Desizing Process: The yarns, particularly the warps running lengthways throughout the fabric, are subjected to a high degree of abrasion during weaving. To prevent breakage or damage of warp yarns due to abrasion, size is imparted to the warp yarn. The presence of size on the fabric makes it stiff and renders its treatment difficult with different liquors used in dyeing and finishing. Therefore, one of the initial steps in wet processing is the elimination of size and water-soluble admixtures, the operation being called desizing. Desizing process is very essential to make the fabric suitable for the next process.
Fig: Cotton fabric desizing
Sizing components such as starch, thin-boiling starch, carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA), vegetable oils, mutton tallow, etc., applied to cotton warp yarn to strengthen it before weaving, make the fabric unsuitable for dyeing as the fabric becomes hydrophobic. Hence this unwanted size has to be removed for successful wet processing by making the fabric hydrophilic in nature.
The object of desizing is to remove size that is applied to the grey fabric as a part of the weaving process so that the fabric becomes free of the added matter and can be effectively scoured and bleached. Hydrolysis of starch:
(C6H12O5)n→ (C6H12O5)x →C12H22O11→ C6H12O16
Starch → Dextrin→ Maltose → Glucose
(high molecular weight) → (low molecular weight)
Purpose of Desizing:
The size applied during sizing is required only while weaving, and has no function once the fabric is woven. The size, if remains in the fabric, does not allow the dyes and chemicals to penetrate inside the fibers. Hence the purpose of desizing is to remove the size applied to the warp yarn and make the fabric suitable for further wet processing such as scouring, bleaching, dyeing, printing and/or finishing.
Methods of desizing:
Different methods of desizing are given in below figure.
The most commonly used methods for cotton are enzymatic desizing and oxidative desizing. Acid steeping is a risky process and may result in the degradation of cotton cellulose while rot steeping, hot caustic soda treatment and hot washing with detergents are less efficient for the removal the starch sizes.
Enzymatic Desizing:
In enzymatic desizing, enzymes liquefy starch in the fabric. An enzyme is essentially a protein produced by living cells. It acts as a catalyst in chemical reactions. In almost all instances, enzymes are very specific in action, i.e., each enzyme catalyses’ only one reaction or type of reaction.
Classification of desizing enzymes:
Enzyme
Concentration
Temperature
pH
Malt extracts
3-20
50-60
6-7.5
Pancreatic
1-3
50-60
6.5-7.5
Bacterial
0.5-1
60-70
5.5-7.5
Enzymes suffer from the disadvantage that if the conditions of temperature and pH are not maintained, little or no desizing takes place.
Oxidative Desizing:
An alternative desizing procedure is oxidative desizing with peroxides. The oxidizing agent is hydrogen peroxide or in some cases a combination of hydrogen peroxide and potassium persulphate, and the process is carried under highly alkaline condition.
Starch size can be removed by oxidative method using persulphates, which becomes handy in some process routes. Oxidative desizing is comparatively unimportant as the oxidative agent used can attack the cotton and damage them as the chemical structure of cellulose and starch are similar.
Process Flowchart of Cotton Fabric Desizing:
Batch
↓
Entry J-scary
↓
Desizing chamber
↓
Steamer (95°C – 100°C)
↓
Metallic guider
↓
Hot wash (95°C)
↓
Hot wash (95°C)
↓
Hot wash (95°C)
↓
Cold wash or oxalic acid wash
↓
Exit J-scray
↓
Batching
Important Information about Cotton Desizing:
Machine used: Menzeldesizing machine
Speed of machine: 70m/min – 100m/min
Length of machine: 220m
Capacity of desizing chamber: 1000L
Temperature maintained while desizing: 65°C – 70°C
Cotton fabric is taken from the batch into the entry J-scray and through the guiders to the desizing chamber.
The fabric is arranged inside the desizing chamber in S-threading. Temperature is maintained at 65°C-70°C. There are 7 rollers above and 8 rollers below the chamber. The length of fabric inside the desizing chamber is 24m, which have a capacity of holding 1000L of liquor.
The fabric is then passed into the steamer using a squeezer and bow roll/expander to remove crease. The capacity of steamer is 350 kg. The threading length is 84m arranged in a double S-threading.
The dwell time varies from 8min for yarn dyed fabric to 12min for bleached fabric. The temperature is maintained at 95°C-100°C. The temperature at lower compartment of steamer is slightly less to avoid formation of cloud.
The fabric is then passed through a metallic guider to washing chamber having water holding capacity of 1000L.
After three consecutive hot wash at 95°C, the fabric is cold washed or washed with oxalic acid at 55°C (if online route is taken) and passed to the exit J-scray.
From where it is taken through the guiders and guide rollers to the winding roller and finally batching is done.
References:
Textile and Clothing Design Technology Edited by Tom Cassidy and Parikshit Goswami
Pretreatment of Textile Substrates by Mathews Kolanjikombil
Handbook of Value Addition Processes for Fabrics By B. Purushothama
Chemical Technology in the Pre-Treatment Processes of Textiles by S. R. Karmakar
A Novel Green Treatment for Textiles: Plasma Treatment as a Sustainable Technology By Chi-wai Kan
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
