Desizing Process in Textile:
Grey cotton fabric contains natural impurities as well as ‘size’ added to the fabric to facilitate weaving. The size is added to grey fabric in a weaving preparatory process called ‘sizing’. The added matter or size is removed from the fabric by a process called ‘desizing’. Desizing is carried out to remove the sizes that have been applied to the warp yarns for reinforcement during the weaving process. Sizes are normally composed of starches or other synthetic chemical agents, like polyvinyl alcohol (PVA). Removal of sizes is traditionally done by some chemical treatments, such as acid steeping, alkali steeping and oxidative treatment, etc., which are quite time consuming and polluting. Nowadays, a more practical and faster method is to make use of biochemical enzymes which decompose and digest the size, especially starch-based ones. The enzymes, together with the decomposed size residues, are then removed by successive hot- and cold-water rinsing.
So we can say that desizing is the process of removing the sizing materials present in the fabric in order to make fabric suitable for further operations in wet processing.
Desizing is more important for cotton fabrics, but in case of synthetic fabric also we have to remove the additions done during spinning which has to be removed before further processes. These are also termed as desizing. There are three types of techniques in desizing and four methods of desizing that has been used in wet processing. Different types of desizing techniques are depending on the kind of sizing agent to be removed.
Natural starch, starch ethers, cellulose ethers and polyacrylates are usually used as sizes. Economical starch-based formulations are effective for cotton yarns. Polyvinyl alcohol is often preferred for sizing polyester/cotton blends. Waxy plasticizers may be added to the size mix. Considering the sizing chemicals used, the desizing chemicals are to be decided.
The typical sizing materials are:
- Natural starches from potatoes, maize, rice, or topioca.
- Chemically modified starches (ethers or esters).
- Organic polymers, for example, polyacrylates, carboxymethylcellulose, methylcellulose, polyesters, or polyvinyl alcohol.
- Solvent-soluble materials, for example, copolymers of methyl methacrylate.
About 75% of the sizing agents used throughout the world consist of starch and its derivatives because of low cost and high effectiveness as a sizing material on cellulosic textiles. Starch is difficult to remove, as it is not soluble in water or in normal scouring liquor. Removal of starch before scouring minimizes the work required in subsequent cleaning processes; hence, reduced concentrations of the chemicals are required in subsequent scouring and bleaching processes.
Under favorable conditions, starch can be progressively hydrolyzed to the following products with varying solubility in water:
Starch (insoluble) → dextrin (insoluble) → dextrin (soluble) → maltose (soluble) → a-glucose (soluble).
In desizing, the hydrolysis reaction is carried out up to the stage of only soluble dextrin and not up to the stage of a-glucose to avoid degradation of cellulose.
Factors to be considered for removing the size are:
- Concentration and viscosity of the size formulation
- Nature and amount of plasticizer present
- Fabric construction
- Ease of dissolution of the size
- Washing-off procedure and temperature
Types of Desizing Methods with Advantages and Disadvantages:
Desizing involves impregnation of the fabric with the desizing agent, allowing the desizing agent to degrade or solubilize the size material and finally to wash out the degradation products. The major types of desizing processes are:
- Enzymatic desizing of starches on cotton fabrics
- Oxidative desizing
- Acid desizing
- Alkali desizing
- Removal of water-soluble sizes
1. Enzymatic desizing:
It is one of the most important and widely used desizing among the types. Enzymatic desizing is the process of degrading starch size on cotton fabrics using enzymes. Enzymes are complex organic, soluble biocatalysts formed by living organisms that catalyze chemical reaction in biological processes. Enzymes are quite specific in their action on a particular substance. A small quantity of enzyme is able to decompose a large quantity of the substance it acts upon. Enzymes are usually named by the kind of substance degraded in the reaction it catalyzes. In a typical enzyme desizing, the grey fabric is first treated with 0.5 g/l non-ionic wetting agent at 90°C which is known as pre-wash.
Amylases are the enzymes that hydrolyze and reduce the molecular weight of amylose and amylopectin molecules in starch, rendering it water soluble enough to be washed off the fabric. Amylase enzymes are highly effective catalysts for the hydrolysis of the amylose and amylopectin components of the starch.
Effective enzymatic desizing requires strict control of pH, temperature, water hardness, electrolyte addition and choice of surfactant. Normal types are applied for several hours at 65–70°C.
Thermostable hydrolytic enzymes have been introduced, allowing brief dwell times at temperatures up to 120°C. Common salt and calcium ions increase the rate of hydrolysis but amylase is deactivated by copper or zinc ions, as well as most anionic surfactants.
