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Indigo derivativesInclude natural and synthetic indigoThe colour of the soluble leucoderivative is usually different from the insoluble dyeform and tends to be pale yellowAlkalinity required for dye reduction is less as compared toanthraquinone dyesDyes are soluble in pyridine

Many dyes may sublime when heatedLeucoderivatives are easily oxidizable by the oxygen in airHave more brilliant colours than anthraquinonePoor light and washing fastness than anthraquinone

Anthraquinone and derivativesMay be derivatives of anthraquinone, indanthrone or benzanthroneMore alkalinity required during reduction than indigoColor of the reduced form different from insoluble form and tends to be different for eachdyesOxidize back by oxygen in air or oxidizing agents.Washing fastness generally better in dark shades than light shades

Washing fastness better in case of flock dyeing before spinning than fabric dyeingLight fastness varies from dye to dye; from excellent to poorLight fastness of some dyes improves in combination with other dyesSome dyes may cause loss in resistance of cellulose to light

Factors affecting rate of dyeingType of the substrate:

Mercerized cotton gives a higher rate of dyeing as compared to un-mercerized bleached cotton

which in turn gives higher rate than the grey materialTemperature,

At low temperature, the rate of exhaustion is lowwhich might promote levelness but the rate ofdiffusion is also low.

At high temperature, the rate of exhaustion is high which might decrease levelness but the rate of

diffusion is high.

Maximum exhaustion, penetration and levelness canbe obtained by starting the dyeing at low

temperatures in the leuco stage and slowly raising the temperature.

Some dyes may not be stable to very high temperatures, so the stability of dyes to temperaturemust be taken into account.

Liquor ratio, The higher the liquor ratio, the slower is the rate of dyeing.

Concentration of dye Most of the dyes exhaust more rapidly at low concentrations, increasing the risk of unlevel dyeing

in light shades. Some have the same rate of dyeing irrespective of the concentration.

Concentration of electrolyte The higher the concentration of electrolyte, the higher is the rate of dyeing.

Rinsing before oxidationPurposeTo remove any loose dye, excess of reducing agent and alkali to lower the pH andestablish conditions favourable for oxidation.Parameters

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The higher the temperature and/or pH of the rinsing bath, the lower is the colourstrength. Very high pH and temperature during rinsing may alsoresult in the dulling of theshade.- The ideal is to do rinsing thoroughly at low temperature at a rinsing bath pH valueof 7.

OxidationPurpose To convert the water-soluble leuco from of the vat dye, back into the insoluble pigment formParameters The type and concentration of oxidising agent, The type of pH regulator and pH during oxidation, Temperature during oxidationOxidizing agentThe oxidizing agent must provide a level of oxidation potential sufficient to oxidize the reducedvat dye into insoluble pigment, with no over-oxidation i.e., beyond the oxidation state of theoriginal pigment form of the dye.

pH Poor control of pH during oxidation may result in uneven oxidation. A pH below 7.5 should be avoided to prevent the possible formation of acid leuco form of vatdyes. The optimum pH for oxidation is 7.5 - 8.5.- The acid leuco form of vat dye is difficult to oxidize, has little affinity for fibre and is easilyrinsed out.TemperatureA lower temperature may result in slower oxidationThe higher the temperature, the faster is the oxidation, the optimum temperature being 50-60C

SoapingPurposeTo remove any dye that is not diffused into the fibreTo stabilize the final shade.This results in improved fastness properties and resistance to any shade changeto a resin or other finish or to consumer use.ParametersTime,Temperature,Type and concentration of soaping auxiliaries

Machine factor on hydrosulfite consumptionFactorsthe sealing of the machinethe air volume in the machinethe liquor turbulence When calculating the recipes, it is necessary to incorporate:2.0 liters caustic soda 36B (66Tw) and1.7 kg hydrosulfite conc .to eliminate the oxygen present in each cubic meter of air.

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