Indian Journal of Fibre & Tex tile Research Vol. 39. March 2004. pp. 100- 109

Review Article

Dyeing of deni m with indigo

J N Chakraborty"

Nati ona l Insti tute of Tecllllology. Jaland lwr 1440 11 , India and

R B Chavan

Ind ian Instituti.! of Technology. New Delhi 11 00 16. India

Reech'ed 25 April 2003; (lcceplcd 30 J IIII C 2003

Indioo is ex tcnsi ve ly uscd for dyeing o f dcnim. Though indigo is a class or va t dye, it cannot be applied follow ing conventi ; nal vat dyeing method due to its very little arfini ty for colton. Thi paper rev iews proper applica tion of indigo on denim as well as various aspects rclating to thi s app licati on.

Keywords: Colton. Denim, Dyeing. Ind igo. Va t dye

IrC Code: In t. Cl 7 D06P 1122

1 Introduction Natural indigo, known in d iffe re nt na mes in

d iffere nt na rt s of the world , has been in use s ince aro und 7000 Be for dyeing or cotto n in attractive a nd bright b lu e shades. Plants be longi ng to the genus lndigofera are most va luab le fo r produc ing natural indi go. T he co lo uring malle I' in the p lant is present as a gl ucoside of indoxyl , known as indican, w hi c h is hydro lyzed to the indoxy l by e nzy matic action a nd ind igo is then ob tained by subseque nt ox idati o n l

. ]n anc ient times, reducti on of ind igo \-vas carri ed o ut by fermentation technique w hi ch included use of ripe n fruit and stale urine ass isted by wood as h o r lime as alkali. T he soluti o n prepared in thi s way was left overn ight fo r red uction and solubili zation of indi go. T he supe rn atant liquo r was the n take n o ut for

colo urati o n through re peated dip-ex posure to a ir seque nce at room te mpe rature l

-J till a deep shade was

produced , a cumberso me as well as time consuming

p rocess. However, ex tens ive work fo r findin g o ut a suitable

al te rnative of natura l indigo to meet growing demand o f it gave birth o f modern indi gotin by Adolf Von Bayer in 1880. Later, BASF AG (Germany) pl aced co mmercial syn theti c indi go in marke t in 1897 (refs 4 , and 5). Indi go (C. 1. Vat Blue 1) has the chemical structure as shown be low:

a To whom all the correspondel',cc should be addressed. Phonc : 2690301 -03 Ext 22 1: Fax: +lJ 1- 18 1-2690320/2690932 : E-mail : chakrabortyjn @hotmail.com

o 0 II II

0)=0) N N H H

T houg h natural indigo was known s ince anc ient times and the sy nthetic one was developed mo re than 100 years back, research work started s low ly in va ri o us areas of indigo dye ing o nly in the 60s w ith an inte nsi fi cation s ince 80s to cope up w ith increase in popularity of indigo dyed denim worldw ide. Eno rmo us develo pme nts were pro posed which mainly compri sed changes in conventional reducti o n a nd app licatio n techniques, modificatio n in des ig n of machine ri es , various ways to enhance indigo uptake and quantitative es timatio n of indi go on cotton de nim . Bes ide, developments have also been made to improve wash-down property th ro ug h efficie nt dyeing6

-J6

. The present pape r reviews proper applicati o n of indigo o n denim as we ll as vario us as pects re lating to this application.

2 Application of Indigo Indigo is excl us ive ly used to produce attracti ve

b lue shades o n de nim along w ith desi red wash-down property. However, in its reduced and solubili sed

for m, ind igo possesses littl e affinity for cotton- the

CHA KRABORTY & CHA VAN: DYEING OF DENIM WITH INDIGO 101

extent of affinity is too less to exercise exhaust dyeing; rather multi dip/nip technique with intermediate air oxidation is followed for gradual build up of shade. A suitable method of dyeing, viz. slasher dyeing (sheet dyeing) or rope dyeing (ball warp dyeing or chain dyeing) or loop dyeing, is fo llowed for dyeing of warp yarn . After dyeing, the dyed warp yarn is woven using white cotton weft to produce mostly a warp faced 3 x I twill fabric with blue face and white back. The fabric is then washed through specific techniques to remove a part of dye in order to develop unique fading properti 7.

In slasher dyeing method, denim yarns in the form of a warp sheet are pretreated in earlier compartments fo llowed by multi dip/nip indigo dyeing; the process completes with after-washing followed by drying, sizing and final drying. Handling of yarn is minimum as the warp sheet is directly processed and then sent to weaving section for its subsequent conversion to f b . 38 - 4 I A I I d . . f a I'!C' . S as ler yelng range may consist 0 . h h . d' d' I 42-45 elt er t e 111 Igo yelng range a one or a

continuous indigo dyei ng range with an integrated sizing range46

. In the first type, yarn sheet moves forward through one or two pretreatment boxes with the stages of dipping, squeezing and skying; washing and drying are the only aftertreatments. On requirement, two or more layers of warp sheet may be processed simultaneously. Schematic diagram of a

2 3 4 5 6

popular slasher dyeing range is shown in Fig. I. The second type includes the steps followed in first type, extra attachments are the sizing and the washing compartments as shown in Fig. 2. Slasher dyeing is suited for better production schedules with a poss ible problem of centre to side variation. Superior quality of yarn is needed to minimize breakage problems. Broken end roll wrap ups can cause catastrophic shut­down problem as well as undesired waste. On the other hand, the dipping, squeezing and oxidation require less time as each yarn is independently subjected to treatments.

