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Effects of different types of bleaching

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Effects of different types of bleaching

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  1. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Effects of different types of bleaching

  2. PREPARED BY ©right Name : MAZADUL HASAN SHESHIR ID: 2010000400008 Batch: 13th Batch (Session 2009-2013) Department: Wet Processing Technology Email: mazadulhasan@yahoo.com Blog: www. Textilelab.blogspot.com Southeast University Department of Textile Engineering

  3. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Bleaching The process of decolorize the natural coloring matter present in the cloth treating with some oxidizing agent and reducing agent and ensure the permanent whiteness is called bleaching. Objects A high uniform absorbency of fabric to water and dye stuffs. Uniform degree of whiteness. Fabric should not damage and DP should remain high. Destruction of natural coloring matters from the fabric. To ensure a level dyeing properties. To make the textile materials suitable for subsequent processing. (Dyeing, printing, etc.) 3 Prof . Engr. Dr. Ayub Nabi Khan Bleaching

  4. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Types of bleaching agents Oxidative bleaching agent Hydrogen peroxide bleaching. Sodium chlorite bleaching. Hypochlorite bleaching. Percretic acid Ozone Bleaching powder Reductive bleaching Zn dust Staneous chlorite Ferous sulphate Sulpher dioxide Sodium hydrosulphate 4 Prof . Engr. Dr. Ayub Nabi Khan Bleaching

  5. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Reducing bleaching is a temporary bleaching process because after bleaching there is possibility that the oxygen in the air may reoxidize and it will return to its original state. But oxidizing bleaching is permanent because it gives almost invariably a more permanent white. Bleaching can be done in various process: 1. Hypochlorite bleaching for 100% cotton (less used) 2. Hydrogen peroxide bleaching for 100% cotton (more used) 3. Sodium Chlorite bleaching only used for special sector and only for polyester and cotton blend. 4. Peracetic acid bleaching can also be done for the better improvement of the process and this is not still used in our country commercially. 5 Prof . Engr. Dr. Ayub Nabi Khan Bleaching

  6. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Hypochlorite bleaching 6 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  7. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering In textile hypochlorite bleaching sodium hypochlorite [NaOCl] or calcium hypochlorite [Ca(OCl)2] may be used as hypochlorite bleaching agent. Differences between Ca(OCl)2 and NaOCl bleaching Ca(OCl)2 NaOCl 1.It is unstable 1.It is stable 2.It produces CaCO3 precipitate 2. It doesn’t produce any precipitate 3.It makes harsh feeling on the fabric 3.It doesn’t make harsh feeling on the fabric 4.Comperatively cheaper than NaOCl 4.Higher cost than Ca(OCl)2 bleaching bleaching 7 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  8. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Sequence of cotton bleaching NaOCl Ca(OCl)2 Antichlore treatment souring Cold wash Antichlore treantmnet Cold was It is seen that souring is used only for Ca(OCl)2 but antichlore treatment is used for both. Cause when Ca(OCl)2 is used, it reacts with atmospheric CO2 to give CaCO3 as white precipitate. CaCO3 deposited on the fabric causing harsh handling and uneven dyeing, hence it has to separate and souring (acid treatment) is done to remove it. Ca(OCl)2 + CO2 + HO2 CaCO3 + HOCl 8 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  9. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Souring The treatment by which the fabrics after processing with alkali (scouring/bleaching) are treated with HCl or dilute H2SO4 for removing alkali or neutralizationfor alkali is called scouring. This process is necessary in case of Ca(OCl)2 bleaching. CaCO3 + 2HCl = CaCl2 + CO2 + H2 O CaCO3 + H2SO4 = CaSO4 + CO2 + H2 O Antichlore treatment In case of hypochlorite bleaching (OCl -) hypochlorous ion is produced, which will react with residual protein to form chloramines > NCl which is corrosive and toxic. Cl2 is also produced which also unhygienic and irritate to skin. Moreover >NCl reacts with moisture and gradually cotton becomes yellowish due to forming of Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching 9 HCl.

