Direct Dyes

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.

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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.

• Many Direct Dyes are bland and dull in color.
• Direct Dyes provide duller color than the color provided by the fiber reactive dyes.
• The wash fastness quality is also low.
• Few Direct Dyes have low light fastness.

There are two major types of Direct Dyes:
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