In rope dyeing method, yarns from creel are pulled by a ball warping machine and then passed through a lease stand consisting of a special comb and lease rods to ensure proper registration of ends. These drawn ends then pass through a condenser tube assembly to merge these into a bundle followed by passing the latter to form a rope comprising 350-400 ends . Ropes are then wound on drums Once the beam has been fully wound, it is unloaded and mounted on d 1747 0 . . db ' ye range cree '. yell1g IS one y passing ropes, lip to 12 at a time, through pretreatment bath foll owed by multiple dipping in separate indigo baths wi th intermediate squeezing and skying, as done in slasher dyeing. Schematic diagram of a rope dyeing unit is shown in Fig. 3 (ref. 48). Rope dyeing method is important for higher production rate with better

7 8 9 10 Creel Pre- Drying & oxidizing Washing

& rinsing Cylinder Synchronizing

Treatment

Sheet dyeing Slasher -dyeing

drier unit

Fig. I-Omez SpA continuous slasher dyei ng range38

~~.~ -'····~I . eo H~~; ee$~ Linked with slasher .....

Fi g. 2-Slasher dyeing with integraled sizing range'S

102 INDIAN J. FIBRE TEXT. RES., MARC H 2004

Ball Warping --- --- After-wash

-- 'tWI Fig. 3--Rope dyeing 7.38 (reproduced fro lll Internatio nal Textile Bulletin)

Fig. 4--Princip le of " Loopdye I for 6" (ref. 49) (reproduced frolll Internat ional Textile Bulleti n)

quality dyeing and better fastness to wet/dry rubbing as well as colour wash down It includes less breakage of ends and better shade consistency but in contrast more handling of yarn to open ropes before sizing. However, the efficiency of slasher and rope dyeing methods depends on back feed to dyebaths to maintain concentration of indigo and other chemicals at a constant rate42-4S

, which is a cumbersome work. Moreover, consumptions of chemicals and dye remain on higher side with greater chance of uneven / tailing effect.

[n loop dyeing, problems associated with slasher and rope dyeing were solved through the development of a new dyeing technique, known as "Ioopdye 1 for 6" , by Eckhardt Theodor Godau In comparison with other indigo dyeing methods with six colour troughs, in loop dyeing there is only one brief indigo dyebath with one squeezing unit. The process, offered by Looptex SA(CH) and under licence by Intes(I) and Texima SA(Brazil), is of modular construction. It comprises 8-16 warp beams (threading in feature), pre-wetting or pre-mercerising unit, twin-pad colour applicator, an integrated skying passage, two rinsing sections and an accumulator scray. Fig. 4 shows the

f . d ' h' h ' 49 -53 sequence 0 operatIOn use tn t IS new tec l11que ,

3 Reduction of Indigo 3.1 Conventional Reducing Agents

Sodium hydrosulphite reduces indigo at 30-S0°C depending on type of indigo used, resulting in formation of biphenols (Ieuco-indigo)54. The reduced form is quite stable in presence of NaOH to carry out dyeing at room temperature. However, hydrosulphite is very unstable, At the time of reduction of dye, it gets decomposed thermally, oxidatively and in various other ways55, requiring 2-3 times higher amount over the stiochiometric requirement56-5s. Attempts have been made to keep control over its use through scientific analysis in course of dyeing59-62.

Other reducing systems, viz . fermentation technique l copperas method I. 63, 64 zinc-lime method I ,

bisulphite-zinc-lime vat method 1,65, thiourea dioxide method 66-{iS and sodium borohydride method 69 -72 have

fallen out of use due to limitations associated with their applications.

3.2 Ecofriendly Reducing Agents In recent times, the use of ecofriendly reducing

systems for indigo has attracted attention of researchers?3. These new reducing systems include hydroxyacetone, iron-pentacarbonyl compounds,

CHAKR ABORTY & CHA VAN: DYEI G OF DENIM WITH INDIGO 103

electrochemical reduction, g lucose-NaOH and iron (II ) salt along with a suitable li gand .

Hydroxyacetone when used as a reducing agent (reduction potenti al --8 10m V) in re fined dyeing 74

process results in 20% higher dye uptake along with less consumption of auxiliary che micals . Other advantages are higher quality dyeing, better ring dye ing effect, better elasticity of yarns, increased producti vity, higher dye uptake and less dyestuff in effl uent. However, in thi s method the required reduction potenti al for vatting is obtained at 100°C wi th higher concentration of NaOH and the produced shade does not co rrespond with that obtained with hydrosulphite. Hydroxyacetone is found to be more suitable for pad - steam method75

.

The detailed report on the use of lron­pentacarbonyl compounds, though sugges ted as reduci ng agent, has not been d isclosed so far76

.

In electrochemical reducti on technique, vat dye is directly reduced by making the contact between dye and electrode. A reducing agent at lower concentration must be added to the reduced dyestu ff to ensure co rresponding stability o f reduced dye liquor. However, the dyestu ff requ irement fo r a specific shade is too high77

-8u

.

Glucose in combination with NaOH can be used fo r the reduction of sulphur dyes [reduction potenti al - (-550)- (-600) mY ]. Indigo requires - -700 mV potenti al for reduction ; hence g lucose-NaOH sys te m can also be used preferably at boi l, producing reduced indigo baths free from sediments and hi ghly stable for several hours. The problem re lated to this reducing system is that the padded tex ti Ie requires more ti me fo r oxidation in between two successive dips and the indigo uptake is less81

•

It has been reported that ferrous hydrox ide is a strong reducing agent in an alkaline medium73

. The reducing effect increases even more with the increase in pH value. Ferrous hydrox ide is poorly soluble in alka line conditions and precipitates. It must be complexed82 in order to ho ld the ferrous hydrox ide in solution . Fe++ has hardly any reducing power as the central ion of complexes has the stable arrangement of krypton shell. However, a stable complex with reducing power is obtained with weaker ligands, e .g. gluconic acid. On dyeing with vat dyes at 60°C, good dye uptake was achieved fo r optimum concentration of iron(II ) salt and molar ratio between thi s salt and gluconic acid. However, dye uptake decreases with the decrease in concentrati on of iron salt, even too

low concentrati on of iron(II ) salt g ives lighter and unlevel dyeings at 1: 1 and 1:2 molar rati os of iron(lI ) salt and gluconic acid due to the insufficient vatting of dye. When molar concentrati on rati o of iron(J I) salt and gluconic ac id was reduced to I :0.5 unlevelled shades were produced with reduced brillianceD

