Bleaching

Bleaching of a textile material

1. Bleaching

2. Bleaching --- What and When?? When fibres are to be dyed to very dark shades , where fibres brightness is not so important no bleaching is required For light and medium shades , fibre brightness is important bleaching is an essential operation before dyeing and printing In case of man-made fibres if the process sequence causes slight dis-colouration that sometimes cannot be accepted as full white, bleaching is required A process to remove The natural colouring material Any other colouring material from natural fibres or man-made fibres

3. The natural colouring material can be destroyed by oxidation by reduction How ???? Oxidation bleaching agents Chlorine Hydrogen Peroxide Reduction bleaching agents Sodium sulphite Sodium hydrosulphite Bleaching Agents

4. Oxidizing bleaching agents Use oxygen directly or indirectly for the bleaching Nascent oxygen decomposes colouring matter completely into simpler compounds Simpler compounds are washed away with water during washing Do not give any trouble at the later stage

5. Reducing bleaching agents The nascent hydrogen combines with the colouring matter in the fibre to produce a colourless compound The compound however remains in the fabric and gets oxidized back to the original colouring matter upon exposure to the air Hence the whiteness produced is not permanent

6. Bleaching – after this treatment the natural yellowish and brownish pigments and the impurities are disappeared. Generally Bleaching can be done in three way Sodium Hypochlorite ( NaOCl ) Bleaching. Chlorite Bleaching Hydrogen peroxide Bleaching.

7. Sodium Hypochlorite ( NaOCl ) It is the oldest industrial method of bleaching cotton Sodium hypochlorite is the strongest agent It is capable of giving oxygen at room temperature It is also economical in use Prior to bleaching with hypochlorite it is necessary to thoroughly scour fabrics to remove fats, waxes and pectin impurities This impurities will reduce the available hypochlorite, reducing its effectiveness for whitening fabrics

8. Mechanism NaOCl + H 2 O NaOH + HOCl HOCl HCl + [O] NaOCl + HCl NaCl + HOCl HOCl + HCl H 2 O + Cl 2

9. Sodium hypochlorite dissolves in water to give the hypochlorous acid (HOCl) HOCl is unstable and dissociates to yield active oxygen and hydrochloric acid (HCl) To avoid the fibre damage it is advisable to maintain a pH of 10.5 to 11 and for that Sodium carbonate is added Small amount of NaOH is also added as pH stabiliser

10. Cellulose textiles are to be bleached very carefully with hypochlorite The four factors affecting the bleaching efficiency are: Concentration of the bleaching liquor Its temperature Its pH (pH between 10 – 11 is to be maintained; pH below 9 will accelerate the degradation of cellulose) The time of bleaching

11. The greater the concentration of the bleach liquor, the better the bleaching result but the greater the risk of chemical damage to the cellulose The time and temperature must be standardized to minimize the chemical damage The pH should be controlled in between 10 and 11, a fall in pH to below 9 accelerating the degradation of the cellulose material through the formation of oxy-cellulose

12. Hypochlorite Bleaching process Sodium Hypochlorite Bleaching Jigger J Box Conveyer Available Chlorine /gl -1 2-4 2-5 Soda Ash /gl -1 2-4 2-4 Time /hour 3-4 1-2

13. Above pH 10 NaOCl Na + + OCl - - Between pH 4 and 8 HOCl H + + OCl - Below pH 3 HOCl + H + + OCl - Cl2 + H2O Effect of pH

14. Advantage Low chemical cost Lower (not zero) risk of catalytic damage. Low energy input (heating cost) Disadvantage The formation of high level of Chloroform. The rapid bleaching process is not available. The danger of yellowing of the bleached fibre on storage. Danger of chemical damage on cellulose The fibre must be prescoured before bleaching. Degrades most dyes and fluorescent brightening agent

15. SOURING It is an acid treatment generally given to hypochlorite bleached goods Dilute hydrochloric acid at room temperature is generally used It is very difficult to remove traces of alkali even after thorough washing. The alkali needs to be neutralized, otherwise it may get concentrated and result in the formation of oxy-cellulose during drying In case of bleaching powder, the calcium carbonate formed during the bleaching is deposited on the fabric and cannot be removed by simple washing. It imparts a harsh feel to the cloth

16. Sulphuric acid may also be used for souring when bleaching is carried out with sodium hypochlorite But it is not suitable when bleaching with calcium hypochlorite because of the formation of calcium sulphate, which will deposited on the fabric yielding a harsh feel The reaction with the HCl will give the soluble chloride

17. Complete removal of acid is required after souring before the cloth is dried Any traces of acid will cause degradation of the cellulose material because of the increased concentration of the acid on drying Also the goods should not contain any hypochlorite because over bleaching may take place during souring Presence of hypochlorite in souring will cause intensive action of hypochlorite by decreasing the pH of the solution to the neutral level To avoid this the goods may be given “antichlor” treatment with a solution of sodium bisulphite or sodium thiosulphite , which destroys the residual hypochlorite in the fabric

18. Precautions during souring Concentration of acid solution to be controlled Control of process temperature Intermediate local drying of the fabric should never be allowed Optimum dwell period should be standardized

19. Antichlor Treatment An antichlor treatment with reducing agent may be given after the hypochlorite treatment to ensure removal of any residual chlorine from the bleached fabric Chloramines are formed which cause after-yellowing of the material Reducing agents used Sodium bisulphite (NaHSO 3 ) Sodium thiosulphate (Na 2 S 2 O 3 ) Sodium hydrosulphite (Na 2 S 2 O 4 )

20. Peroxide Bleaching (H 2 O 2 ) Universal bleaching agent Applicable for almost all textile material Including wool, silk and manmade fibres Bleaching is carried out at raised temperature Hence can be utilised for simultaneous scouring and bleaching H 2 O 2 is colourless liquid, soluble in water, highly stable in acidic condition but stability decreases as the alkalinity increases

