SOFTENING AGENTS

Softening agents are intended to impart a pleasant handle and smooth feel to the treated material. The softeners are of five types viz. anionic, cationic, non-ionic, reactive and emulsion.

Anionic Softeners

Chemically, these are sulphated oils and fatty alcohols of high molecular weight. They have good heat stability and are compatible with all dyes except basic dyes. They do not possess substantivity to fibre and are not wash fast. They are generally used with starch based temporary finishing formulation.

Cationic Softeners

This class of softeners is the most widely used due to their high degree of substantivity (especially to acrylic fibres) and effectiveness at very low concentration. The subclasses of these softeners are long chain amides, imidazo lines and quaternary nitrogen compounds.

Nonionic Softeners

These softeners are generally based on ethoxylates and esters. Compounds of fatty esters are probably the most widely used. In addition to softening effect they impart a high degree of lubrication. However, they do not have substantivity and are not fast to washing.

Reactive Softeners

The softening agents described in the foregoing are merely deposited on the material i.e. they are not fast to washing. Reactive softening agents can partially react with the cellulose on account of their chemical structure and thus give permanent effects. The most common reactive softening agents are:

N-methylol compounds of the higher fatty acid amides, e.g. N-methylol stearic amide.

N-methylol compounds of urea substituted with higher fatty acid, e.g. octadecylethylene urea (Octex EM).

These are popularly used in resin finishing and have a water repellent as well as softening effect.

Emulsion Softeners

These are mostly non-ionic emulsion types. The characteristics of this type of a softener is that it is a highly insoluble compounds (such as high melting wax or

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silicone oil) emulsified in a suitable emulsifying system, to form a stable composition which maintains its finely divided state in all dilutions and at all temperatures for treatment of fabrics. These emulsions also impart excellent lubrication and thereby enable improvement in tear strength and abrasion resistance. There are two main types of emulsions, viz. wax or polyethylene and silicon. Although these products may be used alone as finish, they are more commonly used in combination with other finishing formulation.

Vat Dyes

Vat dyes are characterized by all-round fastness properties. These are water insoluble dyes and cover almost full range of shades. Greens, yellows, blues etc. are most commonly dyed shades with vat dyes. Vat dyes can be broadly grouped into two categories – anthraquinone and indigold; anthraquinone class produce bright shades and change their colour in water soluble form. In the application of vat dyes, first step is to convert the dyes into their water soluble form. This is achieved by reducing or vatting the dye with a reducing agent (sodium hydrosulphite) in a strong alkaline medium created by sodium hydroxide. In this way dye is converted into sodium salt of reduced dyestuff – popularly known as leuco dye – which is soluble in water and possesses great affinity for cellulosic fibres. On exhaustion, oxidation is carried out to convert the dyes back to original form, i.e. vat dyes. These dyes, depending upon their chemical structure, require different dyeing conditions. As a practical approach, however, the class has been divided into four categories or groups namely IN, IW, IK and IN Special. Table-1 gives at a glance vatting and dyeing temperatures and quantities of chemicals required for each of the group.

Table 1 : Conditions in dyeing with vat dyes (powder) in jiggerDyeingMethod

Temperature (oC) Chemicals (g/l)Vatting Dyeing Sod. hydro-

sulphiteCausticSoda

CommonSalt

IN Special Individual Treatment 15-25 2-530 -IN 60 55-60 7.5-15 10-18 -IW 50-55 45-50 7.5-15 7.5-15 15-20IK 40-45 35-40 7.5-15 5-10 15-20

The quantities of chemicals are for dye-bath addition. For vatting, however, higher concentration of chemicals is needed. As a general rule, caustic soda and sodium hydrosulphite are taken 1.0 to 2.0 times the dye amount. The vatting is achieved in about 15 minutes. Here, care needs to be taken in that the dye is not over-reduced, otherwise lower colour yield would result. Sodium hydrosulphite is the most popular and effective reducing agent. Another reducing agent – thio-urea dioxide is now gaining popularity. Claims have been that by the use of this reducing agent substantial savings in chemicals cost can be achieved. It seems however that thio-urea dioxide can replace sodium hydrosulphite only partially; 50% of sodium hydrosulphite can be substituted by taking thio-urea dioxide to the extent of 10% of the total amount of hydrosulphite needed. In India thio-urea dioxide is now manufactured indigenously and one such product is Reduction HF.

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Vat dyes are marked in two forms – powder and paste. The latter is exclusively used in printing. The powders are also available with varying particle size such as – powder, powder fine (P/F), ultra disperse (U/D), highly concentrated (H/C), supra fine (S/F), micro fine (M/F) etc. These differ in their relative dye strength and therefore, while switching over from one form to another shade cards should be referred. The vat dyes can be applied to fabrics by the following methods:

Exhaust dyeing in jigger

Pad-jig develop

Continuous dyeing

Exhaust Dyeing in Jigger

In this method mostly powder and powder fine dyes are used. The dyes are stock vatted and then added to the bath made-up with the required quantities of sodium hydrosulphite and caustic soda. The vatted dyes are added in two parts – preferably 60-65% initially and the remaining after the first end. While dyeing very deep shades, vatting should be carried out in the bath itself. The vatting in the bath (long liquor) is also recommended for black, magenta etc. dyes. Sometimes, leveling agent is also added to the bath. The leveling agent reduces the strike of the dyes and thus helps in getting level dyeings. However, in some cases, the addition of leveling agent reduces the exhaustion of the bath considerably and therefore shop-floor trials should be conducted to ascertain the need and optimum addition of leveling agent. The bath is raised to required temperature preferably by indirect steam heating as direct steam heating causes rapid decomposition of sodium hydrosulphite. After two ends, common salt (if required) is added in two parts and two to four more ends are given. Common salt is not needed for the dyeings of IN Special and IN class. The bath is now dropped and fabric rinsed cold and hot. This is followed by oxidation with an oxidizing agent and a thorough hot soaping with 2 g/l each of detergent and soda ash at 70-80oC. In case of pastel shades, oxidation can be achieved by running the goods in empty jigger i.e. the air oxidation. Sometimes, particularly in deep shades an acetic acid rinse after the cold rinse is given to remove caustic soda. The true tone and depth of shade is attained only after hot soaping.

During dyeing, care should be taken that both sodium hydrosulphite and caustic soda are adequately present in the bath. Their presence can be checked by Caledon Yellow paper – which turns blue – and red litmus paper which turns blue, respectively. Insufficient quantity of sodium hydrosulphite leads to oxidation marks. The oxidized dyes possess greater affinity for the soluble dyes and thus result in deep patches. If caustic soda is insufficient in the bath, insolubilisation of dyes occur leading to specks and loss in depth of shade. Oxidation marks also appear if the fabric is exposed to air without having sufficient liquor. The batching of fabric should also be razor sharp otherwise the problem of deep selvedges is encountered. Enclosed jiggers should be preferred as the loss of sodium hydrosulphite due to oxidation is reduced.

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Pad-Jig Develop

This method is particularly suitable for heavy and light construction fabrics. It also produces level dyeings with minimum batch-to-batch variations. The dyes are spread evenly on the fabric and little time is given for preferential absorption of one component over other. Differences in fabric structure such as reediness is also minimized. The fabric is passed with dispersion of dyes and then either wound on a batch or dried before development in a jigger. The intermediate drying does result in 5-10% more depth on development, in comparison with wet-on-wet development but involves a drying step which is quite expensive these days. Secondly, drying on many occasions leads to migration causing unevenness. Wet-on-wet development under the circumstances is preferred. Strict control on wet-pickup is a must so as to ensure repeatability of shades. The padding of fabric is carried out either with pigment dispersion or with vat acid. The development is done like exhaust dyeing. The strength of pad-liquor should be adjusted in such a way that about 90% of required dyes are applied by padding and 10% added to the developing bath.

For preparing pigment dispersion S/F, U/D, M/F forms of dyes are used. For preparing pigment dispersion the dye is stirred in a large amount of water at 45-50oC. The dispersion …………………..

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DYEING – BACKGROUND NOTES

Dyeing is the process whereby dye molecules, thoroughly dissolved or dispersed in water or some other carrier, are able to penetrate and colour textile materials. Dyeing can be carried out at the polymer, fibre, yarn, fabric and garment or product stage. Dyes are substances with a special capacity for reflecting light. They have a distinctive structure and are made up of a colour bearing component called a Chromophore and an Auxochrome which gives a dye its solubility and ability to attach itself to a fibre.

Each class of dye has a unique chemistry, structure and way of bonding to the fibre. Some dyes react chemically with the fibre forming strong bonds – others are held by physical forces.

CLASSES OF DYES

Direct Dyes

Direct dyes are a relatively inexpensive and easy way of dyeing natural cellulosic fibres like cotton and regenerated cellulosic fibres like viscose rayon although they do not have good fastness to washing or other wet processes. Direct dye molecules are large and enter the cotton or viscose fibre from the dye-bath seeking a place to bind to the fibre. Hydrogen bonding and Van der Waals forces help bind the dye to the fibre.

Fastness properties may be improved by other treatment – a past dyeing chemical treatment.

Reactive Dyes

Reactive dyes form strong covalent bonds with cellulosic fibres like cotton and regenerated cellulosics like viscose rayon. The formation of the covalent bond between dye and fibre means reactive dyes give extremely high wash and wet fastness properties.

Vat Dyes

Vat dyes are used to dye cotton and viscose rayon. Vat dyes are insoluble and so cannot penetrate the fibres in solution. They can however be reduced to a soluble form called the leuco form in the presence of alkali and a reducing agent. Vat dyeing is a multi-stage process:

the insoluble vat dye is reduced to a soluble leuco form to be applied to the textile;

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the leuco molecules are then oxidized to be insoluble once more and develop the colour inside the fibre. Vat dyes have excellent wash fastness properties but the colour range is more limited and more expensive.

Sulphur dyes are similar to vat dyes but are cheaper, less environmentally friendly and are limited to flat dull colours. They also have poor fastness to sodium hypochlorite.

