Azo dyes

Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl. They are a commercially important family of azo compounds, i.e. compounds containing the linkage C-N=N-C. Azo dyes are widely used to treat textiles, leather articles, and some foods.

1. Azo Dyes

2. Submitted to MD. raijul islam SUBMITTED BY Arafat Islam Pranto ID:172-010-0-155 2

3. What is azo dyes? The dyes containing azo group (-N=N) in chemical constitution ( nucleous ) are known as azo dyes. 3

4. EXAMPLE OF AZO DYES Blue Br Yellow 2G Yellow 4G Orange 2R Reactive Red Reactive violet 4

5. CLASSIFICATION OF AZO DYES AZO DYES are further classified according to presence of azo group (-N=N) In the chemical constitution . A. Mono Azo Dyes B . Bis (Dis) Azo Dyes C . Tris Azo Dyes D . Tetra Azo Dyes E . Poly Azo dyes 5

6. PROPERTIES OF AZO DYES Azo dyes are insoluble in water. They are not ready made dyes. The colour is formed in the fibre by two components usually referred to as “ napthols ” and “bases . The dyed goods have very good to excellent light fastness . The dyed goods have good washing fastness . The dyed goods suffer from poor rubbing fastness and this can be overcome by proper soaping treatment . 6

7. BOND IN AZO COMPOUNDS 7

8. Mechanism of Dyeing 8 The steps involved in dyeing cotton is stated below - Impregnation with solution of napthol . This step is called “ naptholation ” Development of the chromophore ( azo group). This is the “ diazotisation ” step Development of coloured compound. This is the “coupling” step Removal of the surface colour from the dyed fibre . This step is called “soaping”

9. Napthols are insoluble in water They can be dissolved in water by converting them into sodium compounds, which are soluble in water For this caustic soda is used Napthol – OH + NaOH Napthol – ONa ( Insoluble in water ) (Sodium salt of napthol – soluble in hot water ) Treat the material with the napthol solution in room temperature for 20 – 30 minutes. 9

10. Time of impregnation • Temperature – Affinity of napthol is more at lower temperature and decreases with increase in temperature – The best temperature of impregnation is 20 – 250C Addition of common salt / glauber salt. 10

11. Coupling or Development The naptholated material is entered and developed in the base solution for 15 – 20 minutes at room temperature This process is called “coupling” or “developing ” The pH should be maintained 5 to 6 during this step Napthol – ONa + Base – N=N- Cl Napthol + HCl 11 N=N-Base ONa

12. SOAPING 12 After dyeing, the material is rinsed in cold water to which 1 g/l HCl is added and then it is soaped at the boil for 30 minutes to remove unfixed dye. • The soaping process is important to increase the rubbing fastness of the dye

13. Fastness properties of Azo dyes Azoic dyes have good to very good to excellent light fastness which is attributed to the very stable electron arrangement of the dyes Their washing fastness is also very good due to the fact that azoic dyes are insoluble in water Azoic dye suffer from poor rubbing fastness due to the formation of insoluble azoic dye on the surface of the fibre 13

14. Advanttage and disadvantage 14

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