Sunday, June 28, 2009

Pre-Treatment of Cellulosic Textiles

Textile fabrics, either in yarn form or fabric form or hank form, are subjected to different processing procedures depending on its textile end use. The grey cloth is chemically and mechanically processed to give a marketable finish. The sequence of the preparatory process depends on the machinery available, the ease of the availability of water, the type of fabric and the composition of the blend.

A good preparatory process has several objectives, which are given as follows:
a) Removal of loose hairy protruding fibres from the surface of the fabric to give a smooth, even and clean looking face.
b) Removal of natural impurities like oils, fats, waxes, greases, natural matter, lignin and sizing material like starches.
c) To obtain an absorbent fabric, which is ready for dyeing or printing process.
d) To obtain softer and proper white fabric, depending on its application.5

A sized warp is a weaver’s necessity, but a great drawback for the processing department. Warp sized removability depends on the film forming size. Size mainly consists of starch, wax and tallow. All these remain on the warp yarns after weaving the cloth. The process of removal of starch sized from the cloth is known as desizing.

Factors affecting desizing:

1. Concentration of desizing agent.
2. Resolution of size.
3. Alkaline and acidic condition
4. Mechanical properties

Mechanism of desizing:
Chemically, starch is a poly-ά- glucopyranose containing straight chain (amylase) and branch chain (amylopectin) water insoluble polymers. The breaking of these long chain compounds to a shorter one is due to hydrolysis or due to oxidative degradation.

Desizing methods:
1. Hydrolytic method of desizing
2. Oxidative method of desizing

Hydrolytic desizing, which is most prefered over the Oxidative desizing, again classified into three methods. These are
1. Rot steep
2. Acid steep
3. Enzymatic desizing

Rot steep: This is the least expensive and oldest method of desizing in which no special chemicals are added. Here starch-sized fabric is passed through a padding mangle and saturated with water at 400 C to give 100% pick up. Then the fabric is allowed to remain for 24 hours, during which starch becomes solubilised in water because of fermentation.

1. This method does not yield uniform results.
2. It requires large floor space area and longer duration of time.

Acid steep: Dilute sulphuric or hydrochloric acid may also be used to hydrolyse the starch from the sized fabric. A 0.25 % (w/v) solution of the acid at room temperature (300 C) is sufficient for the purpose. By using acid solution, the duration of the desizing process may be reduced to 8-12 hours.

1. Local evaporation of water during the storage period may cause hydrolysis of cellulose itself leading to weakening of cotton at the places where evaporation has taken place.
2. This method does not yield uniform results.

Enzymatic Desizing
Enzymatic desizing is a more advanced method for removing the starch. Enzymes are complex, protienaceous substances, which are secreted by the cells of the living organisms and they have a very good water solubility. Three principal types of starch splitting enzymes used for desizing are ά-amylase,ß-amylases and amido glucosidase The origin of these enzymes also plays an important role in the activity of enzymes. These enzymes are highly specific and work at different specific temperature and pH.

1 comment:

Md. Nasir Uddin Shamim said...

Hey Nice blogging. I have also a textileblog.
Nice to see u here.