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Objectives of bleaching process:
* The main objective of the bleaching process is to remove the natural colours and pigments present in the cotton fabric, yarn, and fibres.
* To whiten the fabric.
* If the scoured fabric contains natural colours and pigments, the brightness of the shade, absorbency of the cotton, and uneven shade result after dyeing.
* Pastel shades do not appear prominently on the fabric if the fabric has traces of natural colours and pigments.
Bleaching is the last process of textile wet processing.
The removal of natural colours and pigments present in the cotton by treating the material with bleaching agents is called the “bleaching process” the natural colours and pigments get disappeared from the cotton after bleaching and whiteness in the material results.
Types of bleaching:
There are two kinds of bleaching carried out;
* Bleaching by the oxidation process
* Bleaching by reduction process
Bleaching by oxidation process:
There are three kinds of bleaching processes carried out in this kind of bleaching process.
* Hypochlorite bleaching
* Hydrogen peroxide bleaching
* Sodium chlorite bleaching
Hypochlorite bleaching process:
The calcium compound known as bleaching powder was used exclusively. It is a mixture of calcium hypochlorite Ca (OCI)2, and basic calcium chloride, CaCl2, Ca (OH)2.H2O. Bleaching powder being solid is easy to transport and has a long shelf life but its use involves excessive and careful handling in dissolving and removal of the non-oxidizing solid constituents.
Sodium hypochlorite has now replaced bleaching powder because it is more economical at easier to handle.
A percentage of 15- 18% of available chlorine is generally maintained in the concentrated hypochlorite solution but it may be noted that the concentration falls gradually during storage and needs to be checked the information of use.
Hypochlorites are strong oxidizing agents and it is, therefore, necessary to carry out bleaching under the below-prescribed conditions:
* Hypochlorite bleaching requires well-controlled pH during the bleaching process. The oxidizing capability of the hypochlorite varies with variation in the pH of the bleaching bath.
* The oxidizing component exists as a hypochlorite ion (-OCl- ) at pH above 10.
* The presence of the hypochlorite ion gets almost zero between 5 and 8.5 pH. it gets changed into hypochlorous acid.
* It starts to liberate the chlorine gas at pH below 5.
* The whole hypochlorous acid gets changed into chlorine.
* It is well known that the rate of bleaching or oxidation of cellulose becomes maximum between pH 5 - 9. The maximum generation of the hypochlorous ion during the bleaching process takes place above the pH range.
* Bleaching is, therefore, done at a pH range of 10-11 for the minimum damage to the cotton goods.
* Sodium hypochlorite solution has a pH of 11.5 but during the bleaching reaction, hydrochloric acid is generated. The generated hydrochloric acid causes to start falling off the pH of the solution.
* The hypochlorite solution is buffered with 5-I 0 g/l of sodium carbonate to maintain the pH.
* Rate of bleaching also depends on temperature and rises with an increase in the reaction temperature.
* Normally bleaching is carried out with 5 to 9 g/l of active chlorine at room temperature for about 2-4 hours but it is quite sale to process at 40°C to reduce the production time.
* Contact with copper and iron metals is avoided as these catalyse effects of water quality on reactive dyeing of cellulose.
* The oxidation reaction may tender the goods.
Dechlorination or anti- chlorine treatment
It is necessary to remove traces of the residual chlorine after bleaching otherwise it may damage cotton goods during storage. This is done either by treatment with 5 g of sulphuric or 20 g/l of hydrochloric acids or with 5 to 10 g/1 of sodium bisulphite (NaHSO3) solution at 60°C for 30 minutes. The acid converts hypochlorite into volatile chlorine while the bisulphite reduces it into a harmless sodium or calcium chloride.
In an alternative method, alkaline hydrogen peroxide (3cc/I H2O2 + 0.3 g/ NaOH at 90°C) is used that besides dechlorination improves the quality and stability of the whiteness.
Bleaching with hydrogen Peroxide
* Hydrogen peroxide is most commonly used as an oxidative bleaching agent today. It is used in batch, semi-continuous and continuous processes as well as for bleaching coloured goods and for combined scouring and bleaching of lightweight cotton goods.
* Less amount of water is required in peroxide bleaching in comparison to hypochlorite bleaching.