Azoic Dyes

Azoic dyes are applied to cotton and viscose rayon. The textile is impregnated with a naphthol based, coupling compound and immersed in a dye-bath containing a diazotised base triggering a precipitation reaction. The colour is manufactured inside the fibre by the coupling of the two components.

Since the dye molecules are large and insoluble, they have excellent wash fastness properties. Poor rub fastness can be a problem due to dye formation on the textile surface. Insufficient after-washing will give poor fastness to wet treatments.

Acid Dyes

Acid dyes have a direct affinity for protein fibres and are the main class of dyestuff for dyeing wool. Nylon also has an affinity for acid dyes. The attraction between dye and fibre is the result of negatively charged dye particles called anions associating with positively charged basic groups in the fibre generally under acid conditions.

Disperse Dyes

Disperse dyes are applied to polyester. Polyester has a tightly packed molecular structure called a crystalline structure. It is hydrophobic or water heating. Heat opens up the crystalline structure to allow disperse dye molecules to enter the fibre from solution where they have been held in suspension. The dye is trapped in the fibre upon cooling and held by physical forces to produce good fastness properties. Disperse dyes may be applied at elevated temperatures from pressurized vessels or at the boil with the assistance of a chemical called a carrier.

Basic Dyes

Acrylic fibres are dyed with the brilliant and intense modified basic dyes. Basic dyes are positively charged or cationic. These positively charged cations are attracted to negatively charged anions in the acrylic fibre. The reaction of the cation and anion form salt linkages and the fibre is coloured with good wash and light fastness properties.

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Pigments

Pigments are water insoluble colourants that can be applied to most fibre types. They are coloured particles dispersed in an aqueous paste containing a binder. Through various methods, the pigment paste is then applied to the surface of the textile and the binder is then cured.

DEVELOPING A COLOUR RANGE

What’s involved in the dyeing process and what process is used to select a dye for a given application. Modern dye-houses use a wide range of equipment for formulating dye recipes and dyeing textiles. Many dyeing processes are computerized and highly automated.

Information gathering and the generation of ideas are key elements in the design process. At the information gathering stage, market research – what the market is doing and what it is seeking is undertaken. Working with customers to ensure their needs are met is essential.

Almost all product is subject to seasonal variation and demand. Dyes used in industrial textiles must rise to meet new and exacting technical requirements, whilst apparel must meet the ever changing and fickle fashion markets.

Once the information is gathered and assimilated, samples can be generated. In the lab, colour swatches either collated by designers or supplied by customers are placed in a spectrophotometer – an instrument used to measure the amount of light reflected from a sample at a number of wavelengths in the visible spectrum in comparison to a white standard.

When the sample is placed in the spectrophotometer and a reading obtained, the reflected light is specified mathematically. Different combinations of dyestuffs can be mixed to assimilate the colour of the sample.

The information from the spectrophotometer is transferred to the computer. The computer assesses the reading and then seeks to find and name a combination of dyestuffs that will match the sample. The computer gives a number of dye recipe options that very closely resemble the original sample. The most suitable recipe is chosen but choice will be based on such things as price, colour fastness to washing and light, migration and exhaustion rates and leveling properties.

A recipe sheet is printed and laboratory trials are undertaken to ensure a good colour match and that the recipe formulated will produce properties that meet the customer’s end use specifications. Once a trial is successful, the recipe is ready for use in the dyehouse and production begins.

PRINCIPLES OF DYEING

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Migration of dye molecule from liquor to fibre. This process is assisted by increasing temperatures and using auxiliaries – substances that help the dyeing process.

Diffusion of dye from the fibre surface into the fibre. This process is assisted by agitation of the fibre, dye-bath or both together with heat.

Fixation ensures the dye molecule is attached to the fibre either by physical forces or chemical bonding. These forces may be weak or strong.

Most dyeing processes need heat to provide the energy for the dyeing to take place. This is commonly supplied by direct or indirect steam.

APPLYING DYESTUFFS – MACHINERY USED

Package Dyeing

Dyeing may take place at the yarn stage. Yarn dyeing is generally carried out on package dyeing machinery where the yarn is destined for sewing threads or knitting and weaving into striped or patterned fabrics. Yarn may also be dyed in the hank form. This form is most commonly used in the wool industry, particularly where the yarn is destined for carpet manufacturing.

Package dyeing is a method of dyeing textiles in yarn form. The yarn is first wound onto perforated plastic tubes or spiral springs. In this form the yarn is known as a package. The undyed yarn packages are loaded onto a carrier ready for dyeing in a package-dyeing machine. When the carrier is full, the packages are compressed and secured. The carrier holding the yarn is lowered into the dye vessel via an overhead crane. The vessel is closed so that it can be pressurized. Premixed dye is added to a tank at the side of the machine. During the dyeing cycle, the dye liquor will circulate constantly through the vessel and tank until all the dye is used or exhausted. The perforations in the tube allow the dye to flow through the yarn package.

Once exhaustion is achieved, the carrier of coloured yarn is removed from the vessel. Excess water is removed from the packages in a large centrifuge. The yarn is then dried in an infrared drying oven.

Winch Dyeing

Winch dyeing machines are a low cost design that is simple to operate and maintain, yet versatile in application proving invaluable for preparation, washing or after treatments as well as the dyeing stage itself.

In all winch dyeing machines a series of fabric ropes of equal length are immersed in the dye bath but part of each rope is taken over two reels or the

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winch itself. The rope of fabric is circulated through the dye bath being hauled up and over the winch throughout the course of the dyeing operation.

Dyestuff and auxiliaries may be dosed manually or automatically in accordance with the recipe method.

Jet Dyeing

The jet-dyeing machine moves the fabric together with the dye liquor. This reduces the strain on the fabric. As it is fully enclosed, the jet dyer can be pressurized and heated up to 130oC for dyeing polyester textiles and polyester blends with disperse dyes. The jet dyer uses less water – has a lower liquor ratio than the winch and is therefore more economical of energy, water and chemicals. The more gentle treatment of fabrics is also an advantage for fine or delicate fabric constructions.

Jig Dyeing

Jig dyeing is an effective technique for dyeing woven fabrics in open width to avoid creasing problems. A batch of fabric on one roller is gradually unwound and passes through a dye-bath of relatively low volume. As it moves through the dye-bath it is wound onto a second roller. When the second roller is full, the direction of fabric movement is reversed. In jig dyeing the duration of the process is normally counted in terms of the number of “ends” or passages of the fabric through the dye-bath from roller to roller rather than in minutes.

Atmospheric jigs operate at temperatures and pressures of atmospheric conditions. These machines are well suited to natural fibred goods.

The high temperature jig works in much the same way as the atmospheric jig but is a pressurized vessel designed to operate at 130oC. It is used for dyeing synthetic fibred woven goods with disperse dyes.

Beam Dyeing

In beam dyeing, fabric in open width is rolled onto a perforated beam. The beam is slid into a vessel that can be closed and pressurized. The dye liquor is circulated through the preformations in the beam and colour thus impregnates the fabric.

Padding

In cold pad batch padding fabric is passed through a dye-bath in open width or tubular form and then through padding mangles which squeeze out the excess dye. It is important that the fabric picks up a constant amount of dye liquor otherwise the depth of shade will vary from one part of the fabric to the other. After padding the fabric in open width form is taken up on an A-frame, wrapped in plastic and left to rotate whilst migration and diffusion take place. Fabric in

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tubular form is lapped onto a stillage. The stilage is wrapped in plastic and left for up to 24 hours for diffusion and fixation to take place.

Once the dye is fixed, the fabric is washed to remove any unreacted dye molecules that may cause poor fastness in use.

The cold pad batch padding system is most suited to the dyeing of cotton goods with reactive dyes.

Pad thermo-fixation is another method of padding. After padding, the dye is fixed by passing through a steamer or stenter to provide heat and energy for the dyeing process to take place. There may be an intermediate drying stage between padding and fixation.

Printing

There are two main methods of fabric printing – rotary screen printing and flat bed printing. Pigments and dyestuffs are frequently applied through these printing processes.

The first requirement is a patterned screen. Flat and rotary screens are produced by etching a design onto a metallic screen through a photochemical process to create the desired pattern.

Pigments are generally applied via a white binder or printing paste. This is prepared by mixing in a large vat. The colours to be printed are specified on a sheet for the operator. The various pigments specified in the printing recipe are added to the white binder and mixed thoroughly to achieve the required colour.

In rotary screen-printing the printing paste is supplied through a pipe inside the screen itself. A squeegee forces the paste through the holes in the screen and onto the fabric passing below.

In flat bed printing, the printing paste is poured into one side of the screen. The squeegee is then drawn across the screen to force the paste through the design and onto the fabric.

The printed fabric is dried in an oven at the end of the printing line. This process is similar for both flat bed and rotary screen-printing techniques. After drying, pigments are then cured and dyestuffs are heat fixed.

Garment Dyeing

Garment dyeing is carried out on made up garments or products. These products are often made from fabric, which has been specifically prepared for garment dyeing. Both dyes and pigments are used in garment dyeing. Both dyes and pigments are used in garment dyeing. Pigments are widely used in

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garment dyeing to achieve unusual or random effects such as washed out or distressed looks.

The machines used are related to commercial washing machines as these products require similar handling. Garments are loaded into the machine, the machine is filled with water and the garment wet out.

The dye-bath auxiliary chemicals are added to the machine. Once the auxiliaries have mixed with the water, the prepared dyestuffs are introduced. The dyeing cycle begins and steam is turned on to provide energy for dyeing.

Once completed, the garments are unloaded from the machine and placed in a centrifuge usually called a hydro extractor, to remove excess water. The garments are then unloaded from the centrifuge and loaded into a tumble dryer to dry.

As the garments may become creased and crushed during processing, the customer often requires them to be pressed, making them more presentable for retail sale.

