Cotton fibres:- Explained Completely
by : ---
Cotton fibres are coated with natural waxes and pectins, which are water-insoluble but removable by aqueous alkalies or by solvent-dewaxing process, Removal of these impurities improves the absorbent properties of the natural cotton and its receptivity to dyestuffs. The whiteness of the cotton is improved and seed particles are destroyed by caustic boil-off and bleaching operations.
Cotton may made suitable for dyeing in a variety of forms, such as raw stock, yarn, or piece goods. Raw stock is normally dyed without thorough dewaxing, since the natural waxes aid in subsequent spinning operations. Surfactants (qv) are employed to aid the penetration of dyestuffs through the protective waxes. Flaws in dye levelness are overcome by subsequent carding. Yarn is normally boiled off prior to dyeing. Hard-twisted ply yarns are frequently given a kier boiling prior to dyeing to improve levelness. Careful preparation of cotton piece goods is essential to achieve suitable dye penetration fastness and general appearance. Fabric construction dictates whether the fabrics will be processed in tope or open-width forms. Heavy piece goods, which are subject to rubs and crease marks and certain other constructions must be handled in open width.
The first step in the preparation of singeing, which removes lint and fuzz from the fabric surface. The goods are then impregnated with an enzyme solution for solubilization of the sizing. Desizing may be accomplished by pad-batch, jip, or pad-steam techniques. For material that must be handled in open width, the next step is hot scouring on jigs or open-width boil-off machines, in 10-12 degree Twad caustic soda containing a surfactant. Materials which may be roped are placed either in kiers, when continuous equipment is not available and boiler under 10-25 psi pressure for 10-12 hr. with 3-5% caustic soda based on the weight of the material or preferable for economic reasons, scoured continuously in rope-soaping machines. Kiers are normally lined with cloth and movement of the goods is avoided to prevent chafe marks. Occasionally a second boil-off is given. The goods are then rinsed, scoured with acetic acid or 1 degree Be sulphuric acid, rinsed, neutralized and dried.
Before dyeing in light shades, the goods should be bleached with hydrogen peroxide, sodium hypochlorite, or peracetic acid. This operation helps also in the removal of trace impurities that remain after boil-off. Although kiers are still used for boil-off and bleaching, then have been largely replaced by steam-teated J-boxes or steamers, which allow continuous processing and reduction of processing time from days to hours.
Mercerizing is accomplished by passing the cotton fabric through 15-30% caustic soda. Improved luster and increased dye affinity results. The fabric is normally held under tension during processing. In order to obtain cotton stretch fabrics, mercerization is carried out on fabric in a slack condition.