* The effluent treatment gets lesser than hypochlorite bleaching due to the low amount of water used.
* Oxidative damage is low in peroxide bleaching.
* The bleaching equipment is the same as that used for scouring for both the batch and continuous processes.
* In the latter, two storage chambers can either be installed in tandem for scouring and bleaching or these processes may be done in one chamber alternately.
* Hydrogen peroxide has a low dissociation constant and is a weak acid.
* The peroxide anion is produced In an alkaline solution, which is the active bleaching agent.
* A high concentration of hydroxyl ions has an accelerating effect on the rate of bleaching.
* The rate of reaction is controlled necessarily by adding a stabilizing agent.
* Sodium silicate is used as a stabilizing agent.
* Sodium silicate also provides protection against metal contaminants.
* Silicate is more effective in the presence of salts of magnesium and this provides a rare case where hard water is preferred over the soft type.
* Magnesium salts are sometimes added into the peroxide bath of calculated concentrations.
* A disadvantage of the silicate is that it deposits hard insoluble incrustations on tile sides on the side of the machine, which scuffs the fabric and the scuff marks
A typical recipe for a wet on wet bleaching in a roller bed continuous system is as below:
Hydrogen Peroxide 35%: (50-60 ml/l)
Sodium Silicate: 10 ml/l
Organic Stabilizer: 10 g/l
Sodium Hydroxide: 3 g/l
Wetting Agent: 1-2 g/l
Liquor Pick-up: 100%
Impregnating Temperature: 20-30°C
Reaction Time: 1-2 hours at about 95°C.
The concentration of the reactants and the time of treatment may, however, vary according to the degree of impurities and quality of whiteness required.
Bleaching with Sodium Chlorite:
* Sodium chlorite (NaClO2) was originally introduced for bleaching the synthetic fibres
* Now the use of sodium chlorite is increasing as a bleaching agent for cotton goods because of all the bleaching agents it is the least damaging to cellulose.
* It is sometimes used for bleaching grey cotton goods without prior boil-out but absorbency obtained is just tolerable.
* The impurities of cotton do not get removed in the grey bleaching. The impurities of cotton are oxidized to colourless products
* There is very little loss in weight of the goods after the treatment.
* Sodium chlorite is commonly marketed in the form of a crystalline powder in 80% strength but it is available in a liquid state also.
* The aqueous solution of the chlorite is slightly alkaline.
* It has a pH of about 8.5.
* However, it is acidified to a pH value within a range of 3.5 to 4.5 to liberate the oxidizing agent.
* Doubts exist about the real oxidizing entity and different workers have suggested this to be chlorine dioxide (ClO2) chlorous acid (HClO2) or even atomic oxygen.
* Chlorine dioxide gas gets poisonous and explosive. It is also very corrosive to metals in the aqueous medium.
* Chlorite bleaching is therefore often carried out in an exclusive room in the dye house that is very well ventilated.
* The machines are fabricated from special stainless steels that have a high proportion of molybdenum or titanium.
* Alternatively, bleaching is done in equipment made of stone, PTFE coated steel or wood.
* It is not uncommon to add sodium nitrate in a quantity equal to the chlorite for the protection of the stainless steel metal of the machines that moderates decomposition of the chlorite and inhibits corrosion of metals.
* The pH of around 4 ± 0.2 required for bleaching is maintained with buffers or as termed in industry activators, like sodium acetate or sodium dihydrogen phosphate (NaH2PO4) Latter is usually preferred because it improves whiteness o goods.
* Neutral or slightly acid chemicals that liberate acid on heating are also used occasionally.
* Organic esters like ethyl lactate or titrate and their ammonium salts are also suitable for this purpose.
Common recipes for bleaching are given below:
Chemicals Batch Process Continuous Process
Sodium chlorite (80%) : 20-25 g/l
Sodium nitrate: 2-3 g/l
Sodium dihydrogen phosphate: 0.5-1.0 g/l
Wetting agent: 1-2 g/l
Formic acid to adjust pH: 3.8-4.2 6-6.5
Reaction time: 2-4 Hours
After the treatment, washing with boiling water and dechlorination with 0.5- 1 % cold sodium bisulphate solution is carried out.
