Bleaching process textile

Destruction of natural coloring matter to impart pure permanent & basic white effects suitable for the production of white finishes level dyeing & desired printing shade with a minimum or no tendering Bleaching of textile material is a chemical or commercial process bleaching process mainly performed By following steps.

Bleaching agents

Two types of Bleaching agents are used like:-

A) Oxidative bleaching agents           B) Reducing bleaching agents

      1.O₃                                                           1.Zink dust

      2.H₂O₂                                                      2.Stannous Chloride,SnCl₂

      3.Ca(OCl)₂, NaOCl                               3.FeSO₄

      4.NaClO₂                                                  4.SO₂

      5.K₂Cr₂O₇                                                 5.H₂S

      6.KMnO₄                                                  6.NaHSO₄

      7.Bleaching powder (which is complex mixture of Ca(OH)₂ & Ca(OCl)₂ with distilled Water.

Bleaching process

1.Hypochlorite Bleaching process

2.H₂O₂ Bleaching process

3.Sodium chlorite bleaching process

Hypochlorite Bleaching process

     Ca(OCl)₂ & NaOCl are used as hypochlorite bleaching agents with soda ash to maintain P^H wetting agent & at required time, temp & P^H. Ca(OCl)₂ has a cheap source of Bleaching powder. It is less expensive but unstable. NaOCl have some technical advantages over Ca(OCl)₂ , it is stable soluble & commercially available.

Here OCl^- is the hypochlorus ion which is responsible for bleaching.

OCl^-  + OH-cellulose                         White cotton

In whole reaction, P^H is sensitive. So P^H should be maintained.

Antichlor treatment:

In case of hypochlorite bleaching OCl^- is produced this will react with residual protein to produce Chloramines. These Chloramines is corrosive & unhygienic. The chloramines react with moisture forming HCl resulting yellowing of fabric. Cl₂ is also produced in this process which is toxic & unhygienic, make irrigate to skin. To remove these chloramines & Cl₂ fabric is treated with a reductive agent & this process is known as antichlor treatment.

Chemicals & Condition in H₂O₂ bleaching:

1 .H₂O₂: It is a clear corrosive oxidizing bleaching agent. It ionizes in water to form a hydrogen ion and a per hydroxyl ion. The per hydroxyl ion is the active bleaching agent.

                H₂O₂                                       H⁺ + HO2^-

Hydrogen peroxide can also decompose. This reaction is catalyzed by metal ions e.g. Cu⁺⁺, Fe⁺⁺⁺. This reaction is not desired in bleaching because it is an ineffective use of hydrogen peroxide and causes fiber damage

                H₂O₂                                      H₂O + O₂

2. Stabilizer: Stabilizers must be added to the bleach solution to control the decomposition of hydrogen peroxide & preserve the strength of H₂O₂ by providing buffering action to control the P^H at optimal level. It makes a complex compound with the catalyst & stop Oxygen generation. Sodium silicate, organic compounds and phosphates are used as stabilizer.

3. Alkali or soda ash: To maintain the P^H& getting more whiteness.

4. Wetting agent: To wet the fabric by lowering the interfacial tension.

5. Sequestering agent: Same as scouring.

6. Impurities: fabric or fibre impurities have a stabilizing power. Higher   impurities, higher stability of H₂O₂ & good bleaching action.

7.Time & temperature: Stabilized hydrogen peroxide does not decompose at high temperature therefore faster and better bleaching occurs at 95 to 100 C. Time is adjusted with the process.

8. PH: Decomposition of H2O2 is depending on PH & an optimal PH should be controlled. At PH<9 or 10 H2O2 is inactive. At PH 10 to 11, there is a moderate concentration of per hydroxyl ions. PH 10.2 to 10.7 is optimum for controlled bleaching. At PH> 11, there is a rapid generation of per hydroxyl ions. When the pH reaches 11.8, all of the hydrogen peroxide is converted to per hydroxyl ions and bleaching is out of control.

9. Water & water impurities: water should be slandered quality & free from metal ions. Metal ions work as catalyst & breaks H₂O₂ to produce O₂ which has no bleaching action.

Bleaching Action

The natural color of wool varies from white to pale yellow. The yellow tint is usually a sigh that the protein is undergoing in some reaction on exposure to air & light. To remove this yellow tint & to produce bright pastel shades wool must be bleached. However the white effects after bleaching are not permanent.  On further exposure to air & light this yellow tint produces. Wool cannot be bleached with NaOCl as it degrades wool. SO₂ was used for bleaching but due to some problem H₂O₂ is now used for wool bleaching. Bleaching is usually carried out in a weakly alkaline condition with a stabilizer & a sequestrant. The temp is kept low to avoid yellowing. After bleaching the materials are washed with acidic materials (CH₃COOH or NaHSO₄) to neutralize at room temp for 10-20 mins.

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