Objectives and Classification of Bleaching

Objectives and Classification of Bleaching

Bleaching

Bleaching is a chemical process which removes the natural color of Textile fibres. The fibre/yarns/ fabric become permanent white.

 Objective of bleaching

  1. It removes natural color from textile materials.
  2. It brings the textile materials permanent white.
  3. Improve the absorbency of the textile materials.
  4. Produce the textile materials for the next process.
  5. Some fabrics are sold as bleached goods.
Objectives and Classification of Bleaching

A Modern Bleaching Machine

Types of bleaching process

  • Generally there are two types of bleaching process
  1. Hand Bleaching
  2. Machine Bleaching

Hand Bleaching: It is small scale production process for domestic and cottage industry. Before machine invention, it was practiced till it is in use. It is not defect less and controlled process.

Machine Bleaching: It is continuous and discontinuous large scale production process. Modern textile industries are used this process. Machine bleaching us controlled and defect less. Production cost is less in machine process.

 Bleaching process can be divided into two process depending on bleaching agent:

  1. Bleaching by Oxidizing agents
  2. Bleaching by Reducing agents

Oxidizing bleaching agents

  • Chlorine gas
  • Bleaching powder {Ca(OCl)Cl
  • Sodium hypo-chloride (Na2OCl)
  • Calcium hypo-chloride {Ca(OCl)2}
  • Potassium di-cromate (K2Cr2O7)
  • Sodium di-cromate (Na2Cr2)
  • Potassium permanganate ( KMnO4)
  • Potassium Chlorate (KClO3)
  • Sodium peroxide (Na2O2)

Reducing bleaching agents

  • Hydrogen (H2)
  • Carbon (C)
  • Carbon mono oxide
  • Hydrogen sulphide (H2S)
  • Sulphur dioxide(SO2)
  • Ferrous sulphate (FeSO4)
  • Titenous chloride(TiCl2)
  • Zinc dust (Zn)
  • Sodium Sulphate (Na2SO4)

The bleaching agents mentioned above all are not used only bleaching process. Some bleaching agents are very costly. Some agents cannot bleach completely. Reducing bleaching agents only used for protein fibres.

Hydrogen peroxide can bleach all kinds of fibers. So, it is called universal bleaching agents.

Oxidizing bleaching agents

Oxidizing agents generally is used for cellulosic fibres. If reducing agents are used then after bleaching when fabrics come in contact with air they become yellowish or reddish that is come back to origin color. But oxidizing agents bleach goods free from this defect. So, goods become permanent white. Therefore Oxidizing bleaching agents are generally favored.

Reducing bleaching agents

Protein fibres wool and silk generally bleached with reducing bleaching agents. Because chlorine present in oxidizing agents damage proteins fibres without bleaching action. This is why animal fibres are bleached with reducing agents.

 Fibre types choice the bleaching process to be used. Generally need basis bleaching is made may be half bleach or full bleaching.

 Factors considered in bleaching process

  1. Concentration of chemicals
  2. Liquar ratio (L:R)
  3. pH of the chemical solution.
  4. Time of treatment.

Written by

Engineer Sheikh Nurja

B.Sc engineer of textile

Merchandiser at buying house

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