Textiles Processing

By Manish Solanki

SCOURING

• This cleaning treatment, also referred to as alkali boil-off, removes most of the cottons impurities. This includes sizing residues, wax, oil, protein, and other foreign substances like sand, dust and leafy matters. The scouring efficiency is mainly depends on maturity, fineness and origin of cotton fibers.

KIER BOILING

• Kier boiling is most commonly used for scouring operation. A kier is a cylindrical vessel capable of holding 1 to 5 tons of fabric. It is provided with pressure gauge and safety valve. For kier boiling, a mixture of 5-10 g/l caustic soda solution, 1% sodium silicate, 1% soap along with wetting agent. Boiling takes place at 101oC temperature for 6-8hrs under pressure.

• After completion of boiling the liquor is completely drained out and immediately follows washing with cold water. It is the best way of scouring cotton fabrics as the maximum impurities are removed.

Kier Machine

J-Box

• In continuous machines, scouring can be carried out through a series of J-Boxes for progressive desizing and scouring. The desized and washed fabric is padded with strong alkali solution (5-10 gm/lit NaOH or mixture of NaOH & sodium carbonate), emulsifying agent and wetting agent. After padding the saturated cloth, it enters in to J-Box where it remains for 40-60 mins in plaited form at a temperature of about 100°C. The cloth before entering J-Box is rapidly preheated by means of steam in U- shaped heating tube. The internal surface of the entire J-Box is very smooth to avoid any friction an the moving cloth.

BLEACHING Bleaching is the process of removing natural coloring

matters that cannot be removed from fibers by extensive scouring. Bleaching is also required for fabrics that are to be dyed in pastel shades, in particular light blues and violets, and for materials to be dyed in colors with maximum brightness. The main objective of bleaching process is, to achieve the desirable level of whiteness with minimum damage to the fibers, and within the shortest possible time.

Sodium hypochlorite Bleaching

• In typical batch treatments of cotton fabrics with sodium hypochlorite in kiers, the bleaching bath is prepared as follows:

• Sodium hypochlorite 1-3 gm/lit of available chlorine

• pH of the bath 11 – 11.5• Temperature Room temperature • Time 2 hrs

Since the bleaching is done in alkaline medium (pH 10-11) the alkali, which is present in the material has to neutralized with dilute mineral acid. It is referred as “souring” This treatment with dilute hydrochloric acid or sulphuric acid is known as souring.

• The fabric is impregnated with dilute hydrochloric acid or sulphuric acid at room temperature for 30-60 minutes; the fabrics are then thoroughly washed with water and then dried.

Advantages;

• Bleaching with sodium hypochlorite can be carried at room temperatures.

• Powerful and economical method of processing

• Free from the defects of bleaching powder

Disadvantages

• All protein impurities must be completely removed before bleaching otherwise the fabric may turns yellowish.

• Residual chlorine must be removed otherwise fabric may be damaged.

Anti-chlorine treatment

To remove the residual chlorine, the fabric is to be washed with sodium – thio – sulphate, sodium bi-sulphite (chlorine destroyer) the chlorine will wash away. This treatment is called “antichlor”

Advantages of bleaching with hydrogen peroxide

• Hydrogen per oxide is a universal and is used extensively for the bleaching of cotton materials. It can be employed for bleaching fibers like wool, silk, polyester and other man- made fibers also, under a wide range of application conditions.

• The loss in weight of fabric in bleaching is less than that with hypochlorite bleaching.

• Per oxide bleached are more absorbent than hypochlorite bleached fabrics.

• No possibilities of yellowing of fabric.• In addition, hydrogen per oxide relatively

inexpensive, does not release toxic chemicals or unpleasant odour, and does not cause corrosion of equipment.

CONTINUOUS BLEACHING PROCESS

• In this process desizing, scouring and bleaching are combined together. These are very attractive options since savings in time, energy, labor, etc., are possible. However, combined operations are not always sufficient for preparation of certain cotton fabrics for dyeing and finishing.

Procedure

• In this method, the fabric is padded with the enzyme bath and then passed into a wet steam chamber at 96oC- 100oC where the desizing takes place in less than a minute.

• The fabric after desizing is impregnated a solution of 4-6% NaOH, 1-3 g/l wetting agent, and 1-3 g/l emulsifier at the period of 2-3 minutes.

• The fabric after impregnation is taken out and piled into J-box at 98oC and allowed to remain there for two hours during which period the alkali reacts with the impurities.

• The fabric is then rinsed and impregnated with 1% hydrogen peroxide at pH 10.5-using1% sodium silicate as a stabilizer and wetting agent.

• The cloth after impregnation is again heated to 90-95oC and stored for an hour in J- box.

• It is then rinsed thoroughly.

MERCERIZATION

• The treatment of cotton fabrics or yarns with a cold concentrated solution of sodium hydroxide solution for one minute or less is called mercerization: cotton fibers swell, untwist and their bean shaped cross section changes into round form.

Mercerization improves the following properties.

• Mercerization improves the following properties.

• Strength would be increased 15-25%;• Luster further enhanced; • Greater affinity to water dyes and other

chemical finishes.

• Fabric is padded with about 20-25% NaOH solution containing a wetting agent and then passed over several cans to allow a doweling time of approximately one minute. During this time NaOH will penetrate the fibres and react with them. At this stage the tension is applied lengthwise.

