Лако-красочные материалы — производство

The three most important types of synthetic fibres used commonly as textiles are polyester, polyamides (nylon) and acrylic fibres. Polyester and the semi-synthetic fibre cellulose acetate are dyed almost exclusively with the use of disperse dyes. Polyamide fibres may be coloured using either acid dyes, the principles of which have been discussed in the section on protein fibres, or with disperse dyes. Acrylic fibres are dyed mainly using basic (cationic) dyes.

Polyester (polyethylene terephthalate, PET) has the chemical structure shown in Figure 7.6. Polyester is relatively hydrophobic (non-polar) in character, certainly in comparison to the natural protein and cellulosic fibres, largely as a result of the prominence of the benzene rings and the — CH2CH2- groups. However, the ester groups do confer a degree of polarity on the molecule, so that the fibres are not as hydrophobic as, for example, hydrocarbon polymers such as polyethylene and polypropyl­ene. Nevertheless, it is not surprising that relatively hydrophobic fibres such as polyester and cellulose acetate show little affinity for dyes that are ionic in character. This means that dye application classes containing ionic water-solubilising groups such as the sulfonate (-SO3_) group, for example acid and direct dyes, are inappropriate for application to polyes­ter. The inevitable consequence is that dyes for polyester and cellulose acetate cannot be expected to have substantial water solubility. A further feature of polyester is that it is a highly crystalline fibre that consists of tightly packed, highly ordered polymer molecules. As a result, it is rela­tively inaccessible even to small molecules and it is thus very difficult to dye. Disperse dyes are an application class of dyes of relatively low water solubility, which may be applied as a fine dispersion in water to these relatively hydrophobic synthetic fibres. They were originally developed for application to cellulose acetate, but they have assumed much greater importance for application to polyester, and to a lesser extent to poly­amides.

Figure 7.6 Structure of polyester

Disperse dyes are required to be relatively small, planar molecules to allow the dyes to penetrate between the polymer chains and into the bulk of the fibre. These dyes are commonly applied to the fibre as a fine aqueous dispersion at temperatures of around 130 °C under pressure. At these temperatures, the tight physical structure of the polymer is loosened by thermal agitation, which reduces the intermolecular bonding and facilitates entry of the dye molecules. Disperse dyes are non-ionic mol­ecules which effectively dissolve in the polyester. In the solid solutions formed, the dye-fibre affinity is generally considered to involve a combi­nation of van der Waals’ and dipolar forces and hydrogen bonding. A general feature of disperse dye molecules is that they possess a number of polar, though not ionic, groups. Commonly encountered polar groups include the nitro (NO2), cyano (CN), hydroxy, amino, ester, amide (NHCO) and sulfone (SO2) groups. These polar groups can be thought of as making a number of contributions to the application properties of disperse dyes. One of their roles is to provide an adequate degree of water solubility at the high temperatures at which the dyes are applied. A second function of the polar groups is to enhance affinity as a result of dipolar intermolecular forces with the ester groups of the polyester molecule. Disperse dyes can therefore be considered as compromise molecules that possess a balanced degree of polar (hydrophobic) and non-polar (hydrophilic) character, similar to that of the polyester mol­ecule. The polar groups will also influence the colour of the dye molecules as discussed in Chapter 2.

A consequence of the fact that disperse dyes are relatively small, non­polar molecules is that they may have a tendency to be volatile, and hence prone to sublimation out of the fibre at high temperatures. This can lead to a loss of colour and the possibility of staining of adjacent fabrics when dyed polyester is subjected to high temperatures, e. g. in heat setting and ironing. Increasing the size and/or the polarity of the dye molecules enhances fastness to sublimation. However, as a consequence, the com­promise arises that these larger, more polar dye molecules will require more forcing conditions, such as higher temperatures and pressures, to enable the dyes to penetrate into the fibre.

