Desizing in textilwe industry
Desizing is used for removing the size material from the warp yarns in woven fabrics and is usually the first wet finishing operation performed on woven fabric (BAT for the Textiles Industry, July 2003).
2. FIELD OF APPLICATION
Currently applied techniques can be categorised as follows (BAT for the Textiles Industry, July 2003):
- techniques for the removal of starch-based sizing agents (water-insoluble sizes)
- techniques for the removal of water-soluble sizes
- techniques for the removal of water-soluble and insoluble sizes
3. DESCRIPTION OF TECHNIQUES, METHODS AND EQUIPMENT
Desizing techniques are different depending on the kind of sizing agent to be removed:
1. Techniques for the removal of starch-based sizes
Starch based sizes are difficult to remove and require either catalytic action of an enzyme (catalytic degradation) or other chemical treatment in order to be converted into a washable form. This chemical degradation is mainly achieved by either enzymatic or oxidative desizing.
Enzymatic deszing is the most widely used method for the removal of starch, amylases being particularly suitable. The advantage in the use of enzymes is that starches are decomposed without damaging cellulose fibre.
In order to reduce the number of steps in the pre-treatment process, it is common practice to combine desizing with cold bleaching in a single step. In this case the process is also called “oxidative desizing”. The fabric is impregnated in a bath containing hydrogen peroxide and caustic soda, together with hydrogen peroxide stabilisers and complexing agents. Persulphate is also usually added to the solution.
Due to the action of NaOH, this treatment, beside a desizing/bleaching effect, also serves as a pre-scouring treatment. Furthermore, oxidative desizing is particularly useful when the textile contains enzyme poisons (fungicides) or when sizes are present that are difficult to degrade. However, because starch scarely differs from cellulose in the cross-linking of the cellulose rings, chemical oxidation, if not well-controlled, can damage the fibre. Desizing is usually carried out in patch-process, but discontinuous (e.g. jigger) and continuous (pas-steam) processes can also be applied. In the case of enzymatic desizing, pad-steam is applied only for big lots and with enzymes that are stable under steaming conditions. After the reaction timer, the fabric is thoroughly washed in hot water (95°C).
2. Techniques for the removal of water-soluble sizes
The removal of water-soluble sizes, such as PVA, CMC and polyacrylates, theoretically only requires washing with hot water and sodium carbonate. However, the washing efficiency can be increased by:
- adding suitable auxiliaries (wetting agents) to the desizing liquor (with some restrictions in case of size recovery)
- allowing adequate time for immersion in the desizing liquor (this ensures maximum liquor pick-up and adequate time for the size to swell)
- washing thoroughly with hot water in order to remove the solubilised size.
In this case, the process is carried out in normal washing machines. Continuous washers are often used, but sometimers the treatment time may be too short to allow complete desizing. Pad-batch and pad-steam or discontinuous processes for prolonging the residence time are therefore also in use.
3. Techniques for the removal of water soluble and insoluble sizes
The “oxidative desizing” technique mentioned above, is applicable not only for water insoluble sizing agents, but also for water-soluble ones. This technique is particularly useful for textile finishers, but also for water soluble ones. This technique is particularly useful for textile finishers dealing with many different types of fabrics and therefore sizing agents.
The washing water from desizing of cotton and cotton-blend fabrics may contain 70% of the total COD load in the final effluent. The emission factor can well be in the order of 95 g COD/kg of fabric, with COD concentrations often above 20 000 mg COD/l.
4. COMPETITIVE TECHNOLOGIES AND ENERGY SAVING POTENTIALS
- a) Changes in the process
- Use of low add-on techniques: (BAT for the Textiles Industry, July 2003)
The combination of low add-on techniques such as pre-wetting of the warp yarn or compact spinning, with the targeted selection of sizing agents, helps to reduce the environmental impact of the desizing process. It is now accepted that readily biodegradable or bioeliminable compounds are available, covering all needs. Moreover, latest generation-polyacrylates are highly efficient with lower add-on and can be completely and easily removed from the fabric.
- b) Changes in the energy distribution system
- Water re-use/recycle: (BAT for the Textiles Industry, July 2003)
Reducing water consumption is achieved here, by combining processes (e.g. scouring and desizing) and reusing/recycle water. These measures allow significant savings not only in water consumption, but also in energy consumption, because energy is used to a great extent to heat up the process baths.
- c) Changes in the heat supply system
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