Decatising:

       Decatising or decatizing, also known as crabbing and blowing, is a finishing process. In this process wool or its blends are finished in such a way that that it does not shrink during garment making. The word comes from the French décatir, which means to remove the cati or finish of the wool. Though used mainly for wool, the term is also applied to processes performed on fabrics of other fibers, such as cotton, linen or polyeste. 

       A finishing process in which the fabric is wound tightly onto a perforated roller and either immersed in hot water, which is also circulated through the fabric (wet decatising) or has steam blown through it (drydecatising). Decatized wool fabric is interleaved with a cotton, polyester/cotton or polyester fabric and rolled up onto a perforated decatizing drum under controlled tension. The fabric is steamed for up to ten minutes and then cooled down by drawing ambient air through the fabric roll. The piece is then reversed and steamed again in order to ensure that an even treatment is achieved. There are several quite different types of wool decatizing machines including batch decatizing machines, continuous decatizing machines, wet decatising machines and dry decatizing machines.

How does wool scouring differ from cotton scouring?

First point to be noted that, wool is having very high amount of waxy matter than cotton. The wax matter of cotton is removed by treating cotton with sodium hydroxide solution at boiling temperature. But sodium hydroxide solution can not be used for scouring of wool as wool is very sensitive to alkali. Therefore wool scouring is carried out with detergent or mild alkali like sodium carbonate solution (pH around 8 to 10). 

There are three methods for wool scouring:

i) Emulsion scouring: In this method the raw wool is scoured in a tank filled with detergent having pH 8-10. In the process temperature is maintained around 55-60 degree C and time duration 30-45 minutes. After that wool is washed and dried.

ii) Solvent scouring: In this type of scouring raw wool is treated with solvents like Trichloroethylene, perchloroethylene or carbon tetra chloride (now it is banned in India). These solvent remove wax from the wool. This wax and solvent is recovered. The recovered wax are used various application.

iii) Freezing: As its name suggest, wool is subjected to very low temperature about -30 degree C. Due to this wool wax becomes hard. At same low temperature is is mechanically crushed followed by shaking. Wax is removed.

Scouring and cleaning of raw wool:

Wool is composed with Keratine (approx. 33%), Dirt (Aprrox. 26%), Suint (Dry perspiration: 28%), Fat/grease (12%), Burr (Vegetable matter: 2-10%), Mineral matter (aprox. 1%) and Colouring matter (0.5 to 1%).

1. Wool grease can be removed by scouring. These Grease/waxes are comprised of a variety of monocarboxylic, dicarboxylic and hydrocarboxylic acids as well as steroidal alcohols. It has been determined that unscoured wool contains an unoxidized fraction of wool grease and other contaminants that is easily removed and readily recoverable 

2. Suint is usually considered to be a variable composition of water-soluble materials that is readily removed by scouring. 

3. The dirt that is removed from the scoured wool consist of both inorganic and organic materials.

4. Dirt: It is held by adhesive action of suint and wool fat. It removed during the scouring and washing processes.

5. Burrs: It is vegetable fragments consisting of dried grass, straw, sticks etc. These vegetable fragment come on the body of sheep during grazing and scratching the body against bush or tree to relive itching. It is remove using Carbonizing process. Following is the process of removal of Burrs:

i) Wool is resistant to acid. Burr being a vegetable matter is not resistant to acid.

ii) This property is used for the removal of burr.

iii) In this case wool is treated with 5-7% Sulphuric acid for 2 hours followed by hydro-extraction and then drying at 80-90oC for 20-30 minutes. 

iv) Carbonized burr is removed by washing with mechanical agitation.