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Direct dyes are defined as anionic dyes with substantivity for
cellulosic fibres, normally applied from an aqueous dyebath containing an
electrolyte, either sodium chloride (NaCl) or sodium sulfate (Na2SO4). The
dyeing process with direct dyes is very simple, Direct dyeing is normally
carried out in a neutral or slight alkaline dyebath, at or near boiling
point, but a separate after treatment such as cationic dye fixing, to
enhance wet fastness has been necessary for most direct dyeing.
Chemically they are salts of complex sulfonic acids. More than 75% of all
direct dyes are unmetallised azo structures, great majority of them are
disazo or polyazo types. Their ionic nature is anionic and are soluble in
water. 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.
Direct dyes are widely used on cotton, paper, leather, wool, silk and
nylon. They are also used as pH indicators and as biological stains.
How to solve the wash fastness problem for Direct Dyes
Wash fastness quality is poor unless treated
with suitable dye fixing agent and/or fastness improving finishing agent:-
In most Direct Dyes, the wash fastness quality is poor. After every dyeing,
these dyes bleed a little with every washing. This results in losing
brightness and also damage other clothes which are washed in the same load.
So, there is a process i.e. 'special after treatment' can be used to solve
this problem. For accomplishing this process, a cationic bulking agent i.e.
RETAYNE is used. This agent glues the dyes into the fibers.
Disadvantages of Direct Dyes
- Many Direct Dyes are bland and dull in color.
- Direct Dyes provide duller color than the color provided by the fiber
- The wash fastness quality is also low.
- Few Direct Dyes have low light fastness.
Major types of Direct Dyes
There are two major types of
Anionic Direct Dyes
Anionic Direct Dyes are used for coloring papers, controlling tint and
shade and correction of two sided paper color. The main structural features
of the Anionic Direct Dyes are: the planar vary from each other, the
structural single and double bonds are very much extended, and contains one
or more than one sulfonate groups. These sulfonate groups make them soluble
in the water.
Cationic Direct Dyes
The main structural features of cationic direct dyes are :
- the structural single and double bondings are extended.
- the molecular structure is planar
- positive charges are more than the negative charges