Azoic dyes (Naphthol dyes)

 Azoic dyes (Naphthol dyes)

Introduction
The discovery of diazonium compounds by Peter Griess in 1858 was exploited later on to develop a colored insoluble azo compound. In 1876, he demonstrated that azo dyes were readily obtained by coupling diazotized sulphanilic acid with various bases derived from benzene. However, insoluble azoic dyes were introduced in 1880 by Reed Holiday and generally referred to as ice colors or azoic colors. The first azoic dye was obtained from 2-naphthol and diazotized 4-nitroaniline, known as Para Red. Betanaphthol became a very popular component in the dyeing of azoic dyes.

Azoic colors give wide range from yellows, orange, scarlet, red, blues, blacks, but there are only few greens. Also called Naphthol Dyeing, is recommended to get the bright and fast shades in maximum depth similar to vat dyeing. Lemon yellow, dark orange, blood red, pink, red -orange, red brown, yellow brown, tan, dark red, violet, brown violet, coffee-violet, bright blue, china blue, navy blue, blue-black and royal blue are dyed successfully under this style. Except lemon, gold orange, brown, scarlet, red, blue, violet and black, rest colors have problems of dry and wet rubbing fastness; though this defect is removed by using efficient soaping and washing after dyeing. Few shades which are difficult to dye in vat dyes (due to no dyes available) are successfully dyed in this style. These are orange, scarlet, green and yellow colors. The light and chlorine fastness depends on the combination used and can be as high as 7 and 5, respectively. Naphthol dyeing is employed for cheap quality fabric like khaddar, poplin, twill and net fabric. Blended fabrics are not recommended.

What is Azoic Dye?

 Azoic dye is that which has insoluble azo group. Azoic dyes do not get in ready-made form in the market. The dyes containing azo groups – N = N – are called Azoic Dye. Azoic dye is produced by the combination of Base/Salt and coupling component (Napthol) or Diazo component. Azoic color is different from other dyes and its application process is also different. Dye formation in fiber occurs on the basis of coupling reaction.

RN₂Cl + Rʹ -ONa → R-N=N-Rʹ -OH

 

Azoic Color is called Ice Color or magic color because it needs low temperature and after coupling color is produced within a few seconds. Color fastness properties are good to excellent.

 

General Procedure For Dyeing Cellulosic Material With Azo Dyes

Two bath of individual recipe is used for dyeing with azo dyes.

     - Impregnation Bath.
     - Developing Bath.

 

Why Azo Dye Is So Called?

This color contains insoluble azo groups in its chemical structure.  – N=N – Azo group. That’s why this color is called Azo color.

Azo Color Called ‘Ice’ And ‘Magic’ Color

 

Ice Color

The coupling component is finally soluble in diazotization reaction. Diazotization is a chemical reaction where the base (Aromatic Amine) is to be converted to a solubilized form by the chemical reaction with NaNO₂ at ice temperature (0 – 5˚C).

Magic Color

Two steps are required by dyeing with azoic color. In the first step textile goods are dyed by Napthol color. In the 2nd bath, Base or Salt is used for dyeing. After 2 – 3 minutes dyeing in 2nd bath, the azoic color is seen in the textile goods magically. For this reason, this dye is called Magic color. 

Azo Dyes Are Called ‘Napthol’ And ‘Pigment’ Color

 

Napthol color

Azoic coupling components are insoluble in water. To make them soluble in water the textile materials are impregnated in a solution of Napthol and NaOH. As the first coupling component is Napthol color.

Pigment color

Azoic dyes contain Azo group and final color is insoluble in water, so it is called Aoic Pigment.

                            Base   +   Napthol → Azoic dyes

                            Soluble   Soluble     Insoluble

Properties/Significance of Azo Dyes

 

·         These dye stuffs always contain Azo groups in its chemical structure.

·         Azoic dye is directly insoluble in water.

·         Light fastness property is admirable.

·         Brightness of shade is also admirable.

·         Coupling is necessary for color production

·         These dye stuffs are always used in dyeing cellulosic material.

·         Here dyeing operation is completed by two bath arrangement. One is called impregnation bath and another is called developing bath.

·         Alkali resistance is poor to good. Index is 2 to 4.

·         Suitable for lighter shade dyeing (Light resistance: Poor to very good, Index 2 – 7).

·         This dye stuff is called developed dye due to formation of dye in fiber during dyeing process.

·         Addition of salt increases the substantivity.

·         Napthol dispersed in alcohol, T.R. oil.

 

Trade name of Azoic color                                   

Chemical Structure Of Azoic Dye

 

General dyeing procedure of Naphthol dyes

The application of the naphthols consists of following steps:
1. Dissolution of the naphthol component.
2. Exhaustion of the naphthol solution onto the substrate or absorption of the naphtholate ion by the cotton.
3. Removal of excess naphthol from the material by squeezing, partial hydroextraction or brine washing.
4. Diazotization of the base component.
5. Development or treatment with the diazonium ion solution to bring about coupling.

6. Neutralization, soaping at the boil to remove superficial pigment, followed by rinsing and drying.

The process can be carried out in almost any type of dyeing machine determined by the form of the goods.

 

Advantages of Azoic Color:

 

There are some benefits of azoic color. The following are the advantages of azoic color. They are-

 

Firstly, color fastness of azoic color is good.

Secondly, it has wide color range.

Also, it has ability to absorb light.

Lastly, the environmental impact is low.

Above all, azoic Color uses for coloring small amounts of textile materials dyeing. Therefore, it is suitable for small-scale industries. Besides, the azoic dye dyeing process is not popular as reactive dye or disperses dye dyeing. So, select your dyes as you need.

 

Precautions in Naphthol dyeing

 

1. The alkalinity of the naphthol bath shall not drop below the prescribed limit otherwise the naphthol may precipitate.

2. Formaldehyde shall not be used when working at more than 50°C or when the material is to be dried after naphthol application.

3. Material shall be protected from water spotting, steam, acid and chlorine fumes, and exposure to sunlight after naphthol application.

4. Use of excess salt in naphthol bath may result into precipitation of the bath.
5. The temperature is very important in base preparation stem, otherwise diazotization may not take place.
6. Sodium acetate must be added to the developing bath just before the use, otherwise base will become unstable due to fall in concentration of HCl.

7. Hydroextraction time must not be too long, which may result into light spots after development.
8. Material shall be rinsed without delay after developing, otherwise the mechanically held excess developing liquor will undergo some decomposition and cause deposition of dark colored spots, which will be difficult to remove.

9. It is important to use sufficient amount of alkali binding agents, otherwise it will result into precipitation of developing bath.