  10. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering OCl - + resedual protein >NaCl >NaCl + H2 O HCl 10 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  11. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Treatment procedure: In an experiment the hypochlorite bleaching scoured fabric samples (10 g. each) were treated with sodium hypochlorite solutions (0.5, 1.0, 1.5 g/liter available chlorine) at different pH values (11, 12, and 13) for 15 min., keeping a liquor ratio of 30:1 at 30º, 45º, 60º C. the bleached samples were washed, scoured with 0.1% hydrochloric acid and washed free from the acid. For the other purpose of comparison two pieces of scoured cloth were bleached with sodium hypochlorite solution (3 g/liter available chlorine) at pH 11 and 30ºC for 1 hr and scoured as above. 11 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  12. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Result: Conc. Of NaOCl g/liter avail. Cl2 Temp. ºC Whiteness pH 11 30 0.5 Yellowish white 0.5 45 Dull white 0.5 60 White 1.0 30 Yellowish white 1.0 45 White 1.0 60 White 1.5 45 White 1.5 30 Dull white 12 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  13. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Conc. Of NaOCl g/liter avail. Cl2 Temp. ºC Whiteness pH 12 30 0.5 Good white 0.5 45 Slightly white 0.5 60 Dull white 1.0 30 Yellowish white 1.0 45 White 1.0 60 White 1.5 30 Yellowish white 1.5 45 Dull white 1.5 60 White 13 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  14. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Conc. Of NaOCl g/liter avail. Cl2 Temp. ºC Whiteness pH 13 30 0.5 Dull white 0.5 45 White 0.5 60 White 1.0 30 Dull white 1.0 45 Cream white 1.0 60 White 1.5 30 Slightly white 1.5 45 White 1.5 60 White 14 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  15. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Conc. Of NaOCl g/liter avail. Cl2 Temp. ºC Whiteness pH 11 30 (1hr) 3.0 White 3.0 30 (1hr) White At a given hypochlorite concentration, the bleaching efficiency increases with an increase in either the temperature or in the pH. Good white effects are obtained when bleached for 15 min. at 60ºC with 1.5 g/liter available chlorine. In case of 1 hr process with 3 g/liter available chlorine at 30ºCwhite effects are obtained. 15 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  16. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – Cheapest – Variety of machine can be used – Large batches fabric can be processed – Simple machine can be used Disadvantages – There is a danger of damaging the fabric due to accidental lowering of pH. – The process is slow, it is carried out at low temperature and it is therefore difficult to integrate into rapid continuous operation. – There is danger of yellowish upon storage. – The relatively high salt loads in the process are undesirable for ecological reason 16 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  17. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Hydrogen peroxide bleaching 17 Prof . Engr. Dr. Ayub Nabi Khan

  18. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Hydrogen peroxide is called universal bleaching agent and most widely used for bleaching and over 90% of all fibre can be bleached with H2O2. It can be used for vegetable fibre as well as protein fibre although the cost of H2O2 is greater then hypochlorite. Properties of hydrogen peroxide: (a) H2O2 is a clear colorless liquid. (b) Strength of H2O2 is 35% - 70%. (c) Should store away from light. (d) In presence of heavy metal like gold, silver, platinum it decomposes with the (e) liberation of oxygen. (f) This material is irritant for skin (g) This is non flammable. 18 Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching

  19. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Bleaching methods with hydrogen peroxide: There are two chief methods for bleaching of cotton goods with H2O2. They are: (a) Bleaching in Kier (discontinuous). (b) Bleaching in J-Box. Treatment procedure: In continuous bleaching the cloth was first desized with desizing agent (3 g/liter) at Ph 7 for 2 hrs at 70˚ - 75˚C. This was followed by scouring by boiling in a stainless steel vessel with sodium hydroxide (3 g/liter), sodium carbonate 2(g/liter) and soap 2(g/liter) for 4 hrs. The cloth was then mercerized under tension for 10 min. with sodium hydroxide solution (50˚ Tw) containing wetting agent (15 ml/liter). It was washed , scoured with oxalic acid (10 g/liter) solution. the liquor ratio was 20:1. 19 Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching

