Substantivity, cotton

Congo red is the prototype of the substantive cotton dyes of the benzidine group. The tinctorially valuable property of dyeing cotton directly (i.e. without a mordant) is doubtless to he attributed to the intimate adsorption of the colloidal particles of the dye on the fibre. 

It is this action alone which enables a relatively small number of dyes, the so-called substantive cotton dyes, to be absorbed directly by the unmordanted vegetable fibre. The most important of these dyes are the bis-azo dyes, such as Congo-red and related compounds, which are derived from doubly diazotised benzidine. In aqueous solution they are present as sols and are colloidally adsorbed by the fibres as irreversible gels. 

Compounds 14 to 2-3 are all used as end or middle components for substantive cotton dyes. 

Direct dyes are defined as anionic dyes substantive to ceUulosic fibers (cotton, viscose, etc), when applied from an aqueous bath containing an electrolyte. Before the discovery of Congo Red in 1884, only mordanted cotton could be dyed. Congo Red [573-58-0] (62) (Cl Direct Red 28 Cl 22120) a primary symmetrical disazo dye, which is made readily from bisdiazotized benzidine and naphthionic acid [84-86-6] (4-arnino-l-naphthalenesulfonic acid), was the precursor of a most important line of dyes, including all shades, derived from benzidine and its homologues. Today, no benzidine dye is produced because benzidine is carcinogenic. 

One of the earliest fibre pretreatments for improving the dyeability of cotton is of course mercerisation (section 10.5.4). However, more recent research interest in this area has been generated by environmental concerns about reactive dyeing, aiming to enhance substantivity for the modified fibre so that higher absorption and fixation are obtained. This results in less dye (hydrolysed or still active) in the effluent. A further objective is to minimise the usage of electrolyte in the application process. This area has been thoroughly reviewed [392,393]. 

Processes devised to make cotton hydrophobic are summarised in Table 10.42. These processes are undoubtedly successful in conferring substantivity for disperse dyes but attaining compatibility within a range of dyes across the entire colour gamut and on fibre blends of various blend ratios could be a problem. In addition, ester bonds can be saponified... 

Substituted triazinyl derivatives of DAS are usually chosen for pad-dry-bake application to cotton in conjunction with an easy-care or durable-press finish. In these mildly acidic conditions (pH about 4) the FBA must show appreciable resistance towards the catalyst (usually magnesium chloride) necessary to cure the resin. The less substantive products in the upper half of Table 11.1 are important in this respect, as are compounds of type 11.9 where R = OCH3 or CH3NCH2CH2OH. It is likely that the hydroxyethylamino groups present in many of these compounds participate in condensation reactions with N-methylol groups in the cellulose-reactant resin. The performance of an FBA applied in conjunction with a resin finish can be modified and improved by careful formulation of the pad liquor but this lies beyond the scope of the present chapter. Alternatively, FBA and resin can be applied in two separate steps most DAST-type brighteners would be suitable if applied in this way. 

The FBAs used to brighten wool are mainly DAST types and pyrazolines of the acid dyeing type already discussed in section 11.8. Examples include the three most cotton-substantive DAST brighteners listed in Table 11.1, although on wool these give light... 

Higher substantivity for cellulose. Dye at 45-50 °C with more caustic soda and less salt (none for mercerised cotton or regenerated cellulosic fibres). 

Direct dyes are attracted to the textile, according to their substantivity, by intermolecular forces without the need of mordant. They are used to color cotton and paper leather, silk, and nylon, and are also used as pH indicators or as biological... 

Controversy has arisen at times [52] regarding the apparent synonymity of the terms basic dye and cationic dye . The Society of Dyers and Colourists defines a basic dye as characterised by its substantivity for the acidic types of acrylic fibres and for tannin-mordanted cotton , whereas a cationic dye is defined as one that dissociates in aqueous... 

Cotton and wool, even non-mordanted, are dyed in a dye bath containing sodium sulphate dyed natural wool gives up its Substantive colour in a slightly alkaline medium to white cotton. . . colouring matters... 

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