Лако-красочные материалы — производство

Protein fibres are natural fibres derived from animal sources, the most important of these being wool and silk. The principal component of the wool fibre is the protein keratin, the molecular structure of which is illustrated in outline in Figure 7.1. The protein molecules consist of a long polypeptide chain constructed from the eighteen commonly encountered amino acids that are found in most naturally-occurring proteins. The structures of these amino acids are well documented in general chemical and biochemical textbooks and so they are not reproduced here. As a result of the diverse chemical nature of these amino acids, the protein side-chains (R1, R2, R3 in Figure 7.1) are of widely varying character, containing functionality which includes, for example, amino and imino, hydroxy, carboxylic acid, thiol and alkyl groups and heterocyclic func­tionality. At intervals, the polypeptide chains are linked together by disulfide (-S-S-) bridges derived from the amino acid cystine. There are also ionic links between the protonated amino (-NH3+) and carboxylate (CO2) groups, which are located on the amino acid side-groups and at the end of the polypeptide chains. Many of the functional groups on the wool fibre play some part in the forces of attraction involved when dyes are applied to the fibres. Protein fibres may be dyed using a number of application classes of dyes, the most important of which are acid, mor­dant and premetallised dyes, the structural features of which are dis­cussed in the rest of this section, and reactive dyes which are considered separately in Chapter 8.

Acid dyes derive their name historically from the fact that they are applied to protein fibres such as wool under acidic conditions. They are also used to a certain extent to dye polyamide fibres such as nylon. Acid dyes may be conveniently classified as either acid-leveling or acid-milling types. Acid-leveling dyes are group of dyes that show only moderate affinity for the wool fibres. Because the intermolecular forces between the dye and the fibre molecules are not strong, these dyes are capable of migrating through the fibre and thus produce a level dyeing. Acid-milling dyes are a group of dyes, which show much stronger affinity for the wool fibres. Because of the strength of the intermolecular forces between the dye and the fibre molecules, the dyes are less capable of migration and this can present difficulties in producing level dyeings. However, they give superior fastness to washing.

A characteristic feature of acid dyes for protein and polyamide fibres is the presence of one or more sulfonate (=SO3 _) groups, usually as sodium (Na+) salts. These groups have a dual role. I3irstly, they provide solubility in water, the medium from which the dyes are applied to the fibre. Secondly, they ensure that the dyes carry a negative charge (i. e. they are anionic). When acid conditions are used in the dyeing process, the protein molecules acquire a positive charge. This is due mainly to protonation of the amino (-NH2) and imino (=NH) groups on the amino acid side- chains, to give — NH3+ and — NH2+ groups respectively, and to the suppression of the ionisation of the carboxylic acid groups. The positive charge on the polymer attracts the acid dye anions by ionic forces, and these displace the counter-anions within the fibre by an ion exchange process. As well as these ionic forces of attraction, van der Waals’ forces, dipolar forces and hydrogen bonding between appropriate functionality of the dye and fibre molecules may also play a part in the acid-dyeing of protein fibres. In terms of size and shape, often an important consider­ation in the design of dye molecules, acid-leveling dyes may be described as small to medium-sized planar molecules. This allows the dyes to penetrate easily into the fibre and also permits a degree of movement or migration within the fibre as the ionic bonds between the dye and the fibre are capable of breaking and then re-forming, thus producing a level or uniform colour. However, as the dye is not very strongly bonded to the fibre, it may show only moderate fastness towards wet-treatments such as washing. Acid-milling dyes are significantly larger molecules than acid­leveling dyes and they show enhanced affinity for the fibre, and hence improved fastness to washing, as a result of more extensive van der Waals’ forces, dipolar forces and hydrogen bonding.

Most acid dyes, especially yellows, oranges and reds, belong to the azo chemical class while blues and greens are often provided by carbonyl

