The Dyeing of Cotton Fabrics | Page 4

caustic potash,
of from 2 to 7 per cent. strength, have no action on cellulose or cotton,
in the cold, even when a prolonged digestion of the fibre with the
alkaline solution takes place. Caustic alkali solutions of from 1 to 2 per
cent. strength have little or no action even when used at high
temperatures and under considerable pressure--a fact of very great
importance from a bleacher's point of view, as it enables him to subject
cotton to a boil in kiers, with such alkaline solutions at high pressures,
for the purpose of scouring the cotton, without damaging the fibre

itself.
[Illustration: FIG. 2.--Mercerised Cotton Fibre.] [Illustration: FIG. 2A.]
Solutions of caustic soda of greater strength than 3 per cent. tend, when
boiled under pressure, to convert the cellulose into soluble bodies, and
as much as 20 per cent. of the fibre may become dissolved under such
treatment. The action of strong solutions of caustic soda or caustic
potash upon cellulose or cotton is somewhat different. Mercer found
that solutions containing 10 per cent. of alkali had a very considerable
effect upon the fibre, causing it to swell up and become gelatinous and
transparent in its structure, each individual cotton fibre losing its
ribbon-like appearance, and assuming a rod-like form, the central canal
being more or less obliterated. This is shown in Fig. 2 and 2A, where
the fibre is shown as a rod and the cross section in Fig. 2A has no
central canal. The action which takes place is as follows: The cellulose
enters into a combination with the alkali and there is formed a sodium
cellulose, which has the formula C{6}H{10}O{5}2NaOH. This alkali
cellulose, however, is not a stable body; by washing with water the
alkali is removed, and hydrated cellulose is obtained, which has the
formula C{6}H{10}O{5}H{2}O. Water removes the whole of the
alkali, but alcohol only removes one half. It has been observed that
during the process of washing with water the fibre shrinks very much.
This shrinkage is more particularly to be observed in the case of cotton.
As John Mercer was the first to point out the action of the alkaline
solutions on cotton, the process has become known as "mercerisation".
Solutions of caustic soda of 1.000 or 20° Tw. in strength have very
little mercerising action, and it is only by prolonged treatment that
mercerisation can be effected. It is interesting to observe that the
addition of zinc oxide to the caustic solution increases its mercerising
powers. Solutions of 1.225 to 1.275 (that is from 45° to 55° Tw. in
strength) effect the mercerisation almost immediately in the cold, and
this is the best strength at which to use caustic soda solutions for this
purpose. In addition to the change brought about by the shrinking and
thickening of the material, the mercerised fibres are stronger than the
untreated fibres, and at the same time they have a stronger affinity for

dyes, a piece of cloth mercerised taking up three times as much
colouring matter as a piece of unmercerised cloth from the same
dye-bath.
The shrinkage of the cotton, which takes place during the operation of
washing with water, was for a long time a bar to any practical
application of the "mercerising" process, but some years ago Lowe
ascertained that by conducting the operation while the cotton was
stretched or in a state of tension this shrinkage did not take place;
further, Thomas and Prevost found that the cotton so treated gained a
silky lustre, and it has since been ascertained that this lustre is most
highly developed with the long-stapled Egyptian and Sea Island cottons.
This mercerising under tension is now applied on a large scale to
produce silkified cotton. When viewed under the microscope, the
silkified cotton fibres have the appearance shown in Fig. 3, long
rod-like fibres nearly if not quite cylindrical; the cross section of those
fibres has the appearance shown in Fig. 3A. This structure fully
accounts for the silky lustre possessed by the mercerised fibres. Silky
mercerised cotton has very considerable affinity for dye-stuffs, taking
them up much more readily from dye-baths, and it is dyed in very
brilliant shades.
[Illustration: FIG. 3.--Silkified Cotton Fibre.]
[Illustration: FIG 3A.]
In the chapter on Scouring and Bleaching of Cotton, some reference
will be made to the action of alkalies on cotton.
ACTION OF ACIDS ON CELLULOSE.
The action of acids on cellulose is a very varied one, being dependent
upon several factors, such as the particular acid used, the strength of the
acid, duration of action, temperature, etc. As a rule, organic acids--for
example acetic, oxalic, citric, tartaric--have no action on cellulose or
cotton. Solutions of sulphuric acid or hydrochloric acid of 2 per cent.
strength have practically no action in the cold, and if after immersion
the