. Iron (II ) salts in combinati on with NaOH and a weak ligand, like gluconic ac id, tartaric ac id citri c acid or triethanolamine, at suitable concentrations can produce reduction baths wi th reduction potentia l ranging fro m-800m V to-1000m V. These reducti on baths are capable of reduction and dye ing with indigo at room temperature. The reduction baths showed superi or stability in presence of indigo up to 24 h. Depos ition o f iron on dyed samples was also assessed and it was fo und that g luconic acid as li gand causes least depos ition whereas c itri c ac id causes max imum deposition. Produced shades were bright with no

shifting of Amax. The baths were turbid th roughout dye ing83

.

It may be summarised here that fo r quality dye ing, whatever may be the reducing agent the reducti on potenti al and alkalinity of bath should be accurately adjusted for development of true shade of indi goS4

.90.

4 Factors Influencing Indigo Uptake Reduced and solubili zed indigo, though ion ic in

nature, possesses negligi ble affinity for cotton ; a mUltiple dip / nip technique is fo ll owed with intermedi ate airing fo r gradual build-up of shade. Yarn to be dyed is dipped in indigo bath fo r a specific time fo llowed by a passage through padding unit in order to achieve better penetration as well as di stribution at nearl y 100% ex press ion, succeeded by air ox idation. Thi s dip / nip fo llowed by airing cycle is repeated till des ired depth of shade is produced, succeeded by a fi nal oxidation for 3-5 min for complete conversion of soluble indi go to its insoluble fo rm.

As stated , desired depth of shade is primarily governed by number of dips/nips, but dye uptake during each dip is influenced by a number of other process parameters, viz. pH of dyebath , immersion time, time of intermediate ox idati on, concentrati on of indigo and temperature o f dyebath. These parameters affect build-up of indigo on cotton, degree of penetration and various fas tness properties.

The crucial facto r is pH of indigo bath 54.9 1.95 .

Depending on dyebath pH , indi go may exist in fo llowing fo ur di ffe rent forms as shown in Fig. 5:

104 INDIAN J. FIBRE TEXT. RES. , MARCH 2004

o 0

II II

oo.--<X) N N /I I I

(i)

DNa OH I I

~~ ~ .. /'"V " r-: II II

(iii)

0 11 01-1 I I

~~ ~ /"- .~ N N II II

(ii)

ONa ONa

( iv)

Fig. 5--Different ionic and non-ionic forms of indigo1s . ~2 [(i) indigotin, (ii ) reduced non-ionic form. (iii ) mono-phenolate form. and (iv) bi-phenolate formJ

(i ) indigotin structure at lower alkaline pH, (ii) reduced but non-ionic form at moderate a lkaline

pH, (iii) mono-phenolate fo rm at relatively higher pH, and (iv) bi-phenolate form at very higher pH ,

Structures (i) and (ii) exi st below pH 9-9 .5 ; thei r relative fraction depends on exact pH of bath. With slow increase in pH structure (i) collapses and gets converted to either (ii) or (iii) or mixture of both. Further increase in pH above pH 10 slowly converts all (ii ) to (iii). At around 11.5 pH almost a ll indigo molecules are in their mono-phenolate form . Any further addition of a lkali beyond pH 11.5 slowly attacks second C=O group of indigo in presence of excess hydrosulphite, resulting in the conversion of mono-phenolate form (iii) to bi-phenolate form (iv) . The extent of conversio(1 strictly depends on exact pH and excess hydrosulphite in dyebath. As the presence of excess hydrosulphite is mu st for successful indigo dyeing, if a lkali is added in excess above pH 12.5, the structure (iv) predominates . Indigo in the form of structure (iv) shows reduced affinity for cotton and thus dye uptake falls. Structure (i) shows no affinity whereas structure (ii) has negi ig ible affinity, as both these structures are non-ionic forms of indigo. Mono­phenolate form is the desired structure which possesses highest affinity as well as strike rate to g ive higher dye uptake. In this context it is also to be noted that cotton when dipped in a lkaline bath acquires negative charge. The higher the pH, the higher is the charge on fibre and more is the repul sion between dye and fibre, resulting in reduced dye uptake .

Importance of pH is also related to ease in washing. Indigo shows maximum affinity as well as higher strike rate on cotton at pH 10.5-11.5 due to the lower solubili ty of mono-phenolate form in this pH range.

1.0

~ 0.8

~ 0.4

o

.2 0.2

2 '. .' .

"

.. ...... '"

10 Dy(:oalh pi t

. ::- .... . I

II

/ / ..

12

/ ,

, , , , ,

, , ,

IJ

, / ,

I

I.

Fig. 6--Fract ional form of each reduced species of indigo found in dyebath as a function of pH 9 1 [-" - "-"-"- " Acid leLico form.

Mono-phenolate form, and ___ __ Bi-phenolate formJ (reproduced from AATCC Review)

000 13 .0 12.:" 12.0 I I .' II .n

Ihcba(h pi I

Fig. 7- Distribution of indigo in the cross- section of cotton deni m yarn at different pH conditions 105 (reproduced frolll AATCC Review)

This hinders penetration inside cotton and g ives more intensive surface dyeing, achieving better wash-down effect96

.

