Dyeing With Direct Dyes

• Cheap Dyes

• Very easily dyed onto cellulosic fabrics

• Defination: Anionic dyes which have affinity for cellulosic fibre and are generally dyed from water in the presence of salt.

• Name is ‘Direct Dye’ as dye can be directly applied on fibres/fabrics without any treatment.

IMPORTANT FACTORS

1. DYES

2. PROCESS

3. AFTERTREATMENTS

4. DYE-FIBRE INTERACTIONS

GROUPS THAT SHOW SUBSTANTIVITY

1 OR MORE GROUP CAN BE PRESENT

N = N Azo

Diphenyl

Stilbene

CH=CH

J -acid

NH2 NaO3S

OH

Gamma-acid NaO3S

OHNH2

ThiazoleS

HC

HCN

CH

STRUCTURE OF DYES

1. CONGO RED

This dye is no longer used as (a) at pH 3 or below it gives blue colour

(b) It is made from benzidene amine which is carcinogenic

Congo Red

N=N N=N

NH2

SO3Na

NH2

SO3Na

STRUCTURE OF DYES..contd

2. CHLORANTINE FAST GREEN BLL

Blue dye and yellow dye are connected via triazine ring to get a green dye

Chlorantine Fast Green BLL

OO

SO3Na

H2N NH

NO3S

NH

N N

N

NH

NH N =N OH

COONa

STRUCTURE OF DYES..contd

3. C.I. Direct Yellow 12

This dye can be applied on cotton, wool and silk.

C.I. Direct Yellow 12

H5C2O N=N CH=CH N=N OC2H5

SO3Na

SO3Na

STRUCTURE OF DYES..contd4. COPPER PHTHALOCYANINE

This is a general molecule which can be used as direct dye, acid dye or a reactive dye can be made based on them.

Copper Phthalocyanine

N

NaO3S

N

N

N

N

N

N

SO3Na

N-

-

Cu2+

CHROMOPHORES IN DIRECT DYES

1. Azo (90%)

2. Stilbene

3. Dioxane

4. Quinoline

5. Thiazole

6. AnthraquinoneAll direct dyes have a solubilising group Ex- SO3Na in Congo

Red and so they form anionic molecule in water

DYEING PROCESS

The requirements are –

1. DYE

2. AUXILLARY

3. pH

4. TEMPERATURE

5. MLR

METHODS FOR DYEING

1. BATCH PROCESS

2. SEMI-CONTINUOUS PROCESS

3. CONTINUOUS PROCESS

Only batch process is used for direct dyes as the dyes have a high affinity for fibre.

Dye 1 has high affinity for fibre.Dye 2 has high affinity for waterPartition coefficient = K= [D]f [D]sHigher the K, more is the dye taken up by the fibre, which is preferable for Batch Process.

• For continuous or semi-continuous process, dye should have very low affinity, because if the dye has high affinity it will be preferentially taken up which is called Tailing effect. • At high temperature, dye has low affinity as dyeing is exothermic process. So for direct dyes high temperature should be used.

AUXILLARY

• Salt is used as an auxillary in direct dyeing.

• Commercial dye 40-60%Dye + 40-60%

Deluent

• Here the deluent is salt / electrolyte. So some dyeing occurs even if extra salt is not added.

EFFECT OF ELECTROLYTE

• Zeta Potential• Dye is negatively charged DSO3Na <=> DSO3

- + Na+

• So there is repulsion between dye and fibre. • To overcome charge on surface or to suppress the

dissociation reaction of dye, salt is used.• Ex- NaCl, Na2SO4.10H2O (Glauber’s salt)

pH

• Normally neutral pH is used.

• In viscose rayon, if pH becomes alkaline, dye gets reduced and so lower shade is obtained.

• Optimum pH is 6-7’

TEMPERATURE

• For all direct dyes there is an optimum temperature

Example- C.I.Direct yellow 12 …….20oC

C.I.Direct Red 81………..60oC

C.I.Direct Yellow 28……100oC

• Many shades are obtained by mixing 2-3 dyes.

Dyeing of direct dye is carried out at 90-100oC for 1-2hrs

MATERIAL TO LIQUOR RATIO

• MLR is a function of the machine being used.

• Higher the MLR, lower is the dye uptake.

