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Introduction of bleaching processing

Textile chemical processing - Bleaching

Desizing and scouring make the material more absorbent, however, it still incorporates a dull appearance because of the presence of natural colouring matters.

I.e - sizing removed and applied to size, and scouring removes impurities aside from natural colouring matters. 

The natural colouring matter is not harmful to the fibre except that it diminishes or reduces the whiteness of the material Depending upon the end-use of the material bleaching is decided whether to be carried or not on the basis of the following terms.

- For light, medium and brighter reminder colour application on fabric which will be used for entire purposes should be bleached. I.e - bleaching is a necessary operation before dyeing and printing.

- For dyeing of material with very dark shades where fibre brightness is employed for purposes like carpet then bleaching isn't needed.

- In man-made fibres which are generally white, bleaching isn't needed However, slight discolouration sometimes can't be accepted as full white, in such cases bleaching is the only option.

Bleaching process of fabric
Bleaching process of fabric

Bleaching method -     

It's the method by which the natural colouring matter (unsaturated chromophoric group) and the other colouring matter are converted into colourless matter (Unsaturated chromophoric group) from fabric by employing an agent. 

I.e - colouring matter is destroyed by using - 

(A) Oxidation (Oxidative Bleach)
(B) Reduction (Reduction Bleach)
(C) Combination of both Oxidation and Reduction 

Objective -

  • The process of bleaching gives whiteness to the material.
  • Bleaching decreases the yellowness of the material.
  • Bleaching makes fabric suitable for further processing like dyeing and printing. 

Classification of bleaching agents - 

Bleaching agents are classified into two groups by their bleaching agents. One is Oxidising bleaching agents and another is Reducing bleaching agents.

Oxidising bleaching agents

Reducing bleaching agents

Bleaching agent example -
(i) Chlorine (Cl2)

Bleaching agent example -
(i) Sulphur dioxide (SO2) 

(ii) Ozone(O3)

(ii) Sodium sulfite (Na2SO3)

(iii) Sodium peroxide (Na2O2)

(iii) Sodium hydrosulphite (Na2S2O4) 

(iv) Hypochlorous acid (HOCl)

(iv) Sulphurous acid(H2SO3)

(v) Potassium permanent (KMnO4)

(v) Bisulphites(NaHSO3)

These bleaching chemicals utilize oxygen, either directly or indirectly. 

Nascent oxygen decomposes colouring matter into simpler compounds that can be wiped away with water during washing and do not cause problems later. 

The bleaching agents utilize hydrogen.
Nascent hydrogen reacts with the fibre's colouring component to form colourless molecules that remain in the fabric. 

This whitening effect is persistent and permanent due to the degradation of colouring materials.

When exposed to air, this usually oxidises back to the original colouring component, therefore the whiteness is only temporary.

Properties of bleaching agent

(Bleaching agent should follow given properties)-

(1) Bleaching chemicals should assure a permanent whiteness impact on the fabric. 

(2) It should not cause fabric tenderness or weight loss.

(3) It should not cause tensile strength loss.

(4) It should give level dyeing in subsequent processing.

Agents from the list above Bleaching is done with a variety of chemicals at various temperatures, which will be discussed in parts 2, 3 and 4.

Fabric without bleached
Fabric without bleached

Questions -

  1. What is the bleaching process?
  2. Which principle is used for bleaching action?
  3. Give classification of bleaching agents.
  4. Role of bleaching process in textile chemical processing.
  5. Which properties are needed for bleaching agents for ideal processing?


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(N.d.). from

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Further reading,

Part 1 Introduction to bleaching processing

Part 2 Bleaching types - Sodium Hypochlorite Bleaching

Part 3 Bleaching types - Using Hydrogen peroxide

Part 4 Bleaching types - Using Sodium chlorite

Part 5 Bleaching process of Polyester materials

Part 6 Bleaching process of wool fibres

Part 7 Bleaching process of synthetic fibres

Part 8 Bleaching process of silk and cotton blend fibres

Part 9 Weft-knitted and woven fabric bleaching process

 - Rushikesh Patil (Textile Engineer)
(DKTE Society textile engineering college Ichalkaranji)

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