25 Preparatory Steps Prior to Finishing textiles

S. Karpagam Chinnammal

Learning objectives

To gain knowledge about

Preparatory processes its definition and objectives
Preparatory processes suitable for different fibres
Objects, importance and technique of each preparatory process

1. Introduction

Fabrics received as gray cloth from the loom have unwanted matter present in them. They may be natural or stains, oils, waxes and sizes acquired during manufacture of the fabric. Elimination of these impurities is imperative before applying any other finish as they may hinder withsubsequent processes. The impurities are removed by preparatory processes. Preparation is important as success of all process such as dyeing , printing depend on it. Improper preparation leads to reduced performance and value in the finished product.

2. Preparation -Definition

Preparation is the procedure of preparing fiber, yarn or fabric for the ensuing processes.
Preparation is cleaning or removing impurities from textile materials.

3. Preparation- Objectives

The main aim of preparation is the removal of impurities that will hinder with dyeing and finishing operations.
Improve the ability of fibres to absorb dyes and chemicals rapidly and uniformly
Produce a Substrate that hasUniform standards of white o Minimal fiber damage
A constant pH

4. Preparation Processes

Singeing
Desizing
Scouring
Bleaching
Mercerizing (cotton only)
Heatsetting (synthetics only)
Wool carbonizing ( wool only)
Degumming( silk only)

The list shows the usual preparation processes. The process sequence selected will vary based on fiber content, fabric manufacture, end use and the kind of impurities present. Impurities found on textile materials vary greatly in their composition from fiber to fiber. As the impurities vary the process of removal also varies. eg. Cotton may be kiered, silk is degummed wool is carbonised.

5. COTTON

In preparation for dyeing or printing and finishing, raw cotton fabric is singed, desized, scoured, bleached, and mercerized. These treatments eliminate non-cellulosic impurities and enhance the affinity of fiber for dyes and finishes.

5.1. Singeing

Singeing is the first process to which the grey cloth is subjected. The warp and weft yarns are made by spinning a numberof short fibres and consequently the tiny fibres protrude from the surface. The cloth wovenfrom suchyarns also has the protruding fibres and these are removed by the process known as singeing.

5.1.1. Objects

To remove loose hairy fibre protruding from the surface of the cloth and to impart a smooth even andclean looking surface.

5.1.2. Importance

If the cloth is not singed, it will have a matted, raw appearance that is roughness and dullness
In order to impart a clear defined and sharp design to the cloth in printing
It reduces pilling

5.1.3. Gas singeing

The protruding fibres are burnt by passing the fabric rapidly over gas flames. The machine consistsofone or more burners, giving continues flames produced by a mixture of compressed gas and coal gas.Theflameissues from a narrow slit which is adjustable with width whenthecloth is drawn over the flameata high speed,the flame impinches on its surface and burns the protruding fibres without damaging the cloth.The singed cloth passes through a water trough to extinguish any sparks present.

5.2. Desizing

Warp yarns are sized to make it withstand the stress and strain during weaving .The major portion of the size consists of starch, wax, oil and all these remain on warp yarns even after weaving. This size hinders absorption of dyes and causes uneven dyeing so it is removed using dilute acids or enzymes and this process of size removal is called desizing.

5.2.1. Importance

It is necessary to desize the cloth otherwise, the hydrophobicity of the wax and oils hinder the subsequent dyeing, printing and finishing process.

5.2.2. Objectives

To remove the size from the grey fabric and to make the fabric more absorbent, to facilitate further processes.

5.2.3. Methods

5.2.3.1. Rot steep is the oldest and requires no special chemicals. The cloth is first soaked in water at 35 – 40˚ C and passed through a padding mangle, where it is squeezed and then allowed to stand for 24 hours. The starch liquefying enzymes secreted by the micro organisms in water solublise the starch present in the fabrics. The cloth is finally washed with water when most of the starch originally present on the fabric is removed.

Fig .2 Desizing

5.2.3.2. Acid Steep

A 0.25% – 0.5% solution of dilute Sulphuric acid or Hydrochloric acid at room temperature (30˚ C) is used to hydrolyse the starch from the sized fabric. Hydrolysis of starch is an exothermic process, as the reaction proceeds the temperature of cloth raises even to the temperature of 50˚ C. The rate of hydrolysis increases at this higher temperature. The cloth is kept for 4-12 hrs and then washed. It is efficient and quick. Prolonged exposure with acid will degrade the material.