It was mentioned earlier that the rcl ative fraction of mono-phenolate and bi-phenolate forms of indigo can be positively controlled by adjusting the dyebath

H54. 87.91. 92. 94. 95. 97- 104 · . . P . pH IS such a c rucIal factor 111

dyeing with indigo that it not only contro ls dye uptake but also distribution of indigo on cotton yarn through control over strike rate and hence wash-down properties with removal of unfixed dyes87.105

Out of two solubl e forms of indigo viz. mono­phenolate and bi-phenolate, the former has relatively lower water solubility and higher strike rate, just opposite to that of bi-phenolate species (Fig. 6). Hence, if pH can be kept within a range of 10.5-11.5, most of indigo molecules will exist in bath in their mono-phenolate form, showing higher strike rate for cotton, resulting in poor penetration and higher surface deposition of dyes-{he so called ring dyeing ft' 87 9 1 105 I . d ' Iff' I e ect . . . n nng yell1g, on y a ew sur ace ayers

are dyed leav ing interior of yarn undyed due to the absence of adequate time for penetration as well as thorough di stribution 105 (Fig. 7). If pH is rai sed above 11.5, the relative fraction of mono-pheno late form

CHAKRABORTY & CHA VAN: DYEING OF DENIM WITH INDIGO 105

gets reduced with instant inc rease in that of bi­phenolate form , lowering the strike rate. This consequently increases time for absorption, resulting in dyei ng of few mo re layers than that obtained with mono-phenolate form . With the further increase in pH , mo no-pheno late form goes on diminishing with a substantial ri se in bi-pheno late form and thorough d · f 105 ye lllg 0 cotton occurs .

Efficient wash-down property of indigo dyed yarn depends on the amount of dye present on yarn surface. It is the 10.5- 11 .5 pH range which g ives dye ings with desired washing performance and a deeper shade with less amount of indigo. Dyeing at hi gher pH range gives re lative ly lighte r shades and I 'P IO(i - 108 ower wash-down property - .

Ionic nature of cellulose also affects indigo uptake. With the increase in alkalinity of indigo bath ionisation of cellulose (expressed in g- ions I litre) increases with development of more anio ns, e.g. concentration of

Cell-O' at pH 10 is 3 X 10-3 g-i onsl litre, which increases to 1.1 g-ions/ litre at pH 13. This, in turn , suppresses substantivity of indigo, finally lowering the dye uptakeS7. 105. Calculation of different forms of indigo in bath, though cumbersome, can be made through the study of equilibrium ioni sation constant values l07 .

Effect of immers ion time on final colour depth in indigo dyeing shows that with the increase in immersion time, dye uptake increases initially , remains nearl y same for little further increase in pH and beyond certain limit falls considerably. An immers ion time of 30 s seems to be adequate. Prolonged immers ion reduces colour depth mainly due to the re-reducti o n of oxidised indi go retained by cotton through previous dips and go back to dyebath . To get optimum colour yield oxidation time is fixed at

~60 s. Due to very low affinity of solubili sed indi go in bath for cotton, the increase in indi go concentration in bath though initi a lly can increase the depth of colour, but beyond certain concentration does not show any remarkable change on dye uptake. Increase in indigo concentration up to 3 giL in bath increases its uptake o n cotton but beyond thi s concentration it shows little impact on dye uptake. Optimum depth of shade is obtained in as hi gh as ten dips and beyond that no such inc rease occurs. But keeping in view the limitation in machinery set up and a lengthy process time, generally a 6-dip 6-nip technique is followed%' 97. However, it has been reported elsewhere to use the lowest poss ible concentration of indi go and maximum number of dips . By doing so, colour uniformity ,

fastness and wash-down properties after dye ing improve. A rule of thumb is that mo re the number of dips used for a given shade with low concentration of indigo in bath, the better the co lour uniform ity , fastness and wash-down properties 109 .

Affinity of solubili sed indi go for cotton decreases w ith the increase in temperature, initiall y at a faster rate followed by very slow decrease. At lo wer te mperature, affinity o f indigo is better and also re­reductio n of oxidized indigo does not occur typically . Hence, it is better to carry out dye ing without any application of heat. It has been reported that the maximum dye uptake occurs if dyei ng is carried OLit at room temperature. With the increase in dye ing te mperature from 40°C to 60°C , dye uptake fa ll s re lative ly at a s lower rate and beyond that it becomes almost parallel9697.

5 Techniques to Enhance Indigo Uptake

5.1 Alternate pH Control Techniques Indi go uptake can be enhanced through proper

control over pH in the regi on 10.5-11.5 . NaOH , used in indi go dyeing, is a very strong alkali and makes efficient dyeing impracticable due to the wide fluctuation in pH during dyei ng. Initially , the pH is highly sensitive to NaOH conce ntratio n, a smal l change in NaOH concentration changes pH remarkably 11 0 The normal practice is to add NaOH at higher concentration so that the final dyebath pH does not fall below 10.5 with a result of less dye uptake in initial dips. Thi s is because when less NaOH is used in an attempt to achieve a dyebath pH of 11.0, small changes in NaOH concentration lead to very large changes in dyebath pH. In fact, the dyebath pH may drop suddenly into a range in whi ch mainl y non­io nised indigo is formed and superficially deposited onto the denim yarn . Such deposi tio n inhibits sorption of ioni sed dye, leading to streak ing and extremely poor crock fastness 110. In contrast, the use of excess alkali makes li ghter dyeings of higher alkalinity , which creates trouble in finishing III.

Attempts to substitute NaOH with o ther alkalisl bases deve lo ped buffe red alkali system of undi sclosed composition., This was manufac tured by Virkler Company, USA, which when applied in indigo bath at recommended concentration produced a stabl e pH in the range of 11 .0 and little fluctuati on occurred in dye uptake throughout the period of dye ingl1 2.

