Vat

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7VAT DYES

Except Deep Red

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Introduction1

Chemical Structure of Vat Dyes2

Dyeing Structure of Vat Dyes3

Method of Dyeing4

Treatments After Exhaustion of Vat Dyes5

Overview

Semi and Fully Continuous Method6

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Wet on Wet Process7

Fixation Process8

Package Dyeing9

Dyeing of Knit Goods10

Correction of Faulty Dyeing11

Overview

Indigo Dyeing for Denim12

Solublised VAT Dyes13

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Dyeing produced with the vat dyes have the highest overall fastness properties.

All of these dyes have good fastness against

Wet treatments

Crocking

Light fastness in the region of 6 to 7.

The vat dyes can also withstand oxidizing agents like

Hydrogen peroxide (H202)

Sodium Chloride (NaCl)

Hypochlorite (--OCl)

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the vat dyes are extensively used for dyeing yarnTHIS CLOTH can undergo the pretreatment processes like

Alkali Boiling

Bleaching

Mercerizing

Without any damage to the dyed yarn.

After the introduction of hydrogen peroxide- tolerant reactive dyes

Yarn dyeing has been taken over by the reactive dyes.Because reactive dyes less expensive , easier to apply.

Vat dyes are still popular on account of the all round high fastness properties.VAT DYES ARE COMMONLY USED FOR:

Superior Quality Shirting MaterialMilitary UniformsFurnishings, Curtain, Toweling etc.

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there are about 60 vat dyes in the market

That cover the entire Gamut(range) of shades except Deep Red Colors .

VAT DYES ARE ESSENTIALLY Insoluble Polycyclic Aromatic Compound:

Containing Two or more pair of QUINONE GROUPS

Indigo and Tyrian Purple belongs to this family.

VATTING: In order to apply cellulosic Fibers

VAT PIGMENTS are made water soluble by reducing to HYDROQUINONE Forms in an alkaline media.This process Known as Vatting.

Vatting Term referring to a historical link with the vegetable Indigo Dye that used to reduced in WOODEN VATS with natural reducing agents like sugar.

PRESENT VATTTING CHEMICALS ARE:

Sodium Hydroxide

Sodium Dithionate(Na2 S2 O4) a strong reducing agent one with high reduction potential. Commonly termed in Industry as Sodium Hydro Sulphite or Simply HYDROS.

Sodium Dithionate reduces the KETO GROUPS into the ENOL Form

Sodium Hydroxide forms sodium salts of the ENOL to make the DYES WATER SOLUBLE.

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PRESENT VATTTING / REDUCING CHEMICALS ARE:

Sodium Hydroxide

Sodium Dithionate(Na2 S2 O4) a strong reducing agent one with high reduction potential.

Commonly termed in Industry as Sodium Hydro Sulphite or Simply HYDROS.

Sodium Dithionate reduces the KETO GROUPS into the ENOL Form

Sodium Hydroxide forms sodium salts of the ENOL to make the DYES WATER SOLUBLE.

Caustic Soda Also NEUTRALISE THE ACIDIC Decomposition products of the reducing agent ‘DITHIONITE’ produced during vatting and Dyeing.

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PRESENT VATTTING / REDUCING CHEMICALS ARE:

Quantity of Reducing Agent Depend upon

No. of KETO groups Present in the Dye Molecule

To the extent/ amount of exposure or the dye bath to AIR.

Reduction is Accompanied by

Change in Color that is due to alteration in the conjugation of DOUBLE BONDS.

The reduced indigo has a PALE SHADE and so it was called the LEUCO(white) dye.

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Around 1900 Rene Bohn in GERMANY

accidently prepared a blue dye from ANTHRAscene, which he named as an INDANthrene dye. Soon after this breakthrough

Bohn and his Coworkers synthesize many other VAT DYES.

THE VAT DYES ARE OFTEN CLASSIFIED AS:

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•Low substantive to cellulose

INDIgoid

• Includes the HYDRONE BLUES that are type of HYBRIDS of sulphur

• Often used as substitute of Indigo

• Low substantive to cellulose

Thio-INDIgoid

•Majority of vat dyes belongs to this group.•Having a more complex structure.•Posses high affinity.•Better all- around fastness properties than other two groups.