Sanforising

Principal of Sanforising Machine

The sanforising machine, at present mostly in use, is to shrink the cloth both widthwise and lengthwise by means of an elastic blanket. The cloth is first damped and then enters a short-range stenter to control the selvedges. The damped cloth passes over a small diameter feed roller, which also carries the elastic blanket. Here the cloth is held in position by an electrically heated shoe. As soon as the blanket and the cloth adhering the blanket enters the large diameter of the palmer machine, the blanket gets relaxed and shrinks and as the blanket shrinks, the cloth which is in grip with the elastic blanket also shrinks and in this shrunken state, the cloth is dried and pressed. For producing the finish on both sides of the cloth, there is a similar set with double palmer arrangement and before entering the second palmer the cloth is again damped with spray. This method is known as “compressed shrinkage”. Blankets of different thickness are used depending upon the extent of shrinkage required. Thicker the blanket more is the shrinkage.

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Antishrink and Dimensional Stability

For the antishrink effect to give dimensional stability, the general method is to dry and press the fabric in a shrunken state and the earlier device was made to use the ordinary pin stenter at a speed more than the speed of the stenter so that the cloth gets loose and wavy being pressed by a brush roller on the pins. The cloth gets dried in this slack condition. The cloth after being shrunk lengthwise is again moistened and shrunk widthwise in the palmer drier.

The Modern Sanforising Machine

The sanforising machine, at present mostly in use, is to shrink the cloth both widthwise and lengthwise by means of an elastic blanket, which carries the cloth. The cloth is first damped and then it enters a short-range stenter to control the selvedge. The damped cloth passes over a small diameter feed roller, which also carries the elastic blanket. Here the cloth is held in position by an electrically heated shoe. As soon as the blanket and the cloth adhering the blanket enters the large diameter of the palmer machine, the blanket gets relaxed and shrinks and as the blanket shrinks, the cloth which is in grip with the elastic blanket, also shrinks and in this shrunken state, the cloth is dried and pressed. For producing the finish on both the sides of the cloth, there is a similar set with double palmer arrangement and before entering the second palmer, the cloth is again damped with spray. This method is known as “compressed shrinkage”. Blankets is known as “compressed shrinkage”. Blankets of different thickness are used depending upon the extent of shrinkage required. Thicker the blanket more is the shrinkage.

Finishing of Textile Fibre

The term “finishing” covers all the operations carried on the piece-gods after the loom stage including scouring, bleaching, dyeing, printing and finishing up to calendaring, and in most works the “finishing” department consists of all these sections. Today as the finishing of textile materials has taken as much rapid stride due to the introduction of the various synthetic resin finishes, including compressed shrinkage by special machineries.

Various types of finishing, according to the market demands, will start after the bleaching operation which has already been dealt with. Different types of finishing operations are needed for different types of textile fibres such as cotton and regenerated cellulosic fibres, woolens and worsted natural silk acetate rayon and synthetic fabrics.

Finishing of Cotton Material

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The finishing of cotton piece-goods may be broadly divided into the following three main heads:

(i) Temporary finish(ii) Semi-permanent finish(iii) Permanent finish

Finishing of Textiles

Finishing is a process given to textiles i.e. yarn, fabrics and garments to make it attractive and presentable and to impart certain desirable properties to it. It is often the ‘finish’ which increases the sale value of textile goods.

The finishing of yarn is not of particular interest, for most of the yarn is used by weavers and knitters without finishing, to be finished in fabric or garment form. There are, of course, hand knitting yarns which are twisted and converted into skeins or balls, to be sold to home knitters, however, not much finishing treatment is given to them.

Fabrics after wet processing, that is, scouring, bleaching, dyeing or printing, are in a distorted condition. One of the important functions of finishing is to straighten the fabric and bring it to the required dimensions.

Finishing of fabric is the final operation in wet processing department. It can be classified as:

(a) Temporary finishes(b) Semi-durable finishes(c) Durable finishes

Finishes which are washed away during washing or laundering are called ‘temporary’ finishes whereas those which are not washed away during washing or laundering are called ‘durable’ finishes.

Besides the above three types of finishes, the operation of singeing, shearing, sanforising, heat setting for synthetics, mercerizing, calendaring, moiring, schreinerizing, embossing, tentering, weighting, decatising, optical whitening of synthetics etc. fall under the purview of the finishing department.

Drying forms a part of the finishing process. This can sometimes be carried out at the same time that the fabric is straightened and brought to its desired finished width and length of alternatively it must be dried first and then lightly damped for the final finishing treatment. The most common finishing processes are listed above, which do not represent a sequence nor are all the processes applicable to all kind of gray fabrics. Some fabrics must be put through more than one process. Each fabric is given its own characteristic finish.

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Temporary Finishes

These can be carried out by treating the fabric with a paste consisting of starches and gums, filling materials, softening agents, wetting agents, glazing agents, blueing agents and optical brightener etc. The need for and the quantity of different ingredients depends upon the count and construction of the fabric and on the final feel required.

(a) Starches are used to impart stiffness, body lusture etc. The starches use are maize and tapioca.

(b) Softeners are used for softening and glazing the fabric surface e.g. wax emulsion, glazing paste, polyethylene emulsion.

(c) Wetting Agents help finishing paste wet the fabric quickly, wetting agents are included in the finishing mixture. Wetting agents are mainly of three types, depending on their ionic character.

(d) Weighting Agent such as French chalk, China clay, synthetic softeners etc. are used for filling the thick and thin places and to add weight to the fabric.

(e) Blueing Agent like Victoria blue or acid violet compensate for the slight yellow tinge of the base fabric and thus give a slightly bluish tone to the finished fabric. The optical brighteners, on the other hand, absorb the invisible ultraviolet portion of the day light and in turn emit visible blue light which gives a bright and white appearance to the finished material.

Semi-Durable Finishes

Waterproof and Water Repellent Finishes

The production of waterproof fabric is an important section of textile trade, both for garments and industrial uses. The term ‘waterproof’ is loosely used in the industry for both water repellent and waterproof fabrics. In fact, water resistant fabrics which are permeable to air and water vapours are termed as water repellent fabrics, while those which are not permeable to air and water vapours are termed as waterproof fabrics.

Waterproof Finish

Waterproof finish i.e. impermeable finish may be produced by any substance which beside coating the fabric, will form a water resistant film and close the interstices of the cloth. The substances commonly used are rubber, drying oils etc. Recently substances like polyvinyl chloride, butyl rubber and synthetic resins have been employed to impart waterproofing. These substances are employed by coating techniques. The fabrics can withstand heavy showers or rains but are

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not permeable to air, as such, are commonly used for tents, tarpaulins and wagon covers.

Water-repellent Finish

In this process, a hydrophobic substance is applied in such a way that continuous film is not formed. The interstices between warp and weft are not filled in. Hence the air and water vapour can pass through the fabric.

The agent used to impart water repellency i.e. a combination of aluminium acetate and soap or aluminium salt and wax are applied on the surface of the fabric by pad dry process, to get the desired result.

Nowadays, durable water repellency can be achieved by the application of Octadecyl isocyanate, modified melamine formaldehyde or stearamidomethyl pyridinium chloride (Velan PF) by pad-dry-cure technique.

Flame Proofing

Fabrics cannot be made absolutely fireproof but they can be chemically treated to retard inflammability. Flameproofing is a practical form of fire protection where a fire resistant quality is desirable, as the fabrics used in awnings, mattresses, work clothes or draperies.

Textile fabrics may be given temporary fire resistant quality by simple home method of immersing them in a water solution of borax and boric acid, a mixture of boric acid and diammonium phosphate, sulphamic acid, calcium chloride, sodium phosphate etc. This method does not alter the appearance of the fabric but the treatment is to be repeated after each washing.

In ‘durable’ fire resistant finishing, the chemicals react with the fibre to impart fire retardant properties. Most of them tend to stiffen the fabric. A few finishes are mentioned below:

(i) Ammonia-cured tetrakis-hydroxy-methyl-phosphorous hydroxide (THPOH)

(ii) Tetrakis-hydroxymethyl-phosphonium chloride (THPC)

(iii) Tris (I-aziridinyl) phosphine oxide (APO) etc.

Durable Finishes

We know that cotton is the most widely used textile fibre and due to its inherent properties it could retain its position in spite of the advent of regenerated or synthetic fibres. The superiority of cotton is due to its availability, excellent washing property and comfort during wear, still it lags in poor resistance to

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creases and substantial amount of shrinkage during washing, in comparison to synthetics. On the other hand synthetic garment get easily soiled during wear, which could be overcome by applying anti-soiling treatment with acrylic polymers and fluoro chemicals. Since both cotton and synthetics are neither flameproof nor water-repellent as such, to overcome these shortcomings ‘Durable Finishes’ have been developed. The main type of durable finishes are ‘Anti-shrink’, ‘resin’, ‘flame-resistant’ and water-repellent’ finishes etc. Flame-resistant and water-repellent finishes have been already discussed above.

Anti-shrink Finish (Sanforising)

The main object of this finish is to overcome the problem of shrinkage of cotton and cotton-blended fabrics during washing. The process largely used in textile industry, is a patented process by ‘Cluctt Pabody and Co.’. The cloth finished under this patent is known as ‘Sanforised fabric’. In the process, the cloth is mechanically shrunk between a drum and a rubber belt. Comprehensive shrinkage is achieved by passing the cotton fabrics on to a movable elastic felt blanket, which is in a state of tension. When the tension is released, it assumes a shortened condition and the cotton fabric is forced to conform to the compression, as it is held firmly in contact with the blanket and the surface of a large steam heated metal cylinder. Anti-shrink finishing is the final finishing process. The cloth is finished on a hot air stenter, calendered with one or two nip calender and then passed through anti-crease finishing machine. The fabric is then delivered to folding department.

