Dyeing of denim with indigo

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Dyeing of denim with indigo

  1. 1. DYEING OF DENIM WITH INDIGO R B Chavan , Indian Institute of Technology, New Delhi – 110016 J N Chakraborty, Regional Engineering College, Jalandhar-144011, IndiaINTRODUCTIONNatural indigo , known in different names in different partsof the world has been in use since around 7000 B.C. fordyeing of cotton in attractive and bright blue shades. Plantsbelonging to the genus Indigofera are the most valuable forcultivation to produce natural indigo. The colouring matter inthe plant is present as a glucoside of indoxyl , known asindican which is hydrolysed to the indoxyl by enzymaticaction and indigo is then obtained by subsequent oxidation[ 1 ]. In ancient times, reduction of indigo was carried out infermentation technique which consisted of use of ripe fruit
  2. 2. and stale urine assisted by wood ash or lime as alkali . Thesolution prepared in this way was left overnight for reductionand solubilisation of indigo. The supernatent liquor was thentaken out for colouration through repeated dip - exposure toair sequence at room temperature, till a deep shade wasproduced ; a combersome as well as time consuming process[ 1 , 2 , 3 ].However, extensive research in search of a suitablealternative in place of natural indigo to met growing demandof it gave birth of modern indigotin by Adolf Von Bayer in1880. BASF AG ( Germany ), after a lot of research work fornext 17 years placed commercial synthetic indigo in marketin 1897 [ 4, 5 ]. Indigo has the chemical structure as shownbelow ( C.I. Vat Blue 1 ). O O N N H H Though natural indigo was known since ancient times andthe synthetic one was developed more than 100 years back,research work started slowly in various areas of indigo 2
  3. 3. dyeing only in the 60s with an intensification since 80s tocope up with increase in popularity of indigo dyed denimworldwide. Enormous developments were proposed whichmainly comprised changes in conventional reduction andapplication techniques, modification in design ofmachineries, various ways to enhance indigo uptake,quantitative estimation of indigo on cotton denim. Beside,developments have also been made to improve wash downproperty through efficient dyeing [ 6 - 36 ] .APPLICATION OF INDIGOIndigo is exclusively used to produce attractive blue shadeson denim alongwith desired wash-down property. However,in its reduced and solubilised form, indigo possesses littleaffinity for cotton - the extent of affinity is too less toexercise exhaust dyeing ; rather several dip and nip techniquewith intermediate air oxidation is followed for gradual buildup of shade. A suitable method of dyeing, viz. slasher dyeing(sheet dyeing ) , rope dyeing ( ball warp dyeing or chaindyeing ) or loop dyeing is followed for dyeing of warp yarn.After dyeing, dyed warp yarn is woven using white cotton 3
  4. 4. weft to produce mostly a warp faced 3 × 1 twill with blueface and white back and is then washed through specifictechniques to remove a part of dye in order to develop uniquefading property [ 37 ].In slasher dyeing method, denim yarns in the form of a warpsheet is pretreated in earlier compartments followed by multidip / nip indigo dyeing ; an after-washing, drying, sizing andfinal drying completes the process. Handling of yarn isminimum as once prepared warp sheet is directly processedand is send to weaving section for its subsequent conversionto fabric [ 38 - 41 ].A slasher dyeing range may either consist of the indigodyeing range alone[ 42 – 45 ] or a continuous indigo dyeingrange with an integrated sizing range [ 46 ]. In the 1st type ,yarn sheet moves forward through one or two pretreatmentboxes with the stages of dipping, squeezing and skying ;washing and drying are the only after-treatments. Onrequirement, two or more layers of warp sheet may beprocessed simultaneously. Schematic diagram of a popularslasher dyeing range is shown in fig. 1. The 2nd type 4
