The document discusses the bleaching of cotton textiles, focusing on the purpose of bleaching to remove natural pigments and prepare the fabrics for dyeing. It details various bleaching techniques, primarily oxidative methods using agents like calcium hypochlorite and hydrogen peroxide, and highlights the importance of maintaining proper pH levels during the process. Additionally, it covers the chemical reactions involved in each bleaching agent's application and the advantages of using hydrogen peroxide as a bleaching agent.

1. 1
Bleaching of TextileBleaching of Textile
MaterialsMaterials
Prepared byPrepared by
ROHIT SINGHROHIT SINGH
nitragzb20152019@gmail.comnitragzb20152019@gmail.com
+91 9450316927+91 9450316927
NITRA TECHNICALNITRA TECHNICAL
CAMPUSCAMPUS

2. BleachingBleaching
 To decolorize the natural coloring matter in the cottonTo decolorize the natural coloring matter in the cotton
using bleaching agentsusing bleaching agents
 To facilitate the textile material( yarn/fabric ) ready forTo facilitate the textile material( yarn/fabric ) ready for
dyeing and printingdyeing and printing
 AA bleachbleach is a chemical that removes colors or whitens,is a chemical that removes colors or whitens,
often via oxidation or reductionoften via oxidation or reduction
 Many bleaches have strong bactericidal properties, andMany bleaches have strong bactericidal properties, and
are used for disinfecting and sterilizing.are used for disinfecting and sterilizing.

3. Bleaching of Cotton FibreBleaching of Cotton Fibre
 Bleaching is another important pretreatmentBleaching is another important pretreatment
next to scouring, performed on cotton fibresnext to scouring, performed on cotton fibres
 Bleaching removes the unwanted naturalBleaching removes the unwanted natural
coloring pigments.coloring pigments.
 Bleaching could be performed by two types ofBleaching could be performed by two types of
techniques; reductive and oxidativetechniques; reductive and oxidative
 Reductive technique is not followed nowadaysReductive technique is not followed nowadays
due to its inefficient performance on cottondue to its inefficient performance on cotton

4. Oxidative Bleaching on CottonOxidative Bleaching on Cotton
 Normally oxidative bleaching action is performed in theNormally oxidative bleaching action is performed in the
industries on cotton fibre substratesindustries on cotton fibre substrates
 Though number of bleaching agents are available in theThough number of bleaching agents are available in the
chemical market, few bleaching agents are being usedchemical market, few bleaching agents are being used
extensivelyextensively
 Calcium hypochlorite, Sodium hypochlorite andCalcium hypochlorite, Sodium hypochlorite and
Hydrogen peroxide are the most frequent usedHydrogen peroxide are the most frequent used
bleaching agents in the conventional cotton processingbleaching agents in the conventional cotton processing
unitsunits

5. Methods of BleachingMethods of Bleaching
 Common chemical bleaches include household chlorine bleach, aCommon chemical bleaches include household chlorine bleach, a
solution of approximately 3–6% CaOClsolution of approximately 3–6% CaOCl22 (bleaching powder)(bleaching powder)
 Using dilute Sodium hypochlorite solution (NaOCl)Using dilute Sodium hypochlorite solution (NaOCl)
 Oxygen bleach, which contains Hydrogen peroxide (universalOxygen bleach, which contains Hydrogen peroxide (universal
bleaching agent)bleaching agent)
 A peroxide-releasing compound such as sodiumA peroxide-releasing compound such as sodium
perborate,sodium per sulfate ,tetra sodium pyrophosphate orperborate,sodium per sulfate ,tetra sodium pyrophosphate or
urea peroxide together with catalysts and activators.urea peroxide together with catalysts and activators.
 Using sodium chlorite solution at the boilUsing sodium chlorite solution at the boil

6. Bleaching RequirementsBleaching Requirements
An efficient bleaching must ensure,An efficient bleaching must ensure,
 Pure and permanent whitePure and permanent white
 Level dyeing properties (avoiding over or underLevel dyeing properties (avoiding over or under
bleaching which affects the dye absorptionbleaching which affects the dye absorption
characteristics of the fabric)characteristics of the fabric)
 Fabric does not undergo tendering ( chemical damageFabric does not undergo tendering ( chemical damage
or degradation, results in loss in tensile strength andor degradation, results in loss in tensile strength and
durability)durability)

