Page 411 - Basic Principles of Textile Coloration

400 DYES SYNTHESISED IN THE FIBRE

there are no greens or bright blues. The actual hue depends on the choice of the
diazonium and coupling components. Their use on cotton today is more and more
limited, largely because of the success of fibre-reactive dyes for cotton. Black
shades on polyamide, polyester and acetate fibres are also often azoic combinations
(Section 15.5). For azoic dyeing of these artificially-made fibres, a dispersion of the
primary amine and coupling component is used. The fibres absorb these like
disperse dyes. The amine is then diazotised in the fibre and reacts with the
coupling component to give the azo pigment.

19.2.2 Coupling components

Azoic coupling components are often called Naphtols (note the difference in
spelling from naphthol), after Naphtol AS, the original commercial name of the
anilide of 2-hydroxy-3-naphthoic acid (beta-oxynaphthoic acid or BON acid).
Naphtol AS (CI Azoic Coupling Component 2) was introduced in 1912. Many
other Naphtols are also anilides of BON acid (Figure 19.2). The Naphtols are
phenols, soluble in alkaline solution and substantive to cotton, particularly in the
presence of salt. A cotton fabric impregnated with the Naphtol solution does not
usually require intermediate drying before development with the diazonium ion
solution. Their dyeings have better fastness properties than those of Para Red.
Deep red and bordeaux are the predominant shades obtained with azoic
combinations. The Naphtols used for yellow and orange azoic combinations are
usually arylamides of acetoacetic acid (Figure 19.2), which dissolve in dilute
alkaline solution as the enolate ions.

The anilides of BON acid are soluble in dilute NaOH solution and form the
corresponding naphtholate ion. These relatively small molecules are of only low to
moderate substantivity for cotton, but they diffuse rapidly into the fibres. In
general, the higher the substantivity the better the rubbing fastness as less azo
pigment forms on the fibre surfaces. The naphtholate ions are always coplanar and
preferably have elongated molecular structures. They behave essentially as
colourless, low molecular weight direct dyes. The substantivity increases with
increase in the molecular size of the naphtholate ion, but the diffusion rate in the
fibres and solubility in dilute aqueous alkali decrease. Addition of salt promotes
better exhaustion of the bath, more being needed for Naphtols of lower
substantivity. Low substantivity is preferred in continuous dyeing so that the bath
composition does not vary because of preferential absorption (Section 10.5.2). For