UNIT -1 BLEACHING:

The textile chemical processing sector of the textile industry covers all the processes on the textile that

involve some form of wet or chemical treatment. Processes such as scouring, bleaching, dyeing, printing

and finishing are carried out at different stages during textile manufacturing.

These processes all involve some form of chemical action on the material. The textile wet processing

sector can be divided into three distinct sections.

1. Preparation process

2. Coloration process

3. Finishing process

1. PREPARATION PROCESS:

The pretreatment process is carried out to prepare the textile material for subsequent processing, which

includes dyeing, printing and finishing. It is the heart of textile processing

The processes are: Singeing, Desizing, Scouring, Bleaching, Mercerizing, etc.

2. COLOURATION PROCESSES:

This process is to provide the textile with colour either for aesthetic reasons or for some functional

purpose determined by the end-use of the product.

The processes are: Dyeing, Printing.

3. FINISHING PROCESSES:

This process is to provide the textile with the properties that the end-use demands and which have not

already been provided by any earlier processes. The finishing processes are normally given to the textile

material for aesthetic or functional purpose.

The finishing treatment is given to the quality of the fabric in terms of appearance, handle, functionally

enhanced by some physical means or by chemicals.

The finishing process is two types

1. Mechanical process

2. Chemical process

EXAMPLE:

Mechanical processes: Compacting, Calendaring, Raising

Chemical processes: Softening, stiffeneing,Water Repellency, Flame Repellency, Antisoiling finish etc.

PREPARATORY PROCESS

Pretreatment is a heart of processing of textile. In Pretreatment, all these impurities are removed and

fabric is brought to a stage where it is more absorbent and white and can be easily processed further.

The process which is done to make the textile materials suitable for dyeing and printing such as

singeing, desizing, scouring, bleaching etc.

DEFINITION:

Pretreatment means any treatment, which is done before actual (dyeing and printing) process.

Textile pretreatment is the series of cleaning operations.

All impurities which cause adverse effect during dyeing and printing are removed in pretreatment

process.

OBJECTIVES:

Remove impurities (both natural and/or those added during production) from the fibers.

Improve the ability of the fibers to absorb water solutions of dyes and chemicals.

Impart the proper brightness or whiteness to fibers according to need, especially when brilliant or

certain pastel shades are desired, and

Impart dimensional stability to thermoplastic textile materials.

CONSTITUENTS OF COTTON:

As discussed earlier pretreatment is the series of cleaning operations. In pretreatment all the

impurities present in cotton are removed. Cotton fiber by nature contains:

Cellulose 86.8%

Oil and Waxes 0.7%

Pectin’s 1.0%

Carbohydrates 0.5%

Proteins 1.2%

Salt 1.0%

Moisture 8.5%

Colour Pigments

Others 2.0%

Except cellulose and water all the other are called as natural impurities.

Theses all the natural impurities are removed in pretreatment process.

PROCESS INVOLVED FOR REMOVING THE NATURAL IMPURITIES DURING THE PRE-

TREATMENT PROCESS:

• Short Fibres - Singeing

• Applied Impurities (Size Material) - Desizing

• Artificial Impurities (Oil, Strains, Dust, Dirt) - Scouring

• Natural Impurities (Oil, Wax, Pectin’s, Proteins) - Scouring

• Colour Pigments (Naturally present in cotton) - Bleaching

PRETREATMENT PROCESS OF COTTON FABRIC:

Inspection of grey fabric

Marking of grey fabric

Stitching

Shearing / Cropping

Singeing

Desizing (only for woven fabric)

Scouring

Bleaching

1. INSPECTION OF GREY FABRIC:

After receiving the cloth from grey godown, it should be thoroughly checked and inspected before

subjecting it to wet processing. The cloth is examined for the following:

FABRIC DEFECTS:

Oil, rust, stains, holes, damage selvedge and weaving faults like floats and read marks etc.

FABRIC SPECIFICATIONS:

Width, length, yarn count, ends and picks per inch and fabric composition.

2. MARKING OF GREY FABRIC:

Before carrying out any process, marks are made on the material for identification. For marking,

special inks are used for capable of resisting all subsequent treatment which the material may have to

go undergo.

3. STITCHING:

Small pieces of fabric are stitched together so as to form a continuous length, which is workable in

the subsequent processes. The stitching must be very secure so that the fabric may run smoothly on

various machines.

4. SHEARING / CROPPING:

The tufts and loose threads from the surface of material removed by the cutting them with blades on

shearing or cropping machines. This process is carried out in open width form.

5. SINGEING:

Singeing is the process for burning off of the protruding or short fiber ends on the surface of fabrics

to produce smooth fabric appearance and minimize pilling.

6. DESIZING:

Desizing is the process in which size material is removed.

7. SCOURING:

Scouring is the process in which natural as well as artificial impurities are removed

8- BLEACHING:

Bleaching is the process in which we remove the colour pigments in order to achieve the degree

of whiteness.