7. Calcium hypochloriteCalcium hypochlorite
 In the earlier decades calcium hypochlorite was the onlyIn the earlier decades calcium hypochlorite was the only
bleaching agent dominated in the bleaching sectionsbleaching agent dominated in the bleaching sections
 Calcium hypochlorite is in the powder form calledCalcium hypochlorite is in the powder form called
Bleaching powderBleaching powder
 The application of calcium hypochlorite for bleachingThe application of calcium hypochlorite for bleaching
requires dissolution of bleaching powder and separationrequires dissolution of bleaching powder and separation
of supernatent solution, consumes some more timeof supernatent solution, consumes some more time
 Ca(OCl)Ca(OCl)22 + H+ H22OO  Ca(OH)Ca(OH)22 + CaCl+ CaCl22 + Cl+ Cl22↑↑

8. ……continuedcontinued
 Ca(OH)Ca(OH)22, calcium hydroxide gives the required pH (9.5, calcium hydroxide gives the required pH (9.5
to 11.0) in the bathto 11.0) in the bath
 CaClCaCl22 is a salt present in the solution without anyis a salt present in the solution without any
trouble for bleachingtrouble for bleaching
 ClCl22 is the chlorine available for bleachingis the chlorine available for bleaching
 The available chlorine reacts instantly with water andThe available chlorine reacts instantly with water and
form HOCl, which dissociates to give HCl and (O),form HOCl, which dissociates to give HCl and (O),
nascent oxygen (an powerful oxidising agent); due tonascent oxygen (an powerful oxidising agent); due to
the presence of HCl in the bath, the pH should bethe presence of HCl in the bath, the pH should be
maintained properly by adding buffers like Namaintained properly by adding buffers like Na22COCO33..

9. Sodium hypochloriteSodium hypochlorite
 Sodium hypochlorite is also similar to calciumSodium hypochlorite is also similar to calcium
hypochlorite, with little differencehypochlorite, with little difference
 NaOClNaOCl + H+ H22OO  NaOH + NaClNaOH + NaCl + Cl+ Cl22↑↑
 NaOH is the alkali which gives the required pHNaOH is the alkali which gives the required pH
(9.5 to 11.0) during bleaching(9.5 to 11.0) during bleaching
 NaCl is the salt present with no interferenceNaCl is the salt present with no interference
 ClCl22 is the available chlorine leads to nascentis the available chlorine leads to nascent
oxygen as oxidative bleaching agent.oxygen as oxidative bleaching agent.

10. Comparison between Ca(OCl)Comparison between Ca(OCl)22 andand
NaOClNaOCl
 Ca(OCl)Ca(OCl)22 is available in the powder form whereasis available in the powder form whereas
NaOCl is in the liquid formNaOCl is in the liquid form
 Dissolution of Ca(OCl)Dissolution of Ca(OCl)22 requires additional 3 torequires additional 3 to
6 hours, since Ca(OCl)6 hours, since Ca(OCl)22 is partially only dissolvedis partially only dissolved
in waterin water
 During bleaching there is a possibility of suddenDuring bleaching there is a possibility of sudden
reduction in pH due to the decrease of Ca(OH)reduction in pH due to the decrease of Ca(OH)22
in the bleaching bath and also by the formationin the bleaching bath and also by the formation
of HClof HCl

11. Bleaching processBleaching process
For hypochlorite bleaching,For hypochlorite bleaching,
 Batch processBatch process
 Liquor circulation methodLiquor circulation method
For peroxide bleaching,For peroxide bleaching,
 One stage process( Continuous bleaching J Box)One stage process( Continuous bleaching J Box)
 Two stage method (Continuous scouring andTwo stage method (Continuous scouring and
bleaching process)bleaching process)

12. Process conditions for hypochlorite BleachingProcess conditions for hypochlorite Bleaching
 Material weight – x gMaterial weight – x g
 NaOCl/ Ca(OCl)NaOCl/ Ca(OCl)22 – 2-3 gpl of available Cl– 2-3 gpl of available Cl
 NaNa22COCO33 – 0.5 % owm– 0.5 % owm
 MLR – 1:20MLR – 1:20
 Temp – Room TempTemp – Room Temp
 Time – 2 hrsTime – 2 hrs

