Page 113 - Basic Principles of Textile Coloration

102 ARTIFICIALLY MADE FIBRES BASED ON CELLULOSE

clean them off the fibre surface without digesting the bulk of the more compact
fibre. A second fibrillation is next carried out on the cleared fibre surface but only
on the exposed areas. This avoids any problems of pilling and fibril entanglement.
It is preferable that dyeing follow these processes. If a fabric is required with a
clean surface appearance, fibrillation must be minimised by using high liquor
ratios, a lubricant in the bath or by low temperature processing. To minimise
fibrillation of Tencel, Courtaulds recommend the use of a proprietary chemical
applied under hot alkaline conditions. It functions as a crosslinking agent and
prevents fibrillation.

6.2.5 Dyeing regenerated cellulose fibres
All the various forms of viscose are composed of cellulose and therefore have
similar chemical properties. The different types of viscoses, however, have
different dyeing properties. They can all be dyed with the same types of dyes used
for dyeing cotton, but the dyeing rate and amount of dye absorbed may vary from
one type of viscose to another. If a viscose/cotton blend is dyed with a typical
cotton dye, it is usual that the viscose will be more deeply coloured. Even in the
case of fibrillar viscoses, which closely resemble cotton, it is unlikely that the two
different cellulosic fibres in the blend will be dyed to the same depth. Dyed lyocell
fibres have a colour yield (the depth of colour obtained for a given amount of dye)
that is higher than that for dyed viscose, but lower than for mercerised cotton.
These differences are not surprising since viscose morphology can be quite varied,
depending upon the manufacturing conditions. From the dyeing point of view, all
they have in common with each other, and with cotton, is their cellulosic nature.
Dyeing trials are therefore recommended.

6.3 CELLULOSE ACETATES [2,4]
Cellulose acetates are esters of cellulose in which a large fraction or even all the
hydroxyl groups have been esterified using acetic anhydride (Scheme 6.5). The
two major types of cellulose acetate have about 55 and 62% of combined acetic
acid. These values correspond to cellulose with degrees of substitution (DS) of
2.48 and 3.00, respectively. The latter is correctly called cellulose triacetate, but
the name cellulose diacetate for the former is somewhat misleading, its
composition being intermediate between the di- and triacetate.

These fibres in no way resemble fibres of cellulose. Both cellulose diacetate and