Direct dyes are water-soluble dyes that have an affinity for cellulosic fibers like cotton. They are applied from an aqueous solution and their name comes from this "direct" application method. Direct dyes contain sulphonic acid groups that give them a negative charge in water, allowing attraction to the positively charged sites on cotton fibers. Several factors influence direct dyeing, including electrolyte concentration, temperature, and liquor ratio. After-treatments can be used to improve wash fastness, including treatments with metallic salts or formaldehyde. Cationic dye-fixing agents are also commonly used in after-treatment to form complexes that improve wash fastness.

1. Dyeing of cotton with Direct Dyes
Presented by-
VIKASH KUMAR
M.Tech in TEXTILE TECHNOLOGY(TECHNICAL TEXTILES)
UNIERSITY OF CALCUTTA

2. Introduction
Direct dyes are also called substantive dyes because of their excellent substantivity for cellulosic textile materials like cotton
and viscose rayon.
This class of dyes derives its name from its property of having 'direct' affinity for cellulosic fibres when applied from an
aqueous solution
Chemically, direct dyes are sodium salts of aromatic sulphonic acids and most of them contain an "azo" group as the main
chromophore.
Various manufacturers market direct dyes under different trade names. Some examples are given below
Manufacturer ProductsTrade name
Indokem Incomine
Hindustan Ciba Geigy Chlorantine, Solophenyl
Clariant Solar
Atul products Atul Direct
ICI ChlorazoI
However, there is a German ban on the use of certain azo-dyes that has become effective since 1996.
Other countries in Europe now have similar legislations in force and thus the use of direct dyes is now seyerely restricted.
This is because many of the direct dyes are based on harmful amines such as benzidine, etc. It is therefore absolutely
essential that the eco-friendliness of direct dyes be ascertained before using them in regular bulk dyeing of textiles. To
facilitate this official lists of the red-listed direct dyes are available.

3. The structure of a direct dye is shown below
Note the-N=N-(azo) and the –SO3Na groups

4. Properties of Direct Dyes
1. Direct dyes are soluble in water and have affinity for cellulose and protein fibres, especially wool.
2. Chemically the dyes are represented as Sodium salts of Sulphonic acid (DSO3Na).
3. When the dyes are dissolved in water, the dye molecules get dissociated into ions (DSO3- and Na+).
During dyeing, the textile material absorbs the coloured anions from the dye solution. This is followed by the diffusion of
the molecules into the fibre where they are finally retained or “anchored” by means of physical forces
4. Owing to their water solubility, the dyes possess poor wash fastness.
Light ness, however, is poor to moderate, even good in some cases.
4. The fastness properties of these dyes can be improved slightly by means of certain after treatments.

5. General Properties of Direct dyes
• water-soluble compounds
• cheap and easily applied
• can yield bright colours
• Washfastness is poor may be improved by after treatment.
Disadvantages of Direct dyes
• Few Direct Dyes have low light fastness.
• Many Direct Dyes are bland and dull in color.
• Direct Dyes provide duller color than reactive dyes.
• The wash fastness quality is also low.

6. Classification of Direct Dyes
The direct cotton dyes are classified into the following groups depending on the effect of electrolyte and temperature.
Class A :Self-leveling dyes
While using these dyes, the dyeing may be uneven in the initial stages but they get levelled on prolonged dyeing because of
better migration properties. These dyes do not require salt for their exhaustion.
Class B: Salt-controllable dyes
These dyes do not migrate well and require salt addition for increased exhaustion. If uneven dyeing takes place initially, it is
very difficult to correct the dyeing.
Class C :Temperature-controllable dyes
Similar to the Class B dyes, the levelling properties of these dyes are poor. They are also sensitive to salt and their
exhaustion cannot be controlled with salt alone. These dyes require their exhaustion to be controlled by controlled rise of
dye bath temperature.
When more than one dyestuff is required to match a given shade, dyes belonging to the same class should be preferred.