9- MERCERIZATION:

Mercerization is the optional process or in on the customer requirement. Mercerization is done to

achieve the luster, strength, more absorbent etc.

SINGEING

The verb ‘singe’ literally means ‘to burn superficially’. Technically, singeing refers to the burning-off

of:

– Loose fibres not firmly bound into the yarn and/or fabric structure;

– Loose yarns not firmly bound into the fabric structure;

– Protruding fibre ends sticking out of the textile yarns and/or Fabrics

Objectives:

Burning of protruding fibers from surface of yarn

or fabric

To improve the lustre and smoothness of the

material

Smoother and more uniform surface

Brighter appearance

Singed fabrics allow printing of fine intricate

patterns with high clarity and detail.

Types of singeing machines

There are three main types of singeing machines:

1. Plate singeing machine

2. Rotary-cylinder singeing machine

3. Gas singeing machine

DESIZING

• Desizing is used for removing sizing compounds from woven fabric previously applied to warp

and is usually the first wet finishing operation performed on woven fabric

Objectives:

• Removal of added impurities- startch

• Weight loss

• Improve wettability

Classification of Desizing Methods

The desizing methods can be classified as in

SCOURING:

Scouring is the process by which all natural and additive impurities such as oil, wax, fat, hand dust etc.

are removed to produce hydrophilic and clean textile material. It is one of the vital processes of wet

processing.

The action of scouring is performed by the alkali (sodium hydroxide or sodium carbonate) together with

auxiliaries

The main effects of this treatment are a 5 to 10% loss in mass and a dramatic improvement in wettability

and absorbency

The natural and added impurities have to be removed for the following reasons:

The sizing ingredients have to be removed for obtaining uniform reaction of chemicals during

bleaching operation.

The waxes, proteins, spinning oils need to be removed to get uniform absorbency for dyeing and

printing.

Mineral matter and natural coloring matter have to be removed to get uniform whiteness.

Objects of Scouring:

• To make the fabric highly hydrophilic.

• To remove natural impurities such as oils, waxes, gum, husks as nearly as possible.

• To increase absorbency of fabric or textile materials without physical and chemical damage.

• To produce a clean material by adding alkali.

• To make the fabric ready for next process.

• To remove non-cellulosic substance in case of cotton.

The changes occurring of cotton fibers during scouring:

• Saponifiable oils and free fatty acids are converted into soaps.

• Pectins and pectoses are converted into soluble salts of pectic acid.

• Proteins are degraded to simple soluble amino acids or ammonia.

• Mineral matters are mostly dissolved.

• Non-saponifiable oils are emulsified by the soluble soaps generated from the saponifiable oils.

• Additive dirts are removed.

• Residual sizing materials are broken down into soluble products.

Scouring process depends on:

• The type of cotton. & The color of cotton.

• The cleanliness of cotton.

• The twist and count of the yarn.

• The construction of the fabric.

• The shade % and type of shade of finished product

BLEACHING OF TEXTILES

Introduction

The natural fibre and fabrics even after scouring still contain naturally occurring colouring matter. This

yellowish and brown discolouration may be related to flavones pigments of the cotton flower. The

climate, soil, drought and frost can also cause various degrees of yellowness. Tips of leaves or stalks

coming in contact with the moist ball after opening will cause dark spots and discolouration.

Discolouration may also come from dirt, dust, and insects or from harvesting or processing equipment in

the form of oils and greases.

Definition:

The process in which natural coloring matter is removed from a fiber to make it white. The process may

be used on fiber, yarn, or fabric

Objectives:

To produce white fabrics by destroying the colouring matter with the help of bleaching agents with

minimum degradation of the fibre.

Classification of bleaching agents

The bleaching agents are classified in two types based the principles involved in removing of naturals

colouring matter in the fiber.

The bleaching agents either oxidize or reduce the colouring matter which is washed out and whiteness

thus obtained is of permanent nature.

So that bleaching agents are called as oxidation (oxidizing) bleaching agents and reductive

(reducing) bleaching agents

Chlorine based materials such as sodium hypochlorite, calcium hypochlorite and sodium chlorite,

predominated until the late 1920s when peroxide came into wider use with the cheaper electrolytic

synthesis of hydrogen peroxide.

By 1940, about 65% and to-day about 90% of all cotton is bleached with hydrogen peroxide.

Oxidative bleaching

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

The bleaching agent will destroy the coloring matter by reductive reaction of SO2.

Auxiliaries

It is used for to support and improve the efficiency of bleaching process. The following auxiliaries is

used in bleaching process.