  20. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Temperature ºC Time, hr. Sodium silicate, g 100ºTw solution/liter Peroxide decomposition % Reflectance 2 5 36.0 84 2 40 95.0 88 95 ½ 5 - - ½ 40 - - 2 10 32.5 85.0 80 10 0 66.0 72 2 40 67.0 86.5 60 10 5 - 78.5 20 Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching

  21. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering It is seen that bleaching at 95˚ C for 2 hrs with 40 G/liter Tw solution of sodium silicate gave the best result with the reflectance of 88.0, but 95% peroxide has been decomposed. With the increase in time and temperature improved the bleaching rate but a marked decrease in peroxide decomposition has been observed. 21 Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching

  22. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – H2O2 doesn’t react with residual protein of fiber and no need of antichlore treatment. – Permanent white cotton is obtained and the bleached fabrics are highly hydrophobic since the waxes are solublised and removed by hot alkaline solution. – Its reaction products are relatively non toxic and it decomposes to oxygen and thus reducing greatly the effluent pollution of the bleaching plant.. – Weight of fabric after H2O2 bleaching is higher than that of hypochlorite bleaching. – Bleaching and dyeing can be sometimes combined in a single operation. Disadvantages – It is irritant to skin. – Stabilizer should be used in order to hold the strength of H2O2 – It decomposes in the presence of heavy metal like gold, silver, platinum with the liberation of oxygen. – Higher energy is needed to bleach the fabric 22 Prof . Engr. Dr. Ayub Nabi Khan Hydrogen peroxide bleaching

  23. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Peracetic acid bleaching 23 Prof . Engr. Dr. Ayub Nabi Khan

  24. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering PAA (CH3COOOH) is a strong oxidizing agent with excellent antimicrobial and bleaching properties. Though it is not widely used in textile sector but has some advantages in comparison to HP bleaching. Recipe for exhaustion bleaching with PAA: Persan S15 (PAA) 6 to 240 ml/L Temperature 60 C Time 40 min pH 7.5 M:L 1:20 24 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  25. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Exhaustion Bleaching The cotton fabric samples were bleached with different concentrations of Persan S15, ranging from 6 to 240 ml/l at 60 °C, and at pH 7.5 for 40 minutes, in closed beakers in the Launder-Ometer apparatus at a cotton:liquor ratio of 1:20. The temperature of bleaching baths was raised from 25 °C to 60 °C within 15 minutes. Impregnation Bleaching Two bleaching baths were prepared with 15 and 60 ml/l of Persan S15. The samples of cotton fabric were impregnated with the bleaching baths on a Werner Mathis AG (CH) two-roll padder with a wet pick-up of 100 %. Pad-batch Bleaching The samples impregnated with bleaching solutions were wrapped in a plastic sheet, sealed, and stored at room temperature (23 °C) or at 60 °C for 0.5 to 24 hours. 25 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  26. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Pad-steam Bleaching The samples impregnated with bleaching solutions were steamed for 10 minutes at 100 °C in a 100 % steam atmosphere in a Werner Mathis AG (CH) laboratory steamer. Steaming was performed: (a) (b) (c) (d) Immediately after impregnation After 1 hour of storage at room temperature After 24 hours of storage at room temperature After 1 hour of storage at 60 °C 26 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  27. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Treatment process Bleaching baths were prepared with demineralized water. First, a corresponding amount of Persan S15 was added into water at a temperature of 25 °C, and secondly, the pH was adjusted to 7.5 by the addition of Na2CO3 at 0.5 M. After completing all bleaching processes, the samples were washed once with hot (80 °C) and twice with cold (40 °C) demineralized water, and finally dried in ambient conditions. 27 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  28. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Result Person S15 w L* C* h (ml/l) 6.0 70.3 94.3 3.4 90.8 7.5 74.2 94.5 2.7 89.4 10 74.5 94.5 2.6 89.9 15 79.2 95.3 2.0 89.5 24 82.0 94.9 1.2 80.1 30 83.7 95.0 0.9 73.1 40 85.3 95.1 0.7 52.3 60 88.2 96.3 0.6 58.8 120 92.5 96.5 0.4 154.2 240 94.4 96.7 0.8 301.3 28 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  29. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering The results for the degree of whiteness and the color coordinates in Table 1 reveal that the concentration of the bleaching agent is very important in exhaustion bleaching of cotton fabric with PAA. As expected, with increasing concentration of PAA, the degree of whiteness was also increased. The highest whiteness was obtained with 240 ml/l of Persan S15 (W = 94.7) and the lowest at 6 ml/l of Persan S15 (W = 70.3). Along with the increase of the degree of whiteness, the lightness (L*) was also increasing and chroma (C*) was decreasing. The hue (h) value shows that most samples had a yellow hue, and only the samples bleached with the highest concentration of PAA had a bluish hue. At lower concentrations of Persan S15 (up to 60 ml/l), the whiteness was lower (up to W = 88.2), but still high enough to allow the dyeing in light, medium and dark shades. 29 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  30. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Exhaustion bleaching with PAA at 60 °C and an initial pH of 7.5 for 40 minutes gave whiteness values from W = 70 to W = 95, depending on the concentration of the bleaching agent. The remarkable drop in DP occurred only at very high PAA concentrations. In the cases where preparations with 15 % PAA (Persan S15) were used, the use of 15 to 25 ml/l of bleaching agent can be recommended to obtain a whiteness value of around W = 80 without any damage to the cotton fibers. With 60 ml/l of the bleaching agent, values near W = 90 could be achieved, but with a small degree of damage to the fibers (DP ≈2070). Higher concentrations are not recommended. Prof . Engr. Dr. Ayub Nabi Khan 30 Peracetic acid bleaching

