1. UNIVERSITY OF KARCHI
CHEMICAL ENGINEERING
BLEACHING

2. 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”.

3. 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.

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

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

6. 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.

7. 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

8. 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.

9. 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 as solid 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.

10. 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 –

11. 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 bleaching takes 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 gas is liberated and
the solution has no bleaching effectiveness at all.
• The optimum pH for bleaching is between 9 and 10.

12. Effect of Time and Temperature
Time and temperature of bleaching are interrelated.
• Concentration is also interrelated with time and temperature.
•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 develop a distinctive
chlorine odor
• An antichlor treatment with sodium bisulfite and acetic acid
removes any residual chlorine from the cloth

13. Uses:
Hypochlorite is used mainly to bleach cellulosic fabric
• It cannot be used on wool, polyamides (nylon), acrylics. These
fibers will yellow from the formation of chloramides.
• Bleaching with hypochlorite is performed in batch 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 40 ⁰C
• Drop bath, rinse
• Add antichlor chemicals
• Rinse

14. 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

15. 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 nascent oxygen)
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.

16. • 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.

17. 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 and equipment
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
phosphates

18. Activator
provide alkalinity eg:NaOH, Na2CO3, Na3PO4,etc.
Sequestering agent:
organic stabilizers or separate eg:EDTA, Sodium hexa meta
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
stabilization.

19. • Since it ultimately decomposes to oxygen and water, it
doesn’t create effluent problems.
Advantages of H2O2
• Universal bleaching for Cotton, Rayon, Wool and Silk
• It can be used on continuous equipment.
• Permanent Whiteness
• Simultaneous Scouring/ bleaching and continuous
bleaching possible
• Degradation is less.
• Less water is required with peroxide bleaching and
there is no need for souring after bleaching.
• Peroxide bleached goods are more absorbent than
hypochlorite bleached goods.

21. 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 bleaching specie
• 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.

22. 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.
• At boiling - excessive loss of ClO2 with the steam.

24. Advantages:
• Safe for Rayons, Acrylics and polyester
.• Complete removal of kitties, waxes etc. with bleaching.
• Feel of the fabric extremely good.
• Hard water does not interfere.
• Aqueous solution is extremely stable.
• Chlorite bleaching results in a degree of whiteness which
cannot be achieved with hydrogen peroxide in a single
stage process.
Disadvantages:
• Treatment under acidic conditions so all alkali must be
removed before hand.
• ClO2 liberation causes pollution and corrodes
equipment.
• Continuous methods cannot be used.
• More expensive than H2O2.

27. 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.

28.  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.

29. J-Box

30. Bleaching of Viscose:
• Filament viscose rayon may not require bleaching
since this is normally carried out during manufacture.
However, viscose in staple form requires bleaching as it
may not necessarily include a bleaching treatment
during its manufacture.
•The same reagents as those used for bleaching linen
and cotton fabrics are useful for these fibers.
 For very good whiteness, rayon may be bleached on a
jigger with alkaline hypochlorite or combined scour and
bleach using hydrogen peroxide containing sodium
silicate and alkaline detergents-at a temperature of
about 70°C.

31. Bleaching Jigger

32. Bleaching of Blended Fibre Fabrics
Polyester/Cellulosic Blends:
•Polyester fibre in blends with cellulosic fibres in the ratios of
65/35 and 50/50 are common construction.
• When cellulose portion is rayon, the blends rarely require
bleaching, but when cotton is present bleaching is usually
necessary.
•Bleaching treatments of such blends are normally required to
remove the natural colours of cotton, sighting colours and if the
polyester portion is turned yellow at the time of heat-setting
operation.
•Chlorine bleaching, peroxide bleaching and chlorite bleaching
are employed widely.

33. •If the polyester portion requires bleaching, then chlorite
bleaching is used, as this bleaching agent bleaches both
polyester and cellulose.
•If the polyester portion does not need bleaching, then
peroxide bleaching is more convenient.
•Alkaline hydrogen peroxide bleaching is the most
preferred system for polyester/cotton blends.
Polyester/Wool Blends:
• In general, blends containing wool and polyester fibres can
be bleached with hydrogen peroxide either in acid or
alkaline medium without risks of damage.
• In acid medium, the fabric is treated with a solution
containing 30-40 ml/l H2O2 (35%), 2-4 g/1 organic
stabilizer,0.25 g/1 wetting agent and 0.25 g/1 detergent at
pH 5.5-6 (acetic acid) for 40-60 min at 80°Cor 2-2.5 h at
65°C. The treated fabrics are then given warm and cold
rinse.

34. •In alkaline medium, the bath comprises of (35%), 30-40
ml/l H2O2; sodium pyrophosphate, 2-4 g/l; ammonia to
maintain the pH 8.5-9. The bath is set at 40°C and the
goods are treated for 2-4h, and rinsed well in warm and
cold water.
Nylon/Cellulosic Blends:
•Blends of nylon and cellulosic fibres may be bleached
with either H2O2 or NaClO2. H2O2 does not bleach nylon
and normal methods of bleaching degrade nylon.
•Blends containing 30% or less of nylon may be bleached
by the continuous H2O2 method, and in such cases cotton
will absorb the peroxide preferentially and so protect the
nylon from damage.

