Thursday, December 17, 2009

Bleaching With Sodium Chlorite

Sodium Chlorite
Sodium chlorite, anhydrous is in the form of slightly hygroscopic crystals or flakes , soluble in water.The hydrated material is in the triclinic leaflets. The chemical formula is NaClO, and molecular weight is 90.45.  The molecular weight of hydrated product is 144.5.

Bleaching with sodium chlorite.(NaClO2 )
The sodium chlorite is available as a powder , and it is applied under strongly acidic conditions to textiles. Its application produce a toxic and corrosive gas.
a. Bleaching Mechanism
Sodium Chlorite is an oxidant particularly adapt for synthetic fibre bleaching (polyamidic, acrylic, polyester) and cellulosic (man-made and natural, particularly for linen). Its oxidant action works thanks to Chlorine Dioxide which develops through an acidification process: therefore it is highly recommended to work with closed equipment and to furnish the bleaching departments with aspiration systems.
A general guideline for  bleaching bath at M:L=1:10 is 
b.  pH :- 
The pH of around 4 ± 0.2 required for bleaching is maintained with buffers or as termed in industry activators, like sodium acetate or sodium dihydrogen phosphate (NaH 2PO4) Latter is usually preferred because it improves whiteness o goods. Neutral or slightly acid chemicals that liberate acid on heating are also used occasionally. Organic esters like ethyl lactate or titrate and their ammonium salts are also suitable for this purpose
however the pH range for different substrates is as given below,
Polyamide= 3.5-3.8
Acrylic= 3.0-3.5
Cotton= 3.6-4.0
Polyamide=85-900C/90-120 min
Acrylic=90-95oC/60-90 min
Cotton/Linen/cotton-polyester=950C/45-60 min
d.Quantity of sodium chlorite
Polyamide =1-1.g gpl
Acrylic = 1.5-2.5 gpl
Cotton=0.3-0.5 gpl
Linen=0.3-0.5 gpl
Cotton-polyester=0.3-0.5 gpl
Stabilizers that control chlorine dioxide emission, such as borax or polyphosphates should be used.
e. Effect on metals
Since chlorine dioxide is highly corrosive in nature therefore it is recommended to use it in fully closed system ,and preferably in fully flooded machines . The gas is also corrosive and result into rusting of stainless steel surface, which stain the material to be bleached.A corrosion inhibitor such as sodium nitrate is essential in an quantity equal to the chlorite that moderates decomposition of the chlorite and inhibits corrosion of metals.
f.Water Quality
The water hardness and presence of metal ions does not affect the sodium chlorite bleaching under acidic conditions.
g.Antichlor treatment
Some cationic dyes are very sensitive to traces of chlorine and will rapidly fade giving poor colour yields, particularly when dyeing pale shades. An anti-chlor treatment of fabric bleached with sodium chlorite may be necessary and small additions of sodium bisulphate or thiosulphate to the dyebath will avoid problems with cationic dyes
h.Application process of chlorite bleach on cellulosics and synthetics
i.Advantage of sodium chlorite bleaching
          The principal advantages of Sodium Chlorite/Chlorine Dioxide for this application may                   be summarised as follows:
  • high brightness degree (especially for acrylic fibres)
  • negligible degradation of fibres (1-2% weight loss for cellulosic fibres and no attack to the polymeric chains in the synthetic fibres)
  • lower environmental impact of wastewaters (negligible level of AOX and COD).
  • Versatile bleaching agent for cellulosics , synthetics and blends ,especially poly-cotton blends. It is also safe for those synthetic fibers which are sensitive to alkali.
  • Non sensitive to metal ions such as iron and Water hardness under acidic conditions.
      i. Disadvantages of Sodium Chlorite bleaching
  • It is expensive as compared to hydrogen peroxide.
  • It can not be used for wool and silk.
  • Chlorine dioxide is highly corrosive to metals and toxic in nature.
  • Bleaching in neutral and acidic pH damage cotton. 

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