Dyeing_with_Vat_Dyes.pdf

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Dyeing with Vat Dyes

Dr. Tanveer Hussain

Dean Faculty of Engineering & Technology

National Textile University Faisalabad.

5/9/2012 Dr. Tanveer Hussain 1

The name ‘vat’ dyes

• The name ‘vat’ dyes is given from the recipient/vessel in which dye reduction and dissolving is performed


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General properties of vat dyes

• Insoluble in water

• Are converted into water-soluble leuco-derivatives by reduction in alkaline solutions

• Leuco-derivatives possess substantivity for cellulose on which the colour is developed by subsequent oxidation


Classification of vat dyes - I

• Based on chemistry:

– Indigo derivatives

– Anthraquinone derivatives


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Indigo derivatives

• Include natural and synthetic indigo • The colour of the soluble leucoderivative is usually different

from the insoluble dye form and tends to be pale yellow • Alkalinity required for dye reduction is less as compared to

anthraquinone dyes • Dyes are soluble in pyridine • Many dyes may sublime when heated • Leucoderivatives are easily oxidizable by the oxygen in air • Have more brilliant colours than anthraquinone • Poor light and washing fastness than anthraquinone


Anthraquinone derivatives

• May be derivatives of anthraquinone, indanthrone or benzanthrone

• More alkalinity required during reduction than indigo

• Colour of the reduced form different from insoluble dye form and tends to be different for each dyes

• Oxidize back by oxygen in air or oxidizing agents


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Anthraquinone derivatives…

• Washing fastness generally better in dark shades than light shades

• Washing fastness better in case of flock dyeing before spinning than fabric dyeing

• Light fastness varies from dye to dye; from excellent to poor

• Light fastness of some dyes improves in combination with other dyes

• Some dyes may cause loss in resistance of cellulose to light


Classification of vat dyes - II

• Based on application conditions

– IN* or CI** dyes (Hot)

– IW or CII dyes (Warm)

– IK or CIII dyes (Cold)

– IN special dyes

• * Bayer classification

• **Ciba classification


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Comparison of different vat dyes

IN dyes IW dyes IK dyes IN special dyes

-Require high temperature -Temperature of max. affinity 60°C -High amount of sodium hydroxide -No salt -Poor levelling -Require more levelling agent

-Require medium temperature -Temperature of max. affinity 45°C -Medium amount of sodium hydroxide -Medium amount of salt -Medium levelling -Require some levelling agent

-Require low temperature -Temperature of max. affinity 25°C -Low amount of sodium hydroxide -High amount of salt -Good levelling -Require no levelling agent

-Require higher temperature and caustic soda than IN dyes


Common phases in vat dyeing by exhaust method

• Reduction – Conversion of insoluble vat pigment into soluble sodium leuco-

vat anions

• Diffusion – of sodium leuco-vat anions into cellulosic fibres

• Rinsing – Removal of excess alkali and reducing agents by washing off

• Oxidation – of the soluble dye into insoluble pigmentary form within the

cellulosic fibres, and

• Soaping, – during which the isolated molecules of vat pigments are re-

orientated and associate into a different, more crystalline form


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Reducing agent requirements

• A level of reducing power sufficient to reduce all commercial vat dyes to their water soluble form, quickly and economically

• Conversion of the vat dyes into products from which the original pigment can be restored (no over-reduction)


Exhaust dyeing phase - 1

Commonly used reducing agent

• Sodium hydrosulphite, commonly known as hydros but more correctly known as sodium dithionite, which has the chemical formula Na2S2O4.

• Although a part of the hydros is used up in the reduction of vat dyes, a large part of it may be destroyed by its reaction with oxygen in the air (oxidation), particularly at higher temperatures.



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Factors of rate of dye reduction

• Dye-related: – Amount of dye

– Particle size of the dye,

• Auxiliary-related – Concentration of the reducing agent

– Concentration of alkali/pH during reduction

• Process related – Temperature

– Time



Stability of alkaline solutions of reducing agent

• May decrease with:

– increased temperature,

– greater exposure to air,

– greater agitation and

– lower concentration of the reducing agent



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Over-reduction

• Vat dyes of the Indanthrene type may produce duller or greener shades at dyeing temperatures higher than 60 °C due to over-reduction.

• Over-reduction can be prevented by the use of sodium nitrite if the reducing agent is hydrosulphite



Factors affecting rate of dyeing

• Type of the substrate,

• Temperature,

• Liquor ratio,

• Concentration of dye

• Concentration of electrolyte



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Effect of 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 material.



