Desizing and type of desizing

DESIZING 

Grey cotton fabrics contain natural impurities as well as those added to the fabric such as size to facilitate weaving. Although the sizing of warp is necessary operation to the weaver, it is this size in the grey fabrics which is a source of trouble to the processing of fabric processor.The very first operation of wet processing to fabric therefore, is generally to remove the added ,size.This operation of removal of size is known as desizing.The purpose of desizing is to remove the starches used in sizing by converting them to water-soluble degradation products such as dextrins and glucose without degrading the cellulose.Since the size mixture contains other ingredients for which the starch acts as a binder, most of them are_readily released.and, removed when the starch is hydrolyses and.washed, off.For.any efficient desizing operation , 85 to 90 % of the added size should be

removed during desizing. Desizing should be done uniformly Non- uniform desizing will give uneven scouring and bleaching and hence irregular dyeing and finishing.The reduced size on the fabric after desizing should be brought down to 0.2 % 2 Grey fabrics are almost water-repellant and will not easily take up the desizing solution.Moreover, starch which is the main ingredient in desizing, is not easily accessible to desizing chemicals or enzymes.The fabric, is therefor rug through a bath of boiling water before desizing in order to swell the starch and also reduce antiseptic agents used in sizing.The steeped fabric after a short time is then taken to the desizing bath 6 Two B The methods of desizing can be classified into

(I)  hydrolysis desizing viz.rot steeping, acid steeping and enzyme steeping and

(ii)  oxidation desizing viz. treatments with chlorine, chloride and bromine.

HYDROLYSIS  DESIZING METHODS

Rot Steeping 

This is the oldest and cheapest method because no chemical is used in this treatment.The cloth is soaked in warm water at 35-40° C and passed through a padding mangle and squcezed. It is allowed to stand for 24 hours or overnight.

The padding mangle used for impregnating the material consists of a trough in which hot water is continuously feed.The rollers are of rubber and ebonit. A three bowl padding mangle can be used with two dips and two nips, with the rubberized roller placed between two ebonite rollers.

 The microorganisms present in water multiply and secrete .starch liquefying enzymes which solubilise the starch present in the size.Thus the prolonged soaking helps to remove the starch and some water soluble matter such as pectin. The cloth is finally  washed when most of the starch is removed.

In rot steeping the fermentation taking place is not controlled and if it is too active it may attack cellulose. Mildew may also develop during storage of the cloth.The main disadvantage of this method is that it is a slow process and requires enormous floor

space for storing the water soaked material.In fabrics of heavy construction like drill or sateen, there is no proper penetration which hinders the process.

Acid Steeping

In this method, dilute hydroelectric acid or sulfuric acid or a mixture of both is used to hydrolyses the starch from the sized fabrics .The fabric is steeped in 0.25 % ( w / w ) solution ( or 10 gs per liter) of the acid, passed through a 2-bowl or 3-bowl mangle and kept for 4 to 6 hours, the temperature rises to 50 C as the reaction proceeds and also the rate of hydrolysis increases. In order to avoid evaporation of water during storage (which may raise the concentration of acid) moistened gunny cloth is placed on the fabric during storage. Sulphuric acid decomposes oils an waxes and these products adhere to the fabric tenaciously affecting subsequent processes

 The starch present in the cloth is liquefied and the solubilised starch is removed by washing.Since acid can attack cellulose, care has to be taken to avoid tendering of cotton.It is essential to neutralise the fabric to avoid hydrolytic damage.

The fabric should be washed with hot water after desizing since cold water reduces the solubility of degraded material