7. Mechanism of Dyeing
When cotton is immersed in a solution of a direct dye, the following mechanism takes place.
1. Fibre swelling in the liquor
2. Adsorption of the dye at the fibre surface
3. Diffusion of dye molecules into the interior of the fibre.
4. Fixation of the dye molecules at appropriate places in the fibre structure by means of weak van der Waals’ forces
and hydrogen bonds

8. The phenomenon of adsorption and diffusion can be illustrated by microscopic views at different stages of dyeing of
cotton as shown in Figure (Diffusion of dye into fibre with time )
Undyed cotton Cotton dyed at 100°C Cotton dyed of 100°C (1 day)
(few minutes)
The cause for the affinity of direct dyes to cotton cellulose results from hydrogén -bond like linkages with the
hydroxylic groups of cellulose (proton donors) with the delocalisable pie- electron system of dye (proton acceptor). It
is also held that the affinity of the dye is connected with linearity and coplanarity of the dye molecule. The affinity is
also associated with a system of alternate single and double bonds (conjugated double bonds).Oher interactions such
as van der Waals forces also play a part

9. The effective factors
 Electrolytes  to promote exhaustion of direct dyes
 Temperature  Increase of temperature decrease of amount of dye adsorb
 liquor ratio  Dyeing in short-liquor ratio
 pH value  applied from a neutral solution

10. Effect of Electrolytes
It has been observed that the addition of salt in dyeing with direct dyes promotes exhaustion.
However the effect varies considerably from one dyestuff to another.
When cellulose fibres are immersed in water, they acquire a small negative charge known as "zeta potential“
In an aqueous bath containing both fibre and direct dye, the Latter being anionic will be repelled by the negative surface
charge on the fibre. little exhaustion will therefore take place.
When an electrolyte such as sodium chloride is now added to the bath, it ionizes into sodium cations and chloride anions.
The sodium cations neutralise or reduce the negative charge on the fibre surface and the dye anions in the bath, repelled
by the chloride anions move to the fibre where they are adsorbed. The dye anions are much larger than the chloride
anions, but they have a great substantivity for the cellulose fibre therefore quickly absorbed by the almost neutral fibre
surface.
Once adsorption of dye has occurred, the other steps of dyeing, namely diffusion and fixation take place subsequently.
Thus, in the dyeing of cellulose fibres, electrolytes enhance the exhaustion of direct dyes to a considerable extent.
However, the exhaustion effected by electrolytes varies from dye to dye, a stated
the variations in the extent of exhaustion of various dyestuffs in the presence of have been attributed to the presence of
the sulphonic acid groups in the dye molecule. Effectiveness of salt in promoting exhaustion varies directly with the number
of sulphonic acid groups. Thus greater the number of sulphonic acid groups in a dye, greater the effect of the electrolyte

11. Effect of Temperature
The amount of dye taken up by the fibre (while dyeing a cellulosic fibre with direct dye) depends on the temperature of
dyeing.
The rate of dyeing increases with rise in temperature i.e. dyeing takes place slowly at lower temperature and the dyestuff
rushes on to the fibre with the increase in temperature.
It is therefore imperative for the results to be uneven, when dyeing is carried out at higher temperature.
In the light of above facts, it is concluded that dyeing should start at room temperature, and then the temperature is
raised gradually to the maximum dyeing temperature.
As the temperature rises, the rate at which equilibrium is attained increases until it reaches the maximum.
Affinity however decreases with further increase in temperature.

12. Effect of Liquor Ratio
Figure shows the variation of dye uptake (g/kg of fibre) with changing
dye hath concentration.
It can be seen that as the initial concentration of the dye in the dye
bath increases (or as the material to liquor ratio decreases), the dye
uptake also increases.
Therefore it follows that a deeper shade is obtained by dyeing a 1%
shade at a material to liquor ratio of 1:10 than by dyeing a 1% shade
at a ratio of 1:40.
Therefore deeper shades can be obtained by dyeing given shade
percentages at low ML ratios.
It is therefore essential to keep this ratio constant throughout the
dyeing and also while dyeing many lots of the same shade.

13. Application Procedures :
The dyeing process basically involves two steps:
1. Preparation of the dye bath
2. Dyeing
3. Dye-fixing treatment
Preparation of the Dye bath
The dye is dissolved by pasting it with a small amount of water and soda ash(Na2CO3)(if required).
Bolling water should then be added to the paste with constant stirring.
If pasting is not done prior to the addition of boiling water, the dye may form into lumps, and this turn will result in
speckeled dyeing.
Stock solutions(i.e, concentrated olutions)are usually prepared and the required volume of these lutions is tken for
dyeing.

