The first subjective or 'direct'
dyes discovered in 1884 were diazo dyes obtained from terazotised benzidine.The
direct dyes are called substantive dyes they have strong affinity for
Cellulosic fibers. It was called a
direct dye because it was the first dye to become available for coloring
cellulose 'directly' without the use of a any other chemical such as mordant. The
direct dyes are called substantive dyes as they have strong affinity for cotton
and other Cellulosic fibers.
Properties:
They are available in a full
range of hues but are not noted for their color brilliance. They have poor
moderate fastness to washing but can be improved by after - treatment with
copper salts & cationic fixing agents. The light fastness of with direct
dyes on cellulose fibers varies from poor to fairly good, although some copper
complex direct dyes have good light fastness. SDC committee also classified Direct Dye on
three categories based on their leveling ability
and their response to increase in the dyeing temperature and to added salt
during exhaust dyeing.
Class A (Self Leveling): Dyes
with good migration or leveling properties.
Class B (Salt Controllable): Dyes with poor leveling but which can be controlled by addition of salt.
Class C (Temperature Controllable): Dyes which are not self-leveling and are highly sensitive to salt; the exhaustion of these dyes needs addition of salt and temperature.
Class B (Salt Controllable): Dyes with poor leveling but which can be controlled by addition of salt.
Class C (Temperature Controllable): Dyes which are not self-leveling and are highly sensitive to salt; the exhaustion of these dyes needs addition of salt and temperature.
FACTORS FOR DIRECT DYEING:
The Society of Dyers and Colorists (SDC)
Committee in 1946, on the basis of the dyeing Properties of direct dyes on
cotton determined four factors for direct dyeing of cotton:
migration
(or leveling power)
salt
controllability
the influence of temperature
liquor
ration on exhaustion
Photochromism of direct
dyeing:
On certain direct dyes, hue
changes from bright turquoise to violet or reddish blue on prolonged exposure
to sunlight or UV radiation. This is known as Phototropism. The hue reverts
gradually to the original turquoise when the illuminant withdrawn. An electrode
is a substance that, because of its chemical nature, conducts electricity in
solution. When simple electrolytes like NaCl dissolve in water they spilt up
(dissociate) into separate ions. It is for this reason that they will allow an
electric current to pass through the water.
Mordanting & Fixing of dyeing
Tannic acid is used here as a mordant. Generally cotton is Mordant with
tannic acid which is thus times the dye used. Tannic acid solution is boiled,
where the cellulose is immersed and the temperature is lowered to 600C
within 1–2 hour. For this treatment the affinity of cellulose for tannic acid
is increased. Then the material is withdraw from the bath and squeezed. Then
the Mordanted cotton is dipped into tartar emetic solution (Dye: Tannic acid:
Tartar emetic = 1:2:1) where it is worked for 30 min at room temperature where
tannic acid is fixed on the cotton fibre.
The Mordanted cotton is then dyed with above recipe. The dye stuff is
pasted with equal amount of acetic acid or methylated spirit. Then the dye
solution is made with required amount of hot water. The Mordanted cotton is
then dyed in the dye bath at 700C for 1-1.5 hrs. the dyed material
is then rinsed, squeezed and dried.
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