Pre-treatments of textile fibers

The term ‘‘pre-treatment” includes all operations of preparing the textile materials, such as fibres, Yarn. Woven and knit fabric for the subsequent processes of dyeing, printing and finishing. For all practical purposes pretreatments are carried out incontinuation of dyeing or printing and their equipment is pat of the wet processing plant.The main object of pretreatment is to impart a uniform and high degree of absorptivityfor aqueous liquors with the minimum possible damage to the Fibrous material. Thecotton fabric, for example, after the pretreatment should become free of all natural impurities like pectin, wax, protein and husks and the sizing chemicals comprising of adhesives and softeners. Besides high and uniform absorptivity the textile materialsshould have adequate degree of whiteness so as not to mar colour and brilliance of theapplied colours. Normally achievement of whiteness of about 80% remission (As. c100% reflectance from barium sulphate) and a D.P. of 1,600 to 2,000 are aimed at for thecotton goods.The grey cotton fabric normally undergoes the following pretreatment procedures:

•Inspection and marking
•Shearing or cropping
•Singeing
•Desizing
•Scouring
•Mercerizing
•Bleaching

Shearing and singeing are always carried out in open width but desizing, scouringand bleaching of woven fabric may be done either in rope or in open-width Forms by batch, semi-continuous or hilly continuous systems. The rope processing equipment iscomparatively less expensive but is not quite suitable for the heavier-weight and thewider-width fabrics. Such fabrics tend to develop “rope marks’’ that cause uneven dyeinglater. In the rope form, woven and knit fabrics are scoured and bleached in kiers(autoclaves), winches and jets for batch and in J box or a similar storage system for continuous working. In open width processing, jiggers, pad-batch and pad- roll machinesare used for batch system and J-box, U-box, normal steamer, pressure steamer, roller bedconveyor steamer or perforated belt steamer are employed for the continuous working.Choice of the equipment depends mainly on the volume of cloth to be processed but the present clay trend is to install the open-width continuous pretreatment machines.

Loose cotton Fibres, slubbing and yarn in both hank and packages of cone, cheeseor warp beams are scoured and bleached in package processing equipment. If the yarn isto be mercerized later, pretreatment is carried out on hanks because yarn is commonlymercerized in hank form. Hanks are made into a chain by linking together in the formshown in fig.4. 1 and are then packed in kiers for scouring.

A brief description of the above pretreatment processes and the equipment used inthese is given below. Mercerization of cotton fabric and yarn is a special pretreatmentthat can technically he consider a finishing process.

1.Inspection and Marking

 In composite or vertical mills, the grey cloth is thoroughly checked to identify itsdefects and to determine its quality. Inspection is carried out on slanting ground glass plate tables that are illuminated from below by neon tube-lights. The machines are o lienfitted with ultra-violet ray illumination system that spots oil stains and variation indifferent qualities of the cottons that are blended prior to spinning. The cloth is pulledover the glass plate either manually or by a variable-speed motor and different clothdefects are marked and recorded for the quality control purposes. The fabric pieces arenext marked, for identification purposes and flat stitched end to end to make a continuouslength. Marking done with special ink that would withstand subsequent contact withscouring and bleaching chemicals. This indelible ink is usually made with mineral pigments dispersed in a solution of chlorinated rubber binder.

2.Shearing


Purpose of shearing is to remove loose threads and fibre tufts on the surface of thefabric. These are shaved off by carefully running cloth in close contact with razor-sharphelical blades mounted on steel cylinders that are rotating at a high speed in a directionopposite to the movement of the cloth. Shearing is not always necessary for fabrics meantfor bleaching or dyeing but is useful for printing that requires a fabric with a very smoothface. For the same reason, the cloth meant for printing is sometimes sheared after bleaching.

3.Singeing


Fabrics made from staple fibres show protruding fibre ends at the surface of clothand this nap disturbs appearance of the dyed and the printed fabrics. To improve the surface appearance of fabric, the fibre ends are removed by singeing with a flame or a hot plate. Cotton yarn especially that used for sewing thread is also singed to give a smooth appearance.
 Prior to singeing, the fabric is brushed to loosen and raise the fibre ends andthen rapidly moved at a speed of around 100 meters per minute, over a row of gas burnersto burn projecting fibres without scorching the cloth.Singeing can also be done by contact with hot plates but the flame singeing is preferred because flames penetrate into pores of the material, in some machines indirectheat is provided by a red-hot clay plate that is heated by the gas flame.

