Introduction
◆Vat dyes retain C=O chromophore in structure, which renders water insoluble and nonionic.
◆Rate of exhaustion is excellent with higher strike rate raising chances of unlevel dyeing.
◆Shades possess all round fastness properties and are brilliant.
◆All hues are available with high tinctorial power.
◆These are converted to leuco compounds on reduction followed by solubilisation with alkali in which state these show substantivity towards cellulose.
◆After dyeing, parent dye structure is recovered by oxidising it within fibre, when the dye molecule get trapped in situ and establish linkage with fibre through H-bonds and van der Waals forces (Fig. 8.1).
◆In ancient times, dyeing was often carried out in wooden vat assigning the name 'vat dye'.
◆Carbonyl groups are the chromophores which are changed to >C=OH groups on reduction and then to soluble sodium derivatives (>C=ONa) in presence of alkali.
◆The dyeing process generally involves three basic steps:
(i) Preparation of the vat, i.e. reduction of dye to its leuco form followed by its conversion to sodium salt.
(ii) Dyeing
(iii) Oxidation to parent dye
◆Dyes are non-ionic - develop anion on solubilisation; possess excellent affinity for cellulosics and are retained by fibre with H-bonds and van der walls forces on oxidation.
◆Though molecular size of dye is too small compared to pore size of cellulosics.
◆After diffusion and oxidation, these crystallize to form big stable aggregates.
◆During soaping it can not come out providing superior fastness properties.
Physical form of dye
Vat dyes are available in various commercial forms:-
(i)Powder form
(ii)Fine micro-form
(iii)fine ultra dispersed form
(iv)superfine form
(v)Paste form
◆Powder form has large particle size, (ultra-concentrated or uc grade) and is designed for exhaust dyeing.
◆Fine micro-form (small particle size, fm), fine ultra dispersed form(fd, ud) and superfine (sf) qualities -all are suitable for continuous dyeing.
◆Paste form is suitable for printing.
Properties of vat dyes
◆All vat dyes are water insoluble, anthraquinones are mostly soluble in hot DMSO (dimethyl sulphoxide) and can be extracted from dyed sample at boil without any change in , C=0 groups in anthraquinone dye act as chromophore and groups such as NH2, OH, alkylamino (NHR, NR2), benzamide (NH-CO-RH), alkoxy (-OR) are auxochromes.
◆Sublimation temperature is very high and possesses excellent fastness properties.
◆One characteristic feature of anthraquinone dyes is that on reduction these show typical change in colour and original colour is restored on oxidation.
◆Dyes are costlier, full shade range is available; some of yellows, browns and oranges have marginal poor light fastness.
◆Indigoid dyes contain -c-co-c-co- as chromophore and -NH or -S- groups as auxochrome.
◆Indigoids are soluble in DMSO at room temperature, boiling glacial CH3COOH, boiling C6H5NH2 or C6H5N, partially soluble in CHCI3, C6H5OH and H2SO4.
◆On reduction, indigoids develop a ncarly colourless solution and original colour is restored on oxidation.
◆At 170°C, it sublimes-off and condenses into blue crystals of pure indigotin.
◆Reduced and solubilised sodium compound of indigo has limited affinity for cellulose and very readily oxidized in the fibre to insoluble blue indigotin by atmospheric oxygen.
Classification
Vat dyes are classified based on chemical structure and method of application.
◆This classification is based on conditions and amount of chemicals required for reduction, solubilisation and dyeing.
◆Based on these facts, all vat dyes can be applied in any of these four methods viz.
(i) IK dyes (Indanthrene cold dyes): Dyes belonging to this class require less alkali, less Na2S2O4, low vatting (35-50°C) and dyeing temperatures (30°C) but lacks affinity. To promote exhaustion, large quantities of electrolyte is applied, e.g. C. 1, Vat Blue l (indigo).
(ii) IW dyes (Indanthrene warm dyes): These dyes are applied in presence of moderate amount of alkali, moderate Na2S2O4, lower vatting (45-50°C) and dyeing temperatures (40-45°C).
Dyes possess moderate affinity and
require moderate amount of salt to promote exhaustion, e.g. C.I. Vat Blue 6 (Blue BC)
(iii) IN dyes (Indanthrene normal dyes): Dyes from this class require relatively high alkali concentration, higher amount of Na2S2O4 high vatting (55-60°C) and dyeing temperatures (50-55°C).
