Definition of Desizing:
Desizing is a process of removing sizing agents from the fabrics or garments, which are usually applied on the warp yarns before weaving. Desizing must be done before other wet processes of bleaching, mercerizing, dyeing, printing or finishing are carried out. Sizing agents mostly comprise macromolecular film-forming and fiber-bonding substances such as starch, PVA and polyacrylates, which are applied on warp yarns to increase their strength and reduce yarn breakages during weaving. Some auxiliaries such as wetting agents, softening agents, lubricating agents or hygroscopic agents may also be included in the sizing recipe. Removal of sizing agents after weaving is necessary in order to make the fabric more absorbent for dyes and other chemical processing agents. The choice of method for desizing depends on the type of sizing agents used during sizing. Different desizing methods include enzymatic desizing, oxidative desizing, acid desizing, desizing with hot alkali treatment and desizing with hot detergent solutions. Starch-based sizing materials are usually not water-soluble and require use of amylases enzymes for their removal. Enzymatic method is most commonly used for desizing fabrics which contains starch-based sizing materials. Advantages of enzymatic desizing in comparison to oxidative and acid desizing include no damage to the fiber, no use of aggressive chemicals, high biodegradability and wide variety of application methods.
Water-soluble sizing materials such as polyvinyl alcohol (PVA) or carboxymethyl cellulose (CMC) may be removed by simple washing with hot water, without the use of any enzymes. Important parameters for enzymatic desizing method include liquor pH, temperature, time, type and amount of any wetting agent or detergent. The effectiveness of desizing process can be checked by putting a drop of iodine solution on the desized fabric, which turns into blue colour if there is still some unremoved starch present in the fabric.
Stages of Garment Desizing:
Efficient desizing of garment (i.e., of Denim Jeans, Denim jackets and other cellulosic garments) gives better washing and preshrinking effects.
Process stage 1:
Wet out garments with 0.5–1 g/l wetting agent in drum washing machines or pits.
Process stage 2:
Desize with 1–5 g/l enzyme (e.g., Bactosol AMI liq-Clariant)
2–4 g/l Common salt At 60–90°C, pH 6–6.5 for 60–75 min on drum washing machines or soaked in pits for overnight.
Process stage 3:
Thorough washing, preferably hot wash.
Garment Desizing Process:
Garment desizing is done in below methods.
A. Rot steeping:
In this method, grey cotton fabric is steeped in water in suitable box at a temperature of about 30–40°C or pad the material with warm water and squeezed to 100% expression and left it in open space or box for 24 h at 35–40°C or overnight at 60°C. During the storage micro-organisms naturally present in the water develop excreting enzymes which attack the starch. The swollen and hydrolysed starch is thus partially converted into soluble state which is then removed from the fabric by normal washing with water. The main problems in this method are low-efficiency due to longer treatment time and degradation of cellulose due to cross-infections of mildew if the fermentation process is not properly controlled, requirement of large floor space for storing the goods, etc.
Advantages:
- Rot steeping is the cheapest method.
- No Chemicals are required.
- No special equipment is required.
Disadvantages:
- Large floor space is required.
- The process is slow.
- No control over the process.
- Mildew may attack the cloth during the desizing and cause stains on the fabric.
B. Acid steeping:
Not very well-practiced now, in this method the material is impregnated with 2.5 g/l sulphuric or hydrochloric acid solution at room temperature (25–30°C) followed by storage for about 6–8 h. Faster desizing (within 1–2 h) is possible using stronger solution of acid (e.g., 10 g/l). The process called “souring” is used for low-quality cotton fabric. Hydrolysis of starch by mineral acid is an exothermic process and the temperature may even rise up to 50°C. During storage the fabric should not be allowed to dry, otherwise the concentration of the acid in the fabric will increase with subsequent damage of the fabric. By the acid steeping method, almost all the starch present in the fabric may be converted to soluble form and can be washed-off.
Even though the desizing efficiency is much better than the rot steeping there are many disadvantages for this method. The main disadvantage is the damage of the fabric due to any localised drying during storage which can appear as patches in dyeing or subsequent processes. The main disadvantage of acid steeping is that there is always a chance of tendering of the fabric by acid hydrolysis unless proper care is taken. The loss in weight after acid desizing is always more than that after rot steeping, because the former process dissolves nearly all the mineral impurities.
Advantages:
- Acid desizing is an economical process
- The process is effective and gives fairly uniform desizing as it is a chemical-based process.
- It doesn’t require a specific condition of PH and temperature it is a quicker process than rot steeping.
Disadvantages:
The main disadvantage is that the mineral acid is harmful to cellulose fiber if proper care is not taken especially during the storage period.
C. Solvent desizing:
Solvent can easily remove synthetic sizing agents and with proper selection of the solvent any sizing agent can be removed. Since most of the solvent is able to remove fats, waxes, spinning oils, etc., usually desizing and scouring can be done in one operation. In a typical process the fabric is padded through the solvent formulation, steamed to remove the solvent and to activate the enzyme. A wash-off with water completes the process. These processes are suitable for polyester/cotton, polyester/rayon and other blends as well as for cotton.
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Editor of Fashion2Apparel. She is a fashion designer and ex-lecturer in Fashion Designing. She wants to spread fashion knowledge throughout the world.