35. Nylon/Wool Blends:
• It is difficult to bleach this blends since the method
normally used for nylon degrade wool.
• Alkaline H2O2 bleaching always damages the polyamide
fibres to some extent.
•Normal alkaline H2O2 bleaching process may be used
with safety on blends containing up to 25%polyamide, but
acid bleach must be used when proportion exceeds this
figure.
• The fabric can be bleached with a solution containing 12-
15 ml/1 H2O2 (35%); 2 g/1 tetra sodium pyrophosphate, 1
g/1 EDTA (30%) at 60-65°C for 45-60 min and then rinsed
well in water.

36. Determine the bleaching Efficiency:
Absorbency Test:
•The simple test of measuring the absorbency of sample
consists of allowing a drop of water to fall from a fixed height
(2.5 cm) on to the conditioned fabric sample, which is
mounted on an embroidery frame of about 6 inches diameter.
A stop watch is started as soon as the drop falls on the fabric
and stopped as the water drop is completely absorbed by the
fabric. This complete absorption of drop is ensure by
appearance of a dull spot on fabric i.e. the reflected light
disappear from the edge of drop. This time is termed as the
absorbency time.
•Yet another method for absorbency test is the measurement
of the time required for the sample of about 1 inch size to
sink in water, termed as sinking time. A drop absorbency or
sinking time of about 5 sec is generally considered
satisfactory for well prepared fabric.

37. Developments in Bleaching
1.Activated Bleaching Process
2. Non conventional stabilizers for hydrogen peroxide
bleaching
3. Use of enzymes
Activated Bleaching Process
1st Step (FRP J-Box)
Sodium hypochlorite (3g/l)
Temp. - 30OC; Time - 30-45 min
2nd Step (SS J-Box)
H2O2 (0.5-8%), Sodium silicate,
EDTA Ethylenediaminetetraacetic acid,
pH 10.5
Temp. 95OC
Time 2hr

38. Non conventional stabilizers
• for hydrogen peroxide bleaching based on caprolactum,
ethylene glycol, N-acyllactum and glucamine and glutamic
acid.
Use of enzymes
• Use of glucose oxidase coupled with glycoamylase, has
shown whiteness and yellowness index similar to that of
conventional alkaline bleaching. Use of immobilized
catalases for removing the residual H2O2 leads to savings
in water consumption

39. Optical Brightening
• Optical brighteners (sometimes called
optical bleaches or fluorescent whitening
agents) are fluorescent white dyes that
absorb ultraviolet region (340-370 nm)
and re-emit into longer visible (400–500
nm) wavelengths.
• These OBA are available in different tints
such as blue, violet and red.
• By increasing the amount of blue light
reflected, yellow tones appear whiter.
• Now membrane technology can be applied
to capture and concentrate the optical
brightening agent for reuse to lower the
operating cost in textile industries.

40. Classification of Optical Brightening Agents:
The classification of of OBA can be based either on the
chemical structure of the brightener or on its method of
application. They can be broadly classified primarily in to
two large groups.
•Direct {Substantive} brighteners
Direct optical brightening agents are predominantly
water soluble substances used for the brightening of
natural fibers and occasionally for synthetic materials such
as polyamide.
• Optical brighteners increase the apparent reflectance of
the textile in the visible region by converting UV-radiation
in the visible light and so increase the whiteness or
brightness.
•Optical brightening is used when more white form of
fabric is required.

41. Chemical Constitution of Optical Brighteners
Fluorescence characteristic is associated with their chemical
structure. FBAs resemble dyes in all respects except that they
have no visible color and are thus called colorless dye. They are
substances normally having a system of conjugated double
bonds and must be essentially planer and should contain
electron donating groups such as OH, NH2 etc. and be from
electron accepting groups such as NO2, -N=N - etc.
•Disperse brighteners.
Disperse optical brightening agents are mainly water
insoluble and as with disperse dyes they are applied either
to colored from an aqueous dispersion or they can be used
for mass colouration. They are used for synthetic materials
such as polyamide, polyester, acetate.

42. Application
As far as their behavior towards the fiber is concerned and their
application properties, fluorescent brightening agents are similar
to dye stuffs. The same standards of fastness have to be met by
white goods which have been treated with fluorescent
brighteners as colored dyeings. Exhaustion on to the fibre may be
controlled by the addition of salt and the application temperature.
FBA also have different fastness properties.
Conjugated double bonds are essential in the constitution of
fluorescent molecules
e.g.: -C=C-C=C-C=C- (stilbene type)
-N=C-C=C-C=C- (imidazole type).
A large number of FBAs are based on diamino stilbene
disulphonic acid. There are also products based on coumarin
and benzimidazole. FBAs may be anionic, cationic or a
dispersion.

44. REFERENCE:
http://textilelearner.blogspot.com/
http://www.digilibraries.com/
Encyclopaedia britannica 11th edition
Chemical Technology in the Pre-Treatment Processes
of Textiles By S.R. Karmakar
Textile Preparation and Dyeing By A K Roy Choudhury
Textile Chemicals: Environmental Data and Facts By
Katia Lacasse, Werner Baumann
Textile fiber to fabric by Bernard P. Corbman
http://www.slideshare.net/
http://www.ptj.com.pk/
http://www.fibre2fashion.com/
https://sabeenzulfiqar.wordpress.com/2013/01/
http://www.synderfiltration.com/optical-
brightening-agent