dyes, especially anthraquinones, and to a certain extent by arylcar — bonium ion types. Figure 7.2 illustrates some typical acid dye structures. A notable aspect of the structure of dyes 146-149 is the strong intra­molecular hydrogen-bonding which exists in the form of six-membered rings, a feature which enhances the stability of the compounds and, in particular, confers good lightfastness properties. This has been explained by a reduction in electron density at the chromophore as a result of the hydrogen-bonding, reducing the sensitivity of the dye towards photo­chemical oxidation. For this reason, intramolecular hydrogen-bonding is a feature commonly encountered in the structures of a wide range of dyes and pigments. Intramolecular hydrogen-bonding also reduces the acidity of a hydroxy group, and thus can lead to improved resistance towards alkali treatments. A comparison between the two isomeric monoazo acid dyes C. I. Acid Orange 20 (145) and C. I. Acid Orange 7 (146) illustrates the effect of intramolecular hydrogen bonding. Dye 146 shows signifi­cantly improved fastness to alkaline washing and lightfastness compared with dye 145 in which intramolecular hydrogen bonding is not possible. A comparison of the structurally related monoazo dyes 147a (C. I. Acid Red 1) and 147b (C. I. Acid Red 138), and of the anthraquinone acid dyes 149a (C. I. Acid Blue 25) and 149b (C. I. Acid Blue 138) illustrates the distinction between acid leveling and acid-milling dyes. Dyes 147b and 149b show excellent resistance to washing as a result of the presence of the long alkyl chain substituent (C12H25), which is attracted to hydrophobic or non-polar parts of the protein fibre molecules by van der Waals’ forces. Because of the extremely strong dye-fibre affinity, dyes of this type are often referred to as acid supermilling dyes. Dyes 147a, 149a (C. I. Acid Black 1), 148, a typical disazo acid dye, and C. I. Acid Blue 1 (150), an example of a triphenylmethine acid dye, are acid-leveling dyes. In the case of dye 150, note that while the nitrogen atoms carry a formal single delocalised positive charge, the presence of two sulfonate groups ensures that the dye overall is anionic.

The ability of transition metal ions, and especially chromium (as Cr3+), to form very stable metal complexes may be used to produce dyeings on protein fibres with superior fastness properties, especially towards wash­ing and light. The chemistry of transition metal complex formation with azo dyes is discussed in some detail in Chapter 3. There are two applica­tion classes of dyes in which this feature is utilised, mordant dyes and premetallised dyes, which differ significantly in application technology but involve similar chemistry.

Mordant dyes generally have the characteristics of acid dyes but with the ability in addition to form a stable complex with chromium. Most commonly, this takes the form of two hydroxy groups on either side of (ortho to) the azo group of a monoazo dye, as illustrated for the case of C. I. Mordant Black 1 (151). The dye is generally applied to the fibre as an acid dye and then treated with a source of chromium, commonly sodium or potassium dichromate. As a result of the process, the chromium(vi) is reduced by functional groups on the wool fibre, for example the cysteine thiol groups, and a chromium(iii) complex of the dye is formed within the

fibre by a process such as that illustrated in Figure 7.3. A dye of this type acts as a tridentate ligand, the chromium bonding with two oxygen atoms derived from the hydroxy groups and with one nitrogen atom of the azo group. The complexes formed are six-coordinate with octahedral ge­ometry. It has not been established with certainty how the remaining three valencies of chromium are satisfied in the mordant dyeing of protein fibres. There are a number of possibilities, which include bonding with water molecules, with coordinating groups (-OH, — SH, — NH2, — CO2H, etc.) on the amino acid side-chains on the fibre, or with another dye molecule. The principal problem currently with the use of chrome mor­dant dyes is environmental, associated mainly with the presence of resid­ual chromium, an undesirable heavy metal, in dyehouse effluent.

Premetallised dyes, as the name implies, are pre-formed metal complex dyes. They are usually six-coordinate complexes of chromium(iii) with octahedral geometry, as exhibited for example by C. I. Acid Violet 78, 152, although some complexes of cobalt(iii) are also used. Most premetal­lised dyes are azo dyes, with one nitrogen of the azo group playing a part in complexing with the central metal ion. Since in this case there are two azo dye molecules coordinated with one chromium atom, compound 152 is referred to as a 2:1 complex. 1:1 Complexes are also used, but to a lesser extent. Premetallised dyes of this type, like traditional acid dyes, are anionic in nature even though, as is the case with compound 152, they may not contain sulfonate groups. Indeed, the presence of sulfonate groups can cause the dye anions to be too strongly attracted to the fibre, which leads in turn to levelness problems. The purpose of the sulfone group in dye 152 is to enhance the hydrophilic character of the molecule and hence its water solubility, without increasing the charge on the dye anion. Premetallised dyes are applied to protein fibres as acid dyes and, because of the special stability of chromium(iii) complexes due to their d3 configuration, provide dyeings with excellent fastness properties.