Bleaching, a process of whitening fabric by removal of natural colour, such as the tan of linen, is usually carried out by means of chemicals selected according to the chemical composition of the fibre.

1. BLEACHING

2. Bleaching
• Natural fiber and fabrics even after scouring still contain
naturally occurring coloring matter.
• Natural fibers are off-white in color due to color
bodies present in the fiber
• This yellowish and brown coloration may be
related to flavone pigment of the cotton flower.
• The climate, soil, and frost can also cause various degrees
of yellowness.

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

4. Objectives of bleaching
• The objective of bleaching is to produce white fabrics by
destroying the coloring matter with the help of bleaching agents
with minimum degradation of the fiber.
• The bleaching agents either oxidize or reduce the coloring
matter which is washed out and whiteness thus obtained is of
permanent nature.
• Optical brighteners (OBA) are also used to increase whitness
• brightners improve distribution of light reflected from the fabric
surface in the visible range and increase the total amount of light
reflected from that surface.
• The brighteners thus make the fabric surface appear whiter and
brighter by selective absorption of the ultraviolet component (300-
400 nm) of light and reemission of energy absorbed at
wavelengths (420-470 nm) .

5. Mechanism of bleaching
• Mechanism of bleaching is very complicated and not completely
understood.
• One opinion is that the color producing agents in natural fibers are
often organic compounds containing conjugated double bonds.
• It is known in dye chemistry that conjugation is necessary for a
molecule to perform as a dyestuff.
• Decoloration can occur by breaking up the chromophore, most likely
destroying one or more of the double bonds within the conjugated
system.

6. Mechanism of
bleaching

7. Bleaching agents

8. Bleaching agents
• The major bleaching agents used in textile preparation are
sodium hypochlorite, hydrogen peroxide and sodium
chlorite.
• Other bleaching agents, of lesser importance to textile
preparation but important in consumer laundry products,
are perborates, percarbonates and peracetic acid.
• All of these are oxidative bleaches.
• Known to degrade cellulose so the objective in bleaching is
to optimize whitening and minimize fiber damage.

9. Hydrogen peroxide bleaching
• Most commonly & widely used bleaching agent.
• 90 to 95 % of all cotton and cotton/synthetic blends are
bleached with hydrogen peroxide
• It is available commercially as 35, 50 and 70 % solutions.
• Residual fats, oils, waxes and pectines do not reduce the
bleaching effectiveness of hydrogen peroxide.

10. Hydrogen peroxide
bleaching
• It is a corrosive, oxidizing agent which may cause combustion
when allowed to dry out on oxidizable organic matter.
• Decomposition is accelerated by metal contamination and is
accompanied by the liberation of heat and oxygen, which
will support combustion and explosions in confined spaces.
• The material is an irritant to the skin and mucous
membranes and
• Dangerous to the eyes.

11. Mechanism of Hydrogen peroxide bleaching
• Hydrogen peroxide is a weak acid and ionizes in water to
form a hydrogen ion and a perhydroxyl ion. The perhydroxyl
ion is the active bleaching agent.
• H2o2 + H2O➔ H+ + HOO-
• Hydrogen peroxide can also decompose. This reaction is
catalyzed by metal ions e.g. Cu++, FE+++. This reaction is not
desired in bleaching because it is an ineffective use of
hydrogen peroxide and causes fiber damage.
• H2o2 + H2o ➔ H2o + ½ O2

12. Mechanism of peroxide bleaching
• Though hydrogen peroxide is stable in acidic medium, but
bleaching occurs by the addition of alkali or by increased
temperature. Hydrogen peroxide liberates perhydroxyl ion
(HO2-) in aqueous medium and chemically behaves like a
weak dibasic acid.
• The perhydroxyl is highly unstable and in the presence of
oxidisable substance (coloured impurities in cotton), it is
decomposed and thus bleaching action takes place.
• Sodium hydroxide activates hydrogen peroxide because H+
ion is neutralized by alkali which is favorable for liberation of
HO2-.