Stabilizers

Activators

Wetting agents/detergents

Sequestering agents

Anti-corrosion agents

BLEACHING OF COTTON WITH HYPOCHLORITES• Hypochlorite bleaching (OCl-) is the oldest industrial method of bleaching cotton. Originally,

calcium hypochlorite, Ca(OCl)2 (bleaching powder) was used. Until 1940 most cotton fabrics

were bleached with NaOCl. Today only 10 % of the cotton fabrics bleached with sodium

hypochlorite method.

• Their use is declining because of anti-chlorine lobby and environmental pressures.

• Hypochlorites are excellent cidal agents for mildew and other bacteria and are used as

disinfectants and to control bacteria in swimming pools.

Sodium hypochlorite

Sodium hypochlorite is a sodium salt of hypochlorous acid (HOCI).

Sodium hypochlorite can be prepared by passing chlorine gas into a tank containing prepared solution of

caustic soda, sodium carbonate or mixture of the two (about 4% by weight) at about 27°C

2NaOH + C12 = NaOC1 + NaC1 + H20 + 25 Cal

6NaOH + 3C12 = NaC1 + NaC1O3 + 3H20

Sodium hypochlorite is called as the strongest oxidative bleaching agent used in textile processing

because of it is released the nascent oxygen (bleaching agents) at room temperature in alkali condition

(pH-10.5 to 11)

The strength of hypochlorites is generally expressed as the available chlorine content. The amount of

chlorine present in one liter of sodium hypochlorite solution.

Commercial NaOCl will have 12 to 15 % active chlorine. Household bleach is 5 % active chlorine.

Calcium hypochlorite is sold as a solid material and contains 65 % active chlorine.

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.

Mechanism of bleaching

The mechanism of sodium hypochlorite (NaOCl) bleaching is four step processes.

1. Hydrolysis

2. Release of bleaching agent

3. Maximum development of bleaching active agent (HOCl)

4. Available chlorine

1. Hydrolysis

Sodium hypochlorite is dissolved in water and it’s release the Hypochlorous acid (HOCl)

Hypochlorous acid (HOCl) is the active bleaching agent.

NaOCl + H2O NaOH + HOCl

2. Release of bleaching agent

Hypochlorous acid (HOCl) is a weakest acid in alkali condition. So that it is selfly decomposed in

this condition and disassociates in strongest hydrochloric acid (HCl) and nascent oxygen (O).

The nascent oxygen is a bleaching agent. It is oxdising the natural colouring matters and converts to

simple colourless compound.

HOCl → HCl + (O)

3. Maximum development of bleaching active agent (HOCl)

To develop the maximum amount of amount bleaching active agent (HOCl) with help of

hydrochloric acid (HCl) present solution.

NaOCl + HCl → NaCl + HOCl

4. Formation of free chlorine

To release the chlorine for bleaching process

HOCl + HCl H2O+Cl2

Process:

The process of hypochlorite bleaching is normally batch process.

1. Fabric preparation

2. Bleaching solution preparation

3. Blaching

4. Washing

5. Souring

6. washing

7. Antichlor treatment

8. Washing

1. Fabric preparation

Fabric must be desized and scoured.

2. Bleaching solution preparation

The solution is prepared based on the below recipe

Recipe:

NaOCl – 1.5 to 3 gpl of available chlorine

Na2CO3 – 0.5 gpl

Function of Chemical

NaOCl – Bleaching Agent

Na2CO3 – Its act as a buffer and to control pH of the solution. It’s also called bleaching active agent

3. Blaching

The bleaching normally done in batch process. Jigger, winch, kier, cistern, softflow dyeing machine

normally used for bleaching process

Condition of process:

Temperature : room temperature

pH : 10.5 – 11

Time : 1-2 Hours

Cistern machine:

Parts:

4. Washing

After bleaching, the bleached fabric washing with hot wash and clod wash

5. Souring

Bleached fabric is treated with diluted hydrochloric acid or sulphuric acid with 30 mins to make

neutral condition for bleached fabric.

6. Washing

After bleaching, the bleached fabric washing with hot wash and clod wash

7. Anti chlorine treatment

Anti chlorine treatment is to remove the chlorine content present in the hypochlorite bleached fabric

to avoid the yellowise problem due to formation of chloramines by using sodium bisulphate or

sodium thiosulphate or sodium hydrosulphite.

8. Washing

After bleaching, the bleached fabric washing with hot wash and clod wash

Factors effecting in hypochlorite bleaching operations

Cotton can be bleached with hypochlorite solution at room temperature containing 1-3 g available

chlorine per litre at pH range of 9.5 to 11. The pH is maintained by adding 5 g/1 sodium carbonate in

the bleach bath. After bleaching the cloth is treated with dilute hydrochloric acid to neutralise any alkali

(souring) present in the cloth. An antichlor treatment with sodium thiosulphate or bisulphite is also

recommended to remove any residual chlorine from the cloth. Finally, the cloth is washed with water to

rinse out acid from the cloth.

Effect of pH

In the region of pH 7, when hypochlorous acid and hypochlorite ion are present approximate the

same concentrations, the rate of attack on cellulose is greatly enhanced.

