1. Bleaching of textiles
Faiza Anwar

2. Introduction
The natural fibre and fabrics even after scouring still contain naturally occurring coloring matter.
This yellowish and brown discolouration may be related to flavone(2-Phenyl-4H-chromen-4-one) pigments of the cotton flower.
The climate,
soil, and
weather can also cause various degrees of yellowness.

3. Introduction
Tips of leaves or stalks coming in contact with the moist ball after opening will cause dark spots and discolouration.
Discolouration may also come from
dirt,
dust,
insects or
from harvesting or
processing equipment in the form of oils and greases.

4. Objectives of Bleaching
To produce white fabrics by destroying the colouring matter with the help of bleaching agents with minimum degradation of the fibers.
These agents either
oxidise or
reduce the colouring matter which is washed out.
A consistent white base fabric has real value when dyeing light to medium shades because it is much easier to reproduce shade

5. Mechanism of Bleaching
Very complicated and not completely understood.
Conjugation is necessary for an organic molecule to perform as a dyestuff so breaking it is one of the ideas.
Double bonds are known to be oxidize into single.

7. Bleaching Agents
The major bleaching agents used in textile preparation are
sodium hypochlorite,
hydrogen peroxide and
sodium chlorite

8. Mechanism of HYDROGEN PEROXIDE bleaching
It is a weak acid and ionizes in water in the presence of alkali to form a hydrogen ion and a perhydroxyl ion
H2O2 H+ + HOO-
HOO O* + OH-
O* :it breakes double bond present in carbon atoms of colour impurities which causes bleaching.
Hydrogen peroxide can also be decomposed by metals.
Fe2+ + H2O2 → Fe3+ + OH· + OH−
Fe3+ + H2O2 → Fe2+ + OOH· + H+
This reaction is catalyzed by metal ions e.g. Cu++, Fe+++.

9. Stability of Hydrogen Per oxide
Pure hydrogen peroxide is fairly stable in presence of sulphuric acid and phosphoric acid if stored away from sunlight in a perfectly smooth bottle.
In alkaline medium it is less stable and even traces of alkali (NaOH, Na2CO3) decompose aqueous solution of hydrogen peroxide.
The addition of alcohol, glycerine also stabilise hydrogen peroxide.

10. Stabilizers for HYDROGEN PEROXIDE
To control the decomposition of hydrogen peroxide.
The process of regulation or control of perhydroxyl ion to prevent rapid decomposition of bleach and to minimise fibre degradation is described as stabilisation
Stabilisers for peroxide normally work by controlling the formation of free radicals
Stabilizers function
By providing buffering action to control the pH at the optimum level
To make complex with metals which catalyze the degradation of the fibers
Stabilizers include sodium silicate, organic compounds and phosphates
Sodium silicate is the most conventional, easily available and widely used stabiliser

11. Effect of pH
At pH < 10, H2O2 is the major specie so it is inactive as a bleach.
At pH 10 to 11, there is a moderate concentration of perhydroxyl ions.
pH 10.2 to 10.7 is optimum for controlled bleaching.
At pH > 11, there is a rapid generation of perhydroxyl ions.
When the pH reaches 11.8, all of the hydrogen peroxide is converted to perhydroxyl ions and bleaching is out of control.
hence an stabiliser is frequently added in the bleaching bath.

12. 1.HYDROGEN PEROXIDE Effect of Temperature
The rate of bleaching increases with the increase in temperature, but at the same time solution becomes unstable and degradation of cotton increases.
Better bleaching occurs at 95 to 100 C.
This feature makes it ideal for continuous operations using insulated J-boxes or open- width steamers.

13. Continuous Bleaching Machinery

14. Continuous Bleaching Machinery

15. Continuous Bleaching Machinery

16. Risks Involved with Peroxide Bleaching
Catalytic damage results in
small spots of unevenly dyed fabric
small holes
Injurious effect on skin when used in a concentrated form.

17. Bleaching with Hypochlorites
The hypochlorite ion, also known as chlorate anion is ClO−.
A hypochlorite compound is a chemical compound containing this group.
Hypochlorites are the salts of hypochlorous acid.
Calcium Hypochlorite
Sodium Hypochlorite
Hypochlorites are frequently quite unstable.

18. 2.Sodium hypochlorite

19. Sodium Hypochlorite

20. Mechanism of bleaching

21. The solution of NaOCl contain OCl-
The solution of NaOCl contain OCl-.The hypochlorite ion gives up its oxygen to the coloured dye, thus making it colourless as follows:
OCl-(aq) + dye(aq)  Cl-(aq) + (dye + O)
colored colorless
As oxygen is added to the dye, chlorine bleaches by oxidation.

22. Antichlor Treatment
An antichlor is a substance used to decompose residual hypochlorite or chlorine after chlorine-based bleaching, in order to prevent ongoing reactions with, and therefore damage to, the material that has been bleached. Antichlor's include sodium bisulphite , potassium bisulfite sodium metabisulfite, sodium thiosulfate, and hydrogen peroxide. In the textile industry, the antichlor is usually added right before the end of the bleaching process. Antichlor's are used mainly on fiber and textiles, Rinsing with water should follow the antichlor treatment in order to flush out by-products of the procedure.
Finally, the cloth is washed with water to rinse.

23. Factors effecting in hypochlorite bleaching operations
Cotton can be bleached with hypochlorite solution at room temperature at pH range of 9.5 to 11.
The pH is maintained by adding 5 g/l sodium carbonate in the bleach bath.
After bleaching the cloth is treated with dilute hydrochloric acid to neutralise any alkali (scouring) present in the cloth.

24. Effect of pH
In the region of pH 7, when hypochlorous acid and hypochlorite ion are present approximate the same concentrations, the rate of attack on cellulose is greatly enhanced.
As the pH falls below 5, the liberation of chlorine begins to take place and pH below 1.5, the whole of hypochlorous acid is converted into chlorine.
In the pH range 9 to 11, a level occurs at which little change occurs and is the normal use range for bleaching with hypochlorite solution.
The active species appear to be OCl- ion, or a complex of HOCl and OCl-.

25. Effect of temperature
Generally bleaching of cotton is carried out with 1.5 g/l of hypochlorite solution at about 40C for 1 h.
Higher temperature increases the rate of bleaching but at the same time degradation of cotton is also increased.
Hypochlorite solutions by themselves are quite stable at the boil at pH values 11 or higher, but decomposes at lower pH values.
Hypochlorite solutions if buffered to pH 11, the rate of bleaching is increased by a factor of 2 times for every 10 C rise in temperature.
It is thus, possible to bleach at 60 C in 7 min, but is difficult to control the degradation of cotton in such short period of bleaching.

26. Disadvantages of sodium hypochlorite
Sodium hypochlorite does not produce completely satisfactory whites.
It produces damage to cellulosic fibres.
Bleaching with sodium hypochlorite solution requires corrosion resistant equipment.
Sodium hypochlorite produces unpleasant odors in working environment.
Sodium hypochlorite solution is harmful to skin in concentrated form.
It produces harsh handle on fabric. .

31. Evaluation of Bleaching Process
Whiteness Measurement:
The CIE Whiteness Index value (CIE WI) is determined using AATCC Test method (110–1995)
This gives a measure of how well the yellow impurities were removed by bleaching.
The standard ceramic tile is measured and set to equal 100.
The other specimen are rated against this standard. Unbleached fabrics will give values in the 50 to 60 range. Well bleached fabrics will rank

32. References
“Chemical technology in the pre-treatment processes of textiles” by S.R. Kar Makar