Direct dyes fastness

Stilbene dyes ate classed as a subgroup of azo dyes having excellent colorfastness and typical direct dye wash fastness on cotton and ate arranged iato six categories by the Society of Dyers and Colourists (2), as described ia the foUowiag. 

It is not surprising, therefore, that chitosan and its basic derivatives will complex with anionic dyes. Giles et al. [68,69] researched the use of chitosan for the removal of dyes from effluent as long ago as 1958. The binding capacity of chitosan for anionic dyes is pH-dependent, but it has been reported [65] that in effluent treatment as much as 10 g dye per kg chitosan can be complexed at pH values above about 6.5. Similarly, chitosan has been used for the aftertreatment of direct dyeings on cotton to improve their fastness. 

Amination A/-Methylolacrylamide in presence of Lewis acid catalyst. Further modifications possible by addition to double bond (Scheme 10.61) Amines with durable press resins Improved dyeability with dichlorotriazine dyes at pH 5 without salt, giving 99% fixation Some improvements in dyeability, especially with direct dyes, but light fastness can be a problem... 

Cotton grafted with 2-vinylpyridine followed by quaternisation using an excess of an alkyl bromide or epichlorohydrin showed markedly increased exhaustion with direct dyes [399]. Grafting alone gave a substantial effect, with further slight improvements being conferred by the quaternisation. Improved fastness to washing was also claimed. 

Polyepichlorohydrin and dimethylamine Polymerisation of epichlorohydrin in carbon tetrachloride with boron trifluoride/ether catalyst, then reaction with dimethylamine. Applied to cotton by exhaust method or pad-dry. Scheme 10.65 Good yields with direct dyes using only 2 g/l salt. Excellent build-up with most reactive dyes only 10% of normal salt usage needed for low-reactivity dyes and none for highly reactive types. Washing fastness very good but light fastness impaired. 

The fact that the aftertreatment of direct dyes has a long history is not surprising since wet fastness within this class is not particularly good. Their prime advantages are ease of application and economy compared with dyes of higher fastness (reactive, sulphur or vat) -hence the continued search for highly effective aftertreatments that improve wet fastness... 

A third approach utilised copper salts, especially copper(II) sulphate, in conjunction with dyes containing chelatable groupings such as salicylic acid or o,o -dihydroxyazo moieties. Indeed, special ranges of copperable direct dyes, for which the treatment with copper(II) sulphate was really part of the dyeing process rather than an optional aftertreatment, were introduced. In the past the main use of this chelation treatment was to enhance light fastness, but it is little used for this purpose nowadays. 

The earliest polymeric cationic aftertreatments stemmed from the development of crease-resist finishes for cellulosic fibres. One such, promoted specifically for its colour fastness improvements when applied as an aftertreatment to direct dyeings, was a condensation product of formaldehyde with dicyandiamide (Scheme 10.82). Many similar compounds followed, such as condensation products of formaldehyde with melamine (10.212), polyethylene imine) with cyanuric chloride (10.213) and alkyl chlorides with polyethylene imine) (10.214 R = alkyl). 

Direct dyes are of limited interest for printing because of their restricted wet fastness, resulting in cross-staining of whites or pastel-dyed grounds when the prints are subsequently washed. Somewhat better results can be achieved by treating the prints after steam fixation... 

The application range designated by this generic name in the Colour Index incorporates those acid, direct and mordant dyes with substantivity for leather and satisfactory fastness on that substrate [55]. It is a commercially important sector, the number of products listed being exceeded only by the complete acid or direct dye ranges. As expected from the sources of this selection, about 85% of leather dyes are azo compounds (35% disazo, 30% monoazo, 20% metal-complex monoazo) and the remainder are mainly yellow to orange stilbene dyes and anthraquinone or triarylmethane types in the violet to green sectors. 

Direct dyes have only modest fastness to washing, which may be improved by after-treatments such as metal-complex formation (section 5.5.3) or by diazotisation of the dye on the fibre and further coupling of the diazonium salt with an insoluble coupling component (section 1.6.14). In addition to their use on cotton and viscose, direct dyes are important in the dyeing of leather. The cheapest members of this class are also used in the coloration of paper, since for this purpose fastness properties are largely irrelevant and price is all-important. 

Another terminal bidentate ligand that has been exploited occasionally in bright disazo direct dyes is the sulphated 8-hydroxyquinoline residue (5.15). On aftercoppering, fastness to light and wet tests is enhanced by hydrolysis catalysed by the copper(II) ion and formation of a bidentate 1 2 complex (Scheme 5.3). Apparently, electron withdrawal by sulphur facilitates removal of the sulphite grouping and approach of the copper(II) cation [10]. 

Many of the premetallised direct dyes are symmetrical structures in the form of bis-1 1 complexes with two copper(II) ions per disazo dye molecule. Scheme 5.12 illustrates conversion of the important unmetallised royal blue Cl Direct Blue 15 (5.43), derived from tetrazotised dianisidine coupled with two moles of H acid, to its much greener copper-complex Blue 218 (5.44) with demethylation of the methoxy groups as described above. Important symmetrical red disazo structures of high light fastness, such as Cl Direct Red 83 (5.45), contain two J acid residues linked via their imino groups. Unsymmetrical disazo blues derived from dianisidine often contain a J acid residue as one ligand and a different coupler as the other, such as Oxy Koch acid in Cl Direct Blue 77 (5.46), for example. 

A relatively uncontrolled attempt to enhance the colour yield and fastness to washing of selected direct dyes involved addition to the dyebath of a colourless reactant that was intended to form covalent attachments between these dyes and hydroxy groups in cellulose [135]. Most of the dyes selected contained at least two primary amino groups in aminonaphthol residues. The reactants evaluated were cyanuric chloride (7.3), N,N-diethyl-3-hydroxyazetidinium chloride (7.124) and 2,4-dichloro-6-(4,-sulphoanilino)-s-triazine (7.125), all of which will indeed react readily with primary arylamines. Some of the dyes selected, however, contained only diphenylurea or salicylic acid residues and these are most unlikely to react efficiently under dyeing conditions. 

Leather can be dyed with acid, direct and mordant dyes. Many of the direct dyes were based on benzidine and its congeners but the German Ordinance, covered under the toxicity of certain azo dyes in section 2.3.1.1, has meant that this is no longer an option. To improve the light fastness of the dyed leathers, 1 2 premetallised azo dyes have also been used, but once again the use of metal complex dyes is becoming less favoured. ... 

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