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Scouring
Scouring is the process of removing natural and added impurities from textiles using alkali solutions. It makes fabrics hydrophilic and absorbent. There are two main methods - batch/discontinuous scouring using kier boilers, and continuous scouring using J-boxes. Key steps involve saponification of oils and emulsification of waxes. Souring neutralizes residual alkali on scoured fabrics using acids.
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Scouring
- 1. Scouring: Scouring isthe process by which all natural and additive impurities such as oil, wax, fat, hand dust etc. are removed to produce hydrophilic and clean textile material. It is one of the vital processes of wet processing. Objects of Scouring: To make the fabric highly hydrophilic. To remove impurities such as oils, waxes, gum, husks as nearly as possible. To increase absorbency of fabric or textile materials without physical and chemical damage. To produce a clean material by adding alkali. To make the fabric ready for next process. To remove non-cellulosic substance in case of cotton.
- 2. Mechanism: Saponification: The vegetable oil, which is immiscible with water, is glyceride of fatty acids. When such oils are heated with a solution of sodium hydroxide in water, the oil splits up into its constituents-fatty acid and glycerine. Glycerine is miscible with water easily and the fatty acids reacts with sodium hydroxide present in the solution forming its sodium salt i.e.soap which is also soluble in water. Thus oil is removed. Emulsification: Wax and non saponifiable oils are removed by emulsification as they are immiscible in water. Normal washing soap is used as a emulsifying agent which makes emulsion of them.
- 3. The changes occurringof cotton fibers during scouring: Saponifiable oils and free fatty acids are converted into soaps. Pectins and pectoses are converted into soluble salts of pectic acid. Proteins are degraded to simple soluble amino acids or ammonia. Mineral matters are mostly dissolved. Non-saponifiable oils are emulsified by the soluble soaps generated from the saponifiable oils. Additive dirts are removed. Residual sizing materials are broken down into soluble products.
- 4. Scouring process dependson: The type of cotton. The color of cotton. The cleanliness of cotton. The twist and count of the yarn. The construction of the fabric. The shade % and type of shade of finished product.
- 5. CHEMICALS USED IN SCOURING PROCESS Main chemicals Use Caustic soda Neutralize acidic materials, saponify glycerides (waxes and oil),solubilise silicates Surfactants Reduce surface tension & minimize interfacial tension. Detergents Emulsify oil, fats, waxes and remove oil-borne stains. Chelating agent Deactivate metal ions. Sodium silicate Penetrate & break drown lignins Soda ash Maintain pH Solvent Assist emulsification by dissolving oily materials.
- 6. FORM OF SCOURING Yarnscouring: Hank form Package form Continuous sheet warp form. Fabric scouring: Open width form : Jigger Pad batch Progressive jig Rope form Kier Washer.
- 7. Scouring process: Batch process. Semi – continuous process. Discontinuous process. Modern process.
- 8. Scouring methods ofCotton: Generally, there are two principle methods of cotton scouring. Discontinuous (Kier boiling process or winch dyeing machine) Continuous (Scouring in J or L box)
- 9. Kier boiler process/Scouring of cotton fabric in Batch process/ discontinuous process: In this process of cotton scouring, the fabric is scoured in rope form, by alkali liquor. Kier means specially constructed large cylindrical iron vessel. The kier boiler may be- a. horizontal(yarn scouring) b. vertical(fabric scouring). Kier boiler capacity: maximum 5ton, but in industrially 2-3 ton kier boiler is very popular.
