1.1 TEXTILES:The word textile encompasses anything that is associated with fibres, whether they be of animal, vegetable or mineral origin, manufactured or natural. Fibres can be spun together to form continuous filaments which can then be knotted, woven or felted. When fibres are spun and then woven, the result may be, or can be, a length of fabric (natural or synthetic). Fabrics can be further enhanced by controlled weaving to create textured patterns within the woven form, colouring of different spun fibres to create distinct pattern groupings and finally by adornment through such techniques as creative stitchery, appliqu or beading. Textiles are historically an integral part of many cultures. People have knotted woven, intertwined, felted, knitted, sewn for many reasons. To create functional objects was always the primary aim, but personal adornment and embellishment have grown into cultural traits over the years. People have knotted and intertwined continuous filaments to create everyday usable objects such as carry-bags. They have used the knotting technique to increase the strength of fibres so that their function and life span may be extended and, of course, people have woven fibres into continuous lengths of cloth for personal clothing and soft furnishing. The urge to adorn and beautify has led people to embellish these functional objects. Such embellishment has been handed down through generations so that today we can see and identify pattern and colour combinations in one culture uniquely different from those in another. Today, work with fibres encompasses not only functional forms but also non-functional sculptural and/or pictorial forms. When we look at the extensive range of textile products and techniques used to obtain these products it may all seem quite daunting and you may wonder where to start. However, by grouping according to technique it all begins to become a lot clearer. 1. Fibre to Fibre - spinning, weaving, tapestry, crochet, macram, knitting, felting. 2. Colour to Fabric - tie dye, batik, gutta, dispersal dye, serigraphic printing (silkscreen) relief printing - lino, card, string, hand painting. 3. Fabric to Fabric - patchwork, appliqu, quilting. 4. Fibre to Fabric - creative stitchery. Each technique can be used by itself to create an end product, or they can be combined to create further exciting products.

1.2 Dyes & Fibre Polymer System1.2.1 Wool Fibres:The wool fibre is composed of the protein keratin, which consists of long polypeptide chains built from eighteen different amino acids. Most of these acids have the general formula H2N.CHR.COOH, in which R is a side chain of varying character. The chain structure is of the type:

And at intervals bridges derived from the amino acid cystine connect the chains.

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Some of the side chains end in amino groups and others in carboxyl groups; internal salts are therefore formed and the

Molecules are bound together by electrovalent linkages. The molecules of keratin are very large, with and average molecular weight estimated at about 60,000. The wool fibre is readily destroyed by alkali, but withstands acid conditions fairly well; some hydrolysis of peplide linkages occurs on prolonged boiling with acids, however. The carboxylic acid and amino groups in the keratin molecule confer affinity for basic and acid dyes. Basic dyes are now little used on wool since their fugitive properties render them unsuitable for such and expensive and durable fibre. Acid dyes, however, are extensively used, and the general characteristics of this large class and the related mordant and pre-metallised azo dyes are now described. Since the bonds between dye anions and amino groups in the wool fibre are easily broken and re-formed, dyes attached in this way are liable to migrate. This property is advantageous, in that level dyeing is readily attained, but it leads to low fastness to wet treatments, and any undyed wool present during washing becomes stained. These characteristics are chiefly apparent in dyes of low molecular weight, and fastness to washing is in general much better in more complex dyes. The larger dye molecules are evidently attached the fibre by some means other than the ionic bonds mentioned above, and it is believed that6 they are held by non-polar Van-der Waals forces exerted between hydrophobic dye anions and hydrophobic regions of the wool fibre, their strength being proportional to the area of contact. From an application point of view acid dyes are classed as either Levelling or Milling types. The Levelling (sometimes called Equalizing) dyes have fairly simple chemical structures, migrate readily on wool, and are easily applied from strongly acid baths; their wet-fastness properties are low. The Milling dyes are structurally more complex, have high affinity, and must be applied form weakly acid baths for control of the rate of dyeing, but they show high fastness to milling and other wet treatments. Milling is a felting process applied to woolen cloth by squeezing or beating, usually in a soap solution. It sometimes follows dyeing, and the dyes used must then have high wet-fastness properties in order to withstand these severe conditions. The advantages of good levelling and high milling fastness cannot be fully combined in a single dye, but there are general purpose dyes with intermediate GTFVJ,./properties. The application classes can becorrelated roughly with chemical types, as shown for monoazo and disazo dyes in Table 4 1, which provides a few typical examples. As might be expected from the foregoing generalizations, trisazo and other polyazo dyes are of the milling class, but since shades are usually dull and uneven they are seldom of technical value on wool. 1.2.2 Silk Fibres: Cultivated silk is a natural fibre produced by larvae of the silkworm Bombyxmori, and wild silk is produced similarly by silkdworms of various species. Raw silk consists of the protein fibroin surrounded by silk gum (sericin), and the latter is removed in the process of de-gumming or boiling off which precedes dyeing.

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Fibroin consists of long parallel chain containing about 400 amino acid residues with a structure of the general type

The residues are derived mainly from the amino acids glycine (R = H), alanine (R = CH3), serine (R=CH2OH) and tyrosine (R= --CH2--- OH), but there are numerous others in small quantities. Fibroin differs from keratin in that it contains no sulphur. Its chemical properties are similar to those of keratin, but it is more sensitive to acids than the latter and less sensitive to milk alkalis. Silk can be dyed with dyes of almost every class, but some restrictions arise from the common practice of weighting the fibre with tin salts, which is carried out in order to improve handling properties and reduce cost. So far as azo dyes are concerned the main classes applied to silk are the acid dyes and pre-metallised dyes already described as wool dyes, the direct dyes described in Chapter and the reactive dyes described in Chapter. Mordant dyes applied to silk are mainly of the anthraquinone type. It has never been necessary to develop dyes especially for silk. 1.2.3 Cellulosic Fibres: The earliest cellulosic fibres were lines and cotton, both of which have been used since remote antiquity. Linen, or flax, is derived from bast fibres of plants of the Linum family, especially Linum usitatissimum. After removal of glutinous and pectinous matter the fibre has cellulose content of 82 83%. Cotton, which is fine hair attached to seeds of various species of plants of the Gossypium genus, has a cellulose content which may reach 96%. Cellulose is a polymer of high molecular weight consisting of long chains of D-glucose units connected by B-1, 4- glucosidic bonds, and its structure may be represented as follows:

Each glucose unit contains three alcoholic hydroxyl groups, of which two are secondary and one is primary. The degree of polymerisation of cellulose varies from a few hundred to 3500 or more.Regenerated cellulose fibres were introduced during the last two decades of the 19th century. The first process was that Chardonnet (1884), who produced a fibre by spinning a solution of nitrocellulose in a mixture of alcohol and ether and subsequently removing nitro groups. The cuprammonium process followed (1890), and in 1891 Cross and Bevan introduced the viscose process

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whereby wood pulp cellulose is treated with caustic soda and carbon disulphide to form sodium cellulose xanthate, which, after a ripening stage, is spun into an acid coagulating bath. The nitrocellulose process is now obsolete, but the cuprammonium process, which has the advantage of giving an exceptionally fine filament, is still used. The viscose process is of much greater importance, but it is declining in consequence of the development of the newer synthetic fibres. The dyeing properties of the various cellulosic fibres are broadly similar, but application conditions are affected by differences in physically properties. Thus lines, which has a harder structure than cotton, is less readily penetrated by dyes. There are also differences in dyeing properties between the several types of regenerated cellulose fibres; cuprammonium rayon, for example, having fine filaments, is more easily dyed than viscose. Dyes of many chemical classes are applied to cellulosic fibres. Azo dyes, which predominate numerically, are described here, and others are dealt with in the appropriate chapters. The first substantive or direct dyes discovered in 1884 were diazo dyes obtained from tetrazotised benzidine, but other structure have since been found to confer affinity for cellulose. The azo group itself favours substantivity but for adequate effect either a second azao group or another favourable group must also be present in the dye molecule.The structures which are chiefly important in substantive dyes are as follows

All of the other groupings listed find used in conjunction with the azo chromophore to give a great variety of dyes for cellulosic fibres. Others of smaller importance, such as the residues of pyrazol-5-one, resorcinol and m-phenylenedianine, also confer a measure of cellulose affinity. Apart from the presence of one or more of the favourable components there are other structure requirements for substantivity. Typical substantive azo dyes of the various chemical classes are now described. 1.2.4 Monoazso Dyes: 4

About 35 monoazo direct dyes are in use, most of them containing either a thiazole or a J acid residue. Examples are CI Direct Yellow 8 (CI 13920), CI Direct Brown 30 (CI 17630) and CI Direct Red 118 (CI 17780) (diazotised and developed on the fibre with B-naphtol or 3- methy-1-phenylpyrazol-5-one), with the structures shown:

1.2.5 Dyes with Mixed Chromophores: Polyazo dyes normally contain a single chromophoric system, and a conjugated chain runs through the whole molecule. It is possible, however, for a dye molecule to contain two or more independent chromophoric systems electronically insulated from each other. Such dyes were first introduced by CIBA, who utilized the triazinyl ring as a chromophoric block. This ring serves as a convenient link since it can be introduced 5

by reaction of cyanuric chloride with two or three aminocontaining dyes in succession. Substitution of the first chlorine atom takes place easily in presence of alkali at atmospheric temperature. The second chlorine atom is less easily removed, and reaction with an amino compound may require a temperature of 55o 60oC, the optimum conditions varying with the basicity of the amine. Replacement of the clorine atom calls for still more vigorous conditions, and a temperature of 90o 100oC may be suitable; much higher temperature are needed, however, in the case of weakly basic compounds. The progressive loss of activity at each stage enables condensation to be carried out with three different components to give a substantially homogeneous product. The residues of three dyes (Dye 1)-NH2, (Dye 2)-NH2 and (Dye 3)-NH2 may be linked by a series of reactions in alkaline medium, as shown:

In the resulting product each dye residue contributes its own absorption characteristics; by combining yellow and blue components, green dyes can therefore be obtained that are much brighter than normal polyazo greens. Dyes containing three inde; pendent chromophoric system is of limited interest and the third condensation is often carried out with a suitably reactive colourless compound such as aniline or phenol.An example of a commercial dye containing two electronically insulated chromophores is Chlorantine Fast Green BLL (CIBA) (CI Direct Green 26; CI 34045); it gives bluish green hue on cellulosic fibres. One of the aminoazo dye residues. A commercial dye containing both azo and anthraquinone residues is Chlorantine Fast 5 GLL (CIBA) (CI Direct Green 28; CI 14155), which has the structure (2) and gives bright yellowish green shades on cellulosic fibres.

