Scouring Process for Cotton:

It is a process to remove all undesirable impurities especially natural origin like fats, wax, vegetable matter such as seed coat fragments, pectins, protein etc. Scouring process also removes some added impurities (which are not naturally present) such as oil stains, residual sizing material. Scouring process improves absorbency of cotton material. With the improved absorbency, water can transport dyes and chemical within the cotton fibres. Proper scouring is must for uniform dyeing and printing. The main objective of scouring is to  make textile material highly hydrophilic or improve water absorbency.

Mechanism of scouring: Following are main points related to mechanism of scouring:

i) Saponification: The vegetable oil, which is immiscible with water, is glyceride of fatty acids. When such oil are heated with a solution of sodium hydroxide (Caustic soda) in water, the oil splits up into its constituents fatty acid and glycerine. Glycerine is miscible with water easily and the fatty acids reacts with sodium hydroxide present in the solution forming its sodium salt i.e soap which is also soluble in water. Thus oil is removed.

Saponification process

ii) Emulsification: Wax and non saponifiable oils (Non polar) are removed by emulsification as they are immiscible in water (Polar). Normal washing soap  is used as a emulsifying agent which make emulsion of them. Soap generated from the saponification process also act as a emulsifying agent.

The scouring process brings following changes in cotton fibres:

i) Oils and fatty acids are converted into soaps.
ii) Pectins and pectose are converted into soluble salts of pectic acid.
iii) Mineral matters are mostly dissolved or settle down.
iv) Non saponifiable oils are emulsified by the soluble soaps generated from the saponificable oils.
v) Additive dirt's are removed.
vi) Residual sizing materials are broken down into soluble products.

Natural Impurities:

The cotton fibres are having various types of impurities. Some of them are given below:

• Pectins
• Proteins
• Colouring matters
• Mineral matters

Let us discuss above impurities one by one:

1. Pectins: It is a structural heteropolysaccharide, containing in the primary walls of terrestrial plants.

Here you may corelate it to cotton. Pectins also known as pectic polysaccharides. It is derived from pectic acid. It is high molecular weight polymer and composed of Galactouronic acid. D-Galacturonic acid is sugar acid, an oxidized from D-galactose. It is the main component of pectin, in which it exists as the polymer polygalacturonic acid.

2. Proteins: It is nitrogenous organic compounds and composed of one or more long chains of amino acids. It is present in the primary wall and Lumen. Some of the amino acid used are

• Leucine
• Leucin
• Valine
• Proline
• Alanine etc

  Some time the yellowish colour of cotton are related to Proteins and colouring matters.

3. Colouring matter: The main reason of colour in cotton is due to colouring matter. These                 colouring matter are due to presence of colour pigments.
Examples are:
3,5,7,2',4' Penta hydroxy flavone and 3,5,7,8,3',4' Hexa hydroxy flavones (Gossypetine)                      

(flavus = yellow)

3,5,7,2',4' Penta hydroxy flavone :

Molecular structure of flavone

3,5,7,8,3',4' Hexa hydroxy flavones (Gossypetine):

4. Mineral Matter: The presence of mineral matter in the cotton depends upon the soil in which it is cultivated. The quantity of  mineral matter can be determined by converting cotton  fibres in to ash and then by any suitable technique like Atomic Absorption Spectrophotometeric can be used to quantify the matter.

Q. What is preparation?
Ans: In simple way one can say that the process(s) that can be used to remove impurities from the textile materials (fibre/yarn/fabric/garments) to make them dyeable or printable or any other further use, is called preparation in the chemical processing.

Q. What types of impurities are present in the textile material?
Ans: There are mainly two types of impurities:

• Natural impurities
• Added impurites

Natural impurities: As its heading suggest, the impurities that are present naturally are come under this category. These impurities are present in the natural fibres (Vegetable and animal). Let us take example of Cotton fibre. Following figure indicates the composition of cotton:

From the above figure except cellulose all other constituents are considered as impurities.

To further understand each types of above impurities, let us discuss structure of cotton fibre. Following figure can explain cotton fibre structure:

Q. What is cellulose?

       Ans: It is an insoluble substance which is the main constituent of plant cell walls of vegetable fibres such as cotton. It is polysaccharide consisting of chains of glucose monomers. It is an organic compound with the formula (C6H10O5)n, a polysaccharide consisting of a linear chain of several hundred to many thousands f β(1→4) linked D-glucose unit              

 

          D- Glucose                            β(1→4) linked D-glucose unit

                                                 

Major Desizing Processes: The main desizing processes are given below:

•Rot steeping

•Acid steeping

•Oxidative desizing

•Enzymatic desizing

Rot steeping:   

                         In this process fabric is stored at 40-60 degree Celsius for overnight in water. It                         removes starch and water soluble impurities with natural reaction.

 Following are main drabacks associated with this  process:

                      

                           - It is time consuming process
                           - It is not certain that in the given duration, sizing material will remove or not.

Acid Steeping:

                           Here acid behave as hydrolysing agent. It reacts with bigger molecule and hydrolyze                  it to smaller molecules. For example starch, which is important ingredient of sizing                      recipe react with acid and  is hydrolyzed to glucose. In acid steeping, fabric is treated                  with sulphuric acid (0.5% to 1.0%) or Hydrochloric acid. The main drawback                              associated with this process is that the fabric get tendered or damaged if acid is                            remained in the treated fabric.

Oxidative desizing: 
                
                          Starch and other added impurties are hydrolysed through oxidation process.                                  Following oxidizing agesnts may be used for this purpose:

                           -Sodium hypochlorite: 2.0-5.0 g/l at room temperature at around pH 7.0
                           -Hydrogen peroxide: 3.0- 6.0 g/L  and 7-15 g/l Sodium hydroxide at 40 degree                             Celsius for 12-16 hours. 

Following are the drawbacks involved with this process:

                        
                            -It is a time consuming process
                            -In this process, bleaching effect is also produced which some time not                                          required.

Enzymatic desizing: 

                            Enzymatic desizing is carried out using amylase enzyme as the main                                            constituent of starch is amylose. Following recipe may be used for desizing

                            

                                       -0.5% to 2.0% Enzyme  on weight of fabric (OWF) in water                                                          -Require quantity of Common salt
                               - pH should be neutral and Fabric should run continuously in the machine                                          loaded with enzyme solution

    

Preparation of textiles for wet processing i.e Bleaching or Dyeing or Printing:

The preparation of textiles for bleaching/dyeing/printing is just like same as we do preparation for examination. Why we do preparation for examination? The answer may be  to pass the exam. Therefore when we have certain goal in mind, we do prepare our self to achieve the same. In similar way, in the textile, our goal may be to get bleached or dyed or printed fabric. To achive this goal we ahve to prepare textile material to make it ready to bleach/dye/print.

Following are the processes for preparation of textile material for bleaching/dyeing/printing:

Desizing: Removal of sizing chemicals such as starch/poly vinyl alcohol (PVA) etc from the fabric . To understand desizing we must know sizing process.

           Sizing: Sizing is a process in which we apply starch or PVA or other ingredient on the warp                 yarn (Just like a coating) to improve warp yarn strength without affecting its elongation                       properties much so it can stand stress and strain during weaving process. Please note that                  sizing is carried out only in weaving not in Knitting. 

Desizing process: There are four methods for desizing. These are:

1. Rot steeping
2. Acid steeping
3. Oxidative desizing
4. Enzymatic desizing