Introduction
◆Sulphur dyes are exclusively used in dyeing cotton with medium to heavy black, navy blue, brown and olive green and a few other shades at remarkably cheaper price alongwith good light and washfastness.
◆Dyes are non-ionic and insoluble in water; the desired anion is developed on reducing
and solubilising at higher temperature when it shows affinity for cellulose.
◆Produced shades lack tinctorial brilliance which may be modified by topping while brilliant red, orange and yellow are the limitations.
◆Sodium sulphide (Na2S) performs reduction and solubilisation both severing sulphur linkages into thiols followed by formation of sodium salt of thiol or mercaptides or thiolates, which are soluble in water and substantive towards cellulose.
◆ Higher rate of exhaustion occurs at 90-95°C in presence of electrolyte.
◆Dyed cotton is oxidized to restore parent dye structure in situ cotton and dye is retained by cellulose as aggregates with H-bonds and van der Waals forces.
◆Excess free sulphur is liable to damage cotton in moist atmosphere forming tiny holes.
◆Sodium sulphide, the most effective reducing cum solubilising Ã¥gent pollutes waste water and corrodes concrete discharge pipes.
◆Sulphur dyes are marketed under various brand names viz. Sulphur, Nissen, Asathio, Diresul, Pirocard / Pirosol, Coranil, Kayaku, Mitsui, Sodyeco/ Sodyesul, Sulphol / Sulphosol, etc., and have been assigned CI constitution numbers ranging from 53000 to 53830.
Features of sulphur dyes
◆Sulphur dyes are of variable composition, generally available in paste or powder form, insoluble in water, cheaper and easy to apply; posses good to excellent wash fastness (~4-5), good light fastness (~5-6), except black 1 (~7).
◆Shades are relatively dull and incomplete in hue range: (not fast to chlorinating agents/and hence not applied on swimming costumes on fear of decolourisation.
◆These dyes are important in producing cheap black, navy blue, khaki and olive green hues only which is a dream with other classes of cotton dyes at such a lower cost with good fastness.
◆Brilliant red, orange and violet are not available in this class of dye.
◆Higher alkalinity of dyebath restricts use in dyeing protein fibres due to fear of hydrolysis.
◆Sulphur dyes are importag garments and denim in rope form for high quality and performance.
◆Lack in imposing right dyeing conditions causes-bronziness.
◆H2S liberated during dyeing, which is the characteristic feature of all sulphur dyes, forms corrosive metal sulphides and this restricts use of metal vessels except stainless steél.
Fe+H2-------->FeS+H2
◆Sulphur dyes are identified by boiling a piece of dyed cotton with SnCl2 (alternately Zn) and HCl in a test tube and covering mouth of test tube with a piece of filter paper soaked in lead acetate.
◆H2S formed reacts with lead acetate to form PbS to convert the paper black.
H2S+(CH3COO)2Pb-------> PbS+2CH3COOH
◆Free sulphur, if present, gets oxidized to SO2 during storage in moist atmosphere to form H2SO3; the later develops tiny holes on cellulose through local hydrolysis.
Chemistry of sulphur dyes
◆Excess sulphur is isolateds.
◆Sulphur linkages are the integral part of chromophore and are basically complex mixture of polymeric molecular species comprising of large proportion of sulphur in the form of sulphide (-S-), disulphide (-S-S-) and polysulphide (-Sn-) links in heterocyclic rings.
◆Chromophoric systems are based on thiazole, thiazone, thianthrenes and phenothiazonethioanthrone.
◆Synthesis of sulphur dyes, in general, involves sulphurisation, in which sulphur, polysulphide or both in mixture is heated at around 180-350°C alongwith aromatic amines, phenols or aminophenols or refluxed in solvents under pressure.
◆Sulphurised vat dyes are synthesized in the same way too but their reduction requires a strong reducing agent like sodium
hydrosulphide.
Synthesis of sulphur dyees
◆Sulphur Black BG/ GXE / GXR (Bluish Black, C I Sulphur Black 1, CI 53185) is the most important member having empirical formula
◆C24H16N6O8S7 or C24H16N6O8S8 and is produced by heating 2,4-dinitrophenol with sodium polysulphide (in proper ratio) at 110-120°C for 48-72 h at 130-140°C under pressure.
◆The melt is diluted and black dye is precipitated by means of addition of acid or air-oxidation.
