Bleach Wash of Woven Denims: Analysis of before and after Wash Properties (Part-3)
S.M. Bappy Rana
Dept. of Textile Engineering
Bangladesh University of Business & Technology
2.8 Bleach washing:
This way be considered as a preparatory process of mercerizing, dyeing, or printing .therefore proper bleaching should be carried out other wish the subsequent process would be fault.
- To remove the size material from the garments.
- To remove the starch present on the garments.
- For soft feeling to wear the garments.
- To increase the color fastness and rubbing fastness.
- Especially develop the “Bio-polishing’’ affect cotton/Denim.
Bleaching powder is a mixture of calcium hypochlorite, CaOCl2 and calcium chloride CaCl2. Thus, its chemical composition is CaOCl2 + CaCl2 or can be written as CaOCl2 or Ca(OCl)Cl (Calcium chloro hypochlorite).
Bleaching powder is manufactured by passing chlorine gas over dry slaked lime at 400 C.
Ca (OH)2 + Cl2 ← CaOCl2 + H2O
1. It is an yellowish white powder with strong smell of chlorine and is soluble in water. Aqueous solution of bleaching powder contains chloride and hypochlorite ions.
CaOCl2 ↔ Ca2+ + Cl- + ClO-
2. In presence of small quantities of dilute acids, it liberates nascent oxygen. Thus it act as oxidizing agent.
2CaOCl2 + H 2SO4 → CaCl2 + CaSO4 + 2HClO
HClO ← HCl + [O]
3. When bleaching powder is treated with excess of dilute acid or CO2, whole of the chlorine present in the molecules is evolved. The amount chlorine so liberated is called available chlorine. A good sample of bleaching powder contains 35-38% available chlorine.
CaCl2 + 2HCl → CaCl2 + H 2O + Cl2
CaOCl2 + CO2 → CaCO3 + Cl2
2.10 Mechanism of bleaching action:
Color in most dyes and pigments is produced by molecules, such as beta carotene, which contain chromophores. Chemical bleaches work in one of two ways
- An oxidizing bleach works by breaking the chemical bonds that make up the chromophore. This changes the molecule into a different substance that either does not contain a chromophore, or contains a chromophore that does not absorb visible light.
2.11 Different type of bleaching agent:
1) Oxidizing Agent:
- Ozone (O3)
- Hydogen peroxide (H2O2)
- Sodium hypochloride (NaOCl)
- Sodium chlorite (NaClO2)
- Potassium dichromate (K2Cr2O7)
- Sodium Peroxide
- Sodium perborate
- Potassium permanganate
- Peracetic acid
- Bleaching powder
- Lithium hypochlorite
- Zinc dust (Zno)
- Sodium hypo-sulphite (Na2S2O2)
- Hydrogen sulphide (H2S)