Resin Finish

Durable finish of the type of wash-n-wear and durable press are generally based on thermosetting resins of the type DMDHEU, DMPU, Carbamates etc. The main object of resin finishing is to impart dry and wet crease recovery and sharp crease retention to the fabric. This finish is largely applied to cotton and its blends with other synthetic fibres. There are mainly three types of finishing treatment with resin:

(a) Anti-crease finishing(b) Wash and wear finishing and(c) Durable press treatment

Though the method of imparting anti-crease or wash-n-wear finish is essentially the same except the difference lies in the type and level of crease recovery. In an anti-crease finish one gets satisfied with the dry crease recovery angle of 230 Deg. (warp + weft). However, in wash-n-wear fabrics, it is not only the dry crease recovery, which is important but also the wet crease recovery. Dry crease recovery is responsible to a large extent for the wrinkle resistance during wearing whereas wet crease recovery is important in the smooth drying property after washing. As such a dry as well as wet crease recovery angle of 240 Deg. (warp + weft) is accepted as quite satisfactory for a wash-n-wear fabric.

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Anti-crease and Wash-n-wear Finishing

Pad-Dry-Cure Process

In this process the fabric is padded through the resin finishing solution, comprising of resin, catalyst and softener, dried on a stenter and cured in the polymerizer. The moisture content of the dried fabric should be uniform and range around 5 to 7 per cent. If the cotton fabric after drying is to be calendered the moisture content should be around 10 per cent. This helps in reducing losses in tensile and tear strengths of the fabric. The time and temperature of curing of dried fabric depends upon the nature of resin and type of catalyst used. With highly acidic catalyst such as ammonium salts, organic acids, lower curing temperature is possible. However, with metal salt catalysts, in general, a curing temperature is about 140oC to 160oC with 5 to 3 minutes as the time of curing. After curing the fabric is washed with soap and soda-ash to remove decomposed products from the cured fabrics.

Wet Cross-Linking

In the wet cross-linking process, the cross-linking agent is allowed to react with cotton in wet condition. Wet cross-linking can be achieved in acidic as well as in alkaline medium. For wet cross-linking in acidic medium the fabric is padded with 80% pickup from a solution containing thermosetting resin, using highly acidic catalyst and thermoplastic resin. After padding, the fabric is batched wet and left to react under rotation for 10 to 16 hours, depending on the amount of catalyst. The fabric is then washed, neutralized and rewashed. For wet cross-linking in alkaline medium the resins used are dichloropropanols etc. The fabric is first treated with 15% caustic soda solution. After squeezing, the wet fabric is again padded under tension with the resin solution. It is then batched in roll to react under rotation for eight to ten hours at room temperature. The fabric is then washed, neutralized and rinsed. The process imparts only wet crease recovery and very little dry crease recovery to the fabric.

The anti-crease process has proved exceptionally useful for viscose rayon. Not only are such materials made resistant to creasing but they are given fuller handle and better appearance. The anti-crease process is not suitable for acetate rayon and so far it does not appear of any use for wool or silk. The latter are already sufficiently crease resistant.

Durable Press

Durable Press or Permanent Press

The term durable press or permanent press originated in the USA where it is used to define the ability of a garment to withstand wear and washing without requiring to be ironed or repressed and to retain a sharp pleat or crease for the duration of the life of the garment, without defect such as pilling or seam

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puckering. However in UK, washing conditions in terms of both, mechanical action and temperature tend to be more severe and standards of acceptance are more discriminating than in the USA. It is considered that a more practical definition of durable press is the ability of the garment to resist creasing during wear and when washed, to retain a definite crease and pleat, and to require only an occasional light ……………… in order to satisfy more critical wearers. Presence of pilling, seam puckering or other garment defects is again implicit.

Different methods such as post-cure process, pre-cure process and garment treatment are followed for durable press. Out of these, pre-cure and post-cure processes are largely used.

Post-cure Process

In this process, the fabric is impregnated with a solution containing thermosetting resin, acidic catalyst, softener etc. and dried under conditions which do not cause complete reaction between the cotton and cross-linking agent. The fabric at this stage is called “sensitized” fabric. Garments are stitched from the “sensitized” fabric lightly ironed to shape properly and then cured with the use of garment process or in garment curing ovens. The garments are then washed, dried and packed.

Pre-cure Process

This process follows the usual pad-dry-cure-after wash technique. After the manufacture of the garment from the crease resistant fabric, a solution of an acidic or potentially acidic catalyst together sometimes with a small amount of cross-linking agent is applied to the appropriate area by wetting or spraying. The creases are heat pressed and cured. During the process the acid-catalyst breaks the cross-links in the flat treated and new cross-links are reformed in the crease of the garment, on heating. In another process, instead of applying the catalyst solution to the appropriate area after garment is stitched as mentioned earlier, the creases are set by subjecting that portion of the fabric through the use of high temperature and high-pressure treatment.

The combination of high resin content, and prolong curing at high temperature in durable press treatment to cotton fabric causes severe strength losses and reduced abrasion resistance below acceptable minimum. As a consequence, this treatment is applied mostly to blends of polyester and cotton.

Permanent Pleating

Thermo-plastic fibre has the ability to take on “permanent set” when shaped at high temperatures. It can therefore be imparted pleats durable to wear and laundering. The pleat retention of a thermoplastic fibre blended fabric improves with increasing the fibre content of the thermoplastic fibre. In blend with cotton or viscose rayon, 67% of polyester gives excellent pleat retention during washing.

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Fabrics treated with certain new resin products are easily washed and dried and require little or no ironing. With the increased body resin treated fabrics have greatly improved hand and drape qualities and make possible permanent pleating.

Wet Finish

Polyester fabrics for durable press outlets or which may be used with fusible interlinings must possess colour-fastness, adequate to withstand hot head pressing in making up. Pressing techniques usually employed, involve heating at a temperature in the range 150-180oC (300-355oF) for 5-20 seconds. Dyes for polyester component should be selected carefully to ensure that any colour change or sublimation of dyestuffs under heat-setting or high temperature, hot head pressing conditions are acceptable.

Fabric pH

Polyester/wool worsted fabrics for “durable press” or “fusible interlinking” garments should always be finished out in an acidic condition. If left in an alkaline state the wool component of the blend may undergo considerable yellowing when subjected to hot-head pressing and this can be objectionable even in solid-dyed fabric where it is still possible to detect a noticeable shift towards yellow. To overcome this fault it is recommended that polyester/wool fabric be finished out to approximate pH 3 by adding 2-3% of tartaric acid (calculated on the weight of goods) to the final bath. This is particularly important after the alkaline scouring of slubbing-dyed fabrics.

A large number of garments, including many intended for durable press applications are now made with fusible interlinings. A specially treated interlining is bonded to the body fabric by pressing on a hot head press. This method produces well-shaped garments with good wear performance at a cost less than when employing conventional lining fabrics.

Commonly Used Dyeing Machines for Cloth Dyeing

(1) Winch – It is employed for knitted fabrics as the fabric in this machine is dyed tensionless. Woven fabrics are not dyed. The disadvantage of the equipment is high material to liquor ratio, 1:20 to 25. The consumption of colours, chemicals, steam and water is high. The fabric is dyed in rope form and therefore, the probability of dyeing streaks does exist.

(2) Jigger – This is the most universal dyeing equipment for woven fabrics. The fabric is dyed in open width under tension. Material to liquor ratio varies from 1:3 to 4. The jiggers are available in varying widths so as to handle fabrics of different widths. The size of a jigger is the width of

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batching rollers. Speed of the machine is about 60 m/min. Two types of jiggers are used – ordinary and semi-automatic. In the former, ‘end change’ is done manually whereas in semi-automatic it is done by mechanical means by itself. Also the desired number of ends can be set on the counter, in the latter type. The productivity of the machine varies from 1000-1200 m/shift. For ordinary jigger one worker for one jigger and for semi-automatic jiggers one workers for two jiggers are normally employed.

(3) Padding Mangle – It is mostly used for dyeing reactive dyes and pigment padding with vat dyes. The production of the machine ranges between 6000-8000 m/shift. Generally, two bowl mangles are favoured. The hardness of both the bowls should be nearly same; both the rollers should preferably be soft. The trough of the mangle should be smallest permissible and bowls diameter (12 to 18 normally) should be in accordance with the width (48-64 normally). The rollers should be adequately cambered for uniform wet-pickup from selvedge to selvedge. The mangle must be of pneumatically loading type so that pressure can be maintained uniform. The direct labour complement is usually two workers.

(4) Float Dryer and Hot Flue – The difference between the two equipments is in the passage of fabric. In a float dryer the fabric moves parallel to floor whereas in a hot-flue the passage is vertical. These are used for pigment pad-dry with vat dyes, pigments, phthalogens and in application of naphthols, and reactive dyes. In the case of vat dyes, naphthols and reactive dyes, these machines are normally used when a large yardage of fabric needs to be dyed, so as to obtain uniform shade. The productivity is good – of the order of 8000 m/shift. Usually three workers are kept on this type of machine.

(5) Continuous Dyeing Range (CDR) – CDR is employed for dyeing vat and reactive dyes. The productivity of the machine is high, 8000-12000 m/shift and employs normally four workers in a shift. Cost of the equipment is exorbitant.

The range consists of a mangle, float dryer, chemical padder, steamer, soaper and can dryer. The fabric once entered the range comes out in the final form.

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Sanforising

Principal of Sanforising Machine

The sanforising machine, at present mostly in use, is to shrink the cloth both widthwise and lengthwise by means of an elastic blanket. The cloth is first damped and then enters a short-range stenter to control the selvedges. The damped cloth passes over a small diameter feed roller, which also carries the elastic blanket. Here the cloth is held in position by an electrically heated shoe. As soon as the blanket and the cloth adhering the blanket enters the large diameter of the palmer machine, the blanket gets relaxed and shrinks and as the blanket shrinks, the cloth which is in grip with the elastic blanket also shrinks and in this shrunken state, the cloth is dried and pressed. For producing the finish on both sides of the cloth, there is a similar set with double palmer arrangement and before entering the second palmer the cloth is again damped with spray. This method is known as “compressed shrinkage”. Blankets of different thickness are used depending upon the extent of shrinkage required. Thicker the blanket more is the shrinkage.