  31. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – There is no need of stabilizers, acetic acid, caustic soda, sequestrants and dispersants. – Bleaching takes place at the temperature between 40 °C to 80 °C and at pH 7 to 8 – Neutralization is not required. – Low energy comsumption. Disadvantages – This is not used in commercially. – More experiments are needed. – Not available. 31 Prof . Engr. Dr. Ayub Nabi Khan Peracetic acid bleaching

  32. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Sodium chlorite bleaching 32 Prof . Engr. Dr. Ayub Nabi Khan

  33. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Chlorine dioxide is a non-chlorinating reagent used to obtain a high degree of whiteness on cotton as well as synthetic fibers without degrading them properties of sodium chlorite: (a) At temperature of up to 150ºC sodium does not decompose. (b) When heated to 200ºC for 30 min. less than 50% of sodium chlorite decomposed with the formation of sodium chlorate and chloride; less 5% of the chlorine decomposed into sodium chloride and gaseous oxygen. 3NaClO2 2NaClO3 + NaCl NaClO2 NaCl + O2 than (C) (d) NaClO2. 3H2O (trihydrate). (e) Solid sodium chlorite has been found to be extremely stable. It decomposes in the temperature range 180º – 220ºC. Sodium chlorite is fairly highly soluble in water. Sodium chloride exists in two forms – NaClO2 (anhydrous) and 33 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  34. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering (f) Dilute solution of sodium chlorite containing 0.5% sodium hydroxide are stable for several days if protected from light of the laboratory. Increase in the concentration of sodium chlorite does not further accelerate the chlorite decomposition reaction. Bleaching process: Semi-continuous bleaching Two semi-continuous processes are used for open-width bleaching of cotton fabrics. In the first, called Pad-Roll, the fabric is saturated with a pad (80-100% delivery rate), placed in a steamer at 95-98°C and wound in a box in which steam is injected at a temperature of 95- 98°C to keep the air inside moist and warm. The fabric is spun on its shaft for 1-3 hours. In the second, called Pad-Batch, the fabric is impregnated with a pad, wound around a shaft at ambient temperature, wrapped in a sheet of plastic to avoid evaporation and spun for 15-20 hours. The fabric then passes into the washer, where it is rinsed in hot then cold water and then dried. Before being dried, it may be neutralized with acetic acid. (g) 34 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  35. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Fig: Pad - Roll process 35 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  36. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Fig: Pad – Batch process 36 Prof . Engr. Dr. Ayub Nabi Khan Hypochlorite bleaching

