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Thus lines, which has a harder structure than cotton, is less readily penetrated by dyes. There are also differences in dyeing properties between the several types of regenerated cellulose fibres; cuprammonium rayon, for example, having fine filaments, is more easily dyed than viscose. Dyes of many chemical classes are applied to cellulosic fibres. Azo dyes, which predominate numerically, are described here, and others are dealt with in the appropriate chapters. The first substantive or ‘direct’ dyes discovered in 1884 were diazo dyes obtained from tetrazotised benzidine, but other structure have since been found to confer affinity for cellulose.

Rosaniline is prepared by condensing one molecule each of aniline, p-toluidine, and o-toluidine in nitrobenzene to give a carbinol. Treatment of the carbinol with hydrochloric acid yields the dye. H C H2N H H + H NH2 NH2 3[O] OH NH2 -2H2O H p-toludine Colourless carbinol CH3 O-Toluidine CH3 + HCl -H2O NH2 C H2N NH2 H2N C NH2 CH3 PARAROSANILINE 29 Cl Rosaniline has a greenish metallic lustre. It dissolves in water to give a deep-red colour. Rosaniline is used for dyeing cotton, silk and wool. Addition of sulphur dioxide to the dye solution produces the colourless Schiff’s reagent, which is used to test the presence of aldehydes.

This change in the colour is attributed to change in the structure of its ion. Under alkaline conditions the ion contains the azo chromophore, while under acidic conditions it contains p-quinoid chromophore. Azo Compound p-Quinoid Chromophore N=N N CH3 CH3 H+ OH - - O3S + N CH3 CH3 NH N Red Acidic Solution Yellow (Alkaline Solution) (e) Orange II: It is obtained by coupling diazotised sulphanilic acid with β-naphthol is basic medium. HO3S + - HO3S N2Cl + H HO OH Diazotised Sulphanilic Acid + HCl N=N Orange 2 -Naphthol Orange II is used to dye wool, silk, nylon, paper and leather.

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Advanced Thermal-Hydraulic, Neutronic Codes Vol 2 (csni-r2001-2-vol2)

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