Machine MLR

Jigger 1:4

Winch 1:20

CLASSIFICATION OF DIRECT DYES

• Class A : Self levelling (20%)

• Class B : Salt controllable (40%)

• Class C : Salt & Temperature

Class A- Self leveling direct dyes

Dyes in this group have good leveling characteristics and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation. They may require relatively large amounts of slat to exhaust well.

Class B - Salt Controllable Dyes

These dyes have relatively poor leveling or migration characteristics. They can be batch dyed uniformly by controlled addition of electrolyte, usually after the dye bath has reached the dyeing temperature.

Class C - Salt and Temperature Controllable Dyes

These dyes show relatively poor leveling or migration and their substantivity increases very rapidly with increasing temperature. Their rate of dyeing is controlled by controlling the rate of rise of the dyebath temperature as well as controlling the salt addition. Some of these dyes require very little salt to show good exhaustion in light shades.

TESTS TO FIND CLASS OF DYE

1. Levelling or migration test

(i) 2.5g hank of cotton, 1%shade, 20%NaCl, M:L::1:30, boil, 30mins

(ii) Dyed hank + Undyed hank(2.5g), 10%NaCl, M:L::1:30, boil, 30mins

The 2 hanks are compared,

If almost equal shade Class A

If large difference Class B or Class C

2. Salt test

• 3 dye baths are dyed with 1% shade

Quantity of salt varied- 0.6%, 0.8%, 1%

MLR:1:30, 100oC, 30mins

Hanks squeezed and removed.

• Salt conc increased to 20% in all baths and 2nd set of hanks inserted.

If Set 1 lighter or equal to Set 2 Class B

If Set 1 darker than Set 2 Class C

AFTER TREATMENTS

• Direct dyes have W.F. of 2-3

L.F. of 3-4 generally

Hence, after treatment is required.

• Dye molecule size is low low wash fastness.

• If molecule size is increased, the fastness will improve.

AFTER TREATMENTS…contd.

1. Diazotisation and DevelopmentAmino group of dye can be diazotised and coupled.

NaNO2 + HCl HNO2 + NaCl

Additional azo group change in shade

D-NH2 + HNO2 + HCl --> D-N _

NCl + 2H2O-+

D-N ___ NCl + OH

OH

+ HClN=ND

Developer ( beta napthol)

AFTER TREATMENTS…contd.

• Developers used are-

Β-Napthol, m-phenylene diamine phenol, resorcinol.

• Dyes that are used in such way are called D&D dyed (Dyed & Developed dyes)

AFTER TREATMENTS…contd.

• If Dyes does not have –NH2 group, coupling is done with diazonium salt.

NH2

p-Nitro aniline

C.I. Azoic Diazo Component 37

NO2

O2N N N-Cl

Diazotised p-nitro aniline

+___ -

AFTER TREATMENTS…contd.

2. Treatment with Metal Salts

• CuSO4 or K2Cr2O7 or mixture of both.

• Improves the L.F. & to a extent W.F.also.

• Trade names of dyes- Cuprofix, Cuprophenyl, Cuprantine

AFTER TREATMENTS…contd.Dyes that have the following structure can be

treated with Cu.

OH groups at o,o’ position to azo ring or derivatives of

Treatment is 0.25-2% CuSO4 + 1% CH3COOH, 20-30min at 60oC

Treatment with metal salts

N=N N=N

OH OH O OCu_ _

OH COOH

N=N

OH

COOH

AFTER TREATMENTS…contd.

3. Formaldehyde Treatment

Given to those dyes which have amino group situated at p-position to azo group.

2-3% HCHO(30%)+ 1% CH3COOH(30%), 30min, 30oC

Formaldehyde treatmentNH2

H N

D

+ HCHO +

NH

D

NH

D

NH

H N

D

NH

NHCH2

2

22

2

2

2

AFTER TREATMENTS…contd.

4. Cationic Fixing AgentDifferent kinds-a) Quarternary ammonium compoundsb) Amino triazine condensatesc) Pyridinium derived basesAll have a quarternary NitrogenNowadays, Condensation product of dicyandiamide,

formaldehyde and ammonium chloride are most commonly used.

AFTER TREATMENTS…contd.

Bond is weak, so can withstand only few washes.

Sometimes reduces L.F.

+N O S Dye-

3