Properties of Azoic Dyes    click here

Azo Dyes     click here

Azoic Dyes      click here

Chemistry of dyeing with Azoic Dyes       click here

Sequence of operation for dyeing with Azoic dyes       click here

Method of dyeing with Azoic Dye        click here

Color performance of Azoic dyeing         click here

Problem in dyeing with azoic color         click here

Protein fibres are not dyed with Azoic dyes       click here

Azoic Dyes:

The dyes which contain insoluble azo group(-N=N-) are known as azoic dyes. These dyes are not found in ready-made form. They dyes are produced through an in-situ process which created colored material directly on the fabric by a reaction between two compounds namely

1.      Diazo compound or diazo base

2.      Coupling compound i.e. Naphthol

Azoic dye is one type of ingrain dyes .e. it is produced during dyeing process inside the fibre. The general reaction of azoic dye formation is given below:

          R-N=N-Cl + R`-ONa                      R-N=N-R`

Diazo compound     Naphthol                        Azoic color

The diazo compound is very often said fast base or diazonium salt of fast base.

The colored substance formed from essential colorless compound is insoluble in water and the washing fastness of this shade is excellent. But it has poor rubbing fastness. Johann Peter Griess of German discovered the diazotized reaction and subsequent reactions of a diazotized reaction in 1858. This discovery was the foundation stone for the immerse edifice of azo dyes. He found that Premuline can be diazotized on the fiber which afterward takes part in couple reaction with an aromatic hydroxyl or amino compound to give a new dye of greater wet fastness.the oldest application of this method was in the dyeing of para red. This para red was produced by coupling diazotized p-notro aniline with β-naphthol. the formation of an insoluble azoic pigment was first patented by Thomas and Holliday in 1880.

Properties of Azoic Dyes:

        i.            Azoic dyes are directly insoluble

      ii.            Excellent light fastness

    iii.            Heat resistance moderate

    iv.            Good wash fastness

      v.            cotton, nylon, polyester, etc are dyed

    vi.            poor rubbing fastness

  vii.            They contain water insoluble Azo group (-N=N-)

viii.            They are fund of solid and powder form.

    ix.             Toxic and hazardous

      x.            Salt increases Substantivity.

Commercial names of Naphthols

Name	Manufacturer
Celcot	Sandoz
Ciba Naphthol	Ciba
Naphtho nil	DU pont
Acna Naphthol	Acna
Amar thol	ADC

Azo Dyes:

        i.            Contains soluble azo group

      ii.            Ready made dyes

    iii.            Do not require diazotization and coupling reactions

    iv.            Do not produce bright shades

      v.            Poor wash fastness property

    vi.            Simple dyeing and printing process

  vii.            Less costly

viii.            No ice is used

    ix.            One bath process

Azoic Dyes:

          i.            Contain insoluble azo group

        ii.            Not ready made dyes

      iii.            Requires diazotization and coupling reactions

      iv.            Produce dark, bright shade

        v.            Wash fastness is good

      vi.            Complicated dyeing and printing process

    vii.            Ice is used

  viii.            More costly

      ix.            Two bath process.

 Chemistry of dyeing with Azoic Dyes:

Dyeing process of azoic dyes perform by three stages,

1)      Naphtholation

2)      Diazotisation

Now for the each of discussion they are described below:

1 ) Naphtholation:

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

…………………………..

This salt solution is used for the impregnation of fabric.

2 ) Diazotisation:

A base containing amino group (-NH₂) reacts with NaNO₂ ( Sodium Nitrite )

to form a solution of diazonium chloride of that base in presence of excess HCl at O - 5°C temperature.

…………………….

3 ) Coupling:

The impregnation material is treated in a bath containing diazonium solution to carry out coupling and thus color is produced inside the fabric. here P^H maintaining is important.

…………….

Sequence of operation for dyeing with Azoic dyes:

1 ) Naphtholation (Naphthol is dissolved in water)

2 )Impregnation (Impregnation of fabric with Naphthol )

3 ) Diazotization ( Diazo component is prepared at temp O - 5°C )

4 )Naphtholated fabric is treated with diazo component solution

5 ) Reaction and color production by coupling

6 ) Removal of excess color from fabric surface by washing

7 ) After treatment

Method of dyeing with Azoic Dye:

There are three method of dyeing with azoic dyes:-

1)      Batch/Discontinuous method

2)      Semi continuous method

3)      Continuous method

1 ) Discontinuous dyeing method with Azoic Dyes in Jigger:

The discontinuous methods of dyeing with azoic dye in Jigger machine can be described as below:

        i.            At first the fabric is passed through water.