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1.2.6 Cellulose Acetate Fibres: The D-glucose units in the cellulose polymer contain three hydroxyl groups, of which one is primary and two are secondary. By acetylating all of the hydroxyl group in cellulose a triacetate is obtained with a polymetric structure which may be represented thus:

The triacetate is soluble in chloroform, and a fibre known as Lustron was spun from chloroform solution in early small-scale American manufacture (1914-1924). If the triacetate is partially hydrolysed to give a mixture with an average of 2 acetyl groups per glucose residue the product loses solubility in chloroform, but becomes soluble in acetone. A different product, which is insoluble in acetone, is obtained by direct introduction of 2 acetyl groups; presumably the less accessible hydroxyl groups are the last to be acetylated and the last to be re-formed on hydrolysis. During the First World War incompletely acetylated cellulose was produced on a large scale for use as a dope for aircraft fabric. After the war efforts to find a new use for it led to the production of cellulose acetate fibre by British Celanese Corporation. The commercial product contains an average of 2-3 acetyl groups per glucose residue, and 7

is known as secondary acetate, or simply acetate. It is spun from acetone solution. In spite of an inconveniently low melting-point acetate fibre attained great success and is still extensively used, but in recent years it has been partly superseded by other synthetic fibres. It was appreciated in the early days that the dyeing of acetae differs from that of the natural fibres and viscose in that fibre serves as a solvent for the dye. In 1923 work on dyes in the form of aqueous dispersions was carried out independently by British Celanese Corporation and British Dyestuffs Corporation. The SRA colours of British Celanese Corporation were dispersions of aminoazo or hydroxyazo dyes obtained by means of the surface-active agent sulphated ricinoleic acid. Other dispersing agents used included alkyl sulphates, alkaryl sulphonates and fatty alcohol-ethylene oxide condensates (a long alkyl chain being usually present in the molecule), and the dispersions obtained by applying them with various milling techniques were often so fine as to be easily mistaken for true solutions. British Dyestuffs Corporation marketed dispersed aminoazo and hydroxyazo dyes in their Dispersol range, and this is still maintained by ICI (formed in 1926 by a union of British dye stuffs Corporation with other firms). The Duranol range is a parallel range of dispersed dyes of the anthraquinone series. Dyes of these types are now produced by many manufactures. Whereas the dyes were formerly supplied only as aqueous dispersionsthey are now usually marketed in the form of re-dispersible powders which yield suitable dispersions on stirring with water. Hydroxyalkylamino groups impart a small degree of watersolubility and assist dispersibility; coupling components such as N-ethyl- N- B-hydroxyethylaniline or N, N-di (B-hydroxyethyl) aniline aretherefore commonly used. Many of the earlier yellow and orange dyes proved to be phototropic, but it was found that this groublesome characteristic can be largely avoided by introducing nitro or other negative groups into the dye molecule; these substituents restrain trans cis isomerizatrion. Since dye molecules must be fairly small in order that they dissolve readily in the fibre monoazo dyes are commonly used, but a few disazo dyes are included in commercial ranges. Blacks are obtained by diazotising aminoazo dyes on the fibre and developing with a solution of 3-hydroxy2-naphthoic acid. In 1936 a range of water-soluble dyes for acetate was marketed by ICI under the name Solacet. Their solubility was due to the presence of a sulphuric ester group (-- OSO3Na), usually introduced by sulphation of a B-hydroxyethlamino group, which did not seriously impair affinity for the fibre. The unsuitability of conventional water-soluble dyes for acetate fibres is apparently due to the presence of highly ionised SO3-Na+ groups rather than their solubility in water. In consequence of the of the development of dispersed dyes with improved dyeing and fastness properties the Solacet range has now been superseded. Many dyes developed for acetate have now been applied to the newer synthetic fibres, and the manufactured ranges have been extended specially for these outlets. The term Acetate dyes has therefore, been discarded in favour of Disperse dyes so that all applications may be included. Disperse dyes for fibres other than acetate are described later. Examples of azo disperse dyes applied to acetate are shown in Table 4.6; violet and blue dyes are included, but disperse dyes of these shades are derived mainly from the anthraquinone series.

1.2.7 Cellulose Triacetate Fibres: Cellulose triacetate was manufactured in the United States during the period 1914-24, but the process was unsatisfactory because the only suitable solvent then

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commercially available was chloroform, and this was both toxic and expensive. Methylene dichloride is a suitable solvent of low toxicity, and it became available fairly cheaply about 1930, but by the eat time secondary acetate was fully established. Later, however, when the hydrophobic fibres nylon and Terylene had achieved great success, the possibilities of triacetate as an inexpensive fibre sharing some of their good properties became apparent, and it has now been introduced as a commercial fibre under names such as Tricel (Courtaulds Ltd.), Armel (Celanese Corporation of America) and Trilon (Canadian Celanese Ltd.). It has good shrink- and creaseresistance, is quickdrying, shows good fastness to wet treatments and can be heat-set without loss of lustre. As it has a higher melting point the hazards associated with the ironing of fabrics of secondary acetate are largely avoided. Because of its hydrophobic character triacetate is less easily dyed than acetate, but suitable dyes can be selected from existing ranges of disperse dyes. Whereas acetate is dyed at 75o 80oC, triacetate requires a temperature at or near the boil. If the fabric is to be heat-set for pleats the dyes used must be stable at 200oC. 1.2.8 Poyamide Fibres: Nylon 6,6 and Nylon 6 can be dyed by many disperse, acid and direct dyes. Since many suitable dyes are available, commercial ranges are usually selected from products already manufactured for other purposes, and (apart from the reactive dyes discussed later) new structures have not been required. It has proved very difficult to manufacture nylon with uniform dyeing properties, and for this reason dyes with good leveling properties are necessary. In this respect disperse dyes have a great advantage in that they conceal fibre irregularities. For high wet-fastness acid dyes are preferred, but very careful application is necessary in order to secure level dying. These dyes often show better wet-fastness properties on nylon than on wool because of the hydrophobic character of the former. Fastness to light, however, is often slightly lower on nylon than on wool. The affinity of acid dyes for Nylon 6 is higher than that for other types because polycaprolactam fibres contain a higher proportion of free amino groups Ranges of acid dyes for nylon are classified by the makiers so that users may select dyes with good leveling or good wet-fastness properties according to their requirements, there are also ranges with intermediate properties and others specially designed for fabric printing. 1.2.9 Polyester Fibres: In the course of the exploratory work that led to the development of nylon W.H. Carothers examined aliphatic polyesters but abandoned them in favour of the more promising polyamides. Subsequently, however, the late J.R. Whinfield and J.T. Dickson of The Calico Printers Association re-examined polyesters for the purpose. They extended the work to aromatic compounds and obtained a polymer with excellent fibre-forming properties from terephthalic acid and ethyleneglycol. This has the structure (3). The important fibre Terylene was based on this

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Work, and was first prepared in the laboratory in 1941. Development and production of the fibre were carried out by ICI, and this fibre is now manufactured under various names on a very large scale in many parts of the world. Dimethyl terephthalate is now commonly used in place of the free acid so that the terminal carboxlic acid group is esterified. Since n has a value of about 80 the properties of the fibre are not greatly affected. The ester is preferred as starting material because it is more easily purified, and purity is essential in the manufacture of high polymers. Terylene fibre is highly hydrophobic, withstands attack by bacteria, moulds, months, acid and alkali, has high strength even in wet conditions, is superior to nylon in resistance to light, can be heatset and has glood di-electric properties. It is largely used for manufacture of net curtains, in blends with wool for suitings and other outerwer,k and for many industrial purposes. The hydrophobic nature of Terylene, its tightly packed molecular structure and its lack of reactive groups all render it unreceptive to dye molecules. Certain disperse dyes can be applied, but under normal conditions adsorption is slow and only pale shades are obtained. Much better penetration is obtained by dyeing at about 12Oc under pressure, and the special dyeing machinery required is now in general use. Since diffusion is slow in conditions of normal use, dyeings obtained in this way have good wet-fastness properties. Suitable dyes must be selected for this process since not all will withstand the high temperature without change of shade. Disperse dyes can also be applied at temperatures below 100oC by the aid of carriers added to the dyebath. These agents are supplied under various brand names, such as Tumescal (ICI), and usually consist of compounds such as o- or phydroxydiphenyl. Their presence greatly facilitates the dyeing process, but the mode of action is not fully understood. This process enables dyeing to be carried out in standard machines, but it is somewhat expensive and has disadvantages in that the carriers are often difficult to remove completely; their presence may cause a noticeable odour and sometimes impairment of light-fastness. Polyester fibres can be dyed by the Thermosol process (DuPont), which consists in padding with disperse dye and a thickening agent, then drying and heating at 175o 200oC for about one minute. Under such conditions the dye is absorbed rapidly; after scouring to remove loose colour the dyed material is finished in the usual way.