The best result is achieved by singeing after desizing but it is not practiced because itinvolves an extra drying of the fabric. The burner flame should be free from soot to avoiddiscoloration of the material and so only the more volatile and non-aromatic fuels areused for this purpose. An exhaust fan continuously removes fumes of the burnt fibres.Modern singeing machines have proper safety arrangements against fire hazardsand fuel supply is cut oft immediately on an unforeseen stoppage of the machine. Themachine room has also fireproof doors that automatically close in ease of an accidentalfire to eliminate any chance of spreading fire in the entire department. To quench anylingering flame, the singed fabric is directly fed into a trough full of water that usuallycontains a desizing agent also.

4.Scouring


Cotton goods as yarn and woven and knit fabrics contain about 8 to 10% of natural impurities and the woven fabric carries additional 5 to 7% of the sizing material.While most of the sizing material is removed from the cloth during desizing, the naturalimpurities require treatment with strong alkalis like caustic soda at elevated temperaturefor their breakdown and dispersion in water.For this purpose, a variety of equipment is available to process the materials indifferent forms and by batch, semi-continuous and continuous processes. The ultimateaim of scouring is to make the material tin formally and highly absorbent in a costeffective manner so that there are no difficulties in the later processes of dyeing, printingand finishing. Some of the more common batch and continuous scouring machines aredescribed below.

a.High Pressure Kier


A kier is cylindrical steel autoclave that is capable of withstanding high steam andhas a capacity varying between 1 to 3 tons. To eliminate formation of rust marks. kier iscoated with a mixture of lime and sodium in silicate that is periodically renewed. Kier hasa false bottom at the base on which the cloth pieces, already sewn end to end to loon arope, are piled up to its entire height. A manhole with a pressure lid is provided at the topfor entry of the fabric rope that may be fed manually or mechanically by an overhead.After closing the manhole the scouring liquor is circulated by a centrifugal pumpafter collecting it from the bottom and sprayed at top of the fabric pile through a circular perforated pipe.

On its way up, the liquor is heated with steam indirectly in a multi-tubular heater, an air escape valve at top is left open till the liquor starts boiling and steam starts issuing from it for a few minutes. This is done to remove all the air from the kier otherwise oxygen in the air will react with cellulose to for oxycellulose that causes severe tendering of the material. Composition of the scouring liquor, its temperature and duration of the treatment depend upon density of the fibre to be processed and ultimatedegree of absorption required.A typical recipe, based on the dry weight of the fabric and a liquor ratio of 4:1, is as under:

•Sodium Hydroxide3.0%
•Sodium Silicate0.5%
•Wetting Agent0.1 %

At elevated temperatures sodium hydroxide completely breaks down proteins and pectates that are mainly present in the cuticle layer and converts these into water-soluble products. The oils and fats are converted into soap and this in turn emulsifies waxes thatare removed by washing later. Residues of leaves and husks are degraded but are notcompletely removed by hot caustic soda solution. However, the degraded substances aredestroyed in the subsequent hypochlorite or peroxide bleach. Sodium silicate by virtue of its colloidal nature serves to keep the reacted impurities suspended and reduces their tendency to settle on the scoured fabric.

The wetting agent that may be amixture of anionic and non-ionicsurfactants for having synergisticeffect reduces surface tension of the scouring liquor and so helps inits quick penetration into the fibres.The wetting agents should be ableto withstand high temperature andstrong alkaline solutions. Theliquor is rapidly raised to atemperature of 130°C and is keptcirculating for 6 to 8 hours. Whenscouring is complete the brown colored liquor is discharged, preferably in stages with additionof hot water in to the kier so that the impurities do not settle and stick to the fibres and are removed morecompletely. After washing fabric in the kier, the remaining impurities are removed in arope washing machine (Fig.) preferably with a counter-current flow of water.

