5 Vat Dyes and Fibre Reactive Dyes

R. Sukanya Devi

epgp books

 

 

Vat Dyes

 

Introduction

 

Vat dyes owe their name to the fact that the foremost member in this series, indigo, was applied to textiles by means of a fermentation process in wooden vessels commonly known as ‘vats’. The dyes are amongst the oldest natural colouring matter derived from origins like vegetable (plants, stems, etc.), insects and animals.

 

Vat dyes provide textile materials with the best colour-fastness of all the dyes in common use. The fibres most readily coloured by them are the natural and man-made cellulosic fibres, like cotton, viscose rayon, etc.

 

Properties of Vat Dyes

  1. Vat dyes are insoluble in water.
  2. They are generally converted to their soluble “leuco-state” by means of sodium hydrosulphite (reducing agent) in the presence of caustic soda.
  3. Vat dyes have excellent washing and light fastness.
  4. Vat dyes are very expensive compared with the other classes of dye.
  5. They are available commercially in different forms such as powder fine, micro-fine, ultra-disperse, highly concentrated, supra-paste and double-paste.

Application of vat dyes

 

The application of vat dyes to cotton goods involves the following four basic steps.

 

1.Vatting

 

This step converts the insoluble commercial vat dye powder into its soluble sodium salt.

 

2. Dyeing

 

This step brings about absorption of the reduced and dissolved vat dye by the fibre from an alkaline reducing bath in the presence of exhausting or retarding agent, as necessary.

     3.Oxidation

 

The soluble sodium salt of leuco-vat dye absorbed by the fibre is converted into its original insoluble form. This is done either by air or chemical oxidation.

 

4. After-treatment

 

The dyed material is subjected to a treatment with boiling detergent solution to get the proper shade (hue) with brilliance and good fastness properties.

 

Step1: Vatting

 

The conversion of insoluble vat dyes into their soluble form involves the following steps:

 

a)  Reduction of the original insoluble vat dye into weakly acidic leuco-vat (or vat-acid) form.

b)  Neutralising the leuco-vat dye with sodium hydroxide to give a water-soluble sodium salt of the leuco-vat dye.

 

In order to keep the dye in soluble sodium salt form, a sufficient excess of both caustic soda and hydrosulphite of soda is required to be maintained in the bath. When either alkali or hydros or both are present in insufficient quantities a complete solution is not obtained.

 

The stability of the vatted dyed solution is greatly affected by improper vattting conditions. It is therefore very much important to maintain (i) therecommended conditions of temperatureand (ii) theoptimum concentrations of hydros and caustic soda during vatting and dyeing.

 

Methods of vatting

 

There are different methods of converting vat dyes into their substantive form i.e. sodium salt of leuco-vat dye. Generally vatting is carried out either by,

 

(i) Stock vat method or

(ii) Long liquor method (in full volume in the dye bath itself).

 

Stock vat method: In the stock vat method, the dye powder is pasted with an equal amount of Turkey Red Oil and hot water at 50 – 60º C is added. This is followed by addition of the required amounts of caustic soda and hydros; the solution is allowed to stand for the recommended time (10 – 20 minutes) with occasional stirring. The vatted dye solution is a concentrated solution and is hence called the ‘stock vat’. It is added in two or four portions to the blank bath. A blank bath is a bath that is usually prepared in the machine vessel; it contains the required quantity of water and the required amounts of caustic soda and hydros kept at the recommended dyeing temperature; a blank bath contains no dye.

 

Long liquor vatting: In the long liquor method of vatting, a dispersion of the vat dye is added to the full volume of the dye bath in the machine vessel itself. (A dispersion is a mixture of dye particles in water.) The recommended quantity of caustic soda is then added and the solution is mixed well. It is then brought to the recommended vatting temperature (50-60ºC), usually by means of steam. The recommended quantity of hydros is then slowly and uniformly added all around the bath with stirring. Both the colour and consistency (viscosity) of the bath will change with the addition of hydros. The colour change will depend upon the dye used and the consistency becomes more viscous. The bath is allowed to stay, with occasional stirring, until the vatting is complete. Vat Blue RSN is best vatted by this method.

 

Step 2: Dyeing

 

Actual dyeing takes place in the second step when the sodium salt of the leuco-vat dye is adsorbed on the surface of the fibre and then diffuses into the interior of the fibre. The adsorption and diffusion are possible only when the dye is completely dissolved, since the insoluble vat dye has no affinity for the fibre. It is therefore very much essential to maintain recommended concentrations of caustic soda and hydros, and the temperature of dyeing.

 

Precautions in dyeing: Truly speaking, a slight excess of caustic soda and hydros is essential to keep the dye in soluble and substantive form during dyeing. (Refer to the table on Page___ for the recommended quantities of these chemicals for the different classes of vat dye.)

