Scouring process in textile processing
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The document discusses the scouring process used to clean fabrics prior to dyeing or finishing. Scouring removes natural and mechanical impurities through processes like saponification, solubilization, and emulsification. Saponification converts oils and fats to soap and glycerin. Solubilization and emulsification remove other impurities like pectins, proteins, waxes, and minerals by making them water soluble or dispersing them. Selection of scouring agents depends on fiber type, fabric properties, and impurity level. Effectiveness is tested through measures like absorbency, weight loss, and residual wax content.
Scouring process in textile processing
- 2. Grey fabric contains mainly three type of impurities: ◦ Waxes, proteins, pectic substances and mineral matters ◦ Mechanically held impurities e.g. motes, leaves ◦ Spin oils, waxes, grease ◦ Machine oils, tars, greases, dirt, dust
- 3. Cellulose 85-96% Oil and Wax 0.5 -1% Proteins, Pectoses 3-5% and colouring Matter Mineral Matter 0.5-1% Moisture 7-8%
- 4. • Natural fibers contain oils, fats, waxes, minerals, leafy matter and motes as impurities that interfere with dyeing and finishing. • Synthetic fibers contain producer spin finishes, coning oils and/or knitting oils. • Mill grease used to lubricate processing equipment mill dirt, temporary fabric markings and the like may contaminate fabrics as they are being produced.
- 5. Scouring is a purifying treatment of textiles. Also known as kiering, kier boiling or boiling out. The objective of scouring is to reduce the amount of impurities sufficiently to obtain level and reproducible results in dyeing and finishing operations.
- 7. Selection of Scouring Agents depends on: ◦ Kind of fiber Wool and silk are dissolved by alkali Acetate and triacetate are converted to cellulose Cotton absorbs it ◦ Fabric type i.e. woven or knitted ◦ Fabric density i.e. thick or thin ◦ Texturised or non-texturised ◦ Extent of impurities present in the fiber
- 8. Scouring mechanism involves the following processes: (for vegetable oils, animal fats, mineral oils) (for pectins, minerals, heavy metals, amino acid, proteins) (for mineral oils, waxes) (for dirt, dust, china clay)
- 9. Sponification ◦ The name given to the chemical reaction that occurs when a vegetable oil, animal fat and mineral oils (not soluble in water) are mixed with a strong alkali. The products of the reaction are two: soap and glycerin ◦ Fats, oils and waxes (glycerides of fatty acids) ◦ Soap formed enhances scouring ◦ Quantity of residual waxes in particular decide the absorbency of fabric
- 10. Oil + Caustic Soda Glycerine + Soap
- 11. Solublisation ◦ NaOH swells the impurities & makes them water soluble For Pectins, protiens and motes ◦ Minerals and heavy metals are converted to soluble salts by use of acids ◦ Amino acids and protiens are removed by formation of their sodium salts
- 12. Emulsification ◦ An emulsion is a mixture of two or more liquids that are normally immiscible ◦ Non-sponifiable mineral oils and waxes which are esters of higher fatty alcohols and fatty acids (natural and added) can be removed by emulsifying them with a surfactant (emulsifying agent) ◦ Ordinary soap (washing soap) is a good emulsifying agent
- 13. Emulsification An emulsion consists of a base liquid, with the other liquid contained and spread within the base liquid in tiny droplets. The base liquid is called the ‘continuous phase’ and the contained liquid is called the ‘dispersed phase.’
- 14. In order to keep the liquids uniformly distributed and stable, an agent used is called emulsifying agent. These agents have both a hydrophilic and a lipophilic part in their chemical structure. They concentrate at and are adsorbed onto the oil:water interface to provide a protective barrier around the dispersed droplets. They also reduce the interfacial tension of the system. Some agents enhance stability by imparting a charge on the droplet surface. In scouring it is a soap formed already.
- 15. The remaining constituent of size and other dust and dirt particles, stains, are removed by detergency. Detergency is closely bound up with surface tension.
- 16. When a piece of greige fabric is placed on the surface of water in a beaker, the piece does not sink but floats on its surface. This is because every molecule of water is attracted by every other molecule of water. The molecules inside the water counterbalance the forces from all sides and cancel out the overall effect on them. While the surface molecules are only pulled sideways and downward, due to which they form and interface between liquid and solid.
- 17. Generally, surface tension between textile fibres, and water is high, and hence wetting of the fibre surface does not take place thoroughly and quickly. A surfactant is a substance which, when applied in low concentration, markedly reduces the surface tension of a solution. A good surfactant, for example, will reduce the surface tension of water from 72 dynes/cm to 30 dynes/ cm at a concentration of less than 0.1%. The concentration at which no further reduction in surface tension occurs is known as 'critical micelle concentration'.
- 18. When a surfactant (soap) is dissolved in water the hydrocarbon chain (hydrophobic part tail, ==) tries to get away from the water medium due to its hydrophobicity and concentrate at the surface. The hydrophilic grouping (carboxyl head, O) just dip in the water at the surface and the sodium cation is in the vicinity of negatively charged carboxyl head. Thus this compound, sodium stearate (soap), distort the structure of water and decrease the free energy of the system.
- 19. ◦ The reason for reddish shade after bleaching with peroxide is the high content of manganese. ◦ The contents of Fe, Mn, Ca and Mg varies depending upon the origin of cotton ◦ Can be done by complexion and consequent solublization using an acid or alkali ◦ Usually done by sequestering action. ◦ They form complexes with metal ions and keep them away from fabric. Generally good pre-treated materials should have 100- 300 p.p.m, of Ca and Mg and 4-8 p.p.m, iron.
- 20. ◦ Advantages Better Levelness and more Brilliance in textile dyeing lower peroxide consumption in bleaching Increase in degree of Whiteness regular decomposition of peroxide and No catalytic damage
- 21. Absorbency: Drop Test ◦ Min 3 Sec and max 10 sec Weight loss is also checked Residual Wax Content ◦ Wax is dissolved in a solvent ◦ Solvent is distilled off ◦ The sample is reweighed
- 22. Dyeing and Chemical Technology of Textile Fibers, By: E.R. Trotman Chemical Technology in the pretreatment processes of Textiles, By: S.R. Karmakar.
- 23. Absorbency: ◦ Drop Test Less than 5 sec ◦ Other tests Wicking Height Sinking Test Weight loss is also checked Residual Wax Content ◦ Wax is dissolved in a solvent ◦ Solvent is distilled off ◦ The sample is reweighed Removal of Motes ◦ Observed visually and given a rating of 1-5