5.Desizing

To improve efficiency of weaving, warp yarns are ‘sized’ i.e. coated with anadhesive containing an oily product to prevent their fraying and breaking during weaving.The sizing materials hinder absorption of water or dye-liquor and so their removal isessential prior to attempting any wet process. The cotton warps are usually sized with a preparation based on starch. Starch swells hut does not dissolve in water and dispersible products by either certain enzymes so it is removed by breaking it down into water or oxidizing chemicals. The latter products are comparatively expensive but are faster acting and so are suitable for the continuous processing.

1.Enzyme Desizing

The enzyme preparations are of 3 types. Diastastic, Pancreatic and Bacteria!.These stereospecific enzymes split a-glucoside bonds of starch but not the 3-glucoside bonds of cellulose. The enzymes break down starch (amylose) chains into water-solubleor water- dispersible products. Activity of the enzymes depends on pH temperature and presence of activators (Na+, K +and Ca++ ions) and also the wetting agents.Of all the enzymes the bacterial amylase are the most popular because these arefaster in action and more tolerant to pH and temperature variations. The quenching box of singeing machine contains about 1 % solution of the enzyme with 5g sodium chloride.The optimum pH and temperature of the solution is 6-7 and 60-70 at 18°C respectively.The fabric saturated in the solution is batched on a roller rotating at about 4 rpm. Toimprove penetration, the material is padded twice with an intermediate passage on thetiming-guide rollers before batching.The roll of cloth is wrapped in plastic sheet to avoid drying of fabric and slowlyrotated for a couple of hours. The degraded starch products are then removed by washingthe fabric in boiling water in a washing range. Addition of 5 to 10 g/l of sodiumhydroxide in the first two washing cisterns promotes the washing efficiency. In the earlier practice, the amylase soaked fabric rope was piled in wooden tanks or cement pits andafter about 24 hours the degraded starch was washed off with boiling water in a rope washing machine.

2.Oxidative Desizing

Under controlled conditions, certain oxidizing agents decompose starch into water dispersible particles without damaging cellulose of the fabric. The starch degradingaction is sufficiently rapid to make the process compatible with the continuous fabricscouring and bleaching system. Another advantage of the process is that motes and other non-cellulosic impurities present in fibers are also attacked resulting in partial break down and this helps in their removal in the subsequent scouring and bleaching processes.The more popular oxidizing agents are Sodium Bromite (NaBrO3), Hydrogen Peroxide (H2O2) and Ammonium or potassiumPersulphate (NH4SO4)2Sodium Bromite has provedvery effective.

For rapid oxidation of starch and is applied in a concentration oil 1 g/l of active bromite at a pH of 10 to 10.5(brought with Disodium Hydrogen phosphate-Na2HPO4) at70-90 temperature at 180°C. After a short dwell time of about 20 minutes, thedecomposed starch products are thoroughly washed off with boiling water’ in a washingrange. Alternatively the oxidative desizing agent is added into the scouring liquor toreduce the production time.The water soluble sizes like carboxymethyl cellulose (CMC), Polyvinyl alcohol(PVA) and acrylates need no chemical desizing and only thorough washing with hotwater is enough for their removal, Addition of 1-5 g of a wetting agent and provision of alittle dwell time is helpful in removal of the size especially those containing PVA thattakes comparatively longer time to swell and dissolve in water.

6.Bleaching 

Scouring removes almost all the impurities of cotton fibres except husk amnatural coloring matter that are ultimately removed by oxidizing agents. The oxidationtreatment or bleaching is necessary for producing white goods as finished products or for dyeing paste shades. Even for dark shades, bleaching improves the brilliance andevenness of the shade. Bleaching is normally carried out with hypochlorites, hydrogen peroxide or sodium chlorite.

1.Bleaching with Hypochlorite

The active constituent of the hypochlorites is chlorine o hypochlorous acid that isstabilized as calcium or sodium salts. In earlier times, the calciun compound known as bleaching powder was used exclusively. It is a mixture of calcium hypochlorite Ca (OCI)2, and basic calcium chloride, CaCl2Ca (OH)2.H2O .Bleaching powder being solid is easyto transport and has long shelf life but its use involves excessive and careful handling indissolving and removal of the non-oxidizing solid constituents.Sodium hypochlorite has now replaced bleaching powder because it is moreeconomical at easier to handle. 