In another attempt, NaOH was substituted with different organic bases . Before application, alkalinity of these bases was evaluated and then these were

106 INDIAN J. FIBRE TEXT. RES., M ARCH 2004

applied in various indigo baths, keeping equi va lent alkalinity with that produced with NaOH. It has been reported that the dye uptake with these new systems was on quite higher side than th at obtained with NaOH method. Stability of indi go bath was superior to that o f NaOH baths even after a lapse of 8 h. Change in pH with time was also studied and it was found that most of the bases are capable of maintaining pH in 10.5-11 .5 range upto 4 h (ref. 11 3).

5.2 Pretreatment of Cotton Indigo in solubili sed state is anio nic in nature and

will be attracted by cotto n if the latte r is pretreated with cationic compounds. Cotton was pretreated by immersing it in a soluti on contain ing 200 giL of 12% aqueous Hercosett 57 (cati onic water soluble polyamine-epichlorohydrin res in) and 10 giL of Sandozin NI for 5 min followed by squeezing the material in padding mang le to achieve 80% pick-up and drying at 100°C fo r 3 min. Treated cotto n was padded with indigo th rough 6-dip 6-nip technique. It has been reported that dye uptake on pretreated cotton remain on higher s ide than that o n untreated cotton under identical dyeing conditio ns96

.

Pretreatment o f cotton with metal sa lts prior to padding with indigo (wh ich may be done in several ways to synchronize with batch or continuous dyeing methods) results in substantia l increase in indigo uptake. It is also observed that iron and cobalt (II ) salts are more e ffecti ve even at very low concentration of I giL. Formation of insoluble meta ll ic hydroxides ill situ colto n in presence of NaOH and air is responsible to attract more anionic indigo molecules in bath , causing increase in dye uptake l

i 4 .

Introduction of a merceri zing step prior to indigo padding enhances ind igo uptake considerably. A DIM ER was shown at ATM E- I, Greenville, October 1988, which is bas ica lly a loop dyei ng system that incorporates real premerceri zing in the loop dye process for indigo warp dyeing. The technique was originally developed in Switzerl and by Loopdye, but is now being produced and marketed by Kuesters. The unit simpli fies warp production with advantages like improved fabric appearance and handle, higher yarn strength , higher dye uptake and more intensive ring­spun yarn dyeing with shorter stone washing. Even it is also poss ible to botto m with sulphur, indanthrene, reacti ve and naphthol colours 11 5 .

6 Cost Effective Production in Indigo Dyeing Multi-vat machine concept has regained its

importance throug h fl ex ible adaptation o f machine

techno logy. Accurate dos ing, targeted dyes tu ff use, lo w dyestuff and chemical losses a re the mai n aspects of such productio n system.

Acco rd ing to lite rature, adequate dwell times in the steeping bath (approx. 6 m/bath) and ox idati o n zones (>60 s) together with a high c irc ulati o n rate with low dyestuff concentratio n have been pursued in order to

h· . d ' 10-1 116· 118 D I ac leve o ptimum yell1g' . ue to ower dyestuff concentratio n in the effluent, ultra-filtrati o n devi ce can then be equipped with smalle r f il te r surfaces to red uce in vestment and operating cost. Washing water consumpti on can be kept low at 3 Llkg o f warp.

Indi go vat, hydrosul phite and NaOH are continuous ly dosed in propo rtio n to speed of c ircul atio n avoiding shade variati o n. A Semen Op 20 auto mati o n unit , confirming a hi gh degree of process ing re liability and reproducibility, contro ls dos ing. Dosing of dye is controll ed via a special f requency contro l pump. Measuring systems are ava il able for bath concentratio n, pH va l ue, redox potenti a l, bath temperature, etc .

Another unique way o f cost red ucti on is 'zero discharge concept ', which works in ' Ioopdye I fo r 6 process' using hydroxyacetone as reduc ing agent to p roduce waste-free improved quality dyeing at reduced cost thro ugh recovery and reuse of unused dye and was tewater I 19.

7 Quantitative Estimation of Indigo Indi go in dyed textile/dyebath/wash liquo r is

determined spectro photometrica lly at max imum wave

length (I"m",)' When indigo is in solid state, each of its mo lecules fo rms H-bo nds w ith 4 adj acent dye mo lecul es (between C = 0 and N H groups) and makes it insoluble in water as we ll as in most o f the

. I 120 P 'd' d I orgal11 c so vents . yn me acts as a goo so vent For spectroscopic estimatio n of indi go, a clear and stable solutio n of indigo is an ind ispensable pre­conditio n for consistent and reprod ucible results. Alte rnate ly, turbidity o f oxidi sed form of indi go may be a measure of quantitati ve estimation . Several analytical methods have been devised in this context, viz. colorimetry of leuco indigo, turbidity of indigo in oxidi sed form and colorimetry of sulpho nated indi go. In solvent technique, a few methods based on colorimetry o f chloroformic, pyridinelrwater and dimethyl sulphoxide extracts have al so been reported 12 1 - 124 .

The colorimetry o f leuco indigo method consists of reducti on of pure indigo with hydrosulphite in

CHA KRABORTY & CHA VAN : DYEING OF DENIM WITH INDIGO 107

alkaline medium. Absorbance is instantly observed at 41 2 nm, as reduced indi go has time-bound stability ; its spectrum changes considerabl y in very short time intervals. Thi s technique lacks reproducibili ty l25

However the technique may be di spe nsed with other methods if stability o f reduced indigo can be increased It has been mentioned in literature that iron(ll) complex with tri ethanolamine can reduce vat dyestuffs and the reduced dyebath has high degree o f stability to temperature, time and atmospheric oxygen l 26 In thi s technique, iron(I1) sa lt concentrati on should be kept under contro l as iron ions below 400 nm tend to go fo r self absorpti on.

