Sustainable developments in printing cotton fabric with indigo

Indigo is insoluble in water and most dyeing solvents. Hence, its use in textile coloration is achieved by alkaline reduction to a soluble leuco-compound and then applying this solution followed by oxidation. Printing with indigo on cotton commonly employs sodium hydroxide (caustic soda) as the alkali and sodium hydrosulphite (hydros) as the reducing agent. Caustic soda poses effluent disposal problems. Hydros rapidly loses potency by reacting with atmospheric oxygen during the printing / padding process. This causes premature oxidation of print paste and necessitates constant monitoring. Further, the degradation products of hydros pose effluent hazards. Hence, there is a constant search to find better alternatives in terms of chemical stability and effluent management. In the above context, the present research used thiourea dioxide (TUD) as a more stable reducing agent and sodium carbonate as a milder alkali. The resulting effect on printing cotton fabric with both natural and synthetic indigo has been evaluated. The shades obtained using synthetic indigo were about 20% darker than those obtained using natural indigo. When caustic soda was used as the alkali, a steaming time of 60 seconds was found to be ideal, for both natural and synthetic indigo. However, when sodium carbonate was used darkest shades were obtained at steaming times of 2 and 6 minutes for natural and synthetic indigo respectively. The printed samples showed good colour fastness ratings when evaluated according to Australian standards. The study concludes that indigo can be printed on cotton fabric using chemicals with a lesser environmental impact