AnthraQuinonOid

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Sodium Salts (Nacl) of the Reduced or Leuco VAT Dyes behave like:

Direct dyes

As for as adsorption on the cellulose fibres is concerned

But the ANTHRAquinone derivatives have

High intrinsic substantivity

In spite of lack of linearity and

Hydrogen bonding groups attributed

to the molecular complexity

And CO-PLANARITY of the VAT dyes.

As a High Concentration of SODIUM IONS from Caustic Soda(NaoH) and Sodium Dithionate(Na2 S2O4)

Already exist in the Reduced Bath.

The VAT dye tend to have HIGH RATE of adsorption

And EXHAUSTION in the initial stage of Application.

The RAPID ADSORPTION of dye in the fi rst few minutes:

Technically termed as “HIGH STRIKE” value.

This tendency require a careful attention as well as ADDITION of RETADING agents to obtained level dyeing.7/2/2008

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After the adsorption of the LEUCO DYES

AN OXIDIZING AGENT IS ADDED TO THE BATH

That regenerates the VAT PIGMENT inside the fibres.

Finally, to achieve full fastness properties and to develop the proper shade, the dyed goods are

THOROUGHLY SOAPED AT BOIL.

VAT dyes three main classes for APPLICATION PUPOSES.

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CLASS I or IN

CLASS II or IW

CLASS III or IK

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CLASS I or IN

Where I stands for Indanthrene and N for NormalIn their Leuco forms Have High substantively for the cellulosic fibres.These needs High Alkalinity for DissolutionAnd are dyed at about 75o c.

CLASS II or IW

Where I stands for Indanthrene and W for WarmIn their MODERATE substantivity for cellulosic fibres.These needs MEDIUM Alkalinity for DissolutionAnd are dyed at about 45o c TO 75o c.Needs Electrolytes for complete EXHAUSTION of the dye.

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CLASS III or IK

Where I stands for Indanthrene and K for KALT / COLDHave LOW substantivity for the cellulosic fibres.These needs moderate Alkalinity for DissolutionAnd are dyed at low temperature about 20o c to 30o c..Needs in a Higher concentration Electrolytes than class II for complete EXHAUSTION of the dye.

Dyes of the IK Group other than the Indigo, are Not much used now a days.Some vat dyes like BLACKS do NOT into the above mentioned three groups and are dyed by a special method

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In Batch dyeing of fabric, VAT dyes are normally applied in the

Reduced in a JIGGER.

For continuous processes,

Fabric is PADDED in stable dispersion of very finely divided vat pigments often termed as MICRO-DISPERSIONS.

After drying the dye padded fabric, it is reacted with an ALKALINE REDUCING SOLUTION in a padder to VAT PIGMENT into water-soluble LEUCO FORM

LEUCO FORM diffuses into the fibre.

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REDUCED VAT DYEING:

This process is applicable for

Batch wise dyeing WOVEN and KNIT fabrics

Also for YARNs in hank or package form.

REDUCTION OF the dye is faster in the STOCK VAT

BECAUSE

Concentration of the dye and reducing chemicals is higher than LONG BATH.

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The dye is first reduced into SOLUBLE FORM.In the bath(long liquor)Or in the stock vat

Before bringing it in contact with

substrate

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REDUCED VAT DYEING:

THE MORE Diffi cult to reduced CLASS I dyes:

Vatted by the STOCK VAT method.

This dispersion at optimum temperature

Stirred for 10 minutes. When reduction is COMPLETE.

SOFT WATER must for dissolving and as far as dyeing.

Dyeing in HARD WATER causes precipitation (useless product) COUNTER THIS PROBLEM by use of SQUEStering AGENT.7/2/2008

Dye is pasted with the

dispersing agent. And soft

water

And then the required quantities of

Caustic Soda(NaoH)And

DI-THIONATE(S2O4) Are added as per manufacturer

literature.

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WETTING AGENT

CAUSTIC SODA &HYDROS(dithionate)

Goods are thoroughly wetted in it before reduced dye.

Concentration of the castic and hydros depend upon class of dye and the liquor Ratio and literature of mfr should be consulted.

