CHEMICAL CONSTITUTIONS OF DISPERSE DYES

The majority of disperse dyes are low molecular weight, non-ionic mono-azo and anthraquinone derivatives. Polar substituents are usually present in the dye molecule so that the dye has the slight solubility in water required for dyeing. Hydroxyethylamino groups (NHCH2CH2OH) are typical of such substituents. The interaction of such polar groups with the water, by dipole interactions and hydrogen bonds, is crucial for water solubility. Dipole forces and hydrogen bonds, as well as dispersion forces, also bind the dye molecules to polar groups in the fibres.

There are many thousands of azo disperse dye structures because of the numerous substitution patterns possible in the diverse diazonium ion and coupling components. Colours that are less typical of simple azo compounds, such as greenish-yellow and blue, are also possible using more specialised components. These may have heterocyclic units or cyano substituents. There is also a limited number of other chromophores providing disperse dyes with particular properties. Figure gives some typical disperse dye structures.

Some typical disperse dye structures

Anthraquinone disperse dyes are usually 1-hydroxy or 1-amino derivatives. These have bright colours ranging from red through to blue. Simple anthraquinone dyes have low molar absorptivities compared to azo compounds and therefore give dyeings of lower colour yield. Apart from a few bright pinks and blues, anthraquinone disperse dyes are gradually being replaced. In their manufacture, the production of the required intermediate chemicals, and of the dyes themselves, often involves complex reactions under pressure. The reaction equipment is more sophisticated than that used for the simpler azo coupling reaction. In addition, anthraquinone-1-sulphonic acids are key intermediates and the sulphonation reactions for their preparation use a mercuricion catalyst. The environmental threat of mercury in the chemical plant effluenthas led to increasingly stringent regulations for its containment and therefore increased production costs.

There are no true green or black disperse dyes. Dyes with both red and blue light absorption bands for greens, or with several overlapping absorption bands for blacks, are difficult to prepare. A major constraint for disperse dye structures is the relatively low molecular weight that the dye must have to be slightly water-soluble and to be able penetrate into hydrophobic synthetic fibres. A combination of blue and yellow dyes gives green dyeings. Blacks require an aftertreatment of the dyeing involving diazotisation of the absorbed dye containing a free primary amino group followed by reaction with a coupling component. Black disperse dyes may also be mixtures of dull orange, rubine and navy dyes. Many disperse dyes are mixtures generated by the reactions used in their synthesis. Techniques such as thin layer chromatography are useful for establishing the number of components.