  5. 5. includes the steps followed in 1st type , extra attachments area sizing and a washing compartment as shown in fig. 2 .Slasher dyeing is suited for better production schedules , witha possible problem of centre to side variation . Superiorquality of yarn is needed to minimize breakage problems,broken end roll wrap ups can cause catastrophic shut-downproblem as well as undesired waste. On the other hand ,dipping , squeezing and oxidation - all require less time aseach yarn is independently subjected to treatments.In rope dyeing method, yarns from creel are pulled by a ballwarping machine, pass through a lease stand consisting of aspecial comb and lease rods to ensure proper registration ofends. These drawn ends then pass through a condenser tubeassembly to merge these into a bundle followed by passingthe latter to form a rope comprising 350 - 400 ends. Ropesare then wound on drums . Once the beam has been fullywound , it is unloaded and mounted on dye range creel [ 7 ,47 ]. Dyeing is done by passing ropes, upto 12 at a time,through pretreatment bath followed by multiple dipping inseparate indigo baths with intermediate squeezing and 5
  6. 6. skying, as done in slasher dyeing . Schematic diagram of arope dyeing unit is shown in fig. 3 [ 48 ].Rope dyeing method is important for higher production ratewith better quality dyeing and better fastness to wet / dryrubbing as well as colour wash-down . Less breakage ofends, shade consistency is better ; in contrast , more handlingof yarn to open ropes before sizing. However, efficiency ofslasher and rope dyeing methods depend on back feed todyebaths to maintain concentration of indigo and otherchemicals at a constant rate, which is a combersome work.Moreover , consumption of chemicals and dye remain onhigher side with greater chance of uneven / tailing effect.In loop dyeing , problems associated with slasher and ropedyeing was solved through development of a new dyeingtechnique, known as "loopdye 1 for 6", developed andpatented by Eckhardt Theodor Godau . In comparison withother indigo dyeing methods with six colour troughs , herethere is only one brief indigo dyebath with one squeezingunit. The process is offered by Looptex SA(CH) and underlicense by Inters(I) and Texima SA(Brazil), is also ofmodular construction. It comprises 8 - 16 warp beams , 6
  7. 7. threading in feature , pre-wetting or pre-mercerising unit, thetwin - pad colour applicator, an integrated skying passage ,two rinsing sections as well as an accumulator scray . Fig. 4shows the sequence of operation used in this new technique[ 49 - 53 ].REDUCTION OF INDIGOSodium hydrosulphite reduces indigo at 30oC-50oC,depending on type of indigo used, resulting in formation ofbiphenols ( leuco-indigo) [ 54 ]. The reduced form is quitestable in presence of NaOH to carry out dyeing at roomtemperature.However, hydrosulphite is very unstable. At the time ofreduction of dye , it gets decomposed thermally, oxidativelyand in various other ways [ 55 ], requiring 2 - 3 times higheramount over the stiochiometric requirement [ 56 - 58 ].Attempts were made to keep control over its use throughscientific analysis in course of dyeing [ 59 - 62 ].Other reducing systems, viz. fermentation technique [ 1 ],copperas method [ 1, 63, 64 ], zinc - lime method [ 1 ],bisulphite - zinc - lime vat[ 1 , 65 ], thiourea dioxide [ 66 – 7
  8. 8. 68 ], Sodium borohydride [ 69 -72 ], have fallen out of usedue to limitations associated with their application.Eco-friendly reducing agentsIn recent times, use of eco-friendly reducing systems forindigo have attracted attention of researchers [ 73 ]. Thesenew reducing systems include hydroxyacetone, iron -pentacarbonyl compounds, electrochemical reduction,glucose – NaOH and iron (II) salt alongwith a suitableligand.Hydroxyacetone, when used as a reducer ( reduction potential~ - 810 mV) in RD (refined dyeing ) process [ 74 ], results20% higher dye uptake alongwith less consumption ofauxiliary chemicals. Other advantages associated are higherquality dyeing, better ring dyeing effect, better elasticity ofyarns, increased productivity, higher dye uptake, lessdyestuff in effluent. However, required reduction potentialfor vatting is obtained at 100oC with higher concentration ofNaOH in this method and produced shade does notcorrespond with that obtained with hydrosulphite and moresuitable for pad - steam method [ 75 ]. 8