Antishrink and Dimensional Stability

For the antishrink effect to give dimensional stability, the general method is to dry and press the fabric in a shrunken state and the earlier device was made to use the ordinary pin stenter at a speed more than the speed of the stenter so that the cloth gets loose and wavy being pressed by a brush roller on the pins. The cloth gets dried in this slack condition. The cloth after being shrunk lengthwise is again moistened and shrunk widthwise in the palmer drier.

The Modern Sanforising Machine

The sanforising machine, at present mostly in use, is to shrink the cloth both widthwise and lengthwise by means of an elastic blanket, which carries the cloth. The cloth is first damped and then it enters a short-range stenter to control the selvedge. The damped cloth passes over a small diameter feed roller, which also carries the elastic blanket. Here the cloth is held in position by an electrically heated shoe. As soon as the blanket and the cloth adhering the blanket enters the large diameter of the palmer machine, the blanket gets relaxed and shrinks and as the blanket shrinks, the cloth which is in grip with the elastic blanket, also shrinks and in this shrunken state, the cloth is dried and pressed. For producing the finish on both the sides of the cloth, there is a similar set with double palmer arrangement and before entering the second palmer, the cloth is again damped with spray. This method is known as “compressed shrinkage”. Blankets is known as “compressed shrinkage”. Blankets of different thickness are used depending upon the extent of shrinkage required. Thicker the blanket more is the shrinkage.

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Finishing of Textile Fibre

The term “finishing” covers all the operations carried on the piece-gods after the loom stage including scouring, bleaching, dyeing, printing and finishing up to calendaring, and in most works the “finishing” department consists of all these sections. Today as the finishing of textile materials has taken as much rapid stride due to the introduction of the various synthetic resin finishes, including compressed shrinkage by special machineries.

Various types of finishing, according to the market demands, will start after the bleaching operation which has already been dealt with. Different types of finishing operations are needed for different types of textile fibres such as cotton and regenerated cellulosic fibres, woolens and worsted natural silk acetate rayon and synthetic fabrics.

Finishing of Cotton Material

The finishing of cotton piece-goods may be broadly divided into the following three main heads:

(iv) Temporary finish(v) Semi-permanent finish(vi) Permanent finish

Finishing of Textiles

Finishing is a process given to textiles i.e. yarn, fabrics and garments to make it attractive and presentable and to impart certain desirable properties to it. It is often the ‘finish’ which increases the sale value of textile goods.

The finishing of yarn is not of particular interest, for most of the yarn is used by weavers and knitters without finishing, to be finished in fabric or garment form. There are, of course, hand knitting yarns which are twisted and converted into skeins or balls, to be sold to home knitters, however, not much finishing treatment is given to them.

Fabrics after wet processing, that is, scouring, bleaching, dyeing or printing, are in a distorted condition. One of the important functions of finishing is to straighten the fabric and bring it to the required dimensions.

Finishing of fabric is the final operation in wet processing department. It can be classified as:

(d) Temporary finishes(e) Semi-durable finishes

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(f) Durable finishes

Finishes which are washed away during washing or laundering are called ‘temporary’ finishes whereas those which are not washed away during washing or laundering are called ‘durable’ finishes.

Besides the above three types of finishes, the operation of singeing, shearing, sanforising, heat setting for synthetics, mercerizing, calendaring, moiring, schreinerizing, embossing, tentering, weighting, decatising, optical whitening of synthetics etc. fall under the purview of the finishing department.

Drying forms a part of the finishing process. This can sometimes be carried out at the same time that the fabric is straightened and brought to its desired finished width and length of alternatively it must be dried first and then lightly damped for the final finishing treatment. The most common finishing processes are listed above, which do not represent a sequence nor are all the processes applicable to all kind of gray fabrics. Some fabrics must be put through more than one process. Each fabric is given its own characteristic finish.

Temporary Finishes

These can be carried out by treating the fabric with a paste consisting of starches and gums, filling materials, softening agents, wetting agents, glazing agents, blueing agents and optical brightener etc. The need for and the quantity of different ingredients depends upon the count and construction of the fabric and on the final feel required.

(f) Starches are used to impart stiffness, body lusture etc. The starches use are maize and tapioca.

(g) Softeners are used for softening and glazing the fabric surface e.g. wax emulsion, glazing paste, polyethylene emulsion.

(h) Wetting Agents help finishing paste wet the fabric quickly, wetting agents are included in the finishing mixture. Wetting agents are mainly of three types, depending on their ionic character.

(i) Weighting Agent such as French chalk, China clay, synthetic softeners etc. are used for filling the thick and thin places and to add weight to the fabric.

(j) Blueing Agent like Victoria blue or acid violet compensate for the slight yellow tinge of the base fabric and thus give a slightly bluish tone to the finished fabric. The optical brighteners, on the other hand, absorb the invisible ultraviolet portion of the day light and in turn emit visible blue light which gives a bright and white appearance to the finished material.

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Semi-Durable Finishes

Waterproof and Water Repellent Finishes

The production of waterproof fabric is an important section of textile trade, both for garments and industrial uses. The term ‘waterproof’ is loosely used in the industry for both water repellent and waterproof fabrics. In fact, water resistant fabrics which are permeable to air and water vapours are termed as water repellent fabrics, while those which are not permeable to air and water vapours are termed as waterproof fabrics.

Waterproof Finish

Waterproof finish i.e. impermeable finish may be produced by any substance which beside coating the fabric, will form a water resistant film and close the interstices of the cloth. The substances commonly used are rubber, drying oils etc. Recently substances like polyvinyl chloride, butyl rubber and synthetic resins have been employed to impart waterproofing. These substances are employed by coating techniques. The fabrics can withstand heavy showers or rains but are not permeable to air, as such, are commonly used for tents, tarpaulins and wagon covers.

Water-repellent Finish

In this process, a hydrophobic substance is applied in such a way that continuous film is not formed. The interstices between warp and weft are not filled in. Hence the air and water vapour can pass through the fabric.

The agent used to impart water repellency i.e. a combination of aluminium acetate and soap or aluminium salt and wax are applied on the surface of the fabric by pad dry process, to get the desired result.

Nowadays, durable water repellency can be achieved by the application of Octadecyl isocyanate, modified melamine formaldehyde or stearamidomethyl pyridinium chloride (Velan PF) by pad-dry-cure technique.

Flame Proofing

Fabrics cannot be made absolutely fireproof but they can be chemically treated to retard inflammability. Flameproofing is a practical form of fire protection where a fire resistant quality is desirable, as the fabrics used in awnings, mattresses, work clothes or draperies.

Textile fabrics may be given temporary fire resistant quality by simple home method of immersing them in a water solution of borax and boric acid, a mixture

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of boric acid and diammonium phosphate, sulphamic acid, calcium chloride, sodium phosphate etc. This method does not alter the appearance of the fabric but the treatment is to be repeated after each washing.

In ‘durable’ fire resistant finishing, the chemicals react with the fibre to impart fire retardant properties. Most of them tend to stiffen the fabric. A few finishes are mentioned below:

(iv) Ammonia-cured tetrakis-hydroxy-methyl-phosphorous hydroxide (THPOH)

(v) Tetrakis-hydroxymethyl-phosphonium chloride (THPC)

(vi) Tris (I-aziridinyl) phosphine oxide (APO) etc.

Durable Finishes

We know that cotton is the most widely used textile fibre and due to its inherent properties it could retain its position in spite of the advent of regenerated or synthetic fibres. The superiority of cotton is due to its availability, excellent washing property and comfort during wear, still it lags in poor resistance to creases and substantial amount of shrinkage during washing, in comparison to synthetics. On the other hand synthetic garment get easily soiled during wear, which could be overcome by applying anti-soiling treatment with acrylic polymers and fluoro chemicals. Since both cotton and synthetics are neither flameproof nor water-repellent as such, to overcome these shortcomings ‘Durable Finishes’ have been developed. The main type of durable finishes are ‘Anti-shrink’, ‘resin’, ‘flame-resistant’ and water-repellent’ finishes etc. Flame-resistant and water-repellent finishes have been already discussed above.

Anti-shrink Finish (Sanforising)

The main object of this finish is to overcome the problem of shrinkage of cotton and cotton-blended fabrics during washing. The process largely used in textile industry, is a patented process by ‘Cluctt Pabody and Co.’. The cloth finished under this patent is known as ‘Sanforised fabric’. In the process, the cloth is mechanically shrunk between a drum and a rubber belt. Comprehensive shrinkage is achieved by passing the cotton fabrics on to a movable elastic felt blanket, which is in a state of tension. When the tension is released, it assumes a shortened condition and the cotton fabric is forced to conform to the compression, as it is held firmly in contact with the blanket and the surface of a large steam heated metal cylinder. Anti-shrink finishing is the final finishing process. The cloth is finished on a hot air stenter, calendered with one or two nip calender and then passed through anti-crease finishing machine. The fabric is then delivered to folding department.

Resin Finish

Durable finish of the type of wash-n-wear and durable press are generally based on thermosetting resins of the type DMDHEU, DMPU, Carbamates etc. The

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main object of resin finishing is to impart dry and wet crease recovery and sharp crease retention to the fabric. This finish is largely applied to cotton and its blends with other synthetic fibres. There are mainly three types of finishing treatment with resin:

(d) Anti-crease finishing(e) Wash and wear finishing and(f) Durable press treatment

Though the method of imparting anti-crease or wash-n-wear finish is essentially the same except the difference lies in the type and level of crease recovery. In an anti-crease finish one gets satisfied with the dry crease recovery angle of 230 Deg. (warp + weft). However, in wash-n-wear fabrics, it is not only the dry crease recovery, which is important but also the wet crease recovery. Dry crease recovery is responsible to a large extent for the wrinkle resistance during wearing whereas wet crease recovery is important in the smooth drying property after washing. As such a dry as well as wet crease recovery angle of 240 Deg. (warp + weft) is accepted as quite satisfactory for a wash-n-wear fabric.

