Industry And Cluster | News & Insights

Scouring of Cotton

Published: October 13, 2020
Author: seemanagrath

Desized cloth is free from size or ‘added matter’, but it still contains natural impurities like oils, fats, waxes and colouring matter. These oils and fats are higher fatty acids of inorganic compounds. They are hydrophobic (or water-hating) materials that affect the absorbency of the cloth. Their presence leads to improper dyeing, printing and finishing as it would be difficult for dyes and chemicals to go uniformly into the fibre structure. The oils, fats and waxes make up the outermost cuticle layer of the cotton fibre. As they prevent the easy entry of water, chemical and dyes, etc into the fibre they have to be removed to make it absorbent.
A process called scouring removes these impurities. In the case of cotton textiles, the process is also referred to as ‘kiering’, ‘kier boiling’, etc as cotton yarn or cloth is boiled with alkaline liquor in a pressure vessel called a ‘kier’.

Objectives of Scouring:
• To remove natural impurities of essentially hydrophobic character (oils, fats, waxes, etc) as completely as possible and leave the fabric in a highly absorptive condition without undergoing significant chemical or physical damage.
• To make the goods ready for the next process, i.e. bleaching, where the natural colouring matter of theb) cotton is removed.

What happens during scouring (kier boiling) of cotton goods?
The chemical actions that take place during the scouring of cotton are listed below:
• The saponifiable oils are converted into soaps.
• The unsaponifiable oils and waxes are emulsified.
• The proteins are hydrolysed into soluble products.
• Pectose and pectin are changed into soluble salts of pectic of metapectic acid.
• Simpler amino-compounds are dissolved or hydrolysed to ammonia.
• Mineral matter is dissolved.
• Dirt particles are removed and held in a stable form in the scour liquor.

Saponification: Vegetables oils, animal fats and mineral oils are water-insoluble. Vegetable oils are glycerides of fatty acids like stearic and palmitic acids. When such oil is heated with a solution of sodium hydroxide in water, it is split up into two components, fatty acid and glycerine. The glycerine dissolves freely in water. The fatty acid reacts with the sodium hydroxide present in the solution to form the sodium salt i.e. soap, which is soluble in water. Conversion of glycerides of fatty acids into soap by means of sodium hydroxide is called saponification. Thus, the water-insoluble oils and fats in the cotton textile are saponified during the scouring process into water soluble products.
Waxes presents in the fibre are esters of higher fatty alcohols and fatty acids and cannot be removed by saponification. They are hydrophobic in character. They can be removed by converting them into emulsions, which can be held in the kier liquor. (An emulsion is a fairly stable mixture of two liquids, which normally do not mix with each other.) Therefore, an emulsifying agent (e.g. soap) is added to the kier liquor. This is over and above the soap formed during the saponification action in the kier. Water + Soap (added and formed by saponification)

Suspension or Emulsion
Types of Kiers: Kiers are basically of two types:
• Open Kier
• Closed Kier
Most kiers are vertical kiers, but horizontal kiers have also been used. The older kiers were made of wood. The kiers of today are made of metal with special inside lining.
Methods of heating the scouring liquor in kiers: With regard to this, kiers are divided into three types, viz. those heated by direct steam within the kier
• those heated by indirect steam (closed coil) within the kier and
• those heated by indirect steam outside the kier.
• those heated with direct steam.

Modern Kiers:
Modern kiers are characterised by the following main features:
• Heating is done using steam under pressure, generated by means of an independent boiler, instead of heating by direct fire. Iron and steel take place of wood as the material of construction of the kier.
• Improved methods of circulation of kier liquor through the material by the use of powerful pumps.
• Improvement in the handling of the goods (rope form or open-width form) by the use of automatic piling devices.
• Use of high-pressure in the kier to shorten the duration of boiling off and hence giving savings in the cost of5. steam, power etc.

Pressure kier: A kier is a cylindrical vessel (mostly vertical) capable of holding 1 to 3 tonnes of cloth. A 2- tonne kier having dimensions of 2.70 m in height and 1.95 m in diameter is commonly used for alkali boiling. Either open kiers (working at atmospheric pressure) or pressure kiers are used, depending upon the material and process required. Coloured woven goods, for example, are given an ‘open-boil’ and a milder alkali treatment. When the lid of the kier is removed, it serves as an open kier working at atmospheric pressure. Kiers are made of mild steel plates 0.6 cm thick joined by welding or rivetting. They are provided with a pressure gauge and a safety valve. High-pressure kiers have manholes through which the cloth is introduced. As the kier is made of iron, a ‘lime wash’ (with a mixture of lime, cement and sodium silicate) is given to its interior to avoid the formation of rust stains on the cloth when it comes in contact with the inner wall of the kier.