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1.2.10 Polypropylene Fibres: Polyethylene has many applications in the plastic industry, but its low meltingpointrenders it unsuitable for fibre formation. Its homologure polypropylene exists in various forms according to the disposition of the substituent methyl groups; the isotactic polymers, in which these groups are all attached on the same side of the main carbon chain, can be spun and drawn into fibres. Catalysts promoting formation of isolactic polymers were discovered by Zeigler, and the process was further developed by Natta and others Polyproplene fibres and hydrophobic and resist chemical attack, but they are not readily dyed. Certain disperse dyes can be applied, but only pale and medium shades are obtainable. Many methods have been described in the paten literature whereby the fibre may be modified to confer affinity for acid or basic dyes. Side chains carrying polar groups may be grafted to the main polymer chain, or basic substances may be included in the melt from which the fibre is spun. Much attention has been paid to a method whereby a compound of a polyvalent metal such as nickel, zinc or aluminium is incorporated in the fibre, which can then be dyed with metalisable dyes. 1.2.11 Polyurethane Fibres: Several elastomeric fibres developed in America are based on polymeric structures containing urethane (--NHCOO--) linkages. Full details of the processes used are not available but complex cross-linked polymers with rubber-like properties are obtained from polyesters or polyethers containing terminal hydroxyl groups by means of a series of reactions involving di-isocyanantes and diamines. Typical examples of such fibres are Lycra (Du Pont) and Vyrene (U.S. Rubber Co.), which are extensively used for foundation garments and swimsuits. These fibres have advantages over rubber in strength, resistance to oxidation, perspiration and cosmetic oils, also in whiteness and affinity for dyes. They are readily dyed by acid, basic and disperse dyes, but fastness properties are in general rather low. 1.2.12 Polyacrylonitrile Fibres: The simplest, polyacrylonitrile fibres are straight polymers of

acrylonitrile with structure where n varies from 600 to 2000. The first commercial fibre of this type was Orlon, introduced by DuPont in 1948. It can be dyed by basic dyes or by acid dyes in presence of copper sulphate. Various modified acrylic fibres (often called modacrylic fibres) are now obtained by copolymerizing acfrylonitrile with other substances, and dyeing properties are thereby improved; Orlon as now manufactured is a copolymer, but the identity of the second component has not been disclosed. Acrilan (Chemstrand Corporation), Dynel (Union Carbide Corporation) and courtelle (Courtaulds Ltd.) are other modified acrylic fibres. In general acid, disperse, basic and vat dyes can be applied to these fibres, but acid dyes are not recommended for Courtelle. The older basic dyes often show better light

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fastness on acrylic fibres than on natural fibres, but new basic dyes have been developed with fastness properties on polyacrylonitrile that are fully compatible with modern standards.

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2.1 DyestuffDyestuff is organic or inorganic substances which can absorb light and reflect some lights to show color. Actually, the dyestuff is water soluble substances.

2.2 PigmentPigment is a substance that can absorb light and reflect some lights to show color but it is water insoluble substances. Normally it is used for printing (with the presence of binder) or mass-coloration of the synthetic fibers.

2.3 CRITERIEA FOR A SUITABLE DYESTUFF1. Cheap 2. Non-toxic 3. Compatible to other dyes and chemicals 4. High color strength 5. Better brightness 6. Better fastness 7. Good levelness on the materials

2.4 Exhaustion of the dye stuff to the fibers is as follows:1. Moving of dyestuff from dye bath to surface of the fiber 2. Adsorption of the dyestuff into the surface of the fiber 3. Diffusion of the dyestuff into the center of the fiber.

2.5 Classification of DyestuffsDyestuffs can be classified according to two ways 1. According to common parent structure 2. According to application

2.5.1. Classification according to common parent structure or Chemical classification of dyes:The chemical constitution of dyes are varied that it is difficult to classify them into distinct groups. The colour index classifies dyes as shown. 1. 2. 3. 4. Nitro dyes. Nitroso dyes. Azo (Monazo) dyes. Azoic dyes. 13

5. Stilbene dyes 6. Tgri0aryl methane dyes. 7. Xanthenei dyes. 8. Quinoline dyes. 9. Methine dyes. 10. Acridine dyes. 11. Nitro dyes. 12. Sulphur dyes. 13. Thiazole dyes. 14. Thizine dyes. 15. Indamine dyes. 16. Oxazine dyes. 17. Azine dyes. 18. Lectone dyes. 19. Anthra quinine dyes. 20. Indigoid dyes. 21. Phthalocyanin dyes.

2.5.2 CHEMISTRY OF SOME DYESTUFF(1) Nitro Dyes: Nitro dyes are polynitro derivatives of phenols containing at least one nitro group ortho or para to the hydroxyl group. They are of relatively little importance industrially, because the colours are not very fast. Examples of this class are picric acid (2,4,6-trinitrophenol), Maritus yellow (2, 4-dinitro-1-naphthol), and Naphthol yellow S (2,4-dinitro-1- naphthol-7-sulphonic acid). Naphthol yellow S can be used to dye wool, and is one of thecolours permitted in foods. Naphthol yellow S can be used to dye wool, and is one of the colours permitted in foods. 1. Naphthol yellow S can be used to dye wool, and is one of the colours permitted in foods.

(2) Azo Dyes: The azo dyes represent the largest and the most important group of dyes. They are characterised by the presence of one or more azo groups (N=N), which form bridges between two or more aromatic rings. Preparation of azo dyes involves the following two steps.

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Step 1. Conversion of primary aromatic amines into diazonium compounds by treatment with sodium nitrite in excess hydrochloric acid (Diazotisation)

Step2. Coupling of diazonium compounds with phenols, naphthols, or other aromatic amines. Coupling with phenols and naphthols is carried out in basic solution; coupling with amines is carried in acid solution.

The above reactions are carried out at low temperatures (0 5o) because diazonium compounds are usually unstable. In the resulting dyes an aromatic system joined to the azo group is the chromophore, and the hydroxyl group or amino group is an auxchrome. To simplify the description of azo dyes, only the coupling step will be shown. It will be assumed that the diazonium compound was obtained by diazotisation of the corresponding amine. (a) Aniline Yellow: Solvent Yellow 1. It is p-aminoazobenzene. Aniline yellow is the simplest azo dye and is obtained by coupling benzenediazonium chloride with aniline in acidic medium

Aniline yellow is used as a dye for oils and lacquers, and is also an intermediate for other dyes. (b) Chrysoidine: Basic Orange 2. It is 2, 4-diaminoazobenzene. Chrysoidine is prepared by coupling benzenediazonium chloride with mphenylenediamine.

(3) Diphenlymethane Dyes: Auramine O: It is one of the most valuable of the diphenylmethane dyes. It is obtained by heating Michlers ketone with ammonium chloride and zinc chloride at 160oC.

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Auramine O is used to dye wool, silk, silk, nylon, rayon and paper. (4) Triphenylmethane Dyes: Triphenylmethane dyes can be identified by common structural cature shown below. Notice that the central carbon atom is joined to two benzene rings and to p-quinoid group. Triphenylmethane dyes are not fast to light or washing, however, except when applied to acrylic fibres. They are used in large quantitiesfor colouring paper, and typewriter ribbons where fastness to light is not so important. (a) Malachite Green: It is obtained by condensing benzaldehyde (1 molecule) with N, Ndimethylaniline (2 molecules) in the presence of concentrated sulphuric acid to give a leuco base (Gr. Leuco, colourless). Oxidation of the leuco base with lead peroxide followed by treatmen with hydrochloric acid yields the dye.

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(b) Pararosaniline: It is the simplest triphenylmethane dye. Pararosaniline is obtained by consdensinig p-toluidine (1 molecule) with aniline (2 molecules) in the presence of nitrobenzene to give a colourless carbinol. Nitrobenzene serves both as a solvent and an oxidising agent. Treatment of the carbinol with hydrochloric acid yields the dye. Pararosaniline has been used to dye cotton, wool and silk.

(5) Xanthene Dyes: Xanthene dyes can be identified by a common structural feature shown below. They are obtained by condensing phenols with phthalic anhydride in the presence of zinc chloride, sulphuric acid, or anhydrous oxalic acid. Examples of this class are fluorescein, eosin, and rhodamine B.

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(a) Fluorescene. It is prepared by heating resorcinol (2 molecules) and phthalic anhydridge (1 molecule) with zinc chloride at 190oC Fluorescein is of no value as a dye. It is red powder, which is insoluble in water. A dilute solution of fluorescein in sodium hydroxide gives a strong yellowgreen fluorescence when exposed to light. It is used to trace pollution of water supplies by sewerage, since if a small quantity of it is put in at the suspected source of pollution, the colour will be detectable at some distance from the source, even after extensive dilution. During World War II, fluorescein was used as a sea marker for airmen who had to bail out from aeroplanes over water. It also aided searchers in locating them. Fluorescein is also used as a mild purgative. The sodium salt of fluorescein is called Uranine. It is used to dye wool and silk. (6) Phthaeleins: Phthaleins are related to xanthene dyes and are made in the same way. Phenolphthalein: It is prepared by condensing phenol (2 molecules) with phthalic anhydride (1 molecule) in the presence of zinc chloride at 120oC

Phenollphthalein is not a dye. It is colourless solid, mp 261oC.Phenolphthalein is insoluble in water, but dissolves in alkalis to form deep red solutions. This is due to the formation of a disodium salt, the ion of which is coloured because of resonance. When excess of strong alkali is added, the solution of phenolphthalein becomes colourless. This is attributed to the formation of a trisodium salt, the ion of which is colourless because of loss of resonance and quinoid structure.

