J N, Text Chelll Color, 27 (2 )( I t)Y5 ) 17 .