Jhilam_1004_C1

Jhilam_1004_C1

by M.M. Shah Zinnat Jhilam -
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Azoic Colors (Azoic Dyes) – A Detailed Discussion

Azoic dyes, also known as Napthol dyes or ice colors, are azo-based dyes formed directly on the fiber through a chemical reaction between two components: a coupling component and a diazo component. This type of dyeing is called azoic dyeing, and it is mainly used on cellulosic fibers like cotton to produce bright, fast colors, especially reds, oranges, and maroons.

1. Principle of Azoic Dyeing

Azoic dyeing is a two-step process where the actual dye molecule is synthesized inside the fiber:

a. Coupling Component (Napthol)

  • Applied to the fabric first.
  • Examples: Napthol AS, Napthol AS-BO, Napthol AS-G, etc.
  • Forms a stable base inside the fiber.

b. Diazo Component (Diazotized Base)

  • Usually aromatic amines (e.g., Fast Red B, Fast Orange GC) that are diazotized in acid and ice water.
  • When reacted with napthol on the fiber, it forms an insoluble azo dye.

2. Application Process

The dyeing process involves:

Step 1: Impregnation with Napthol

  • The fabric is soaked in a solution of napthol and sodium hydroxide.
  • The fabric is then dried or semi-dried.

Step 2: Diazo Development

  • The dried fabric is treated with a diazotized amine in a cold bath (0–5°C).
  • A coupling reaction takes place on the fiber, forming the final azo dye.

Step 3: Washing and Soaping

  • Excess reagents are removed.
  • Soaping improves fastness and color brilliance.

3. Characteristics of Azoic Colors

  • Bright and vibrant shades, especially reds, oranges, and yellows.
  • Excellent wash and light fastness.
  • Forms insoluble dye inside the fiber, ensuring durability.
  • Used for printing and dyeing.

4. Advantages

  • Very bright and fast colors not easily achievable by other dyes.
  • Good resistance to washing, rubbing, and light.
  • Can be used for resist and discharge printing.

5. Disadvantages

  • Complex application process involving multiple steps and low temperatures.
  • Use of aromatic amines, some of which are carcinogenic or toxic (e.g., banned azo dyes).
  • Requires cold temperatures and careful handling.
  • Environmental and health risks associated with aniline derivatives.

6. Environmental and Safety Concerns

  • Azoic dyes are partially banned or restricted in many countries due to the release of carcinogenic amines like benzidine and naphthylamine.
  • Many azoic components are listed under REACH and OEKO-TEX banned substances.
  • Safer alternatives or modified azoic systems are under development.

7. End Uses

  • Printed and dyed cotton fabrics
  • Traditional textiles (e.g., Batik, tie-dye in South Asia)
  • Flags, bandanas, and garments requiring bright fast shades

Conclusion

Azoic colors offer excellent brightness and fastness, especially for reds and oranges, but are falling out of favor due to environmental and health concerns. Safer dyes like reactive dyes are now more widely used as alternatives in most modern textile applications.