Лако-красочные материалы — производство

In Chapters 3-6, the commercially important chemical classes of dyes and pigments are discussed in terms of their essential structural features and the principles of their synthesis. The reader will encounter further examples of these individual chemical classes of colorants throughout Chapters 7-10 which, as a complement to the content of the earlier chapters, deal with the chemistry of their application. Chapters 7, 8 and 10 are concerned essentially with the application of dyes, whereas Chap­ter 9 is devoted to pigments. The distinction between these two types of colorants has been made previously in Chapter 2. Dyes are used in the coloration of a wide range of substrates, including paper, leather and plastics, but by far their most important outlet is on textiles. Textile materials are used in a wide variety of products, including clothing of all types, curtains, upholstery and carpets. This chapter deals with the chemical principles of the main application classes of dyes that may be applied to textile fibres, except for reactive dyes, which are dealt with exclusively in Chapter 8.

Textile fibres may be classified into three broad groups: natural, semi­synthetic and synthetic. Unlike dyes and pigments, which are now almost entirely synthetic in origin, natural fibres continue to play a prominent part in textile applications. The most important natural fibres are either of animal origin, for example the protein fibres, wool and silk, or of vegetable origin, such as cotton, which is a cellulosic fibre. The only significant semi-synthetic fibres used today are derived from cellulose as the starting material, the two most important of these being viscose rayon and cellulose acetate. Viscose rayon is a regenerated cellulosic fibre. It is manufactured by reacting cellulose, e. g. from wood pulp, with carbon disulfide in alkali to give its water-soluble xanthate derivative. This is followed by regeneration of the cellulose in fibrous form using sulfuric acid. Cellulose acetate is a chemically modified cellulose derivative, manufactured by acetylation and partial hydrolysis of cotton. The most important completely synthetic fibres are polyester, polyamides (nylon), and acrylic fibres. Textile fibres share the common feature that they are made up of polymeric organic molecules, but the physical and chemical nature of the polymers involved vary widely and this explains why each type of fibre essentially requires its own ‘tailor-made’ application classes of dyes.

Dye molecules are designed to ensure that they have a set of properties that are appropriate to their particular applications. The most obvious requirement for a dye is that it must possess the desired colour, in terms of hue, strength and brightness. The relationships between colour and con­stitution of dyes has been discussed principally in Chapter 2, although the reader will find specific aspects relating to particular chemical classes in Chapters 3-6. A further feature of dye molecules, which is of some practical importance, is their ability to dissolve in water. Since textile dyes are almost always applied from an aqueous dyebath solution, they are required to be either soluble in water or, alternatively, to be capable of conversion into a water-soluble form. Many dye application classes, including acid, mordant, premetallised, direct, reactive and cationic dyes, are readily water-soluble. Disperse dyes for polyester, in contrast, are only sparingly soluble in water, but they have sufficient solubility for their application at high temperatures. A few groups of dyes, including vat and sulfur dyes for cellulosic fibres, are initially insoluble in water and are thus essentially pigments. However, they may be converted chemically into a water-soluble form and in this form they can be applied to the fibre, after which the process reversed and the insoluble form is regenerated in the fibre.

Dyes must be firmly attached to the textile fibres to which they are applied in order to resist removal, for example by washing. This may be achieved in a number of ways. The molecules of many dye application classes are designed to provide forces of attraction for the polymer molecules which constitute the fibre. In the case of reactive dyeing, the dye molecules combine chemically with the polymer molecules forming covalent bonds (Chapter 8). In further cases, for example vat, sulfur and azoic dyes for cellulosic fibres, an insoluble pigment is generated within the fibres and is retained by mechanical entrapment. In other cases, a set of dye-fibre intermolecular forces operate which, depending on the par­ticular dye-fibre system, is commonly a combination of ionic, dipolar, van der Waals’ forces and hydrogen bonding. An additional feature of textile dyeing is that the dye must distribute itself evenly throughout the material to give a uniform colour, referred to as a level dyeing. Finally, the dye must provide an appropriate range of fastness properties, for example

to light, washing, heat, etc. This chapter provides an overview of the most important application classes of dyes for textiles, with a range of selected examples that illustrate how the dye molecules are designed to suit their particular application. The discussion is organised according to fibre type in three sections: dyes for protein fibres, dyes for cellulosic fibres and dyes for synthetic fibres. In each case there is a description of the structure of the polymer, followed by a discussion of the structural features of the dyes that determine their suitability for application to the particular type of fibre. The subject of reactive dyes is of sufficient interest and importance to warrant separate treatment (Chapter 8).