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Bleaching:

• Bleaching is chemical treatment employed for the decolorize of natural coloring matter from the substrate. The material appears whiter after bleaching.

• It improves whiteness by removing natural coloration and remaining trace impurities from the cotton; the degree of bleaching necessary is determined by the required whiteness and absorbency

The aim of bleaching can be described as following: 

• 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 Agent

• A bleaching agent is a substance that can whiten or decolorize other substances. Bleaching agents essentially destroy chromophores via the oxidation or reduction. Thus, bleaches can be classified as either oxidizing agents or reducing agents .

Bleaching agents

Bleaching agents…

• 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.

Bleaching:• Cotton being a vegetable fiber will be

bleached using an oxidizing agent, such as dilute sodium hypochlorite or dilute hydrogen peroxide. If the fabric is to be dyed a deep shade, then lower levels of bleaching are acceptable. However, for white bed sheets & medical applications, the highest levels of whiteness & absorbency are essential.

• Reductive bleaching is also carried out, using sodium hydrosulphite. Fibers like polyamide, polyacrylics and polyacetates can be bleached using reductive bleaching technology.

Type of Bleaching Agents:

Oxidative Bleaching Agents

Reductive Bleaching Agents

Enzymatic Bleaching Agents

Bleaching agent:

Auxiliaries used for bleaching:StabilizersActivatorsWetting agents/detergentsSequestering agentsAnti-corrosion agents

Calcium Hypochlorite Bleaching

• Calcium hypochlorite is usually a white or grayish-white powder free from lumps , contains about 35-37% active chlorine,

• Calcium hypochlorite is a yellow white solid which has a strong smell of chlorine. It is not highly soluble in water and is more preferably used in soft to medium-hard water. It has two forms: dry and hydrated.

Calcium Hypochlorite Bleaching

• 2Ca(OH)2+2Cl2 Ca(ClO)2 +CaCl2+H2O

• Calcium hypochlorite is the main component of bleaches

• *Bleaches cannot be stored with sunshine directly

• Ca(ClO)2+H2O+CO2 CaCO3↓+2HClO

• *If bleach is exposed to the sun, it will lose its function.

Sodium Hypochlorite Bleaching:

• It is done by using sodium hypochlorite (NaOCl) as a bleaching agent. NaOCl is a highly unstable compound at normal conditions of temperature & pH. It doesn’t exist as solid form. As it is highly unstable so it undergoes self decomposition by following reactions:

• 2NaOCl NaCl + NaClO2

• 3NaOCl 2NaCl + NaClO3

• 2NaOCl 2NaCl + O2

• The bleaching mechanism of sodium hypochlorite consists of the following reaction:

• NaOCl NaCl + (O)

• Relatively it is less costly.

• No need of stabilizers in hypochlorite bleaching.

• It is mainly used for cellulosic fibres not for protein fibres like wool, silk.

• Hypochlorite is having degrading affect on cellulose.

• It doesn’t give any scouring action.

• It can’t be used over coloured material.

• There is a problem of corrosion and unpleasant odours.

• Hypochlorite bleaching needs an antichlor treatment.

Hydrogen Peroxide (H2O2) Bleaching• Properties of H2O2

• It is a colourless syrupy liquid

• It is absolutely stable under acidic conditions

• It is sensitive to sunlight.

• It decompose if allow to react with heavy metals.

• It is highly unstable to alkali like NaOH, Na2CO3, rapidly decomposition takes place.

• 2 H2O2 2 H2O + O2

Mechanism of peroxide bleaching

Hydrogen peroxide is stable in acidic medium,

Bleaching occurs by the addition of alkali or by increased temperature, when perhydroxyl ion (HO2

-) liberates.

Perhydroxyl is highly unstable and in the presence of oxidisable substance (coloured impurities in cotton), it is decomposed and thus bleaching action takes place. Sodium hydroxide activates hydrogen peroxide because H+ ion is neutralized by alkali which is favorable for liberation of HO2

-.

• H2O2 + NaOH H2O + HO2-

Recipe for Bleaching:

• H2O2 Concentration - 2-4%

• Sodium silicate - 0.5-1%

• NaOH/Na2CO3 - 0.5-1%

• Sequestering agent - 0.1-0.3%

• pH - 9.5-10.5

• Temperature - 80-85°C

• Time - 90 mins

• Peroxide is universal bleaching agent can be employed to wool, silk as well as cotton.

• Peroxide is milder agent so degrading affect on cellulose is less.

• Peroxide also gives mild scouring action so simultaneous scouring and bleaching is possible in continuous process.

• It doesn’t affect the coloured material so it can be used for coloured materials.

• With H2O2 there is no need of danger of equipment corrosion and no unpleasant odours.

• Only rinsing after bleaching is sufficient

• Bleaching with peroxide is a costlier than hypochlorite.

• Hydrogen peroxide bleaching requires stabilization usually with silicates which have problem of stains on subsequent dyeing.

Comparison of Bleaching Efficiency:

pH Temp. Time Wax Ash Fluidity %Reflect.

NaOCl 11 40 0C 2h 0.23 0.05 6.2 83

NaClO2 3.8 98 0C 2h 0.21 0.02 6 86

H2O2 10.5 90 0C 2h 0.09 0.41 5.1 90

Image of Bleaching:

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.

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