◆Sulphur Blue FBL (Blue, C I Sulphur Blue 10, CI 53470) is produced by heating N-[p-(p-hydroxyanilino) phenyl] sulphanilic acid with aqueous sodium polysulphide at 105-106°C for 100 h, precipitated by air blowing and salt is added at 70-80°C.
Synthesis of sulphurised vat dyes
◆Sulphurised vat dyes are produced through sulphurisation process that is used for other sulphur dyes but are reduced with Na2S2O4 and applied like vat dyes.
◆One such commonly used dye is Hydron BluÄ™ R (CI Vat Blue 43, CI 53630).
◆Indophenol produced from condensation of p-nitosophenol and carbazole in presence
concentrated H2SO4 at -20°C is reflxed at 107°C for 24 h with sodium polysulphide in butanol to produce this dye.
◆Hydron Blue R is an inexpensive substitute for indigo and can not he completely reduced by Na2S, so it is reduced invariably by Na2S2O4 in combination with NaOH.
Classification
◆Sulphur dyes are classified based on chemical structure and application.
Chemical structure
(i)Sulphur dyes and
(ii)Sulphurised vat dyes.
◆Sulphur dyes having only sulphur linkages and are reduced with Na2S at boil.
◆Sulphurised vat dyes retain both sulphur linkage as well as carbonyl group as chromophore and are reduced with Na2S2O4 and NaOH combination at specific temperature.
Application
Dyes are available in three main forms,
(i)Powder,
(ii)Reduced and
(iii)Water soluble
◆Powder form is the conventional
insoluble type; these are reduced and solubilised with Na2S at boil before
application.
◆Leuco sulphur dyes are physical mixture of dye and Na2S; requires heating in water with little excess Na2S to reduce and solubilise
dye.
◆Water soluble liquid dye or ready to apply type belongs to sulphurised vat class and is directly used in dyeing.
The last type is free from fly-out, suitable for continuous dyeing but too costly.
Batch application
Reduction
◆Reduction involves conversion of sulphide, disulphide or polysulphide into leuco thiols.
◆Dye is pasted with little turkey red oil to improve wetting and prevent floating of dye molecules.
◆Water is added to it followed by a little Na2CO3 to counteract any H2SO3 that might has been formed during storage of dye.
◆Na2S is added gradually with constant stirring the mixture at 90-95°C for 10-15 min for complete reduction and solubilisation of dye which is marked by formation of stable foam on surface of liquor.
◆Reduction occurs through formation of NaHS (sodium hydrosulphide) and NaOH during hydrolysis of Na2S; NaHS reduces dye while NaOH solubilises leuco dye to thiolates.
◆Stable reduced dyebath is highly alkaline; addition of little Na2CO3 neutralizes this acid prior to reduction, avoids wastage of Na2S and stabilizes reduced form of dye.
◆Concentration of Na2S required is based on its purity.
◆Crude and flake grades have a purity of 30-35% and 60% respectively and so almost double amount is used for reduction and solubilisation; increase in purity will result in lesser dosing proportionately.
◆Over-reduction of dye with excess Na2S must be avoided as it leads to product having lower affinity for cellulose. A chelating agent, like EDTA counteracts interference of metals in hard water.
Dyeing
◆A commercial recipe for dyeing cotton with sulphur dye consists of
Dye - x%
Turkey red oil - a little
Na2CO3 - 2 g/l
Na2S - 2x% for crude and 1.5x% for flakes
NaCl - 50 g/l
◆Once reduction and solubilisation is over, wet cotton is dyed at 90-95°C for 30-60 min after which salt is added and dyeing is continued for further 1.5-2 h; the bath is drained out and dyed cellulose is washed thoroughly.
◆Due to presence of large amount of dye required to produce deep shades and its moderate affinity for cellulose, dyeing time before and after salt addition should be kept on higher side to promote exhaustion of bath.
◆Reduction as well as dyeing of cotton is preferably carried out in jigger or winch.
◆At least four turns are imparted in dyebath after which salt is added and further eight turns are imparted at boil.
◆The bath is dropped, dyed cotton is cold washed for 2 turns, oxidized for 2-4 turns followed by soaping, washing and padding with CH3COONa.
Oxidation
◆Dyeings are oxidized in K2Cr2O7 (1-2 g/l) and CH3COOH (1-2 ml/l) at 50-60°C for 30 min, when the dye restores its parent oxidized form.
◆Reaction of dichromate and acid produces nascent oxygen required for oxidation.