Dyes show very good affinity for cellulosics and application of salt at very lower concentration completes exhaustion, e.g. C.I. Vat Violet 1(Purple 2R).
Structure of some important vat dyes
Vat dyes are marketed under various brand names, viz. Navinon, Novatic, Algol, Arlanon, Arlanthrene, Arlanone, Chemithrene, Cibanone, Helanthrene, Intravat, Mikethrene, Nihonthrene, Palanthrene, Paradone, Patcovat, Sandothrene etc. Structure and C I specification of some commonly used vat dyes are shown fig.
Application on cellulose
Typical recipe
Vatting and Dyeing
Dye -1%
Turkey red oil - a little
Na2S2O4 - 'A'g/l (Table 8.2)
NaOH - 'B'g/l (Table 8.2)
Oxidation: H2O2 (35%)- 1-2 ml/l at 50-60°C for 15-20 min
Soaping: Na2CO3 and soap (0.5 g/l cach) at boil for Q15-30 min or steaming only.
◆Dye is pasted with turkey red oil.
◆Water is added to it and the mixture is heated up to required vatting temperature depending on class of dye.
◆One-third of total NaOH is added followed by addition of one-third of Na2S2O4 the solution is stirred till colour of solution is changed (vat dyes on reduction show complete change in colour) with coloured foam at the top layer of solution. 10 min is allowed for complete reduction.
◆Dyebath is prepared with reduced and solubilised dye, one-third of total NaOH and Na2S2O4 each.
◆Pretreated wet cotton is dyed in this bath for 30min after which salt is added (if required, as IN special dyes require no salt) and dyeing is continued for further 1-2 hrs depending on depth of shade.
◆Throughout dyeing, remaining one-third of Na2S2O4 and NaOH are added in small amounts at regular intervals to retain dye in reduced and solubilised form and to prevent precipitation of oxidized dye on goods.
◆A too low vatting temperature causes incomplete reduction; if high, over
reduction of dye may occur.
◆IN special dyes are not generally over-reduced if dose of Na2S2O4 and
NaOH are in excess.
◆After dyeing, a cold wash is imparted followed by oxidation of dyed material at 50-60°C with 1-2 ml/l H2O2 (35%) for 15-20 min, soaped or steamed at boil for 15-30 min, finally a thorough wash completes the process.
Reduction of vat dyes
◆All vat dyes are to be essentially reduced and solubilised before dyeing.
◆Indigo requires around (-700 to -750) mV and anthraquinoid dyes require (-800 to -1000) mV for complete reduction depending on class of vat dye, i.e. IK, IN etc; IK requires the lowest range of reduction potential while IN special the highest.
Na2S2O4 as reducing agent
◆Na2S2O4 (sodium hydrosulphite or sodium dithionite) is the universally accepted reducing agent in vat dyeing.
◆It reduces insoluble vat dye to partially soluble leuco dye, counter-acts effect of dissolved oxygen in water, which otherwise may precipitate a part of reduced dye through oxidation.
◆Little excess of it retains stability of reduced liquor required for levelled dyeing, and it produces a sediment free clear reduction bath.
◆Na2S2O4, if in too excess, retards rate of dyeing and large excess results over reduction as well as wastage of dye.
◆It releases nascent hydrogen required for reduction when added in water or alkali.
Na2S2O4 + 4H2O = 2NaHSO4 +6H
Na2S2O4 + 2NaOH = 2Na2SO3 + 2H
◆Na2S2O4 reduces indigo at 30-50°C, resulting in formation of mostly biphenols (leuco-indigo).
◆The reduced form is quite stable in presence of NaOH to carry out dyeing at room temperature.
Over-reduction of vat dyes
◆When Na2S2O4 is applied in too excess or temperature of vatting is not maintained properly, few vat dyes get over-reduced and pose problem in developing true shade.
◆Over-reduction is of several types and depends on structure of dye.
◆Most of these over-reduction processes are irreversible and cause permanent change in dye structure.
◆Simple over reduction of some vat dyes occur either at high concentrations of Na2S2O4 or at high temperature.
◆Dyes containing heterocyclic rings with nitrogen atoms and where not all the keto (>C=O) groups present in the molecule are to be reduced are liable to be over-reduced (Fig. 8.2).
◆Indanthrene blue and yellow dyes are susceptible to this type of defect, e.g. only two keto groups of indanthrene blue dyes should be reduced under ideal conditions of reaction, but when over-reduction occurs, all four keto groups are reduced.
