Wet processing

• Desizing

Desizing is the first wet processing technology employed to remove sizing material from the surface of the fabric.

• Scouring

The term scouring applies to the removed of impurities such as oil, fats, waxes, soluble impurities and any particles or solid dirt adhering to the fibres. This is the heart of the wet process applied to textile material.
If the appropriate reagents are used , scouring will also remove size from the fabric although desizing often precedes scouring and considered to be a separate process known as fabric preparation.

• Bleaching

Bleaching is a process of destruction of natural coloring matter to produce white fabric band be accomplished with a minimum damage to the cloth being bleached.Cotton being a vegetable fiber will be bleached using an oxidizing agent, such as dilute sodium hydrochloride or dilute hydrogen peroxide. If the fabric is to be dyed a deep shade, then lower levels of bleaching are acceptable.

This is done by bleaching in different ways-
        i) By using dilute hydrochloride solution at room temperature.
        ii) By using hydrogen peroxide at 80-85 degree celcious.
        iii) Using sodium chlorate solution.

        Bleaching agent:
1) Oxidizing agent
        i) Sodium hydrochloride (NaOCl)
        ii) Calcium hydrochloride
        iii) Hydrogen peroxide
        iv) Sodium chloride
        v) Ozone
        vi) Potassium permanganate
        vii) Potassium dichromate
2) Reducing agent
        i) Zinc dust
        ii) Spontaneous chloride
        iii) Ferrous sulfate
        iv) Sodium hydrosulphate

• Mercerizing

Mercerization is a physio-chemical process where cotton yarn or fabric is treated with 15-22% caustic soda solution at the temperature 20-30 degree celcious. It is necessary to hold the fabric under tension during the process and wash thoroughly while under tension.

Benefits obtained by mercerization-
         i) Improved strength
         ii) Improved hygroscopicity
         iii) Improved dye affinity
         iv) Improved smoothness
         v) Improved luster

• Singeing

Singeing is an operation carried out to remove hairy fibers protruding from surface of the cloth.

• Raising

Another finishing process is raising. During raising, the fabric is treated with sharp teeth to lift the surface fiber, thereby imparting hairiness, softness and warmth.

• Dyeing

The molecules of the organic compounds called dyes are responsible for the color of dyed and printed textile fiber materials. For improved fastness and washing, rubbing and light other dyes such as vat dyes and reactive dyes are commonly used.

The dye molecules

Dye molecules are colored because they are selectively able to absorb and reflect incident light. Light is a form of energy; it is also the visible portion of the electromagnetic spectrum.

Organic molecules become colored and thus useful dye molecules, if they contain at lest one of each of the radicals called chromophores and auxochromes.

Classification of dye:
1) According to chemical structure
          i) Nitroso dye
          ii) Nitro dye
          iii) Azo dye
          iv) Stilbene dye
          v) Anthraquinone dye
          vi) Indigoid dye
2)According to mode of application
          i) Acid dye
          ii) Basic dye
          iii) Direct dye
          iv) Mordant dye
          v) Vat dye
          vi) Ingrain dye or developed dye
          vii) Reactive dye
          viii) Disperse dye
          ix) Pigment dye

Acid dyes

Acid dyes are so called because they are usually applied under acidic conditions. The fibers most readily colored with acid dyes are man-made, synthetic, nylon fibers and natural protein fibers.

Azoic dyes

Azoic dyes are so called because their molecules contain an azo group. Azoic dyes are also called napthol dyes, ice colors or developed colors.

The fibers most readily colored with azoic dyes are the man-made and natural cellulose fibers, e.g. viscose, cotton, etc.

Basic dyes

These are also called cationic dyes, because in solution the basic dye molecule ionizes, causing its colored component to become a cation or positively charged redical.

When they were first synthesized, the basic dyes were used on wool and silk but they had very poor color fastness properties. They were therefore displaced for these fibers by acid dyes.

Direct dyes

Direct dyes are also called substantive colors because of their excellent substantivity for cellulosic textile materials.
The fibers most readily colored with direct dyes are the man-made and natural cellulose fibers; that is, cotton and viscose fibers.

Disperse dyes

These dyes derive their name from their insoluble aqueous properties and the need to apply them from an aqueous dispersion.
The fibers most readily colored by disperse dyes are the man-made ester-cellulose and synthetic fibers, especially the acetate fibers and polyester, and less often acrylic and nylon.

Mordant dyes

The term mordant is derived from the Latin mordeo, which means to bite or to take hold of. The mordant dye is attached to the textile fiber by a mordant, which can be an organic or inorganic substance. The most commonly used mordant is inorganic chromium. Other inorganic mordants, such as aluminium, copper, iron and tin, and organic mordants, such as tannic acid, are rarely used. Since chromium is used so extensively, mordant dyes are sometimes called chrome dyes.
Fibers most readily dyed with mordant dyes are the natural protein fibers, particularly wool; and sometimes the synthetic fibers modacrylic and nylon.

Reactive dyes

Reactive dyes are so called because their molecules react chemically with the fibre polymers of some fibres to form a covalent bond between the dye molecule and fibre polymer.
 The fibres most readily colored with reactive dyes are the man-made and natural cellulose fibers, synthetic nylon and natural protein fibers.

Sulphur dyes

These dyes are so called because they contain sulphur atoms in their molecules. The fibers most readily colored with sulphur dyes are the natural and man-made cellulosic fibers.

Vat dyes

The name vat was derived from the large wooden vessel from which vat dyes were first applied. Vat dyes provide textile materials with the best color fastness of all the dyes in common use.
The fibers most readily colored with vat dyes are the natural and man-made cellulosic fibers.       

• Printing

Printing, on the other hand, is the application of color in the form of a paste or ink to the surface of a fabric. It may be considered as localized dyeing. Printing designs on to already dyed fabric is also possible.