  9. 9. Use of iron-pentacarbonyl compounds, though has beensuggested as reducer, detailed report of this system has notbeen disclosed so far [ 76 ] .In electrochemical reduction technique, vat dye is directlyreduced by contact between dye and electrode , though areducing agent must be added to the reduced dyestuff( though lower in quantity) in order to ensure correspondingstability of the reduced dye - liquor . However, dyestuffrequirement for a specific shade is too higher [ 77 - 80 ].Glucose in combination with NaOH can be used forreduction of sulphur dyes ( reduction potential ~ - 550 to -600 mV). Indigo requires nearly ~ -700mV for reduction ;hence glucose - NaOH system can also be used , preferablyat boil, producing reduced indigo baths free from sedimentsand highly stable for several hours. One problem related tothis reducing system is that padded textile requires more timefor oxidation in between two successive dips and indigouptake is less [ 81 ].It has been reported in the literature that ferrous hydroxide isa strong reducing agent in an alkaline medium [ 73 ]. Thereducing effect increases even more with increase in pH 9
  10. 10. value. Ferrous hydroxide is poorly soluble in alkalineconditions and precipitates. It must be complexed [ 82 ] inorder to hold the ferrous hydroxide in solution . Fe++ hashardly any reducing power as the central ion of complexeswhich have the stable arrangement of the krypton shell.However a stable complex with reducing power is obtainedwith weaker ligands , e.g. gluconic acid. When dyeing withvat dyes at 60oC, good dye uptake was achieved for optimumconcentration of iron(II) salt and molar ratio between this saltand gluconic acid. However, dye uptake decreases withdecrease in concentration of iron salt , even too low aniron(II) salt concentration gives lighter and unleveldyeings at iron(II) salt : gluconic acid - 1 : 1 and 1 : 2(molar) due to insufficient vatting of dye. When molarconcentration of gluconic acid was reduced to 1 : 0.5 ,unlevelled shades were produced with reduced brilliance [ 73].Iron (II) salts in combination with NaOH and a weak ligandlike gluconic acid, tartaric acid , citric acid or triethanolamine, at suitable concentrations, can produce reduction baths withpotential ranging from –800mV to –1000mv. These reduction 10
  11. 11. baths are capable of reduction and dyeing with indigo atroom temperature. The reduction baths showed superiorstability in presence of indigo upto 24 hours. Deposition ofiron on dyed samples was also assessed and it was found thatgluconic acid as ligand causes least deposition , whereascitric acid causes maximum deposition. Produced shadeswere bright with no shifting of λmax . The baths were turbidthroughout dyeing [ 83 ].It may be summarised here that, for quality dyeing, whatevermay be the reducing agent, reduction potential and alkalinityof bath should be accurately adjusted for development of trueshade of indigo [ 84 - 90 ].FACTORS INFLUENCING INDIGO UPTAKEReduced and solubilised indigo , though ionic in nature,possesses negligible affinity for cotton ; a multiple dip / niptechnique is followed with intermediate airing for gradualbuild-up of shade. Yarn to be dyed is dipped in indigo bathfor a specific time followed by a passage through padding unitin order to achieve better penetration as well as distribution atnearly 100% expression, succeeded by air oxidation. This 11
  12. 12. dip / nip followed by airing cycle is repeated till desired depthof shade is produced, succeeded by a final oxidation for 3 - 5minutes for complete conversion of soluble indigo to itsinsoluble form.As stated , desired depth of shade is primarily governed bynumber of dips / nips , but dye uptake during each dip isinfluenced by a number of other process parameters , viz. pHof dyebath , immersion time , time of intermediate oxidation ,concentration of indigo and temperature of dyebath. Theseparameters affect build-up of indigo on cotton , degree ofpenetration and various fastness properties.The crucial factor is pH of indigo bath [ 54, 91 - 95 ] .Depending on dyebath pH , indigo may exist in fourdifferent forms as shown in fig. 5.i) indigotin structure at lower alkaline pH .ii) reduced but non-ionic form , at moderate alkaline pH.iii) Mono-phenolate form , at relatively higher pH.iv) Bi-phenolate form , at very higher pH.Structures (i) and (ii) exist below pH 9 - 9.5 ; their relativefraction depends on exact pH of bath. With slow increase in 12