5.2.3.3. Enzymatic Desizing

The most popular and most effective method of removing starch from fabric is by the use of enzymes. Enzymes give a gentle reaction with no danger of destroying the fabric. Control of temperature and pH is very important. Most of the enzymes function at pH 5.5 – 7.5 and temperature 50 – 60 ˚C. If the temperature is more than 70 ˚C the enzymes will become inactive. Amylase enzymes are used in desizing.

The fabric is passed through the two bowled padding mangles, the trough contains the enzyme bath composed of 1 -2 % enzyme, 0.5-1 % common salt and 1 – 3 g/l nonionic wetting agent. pH of 6 to 7.5, temperature 50 -60 ˚C are maintained for 3 to 4 hours. Starches are hydrolyzed to water soluble compounds by enzymes and are removed by washing.

The starches are converted to simple water soluble product such as dextrins and glucose without degrading the cellulose

5.3. Scouring

After desizing the cloth still contains oils, waxes, fats and other foreign matters which are hydrophobic in nature. The presence of these affects the absorbency of the material badly and results in improper dyeing, printing and finishing. Scouring/cleaning process also called “kiering” (or) “kier” boiling removes impurities from the fabric.

5.3.1. Objectives

To remove natural as well as added impurities which are hydrophobic in character (oils, waxes, fats etc) and leave the fabric in a highly absorptive condition without undergoing chemical or physical damage.

5.3.2. Alkaline Scouring

Scouring is carried out in a boiler called a “kier”. The process consists of circulating hot alkaline liquor Caustic soda under high pressure (20-30 pounds per square inch) through a regularly packed column of desized fabric at 120-130˚C for 8hours. Scouring liquor is prepared with Sodium hydroxide =2-4%, Sodium carbonate =0.5-1%, Soap 0.5%, wetting agent 0.5%, Sodium silicate 1% , Material: liquor ratio= 1:4.This is a batch process.

Fig.3 Kier

In continuous scouring, the desized fabric is passed through scouring solution and then passed on to ‘J’ box where the fabric is heated with live steam at a temperature of 100˚C for 1hour. After cleaning, the smoothness, neatness and absorbency of the fabric are enhanced.

Fig.4 Continuous scouring

The process of converting water insoluble form of oil, fatty acids to water soluble form using strong alkali is saponification. The conversion of non saponifiable oils and waxes into stable dispersion using emulsifying agent (soap) is called emulsification.

5.3.3. Enzymatic scouring is done using alkaline pectinase and is an ecological friendly substitute to usual scouring. It provides better whiteness apart from saving water and energy.

5.4. Bleaching

Bleaching is a chemical process by means of which coloured or discoloured fabrics are made white. Bleaching should be carried out carefully as the chemical may damage the fabric.

5.4.1. Objectives

* To decolourize the natural colouring matter

* Make the fabric white or colourless

* To remove the stains which have occurred in the previous process

* To make the fabric more absorbent

5.4.2. Bleaching agents

Bleaches can be divided in to two groups

Oxidising Reducing

The most commonly used bleach in the bleaching industry is oxidizing bleach. The liberation of oxygen which combines with stain to form a colourless compound is the principal of this bleach.

H2O2——–› H2O+ [O] ↑Cl2+H2o ———-› 2 Hcl+[O] ↑

Nascent oxygen[O] reacts with the colouring matter decomposes or breaks up the colouring matter in to simple colourless compounds.

5.4.3. Chemicals used in cotton bleaching

·Sodium Chlorite- less harmful to the fibre.
Sodium Hyphochlorite – ideal for bleaching cotton .
Hydrogen peroxide- least injurious bleach used in cotton
Chlorinated lime
Chlorine – weakens the fabric.
A commerical preparation such as “javel water “

5.4.4. Hydrogen peroxide

Hydrogen peroxide is the most commercially used bleach. It is suitable for all kinds of fabric and it is called the universal bleaching agent. Less weight loss, less water requirement, superior fastness, increased absorbency of fabrics, safety, less tendency of yellowing are claimed to be the advantages of this bleach.

H2O2 splits up into H2O and O and so colouring matter is oxidized. The action takes place most rapidly with alkaline solution. Addition of stabilizer sodium silicate is required as it fixes the pH value of bleach liquor and produces good peroxide stability for efficient bleaching.