Hydrolysis
◆Vat dyes containing benzyl amino or acyl amino group (yellow GK and gold orange 3G) are hydrolysed at high temperatures and prolonged vatting time resulting duller and less fast shades.
◆During hydrolysis,-NHCOC6H5 group is changed into-NH2 group (Fig. 8.3).
Dehalogenation
Halogenated dyes such as violet 2R, violet 3B, violet 2RN, blue RC, blue BC and green 2G looses halogen in its structure at high temperature vatting/dyeing. Dehalogenation results poor bleach fastness of dyeings.
Molecular rearrangement
◆Certain vat dyes like khaki 2G, olive T, grey M, when dyed at or above 66°C give permanent change in hue ; this is caused by the irreversible change of some keto groups to >CH or >CH2 groups (Fig. 8.4).
◆Chances of over-reduction may be reduced partially by adding HCHO or glucose in bath.
Other reducing systems
Wooden vat or copperas method
◆Iron (II) salts in presence of milk of lime produces iron (II) hydroxide, the latter is the reducing agent.
The bath has a reduction potential around (-700 mv to -750 mv), but due to very poor water solubility of Fe(OH)2, partial reduction of indigo is only posible.
FeSO4 +Ca(OH)2 = Fe(OH)2 + CaSO4
Fe(OH)2 + 2H2O = Fe(OH)3 + 2[H]
◆Insoluble Fe(OH)2 is precipitated and forms huge sediment, five times bulkier than that of zinc-lime vat.
Zinc-lime method
Zn + Ca(OH)2 = CaZnO2 + 2H
Nascent hydrogen reduces indigo and the excess slake dissolves reduction product; precipitation of CaZnO2 interferes seriously with dyeing.
Sodium borohydride method (NaBH4)
NaBH4 + 2H20 = NaBO2 + 8H
It is a bi-component reducing system and is suitable for pad - steam process. It can be used only along with accelerator (NaCI). However, it fails to stabilize leuco vat dye.
Eco-friendly reducing systems
◆In recent times, use of eco-friendly reducing systems for indigo and anthraquinone vat dyes have been developed, viz. hydroxyacetone, electrochemical method, glucose-NaOH and iron (II) salt-ligand complex.
◆Hydroxyacetone, (rp~ -810 mV) results in 20% higher indigo uptake along with less consumption of auxiliary chemicals.
◆Other advantages are better quality dyeing, ring dyeing effect, better elasticity of yarns, increased productivity, higher dye uptake and less dyestuff in effluent.
◆Required reduction potential for vatting is obtained at 100°C with higher concentration of NaOH and the produced shade does not correspond does not correspond with that obtained with Na2S204.
◆Hydroxyacetone is found to be more suitable for pad-steam method and is not suitable for reduction of anthraquinoid vat dyes.
Solubilisation
◆Reduced leuco dye is converted to its sodium salt with NaOH to make it water soluble and develop affinity for cellulosics.
◆NaOH is the popular and accepted solubilising agent for leuco vat dyes and is multi-functional in nature, viz.
(i) it dissolves leuco dye to its Na-salt,
(ii) develops aftinity of dye for fibre,
(iii) neutralizes acidic by-products of Na2S2O4 in bath to maintain pH and
(iv) suppresses hardness of water,
◆While use in excess increases negative potential of bath and causes lesser colour yield due to more repulsion between dye and cellulose.
◆To keep dye in its water soluble salt form, sufficient excess of both Na2S2O4 and NaOH must be present in bath.
◆Oxidation of Na2S2O4 by atmospheric oxygen as well as Oxygen dissolved in water leads to production of acidic products (Na2SO3, NaHSO3) neutralizing a part of alkali, necessitating compensation of it in bath.
2NaHSO3+ 2NaOH = 2Na2SO3+2H2O
◆Also, excess alkali is required to combine CO2 from air getting into liquor to retain reduced form of dye thus making use of NaOH around 2-3 times higher over its stiochiometric requirement throughout dyeing.
◆Presence of excess NaOH in bath can be checked by dipping piece ot filter paper soaked in phenolphthalein, turns pink if excess NaOH is present in bath.
Dyeing
◆Cellulose has higher affinity for most of sodium salts of leuco vat dyes necessitating control over temperature and salt; dye solution is to be added in two installments.