13. Problems in Hypochlorite BleachingProblems in Hypochlorite Bleaching
 In Ca(OCl)In Ca(OCl)22 bleaching Ca(OH)bleaching Ca(OH)22 is converted to CaCOis converted to CaCO33 by theby the
reaction with COreaction with CO22. [Ca(OH). [Ca(OH)22 + CO+ CO22  CaCOCaCO33]]
 This conversion reduces the alkali content in bleaching bath andThis conversion reduces the alkali content in bleaching bath and
decreases the pH, since CaCOdecreases the pH, since CaCO33 is insoluble in wateris insoluble in water
 In NaOCl bleaching NaOH is converted to NaIn NaOCl bleaching NaOH is converted to Na22COCO33 which iswhich is
soluble in water [NaOH + COsoluble in water [NaOH + CO22  NaNa22COCO33]]
 In both the bleaching, due to HCl formation during bleachingIn both the bleaching, due to HCl formation during bleaching
pH decreases commonlypH decreases commonly
 To avoid this problem buffer like NaTo avoid this problem buffer like Na22COCO33 is required to be addedis required to be added
to maintain pH 9.5 to 11.0, the decrease of pH below 9 willto maintain pH 9.5 to 11.0, the decrease of pH below 9 will
cause damage to the polymercause damage to the polymer

14. Hydrogen PeroxideHydrogen Peroxide
 Hydrogen peroxide is considered as an universalHydrogen peroxide is considered as an universal
bleaching agent, as it is suitable for all sort of textilesbleaching agent, as it is suitable for all sort of textiles
 Hydrogen peroxide is stable at neutral / near neutralHydrogen peroxide is stable at neutral / near neutral
pHpH
 The pH of hydrogen peroxide solution can be modifiedThe pH of hydrogen peroxide solution can be modified
based on the suitabilitybased on the suitability
 Processing pH of cellulosic is between 9 and 11.5;Processing pH of cellulosic is between 9 and 11.5;
proteins 2.5 to 6.0; and synthetics near neutral acidicproteins 2.5 to 6.0; and synthetics near neutral acidic
pHpH

15. ..continued..continued
 For cotton bleaching, the pH of hydrogen peroxideFor cotton bleaching, the pH of hydrogen peroxide
solution is maintained at around 11solution is maintained at around 11
 When alkali is added the stability of hydrogen peroxideWhen alkali is added the stability of hydrogen peroxide
is reduced and decomposed fastlyis reduced and decomposed fastly
 To control the rapid decomposition stabilizer is addedTo control the rapid decomposition stabilizer is added
 The ingredients added during peroxide bleaching areThe ingredients added during peroxide bleaching are
normally; hydrogen peroxide (bleaching agent, 1-normally; hydrogen peroxide (bleaching agent, 1-
3%owm), sodium hydroxide or carbonate (bleaching3%owm), sodium hydroxide or carbonate (bleaching
promoters, 0.25 to 1.0%owm), sodium silicate (bufferpromoters, 0.25 to 1.0%owm), sodium silicate (buffer
or stabilizers, 0.5 to 1.0%owm)or stabilizers, 0.5 to 1.0%owm)

16. ……continuedcontinued
 HH22OO22 ↔ H↔ H++
+ HO+ HO22
--
[stable][stable]
 HOHO22
--
↔ [OH]↔ [OH]--
+ (O) [unstable]+ (O) [unstable]
 In the alkaline condition the unstability ofIn the alkaline condition the unstability of
peroxide is continued by the concentrationperoxide is continued by the concentration
 The liberation of nascent oxygen is utilized forThe liberation of nascent oxygen is utilized for
the oxidation reaction in a controlled manner bythe oxidation reaction in a controlled manner by
selecting the stabilisers to get uniformselecting the stabilisers to get uniform
applicationapplication

17. Advantages of Hydrogen peroxideAdvantages of Hydrogen peroxide
BleachingBleaching
 Application of hydrogen peroxide is simple, sinceApplication of hydrogen peroxide is simple, since
available in liquid formavailable in liquid form
 Risk on textiles during bleaching is minimized, due toRisk on textiles during bleaching is minimized, due to
its stability at near neutral pHits stability at near neutral pH
 No solid deposits from hydrogen peroxide duringNo solid deposits from hydrogen peroxide during
bleaching is formedbleaching is formed
 Cost of processing is considered less, because of lessCost of processing is considered less, because of less
sequence and less time during applicationsequence and less time during application
 Quality of bleached textiles is high due to efficientQuality of bleached textiles is high due to efficient
application and less risk of damage on materialapplication and less risk of damage on material

18. Process conditions for peroxide BleachingProcess conditions for peroxide Bleaching
 Material weight – x gMaterial weight – x g
 HH22OO22– 3% owm– 3% owm
 NaNa22SiOSiO33 – 2 % owm– 2 % owm
 NaOH – 1% owmNaOH – 1% owm
 MLR – 1:20MLR – 1:20
 Temp – 95° CTemp – 95° C
 Time – 2 hrsTime – 2 hrs

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