There is some interest in the use of pectinases as scouring agents and lignases to degrade the lignin in bast fibres, but as yet no commercial processes have been developed.
Advantages of enzymatic desizing:
- No fibre damage
- No use of aggressive chemicals
- A lot of process possibilities
- High biological degradability.
Disadvantages of enzymatic desizing:
- No additional cleaning and cracking effect
- Low-effects on certain starches (e.g., Tapioca starch)
- Effects can be reduced by certain size additives and other impurities.
2. Oxidative desizing:
In oxidative desizing, the risk of damage to the cellulose fiber is very high, and, hence, its use for desizing is rare. Oxidative desizing uses potassium or sodium persulphate or sodium bromite as an oxidizing agent.
Oxidative desizing reduces the number of fabric preparation stages minimizing the overall energy consumption. The oxidant can be added to the hot caustic scour liquor and little or no magnesium silicate or organic stabilizer is needed. Rapid desizing treatments require more critical control of alkali and oxidant concentrations. Increased oxidant above the quantity necessary for effective desizing and increasing the alkalinity for a given oxidant concentration both tend to increase the degree of chemical damage. Persulphate promotes desizing rather than bleaching and requires more critical control of concentration than does hydrogen peroxide.
Advantages of oxidative desizing:
- It degrades and removes starch, PVA, and CMC so the type of size on the fabric or size blends do not affect the procedure, effective for tapioca starches and no loss in effectiveness due to enzyme poisons.
- Some scouring and bleaching actions are also obtained, and
- In some cases the scouring process can actually be combined with desizing.
Disadvantages of oxidative desizing:
- The high-risk of degradation of cotton resulting in strength loss, oxycellulose formation, and possible strength loss of polyester in polyester/ cotton blend.
- With problematic sizes it is advisable to carry out combined treatment, i.e., enzymatic desizing followed by an alkaline stage desizing with persulphate.
- Normally sodium, potassium, or ammonium persulphate can be used. Sodium persulphate is most widely used because of its good solubility and because it has no disturbing odour as is the case of ammonium persulphate.
3. Acid desizing:
In acid desizing process, cotton fabric is treated with dilute sulphuric acid with a concentration of 5-10 g/1 at a temperature of about 40°C for 3-4 h. Dilute acid attacks the polymer chain of starch and due to chain cleavage of starch molecule short water soluble or dispersible chain segments are formed. With sulphuric acid higher than 10 g/1 and above 50°C there is always the possibility of weakening the cloth or causing holes. The treated cloth must not be allowed to dry at all otherwise degradation of cotton will occur at the dried area. Rise in temperature increases the rate of reaction, but at the same time there is possibility of attacking the cellulose chain. Generally, the rate of reaction doubles for each 10°C rise in temperature. The acid-steeping method is particularly suitable for cotton varieties containing large metal contents as the mineral acid converts the metals to their corresponding sulphate which are water soluble. The degraded starch is removed from the fabric by normal washing treatment.
Cold solutions of dilute sulphuric or hydrochloric acids are used to hydrolyze the starch; however, this has the disadvantage as it also affects the cellulose fiber in cotton fabrics.
Advantages of acid desizing:
- The amount of size removed is comparatively more tremendous.
- Not possible to shrinkage.
Disadvantages of acid desizing:
- Large floor space required.
- Discontinuous process.
- Risky process, hydrolytic degradation falls the strength of cotton.
4. Alkali desizing:
In this method, size material is removed from the fabric through alkaline hydrolysis. It is stored for 8-10 hours in a solution of 0.4-0.6% sodium hydroxide at a temperature of about 60-70°C. Then wash the fabric with water to remove the size materials. However, care must be taken that the fabric does not dry out during desizing. It is a very cheap and widely used process.
5. Removal of water-soluble sizes:
Fabrics containing water-soluble sizes can be desized by washing using hot water containing wetting agents (surfactants) and a mild alkali. The water replaces the size on the outer surface of the fibre, and get absorbs within the fiber to remove any fabric residue.
Bioscouring is a process in which alkali-stable pectinase as the enzyme is applied to selectively remove pectin and waxes from cotton fibers. By hydrolyzing the pectin material between the waxes and the fibre surface, the enzyme exposes the waxes to emulsification when the scouring bath temperature exceeds their melting range. Bioscouring does not eliminate motes (cottonseed fragments) or the natural color of the cotton, which can be beneficial when scouring for a natural look.
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