  13. 13. pH , structure (i) collapses and gets converted to either (ii) or(iii) or as a mixture of both. Further increase in pH slowlyconverts all the (ii) to (iii) structures ( above pH 10 ). A pHaround 11.5 indicates that almost all indigo molecules are intheir mono-phenolate form. Any further addition of alkalibeyond pH 11.5 slowly attacks second C=O group of indigoin presence of excess hydrosulphite with a result ofconversion of mono-phenolate to bi-phenolate form (iv). Theextent of conversion strictly depends on exact pH and excesshydrosulphite in dyebath. As presence of excesshydrosulphite is a must for successful indigo dyeing, onlyaddition of alkali , if made in excess above pH 12.5 , thestructure (iv) predominates. Indigo in the form of structure(iv) shows reduced affinity for cotton and thus dye uptakefalls. Structure (i) shows no affinity , whereas structure (ii)has negligible affinity , as (i) and (ii) are non-ionic forms ofindigo. Mono-phenolate form is the desired structure whichpossesses highest affinity as well as strike rate to give higherdye uptake. In this context , it is also to be noted that cottonwhen dipped in alkaline bath acquires negative charge. Higherthe pH , higher the charge on fibre and more is the repulsion 13
  14. 14. between dye and fibre resulting in reduced dye uptake.Another importance of pH is related to ease in washing. AtpH 10.5 - 11.5 , indigo shows maximum affinity as well ashigher strike rate on cotton due to lower solubility of mono-phenolate form of indigo in this pH range, hinderingpenetration inside cotton and results more imtensive surfacedyeing, facilitates achieving better wash-down effect [ 96 ].It was mentioned earlier that relative fraction of mono-phenolate and bi-phenolate forms of indigo can be positivelycontrolled by adjusting the dyebath pH [ 54 , 87 , 91 , 92 , 94 ,95, 97 - 104 ] . pH is such a crucial factor in dyeing withindigo that it not only controls dye uptake, but alsodistribution of indigo on cotton yarn through control overstrike rate and hence wash-down properties with removal ofunfixed dyes [ 87 , 105 ].Out of two soluble forms of indigo , viz. mono-phenolate andbi-phenolate , the former has relatively lower water solubilityand higher strike rate - just opposite to that with bi-phenolatespecies ( fig. 6 ). Hence, if pH can be kept within a range of10.5 - 11.5 , most of indigo molecules will exist in bath intheir mono-phenolate form showing higher strike rate for 14
  15. 15. cotton , resulting poor penetration and higher surfacedeposition of dyes , the so called ring dyeing effect [ 87 ,91 , 105 ]. In ring dyeing , only a few surface layers are dyedleaving interior of yarn undyed due to absence of adequatetime for penetration as well as thorough distribution ( fig. 7 )[ 105 ]. If pH is raised above 11.5 , relative fraction of mono-phenolate form gets reduced with instant increase in that ofbi-phenolate form, lowering strike rate and consequentlyincreases time for absorption - resulting dyeing of few morelayers than that obtained with mono-phenolate form . Withfurther increase in pH , mono-phenolate form go ondiminishing with a substantial rise in bi-phenoate form andthorough dyeing of cotton occurs [ 105 ].Efficient wash-down property of indigo dyed yarn depends onamount of dye present on yarn surface. It is the 10.5 - 11.5 pHrange , which gives dyeings with desired washingperformance and a deeper shade with less amount of indigo.Dyeing at higher pH ranges gives relatively lighter shadesas well as lowers wash-down property of dyeings [ 92 , 106- 108 ]. 15
  16. 16. Ionic nature of cellulose plays another key role to affectindigo uptake. With increase in alkalinity of indigo bath ,ionisation of cellulose ( expressed in gms - ion / l ) increaseswith development of more anions, e.g. concentration of (Cell-O-) at pH 10.0 is 3 ×10-3 gm-ions/ l, increases to 1.1 gm-ions/ lat pH 13.0 . This in turn suppresses substantivity of indigo,finally lowering dye uptake [ 87 , 105 ]. Calculation ofdifferent forms of indigo in bath, though combersome, can bemade through study of equilibrium ionisation constant values[ 107 ].Effect of immersion time on final colour depth in indigodyeing shows that with increase in immersion time, dyeuptake increases initially , remain nearly same for little furtherincrease, but beyond certain limit falls considerably. Animmersion time of 30 seconds seems to be adequate,prolonged immersion reduces colour depth mainly due to re-reduction of oxidised indigo already on cotton throughprevious dips and go back to dyebath . To get optimumcolour yield , oxidation time is fixed at 60 seconds or beyondthat. Due to very low affinity of solubilised indigo in bath forcotton , increase in indigo concentration in bath though 16