  37. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Continuous bleaching Continuous bleaching lines generally utilize the Pad-Steam process and include continuous desizing and boiling steps. Continuous processes are suitable for large amounts of fabric of the same, or at least similar, quality. The machines used are a saturator (pad), a preheater and a steamer in the shape of a ‘U’ or ‘J’ (called a U- box or J-Box). The fabric travels through these chambers for a relatively long period of time (30 minutes on average) in folded (cuttled) rope form (open-width boxes may be used, but they are expensive and bulky) in a 100-102°C steam atmosphere. Several units may be placed side by side so that the fabric may be desized, boiled and bleached once or twice without stopping. A washer is placed at the end to rinse and neutralize the fabric. Hydrogen peroxide is particularly suited to this type of process. Prof . Engr. Dr. Ayub Nabi Khan • 37 Sodium chlorite bleaching

  38. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Batch bleaching Batch processes are used for small amounts of fabric. The fabric is bleached in machines called winches with a high liquor ratio (1:20) or jet or overflow machines with a low liquor ratio (1:5 to 1:10). If the latter are used, non-foaming auxiliary agents are necessary on account of the agitation. Jiggers are best for crease- sensitive fabrics (heavy fabrics or those with a high thread /count). The liquor ratio is around 1:5. Kiers and beam machines are used more for delicate fabrics such as gauze and bunting as well as knitted fabrics. The liquor ratio is nearly 1:10. 38 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  39. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Result Sample Concentratio Temp. Time required for no. n of ºC complete Remarks decomposition of NaOCl2, NaOCl2, min. g/liter 1 10 50 30 Not completely white, motes and shieves still present. 2 20 60 30 Not completely white, motes and shieves still present. 3 30 70 35 Fairly good white, shieves still present. 4 40 80 40 Good white, almost free from shieves. 5 50 90 40 Good white, almost free from shieves. 39 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  40. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering The cloth is bleached with sodium chlorite solution (40 g/liter) at 50º C the yellowness persisted and so also motes and shieves. But its feel was better than that of grey cloth. When it was bleached at 60ºC there was slight improvement, but yellowness was not completely removed. Yellowness was absent on cloth bleached at 70ºC but some motes and shieves still remained. When bleached at 80ºC and 90ºC, the cloth acquired good whiteness with less no. of shieves. The sample bleached at the boil had the maximum whiteness and was almost free from motes and shieves. When the bleaching process was repeated once again, samples bleached at all the temperatures had very good whiteness and were completely free from motes and shieves. The feel was further improved by the second bleaching process and improved gradually as the temperature increased. 40 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  41. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – Permanent whiteness. – High brightness degree (especially for acrylic fibers) – Negligible degradation of fibers (1-2% weight loss for cellulosic fibers and no attack to the polymeric chains in the synthetic fibers) – Lower environmental impact of wastewaters (negligible level of COD). – Versatile bleaching agent for cellulosics, synthetics and blends, especially poly-cotton blends. It is also safe for those synthetic fibers which are sensitive to alkali. – Non sensitive to metal ions such as iron and Water hardness under acidic conditions. Disadvantages – It is expensive as compared to hydrogen peroxide. – It cannot be used for wool and silk. – Chlorine dioxide is highly corrosive to metals and toxic in nature. – Bleaching in neutral and acidic pH damage cotton. 41 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

  42. SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Conclusion: In textile factory before dyeing, pretreatment i. e. singeing, desizing, scouring and bleaching play an important role for the convenience in dyeing procedure A bleaching agent can whiten or decolorize a substance by reacting with the chromophores that are responsible for the color of the substance. Depending on the nature of the chromophores, the bleaching agent will either be an oxidizing or reducing agent. That is, the chromophore is either oxidized or reduced to produce a colorless or whitened substance. As there is various types of bleaching agent, selecting the suitable one is the major task to conduct. What may be the bleaching agent there are some parameters which have to be maintained strictly, they are: time, temperature and pH, otherwise the whole procedure would be faulty. 42 Prof . Engr. Dr. Ayub Nabi Khan Sodium chlorite bleaching

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