General

Bleaching agent: H2O2 35 or 50 %

(Redox potential: 810-840 mV)
Bleaching of Cotton with Molecular mass: 34 g/mol
Hydrogen Peroxide Powerful oxidizing agent and can cause combustion.
Storage
Heavy metals
pH value
Muhammad Zubair Conc. of H2O2
Lecturer
Temperature
Influence of light
Activation by temperature, alkali, impurities

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Hydrogen Peroxide Bleaching Hydrogen Peroxide Bleaching

Available commercially as a stabilized liquid of pH 4.5-5.5. Advantages Disadvantages
Requires activation to produce a bleaching effect.
Stability and consistency of supply Fibre damage
Temperature controls the bleaching action.
Permanent white Quality of whiteness
Bleaching best carried
Environmental friendly Cost-effectiveness
out at pH 10.5-11.
Versatility of application

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Mechanism of Hydrogen Peroxide Bleaching Mechanism of Hydrogen Peroxide Bleaching

Two reactions carried out simultaneously

STEP-I
Main reaction : generation of bleaching specie

Side reaction: 2H2O2 2H2O + O2

ionization of the hydrogen peroxide molecules in the presence of hydroxide
ions to perhydroxide ions (HOO-) and protons (H+)

H2O2 + OH- HOO- + H+

H+ + OH- H2O

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Mechanism of Hydrogen Peroxide Bleaching Damage during Bleaching

STEP-II

Colour destruction and breakdown of other non-cellulosic impurities result

from oxidation reactions by the release of an active oxygen atom from
the decomposition of the unstable perhydroxide ion

HOO- OH- + [O]

Chromophore
Oxirane Diol

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Damage during Bleaching Control of pH and temperature in peroxide bleaching
The rate of decomposition of hydrogen peroxide is dependent on the
following variables:
pH

Temperature

Types and concentrations of impurities

Type and concentration of stabilizer

Aldehyde Carboxylic acid Keto

Diketo Tartaric acid Oxalic acid

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Stabilization of peroxide Stabilization of peroxide

To avoid undesirably rapid decomposition an additive is required known as
Stabilizer or Regulator.
Stabilizer
Stabilizer preserve the perhydroxy anions (HOO-) and largely suppress
decomposition.
A Stabilizer has to fulfill following main requirements:
Predominant stabilization of perhydroxy anions
Anti-catalytic effect by means of complex formation
To maintain alkaline earth metal ions in a sufficient
Quantity in homogeneous distribution by means of dispersion
Stability in bleaching conditions (oxidation, pH and temperature)
Biological degradability
Economic efficiency

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Stabilization of peroxide Types of Stabilizer
Stabilization is essentially dependent on two factors:
The alternating attraction forces which are active between the activated HOO-
ion and the alkaline earth metal ion
The extent to which the heavy metal ion is masked by the stabilizer, or the
formation of hydrogen peroxide radicals is prevented.

Stabilize helps in:

Increasing the degree of whiteness.
Avoiding chemical damage to the fibre.
Degradation of Impurities.
Suppressing the release of oxygen.

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Types of Stabilizer I. Silicates

Inorganic: The water glass frequently used in peroxide immersion bleaching is a
Silicates (water glass) in association with alkaline earth metal ions water-soluble sodium silicate with a ratio of:
Phosphoric acid, alkali phosphates, polyphosphates and pyrophosphates Na2O: SiO2 = 1: 3.3 = Na2SiO3 .2H2O.
Stannates
Borates
Organic:
Products based on complexing agents, e.g.
Polyaminocarboxylic acids
Polyoxycarboxylic acids
The ratio of sodium oxide to silica
Phosphonic acids
Products based on dispersion agents, e.g. 1 in sodium metasilicate (Na2O:SiO2)
crystalline forms, > 1
Fatty acid condensation products 2 in the orthosilicate (2Na2O:SiO2)
Fatty alcohol sulphates
Ethoxylates Colloidal forms preferred <1
Organic sulphonic and phosphonic acids

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I. Silicates II. Polyphosphates

Advantages Disadvantages These are produced as a result of the condensation of orthophosphates.

Low cost and high performance. Tendency to dehydrate at high
Best stabilizer in combination with temperatures.
alkaline earth ions in pad-steam Deposits on fabric and machinery
process. parts.
It is resistant to oxidization and Dye resist areas and stains.
heat

Condensed Polyphosphate

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II. Polyphosphates III. Amino polycarboxylic acids

Advantages Disadvantages Compounds in this group include the following:

Detergent and soil-suspending At temperatures above 80C Nitrilotriacetic acid (NTA)
properties, solubility properties. and in a strongly alkaline
Ethylene diaminetetraacetic acid(EDTA)
Completing characteristics for medium above pH 11:
calcium, magnesium and Low stability Diethylenetriaminepentaacetic acid (DTPA)
polyvalent heavy metal ions. Lack of selectivity for heavy metal Hydroxyethylene diaminetriacetic acid (HEDTA)
ions in the presence of alkaline
earth Form water-soluble complexes in combination with alkaline earth
Water eutrophication. and heavy metal ions.