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Because of the colour changes shown above, phenolphthalein is used as an indicator in acid-base titrations. Phenolphthalein is an extremely powerful laxative, and this accounts for its widespread use as a denaturant for laboratory alchohol. (7) Indigoid and Thioindigoid Dyes: Indigoid. It is the parent compound of indigoid dyes and has been used in this country from times immemorial. Egyptian mummy clothes, which are 5000 years old, were dyed with it. Indigo was originally obtained from plants of indigofera group. The leaves from these plants were covered with water and allowed to stand for several hours. Enzymes present in the plants brought about fermentation, as a result of which the -glucoside of indoxyl (known as indican) in the leaves was converted into indoxyl and glucose. Upon exposure to air the indoxy was oxidised to indigo.

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Natural indigo contains an isomer of indigo known as indirubin(Indigo Red), and other impurities in varying proportions. Most of the indigo used at the present time is a synthetic product. It has replaced the natural stuff because of cheapness, purity, and uniformity of the manufactured dye. Preparation of Indigo. It may be obtained: (1) By Heumanns First Indigo Synthesis (1890). This involves the condensation of aniline with chloroacetic acid to give Nphenylglycine. The phenylglycine is then fused with sodium hydroxide and sodamide at 250oC to form indoxyl, which on oxidation by air yields the dye.

N-Phenylglycine is now obtained in much higher yield by interaction of aniline and the bisulphite compound of formaldehyde at 50-60oC, followed by treatment with aqueous sodium cyanide, and hydrolysis of the resulting nitrile.

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(2) By Heumanns Second Indigo Synthesis (1896): This involves the condensation of anthranilic acid with chloroacetic acid to form N-phenyl-o-carboxylic acid. This is then fused with sodium hydroxide and sodamide to produce unstable indoxylic acid, which decarboxyhlates to give indoxyl. Oxidation of indoxyl by air yields the dye.

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Structure of Indigo: The structure of indigo has been deduced from consideration of facts and conclusions such as the following. 1. Elemental analysis and molecular weight determinations show that the molecular formula of indigo is C16H10O2N2. 2. Fusion of indigo with sodium hydroxide at low temperature produces anthranilic acid. This indicates the presence of a benzene ring attached to one carbon atom and one nitrogen atom in the ortho position. 3. Oxidation of indigo with nitric acid yields only two molecules of isatin. This indicates that the indigo molecule contains two identical units joined together, and that each unit on oxidation produces a molecule of isatin. The following two possible structures of indigo meet this requirement. 4. Oxidation of indoxyl by air yields indigo. From above, it follows that (A) represents the structure of indigo. This has been confirmed by the following synthesis by Baeyer (1872).

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X-Ray analysis shows that mostly indigo exist in the more stable transform. However, derivatives of both are known. (a) Tyrian Purple. It is 6, 6-dibromoindigo. Its discovery was later than that of indigo but it is believed to have been known in 1600 BC.

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Tyrian purple occures naturally in the purple snail, Murex Brandaris. It takes about 12000 snails to produce 1.4 g of the dye. This explains its rarity and why the colour of mercial value. (b) Thioindigo. It is analogues to indigo in which the two NH groups are replaced by sulphur atoms. Thioindigo may be obtained by the reaction of thiosalicylic acid, with chloroacetic acid, followed by fusion with sodium hydroxide and oxidation by air. Thiosalicylic acid itself is made by diazotisation and reaction with hydrogen sulphide.

Thioindigo is used to dye cotton, wool and polyesters. (8) Anthraquinoid Dyes: Anthraquinoid dyes can be identified by a common structural feature shown below. Notice that a p-quinoid group is fused to two other benzene rings.

Anthraqluinoid dyes are used for dyeing wool, silk, nylon, cotton, leather and paper. The most important dye in this group is alizarin. Alizarin. It is 1,2dihydroxyanthraquinone. Alizarin derives its name from the fact that it was first obtained from the roots of the madder plant (Fr. Alizari, madder). It is now prepared from phthalic anhydride by the following six steps. Step 1. Phthalic anhydride is treated with benzene in the presence of AlCl3 to give obenzoylbenzloic acid. (Friedel-Crafts Reaction)

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Step 2. OBenzoylbenzoic acid is cyclized by treatment with concentrated sulphuric acid to form anthraquinone.

Step 3. Anthraquinone is heated with fuming sulphuric acid at 180oC to give anthraquinone-2-sulphonic acid.

Step 4. Anthraquinone-2-sulphonic acid is converted into its sodium salt by treatment with sodium hydroxide.

Step 5. Sodium salt of anthraquinone-2-sulphonic acid is fused with sodium hydroxide in the presence of potassium chlorate at 200oC under pressure to give sodium salt of 1, 2-dihydroxyanthraquinone. The purpose of potassium chlorate is to provide oxygen for the oxidation of the carbon atom at C-1.

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Step 6. Sodium salt of 1,2-dihydroxyanthraquinone is treated with sulphuric acid to yield 1,2-dihydroxyanthraquinone (Alizarin).

Alizarin can also be made by condensing phathalic anhydride with catechol in the presence of sulphuric acid at 180oC (See Below). Alizarin forms ruby red crystals, mp 290oC. It is insoluble in water and ethanol. It dissolves in alkalis to give purple solutions. Alizarin is used to dye cotton and wool. It is also used for making printing inks. Structure of Alizarin: The structure of Alizarin has been deduced as follows: 1. Elemental analysis and molecular weight determinations show that the molecular formula of alizarin is CI4H3O4: 2. Reduction of alizarin with zinc dust at 400oC produces anthracene. This indicates that it has the same carbon skeleton as that of anthracene. 3. Alizarin reacts with acetic anhydride to form a diacetate. This indicates the presence of two hydroxyl groups in the molecule. 4. Alizarin can be prepared by condensing phthalic anhydride and catechol in the presence of sulphuric acid at 180oC. This shows that alizarin is a dihydroxy derivative of anthraquinone and both OH groups are substituted in the same benzene ring.

The above facts limit our choice to two structural formula for alizarin. 26

5. Alizarin on nitration yields two isomeric mononitro derivatives, both of which on oxidation give phthalic acid. This shows that in each of these derivatives, the NO2 group is substituted on the hydroxylated benzene ring. Let us now write down the possible mononitro derivatives from (A) and (B). Mononitro derivatives from (A):

2.5.3 DYE CLASSES ACCORDING TO APPLICATION1. BASIC DYES The colours are very bright, but not very fast to light, washing, perspiration. Fastness is improved if they are given an after-treatment or steaming, e.g. French Silk dyes are basic dyes and should be steamed to fix. 2. ACID DYES These are acidified basic dyes, intended for use on protein fibres but can be used on nylon and acrylics. They have a fair light fastness but poor wash fastness 3. PREMETALLIZED DYES These are an acid dyes with the addition of one or two molecules of chromium. The dyesgive mutetonings, not unlike those of natural dyes. They are the synthetic dyes mostly used by weavers who dye their own yarns.

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4. DIRECT DYES These substantive dyes colour cellulose fibres directly in a hot dyebath without a mordant, to give bright colours. They are not very fast to light or to washing. Direct dyes are generally any dyes which use salt as their only fixative, e.g. Dylon dyes (not to be confused with reactive dyes, which use salt plus other chemicals). 5. AZOIC (NAPHTHOL) DYES These are another sort of direct dye, but ones that are extremely fast to washing, bleach and light. They are intended for cellulose fibres and can be used successfully on protein fibres, although the colours are different. These dyes are widely used all over Asia and Australia for batik and direct application. They can be used to give interesting texture colour effects on fabric, thread or paper. Their use for straight silk painting is minimal because of the difficulty in achieving evenness of painted colour. 6. DISPERSE DYES Originally developed for acetate fibres, these are now the major dyes for synthetics. They are not soluble in water, but in the actual fibres themselves. They require a carrier to swell the fibres so that the finely ground particles can penetrate. They are dyed hot, like direct dyes, but do not use salt. Disperse dyes are widely used for heat transfer printing (Polysol). Dye is printed or painted onto paper and heat pressed onto fabric. Prints have excellent light and wash fastness and strong bright colours. Their major disadvantage is that only synthetic fabrics can be used. 7. VAT DYES Vat dyes are the fastest for cellulose fibres. The dye is made soluble with alkali, put in a 'vat' with a reducing agent, usually sodium hydrosulphite, which removes all oxygen from the liquid, and the fabric is dyed, then oxidized in the air to achieve the true colour. Synthetic indigo is a characteristic vat dye, but there are many colours available 8. REACTIVE DYES Reactive dyes are the most recent of dyes. These are the most popular in the world among fibre and fabric artists, used at first only by surface designers, but recently by weavers as well. There are now reactive dyes for a wide range of fibres, e.g. cotton (PROCION), silk and wool (PROCILAN). The dye actually reacts with the fibre molecules to form colour and is, as a result, extremely fast to both light and washing. There are hot and cold water reactive dyes, in fact there is a dye for almost every need. They can be most successfully used for silk painting, with a much better colour fastness than the traditional basic dyes, and are already used by batik artists. we can identify a reactive dye by the alkali used to set off the fixation process, which requires time to take place (silk and wool reactives uses acetic acid). Assistants used are salt, soda ash and resist salt, and sometimes bicarbonate of soda and urea. Reactive dyes are equally suited to screen printing polychromatic printing, fabric painting yarn and piece dyeing.

