Preparatory Process of Cotton
Pursuing M.Sc. in Fabric and Apparel Science
Delhi University, India
Scouring process of cotton
Other natural fibers such as cotton and silk contain impurities that are easier to remove than those that occur in wool, though still preparatory process of cotton is necessary to scour them to ensure uniform bleaching, dyeing and finishing as well as to enhance their wettability and absorbency.
Cotton may contain from 4-12% by weight impurities in the form of the waxes, proteins, pectins, ash and miscellaneous substances such as pigments, hemicelluloses and reducing sugars. The hydrophobic nature of the waxes makes their removal difficult relative to the removal of other impurities.
Effective removal of impurities in cotton, particularly waxes, is achieved by boiling in 3-6 % sodium hydroxide or less frequently in dilute solutions of calcium hydroxide (lime) or sodium carbonate (soda ash).
The proper choice of textile auxiliaries in the alkaline bath is essential for good scouring. These include sequestering or chelating agents such as ethylenediaminetetraacetic acid (EDTA) to solublize insoluble inorganic substances present in hard water and surfactants such as the anionic sodium lauryl sulfate that serves as a detergent, dispersing agent, and emulsifying agent to remove unsaponified waxes.
Synthetic fibers are scoured with milder formulations such as soap or detergents containing comparatively small amounts of alkali. Cotton/synthetic fiber blends (such as cotton/polyester) require alkaline concentrations and conditions intermediate between those of all cotton and all synthetics for effective scouring.
Techniques and machinery for scouring different fiber types and fabrics have evolved from batch processes utilizing various types of open and pressure kiers to continuous processes in which fabrics are passed through roller steamers under pressure. Continuous scouring processes are similar to those used to continuously bleach fabrics.
Key points of cotton scouring:
A. Open or pressure kiers are stainless steel containers that circulate the alkali at elevated temperatures from 100oC at atmospheric pressure to over 130oC at reduced pressure.
- The alkaline solution is injected or introduced from the bottom of the kier and air removed by displacement by steam.
- After scouring for sufficient time, the fabric is rinsed by continually introducing hot water to dilute the alkali in the bath.
B. In semi continuous process where the fabric is scoured in rope or open width form
- A device called a saturator serves as scouring bath. The fabric is then rapidly preheated by steam and passed into a J-box storage, chamber or alternatively, the fabric is directly heated in the storage chamber
- After storage in the J-box the fabric is then washed with hot water to remove alkali and impurities
C. Continuous processes that rebatch the fabric or utilize roller steamers under pressure, reduce the dwell time of the fabric considerably and allow for higher proceeding speeds.
D. Solvent scouring of cotton and cotton/polyester blends has been employed, with trichloroethylene frequently used as the scouring solvent.
- However, only waxes are removed by this method and some form of aqueous alkaline scouring is still required.
- When the only impurities in fabrics are oils incorporated to assist in knitting or weaving, trichlorotrifluoroethane is claimed to be very advantageous in solvent scouring.
Process and effects of mercerization of cotton:
Mercerization is one of the most important finishing processes of cotton with a strong caustic alkaline solution in order to improve the luster, hand and other properties.
It imports gloss to the fiber, increases its hygroscopicity, strength and improves its dye affinity.
Mercerizing improves the reactions with a variety of chemicals and elongation of the fibers and also improves the stability of form. Mercerizing process consists in treatment of cellulosic materials with concentrated solutions of caustic soda at a temperature of 15 to 18°C.
Mercerized cellulose is hydrated cellulose, i.e., a product which from the chemical point of view is identical to the original cellulose, but differing from it in physical properties. This method was patented in 1850 by the English calico printer John Mercer and hence forth this process has been called as mercerization.
Cellulose mercerizing process:
Stages at which cellulose mercerization is possible are- On greige goods, after desizing and scouring, after bleaching, after dyeing usually sodium hydroxide concentration varies from 20% – 30%.
The process, done in a continuous way, involves four subsequent steps:
- Impregnation of the material in relaxed state, cold caustic solution of required strength and wettability.
- Stretching while the material is still impregnated in the caustic solution.
- Washing off the caustic soda from the material while keeping the material still in the stretch state.
- Neutralizing with acids and rinsing.
Physio-chemical changes during mercerization:
Under the action of concentrated alkaline solutions chemical, physico-chemical and structural modifications of cellulose take place. Native cellulose (Cellulose I) forms alkali cellulose I with concentrated sodium hydroxide. On washing and neutralization cellulose II is formed.
As a result of the penetration of the alkali into the lattice, internal hydrogen bonds are broken and in Cellulose II the number of available hydroxyl groups (-OH) is increased by around 25%. The treatment with alkali and subsequent washing may be performed so that the fabric or yarn may either freely contract or they may be held under tension. In both cases the mercerized cotton has an increased affinity for both reactive and direct cotton dyes, water and an increased strength. Cotton yarn or fabric mercerized without tension contracts, but if held under tension it retains its original dimensions and the luster is increased. Major changes during Mercerization can be divided into three levels.