In the case of bleaching powder the pH has fallen to within the danger zone after 15 min and after l

hours when sodium hypochlorite is used.

As the pH falls below 5, the liberation of chlorine begins to take place and pH below 1.5, the whole

of hypochlorous acid is converted into chlorine.

In the pH range 9 to 11, a plateu occurs at which little change occurs and is the normal use range for

bleaching with hypochlorite solution.

Effect of temperature

Generally bleaching of cotton is carried out with 1.5 g/1 of hypochlorite solution at about 40°C for 1

hours. Higher temperature increases the rate of bleaching but at the same time degradation of cotton

is also increased.

A hypochlorite solution by themselves is quite stable at the boil at pH values 11 or higher, but

decomposes at lower pH values.

During the decomposition of hypochlorite solutions, chlorate formation predominates in the absence

of cotton, while in the presence of cotton, it is practically absent under alkaline condition.

Hypochlorite solutions if buffered to pH 11, the rate of bleaching are increased by a factor of 2.3

times for every 10°C rise in temperature. It is thus, possible to bleach at 60°C in 7 min, but is

difficult to control the degradation of cotton in such short period of bleaching.

Effect of concentration

The commercial solution of sodium hypochlorite contains 14-15% available chlorine, compared to

35-36% in bleaching powder. The concentration of hypochlorite in the bleaching bath generally

varies from 1 to 3 g/1 available chlorine.

The optimum bleaching conditions, however, depend on the degree of discolouration of the cloth and

thus the temperature and time of reaction should be adjusted according to the requirement. The

concentration of hypochlorite solution is normally estimated by means of standard thiosulphate or

arsenite titration.

Effect of electrolyte

The addition of salt in the hypochlorite bleaching bath increases the activity in the initial period due to

the production of nascent chlorine. The addition of chloride moves the equilibrium to the left-hand side,

but after, say 15 rain, when the new equilibrium is reached, the sudden stimulated effect is ceased and

bleaching resumes its normal course and hence action of salt is only temporary and confined to the first

stage of bleaching.

Advantages of sodium hypochlorite bleaching over bleaching powder

Sodium hypochlorite has practically replaced bleaching powder due to its various advantages although

solutions of hypochlorite are more expensive than calcium hypochlorite.

Bleaching powder is a mixture of calcium hypochlorite with lime. Thus, calcium carbonate

settles on the fibre during bleaching process. On the contrary, sodium hypochlorite is free from

any such danger of specks of lime being deposited on the cloth.

Bleaching powder is partially soluble in water and exists in a solid form. However, sodium

hypochlorite, being sodium salt of hypochlorous acid, does not require any dissolving

arrangement and are ready for immediate use.

Hypochlorites are easy to handle.

Chlorinated lime requires higher alkalinity than that of sodium hypochlorite solutions for active

bleaching conditions.

Sodium hypochlorite solutions have fewer tendencies for the pH value to fall during bleaching.

Caustic soda is liberated by the hydrolysis of sodium hypochtorite form sodium carbonate under

the action of carbon dioxide from atmosphere. Sodium carbonate thus formed can be easily

washed away with water and reduces acid requirement for souring. In such case, sulphuric acid

may be used as souring agent in place of the more costly hydrochloric acid.

Sodium hypochlorite can penetrate into the fabric more thoroughly than in the case of bleaching

powder and therefore shorter time of bleaching is possible in the case of sodium hypochlorite.

Disadvantages of sodium hypochlorite

Sodium hypochlorite does not produce completely satisfactory whites inspite of many

advantages.

Bleaching with sodium hypochlorite produces slight damage to cellulosic fibres.

Sodium hypochlorite cannot be used for the bleaching of synthetic fibres as it produces greater

damage to such fibres.

Bleaching with sodium hypochlorite solution requires corrosion resistant equipment.

Sodium hypochlorite produces unpleasant odours in working environment.

Sodium hypochlorite solution is harmful to skin in concentrated form.

It produces harsh handle on fabric. Furthermore, it cannot be used on natural animal fibres.

Stabilisation of sodium hypochlorite is difficult to achieve where pH varies.

The formation of highly toxic chlorinated organic biproducts (AOX) during the bleaching

process has limited its use because these compounds are a potential hazard to the drinking water

resources when discharged

Bleaching with Hydrogen Peroxide The volume strength of hydrogen peroxide is expressed as the volume of liberated oxygen at on

heating one volume of hydrogen peroxide sample.

Hydrogen peroxide is generally made of 10, 12, 20, 100 and 130 volume strength.

A 10 volume peroxide solution is one which will liberate 10 times its own volume of oxygen. The

percentage concentration is expressed as the quantity of pure hydrogen peroxide in 100 parts of

sample and is expressed as x%, y% etc.

