OXIDATION

BLEACHING

Bleaching

Definition

“After the removal of the waxes and other hydrophobic
type of impurities from grey fabric by the desizing
and scouring the fabric is now in a more
absorbent state.
But still have the pale appearance due to the
presence of natural coloring material like pigment
etc, these pigment cannot be removed the only
way to tackle these pigment is to decolourise
them using suitable oxidizing agents. This will
make the fabric in a super white form. This
process of decolouration of natural
pigments is called the bleaching”.

Mechanism of Bleaching

The 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.
Decoloration can occur by breaking up the chromophore,
most likely destroying one or more of the double bonds
within the conjugated system. The bleaching
agents either oxidize or reduce the coloring matter …thus
whiteness obtained is of permanent nature. Primitive
bleaching
- expose scoured fabric to the sun - light served as an
oxidation catalyst.

Objectives of Bleaching

•Removal of colored impurities.
•Removal of the seed coats.
•Minimum tendering of fibre.
•Technically reliable & simple mode of
operation.
•Low chemical & energy consumption.
•Increasing the degree of whiteness.

Bleach goods:

When cloth has been bleached
for finishing, it is called bleach
goods.

Bleaching Agent

A bleaching agent is a substance that can whiten or
decolorize other substances.

Oxidative bleaching

The Oxidative bleaching agents are used much more
than Reductive bleaching agents . The bleaching agent
is a chemical reagent which decomposes in alkali
solution and produce active oxygen. The active oxygen
is in fact the intrinsic bleaching agent as it will further
destroy partly or completely the coloring matter present
in the textile material.

Reductive bleaching

Many colouring matter can be reduced to colourless
compound by reducing agents. Before the
invention of hydrogen peroxide, sulphur dioxide
was the only bleaching agent for wool. Fibers like
polyamide, polyacrylics and polyacetates can be
bleached using reductive bleaching technology.

Auxiliaries used for bleaching:

Anti-corrosion agents
 Sequestering agents (to removes ion from a

solution
system by forming a ring which does not have the
chemical reactions of the ion which is removed. It
can be a complexing or a chelating agent.)
 Wetting agents/detergents
 Activators
 Stabilizers

Bleaching with Hypochlorites

Hypochlorite bleaching (OCl-) is the oldest
industrial method of bleaching cotton.
Until 1940 most cotton fabrics were bleached with

NaOCl
…today only 10 %of the cotton.
It is however the main stay of home laundry
bleaching products.
Their use is declining because of anti-chlorine
lobby and environmental pressures.

Sodium hypochlorite (NaOCl) Bleaching:

Sodium hypochlorite is the strongest oxidative bleach -
used in textile processing.
NaOCl is a highly unstable compound at normal
conditions of temperature and pH. It doesn’t exist assolid
form.
Prior to bleaching with hypochlorite, it is necessary to
thoroughly scour fabrics to remove fats, waxes and
pectin impurities. These impurities will deplete the
available hypochlorite, reducing its effectiveness for
whitening fabric.

Product strength of hypochlorites is generally
expressed as the available chlorine content.

Commercial NaOCl will have 12 to 15 % 

active chlorine. Household bleach is 5 %

active chlorine.

Bleaching Mechanism:

NaOCl is the salt of a moderately strong base
(OCl-) and a weak acid (HOCl(
NaOCl solution is strongly alkaline (pH ~ 11.55)

and the free caustic present in the solution acts as a

stabilizer.
Stability of sodium hypochlorite solution is

also improved by storing it in a dark room
below 30 °C. NaOCl + H2O→ Na+ + OCl -

(hypochlorous ion)
OCl - + H2O→ HOCl + OH –

Factors Affecting Hypochlorite

Bleaching:

Effect of pH

pH has a profound effect on bleaching with

hypochlorite.
• Addition of caustic favors the formation of OCl- ion.
• Na2CO3 is used to buffer the bleach bath to pH 9 - 10.
• At pH > 10, little to no bleachingtakes place.
• When acid is added, the HOCl concentration

increases.
•pH 5 - 8.5, HOCl is the major species present …very
rapid bleaching takes place, …but rapid degradation of

fiber.
•When the pH drops below 5, chlorine gasis liberated and

the solution has no bleaching effectiveness at all.
• The optimum pH for bleaching is between 9 and10.

