Monday, May 8, 2017

Textile Bleaching | Bleaching Agent | Estimation of Bleaching Effect

Textile bleaching process:
Textile bleaching is one of the chemical treatments in textile manufacturing process. All textile goods become greige in natural form and contain different types of odor and impurities that are not suitable for apparel manufacturing. In textile bleaching process, natural coloring matters and remaining trace impurities are removed from the grey fabric. So, the fabric gets a permanent white color which is very important for the following dyeing or printing process. The degree of bleaching necessary is determined by the required whiteness and absorbency.
Textile bleaching process
Fig: Textile bleaching process
Objectives of bleaching:
  1. By removing natural color and impurities to increase the degree of whiteness of the textile material.
  2. To make the textile goods to be suitable for dyeing and printing with pale or bright shade.
  3. To be follow with adding optical brightening agent process (super white).
  4. Bleaching agent may occur damage of textile product. So, bleaching must be accompanied with minimum fiber damage.
Types of bleaching agent:
Bleaching process can be performed by three types of bleaching agents.

  1. Oxidizing agents
  2. Reducing agents
  3. Enzymatic bleaching agents
1. Oxidizing bleaching agent:
There are several oxidizing bleaching agents. Such as Cl2, O3, Ca(OCl)Cl, H2O2, NaOCl, Ca(OCl)2, K2Cr2O7, Na2Cr2O7, KMnO4, KClO3, Na2O2 etc.

Why natural cellulosic fibers are bleached by oxidizing agents?
Oils, waxes and other coloring matters are present in the natural fibers as impurities. Natural coloring matters are mostly nitrogenous compounds. To remove these impurities oxidizing agents are used. Because if we use reducing agents for bleaching, afterwards they can react with oxygen of the air and can make yellowish or reddish color on the material. In case of oxidizing agent there is no such chance of yellowish color formation.

2. Reducing bleaching agents:
The reducing agents which are used for bleaching action are H2, C, CO, H2S, SO2, FeSO4, SnCl2, TiCl2, Zn, Na2SO4 etc.

Why protein fibers are bleached by reducing agents?
Most of the oxidizing bleaching agents contain Chlorine (Cl) in their composition. This chlorine can react directly with the protein of fiber and can damage the fiber. Thus, protein fibers are bleached with reducing agents (mainly SO2).

Typical bleaching recipe for cotton:

Peroxide (H2O2) bleaching:

  • Wetting agent ---------------------------------------0.5-1.0 g/L
  • Sequestering agent ---------------------------------as required
  • Detergent --------------------------------------------1.0-2.0 g/L
  • Stabilizer --------------------------------------------0.3-0.8 g/L
  • Caustic soda (36oBe) ------------------------------1.0-3.0 g/L
  • H2O2 (35%) ----------------------------------------4.0-8.0 g/L
  • Temperature -----------------------------------------90-95oC
  • Time --------------------------------------------------20-45 minutes
  • pH ----------------------------------------------------10-11
  • M:L --------------------------------------------------1:10
Sodium hypochloride (NaOCl) bleaching:
  • Wetting agent -----------------------------0.3-0.5 g/L
  • Caustic soda (36oBe) --------------------0.5-1.0 g/L
  • Soda ash -----------------------------------2.0-4.0 g/L
  • NaOCl solution --------------------------5.0-15.0 ml/L
  • Temperature ------------------------------25-60oC
  • Time ---------------------------------------30-60 minutes
  • pH -----------------------------------------10.5 +/-0.5
  • M:L ----------------------------------------1:10
3. Enzymatic bleaching agents:
Enzymes are a new category of bleaching agents. They are used for textile, paper, and pulp bleaching as well as for home laundering. Enzymatic bleaching is also eco-friendly because it reduce water, energy and chemicals consumption.

Estimation of bleaching effect:
Two tests are performed for the assessment of bleaching effect.

  1. Degree of whiteness
  2. Permanency of whiteness
1. Degree of whiteness/Measurement of reflectance:
When a fabric is bleached, the capacity of reflecting incident light upon it increases. Bleaching effect is measured by reflectance. Reflectance can be measured by “Spectrophotometer” or “Reflectometer”. Here, reflectance 100 means a perfect white whereas 0 means perfect black.

For testing, the standard range of reflectance is considered as follows. Reflectance 68-70 means a “Good”, 70-72 means a “Very Good” and a reflectance value of 73 means an “Outstanding” whiteness.

2. Permanency of whiteness:
Degree of whiteness of bleached goods is measured. If the degree remains same for 7 days- then the whiteness should be considered as permanent.

You can also read:

  1. Textile Dyes and Dyeing
  2. Textile Chemicals and Auxiliaries Used in Wet Processing 
  3. Dyeing Defects and Their Remedies
  4. Typical Dyeing Recipe for Different Fibers and Dyes
  5. Cotton Garment Dyeing with Direct Dyes