K2Cr2O7+8CH3COOH-----> 2CH3COOK +2(CH3COO)3Cr + 4H2O+3[O]
◆Dichromate oxidation is cheaper but stiffens dyed cotton through deposition of chromium and changes tones of blue dyeings.
◆Stiffness results more needle cuttings of sewing threads forcing to oxidize these with H2O2.
After-treatment
◆After oxidation and soaping, dyeings are treated with CH3COONa at low liquor ratio and dried to counteract inorganic corrosive acid, if formed in post-dyeing stage.
◆However, deposition of salt (NaHSO3) makes dyeings marginally stiffer.
◆To improve light fastness, dyeings may be treated with metal salts in presence of acid and light fastness may be improved by treatment with dye-fixing agents.
◆Dullness of shades may be improved to some extent by topping with little basic dye.
Continuous dyeing
◆Conventional sulphur dyes are not suitable for continuous application due to inconsistency in penetration of large amount of dye to produce deep shades within reasonably short time.
◆Micro-dispersed powdered sulphur dyes or solubilised sulphur dyes, viz. hydron stabilisol dyes are most Suitable and may be applied on mercerized cotton in pad-steam process With intermediate drying using caustic lye and hydrosulphite.
◆Polyester-cotton blends can be dyed in thermosol/ pad-steam in single or two stage
process.
◆The blend is padded with samaron dyestuff, followed by drying and thernmosoling; repadded with hydron stabilisol dyes with 40-60% liquor pick up and steamed.
◆Washing, oxidation and soaping are carried out as usual.
◆However, cost of dyeing and chances to develop well levelled shade poses problem in continuous dyeing with sulphur dyes.
Reducing systems
◆Sulphur dyes require reduction potential around (-550 to -650) mV for proper reduction and to retain its reduced form throughout dyeing.
◆Various effective reducing systems are available, few are cheaper but hazardous to ecological balance.
Conventional reducing agents
◆Either Na2S alone or glucose (4 g/l) along with NaOH (2 g/l) are two effective formulations to reduce and solubilise sulphur dyes at 90-95°C and produced thiolates are very much stable for prolonged time.
◆Sodium hydrosulphide (NaHS) is another important reducing agent and the dosing is equal to that of Na2S flakes but require addition of either of Na2CO3 (10 g/l) or NaOH (5 g/l).
◆In fact, Na2S produces NaHS and NaOH when dissolved in water.
◆Sodium hydrosulphite or dithionite (Na2S2O4 3 g/l) with NaOH (1.5 g/l) is more powerful reducing agent than Na2S with reduction potential around -700 mV.
◆Reduction with hydrosulphite may cause over-reduction of dye leading to a product having poor affinity for cellulose.
◆A few red-brown, green and olive dyes decompose in presence of hydrosulphite, while a few dyes give better yield but have not been used for dyeing sulphur blacks because of reduced colour
yield and poor reproducibility.
◆Solubilized sulphur blacks are applied using hydrosulphite only at higher cost.
◆Leuco sulphur dyes and sulphurised vat dyes are often reduced with Na2S2O4 with a consequence of inadequate stable bath and dyeings possess lower wash fastness.
◆Sodium bisulphide reduces scopes of bronziness by keeping dyebath in reduced condition consistently and prevents surface oxidation.
◆Sodium sulphide, the cheaper and most efficient reducing agent is associated with huge waste water load.
Sulphur-free reducing agents
◆Glucose supresses dye uptake in jiggers and winches due to inconsistency in maintaining temperature beyond 90°C.
◆It is a strong hydrocarbon and so though readily degradable, produces higher BOD, and COD.
◆β-mereaptoethanol alongwith NaOH results efficient dye uptake in dyeing with leuco sulphur dyes in exhaust and pad-dry-steam processes without releasing any odour but costly.
◆Iron salts along with alkali may act as efficient reducing agent at room temperature, but the extent of reduction of dye remains proportional to solubility of iron.
◆Iron (II) salts on reaction with NaOH produces Fe(OH)2 generating a reduction potential around (700-750) mV; the potential is well above that required for reduction of conventional sulphur dyes.
◆Negligible solubilisation of iron can not reduce dye upto any remarkable extent till Fe(OH)2 is not complexed with weaker ligands, viz. tartaric acid, citric acid, triethanolamine, ete.
◆However, the reduction potential becomes too higher to cause over reduction of dye.
