Bleaching

Bleaching used for:

Destruction of color impurities for
1. producing base white for subsequent dyeing
2. Producing permanent white

But it may not always be necessary:
1. For synthetic fiber
2. For deep shade
3. In some cases it may require partial bleach.

Parameter for H2O2 Bleaching:
1. Concentration of H2O2
2. Concentration of alkali
3. pH
4. Temperature
5. Time
6. Nature & quality of goods
7. Water hardness & other impurities
8. Types & concentration of auxiliaries
9. Available machine & stabilizer system employed
10. Desired bleaching effect

Status nascendi: In which point where active oxygen will get.

pH < 10 practically no HOO-
pH 10-11 Low to moderate HOO-
pH > 11 Moderate to high HOO-

Major bleaching agent:

Oxidative bleaches:
1. Hydrogen peroxide H2O2
2. Persulfate (NH4)2 S2O8, K2S2O8, Na2S2O8
3. Peracetic acid CH3CO.O.OH
4. Sodium hypochloride, NaOCl
5. Sodium hypochloride, available in bleaching powder with approximate formula 3Ca(OCL)Cl.Ca(OH)2.5H2O
6. Sodium chloride NaClO2

Reactive bleaches:
1. Sulfur dioxide SO2
2. Sodium hyposulfite NaS2O4

Advantages of H2O2 or why called universal bleaching agent

1. Most widely used for the natural cellulosic fibers( cotton, flax, Linen, jute etc) as well as the protein fibers (wool, Silk etc) regenerated cellulose fiber &natural fiber blends.

2. Extremely versatile bleaching agent, applicable over a very wide range of bleaching temperature , time & with wide range machineries.
3. Environmentally friendly
4. Decomposition products are oxygen & water
5. Compatible with most dyes & optical brightening agent (OBA)
6. Excellent storage stability
7. Produces a stable white fiber with good absorbency
8. Allow saving by combining stages such as desizing &scouring of over 50% in labour, water & energy cost.
9. Small amounts of impurities can acts as stabilizer.
10. Give versatile processing (batch\ continuous, hot\cold, rapid\ long dwell).

pH < 10 – no HOO-
pH - 10-11 -- Low to moderate HOO-
pH >11 -- Moderate to high HOO-

HOO- is very important , without it bleaching is not possible,
Commercially we get H2O2 in %
35% (W/W) weight/weight
50% {W/W}
70% (W/W) – conc. Available active oxygen

Hypochloride bleaching agent:
Calcium hypochlorite or sodium hypochlorite are used a hypochlorite bleaching agent. Calcium hypochlorite\rite has cheap source of bleaching powder. It is cheap but unstable. NaOCl is stable & soluble.
Mechanism:

Ca(OCl)2 + H2O ---- Ca (OH)2 + HOCl
Hypochlorous acid
NaOCl + H2O ------- NaOH + HOCl
HOCl ----- H+ + OCL-
Hypochlorous ion
OCl - + OH -- Cellulose ----------- White cotton

pH is need about 10-11

Most disadvantages:

(AOX) – Absorbable Organic Halogen
If AOX is more pollution is more.


Scouring:
Calcium hypochlorite forms CaCO3 with CO2 & water. To remove this acid is used. Which is known as scouring.

Mechanism:
Ca(OCl)2 + H2O +CO2 --- CaCO3 + HOCl
CaCO3 + 2HCl ---- CaCl2 + H2O + CO2
CaCO3 + H2SO4 --- CaSO4 + CO2 + H2O

As CaCl2 is more soluble in water then CaSO4. HCl is preferable.

Antichlor treatment :
Antichlor, process is required
To destroying active chlorine bleach liquor
To remove chloramines from textile

R – NH2 + NaOCl ---- R – NHCl + NaOH
Chloramines

Disadvantage of chloramines:
Corrosive & un hygienic yellowing of fiber
To remove the chloramines & Cl2 fabric is treated with reduce in agent for polyamide & peroxide for cellulosic fibers. This process of removal known as Antichlor treatment.

NaOCl + H2O2 -------- NaCl + H2O + O2
R – NHCl + H2O2 --- R – NH2 + HCl + O2


Test for bleaching:

1. Whiteness Index (WI) : AATCC – 110
2. Physical or chemical damage :

Fluidity test

1. Whiteness is get by reflectance value 72-80 acceptable for cotton

The reflectance values of well bleached samples are:
Cotton – polyester/cotton 80 – 90 %
Wool 50 – 60 %
Linen 60 – 80 %

Wish you good luck-------------------------------------------
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