Introduction of Direct Dyes | Classification of Direct Dye | Application of Direct Dyes | Aftertreatment of Direct Dye

Direct Dye:
Direct Dye is a class of dyestuffs that are applied directly to the substrate in a neutral or alkaline bath. They produce full shades on cotton and linen without mordanting and can also be applied to rayon, silk, and wool. Direct dyes give bright shades but exhibit poor washfastness. Various aftertreatments are used to improve the washfastness of direct dyes, and such dyes are referred to as “aftertreated direct colors.”Direct Dyes are molecules that adhere to the fabric molecules without help from other chemicals. 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)..
Direct Dyes 
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 aftertreatment such as cationic dye fixing , to enhance wet fastness has been necessary for most direct dyeing .

Direct dyes are used on cotton, paper,leather, wool, silk and nylon. They are also used as pH indicators and as biological stains.

Chemicals Nature of Direct Dyes:

Chemically they are salts of complex sulfonic acids. 
Structure:-More than 75% of all direct dyes are unmetallised azo structures, great majority of them are disazo or polyazo types.

Ionic Nature:-Their ionic nature is anionic. 

Solubility:-They are soluble in water .

Affinity:-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. 

Types of Direct Dyes:

The SDC classification of direct dyes is follows

(1) Class A – dyes that are self-levelling, i.e. dyes of good migration or leveling properties.

(2) Class B – dyes that are not self-levelling, but which can be controlled by addition of salt to give level results; they are described as salt-controllable.

(3) Class C – dyes that are not self-levelling 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.

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.

Dyeing Method
The color is pasted well and dissolved in boiling water to get a lump free solution .An addition of 0.5–2 g l–1 sodium carbonate may be advantageous when applying dyes of only moderate solubility in full depths.
  • The dyebath is set at 40°C,
  • Raise to the boil at 2 degC min–1
  • Hold at the boil for 30–45 min, 
  • During hold add 10–15 g l–1 of sodium chloride or calcined Glauber’s salt. Light shades are dyed without or lesser addition of salt.
  • Improved yields can be achieved when applying full depths by cooling to 80°C at the end of the period at the boil, adding a further 5 g l–1 salt and rising to the boil again
  • Dye bath variables which must be considered for level dyeing,
1.Temperature of Dyeing and rate of heating
2.Electrolyte concentration and addition
4.Dye solubility
5.Use of leveling agent 

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 
Stripping:- Most direct dyes can be stripped of the use of stripping salts (Sodium Hydrosulphite) and/or by using a chlorine bleaching agent such as sodium hypochlorite, without harmful effects on the fibres.

Color fastness properties of Direct Dyed material: Generally these dyes are used where high wash fastness is not required.

Wash Fastness: Poor unless treated with suitable dye fixing agent and/or fastness improving finishing agent.

Light Fastness:-Good

Rubbing Fastness: Moderate to Good

Chemical Wash Fastness:- Poor
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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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