It is often prepared in the mills by dissolving chlorine gasslowly in a cold solution of caustic soda or sodium carbonate as per following reactions

2 NaOH + Cl2 → NaOCl + NaCI +H2O
Na2CO3+ Cl  →   NaOCI + NaCI + CO2

Chlorine is abundantly available as a by-product in the electrolytic manufacture of sodium hydroxide. A percentage of 15- 18% of available chlorine are generallymaintained in the concentrated hypochlorite solution but it may be noted that theconcentration falls gradually during storage and needs to he checked he lore use.

Hypochlorites are strong oxidizing agents and it is, therefore, necessary to carryout bleaching under the prescribed conditions of pH, temperature and time so as to avoiddamage to the cellulose. Careful control of the pH is very necessary because theoxidizing entity changes with variation in pH. At pH above 100 the oxidizing componentexists as hypochlorite ion (-OC1-) but at lower nil between 5 and 8.5, it is converted intohypochlorous acid. At pH below 5 liberation of chlorine gas starts and at pH 3 whole of the hypochlorous acid is converted into chlorine. It is well known that the rate of bleaching or oxidation of cellulose becomes maximum between pH 5 and 9 and that pHrange corresponds to maximum generation of the hypochlorous ion.For the minimum damage to the cotton goods bleaching should, therefore, bedone at pH 10-11. Sodium hypochlorite solution has a pH of 11.5 but during the bleaching reaction, hydrochloric acid is generated and pH of the solution starts falling. Tocounter this, tile hypochlorite solution is buffered with 5-I 0 g/l of sodium carbonate.Rate of bleaching also depends on temperature and rises with increase in the reactiontemperature. Normally bleaching is carried out with 5 to 9 g/l of active chlorine at room temperature for about 2-4 hours but it is quite sale to process at 40°C to reduce the production time. Contact with copper and iron metals should be avoided as these catalyze the oxidation reaction and may tender the goods. Equipment for bleaching may be same as that used for scouring.

2.Discolorination or anti-color treatment

It is necessary to remove traces of the residual chlorine alter bleaching otherwiseit may damage cotton goods during storage. This is done either by treatment with 5 g of sulphuric or 20 g/l of hydrochloric acids or with 5 to 10 g/1 of sodium bisulphite(NaHSO3) solution at 60°C for 30 minutes. The acid converts hypochlorite into volatilechlorine while the bisulphite reduces it into a harmless sodium or calcium chloride.In an alternative method alkaline hydrogen peroxide (3cc/I H2O2+ 0.3 g/ NaOHat 90°C) is used that besides dechlorination improves quality and stability of thewhiteness. This principle is also a basis for a continuous bleaching process for cottongoods the reaction may be expressed as under:

HOCl + H2O→2HCl + H2O +O2

3.Bleaching with hydrogen Peroxide

The oxidizing agent most commonly used today in bleaching of textiles ishydrogen peroxide. It is used in batch; semi-continuous and continuous processes as wellas for bleaching colored goods and for combined scouring and bleaching of thelightweight cotton goods. Other advantages of peroxide over the hypochlorites are lower loss in weight of goods, reduced oxidative damage and economy in water usage and solesser cost of effluent treatment t. The bleaching equipment is the same as that used for scouring for both the batch and continuous processes.In the latter, two storage chambers can either be installed in tandem for scouringand bleaching or these processes may be done in one chamber alternately. Hydrogen peroxide has a low dissociation constant and is a weak acid. In an alkaline solution the peroxide anion is produced that is the active bleaching agent (Reaction 1 and 2). In asecondary reaction (3) molecular oxygen is formed that has no bleaching action.

H2O2+ OH = H2O + HO2-………………………………….1
HO2-= OH-+ O………………………………………………2
2H2O2+ 2H2O + O2………………………………………….3

A high concentration of the hydroxyl ions has an accelerating effect on the rate of bleaching and so to control the tale of reaction it is necessary to add a stabilizer. The most common stabilizer is sodium silicate and it also gives protection against metalcontaminants. Silicate is more effective in the presence of salts of magnesium and this provides a rare case where hard water is preferred over the soft type. Magnesium salts are sometimes added into the peroxide bath in calculated concentrations.A disadvantage of the silicate is that it deposits hard insoluble incrustations ontile sides on the side of machine, which scuff the fabric and the scuffmarks show in dyeing. For this reason silicate is sometimes replaced partly or completely with tetra sodium pyrophosphate (Na2P2O2) or organic proprietary stabilizers.

