6 Vat dyes and Azoic dyes

V. RameshBabu

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
3. hydrosulphite (reducing agent) in the presence of caustic soda.
4. Vat dyes have excellent washing and light fastness.
5. Vat dyes are very expensive compared with the other classes of dye.
6. 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), 2 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. Quick test for vatting: In order to check whether vatting is complete or not, a drop of the vatting solution is placed on a filter paper and observed. If the liquor is only incompletely vatted, insoluble particles i.e. unvatted dye, will be visible at the centre of the place that was spotted. When the dye is perfectly solublised, the dye solution spreads out uniformly in a radial direction. The observation of the extent of vatting should be done quickly, when the dye is still in the reduced state, i.e. before it gets oxidised by air.

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.

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.

Dye bath assistants or dye bath auxiliaries

These are chemicals or commercial products that are added to the dye bath to facilitate effective and quality dyeing.

An exhausting agent, e.g. common or Glauber’s salt, is added, especially for the IK and IW classes of dye, in dissolved form during dyeing. Such additions are made in two equal instalments; dyeing after each addition of salt is usually for at least 15 minutes.

Vat dyes vary in their affinities for the fibre and hence vary in their rates of dyeing. Levelling agents are used for dyes that tend to rush into the fibre and result in uneven dyeing. These agents retard and control the rate of dyeing such that level dyeing results are produced.

Retarding the initial rate of dyeing may be achieved by adopting the following methods.

  a)Starting the dyeing at lower temperatures and then heating the dye bath slowly to the required temperature.

b)By adding the dye in two/four instalments, thereby reducing the initial concentration of the dye bath.

c)By incorporating suitable auxiliaries or dyeing assistants called levelling agents or retarders. These chemicals essentially ‘retard’ or slow down the dyeing and function via two different mechanisms.

i) They compete with the dye molecules for the available sites in the fibre and thereby reduce the effective rate of dyeing.

ii)Alternatively, they form loose complexes by combining with dye molecules; the fibre cannot absorb these complexes, as they are very large. In the later stages of dyeing, when the temperature is higher, these complexes are broken down and the dye molecules are set free for adsorption by the fibre surface.

Some of the commonly used retarders in vat dyeing are, Dispersol VL (ICI) Remol OK Albatex PO Evasol N (Indokem), etc

Right up to the end of the dyeing step, the dye absorbed by the fibre and the dye in dye bath must be kept in a completely reduced state, otherwise, the dye is oxidised and this leads to the production of dull or even uneven shades. Sufficient amounts of sodium hydrosulphite and sodium hydroxide are thus maintained in the bath even during the completion stage of the dyeing process.

Another very important point to note is that the material is always kept immersed in the dye bath during dyeing, so as to prevent local oxidation, i.e. oxidation in some portions of the material. This is most important for open beck dyeing of yarn.

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.

Azoic dyes

Introduction

•  The dyes containing insoluble azo group (-N=N-) are known as azoic dyes.
• These dyes are not found in readymade form. Azoic dyes are produced by a reaction between two components. The components are:

    1.  Coupling Compound (Naphthol)

2.  Di-azo –compound or diazo base or diazo salt.

• The colored substance formed from this colorless compound is insoluble in water and washing fastness of this shade is excellent.

Azoic Dye – Properties

• The word ‘Azoic’ is the distinguishing name given to insoluble azo dyes that are not applied directly as dyes, but are actually produced within the fibre itself.
• This is done with impregnating the fibre with one component of the dye, followed by treatment in another component, thus forming the dye within the fibre.
• The formation of this insoluble dye within the fabric makes it very fast to washing.
• The deposition of the dye on the surface of the fibre produces poor rub fastness, but once the loose dye is removed by boiling the fabric in soap, the dyeing becomes one of the fastest available.
• Normally it is dyed in cold for all natural fibers
• Naphtol dyes are not sold in the form of a “finished dye” but in form of their components (Insoluble azo base & fast colour coupling compound) which combine on the fibre to produce a water insoluble azo dye of exceptional fastness properties.

Reaction of Azoic Dye

Naphtholation:

Naphthols are insoluble in water and they are converted into water soluble compound by treating with alkali.

Diazotization:

A base containing amino group (-NH2) reacts with the NaNO2 (Sodium Nitrite) to form a solution of diazonium chloride of that base in presence of excess HCl at 00-50C temperature. Coupling/ Developing:

The impregnated material is treated in a bath containing diazonium solution to carry out to coupling and thus color is produced inside the fabric. The PH maintaining is important.

What are azo compounds?

• Contain the -N=N- group.
• Where R and R’ are arene groups more stable than alkyl groups.
• Azo group is stabilised by becoming part of extended delocalised system.
• Result of a coupling reaction between a diazonium salt and a coupling agent.

Fastness Properties of Azo Dyes

• The formation of this insoluble dye within the fabric makes it very fast to washing.
• The deposition of the dye on the surface of the fibre produces poor rub fastness, but once the loose dye is removed by boiling the fabric in soap, the dyeing becomes one of the fastest available.
• They have excellent coloring properties, again mainly in the yellow to red range, as well as good lightfastness.
• The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been absorbed on the pigment carrier.

Application of Azoic Dye

• This dyes are mainly applied to cellulosic materials.
• Because of high degree of brightness, ease of application and excellent fastness properties azoic combinations (naphthol and bases) are widely used in printing on cellulose fiber fabrics.
• Normally it is dyed in cold for all natural fibers.
• Azoic dyes are used for producing bright shade of color such as orange, red, scarlet, navy blue, and black color.

REFERENCES and URLs

1. Colour Chemistry – Synthesis, Properties and Applications of Dyes and Pigments, Zollinger H., 2nd ed., Weinheim – VCH, 1991
2. Industrial Dyes – Chemistry, Properties, Applications, Hunger K. (Ed), Wiley-VCH, Weinheim, 2003
3. Chemistry of Synthetic Dyes and Pigments, Lubs H. A., Robert E Krieger Publishing Company, New York, 1977
4. Colour and Chemical Constitution of Organic Dyes, Griffiths J., Academic Press, 1976