Bleaching by reduction process:
* Many coloured materials are decolourised by reducing agents such as sulphites, bisulphites and hydrosulphites.
* The colourless form is sometimes insoluble in water and may be easily oxidised to the coloured form.
* These reasons limit the use of reducing agents as bleaching agents.
* Sodium hydrosulphite (hydros) is used for the bleaching of wool but only in combination with hydrogen peroxide.
* Sodium hydrosulphite gets available in powder form.
* It does not dissolve in water easily and rapidly decomposes unless the solution is alkaline.
* Stabilised sodium hydrosulphite has alkali added to the powder to improve solution stability.
* Reducing agents are used as antichlors (chemicals that remove the last traces of chlorine from the fibre after a hypochlorite or chlorite bleach).
* Their use in this area is diminishing, often on environmental grounds (odour, toxicity), in favour of hydrogen peroxide.
Continuous bleaching range (CBR):
In continuous bleaching range, it combines the pre-treatment processes of textiles like desizing, scouring and bleaching in this machine itself. This minimises energy consumption and reduces the number of operations.
The three Processes involved in the Continuous bleaching Range machine are:
1. Desize Wash (To remove starch content from fabric)
2. Scouring (To remove the impurities present in fabric & to improve absorbency)
3. Bleaching: (To bring whiteness to the fabric.)
Fabric passage in continuous bleaching range:
Structure and working principle continuous bleaching range(CBR):
The structure and working principle of the continuous bleaching range( CBR) are given below:
Desized fabric batch:
The desized fabric wound on the batch fed to CBR for the bleaching process.
This unit controls the fabric feeding. It helps to make the process efficient and continuous. The fabric at the feeding side is collected in the J-scray during batch change. The J-scray avoids machine stoppage during batch change.
It consists of the tension roller, pressure roller, and compensator. The tension roll helps to provide the required feeding tension. The compensator compensates for the fabric tension when the feeding tension gets varied. Fabric feeding without crease is ensured by inlet J- scary unit.
Steam and hot water injection unit:
The desized fabric has residues of starch and other chemicals. The prior removal of these chemical residues is required before the bleaching process. The fabric enters the first pre-washing compartment. The steam and hot water are injected into the fabric in this compartment.
Now, the fabric enters the second pre-washing compartment. The starch and other impurities are washed off from desized fabric by using hot water.
Bleaching Chemical trough:
Next, the washed fabric enters the bleaching chemical trough. The solution of a mixture of various chemicals according to recipes is filled in this trough. The fabric padding with bleaching chemicals is carried out in this trough. This unit helps to pad the fabric uniformly with the bleaching chemicals.
Chemical dosing system:
A computerized chemical dosing system is provided in this machine. the rate of dosing of each chemical is programmed according to the recipes, fabric quality and machine. It sends each chemical to the bleaching chemical trough. The chemical solution level is regularly maintained by this system.
The main objective of the steamer is to give the reaction time to the fabric with bleaching chemicals at high temperatures(95 degrees C). The steamer house is mounted with a Solid Stainless steel frame. Its roof gets insulated and prevents unnecessary temperature loss. The condensed water comes out of the steamer through condensation drainage and a collecting pipe. A hinged window is provided in the steamer compartment for maintenance and cleaning. Halogen lamps are mounted inside the steaming chamber. These lamps provide enough light during cleaning and maintenance.
Post washing and neutralizing Compartment
When the fabric comes out of the steamer, it enters the pre-washing chamber. It is washed with hot water in this chamber. The bleaching chemicals present in the fabric are washed out here. Now, the fabric enters the neutralising chamber. The fabric gets neutralised with mild acetic acid in this zone. The neutralised fabric is washed again with normal water in this zone. Finally, the rinsing of the fabric is done to wash off the impurities in the rinsing compartment.
The washed fabric is squeezed and dried in this unit. Teflon-coated steam cylinders and stainless steel cylinders are vertically mounted on the iron frame. The saturated steam is supplied inside the cylinders. The fabric gets dried on the heated surface of steam cylinders.
The main objective of the outlet unit is to collect the fabric during the batch change to avoid machine stoppage and provide a perfect batch of fabric without any crease.
Please click on the below video article to watch the full lecture in Hindi:
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