  17. 17. initially can increase depth of colour , but beyond certainconcentration , does not show any remarkable change on dyeuptake. Increase in indigo concentration upto 3 g/l in bathincreases its uptake on cotton , beyond which it shows littleimpact on dye uptake . Optimum depth of shade is obtained inas high as ten dips and beyond that no such increase occurs.But keeping in view limitation in machinery set up and alengthy process time , generally a 6-dip 6-nip technique isfollowed [ 96 , 97 ]. However, it has been reported elsewhereto work with lowest possible concentration of indigo and touse maximum number of dips. By doing so , colouruniformity , fastness properties and wash-down after dyeingare improved. A rule of thumb is that more the number of dipsused for a given shade , the better [ 109 ]. Affinity of solubilised indigo for cotton decreases withincrease in temperature , initially at a faster rate followed byvery slow decrease. At lower temperature , affinity of indigois better and also re-reduction of oxidised indigo does notoccur typically. Hence , it is better to carry out dyeingwithout any application of heat. It has been reported thatmaximum dye uptake occurs if dyeing is carried out at room 17
  18. 18. temperature. With increase in dyeing temperature, dye uptakefalls relatively at a slower rate from 40oC - 60oC and beyondthat become almost parallel [ 96 , 97 ].TECHNIQUES TO ENHANCE INDIGO UPTAKEAlternate pH control techniquesIndigo uptake can be enhanced through proper control overpH in the region 10.5 - 11.5. NaOH , which is used in indigodyeing is a very strong alkali , makes efficient dyeingimpracticable due to wide fluctuation in pH during dyeing .Initially pH is highly sensitive to concentration of NaOH ,a small change in NaOH concentration changes pHremarkably [ 110 ]. The normal practice is to add NaOH athigher concentration, such that final dyebath pH does not fallbelow 10.5 , with a result of less dye uptake in initial dips.This is because when very low levels of NaOH is used in anattempt to achieve a dyebath pH of 11.0 , small changes inNaOH concentration lead to very large changes in dyebathpH. In fact , the dyebath pH may drop suddenly into a rangein which mainly non-ionised indigo is formed andsuperficially deposited onto the denim yarn. Such a sorptioninhibits sorption of ionised dye , leading to streaking and 18
  19. 19. extremely poor crock fastness [ 110 ]. In contrast, use ofexcess alkali makes lighter dyeings of higher alkalinity,which creates trouble in finishing [ 111 ].Attempts to substitute NaOH with other alkalis / basesdeveloped buffered alkali system of undisclosed composition ,manufactured by Virkler company, USA, which whenapplied in indigo bath at recommended concentration,produces a stable pH in the range of 11.0 and littlefluctuation occurs in dye uptake throughout the period ofdyeing [ 112 ].In another attempt, NaOH was substituted with differentorganic bases. Before application, alkalinity of these baseswere evaluated and applied in various indigo baths keepingequivalent alkalinity with that produced with NaOH. It hasbeen reported that dye uptake with these new systems were onquite higher side than those obtained in NaOH method.Stability of indigo baths were superior to NaOH baths evenafter a lapse of 8 hours. Change in pH with time was alsostudied and it was found most of the bases were capable ofmaintaining pH in 10.5 - 11.5 region upto 4 hours [ 113 ]. 19
  20. 20. Pretreatment of cottonIndigo in solubilised state is anionic in nature and will beattracted by cotton, if the latter is pretreated with cationiccompounds. Cotton was pretreated by immersing it in asolution containing 200 g/l of a 12% aqueous Hercosett 57( cationic water soluble polyamine - epichlorohydrin resin )solution and 10 g/l of Sandozin NI for 5 minutes followed bysqueezing the material in padding mangle to achieve 80%pick-up and dried at 100oC for 3 minutes. Treated cotton waspadded with indigo through 6-dip 6-nip technique. It has beenreported that dye uptake on pretreated cotton remained onhigher side than that on untreated cotton under identicaldyeing conditions [ 96 ].Pretreatment of cotton with metal salts prior to padding withindigo ( which may be done in several ways to synchronisewith batch or continuous dyeing methods) results substantialincrease in indigo uptake. It was also found that iron andcobalt (II) salts were more effective even at very lowerconcentration, viz. 1 g/l. Formation of insoluble metallichydroxides in situ cotton in presence of NaOH and air was 20
