Dyeing with Vat Dyes

Demonstration Experiment on Video

Dyeing with Vat Dyes

Objectives: Indanthren Yellow G, Light and Washing Fastness

Peter Keusch

German version

Material and chemicals:
cotton yarn
Indanthren Yellow G (BASF)
sodium hydrosulfite (sodium dithionite)
sodium perborate
calcined soda (sodium carbonate anhydrous)
sodium hydroxide solution (32.5 %)

Apparatus and glass wares:
hotplate
thermometer

glas stirring rod
metal forceps
crystallizing dish d = 12 cm
2 beakers 500 mL
beaker 50 mL
2 snap-cap vials 10 mL

Hazards and safety precautions:

Sodium hydroxide solution is corrosive! Contact with skin can cause irritation or severe burns and scarring with greater exposures. Swallowing may cause severe burns of mouth, throat, and stomach.

Sodium dithionite and sodium perborate are harmful if swallowed or ingested, and through skin contact. Skin and eye irritant.

Safety goggles and protective gloves must be worn.

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.

References:
Demonstration Experiment on Video Dyeing with Substantive Dyes
Vat Dye

Index of Lecture Experiments