 

Sodium hydrosulphite gets decomposed by atmospheric oxygen and produces acidic products, which in turn neutralise a part of the alkali. Hence the concentration of sodium hydroxide in the dye bath decreases. To maintain the dyestuff in soluble form, fresh quantities of caustic soda and hydros are added.

 

If these conditions are allowed to worsen even towards the end of the dyeing process, though exhaustion has taken place, premature oxidation of the leuco-vat dye taken up by the fibre results in patchy dyeing, showing dark and light areas in the dyed material. It is therefore essential to maintain fairly alkaline and sufficiently reducing conditions in the dye bath from the beginning to the end of dyeing.

 

Test for ideal dyeing conditions: The presence of caustic soda and hydros during dyeing is checked respectively by means of phenolphthalein paper and vat yellow paper as follows.

 

Hydros: Satisfactory presence of hydros is indicated by the appearance of a blue colour within seconds of spotting the vat yellow paper with the dye bath solution. If the blue colour does not appear, a fresh quantity of hydros needs to be added.

 

Caustic soda: The alkalinity, i.e. presence of caustic soda, is checked with phenolphthalein paper, which changes from white to intense pink in the presence of alkali.

 

Dyeing temperature: The dye bath temperature is also checked frequently with an accurate mercury thermometer and kept at the recommended temperature.

 

Dyeing procedure

 

a)   The dye bath is set with the required amount of water, caustic soda and hydros and maintained at the recommended temperature for the particular class of dye being used. The thoroughly prepared textile material is introduced and worked for 10 minutes in this blank bath. As this treatment prepares the material with the chemicals that keep the dye in solution, the blank bath is also called the ‘sharpening bath’.

b) The vatted dye solution is divided into two equal portions. One of these is added to the blank bath and mixed well. The material is run in the dye bath for 10 minutes. The second portion of vatted dye is then added to the bath and the dyeing is continued at the recommended temperature for 45 – 60 minutes. The dye is added in portions to control the initial rate of dyeing and bring about level dyeing, In some cases, the dye is added successively in four portions.

 

Step 3: Oxidation

 

In this step, the dyed goods are subjected to an oxidation treatment for conversion of the sodium salt of the leuco-vat dye taken up by the fibre into the original insoluble vat dye. The oxidation process plays an important role and needs to be carried out under controlled conditions. Improper oxidation leads to faulty dyeing, especially unevenness, inadequate fastness properties and shade deviations in terms of hue, tone and depth of shades.

 

On removal from the dye bath, the material is squeezed or hydro-extracted for complete removal of excess liquor containing unexhausted dye, sodium hydroxide and sodium hydrosulphite. It is then subjected to oxidation treatment, which can be done in one of the following ways:

 

a) Air oxidation or b) Chemical oxidation

 

During airing, the dyed goods are exposed to atmospheric oxygen while in chemical oxidation, use is made of various chemicals to accelerate the oxidation process.

 

Some of the important oxidising agents used for the purpose are hydrogen peroxide, sodium perborate, and sodium hypochlorite. The concentration depends on the agent used, e.g. if 35% hydrogen peroxide is used, a concentration of 1-2 ml/l may be used.

 

The oxidation treatment has to be carried out under mild conditions depending upon the chemical chosen (i.e. low temperature, minimum time and low concentration) to prevent the oxidation of cellulose.

 

Step 4: After-treatment

 

The after-treatment for vat-dyed material consists of soaping it at the boil and then washing it to remove the soap.

 

Why is soaping of vat-dyed goods necessary? The leuco-vat dye absorbed by the fibre is converted into its original insoluble form inside the fibre during oxidation. At the same time dye particles in the residual liquor (left over after the squeezing of the dyed material) are also oxidised and get loosely deposited on the fibre surface. These dye particles are not firmly fixed to the fibre, and if not removed, will contribute to poor rubbing fastness properties of the dyed material.

 

It is therefore necessary to treat the dyed goods in a hot detergent bath to develop the true shadeandgive good fastness properties. During soaping, the loosely held dye particles are removed and held in suspension to prevent them from depositing back on the material.

 

Apart from contributing to the fastness properties, the soaping treatment allows the dye molecules to aggregate and form large crystals. This imparts the true shade to the material. Moreover the resulting crystals are more resistant to subsequent chemical or physical treatments. The aggregation of dye particles is accelerated in the presence of detergent at boiling temperature.

 

The soaping process: A boiling solution of soap or a synthetic detergent is used in this process. It is advantageous to use soda ash in the soaping bath when soap is used. The treatment is carried out at the boil in order to achieve the true fastness properties of the dyed material. In a typical soaping treatment, the goods are treated at the boil in a bath containing 3 g/l soap and 1 g/l soda ash.

