DYEING
Dyeing is process to give color to the fabric. There are different methods of dyeing.
- Continuous dyeing.
- Exhaust dyeing.
DYEING PREPARATIONS
Before dyeing the fabric is prepared to dye. There are different pretreatments before dyeing. These pretreatments are as under.
· Singeing
· Desizing
· Scouring
· Bleaching
· Mercerizing
Singeing
The gereige fabric contains hairiness or protruding fibers on its surface. These fibres create problems during dyeing. For uniform dyeing these protruding fibres are removed from the surface of the fabric. This process is called singeing. In singeing process fabric is passed through the fire flame with the fabric speed of 80-150 m/min.
Desizing
After spinning oil, starch and wax is applied to the yarn.
These sizing materials are applied to the warp yarn to produce strength in the yarn so it does not break in the weaving process. These sizing materials are some time natural like rice starch, potato starch or synthetic like PVA, Poly acrelates etc. These sizing materials produce problems in dyeing like uneven dyeing and color patches. The removal of these sizing materials is necessary to produce absorbency in the fabric. In desizing process these sizing materials are removed. The desizing chemicals depend upon the type of sizing materials used on the fabric. The desizing methods are
· Hot water desizing
· Acidic desizing
· Enzymatic desizing
Scouring
The gereige fabric contains impurities i.e. seed particles, leaf particles etc. These impurities are harmful for dyeing. These impurities produce undesired effect that is called frost. In scouring process fabric is treated with caustic soda (NaOH) to remove these impurities.
Bleaching
Fabric coming from the weaving contains colour matters. For uniform dyeing these color matters should be removed from the fabric. In bleaching process these colour matters are removed from the fabric. Hydrogen per oxide is used as bleaching agent to remove color matters from the fabric.
Mercerizing
Jhon mercer observed that if fabric is treated with NaOH before dyeing it produces strength in the fabric and creates luster also. This process is called mercerizing.