◆Non-ionic dispersing agents promote diffusion and decrease rate of exhaustion at equilibrium; these are not to be essentially added in bath but can be used to promote level dyeing; no other chemicals are to be added during reduction and solubilisation except Na2S2O4 and NaOH.
◆Dyes in combination for composite shades must belong to identical application class, even after that compatibility is to be ensured before application.
◆Control over liquor ratio is a must to control strike rate to achieve levelled shades.
◆Too higher a liquor ratio will definitely produce brilliant levelled shades by reducing effective dye concentration in bath as well as strike rate, but in expense of high power, water, Na2S2O4 and NaOH consumption; whereas a lower ratio gives a dull shade and chances of unevenness is high due to higher strike rate.
◆The working condition is, therefore, a compromise among all dyeing parameters.
Oxidation
◆Dyed textile is oxidized to restore original dye structure, to trap dye molecules inside fibre and to develop actual shade.
◆It is essential to impart a thorough wash to dyeings to remove last trace of alkali for proper oxidation.
◆Oxidation may be carried out in open air, hydrogen peroxide (50-60°C), sodium perborate (50-60°C) or hypochlorite at room temperature for 15-20 min.
◆Air oxidation is suitable for loose structures but time consuming and practicable for smaller production schedule, whereas compact structures are chemically oxidised.
◆Improper oxidation may lead to faulty shades called patchy dyeing, which is mainly due to inadequate washing of dyed textiles - alkali carried forward by textile does not allow oxidizing agents to release free oxygen; shades produced in this way lack desired fastness.
◆It is preferred to oxidize dyeings at lowest possible temperature for minimum time; otherwise cotton may get oxidized, especially when hypochlorite is used as oxidizing agent.
◆If the dyed textile is not imparted intermediate wash before oxidation or the wash is not adequate for complete removal of Na2S2O4 and NaOH, dose of hydrogen peroxide for oxidation is to be increased accordingly, because a part of it gets wasted to consume Na2S2O4 and NaOH.
Na2S2O4 +2NaOH + 3H2O2 ----> 2Na2SO4 + 4H2O
Soaping
◆A final treatment with soap and soda ash (0.5 g/l dalch) at boil or only steaming for 10-15 min is a must for all vat dyed textiles to develop desÃred bright shade.
◆Only steaming is also efficient but fails to remove superficial dyes.
◆Dye-fibre linkage is established through physical forces.
◆Size of a vat dye molecule is too smaller compared to pore size of cotton implying poor wash fastness but soaping promotes aggregation to remarkable extent to develop excellent wash fastness.
◆This aggregation is accelerated in presence of salt and / or at lower temperature.
◆Tone of shades is attributed to location of dye in respect of fibre axis and when dye molecules are positioned parallel to fibre axis, the true tone of dyeings appear.
◆This positioning is enhanced by rise in temperature at least for a short period.
The function of soaping is thus three folds:
(i) It removes loosely attached superficial dyes.
(ii) It develops right tone of shade placing dye crystals perpendicular to fibre axis.
(iii) It helps formation of large dye crystals to offer superior fastness.
Photochemical degradation
◆Few yellow, orange and brown vat dyes show deterioration in strength of dyeings and fading of shade on prolonged exposure to light.
◆Absorbed sunlight, atmospheric oxygen and moisture cause slower decomposition of dye structure; presence of metal in cellulose or dye may catalyze this fading.
◆The effect is also pronounced when specific dyes are in mixture one shows catalytic action on another.
◆Dyed rayon shows intense catalytic oxidation than cotton. However, the overall light fastness remains above satisfactory level.
Methods for practical applicatioon
Exhaust dyeing
◆Dye solution is prepared outside with dye, Na2S204 (200-300 g) and NaOH (200-300 g) at required vatting temperature in 8-10 l water and 10-15 min is allowed for complete reduction and solubilisation.
◆A fresh bath is prepared with excess Na2S204 and NaOH in jigger or winch at dyeing temperature.
◆Cellulosics in wet state is loaded and one-half of the reduced and solubilised dye solution is added to dye bath.
◆After one complete passage of textile or desired time, left dye solution is added and the material is run in this solution for one more passage.
◆Due to higher affinity of dye, jigger is preferred as dyeing is done in open width and maximum surface is offered against dye molecules.
◆Dyeing is done for two turns (~30 min) with half dye at the start of each turn, after which salt (10-20g/l) is added and dyeing is continued for further 1 hr followed by oxidation and soaping.
◆This method is suitable for light to medium weight fabrics up to around 120 g/m2.