THE DYE BATH is set

with

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THE FOLLOWING TABLE WILL GIVE A

GENERAL IDEA OF CONCENTRATION

OF rducing and exhausation chemicalsrequired in the bath at a liquor ratio of 10:1

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DYE GROUP Caustic soda (g/l) Hydros (g/l) Sod.Sulphate (g/l)

IN 8.8 5.0 NIL

IW 4.8 4.0 12.0

IK 3.6 3.0 12.0

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For the Class I dyes,

a nonionic retarding agent (ALKO-Xylated type)

Is often added that forms a complex with the REDUCED dye and release it gradually as the bath is exhausted,

addition of the Retarding or LEVELLING agent is more necessary

When dyeing mercerized COTTON, Regenerated RAYON

And for PALE SHADES

And also in LOW LIQUOR ratios.

These LEVELLING Agent reduce the ultimate dye- uptake slightly,

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During DYEING•Dithionate•Keeps on oxidizing due to contact with air.•And their oxidation products•Being acidic•Neutralize Caustic Soda

• IF concentration of ALKALI falls below the certain LEVEL

• The ACID LEUCO is formed, that PRECIPITATE cause a Weaker SHADE with Lower Fastness To RUBBING.

Desired Excess Checked With

• Phenol-Ph-Thalein• Yellow Papers (carries a yellow VAT Dye –

Deep BLUE when Leuco state )• If the PAPER turn BLUE (Adequate Quantity of

DITHIONATE in the Bath)

Metering Devices

• That may automatically control INPUTS OF REDUCING CHEMICALS into the DYEBATH.

• These have improve level of REPORODUCEABILITY OF THE SHADE.

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Some Dyes during dyeing

OVER – REDUCED

Their SHADES becomes LIGHTER or even ALTERED.

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THIS is checked by adding RADOX BUFFOERS

Like SODIUM NITRITE

Or by replacing part of DiThionate with GLUCOSE

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Dyeing is started at temperature of 20 o C.

Which is gradually raised to the optimum.

Dyes of IN and IW groups temperature

can be raised to 80o C

The Dyeing Continued for 45 TO 60 minutes.

ELECTROLYTE is added in portions about 20 Minutes

After the dye Liquor reaches the MAXimum Temperature.

In case or CLASS III

ELECTROLYTE added at beginning

And dyeing is carried out in a cooling BATH after attaining TEMP of 45o C.

However SPECIAL REDUCING AGENTS of the HYDROXY-Alkyl-Sulphinate type may have to be used that can withstand decomposition at these HIGH Temperature.7/2/2008

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AFTER EXHAUSTION OF THE VAT DYES

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•The substrate is usually first washed free of ALKALI

Rinsing

•And REDUCING CHEMICALS before regeneration of the DYE PIGMENTS inside the fibers by oxidation.

OXIDATION

OXIDATION IS FOLLOWED BY:SOAPING: to remove the pigments sticking to the surface layers of the dyed material.

SOAPING

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•After Exhaustion.•The Class I & II dyes are rinsed with COLD WATER.•TO remove Chemicals and surface dye from the fibers.•But the LOW AFFINITY CLASS III DYES are not rinsed prior to their OXIDATION, to avoid their Bleeding.•RINSING also oxidizes part of the DYE but except for the class III DYES.

Rinsing

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•COMPLETE regeneration or the VAT PIGMENT is done with the oxidizing agents.•Common example•H2O2 (hydrogen peroxide)•NaCo3 ( Sodium Carbonate)•Sodium Meta-NITRO-Benzene Sulphonate (Ludigol of BASF)•Dichromate(are Now rarely used due to Environmental issues)

oxidation

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•Concentration of the Oxidizing agent Depend on •Depth of shade•Liquor ratio ( may range from 1-3ml/l Hydrogen peroxide(50%) or 1-4 g/l Perborate or LUDIGOL.•Oxidation time 20 minute at 40-60o C.•PROCESS ACCELERATE through (higher temp--- lower Ph)•The class III Dyes are oxidized by exposure to air.

oxidation

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•AFTER OXIDATION, THE PIGMENT IS IN MOLECULAR FORM AND IS loosely held in between the cellulose molecules.•DURING THE SOAPING the dye molecules , under the influence of HEAT and lubrication by the detergent, migrate and come close to one an other to form SUBMICROSCOPIC CRYSTALS in the fissure/gap of the fiber.•Crystals have different absorption spectra so true solid shade produced after soaping. •Crystallization process increase SIZE OF THE DYE PARTICLE. Increased wet fastness.•Detergent 1 – 2 g/l•Sodium Carbonate 2 g/l•Boils for 30 minutes•Then rinsed hot and cold followed this.