The commercial supply of hydrogen peroxide is generally 35% and 50%. A 10 volume hydrogen

peroxide contains 3% hydrogen peroxide and thus, 1% H2O2 = 3.3 volume concentration.

Mechanism of peroxide bleaching

Though hydrogen peroxide is stable in acidic medium, but bleaching occurs by the addition of alkali or

by increased temperature.

1. Autolysis

H2O2 → H2O + (O) + x kcal

2. Dissolved in Aquesious Medium

H202 H+ + HO2-

3. Bleaching Reaction

HO2- → OH- + (O) atomic oxygen (bleach active)

4. Bleaching Reaction in alkali medium (activation)

NaHO2 Na+ + HO2-

5. Decompostion

2H2O2 → 2H20 + O2

Stabilisers for peroxide bleaching

The process of regulation or control of per hydroxyl ion to prevent rapid decomposition of

bleach and to minimise fibre degradation is described as stabilisation.

Stabilisers for peroxide normally function by controlling the formation of free radicals.

The commonly used stabliser is sodium metasilicate. Sodium silicate is the most conventional,

easily available and widely used stabiliser.

Sodium silicate forms a complex compound with perhydroxyl ions which are liberated slowly at

higher temperature during bleaching process.

Parameters in peroxide bleaching operations

1. Effect of pH

The stability of hydrogen peroxide depends on pH.

At pH 1 to 3 it is stable ;

At highly alkaline pH 11.5 to 13 it has least stability.

The bleaching takes place around 10.5 due to accumulation of perhydroxyl ions in the bleaching

bath. At neutral or weak alkaline media, hydrogen peroxide does not produce any whitening

effect and may cause degradation of cellulose.

2. Effect of Temperature

In practice cotton bleaching with hydrogen peroxide is carried out at 90-100ºC but the

temperature may be increased to 120ºC in the case of pressurised equipment with a

corresponding reduction in process time.

The rate of bleaching increases with the increase in temperature, but at the same time

solution becomes unstable and degradation of cotton increases.

Below 80ºC the evolution of perhydroxyl ion is very slow so also the rate of bleaching.

3. Effect of concentration of liquor

The optimum concentration of hydrogen peroxide depends on number of factors namely

1) Liquor Ratio,

2) Temperature and

3) Class of Fibre.

In the batch process using kiers 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.

4. 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. Cotton may be bleached in open kiers by circulating heated hydrogen

peroxide solution (88-95 ºC ) for 6 to 10 hours.

Continuous bleaching of hydrogen peroxide bleaching

The process is combination of both scouring and bleaching process. The following step are involved

1. Scouring

PADDING

PREHEAT

JBOX

WASHING

2. Bleaching

PADDING

PREHEAT

JBOX

WASHING

Scouring

Padding (caustic saturator)

Fabric Padded With 4 -5 % concentrated solution of caustic soda at 75ºC run at 6 to 8 times

The cloth is squeezed at 80 – 90% expression through the pair of rolls.

Recipe:

NaOH – 4-5%

Na2Sio3 – 1%

Wetting agent – 1%

Soap – 0.5%

Preheat

Heating chemically padded fabric to 93 to 100ºC by applying steam.

J - Box

Storing heated cloth for sufficient period of time to allow the bleaching reaction to take place

Temperature: 100ºC

Time: 60 – 90 mins

Washing

Wash the scoured fabric to remove the chemicals and products of decomposition

Bleaching (Peroxide saturator)

Padding

Fabric Padded With 2 -8 % concentrated solution of Hydrogen Peroxide run at 6 to 8 times

The cloth is squeezed at 80 – 90% expression through the pair of rolls.

Recipe:

H2O2 – 2 -8 %

NaOH – 0.4-0.6%

Na2Sio3 – 3%

Na2Co3 – 0.8 - 6%

Preheat

Heating chemically padded fabric to 90 to 100ºC by applying steam

J - Box

Storing heated cloth for sufficient period of time to allow the bleaching reaction to take place

Temperature: 100ºC

Time: 60 – 90 mins

Washing

Wash the scoured fabric to remove the chemicals and products of decomposition

Peroxide killer

To remove the traces of the peroxide content in bleached fabric using vigorous rinsing or using

inorganic reducing agents.

Washing

Wash the bleached fabric to remove the chemicals and products of decomposition

Advantages and Disadvantages of Peroxide over Hypochlorite Bleaching

Advantages:

i. Peroxide is a universal bleaching agent and can be employed for wool, silk as well as cotton.

