The Dyeing of Cotton Fabrics | Page 8

boiling,
chromic acid oxidises cellulose into oxycellulose and other products.
It is therefore always advisable in carrying out any technical process
connected with cotton which involves its treatment with oxidising
agents of any kind, and where it is desired not to alter the cotton, to
allow these actions to be as short as possible.
=Dyes and Cotton Dyeing.=--An account of the chemistry of the cotton
fibre would not be complete unless something is said about the
reactions involved in the processes of dyeing and printing cotton. This
is a most interesting subject and opens up quite a number of problems
relating to the combination of the fibre with colouring matter of various
kinds, but here only a brief outline of the principles that present
themselves in considering the behaviour of the cotton fibre as regards
colouring matter will be given.

When the question is considered from a broad point of view, and
having regard to the various affinities of the dyes for cotton; we notice
(1) that there is a large number of dye-stuffs--the Benzo, Congo,
Diamine, Titan, Mikado, etc., dyes--that will dye the cotton from a
plain bath or from a bath containing salt, sodium sulphate, borax or
similar salts; (2) that there are dyes which, like Magenta, Safranine,
Auramine and Methyl violet, will not dye the cotton fibre direct, but
require it to be mordanted or prepared with tannic acid; (3) that there
are some dyes or rather colouring matters which, like Alizarine,
Nitroso-resorcine, barwood, logwood, etc., require alumina, chrome
and iron mordants; (4) that there are some dyes which, like the azo
scarlet and azo colours in general, cannot be used in cotton dyeing; (5)
that there are a few dyes, i.e., indigo, which do not come under this
grouping.
From the results of recent investigations into the chemistry of dyeing it
is now considered that for perfect dyeing to take place there must be
formed on the fibre a combination which is called a "colour lake,"
which consists of at least two constituents; one of these is the dye-stuff
or the colouring matter itself, the other being either the fibre or a
mordant, if such has to be used. The question of the formation of colour
lakes is one connected with the molecular constitution of the colouring
matter, but much yet remains to be done before the proper functions
and mode of action of the various groups or radicles in the dye-stuffs
can be definitely stated. While the constitution of the dye-stuff is of
importance, that of the substance being dyed is also a factor in the
question of the conditions under which it is applied.
In dealing with the first of the above groups of dyes, the direct dyes, the
colourist is somewhat at a loss to explain in what manner the
combination with the cotton fibre is brought about. The affinity of
cellulose for dyes appears to be so small and its chemical activities so
weak, that to assume the existence of a reaction between the dye-stuff
and the fibre, tending to the formation of a colour lake, seems to be
untenable. Then, again, the chemical composition and constitution of
the dyes of this group are so varied that an explanation which would
hold good for one might not do so for another. The relative fastness of

the dyes against washing and soaping precludes the idea of a merely
mechanical absorption of the dye by the fibre; on the other hand the
great difference in the fastness to soaping and light between the same
dyes on cotton and wool would show that there has not been a true
formation of colour lake.
The dyeing of cotton with the second group of dyes is more easily
explained. The cotton fibre has some affinity for the tannic acid used in
preparing it and absorbs it from the mordanting bath. The tannic acid
has the property of combining with the basic constituents of these dyes
and forms a true colour lake, which is firmly fixed on the fibre. The
colour lake can be formed independently of the fibre by bringing the
tannic acid and the dye into contact with one another.
In the case of the dyes of the third group, the formation of a colour lake
between the metallic oxide and the colouring matter can be readily
demonstrated. In dyeing with these colours the cotton is first of all
impregnated with the mordanting oxide, and afterwards placed in the
dye-bath, the mordant already fixed on the fibre then reacts with the
dye, and absorbs it, thus dyeing the cotton. To some extent the dyeing
of cotton with the basic dyes of the second group and the mordant dyes
of the third group is almost a mechanical one, the cotton fibre taking no
part in it from a chemical