Dyeing with Vat Dyes

Objectives: Indanthren Yellow G, Light and Washing Fastness

Peter Keusch
 

Experimental procedure: 0.3 g of Indanthren Yellow G, 3 mL of sodium hydroxide soultion (32.5 %) and 1 g of sodium ditionite are dissolved in 200 mL of dist. water heated to 50°C. After 10 minutes 10 g of cotton yarn are added to the solution. The dyeing takes place at about 50°C. After 30 minutes the yarn is washed. The washed substrate is placed in a beaker containing 200 mL of dist. water in which 0.5 g sodium perborate had been dissolved. The temperature of the solution is 50°C. After 15 minutes the impregnated yarn is washed with cold water. Now the textile fiber is soaped in a solution of soap (0.5 g) and calcined soda (0.2 g) in 200 mL dist. water. The yarn is washed again. Result:     Video clip (Download RealPlayer .rm file)

Discussion and background:

"Vat dyeing" means dyeing in a bucket or vat. Vat dyes are insoluble in water and incapable of dyeing fibers directly. Reduction in alkaline medium produces the water soluble alkali metal salt of the dye. In the reduced form the dye is applied to impregnate the fiber. The dye absorbed on the fiber is converted to its original water insoluble form by a subsequent reoxidation. Vat dyes have excellent wash and light fastness properties.

In the experiment described above the anthraquinone dye Indanthren Yellow is reduced with sodium hydrosulfite (sodium dithionite). The reduction occurs at the keto groups (C=O) in the vat dye molecule. These groups are reduced to the acid form (C-OH) (1) and, in the presence of excess alkalinity, form the soluble sodium salt (C-ONa).

 

Perborate is used as oxidising agent to regenerate and fix the insoluble dye on the fiber. Calcined soda (sodium carbonate), also known as washing soda, helps the dye bond with the fibers.