Turbidity of ox idi zed indigo method shows no linear relation between turbidity and concentration. In some cases, the hi gher va lues of turbid ity were also achieved on reducing the concentrati on. Thi s behav iour is poss ibl y due to the excessive dimensions of indigo particles as well as di stribution of uneven sizes. Additi on of g lycerine to stabili ze the suspension did not improve the results (g lycerine increases bath density and retards particle fa ll ). Maximum absorbance was found to be at 666 nm but the slope of the calibration curve vari ed according to the statu s of initi al dye ing bath l26

.

In sulphonated indi go method, incorporation of solubili zing groups of sulphuric ac id in indigo structure makes indi go water solubl e, stable and deep blue coloured. T he absorbance is max imum at 605 nm (ref. 125). Beer's law is fulfill ed up to 15 ppm of concentration and the reproduc ibility of the method is 0 .3-2%. However, before sulphonation, indigo should be in ox idi zed and dried fo rm otherwise a greeni sh­brown solution is obtained Indi go should also be free from hydrosulphite. This analyti cal technique is hardly suited for instantaneous and continuous contro l of indi go liquorsl26. Sulphonation of indi go with concentrated sulphuric acid at 75°C is also troublesome.

7.1 Solvent Techniques The colorimetry of chloroformic ex tract g ives

max imum absorbance at 605 nm and good linearity is achi eved in the calibration curve up to indi go concentration of 10 ppm. At thi s concentration solubility limit of dye in chloro form is reached and a blue interphase between the two layers is formed d

. . 125 unng extractI on

Indi go structure has a simil arity with that of pyridine. So lubilization occurs through H-bonding between these two Oxidi zed indi go can be di ssolved

up to a concentration of 40 ppm by bo iling in pyridine-water (50 :50) soluti on which is then photometricall y measured at 625 nm. Measurement must be done when the solution is hot to avo id precipitati on of indi go on cooling the solution. Ind igo should be completely free from hydrosulphite; if not, the later decomposes indigo during heating with reagent solution. Presence of NaOH with dye also changes absorbance as aggregati on of dye alters wi th pH . Thi s method has a reproducibility of 2-4 % (ref. 125).

A completely new approach has been made by solubili zing oxidized indigo in dimethyl sulphox ide (DMSO). The method does not have any problem as shown with earlier methods. It is time saving, safe, without interfe rence and largely reproducible. Solubility of indigo is excellent even at room temperature; the max imum absorbance is achieved at 6 19 nm. The soluti on is highly stable fo r several days

and for indi go concentration of up to 250 Ilg/ml, complete dissolution takes pl ace. The onl y problem lies in handling of DMSO is that it is sli ght ly detrimental to health , causes irritati on in contact with k· d' h' 127 S 111 an ItC 1I1g, etc. .

In some cases, denim is dyed with a combination of indigo and sulphur or hydron blue dyes. To estimate contribution of these two in the shade developed, indigo is first ex tracted complete ly from sample at bo il in pyridine-water solution. The dyed sample under test is rinsed with methyl alcohol and dried at 40°C to remove res idual pyridine and methyl alcohol. Refl ec tance of the sample is measured. Colour strength calcul ated from refl ectance value at each wavelength is integrated over the wavelength range, the result is an integrated value l28.

8 Conclusion Though indigo is a class of vat dye, it cannot be

applied according to conventional vat dyeing techniques due to its neglig ible affinity for cotton, rather multiple dip/nip with intermedi ate airing is to be fo llowed. During dye ing, the pH of dyebath pl ays a crucial ro le to affect indigo uptake as well as location of dye on cotton structure, though influence of other factors cannot be overlooked. Dyeing mostly takes pl ace in yarn stage, necessitating selection of specific machine depending on form of yarn . Beside dye ing parameters, special techniques through use of buffer/other alkali sibases, pretreatment of cotton, e tc. may be introduced to enhance indi go uptake. As fa r as quantitative assess ment of indi go is concerned,

108 INDIAN 1. FIBRE TEXT. R.ES., MARCH 2004

DMSO method has been found to be the most effective one.

References I Fox M R & Pierce J H, Texi Chelll Color. 22 (4) (1990) 14. 2 Schmidt H, Melliand 1111. 78 (2) ( 1997) 89. 3 Hughey C S, Texl Chelll Calv I' , 15 (6) ( 1983) 103/13. 4 Cassebaum H, Melliond Texlilber, 46 ( 12) ( 1965) 133 1. 5 Pi erce J H, 1111 Dyer Texl Prilller. 180 (7) ( 1995) 20. 6 Klei n R, Texlilber. 9 (9) ( 1976) 74. 7 Stromberg W, In l Texl BIIII - DyeingIPrintingIFinishing, 40

( 1) (1994) 19. 8 Techni cal report. Text World, 145( 1) (1995) 44. 9 Patil M R, Indic/II Text J, 9 1 (6) ( 198 1) 11 9. 10 Technica l report (BASF AG), Int Text BIIII - Dyeing I

Printing I Finishing, 40 ( I) ( 1994) 12. 11 Hoechst Technical info rlllation OF 0006E. and OF 0029 E. 12 BASF. Cololl rage, 32 (16) ( 1985) 15. 13 Kramri sch B, Alii Dyesl Rep, 69 ( 1 1) ( 1980) 34. 14 Harper R J & Lambert A H. Texl Chelll Color, 24 (2) (1992)

13. 15 Pau l R & Naik S R, Indiall Texl J. 108 (5) ( 1997) 28. 16 Tech ni cal report , Text Prax Int , 48 (3) ( 1993) 227. 17 Ri az A R & Riaz J, Pakistan Text J, 4 (1992) 39. 18 Patil M R. lndian Text J, 9 1 (8) (198 1) 12 1. 19 Etlc rs J N, Alii Dyest Rep , 84 (5) (1995) 20. 20 Kochav l 0 Videbaek T & Cedronl 0 , Alii Dvest Rep, 79 (9)

