Page 252 - Basic Principles of Textile Coloration

CLASSIFICATION OF ACID DYES 241

the dye with hydrophobic groups in the fibre. Dyeing is therefore not solely a
consequence of simple ionic attraction.

The colour gamut of acid dyes is complete, including greens and blacks. The
dyes are available as powders, grains and liquids for continuous dyeing, and as fine
dispersions of the less soluble types. The selection of acid dyes for dyeing a
particular material is not an easy matter, given the wide range of textile products
and fastness properties demanded. Manufacturers recommend groups of selected
acid dyes for each type of application. Compatible dyes are selected to have similar
rates of dyebath exhaustion, when applied together by the recommended
procedure, and similar fastness properties.

13.2 CLASSIFICATION OF ACID DYES

13.2.1 Classification according to chemical structure
Acid dyes are of many different chemical types. Sulphonated azo dyes constitute
the major group and are mainly mono- and bis-azo compounds ranging in colour
from yellow, through red to violet and brown. There are some navy blue bis-azo
dyes that can build up to give blacks. The substantivity of azo dyes for polyamide
and protein fibres is greater the higher their molecular weight and the lower the
number of sulphonate groups per dye molecule. Many mordant and metal-complex
dyes are also azo compounds. Their molecular structures are discussed later in this
chapter (Sections 13.7.2 and 13.8).

Acid dyes usually have low substantivity for cellulosic fibres and do not give
appreciable exhaustion onto them. Some of the higher molecular weight azo acid
dyes, however, are difficult to distinguish from direct dyes for cotton (Chapter 14).
The latter are often sulphonated bis-azo and polyazo dyes. There are therefore a
number of acid and direct dyes with similar structural features that could be
classified in either group. These dyes will dye wool, nylon and cotton. One feature
that distinguishes acid dyes from direct cotton dyes, apart from their lower
molecular weights, is that molecular planarity is not essential for acid dyes, but is
for direct cotton dyes.

Anthraquinone acid dyes complement the azo dyes, ranging in colour from
violet through blue to green. These dyes often have very good light fastness. Acid
dyes with triphenylmethane (blues and greens) and xanthene (reds and violets)
chromophores are less important types noted for their brilliant colours. These