Effect of Time and Temperature
Time and temperature of bleaching are interrelated.
• Concentration is also interrelated with time andtemperature.
•1 hr at 40 ⁰C is satisfactory for effective bleaching.

Effect of Metals
•Copper and iron catalyze the oxidation of cellulose by sodium
hypochlorite degrading the fiber.
.• Stainless steel equipment is required and care must be taken
that the water supply be free of metal ions and rust from
pipes.

Antichlor
•Fabrics bleached with hypochlorite will developa distinctive
chlorine odor
•An antichlor treatment with sodiumbisulfite and acetic
acid removes any residual chlorine from the cloth

Uses:

Hypochlorite is used mainly to bleach cellulosic fabric
• Itcannot be used on wool, polyamides (nylon), acrylics. These

fibers will yellow from the formation of chloramides.
•Bleaching with hypochlorite is performed inbatch equipment.

It is not used in continuous operations because chlorine is

liberated into the atmosphere.
Typical Batch Procedure:
• NaOCl - 2.5% active bleach
• Na2CO3- 1.0% pH buffer (5 g/l)
Bleach Cycle:
• Run 1 hr at 40⁰C
• Drop bath, rinse
• Add antichlor chemicals
• Rinse

Bleaching with Hydrogen Peroxide (H2O2)

H2O2 was discovered in 1818.
By 1940, about 65% and to-day about 90 - 95 %
of all cotton and cotton/synthetic blends are
bleached with H2O2.
 It is available commercially as 35, 50 and 70 %

solutions.
It is a corrosive, oxidizing agent which may cause
combustion when allowed to dry out on oxidizable
organic matter.
H2O2 is an irritant to the skin and mucous
membranes and dangerous to the eyes.
 H2O2 - Ecologically acceptable

Bleaching Mechanism:

H2O2 is a weak acid and ionizes in water to
form: H2O2+H2O→H+ + HOO- (active bleaching
agent)
HOO- (unstable) →OH+ O* (active or nascentoxygen)
O* + X→X-O, X- oxidizable substance
H2O2 decomposition is catalyzed by metal ions e.g.
Cu++, Fe+ + ...undesired rxn: no bleaching effect and
causes fiber damage
H2O2→H2O + 1/2O2

Effect of pH
pH has a profound effect on bleaching with

hydrogen peroxide..
• H2O2 is an extremely weak acid.
•Caustic neutralizes the proton and shifts the reaction to
the right.

• pH <10, H2O2 is the major species so no bleaching.
• pH = 10 -11, moderate conc. of perhydroxyl ions.
• pH = 10.2-10.7 ( with NaOH) is optimum.
• pH >11, rapid generation of perhydroxyl ion.
•pH 11.8 all H2O2 is converted to HOO- and rxn is out
of control.

Effect of Time and Temperature
•Stabilized H2O2 does not decompose at high
temperature therefore faster and better bleaching
occurs at 95 to 100 ⁰C…ideal for continuous
operations.
•Temp↑ - rate of bleaching ↑ …but solution becomes
unstable and degradation of cotton increases.
•Below 80 ⁰C the evolution of perhydroxyl ion is
very slow so also the rate of bleaching.

Effect of concentration of liquor
• Batch process = 2-4% H2O2
• In the continuous process=1-2%
• H2O2 Very high concentration may damage the fiber.
Effect of time
•The time depends on temp, class of fiber andequipment
used for bleaching.
• temp. of bleaching↑ bath time of bleaching↓

Auxiliaries for Bleaching With H2O2

1. Stabilizers
To control the decomposition of H2O2.
It provide buffering action to control the pH and to
complex with trace metals which catalyze the
degradation of the fibers.
Major types are Sodium silicate, organic

compounds and

Activator

provide eg:NaOH, Na2CO3,

alkalinity Na3PO4,etc.