Anti-crease and Wash-n-wear Finishing

Pad-Dry-Cure Process

In this process the fabric is padded through the resin finishing solution, comprising of resin, catalyst and softener, dried on a stenter and cured in the polymerizer. The moisture content of the dried fabric should be uniform and range around 5 to 7 per cent. If the cotton fabric after drying is to be calendered the moisture content should be around 10 per cent. This helps in reducing losses in tensile and tear strengths of the fabric. The time and temperature of curing of dried fabric depends upon the nature of resin and type of catalyst used. With highly acidic catalyst such as ammonium salts, organic acids, lower curing temperature is possible. However, with metal salt catalysts, in general, a curing temperature is about 140oC to 160oC with 5 to 3 minutes as the time of curing. After curing the fabric is washed with soap and soda-ash to remove decomposed products from the cured fabrics.

Wet Cross-Linking

In the wet cross-linking process, the cross-linking agent is allowed to react with cotton in wet condition. Wet cross-linking can be achieved in acidic as well as in alkaline medium. For wet cross-linking in acidic medium the fabric is padded with 80% pickup from a solution containing thermosetting resin, using highly acidic catalyst and thermoplastic resin. After padding, the fabric is batched wet and left to react under rotation for 10 to 16 hours, depending on the amount of catalyst. The fabric is then washed, neutralized and rewashed. For wet cross-linking in alkaline medium the resins used are dichloropropanols etc. The fabric is first treated with 15% caustic soda solution. After squeezing, the wet fabric is again padded under tension with the resin solution. It is then batched in roll to react

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under rotation for eight to ten hours at room temperature. The fabric is then washed, neutralized and rinsed. The process imparts only wet crease recovery and very little dry crease recovery to the fabric.

The anti-crease process has proved exceptionally useful for viscose rayon. Not only are such materials made resistant to creasing but they are given fuller handle and better appearance. The anti-crease process is not suitable for acetate rayon and so far it does not appear of any use for wool or silk. The latter are already sufficiently crease resistant.

Durable Press

Durable Press or Permanent Press

The term durable press or permanent press originated in the USA where it is used to define the ability of a garment to withstand wear and washing without requiring to be ironed or repressed and to retain a sharp pleat or crease for the duration of the life of the garment, without defect such as pilling or seam puckering. However in UK, washing conditions in terms of both, mechanical action and temperature tend to be more severe and standards of acceptance are more discriminating than in the USA. It is considered that a more practical definition of durable press is the ability of the garment to resist creasing during wear and when washed, to retain a definite crease and pleat, and to require only an occasional light ……………… in order to satisfy more critical wearers. Presence of pilling, seam puckering or other garment defects is again implicit.

Different methods such as post-cure process, pre-cure process and garment treatment are followed for durable press. Out of these, pre-cure and post-cure processes are largely used.

Post-cure Process

In this process, the fabric is impregnated with a solution containing thermosetting resin, acidic catalyst, softener etc. and dried under conditions which do not cause complete reaction between the cotton and cross-linking agent. The fabric at this stage is called “sensitized” fabric. Garments are stitched from the “sensitized” fabric lightly ironed to shape properly and then cured with the use of garment process or in garment curing ovens. The garments are then washed, dried and packed.

Pre-cure Process

This process follows the usual pad-dry-cure-after wash technique. After the manufacture of the garment from the crease resistant fabric, a solution of an acidic or potentially acidic catalyst together sometimes with a small amount of cross-linking agent is applied to the appropriate area by wetting or spraying. The creases are heat pressed and cured. During the process the acid-catalyst breaks

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the cross-links in the flat treated and new cross-links are reformed in the crease of the garment, on heating. In another process, instead of applying the catalyst solution to the appropriate area after garment is stitched as mentioned earlier, the creases are set by subjecting that portion of the fabric through the use of high temperature and high-pressure treatment.

The combination of high resin content, and prolong curing at high temperature in durable press treatment to cotton fabric causes severe strength losses and reduced abrasion resistance below acceptable minimum. As a consequence, this treatment is applied mostly to blends of polyester and cotton.

Permanent Pleating

Thermo-plastic fibre has the ability to take on “permanent set” when shaped at high temperatures. It can therefore be imparted pleats durable to wear and laundering. The pleat retention of a thermoplastic fibre blended fabric improves with increasing the fibre content of the thermoplastic fibre. In blend with cotton or viscose rayon, 67% of polyester gives excellent pleat retention during washing.

Fabrics treated with certain new resin products are easily washed and dried and require little or no ironing. With the increased body resin treated fabrics have greatly improved hand and drape qualities and make possible permanent pleating.

Wet Finish

Polyester fabrics for durable press outlets or which may be used with fusible interlinings must possess colour-fastness, adequate to withstand hot head pressing in making up. Pressing techniques usually employed, involve heating at a temperature in the range 150-180oC (300-355oF) for 5-20 seconds. Dyes for polyester component should be selected carefully to ensure that any colour change or sublimation of dyestuffs under heat-setting or high temperature, hot head pressing conditions are acceptable.

Fabric pH

Polyester/wool worsted fabrics for “durable press” or “fusible interlinking” garments should always be finished out in an acidic condition. If left in an alkaline state the wool component of the blend may undergo considerable yellowing when subjected to hot-head pressing and this can be objectionable even in solid-dyed fabric where it is still possible to detect a noticeable shift towards yellow. To overcome this fault it is recommended that polyester/wool fabric be finished out to approximate pH 3 by adding 2-3% of tartaric acid (calculated on the weight of goods) to the final bath. This is particularly important after the alkaline scouring of slubbing-dyed fabrics.

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A large number of garments, including many intended for durable press applications are now made with fusible interlinings. A specially treated interlining is bonded to the body fabric by pressing on a hot head press. This method produces well-shaped garments with good wear performance at a cost less than when employing conventional lining fabrics.

Commonly Used Dyeing Machines for Cloth Dyeing

(6) Winch – It is employed for knitted fabrics as the fabric in this machine is dyed tensionless. Woven fabrics are not dyed. The disadvantage of the equipment is high material to liquor ratio, 1:20 to 25. The consumption of colours, chemicals, steam and water is high. The fabric is dyed in rope form and therefore, the probability of dyeing streaks does exist.

(7) Jigger – This is the most universal dyeing equipment for woven fabrics. The fabric is dyed in open width under tension. Material to liquor ratio varies from 1:3 to 4. The jiggers are available in varying widths so as to handle fabrics of different widths. The size of a jigger is the width of batching rollers. Speed of the machine is about 60 m/min. Two types of jiggers are used – ordinary and semi-automatic. In the former, ‘end change’ is done manually whereas in semi-automatic it is done by mechanical means by itself. Also the desired number of ends can be set on the counter, in the latter type. The productivity of the machine varies from 1000-1200 m/shift. For ordinary jigger one worker for one jigger and for semi-automatic jiggers one workers for two jiggers are normally employed.

(8) Padding Mangle – It is mostly used for dyeing reactive dyes and pigment padding with vat dyes. The production of the machine ranges between 6000-8000 m/shift. Generally, two bowl mangles are favoured. The hardness of both the bowls should be nearly same; both the rollers should preferably be soft. The trough of the mangle should be smallest permissible and bowls diameter (12 to 18 normally) should be in accordance with the width (48-64 normally). The rollers should be adequately cambered for uniform wet-pickup from selvedge to selvedge. The mangle must be of pneumatically loading type so that pressure can be maintained uniform. The direct labour complement is usually two workers.

(9) Float Dryer and Hot Flue – The difference between the two equipments is in the passage of fabric. In a float dryer the fabric moves parallel to floor whereas in a hot-flue the passage is vertical. These are used for pigment pad-dry with vat dyes, pigments, phthalogens and in application of naphthols, and reactive dyes. In the case of vat dyes, naphthols and reactive dyes, these machines are normally used when a large yardage of fabric needs to be dyed, so as to obtain uniform shade. The productivity is good – of the order of 8000 m/shift. Usually three workers are kept on this type of machine.

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(10) Continuous Dyeing Range (CDR) – CDR is employed for dyeing vat and reactive dyes. The productivity of the machine is high, 8000-12000 m/shift and employs normally four workers in a shift. Cost of the equipment is exorbitant.

The range consists of a mangle, float dryer, chemical padder, steamer, soaper and can dryer. The fabric once entered the range comes out in the final form.

Direct Dyes

These dyes possess strong affinity for cellulosic fibres and are therefore, also known as substantive dyes. Molecular structure of many direct dyes first exactly upon the cellulose molecule and the dyes are strongly adsorbed. The exhaustion of direct dyes depends upon temperature of dyeing and concentration of common salt in the dye-bath. At the same time different dyes exhibit varied sensitivity towards temperature and common salt. Therefore, depending upon the effect of temperature and common salt on dyeing, the dyes have been classified into three groups – Group A, B and C. Group A dyes are self-leveling and do not necessarily need common salt for exhaustion. The dyes in Group B have poor leveling properties and need controlled addition of common salt for exhaustion. The exhaustion of dyes in Group C is controlled not only by the addition of common salt but also by temperature. The foregoing discussion necessitates the selection of dyes from the same group while dyeing compound shades. The use of direct dyes is limited, because of poor fastness properties, to low quality of goods only.

For dissolving, the dye is pasted with an anionic wetting agent or with cold water and then small amount of soda ash is added. Hot boiling water is then poured on the paste slowly but with constant stirring to bring the dye into solution. Addition of soda ash increases the solubility and in some cases aids solubilising of dyes. Further, boiling water should never be poured on dye powder as it leads to lump formation resulting in specky dyeings. For commonly used dyes, stock solution can be prepared and stored for convenience. The dyeing is mostly carried out in a jigger with the addition of 5-20% common salt and 1-3% soda ash as dye-bath assistants. For dyeing with Group A dyes, dissolved dye, common salt and soda ash are added to the dye-bath initially. The material is entered at 40-50oC and two ends are given. The liquor is raised to boil slowly and dyeing continued at boil, 4 to 6 ends are adequate. For the dyes in Group B common salt is not added initially, but during the period when dye-bath is heated to attain boiling temperature. Dyeing with the dyes of Group C is started at room temperature but without common salt. Two ends are given. The dye-bath is raised to boil slowly and material is run for two ends. Now, common salt is added in parts and minimum four ends are given at dyeing temperature. At the end of dyeing, dye-bath is drained. The fabric is given cold wash and then soaping at 60-70oC.

