What is scouring process of textile fabric?

Scouring Process

Scouring:

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.

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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.

the reaction in the scouring process is following

CnH_2n+1COOH +NaOH = CnH_2n+1COONa + H2O

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Objects of scouring:

To remove natural fat, wax and oil materials containing in the fabrics without damaging the fibers.
To accelerate dye and chemical absorption of the fabrics.
To improve the handle of the goods.
To remove natural color and make the fabric for next process.
To remove non-cellulosic substance in case of cotton.

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%

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

There are basically three scouring systems for cotton:

1. Conventional system

Most widely used system. It consists of an alkali and a combination of surfactants.

Alkali	Surfactant	Emulsifiers
NaOH	Wetting Agents	Non-ionic surfactants

Both wetting agents and emulsifiers are surfactants but they perform different roles. This system removes almost all the impurities except motes. It is a high temperature, high alkalinity, chemical and energy intensive process.

2. Solvent based

Two types of solvents may be used

Chlorine Based	- CCl 4, PERC, Trychloroethylene
Hydrocarbon	- Benzene, White Spirit,

Solvent based systems rely on their solvating power to remove wax by dissolving it. This system removes only hydrophobic impurities and not others. It’s a low temperature, low energy process.

3. Enzyme based

There have been attempts to remove impurities from cotton by use of enzymes. It uses mild temperature and pH.

Conventional system

The agents used are:

Alkali – Mainly Sodium hydroxide (NaOH), sometimes mix of NaOH and Na 2CO 3 (washing soda)

Wetting agent - to reduce the surface tension of scouring liquor so as to wet out the goods uniformly, generally anionic surfactants, mostly –

Emulsifier – to emulsify non emulsifiable wax, generally nonionic surfactants,

In this system, the combination of alkali and surfactants acts on various impurities in the following manner:

Saponifiable oils are converted to Soaps by the action of alkali.
Proteins – Hydrolyzed to soluble degradation products by alkali.
Simple amino compound – hydrolyzed to ammonia by alkali.
Pectose & pectins are converted into soluble salts of pectic/metapectic acid By alkali.
Mineral Matter – either dissolves in water or removed by souring or sequestering agents.
Unsaponifiable oils & waxes – Emulsified by emulsifying agent.
Modern mineral oil formulations usually contain self-emulsified system which can be removed easily with water.
Motes - Cellulose of low crystallinity swell in alkali & became sodium cellulosate which is water soluble. Residual motes are destroyed in bleaching.

While alkali removes majority of the impurities like, pectins and proteins, the fibre does not become absorbent until the wax is removed significantly.

Hence removal of waxes is important for providing absorbency to fabric.

SAPONIFICATION

Wax, fats, oils & lubricants are present mostly as esters in the form of triglycerides.
The triglycerides form glycerin and soap when reacted with NaOH (alkali).

Emulsification

An emulsion is a dispersion of a liquid in another liquid in which it is not soluble.

When two immiscible liquids are mixed & shaken vigorously, one of them breaks into small droplets& gets dispersed in another. However, such a dispersion is thermodynamically unstable hence the liquids again for two distinct continuous phases.

To stabilize such dispersions, emulsifiers are used. Emulsifier is surfactants whose molecules cover the surface of two droplets. One portion of the surfactant molecule becomes compatible with one phase &other part with second phase. This way of dispersion of one liquid phase in another becomes stable, which is known as emulsion.

Milk is a common example of emulsion of fat in water.

In an emulsion the size of dispersed droplets is not defined & may vary to large extent.

Removal of hydrophobic impurities from textiles using surfactant in a aqueous medium by emulsification is a common mode. Agitation helps & is provided either moment of goods, liquor or both.

Milan Tomic

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