Effect of temperature

• At low temperature, the rate of exhaustion is low which might promote levelness but the rate of diffusion 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 can be 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 temperature must be taken into account.



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Effect of liquor ratio

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



Effect of dye concentration

• 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.



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Effect of electrolyte

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



Rinsing before oxidation

• Purpose – To remove any loose dye, excess of reducing agent

and alkali to lower the pH and establish conditions favourable for oxidation.

• Parameters – The higher the temperature and/or pH of the rinsing

bath, the lower is the colour strength. – Very high pH and temperature during rinsing may also

result in the dulling of the shade. – The ideal is to do rinsing thoroughly at low

temperature at a rinsing bath pH value of 7.



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Oxidation

• Purpose

– To convert the water-soluble leuco from of the vat dye, back into the insoluble pigment form

• Parameters

– The type and concentration of oxidising agent,

– The type of pH regulator and pH during oxidation,

– Temperature during oxidation



Oxidation…

• Oxidizing agent

– The oxidizing agent must provide a level of oxidation potential sufficient to oxidize the reduced vat dye into insoluble pigment, with no over-oxidation i.e., beyond the oxidation state of the original pigmentary form of the dye.



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Oxidation…

• 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 vat dyes.

– 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 easily rinsed out.



Oxidation…

• Temperature

– A lower temperature may result in slower oxidation

– The higher the temperature, the faster is the oxidation, the optimum temperature being 50-60°C



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Soaping

• Purpose – To remove any dye that is not diffused into the fibre

– To stabilize the final shade. • This results in improved fastness properties and resistance

to any shade change to a resin or other finish or to consumer use.

• Parameters – Time,

– Temperature,

– Type and concentration of soaping auxiliaries



Vatting process


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Example: NOVASOL RED 6B


Vat dye reducing agents


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Machine factors for consumption of reducing agent

• Factors

– the sealing of the machine

– the air volume in the machine

– the liquor turbulence

• When calculating the recipes, it is necessary to incorporate:

– 2.0 liters caustic soda 36°Bé (66°Tw) and

– 1.7 kg hydrosulfite conc.

• to eliminate the oxygen present in each cubic meter of air.


Preparing dye dispersions

• Strew dye into 5-8 times its weight of soft water at 20-25°C, while stirring at a high speed.

• Alternatively, mix dye to a smooth paste with the gradual addition of about 5 times its weight of soft water at 20-25°C and then dilute as required.

• Dispersions should not be made up with hot water or boiled up with live steam as this can cause dye agglomeration.


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Novasol Exhaust Vat dyeing methods

1. Semi-pigmentation method

2. Hot pigmentation method

3. HT method

4. Leuco method

5. Pigment dispensing

6. Special dyeing process for black


Semi-pigmentation dyeing method – yarn package dyeing


A -Dye -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent

B -Caustic Soda 36Be -Sodium dithionite -Non-ionic Leveller

C Salt for CII dyes only

D Re-sharpen with Hydros if necessary

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Semi-pigmentation dyeing method – piece jet dyeing



B - 2/3Caustic Soda 36Be

C - 1/3Caustic Soda 36Be -Sodium dithionite -Non-ionic Leveller

D Salt in case of CII

E Re-sharpen with Hydros if necessary

Semi-pigmentation dyeing method – yarn package dyeing

Merits

• With most dyes, slow vatting means leveling through slower uptake of the sodium leuco compound

• .“All-in” method is readily integrated into automation concepts

• Levelness is further improved by raising the dyeing temperature above 60°C

Demerits

• Low package permeability in the initial phase of dyeing


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Hot pigmentation method – yarn package dyeing



B -Caustic Soda 36Be -Sodium dithionite -Non-ionic Leveller


D Re-sharpen with Hydros if necessary

Hot pigmentation method – piece jet dyeing



B - 2/3Caustic Soda 36Be

C - 1/3Caustic Soda 36Be -Sodium dithionite -Non-ionic Leveller

D Salt in case of CII

E Re-sharpen with Hydros if necessary

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Hot pigmentation method – yarn package dyeing

Merits

• High package permeability at high initial temperature in neutral bath

• Suitable for all shade depths (except black)

• Problems solved in critical cases

Demerits

• High energy cost


HT dyeing method (90−105°C) – yarn package dyeing


A -Dye -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent -Caustic Soda 36Be -Rongal HT (BASF) -Non-ionic Leveller

B Re-sharpen with Hydros if necessary


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A -Dye -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent -Caustic Soda 36Be -Rongal HT (BASF) -Non-ionic Leveller

