Saturday, May 20, 2017

Scouring Process in Textile

Textile scouring is a process in which the fabric is treated with alkali at room temperature or at suitable higher temperatures and by this process oil, fat, wax and other natural and added impurities are removed. Absorbency of the fabric also increases a greater extent in this process i.e. makes the fabrics highly hydrophilic. It also helps to clean textile material by adding alkali. It is a vital process of wet processing.
Textile scouring process
Fig: Textile scouring process
Scouring reaction:
Raw cotton is hydrophobic because, fiber surface is protected with cuticle and cuticle consists of oil, fat, wax. These are high molecular fatty acid i.e. steeric acid (CnH
2n+1COOH, where n:17-35). If NaOH (alkali) reacts with steeric acid, cuticle is removed from fiber surface and regular cellulose chains are opened and the fiber became hydrophilic.

2n+1COOH +NaOH = CnH2n+1COONa + H2O

Impurities of raw cotton:
Raw cotton contains a wide range of organic and mineral impurities in such amount that vary according to the origin of the cotton.
  • Pectins 0.7-1.2%
  • Waxes 0.4-1%
  • Proteins 1.1-1.9%
  • Inorganic components 0.7-1.6%
  • Other organic compounds 0.5-1.0%
Objects of scouring:
  1. To remove natural fat, wax and oil materials containing in the fabrics without damaging the fibers.
  2. To accelerate dye and chemical absorption of the fabrics.
  3. To improve the handle of the goods.
  4. To remove natural color and make the fabric for next process.
  5. To remove non-cellulosic substance in case of cotton.
Changes occurring of cotton fiber during scouring process:
  • Saponifiable oils and free fatty acids are converted into soaps.
  • Pectins and pactoses are converted into soluble salts of pectic acid.
  • Proteins are degraded to simple soluble amino acids or ammonia.
  • Mineral matters are mostly dissolved.
  • Un saponifiable oils are emulsified by the saponifiable matters.
  • Adventitious dirt are removed and forms in suspension by the soap.
  • Residual sizing materials are broken down into soluble products.
Scouring process of cotton depends on:
There are some factors which are responsible for scouring effects of cotton. They are:
  • The type of cotton
  • The color of cotton
  • The cleanliness of cotton
  • The twist and count of the yarn
  • The construction of fabric
Chemical used:
Caustic soda and soda ash are mainly used for scouring of cotton fabric. There are also used surfactants, detergents, chelating agents, sodium silicates, builder, solvent etc.

Auxiliary chemicals:
  • Leveling agent
  • Tinting agent
  • Wetting agent
  • Solvent
Form of scouring:
Yarn and fabric scouring is done in different form. They are:

Yarn scouring:
  • Hank form
  • Package form
  • Continuous sheet warp form
Fabric scouring:
  • Open width form: a. Jigger, b. Padbatch, c. Progressive jigger
  • Rope form: a. Kier, b. Washer
Scouring process:
Generally 4 types of way scouring process are done. They are:
  1. Batch process
  2. Semi-continuous process
  3. Discontinuous process
  4. Modern process
Mechanism of removal of impurities in scouring process:
The action of alkaline scouring agent is to saponify any residual oils, to neutralize carboxylic acids, to solubilise any sizing materials and to cause dispersion of naturally occuring impurities in natural fibers.

Techniques for removing natural impurities of cotton during scouring:

Method of removal
Fats and waxes
Levels are reduced to acceptable limits by the
action of alkali and surface active products, in
extreme cases the use of solvent and surfactant
mixtures may be necessary.
Pectins and related substances
Solubilised by the action of alkali, usually caustic soda, which also acts as a swelling agent to facilitate removal.
Minerals and heavy metals
(a) By producing more soluble salts e.g. acid
demineralisation (b) By use of sequestering agents.
Lubricants/Knitting oils
Modem mineral oil formulation usually contain
their own self-emulsification system.
Amino acids or proteins
Solubilised by producing corresponding sodium
Typical scouring recipe of cotton, jute and polyester fiber:
Wetting agent: 0.5-1.0 g/L
Detergent: 1.0-2.0 g/L
Sequestering agent: 1.0-3.0 g/L
Caustic soda: (36oBe) 2.0-4.0 g/L
Temperature: 95-100oC
Time: 15-45 minutes
pH: 10.5-11.0
M:L – 1:10
Wetting agent: 1.0-2.0 g/L
Detergent: 0.5-1.0 g/L
Sequestering agent: 1.0-2.0 g/L
Caustic soda: 0.5-1.0 g/L
Temperature: 90-95oC
Time: 45-60 minutes
pH: 10.5+/-0.5
M:L – 1:10-1:20
Wetting agent: 0.5-1.0 g/L
Detergent: 1.0-2.0 g/L
Sequestering agent: 1.0-2.0 g/L
Soda ash: 2.0-3.0 g/L
Caustic soda: 0.5-1.0 g/L
Temperature: 80-90oC
Time: 15-30 minutes
pH: 9-10
M:L – 1:10
Souring process in textile:
The treatment by which the fabric, after processing with alkali or scouring is treated with hydrochloric acid (HCl) or dilute sulfuric acid (H2SO4) for removing alkali or neutralization of alkali is called souring. Souring process is done after scouring. Because scouring process is done by alkali, so after scouring process it is necessary to neutralize the fabric for next process.

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