- 10. Standard Recipe: Theamount of different chemicals used in a kier boiler may vary according to Hardness of water. Quality / Quantity of cotton. Fabric construction/ texture, yarn twist etc. But in generally, the following amounts of the chemical are used. Caustic Soda (Strong)→ 2-5% owf Soda ash (mild)→ 0.3-0.5% Sequestering agent→0.05-0.5% Wetting agent→ 0.25-1.0% M:L→1:4 to1:5 Temp→100˚C-120˚C
- 11. Main parts ofkier boiler: Cylindrical vessel Mixing tank(liquor prepared according to recipe) Multi tubular heat exchanger(heating the liquor) Perforated false bottom Circular tube(spray liquor on fabric)
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- 13. The fabric isloaded in the m/c and kept in rope form. The hot liquor is pumped and sprayed by circular tube on to the fabric The liquor passes slowly over the packed cloth and collects at the false bottom of the kier. The liquor again pumped into the heater by a centrifugal pump and this cycle is repeated After scouring ,the fabric is washed with 800C water to remove impurities on the fabric
- 14. Continuous scouring processof cotton (Scouring in J- box)The scouring vessel is looks like the English letter ‘J’ hence, this process is called j box process. In the process, desizing, scouring and bleaching can be performed at a time. Standard recipe Alkali (NaOH) 4-5gm/L Wetting agent +Detergent 4-5gm/L M:L 1:3 Pick up 90-100% Impregnation Temp 70-800C Impregnation Time 45-90sec Storing time in J-box 2-4hr Temperature in J-box 102-105oC
- 15. Process: The workingprocess in J-box can be divided into four units. There are Impragnaion box Pre – heater. J – box. Washing unit. Impragnaion box: The fabric is passed through the guide roller and immersed into the solution at temperature 70˚C – 80˚C for about 40-90 sec Pre heater: Material is passed into the thermostatic controlling system at temperature 90˚C – 100˚C for 30 sec J-box: In j-box, fabric is stored in this solution of NaOH for about 1-4 hrs at a temperature of 102˚C.
- 17. CH2-OOC-C17H33 CH2-OH CH2-OH + 3C17H33COONa CHOOC-C17H33 + NaOH CH2-OH Sodium salt CH2-OOC-C17H33 Washing unit: The impurities are removed here. First the materials are washed in hot water then cold water.
- 18. Scouring process ofsilk: Impurities present in silk: Sericin up to 30% (main impurities) Mineral matter Coloring matter. 1 − 1.5% Traces of waxes, fats The removing of above impurities in silk are called de- gumming.
- 19. Principle: In Silk, there is protein which is damaged by alkali, so for scouring soap wash is done. The main impurities of silk are one kind of gum which is soluble in hot soap solution. During soap washing, alkali is produced which reacts with carboxyl group of sericine and convert it into soluble form and remove impurities. The different types of silk on the basis of scouring: Ecru silk: Ecru silk is obtained by removing of 3-4% impurities (Sericin)
- 20. Soap solution 2-3% Temp Room temperature Time 40-60 min Used for warp yarn and for dark shade 2. Souple silk: Souple silk is obtained by removing of 10% impurities (Sericin) Soap solution 10% Temp Room temperature Time 1-2 hrs. Used for medium shade
- 21. 3. Boiled offsilk: Sericin is removed up to 30% This process required two bath processes: 1st Bath 2nd Bath Soap solution 30% 10-15% Temp 90-950C 95˚C Time 60-90mins. 1-3 hrs Soda ash 1-2% Used for white and light shade
- 22. Scouring of Jute:(Bast fibre) Bast fibre, jute, linen etc. are multicellular fiber, having polygonal shape. The cells are cemented with each other by lignin which is non-cellulosic unit and is about 11-14%. If strong alkali is used for jute scouring, its strength is reduced as lignin is removed and cells are separated. So, mild alkali is used in jute scouring at low temperature.
- 23. Standard recipe Na2CO3 4-6gm Wetting agent + Detergent 0.5-1gm/ltr Water softening agent 0.5-1.0gm/ltr M:L 1:15-1:20 Temp Boiling(1000C) Time 2-4 hrs.
- 24. Assessment of scouring:/Absorbencytest: 1. Drop/Spot test: In a pipette a solution of0.1% direct red or Congo red is taken and droplet of solution put on the different places of the fabric. Then the absorption time of the fabric is observed. The standard time for the absorption of one drop of solution is 0.5-0.8 sec up to 1 sec.
- 25. Good scouring Uniform scouring Uneven scouring
- 26. Souring: Thetreatment/the process by which the fabric, after processing with alkali or scouring, is treated with Acetic Acid, Hydrochloric acid or dilute H2SO4 for removing alkali or neutralization of alkali is souring.
- 27. DIFFERENCE BETWEEN SCOURING AND SOURING Scouring Souring 1. To remove oil, waxes gum 1. Not to remove any soluble impurities. impurities, only for alkali neutralization. 1. Scouring is done in alkali • Souring is done dilute solution. HCl or H2SO4 1. Required heat to boiling. 1. No need of heat. 1. Need of definite time. 1. No need of definite time.