Which is the process in which cotton is treated with a concentrated solution of sodium hydroxide ?

Question

Accepted Answer

The correct option is 1:
Mercerising is the correct answer. This process is a crucial textile finishing treatment applied to cotton thread or fabric to improve its performance and aesthetic qualities. The core of mercerisation involves soaking the cellulosic material (cotton) in a concentrated solution of Sodium Hydroxide (NaOH), also known as caustic soda, typically at concentrations ranging from 15% to 25%. A vital aspect of this modern method, often attributed to the improvements by Horace Lowe, is that the cotton must be kept under *tension* throughout the treatment and subsequent washing. When the strong alkali solution acts on the cotton's cellulose structure, it causes the naturally twisted, kidney-shaped fibres to swell and change permanently into a more rounded, smooth, and circular cross-section. This physical and chemical change makes the cotton appear more lustrous, gives it a silk-like sheen, significantly increases its tensile strength, and, crucially, improves its affinity for dyestuffs, leading to brighter and more permanent colours.

The incorrect option is 2:
Scouring is a textile wet process primarily used to cleanse cotton materials of their natural impurities before dyeing or finishing. Raw cotton contains numerous substances like natural waxes, pectins, mineral matter, and dirt acquired during farming and ginning. Scouring involves treating the material with a hot, dilute alkaline solution, often including a weak sodium hydroxide and a wetting agent. The main objective is not to change the fiber's structure for luster, but rather to emulsify and dissolve the non-cellulosic impurities. This purification step ensures uniform absorbency, which is essential for subsequent processes like mercerising or dyeing, as residual waxes would otherwise repel the dye.

The incorrect option is 3:
Singeing, also called gassing, is a *dry* preparation process and does not involve any chemical solution like sodium hydroxide. The primary function of singeing is to remove the minute, protruding fibers or 'fuzz' from the surface of the cotton fabric. This is achieved by quickly passing the fabric over open gas flames or hot copper plates. The high temperature rapidly burns off the surface fuzz without damaging the main yarn structure. Singeing results in a very smooth surface, which is beneficial for achieving clear prints and an aesthetically clean finish, making the fabric less prone to pilling, but it does not impart strength or permanent luster.

The incorrect option is 4:
Degumming is a process that is specifically associated with the preparation of natural silk, not cotton. Raw silk consists of two fibroin filaments held together by a gummy protein called sericin. Degumming involves boiling the silk in a mild, slightly alkaline soap and water solution to dissolve and remove this sericin. The removal of sericin is essential because it makes the silk fiber soft, lustrous, and flexible, allowing it to take dyes beautifully. Although a mild alkaline solution is used, the process is fundamentally different in purpose and material; its goal is to strip an external coating (sericin) from a protein fiber (silk), whereas mercerising structurally modifies a cellulose fiber (cotton).