The bleaching solution contains hydrogen peroxide 3 to 4%, Soda ash – 1%, Wetting agent- 1%, Sodium silicate- 3%, and Sodium hydroxide-0.4 – 0.6% , Temperature – 100°C, pH – 10.3 – 10.8,Time – 1 hour, Material liquor – 1:4 – 1:5

The fabric is passed through the saturator where it is impregnated with a bleaching solution. The fabric after leaving the saturator is squeezed by a set of rollers to remove the excess chemical and to ensure proper penetration of the bleach liquor. Then the cloth passes in to the heated tube, an integral part of J-box. Dry steam at atmospheric pressure is then admitted in the heater tube. After the cloth is heated, it falls into the J-box which is a large stain less steel box provided with a cloth piling device and large enough to hold the cloth to be processed for an hour at the operating temperature of boiling point. The heated cloth is taken out from the J-box and fed into the washer where it is thoroughly washed.

Fig 5 Continous Bleaching

5.5. Mercerization

Mercerization is a chemical finish where cotton is treated with concentrated solution of Sodium hydroxide under tension. When the cotton boll bursts the water in the fabric dry out and the fiber collapses, in to a twisted or convoluted flat ribbon. When treated with strong solution of caustic soda the cotton fibers swells, up to the same diameter that it had while growing in the boll and shrinks longitudinally.

5.5.1. Objectives

To improve the lustre of the fabric.
To increase the dye ability
To improve the brilliancy of shade
To increase the tensile strength of fabric
To improve the pliability and elasticity of the fabric.

Mercerisation is a process of treating cotton fabric or thread with a strong solution of sodium hydroxide /caustic soda (18-24%), at room temperature for 30- 45 seconds. The yarns or fabric are held under tension then washed when still under tension. The caustic soda penetrates the cotton fiber, makes it to swell and achieve a round cross section that reflects light to create a gloss or sheen. The natural twist of cotton fibre is removed. The swelling also results in better moisture absorption properties, increased color yields, and chemical reactivity. This process increases the tensile strength of the cotton fiber by 20%.Chemical finishes show improved performance on mercerized fabrics.

5.5.2. Mercerisation range

The fabric is saturated with the mercerization chemicals and passed through rolls to squeeze out excess chemicals. Adequate tension is imparted to the fabric and sent to the stenter. The selvedges of the fabric are gripped by two parallel clips or pins in the stenter and the fabric is pulled to required width and tension is maintained to ensure optimum results. When the process is complete the chemicals are washed while still under tension by counter current washing system. It operates reusing water flowing in the opposite direction to the fabric movement. Fresh water is sprayed over the fabric in the last washer, the washed liquor is collected beneath and sprayed over the fabric in the last before washer. The process is repeated so the dirtiest water is used in the first washer. In the recuperator it is again washed with fresh water.

Fig 6 Mercerisation

6. WOOL

Wool is an animal hair fiber. It has a lot of impurities from the body of the animal as well as from its environment. So, it is much dirtier than cotton. The raw wool is composed of Protein-50-60%, Dirt-10-20%, Suint-6-8%, Grease-10-15%, Plant-5-10%, Water-10-12%. Protein represents the actual wool fiber, 50% of the weight of the raw fiber is impurities. The wool fibres are cleaned before it is processed into yarn or fabric ie in fiber stage because of the high proportion of impurities present.

6.1. Wool Scouring

Raw wool is steeped in water at a temperature of 32-42°C to remove the water soluble suint and other heavy dirt particle. Wool is sensitive to alkali. The wool grease which is water insoluble is removed by treating in a solution containing a mild detergent or soap and a mild alkali such as soda ash/ sodium carbonate. The temperatures of the solution should be maintained below the boiling point. Wool should not be moved vigorously as felting occurs.

6.2. Wool Bleaching

Wool is bleached with 0.3-0.6% hydrogen peroxide, pH is adjusted to 8-9 at temperature between 50-55°C.

6.3. Wool Carbonizing

This is the process applied to wool for removal of burrs and other vegetable impurities in it. Woolen fabrics is impregnated with 4-7% sulphuric acid for a period of 2-3 hours and then dried in an oven at 82° to 88°C and baked at 100° C-104°C for 15-30 minutes. The burrs which are cellulosic are degraded by acid and converted to hydrocellulose and are charred. The woolen cloth is passed between pressure rollers where the carbon residues are crushed. The loosened residual powder can be shaken off by mechanical agitation. The fabric is then rinsed with water and neutralized in a solution containing sodium carbonate to neutralize the left over acid, washed and dried.

