DIRECT DYES

ð  Direct dyes are mainly applied on cellulosic fibres such as cotton, viscose rayon , cuprammonium rayon etc.

ð  Generally, these dyes are easily applied on cellulosic fibres and can be directly dyed from simple solution in water.

ð  Hence, these are called Direct Cotton Dyes.

ð  These dyes have an excellent substantivity for cellulose and therefore they are also called Substantive Dyes.

ð  Generally, all direct dyes possess at least two azo groups in their structure and therefore they are also called Diamine Dyes.

ð  The first manufactured direct dye was Congo Red.

PROPERTIES

ð  Direct dyes are soluble in water.

ð  Direct dyes are relatively inexpensive (cheap).

ð  Direct dyes are available in a full range of colours but are not high colour brilliance.

ð  Direct dyes show poor to moderate washing fastness.

ð  Direct dyes show poor to fairly good light fastness.

PRINCIPLE & MECHANISM

ð  When Direct Dye is dissolved in water, it ionizes giving D^-, dye anions and Na⁺, Sodium cations.

ð  These dyes are manufactured as sulphonic acids and converted generally into their sodium salts.

ð  The suphonic acids are less soluble in water than their sodium salts.

ð  Also, the sulphonic acids have less affinity for cellulose than their sodium salts.

ð  Therefore, if the sulphonic acids are not completely converted in to their salts some of the dye is wasted during their application.

ð  Therefore, while preparing a solution of a direct dye, it is customary to add a small amount of soda ash (sodium carbonate) to convert the remaining sulphonic acid into the corresponding sodium salt.

    

      

ð  Direct dye molecules contain some groups such as amino(-NH₂), which can get attached to cellulose macromolecule at the hydroxyl groups by weak hydrogen bonds.

ð  Electrolytes (Salts) :- It is found that when direct dyes are dyed on cellulosic fibres, the amount of dye taken up is considerably more when a salt such as sodium chloride or Glauber’s  salt is present in dye bath.

·         Therefore, the salt is called an exhausting agent.

·         This effect varies from dye to dye.

ð  Temperature:- The rate of dyeing increases with increasing temperature, i.e., dyeing takes place quickly at high temperature.

·         As a result uneven dyeing take place at high temperature.

·         From this it follows that for obtaining even dyeing, the dyeing process should be started at a low temperature and then the temperature of dye bath should be raised gradually.

ð  Material to Liquor Ration (M:L Ratio):- The amount of dye only depends on the percent shade and the quantity of the material to be dyed.

·         Dye uptake, which determines the depth of the final dyeing depends on the M:L ration, this ratio has to be kept constant throughout dyeing.

·         Generally, M:L ratio for cotton dyeing is 1:20.

CLASSIFICATION OF DIRECT DYES

Self Levelling Dyes (Group A)	Salt Controllable Dyes (Group B)	Temperature Controllable  Dyes (Group C)
Dyes belonging to this group migrate well and therefore have high levelling power.	These dyes have poor levelling properties.	These dyes have poor levelling properties.
When dyeing these dyes, dyeing may be uneven initially but continued dyeing levels out the shade.	Dye uptake is not uniform initially, it is very difficult to even out the shade	These dyes are highly sensitive to salt and their exhaustion on to the fibre adequately controlled by addition of salt alone
These do not need the addition of salt for exhausting.	Their exhaustion has to be brought by controlled addition of salt.	The exhaustion is controlled by the proper rate of raising the temperature.

APPLICATION

ð  Take the require amount of direct dye and dissolve it using cold or hot water with continuous stirring.

ð  Filter the dissolve dye to remove insoluble dye particles.

ð  Prepare dye solution as follows:-

1 : 25               -           M:L  Ratio

0.5 – 2.0 %      -           Soda ash

10.0 – 30. 0 % -           Glauber’s salt or Common salt

Required amount of dye solution

ð  Enter well wetted cloth in this dye solution at 50 – 60 ⁰C, carry out dyeing process at this temperature for 10 – 15 minutes.

ð  Then raise the temperature to boil and continue dyeing for 1hour.

ð  During dyeing salt is added in two parts.

ð  After completion of dyeing process, take out the fabric and  wash in hot and cold water.

AFTER TREATMENT

ð  To improve washing & light fastness of water some after treatment are also required,  which are as follows:-

·         0.5 – 20%              Copper Sulphate

0.5 – 20 %             Acetic acid (30%)

After dyeing fabric is treated in above bath at 80 ⁰C for 30 minutes.

·         2.0 – 3.0 %            Bichromate or chrom alum

2.0 – 5.0 %            Acetic acid (30%)

After dyeing fabric is treated in above bath at 80 ⁰C for 30 minutes.

·         0.5 – 2.0 %            Bichromate

0.5 – 2.0 %            Copper Sulphate

1.0 – 5.0 %            Acetic acid (30%)

After dyeing fabric is treated in above bath at 80 ⁰C for 30 minutes.

·         2.0 – 3.0 %            Formaldehyde

         1.0 %            Acetic acid (30%)

After dyeing fabric is treated in above bath at 65 ⁰C for 30 minutes.

·         Diazotisation Process

1.5 – 3.0 % Sodium Nitrate

5.0 – 7.5 % Hydrochloric acid (HCl)

After dyeing fabric is treated in above bath for 20 minutes. Then developed  in following bath

Ø  0.75 – 1.5 %          m – Toluene diamine

(Dissolve with half weight of soda ash)

Ø  0.75 – 1.5 %          β Naphthol

(Dissolve with half weight of caustic soad)

Ø  0.75 – 1.5 %          m – Phenylene diamine

(Dissolve with half weight of soda ash)