Desizing lecture 3

desizing process of cotton fabric

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DESIZING PROCESS OF COTTON FABRIC
LECTURE -3
By Robel L.
Faculty of Textile and Fashion Technology
Aksum University



2020
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2. DESIZING
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3. INTRODUCTION
 In the production of fabrics by weaving process the warp yarns are
coated with size material mainly to avoid yarn breakages in the
making up process.

 SIZING MATERIALS: (Starch, Modified Starch or Synthetic Polymers) + Additives

 The conventional sizing process for cotton warp yarns utilizes
starch with additives mainly humectants and lubricants.




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 The sizing material on woven fabric forms a strong film which
acts as a barrier for penetration of chemical solutions in further
processing.
 For smooth further chemical processing the wettability has to be
increased by removing the size material applied in the weaving
process.
The process of size removal is known as DESIZING.
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5. STARCHES
Starches are carbohydrates composed of repeating anhydroglucose
units linked together by glycosidic linkage.
Starch is a mixture of amylose and amylopectin [70 – 80%].
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 To remove the starch material from the fabric.
To increase the absorbency power of the fabric.
To increase the affinity of the fabric to the chemicals.
To make the fabric suitable for the next process.
To increase the luster of the fabric by increasing
dyeing and printing efficiency.
Objectives of desizing
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7. MECHANISM OF DESIZING
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Desizing involves treatment of the substrate with a solution of the desizing
agent, allowing the desizing agent to Degrade or Solubilize size material, and
finally to wash out accompanying degradation products.
HYDROLYSIS & OXIDATION
Polymeric starches are converted into low molecular weight compounds [such
as glucose, maltose or oligosaccharides] to ease their removal. Then washing is
carried out to remove the soluble components from the fabric.
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8. Viscosity of size in solution
Ease of dissolution of size film
Amount of size applied
Nature and amount of plasticizers
Fabric construction
Washing off conditions



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FACTORS AFFECTING SIZE REMOVAL
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METHODS OF DESIZING
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10. ACID DESIZING

COTTON FABRIC IS TREATED WITH DILUTE MINERAL ACIDS
[HCL OR H
2SO
4]
 Dilute acid attacks the polymer chain of starch
and short water soluble chain segments are formed
 Reduction in molecular weight of starch




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 Good demineralization of cotton
 Cellulose may damaged by acid
Important parameters in acid desizing:-

Optimum concentration, pH and temperature/time

Example: HCl (0.5-1%) at 30-60
0
C for 2-8 h
H
2SO
4 (0.5-1%) at 40
0
C for 3-4h
Cont.…
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Most widely practiced method of desizing
Use of enzymes for starch removal
Enzymes are biocatalysts with protein structure
Enzymes are specific :
Reaction and substrate specific

In desizing enzymes are used to catalyze hydrolysis of
starch into simple sugars.
ENZYMATIC DESIZING
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Amylase Enzyme
Degradation of amylose & amylopectin in starch
The effect is breaking of the polymeric chains and converting
into low molecular weight products which are easily removed
from the fabric by normal hot washing.
The most commonly used enzyme is


Enzymatic desizing is the most effective desizing method mainly
due to specific action on starch
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Cont.…

14. Starch hydrolysis takes place in the active site of
amylase by the action of amino acid residues.
ACTION OF ENZYME
Conditions that affect the protein
structure affect the enzyme activity 3/5/2020
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16. MAJOR AMYLASE ENZYMES
Malt Enzymes extracted from plants
Pancreatic Enzymes are extracted from animals
Bacterial Enzymes are prepared from bacteria
Effective enzymatic desizing requires a strict control: -
 BATH pH
 TEMPERATURE
 WATER HARDNESS
CHOICE OF SURFACTANT
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Time for starch digestion depends on:
 Concentration of enzyme
 Temperature of desizing bath
 Type of goods being desized
 Methods of application

Example: Bacterial amylase at a concentration of 5gpl [pH = 5-7,
Temp.= 60 -70
0
C] in the presence of common salt and non-ionic
wetting agent is used in conventional cotton desizing.
NO DAMAGE TO
COTTON
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ROT STEEPING
Traditional desizing
Stepping in water at room temperature
Micro-organisms develop excreting enzymes which attack the
starch.
The starch swells and hydrolysis takes place.
Soluble compounds which are then removed from the fabric by
normal washing with water.
Low efficiency (longer treatment time) and cellulose degradation
due to cross-infections of mildew.
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21. OXIDATIVE DESIZING
This method is based on oxidation of starch using
oxidizing agents: Sodium hypochlorite, hydrogen
peroxide and persulphates.


Principle of desizing with oxidizing agents:

 Opening anhydroglucose rings

 Conversion into carboxylic acid groups

 Intensive alkaline after-treatment
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23. Optimum oxidative desizing without fiber damage:
 Control of pH, temperature & time of treatment
 Control of auxiliary additions
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Fiber damage and pollution
Best suitable for mixed & unknown sizes
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Other Size Materials and Desizing

For the removal of water-soluble sizes such
as PVA, theoretically, only washing with hot
water is needed for desizing.

However, the washing efficiency can be increased
by adding wetting agents, allowing adequate
immersion time in desizing liquor and by thorough
washing to remove solubilized size.
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27. TEST FOR DESIZING EFFICIENCY
A drop of iodine solution placed on a test specimen resulting in a
characteristic blue color is the universal test for identifying starch.
It can be used as a qualitative test to show whether all the starch
was removed.
Absence of the blue color signifies that all the starch has been
removed.

Other tests include calculation of the weight loss, capillary
measurement and drop test.
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Tegewa scale

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A drop of water is placed on fabric and the time it takes for the drop
to penetrate fabric is recorded. The wettability is determined by
counting the elapsed seconds between the contact of the water drop
with the fabric and disappearance of the drop into the fabric. The
faster the wetting time, the more wettable is the fabric.
Each specimen is cut vertically into a 20 x 2cm strip, hung longways
with the bottom end dipped in water in order to measure the length
of fabric by which water is absorbed in 5 minutes.
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END
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