A typical recipe for a wet on wet bleaching in a roller bed continuous system is asunder:

Hydrogen Peroxide 35% (135 volume) 50-60 ml/l
Sodium Silicate 30°Be 10 ml/l
Organic Stabilizer 10 g/l
Sodium Hydroxide 3 g
Wetting Agent 1-2 g/1
Liquor Pick-up 100%
Impregnating Temperature 20-30°C
Reaction Time 1-2 hours at about 95°C.

Concentration of the reactants and the time of treatment nay however, varyaccording to the degree of impurities and quality of whiteness required. Concentration of the chemicals in the feeding bath is about 3 times of the above mentioned concentrationsin the padding bath but that of hydrogen peroxide needs to he controlled by checkingwith periodic titration, usually with the standard permanganate solution. In the semi-continuous pad-roll process time of treatment is usually 4 to 5 hours.

Addition of certain peroxide ‘activators’ like tetra acetyl ethylene diamine in the bleaching bath is claimed to achieve good bleaching at comparatively lower temperaturesand pH. It also causes minimum damage to the fibres and is specially recommended for bleaching viscose and spandex blends to protect the fibres from fibrillation.

4.Cold Pad-Batch Process

This process is very simple and consists of padding fabric with 40- 50 ml/lhydrogen peroxide (35%), 15-20 ml/l sodium silicate 38° Be, 10-15 g/l caustic soda and3-5 g/l of a wetting agent to a pick-up of 100%. The batch roil is covered with a plasticsheet and slowly rotated (4 rpm) for 15 to 24 hours and then washed in an open width washing range with boiling water. The process has the advantages that it requires inexpensive equipment and is free of danger of catalytic damage to the fibres. However,the degree of whiteness is inferior to that obtained by the steaming process.
If deep shades are to be dyed, bleaching of the grey fabric is occasionally done inone step by the cold-hatch or the pad-roll process. However, the process is not always satisfactory because impurities like traces of the heavy metals in cotton have decomposing effect on the peroxide and may also cause tendering on the fabric.


5.Bleaching with Sodium Chlorite: 

Sodium chlorite (NaClO2) was originally introduced for bleaching the synthetic fibres but is now finding increasing use for cottongoods because of all the bleaching agents it is the least damaging to cellulose. It is sometimes used for bleaching grey cotton goods without prior boil-out but absorbency obtained is just tolerable. In the grey bleaching the impurities of cotton are not removed but are oxidized to colorless products and so there is very little loss in weight of the goods after the treatment.Sodium chlorite is commonly marketed in the form of a crystalline powder in80% strength but it is available in liquid state also. The aqueous solution of the chlorite isslightly alkaline and has a pH of about 8.5. However, it must be acidified to a pH valuewithin a range of 3.5 to 4.5 to liberate the oxidizing agent. Doubts exist about the realoxidizing entity and different workers have suggested this to be chlorine dioxide (ClO2)chlorous acid (HClO2) or even atomic oxygen.

Chlorine dioxide gas is poisonous and explosive and is also very corrosive tometals in the aqueous medium. Chlorite bleaching is therefore often carried out in anexclusive room in the dye house that is very well ventilated. The machines are fabricatedfrom special stainless steels that have a high proportion of molybdenum or titanium.

Alternatively bleaching is done in equipment made of stone, PTFE coated steel or wood.For protection of the stainless steel metal of the machines, it is not uncommon to addsodium nitrate in a quantity equal to the chlorite that moderates decomposition of thechlorite and inhibits corrosion of metals.The pH of around 4 ± 0.2 required for bleaching is maintained with buffers or astermed in industry activators, like sodium acetate or sodium dihydrogen phosphate (NaH2PO4) Latter is usually preferred because it improves whiteness o goods. Neutral or slightly acid chemicals that liberate acid on heating are also used occasionally. Organicesters like ethyl lactate or titrate and their ammonium salts are also suitable for this purpose. Common recipes for the batch and the continuous bleaching are given below:

Chemicals                                        Batch Process           Continuous Process

Sodium chlorite (80%)                      5-7                              20-25g/l

Sodium nitrate                                  2-3                               2-3g/l

Sodium dihydrogenphosphatce         2-4                                0.5-1.0g/l

Wetting agent                                   1-2                                1-2g/l

Formic acid to adjust pH to              3.8-4.                            26-6.5

Temperature                                    85-90°                           85-90°C

Reaction time                                  1-3                                  2-4 Hours

After the treatment, washing with boiling water and dechlorination with 0.5-I %cold sodium bisulphate solution is carried out.