2.6 Dyestuff SelectionFastness is always influencedby the type and constructionand the treatment (heat setting of the fiber, and the typeand family of dyestuff.Dyes should show similar

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diffusionand trichromatic behaviorand fixation rates. The combi-nation of fibers, dyes and auxiliaries must be selected carefully

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2.7 Dyes used for Textile IndustryDyes for the textile dyeing can also be grouped according to the fibres to which they can be applied. The strength of the dye-fibre attraction is called Affinity. Each class of dye has a unique chemistry, structure and way of bonding to the fibre. Colour consents are now being enforced and therefore dye fixation levels are becoming more important. Selection of the right dyes helps to minimise effluent losses. Typical dye fixation levels for most of the dye classes are shown in the table

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3.1 DyeingDyeing is the application of color to the whole body of a textile material with some degree of color fastness. Textiles are dyed using continuous and batch processes and dyeing may take place at any of several stages in the manufacturing process (i.e., prior to fiber extrusion, fiber in staple form, yarn, fabric, garment). Most of textile dyeing is done in finishing departments of basic textile manufacturing facilities, although there are also several commission dye houses. From an environmental perspective, dyeing has typically been viewed as a waste water issue due to large quantities of water, chemicals, and auxiliaries (such as salt). Dyeing is essentially a mass transfer process where the dye diffuses in solution, adsorbs onto the fiber surface, and finally, within the fiber. Dyeing is complicated by the fact that there are many sources of color variations, such as dyes, substrate, and preparation of substrate, dyeing auxiliaries used, and water. Processing variables such as time, temperature, and dye liquor ratio (pounds of dye bath to pounds of cloth) also affect dyeing results. There are hundreds of dyes within several dye classes, each of which exhibits different results when applied to different types of fabric. Various types of dyeing machines are used for both continuous and batch processes. Every dye system has different characteristics in terms of versatility, cost, tension of fabric, use of carriers, weight limitations, etc. Dyeing systems can be aqueous, non-aqueous (in organic solvents), or use sublimation (thermosal, heat transfer). Hydrophilic fibers such as cotton, rayon, wool, and silk, are typically easier to dye as compared with hydrophobic fibers such as acetate, polyesters, polyamides, and polyacrylonotriles. The four basic steps in the dyeing process are: dissolving or dispersing dye; diffusing dye onto the fiber surface; absorbing dye onto the fiber surface; and diffusing dye into the fiber. Batch dyeing involves moving the dye liquor through the goods or moving the goods through the dye liquor. The textile material is immersed in the dyebath during the entire period of dyeing. In batch dyeing, a certain amount of textile substrate, usually 220 to 2200 pounds, is loaded onto a dyeing machine and is brought to equilibrium or near equilibrium with a solution containing the dye. Once immersed in the dyebath, because the dyes have an affinity for the fibers, the dye molecules leave the dye solution and enter the auxiliary chemicals and controlled dye bath conditions (mainly temperature) accelerate and optimize the action. The dye is fixed in the fiber using heat and/or chemicals after which the substrate is washed to remove unfixed dyes and chemicals. There is a trend to use of lower liquor ratios (pounds of dye bath to pounds of cloth) in batch dyeing, which lends benefits such as faster heating/cooling and less waste. Batch equipment can usually be purchased as atmospheric (operated below 212 F) or pressurized (operated to about 280 F) machines. Most batches dyeing are being done using pressurized machines, although some facilities use atmospheric machines, especially for fabric dyeing. Atmospheric dyeing might be required for fleeces and stretch fabrics, such as Lycra, which typically cannot be dyed using jet equipment. Dyeing processes in pressurized machines release no HAP emissions to the atmosphere since the process is totally enclosed and the pressure is released at the end of the dyeing process by cooling the dye bath which is subsequently drained before opening the dyeing machine. However, in some cases, the drying of the pressure-dyed substrate releases HAP emissions. Continuous processes typically consist of dye application, dye fixation with chemicals or heat, and washing. Almost all continuous dyeing is done at atmospheric pressure. Continuous dyeing is usually used for long runs of polyester/cotton fabrics and involves immersing fabrics in a relatively concentrated dye bath for short periods. Textiles are fed continuously into a 31

dye range at speeds usually between 540 and 2690 feet per minute and a concentrated solution of dyes and chemicals (held in pads) is moved evenly and uniformly to the goods with thorough penetration. A pad mangle helps apply pressure to squeeze dye solution into the fabric and the dye is usually diffused or fixed by heating in a steamer or oven. Dye fixation on fiber occurs much more rapidly in continuous dyeing as compared to batch dyeing. After fabrics are dyed, they are dried in ovens or tenter frames after washing to remove un-reacted chemical or loose dye. Fabric that is processed through atmospheric batch dyeing is not dried at the dye range; it is sent to finishing and may be finished wet or dry. Various classes of dyes can be used, e.g. disperse for synthetics and direct for cellulosics. Dyes used in the textile industry are mostly synthetic and are derived from coal tar and petroleumbased derivatives. Dyes are sold as powders, granules, pastes, liquid dispersions, and solutions.Not only are d yes applied in different ways, and they also impart color using different mechanisms. Dyes can be classified according to chemical constitution or method of application. Dyestuffs can work on principles of electrostatic bonding, covalent bonding, or physical entrapment. For example, acid dyes work through the mechanism of electrostatic bonding, whereas disperse dyes work by physical entrapment. Different dye classes exhibit different affinities depending on the type of fiber, although even dyes within the same classes can show wide affinity variations. They also exhibit different properties such as their fastness under end use conditions such as light, laundering, or dry cleaning. Various combinations of chemical auxiliaries and process conditions (temperature and pressure) may be used to better fix the dye on the fabric or impart specific characteristics. For example, a dye bath may contain the dyestuffs along with appropriate auxiliaries such as wetting agents and also specific chemicals such as acetic acid or sodium hydroxide. The use of higher temperatures and superatmospheric pressures have reduced the need for dye carriers (chemical accelerants) that were required at lower temperatures for the use of disperse dyes on synthetic substrates, such as polyester. Staining: Staining is an unpleasant of dyeing in the area that we do not want

3.2 A BRIEF HISTORY OF DYEING.Ever since primitive people could create, they have been endeavoring to add color to the world around them. They used natural matter to stain hides, decorate shells and feathers, and paint their story on the walls of ancient caves. Scientists have been able to date the black, white, yellow and reddish pigments made from ochre used by primitive man in cave paintings to over 15,000 BCE. With the development of fixed settlements and agriculture around 7,000-2,000 BCE man began to produce and use textiles, and would therefore add color to them as well. Although scientists have not yet been able to pinpoint an exact time where adding color to fibers first came into practice, dye analysis on textile fragments excavated from archaeological sites in Denmark have placed the use of the blue dye wood along with an as yet unidentified red dye in the first century CE. In order to understand the art and history of dyeing, we must first understand the process of dyeing itself. According to Websters dictionary, dyeing is the process of coloring fibers, yarns or fabrics by using a liquid containing coloring matter for imparting a particular hue to a substance. There are three basic methods of

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imparting a particular hue to a substance. The first is by staining an item, a without the benefit of some sort of chemical fixative to preserve the color. The next is the use of pigmentation, wherein the color is fixed to the surface of an object by another adhesive medium. A true dye is when the color of a substance is deposited on another substance in an insoluble form from a solution containing the colorant. Natural dyes can be broken down into two categories: substantive and adjective. Substantive, or direct dyes, become chemically fixed to the fiber without the aid of any other chemicals or additives, such as indigo or certain lichens. Adjective dyes, or mordant dyes, require some sort of substance, (usually a metal salt) to prevent the color from washing or light-bleaching out. Most natural dyes are adjective dyes, and do require the application of a mordant (the metal salt) solution to the fibers at some point in the dyeing process. Aluminum and iron salts were the most common traditional mordants, with copper, tin and chrome coming into use much later. In rural areas where these metals were not widely available, plants were also used as mordants, especially those that have a natural ability to extract such minerals from the earth, such as club moss. Most ancient and medieval dyers mordanted their yarns and fabrics before dyeing them. Alum and Iron were used as mordants in Egypt, India and Assyria from early times, as there are many alum deposits in the Mediterranean region. Medieval dyers used alum, copper and iron as mordants, and cream of tartar and common salt were used as to assist in the dyeing process. B> Different fibers also have different tendencies to absorb natural and synthetic dyes. Protein and cellulose fibers (the two main divisions for fibers used historically in spinning and dyeing) need to be mordanted differently because of their structural and chemical composition. Mordants to cellulose fibers such as cotton and linen usually involve the use of washing soda or tannins to create an alkaline dye bath. Tannins (plantstuffs, such as oak galls containing tannic acid) are widely used in dyeing cellulose fibers as they attach well to the plant fibers, thus allowing the dyes to attach thmselves to the tannins, wheras they might not be able to adhere to the fibers themselves (Tannins are sometimes classified as mordants in and of themselves, but are usually considered a chemical to assist in the dyeing process.) Mordants for protein fibers, like wool and silk, are usually applied in acidic dye-baths. Alum with the assistance of cream or tartar, is the most common mordant used to assist the dyes inn taking to the fibers. Since the difference in mordanting different fibers has been mentioned, it would remiss not to spend a moment on the historic nature of the fibers themselves. Wool, a protein-based fiber, has been found in Europe dating back to 2000 BCE. It was a common medieval fabric in both dyed and natural colors, and was processed by both professional manufacturers and housewives. Silk, another protein-based fiber, was imported from China to Persia as early as 400-600 BCE. It became quite popular in the Late Middle Ages, and major silk manufacturing centers were set up in France, Spain and Italy. These silk production centers also became centers of dye technology, as most silk was dyed and required the highest quality dyes available. Cotton was considered a luxury fabric, as it was imported all the way from India and usually dyed or painted before it was shipped. Cotton was also valued because of the brightness and colorfastness of the dyes used to color it, and also for its use in making candle wicks. Samples of cotton fabrics have been found in India and Pakistan dating to 300

