Mercerised cotton yarn is cotton yarn that has been specially treated to make it smoother, stronger, brighter, and more silk-like in appearance. Ordinary cotton yarn has a soft, slightly dull look because cotton fibres are naturally flat, twisted, and ribbon-like. In mercerisation, this natural fibre structure is changed by chemical treatment and controlled stretching.
The process begins by passing the cotton yarn through a cold and strong solution of caustic soda, also known as sodium hydroxide. This solution is quite concentrated. When the cotton yarn comes into contact with it, the fibres swell and the yarn contracts, usually by about 20 percent. This contraction happens because the caustic soda penetrates the cotton fibres and changes their internal structure.
At this stage, the cotton fibres no longer remain flat and twisted like ribbons. They swell, become more rounded, straighter, and more transparent. This change is very important because rounder and smoother fibres reflect light more evenly. That is why mercerised cotton develops a bright, silky lustre.
However, lustre does not develop fully by chemical treatment alone. The yarn must also be stretched. After the yarn contracts in the caustic soda solution, it is stretched back close to its original length. This stretching is done while the yarn is still impregnated with alkali. The tension helps align the fibres and creates the permanent shine associated with mercerised cotton. If the yarn is allowed to shrink freely without being held under tension, the lustre will be much less.
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The tension is maintained while the caustic soda is washed out. This is important because the yarn must remain straight and controlled during the removal of alkali. After washing, the yarn is passed through a dilute solution of sulphuric acid. The purpose of this acid bath is to neutralise the remaining caustic soda. Since caustic soda is strongly alkaline, it must be neutralised properly so that it does not damage the yarn later. After neutralisation, the yarn is washed again and then dried.
The best mercerised effect is obtained when the yarn used is already of high quality. Combed yarn gives better results than carded yarn because combing removes short fibres and impurities, leaving longer, smoother, and more uniform fibres. Gassed yarn gives even better lustre because the tiny projecting fibres on the yarn surface are burned off before mercerising, making the yarn surface cleaner and smoother.
Two-fold yarn is often preferred because it is more uniform, stronger, and rounder than single yarn. A slightly lower twist than ordinary two-fold yarn is useful because too much twist can prevent the fibres from swelling evenly and reflecting light properly. High-quality cotton is also important because long, fine, mature fibres respond better to mercerisation.
Earlier, Egyptian cotton was commonly used for mercerised yarn because of its long staple length, fineness, and superior quality. Such cotton produced excellent lustre and strength after mercerisation. Later, improved processing methods made it possible to obtain good mercerised results from better grades of American cotton as well.
In simple terms, mercerisation changes cotton from a soft, dull, ribbon-like fibre into a smoother, rounder, shinier, and more silk-like fibre. The caustic soda causes swelling, the stretching creates lustre, the acid neutralises the alkali, and washing and drying complete the process. The final yarn looks richer, takes dye better, has improved strength, and gives fabrics a more polished and premium appearance.
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