(1990) 24. 2 1 Harper R J & Lambert A H, Alii Dyesl Rep, 82 (5) ( 1993) 17. 22 Chong C L & Ma C W, Alii Dyesl Rep , 85 ( 11 ) ( 1996) 15. 23 Ty ndall R M, Alii Dyesl Rep, 79 (5) ( 1990) 22. 24 Ki crulff J V, Novo Nordisk TH 33. 25 Technical report , Alii Dyes! Rep, 83 (10) ( 1994) 42 . 26 Kerr P L, 1111 Dyer, 182 (10) ( 1997) 4 1. 27 Lantoo R, Oinonen A M & Suominen P, Alii Dyest Rep , 85

(8) (1996)64. 28 Reidi es A H, Jensen D & Gui sti M, Text Chelll Color, 24 (5)

(1992) 26. 29 Rucker J W Freeman I-I S & Hsu W, Texl Chelll Color, 24 (9)

( 1992) 66. 30 Rucker J W, Freeman I-I S & Hsu W, Text Chelll Color, 24

( 10) ( 1992) 2 1. 3 1 Hoffer J M, Text Chelll Color, 25 (2) ( 1993) 13. 32 Grady R, Texl Chel/I Color, 23 (5) ( 1991 ) 4 1. 33 Chatteljee C, Clothsiinelll , 9 (6) ( 1996) 49. 34 Shah 0 L Clothsline, 9 (6) ( 1996) 53. 35 Dey P, Man - Made Text India, 41 (3) ( 1998) 129. 36 Etters J N & Anni s P A, Alii Dyest Rep , 87 (5) (1998) 18. 37 Continuous dyeing wi th indigo, Technical Report (BASF),

September 1995, 3. 38 Parmer M S, Satsangi S S & Prakash J, Denilll -A Fabric for

All, 151 edn (Northern India Tex tile Research Association, Ghaziabad, India), 1996,20.

39 Voswinkel G, Text Asia, 24 (8) ( 1993) 54. 40 Vosw inkel G, lilt Texi Bull-Dyeillg I Prilltillg I Fillishillg. 40

(1) (1994) 16. 4 1 Haas L, lilt Text Bull - Dyeillg IPrillling I Finishing . 36 (2)

(1990) 45. 42 Technical report, Tex! Ind. 14 (2) ( 1976) 76. 43 Techni cal report, Tex/A sia , 7 (3) (1976) 11 4. 44 Technical report, lil t Dyer Text Printer, 155 ( 15) ( 1976) 253.

45 Techni cal report, lilt Text Bull-Dyeillg IPrintillg I Fillishillg. 23 ( I) (1977) 8 1.

46 Tec hni cal report , Text Ill st Iml, 14 ( I) (1976) 5. 47 Fulmer TO, Alii Textln.l t , 22 (8) ( 1993) 58. 48 Hinkle J G, Techllical Text fill , 38 (1995) 153. 49 Godau E T, 1111 Text Bull - Dyeillg I Prillt illg I Fillishing. 40

( I) ( 1994) 23. 50 Parmar M S Satsangi S S & Prakash J. Colouroge. 43 (9)

(1996) 45. 5 1 Dierkes G, Godau E T & Krefeld, Chelll ie(asem I Texl- Illd

34/86 (5) (1984) 350. 52 Paul R Naik S R & Kumar R, Texl Dyer Prillter 29 (2S)

( 1996) IS . 53 Technical report , lilt Dyer Text Prilller, 170 (2) ( 198S) 32 54 Anni s P A, Etlers J N & Baughman G L, Canadi(/II Text J.

108 (5) ( 199 1) 20. 55 Nair G P & Trivedi S S, Cololl rage, 17 (27) ( 1970) 19. 56 Trotman E R, Dyeillg & Chelllical Technology of Texlile

Fibres (Charles Griffi n & Co. Ltd. , London), 1984407. 57 Patil M R, Illdiall Text J, 9 1 (S) (198 1) 109. 58 Shadov F Korchagin M & Matctsky A, Chelllical Techllolog.'·

o./Fibrous Materials (Mil' Publ ishers Moscow), 1978430. 59 ICI Technica l1nformati on 0 1069. 60 Baumgarte U Z, Ges Textililld, 72 ( 1970) 640. 6 1 Lem & Wayman, J Soc Dyers Colollr, 83 (1967) 277. 62 Strafelda, Sb Vys Sk Chelll Techllol Pro ze, A llal. Chelll. Vo l.

2 ( 1967) 209. 63 Chackravertty R R, A Glill/pse all the Chelllical Tecl/l /O logy

o./Te.rlile Fibres (The Cax ton Press Pvt. Ltd, UK), 1972 77. 64 Paul R & Na ik S R, Text Dyer Printer, 30 (4) ( 1997) 13. 65 Knecht E & Fothergri ll J B, Prillciple & Practice of Texlile

Prillting, 3"1 ed n (Charles Gri ffin & Co. Ltd ., London) 1936 267.

