4.1 Introduction to Bleaching
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After desizing and scouring most of the textile fabrics especially the natural ones, retain slight coloration. The source of the colour may be: |
- Flavone pigment in case of cotton
- Soil and dirt acquired from atmosphere
- Contact with plant parts and seeds etc.
- Colour acquired during mechanical processing e.g., grease, oil etc.
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| The objective of bleaching is to destroy this natural or acquired color to bring the textiles in a white state. |
This may be necessary when:
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- The fabric has to be supplied in white colour
- It needs to be dyed in pastel shades
- It needs to be printed
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| If the fabric is to be dyed in dark shades, then bleaching becomes an optional process.
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In case of cotton, the motes or the seed coat fragments are visible on woven or knitted fabrics after scouring. These appear as specks of brown or black colors on fabric surface. The color of these motes is also destroyed by bleaching. |
| Dyeing an unbleached fabric in pastel shades might mask the brightness of applied color. Many times these bleached fabrics are given treatment with Optical brightening agents to confer extra whiteness. Bleaching also removes residual impurities left by other pretreatment processes like desizing, scouring etc. In that sense, the absorbency of a cotton fabric may increase after bleaching. |
Although reductive bleaching has been used in past to bleach wool and silk, today almost all textile material are bleached by oxidative desizing agents. |
| Broadly speaking, bleaching agents can be classified as: |
Oxidative bleaching agents and reductive bleaching agents |
| Oxidative bleaching agents can be further divided into |
Peroxy compounds and Chlorine based compounds
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Examples of Peroxy compounds |
Examples of Chlorine based compounds |
Hydrogen peroxide
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Bleaching powder
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Sodium hypochlorite |
Sodium hypochlorite |
Sodium perborate |
Lithium hypochlorite |
Potassium permanganate |
Sodium chlorite |
Peracetic acid |
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Examples of reductive bleaching agents
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Sulphur dioxide
Sodium hydrosulphite
Sulphoxylates
Acidic sodium sulphite
Sodium bisulphites
thioureadioxide increases |
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| The main bleaching agents are: |
- Sodium hypochlorite (NaOCl)
- Sodium Chlorite (NaClO2)
- Hydrogen peroxide (H2O2)
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The use of oxidative bleaching dates back to the end of eighteenth century, when solid bleaching powder (based on chlorine) was used for bleaching of cellulosic textiles. The discovery of peroxide bleaching in 1925 has gradually resulted in replacement of other bleaching processes with peroxide/peroxygen based bleaches. Out of the main bleaching agents mentioned above, H2O2 has virtually replaced chlorine based bleaching. It must be mentioned here that hypochlorite and chlorite based bleaching is still prevalent in some developing countries to some extent. |
Bleaching with sodium Hypochlorite (NaOCl):
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| NaOCl is sodium salt of Hypochlorus acid (HOCl). It can be prepared by passing chlorine gas in a solution of NaOH or by electrolysis of NaCl solution (aqueous). Sodium hypochlorite is a strong oxidising agent, with a redox potential of 1400-1550 mV. Cl2 acts as an oxidising agent in presence of water (moisture) only. Dry chlorine gas does not have oxidising power. |
| Cl2 + H2O →HCl +HOCl HOCl →HCl + [O] ↑ |
| Formation of HOCl is important as it provides the nascent oxygen molecule responsible for oxidation (bleaching). NaOCl solution in water undergoes the following reaction: |
NaOCl + H2O →NaOH + HOCl |
| HOCl is a weak acid. Aqueous NaOH solution has a pH of 11-11.5 due to the presence of strong NaOH.
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Bleaching occurs due to the following reaction:
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| HOCl →HCl + [O] ↑ |
| In acidic medium, the consumption of hypochlorite is very fast and can lead to cellulose degradation by oxidation. The reaction is given below: |
| NaOCl + HCl → NaCl + HOCl HOCl + HCl → H2O + Cl2 |
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