Dyeing with Direct dyes

Dyeing Method With Direct Dyes

 Direct Dyes

Direct Dyes are used in wide range of dyeing applications such as on Cotton, Silk and Viscose Fabrics. Direct dyes are Famous for their easy application, leveling property, good brightness and fastness. These dyes can be safely applied on Textiles and Leather.

PRE - TREATMENT

Good pre-treatment is essential if satisfactory dyeing is to be obtained. To get better depth and fair brightness, Grey Fabrics must be free from impurities like sizing materials and spinning oil by boiling the Grey with alkline or some times bleaching also carried out in case of pale shades.

METHOD OF DYEING

(1) DISSOLVING THE DYESTUFF

The dyestuff is pasted with cold water and dissolved by pouring 50 to 100 times its weight of Boiling water while stirring. The solution may be boiled to ensure complete dissolution. For Certain dyes, which are dyes from and alkline dye bath, it is preferable to add half its weight of soda ash while pasting with water.

(2) YARN DYEING

Set the Dye Bath at 50°-60° C temperature with dyes solution, salt and soda ash. Run for 15 Minutes. Gradually raise Temperature to 90°-95° C. Run for 30 to 45 minutes. In order to Overcome un level dyeing salt may be added in installments after about 15 minutes of initial dyeing prior to raising the dye bath temperature.

In the case of dyestuff which possess maximum affinity at temperature below boil, it is preferable to start dyeing at boil, dye for 15 to 20 minutes and then add salt in two installments to exhaust the bath and continue dyeing in a cooling bath (60° c) for 15 to 20 minutes.

 JIGGER DYEING

The dyeing on the Jigger is carried out as follows:-
First End : 1/2 quantity of Dyes solution, soda ash and 1/4 quantity of Salt at room Temperature. Second End : 1/2 quantity of Dyes solution, 1/4 quantity of Salt at 60°)c, raised to 80°c.Third End : 1/4 quantity of Salt.
Fourth End : 1/4 quantity of Salt.

The temperature is raised to raised to boil and four ends are given at boil. Finally the dye-liquor is Drained and further two ends are given in cold water.

AFTER TREATMENT

Fastness to wet treatments can be improved by after treatment with 1-2% Cationic Dye Fixing Agents at 40-50° c for minutes.
In case of some selected dyes, light fastness can be improved by after treatment with:-1-2% Copper Sulphate1-2% Actic (30%) at 80° c Temperature.

Direct Dyes

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).. 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

3.Time

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.

1. Diazotisation and development
2. Metal salt treatments
3. Cationic fixing agents
4. Formaldehyde treatment
5. Crosslinking 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

How to Identify a Direct Dye on a Substrate Dye class identification

How to test comparative strength of a cationic dye fixer

Azo Group A slide show

Chemistry of dyes

Direct Dyes

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).. 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

3.Time

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.

1. Diazotisation and development
2. Metal salt treatments
3. Cationic fixing agents
4. Formaldehyde treatment
5. Crosslinking 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

How to Identify a Direct Dye on a Substrate Dye class identification

How to test comparative strength of a cationic dye fixer

Azo Group A slide show

Chemistry of dyes

Quick and level dyeing dyeing with Direct dyes