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BCE, but it did not appear in Europe until the 4th century. Cotton waving establishments were formed in Italy in the 13th & 14th centuries ut they did not make a significant economic impact on the industry as they produced a coarser quality of fabric than the imported fabric, and therefore had difficulty in obtaining a good supply of cotton fiber. Scientists are almost certain that dyeing was practiced throughout the world, but it is difficult to obtain proof on this for two reasons. First, not all cultures left written records of their practices. Second, because of the wide variance of environmental conditions and degree of geological disturbance, it is not easy to fine well-preserved evidence of dyed textiles in many archaeological sites. A Chinese text from 3,000 BCE lists dye recipes to obtain red, black and yellow on silks. Ancient Indian texts describe several different yellow dyestuffs, how to obtain reds from the wood and bark of certain trees, and also notes the use of indigo to create blues on cotton. In central and South America they dyed bast fibers (plant fibers) in shades of red and purple with the bodies of the cochineal insects. A Greek artifact known as the Stockholm Papyrus details dyestuffs and techniques in almost a recipe fashion as it was practiced Egypt in the third and fourth centuries CE. The great detail in which the preparation of the fibers and the dyeing materials and the dyeing process itself are recorded has led scholars to believe that it had to have been practiced for thousands of ears previously in order to raise the process to such a science and art. It discusses mordanting the fibers using alum, copper and iron oxides to darken or sadden the red, blue, green and purple dyes, as well as the occasional use of tin and zinc. It describes over ten different recipes for using alkanet (Anchusa tinctoria) root as a dye employing camel and sheep urine, lentils, vinegar, wild cucumber and barley malt among others as aids to producing color. It also gave rcipes on obtaining purple hues by over dyeing the alkanet with woad (Isatis tinctoria), madder (Rubia tinctrum), kermes (made from the dried bodies of the female shield louse or scale insect (Kermers ilicis)) and the heliotrope plant (Heliotropium arborescens). Excavated coptic textiles dating from the fourth to the sixth century CE show use of weld (Reseda luteola) to produce yellow, madder and woad for dark purple, and blue from indigo (Indigofera tinctoria). Scientists have been able to date a red obtained from Egyptian madder root from the fourteenth century BCE. In the Mediterranean before the advent of Christianity, a whole dyeing industry arose around Tyrian purple. Tyrian purple is produced from the mucous gland adjacent to the respiratory cavity within some species of Purpura and Murex species of shellfish (Schetky, 4). The shells were crushed to extract this fluid, which only turns purple once it has been applied to the fiber and exposed to light and oxidation with the air. The Phoenicians, skillful shipbuilders and sailors that they were, scoured the coastlines for sight of these whelk shells, and established a dye works and trading station wherever they found a plentiful population of these shellfish. Coastal Indians of Mexico were also using shellfish, but their delicate method involved blowing and tickling the shellfish to get them to spit out the dye precursor directly onto the cotton fibers. Even Ireland can produce archaeological evidence of dyeing with the native dog-whelk shells in the seventh century CE. Both Discorides, the Greek physician and pliny the Elder, the Roman naturalist, mention in their first century works the preparation and dyeing of wool with various shellfish to produce colors of red, blue, 34

purple and violet after first being mordanted with soapwort (Saponaria officinalis), oxgall or alum. Both authors also mention the use of Indigo from the Orient to obtain blues, and Herodotus describes its use in a 450 BCE text. Dioscorides also mentions other dye palants of the ancient world, including madder, saffron (Crocus sativus) and weld for yellow, and woad for blue. (Walnut shells (Juglans nigra), oak bark (Quercus sp.), pomegranate flowers (Punica granatum) and broom (Genista tinctoria) were also used in conjunction with various mordants; but galls formed on trees could mordant themselves, being high in tannic acid. In Europe the art of dyeing rose to new heights with the diversity of climate, culture and migration/invasion waves. This further influenced by the direct impact of trade instigated by the Crusades and furthered by the growing cultural awareness of the Renaissance period-everyone in Europe wanted the exotic, colorful dyestuffs from the Orient, and later rom the Americas. Caravans of camels would cross the Gobi desert for centuries bringing goods from China to the Mediterranean. By the 12th century the two main trade routes for imported dyestuffs headed through Damascus: the first led rom Baghdad to Damascus to Jerusalem and Cairo, the other went to Damascus to Mosul to the Black Sea to Byzantium (Istanbul). Venice was one of the major early centers for imported dyestuffs, supplying Brazilwood (Caesalpinia sappan) from the East, lac (another insect dye) and indigo from India from the fifteenth century CE onward. Dyers of Italy soon became adept in their use, in 1429 the Venetian dyers guild wrote a book for its members containing a number of different dye recipes, including Brazilwood and lac. The Plictho de Larti de Tentori by Venetian author Giovanni Ventur Rosetti (sp-also listed as Giovanventura Rosetti) in the 1450s lists instructions for using both lac and indigo, as well as 217 other recipes for dyeing cloth, linen, cotton and silk with many varieties of dyestuffs. It would remain the best source for dyeing instruction for the next 200 years. From Venice the dyestuffs were traded by ship around the coast of France to Flanders, Southampton and London; in the Mediterranean at Florence, Pisa and genoa; and northward on the continent to the distribution centers of Basle and Frankfurt. Basle was a noted center of trade for saffron, the expensive yellow obtained from certain species of crocus. In later years crocus were grown in that area directly, and the crop became such a vital part of the local economy that they crocus was featured on the citys coat of arms. Frankfurt housed trade fairs from the twelfth to fourteenth centuries that dominated the trade of man dyestuffs, but mainly that of locally grown woad, the only blue dyestuff available to European dyers before the coming of indigo. Many regions in Germany specialized in growing and processing the woad through its complex fermentation process, and strict legislation was placed on every aspect of the trade. The government of Spain controlled the trade of cochineal, the red dye from the bodies of the Cochineal bugs of Central America. In 1587 approximately 65 tons were shipped to Spain, and from there northward throughout Europe. Italian dyers shunned cochineal in favor of the already established dye kermes, made from the dried bodies of the female shield louse or scale insect. Its use was first recorded in 1727 BCE and it was long the standard red dye for silk, wool and leather, but the intense colorific value and relative cheapness of cochineal soon eliminated most of the kermes use in England, so Spain hung on to control of their lucrative monopoly.

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European dyers reached their height of skill in the thirteenth century, mainly use to the guild systems who vigilantly maintained a high standard of quality. In many countries dyers were graded by the guild system, the master dyers being allowed to use the major fast dyes while their lesser colleagues were restricted to the slower, fugitive dyes. In some places it was forbidden to possess, let alone use, major dyestuffs unless you were a member of a guild. In Germany, the dyers and wood workers were regulated by the guilds, each grower having to present his crop to a sworn dyer to determine its quality, weight and condition before it could be sold. English producers of wood had fewer restrictions mainly that of a proclamation in 1587 to restrict growers to certain field size and ensure that no wood mills were sited within three miles o a royal residence, market town or city because of the highly offensive odor they emit. Even the local doctors in Venice in 1413, city fathers, to prohibit dyeing with either wood or ox-blood after March first because of the unhealthy smell.. France had developed an extensive and efficient textile industry by the 13th century and also increased the dyers craft by developing varied techniques to achieve additional colors from the basic dyestuffs. At the end of the 16th century, there were over 220 master dyers listed in Paris alone. While the powerful guild system had numerous dyestuffs with which to blend their color palates of fiber for the bluebloods and wealthy merchants, dyeing in the lower classes was a bit more restrictive. Without the money (or connections) to buy indigo, cochineal and turmeric, clothing in the country tended to natural colourswhites, blacks, browns, grays, and tans of the natural colors of the fibers themselves, with the reds, greens and yellows of local plants used for both food, medicine and dyes. In short, home dyers used any plants they could lay their hands on that world give a good color. Some colors were even derived accidentally. Washing bee hives in preparation for making mead could yield yellows and golds. Blackberries and Bilberries that stained the fingers of pickers could also be used to achieve pale blues and purples, although these were not often color or lightfast. In England, the multitudinous variety o lichens and mosses produced greens, grays and browns. By the seventeenth century a world-wide shipping and trading network was in place, allowing dyestuffs from all parts of the world to be brought to Europe. Legislation from earlier centuries to protect the growers and users of specific dyestuffs was overturned in favor of new demands and standards set by the growing consumer-focused society who wanted more colors and better quality. In the eighteenth and nineteenth centuries the practice of colonialism insured that there would always be a supply of foreign dyestuffs, and the Industrial Revolution met the demands of large-scale productions while finding new ways to make he colors brighter and longer-lasting to wear and washing. As textile weaving technology advanced with the advent of machines to spin, design and weave fabric, dyers were forced to be able to produce dyes with exact shades, matching color lots and most importantly, ones that would stand to the new mechanical and chemical processing. In addition, exporters wanted colors that would stand up to tropical sunlight and still be exotic enough for foreign tastes. Dyers in turn demanded from their suppliers purer chemicals and dyestuffs of consistent quality. Hand in hand, dyers, manufacturers, chemists, and dyestuff producers worked hand in hand to keep up with the progress of technology. Chemists in many countries had