• The fabric is then placed on a stenter frame (tension now is applied in both warp and weft directions) and is pulled to its desired dimensions. While on the tenter frame, the fabric is washed by spraying water until the amount of sodium hydroxide on the fabric is reduced to only a few present.

OPTICAL BRIGHTENING

The fluorescent brightening agents called ‘optical brightening agents, increase the apparent whiteness or brightness of materials, and are used in textiles, detergent, paper, paints, etc.

Application of Optical Brighteners

• The application of optical brighteners are depends on the types of fibres on which is applied and accordingly can be classified as direct, disperse and cationic types. The direct brightening agents are mainly used for the brightening of cotton, linen, viscose and nylon materials. The acid florescent brightening agents serve mainly for the brightening of silk and wool fabrics. The disperse florescent brightening agents are used primarily for polyester, acetate and acrylic materials.

CLEANING OF WOOL

• Raw wool contains the following impurities; Wool grease, soil, dust, cellulose fragments and suint (dried perspiration, mostly soluble potassium salts.)

Keratain 33%Dirt 26%

Suint 28%Fat 12%

Mineral matter 1%

Wool scouring

Scoured with detergent solution at pH of up to 11 at below 60 o C temperature with anionic/non ionic detergent and small amount of weak base (0.4% Na2Co3) sodium carbonate / ammonia.

Carbonizing

• Cellulose material such as leaves, grass, seeds, and vegetable fibers are removed. The fabric (some time’s loose fibers) is treated with a dil. Solution. Of H2So4, then dried, and cured. Cellulose materials decompose to form a very fine black powder, which can be easily dusted out.

Wool bleaching

• Two methods of bleaching are commonly used, viz. (i) the oxidation method using hydrogen peroxide and (ii) the reduction method employing sulphur dioxide, sodium bisulphate or sodium hydrosulphite. Sometimes the wool is bleached with peroxide followed by bleaching with sulphur dioxide.

• A method of peroxide bleaching under acidic conditions has been devised; the bath is made up of 2 g/l of H2O2 containing 0.25% formic acid and adjusted to pH 3 to 3.5 at room temperature. The scoured material is impregnated with the peroxide solution on a padding mangle, and allowed to stand overnight, it is then washed.

Degumming of Silk Silk fiber in its natural state, contains a heavy

coating of a glue-like substance or gum called sericin which gives it a harsh and stiff feel and hides the rich luster and whiteness of the real fiber. This silk-gum amounts to about 25 per cent of the weight of raw silk. Wild silks such as Tasar silk, Eri silk and Muga silk also contain gum. Unless the gum is removed, silk does not exhibit its characteristic luster, smoothness as well as softness and tends to dye unevenly.

• Degumming with Synthetic Detergents Synthetic detergents are now being

increasingly used in place of soap. Their main advantage is that they permit continues processing of piece goods and the time of treatment is shorter viz. 40 minutes.

• In this method, the material is treated with 2.5 to 5 g/l detergent (Sandopan SRS liq. of Sandoz) at pH 11.2 to 11.5 for 30 to 60 minutes at boil and then rinsed thoroughly first with hot water and then with cold water.

• The disadvantage of this method is that soft feel is not always obtained.

• Degumming with Enzymes• This treatment is longer than that with

synthetic detergents and involves a special pre-treatment before enzyme treatment requiring swelling of the gum.

• The material is first treated with• 0.5 g/l soda ash ; 0.5 g/l Glauber’s salt ; 3 g/l

penetrating agent like Imerol XN liq. (Sandoz); at boil for about 1 hour.

• It is then treated with 1-5 g/l protein enzyme; 1 g/l soda ash and ; 2 g/l Glauber’s salt; at 45°C for about 1 hour.

• It is further treated in a third bath containing 0.5 g/l soda ash; 2 g/l sodium silicate; 2 g/l Imerol XN liq. (Sandoz) 0.5 g/l Glauber’s salt at boil for 1 hour.

• It is a three-step process. Improper degumming can give rise to mechanical damage causing chafe marks at the surface; as a result, the ends dye a lighter shade.

Bio-PolishingIn recent years, there has been tremendous use of enzymes in wet processing of textiles Cotton, other natural and man-made cellulosic fibres can be improved by an enzymatic treatment called Bio-Polishing. cellulases based enzymes are used for bio-polishing on cotton & lyocell(Tencel) fabrics.

Objectives

• Smother & softer surface• Prevention of pilling• Superior color brightness

Applications

• Cellulase based enzymes: 2-3 g/lpH 5.5 - 6.0 (acetic acid used to

maintain ph)Temperature 40 - 55 °C, Time 30-45 min.

• Cellulases hydrolyse the microfibrils (hairs or fuzz) protruding from the surface of yarn as they are most susceptible to enzymatic attack. This weakens the microfibrils, which tend to break off from the main body of the fibre and leave a smoother yarn surface. A ball of fuzz is called a 'pill' in the textile trade. These pills can present a serious quality problem since they result in an unattractive, knotty fabric appearance.

• After Bio-Polishing, the fabric shows a much lower pilling tendency. Other benefits of removing fuzz are a softer, smoother feel and superior colour brightness. Similarly enzymes can also be used for wool(protease) & silk (Sebrite) fabric