These colors
are known as
substantive dyes or
Salt dyestuffs or
even simply cotton colors. The colors are well known for
its use in dyeing cellulose fibres like cotton,
viscose rayon, animal fibres such as wool and silk. Selected substantive
dyes can be used to give solid shades on
wool and cotton mixtures. Direct dyes are also called Substantive dyes because of their excellent substantively
for cellulose textile materials like cotton and viscose rayon.
Properties Of Direct Dyes
- Direct dyes are readily soluble in water.
- Some of the dyes need a little amount of soda ash (Sodium Carbonate) for complete dissolve them in water.
- Direct dyes are used for dyeing cotton, viscose rayon, wool and silk fibres.
- If common salt or Glauber‟s salt (Sodium sulphate) is added to the dye during dyeing, more dye is taken up by the fiber i.e. the dye bath gets exhausted to a greater extent. Hence these salts are exhausting agents for direct while dyeing or cotton.
- Direct dyes have poor washing fastness and poor light fastness. These fastnessproperties can be improved by certain after-treatments. If a piece of cotton dyedwith a direct dye is boiled with soap and water which contains also a small pieceof un-dyed cotton, the latter generally becomes stained with the dye from former.This is known as bleeding.
Application of Direct
Dyes to Cotton/Viscose
The dye bath is usually made
up with twenty items as much water as the material. Take for example, for 100
kilograms of cotton fabric 2,000 liters of water can be taken. For dyeing only
soft water should be used.
From 0.5 to 5% of the dyestuff
according to the shade required to be taken. The dyestuff
is pasted with
cold water and sufficient boiling
water is then
added, with constant
stirring to bring
it into solution.
It is then
added to the
dye liquor through
a strainer.
Some direct
dyestuffs give better
results in a
slightly alkaline bath.
In fact it
is advantageous in most cases,
since it renders the dyestuff more completely soluble in the dye bath. From 0.5 to 2% of Sodium carbonate
may be used.
The cotton material is wetted
out in water and entered into the dye bath at 41ºC to 50 ºC and
the liquid is
raised to the boil
over a period
of 30 to
40 minutes and
then common salt or Glauber‟s
salt is added to the dye bath for the complete exhaustion of the dye bath. These salts are exhausting agents for direct
dyes while dyeing on cotton. The dyeing is continued at the boil for ¾ to 1
hour. At the end of the dyeing the
material is rinsed in cold water and dried without soaping. Direct
dyes have poor
washing fastness and
poor light fastness.
However, these fastness
properties can be improved by certain after treatments.
A dye fixing treatment may
give in a fresh bath
containing a cationic dye-fixing agent
before drying; this process improves the wet fastness of the dyed goods.
Fastness properties of
Direct dyed goods - Both
washing and light
fastness of most of the direct
dyed goods are generally poor; to improve the fastness properties after treatments may be given improve the fastness
No comments:
Post a Comment