Textile Desizing Slide.pptx

Desizing in Textile Industry

1. D e s i z i n g Desizing is done in order to removes the size from warp yarn of the woven fabric. Warp yarn are coated with sizing agents period to weaving in order to reduce their fractional properties, decrease yarn breakage in loom and improve weaving productivity by increasing weft insertion speed. The sizing materials present on warp yarn acts as a resist toward dye and chemicals in textile wet processing . it must therefore be removed before any subsequent wet processing of the fabric.

2. Objectives of desizing The object is to remove from the grey fabric the size that has been applied during weaving and thus to make the fabric ready for further processes. The main ingredient in size that is not water-soluble is usually starch. Chemically starch is poly-glucopyranose in which straight chain and branched chain polymers are present. .

3. Chemistry of desizing The main ingredient in size that is not water-soluble is usually starch. Chemically starch is poly-glucopyranose in which straight chain and branched chain polymers are present. Both the constituents of starch are insoluble in water but they can be made soluble by hydrolysis of these long chain compounds to shorter ones.

4. Mechanism The main ingredient in size that is not water-soluble is usually starch. Chemically starch is poly-glucopyranose in which straight chain and branched chain polymers are present. Both the constituents of starch are insoluble in water but they can be made soluble by hydrolysis of these long chain compounds to shorter ones. Thus, under suitable conditions, the following steps show the progressive hydrolysis of starch. However, in desizing, the hydrolysis of starch is carried out only up to the soluble dextrin stage, as this can be removed off the desized fabric by means of an aqueous wash.

5. Mechanism contd.

6. Methods of desizing Rot steeping Enzymatic desizing Acidic desizing Oxidative desizing

7. Rot Steep This is the oldest and cheapest method of desizing. Here no special chemical is used. The cloth is first passed through warm water at 40C in a padding mangle where the cloth is squeezed to about 100% expression. The cloth is then allowed to stand for 24 hours. The microorganisms, naturally present in water, multiply and secrete starch-liquefying (hydrolysing) enzymes, which break down the starch present in the size to w ater-soluble products The cloth is then washed to remove these products.

8. Rot Steeping Wetting & s q uee z i n g S t ee p i n g W a s h i n g

9. Rot steep Advantages Rot steeping is the cheapest of all the desizing methods. No chemicals are required. Disadvantages A large floor space is required for this process. The process is slow, so desizing time is long. Mildew may attack the cloth during steeping and cause stains on the fabric.

10. Acid Desizing Dilute sulphuric acid or hydrochloric acid may be used to hydrolyse the starch from the sized fabric. A 0.25% - 0.5 % solution of the acid at room temperature (30o C) is suitable for this process. The cloth is impregnated with the dilute acid solution in a two-bowl or three- bowl padding mangle and then stored for 8-12 hours in a closed concrete pit

11. Acid Desizing Advantages Acid desizing is an economical process. The process is effective and gives fairly uniform desizing, as it is a chemical- based process. It does not require specific conditions of pH and can be done at room temperature. It is a much quicker process than rot steep desizing.

12. Acid desizing Disadvantage The main disadvantage of the process is that mineral acid is harmful to cellulose fibres if proper care is not taken. Especially during the storage stage, the acid-wet fabric must not be allowed to dry. This would cause the formation of hydrocellulose, which will weaken the fibre.

13. E n z y m a t i c D e s i z i n g The hydrolysis of starch using enzymes under particular concentration, temperature and duration is called enzymatic desizing. Enzymatic desizing is the most widely experienced method to desize the starch.

14. Although traditionally enzymes are used under mild process conditions, genetic engineering has allowed development of robust enzymes which can be used at higher temperatures to increase the rate of desizing. High temperature enzymatic desizing Novozymes, an enzyme producing major, introduced an alkaline amylase with a broad activity spectrum, capable of application over the pH range 5-10 and from 20-85 ° C. This has enabled combined desizing and bioscouring to achieve commercial acceptance in a number of textile mills. They have also developed enzymes for use in high temperature. The details are given below: Aquazym® PS ( Novozymes) A heat-stable alpha-amylase used in high-temperature continuous desizing processes (Pad Steam).

15. Enzymatic desizing Advantages Time required for the desizing process is less. It is continuous process, so greater production can be achieved. Closely constructed fabric can be easily desized, due to the effective enzyme action. T h e r e is n o c h a n c e f or t h e ce l l u l o s e to g e t hy d r ol y s e d, as in acid desizing.

16. Enzymatic desizing Disadvantages Lower additional cleaning effect towards otherimpurities. No effect on certain starches (e.g. tapioca starch)

17. Oxidative desizing Desizing with Oxidizing agents Though the use of oxidants for desizing of cotton fabric is widely accepted but their large scale industrial application is yet to be exploited. The most important aspects of oxidizing agents are that they can be applicable to wide range of fabrics, the size content of which is often not known.

18. Oxidative desizing Advantages  The advantages of oxidative desizing are supplementary cleaning effect, effectiveness for tapioca starches but oxidizing agents may damage to fibres.

19. Assessment of desizing efficiency TEGEWA violet scale Reagent preparation Put potassium iodide [10g of KI (100%)] in 100 ml of water; then add 0.6358 g of iodine (100%); st ir well until iodine is completely dissolved in the KI solution. After this, add 800 ml of ethanol. Then by adding water the volume should be raised to 1000 ml. Method of testing Put one or two drops of the above solution on a fabric. Rub it gently and then assess the colour change as per the Tegewa scale. Before testing, the fabric should be cold and there should not be any residual alkalinity in it . Assessment No Colour Change = No starch is present Pale blue to bluish violet = Presence of starch size or a blend of starch+ synthetic size Brown = Presence of modified starch or a blend of starch/PVA size