Bleaching Process in Textile and Required Agents

In this article, we are going to learn about the types of bleaching and its various agents. There are many objectives of the bleaching process. This is one of the primary stages of the dyeing process of knit fabric. So, let's get started.

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Bleaching

Bleaching of textile material is a chemical or commercial process that can be defined as the destruction of natural coloring matters to impart pure, permanent, and basic white effects, suitable for the production of white finishes, level dyeing, and desired printed shade with the minimum or no tendering (degradation) or without diminishing the tensile strength.

Objectives of Bleaching

• Ensure a pure and permanent basic white color.
• Destruction of natural coloring matters from the fabric.
• Increased absorbency for dyeing operations.
• Ensure level dyeing property.
• Make textile materials suitable for subsequent processing (dyeing, printing, etc.)

Bleached Fabric

Bleaching Agents

Two types of bleaching agents-

1. Oxidizing bleaching agent
2. Reducing bleaching agent.

Oxidizing Bleaching Agents

1. Ozone - O₃

2. Hydrogen peroxide- H₂O₂
3. Calcium hypochlorite - Ca(ClO)₂
4. Sodium hypochlorite - NaClO
5. Sodium chlorite - NaClO₂
6. Potassium permanganate- KMnO₄
7. Per acetic acid- CH₃CO₃H
8. Bleaching powder-Ca(OCl)₂
9. Potassium dichromate- K₂Cr₂O₇
10. Sodium dichromate- Na₂Cr₂O₇
11. Potassium chlorite - KClO₃
12. Sodium peroxide- Na₂O₂

Reducing Bleaching Agents

1. Zinc dust-Zn

2. Stannous chloride - SnCl₂
3. Ferrous sulfate - FeSO₄
4. Sulfur dioxide - SO₂
5. Sodium bisulfate - NaHSO₄
6. Hydrogen sulfide- H₂S
7. Sodium sulfate- Na₂SO4
8. Hydrogen - H₂
9. Carbon - C

Bleaching Process

1. The most common bleaching processes are-
2. Hypochlorite bleaching for 100% cotton (Less used)
3. Hydrogen peroxide bleaching for 100% cotton (Most used)
4. Sodium chloride bleaching: Only used for special sectors and only for polyester and cotton blends.

Effects of pH

1. The effect of pH is very important during hypochlorite bleaching.
2. When pH 2-4: Produces more chlorine, which is toxic and corrosive.
3. If pH 4-6: More stable HOCl. HOCL does not divide into H⁺ and OCl^-
4. When pH 7 is neutral: The decomposition of NaOCl is very high. So it breaks down cellulose. This point is called “The Damage Point”.
5. Quick bleaching at pH 7-8: Quick bleaching tends to cause degradation of cotton fabric.
6. When the pH is 9.2-11: The fabric becomes neutral and more stable.
7. While pH 11-13: is the perfect range, it requires more time. If pH= 11, bleaching needs 130 hours, and if pH= 13, bleaching needs 40 hours to be finished.

From the above chart, it is clear that bleaching with hypochlorite should be done in an alkaline medium and not between the values of pH 2-7. The ideal pH for hypochlorite bleaching is 9.2-11.0, and the pH is maintained at 10. For pH maintenance, 0.2-0.8% Caustic soda and 2-3% Soda ash are used.

Hypochlorite Bleaching Recipe

• Hypochlorite solution - 3-5° TW
• Caustic soda- 0.2-0.8%
• Soda ash- 2-3%
• M: L= 1:4
• Temperature- Below 40℃
• Sequestering agent: If required
• Time- 4 hours

Anti-chloric Treatment

In the case of hypochlorite bleaching, hypochlorite ions are produced during bleaching. This (OCl) ion will react with residual protein in the fiber and produce Chloramine which is corrosive and unhygienic. After bleaching, the chloramine reacts with moisture, and gradually the cotton becomes yellowish due to the formation of HCl.

OCl + Residual protein - Chloramine

Chloramine + H₂O-HCl

For removing Chloramine, Antichlore treatment is done.

For the anti-chloric treatment of cellulose fiber, the general recipe is as follows-

NaHSO₄– 0.2-0.6%

Temperature– Room

Time– 10–20 minutes.