Acid dye     click here

Properties of acid dye      click here

Classification of acid dye     click here

Theory of dyeing of acid dye    click here

Acid dye:

Aciddyes are sodium salts of sulphonic acid and a few carboxyl groups present. They are soluble in water. Acid dyes are so called due to:

The original number of the class are applied in a bath containing mineral or organic acid.

As they are sodium salts of organic acids and active color component is anion.

The acid dyes have a direct affinity towards protein fibres and polyamide fibres.

The first acid dye was made by Nicholson in 1862, when he sulphonated aniline blue, it was known at that time as Blue De Lyons. Other basic dyes were subsequently sulphonated to convert them into acid dyes, making them more easily applicable to wool than the present basic dyes.Acid Magenta(C.I. Acid Violet 19) is obtained by the sulphonation of magenta.

Properties of acid dyes:

        i.            Acid dyes are water soluble.

      ii.            Good light fastness

    iii.            Direct affinity toward protein fibres and polyamide fibres.

    iv.            Bright shade possible

      v.            Sodium salt of organic acid.

    vi.            They are always applied in organic or inorganic acid medium.

  vii.            Most of cellulose fibres will not exhaust acid dyes.

viii.            They produce brilliant shades.

Classification of acid dyes:

The classification of acid dyes are varies way they are described below:

A)    According to the application:

Class-1 : Strong acid dye

Class-2 : Weak acid dye

Class-3 : Natural acid dye

Class-1(Strong acid dye):

        i.            Good leveling property.

      ii.            Good light fastness.

    iii.            Wet fastness is also well

    iv.            During dyeing pH maintained 2-3

      v.            Example of strong acid H₂SO₄.

    vi.            HCl is normally used in dyeing

Class-2(Weak acid dye):

          i.            These are less level dyeing.

        ii.            Well wet fastness.

      iii.            P^H  5.2-6.2 is maintained.

      iv.            Cellulose fibres can be dyed.

        v.            Weak organic acid is used such as CH₃COOH.

      vi.            Glaubers salt should not be used

Class-3(Neutral acid dye):

          i.            Applied in neutral bath.

        ii.            Good light fastness

      iii.            Not leveling dyeing

      iv.            Wet fastness good.

        v.            lower temperature used

B ) According to the chemical structure:

        i.            Nitro:

Group – NO₂

Example : Naphthl Yellow

      ii.            Nitroso:

Group : -NO

Example : Naphthol Green B

    iii.            Mono azo :

Group : -N=N-

Example : Mentanil Yellow

    iv.            Di-azo:

Example :Cloth Red G

      v.            Tri pkenyl methane:

Example : Acid Magenta

    vi.            Xanthene :

Example : Lissamine Rhodamine B

  vii.            Azine :

Example :

Royal Fast Blue EB

viii.            Quinoline :

Example : Quinoline Yellow PN

    ix.            Anthraquinone :

Example : Carbolan Green G

C ) according to the dyestuff particles:

1)      Molecular Split acid dyes.

Example : Acid Orange GG, Alizarine Light Blue etc

2)      Aggregated acid dyes. Example : Polar Yellow R

Trade Name of Acid Dyes:

Trade Name	Manufacturer	Country
Xylene	L.B. Holiday & Co.	UK
Anthralan	Hoechst	Germany
Kiton	Ciba-geigy	Switzerland
Sulfonine	ICI	UK
Supranol	Bayer	Germany

Theory of dyeing of acid dye:

Acid dye are easily applied on wool, silk and nylon fibres. During dyeing, H₂SO₄ or CH₃-COOH is present due to perform dyed. They are water soluble and produce color anion and sodium cation.

RSO₃Na ---------- RSO₃^- +Na⁺

Wool fibre polymer may be represent as:

H₂N-W-COOH ------------- Wool

So there are present amino group of polymer part and other part contain carboxyl group.

Under certain condition, The hydrogen ion attached to the carboxyl group is transferred to the amino group at the other end of the macromolecule so that the two ends of the keratin molecular chain acquire opposite electrical charges:

H₂N-W-COOH----------------H₃N⁺-W-COO^-

Thus when wool fibre is entered in a acid solution, negatively charged carboxylate group take up hydrogen ions released from hydrochloric acid group and the chloride anions, also released from HCl are absorbed by positively charged amino ends of the keratin macromolecule.

HCl ====H⁺ + Cl^-

NH₃⁺-W-COO^- H⁺ Cl^-  ========= ClNH₃-W-COOH

When wool is immersed in acid solution, the concentration of acid is decreased by the absorption of acids with the formation of an electrically positive site whenever an amino group is present in the keratin molecule i.e. acids act as exhausting agents while dyeing acid dyes on wool. This absorption continues until equilibrium absorption and sodium chloride is formed as a result of forward reaction.Thus if the salt reaction is increased in the dye bath, the rate of backward reaction is increased. In fact act as a retarding agent in the dyeing of acid dyes on wool. In order to get uniform dyeing results the dyeing is started in the presence of salt with a small amount of an acid in the cold, followed by raising the temperature gradually. In order to exhaust the dye bath more of the acid is added to the dye bath and the dyeing aontinued..