Bio carbonizing: In this process goods are treated with cellulase enzyme instead of acid

7. SILK

Silk is a protein fiber similar in chemical properties to wool but has much lower impurity content. The composition of silk is fibroin 70-75 %, sericin 25-30 %, waxy substance-2-3%, Natural colours-1-1.5 %, Mineral matter-0.5-1 %.

7.1. Degumming/Scouring

Degumming is the process of removal of sericin a natural gum in silk. It is usually done in a slightly alkaline condition with soap or enzyme to improve its luster and softness.

7.1.1. Degumming with soap

Majority of silk is degummed using soap. Commonly an olive oil soap called as Marseille soap (25% owm) is used. Temperature is maintained at 90°C -95°C for 1-2 hours. Sodium bicarbonate or sodium carbonate (pH 10) can also be used along with soap. Then the fabric is washed with hot and cold water to remove hydrolyzed gum. The alkalinity of soap and its detergency action hydrolyzes sericin and also removes other impurities.

7.1.2. Enzyme degumming

Proteins in silk are converted to water soluble amino acids using Protease enzyme. Sericin is removed by treating silk in 1-2 g/l Trypsin at 50-60°C for 45- 60 minutes .Then it is rinsed in hot water. The impurities like waxes and stains are removed by treating silk in1 g/l soda ash at 80-90°C for 20 minutes. It is then washed with cold water.

7.2. Bleaching

Naturally silk is cream in colour it is bleached with hydrogen peroxide to get pure white colour . Degummed material is entered at 75°C in a bath containing 15-20 ml/l hydrogen peroxide and 2gm/l Sodium silicate using m:l ratio 1:20.Temperature is raised to 80°C in 1 hour and the material is worked for 1 hour then rinsed in warm and cold water and dried. pH is maintained at 8.5-9.

8. RAYON

Is a manufactured fibre and so does not have any inherent impurities. Rayons do not require to be bleached unless bright shades are required.

8.1. Viscose

Oil and grease is removed by Trichlro ethylene and rust marks by treatment in ammonium oxalate at 50˚C for 20 min. It is bleached with cold hypochlorite solution containing 1-5gm available chlorine, pH 10-11 with soda ash for 1 hour. Then washed and soured with dilute hydrochloric acid and washed. Hydrogen peroxide may safely be used on viscose rayon.

8.2. Cuprammonium rayon

Although cupprammonium used on it. A very mild solution of thoroughly rinsed immediately. is similar to viscose, Hydrogen peroxide should not be sodium hyphochlorite may be used and should be

POLYESTER

It is a chemical fibre and so does not have much impurity.

9.1. Scouring

The impurities which have to be removed from man-made fibers are usually oils incorporated during spinning and applied in weaving or knitting as well adventitious dirt. Mild detergent action is sufficient to clean these materials. Scouring can be done with Lissapol 1.5 parts, soda ash- 2parts at 70°C for 30 minutes.

9.2. Bleaching

Polyester needs bleaching only if pure whiteness is required .Sodium chlorite can be used for bleaching polyester. Bleaching is carried out in a bath containing 2-3 gm/l Sodium chlorite, 2ml/l formic acid and 1gm/l sodium nitrate at boil for 1 hour at pH-4. Optical brighteners can be used to give a brilliant white look.

Summary

Preparatory process is removal of impurities from the fabric to make it suitable for further processing. Singeing, Desizing, Scouring, Bleaching, Mercerizing , Heatsetting , Carbonizing ,Degumming are the common preparatory steps but varies from fiber to fiber based on the impurities present in the fiber .

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REFERENCE

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Steiner N, 1995, ‘Evaluation of peracetic acid as an environmentally friendly alternative for hypochorite’, Textile Chemist Colorist27(8), 29-32)
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Batra, S.H. 1985. Other long vegetable fibers: abaca, banana, sisal, henequen, flax, ramie, hemp, sunn, and coir. p.15-22. In M. Lewin and E.M. Pearce (ed.). Handbook of fiber science and technology. Vol. IV, Fiber Chemistry.Marcel Dekker, New York.
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Meenakshi Rastogi,Plant textile,Sanoli Publication,New delhi 2009 p.198