66 Cook M M, Alii Dyest Rep, 68 (3) (1979) 4 1. 67 Baumgarte U, Review Prog Color, 17 (1987) 29. 68 Miles L W C. Textile Printillg (Dyers Company Publication

tru st, Bradford), 198 1, 202. 69 Baumgarte U & Keuser U, Mellialld Texlilber, 50 (1969)

943. 70 Daruwalla E H, Dyer, 154 (1975) 537; Texl Pmx. 30 (1975)

38 1. 71 Baumgarte U & Schluter H, Melliand Textilber, 62 (7)

( 198 1) 555. 72 Technical report, Text Prax, 30 ( 1975) 998. 73 Semet B, Sackingen B & Gurninger G E, Me/liand Textilber,

76 (3) ( 1995) 16 1. 74 Marte E, Text Pmx lilt, 44 (7) (1989) 737. 75 Baumgarte U, Mellialld Textilber, 68 (3) ( 1987) 189. 76 BASF Nahr DEA 4108240. 77 Bechtold T, Texti/ veredlung , 25 (1990) 22 1. 78 Bechtold T, Burtscher E, Gmeiner 0 & Bobleter 0 , Mellialld

Textilber, 72 (1) (199 1) 50. 79 Bechtold T Burtscher E Kuhnel G & Bobleter 0 , J Soc Dyers

Colour, 11 3 (4) (1997) 135. 80 Bechtold T, BUl1scher E, Turcanu A & Bobleter 0. Text Res

J, 67 (9) (1997) 635. 8 1 Nowack N Bracher H, Gering U & Stockhorst T, Me/lialld

Textilber, 63 (2) ( 1982) 134. 82 Kruger R & Ruttiger W, BASF DEA 4206929. 83 Chavan R B & Chakraborty J N. Illdiall J Fibre Text Res, 25

(6) (2000) 130.

CHAKRABORTY & CHA VAN: DYEING OF DENIM WITH INDIGO 109

84 Mercer H, Book of Papers, Illtem atiollal COllferell ce & Exhibitioll (AATCC), 1992, 333.

85 Paul R, Naik S R & Sharma U, Text Dyer Prilller, 29 (26) (1996) 18.

86 Paul R & Naik S R, Text Dyer Prillter, 30 ( I) ( 1997) II . 87 Etters J N, Alii Dyest Rep, 8 1 (3) (1992) 17. 88 Etters J N, Alii Dyest Rep, 78 (3) ( 1989) 18. 89 Process Datasheet, BASF. 90 Etters J N, Alii Dyest Rep, 83 (6) ( 1994) 26. 9 1 International Technical Paper Competition--A report, Text

Chelll Color, 2 1 ( 12) ( 1989) 25. 92 Etters J N, Alii Dyest Rep, 82 (2) (1993) 30. 93 Paul R & Naik S R, Text Dyer Prillter, 30 (7) ( 1997) 16. 94 Etters J N, Colollrage AlIlIlIal, 37 ( 199 1) 39. 95 Etters J N & Hou M, Text Res J, 61 ( 12) ( 1991 ) 773. 96 Chong C L, Poon L W & Poon W C, Text Asia, 26 (I) (1995)

55. 97 Etters J N, Proceedillgs, AA TCC Illtematiollal Dyeillg

SYlllposiulII (A merican Association of Textile Chemist & Colorist), 1992, 11 6.

98 Etters J N, Alii Dyest Rep, 8 1 (8) ( 1992) 26. 99 Etters J N, Alii Dyest Rep, 8 1 (2) (1992) 42. 100 Etters J N, Alii Dyest Rep, 79 (9) ( 1990) 19. 101 Richer P, Textilveredlllllg, 10 (8) (1975) 3 13. 102 Golomb L M & Shalimova G V, Tee/lIIol Text IlId, 48 (5)

(1965) 105 . 103 Mishchenko A V & Artym M I, Text IlId, 54 (4) (1971) 95. 104 Peter J H & Bairathi A, Alii Dyest Rep , 85 (8) ( 1996) 55. 105 Etters J N, Text Chelll Color, 27 (2) ( 1995) 17. 106 Etters J N, Text Asia , 24 (II) ( 1993) 34. 107 Etters J N, J Soc Dyers ColOllr, 109 (7/8) ( 1993) 25 1.

108 Etters J N, Alii Dyest Rep, 83 (5) (1994) 15. 109 GreerJ A & Turner G R, Text Chem Color, 15 (6) ( 1983) II. 110 Etters J N, Illdiall J Fibre Text Res, 21 (3) ( 1996) 33. I II Hauser A & Montagnino J C, Am Dyest Rep, 87 (5) ( 1998)

24. 11 2 Etters J N, Am Dyest Rep, 87 (9) ( 1998) 15 11 3 Chavan R B, Jahan S & Chakraborty J N, Proceedillgs,

Environmental Issues: Technology Option for the Textile IndustlY (liT, Delhi), 199863.

114 Chavan R B & Chakraborty J N, Coloration Tee/lIIol, 117 (2001) 88.

115 Kerr P L, Text World, 139 (2) (1989) 6 1 11 6 Press Release (Sucker + Muller, GmbH & Co), Mellialld

Textilber, 75 (3) (1 994) 214. 117 Trauter J & Stegmaier T , Melliand Textilber, 77 ( I +2) (1996)

27. 11 8 Daruwalla E H, Colourage, 23 ( I) ( 1976) 2 1 11 9 Koleg P, Text Techllollnt , 1990 199 120 Von der Eltz H & Kuhn R, Mellialld Textilber, 67 (5) ( 1986)

336. 12 1 Toyo Rayon Co. Ltd, Jap Pat 15270/ 66. 122 Ramaszeder K, Mellialld Textilber, 68 (9) (1987) 680. 123 Von der Eltz H, Seifert A & Birke R, Tillctoria , 70 ( 1973) 8. 124 Baumgarte U, Text Prax lilt 33 (1978) 747. 125 Gutierrez M C, Crespi M & Gibello C, Mellialld Textilber,

71 (I) (1990) 44. 126 Behtold T , Burtcher E & Bobleter 0 , Text Prax lilt, 47 ( I)

(1992) 44.

127 Kaikeung M & Tsim S T , lilt Text Bull- Dyeillg/Prilltillg / Finishing , 40 (3) (1994) 58.

128 Etters J N & Hurwitz M D, Alii Dyest Rep, 74 (10) ( 1985) 20.