21. Stabilizers H 2 O 2 will decompose readily to form the bleach active nascent oxygen To ensure a fairly uniform availability of bleaching action, stabilizers are used The commonly used stabilizer is sodium silicate (NaSiO 3 ) The use of slightly hard water is preferred for hydrogen peroxide bleaching as the presence of magnesium salts in water improves the effectiveness of sodium silicate stabilizer

22. Recipe for peroxide bleaching Hydrogen peroxide = 2 – 8% Sodium silicate = 3% Soda ash = 0.8 to 6% Sodium hydroxide = 0.4 – 0.6% Initial pH – 10.3 – 12 Temperature –90 degree celcius Liquor Ratio : 1:5 to 1:10

23. Effect Of pH on Bleaching

24. Hydrogen Peroxide (50%) – 40 ml/kg Sodium Hydroxide (96 o TW) – 30 ml/kg (50% purity) Kierlon MFB (Wetting Agent ) - 50ml/kg Stabilizer – 6 ml/kg Sirrix 2UDI ( Complexing Agent) – 30ml/kg Time : 17 – 30 min Temperature - 100 o C by Saturated Steam . Continuous Bleaching Recipe

25. J Box Process

26. Continuous Bleaching Range ( Benninger )

27. Continuous Bleaching Range

28. Ben Impacta

29. Cold Pad Batch Bleachinng

30. Peroxide Killers After peroxide bleaching and rinsing, if traces of peroxide are left on the fabric, it will affect the dye uptake and give rise to uneven dyeing Hence peroxide killers are used to remove the residual peroxide from the fabric The various methods for removal of peroxide are Vigorous rinsing Treatment with inorganic reducing agents Enzyme treatment E.g – Finostab, Basopal etc.

31. Advantages There is generally no need for thorough scouring before peroxide bleaching It is environmentally acceptable as it has no absorbable halogen Hydrogen peroxide is compatible with optical brightening agents It involves low risk of chemical damage to cotton

32. Disadvantages High cost High risk of local Catalytic damage to cotton and wastage of peroxide due to possible metallic contamination in water

33. Sodium Chlorite (NaClO 2 ) It is versatile Can be used for cotton as well as other fibres and mixtures with manmade fibres Cannot be used for silk and wool More expensive than peroxide Little tendering due to the presence of metallic ions

34. Mechanism Sodium chlorite is soluble in water to give a stable solution with a pH of about 10 An acidified solution of sodium chlorite contains chlorous (HClO 2 ) acid, the amount of which depends upon the pH and the temperature of the bath It is to be noted that acidified chlorite solutions are very reactive and cause corrosion to commonly used metallic vessels For this reason vessels lined with resistant materials such as glass or ceramics are used Corrosive inhibitors like sodium nitrate can also be used in the bleaching solution

35. Bleaching process The bleaching liquor may be made up with the sodium chlorite ( 1 – 2 %) and a wetting agent and brought to a pH around 4.0 with addition of acetic or formic acid at 80 C for 2-3 hours of bleaching action 5ClO 2 - + 2H + 4ClO 2 + Cl - + 2OH - 3ClO 2 - 2ClO 3 + Cl - ClO 2 - Cl - + 2(O)

36. Advantages Pre-scouring is not required Soft fabric handle and good sewability due to non-removal of fats Least risk of chemical damage to cotton in chlorite bleaching Least sensitive to metallic contamination in the process water

37. Disadvantages Possibility of liberation of toxic chlorine dioxide gas Equipment is expensive because of the need of corrosion resistant material Chlorite bleached fabric has poor absorbency and thus is not suitable for further dyeing and printing Sodium chlorite is very expensive It is incompatible with optical whitening agents

38. Scouring Absorbency test More absorbency means the scouring is very effective Various methods are available for testing the absorbency the fabric / fibre / yarn Tests based on the time for water to be absorbed by the sample Time taken by a sample to sink in water Fabric absorbency is assessed using water drop absorbency times The fibre and yarn absorbency are assessed by time taken by a sample to sink in water

39. Bleaching Chemical Damage Require to find out how much chemical damage has occurred during the bleaching operation Damage is due to the degradation of the polymers The degree of polymerization of a polymer is directly proportional to the viscosity of its solution In this test sample of cellulose (cotton) is taken and dissolved in standard cuppramonium hydroxide The solution is taken in a ‘viscometer’ to find out the fluidity of the solution Viscosity is inversely proportion to the fluidity So, a value of low fluidity means less chemical damage has occurred

40. The principle of the test is that during dissolution no further breakdown of the cellulose molecular chain structure will occur Long-chain material, i.e. with low damage, will high viscous solution and hence low fluidity Interpretation of test Results Normally unbleached cotton shows a fluidity of 2 Therefore, the fluidity of bleached cotton will be more than 2 depending upon the degree of chemical damage As a general guide, bleached cotton with a fluidity of 5 or less is accepted normally

41. 2) Whiteness Through the whiteness the degree of bleaching could be assessed The whiteness of a substance may be assessed by Comparing it visually with a standard Using a reflectance photometer Photometer is calibrated at different wavelengths using a white standard An ideal standard will give 100% reflectance at all the wavelengths In order to assess the whiteness of a non-fluorescent cellulosic fibre or blend, the amount of light reflected in the blue region of the visible spectrum is usually measured

42. Interpretation of Results Unbleached 100% woven cotton fabrics usually have a reflectance of about 55% After bleaching the reflectance value increases to 83 – 85% Lower reflectance of 78 – 82% is generally acceptable as a preparation for subsequent dyeing