Page 264 - Basic Principles of Textile Coloration

PROBLEMS OF DYEING WOOL LEVEL 253

Damaged wool fibres give two main types of unlevel dyeing:
(1) dyeings with coloured patches of different depths caused largely by uneven

treatment with chemicals during processes such as scouring, bleaching or
chlorination, or incomplete and non-uniform removal of the residual
chemicals;
(2) skitteriness – the uneven dyeing of individual wool fibres whose tips have
degraded more from the greater exposure to the elements during the growth
of the wool fleece.

Wool fibres with damaged scales absorb dye more rapidly than undamaged fibres.
Weathered tips of wool fibres tend to give skittery or so-called ‘tippy’ dyeings. The
fibre tips may have surface damage and have undergone chemical modifications.
They may dye lighter or darker in depth. In the more usual case, the damaged tips
absorb more dye than the rest of the fibre and are darker. If the underlying protein
is chemically changed because of weathering, lighter dyed tips sometimes arise
because of their lower dye absorption at equilibrium compared to the undamaged
fibre root. When dyeing with mixtures of dyes containing different numbers of
sulphonate groups per dye molecule, the wool fibre tip may even dye a different
shade than the root.

Skitteriness is much easier to see in dyed loose fibre. The colour of skittery
dyeings is often weak. This is because a mixture of undyed and dyed fibres always
gives a paler shade than the same total mass of evenly dyed fibre containing the
same amount of dye. The colour strength of a skittery dyeing therefore improves if
the dyes can migrate and the colour becomes more level. When dyeing with non-
migrating dyes, the use of specific cationic or amphoteric auxiliary levelling agents
avoids ‘tippy’ dyeings (Section 13.5.3).

Wool is a sensitive protein. Even dyeing with acid dyes at the boil causes some
degradation, particularly on prolonged boiling to promote dye migration. This
causes hydrolysis of various wool proteins. This is slowest around the isoelectric
point at about pH 5. It is particularly rapid with alkaline solutions. Hydrolysis
impairs the mechanical properties of the wool, giving more brittle fibres with
decreased resistance to abrasion. If the goods are in the form of loose fibre, fibre
brittleness can complicate carding and spinning. Hydrolytic damage of wool fibres
also produces unacceptable yellowing. Certain protective auxiliary products are
used to minimise degradation and yellowing of the wool. These protective agents
are most useful for high temperature dyeing processes, such as the dyeing of wool/
polyester blends at 120 °C, or when dyeing has to be extended or repeated. In