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The fastness of dyeings to washing and light is generally poor and is improved by after-treatments. Fastness to light can be improved by a treatment with 0.5-2% copper sulphate at 60oC by working the fabric two to four ends in a jigger. This treatment also improves fastness to washing. The improvement in fastness is, however, associated with dulling of dyeings. The mechanism is that the dye forms a complex with copper and it is dye-copper complex, which results in improvement of fastness properties. Further, sometimes in order to improve colour yield and fastness properties of dyeings, a treatment with cationic dye-fixing agent is given. Such a treatment indeed improves light fastness in low depth shades by promoting the formation of dye-dye complex. But in higher depths, dye-agent complex is formed, and this being unstable leads to lowering of light fastness. Fastness to washing can be improved by treating dyed fabric with 1-2% dichromate and 3% acetic acid at ambient temperature.

Reactive Dyes

The reactive dyes are different from other classes of dyes in that they enter into chemical reaction with cotton cellulose. The chemical reaction of dye takes place with the hydroxyl group of celluloses with the formation of a covalent linkage between the two. This linkage is responsible for high fastness properties of reactive dyes. As the name suggests, the dye contains a reactive group attached to the colouring matter and the reactivity of dye depends upon the reactivity of reactive group. While dyeing, however, reaction between the dye and hydroxyl group of water also takes place resulting in hydrolysis of the dye. The hydrolysed dye does not behave like reactive dye but like a direct dye. Its removal, therefore, from fabric is very important in order to obtain achievable fastness properties.

The molecules of reactive dyes have simple structure and are of low molecular weight. This leads to three distinct features:

Low affinity of these dyes towards cellulose

Production of bright shades

Outstanding leveling and diffusion properties

The advantage of low affinity is the ease of removal of hydrolysed dye from the fibre. These days, dyes containing an array of reactive groups are manufactured. In India, however, dyes based on triazine mono- and dichloro- and vinyl sulphone reactive groups are only manufactured.

DCT based (M brand) dyes are highly reactive and are applied at room temperature. The reactivity of MCT (H brand) based dyes is low and these days are applied only at elevated temperature – 70oC upward. In so far as Navictive, Reactofix Supra and Remazol dyes (VS) are concerned their reactivity falls between DCT and MCT based dyes. These are also applied at about 60oC. Currently, however, the manufacturers of these dyes have made efforts to

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standardize conditions suitable for application at room temperature or more correctly at about 40oC. For dyeing with reactive dyes, addition of common salt and alkali is mandatory to the dye-bath. Common salt performs two functions: (i) equalize the external and internal pH of the fibre; and (ii) helps in the exhaustion of dye-bath. The addition of alkali is required for the fixation of dyes; a part of it is consumed in neutralizing the acid liberated as a result of reaction of dye(s) with cellulose or water or both. M-brand dyes need mild alkaline conditions for fixation whereas H-brand are fixed in strongly alkaline medium; the fixation of VS based dyes is also carried out in strong alkaline conditions. The dyeing proceeds as follows:

First the dye is applied to the fibre for sometime and then common salt is added. The material is worked for some more time. During this time exhaustion of the dye-bath takes place. The molecules of dye absorb and desorb continuously till the equilibrium is attained. On addition of alkali, the dye molecules inside the fibre react and get fixed. The equilibrium is disturbed and fresh molecules rush towards the fibres. At the same time, however, hydrolysis of the dye is accelerated because of alkali. This process continues until all the dye that is present has partly reacted with the fibre and rest converted into hydrolysed form.

Sulphur Dyes

Sulphur dyes being water insoluble are applied to cellulosic fabrics in their reduced form and then oxidized to obtain the parent dyes. This class of dyes is very popular for shades such as dull green, brown, blue and black. The dyeings are fast to washing and light but fastness to bleaching is poor. The exhaustion of these dyes is around 30% and therefore they can be dyed employing standing baths. The dyeing is carried out in a jigger at near boil using minimal liquor ratio. The reduction of dyes is achieved with sodium sulphide; soda ash is also added to neutralize sulphuric acid formed during the storage of dyes.

For dissolution, dye is pasted with a little wetting agent and water. To this soda ash equal in weight of dye is added and stirred to make a smooth paste. This is followed by the addition of sodium sulphide usually twice the weight of dye and sufficient boiling water. The liquor is kept for 15 minutes and then added to the bath. In case of very deep shades, the reduction should be carried out in the bath of jigger at boil.

The fabric is run for two ends at boil and then common salt is added. This is followed by 6-8 more ends at near boil. Afterwards the bath is drained, the fabric is rinsed and given an oxidizing treatment with hydrogen peroxide to precipitate the original dye. This is followed by soaping at boil, warm wash, rinsing and after-treatments.

Two serious defects that a sulphur dyed fabric may suffer from are: tendering on storage and bronziness – especially in black shade. The tendering of fabric is due to sulphuric acid which is formed by the conversion of loosely bound sulphur. Sometimes this causes tremendous strength losses. This can be avoided by

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treating the fabric with sodium acetate after soaping of the dyed goods has been carried out. The fabric can also be treated by passing through sodium acetate solution at the time of drying. Another beneficial treatment can be performed with potassium dichromate and acetic acid. This treatment improves the light fastness of dyeings in addition. For the treatment both potassium dichromate and acetic acid (60%) are taken 1-3% on the weight of fabric and the dyed fabric is given 2-4 ends at 60oC. This is followed by rinsing and drying.

The other defect viz. bronziness is mainly due to insufficient sodium sulphide in the bath. This can be rectified by treating the defective fabric with 1% sodium sulphide in a jigger. Another method of removing bronziness is to treat the material with 1% olive oil soap, 0.5% olive oil and 0.5% soda ash – all concentrations on weight of fabric – at about 60oC. Two ends are sufficient. The fabric after this treatment should not be rinsed and taken directly for drying.

Blackness of sulphur black shade can be enhanced by a treatment with 1 g/l Turkey Red Oil (TRO) in a jigger. The fabric is worked for two ends at ambient temperature and then dried without intermediate rinse. As a matter of fact, treatment with sodium acetate (to avoid tendering) and TRO can be carried out simultaneously. Sometimes free sulphur also appears in the bath and gets deposited on the fabric being dyed. This results in specks and spoils the appearance of dyed fabric. In order to overcome this problem, sodium sulphite should be added to the bath the moment free sulphur is sported. The addition should be continued until free sulphur disappears. The chemical reaction involved herein is given below:

S + Na2SO3 Na2S2O3 (water soluble)

Azoics

Azoics are also known as naphthol or ice colours and are produced on the fibre by reacting two components – naphthols and bases – during dyeing process. The azoics are employed to produce deep shades such as blue, violet, yellow, orange, scarlet, bordeaux, etc. economically. The dyeings exhibit satisfactory fastness to washing and bleaching but fastness to rubbing is poor. The dyeing with azoics involves two distinct steps: (i) naphtholation; and (ii) diazotisation and coupling.Naphtholation

Naphthols are insoluble in water. But for application on cellulosic textiles first requirement is the solubility. The dissolution is brought about with the help of caustic soda and can be achieved by two methods hot and cold. The former is more popular than the latter.

In hot method naphthol is pasted with a dispersant like TRO and boiling water. This is followed by the addition of caustic soda in required quantity (which varies from naphthol to naphthol) and sufficient boiling water. The whole mass is then boiled till a clear solution – which indicates the conversion of naphthol to its

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sodium salt, is obtained. However, for some Naphthols such as AS-SW, AS-SG, AS-SR, is it necessary to heat the paste containing naphthol, TRO and caustic soda at 85-90oC for about five minutes before the addition of boiling water. The whole mass is then continued to be heated till a clear solution is obtained. Care should, however, be taken that naphthol is made into a smooth paste before the addition of boiling water. This avoids the formation of lumps which otherwise remain as undissolved mass and gets deposited on the fabric.

In cold method, naphthol is pasted with required amount of methylated spirit and cold water. This is followed by the addition of caustic soda first at ambient temperature with constant stirring and then sufficient amount of water to make a clear solution. The method is simple and naphthols difficult to dissolve namely, AS-SW, AS-TR can be easily dissolved. The amount of caustic soda required is less than in hot method but overall cost is more because of methylated spirit. This coupled with handling of methylated spirit has restricted its application.

Naphtholation of fabric can be carried out either in a jigger or by padding method. In jigger method the dissolved naphthol is added to the bath previously set with caustic soda in 4, 3, 2 g/l concentration for deep, medium and light shades respectively at about 30-40oC. In case of AS-G lower concentration of caustic soda (1.5 to 3 g/l) is needed. Common salt, when added in 10-20 g/l concentration, increases the substantivity of naphthols significantly. Its addition is, however, not required for a few naphthols, namely, AS-SG, AS-SR, AS-BT and AS-RT. The liquor ratio is maintained at minimal level. The fabric is run for two ends and then common salt, if required, is added. This is followed by 4 more ends at about 60oC. The fabric is then taken on a roller for squeezing action. It is advisable to give this fabric 2 ends in about 25 g/l solution of common salt. This helps in removing superfluous naphthol from the fabric.

Naphtholation by padding process is carried out only when large metreage is available. The naphthols most suited for padding are those having low substantivity. However, for high substantivity naphthols, time of contact between fabric and liquor should be reduced to minimum possible. The temperature of naphthol solution is kept at about 70oC and a low capacity trough is ideal. The temperature, however, should be uniform throughout the operation; otherwise tailing results. The fabric is padded and dried preferably in a float dryer. The naphtholated goods should be taken for development as soon as possible. If need to be stored, the goods should be covered adequately to avoid exposure to air. On many occasions, in order to obtain a particular shade two naphthols are padded together. In such cases it is advisable not to take naphthols which possess widely different affinity so as to avoid preferential absorption of one naphthol, which would result in uneven shade on development. Naphtholation with such naphthols should be carried out in a jigger and dissolved naphthols should be added in two parts to the bath.

