The Dyeing of Cotton Fabrics | Page 5

cotton or cellulose be well washed there is no change of any kind.

This is important, as in certain operations of bleaching cotton and other
vegetable fibres it is necessary to sour them, which could not be done if
acids acted on them, but it is important to thoroughly wash the goods
afterwards. When the acid solutions are used at the boil they have a
disintegrating effect on the cellulose, the latter being converted into
hydrocellulose. When dried, the cellulose is very brittle and powdery,
which in the case of cotton yarn being so treated would show itself by
the yarn becoming tender and rotten. The degree of action varies with
the temperature (the higher this is the stronger the action), and also
according to the strength of the acid solution. Thus a 10 per cent.
solution of sulphuric acid used at a temperature of 80° C. begins to act
on cotton after about five minutes' immersion, in half an hour there is a
perceptible amount of disintegration, but the complete conversion of
the cotton into hydrocellulose requires one hour's immersion. A dilute
acid with 8 volumes of water, used in the cold, takes three hours'
immersion before any action on the cotton becomes evident.
ACTION OF SULPHURIC ACID ON COTTON.
When cellulose (cotton) is immersed in strong sulphuric acid the cotton
becomes gradually dissolved; as the action progresses cellulose
sulphates are formed, and some hydrolytic action takes place, with the
formation of sugar. This fact has long been known, but only recently
has it been shown that dextrose was the variety of sugar which was
formed. On diluting the strong acid solution with water there is
precipitated out the hydro or oxycelluloses that have been formed,
while the cellulose sulphates are retained in solution.
By suitable means the calcium, barium, or lead salts of these
cellulose-sulphuric acids can be prepared. Analysis of them shows that
these salts undergo hydrolysis, and lose half their sulphuric acid.
The action of strong sulphuric acid has a practical application in the
production of parchment paper; unsized paper is immersed in strong
acid of the proper strength for about a minute, and then immediately
rinsed in water. The acid acts upon the surface of the paper and forms
the cellulose-sulphuric acid which remains attached to the surface. On
passing into the water this is decomposed, the acid is washed away, and

the cellulose is deposited in an amorphous form on the paper, filling up
its pores and rendering it waterproof and grease-proof. Such papers are
now largely used for packing purposes.
ACTION OF HYDROCHLORIC ACID.
Dilute hydrochloric acid of from 1° to 2° Tw. in strength, used in the
cold, has no action on cellulose. Cotton immersed in acids of the
strength named and then well washed in water is not materially affected
in any way, which is a feature of some value in connection with the
bleaching of cotton, where the material has to be treated at two points
in the process with weak acids. Boiling dilute hydrochloric acid of 10°
Tw. disintegrates cellulose very rapidly. The product is a white very
friable powder, which if viewed under the microscope appears to be
fragments of the fibre that has been used to prepare it. The product has
the composition C{12}H{22}O{11}, and is therefore a hydrate of
cellulose, the latter having undergone hydrolysis by taking up the
elements of water according to the equation 2C{6}H{10}O{5} +
H{2}O = C{12}H{22}O{11}. By further digestion with the acid, the
hydrocellulose, as it is called, undergoes molecular change, and is
converted into dextrine. In composition hydrocellulose resembles the
product formed by the addition of sulphuric acid which has received the
name of amyloid. It differs from cellulose in containing free carboxyl,
CO, groups, while its hydroxyl groups, HO, are much more active in
their chemical reactions.
Hydrocellulose is soluble in nitric acid, 1.5 specific gravity, without
undergoing oxidation. Nitrates are formed varying in composition.
The formation of hydrocellulose has a very important bearing in
woollen manufacture. It is practically impossible to obtain wool free
from vegetable fibres, and it is often desirable to separate these
vegetable fibres. For this purpose the goods are passed into a bath of
hydrochloric acid or of weak sulphuric acid. On drying the acid
converts the cotton or vegetable fibre into hydrocellulose which, being
friable or powdery, can be easily removed, while the wool not having
been acted on by the acid remains quite intact. The process is known as
"carbonising". It may not only be done by means of the acids named

but also by the use of acid salts, such as aluminium chloride, which on
being heated are decomposed into free acid and basic oxide. For the
same reason it is important to avoid the use of these bodies, aluminium
chloride and sulphate, zinc