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found a means of extracting highly concentrated powders or pastes from traditional dyestuffs that made stronger colors, such as cochineal carmine and madder garancine. Other procedures were used to extract indigo that gave us sulphonated indigo and Saxon blue. A few novel dyes (precursors of future chemical dyes) such as the yellow obtained from picric acid also made an appearance. With the tremendous rise in the interest of Chemistry in the mid nineteenth century, several important innovations in dyeing came about. W.H. Perkins, a student of celebrated European scientist Wilhelm von Hoffman, accidentally discovered the first synthetic dye in an attempt to synthesize quinine. The 18-year old students purple precipitate, later called mauviene, was quickly put into industrial application, allowing the young Perkins to start his own factory in London to commercially produce his dyestuff. Two years letter a synthetic red dye called magenta or fuchsine was patented in France, and hardly a year passed until the end of the century without a new synthetic dye being patented. Eventually, the old natural dyes lost popularity in favor of the newer synthetic ones. By the end of the nineteenth century a few Scottish tweed producers were the only ones still using natural dyes, and now the use of natural dyes on a commercial scale barely exists, mainly in remote areas where people have either little access to synthetic dyes or a vested interest in retaining their ancient dyeing customs. Use of natural dyes is gaining popularity again with the renaissance in hand crafting, most notable in the fields of spinning and weaving, basketry, papermaking and leather craft. There is also renewed scientific and historic interest in natural dyeing, both to help identify dyestuffs in recently discovered archaeological finds and to preserve the dyed textiles housed in museums and private collections. As Su Grierson says in her book Dyeing and Dyestuffs, Whilst the dyeing industry of today keeps pace with modern science, the future use of natural dyes will also follow a new path, but one firmly rooted in tradition.

3.3 MODERN CONCEPTS OF THE THEORY OF DYEINGIt appears that the mechanism of dyeing depends on the nature of both the dye and the fibre. Textile fall into two main groups : vegetable and animals vegetable fibre is cellulose fibre, e.g. cotton, lined, flax, hemp and jute. Animal fibres are protein fibre, e.g. wool, silk and leather. There is also a third type of fibres, the artificial and synthetic fibres e.g. rayons (cellulose-type) and nylons (protein-type). Dyeing was already practiced inn ancient times and has undergo many changes in its development. The first hypothesis of purely mechanical character attempting to explain the processes underlying dyeing was advanced in 1740-41. According to this hypothesis the fibre has pores which particles of dyes penetrate at high temperature; when the dye bath cools, the pores of the fibre contract, thus fastly retaining the dye. The problem of the different behavior of dyes in respect to various kinds of the fibre was not referred to. For about one and half a century, the dyeing process was considered was only from the mechanical stand point without paying due regard to chemical essence o the

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process, only at the end of the 19th century, when chemistry was developed, the chemical nature of dyeing process was revealed. New theories is come replace the old erroneous ones. thus, as a result of the work carried out by two groups of English chemist-S Neale collaborators and J. Belton, it was found that inn the process of dyeing, the was first absorbed by the external surface of the fibrous materials and than started to diffuse inside in the molecular-disperse state. this was the determining feature of the kinetics of the process. At organic fibres, cellulose,, protein and synthetic have one feature in common; they are all macromolecules having a linear structure and more or less oriented along the fibre axis. Macromolecules contain a great number of identical or similar atoms linked by covalent bond. Most high polymers from which the fibres are composed contain active functional groups, such as hydroxyl, carboxyl, nitrile and amino groups. In cross longitudinal directions, macromolecules are interconnected by the force of intermolecular attraction and in separated cases, by chemical bonds (wool keratin). The super molecular structure of the fibrous polymer is characterized y chain having in some portion a very regular arrangement crystalline region) and a maximum density of packing and in other portions, regions with less regular arrangement (amorphous region) and a maximum density of packing and in other portions, regions with less regular arrangement (amorphous region) with loose structure.

3.3.1 Sticking of dyes to fibres:This depends on the dye and the fibre to which the dye is attached. Protein-based fibres such as wool and silk have free ionisable CO2H and NH2 groups on the protein chains which can form an electrostatic attraction to parts of the dye molecule. For example the sulphonate group, SO3 -, on a dye molecule can interact with a NH3 + group on the protein chain.

Cotton is a polymer with a string of glucose units joined together. Indigo which is used to dye denim jeans is a vat dye. Indigo is insoluble in water. The reduced form of indigo is soluble. Cotton is soaked in a colourless solution of the reduced form. This is then oxidized to the blue form of Indigo which precipitates in the fibres. Direct dyes are applied to the cotton in solution and are held to the fibres by hydrogen bonds and instantaneous dipoleinduced dipole forces. These are weak compared with covalent bonds hence these dyes are only fast if the molecules are long and straight. Fibre reactive dyes actually form covalent bonds with fibre molecules and are therefore extremely colour fast. A dye molecule is reacted with the molecule trichlorotriazine:

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Trichlorotriazine can react with either OH groups (present in cotton) or NH groups (present in wool and nylons), thus effectively bonding the dye to the fabric.

3.2.2 DYE AND DYE SOLUTIONSAll water-soluble dyes use in the textile industry is electrolytes. In the aqueous solution dissociate into ions with positive and negative charge :(a) into the dye coloured ion and (b) into the usual colorless compensation ion with an opposite charge. The solution properties and the behavior or dyes in course of the dyeing process are of great interest. It is established that the salt of the dyes in the solution are electrolytically dissociated and their ions may from more or less large aggregates. The possible types of dye ions association in aqueous solution, according to the data presented by E. Valko, may be represented by the following three schemes (where A and B are an anion and a cation respectively) For the dye cation For the dye anion I C+ AII (nC)n+ (nA)nIII m(CA)nCn+ m(CA)nAn-

Scheme I represent the complete dissociation of the dye into ions, scheme II association of the dye into ions and scheme II aggregate particles including molecules.

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Scheme I is typical of an electrolyte and scheme III of a colloidal particle. The intermediate from (scheme-II) called the ionic micelle is characteristic of colloidal electrolytes. The size of dye particles in solution usually depends on the temperature as well as on the concentration of the electrolytes contained in the solution.

3.2.3 SURFACE POTENTIAL OF FIBRESWhen fibrous materials is immersed in water or in aqueous solution, important electrochemical phenomena take place due to the absorption of certain ions from the solution or to the dissociation of ionic groups contain in the fibres. Electrical change formed on textile fibre at their immersion into water due to the electrostatic attractive or repellent forces influence the distribution of coloured ions in the solution, thus promoting the interaction with the fibre of ions having an opposite charge and on the contrary, creating an energy barrier the approach to the fibre of ions with the same charge, e.g. the protein fibre saturated with (positively charged)- NH3 groups is attract by the negatively charged (acid) dye anions. + -NH3 + D NH2D. Conversely, the negatively charged ions of a direct dye will be repelled by the surface potential of the cellulose fibre, this potential barrier will have to be overcome before the ions enter the fibre. Whenever a dye anion will be energy-content of dye anions can be raise by increasing the temperature of the dye solution. Hence, at higher temperature, a greater portion of dye ions will e able to overcome the potential barrier and penetrates into the fibre.

3.2.4 THE NATURE OF THE FORCE BETWEEN THE DYE AND THE FIBRE MOLECULESThe fixation of dye depends on the physical and chemical properties of the dye and on the kind and properties of the fibrous material. Evidently, the dye is retained on the fibre by the physical and chemical forces of interaction. The attractive forces between dye and the fibre molecules may be classified as follows: 1. Electrostatic attraction between charged sites exists in protein and polyamide fibres. 2. Attraction by induction between ionic dyes and non-ionic dyes fibre. Induction forces of second type above are very small and may be ignored. 3. Polar van der walls forces such as hydrogen bonds, e.g. such polar force are to Hydrogen adjacent molecules of water as follows : H H-O H H-O H H

H-O H- O

Non-polar van der walls dispersion force, e.g. in aqueous dye-bath, non-polar bonding will occur between surface which are predominately hydrophobic. 4. Chemical forces, e.g. the sole example of a chemical bond between cellulose and de molecules is in the case of reactive dyes where a covalent bond is performed

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between hydroxyl groups and reactive halogen containing groups in the dye molecules.

3.3 Flow diagrammatic representation of dyeing process:

3.4 CLASSIFICATION OF DYEING PROCESS1. Mass-coloration of the molten fibers This method is for dyeing the molten fibers or plastic chips or textile polymers with pigment dyes. After that, the molten or polymers will extrude from a spinneret to form fibers. Normally, the synthetic fibers are added with white pigment in order to give a hiding power (non-see through fabrics). Advantage: fastness give excellent

Disadvantage: very difficult to clean

2. Fiber Dyeing is the method of dyeing fibers before blending with other colors to give fancy yarns or fabrics. Note: This is used for special purposes only.