14. Typical recipe for Class A (self-levelling) type of direct dye
Direct dyestuff  X% (on weight of fibre or owf)
Soda ash  0.5 to 1% (owf)
Common salt  5% (owf) for light shades;
10% (owf)-medium shades;
20% (owf) -dark shades
Temperature  Boil
Dyeing Time  45 to 60 minutes

15. Grey Cotton Fabric
Grey Preparation – Desizing, Scouring and Bleaching
Dye material at 40C for 10-15 min
Direct dye solution at
40C + 0.5-1% Na2CO3 as
required
While dyeing, raise dye bath to boil @ 2C/min
Continue dyeing at the boil for 45-60 min
Steam or other
means of heating
NaCl (5 to 20%) is added in
two instalments over 10-15
min
Drain (drop) the bath
Treat in the dye-fixing bath @ 50-60C for 15-20 min
Cationic dye-
fixing agent
1 to 2 %
Squeeze the fabric and dry it
Direct Dyes on Cotton

16. The dye bath is set with the required volume of the stock solution of the dye, 0.5 to soda ash and water to make up
the desired M:L ratio. The well prepared material (desized, scoured and bleached) is entered in the dye bath at 40
degree c and dyeing is carried out for 15-20 minutes. The prescribed quantity of common salt is added in an even
number of installments at intervals of 10-15 minutes. Common salt is generally preferred to Glauber’s salt for reasons
of economy.(Glauber’s salt being anhydrous requires the use of larger quantity). The quantities of salt used are
dependent on the shade being dyed and varies between 5-20@ owf for light to heavy shades.
The temperature od the dye bath is slowly raised to the boil ( for the recommended maximum dyeing temperature).
The dyeing is continued at this temperature, for a period of 45-60 minutes. The material is then allowed to remain in a
cooling bath for 15-20 minutesfor obtaining better exhaustions. Finally, the goods are removed from the dye solution,
squeezed/hydro extracted and dried. An after-treatment in a solution of a dye- fixing agent often proceeds the drying
step.
For Classes B and C the procedure is modified according to the dye used.
m/c like winch and jigger for fabric and hank dyeing m/c for yarn are generally used for dyeing of cotton goods with
direct dyes. It is however not customary to use direct dye on cone/cheese dyeing for yarn padding mangles for fabric
because of tailing problems associated their their high exhaustion property.

17. After Treatments of direct dyes
After treatment methods :increase the molecular weight of the dye
Methods:
After-treatment with metallic salt
After-treatment with Formaldehyde
Coupling with diazonium salt
After treatment with Potassium bichromate
Cationic fixing agents

18. After-treatment of Direct Dyed Goods
The reasons for the popularity of direct dyes in the dyeing of cellulosic fibres are the low cost of dyeing and the simple
dyeing procedure. As a matter of fact, these dyes do not possess adequate washing fastness properties and most of
them fade in light. The use of direct dyes therefore becomes undesirable for material that will subsequently be washed
frequently or continuously exposed to light.
A number of methods has been devised to improve the fastness properties of direct dyes. These after-treatments are
based on principles like increasing the molecular weight of the dye and thus decreasing their solubility in water after
dyeing. All the direct dyes, however, are not capable of such after-treatment since, in many cases the colour of the final
product changes i.e. it tends to become duller and sometimes the hue too is altered.

19. Some after-treatments that have been in use are summarised below
1. Treatment with metallic salts
(a) Treatment with copper salts
Certain dyestuffs are capable of reacting with copper and showing remarkable improvement in light fastness
properties. The dyed material is treated in a bath containing :
Copper Sulphate (owf) 0.5-2% on weight of the fibre (owf)
Acetic acid (30%) (owf) 0.5-2% (owf)
The actual concentrations of the chemicals used depend on the depth of shade of the dyeing.
The treatment is carried out at 80°C for 30-45 minutes.
The material is then rinsed and dried.
Commercial products such as Cuperantine, Cuprofix, Cuprophenyl, etc. have also been used instead of the above
chemicals.

20. (b) Treatment with chromium compounds
Chromium atoms can be introduced in the molecular structure of certain direct dyes, resulting in more complex structures
than those with copper compounds and hence provides a way of improving the washing fastness properties of the dyes.
This treatment however does not alter their light fastness properties.
The dyed goods are treated in a bath containing :
Potassium dichromate (K2Cr2O7) 2-3% (owf)
Acetic acid (30%) (CH3COOH) 2-5% (owf)
The treatment is for 30 minutes at the boil.
The goods are then rinsed and dried.
A combination of the treatments with copper and chromium compounds can be employed to get improvements in washing
as well as light fastness properties.
The dyed goods are worked in a bath containing the following:
Potassium dichromate 0.5-2% (owf)
Copper sulphate 0.5-2% (owf)
Acetic acid (30%) 1-5% (owf)
The treatment is for 30 minutes at 80°C followed by rinsing and drying.