The chemical structures of some typical disperse dyes are illustrated in

provided by increasing the number of electron-accepting and electron — donating groups in appropriate parts of the molecules. There are two further groups of disperse dyes that are heterocyclic analogues of the aminoazobenzenes. Derivatives based on heterocyclic diazo components provide bright intense colours and are bathochromically shifted so that they serve the purpose of extending the range of blue azo disperse dyes available. An example of such a product is C. I. Disperse Blue 339 (160). Derivatives based on heterocyclic coupling components are useful for their ability to provide bright intense yellow azo disperse dyes. An example is C. I. Disperse Yellow 23, (161), which, as illustrated in Figure 7.7, exists as the ketohydrazone tautomer. The fourth group are disazo dyes of relatively simple structures, for example C. I. Disperse Yellow 23 (162). Carbonyl disperse dyes, especially anthraquinones, are next in importance to the azo dyes and there are a few products belonging to the nitro and polymethine chemical classes. C. I. Disperse Red 60 (163) and C. I. Disperse Green 5 (164) are examples of typical anthraquinone disperse dyes, while compound 165 is an example of the more recently-introduced benzodifuranone carbonyl type. C. I. Disperse Yellow 42, (166) and C. I. Disperse Blue 354, (167) respectively provide commercially relevant examples of the nitro and polymethine chemical classes.

Acrylic fibres are synthetic fibres based essentially on the addition polymer polyacrylonitrile, the basic structure of which is illustrated in Figure 7.8. However, most acrylic fibres are rather more complex and contain within their structure anionic groups, most commonly sulfonate (-SO3~), but also carboxylate (-CO2~) groups either as a result of the incorporation of co-polymerised monomers in which these groups pres­ent, or due to the presence of residual amounts of anionic polymerisation inhibitors. The anionic character of these acrylic fibres explains why the principal application class of dyes used for their coloration is cationic dyes. These dyes are classified by the Colour Index as basic dyes, a term which originated from their use, now largely obsolete, to dye protein fibres, such as wool, from a basic or alkaline dyebath under which conditions the protein molecules acquired a negative charge. Cationic dyes are found to exhibit rather poor fastness properties, especially lightfastness, on natural fibres but give much better performance on acrylic fibres.

Some examples of the structures of cationic dyes used to dye acrylic fibres are shown in Figure 7.9. These include the azo dye, C. I. Basic Red 18 (168), the arylcarbonium ion (triphenylmethine) dye, C. I. Basic Green 4 (169) and the methine derivative, C. I. Basic Yellow 11, (170). As the name implies, the dyes are coloured cationic species, generally as a result of the presence of positively-charged quaternary nitrogen atoms (as =NR3+, or = NR2+). In the case of dyes 168 and 170 the positive charge is localised on the nitrogen atom, whereas in dye 169 it is delocalised by resonance. These groups serve two purposes. Firstly, they provide the water solubility necessary for the application of the dyes, due to their ionic character. Secondly, they provide affinity for the acrylic fibres as a result of ionic attraction between the dye cations and the anionic groups (=SO3~ and =CO2) which are present in the acrylic fibre polymer molecules. In a sense, the means of attachment of cationic dyes to acrylic fibres may be considered as the converse of that involved in the acid dyeing of protein fibres, discussed previously in this chapter, which involves the attraction of dye anions to cationic sites on the fibre.

The two most important polyamide fibres are nylon 6.6 (171) and nylon 6 (172) whose structures are illustrated in Figure 7.10. A comparison with Figure 7.1 reveals the structural analogy between natural protein fibres such as wool and polyamide fibres. Polyamides may be dyed using acid

dyes. These are attracted to the fibres by a mechanism similar to the acid dyeing of wool, involving attraction of the dye anions to amino groups, for example at the end of the polyamide chains, which are protonated under acidic conditions. Alternatively, because polyamide fibres are rela­tively hydrophobic, they may be dyed using disperse dyes by a mechan­ism analogous to the dyeing of polyester.