SOAPING

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THE PRINCIPLE OF THIS TECHNIQUE

TO pad the well prepared fabric

With a very fine dispersion (PARTICLE SIZE 0.5 – 1 um) of VAT PIGMENT.

Then to convert the absorb pigment into LUECO FORM

This is followed by

OXIDATION

SOAPING in a Washing Range

The MAIN ADVANTAGE of this process over the reduced-VAT method are as under.

A) HIGH PRODUCTIVITY

B) BETTER LEVELLING

C) GOOD PENETRATION

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Different steps involve in the APPLICATION are:

1. PADDING:

Padder should be mechanically perfect

And exert even pressure throughout the

Width of the fabric.

The pigment dispersion normally contains

a migration inhibitor and

a wetting agent.

The latter is added for quick wetting of the fabric.

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2. DRYING:

It is not necessary for Light Shades

But it HELPS IN LEVELLING and improving colour yield.

It may be done at

Combination of INFRA- RED

HOT FLUE or

CYLINDER DRYER

Incase the cylinder are used

The IR-predryer is must

First few cylinder Teflon Coated as to reduce staining.

Temp 100oC first few cylinders to minimize Migration and two sided effect.

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Diff erent steps involve in the APPLICATION are:

3. DEVELOPMENT:

Method are used for both Development of the DYE PIGMENT:

BOTH the batch and continuous.

In the BATCH PROCESS

Reduction and

Adsorption

Of the dye can be carried out on a Jigger.

DONE in the reduced dyeing method, after ADDING

Alkali and Reducing Agent

IF variation in SHADE due to BLEEDING in not totally eliminated

In the CHEMICAL PADDER 5 – 10 % of the bath is made with the pad liquor7/2/2008

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3. DEVELOPMENT: CONTINUOUS PROCESS

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Dried and Cool Pigment PADDED FABRIC is

• Padded in solution containing• 40 to 80 ml / l 30% Caustic Soda Solution------And 15 to 40 g / l Dithionate in COLD• Also 1-2 G/L WETTING AGENT ------- 0-30 G/L Glauber’s Salt• 30 – 40 ml/l PIGMNET PADDING LIQUOR• PICKUP 70 -80% maintained at Temperature 20-30oC • THE CONCENTRATION OF THE CHEMICALS DEPENDING ON THE DEPTH OF SHADE

STEAMER

• THE FABRIC IS THEN IMEDIATELY ENTER INTO STEAMER• For 20 ---- 40 SECONDS for reduction, sublimation of the dye and its diffusion into fiber.• Steam should be free of air and water droplets and its temp should 102----105oC.• Temp of water seal should be kept low(below 40oC) other wise it becomes a reducing DYEBATH. • Brief Airing passage may follow after steaming to complete exhaustion and partial oxidation of dye.

In Some Cases

• Dithionate is REPLACED with• More stable SODIUM SULPOXY LATE FORMALdehyde (Na HSO2..CH2O)

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OXIDATION AND SOAPING In the Continuous Process,

In a WASHING RANGE

In which fabric is treated in the following different sequences:

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1&2 •Cold Water•Warm water at 40OC

3&4 •Hydrogen Peroxide (50%) 1—3 ml/l Plus 1 to 2 ml/l acetic acid in 2 cisterns•RINSING at 60oC.

5&6 •Soaping in 2 cisterns with 2 g/l of a Detergent and 2g/l sodium carbonate•Rinsing at 60oC…….80oC.

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Fauls are usually corrected by Levelling Out

Partial Stripping or Cross Dyeing

However kept in mind LEUCO VATS have a very high affi nity with cotton fibers and so diffi cult to strip off the dyed goods.

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LEVELLING OUT•Defective material treated in a Blank bath containing 10-20 ml/l Caustic Soda(30%)•50 – 100 G/L dithionate•1-2 g/l wetting /and Dispersing Agent•At 70-80OC for about 30 minutes•After this color removal treatment (15% of the original dye is used is ADDED) and the normal dyeing and oxidation is carried out.

PARTIAL STRIPING•SIMILAR TO THE LEVELLING OUT BUT ADDITION OF DYE complexing agent•2-3 G/L poly vinyl pyrol idine (PVP)•COMPUND FORMS A STABLE COMPLEX with the Reduced dye•Does not allow to exhaustion on the fibre

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