Hydrogen peroxide is a milder reagent than hypochlorite and the degrading effect of peroxide

bleaching on cellulose is less influenced than is the case with hypochlorite.

ii. Peroxide is capable of continuing the scouring action simultaneously with the bleaching action,

thus a single stage combined scour and bleach or a continuous method is possible using

hydrogen peroxide.

iii. Peroxide bleaching is in general less liable to have adverse effect on dyed threads. The white

effect is good and permanent and there is less risk of yellowing at a later stage.

iv. Thorough rinsing followed by scouring or antichlor treatment is required with hypochlorite

bleaching, whereas with peroxide a comparatively short rinsing suffices.

v. With hydrogen peroxide, there is no danger of equipment corrosion, no unpleasent odours and no

limitations as to processing techniques.

vi. Increasing strict control over the discharge of AOX from chlorine bleaching liquors has led to a

greater advantage of peroxide processes for bleaching cellulosic fibres

Disadvantages

i. Bleaching with peroxide is costlier than that of hypochlorite or bleaching powder.

ii. Hydrogen peroxide bleaching requires stabilisation usually with silicates which brings the risk of

forming resist stains in subsequent dyeing, and causes a build-up of hard crystalline deposits on

plant and machinery causing abrasion damage to the fabric during passage.

iii. Catalytic damage' occurs during hydrogen peroxide bleaching of cotton fabrics and results in

small spots of unevenly dyed fabric or even, in severe cases, the formation of small holes.

iv. There is limitation in white obtained on acrylic fibres. It also causes deleterious effect on skin

when used in a concentrated form.

Bleaching of cotton with sodium chlorite

Cotton can be effectively bleached with sodium chlorite (1-2%) at a pH 4.0 •

Sodium Dihydrogen phosphate (0.2-0.5%), Stable Foaming and Wetting agent (0.1-0.25%),

Nitrogenous chlorine absorber and formic acid to maintain the pH.

The temperature of the bleach bath is raised to 82-90ºC and maintained at this temperature for 1-

3 h depending upon the machines (batch wise) employed for bleaching.

Chemicals should be added in the following order:

Water, previously dissolved sodium nitrate, buffer salts or other chlorite stabiliser, surfactants,

sodium chlorite (pre-dissolved, if solid product) and lastly the diluted acid.

The addition of acidic materials to concentrated sodium chlorite solutions must be avoided.

Sodium nitrate is used as a corrosion inhibitor.

It is not essential to use acid chlorite solutions in the semi-continuous processes (pad-roll or pad-

stack) where effective liquor ratio is low and neutral chlorite solutions (10 g/l) are recommended.

When long batching time are used, soda-ash (1 g/l) is added to maintain stability of the bath.

WINCH DYEING M/C

Suitable for Knitted Fabric Dyeing

Material to Liquor Ratio is 1:20 – 1:40

Fabric dyeing with free from of tension

Capacity of winch m/c 10kg to 1000kg

CALENDERING

Definition

Calendering is a process where fabric is compressed by passing it between two or more rolls

under controlled conditions of time, temperature and pressure.

Calender is a machine consisting of two or more massive rollers which are compressed by means

of hydraulic cylinders applying pressure at the journals

The reason fabrics are calendared is to improve aesthetics.

Function of roller:

One roll is considered the pattern roll and is responsible for the finished appearance of the fabric

The other roll is called a bowl and serves as the pressure back-up for the pattern roll and also

serves to transports the fabric through the machine.

Temperature

The temperature ranges from cold to 500°F

Pressure

Pressure may range from 200 psi to 2500 psi.

Moisture in the form of water or steam may be used to achieve a desired luster.

Effects:

The composition of the rollers, number of passes, temperature controls, moisture control , fabric speed,

number of bowls, arrangement of bowls, and pressure can vary to fit the desired effect

The major fabric changes are:

1. To compress the fabric and Reduced fabric thickness.

2. Increased fabric luster

3. Increased fabric cover

4. To up grade the fabric and impart smooth silky surface feel

5. Reduced air porosity and

6. Reduced yarn slippage.

Types of Calenders

The type of calendar used depends on the type of cloth to be run and what the desired effect is to be.

There are

Embossing calenders

Friction calenders

Swizzing calenders

Chase calenders

Compaction calenders and

Schreiner Calender

The difference between them is the number of rolls and the drive system.

The use of different types of calenders gives different effects such as:

Sheen appearance:

It can be obtained by smoothing the cloth surface, which ensures a better reflection of light.

Better coverage:

It is due to the compression of the cloth, which generates a flattening of each single yarn.

Softer hand:

It is obtained thanks to a slight ironing effect, which produces a smoother, and softer cloth

surface

Surface patterns:

They can be obtained by means of special effects ("embossing" for example) for decorative

purposes or to modify the surface smoothness.

Yarn swelling and rounding effect:

They give a modest glaze finishing to the fabric, a surface smoothness and above all a full and

soft hand.

UNIVERSAL CALENDERS:

These calenders, equipped with 3-5-7 or even more rollers, are referred to as universal calenders. They

can give the fabric different effects; some of them are detailed below:

i. Roll Effect: Flattened fabric, high coverage ratio, soft hand and moderate glaze;

ii. Matt Effect: High coverage ratio, soft hand and matt effect;

iii. Lustrous effect: This effect originates from friction created during the passage between a

smooth and a steel roller.