 

The soaping treatment is followed by a washing treatment for 20 – 30 minutes until the material is free from alkali. The material is finally hydro-extracted and dried.

 

Reactive Dyes

 

Properties of Reactive Dyes

  • The reactive dyes, on account of the sulphonic acid groups in their molecules, are readily soluble in water.
  • Unlike direct dyes, the reactive dye molecules tend to be much less substantive to cotton and require much larger quantities of salt for exhaustion.
  • These dyes, unlike any other class of dyestuff, react and combine chemically (forming covalent bonds) with cellulose. It is this characteristic that gives them the name “reactive dyes”.
  • The highly reactive dye molecule, called the cold-brand type of reactive dye, has two chlorine atoms. Its reactivity can be reduced when desirable by blocking one of the reactive chlorine atoms, giving the hot-brand type of reactive dye.
  • Reactive dye molecules are not as long as direct dye molecules. Short molecules have two advantages: (a) clarity and brightness of hue and (b) easy penetration and therefore good levelling.
  • Textile materials coloured with reactive dyes have moderate to good light fastness and washing fastness.
  • Textile materials coloured with reactive dyes have to be thoroughly rinsed and soaped. Reactive dyes can react with the hydroxyl groups of the water molecule to produce dye molecules with poor substantivity for the fibre. In fact it is these molecules, which have to be removed by the washing-off process, involving soaping at the boil and rinsing. If these ‘hydrolysed dyes’ are not removed, poor rubbing fastness will result.
  • The formation of the covalent bond between the dye and fibre occurs under alkaline conditions. The presence of acids may reverse this process. Perspiration and atmospheric pollution, which are both slightly acidic may affect textile materials coloured with reactive dyes and result in some fading.
  • Reactive dyes can be applied to cellulose fibres by the exhaust dyeing (batch dyeing), semi-continuous dyeing (pad-batch) and continuous dyeing techniques.

Dyeing procedure

 

a)    Preparation of cotton for dyeing

 

Bleached material prepared for dyeing must be free from alkali. Where necessary, treat it in a liquor containing 0.5 – 1.0 part of acetic acid (30%) per 1000 parts of water prior to adding dye solution. This treatment prevents premature localised fixation and improves levelling of the dyes.

 

b)    Preparation of dye solution

 

The M-brand dye powder is pasted with cold water and dissolved by adding water at 50°C. The required quantity of water is then added with stirring to dissolve the dyestuff.

 

c)    Dyeing Process

 

The M-brand dyes are cold-dyeing dyes and are therefore dyed at room temperature.

 

(i)   Exhaustion step: The bath is set with the required quantity of water made up to the M:L ratio suitable to the machine being used. The bath pH is then adjusted to 6.5 to 7.0 with acetic acid. The material is run in this bath for 10 minutes at room temperature. The function of acetic acid is to ensure that there is absolutely no alkalinity in the bath during the exhaustion step. Any alkalinity in the incoming fabric will be neutralised by the acetic acid. Alkalinity is avoided here to prevent hydrolysis of the dye in the bath.

 

After the run in the acidic-pH bath, add pre-dissolved dye in two portions, giving 15 minutes after each addition. Then add pre-dissolved common salt or Glauber’s salt to the bath in two portions and run the material for 15 minutes after each addition.

 (ii) Alkali-fixation step: When as much exhaustion of dye into the material has taken place, add the required quantity of pre-dissolved alkali (soda ash) in two portions and dye for 15 minutes after each addition. Then drain the dye bath. The quantities of salt and soda ash for different liquor ratios are given in the following table.

(iii) Washing-off and Soaping(After-treatment)

 

Washing: For maximum fastness, wash the material efficiently after dyeing. The material is washed in cold running water for 10 minutes.

 

Soaping: Treat the washed material with 1 – 2 g/l of neutral soap at the boil for 15 minutes and wash it free of soap, first with hot water, then with cold water. Ordinary soaps mostly contain alkali and such soaps tend to strip (remove) the dye from the fibre, hence neutral soap is used.

 

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REFERENCES and URLs

 

  1. Clark M, Handbook of textile and industrial dyeing: principles, processes and types of dyes, wood head Publishing, 2011
  2. Richard Aspland J, Textile Dyeing and Coloration, AATCC, 1997, ISBN 0-9613350-1-7
  3. Arthur D Broadbent, Basic Principles of Textile Coloration, Society of Dyers and Colourists, 2001
  4. Wilfred Ingamells, Colour for textiles A user’s handbook, Society of Dyers anColourists,1993
  5. Shenai V A, Technology of Textile Processing : Technology of Dyeing, Sevak Publications, 1985