Direct Dye
Direct Dyes
Direct Dye is a class of dyestuffs that are applied directly to the substrate in a neutral or
alkaline condition. Direct Dyes are molecules that adhere to the fabric molecules without
help of any auxiliaries chemicals. Direct dyes are also called Substantive dyes because of
their excellent substantively for cellulose textile materials like cotton and viscose rayon.
Direct dyes are sodium salt of sulphonic acid and most of them contain an azo group as
the main chromophore. Direct dyes are anionic in nature. They have substantivity to
cellulosic fibers when applied from an aquous dyebath containing an electrolyte, either
sodium chloride (NaCl) or sodium sulfate (Na2SO4).
Properties of direct dyes
1. Direct dyes are water solube
2. They are anionic in nature
3. They required no auxiliaries
4. They required electrolyte to increase the exhaustion
5. Dyeing process is carried out in neutral or slightly alkaline condition.
6. They can be applied for cellulosic as well as protein fibers
7. Fastness properties are not good compared to reactive dyes.
8. Fastness properties can be improved by aftertreatment
9. Comparatively cheap in price
10. It is not widely used as compared to reactive dyes.
11. Direct dyes are used for dyeing cheap goods like local products.
Affinity of direct dye
They have an affinity for a wide variety of fibers such as cotton, viscose, silk, jute, linen
etc.. They do not make any permanent chemical bond with the cellulosic fibers but are
attached to it via very week hydrogen bonding as well as vander waals forces. Their flat
shape and their length enable them to lie along-side cellulose fibers and maximize the
Van-der-Waals, dipole and hydrogen bonds.
Fastness properties of direct dyes
Generally these dyes are used where high wash fastness is not required.
Wash Fastness : Poor unless treated with suitable dye fixing agent or
fastness improving agent.
Light Fastness : Poor
Rubbing Fastness : Moderate to Poor
Wash Fastness : Poor
Types of Direct Dyes
The SDC (Society of dyers and colorists) classification of direct dyes is follows
I. Class A: Dyes that are self-leveling, i.e. dyes of good migration or leveling properties.
Direct Dye
II. Class B: Dyes that are not self-leveling, but which can be controlled by addition of
salt to give level results; they are described as salt-controllable.
III. Class C: Dyes that are not self-leveling and which are highly sensitive to salt, the
exhaustion of these dyes cannot adequately be controlled by addition of salt alone and
they require additional control by temperature; they are described as temperature-
controllable.
Recipe for dyeing cotton fabric with direct dye
Direct dye : 2% (OWF)
Glauber salt : 20 gm/l
Soda Ash : 2 gm/l (if alkaline condition required)
Wetting agent : 1 gm/l
Sequestering agent : 1 gm/l
PH : Neutral or slightly alkaline
Temperature : 60 – 700c or sometimes near at Boil.
Time : 30 – 60 min.
M: L : 1:10
Dyeing Procedure
After completing scouring or scouring and bleaching add required amount of water in the
machine. Then add required amount of dye, salt, soda and auxiliaries into the liquor.
Raise the temperature up to 900c. Run the machine in this temperature for 60 min. Then
drain the liquor and wash the fabric with water and detergent if necessary.
Electrolyte and its function
The most common sources of sodium ion are sodium chloride, NaCl (Common salt);
anhydrous sodium sulfate, Na2SO4 and glauber’s salt, sodium sulfate decahydrate,
Na2SO4.10H2O. one gram of sodium chloride contains the same amount of sodium ion as
1.22 grams of sodium sulfate or 2.78 grams of glauber’s salt. It should be clear that the
name glauber’s salt should not be used to include both anhydrous sodium sulfate and
sodium sulfate decahydrate, for this could easily lead to serious errors in making up
dyebaths.
Sodium sulfate and sodium chloride can be used interchangeably. But the more
economical sodium chloride has a much higher tendency to corrode stainless steel under
the high temperature conditions possible in package or jet dyeing machines. Sodium
sulfate is preferred for these applications despite its slightly higher cost.
