Typically, hydrogen peroxide, sodium chlorite, sodium hypochlorite are used for bleaching.
Hydrogen peroxide bleaching is becoming a mainstream, taking environmental problems into consideration. In general, reduction bleaching is not suitable for cotton.
(Hydrogen Peroxide (H2O2) Bleaching)
The H2O2 bleaching is the most common bleaching method, and is environmentally friendly (degradable into water and oxygen). The best bleaching whiteness can be obtained at pH 11 adjusted using caustic soda.
However, if metal ions, such as iron ions and copper ions are present in fabrics or in water, catalytic action of these metal ions causes abnormal degradation of H2O2, which may sometimes cause embrittlement of fabrics until holes (pin-holes) are generated due to the excessive degradation.
Usually, the abnormal degradation of H2O2 and the resulting generation of pin-holes are prevented by adding a chelating agent. Also, as a stabilizer for H2O2, sodium silicate has been conventionally employed. The obtained bleaching whiteness is favorable. However, the use of sodium silicate has the following disadvantages. Silica scales are generated due to calcium ions and magnesium ions reacted with silica ions. The generated silica scales are adhered to machinery, damaging fabrics, or undermining hand of the fabrics.
This case, however, suffers from a contradiction that complete removal of calcium ions and magnesium ions will undermine the stability of H2O2.
To solve the disadvantages and contradiction, a non-silicate type stabilizer containing no sodium silicate has been developed, although the cost is higher than sodium silicate.
Furthermore, when dyeing is performed without a drying process after the H2O2 bleaching, the problem of residual H2O2 arises. 10ppm of residual H2O2 or more causes dyeing problems, although it depends on a type of dye. To degrade the residual H2O2, a reducing agent or the like, or an enzyme (catalase) can be used. The enzyme appears as a current mainstream.
Hydrogen peroxide stabilizer
| BRITE NIK |
Contains sodium silicate; Effective in dispersing silica scales. |
| BRITE W |
Contains no sodium silicate |
| APPEAGEN TOP |
Improved version of BRITE W |
Chelating agent
| CHELET G-8 |
DTPA system
|
| CHELET G-5 |
Fermented organic amino-carboxylate
|
| FN-5000 |
Chelate dispersing agent
|
Residual hydrogen peroxide dispersing agent
| BK-7 |
Reduction system |
| ENZYLON KAT-50 |
Catalase |
(Sodium Chlorite (NaClO2) Bleaching)
The NaClO2 bleaching is performed after pH adjustment to 3.5 with an acid such as a formic acid. It provides soft hand with less embrittlement of cotton. Compared with the H2O2 bleaching, some insist that even bluish (?) whiteness can be achieved with the NaClO2 bleaching.
Also, the NaClO2 bleaching is free from worries about pin-hole generation, as often occurs in the H2O2 bleaching. However, a harmful chlorine dioxide gas is generated during the bleaching process, and therefore, suitable measures including the equipments need to be taken for metallic corrosion problems and dechlorination purposes. After bleaching, a dechlorination process needs to be provided. A trace chlorine which exists in fabrics reacts with nitrogen compound to form chloroamine, or cause yellowing of the fabrics when exposed to light and heat.
Note here that a special reducing agent, sodium thiosulfate (hypo), or hydrogen peroxide (so-called, `cosmetic bleaching`) is used for the dechlorination purposes.
| Sodium chlorite low-temperature activator |
BRITE FB CONC |
One-bath desizing and bleaching
Desizing agent
Bleaching auxiliary |
BIOGEN 300
BIOGEN NN |
| Dechlorination agent |
BRITE CL-B |
(Sodium Hypochlorite (NaClO) bleaching)
The NaClO bleaching is characterized in that the bleaching is performed at room temperatures without requiring a heating apparatus such as a steamer, and that the chemical cost is low. However, the disadvantages of the NaClO bleaching include relatively low bleaching whiteness and embrittlement of fabrics.
No particular pH adjustment is required in the NaClO bleaching. The NaClO bleaching is usually employed for bleaching fabrics after Kier-scouring, or for bleaching towels. For towels, the two-step bleaching comprising the NaClO bleaching and the H2O2 bleaching is preformed to obtain full whiteness. The NaClO bleaching is also employed for jeans fade adjustment, etc.
Note here that a dechlorination process needs to be provided as in the NaClO2 bleaching. Since `chlorine free` is a recent trend, like PRTR, a method without using chlorine is being examined. The same applies to NaClO.