SWIZZING CALENDERS

Swizzing is a British term used to denote that the fabric runs through all of the nips at the same surface

speed as the rolls.

A Swizzing calender usually consists of seven to ten bowls and is run at ambient temperatures.

The fabric effect is closed interstices, a smooth appearance and gloss without the high glaze

characteristic of a friction calender.

Hot roller- More lusture and smoothness

7 - Closed-grained iron bowl

6 - Compressed cotton bowl

5 - Highly polished, chilled iron, steam – heated bowl

4 - Compressed cotton bowl

3 - Compressed cotton bowl

2 - Highly polished, chilled iron, steam – heated bowl

1 - Compressed cotton bowl

Friction calendaring

• Usually a three roll process

– A central cotton fabric or paper roll is sandwiched between two metal rolls which are turned

at very fast speeds as compared to the cotton roll.

– The fabric to be calendered is laced between the metal rolls and the cotton roll, and the

surface of this cloth is brought to a very highly polished state.

• Starches and waxes give a temporary glaze, while durable glazes are generated from fabrics treated

with resins

• It’s produces high degree of lusture on one side of the fabric and also closing up the interlacing

threads

• Third roll – Polished chilled iron bowl with steam and surface speed is double that of the fabric and

those lower two bowl. Speed variation b/w first and third rolls is one and half time higher than

lowest bowl.

• Friction effect is exerted on the fabric due to different speed level of bowl

Speed – 30m/mins for friction , 60m/mins for with out fiction. 7bowl roller run at 80m/mins, light finish

– 130-230m/mins

CHASING CALENDER

The cotton fabric passes normally through the nips of a 7-bowl calender, then chasing rollers. The cloth

is then fed again into the cotton nip of the calender.

Chasing gives the cloth a thready –linen appearance and a special handle

WITH WATER MANGLE

Under high pressure, threads are well flattened and the spaces b/w them are closed

Different effects comes from different bowl

Hollow, steam-heated chilled iron rolls provide hard surface --- lusture produced

Two cotton rolls in contact with each other ( nip ) – flat, soft effect

Steam calendering

The chilled iron roll, which is a hollow cylinder, is usually heated by using high-pressure steam.

This calender roller called steam calendering

SCHREINER CALENDER

The schreiner finish can be imparted to textile fabric by means of schreiner calender

Purpose

It’s gives silk-like brilliance to cotton fabric.

It’s gives the nearest possible resemblance to silk when produced on mercerized fabric

Uses

Cotton linings, sateen and printed fabric

Mechanism

The silk like finish due to the fine lines engraved on the steel roller of the schreiner calender

Lines – 5 to 20 lines/mm

Lines are at small angle with respect to the direction of the warp and weft threads in the fabric.

Weft sateen – angle of about 20° to the weft and in directions of the twist.

Plain weave – course line ( 6-7/mm)

Bowl

Dia – 7.5 cm for both bowl

Top bowl – Special, Fine grained steel and gas heated from the inside

Wide – 325 cm

Pressure – up to 140 tonnes

EMBOSSING CALENDER

Usually a two roll special calender.

Uses a heated metal roll with an engraved pattern surface and a paper roll with the "negative" of the

pattern on the metal or without a pattern.

Fabric is passed between these two rolls and the pattern is set into the fabric by heat and pressure.

Thermoplastic fibers can be set by heat.

Cotton fabrics must have a resin finish to give a durable effect.

To produce a damask effect on cotton fabric but the effect is temporary

INTRODUCTION TO DIFFERENT DYEING MACHINES

There are several dyeing machines used for dyeing of fibre, yarn, fabric and garments.Some of the important of them are

(1) Jigger Dyeing Machine(2) Winch Dyeing Machine(3) Jet Dyeing Machine(4) Padding Mangle(5) Package Dyeing Machine(6) High Pressure High Temperature Dyeing Machine. (Beam Dyeing Machine)

Jigger Dyeing Machine

Jigger dyeing machine is the most commonly used for dyeing all kinds of cotton fabric. There are mainly two types of jigger dyeing machine. One is open jigger dyeing machine and other is closed jigger dyeing machine. The open jigger dyeing machine is shown in the figure. This machine consists of V shaped stainless steel vessel. Two rollers are fitted above the vessel called as cloth rollers. These rollers

are rotated by power. Out of these two rollers one roller is driven by a motor which is called take up roller and the other roller from which the cloth is delivered is called let off roller. When all the cloth is passed from the let off roller to the take up roller, it is called as one end or one turn. The number of ends or turns depends upon the type of the fabric and also the percentage of the shade. Initially, a large length of (50 kg) cloth is wound on the let off roller and take up roller is then driven by the power. After one end is taken, the take up becomes let off roller. These backward and forward movements of cloth through the dye liquor absorb more and more dye. The capacity of the jigger is 100 to 150 gallons. In the modern jigger, automatic devices are fitted along with the timing switch by using reversing will take place automatically. When dyeing all the dye liquor should not be added at one time. The dye liquor should be added in batch wise, in order to get even shade on the cloth. In the present scenario,  closed types of jiggers are used. The main advantage is to prevent heat loss and chemical loses by evaporation. This type of jigger is very important for dyeing vat, Sulphur etc.