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III. Amino polycarboxylic acids IV. Polyhydroxycarboxylic acids

Advantages Disadvantages Include citric acid, oxalic acid and gluconic acid.
Stability at high temperatures At high temperatures (> 90C)
and in strongly alkaline Advantages Disadvantages
High selectivity for heavy metal
ions solutions, limited resistance Exceptional sequestering Working strongly dependent on
against oxidation properties the pH
Do not have protective colloids High selectivity for individual
or dispersing action metal ions
Only function in stoichiometric
numbers, in relation to the
heavy metal ions.

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V. Phosphonic acids V. Phosphonic acids

The salts of phosphonic acids are known as phosphonates,
Derived from the following group of phosphonic acids:

Many different phosphonic acids:

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V. Phosphonic acids Commercial stabilizers

Advantages Disadvantages CLARITE (Huntsman)

High level of thermal stability High cost as compared to water Clarite A
glass. Clarite MAX
Stable in strongly alkaline or
CONTAVAN (CHT)
oxidizing liquor
Stabilizer X(Clariant)
Act selectively on heavy metal
Stabilizer SIFA
ions.
Prestogen (BASF)
Average to good detergent and
dispersing properties.
Can be used in broad pH range.

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Preparing Peroxide Bleach liquors Catalytic Damage

The correct sequence for the addition of chemicals is:
1. Soft water; transition metal ions (iron, copper) should have been removed by
ion exchange
2. All organic auxiliaries, e.g. organic stabilizer, surfactants, sequestrants, if
required
3. Magnesium sulphate, to complex with silicate and/or organic stabilizer
4. Caustic soda
5. Sodium silicate, always added to an alkaline solution
6. Hydrogen peroxide.
For continuous bleaching a minimum of two chemical stock solutions is
recommended:
alkali, stabilizer and surfactant
hydrogen peroxide alone
Modern continuous feed systems use separate tanks for each chemical.

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Catalytic Damage Catalytic Damage

Metal ions can lead to the severest damage Catalytic damage presents itself to the processor in the form of
Formation of holes where metal particles are enclosed in the fabric. Broken yarns
Formation of oxycellulose combined with severe loss in DP value. Tears
Holes
Loss in degree of whiteness due to the loss of hydrogen peroxide.
Catalytic damage occurs when, for example, iron particles are bound up in
the cellulose fibres (e. g. during spinning or weaving)
There are two kinds of catalytic damage
Catalytic damage by oxidative degradation of cellulose
Catalytic damage by hydrolytic decomposition of cellulose

residual amount of peroxide for

bleaching

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Catalytic damage by oxidative degradation Catalytic damage by hydrolytic decomposition

Anodic Reaction

Fe 0 Fe 2+ + 2 e-

Fe 2+ + H2O Fe(OH)2 + 2 H+

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Continuous Bleaching Process Continuous Bleaching Range
To ensure good uniformity of bleaching and good reproducibility strict
control of the concentrations of the chemicals such s caustic soda,
hydrogen peroxide and the other chemicals is essential.

High wet Pick-up: 130-150%

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Advantages of High Wet Pickup

Lower tendency of the cloth to crease during the steaming pass with
tension-free cloth travel
More effective alkali activity on the primary wall and seed coat
Less fibre damage
One-stage bleaching only with peroxide

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High wet-pickup application units Benninger Impacta
Impacta BENNINGER
Flex Nip KSTERS
Dipsat Vario & Dipsat Plus - GOLLER

Liquor volume: 100 - 130 liter

Liquor exchange rate: 0.6 - 0.8
Liquor pick up: 90 - 130 %

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Ksters FlexNip Goller DipSat

Liquor volume: 20 - 30 liter

Liquor exchange rate: 0 - 0.2
Liquor volume: 20 - 30 liter
Liquor pick up: > 100 %
Liquor exchange rate: 0 - 0.2
Liquor pick up: > 100 %

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Steamer Steamer

For high liquor Cloth transport on

roller section
exchange tight strand
rates

Vertical inlet

Impregnation
unit For low liquor Cloth transport on
exchange rates roller bed
conveyor-belt
Horizontal inlet

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Washing

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Semi-continuous Bleaching Processes CPB Bleaching
In the semi-continuous technique we differentiate between cold pad batch Cold bleaching offers the following advantages:
and hot pad roll processes. simple, inexpensive installation
The difference between these two processes is the temperature and the Reduced consumption of energy, reaction at room temperature
type of batching. Direct application on grey fabric straight after singeing
Adequate degree of whiteness and absorbency for various dyeing processes.
In the pad batch process the rolled material is rotated at room temperature
carefully covered with plastic film to avoid drying.
The application itself is simple:
In the pad roll process the impregnated and padded material is rolled in a Singeing.
hot chamber(90-100C) for 1-2 hours.
Cold impregnation.
The quality of rinsing/washing off after bleaching is decisive for high Padding at 80-100% pickup.
degrees of whiteness and good absorbency. Rolling on rollers, a plastic film protects against drying.
batching at room temperature for 16-24 hours with continuous rotation.
Intensive washing on an open-width washer.

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CPB Bleaching Discontinuous bleaching processes

A certain quantity of material is submitted to an oxidizing chemical

process for a certain length of time at a well defined liquor ratio.
Long liquor ratio (20:1 -50:1), short liquor ratio (2:1 -6:1).
Either the material moves(the bath stands still) or the bath
circulates(the material stands still).
Grey Fabric
Batches of different weights can be treated on small, medium or
large machines.
The following machines can be used:
Jet/Overflow
Jig
Beam Dyeing Machine
Package Dyeing Machine
Batching
Winch
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