The piling of the (yarn or fabric) goods in the kier should be done uniformly by well-trained manpower.
• With manual pilling, it is possible to load about 80% of the kier capacity. Automatic piling equipment is also available in modern kiers and with an automatic piler, about 70% of the kier capacity is utilised.
• In any case, the kier should never be filled beyond 80 to 85% of its capacity. Overfilling will compress the fabric and prevent proper circulation of the liquor.
• Due to this the temperature difference between the top and bottom of the kier could vary considerably. Small quantities of cloth should not also be boiled in a large kier; it should at least be half-full; otherwise the cloth may turn and get hopelessly entangled.
• After the material is piled into the kier, heavy stones are placed on it to prevent it from being tossed up during boiling thereby forming channels, which lead to uneven results.
• Clean jute cloth or waste cotton sheeting may be placed between the material and the stones.
• Before the fabric to be scoured enters the kier, it may be impregnated with alkaline liquor in a trough and mangled to ensure even treatment of the cloth.
• After loading the fabric, the kier liquor is let in from the bottom of the kier to sweep out the air in the kier.
• Air is completely removed from the kier by closing its lid and allowing the kier liquor, as it is being heated, to come out of the air valve at the top of the kier.
• All the air in the kier would have come out of it before the liquor comes out. The M:L ratio in a kier is usually 1:4 or 1:5.
• There should be enough kier liquor for the efficient circulation by the pump.
• Complete immersion of the material in the liquor and the removal of air is most essential in order to prevent formation of oxycellulose which is rapidly produced by the action of the oxygen in the air on cotton.

Kier-boiling of cotton in yarn form:
In the case of cotton yarn in hank form, the hanks are usually linked together with pieces of cotton twine (see figure) so as to form a continuous chain or rope and loaded into the kier as if it is a rope of fabric. Alternatively, the hanks are made into small loosely tied bundles and arranged neatly and uniformly in the kier. In either case the loading precautions are followed.

Scouring of coloured woven cotton fabric: A variety of textiles are produced today that contain coloured threads along with grey yarn. Examples of this kind of fabric include dhotis, towels, shirting (striped and checked), saris, etc. Such coloured woven goods may require to be scoured (and bleached) for the grey yarn they contain. Coloured woven goods are generally given a scouring treatment in an open kier using a recipe similar but milder to that used for a pressure boil. The alkali content is lower, the time is much less and the boiling is under atmosphere.
Precaution with goods containing vat dyes:
• When the coloured threads in the grey fabric contain vat dyes, the scouring recipe includes a mild oxidising agent such as Ludigol, Resist Salt, Polysalt G, etc.
• The scouring conditions give degradation products that have reducing power and these in the presence of strong alkali would cause the vat dyes to dissolve and mark adjacent white yarn.
• The oxidising agent takes care of this unwanted side reaction by preventing the dissolution of vat dyes.

Scouring cotton textiles in batches using other machines: The machines and processes in the wet processing of textiles can usually be organised to have three modes of operation:
• Batch Process
• Semi-Continuous Process
• Continuous Process.

Kier-boiling is a batch process for the scouring of cotton goods. Cotton goods can be scoured on other machines too. For example, woven fabric meant for dyeing can be scoured in a jigger machine. Lightweight goods and knitted fabric can be scoured by means of a winch machine or a soft-overflow jet (dyeing) machine. All these machines are for processing fabric in batches, i.e. fabric of short lengths in the range 500-1000 metes, depending upon the type of fabric in process. The jigger and winch machines are described at the end of this unit. Cotton yarn in cone or cheese form can be scoured in a package (dyeing) machine. A warp beam of grey cotton yarn can be scoured in a beam (dyeing) machine. In the small-scale dyeing industry, open-beck scouring is practised. Hank yarn is hung from bent rods or wooden sticks into hot scouring liquor contained in galvanised-iron rectangular tanks and turned around manually in the scouring liquor.

Note that when winch machines are used for scouring, especially when the fabric is processed in rope form, the conditions of scouring are milder than those used in a jigger. This is mainly to avoid the formation of permanent creases in the fabric. The temperature is below 75°C and the concentration of alkali is roughly half that used in a jigger.

Written By:
Gargi Dandegaonkar
B.Tech. (Institute of Chemical Technology, Mumbai)
Intern At TVC

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