Dyeing Pretreatment :
Pretreatment processes of dyeing consist of cleaning operations to rid the fabric of all soil and additives that have accumulated during the weaving or knitting process. These processes are usually the first treatments a fabric undergoes after leaving the loom or knitting machine, and are required before any dyeing, printing, or finishing can be accomplished. In the strict definition of finishing, pretreatment processes do not quality as textile finishes because they are performed prior to dyeing or printing
Grey goods contain warp starches or other sizing (to add stiffness and strength to warp yarns during weaving), as well as oils, waxes, and other lubricants, plus floor dirt or other soils picked up during processing. Complete removal of all these impurities is necessary before any dyeing; printing, or finishing can be done. The processes for cleaning are varied depending on the fiber, the impurities present, and the fabric construction. In cotton, cotton-blend, silk, and manufactured-fiber fabrics, the processes are generally known as the boil-off. In woolens and worsteds, it is called a scour or scouring. Boil-off or scouring is similar to ordinary laundering. Fabrics are treated with soaps or detergents, rinsed, and then dried. If warp starches are present, the fabric is treated with an additional process known as desizing. Enzyme solutions are used to dissolve the starch.
Woolen and worsted fabrics may undergo an additional pretreatment process known as carbonizing. This process removes leaf particles and bits of grass and other cellulosic impurities that become embedded in the wool white sheep are grazing. The treatment consists of steeping the wool fabric in sulfuric acid, which destroys the cellulose but leaves the wool unharmed.
Another dyeing pretreatment process is an operation called singeing, which involves burning off projecting fibers or filament splinters from the surface of a fabric. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics. . Singeing is accomplished by passing gray goods rapidly over gas flames, usually two burners to a side, at a speed of 100 to 250 yards per minute. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics.
Dyeing pretreatment processes may also include bleaching. Fabrics to be dyed in light to medium shades, as well as most prints, are first bleached. Bleaches are required to obtain pure whites because natural fibers are rarely pure white in their natural state; they are usually slightly yellowish or grayish. Bleaches are chemical agents that react with the color compounds in the fiber and render them colorless. Our eyes see the fabric as pure white.
The effectiveness of optical brighteners after dyeing is dependent on an ultraviolet source of light. Natural sunlight is such a source. Fluorescent lighting is also, but to a small degree. Ultraviolet lamps (also called black-light lamps) are another source. Incandescent lighting (regular household bulbs) contains no ultraviolet, so optical brighteners have no effect on fabrics used under this lighting.