Dyeing Process of Cotton Fabric with Azoic Dyes

by

Introduction

Azoic dyes, characterized by their insoluble azo group (-N=N-), are unique in that they are not available in a ready-made form. These dyes are produced through a chemical reaction between two components: a coupling compound (naphthol) and a diazo compound or diazo salt. The resulting colored substance is insoluble in water and exhibits excellent wash fastness.

Components and Reaction

  • Coupling Compound (Naphthol)
  • Diazo Compound (Diazo Base or Diazo Salt)

The dyeing process involves three main steps: naphtholation, diazotization, and coupling/developing.

Naphtholation

Naphthols, which are insoluble in water, are converted into a water-soluble form by treating them with alkali. This solution is then used to impregnate the fabric.

Recipe for Naphtholation:

  • Naphthol: 3%
  • Glycerine: 1 g/L
  • Caustic Soda: 6 g/L
  • Common Salt: 15 g/L
  • Time: 20-30 minutes
  • Temperature: Room temperature
  • M:L Ratio: 1:30

Procedure:

  1. Mix naphthol, glycerine, and caustic soda in a bath.
  2. Add common salt and mix well.
  3. Immerse the fabric in this solution for a few minutes.
  4. Keep the fabric in the solution for 20-30 minutes at room temperature.

Diazotization

A base containing an amino group (-NH2) reacts with sodium nitrite (NaNO2) to form a diazonium chloride solution in the presence of excess hydrochloric acid (HCl) at 0-5°C.

Recipe for Diazotization:

  • Base Color: 3%
  • HCl: 3%
  • NaNO2: 2%
  • Acetic Acid: 2%
  • Sodium Acetate: 1%
  • Temperature: 0-5°C (with ice)
  • Time: 20-30 minutes

Procedure:

  1. Mix base color, HCl, and NaNO2 in a bath at 0-5°C (with ice).
  2. Add sodium acetate and acetic acid to maintain the proper pH.
  3. Mix the chemicals properly.
  4. Immerse the naphthalate fabric in this solution to produce the diazonium salt.

Coupling/Developing

The impregnated material is treated in a bath containing the diazonium solution to carry out the coupling reaction, producing the final color inside the fabric. Maintaining the pH is crucial during this step.

Procedure:

  1. Treat the impregnated fabric in the diazonium solution.
  2. The color is produced almost instantly.
  3. Immerse the fabric for a few minutes to ensure even dyeing.

After Treatment / Soaping

To ensure the removal of any unfixed dye and to enhance the color fastness, a soaping step is included.

Recipe for Soaping:

  • Anionic Soap: 1 g/L
  • Na2CO3: 0.5 g/L
  • Temperature: Boil
  • Liquor Ratio: 1:30
  • Time: 15 minutes

Procedure:

  1. Prepare a bath with anionic soap and Na2CO3.
  2. Heat the bath to boiling temperature.
  3. Immerse the dyed fabric in the bath for 15 minutes.
  4. Rinse the fabric thoroughly in cold water.
  5. Dry the fabric in a dryer.

Conclusion

Azoic dyes are used to produce bright shades such as orange, red, scarlet, navy blue, and black. This practical experiment has enhanced our understanding of the dyeing process with azoic dyes, which will be beneficial in both academic and professional settings. 

Leave a Comment