  21. 21. responsible to attract more anionic indigo molecules in bathcausing increase in dye uptake [ 114 ].Introduction of a mercerising step prior to indigo paddingenhances indigo uptake considerably. In October , 1988 ,ATME -1, Greenville, a DIMER was shown, which is a unitthat incorporates real mercerising in the loop dye process forindigo warp dyeing. The technique was originally developedin Switzerland by loop-dye , but is now being produced andmarketed by Kuesters. The unit simplifies warp productionwith advantages like improved fabric appearance , handle ,higher yarn strength , higher dye uptake and more intensivering spun yarn dyeing with shorter stone washing. Even it isalso possible to bottom with sulphur, indanthrene, reactiveand naphthol colours [ 115 ].COST EFFECTIVE PRODUCTION IN INDIGODYEINGMulti-vat machine concept has regained its importancethrough flexible adaptation of machine technology. Accuratedosing, targeted dyestuff use , low dyestuff and chemicallosses are the main aspects of such production system. 21
  22. 22. According to literature, adequate dwell times in the steepingbath ( approx. 6 m / bath ) and oxidation zones(>60 seconds ),together with a high circulation rate with low dyestuffconcentration has been pursued in order to achieve optimumdyeing [ 104 , 116 - 118 ] .Due to lower dyestuff concentration in the effluent , ultra-filtration equipment can then be equipped with smaller filtersurfaces to reduce investment and operating cost . Washingwater consumption can be kept low at 3 l / kg of warp.Indigo vat , hydrosulphite and NaOH are continuously dosedin proportional to speed in accordance with specificallycirculated quantities avoiding shade variation. Dosing iscontrolled by a Semen Op 20 automation unit confirming ahigh degree of processing reliability and reproducibility.Dosing of dye is controlled via a special frequency controlpump. Measuring systems are available for bathconcentration, pH value, redox potential, bath temperatureetc.Another unique way of cost reduction is ‘zero dischargeconcept’, which works in ‘loopdye 1 for 6 process’ usinghydroxyacetone as reducing agent to produce waste-free 22
  23. 23. improved quality dyeing at reduced cost through recoveryand reuse of unused dye and waste- water [ 119 ].QUANTITATIVE ESTIMATION OF INDIGO ANDOTHER VAT DYESIndigo in dyed textile / dyebath / wash liquor is determinedspectrophotometrically at maximum wave length ( λmax ).When indigo is in solid state , each of its molecules form H -bonds with 4 adjacent dye molecules ( between C = O andNH groups ) , makes it insoluble in water as well as in mostof the organic solvents [ 120 ]. Pyridine serves as a goodsolvent . For spectroscopic estimation of indigo , a clear andstable solution of indigo is an indispensable pre-condition forconsistent and reproducible results. Alternately, turbidity ofoxidised form of indigo may be a measure of quantitativeestimation. Several analytical methods have been devised inthis context, viz. colorimetry of leuco indigo , turbidity ofindigo in oxidised form , colorimetry of sulphonated indigo.In solvent technique, a few methods based on colorimetry of 23
  24. 24. chloroformic , pyridine / water and dimethyl sulphoxideextracts have also been reported [ 121 - 124 ].The ‘colorimetry of leuco indigo method’ consists ofreduction of pure indigo with hydrosulphite in alkalinemedium. Absorbance is instantly read at 412 nm, as reducedindigo has time-bound stability ; its spectrum changesconsiderably in very short time intervals. This technique lacksreproducibility [ 125 ]. However , the technique may bedispensed with other methods , if stability of reduced indigocan be increased . It has been mentioned in literature thatiron(II) complex with triethanolamine can reduce vatdyestuffs and the reduced dyebath has high degree of stabilityto temperature , time and atmospheric oxygen [ 126 ]. In thistechnique, iron(II) salt concentration should be kept undercontrol , as iron ions below 400 nm tend to self absorption .Turbidity of oxidized indigo has no linear relation betweenturbidity and concentration, in some cases, higher values ofturbidity were also achieved on reducing the concentration.This behaviour is possibly due to the excessive dimensions ofindigo particles as well as distribution of uneven sizes.Addition of glycerine to stabilise the suspension did not 24