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3. Yarn Dyeing is the method of dyeing yarns in forms of hanks or packages dyeing. This will give Scottishs style fabrics, carpet with many colors and styles. Note: This is used in hand loom weaving in the Northern and North-eastern parts of Thailand. 4. Fabric Dyeing is the method after weaving, knitting, or non-woven to make fabrics. This is very popular method of dyeing as the dyed fabrics will be processed further to

garment industries very easily. Dyeing forms of the fabric dyeing can be used in 2 ways: 1. Open width form using the fabrics to spread without any creases and dye them. 2. Rope form using the fabrics with the form like a rope (many creases and look like a rope)

3.5 Some people classify into:1. Exhaustion Process This method is using lot of water as shown in Liquor Ratio (ratio between water and goods) This should immerge the goods into dye solution for a long time in order to let the dye penetrate into the goods. This will lead to produce more waste water than the continuous process. Advantage: inexpensive, no need to train the worker to look after and run them properly. Disadvantage: lots of water needed, very slow process (60-120 mm/batch.) 2. Continuous Process This method is designed by putting different machinery into a sequence so that it can produce the dyed fabric in one pass. Advantage: very fast process (10-100 m/min), small amount of water in the process. Disadvantage: very expensive, need to train the worker to look after and run them properly. Example of the open width form fabric dyeing

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. Examples of the rope form fabric dyeing loading and unloading reel heat exchange

5. Garment Dyeing: This method is the last process of the dyeing of goods. However, the penetration of the dye solution may not be completely passed to the fibers such as between the seams, buttons, zippers etc. Normally, it is used for

lingerie, socks, sweater dyeing etc.

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4.1 DEFINATION:a chemical or formulated chemical product which enables a processing operation in preparation, dyeing, printing or finishing to be carried out more effectively, or which is essential if a given effect is to be obtained.

4.2 NECESSITY OF AUXILIARIES IN TEXTILE DYEING(a) To prepare or improve the substrate in readiness for coloration by scouring, bleaching and desizing wetting enhancing the whiteness by a fluorescent brightening effect (b) To modify the sorption characteristics of colorants by acceleration retardation creating a blocking or resist effect providing sites for sorption unifying otherwise divergent rates of sorption improving or resisting the migration of dyes (c) To stabilize the application medium by improving dye solubility stabilizing a dispersion or solution thickening a print paste or pad liquor inhibiting or promoting foaming forming an emulsion scavenging or minimizing the effects of impurities preventing or promoting oxidation or reduction (d) To protect or modify the substrate by creating or resisting dye ability lubricating the substrate protecting against the effects of temperature and other processing conditions (e) To improve the fastness of dyeings, as in the after treatment of direct or reactive dyes the after treatment of acid dyes on nylon the chroming of mordant dyes on wool or nylon giving protection against atmospheric influences, as in UV absorbers or inhibitors of gas-fume fading back-scouring or reduction clearing (f) To enhance the properties of laundering formulations (fluorescent brightening agents).

4.3 Some have more than one purposeAn auxiliary to improve dye solubility may also accelerate (or retard) a coloration process, or an emulsifying agent may also act as a thickening agent; pH-control agents may both stabilize a system and also affect the rate of dye sorption. Undesirable effects during handling, through effluent discharge to surface waters, through discharge to the atmosphere (e.g. via stenter gases), through consumer contact with the finished product (e.g. skin sensitivity) or during the eventual disposal of solid wastes (e.g. incineration or landfill)

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4.4 CLASSIFICATION:A) Non-surfactants B) Surfactants, Non-surfactants Simple electrolytes, acids and bases, both inorganic and organic. E.g., sodium chloride, sodium acetate, sulphuric acid, acetic acid and sodium carbonate, together with complex salts (such as sodium dichromate, copper(II) sulphate, sodium ethylenediaminetetra-acetate, sodium hexametaphosphate), Oxidising agents(hydrogen peroxide, sodium chlorite) Reducing agents (sodium dithionite, sodium sulphide). Anionic polyelectrolytes such as sodium alginate or carboxymethylcellulose, used mainly as thickening agents and migration inhibitors, also fall within the class of nonsurfactants; So too do sorption accelerants such as ophenylphenol, butanol and methylnaphthalene, although they normally require an emulsifier to stabilise them in aqueous media. Fluorescent brightening agents (FBAs) form another large class of nonsurfactant auxiliaries Surfactants: Organic in nature and more complex structuresDEFINITION:

An agent, soluble or dispersible in a liquid, which reduces the surface tension of the liquid In coloration processes this reduction in surface tension usually takes place at a liquid/liquid or liquid/solid interface, although liquid/gas interfaces are also occasionally important

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4.5 Classification of Dyeing Auxiliaries according to functionDyeing Auxiliaries 1. Sequestrant. 2. Lubricants / Anticreasant. 3. Pretreatment Chemicals 4. Levelling and Dispersing Agent. 5. Sewuestering, Dispersing and Levelling Agentfor Reactive dyeing. 6. Antifoam. 7. pH Control and buffer system. 8. Desizing Agent 9. Yarn Lubricant 10. Mercerising agent 11. Dyefixing agent 12. Optical Brightener. 13. Soaping Agent / Washing off Agent.

4.5.1 Sequestrant:The most undesirable impurities in Fibre, Common salt, Glauber salt, Caustic Soda and Soda ash are the di- and tri-valent cations, e.g., Ca ++, Mg ++ Cu ++, Fe+++ etc. These ions increase hardness of the process bath and generate iron oxides in the bath. Calcium and Magnesium reacts with alkali and precipitates as a sticky substance on the textile material, which creates patchy dyeing and discoloration of the fibre. The ferric oxide with cellulose and creates small pinhole on the fibres also damages the machinery by scale formation in the nozzles and base. To overcome these deleterious effects in the scouring and bleaching bath adequate amount of sequestrant must be used. Sequestrants prevent di-and tri-valent metal ions, e.g., Cu++, Fe +++ , Mn ++, Ca++, Mg++ etc from interfering with the chemical processing of the textile material. It prevents catalytic damage of cellulosic fibres in bleaching hath during hydrogen peroxide bleaching.

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In Dye-bath Ca++, Mg ++, Fe +++ attack the dye molecules and forms aggregates of molecules which deposits on the fabric as dye spots and also they prevent the reaction process. Dye bath sequestrant should be of different strength than that of the scouring and bleaching baths. Because some dyes have metal compounds and if powerful chelating agents are used than it will attack the metal compound of dye.

4.5.2 Lubricating / Anticreasant.Wet processing and dye-bath lubricants are used in any operation in which rope marks, creases, crows feet buffing, abrasion can occur on textiles. In low liquor ratios, full-loaded winches and jets when heavy materials re processed, fiber lubricant is essential. The basic requirement is that is it should from a thin uniform protective coating around the fiber to lower the surface friction and flexural rigidity, thus minimizing the formation of durable creases during high temperature processing. The most suitable lubricant should have the properties so that it helps to emulsify, it does not undergo phase separation with extreme changes in pH and temperature, e.g. it is stable in high temperature and over a wide range of pH; and it should have excellent compatibility with all the chemicals in treatment bath, Suitable products are relatively hydrophobic surfactants, many of which also contain a proportion of solubilized or emulsified oil or wax.

4.5.3 Pretreatment Chemical :a. b. c. d. e. f. Detergent & Wetting agent for cellulosic fibers. Oil stain remover Hydrogen peroxide Stabilizer. Single0Bath Scouring & bleaching Agent. Hydrogen Peroxide Killer; Biopolish.

In both 1-bath and 2-bath process, a number of important auxiliaries with specific properties are necessary along with 2-basic chemicals, as shown in the blocks below, Pretreatment process Bleaching bath 2. Bath process 1. Bath process

Scouring/Boiling offDetergent & Wetting Agent Oil stain remove Basic Chemical: Alkali

Bleaching BathDetergent & Wetting Agent Antifoaming Agent Sequestrant Lubricant/Antecreasant Oil stain remove Peroxide Stabilizer Basic Chemical: Alkali Hydrogen Peroxide

Bleaching BathDetergent & Wetting Agent Antifomking Agent Sequestrant Lubricant/Antecreasant Peroxide Stabilizer Basic Chemical: Alkali Hydrogen Peroxide

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Peroxide killer is used after both processes for eliminating residual hydrogen peroxide. Detergent & Wetting agent or Cellulosic fibres: The purpose of scouring is to reduce the level of fats, waxes, oils, dirt and so forth on the substrate. Apart from the aesthetic benefits of clean fabric, the major technical reason for scouring is to improve the extent and uniform of absorbency for subsequent processes, especially coloration. In scouring 1, surface-active products function as primary, rather than auxiliary agents as the basic requirements are for good wetting power and detergency, the latter property generally including the ability to remove, emulsify and suspend the extraneous matter in the liquor. Not all good detergents possess good wetting properties; hence a combination of surface-active agents to provide both wetting and detergency may be preferable. Detergency can be significantly improved with the use of additional chemicals usually referred to as builders the chief of which undoubtedly alkali in the form of sodium carbonate or hydroxide.

4.5.4 Levelling and Dispersing Agent:Unleveled dyeing problems can be of two categories: Gross unlevelness throughout the material or localized unlevelness e..g. barriness, skitteriness. There are two fundamental mechanisms that can contribute to a dyeing. a. Control of the exhaustion dye so that it is taken up evenly. b. Migration of dye after initially unleveled absorption on the fibre. c. Non-ionic agent usually from water soluble complexes with the dye, some degree of solubilization being involved. d. Ionic agent are primarily dye-of fibre-substantive ; in the former case they tend to form complexes with the dye and there is co