Textile Dyeing Auxiliaries

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  • 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 System

    1.2.1 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 asfollows:

    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 cellulo