Bifunctional Reactive Dyes

There is no doubt that the major weakness of the reactive dyeing process is the hydrolysis reaction and the consequent need for a wash-off process. The extent to which dye hydrolysis takes place in competition with dye-fibre reaction varies quite markedly within the range 10 40% depending upon the system in question. A considerable amount of research has therefore been devoted to the search for reactive dyes with improved fixation. The most successful approach to addressing this issue has involved the development of dyes with more than one fibre-reactive group in the molecule, which statistically improves the chances of dye fibre bond formation. Examples of products of this type are the Procion H-E 

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A development reported recently [519] involves reduction of the cystine disulphide bonds in wool with either thioglycolic acid or tetrakis(hydroxymethyl)phosphonium chloride to form thiol groups, followed by crosslinking with bifunctional reactive dyes. This gave improved insect resistance but had adverse effects on physical properties such as strength, shrinkage and stiffness, thus limiting the potential of the process for commercial use. 

When analysing reactive dyes care needs to be taken in the interpretation of the results. Certain ranges of reactive dyes may be supplied in the form of a precursor of the actual reactive dye. Reactive dyes can undergo hydrolysis in aqueous environments (especially if alkaline). In recent years there has been an increase in the number of bifunctional reactive dyes. For aU of the reasons mentioned, a multitude of peaks may be detected from the different derivatives of dyes such as Remazol Black B (C.L Reactive Black 5) The full chemistry of reactive dyes has not always been appreciated by workers analysing them leading to misinterpretations, for example in the work of Oxspring et al. ... 

Tapley K N, Capillary electrophoretic analysis of the reactions of bifunctional reactive dyes under various conditions including a study of the analysis of the traditionally difficult to analyse phthalocyanine dyes , 7. Chromatography A, 1995, 706, 555. 

FT-Raman has been used as an alternative to TG techniques to determine filler content in HDPE/ CaC03 composites and provides comparable results [400]. As most pigments (apart Ifom carbon-black) and glass are poor Raman scatterers, in principle Raman spectra are obtainable Ifom these samples without removal of the fillers or difficult sample preparation. Conventional visible Raman spectroscopy has failed in attempting to analyse dyesmffs. Conventional Raman spectra of dyed textiles tend to be dominated by the (fluorescent) spectrum of the dye [401]. Consequently, FT-Raman spectroscopy may be a more useful tool for direct observation of low levels of dyestuffs in polymeric materials. Indeed, by using NIR excitation dramatic improvements in the Raman spectra of these dyes can be achieved [392]. FT-Raman was quite useful for the discrimination of differently dyed cotton-cellulose fabrics with the bifunctional reactive dye Cibacron C, provided that the interpretation was facilitated by chemometrics [402]. Schrader et al. [403] have used FT-Raman spectra to distinguish non-destructively the main dye components in historical textiles. Bourgeois et al. [401] have successfully used FT-Raman in the characterisation of... 

Bifunctional Dyes. There are many examples of dyes with two or more reactive groups, including many mixed reactive systems. Dye fixation is increased significantly with increasing number of reactive groups as shown by the hypothetical situation in Figure 4. Theoretically, fixation in this example could increase from mono- to di- to trireactive from 60 to 84 to 93.6%. Some multiple reactive dyes are claimed to have as high as 95% fixation. 

The dioxazine ring system is the source of some valuable violet pigments, such as Cl Pigment Violet 23 (6.211). This colorant is obtained by condensing 3-amino-9 ethylcarbazole with chloranil. Sulphonation of the pigment gives the dye Cl Direct Blue 108. Triphenodioxazines have recently been the source of some blue reactive dyes [241-Examples are known of symmetrical bifunctional structures (6.212 NHRNH = alkylenediamine, Z = haloheterocyclic system) and unsymmetrical monofunctional types such as 6.213 [37]. 

Because of the relatively low sensitivity of TLC methods they have not been frequently applied for the analysis of samples containing trace amounts of synthetic dyes. However, TLC techniques have been proven to be a rapid, reliable and valuable tool for the easy following of synthetic procedures for the preparation of new dyes. Thus, the synthesis of some bifunctional reactive triazine dyes has been monitored by TLC. The synthetic pathways a and b are shown in Fig. 3.12. The purity of intermediates was controlled by TLC using a silica stationary phase (Rp value of 2-allylamino-4,6-chloro-l,3,5-triazine = 0.48... 

T. Konstantinova and P. Petrova, On the synthesis of some bifunctional reactive triazine dyes. Dyes Pigm. 52 (2002) 115-120. 

The earliest reactive dyes were of the mono-functional type, e.g. Cl Reactive Red 3 (2.36), but very soon in the development phase homo-bifunctional dyes, e.g. Cl... 

Mixed-anchor systems in reactive dyes were first described in 1959. Such bifunctional dyes were first marketed in the early 1980s. These products are characterized by two anchors with differing reactivities a more reactive 2-sulfo-hydroxyethylsulfonyl group and a less reactive monochlorotriazinyl residue. An example of such a system is 14 ... 

In the outstandingly successful Cl Reactive Black 5, two such precursor-bearing units are used in the synthesis of this near-symmetrical bifunctional structure (7.36). Following this precedent, competing bifunctional dyes of analogous structure were designed with two phenylene-l,3-diamine-4-sulphonate groupings to accommodate the reactive systems... 

The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye Cl Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124] ... 

The trichlorotriazine molecule was the first reactive compound that was found to be able to form a reactive bridge between dye and substrate. One chlorine atom reacts with the amino group of the dye, and the other two chlorine atoms can then react bifunctionally with the substrate (or water) to form covalent bonds. An example is C.I. Reactive Yellow4, 13190 [1222-45-8] (20). 

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