B Re-sharpen with Hydros if necessary



Merits

• Perfect levelness

• Almost an “all-in” method

Demerits

• Slightly limited dye selection

• Less suitable for deep shades (rubbing fastness)

• Special reducing agent necessary (Rongal HT, BASF)

• DP decrease possible

• Time-consuming


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Leuco dyeing method


A -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent -Caustic Soda 36Be -Rongal HT (BASF) -Non-ionic Leveller

B Stock Vat


D Resharpen Hydro if needed

Leuco dyeing method

Merits

• Preferred method for dark shades, including black

• Better leveling with graduated temperature method (for example 20−80°C)

• Suitable for all vat dyes

Demerits

• Pre-vatting is laborious

• Success depends on the leveling power of the individual dyes


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Pigment dispensing method


A -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent -Caustic Soda 36Be -Rongal HT (BASF) -Non-ionic Leveller

B Dye

C Resharpen Hydro if needed

D Salt for CII dyes only

Merits .. Novel, reliable way to solve levelling problems with critical shades .. Suitable even for “incompatible” combinations Demerits .. Dispensing equipment necessary

Special method for blacks


A -Anionic wetting agent -Anionic Sequesterant -Anionic dispersing agent -Caustic Soda 36Be -Rongal HT (BASF) -Non-ionic Leveller Dye

B Resharpen Hydro if needed

NOVASOL® black dyes are vatted only in long liquors. The dye is dispensed, then added to a bath which already contains the full amount of caustic soda 36°Bé (66°Tw) and sodium hydrosulfite. Vatting time is about 15 min at 60°C.

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Example amounts of Aux. for CI dyes


Wetting agent 0.5-1 g/l Dispersing agent 1-3 g/l Sequestering agent 0.5-2 g/l Leveling agent 1-2 g/l

Oxidation of Novasol vat dyes


Anionic dispersant

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Oxidation of Novasol vat dyes


Anionic dispersant

Soaping off Novasol vat dyes


Non-ionic Detergent

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Soaping off Novasol vat dyes


Jigger dyeing

• Pre-pigmentation – Dispersing agent 2 g/l – Wetting agent 0.5-1 g/l – Sequestering agent 1 g/l – Dye 1/2 , 1 end; ½ 1 end

• Development – Caustic soda y g/l – Sodium dithionite z g/l – Leveling agent 2 g/l

• Dyeing – ½ amount 2 ends, ½ amount 2 ends – Temp. depends on type of dyes

• Rinsing • Oxidation • Soaping


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Pad dyeing vat dyes

• Pad-dry-pad steam

• Pad-jig


Pad-dry-pad-steam

• Dye padder – Sequestering agent 1-2 g/l – Wetting agent 1-2 g/l – Dispersing agent 1-2 g/l – Anti-migrant 5-10 g/l – Dye x g/l – Padding temp. 20-30C – Pick-up 60-80%

• Drying – 120C

• Cooling


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Pad-dry-pad-steam

• Chemical padder – Sequestering agent 1-2 g/l

– Wetting agent 1-2 g/l

– Dispersing agent 1-2 g/l

– Caustic soda y ml/l

– Hydros z g/l

– Glucose 2 g/l (for dyes prone to over-reduction)

– Dye padding liquor 20-40 ml/l

– Temp. 20-30C

– Pick-up 80-100%


Pad-dry-pad-steam

• Steaming

– 40-60 seconds

– 102C saturated steam


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Pad-dry-pad-steam

• Rinsing

• Oxidation

• Soaping

• Rinsing

• Drying


Pad-jig method

• Dye padder

– Sequestering agent 1-2 g/l

– Wetting agent 1-2 g/l

– Dispersing agent 1-2 g/l

– Anti-migrant 5-10 g/l (only for intermed. Drying)

– Dye x g/l

– Padding temp. 20-30C

– Pick-up 60-80%


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Pad-jig method

• Jigger development – Sequestering agent 1-2 g/l – Wetting agent 1-2 g/l – Dispersing agent 1-2 g/l – Caustic soda y ml/l – Hydros z g/l – Leveling agent – Dye padding liquor 20-40 ml/l – Salt for low affinity dyes

• Dyeing – 50-70C


Pad-jig method

• Rinsing – Light to medium shades

• rinse with cold, soft water with overflow during one end

– Dark shades • rinse with a cold, reductive rinsing bath with 2 ml/l caustic

soda solution 32.5 % (38° Bé), 2 g/l sodium hydrosulphite and 1 g/l dispersing agent

• Oxidation – 5 g/l oxidising agent, 60C, 2 ends

• Soaping – 2 g/l detergent, boiling, 2 ends


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