Effects of mercerization on cotton:
1. Mercerization increases fiber luster:
Concentrated solutions of caustic soda cause considerable swelling of cotton fiber. The changes in cellulose physical properties are being irreversible. When the fiber swells, its volume undergoes considerable changes; at maximum water absorption, the cross section of cotton fiber is increased by 40 to 50% with inconsiderable increase in length (about 1 to 2%). Swelling of fiber changes its cross section from squashed circular pipe shape to an oval shaped.
Luster of a fiber is due to the regular reflection of light incident on the fiber surface, which depends on the cross-section of the fiber. If the fibers are placed under a tension or stretched position in the swollen state and then washed to reduce the caustic concentration below a particular limit, then there is an increase in the luster of the fiber.
1. Before mercerizing
2-5. Swelling stage with 18% sodium hydroxide
6. Washing stage after mercerization
7. Final stage
The main factors influencing the factors of selling are temperature of treatment, the concentration of the alkali in the solution and additions made to the solution.
2. Mercerization increases tensile strength:
When cotton fiber, yarn or cloth is mercerized, its strength increased by 10-50%. The tensile strength increase depends on various factors, such as temperature of impregnation, concentration of alkali in impregnating bath, construction of yarn etc. Lower the temperature of mercerization, greater is the tensile strength (breaking load of the yarn). Increase of alkali concentration up to 520 Tw, tensile strength increased gradually and further increase of alkali concentration decrease the tensile strength. For long staple fiber yarn, greater the twist, greater is the tensile strength of mercerized material.
3. Ecological impact and recovery of Sodium hydroxide:
The main ecological impact in mercerizing is the high concentrated residual lye. Treating cotton materials with strong sodium hydroxide and washing it off gives a large volume of dilute NaOH solution, which cannot be discharged into the drain for economy and pollution points of view. By suitable means it is possible to recover/reuse 90-95% of NaOH used in mercerizing. The alkaline load of waste water is reduced drastically and acid required for waste water neutralization is minimized. Wash liquor may be used for the preparation of sodium hypochlorite solution (for use in bleaching). NaOH from the impregnated fabric may be recovered by washing using counter-current principle and by using steam in a recuperator.
4. Compare the process of enzymatic and oxidative desizing:
Most earlier desizing utilized enzymes such as alpha and beta amylase at temperatures around 40-70oC and pH values of 4.6-7.0 to remove starch sizes from cellulosics or by steeping the yarns in dilute solutions of acid.
|1. It is effective, though limited to the removal of natural polymers, such as starches and cellulose derivatives because an enzyme will only be active or affect a particular substrate
2. It consists of three main steps: application of the enzyme, digestion of the starch and removal of the digestion products. The common components of an enzymatic desizing bath are as follows:
|1. It may be employed to remove both natural and synthetic polymers
2. Oxidative desizing can be effected by hydrogen peroxide, chlorites, hypochlorites, bromites, perborates or persulphates.
3. Two important oxidative desizing processes are: the cold pad-batch process based on hydrogen peroxide with or without the addition of persulphate; and the oxidative pad-steam alkaline cracking process with hydrogen peroxide or persulphate.
Bleaching of cotton with hydrogen peroxide (H2O2):
The bleaching bath is composed of hydrogen peroxide (35% or 50% by wt.) as the bleaching agent, an activator (usually alkali) and stabilizer
Hydrogen peroxide bleaching can be done by:
- Batch or Discontinuous process e.g. Kier, Jigger, Winch.
- Continuous process e.g. J – box, Vaporloc
- Semi – continuous process e.g. pad – roll.
1. Batch or Discontinuous process
A. Kier boiling – it consists essentially of circulating hot alkaline liquor through the cloth, which is commonly manipulated in ‘rope form’. The kier consists of a vertical cylindrical iron vessel with perforated false bottom in which cloth is laid down in a regular fashion either manually or by mechanical plaiting. It is frequently used and as able to carry out high pressure scoring. The sizes of kiers vary and in general are capable of holding from 500 lbs to 5 tons of cloth. In its simple form, steam is injected through a hole at the center of the bottom of the kier and is able to pass up the tube thereby forcing liquor up with it. Kier boiling may be carried out either at atmospheric pressure in an open kier and under pressure. Temperature in kier is around 130oC. cotton goods are normally treated as kier a period of time varying from 2-12 hours depending on the type of cloth and the effect which is required.
2. Continuous process
A. J-box – in this the cloth is impregnated by passage through a caustic soda solution (3-4%) in a saturator and then squeeze rolls so that the cloth retains its own weight of liquor. The cloth is then heated in steam and stored in J-box for one hour. J- box consists essentially of a stainless chute is piled at the bottom of the chute where it is able to reside for periods up to an hour. Temperature in J-box is 95oC. successful scouring by continuous methods requires good and rapid wetting in order to obtain a good alkali penetration continuous alkaline treatment is convenient when used as preparation for a continuous peroxide bleaching process.
B. Vapor lock- it is a high temperature high pressure machine used for continuous scouring. In this, fabric is padded and passes through machine treated at very high temperature pH of the liquor is around 14 and fabrics are treated in it for 1-2 minutes.
Differentiate between alkaline and solvent scouring:
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- Concept Development of Mercerization with Different Parameter and Comparison
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.