ð Direct
dyes are mainly applied on cellulosic fibres such as cotton, viscose rayon ,
cuprammonium rayon etc.
ð Generally,
these dyes are easily applied on cellulosic fibres and can be directly dyed
from simple solution in water.
ð Hence,
these are called Direct Cotton Dyes.
ð These
dyes have an excellent substantivity for cellulose and therefore they are also
called Substantive Dyes.
ð Generally,
all direct dyes possess at least two azo groups in their structure and
therefore they are also called Diamine Dyes.
ð The
first manufactured direct dye was Congo Red.
PROPERTIES
ð Direct
dyes are soluble in water.
ð Direct
dyes are relatively inexpensive (cheap).
ð Direct
dyes are available in a full range of colours but are not high colour
brilliance.
ð Direct
dyes show poor to moderate washing fastness.
ð Direct
dyes show poor to fairly good light fastness.
PRINCIPLE
& MECHANISM
ð When
Direct Dye is dissolved in water, it ionizes giving D-, dye anions
and Na+, Sodium cations.
ð These
dyes are manufactured as sulphonic acids and converted generally into their
sodium salts.
ð The
suphonic acids are less soluble in water than their sodium salts.
ð Also,
the sulphonic acids have less affinity for cellulose than their sodium salts.
ð Therefore,
if the sulphonic acids are not completely converted in to their salts some of
the dye is wasted during their application.
ð Therefore,
while preparing a solution of a direct dye, it is customary to add a small
amount of soda ash (sodium carbonate) to convert the remaining sulphonic acid
into the corresponding sodium salt.
ð Direct
dye molecules contain some groups such as amino(-NH2), which can get
attached to cellulose macromolecule at the hydroxyl groups by weak hydrogen
bonds.
ð Electrolytes
(Salts) :- It is found that when direct dyes are dyed on cellulosic fibres, the
amount of dye taken up is considerably more when a salt such as sodium chloride
or Glauber’s salt is present in dye
bath.
·
Therefore, the salt is called an
exhausting agent.
·
This effect varies from dye to dye.
ð Temperature:-
The rate of dyeing increases with increasing temperature, i.e., dyeing takes
place quickly at high temperature.
·
As a result uneven dyeing take place at
high temperature.
·
From this it follows that for obtaining
even dyeing, the dyeing process should be started at a low temperature and then
the temperature of dye bath should be raised gradually.
ð Material
to Liquor Ration (M:L Ratio):- The amount of dye only depends on the percent
shade and the quantity of the material to be dyed.
·
Dye uptake, which determines the depth
of the final dyeing depends on the M:L ration, this ratio has to be kept
constant throughout dyeing.
·
Generally, M:L ratio for cotton dyeing
is 1:20.
CLASSIFICATION
OF DIRECT DYES
|
Self Levelling
Dyes
(Group A)
|
Salt
Controllable Dyes
(Group B)
|
Temperature
Controllable Dyes
(Group C)
|
|
Dyes belonging
to this group migrate well and therefore have high levelling power.
|
These
dyes have poor levelling properties.
|
These
dyes have poor levelling properties.
|
|
When dyeing
these dyes, dyeing may be uneven initially but continued dyeing levels out
the shade.
|
Dye
uptake is not uniform initially, it is very difficult to even out the shade
|
These
dyes are highly sensitive to salt and their exhaustion on to the fibre
adequately controlled by addition of salt alone
|
|
These do not
need the addition of salt for exhausting.
|
Their
exhaustion has to be brought by controlled addition of salt.
|
The
exhaustion is controlled by the proper rate of raising the temperature.
|
APPLICATION
ð Take
the require amount of direct dye and dissolve it using cold or hot water with
continuous stirring.
ð Filter
the dissolve dye to remove insoluble dye particles.
ð Prepare
dye solution as follows:-
1 : 25 - M:L Ratio
0.5 – 2.0 % - Soda
ash
10.0 – 30. 0 % - Glauber’s salt or Common salt
Required amount
of dye solution
ð Enter
well wetted cloth in this dye solution at 50 – 60 ⁰C,
carry out dyeing process at this temperature for 10 – 15 minutes.
ð Then
raise the temperature to boil and continue dyeing for 1hour.
ð During
dyeing salt is added in two parts.
ð After
completion of dyeing process, take out the fabric and wash in hot and cold water.
AFTER
TREATMENT
ð To
improve washing & light fastness of water some after treatment are also
required, which are as follows:-
·
0.5 – 20% Copper Sulphate
0.5 – 20 % Acetic acid (30%)
After dyeing
fabric is treated in above bath at 80 ⁰C for 30 minutes.
·
2.0 – 3.0 % Bichromate or chrom alum
2.0 – 5.0 % Acetic acid (30%)
After dyeing
fabric is treated in above bath at 80 ⁰C for 30 minutes.
·
0.5 – 2.0 % Bichromate
0.5 – 2.0 % Copper
Sulphate
1.0 – 5.0 % Acetic acid (30%)
After dyeing
fabric is treated in above bath at 80 ⁰C for 30 minutes.
·
2.0 – 3.0 % Formaldehyde
1.0 % Acetic
acid (30%)
After dyeing
fabric is treated in above bath at 65 ⁰C for 30 minutes.
·
Diazotisation Process
1.5 – 3.0 %
Sodium Nitrate
5.0 – 7.5 %
Hydrochloric acid (HCl)
After
dyeing fabric is treated in above bath for 20 minutes. Then developed in following bath
Ø 0.75
– 1.5 % m – Toluene diamine
(Dissolve with
half weight of soda ash)
Ø 0.75
– 1.5 % β Naphthol
(Dissolve with
half weight of caustic soad)
Ø 0.75
– 1.5 % m – Phenylene diamine
(Dissolve with half
weight of soda ash)
