Page 314 - Basic Principles of Textile Coloration

DYEING DIFFERENT TYPES OF CELLULOSIC FIBRES 303

14.6 DYEING DIFFERENT TYPES OF CELLULOSIC FIBRES

Cellulosic fibres include cotton, mercerised cotton, linen and the various forms of
viscose, each with their own particular dyeing characteristics. Because of the
differences in molecular weight and morphology between native and regenerated
cellulose fibres, there are large differences in their physical properties. Wet cotton
is a strong, rigid fibre and most woven cotton fabrics will withstand considerable
stress. This is not true of fabrics made from regular viscose. Their wet strength is
only moderate and they require careful handling during dyeing. Viscose also swells
much more than cotton when wetted. This can lead to limited solution flow and
dye penetration problems in dyeing packages or packed loose fibre.

The different types of cellulosic fibre are all dyed by direct dyes using essentially
the same dyeing method. The temperature increase of the dyebath, and the
amount of salt added to it, control the rate of exhaustion and levelling. In the
dyeing of blends of these cellulose fibres with direct dyes, they do not absorb dyes
at the same rate or to the same extent because of the differences in their
morphology. Even the extent and conditions of drying after preparation can
influence the dyeing of a fabric of cellulosic fibres. Despite these differences, the
standard affinity of a given direct dye is remarkably constant for the different
cellulose fibres.

The effects of carboxylate groups are important at low salt concentrations
where there is little effective shielding of their negative charge by sodium ions.
The higher negative surface charge of viscose compared with cotton is partly
because of its additional carboxylate groups. In general, with more than 2 g l–1
NaCl, the extent of dye absorption increases in the order cotton, mercerised
cotton, regular viscose; in the same order as increasing fibre accessibility. Even
different grades of cotton fibre have different dye absorption characteristics. This
is one reason why intimate blending of cotton fibres before spinning is so
important.

The rate of dyeing increases as the diameter of a fibre decreases, even though
the equilibrium exhaustions for chemically identical fibres with different diameters
hardly vary. This is readily demonstrated for viscose filaments of different denier.
The internal volume of the pores is different for the various cellulosic fibres. These
internal volumes have been estimated to be 0.30, 0.50 and 0.45 l kg–1 for cotton,
mercerised cotton and regular viscose. A value of 0.22 l kg–1 is widely used for
cotton in estimating dye affinities (Section 11.1.2). In dyeing, these fibres are far