Diazotisation and Coupling

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The naphtholated fabric is passed through a bath of diazotised base to effect coupling of naphthol and base in order to obtain the dyeing.

Diazotisation is carried out by two methods – direct and indirect. In the direct method, fast base is pasted with hot water and then cooled to about 10-15oC by the addition of ice. Now, required quantity of hydrochloric acid is added. In a separate container sodium nitrite is dissolved in cold water and the solution added to the above slowly and with constant stirring. The temperature should be maintained at 10-15oC. After the addition is over, the liquor is left for 15-20 minutes to complete the diazotisation reaction. In indirect method, the fast base is pasted with water and sodium nitrite dissolved separately is added to the paste. This is then cooled to about 10-15oC and then added to cooled solution of hydrochloric acid slowly and with constant stirring. The whole is kept for 20-25 minutes to complete the diazotisation reaction. Commonly used fast bases diazotised by indirect method are Brodeaux GP, Red B, Violet B, Red 3GL and Orange etc.

Diazotisation should preferably be carried out in wooden vessels and throughout the process, excess of hydrochloric acid and sodium nitrite should be maintained. The presence of the former can be checked with Congo Red paper which turns blue and that of the latter by starch iodide paper which exhibits deep blue colouration.

Coupling is carried out either in a jigger or in an open width soaper. Before coupling sodium acetate and acetic acid are added to the diazotised base in order to (i) neutralize free acid; and (ii) to maintain pH at the desired level. Acetic acid also acts as an alkali binding agent; sometimes alkali binding agent such as aluminium sulphate is added. For majority of the bases, acidic pH is maintained but for Violet B and Blue BB the pH should be near to neutral. While developing in a jigger, common salt is added to arrest the bleeding of naphthol to the bath which otherwise would react with diazotised base forming coloured pigment. The pigment particles are deposited on the fabric and if not removed thoroughly affect rubbing fastness seriously. The temperature in the bath is maintained at about 20oC and 4 ends are sufficient. On an open width soaper, airing after the passage through diazotised base is advisable. Very low concentration of bases, 0.5 g/l and below, does not give satisfactory results. In continuous method of development on open soaper, care should be taken that ice is not left in the reservoir holding diazotised base(s). This otherwise will continuously go on diluting the liquor resulting in shade variation. Here, for attaining the temperature crushed ice should be used and unmelted ice removed before making the final volume.

The quantities of sodium nitrite, hydrochloric acid, sodium acetate and acetic acid depend upon the base used. Also, there is a definite coupling ratio of a naphthol with a particular base and therefore, excess amount of base if taken is a waste. For all this manufacturers shade cards should be referred.

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After coupling, the fabric is given a thorough soaping at boil to remove the pigment deposited on the fabric as far as possible. Thorough soaping also leads to aggregation of particles and thereby the attainment of true shade in some cases. This also improves fastness to light and chlorine. Some combinations such as Naphthol AS with Orange GC, and Red GG bases, Naphthol AS-SW with Yellow GC and Red GC bases should, however, not be soaped above 65-70oC.

Phthalogen Blue

Phthalogen Blue is not a dye by itself but is produced in the fibre like azoics. It belongs to phthalocyanine class and produces a very bright and pure blue shade having good tinctorial strength. The first dye of this class was put in the market by Bayer under the name Phthalogen Brilliant Blue IF 3G. This is a monomeric product which on condensation in the presence of a metal atom forms a ring structure with metal atom at the centre. During application, therefore, a metal salt of either copper or nickel needs to be incorporated in the recipe. Theoretically, the monomeric substance is supposed to be soluble in water but the fact that the product contains partially polymerized product makes it sparingly soluble. This, therefore, necessitates the use of emulsifiers. Also some solvent required during condensation needs to be added. Suitable solvents are the mixture of dihydric alcohols (ethylene glycol, diglycol etc.) and amides (formamide, dimethyl formamide). However, these days ingrain products are claimed to be available which do not require a solvent. Another development in this field is the availability of product like Phthalogen Brilliant Blue IF 3GM, which is a physical mixture of Phthalogen Brilliant Blue IF 3G and Phthalogen K. The latter is an organic complex which releases copper at high temperature and not at the temperature of application. This obviates the need of adding a copper complex separately.

The application to fabric is done through pad-dry-polymerise-after treatment sequence. The padding liquor is prepared as follows:

Take required quantity of the monomeric product in a dry vessel, wet it with equivalent quantity of methanol and prepare a smooth paste.

Add the required amount of emulsifier with stirring and amine containing solvent (proprietary products such as Ahurasol TRAF, Levasol TR, Glycine A may be used).

Follow it up with urea dissolved in water, acetic acid, and copper complex if required.

Make up the volume with cold water.

The temperature of this prepared liquor should be maintained at 18-20oC. The increase in temperature beyond this level leads to the loss of active ingredient. For maintaining the temperature, however, solid ice should not be added. If added, it would be diluting the liquor during application leading to tailing effect.

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For concentration of ingredients, manufacturers’ recommendations should be followed in general but a few small trials in the laboratory are advisable for standardization before large scale runs. The preparation of liquor should not be done in copper and zinc vessels and use of dry vessels checks hydrolysis of the product.

The fabric is padded preferably with dip-nip-dip-nip system. The hardness of mangle rollers should be nearest otherwise face to face variation in depth is likely. Drying of the fabric is carried out in a hot flue or preferably a float dryer. The temperature during drying should not be kept more than 100oC. This on one hand ensures satisfactory removal of moisture and retention of the amine-solvent vital for condensation, on the other. While drying it is advisable to reduce the speed of fans, if possible, in the first chamber. This minimizes the risk of migration.

A migration inhibitor may also be added to the padding liquor as a precautionary measure. The colour of the dried fabric is a measure of appropriateness of drying operation. A distinct green shade indicates correct drying. If the colour of dried fabric is that of straw it indicates the exposure of padded fabric to higher temperature than around 100oC in drying. The dyeings thus obtained are of weaker depth.

The dried goods should be protected against humid air and polymerized preferably not later than 4-6 hours after drying. The polymerization is effected at 150oC with a residence of five minutes. The colour of the fabric becomes blue on polymerization. However, during the process some by-products are also formed and cause greenish tinge. The by-products are removed by an acid treatment.

The acid treatment can be carried out either in a jigger or in an open-width soaper. Normally, the treatment is carried out in jiggers. After a short hot wash, the fabric is treated with 3-5 g/l hydrochloric acid (83o Tw) at about 60oC for two ends. Then 1-2 g/l sodium nitrite dissolved in water is added to bath and two more ends are given. This treatment brightens the shade considerably. This is followed by cold wash, hot wash and hot soaping with soda ash and detergent. The colour of the dyed fabric is rich blue or peacock blue.

Pigments

Pigments are not dyes in true sense as they do not contain chromophore and auxochrome, the qualification for an organic compound to be known as a dye. These, therefore, do not possess affinity for textile fibres but are held on the fibre mechanically. The usage of pigments is extensive in printing of all cellulosic and polyester blended textiles. In dyeing, however, their use is confined to pastel and light shades and that too not for quality fabrics. The pigments are held on the surface by a polymeric film which is strong but at the same time resilient. The essential ingredients of pigment colouration system are pigment(s), binder (a

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copolymer product), catalyst – an acid liberating agent, and acetic acid to maintain pH in acidic side. The catalyst is needed to effect cross-linking and thereby fixation of binder film on the fabric; ammonium sulphate, ammonium chloride, diammonium hydrogen phosphate (DAP) and Catalyst LCP are frequently used. Catalyst LCP has an advantage over others as it is effective even at low temperatures around 110oC. the colouration process involves three steps: pad-dry-cure.

The padding liquor is prepared as follows:

Dilute the binder (Acramine SLN or any other similar product) with 2-3 times water and strain.

Dilute the pigment emulsion, filter, and add diluted binder to it slowly but with stirring.

Now add acid liberating agent – the catalyst, predissolved in water and then acetic acid (2-3 g/l).

Finally make up the volume with water at room temperature.

Sometimes antimigrant (gum) is also added to the liquor. The concentration of binder ranges between 10-20 g/l depending upon depth and that of catalyst around 0.5 to 1.0 g/l. While preparing liquor care should be taken that acetic acid is not added to binder as it will lead to the formation of lumps. The padding should preferably be done on a two bowl mangle with wet-pickup of not above 70%. Higher wet-pickup leads to migration during drying.

The drying is carried out either on a hot air equipment (float dryer, hot flue) or on a cylinder drying range (CDR), the latter being more popular. Care, however, should be taken that first two-three cylinders are either Teflon coated or covered with 3-4 layers of fine cotton fabric. The drying in float dryer or hot flue should be carried out at about 120oC with temperature in the first chamber around 100oC.

The dried goods are cured at about 150oC for 2-3 minutes in a polymeriser. However, if catalyst LCP is used as an acid liberating agent curing can be omitted without affecting fastness properties. In this case one more passage on CDR is sufficient. Even otherwise also curing is seldom carried out but it does affect fastness properties. Understandably, curing at this stage is not necessary if the fabric is to be printed later with pigments. A subsequent warm wash, if given to dyed fabric, accords better hand. While padding large yardage of fabric, binder film slowly tends to form on the bowls of the mangle. This, if not removed, creates the problem of migration, specky dyeing and tailing. The bowls, therefore, should be cleaned regularly after every 500 metres with a fabric soaked with acetic acid. Another precaution worth taking is that binder should always be diluted and strained before addition to diluted pigment emulsion. Thirdly, fabric intended for pigment padding should be dried through acetic acid to neutralize any alkali left on the fabric after bleaching.

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