GATE (TF) Textile 2015 Question Paper Solution | GATE/2015/TF/42

Viscose rayon, also known as rayon or artificial silk, is a type of regenerated cellulose fiber that is produced from wood pulp or other sources of cellulose. The production of viscose rayon involves several steps, including:
Pre-treatment of cellulose: The cellulose is obtained from wood pulp or other sources and is typically treated with chemicals to remove impurities and increase its reactivity. This may involve processes such as bleaching, alkalization, and aging to achieve the desired cellulose quality for the production of viscose.
Xanthation: The treated cellulose is then mixed with a solution of sodium hydroxide (NaOH) and carbon disulfide (CS2) to form a viscose solution. The cellulose reacts with CS2 to form cellulose xanthate, which is a derivative of cellulose that is soluble in aqueous NaOH.
Ripening: The viscose solution is allowed to “ripen” or age for a period of time, typically several hours to several days, to allow the cellulose xanthate to fully develop and mature.
Dissolving: The ripened viscose solution is then mixed with water to dilute it and reduce its viscosity, resulting in a viscose dope. The viscose dope is a thick, brown liquid that contains dissolved cellulose xanthate.
Filtration and deaeration: The viscose dope is filtered to remove any remaining impurities or undissolved cellulose particles. It is then deaerated to remove air bubbles, which can affect the quality of the resulting rayon fiber.
Spinning: The deaerated viscose dope is then extruded through spinnerets, which are typically made of metal, into a coagulating bath. The coagulating bath typically contains sulfuric acid (H2SO4) or another acidic solution, which causes the cellulose xanthate to regenerate back into cellulose fibers through a process called coagulation. The regenerated cellulose fibers solidify in the coagulating bath and are then washed to remove residual chemicals.
Washing and neutralization: The regenerated cellulose fibers are thoroughly washed to remove any residual chemicals, such as sulfuric acid and sodium sulfate, which are produced during the coagulation process. The fibers are then typically neutralized with an alkaline solution, such as sodium carbonate (Na2CO3), to remove any remaining traces of acidity.
Bleaching: The neutralized and washed cellulose fibers are then bleached to improve their brightness and whiteness. Bleaching agents, such as hydrogen peroxide (H2O2), chlorine, or sodium hypochlorite (NaClO), may be used in this step.
Stretching and drying: The bleached cellulose fibers may be mechanically stretched or treated with heat to improve their tensile strength and other properties. The fibers are then dried to remove moisture and obtain the final viscose rayon fiber.
Finishing: The dried viscose rayon fibers may undergo additional finishing processes, such as crimping, cutting, or applying coatings or finishes, to further modify their properties and make them suitable for specific textile applications.
It’s worth noting that the exact process flow of viscose rayon production can vary depending on the specific manufacturer, equipment, and process conditions used. However, the steps outlined above provide a general overview of the typical process for producing viscose rayon fibers.