◆Hydroxyacetone generates a reduction potential as high as -810 mV, biocompatible and is applied alongwith NaOH, but chances of over reduction of dye exists and is costlier.
Oxidizing agents
◆Two most commonly used Oxidizing agents are Na2Cr2O7 or K2Cr2O7 in combination with an acid.
K2Cr2O7 + 4H2SO4 = K2SO4 + Cr2(SO4)3 + 4H2O+ 3[O]
◆Chances of over-Oxidation exists as oxygen is formed rapidly; H2SO4 may be replaced with a weak acid like CH3COOH to retard the process and temperature is maintained at 50-60°C for 15-30 min.
◆Before oxidation, a thorough wash is imparted to ensure absenee of alkali; presence of little alkali in dyeings may resist oxidation as K2Cr2O7 produces CO2 and not oxygen when reacts with alkali.
K2Cr2O7 + Na2CO3 = 2KNa CrO4 + CO2
◆Both Na2Cr2O7 and K2Cr2O7 are cheaper, precipitate chromium on cellulose and impair handle that is why sulphur dyed sewing thread is invariably oxidized with H2O2 to reduce needle cutting and to preserve suppleness of yarn.
◆Sulphur blue shades are oxidized with H2O2 to avoid reddish impression imposed with dichromates.
◆H2O2 is generally not used on cost ground and chances of over-0xidation exists if used alone at pH 7; but H2O2 (1 ml/l, 35% or 130V) may be used at pH little over 7 and 50-60°C to release required nascent oxygen.
◆Acidified KIO3 is costlier but does not change tone of dyeings.
◆Both H2O and KIO3 are eco-friendly oxidizing agents; in fact KIO3 was first established as the ideal oxidizing agent for sulphur dyes.
Bronzing of shade
◆Sulphur dyes are liable to develop bronze like look mainly on black and blue shades.
◆There may be several reasons behind this problem, viz.
(i)Excessive heavy dyeing, due to which most of dye molecules occupy surface areas only reflecting back more incident light,
(ii)Exposure of goods to air during dyeing, when a layer of reduced dye is formed on surface of cotton and the same gets dried and oxidized; the same is repeated during next passing through dyebath and so on,
(iii) use of excess dye in dyeing, and
(iv) inadequate Na2S in dye bath causing incomplete reduction of dye.
◆The root of this problem lies on insufficient diffusion of dye with more surface deposition and to rectify it, dye molecules are to be distributed homogeneously which can be achieved by redyeing in a fresh bath with Na2S at 90-95°C for 30-60 min at lower liquor ratio adding 10% of total dye earlier used.
Fastness of dyeings
◆Light and wash fastness of sulphur dyed cotton, in general, are in the range of 5-6 and 4-5 respectively.
◆Light tastness may be enhanced by topping with either of vat dye or aniline black which helps to improve wash fastness too.
◆Various washing cycles discharge a part of dye progressively beyond 50°C due to presence of bleach formulation in modern detergents, but decolourised dye does not stain adjacent apparels because of its non-ionic nature.
◆Treatment with polymeric or conventional cationic dye-fixing agents facilitates to improve washfastness in both batch and continuous methods.
◆Fastness to chlorine is mostly poor; hypochlorite bleach decolourises sulphur dye permanently with exceptions of sulphurised vat dyes and few red, green and black.
◆However, in latter cases, a remarkable part of shade is discharged.
◆Peroxide bleach too decolourises sulphur dyed shades but the action is not so severe like that with hypochlorites.
◆Dry crock fastness is reasonably good (4-5) while wet crock fastness is inferior (2-3).
◆Perspiration fastness grades are generally very good.
Tendering of cellulose
◆Free sulphur, if remains present in sulphur dyed cotton, is oxidized to SO2 which in turn absorbs moisture to form H2SO3 in humid atmosphere and develops tiny holes on cellulose due to local hydrolytic action.
◆The problem can be handled by passing soaped and washed dyeings through CH3COONa bath at too lower liquor ratio so that H2SO3 as and when produced will be converted to harmless CH3COOH
H2SO3 + CH3COONa---->CH 3COOH + Na2SO3
◆To assess extent of tendering, dyed sample is covered on both sides with two white pieces of cotton fabric, stitched, treated at 140°C for 1min, is kept in open air when loose sulphur is oxidized and in contact with moisture form H2SO3.
◆Stitches are removed, three pieces are separated out and is tested for presence of loose sulphur.
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