Sequestering agent: eg:EDTA, Sodium hexa

organic stabilizers or meta

separate phosphate

Wetting agent

to provide wetting and detergency

Uses:

H2O2 is the bleach most widely used for cellulosic

fibers [cotton, flax, linen, jute etc.) and well as wool,

silk, nylon and acrylics.
•Unlike hypochlorites, peroxide bleaching does not

require a full scour.
•Residual fats, oils, waxes and pectines do not reduce
the bleaching effectiveness of H2O2….Impurities help in

•Since it ultimately decomposes to oxygen and water, it
doesn’t create effluent problems.

Advantages of H2O2
• Universal bleaching for Cotton, Rayon, Wool andSilk
• It can be used on continuous equipment.
• Permanent Whiteness
• Simultaneous Scouring/ bleaching and continuous
bleaching possible
• Degradation is less.

•Lesswater is required with peroxide bleaching and
there is no need for souring after bleaching.
•Peroxide bleached goods are more absorbent than
hypochlorite bleached goods.



Sodium Chlorite (NaClO2) Bleaching:

•Bleaching with NaClO2 is carried out under acidic

conditions which releases ClO2, a toxic and

corrosive yellow-brown gas.
• ClO2 is thought to be the active bleachingspecie
•One advantage of sodium chlorite bleaching is that it

leaves the fabrics with a soft hand.
•When a solution of NaClO2 is acidified, ClO2,

hypochlorous acid (HClO2), sodium chlorate

(NaClO3) and NaCl are formed.
•ClO2 and HClO2 are bleaching species, NaClO3 and NaCl

are not.

Effect of pH
• ClO2 is favored at low pH 1 - 2.5.

It is a more active bleaching agent than HClO2.
•ClO2 is a corrosive and toxic gas. When generated too rapidly,

it escapes from the bleaching bath into the atmosphere

creating an explosion and health hazard. Once the ClO2 is

out of solution, its effectiveness as a bleach is lost.
• Optimum pH = 3 - 5 by addition of an acid such as formic,
acetic or phosphoric acid.
•If the pH drops below 3.0, cotton fiber is severely damaged

because weak acid (HClO2) and strong acids (HCl and

HClO3) are

formed which hydrolyze the cellulose.
Effect of Temperature
•< 50 ⁰C little or no bleaching takes place, however

the bleaching rate increases considerably up to 90
⁰C.



Advantages:
• Safe for Rayons, Acrylics and polyester
.• Complete removal of kitties, waxes etc. with bleaching.
• Feel of the fabric extremelygood.
• Hard water does not interfere.
• Aqueous solution is extremely stable.
•Chlorite bleaching results in a degree of whitenesswhich

cannot be achieved with hydrogen peroxide in a

single stage process.

Disadvantages:
•Treatment under acidic conditions so all alkali mustbe

removed before hand.
•ClO2 liberation causes pollution and corrodes

equipment.
• Continuous methods cannot be used.
• More expensive than H2O2.





Bleaching of Cotton:

•Cotton is bleached in the raw state, as yarn and
in the piece.
•In principle, the bleaching of cotton is a comparatively
simple process in which three main operations are
involved,
viz.
(1)boiling with an alkali; (2) bleaching the organic
colouring

matters by means of a hypochlorite or some other
oxidizing agent; (3) souring i.e. treating with weak
hydrochloric or
sulphuric acid.

For loose cotton and yarn these three
operations are sufficient, but for piece goods a
larger number of
operations is usually necessary in order to
obtain a satisfactory result.

In the case of cotton goods, 85 percent of these
fabrics are bleached by continuous peroxide
methods. In this system, the singed goods are put
through a rapid de-size steamer, washed,
impregnated with a mild 3 percent solution of
caustic soda, and pulled up into the top of a huge

J – shaped container (called a J box) that is
equipped to maintain a temperature close to 2120f
(1000 c). The J box is big enough to hold the goods
for atleast an hour. After this time period, the fabric
is hauled out of the J, given a hot wash,
impregnated with a 2 % solution of hydrogen
peroxide, and put in a second J for another hour.
Washing follows, and the fabric goes to the dryer
fully bleached.

J-Box