13. Mechanism of
peroxide bleaching

14. Mechanism of peroxide bleaching
• However, at higher pH (above 10.8) the liberation of HO2- ion
is so rapid that it becomes unstable with the formation of
oxygen gas which has no bleaching property.
• If the rate of decomposition is very high, the un-utilised
HO2- may damage the fiber. A safe and optimum pH for
cotton bleaching lies between 10.5-10.8, where the rate of
evolution of perhydroxyl ion is equal to the rate of
consumption (for bleaching).
• At higher pH, hydrogen peroxide is not stable and hence a
stabilizer is frequently added in the bleaching bath.

15. Stabilization
• The process of regulation or control of perhydroxyl ion to
prevent rapid decomposition of bleach and to minimize fiber
degradation is described as stabilization.

16. Stabilization
• They could include any of the followings:
• Silicates
• Dispersent
• e.g. Acrylates/phosphonates
• Sequestrants
• e.g. Edta/tpa/heptonates/gluconates
• Inorganics
• e.g. Magnesium salts

17. Stabilizers
• To control the decomposition of hydrogen peroxide.
• The process of regulation or control of per-hydroxyl ion to
prevent rapid decomposition of bleach and to minimize
fiber degradation is described as stabilization
• Stabilizers function
• Stabilizers for peroxide normally work by controlling
the formation of free radicals
• By providing buffering action to control the pH at the
optimum level
• To complex with trace metals which catalyze the
degradation of the fibers
• Stabilizers include sodium silicate, inorganic and organic
compounds and phosphates.
• Sodium silicate is the most conventional, easily available and
widely used stabilizer

18. Sodium silicate as a stabilizer
• Sodium silicate is the most conventional, easily available
and widely used stabilizer.
• Sodium silicate is mildly alkaline in nature
• Of the various phosphates only tetrasodium
pyrophosphate, Na4P2O7 and hexametaphosphates are of
interest as stabilizers in alkaline bleach bath.

19. Alkali selection
• Alkali to be used in peroxide bleaching is dependent on
• The fibers or blends being bleached.
• Sodium hydroxide, sodium carbonate are generally used on
cellulosic fibers
• Ammonia and various phosphates are used when bleaching
protein fibers.

20. Parameters for peroxide bleaching
• It is very difficult to specify strict guidelines for optimum
bleaching conditions for hydrogen peroxide as the
operation is normally affected by the nature and quality of
the goods to be bleached, the amount of bleaching required
and on the equipment available.

21. Effect of pH
• The stability of hydrogen peroxide depends on pH.
• At pH 1 to 3 it is stable, at pH < 10, hydrogen peroxide is
the major specie so it is inactive as a bleach
• At highly alkaline pH 11.5 to 13 it has least stability, there
is a rapid generation of perhydroxyl ions
• At pH 10 to 11, there is a moderate concentration of
perhydroxyl ions. The bleaching takes place around 10.5
due to accumulation of perhydroxyl ions in the bleaching
bath.
• pH 10.2 to 10.7 is optimum for controlled bleaching.

22. Effect of temperature
• In practice cotton bleaching with hydrogen peroxide is carried out at 90-
100°c
• The rate of bleaching increases with the increase in temperature, but at
the same time solution becomes unstable and degradation of cotton
increases.
• Stabilized hydrogen peroxide does not decompose at high temperature
• Better bleaching occurs at 95 to 100 °c.
• This feature makes it ideal for continuous operations.
• Below 80°c the evolution of perhydroxyl ion is very slow, also the rate of
bleaching.

23. Effect of concentration
• The optimum concentration of hydrogen peroxide depends
on number of factors namely liquor ratio, temperature and
class of fibre.
• In the batch process about 2-4% (o.w.f.) hydrogen peroxide
is sufficient for cotton fabrics with a liquor ratio of 10:1 to
20:1.
• In the continuous process, the cotton fabrics are saturated
with bleach bath containing 1-2% (o.w.f.) hydrogen peroxide.
• Very high concentration may damage the fibre.

24. Effect of time
• The time required to bleach with hydrogen peroxide depends
on temperature, class of fibre and equipment used for
bleaching.
• In general, the time of bleaching is inversely proportional to
the temperature of the bleaching bath.

25. Effect of temperature and time on hydrogen peroxide
at pH 10.5
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