21. 2. Treatment with formaldehyde
Increasing the relative size of the dye molecules of some direct dyes can also be achieved by treatment with formaldehyde,
thereby yielding improved washing fastness properties.
The dyed goods are treated in a bath containing:
Formaldehyde (40%) 2-3% (owf)
Acetic acid (30%) 1% (owf)
The treatment is carried out at 60-70°C for 30 minutes followed by rinsing and drying.

22. Special emphasis on cationic dye fixing agents:
3. Treatment with cationic dye fixing agents
Different types of dye-fixing agents are available for use in the after-treatment of direct dyed goods to bring about
improved washing fastness proportion; the light fastness, however, is impaired. These products are capable of ionising
into long cations and small anions. On the other hand, direct dyes lonise into long anions and short cations.
Thus when solutions of these two are mixed, bigger complexes are formed as shown below.
The mobility of the complex is much reduced, thus the wet fastness of the dyed material is improved.
The treatment of the dyed goods with 1-2% (owf) of cationic dye-fixing agent is carried out at recommended
temperature (varying for different products) for half an hour and this is followed by rinsing and drying.

23. 4. Topping with basic dyes
Basic dyes do not have any affinity for cellulosic fibres. But special methods have been devised to apply them to cellulosic
material as bright shades are obtained. The process involves the use of certain mordants( mordant or dye fixative is a
substance used to set (i.e. bind) dyes on fabrics by forming a coordination complex with the dye, which then attaches to
the fabric (or tissue).), like tannic acid, which act as a bridge between the dye and the fibre as they have affinity for both
cellulose and basic dyes .
With direct dyed material the direct dye acts as the mordant and forms a complex when the material is treated with a
solution of a basic dye. Such after-treatment of direct dyed material with basic dyes is generally referred to as "topping
with basic dyes".
Theoretically any direct dye can be after treated with basic dyes, but as topping is carried out in an acidic medium, direct
dyes sensitive to acid (e.g. Congo Red) should not be subjected to this treatment.
Usually, for topping, a basic dye of similar hue as the direct dye is used. A very small quantity of basic dye say of the
order 0.1-0.3% is sufficient to produce brilliant shades on cotton dyed with direct dyes
Topping with basic dyes is carried out in a cold bath containing a definite portion (half or even quarter) of the dye solution
and the temperature is raised gradually to 60°C, and then adding the remaining dye portions at regular intervals. A small
quantity of acetic acid (0.5 to 2%) is added to the dye bath for obtaining uniform shades. When all the basic dye has been
added the treatment is continued at the same temperature for a further 45-60 minutes. This is followed by rinsing and
drying.
For obtaining very fast dyeing, the direct dyed goods are treated with 2-4% tannic acid in a hot bath for 20-30 minutes
followed by squeezing, and treatment with tartar emetic (1-2%) in a cold bath for 20-30 minutes. The tannic acid treated
cotton has high affinity for basic dyes. Topping is then carried out as explained above.

25. 5. Diazotisation and development of direct dyed goods
As discussed earlier, it is possible to enhance the wet resistance of water soluble dyes by increasing the size of their dye
molecules. Some of the direct dyes contain free amine (-NH2) groups, which can be diazotised by chemical reaction with
nitrous acid (HNO3). The resulting diazonium salt is then treated with a coupling component (Naphthol), which results in
the formation of an azo (-N=N-) group. The size of the direct dye molecule is now greatly increased and it will show good
resistance to wet treatments.
The chemical reactions are shown below.
Since the azo group itself is a
chromophore, the colour of the
diazotised and developed direct
dyed material will change. Different
colours can thus be produced by
using different coupling
components.
In practice the direct dyed material is
treated in the cold for 30 minutes in
a bath containg
NaNO2(Sodium Nitrite) = 1-3%(owf)
HCI-5-10% (owf).The goods are
rinsed and developed with a
solution of the coupling component
for 15-20 minutes. The material is
then washed, soaped and dried.

26. Stripping of Direct Dyes
In case of uneven dyeing, patchy dyeing and also when the shade is unacceptably deeper than desired, the dyes may
be required to be stripped.
Almost all direct dyes can be stripped with a hot, dilute solution of caustic soda and sodium
hydrosulphite(Na2S2O4).
However, it may not be possible to strip all dyes to a complete white, but the treatment may be good enough for the
correction of faulty dyeing.