Direct Dye
How much of the chosen salt should be added to a dyebath?
The salt is required just enough to ensure good exhaustion, but not enough to precipitate
the dye. The amount will vary with the depth of shade being dyed, the weight of the
goods and the liquor to materials ratio.
Traditionally, both the dye and the salt requirement of a dyeing have been expressed as
percentages on the weight of the goods being dyed (% owf). But the amount of salt must
be increased with increasing dye concentration, and also increase with increasing liquor
ratio to ensure the same percentage exhaustion. If the amount of salt is pegged to both the
percentage of dye being used and the amount of liquor, these problems will both be taken
care of. This can be expressed in terms of a simple equation.
Sulfonate groups in direct dye
Vital substituent’s of direct dyes are the sodium sulphonate groups, -SO3Na, attached to
the benzene or naphthalene rings. It is the sulfonate group which gives most direct dyes
the property of water solubility so imperative to dyeing. When dyes containing these
groups are placed in water they ionize into sodium cations and colored sulfonate anions
which are positively and negatively charged respectively- i.e. Na+ and –SO3- both of
which are extremely attractive to water and become heavily hydrated (surrounded) by
water molecules. There are usually from two to four but can be from one to five sulfonate
groups as an intrinsic part of any direct dye molecule. The number of these groups is a
major factor in determining how strongly the dye is attracted to water. This roughly
translate into how soluble the dye is, and how much electrolyte is needed to force the dye
out of the water and onto the fiber during dyeing. Most direct dye molecules could be
described by a general formula:
Dye-(SO3Na), where n= 1 to 5.
Other substituent chemical groups to be found in direct dyes include the hydroxyl group,
the amino group, and the amido group, -NH-CO-. An amino group is necessary for
aftertreatment by diazotization and coupling.
Chromophore groups in direct dyes
Dyes are often subdivided by organic chemists according to the chemical nature of the
chromophore, which is the group (or groups) within the molecule largely responsible for
the color of the product. These chromophores can be azo, stilbene, oxazine or
phthalocyanine..
Direct Dye
Fig.: Reaction of direct dye with cotton fiber
After Treatment of Direct Dyed Material
The wet fastness properties (particularly washing, water and perspiration) of virtually all
dyeing of direct dyes are inadequate for many end uses but notable improvements can be
brought about by after treatments.
Diazotisation and development
Metal salt treatments
Cationic fixing agents
Formaldehyde treatment
Cross linking agents and resin treatments
Cationic Fixing agent
Cationic fixing agents are organic chemicals with large molecules which dissolve in
water, dissociating into a large positively charged or cationic fragment and a small
negative ion such as a chloride ion. They may be resinous derivatives of cyanamide or
quaternary ammonium compounds with long hydrocarbon chains.
The positive ion is attracted to the direct dye anion to form a large, complex salt molecule
with very low solubility and in consequence, much improved wet fastness. Once the
number of cations has exceeded that of the dye anions, no more need be added. The
amount depends o the depth of shade. Potentials problems include shade changes and
lowered light fastness. Wet fastness can be markedly improved.
Direct Dye
Application of Direct Dyes
Direct dyes are usually applied with the addition of electrolyte at or near the boil in the
machines capable of running at atmospheric pressure .But in HTHP dyeing machines it is
carried out at temperatures above the boil in case of pure as well as blended yarns.
An addition of alkali, usually sodium carbonate, may be made with acid-sensitive direct
dyes and with hard water as well as to enhance the dye solubilisation. When cellulose is
immersed in a solution of a direct dye it absorbs dye from the solution until equilibrium is
attained, and at this stage most of the dye is taken up by the fibre. The rate of absorption
and equilibrium exhaustion vary from dye to dye. The substantivity of the dye for
cellulose is the proportion of the dye absorbed by the fibre compared with that remaining
in the dyebath.
