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 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 PH wetting agent & at required time, temp & PH. 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, PH is sensitive. So PH 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 PH 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 PH& 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.
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.
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