      ii.            Then it is impregnated in Naphthol solution containing caustic soda, wetting agent, protective colloid, formaldehyde(if require) and common salt.

    iii.            Then the Naphtholated fabric is wound on a roller.

    iv.            In another bath base, NaNO₂ and HCl are taken to carry out diazotization reaction at low temperature.

      v.            Then the Naphtholated fabric is immersed in that cold diaznium liquor and there the developing of colour occurs by coupling reaction.

    vi.            After dyeing

Ø  It is treated with 3ml/lit HCl

Ø  Rinsed with cold water

Ø  Soap washed with 2gm/lit soap and 1gm/lit soda at boil

Ø  Again rinsed with cold water

2 ) Semi-continuous dyeing method:

In this method the following sequence is maintained:

        i.            Naphtholation in padding mangle

      ii.            Drying

    iii.            Developing color in Jigger

    iv.            After treatment

OR,

        i.            Naphtholation in Jigger

      ii.            Drying

    iii.            Developing color in padding mangle

    iv.            After treatment

3 ) Continuous dyeing method:

        i.            At first fabric is Naphtholation in padding mangle ( 3 bowl padding mangle is used )

      ii.            Then the Naphtholated fabric is passed through a dryer ( two types of dryer : 1 ) Hot air dryer  and 2) float dryer )

    iii.            Then the dried fabric is padded with base solution. Here color is developed.

    iv.            Then the padded fabric is drief by air dryer.

      v.            Then the dried fabric is treated with acid to neutralize excess bases.

    vi.            Then soap solution wash – hot wash and cold wash is performed one after another.

  vii.            Then the rinsed fabric is dried in a dryer.

viii.            Then it is passed through a finishing bath (for example to add optical brightening agent )

    ix.            Then it is passed through a stenter machine to keep its required width. Then the fabric is Calendered.

 Color performance of Azoic dyeing:

1 ) Characteristics of Naphthol:

        i.            Absorption and penetration of Naphthol

      ii.            Concentration of Naphthol  solution

    iii.            Strength of naphthol solution

    iv.            Inherent substantivity of naphthol

2 )  Characteristics of base:

          i.            Strength of base solution

        ii.            Temperature of base (8-12°C)

      iii.            P^H of base

3 ) Salt addition: 

It is used to increase subantivity of naphthol to fibre and mainly to prevent the migration of naphthol into dye bath.

4 ) Produced hue:

          i.            Red color – Produce quickly

        ii.            Blue color – Produce slowly

      iii.            Yellow color – Produce quickly

5 ) P^H :

Recommended range 5-6 . If p^H   increases the rate of production decreases.

6 ) Rate of Couple:

        i.            Fast at higher p^H

      ii.            Slow at lower p^H

    iii.            Normal at p^H =5-6

 7 ) Time: Recommended time 20-30 minutes.

Problem in dyeing with azoic color:

1 ) Uneven shade:

The reason for uneven shade are :

        i.            Faulty liquor movement

      ii.            Faulty fabric movement during naphtholation

    iii.            Improper naphtholation

    iv.            Improper hydroextraction

2 ) Naphthol Migration:

The naphthol in fabric goes to base solution of the bath and reacts with base there other than in the fabric. This is called naphthol migration. This occurs due to:

            i.            Improper selection of naphthol

          ii.            More moisture in fabric

3 ) Slow coupling reaction:

The reasons for slow coupling reaction are:

            i.            Improper selection of base to mix with naphthol

          ii.            Wrong range of p^H in bath, i.e. more acidity.

4 ) Affinity of base towards cellulose:

Azoic base should have affinity towards naphthol. But sometime it shows affinity towards cellulose. This hampers deing.

5 ) Blinding effect:

In case of rayon dyeing a blinding effect may occur, due to

              i.            Presence of alkali during naphtholation

            ii.            Improper value of p^H

          iii.            Presence of alkali in coupling bath.

This blinding effect may be presented by using alkali binding agent (e.g. alum)

6 ) Pigment on fibre surface:

The reason for this are:

              i.            Unfixed dye on fibre surface

            ii.            Improper washing after dyeing

This can be prevented by:-

              i.            Using binding agent for alkali

            ii.            Curing process.

Protein fibres are not dyed with Azoic dyes:

Generally protein fibres are not dyes with azoic dyes, because in naphtholation process a large amount of alkali is used in bath. But protein fibres are weak to alkali. So the alkali of naphtholation bath may degrade fibres. Again naphtholation protein fibres take part in coupling reaction with amino group. So they can not take part in coupling reaction with diazo component again. For these reason protein fibres(silk, wool) are not dyed with azoic colors.