Advantages of Jigger Dyeing Machine

1. The cloth can be dyed in open width form of full width form.2. Chemical and heat loses are less when compared to winch dyeing machine3. The material to liquor ratio is 1:3 (or) 1:4 which saves considerable amount ofchemical cost and steam cost.

Disadvantages of Jigger Dyeing Machine

It exerts lot of tension in the warp direction and because of this normally woolen, knittedfabrics, silk etc are not dyed in jigger dyeing machine.

Winch Dyeing Machine

Winch dyeing machine is normally used for dyeing light weight fabrics like knitted fabrics as well as woven, silk etc. The winch dyeing machine is different in construction from the jigger. The machine

consists of elliptical winch (or) circular winch which is driven by hand or motor. Elliptical winches are most widely used. It has a perforated portion dividing the machine in to two compartments. One is small and the other one is bigger in size. Dye- liquor pipe, steam pipe, water inlet are provided in small compartment. Dyeing is carried out in the larger compartment. During working, one end of the fabric is passed over the guide rollers and winch and under the dye liquor and passed in between the pegs. After the sufficient amount of cloth is fed the ends are stitched and making a long continuous loop formation. Many such loops run together separately by a series of pegs, to avoid entanglements. The loops are drawn in and out of the bath by rotating the winch. After dyeing is over the pieces are separated. The winch is made up of stainless steel. Now-a-days closed type of winch is also used to prevent the temperature loss and pressure loss.

AdvantagesThin and light weight fabrics like knitted fabrics, voile cloth etc, can be dyed successfully in winch dyeing machine without exerting any tension. Embroidery fabrics may be dyed in winch dyeing machine without affecting the embroidery designs.DisadvantagesFabrics are dyed in rope form, so there are more chances for uneven dyeing. Therefore, we will get even dyeing only when retarding / leveling agent is added to the dye liquor. The material to liquor ratio is very high i.e. 1:30 or 1:40. Therefore the heat and the additional chemicals are provided more when compared to jigger. Heat and chemical losses are more because of the wider open tank. Padding MangleContinuous dyeing methods depend upon the impregnation with the padding mangles. The padding mangles are equipped with two, three or even four mangles. The rollers are arranged one above the other. Three bowl padding mangle are usually prepared for dyeing of cloth. The padding mangle consists of two ebonite rollers and one rubber covered roller. To reduce the material to liquor ratio, low capacity troughs are generally used with the different shapes. Some of the machines are provided with jacket troughs for maintaining the temperature of pad liquor. An over head tank is connected to the mangle for feeding the stock solution through the perforated pipe. The supply of the dye-liquor is controlled automatically to maintain a constant level. The pressure on the padding mangle is applied by hydraulic or pneumatic system which enables heavier and uniform pressure. The trough is fitted with

dye-liquor and the cloth passes through the dye solution, which is heated by steam. After that the cloth is passed between the ebonite and rubber covered roller to give uniform pressure throughout the fabric. The pressure can be altered depends upon the type of the fabric. The speed is 200 yards/minute. It is used for dyeing vat, Sulphur, Naphthol etc. The solution used in the trough is divided in two equal portions to avoid uneven dyeing. The padding mangles may also be used for desizing purposes.

Jet Dyeing Machine

This is the most modern machine used for the dyeing of polyester using disperse dyes. In this machine the cloth is dyed in rope form which is the main disadvantage of the machine. In this machine, the dye tank contains disperse dye, dispersing agent, leveling agent and acetic acid. The solution is filled up in the dye tank and it reaches the heat exchanger where the solution will be heated which then passed on to the centrifugal pump and then to the filter chamber. The solution will be filtered and reaches the tubular chamber. Here the material to be dyed will be loaded and the winch is rotated, so that the material is also rotated. Again the dye liquor reaches the heat exchanger and the operation is repeated for 20 to 30 minutes at 135o C. Then the dye bath is cooled down, after the material is taken out. Metering wheel is also fixed on winch by external electronic unit. Its purpose is to record the speed of the fabric. The thermometer, pressure gauge is also fixed in the side of the machine to note the temperature and pressure under working. A simple device is also fixed to note the shade under working.

AdvantagesDyeing time is short compared to beam dyeing.Material to liquor ratio is 1:5 (or) 1:6Production is high compared to beam dyeing machine.DisadvantagesCloth is dyed in rope formRisk of entanglementChance for crease formation.