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CHAPTER 14

Dyeing cellulosic fibres with
direct dyes

14.1 INTRODUCTION

Direct cotton dyes have inherent substantivity for cotton, and for other cellulosic
fibres. Their aqueous solutions dye cotton usually in the presence of an electrolyte
such as NaCl or Na2SO4. Direct dyes do not require the use of a mordant and, as
their name implies, the dyeing procedure is quite simple. The goods go into the
bath followed by the dissolved dyes. The bath is then gradually heated, usually to
the boil, and additions of salt promote dyeing.

Many direct dyes are relatively inexpensive. They are available in a full range of
hues but are not noted for their colour brilliance. Their major drawback is their
poor to moderate fastness to washing. This limits their use to materials where good
washing fastness is not critical. The light fastness of dyeings with direct dyes on
cellulosic fibres varies from poor to fairly good, although some copper complex
direct dyes have very good light fastness. As usual, the deeper the colour of the
dyeing, the lower the fastness to wet treatments, and the higher the fastness to
light. Various aftertreatments of the dyeings improve the fastness to washing. In
some cases, however, such aftertreatments decrease the light fastness. They also
invariably cause a change in hue that makes shade correction and colour matching
more difficult.

Cotton, and other cellulosic fibres, are dyed with direct, sulphur, vat, reactive or
azoic dyes – more types than for any other fibre. Each of these classes of dye has its
own application methods, dyeing characteristics, cost, fastness properties and
colour range, and therefore its own particular advantages and disadvantages.
Within each group, application and performance properties vary considerably so
the choice of which dyes to use is often not easy. Direct dyes generally cannot
meet today’s more stringent washing fastness requirements for apparel and linens.
In recent years, their share of the market has gradually declined in favour of
reactive dyes. The latter have very good washing fastness on cellulosic materials
and often have bright colours.

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