Introduction – Sizing & Desizing

Size is the name given to the substance or mixture of substances that is applied to the warp thread before weaving. The size forms a coating around the surface of the thread before weaving. This coating provides the lubrication & prevents the breakage of warp thread during the weaving operation.
Desizing process removes the size after weaving. The method of desizing depends on the composition of the size. This article mainly deals with enzymatic desizing, which means the use of µ - amylase enzymes to remove starch size.

Size Composition

The choice of size components not only depends on the type of fibre, but also on factors such as economics, weaving technique and ease of size removal. It is important that the size should give a strong and even film and should adhere to the property of fibre. As the surface property of different fibres is different, it is understandable that the composition of size will not be the same for all fibre types. For e.g., a good size for cotton will not be a good size for polyester and vice versa.

Some common chemicals used to prepare sizes are Polyacrylic Acid (PA), Polyvinyl Alcohol (PVA), Starch, Modified Starch, Carboxymethyl cellulose (CMC) & Paraffin Waxes. As a general rule, cellulosic fibres including cotton, rayon and blend of these with synthetic fibres such as polyester, is sized with starch based sizes. Starch has good sizing properties on cellulosics and is economical. Thus, in the following discussion of Enzymatic Desizing, the main fibre type involved is cellulosic.

Desizing – Part of the finishing operations

Cloth as it comes from the loom is not a finished product. It has to be treated in a series of processes before the final article is obtained. These processes are collectively known as finishing operations. There are basically two types of processes involved :-

1.Removal of impurities ( this includes Desizing )

2.Application of various chemicals e. g. dyes, softeners, etc.

In order to choose good methods for removing the impurities, it is very important to have an idea of what the impurities are. We must consider both the impurities that are present in the fibre itself and the impurities that have been deliberately applied during the processing, i.e., the size.

Removal of Starch Sizes

Starch based size can not be removed simply by washing the cloth with hot water. This is because starch is not soluble in water. Starch can be made soluble by a hydrolysis reaction in which the starch polymer is broken down into small soluble fragments. Complete hydrolysis of starch leads mainly to glucose since starch is a polymer made up of glucose. However, complete hydrolysis is not needed: partial hydrolysis gives sufficient solubility.

Here it is important to remember that cellulose is also a polymer of glucose. The only chemical difference between starch and cellulose is the way in which the glucose units are joined together. It is important to find a way of hydrolysing the starch with out hydrolysing the cellulose and this can be a little tricky as both are chemically so similar. For e.g., use of hot acid as an efficient way of hydrolysing the starch would not be a good idea because there would also be a lot of breakdown of cellulose.

This problem can be solved using enzymes. Enzymes are selective catalyst which only act on specific chemical compounds. The µ - amylases are enzymes that catalyse the hydrolysis of starch. These µ - amylase enzymes have no effect on cellulose.

Amylase enzymes have been used for many years in textile industry for removal of starch sizes. Originally enzymes from plant or animal sources were used but these have long since been completely replaced by enzymes of bacterial origin obtained by fermentation processes. The first bacterial enzymes made available for this purpose were the bacillus subtilis type of a - amylase. This product has already been used for more than 30 years. During the last decade, the superior a - amylase produced by bacillus licheniformis is becoming increasingly popular with the textile industry.

Enzyme properties

To ensure good conditions for desizing, it is necessary to understand some of the properties of enzymes being used. In particular, the influence of temperature, pH and stabilisers on enzyme is of great importance as these factors will determine how much enzyme is needed and how long the desizing reaction will take.

1. Enzymes are proteins - An enzyme is a protein having catalytic properties. These catalytic properties are related to the amino acid sequence of protein chain and to the way the chain is folded up i.e., to the three dimensional structure of the protein. If the three-dimensional structure of an enzyme is changed, it will lose its catalytic activity.

2. Heat Inactivation - At high temperature, this three-dimensional structure of the enzyme will be changed. This is irreversible and leads to loss of enzyme activity. It is called heat inactivation or denaturation.By applying high enough temperature, all enzymes can be inactivated. However, there is big difference between various enzymes as to the temperature they can withstand before this heat inactivation takes place.

3. Stabilizers - Certain metal ions can act as stabilizers for enzymes by holding the protein in the active shape. In the case of the µ - amylase enzymes, calcium is the most effective stabilizing metal ion. Starch also acts as a stabilizer for µ - amylase. Thus the µ - amylase can withstand higher temperatures without heat inactivation when calcium and starch are present than when these stabilizers are not present.

4. pH - The catalytic activity and the stability of an enzyme are related to the pH value of the solution. In order to get the most efficient enzyme performance, it is important to control pH to suit the enzyme. With the a amylase, the best pH is close to neutral.

5. Temperature - Provided the temperature is low that causes heat inactivation, the activity of the enzyme will increasing temperature. From this, it is obvious that there will be an "Optimum temperature", at which enzyme efficiency is at maximum. This optimum will be just below the temperature that causes rapid heat inactivation.

An understanding of the chemistry involved in the sizing/desizing process will enable a processor to ensure a streak-free garment.

Team S&A