Semi-continuous dyeing
◆In pad-exhaust method, a dye solution is prepared with only ultra-disperse or ultra-fine grade dye (remain in well dispersed state) and cellulose is padded with this solution, dried at lower temperature to avoid migration followed by loading in jigger.
◆A reduced and solubilised dyebath is prepared in jigger with excess pad liquor (in case pad liquor is not available, one-tenth of total dye may be added), Na2S204 and NaOH.
◆Padded textile is dyed in this bath for desired time after which salt is added.
◆Compact and heavy weight fabrics are best dyed in this method as pressure applied during padding ensures dye penetration and thorough dyeing which otherwise becomes difficult in exhaust method.
Continuous dyeing
◆Continuous dyeing of cellulose with vat dye could not achieve commercial success on technical grounds.
◆However, cellulose may be padded with ultra-disperse vat dye followed by drying, repad in Na2S204 and NaOH solution, re-drying and finally passed through a steamer at 100-105°C for 5-10 min for fixation of dye.
◆Better dyeing results are obtained if rongolite is included in padding liquor and padding with Na2S204 and NãOH is omitted.
Defects in dyeings
Various dyeing faults, reason and probable solutions are listed as follows:
Dark selvedge
This is due to
(i) thicker selvedge which absorbs more dye, and
(ii) in Jigger dyeing, selvedge remains exposed to air causing rapid oxidation, partially dries up and absorbs more dye during subsequent passing through bath.
Spraying of Na2S204 and NaOH solution on selvedge during dyeing reduces this problem by stopping oxidation.
Light spot
This arises due to
(i) faulty preparation of fabric,
(ii) local oil stain adhered on textile during spinning and was not effectively removed during pretreatment,
(iii) partial degradation of cellulose to oxy or hydrocellulose during scouring.
A mild scouring in post-dyeing stage followed by redyeing reduces this problem.
Oxidation marks/ dark patches
◆This is the most common fault realized on vat dyed goods.
◆Washing after dyeing should be thorough and eliminate last trace of alkali.
◆Presence of alkali during oxidation with H202 develops these marks as dissociation of H202 to release oxygen occurs around a neutral pH.
◆Presence of alkali at places on dyeings forces H202 to release perhydroxyl ions, not oxygen.
◆Dyeings with such problems are to be treated with a solution of Na2S204 and NaOH for 15-30 min at reducing temperature of dye followed by thorough washing and re-oxidation.
◆Striping out of a part of dye in bath may be compensated accordingly.
Streaky dyeing
This may be due to
(i) reed marks on fabric,
(ii) variable tension in weft and warp causing prominence of either of these two and
(iii) faulty mercerisation.
Dyés possessing good coverage power are to be used to avoid this problem.
Poor rubbing fastness
(i) dyeing in too short liquor causing several dye layer formation on surface of cellulose,
(ii) storing solubilised dye for prolonged time causing partial oxidation of dye and subsequent surface deposition on dyeings,
(iii) incomplete reduction of dye towards end of dyeing,
(iv) inefficient soaping after dyeing and
(v) dyeing in hard water when metal salts may react with anionic dye and this complex is deposited on dyeings.
◆Most of the described faults can be rectified by treating dyeings with NaOH and Na2S2O4 at the temperature of vatting in presence of levelling agents, but with a chance of striping of a part of dye in bath.
◆Addition of 10-15% of dye of original recipe is required in bath.
◆A higher liquor ratio at all stages of dyeing and proper control over vatting and dyeing temperatures is a solution to most of the faults occurs in vat dyeings.
Stripping and correction of faulty dyeings
◆When uneven dyeing occurs or the desirable shade is not produced, it is required to correct these through partial or complete stripping.
◆Vat dyes produce very fast shades and complete extraction of dye is impracticable due to strong affinity of dye for cotton.
◆Dyeings are treated with Na2S2O4 and NaOH at specific temperature of reduction for 30 min for levelling of shade as well as partial stripping.
◆Application of non-ionic surfactant in bath promotes rate of levelling.
◆For complete stripping, method used for partial stripping may be carried out with addition of a cationic surfactant.
◆The later form complexes with reduced and solubilised anionic dye, looses affinity for cotton and comes out from dyeings.
◆Most of the vat dyes are soluble in hot DMSO and so dyes can be extracted with it from dyeings.
◆However, the does not have commercial importance on cost ground and is only of research interest.
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