UNIVERSITY OF KARCHICHEMICAL ENGINEERING

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.

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.

Bleaching AgentA 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 bleachingMany 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 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.

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 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.

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

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

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 nascent oxygen) O* + X →X-O, X- oxidizable substanceH2O2 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 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

Activator provide alkalinity eg:NaOH, Na2CO3, Na3PO4,etc. Sequestering agent: organic stabilizers or separate eg:EDTA, Sodium hexa meta phosphateWetting agent to provide wetting and detergencyUses: 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.

• 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.

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.

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.

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.

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 

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.

Bleaching Jigger

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.

•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.

•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.

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.

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.

Developments in Bleaching1.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

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

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.

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.

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.

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.

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