Naphthol Dyes and Dyeing Process, Stripping and Fastness Properties

Last Updated on 26/12/2021

Basic Concept of Naphthol Dyes:
A special group of dyes is formed by the naphthol dyes (Naphthol AS®), where the azo dye is synthesised directly on the fabric. The name of this group of dyes origins from β-naphtholate as one of the first naphthol components used for the azo coupling. Naphthols dyes are also called azoic dyes because their molecules hold an azo group.

The naphthol dyeing process consists of two main steps. In a first step the activated aromatic component (naphthol-component) is deposited on the fabric (= impregnation). The affinity of the naphthol component is rather low, which favors the application in continuous processes and requires attention in exhaust processes. A general scheme is shown in below Figure 1.

Figure 1: General reaction scheme for the application of naphthol dyes

Naphthol dyes are insoluble azo dyestuffs that are produced on the fiber by applying a Naphthol to the fiber and then combining it with a diazotized base or salt at a low temperature to produce an insoluble dye molecule within the fiber. Naphthol dyes are classified as fast dyes, usually slightly cheaper than Vat dyeings; the methods of application are complex and the range of colors limited. A particular strength of naphthol dyes is in the color range of yellow, red, blue and black dyeing.

Azoic combinations are still the only class of dye that can produce very deep orange, red, scarlet and Bordeaux shades of excellent light and washing fastness. The pigments produced have bright colors, and include navies and blacks, but there are no greens or bright blues. Crocking fastness varies with shades but washing fastness is equal to Vat dyeing, generally with less light fastness than the Vats.

Naphthols:
The Naphthols are phenols, soluble in alkaline solution and substantive to cotton, particularly in the presence of salt. The anilides of BON acid (beta-oxynaphthoic acid or BON acid) are soluble in dilute NaOH solution and form the corresponding naphtholate ion. These relatively small molecules are of only low to moderate substantivity for cotton, but they diffuse rapidly into the fibers. In general, the higher the substantivity the better the rubbing fastness as less azo pigment forms on the fiber surfaces. The naphtholate ions are always coplanar and preferably have elongated molecular structures. They behave essentially as colorless, low molecular weight direct dyes. The substantivity increases with increase in the molecular size of the naphtholate ion, but the diffusion rate in the fibers and solubility in dilute aqueous alkali decrease. Addition of salt promotes better exhaustion of the bath, more being needed for Naphtols of lower substantivity.

Bases:
These are available as the free amine base or as amine salts such as the hydrochloride. Many of the amines used are simple substituted aniline derivatives with no ionic substituents. The so-called Fast Color Bases require diazotisation. This usually involves reaction of the primary aromatic amine in acidic solution or dispersion with sodium nitrite, at or below room temperature. Successful diazotisation requires careful weighing of all the chemicals and regard for the supplier’s recommendations. Diazotisation of a primary aromatic amine is often difficult and solutions of diazonium ions are inherently unstable. They undergo decomposition even at low temperature and particularly on exposure to light. Storing prepared diazonium ion solutions is not usually possible.

Properties / Characteristics of Naphthol Dyes:

Naphtol are the only truly cold water dyes available to home dyers which makes them ideal for wax resist (batik) application.
These are sometimes referred to as ice dyes (or) developed colors. These are also called as azoic dyes because of the reason that their molecules contain an azo group.
These dye stuffs are characterized by their beauty, brightness of shades and their all round fastness properties.
Yellow, golden yellow, orange, brown, scarlet, red, maroon, blue, violet and black shades are produced regularly by the cotton dyers.
The first azoic color was discovered by Thomas and Robert holiday who produced a bluish red shade.

General Dyeing Procedure of Naphthol Dyes:
The application of the naphthols is consists of following steps,

Dissolution of the naphthol component.
Exhaustion of the naphthol solution onto the substrate or absorption of the naphtholate ion by the cotton;
Removal of excess naphthol from the material by squeezing, partial hydroextraction or brine washing.
Diazotization of the base component.
Development or treatment with the diazonium ion solution to bring about coupling.
Neutralisation, 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.

Naphthol Dyeing Flowchart:

Precautions in Naphthol Dyeing:

The alkalinity of the naphthol bath shall not drop below the prescribed limit , otherwise the naphthol may precipitate.
Formaldehyde shall not be used when working at more than 50^oC or when the material is to be dried after naphthol application.
Material shall be protected from water spotting, steam, acid and chlorine fumes, and exposure to sunlight after naphthol application.
Use of excess salt in naphthol bath may result into precipitation of the bath.
The temperature is very important in base preparation stem, otherwise diazotization may not take place.
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.
Hydroextraction time must not be too long, which may result into light spots after development.
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.
It is important to use sufficient amount of alkali binding agents, otherwise it will result into precipitation of developing bath.

Stripping Process in Naphthol Dyeing:
Treat the dyed material with Non ionic detergent and 3-5 gpl caustic soda at boil for 15 min cool to 85^oC.

Add 3-5% sod. hydrosulphite for 30-45 min at 85^oC.
Rinse hot and cold
Bleach with 1-2 Gpl Available chlorine for 20 min.
Antichlore and neutralise.
Soap at Boil for 15-20 min.
Cold rinse.

Roles of Different Chemicals in Naphthol Dyeing:

Turkey Red (T.R.) Oil:
Wetting agents for naphthol pasting and dissolution and penetrating agent in fiber in naphthol application.

Caustic Soda:
For solubilising of naphthols and keeping proper alkalinity of naphthol bath.

Formaldehyde:
Protective agent of naphthol impreganated material from effect of air.

Salt:
Electrolyte for exhaustion of naphthol during naphtholation and to prevent the desorption of naphthol in the bath during brine rinsing and development phase.

HCl Acid:
Dissolution of base and to produce nitrous acid in diazotization phase.

Sodium Nitrite:
Producing nitrous acid in diazotization process.

Sodium Acetate:
For neutralization of excess HCl in developing bath.

Acetic Acid:
As an alkali binding agent in developing bath.

Non Ionic Dispersing Agent:
To keep the azoic pigments in fine dispersion phase, which are formed by the coupling of free naphthol in developing bath. Also helps in better color fastness during soaping operation.

Fastness Properties of Naphthol Dyes on Cotton:
Correctly prepared dyeings with azoic combinations on cotton have fastness properties often comparable, or only slightly inferior, to those produced using quinone vat dyes. They complement the vat dyes because of the wide range of orange, red and Bordeaux shades that they provide.

The fastness to washing of azoic combination dyeing on cotton is usually very good to excellent but only after careful elimination of particles of azo pigment loosely adhering to exposed fiber surfaces. Intermediate drying or rinsing of fabric containing the Naphtol, and the soaping of the final dyeing, are key processes ensuring optimum fastness. The same argument applies to rubbing fastness. Deep dyeing that have not been well soaped easily transfer color onto adjacent white fabric, even under conditions of gentle rubbing.

There are two other problems associated with the fastness properties of azoic combinations on cotton. In pale shades, the dyeings often have much reduced light fastness, particularly under humid conditions. Some sensitive azoic combinations also give dyeings of only fair resistance to chlorine and peroxide bleaching.

Uses of Naphthol Dyes:
Naphtol are the only truly cold water dyes available to home dyers which makes them ideal for wax resist (batik) application. This dyes are mostly used in the handloom industry for dyeing of cellulosic fibers. Naphthol dyes are suitable for dyeing of Cotton, Silk and Rayon.

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Mazharul Islam Kiron

Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.