  25. 25. improve the results ( glycerine acts by increasing bath densityand retards particle fall ). Maximum absorbance was found tobe at 666 nm , but the slope of the calibration curve variedaccording to the status of initial dyeing bath [ 126 ].In ‘sulphonated indigo method’, incorporation of solubilisinggroups of sulphuric acid in indigo structure makes indigowater soluble, stable and deep blue coloured. The absorbanceis maximum at 605 nm [ 125 ]. Beers law is fulfilled upto 15ppm of concentration and the reproducibility of the method is0.3 - 2%. However, before sulphonation, indigo should be inoxidised and dried form - otherwise a greenish- brownishsolution is obtained . Indigo should also be free fromhydrosulphite. This analytical technique is hardly suited forinstantaneous , continuous control of indigo liquors [ 126 ].Sulphonation of indigo with concentrated sulphuric acid at75oC is also troublesome.Solvent techniquesThe colorimetry of chloroformic extract gives maximumabsorbance at 605 nm and good linearity is achieved in thecalibration curve upto indigo concentration of 10 ppm. At this 25
  26. 26. concentration , solubility limit of dye in chloroform isreached and a blue interphase between the two layers isformed during extraction [ 125 ].Indigo structure has a similarity with that of pyridine.Solubilisation occurs through H - bonding between these two .Oxidised indigo can be dissolved upto a concentration of 40ppm by boiling in pyridine - water ( 50 : 50 ) solution , whichis then photometrically measured at 625 nm . Measurementmust be done when the solution is hot to avoid precipitationof indigo on cooling the solution. Indigo should be completelyfree from hydrosulphite ; if available with dye , decomposes itduring heating with reagent solution . Presence of NaOH withdye also changes absorbance as aggregation of dye alters withpH. This method has a reproducibility of 2 - 4% [ 125 ] .A completely new approach has been made by solubilisingoxidised indigo in Dimethyl Sulphoxide ( DMSO ). Themethod does not have any of the problems as with earliermethods. It is time saving , safe , not subjected to interferenceand largely reproducible. Solubility of indigo is excellenteven at room temperature ; maximum absorbance is achievedat 619 nm . The solution is highly stable for several days and 26
  27. 27. for indigo concentration upto 250 µg / ml , completedissolution takes place . The only problem lies in handling ofDMSO is that it is slightly detrimental to health, irritation incontact with skin, itching etc.[ 127 ].In some cases , denim is dyed with a combination of indigoand sulphur or hydron blue dyes . To estimate contribution ofthese two in the shade developed, indigo is 1st extractedcompletely from sample at boil in pyridine - water solution.The dyed sample under test is rinsed with methyl alcohol ,dried at 40oC to remove residual pyridine and methyl alcohol.Reflectance of the sample is measured. K/S calculated fromreflectance value at each wavelength are integrated over thewavelength range , the result is an integrated value( I.V.) [ 128 ].CONCLUDING REMARKSThough indigo is a class of vat dye, it can not be appliedaccording to conventional vat dyeing techniques due to itsnegligible affinity for cotton, rather multiple dip / nip withintermediate airing is to be followed. During dyeing, pH of 27
  28. 28. dyebath plays a crucial role to affect indigo uptake as well aslocation of dye on cotton structure, though influence of otherfactors can not be over-looked. Dyeing mostly takes place inyarn stage, necessitating selection of specific machinedepending on form of yarn. Beside dyeing parameters, specialtechniques through use of buffer / other alkalis / bases ,pretreatment of cotton etc. may be introduced to enhance indigouptake. As far as quantitative assessment of indigo is concerned,DMSO method has been found to be the most effective one.REFERENCES 1. M R Fox and J H Pierce , Textile Chemist & Colorist ,22 , 4 ( 1990 ) 14. 2. H Schmidt , Melliand International , 78 , 2 ( 1997 ) 89. 28
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  30. 30. 13. B Kramrisch , American Dyestuff Reporter , 69 , 11( 1980 ) 34. 14. R J Harper and A H Lambert , Textile Chemist & Colorist , 24 , 2 ( 1992 ) 13. 15. R Paul and S R Naik , Indian Textile Journal, 108 , 5 ( 1997 ) 28. 16. Technical Report , Textil Praxis International , 48 , 3 ( 1993 ) 227. 17. A R Riaz and J Riaz, Pakistan Textile Journal, 4 ( 1992 ) 39. 18. M R Patil , Indian Textile Journal, 91 , 8 ( 1981 ) 121. 19. J N Etters , American Dyestuff Reporter , 84 , 5 ( 1995 ) 20. 20. D Kochavl , T Videbaek and D Cedronl, American Dyestuff Reporter, 79, 9 (1990) 24. 30
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