6.Bleaching of Knit Fabrics

Cotton tubular knit goods are commonly scoured and bleached by a one step process in winch or jet machines using 4-5 ml/l caustic soda solution (38° Be), 5-8-ml/lhydrogen peroxide (35%) with a peroxide stabilizer and a wetting agent. Semi-continuouscold pad-batch method has also been developed in which the fabric is first padded withreactants under a minimum of tension, wrapped on a perforated stainless steel tube and covered with a plastic sheet. After the due reaction time, water is forced through the tube
into the batch of the fabric and the oxidation-degraded products are washed off. M/sDornier has modified their “floating cigar’ mercerizing machine. For continuous scouringand bleaching of the knit fabric. The fabric may be mercerized first leaving about 2%alkali in the material during washing off The wet fabric is next padded with hydrogen peroxide its stabilizer and other auxiliaries and stored in a roller-bed steaming unit for 1-2 hours. The bleached fabric is finally washed again on the floating cigars thus making the bleaching process a semi-continuous one.It may not be quite out of place to mention that certain direct dyes are applied tothe unbleached (grey) knit fabrics in the presence of hydrogen peroxide and caustic soda,which is a very economical process as scouring, bleaching and dyeing are combined.Recently it has been suggested that combined scouring and bleaching at a temperature of 130°C in a high-pressure jet machine can save time, steam, water and electric power without sacrificing whiteness and absorbency of the finished material. The maindifference of the process from a conventional one is that rinsing of the fabric is started atabout 120°C by simultaneous feeding of fresh water and draining of reactant liquor whilethe system is still tinder pressure.This technique reduces a considerable quantity of water that is normally requiredfor washing off Moreover treatment with hydrogen peroxide and washing off attemperatures above the boiling point of water reduces the processing time from 170minutes required for the conventional process to only 70 minutes. The method essentiallyconsists in raising temperature of the liquor to 130°C in 40 minutes, treating for 5minutes at that temperature and then adding a hydrogen peroxide killer. To save time rinsing is started during the liquor cooling stage and continuous addition of cold water and discharge of hot liquors is carried out simultaneously. The rinsing stage is completedin 25 minutes. It has been calculated that the high temperature process saves about 25%water, 60% electric power, 6% steam and 59% time as compared with the conventionalsystem.

7.BLUEING

After bleaching, the cellulosic materials retain a slight yellowish tone to which the human eye is very sensitive to detect. The principle of blueing is to neutralize this tone with a blue or violet light and this is achieved by the following two procedures.

1.Blueing by Subtractive Effect

In this cotton is shaded with a blue or violet disperse pigment dispersion that arenot substantive to cellulose. This application suppresses the yellow tone but slightlyreduces brightness of the white. Disperse dyes; vat pigments and Ultramarine Blue arecommonly used for this effect.
2.Blueing by Additive Effect or Optical Brightening

Certain organic compounds have the property of fluorescence i.e. they can absorblight of short wavelengths and re-emit it at the longer ones. Many of such compoundsabsorb ultra rays and re-emit as visible light in the range of 4,000 to 7,000 A0. Textilefibres containing a fluorescent compound reflect more light than an untreated one andthus increase its brightness and whiteness.
During the last half century a large number of fluorescent chemicals, also termed as optical brighteners or FBA have been developed for both natural and man-made fibres that have affinity to the fibres and most of these can also he applied during the bleaching process.Chemically these can be classified as derivatives of stilbene and behave like thedirect dyes for the cellulosic fibres. These can be applied by both exhaustion and paddingtechniques in concentrations that would fix about 0.2-0.4%. of it on the fibres. For cottonfibres, these are generally applied along with the peroxide during bleaching but only aselected few can withstand the hypochlorite. It may be kept in view that the degree of brightness obtained with the FBA depends on proportion of the ultraviolet light in theincident light and is less apparent in the artificial light than the sunlight