Now You Know Performance Analysis of Denim Garments by Bleach Wash (Part-2)

Performance Analysis of Denim Garments by Bleach Wash (Part-2)

Authors: Md. Anwarul Haque¹
Ariful Hassan²
Shantonu Paul Shuvra
Md. ShagorHossain
Textile Engineering College, Begumgonj, Noakhali.
Email: haquebd2014@gmail.com¹, arifulhassan034@gmail.com²

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Bleaching
Bleaching is chemical treatment employed for the removal of natural coloring matter from the substrate. The source of natural color is organic compounds with conjugated double bonds , by doing chemical bleaching the discoloration takes place by the breaking the chromospheres , most likely destroying the one or more double bonds with in this conjugated system. The material appears whiter after the bleaching.

Natural fibers, i.e. cotton, wool, linen etc. are off-white in color due to color bodies present in the fiber. The degree of off-whiteness varies from batch-to-batch. Bleaching therefore can be defined as the destruction of these color bodies. White is also an important market color so the whitest white has commercial value. Yellow is a component of derived shades. For example, when yellow is mixed with blue, the shade turns green. A consistent white base fabric has real value when dyeing light to medium shades because it is much easier to reproduce shade matches on a consistent white background than on one that varies in amount of yellow.

Bleaching may be the only preparatory process or it may be used in conjunction with other treatments, e.g. desizing, scouring and mercerizing. The combination of such treatments for an individual situation will depend on the rigorousness of the preparation standard and economic factors within the various options. Other chemicals will be used in addition to the bleaching agent. These serve various functions such as to activate the bleaching system, to stabilize or control the rate of activation, to give wetting and detergent action, or to sequester metallic impurities. This section gives consideration to the selection of bleaching agents and to the role of the various chemicals used in conjunction.

The purpose of bleaching is to remove coloured impurities from the fibre and increase the whiteness level of fabric.

Manufacturing of Bleaching Powder
The manufacture of bleaching powder is carried out in Backmann's plant as follows:

It consists of a vertical cast-iron tower. The tower is provided with a hopper at the top, two inlets near the base (one for chlorine and other for hot air) and an exit for waste gases near the top.

Performance Analysis of Denim Garments by Bleach Wash Now You Know Performance Analysis of Denim Garments by Bleach Wash (Part-2)

Bleaching powder

The tower is fitted with eight shelves at different heights each equipped with rotating rakes. The slaked lime is introduced through the hopper and it comes in contact with chlorine, which slowly moves upwards. Bleaching powder is collected in a barrel at the base. The chlorine used in the manufacture of bleaching powder should be dilute and the temperature should be maintained below 40^oC.

Mechanism of bleach action
Color in most dyes and pigments are produced by molecules, such as beta carotene, which contain chromospheres. Chemical bleaches work in one of two ways:

Oxidizing bleach works by breaking the chemical bonds that make up the chromosphere. This changes the molecule into a different substance that either does not contain a chromosphere, or contains a chromosphere that does not absorb visible light.

Reducing bleach works by converting double bonds in the chromosphere into single bonds. This eliminates the ability of the chromosphere to absorb visible light.

Chemical interactions
Mixing hypochlorite bleach with an acid can liberate chlorine gas. Hypochlorite and chlorine are in equilibrium in water; the position of the equilibrium is pH dependent and low pH (acidic) favors chlorine.

Cl2 + H2O → H+ + Cl− + HClO

Chlorine is a respiratory irritant that attacks mucous membranes and burns the skin. As little as 3.53 ppm can be detected as an odor, and 1000 ppm is likely to be fatal after a few deep breaths. Exposure to chlorine has been limited to 0.5 ppm (8-hour time-weighted average—38 hour week) by OSHA in the U.S.

Sodium hypochlorite and ammonia react to form a number of products, depending on the temperature, concentration, and how they are mixed. The main reaction is chlorination of ammonia, first giving chloramine (NH2Cl), then dichloramine (NHCl2) and finally nitrogen trichloride (NCl3). These materials are very irritating to the eyes and lungs and are toxic above certain concentrations; nitrogen trichloride is also a very sensitive explosive.

NH3 + NaOCl → NaOH + NH2Cl

NH2Cl + NaOCl → NaOH + NHCl2

NHCl2 + NaOCl → NaOH + NCl3

Additional reactions produce hydrazine, in a variation of the Olin Raschig process.

NH3 + NH2Cl + NaOH → N2H4 + NaCl + H2O

The hydrazine generated can react with more chloramine in an exothermic reaction to produce ammonium chloride and nitrogen gas:

2 NH2Cl + N2H4 → 2 NH4Cl + N2

However, the place of atomic oxygen in accounting for the formation of chlorine is not as plausible as another theory based on the so-called 'chloride system' employed in modern hydrometallurgy to dissolve ores with weak acids in highly ionic and concentrated salt solutions. Salts particularly effective, in this regard, include MgCl2, CaCl2, FeCl3 and, to a lesser extent, the mono-valent NaCl. This is, in effect, an application of the non-common ion theory, or as discussed in Wikipedia under Solubility Equilibrium as the 'salt effect'. With respect to bleaching powder, which has been described as a compound salt of the form Ca(ClO)2.CaCl2.Ca(OH)2.xH2O, the presence of CaCl2 in very concentrated solutions can greatly increase the 'activity level' of weak acids. So, in this particular proposed application, H2CO3 from CO2 and moisture on the bleaching powder, acts on the CaCl2 to release some HCl which acts on the HClO releasing Chlorine:

HClO + HCl → H2O + Cl2

or, the increasing acidity creates more HClO which moves the following known (and old, see Watt's Dictionary of Chemistry) equilibrium reaction to the right:

CaCl2 + 2 HClO = Ca(OH)2 + 2 Cl2

Properties of the Bleaching powder

1) Bleaching powder is yellowish in color and white powder and smells chlorine.

2) Bleaching is soluble in water. The lime present is left behind as an insoluble salt so it is called chloride of lime.

3) When Bleaching powder is exposed to air ,it is oxidized to chlorine.
CaOCl2 + CO2———> CaCO3 + Cl2

4) Chlorine gas is produced ,when Bleaching powder is treated with excess of dilute acid.
CaOCl2 + H2SO4 (dilute) ———> CaSO4 + H2O + Cl2

Objectives of bleaching